bai giang quan tri sx dv

CHNG I M U V QUN TR SN XUT V DCH VI. Khi nim v sn xut, v qun tr sn xut, lch s hnh thnh l thuyt qun tr sn xut. 1. Khi nim v sn xut : Sn xut l qu trnh to ra sn phm v dch v. S khc bit gia sn xut sn phm v to ra dch v Sn xut sn phm Qu trnh to ra sn phm vt cht C th d tr trc c Cn nhiu my mc thit b t tip xc vi khch hng Cn s vn ln khi thit lp Phn b sn phm khng b gii hn v a l nh gi cht lng sn phm n gin hn To ra dch v Khng to ra sn phm vt cht Khng th d tr trc c Cn nhiu nhn vin Thng xuyn tip xc vi khch hng Khng nht thit cn s vn ln C gii hn v a l khi phn phi dch v Kh nh gi cht lng dch v hn

2. Khi nim v qun tr sn xut : L qu trnh t chc phi hp v s dng cc yu t u vo nhm chuyn ha thnh kt qu u ra l sn phm v dch v vi chi ph thp nht v hiu qu cao nht 3. Lch s hnh thnh l thuyt QTSX Phn cng lao ng Tiu chun ha Khoa hc v qun tr ng c lao ng Dy chuyn sn xut Kim sot cht lng p dng computer PERT GERT MRP CAD MAP

CIM

1776 1800 1886 1890 1913 1924 38

57

65

70

75

80

85

90

II. Ni dung nghin cu mn hc QTSX v dch v Chng I Chng II Chng III Chng IV Chng V Chng VI Chng VII Chng VIII Chng IX : M u v qun tr sn xut vdch v : D bo trong qun tr sn xut v dch v : Qun tr tn kho : Hoch nh nhu cu vt t : Hoch nh vic s dng cc ngun lc trong doanh nghip : Hoch nh lch trnh sn xut : p dng l thuyt xp hng trong vic b tr nhn lc v phng tin : La chn a im ca x nghip : B tr mt bng

III. Ti liu tham kho cho mn hc - Qun tr sn xut v dch v L thuyt v bi tp ng Th Thanh Phng - Fundamentals of Operation Management Davis, Aquilano and Chase 1999

CHNG II D BO TRONG QUN TR SN XUTI. Khi nim v d bo, cc loi d bo, trnh t thc hin d bo 1. Khi nim v d bo : D bo l khoa hc ngh thut tin on trc cc hin tng trong tng lai cn c vo : Dy s liu ca cc thi k qu kh Kt qu phn tch cc nhn t nh hng n kt qu d bo Kinh nghim thc t 2. Cc loi d bo D bo di hn >3 nm Cn c vo thi on d D bo trung hn >3 thng 3 nm D bo ngn hn < 3 thng 3. Trnh t tin hnh d bo Bc 1 : Xc nh mc tiu d bo (mc tiu khc nhau, phng php khc nhau) Bc 2 : Xc nh thi on d bo Bc 3 : La chn phng php d bo Bc 4 : La chn i tng thu thp thng tin Bc 5 : Thu thp thng tin bng: Phng vn / Bng cu hi / Da vo i ng cng tc vin Marketing Bc 6 : X l thng tin Bc 7 : Xc nh xu hng d bo - Xu hng khuyn khch - Xu hng chu k - Xu hng thi v - Xu hng ngu nhin Bc 8 : Phn tch Tnh ton http://www.ebook.edu.vn Ra quyt nh v kt qu d bo 2 Cn c vo lnh vc d bo D bo kinh t

D bo cng h D bo nhu cu

II. CC PHNG PHP D BO THEO CHUI THI GIAN A. Cc phng php nh tnh 1. Phng php ly kin ca hi ng iu hnh (Hi ng bao gm cc chuyn gia gii nht trong lnh vc sn xut, marketing, cht lng, ti chnh, nhn s ) u im Nhc im - Nhanh - Khng hon ton khch quan - R - St vi tnh hnh 2. Phng php ly kin ca nhn vin bn hng cc khu vc (dng d bo v nhu cu sn phm, dch v) Nhc im : thng c hai xu hng u im - R - Lc quan qu - St vi nhu cu khch hng - Bi quan qu 3. Phng php ly kin ca khch hng bng : - Phng vn trc tip - Bng cu hi in sn - i ng cng tc vin v marketing u im Nhc im - Khch quan v tng hp t kin - Kh thu thp thng tin khch hng 4. Phng php Delphi (phng php ly kin chuyn gia trong v ngoi x nghip) Ban nhn s : - Son v in sn cc cu hi v lnh vc d bo - a cc cu hi n cc chuyn gia - Tp hp v tng hp kin tr li ca cc chuyn gia - Nu xut hin vn mi hoc mu thun qu trnh trn c lp li ln 2, 3, 4, cho n khi thng nht u im Nhc im - Khch quan - Tn km, chi ph cao - Chnh xc

B. Cc phng php nh lng 1. Phng php tip cn gin n D bo cho thi k th n = S thc t ca thi k th (n-1) u im Nhc im Phm vi p dng - n gin - p t tnh hnh thi k - X nghip c quy m nh - R trc cho thi k sau - X nghip tip cn vi cng tc d n http://www.ebook.edu.vn 3

2. Phng php bnh qun di ng Ly con s bnh qun trong tng thi gian ngn, c khong cch u nhau lm kt qu d bo cho thi k sau

y4 =

y1 + y2 + y3 =A 3

u im Nhc im Phm vi p dng - n gin, r - Hon ton da vo qu kh - Dy s liu qu kh - Khng p t tnh hnh cha c yu t tng lai n nh thi k trc cho thi - Cha phn bit tm quan k sau trng ca cc s liu cc thi k khc nhau - Cn nhiu s liu qu kh 3. Phng php bnh qun di ng c h s (c trng s) D bo cho thi k (S thc t ca thi k th n x H s ca thi k th n) th (n+1) =----------------------------------------------------------------------Tng cc h s V d : d bo n=3; h s 3, 2, 1 Thng S thc t 1 10 2 12 3 13 4 16 5 x Kt qu d bo

(13x3+12x2+10x1):6=12,16 (16x3+13x2+12x1):6=14,33

u im Nhc im Phm vi p dng - C biu th xu hng pht - Cn nhiu s liu qu - Dy s liu qu kh n trin trong tng lai kh nh - C phn bit tm quan trng ca cc s liu cc thi k khc nhau 4. Phng php san bng s m Ft = Ft-1 + (At-1 Ft-1) (1) Ft = At-1 + (1-)At-2 + (1-)2At-3 ++ (1-)n-1At-n (2) Ft : s d bo thi k t Ft-1 : s d bo thi k t-1 : h s san bng s m (0 1) At-1 : s thc t ca thi k t-1 V d : d bo vi =0,2; F1 = 11 Thng 1 2 3 u im C biu th xu hng pht trin trong tng lai Cn t s liu qu kh Thun tin trong vic p dng my tnh S thc t 12 17 x Kt qu d bo F1 = 11 F2 = 11+ 0,2(12-11)=11,2 F3 = 11,2+0,2(17-11,2)=12,36 Phm vi p dng Mi trng hp ca dy s (tr trng hp tuyn tnh)

Nhc im Phi tnh tng thi k mt v d b sai lin i

http://www.ebook.edu.vn

4

5. Phng php san bng s m bc 2 (san bng s m c nh hng) Bc 1 : Ft = Ft 1 + (At 1 Ft 1) Bc 2 : Tt = Tt 1 + (Ft Ft 1) Bc 3 : Ft (h) = Ft + Tt Ft : s d bo ca thi k t Ft 1 : s d bo thi k (t-1) : h s san bng s m bc 1 : h s san bng s m bc 2 Tt : i lng nh hng ca thi k t Tt 1 : i lng nh hng ca thi k t-1 Ft (h) : s DB c nh hng ca thi k t Nhc im Mc chnh xc c hn ch Phm vi p dng Mi trng hp ca dy s (tr trng hp tuyn tnh)

u im - C biu th xu hng pht trin trong tng lai - Ch cn tnh 1 v thi k u s xc nh c cc thi k sau V d : TT 1 2 3 S tt 12 17 x Ft F1 = 11 F2 = 11+0,2(12-11) = 11,2 F3 = 11,2+0,2(17-11,2)=12,36

Tt T1=0 T2=0+0,4(11,2-11)=0,08 T3=0,08+0,4(12,36-11,2)=0,54

Ft (h) F1(h)=11 F2(h)=11,28 F3(h)=12,9

6. Phng php bnh qun b nht Y=ax + b x : s th t cc thi k y : s thc t (thi k qu kh) s d bo (thi k tng lai)a =

x. y n.x. y x n( x)2 2

b = y a.x

x=

xn

y=

yn

V d : Thng X 1 2 3 4 5 6 7 28

S TT Y 74 79 80 90 105 142 122 692

XY 74 158 240 360 525 852 854 3063

X2 1 4 9 16 25 36 49 140

x=

28 =4 7a =

y=

692 = 98,86 7

3063 7 . 4 . 98 ,86 = 10 ,54 140 7 . 4 2

b=98,86-10,54.4=56,7

Y =10,54x + 56,7 D bo thng 8 : Y8 =10,56.8+56,7 = 141 Phm vi p dng : dy s thc t tuyn tnh vi thi gian 7. Phng php h s thi v H s thi v ca thi k th n = Nhu cu bnh qun ca thi k th n : Nhu cu bq ca 1 thi k http://www.ebook.edu.vn 5

V d : Thng 1 2 3 . 12

1996

1997

80 75 90 . 80

90 85 110 . 80

Nhu cu bq ca thi k th n (80+90):2=85 (75+85):2=80 (90+110):2=100 . (80+80):2=80

Nhu cu bq ca 1 thi k 94 94 94 94

H s thi v

85:94=0,904 80:94=0,851 100:94=1,064 80:94=0,851

Nhu cu bq 1 thi k=(80+75+90++80)+(90+85+110++80):(12x2)=94 D bo cho cc thng trong nm 1998 nu bit D bo 1/1998=1200:12x0,904=96 nm 1998 sn xut 1200 sp/nm D bo 2/1998=1200:12x0,851=95 D bo 3/1998=1200:12x1,064=106III. CC PHNG PHP D BO THEO NGUYN NHN 1. D bo theo tng nguyn nhn

Y=ax + b x : s th t cc thi k y : s thc t (thi k qu kh) s d bo (thi k tng lai)a=

x. y n.x. y x n( x )2 2

b = y a.x

x=

xn

y=

yn

V d : x mc thu nhp b/q ca dn c ti vng A y doanh thu ca x nghip xy dng nh ti vng A Nm 1 2 3 4 5 6 x 1 3 4 1 2 7 18 y 2 3 2,5 2 2 3,5 15 xy 2 9 10 2 4 24,5 51,5 x2 1 9 16 1 4 49 80 y2 4 9 6,25 4 4 12,25 39,5

x=

18 =3 6

y=

15 = 2,5 6

a=

51,5 6.3.2,5 = 0,25 80 6.3 2

b = 2,5-0,25.3=1,75

Y = 0,25x+1,75

Gi s nm 7 bit mc thu nhp bnh qun ca dn c ti vng A l : 6(108) th c th d bo c doanh thu ca x nghip xy dng nh ti vng A l 3,25(108) y7 = 0,25.6+1,75=3,25 http://www.ebook.edu.vn 6

nh gi hm d bo 1/ Sai lch chun 2/ H s tng quan2

Sy, x =

y

b y a x. y n2

r=

[n x ( x) 2 ].[n y 2 ( y ) 2 ]2

n xy x y

Sy, x =

39,5 1,75.15 0,25.51,5 = 0,306 62

r=

6.51,5 18.15 [6.80 182 ].[6.39,5 152 ]

= 0,901

Y = 0,25x+1,75 bng 2 ch tiu nh sau : R = 1 : h s tng quan hon ho 0 < r : h s tng quan dng R = 0 : khng c tng quan 0 > r : h s tng quan m2. D bo theo nhiu nguyn nhn Y = a1x1 + a2x2 + a3x3++ anxn +b Trong : x1, x2 , x3 xn l cc nguyn nhn IV. KIM TRA KT QU D BO: Kim tra bng 2 ch tiu :

1. Sai s tuyt i b/q (MAD)=S thc t ca thi k th n S d bo ca thi k th n: S thi k tnh ton. Cng thc vit gn : MAD=Sai s : n 2. Tn hiu d bo (theo di)= (S thc t ca thi k th n S d bo ca thi k th n) : Sai s tuyt i bnh qun Cng thc vit gn : Tn hiu d bo = (Sai s): MAD

Tn hiu cho php : -4 +4

+4 -4

V d : kim tra kt qu d bo sau y :Thng S d S thc Sai s bo t 100 90 -10 100 95 -5 100 115 +15 110 100 -10 110 125 +15 110 140 +30

Sai s

Sai s

Sai s

MAD

1 2 3 4 5 6

-10 -15 0 -10 +5 +35

10 5 15 10 15 30

10 15 30 40 55 85

10 7,5 10 10 11 14,2

Tn hiu d bo -1 -2 0 -1 +0,45 +2,46

Tn hiu d bo dao ng t 2 +2,46 do hon ton nm trong gii hn cho php 4 +4. Nn kt qu d bo ny dng c. http://www.ebook.edu.vn 7

CHNG III QUN TR TN KHO I. NHNG KHI NIM LIN QUAN N QUN TR TN KHO 1. Chc nng ca QTTK

Chc nng lin kt : lin kt gia 3 giai on Cung ng Sn xut Tiu th Chc nng phng tng gi, phng lm pht Chc nng khu tr theo sn lng2. K thut phn tch ABC (K thut Pareto)

Gi tr hng tn kho 80 60 40 20

Nhm A

Nhm B

Nhm C

Nhm A : - Gi tr 70-80% - S lng 15% Nhm B : - Gi tr 15-25% - S lng 30% Nhm C : - Gi tr 5-10% - S lng 55%

% tng s hng tn kho Tc dng ca k thut phn tch ABC u t c trng tm khi mua hng (u tin nhm A+B) Xc nh chu k kim ton khc nhau cho cc nhm hng khc nhau : - Nhm A kim ton hng thng - Nhm B kim ton hng qu - Nhm C kim ton hng 6 thng Nng cao trnh ca nhn vin gi kho Cc bo co tn kho chnh xc, mc chnh xc ty thuc vo gi tr hng p dng cc phng php d bo khc nhau cho cc nhm hng khc nhau. (Nhm A+B d bo chnh xc, nhm C c th d bo khi qut hn).3. a. b. Cc chi ph trong qun tr tn kho. Chi ph mua hng (Cmh) = Khi lng hng x n gi Chi ph t hng (Ch) Chi ph hoa hng cho ngi gii thiu Chi ph hnh chnh thc hin 1 n hng Chi ph chun b phng tin thc hin 1 n hng Chi ph khc

S chi ph cho 1 ln t hng

Cdh =

D S Q

Trong : - Ch : chi ph t hng trong nm - D : nhu cu vt t trong nm - Q : s lng hng ca 1 n hng

c. Chi ph tn tr (Ctt) - Chi ph thu kho (khu hao kho) - Chi ph s dng my mc thit b trang b trong kho - Chi ph lao ng - Thu, bo him - Chi ph mt mt, h hng, hao ht http://www.ebook.edu.vn

H chi ph tn tr 1 n v hng trong 1 n v thi gian 8

Q Ctt = .H 2

Trong : - Ctt : chi ph tn tr trong nm - H : chi ph tn tr 1 n v hng

Q 0

Q 0

Q 0Q + 0 2

TC = Ch + Ctt + Cmh : Tng chi ph CA hng tn kho TC = Ch + Ctt : Tng chi ph V hng tn kho4. Cc dng tn kho Cc bin php gim sn lng hng tn kho Cc dng tn kho

Sn xut Cung ng Ngi cung ngNguyn vt liu trn ng vn chuyn Bn thnh phm trn ng vn chuyn Ph tng thay th trn ng vn chuyn

Tiu thThnh phm trong kho thnh phm Thnh phm trong kho ngi bn bun Thnh phm trong kho ngi bn l

D trSn phm d dang

Cc bin php gim sn lng hng tn kho

1. p dng cc m hnh tn kho

Gim ti a lng vt t d tr

2. p dng k hoch sa cha d phng

Xc nh lng ph tng d tr hp l

3. p dng hnh thc sn xut dy chuyn 4. p dng ch hp ng cht ch vi khch hng 5. p dng k thut phn tch bin thttp://www.ebook.edu.vn

Gim ti a lng sn phm d dang

Xc nh ng s lng thnh phm v thi im giao hng Xc nh khi no cn tng thm hng trnh tn kho9

II. CC M HNH TN KHO 1. M hnh sn lng kinh t c bn (EOQ): (The Basic Economic Order Quantity Model) do Ford W.Harris xut 1915 Gi nh : Nhu cu vt t bit trc v n nh Thi gian vn chuyn khng thay i Q Q Q S lng ca 1 n hng c vn chuyn 1 chuyn Khng c vic khu tr theo sn lng Khng c vic thiu hng trong kho

Q? hoc

TC = Ch + Ctt TC = D.S/Q + Q.H/2

min min

0

0

TCmin

Ch Q* T suy ra :Q* = 2 SD H

Nhn xt : Q* c Ch = OA c Ctt = OA Ch=Ctt Vy mun c Q* Q* cho: Ct = Ctt min phi c iu kin : Ct = Ctt hoc D.S/Q* =Q*.H/2

D=1000 n vQ* =

S=100.000 2.100000.1000 = 200dv 5000

H = 5000/1 n v/nm

Thi im t hng li (ROP) l thi im m sn lng trong kho = Lxd Trong : Q L : thi gian vn chuyn d : lng vt t cn dng trong 1 ngy m Nu L : 3 ngy d : 10 n v/ngy ROP = 3x10 = 30 n v L L

ROP

2. M hnh cung cp theo nhu cu sn xut (POQ) (Production Order Quantity Model)

Qmax

Qmax

t : thi gian cung ng T : chu k cung ng P : lng hng cung ng mi ngy (mc sn xut hng ngy) d : lng hng s dng hng ngy (lng hng tiu th hng ngy)

t T http://www.ebook.edu.vn

10

Qmax = Tng lng hng cung ng trong thi gian t Tng lng hng s dng trong thi gian t Qmax = P.t d.t Q = P.t t = Q/P

Qmax = P.Q/P d.Q/P Qmax = Q(1- d/P) Mun c Q* Q* cho TC = Ch + Ctt Ch = Ctt hoc Q max D .S = .H 2 Q min phi c iu kin : d Q * 1 P .H 2

D S= Q*

T suy ra :

Q* =

2 S .D d H 1 P

Nu D = 1000 v P = 8 v/ngy

S = 100.000 d = 6 v/ngy2.100000.1000 = 400dv 6 50001 8

H = 5000/v/nm

Q* =

3. M hnh sn lng gi li ni cung ng

Q* b*

Q* : lng hng cung ng ti u b* : lng hng mang v ti u Q* - b* : lng hng gi li ti u B : chi ph cho 1 n hng li ni cung ng

TC =

b D .S + H + (Q b).B 2 Q

min

Q* =

2 SD B + H x H B

b* =

2 SD B x H B+H

B Q * b* = Q * 1 B+H

Nu D = 20.000 n v H = 20.000 /v/nm http://www.ebook.edu.vn

S = 150.000 B = 100.000/v/nm 11

Q* =

2.150000.20000 100000 + 20000 x = 600dv 20000 100000

Phm vi p dng : p dng trong trng hp nhu cu khng n nh tng gim tht thng

b* =

2.150000.20000 100000 x = 500dv 20000 100000 + 20000

100000 Q * b* = 6001 = 100dv 20000 + 100000 4. M hnh khu tr theo sn lng Sn lng 1 999 1000 1999 2000 n gi 5 USD 4,8 USD 4,75 USD

Nu D = 5000 n v/nm S = 49 USD H = I.P I = 20% (t l chi ph tn kho tnh theo gi mua.) Q* ?

Bc 1 : Xc nh cc mc sn lng ti u theo cc mc gi khc nhau

Q1* =

2.49.5000 = 700dv 0,2.5

Q 2* =

2.49.5000 = 714dv 0,2.4,8

Q3* =

2.49.5000 = 718dv 0,2.4,75

Bc 2 : iu chnh cc mc sn lng ln mc sn lng c hng gi khu tr :

Q1* = 700 n v (ph hp vi gi 5USD) Q2* = 714 n v, iu chnh ln 1000 n v (ph hp vi gi 4,8USD) Q3* = 718 n v, iu chnh ln 2000 n v (ph hp vi gi 4,75USD) Nh vy iu chnh ta c : Q1* =700v; Q2* = 1000v; Q3* = 2000 v.Bc 3 : Tnh tng chi ph ca hng tn kho cho cc mc sn lng iu chnh, theo cng thc :

TC =

Q D S + .I .P + D.P 2 Q

TC1 =

700 5000 .0,2.5 + 5000.5 = 25700USD .49 + 2 700

TC 2 =

1000 5000 .0,2.4,8 + 5000.4,8 = 24725USD .49 + 2 1000

TC 3 =

2000 5000 .0,2.4,75 + 5000.4,75 = 24822,5USD .49 + 2 2000

TC2 < TC3 < TC1 do chng ta chn Q* = 1000 n v Phm vi p dng : p dng trong trng hp ngi cung ng bn gi khu tr. http://www.ebook.edu.vn

12

5. M hnh xc sut Phm vi p dng : Khng n nh, xc sut thiu ht c th xy ra. Do cn d tr an ton gii quyt s thiu ht . D tr an ton ti u l mc d tr c :

TC = Chi ph tn tr + Chi ph thit hi do thiu hng

min

xc nh mc d tr an ton ti u cn cn c vo cc thng tin nh sau : Xc sut tnh cho cc mc nhu cu trong thi k t hng : S n v hng 30 40 50 60 70 Xc sut xy ra 0,2 0,2 0,3 0,2 0,1

ROP

Thi im t hng li (ROP) = 50 n v Chi ph tn tr 1 n v hng : 5 USD/1v/nm Chi ph thit hi do thiu hng : 40 USD/1v S ln t hng ti u trong nm : 6 ln Mc d tr an ton 20 10 0 Chi ph tn tr tng thm 20x5 = 100 10x5 = 50 0 Ph tn kho do thiu ht gy ra 0 10x0,1x40x6=240 10x0,2x40x6+20x0,1x40x6=960 Tng chi ph 100 290 960

Vy mc d tr an ton ti u l : 20 n v v TC20 = 100 USD l minIII. P DNG K THUT PHN TCH BIN T Q/NH CHNH SCH TN KHO Nguyn tc : Ch tng thm hng khi MP ML (MP li nhun bin t : Marginal Profit) (ML thit hi bin t : Marginal Loss) Nu ta gi P l xc sut tnh cho cc trng hp nhu cu kh nng v (1 P) l xc sut tnh cho cc trng hp nhu cu < kh nng th nguyn tc trn c th biu th di dng biu thc sau : P.MP (1 P).ML P.MP ML P.ML P.MP + P.ML ML P(MP + ML) ML iu kin tng thm hng

P

ML ML + MP

V d : Mt ca hiu bn bnh bng lan, gi mua 1500/1 ci v bn ra vi gi 2500/ci. Nu trong ngy khng bn c th phi loi ra gi uy tn vi khch hng. xc nh khi no cn tng thm bnh bng lan cn tnh iu kin tng thm hng

P

ML ML + MP

P

1500 = 0,6 1500 + 1000

Nh vy ca hng ch tng thm hng khi kh nng bn ht phi 0,6. Mun nh gi kh nng bn ht theo tng mc nhp hng, chng ta xem xt s liu thng k bn ra trong thi gian qua nh sau : http://www.ebook.edu.vn 13

Mc bnh nhp

Nhu cu

Xc sut

-

Nu nhp 160 ci Nu nhp 161 ci Nu nhp 162 ci Nu nhp 163 ci Nu nhp 164 ci Nu nhp 165 ci Nu nhp 166 ci

160 161 162 163 164 165 166

0,06 0,14 0,16 0,20 0,24 0,16 0,04

P : Tng xc sut tnh cho cc trng hp Nhu cu Kh nng C th tng thm hng P=1 > 0,6 P = 0,94 > 0,6 P = 0,8 > 0,6 P =0,64 > 0,6 Khng nn tng thm P = 0,44 < 0,6 hng P = 0,20 < 0,6 P = 0,04 < 0,6

IV. NH GI HIU QU CA CNG TC QUN TR TN KHO 1. Mc p ng nhu cu sn xut v nhu cu khch hng : a. T l % cc n S n hng khng thc hin c hng kh thi = 100 -- --------------------------------------------x 100 Tng s n hng trong nm b. T l % cc n Lng hng tiu th trong 1 thi k v hng kh thi = 100 -- --------------------------------------------x100 Nhu cu trong 1 thi k 2. Mc u t cho QTTK :

a. b.

Gi tr hng tn kho Q phc v sn xut = ------x P (P gi mua mt n v hng) 2 Gi tr hng tn kho Gi tr hng tn kho phc v cho d tr = Gi tr hng tn kho - phc v sn xut iu hnh an ton T l % gi tr ti sn dng cho tn kho Gi tr ti sn u t cho hng tn kho = -----------------------------------------------x100 Tng gi tr ti sn ca x nghip

c.

3. Trnh qun tr tn kho : a. D Chi ph t hng = ----x S Q b. Chi ph tn tr = Chi ph:

- Thu kho, khu hao kho - My mc thit b - Lao ng c. S vng quay ca gi tr hng tn kho

- Thu - Bo him - Mt mt hao ht, h hng.

Gi tr hng bn ra trong nm (gi vn) = ----------------------------------------------Gi tr hng tn kho

d.

T l % cc bo co S bo co khng chnh xc tn kho chnh xc = 100 -- -------------------------------------x100 Tng s bo co trong nm T l gia doanh thu Doanh thu so vi hng tn kho = --------------------------------Gi tr hng tn kho 14

e.


CHNG IV HOCH NH NHU CU VT TNgun d liu : Ho n vt liu Chng trnh hoch nh Thi gian thc hin S liu hng tn kho Lch tin sn xut Cc bo co u ra :

Bo co nhu cu vt liu nh k

NHU CU VT LIU

Bo co nhu cu vt liu hng ngy Bo co v n hng thc hin Khuyn co mua hng Khuyn co c bit : 1. n hng sm tr hoc khng cn thit 2. S lng qu nh hoc qu ln

(phn mm trn my tnh)

S liu v mua hng

I.

Trnh t hoch nh nhu cu vt t :C cu sn phm A : A

B (2)

C (3)

D (2)

E (3)

E (1)

F (2)

G (1)

D (2)


15

Thi gian sn xut cc b phn cu to nn sn phm A :B phn Thi gian (tun) A 1 B 2 C 1 D 1 E 2 F 3 G 2

Bc 1 : Tnh tng nhu cu vt liu cho 50 sn phm ANhm hng Tun l Thi gian phn phi

1

2

3

4

5

6

7 50 100

A. nh k yu cu, nh k a n B. nh k yu cu, nh k a n C. nh k yu cu, nh k a n D. nh k yu cu, nh k a n E. nh k yu cu, nh k a n F. nh k yu cu, nh k a n D. nh k yu cu, nh k a n G. nh k yu cu, nh k a n

8 50

1 tun 2 tun 1 tun 1 tun

100 150 150 200 200 300 300 300 600 600 300 300 2 tun 150 300 3 tun 1 tun 150 2 tun

Bc 2 : Tnh nhu cu rng(Nhu cu rng = Nhu cu tn kho)Lng tn kho sn c Loi hng

10 A

15 B

20 C

10 D

10 E

5 F

0 G

Trc khi tnh ton ta cn thng nht cc k hiu trong k hoch nhu cu vt liu nh sau : (1) KT Kch thc l hng (2) TG Thi gian phn phi (hay thi gian s dng cho mi l hng) (3) Dat Lng d tr an ton (Safety Stock) (4) TK Lng tn kho sn c (on hand inventory) (5) Db Lng d tr c bit trong tng lai (Allocate items) (6) C Cp (hoc nhnh hoc mc ) trong cu trc ca sn phm (level code) (7) LH Loi hng (item identification) (8) NC Tng lng nhu cu (Gross requirement) (9) Nt Lng hng nhn c theo tin (Schedules receipts) (10) TKt Lng tn kho sn c nh trc (Projected on hand) (11) NR Nhu cu rng v vt liu (Net requirement) (12) Ntn Lng hng tip nhn theo k hoch n hng (Planned order receipts) (13) Nvc Lng hng cn vn chuyn n theo k hoch ca n hng (Planned order receipts)KT TG TK Dat Db C LH Ch Tun l


16

Nhn theo l

1

10

-

-

0

A

Nhn theo l

2

15

-

-

1

B

Nhn theo l

1

20

-

-

1

C

tiu NC Nt TK NR Ntn Nvc NC Nt TK NR Ntn Nvc NC Nt TK NR Ntn Nvc NC Nt TK NR Ntn Nvc

1 -

2 -

3 -

4 130

5 65 130 B 0 130 130 -

6 100 -

Nhn theo l

1

0

-

-

2

D

7 40 80A 15 65 65 120 A 20 100 100 -

8 50 10 40 40 -

Bc 3 : Hoch nh nhu cu tng loi vt t cho cc loi sn phm sn xut trong x nghip.V d : Hoch nh nhu cu vt t x nghip c tng hp t 3 nhu cu : Nhu cu B cho sn phm A Nhu cu B cho sn phm S Nhu cu B bn ra ngoi A (4 tun) B C B S (6 tun) C Lch bn ra ca hng B 9 40 10 11 12 13 20 30 1 2 3 Nhu cu B bn ra ngoi

Lch sn xut ca hng A Tun th Sn lng yu cu ca hng B 5 6 40 7 8 50

Lch sn xut ca S 9 10 11 15 8

10 10

Thi k Tng nhu cu ca B

1 10

2 50

3 40

4 50

5 20

6 -

7 45

8

9


17

II. CC M HNH CUNG NG VT T 1. M hnh a hng theo l ng vi nhu cu (lot for lot)V d : Mt cng ty mun xc nh chi ph t hng, chi ph thc hin, chi ph tn tr n hng theo tiu chun a hng theo l ng vi nhu cu. Chi ph thit lp (thc hin) n hng l 100 USD, chi ph tn tr l 1 USD/ 1 n v sn phm/ 1 thi k. Lch nhu cu sn xut cng phn nh nhu cu rng c th hin qua bng sau :

Thi k (tun) Tng nhu cu (NC) Lng sn c (TK) Lng a n (Nvc)

0 35

1 35 0 30

2 30 0 40

3 40 0 0

4 0 0 10

5 10 0 40

6 40 0 30

7 30 0 0

8 0 0 30

9 30 0 55

10 55 0 0

Ch = 7 x 100 USD = 700 USD Ctt = 0 USD TC = 700 + 0 = 700 USD2. K thut xc nh kch thc l hng theo m hnh sn lng kinh t ca n hng (EOQ) Vi k thut EOQ ngi ta c th nh ra kch thc l hngKch thc l hng trong h thng hoch nh nhu cu vt liu bng k thut EOQ Thi k 0 1 2 3 4 5 6 7 8 9 1. Tng nhu cu 35 30 40 0 10 40 30 0 30 2. Lng tn kho sn c 35 0 43 3 3 66 26 69 69 39 3. Lng a n 73 73 73 73

10 55 57

Chi ph tn tr : 1USD/1n v/1 tun Chi ph thit lp n hng : 100 USD Nhu cu trung bnh mi tun l 27 n v Thi gian s dng cho sn xut : 1 tun, xc nh Q* theo m hnh EOQ Tng nhu cu vt liu trong 10 tun l l 270 n v Mc s dng bnh qun 1 tun l 27 n v Mi tun l vic 5 ngy Mi nm c 52 tun do mc s dng trong nm l : 52x27 = 1404 n vQ* = 2 DS = H 2.1404.100 73 1.52

Chi ph t v thit lp n hng s l : 4x100 = 400 USD Chi ph tn tr s l : (43+3+3+66+26+69+69+39+57)x1usd/tun = 375 USD Do tng chi ph s l : 400 + 375 = 775 > 7003. Xc nh kch thc l hng theo k thut cn i cc thi k b phn (part period balancing technique)

K thut cn i cc thi b phn l mt k thut tip cn rt nng ng v hu hiu trong vic tm ra kch thc l hng kinh t nht lm gim c tng chi ph (gm chi ph thit lp n hng v chi ph tn tr). Cch tip cn ny nhm mc ch xc nh cho c sn phm n hng m chi ph thit lp n hng bng vi chi ph tn tr.


18

R rng trong thc t kh tm c mt sn lng m ti chi ph thit lp bng chi ph tn tr cho nn theo phng php ny, chng ta s chp nhn mt sn lng n hng m ti chi ph thit lp v t hng xp x bng chi ph tn tr.K thut tnh ton theo cn i cc thi k b phn Cc thi Ly k Ly k nhu cu tn tr theo Chi ph SC Chi ph HC k kt nhu cu phn k hp rng 2 30 0x1 =0 USD 100 0 2,3 70 40x1= 40 USD 100 40 2,3,4 70 40x1= 40USD 100 40 2,3,4,5 80 40x1+10x3 = 70USD 100 70 2,3,4,5,6 120 40x1+10x3+40x4 = 230USD 100 230 6 40 0x1 = 0 USD 100 0 6,7 70 1x30 = 30 USD 100 30 6,7,8 70 1x30 = 30 USD 100 30 6,7,8,9 100 1x30+30x3 = 120 USD 100 120 10 55 0x1 = 0 USD 100 0

Tng chi ph 100 140 140 170 330 100 130 130 220 100

Qua phng php phn tch trn ta thy trong mt nh k 10 tun chng ta s tin hnh thc hin 3 n hng. n hng 1 : 80 n v cung cp cho 4 tun (2, 3, 4, 5) n hng 2 : 100 n v cung cp cho 4 tun (6, 7, 8, 9) n hng 3 : 55 n v cung cp cho 1 tun (10) Ta c th biu din theo biu sau :nh kch thc l hng p dng k thut cn i thi k b phn Tun 0 1 2 3 4 5 6 7 8 Tng nhu cu 35 30 40 0 10 40 30 0 Lng tn kho sn 35 0 50 10 10 0 60 30 30 c Lng hng a n 80 100

9 30 0 55

10 55 0

Tng chi ph trong nh k ny s l : TC = (100 + 100 +100) + (70 + 120 + 0) = 490 USD M hnh ny cho chng ta mt tng chi ph nh nht


19

CHNG V HOCH NH CC NGUN LC TRONG DOANH NGHIPI. KHI NIM V HOCH NH CC NGUN LC V MI QUAN H CA HOCH NH CC NGUN LC VI CC HOT NG KHC. 1. Khi nim v hoch nh cc ngun lc

Hoch nh cc ngun lc l kt hp vic s dng cc yu t sn xut mt cch hp l vo qu trnh sn xut nhm m bo sn xut n nh chi ph sn xut thp nht v sn lng hng tn kho ti thiu. Khi hoch nh nh qun tr s quyt nh : - Mc sn xut trong gi - Mc sn xut ngoi gi - Mc tn kho - Mc thu ngoi t tng chi ph l thp nht v tn kho ti thiu.2. Mi quan h ca hoch nh cc ngun lc vi cc hot ng khc

NC th trng

Quyt nh v sn phm

NC sn phm v k thut SX

Cc quyt nh v sn xut Nhn lc My mc Hoch nh cc ngun lc Nguyn liu Hng tn kho Hoch nh lch trnh sn xut Hp ng ph

Hoch nh nhu cu vt t

Hoch nh vic s dng my mc thit b


20

II.

NHNG CHIN LC THUN TY (Chin lc n thun)Chin lc tn kho : d tr 1 lng thnh phm khi nhu cu tng c th p ng ngay u im Nhc im - n nh ngun lc - Chi ph tn kho tng - Khng tn thm chi ph o to - B hao mn v hnh 1.

2. Cu tng tng thm lao ng, cu gim sa thi bt lao ng u im Nhc im - Cn bng kh nng v nhu cu - Tn chi ph o to v sa thi - Chi ph tn kho thp - To tm l khng n nh cho ngi lao ng

Cc chin lc th ng (Passive Strategy)

3. Cu tng t chc sn xut ngoi gi, cu gim iu ha cng vic

u im - n nh ngun nhn lc - Khng tn thm chi ph o to v sa thi

Nhc im - Nng sut bin t gim - Chi ph tin lng tng

4. Cu tng thu thm lao ng bn phn u im Nhc im - Khng tng bin ch - Hn ch tinh thn trch nhim - Khng tn thm chi ph o to v sa thi

5. Cu tng thu thm hp ng ph u im Nhc im - Khng tng bin ch - D mt khch hng - Khng tn thm chi ph o to - Kh kim tra cht lng vt. SX v sa thi

6. Chin lc tng gi, ko di chu k phn phi khi cu > cung Chin lc gim gi, tng cng qung co v dch v khi cu < cung u im Nhc im - Cn bng nhu cu kh nng - Thu hp li nhun - Khng tn thm chi ph SX - Khng chc chn Cc chin lc ch ng (Active Strategy)

7. Chin lc hp ng chu (ko di thi gian giao hng) u im Nhc im - Cn bng kh nng v nhu cu - D mt khch hng - Khng tn thm chi ph - Doanh thu v li nhun gim

8. Chin lc t chc SX nhng mt hng i trng (mt hng ngc nhau v thi v) u im Nhc im - Gi vng doanh thu - u t thm thit b - m bo vic lm cho ngi L - Thu thm chuyn gia - Khai thc ht tim nng SX - D ri ro


21

III. CC PHNG PHP HOCH NH TNG HP1. Phng php trc quan Phng php trc quan cn l phng php kinh nghim. Theo phng php trc quan qun tr gia cn c vo kinh nghim x l trong cc thi k qu kh, i chiu vi nhu cu hin ti c tnh iu chnh v ra quyt nh. u im Nhc im - Ra quyt nh nhanh - Khi thay i nhn s th phng php v m hnh - Chi ph thp phi thay i theo

2. Phng php biu ( th) Phng php biu l biu din cc mc nhu cu ca cc thi k ln th, thng qua th s pht hin c cc chin lc phi hp cc ngun lc. u im Nhc im - n gin d hiu - Khng c phng php kim tra bit phng n - C th lp c rt nhiu phng n ti u cha.

3. Phng php bi ton vn ti p dng bi ton vn ti cn bng cc kh nng (cc ngun lc) vi nhu cu nhm t chi ph cc tiu. u im Nhc im - C th chn c phng n ti u - Khng s dng c khi c qu nhiu yu t cng - Chnh xc, logic xt ng thi. - Cc ngun lc phi bit trc mc ti a c th huy ng

V D MINH HA PHNG PHP hc Carnegie Mallon xut 1950 4. Phng php quyt nh tuyn. Do i BIU .L phng php dng 1 nhm ng cong bc 2 v hm s 2 bin thng qua cc s liu thc t thu thp c cc tiu ha chi ph. u im Nhc im - Cung cp phng php gii quyt tng qut - Chi ph cao - i hi k nng tnh ton phc tp

5. Phng php M hnh h s qun l L phng php da vo kinh nghim qun l ca cc chuyn gia trong qu trnh gii quyt cc kh khn xy ra trong sn xut bng phng php phn tch tng quan xc nh t l gia cc yu t. u im Nhc im - n gin - p t kinh nghim qu kh cho tng lai - Vn dng c kinh nghim thc t - Phi c thi gian di mi tng kt c kinh nghim 6. Phng php ng thi do R.C Vergin xut 1960 L m hnh vi tnh nhm tm kim s kt hp gia chi ph cc tiu vi mc sn xut. u im Nhc im - C th kim tra mi quan h gia cc yu t SX - Chi ph cao - Thi gian di.

7. Phng php tm kim quyt nh do W.N.Taubert xut L mu thut ton nhm tm s kt hp gia chi ph ti thiu ca cc ngun lc khc nhau vi cc mc SX. u im Nhc im - Nhng thay i trong quyt nh c th kim chng c. - Chi ph cao - Thi gian xy dng lu.


22

Tnh hnh sn xut ti 1 x nghip c cho theo bng sau : Tng nhu cu mong i trong 6 thng Thng 1 2 3 4 5 6 Nhu cu S ngy Nhu cu b/q ngy (SP) SX 900 22 41 700 18 39 800 21 38 1200 21 57 1500 22 68 1100 20 55 6200 124

6200

Cc chi ph ca cc ngun lc CP tn kho 5 USD/1v/1 thng CP hp ng ph 10 USD/1v CP tin lng trong gi 5 USD/gi CP tin lng ngoi gi 7 USD/gi S gi SX 1 SP 1,6 gi/SP Chi ph o to b/q 10 USD/SP CP sa thi b/q 15 USD/SP

Nhu cu trung bnh = ------------------------------------------------ = ------ = 50 S ngy sn xut trong 6 thng 124 Chin lc 1 : - T chc sn xut trong gi = Mc nhu cu trung bnh 50 SP/ngy - p dng chin lc tn kho.Thn g 1 2 3 4 5 6 Mc SX trong gi 1100 900 1050 1050 1100 1000 Nhu cu 900 700 800 1200 1500 1100 Tn kho mi thng +200 +200 +250 -150 -400 -100 Tn kho cui mi thng 200 400 650 500 100 0 1850 70 60 50 40 30 20 10 0

ng cu trung bnh

1

2

3

4

5

6

Tng chi ph chin lc I: Chi ph SX trong gi : 50 x 124 x 1,6 x 5 USD = 49.600 USD Chi ph tn kho : 1850 x 5 USD = 9.250 USD TC1 = 58.850 USDChin lc tng hp II :

- T chc sn xut trong gi = Mc nhu cu ti thiu 38 SP/ngy - Thng no thiu thu hp ng ph Tng chi ph chin lc II - Chi ph sn xut trong gi : 38 x 124 x 1,6 x 5 USD = 37.696 USD - Chi ph hp ng ph : (6200-38x124) x 10 USD = 14.880 USD TC2= 52.576 USDChin lc tng hp III :Thng D bo nhu cu CPSX(CP lng CN) CP ph tri khi gi tng (thu) CP khi gim SX (sa thi) Tng CP

1 2 3 4 5 6

900 700 800 1200 1500 1100

7200 5600 6400 9600 12000 880049600

100x10=1000 400x10=4000 300x10=30008000

200x15=300 0 400x15=600 0 9000

7200 8600 7400 13600 15000 1480066600

T chc sx trong gi =mc nhu cu. - Cu tng tng lao ng, cu gim gim lao ng. Tng chi ph chin lc III : TC3 = 66.600 USD

-


23

V D MINH HA PHNG PHP BI TON VN TI Tnh hnh nhu cu SX ti 1 XN cho theo bng sau :

Ch tiu Nhu cu Kh nng sn xut - Bnh thng - Vt gi - Hp ng ph - D tr ban u Chi ph : Nu sn xut bnh thng Nu sn xut vt gi Nu sn xut theo hp ng ph Chi ph thc hin (chi ph tn kho)Cung t cc ngun Thng 3 D tr ban u 100 SX bnh thng Vt gi Hp ng ph SX bnh thng Vt gi Hp ng ph SX bnh thng Vt gi 40 700 50 50 70 50 40 700 50 50 70 150 72 52 42 0

Ba 800 SP 700 50 150 100

Cc thi k T Nm 1000 750 SP SP 700 700 50 50 130 150

: 40 USD/SP : 50 USD/SP : 70 USD/SP : 2 USD/SP/thngNhu cu cho Thng 4 Thng 5 2 4 44 54 74 100 42 52 72 40 700 50 50 70 130 0 230 130 2780 0 50 0 0 0 0 700 50 150 700 Kh nng khng s dng 0 0 0 0 Tng kh nng SX (kh nng cung ng) 100 700 50 150

Thng 3

Thng 4

Thng 5

Hp ng ph Tng cu

800

1000

750

700x40 = 28000 50 x 52 = 2600 50 x 72 = 3600 700x40 = 28000 50 x 50 = 2500 150x70 = 10500 700x40 = 28000 50 x 50 = 2500 TC = 105700 USD


24

CHNG VI HOCH NH LCH TRNH SN XUTI. CC NGUYN TC SP XP TH T CC CNG VIC TRN 1 PHNG TIN (1 MY)

1. Theo nguyn tc 1 FCFS (Cng vic no t hng trc b tr lm trc)Cng vic A B C D E Thi gian SX (ngy) 6 2 8 3 9 28 Thi gian hon thnh k c ch i (ngy) 6 8 16 19 28 77 Thi im phi hon thnh yu cu ( ngy th ) 8 6 18 15 23 Thi gian chm tr so vi yu cu (ngy) 0 2 0 4 5 11

Tnh cc ch tiu hiu qu :Tng dng thi gian 77 Thi gian hon tt trung bnh 1 cng vic (ttb) = -------------------------- = ------ = 15,4 ngy S cng vic 5 Tng dng thi gian 77 S cng vic trung bnh nm trong h thng (Ntb) = ------------------------------ = ------=2,74 c/vic Tng thi gian sn xut 28 Tng s ngy tr hn 11 S ngy tr hn trung bnh (TRtb) = ---------------------------- = ----- = 2,2 ngy S cng vic 5 2. Theo nguyn tc 2 EDD (Cng vic no c thi im giao hng sm b tr lm trc)Cng vic B A D C E Thi gian SX (ngy) 2 6 3 8 9 28 Thi gian hon thnh k c ch i (ngy) 2 8 11 19 28 68 Thi im phi hon thnh yu cu ( ngy th ) 6 8 15 18 23 Thi gian chm tr so vi yu cu (ngy) 0 0 0 1 5 6 Tnh cc ch tiu hiu qu : ttb = 13,6 ngy Ntb = 2,42 c/vic TRtb = 1,2 ngy

3. Theo nguyn tc 3 SPT (Cng vic no c thi gian ngn b tr lm trc)Cng vic B D A C E Thi gian SX (ngy) 2 3 6 8 9 28 Thi gian hon thnh k c ch i (ngy) 2 5 11 19 28 65 Thi im phi hon thnh yu cu ( ngy th ) 6 15 8 18 23 Thi gian chm tr so vi yu cu (ngy) 0 0 3 1 5 9 Tnh cc ch tiu hiu qu : ttb = 13 ngy Ntb = 2,3 c/vic TRtb = 1,8 ngy

4. Theo nguyn tc 4 LPT (Cng vic no c thi gian di b tr lm trc) 5.Cng vic E C A B D Thi gian SX (ngy) 9 8 6 3 2 28 Thi gian hon thnh k c ch i (ngy) 9 17 23 26 28 103 Thi im phi hon thnh yu cu ( ngy th ) 23 18 8 15 6 Thi gian chm tr so vi yu cu (ngy) 0 0 15 11 22 48 Tnh cc ch tiu hiu qu : ttb = 20 ,6 ngy Ntb = 3,6 c/vic TRtb = 9,6 ngy

6. nh gi mc b tr hp l cc cng vic v th t u tin trong iu SX.


25

kim tra vic b tr cc cng vic c hp l hay khng ta tnh ch tiu mc hp l nh sau :Thi gian cn li Mc hp l (MHL) = --------------------------------------------------S cng vic cn li tnh theo thi gian

V d : Ti mt cng ty c 3 cng vic c t hng nh bng sau. Gi s thi im chng ta ang xt l ngy 25/12Cng vic Thi im giao hng Cng vic cn li tnh theo ngy

A B C

30 12 28 12 27 12

4 5 2

Theo cng thc trn tnh c MHL nh sau :Cng vic A B C Mc hp l (30 25):4=1,25 (28 25):5=0,6 (27 25):2=1 Th t u tin 3 1 2

Nhn thy : Cng vic A : MHL > 1 chng t s hon thnh sm hn k hn. Khng cn u tin xp u tin 3. Cng vic B : MHL < 1 chng t s b chm cn xp u tin 1 tp trung ch o. Cng vic C : MHL = 1 chng t s hon thnh ng k hn. Xp u tin 2. Cng dng ca ch tiu MHL khi lp lch trnh : Quyt nh v tr cc cng vic c bit. Lp quan h u tin ca cc cng vic. Lp quan h gia cc cng vic c lu li v cc cng vic phi thc hin. iu chnh th t u tin thay i theo yu cu trn c s s tin trin ca cc cng vic. Theo di cht ch s tin trin v v tr ca cc cng vic. II. NGUYN TC JOHNSON 1. Lp lch trnh N cng vic trn 2 my. Mc tiu b tr cc cng vic : tng thi gian thc hin cc cng vic l min. V d : C 5 cng vic phi thc hin ln lt trn my khoan v my tin, c thi gian cho theo bng sau : Cng vic Thi gian thc hin cc cng vic (h) 1 My khoan 2 My tin 5 2 3 6 8 4 10 7 7 12

A B C D E

Hy sp xp th t cc cng vic c tng thi gian thc hin chng l min ? http://www.ebook.edu.vn 26

Bc 1 : Sp xp cc cng vic theo th t thi gian min tng dn. (V d ny th t cc cng vic tun theo nguyn tc thi gian min tng dn, nn khng cn xp li) Bc 2 : B tr cc cng vic theo nguyn tc Johnson. Theo th t xp bc 1 ln lt b tr nh sau : - Cng vic no c thi gian min nm ct 1 b tr bn tri ( u) - Cng vic no c thi gian min nm ct 2 b tr bn phi ( cui)

Kt qu cc cng vic c b tr nh sau :B My 1 My 2 3 6 E 7 12 D 10 7 C 8 4 A 5 2

Bc 3 : V biu v tnh tng thi gian thc hin cc cng vic : 35 gi l min 0 3 10 20 28 33B=3 E=7 B=6 D=10 E=12 C=8 D=7 A=5 C=4 A=2

0

3

9 10

22

29

33

35

2. Lp lch trnh N cng vic cho 3 my Sp xp th t N cng vic cho 3 my c th s dng nguyn tc Johnson nu c hai iu kin sau :

Thi gian ngn nht trn my 1 phi ln hn hoc bng thi gian di nht trn my 2 Thi gian ngn nht trn my 3 phi ln hn hoc bng thi gian di nht trn my 2

V d : C bng sau, hy chuyn i c th p dng nguyn tc JohnsonCng vic

A B C D

My 1 (t1) 13 5 6 7

Thi gian My 2 (t2) 5 3 4 2

My 3 (t3) 9 7 5 6

Hai iu kin k trn u tho mn. Ta lp bng chuyn i nh sau :Cng vic A B C D t1 + t2 18 8 10 9 t2 + t3 14 10 9 8

By gi ta s dng nguyn tc Johnson i vi trng hp N cng vic cho 2 my v s nhn c th t sau BACD. Kt qu ny l kt qu gn ng, nhng c dng tt trong thc t.III. TRNG HP TNG QUT SP XP LCH TRNH CHO N CNG VIC TRN M MY C s ca thut ton

Thut ton ny m bo cho cc my (trong M my) u lm vic lin tc vi cc cng vic khc nhau v tng thi gian thc hin tt c cc cng vic trn tt c cc my l nh nht. http://www.ebook.edu.vn 27

Ta xt trng hp N = 3, M = 4 khi thay i N v M thut ton khng c g thay i. Ta c thi gian thc hin cc cng vic trn cc my theo bng di y : My C.vic A a1 B b1 C c1 S tnh ton : x1 A b1 x1 B b2 x3 T x1 x3 c2 x2 b2 I x1 II a2 x1 x2 x3 III a3 b3 c3 x1 x2 x3 IV a4 b4 c4

a1

a2 x2

a3 x2

a4 x2

D

C c1

b3 x3

b4 x3 c4

c2

c3

Trong s cc x, x, x l thi gian phi ch i ca cc cng vic khi chuyn t my ny sang my kia. Cc x, x, x u c th hin trn s v trn bng tnh. Nhn trn s thy hnh ABCD l 1 hnh ch nht. Do : x1 + a2 = b1 + x2 x1 + a3 = b2 + x2 x1 + a4 = b3 + x2 x2 + b2 = c1 + x3 x 2 + b3 = c2 + x 3 x2 + b4 = c3 + x3 gii cc h phng trnh ny ta cn lu rng trong cc trng hp b tr tt nht th trong x1 x2 x3 s phi c t nht 1 ci = 0, gia cc x1 x2 x3 v x1 x2 x3 cng phi c t nht 1 ci = 0. Gi thit mt x no = 0 s gii ra cc x khc. Trong qu trnh gii nu xut hin x < 0 chng hn x = -3 th ta cng thm 3 bin chng bng 0. Kt qu tnh c tt c cc x 0 ta xc nh c T. Thay i th t ta s c T khc. T xc nh c Tmin vi phng n th t ti u.V D MINH HA CHO VIC SP XP LCH TRNH B TR N CNG VIC TRN M MY

My C.vic A 2 B 2 C 3 -

I x1=0 x2=0 x3=1

II 2 4 5 x1=3 x2=3 x3=0

III 4 2 3 x1=0 x2=1 x3=2

IV 3 4 2

S lng cc phng n kh nng : N! = 3! = 6 28


C th c cc phng n th t sau y : ABC, BAC, ACB, BCA, CAB, CBA. - Xt phng n ABC. Chnh l bng trn. Tnh cc x T s tnh ton ta c cch lp cc h phng trnh nh ni trn. Suy ra cch lp cc h phng trnh t bng tnh nh sau : x1 + a2 = b1 + x2 x1 + 2 = 2 + x2 Tng t x1 + 2 = 2 + x2 x2 + 4 = 3 + x3 Gi thit x1 = 0 x2 = 0; x3 = -3. V cc x khng c quyn m nn ta cng chng thm 3. C : x1 = 3; x2 = 3; x3 = 0Tnh cc x

x1 + 3 = 2 + x2 x2 + 4 = 3 + x3 x2 = 1; x3 = 2; cc x 0

Gi thit x1=0

By gi i t AI n C.IV bng bt c con ng no cng s nhn c T ging nhau. Chng hn theo hng trn cng v ct cui cng : T = 2 + 0 + 2 + 3 + 4 + 0 + 3 + 4 + 2 = 20 gi Theo ct u tin v hng di cng : T = 2 + 2 + 3 + 1 + 5 + 0 + 3 + 2 + 2 = 20 gi Ch trn ng i nu gp cc x, x, x dng th ta nh cng c chng vo. Kt qu : T(ABC) = 20 gi By gi ta thay i th t v tnh li s c cc kt qu sau y ( ngh cc bn t tnh) : T(BAC) = 18 gi T(BCA) = 21 gi T(ACB) = 20 gi T(CAB) = 22 gi T(CBA) = 21 gi

Vy Tmin = 18 gi

IV. PHNG PHP PHN CNG CNG VIC TRN CC MY V CHO TNG NHN VIN

1. Bi ton 1 mc tiu: 2. V d c 3 cng vic R-34, S-66, T-50 v c 3 my A, B, C. Chi ph cc cng vic thc hin trn cc my cho nh bng sau. Tm phng n b tr cc cng vic trn cc my sao cho tng chi ph l nh nht. n v tnh : USDMy C.vic R-34 S-66 T-50 A 11 8 9 B 14 10 12 C 6 11 7

Gii Bc 1 : Chn trong mi hng 1 s min, ly cc s trong hng tr i s min .My C.vic R-34 S-66 T-50 A 5 0 2 B 8 2 5 C 0 3 0


29

Bc 2 : Chn trong mi ct 1 s min, ly cc s trong ct tr i s min .My C.vic R-34 S-66 T-50 A 5 0 2 B 6 0 3 C 0 3 0

Bc 3 : Chn hng no c 1 s 0, khoanh trn s 0 , k ng thng xuyn sut ct. Chn ct no c 1 s 0, khoanh trn s 0 , k ng thng xuyn sut hng. Nu s s 0 khoanh trn = S p n cn tm th bi ton gii xong. Nu s s 0 khoanh trn cha bng s p n cn tm th ta phi thc hin tip bc 4.My C.vic R-34 S-66 T-50 A 5 0 2 B 6 0 3 C 0 3 0

Trong v d ny sau khi thc hin bc 3 ta mi c 2 s 0 khoanh trn cha bng s p n cn tm do ta phi lm bc 4.Bc 4 : Ta to thm s 0 bng cch :

Chn trong cc s khng nm trn cc ng thng 1 s min, ly cc s khng nm trn cc ng thng tr i s min . Ly s min cng vo cc s nm trn giao im ca cc ng thng. Sau ta li b tr cng vic nh trnh by bc 3 c tip tc nh vy cho n khi no s s 0 khoanh trn bng s p n cn tm th bi ton mi gii xong. Cc cng vic s c b tr vo cc c s 0 khoanh trn. Nh vy chng ta s c tng thi gian thc hin hoc tng chi ph thc hin cc cng vic l ti thiu.My C.vic R-34 S-66 T-50 A 3 0 0 B 4 0 1 C 0 3+2=5 0

Sau khi thc hin bc 4 ta b tr li cc cng vic nh bc 3 v s c kt qu nh sau : Cng vic R-34 b tr vo my C : 6 USD Cng vic S-66 b tr vo my B : 10 USD Cng vic T-50 b tr vo my A : 9 USD Tng chi ph thc hin cc cng vic : 25 USD l chi ph ti thiu.3. Bi ton 2 mc tiu V d : c 4 cng vic v 4 my I, II, III, IV. Hy b tr cc cng vic vo cc my sao cho Tng thi gian thc hin chng l min Thi gian thc hin mi cng vic ) 3.1.2 Cc cng thc s dng : Gi : - S lng trung bnh khch hng n trong mt n v thi gian, chng hn trong mt gi, tc l ch s dng vo. - S lng trung bnh khch hng phc v c trong mt n v thi gian, tc l nng sut phc v. Ngi ta lp c nhng cng thc sau y : 1. S lng trung bnh khch hng nm trong h thng Ls , bng s ang xp hng cng vi s ang c phc v.

Ls =

2. Thi gian trung bnh mt khch hng phi chi ph trong h thng Ws , bng thi gian xp hng cng vi thi gian c phc v.

Ws =

1

3. S lng trung bnh khch hng xp trong hng Lq

Lq =

2 ( )

4. Thi gian ch i trung bnh ca mt khch hng xp hng trong hng Wq

Wq =

( )

5. T l hot ng c ch ca h thng tc xc sut cho hot ng dch v ang bn vic

=

6. T l thi gian ri (ngng vic) ca h thng Po , tc l xc sut khng c mt khch hng no trong h thng. Po = 1

7. Xc sut s lng khch hng nm trong h thng ln hn k (Pn > k) Pn > k = k +1


34

V d 1 : Mt ca hng lp thit b gim m cho t. Lu lng t vo lp thit b gim m 2 xe/ gi. Ngi ch ca hng phn vn gia hai phng n : Phng n 1 : Thu mt cng nhn bc 3 lp c 3 ci/gi v chi ph tin lng 7USD/gi. Phng n 2 : Thu mt cng nhn bc 4 lp c 4 ci/gi v chi ph tin lng 9USD/gi. Bit rng chi ph ch i ca khch l 10USD/gi. Hy la chn gip ng ch ca hng mt phng n hp l. Phng n 1 : = 2 xe/gi = 3 xe/gi

Tnh ton cc thng s ca h thng :

Ls =

=

2 = 2 xe 3 2 1 1 = = 1h 3 2

Vy s xe trung bnh nm trong h thng l 2 xe.

Ws =

Tc l mt xe trung bnh phi nm trong h thng 1 gi.

2 22 Lq = = = 1,33xe ( ) 3(3 2)Wq =

Tc l trung bnh phi c 1,33 xe phi xp hng ch.

2 2 = = = 40' ( ) 3(3 2) 3

Tc l trung bnh mt xe phi xp hng ch 40 pht.

=

2 = = 0,67 3

Tc l t l (hoc xc sut) thi gian h thng bn vic (hoc th my bn vic) bng 0,67.Po = 1 Phng n 2 : = 2 xe/gi = 4 xe/gi

= 1 0,67 = 0,33

Ls =

=

2 = 1xe 42

1 xe

Ws =

1 1 1 = = h gi 42 2

Lq = Wq =

2 22 1 = = xe xe ( ) 4(4 2) 2

2 1 = = = 15' 15 pht ( ) 4(4 2) 4 Po = 1 = 1 0,5 = 0,5 0,5 = 2 = = 0,5 4


35

Phn tch kinh t :S xe trong mt ngy : 2 xe/1h x 8h = 16 xe/ngy S gi ch i ca khch hng : 2/3 x 16 = 10,66 h/ngy Chi ph ch i ca khch hng : 10USD x 10,66 = 106USD/ngy Lng ca th bc ba l 7USD /gi hoc 56USD/ngy. Tng chi ph : 106 + 56 = 162 USD/ngy 10USD x 4 = 40USD/ngy 9USD x 8h =72USD/ngy 40 + 72 = 112 USD/ngy

Theo nh phn tch trn th ng ch nn chn thu cng nhn bc 4 tc l chn phng n 2. 1 xe/ h x 8h = 16 xe/ngy x 16 = 4h/ngy

3.2 M hnh B :Hot ng dch v c nhiu knh, mt pha, dng vo Poisson, thi gian dch v phn b gim dn.y l h thng dch v c nhiu knh, trong c hai hoc nhiu hn cc dch v vin hoc cc knh ging nhau c th tip nhn cc khch hng. Cc khch hng ch xp mt hng chung. Khch hng ng u hng nu thy c mt dch v vin no ri th s vo ngay c phc v. Gi s nng sut phc v ca cc dch v vin nh nhau. Cc iu kin khc ging nh m hnh A. Cc cng thc s dng : Gi : M s knh c m - ch s dng vo. - nng sut dch v. Ta c cc cng thc sau y : (1) Xc sut c 0 khch hng trong h thng :Po = 1 M 1 1 n = 0 n! n

1 M M + M ! M

Vi M > http://www.ebook.edu.vn 36

(2) S lng khch hng nm trong h thng :

Ls = Po + 2 (M 1)!( M )

M

(3) Thi gian trung bnh mt khch hng nm trong h thng :

Ls 1 Ws = = Po + 2 ( M 1)!( M ) (4) S khch hng trung bnh xp trong hng ch Lq = Ls (5) Thi gian trung bnh khch hng xp trong hng ch : (6) T l c ch :

M

Wq = Ws

1

= M

=

Lq

V d 2 : Ti ca hng lp thit b gim m ni trn, ng ch c sng kin thu hai cng nhn bc 3 vi nng sut mi ngi lp 3 ci/gi v tin lng mi ngi 7USD/gi. Lu lng khch vo = 2 xe /gi. Chi ph ch i ca khch hng l 10 USD/gi. Nh vy phng n 3 ny c tt hn hai phng n xt v d 1 khng ? Bn hy cho ng ch mt li khuyn. Gii Ta c : M = 2, = 2, = 3Po = 1 M 1 1 n = 0 n! n

1 M + M M ! M

=

1 1 1 2 n = 0 n! 3 n

1 2 2 .3 + 2! 3 2.3 2 2 2

=

1 = 0,5 2

Tc l xc sut c 0 xe nm trong h thng l 0,5 Tc l s xe trung bnh nm trong h thng l 0,75 Thi gian trung bnh mt xe nm trong h thng :

2 2.2 1 2 3 Ls = . + = 0,75 2 (2 1)!(2.3 2) 2 3

3 3 Ws = = 4 = h = 22,5' 2 8Ls

S xe trung bnh xp trong hng :

Lq = Ls

3 2 1 = = = 0,083xe 4 3 12Wq = Ws

Thi gian trung bnh 1 xe phi ch trong hng : Phn tch chi ph cho phng n 3 (2 knh) Thi gian khch hng xp hng ch : 16 xe/ngy x 0,0415h/xe = 0,664h Chi ph ch i ca khch hng : 10USD/h x 0,664h = 6,64USD/ngy Lng ca hai th my : 2 x 8h/ngy x 7USD/ngy = 112USD/ngy Tng chi ph : 6,64USD + 112USD = 118,64USD http://www.ebook.edu.vn

1

=

Lq

=

0,083 = 0,0415h = 2,5' 2

37

Thng k ba phng n ta c : Phng n 3 (2 knh) : 118,64USD/ngy Phng n 2 (1 knh vi th bc 4) : 112USD/ngy Phng n 1 (mt knh vi mt th bc 3) : 162USD/ngy Do chng ta nn khuyn ng ch ca hng thu mt cng nhn bc 4 v phng n ny c chi ph thp nht.

gim bt tnh ton ngi ta lp bng tnh sn Lq theo . Trong = / (M = 15 knh) Bng tr s Lq ca h thng c M = 1 5 knh vi cc tr s = / khc nhau :

162USD > 118,64USD > 112USD

0,10 0,15 0,20 0,25 0,30 0,35 0,40 0,45 0,50 0,55 0,60 0,65 0,70 0,75 0,80 0,85 0,90 0,95 1,00 1,20 1,40 1,60 1,80 2,00 2,20 2,40 2,60 2,80 3,00 3,20 3,40 3,60 3,80 4,00 4,20 4,40 4,60 4,80

Dng vo Poisson, thi gian phc v phn b gim dn S lng cc knh M 1 2 3 4 0,0111 0,0264 0,0008 0,0500 0,0020 0,0833 0,0039 0,1285 0,0069 0,1884 0,0110 0,2666 0,0166 0,3681 0,0239 0,0019 0,5000 0,0333 0,0030 0,6722 0,0449 0,0043 0,9000 0,0593 0,0061 1,2071 0,0767 0,0084 1,6333 0,0976 0,0112 2,2500 0,1227 0,0147 3,2000 0,1523 0,0189 4,8166 0,8173 0,0239 0,0031 8,1000 0,2285 0,0300 0,0041 18,0500 0,2767 0,0371 0,0053 0,3333 0,0454 0,0067 0,6748 0,0904 0,0158 1,3449 0,1778 0,0324 2,8444 0,3128 0,0604 7,6734 0,5320 0,1051 0,8888 0,1739 1,4907 0,2770 2,1261 0,4305 4,9322 0,6581 12,2724 1,0000 1,5282 2,3856 3,9060 7,0893 16,9366

5

0,0059 0,0121 0,0227 0,0398 0,0659 0,1047 0,1609 0,2411 0,3541 0,5128 0,7365 1,0550 1,5184 2,2164 3,3269 6,2675 9,2885 21,6384 38


V d 3 : Chng hn mt ngn hng cn m bao nhiu ca s nu = 18/h, mi nhn vin u c = 20/h. Ta c = 18/20=0,9. Tra bng vi mt ca s (1knh) c Lq = 8,1, vi ca s c Lq = 0,2285, vi 5 ca s th khng c khch hng no phi xp hng c. C Lq ta tnh ngay c thi gian xp hng Wq = Lq/ v suy ra cc ch tiu khc. Do tuy trng hp nhiu knh cc cng thc kh phc tp nhng dng bng tnh sn ta c th tnh c nhanh chng cc thng s ca h thng.

2.3 M hnh C :y l m hnh c thi gian dch v l mt hng s. Trong thc t cng c nhng h thng m thi gian phc v mt khch hng l mt hng s hoc c coi l hng s. in hnh l trng hp ra xe bng my t ng. Lc ny ta s dng m hnh C. Cng thc s dng : 2 (1) S lng khch hng trung bnh xp hng ch : Lq = 2 ( ) (2) Thi gian trung bnh mt khch hng phi ch trong hng : (3) S khch trung bnh nm trong h thng :Ls = Lq + Wq =

2 ( )

Ws = Wq +

(4) Thi gian trung bnh mt khch hng nm trong h thng :

1

V d 4 :Cng ty G chuyn thu thp v chnh trang cc loi can nhm v chai thy tinh. Cc xe ti n y d hng u phi ch trung bnh mt 15 pht trc khi hng ca h c ra sch. Chi ph cho thi gian li xe v xe xp hng ra l 60 USD/gi. Cng ty ny mun mua mt thit b t ng lm cc vic trn vi cng sut khng i l 12 xe/gi (tc 5 pht/xe). Cc xe n tun theo lut Poisson v vi ch s trung bnh l = 8 xe/gi. Nu a thit b mi ny vo s dng th chi ph bo tr tnh bnh qun trn mt u xe vo d hng l 3 USD. Hy tnh xem c nn mua thit b t ng ny khng ?Gii

Vi thit b hin c : Chi ph ch i/chuyn = gi x 60 USD = 15 USD/chuyn Vi thit b t ng d kin mua : 8 1 Thi gian trung bnh mt xe ch trong hng : Wq = = = h 2 ( ) 2.12(12 8) 12 USD USD Chi ph ch i/chuyn = 1/12 gi x 60 = 5 /chuyn Tit kim c 15USD 5USD = 10USD/chuyn Chi ph bo tr = 3USD/chuyn Tit kim rng = 10USD 3USD = 7USD/chuyn Nh vy cng ty nn mua thit b t ng ny v mi chuyn tit kim c 7USD l mt con s rt ng k.

2.4 M hnh D :y l m hnh c s khch hng hu hn. M hnh ny thng dng khi cn sa cha cc my ca mt nh my m trong nh my ch c mt s lng my nht nh. V d i tu ng c my bay cho mt hng c 10 my bay, phc v trong mt trm x ch c 20 ging. Cng thc tnh ton Trong m hnh ny dng cc k hiu khc vi ba m hnh trn nh sau : D Xc sut mt khch hng s xp hng ch. F H s hiu qu http://www.ebook.edu.vn 39

H S khch hng trung bnh ang c phc v. J S khch hng khng xp hng, khng vo h thng. L S khch hng ang ch i c phc v. M S knh dch v. N S khch hng c th c (tim nng) T Thi gian dch v trung bnh. U Khong thi gian trung bnh gia 2 ln khch hng cn n h thng c phc v. W Thi gian trung bnh mt khch hng phi i trong hng. X H s dch v Ta c cc cng thc sau y : (1) H s phc v :X = T T +UL = N (1 F ) W = L(T + U ) T (1 F ) = N L X .F J = N .F (1 X )H = F .N . X

(2) S khch hng ch trung bnh : (3) Thi gian ch trung bnh : (4) S khch hng khng vo h thng :

(5) S khch hng trung bnh ang c phc v : (6) S khch hng trong tp hp khch hng :

N = J +L+H

V d 5 : Mt c s my vi tnh c 5 my in Laser. Mi my c sau 20 gi lm vic th phi a i bo tr. Dng my vo bo tr tun theo lut Poisson. Mt th my bo tr 01 my mt 2h. Thi gian bo tr tun theo lut phn b gim dn. Mt my in ngng vic phi chi mt 120USD, lng ca th my l 25USD/h. Hi c cn n 2 th my hay khng ? Gi s hai th my c cng nng sut v tin lng. Ta s dng bng tnh sn, trong n = 5 tc l tp hp khch hng hu hn = 5. (1) c T = 2h, U = 20h.

Do :

X =

2 T = = 0,091 T + U 2 + 20

(Ly trn = 0,09)

(2) Tra bng vi M = 1 th (1 knh) c : D = 0,350; F = 0,960 (3) Tra bng vi M = 2 th (2 knh) c : D = 0,044; F = 0,998 (4) S lng trung bnh cc my in ang lm vic (khng vo h thng bo tr) : J = N.F(1-X) Vi M = 1 : J = (5 x 0,960)(1 0,091) = 4,36 Vi M = 2 : J = (5 x 0,998)(1 0,091) = 4,54 (5) Phn tch chi ph : S th S my bo tr t/b (N-J) 1 0,64 2 0,46Chi ph t/b do my ngng vic/h 76,8USD 55,2USD Lng th trong 1h 25USD 50USD Chi ph tng cng/h 101,8USD 105,2USD

Ch cn 1 th my


40

CHNG VIII LA CHN A IM CA DOANH NGHIP1. Phng php cho im c trng s Cc bc tin hnh

Lp bng k cc yu t nh hng cn xem xt. Xc nh trng s cho tng yu t ty theo mc quan trng ca chng. Quyt nh thang im t 1 10 hoc t 1 100. Hi ng qun tr tin hnh cho im theo thang im quy nh. Ly s im ca tng yu t nhn vi trng s. Tng hp s im ca tng a im nh la chn v chn a im no c tng s im cao nht. V d : Mt doanh nghip cn chn mt a im xy dng nh my. Qua nghin cu s b thy c th chn 1 trong 2 a im thuc hai tnh A v B. Dng phng php cho im c trng s so snh hai a im ny v cho bit nn chn a im no ? Kt qu trnh by trong bng di y. Dng thang im t 1 n 100. 1. 2. 3. 4. 5.Cc yu t Trng s im s A B 70 60 im c trng s A B 0,25 x 70 = 17,5 0,25 x 60 = 15,0

Gi nhn cng v thi Giao thng vn ti Gio dc, chm sc sc khe Cu trc thu Ti nguyn v nng sut Cng

0,25 0,05 0,10 0,39 0,21 1,00

50 85 75 60

60 80 70 70

0,05 x 50 = 2,6 0,1 x 85 = 8,5 0,39 x 75 = 29,3 0,21 x 60 = 12,6 70,4

0,05 x 60 = 3,0 0,1 x 80 = 8,0 0,39 x 70 = 27,3 0,21 x 70 = 14,7 68,0

Qua bng trn ta thy v tr A c tng s im xt n trng s cao hn v tr B. Nhng s chnh lch v im s khng ln. T m hn ta c th nh gi nhy ca tng s im i vi trng s hoc thang im bng cch thay i trng s hoc thang im ri tnh li tng s im mi. Nu kt qu vn xp x th ta c th a thm vo hoc gim bt i cc yu t cn xem xt.2. Phng php im ha vn Phng trnh xc nh im ha vn nh sau : y1 = a.x (1)

y2 = b.x + c (2)

Trong :

a gi bn 1 sn phm (/ci) b bin ph cho 1 sn phm (/ci) c nh ph tnh cho 1 nm (/nm) x s sn phm bn ra trong 1 nm (ci/nm)

Nh vy vic la chn a im doanh nghip chu nh hng ch yu 2 yu t nh ph v bin ph, nn ta dng phng trnh (2) xc nh a im. V d : Cng ty A cn chn mt a im xy dng nh my sn xut my cng nghip loi nh. C 3 im c a ra so snh l I, II, III. Qua iu tra, tnh ton c c bng cnh bn. Hi cng ty A nn chn a im no ? http://www.ebook.edu.vn

a im

I II III

nh ph hng nm 300 600 1100

Bin ph 1 sn phm 0,75 0,45 0,25

41

Trng hp 1 Khi cng sut c xc nh Chng hn cng sut xc nh bng 2000 sn phm/nm Theo (2) c : yI = 0,75 x 2000 + 300 = 1800 triu ng yii = 0,45 x 2000 + 600 = 1500 triu ng yiii = 0,25 x 2000 + 1100 = 1600 triu ng a im II cho tng chi ph nh nht. Vy nh my nn t ti II. Trng 2 Khi cng sut cn cha khng nh

Vn s dng (2) cho x bin thin c : yI = 0,75x + 300 yii = 0,45x + 600 yiii = 0,25x + 1100

Chi ph trong nm (tr.)

Biu din th

Vng I thch hp 1000

Vng II thch hp 2500

Vng III thch hp

Cng sut

3. Phng php ta 1 chiu v ta 2 chiu Phng php ta 1 chiu

L=

1 W

Widi

p dng trong trng hp cc c s i nm trn 1 trc no . Phng php ta 2 chiu 1 Cx = W Cy = 1 W

Trong : L Ta ca a im mi (Km) Wi Lng hng vn chuyn n c s i (1, 2, 3, , n) di Ta ca c s i so vi 1 im no ly lm gc ta . W Tng lng vn chuyn phi ch n n c s.

dix.Wi diy.Wi

(1)

(2)

Cx Ta x ca c s mi Cy Ta y ca c s mi dix Ta x ca c s I hin c diy Ta y ca c s i hin c WI Lng vn chuyn n c s i W Tng lng vn chuyn n tt c cc c s i.

V d : Nh my bia A c kho phn phi ta ly theo bn (59,40) kho ny cp hng cho 6 i l c cho theo bng di y. C s hin c (i) Ta d(x,y) Lng vn chuyn i l I (58;54) 100 i l II (60;40) 400 i l III (22;76) 200 i l IV (69;52) 300 i l V (39;14) 300 i l VI (84;14) 100 Cng W = 1400

p dng cng thc (1) v (2) ta c : 58 x 100 + 60 x 400 + 22 x 200 + 69 x 300 + 39 x 300 + 84 x 100 Cx= --------------------------------------------------------------------------------- = 60 1400 http://www.ebook.edu.vn

42

54 x 100 + 40 x 400 + 76 x 200 + 52 x 300 + 14 x 300 + 14 x 100 Cy= --------------------------------------------------------------------------------- = 41,28 1400 Nh vy kho mi xc nh nm gn kho hin c, do khng cn xy kho mi. 4. Phng php bi ton vn ti Phng php ta 1 chiu hoc 2 chiu l phng php ly trng tm mi xt n lng vn chuyn nhng cha xt n chi ph vn chuyn. gii quyt vn ny ta cn s dng bi ton vn ti.V d : Cng ty X hin c 2 nh my t ti thnh ph A v th x B. Sn phm ch yu c cp cho cc i l nm 2 th x I v II. Do nhu cu th trng ngy cng tng cng ty quyt nh lp thm 1 nh my th 3. D kin c th t th x C hoc th x D. Chi ph sn xut v chi ph vn chuyn t cc nh my n cc i l cho theo bng di y : Nh my Chi ph SX tr./T 5,3 5,2 5,0 4,8 Chi ph vn chuyn tr./T i l I i l II 1,7 1,8 3,8 1,0 0,9 2,0 1,8 1,2 8 12 Sn lng T/ngy 6 9 5 5 20

A B D kin C D Nhu cu (T/ngy)

Hin c

Hy tnh xem nn chn a im cho nh my ti C hay D ? quyt nh chn C hay D chng ta cn gii 2 bi ton vn ti.i l Nh my

i l I 7 9 5,9 8

i l II 7,1 6,2 7 12

Sn lng 6 9 5 20

i l Nh my

i l I 7 9 6,6 8

i l II 7,1 6,2 6 12

Sn lng 6 9 5 20

A B C Nhu cu

A B D Nhu cu

Tng chi ph ca bi ton 1 : 127,6 triu

Tng chi ph ca bi ton 2 : 129 triu

Nu so snh tng chi ph ca 2 bi ton chng ta d dng rt ra kt lun cn chn bi ton 1, c ngha chn th x C v tng chi ph bi ton 1 < tng chi ph bi ton 2.

CHNG IXB TR MT BNG I. B TR MT BNG THEO NH HNG CNG NGHn n

Tiu thc la chn : Trong :

C = Cij. Xiji =1 j =1

C chi ph vn chuyn ni b Cij chi ph vn chuyn 1 n v hng t b phn i sang b phn j. Xij khi lng hng vn chuyn t b phn i sang b phn j. http://www.ebook.edu.vn 43

V d : Mt phn xng c mt bng chiu di 18m, chiu ngang 12m, c b tr thnh 6 b phn c kch thc bng nhau v bng 6x6m theo s di y : I IV 1 2 3 4 5 6 1 II V III VI 50 Chi ph vn chuyn 1 n v hng gia 2 b phn lin k 1 USD. Chi ph vn chuyn 1 n v hng gia 2 b phn khng lin k 2 USD. Khi lng hng vn chuyn c cho theo bng di y. 100 30 0 50 20 0 10 0 50 5 20 0 100 0 0 6Gii Bc 1 : V s gin lc biu th khi lng hng vn chuyn gia cc b phn v tnh tng chi ph vn chuyn ni b. TC1 = (50 x 1) + (100 x 2) + (20 x 2) + (1 x 30) + (50 x 1) + (10 x 1) + (20 x 2) + (100 x 1) + (50 x 1) = 570USD

Hy xem xt vic b tr cc b phn hp l cha ? Cn b tr li nh th no ?

2

3

4

Qua s chng ta nhn thy cc khi lng vn chuyn ln (> 50 n v) u c t gn nhau, nhng gia b phn I v III c khi lng vn chuyn ln 100 n v nhng hai b phn ny xa nhau. Do cn b tr li nh sau :

Bc 2 : V s gin lc biu th khi lng hng vn chuyn gia cc b phn theo phng n mi. TC2 = (50 x 1) + (100 x 1) + (20 x 1) + (30 x 2) + (50 x 1) + (10 x 1) + (100 x 1) + (50 x 1) = 480 USD

Vy phng n b tr li so vi phng n c tit kim c chi ph vn chuyn ni b : 570 USD 480 USD = 90 USD II. B TR MT BNG THEO NH HNG SN PHM V d : lp rp mt sn phm A, thi gian hon thnh l 66. Cc cng vic c thi gian v th t thc hin theo bng sau. Hy b tr mt bng lp rp sn phm A ?Cng vic A B C D E F G H I Tng cng T (pht) 10 11 5 4 12 3 7 11 3 66 Th t thc hin

-

Sau A Sau B Sau B Sau A Sau C, D Sau F Sau E Sau G, H

Bc 1 : Xc nh chu k SX Thi gian lm vic trong ngy Chu k SX=--------------------------------------S sn phm cn SX trong ngy 8 x 60 Chu k SX = ----------= 12 40


44

Bc 2 : Xc nh s khu ti thiu trn dy chuyn. Tng thi gian thc hin cc cng vic Nmin=-----------------------------------------------Chu k SX 66 Nmin=----= 5,5 = 6 khu 12 Bc 3 : B tr cc khu trn dy chuyn Nguyn tc : 1. Cng vic no c thi gian bng hoc xp x thi gian chu k b tr ring 1 khu 2. Nhng cng vic no c thi gian < thi gian chu k v c lin h vi nhau v cng ngh, ghp thnh 1 khu sao cho tng thi gian ca chng bng hoc xp x thi gian chu k. 3. Cng vic no c thi gian > thi gian chu k gii quyt bng 2 cch : Phn chia cng vic thnh cc cng vic b phn c thi gian bng hoc xp x TG chu k. B tr nhiu my ng thi thc hin cng vic , thi gian thc hin s bng c s ca s my b tr.C D

B A E

F

G I H

Bc 4 : Xc nh hiu nng ca dy chuyn : Tng thi gian thc hin cc cng vic E = -------------------------------------------------- x 100 Chu k SX x S khu c b tr trn /c 66 E = ----------- x 100 = 91,7% 12 x 6

CHNG X QUN TR TN KHOI. PHN TCH NHY Trong m hnh EOQ chng ta xc nh lng hng ti u theo cng thc :

Q* =

2.S .D H

vi gi nh cc tham s u bit trc v khng i. c cch nhn y hn cn phn tch nhy ca Q* bng cch thay i mt cch c h thng cc tham s nhm xc nh s nh hng cc tham s vi Q*.(1) iu g s xy ra nu D tng ln ?

Nu D tng th Q* s tng theo t l cn bc hai ca D nhng thp hn nhu cu thc t. Nu D tng nhng tng s hng chun m khng tng s hng lm theo yu cu khch hng th Q* tng nhng khng bng nhu cu thc t. (C ngha nu D1 v D2 u thuc s hng chun th Q* < Q1* + Q2*) http://www.ebook.edu.vn 45

(2) iu g s xy ra nu ph t hng gim ?

S gim s lm cho Q* gim tc l gim mc tn kho theo chu k ca c l hng nh hn s hiu qu hn do cc NSX lun ch n vic ct gim thi gian v chi ph t hng. Nu chi ph hiu chnh khng ng k th ngi ta s gim chi ph t hng c l hng nh.(3) iu g s xy ra khi mc li sut gim (interest rates) ?

Li sut hay ph tn vn (cost of capital) l mt b phn ca chi ph tn tr (H), nn li sut gim s lm H gim v Q* s tng ln. Nh vy lng hng tn kho theo chu k s gim theo cn bc hai ca H.(4) Cc sai s c tm quan trng nh th no trong vic nh gi D, H v S ?

Q* l hm s ca cn bc hai ca cc tham s D, H, S nn sai s ca chng khng nhy c th nh hng n Q*. Hn na mt sai s ca bin c ny c xu hng trit tiu sai s ca mt bin s khc. Nn dn n Q* c tnh vng chc.II. NHNG HN CH CA M HNH EOQ :

1. M hnh EOQ ch p dng khi nm gi nh nu c tho mn. S gii hn ny dng nh lm mt hiu lc s dng m hnh bi v m hnh c gi nh trong tnh hung qu gin n v c xem l khng hin thc. 2. Trong thc t cn xem xt c l hng khc nhau l cn thit phn nh mc chit khu theo sn lng, ch khng nn chn Q* c nh. Tuy nhin m hnh EOQ l m hnh hp l, gn ng u tin v c l hng trung bnh, thm ch khi nhiu trong s cc gi nh khng tho mn th m hnh EOQ vn ng. xem xt mt cch tha ng hn, gi nh nhu cu vt t trong nm bit trc v n nh, chng ta c th p dng 2 h thng kim tra : H thng kim tra lin tc (thng xuyn) H thng kim tra nh k.III. H THNG KIM TRA THNG XUYN : H THNG Q

H thng kim tra thng xuyn s cho chng ta bit s lng mt loi hng cn bao nhiu, khi c mt lng hng c cp pht i. My tnh v cc my ghi trn mt bng in t c ni lin vi h thng ghi chp v hng tn kho. Nu h thng kim tra bo : mc tn kho qu thp th s c lnh t hng mi. V tr hng tn kho IP nh gi kh nng mt loi hng no c p ng nhu cu tng lai phi cn c vo : - Lng hng nhn c theo tin (scheduled receipts) - Lng hng tn kho c sn trong kho (on hand inventory) IP = OH + SR BO IP : V tr tn kho ca mt loi hng (tnh bng n v) OH : S n v hng c sn trong kho SR : S hng nhn c theo tin BO : L n hng chu, c ngha cam kt hoc k hp ng vi khch hng ri nhng cha thc hin hoc giao hng ri nhng b thiu ca khch hng, khch hng chp nhn cho thiu hng v cung cp sau. H thng kim tra thng xuyn cn gi l h thng im t hng li (reorder point system) Cn t hng Q n v khi vic cp pht hng ra khi kho lm cho s lng hng tn kho gim xung bng im t hng li l mc ti thiu cho php.


46

n t hng

Tn kho sn c Rn hng t

L

L

Thi gian

ng dc i xung th hin hng tn kho ang c ly ra t t s dng, khi mc hng tn kho tt n im t hng li th n hng Q mi c t. Khi t hng, lng hng tn kho vn tip tc gim trong sut thi gian vn chuyn L cho n khi n hng nhn c, nh du kt thc thi gian vn chuyn v hng tn kho vt ln theo ng thng ng Q n v. Ch sau khi mt n hng mi c t nh du bt u ca thi gian vn chuyn th IP mi bt u tng ln mt lng Q. L hng mi c nhn (ng t nt) IP > OH = mt lng hng ng bng lng hng tn kho trong sut thi gian vn chuyn. Ti cui thi gian vn chuyn khi lng hng nhn c theo tin bin i thnh lng hng c sn trong kho tr li. im quan trng mu cht y n gin l so snh IP vi R ch khng so snh OH vi R trong vic quyt nh t hng hay khng ?V d 1: Xc nh thi im t hng li. Bit rng tn kho hin ti ca mt loi hng ch c 10 n v v im t hng li R = 100, khng c cc n hng chu, nhng c mt n hng nhn c theo tin l 200 n v. Vy c nn t mt n hng mi hay khng ? Gii

IP = OH + SR BO IP = 10 + 200 0 = 210 IP > R (210 > 100) nn khng nht thit t hng li. La chn im t hng li Trong m hnh EOQ gi nh : Nhu cu bit trc v n nh. Thi gian vn chuyn khng i. Ngi cung ng lun lun vn chuyn chnh xc s lng hng vo ng thi gian. Nn im t hng li l thi im m lng hng trong kho tng ng vi nhu cu cho sut thi gian vn chuyn v mt n hng mi s c nhp v khi lng hng trong kho = 0. Trong thc t khng phi d dng d on nh vy v nhu cu thay i rt ngu nhin. Nn tt hn ht khi lng hng gim n mc > R phi nn t hng ri, to ra mt lng hng d tr an ton phng s bin dng ca cu. C th hn im t hng li l : R = DL + B R : im t hng li DL : nhu cu bnh qun trong sut thi gian vn chuyn (L) B : d tr an ton (safety stock) DL xc nh ph thuc vo nhu cu khch hng, m nhu cu ca khch hng lun bin ng. Do la chn R cn li ch yu phi xc nh cho c mc d tr an ton B. Quyt nh mc d tr an ton ln hay nh ngi ta cn c vo mi tng quan ca 2 chi ph : Chi ph tn kho Chi ph thit hi do thiu hng http://www.ebook.edu.vn 47

Tuy nhin chi ph thit hi do thiu hng (stock out) rt kh xc nh do cc nh qun tr a ra 1 chnh sch v mc dch v hp l xc nh mc d tr an ton tho mn chnh sch ny. La chn chnh sch v mc dch v hp l Xc sut mong mun khng xy ra s thiu hng trong bt k mt chu k tn kho no c gi l mc dch v theo chu k. T l nhu cu hng nm c p ng nh c d tr gi l mc p ng (c tnh bng n v hng, s n hng) S hng b thiu ht trong nm (tolerated) T l s ngy c p ng trong nm i vi mt loi hng (c ngha s ngy khng b thiu hng) S thiu ht thng xy ra vo cui chu k tn kho (thnh thong c xy ra thiu trong thi k vn chuyn) Mt gi nh quan trng : DL c xem nh nhu cu trong sut thi gian vn chuyn L l phn phi chun nh hnh di y :Mc dch v theo chu k Xc sut xy ra thiu hng (1mc dch v theo chu k)

ZL R DL

Phi nh gi c gi tr trung bnh v lch chun ca phn phi ny t s liu qu kh hoc da trn s iu chnh. Tnh ton d tr an ton : B = Z.L Trong : - Z : lch chun t gi tr trung bnh b sung vo mc dch v theo chu k. - L : lch chun ca phn phi xc sut DL (*) Trong chng d bo c trnh by = 1,25 MAD hay MAD =0,866 Gi tr Z cng ln th mc d tr an ton B v mc dch v theo chu k cng cao. Nu Z = 0 th khng c d tr an ton v s thiu hng s xy ra trong sut 50% ca chu k. D bo nhu cu v d bo v thi gian vn chuyn cng chnh xc th gi tr L v B s cng nh. Hay ni cch khc d bo cng chnh xc th d tr an ton cng thp.V d 2 : Gi s nhu cu trong sut thi gian vn chuyn l phi xc sut chun vi DL = 36 n v v L = 15. Hy xc nh mc d tr an ton sao cho mc dch v theo chu k l 90%. Hy xc nh im t hng li ? Gii

u tin phi tnh Z, lch chun v pha bn phi ca DL.

45%DL =

Z 48


Tra bng ph lc 3 v tm 0,90 s gn nht l 0,8997 tng ng vi 1,2 v 0,08. Ta cng cc gi tr ny ta c Z = 1,28. Vi s liu ny ta c th tnh ton c mc d tr an ton B nh sau : B = Z.L = 1,28 x 15 = 19,2 hay 19 R = DL + B = 36 + 19 = 55 Vy R = 55, B = 19 Nh vy la chn im t hng li R i hi s hiu bit v lch chun ca nhu cu trong sut thi gian vn chuyn (L). Nhng tham s ny kh c c t kt qu ghi chp thng k trong qu kh. Nu cc phn phi xc sut nhu cu hng ngy l c lp, ta c : L = t . Lt Trong : t : lch chun v nhu cu c bit trong thi gian t L : lch chun ca nhu cu trong sut thi gian vn chuyn Lt : thi gian vn chuyn L c din t nh mt tch s ca mt phn thi gian t.V d 3 : lch chun v nhu cu cho 1 tun c nh gi t bo co ghi chp qu kh l 50 n v hng, thi gian vn chuyn l 4 tun. Vy hy xc nh lch chun ca nhu cu trong sut thi gian vn chuyn 4 tun. Gii t = 1 tun Lt = 4 tun Vy L l 100 n v (**) (**) Cng thc ny trong gio trnh thng k, lch chun ca hai hay nhiu bin s c lp ngu nhin ging ht nhau.

L = 502 + 502 + 502 + 502 = 50 4 = 100

L = 50 4 = 50 x 2 = 100Nu gi nh DL c xem nh nhu cu trong sut thi gian vn chuyn L l phn phi xc sut phn tn. t R sao cho cc kh nng ca nhu cu tng mc ca n.V d 4 : Tnh R v B cho mt phn phi theo xc sut phn tn (ri rc) DL. Nhu cu trong sut thi gian vn chuyn ca mt loi hng c cho nh sau : DL 10 20 30 40 50 60 Xc sut 0,10 0,15 0,20 0,25 0,20 0,10 1

Thi gian vn chuyn 2 tun, nhu cu bnh qun 18 n v, qun tr gia chn mc dch v theo chu k 90% (9/10 chu k). Ni cch khc, thi gian duy nht m xy ra thiu hng khi DL = 60, m ch xy ra 10% ca thi gian. Xc nh im t hng li R ? Xc nh d tr an ton B ? http://www.ebook.edu.vn 49

Gii

R = 50 xc sut m s lng ny trnh s thiu ht hng trong sut thi gian vn chuyn L = 0,90. 0,90 = (0,10 + 0,15 +0,20 + 0,25 +0,20) Gi tr mong i ca phn phi theo nhu cu trong sut thi gian vn chuyn DL l : DL=10x0,1+20x0,15+30x0,2+40x0,25+50x0,2+60x0,1 DL=36 n v Tnh ton gi tr ca B cho R=50 n v hng l : R = DL + B 50 = 36 + B B = 14 n v B v R c tnh trong v d 4, khc vi B v R trong v d 3 v trong v d 3 tnh theo phn phi chun cn trong v d 4 tnh theo phn phi ri rc. B l thng tin quan trng gip cc nh qun tr xc nh mc d tr an ton no l tn km. Theo phn phi chun ta phi tnh B suy ra R Theo phn phi ri rc phi tnh R ri mi xc nh c BIV. H THNG R SOT NH K H THNG P (Periodic Review Systems or P systems)

Trong h thng r sot nh k mc hng tn kho c kim tra nh k. Mt n hng mi c t vo cui mi k kim tra, s thi k gia cc n hng l c nh. Nhu cu l mt bin s ngu nhin v vy tng nhu cu gia cc ln kim tra s khc nhau do lng hng c t s khc nhau. S khc bit gia hai h thng Q v P c th hin nh sau : Trong h thng Q, Q l c nh nhng thi gian gia cc n hng thay i. Trong h thng P, Q thay i nhng thi gian gia cc n hng c nh. thc hin theo h thng ny cn thc hin nh sau : - Kim tra mc tn kho IP ca mt loi hng no theo nh k. - t mt n hng = T IP trong T l mc tn kho mc tiu.Lng tn kho sn IP T IP

Q Q1 (n hng nhn c

im t hng li

L

Thi gian P


50

ng i xung th hin lng tn kho sn c. Sau kim tra ln 1 mt n hng c t l Q1 = T IP1. Sau kim tra ln 2 mt n hng c t l Q2 = T IP2. Nhng Q2 > Q1 v IP1 > IP2. C ngha chng ta phi nng mc tn kho ln bng mc tn kho mc tiu T sau thi im kim tra.V d 5 : Xc nh s lng t hng li l bao nhiu, nu c mt n hng chu (Back Order) l 5 n v. Loi hng ny khng c hng d tr sn trong kho. Mc tn kho mc tiu T = 400 n v v khng c n hng nhn c theo tin . Gii

IP = OH + SR BO = 0 + 0 5 = -5 n v Vy Q = T IP = 400 (-5) = 405 n v. Vy cn t mt lng hng 405 n v nng mc tn kho ln mc tn kho mc tiu T. La chn khong cch t hng li. (Selecting the reorder interval) H thng P ch c hai tham s P v T Trc tin chng ta cn xc nh P nu P c tnh bng tun, trong nm c 52 tun th ta c : Q P= ----- x52 D Sau la chn mc tn kho mc tiu : Nhn vo th chng ta c th thy rng, phi c mt n hng ln dng trong khong thi gian P, thm ch phi vt qu ln kim tra k tip. Ngoi ra mt n hng mi c t s mt L thi gian vn chuyn do cn c mt lng hng tn kho m bo an ton trong khong thi gian P+L. V vy T t nht phi bng nhu cu mong i trong sut khong thi gian P+L. Khi B c b sung vo, m bo chc chn khng thiu ht trong sut thi gian P+L. T = DP+L + B = DP+L + ZP+LTrong : - DP+L : Nhu cu bnh qun n k kim tra k tip +nhu cu bnh qun tnh cho thi gian vn chuyn. - P+L : lch chun trong sut thi gian P+L - Z : Mc chnh lch so vi gi tr bnh qun. Bi v h thng P i hi mc d tr an ton cho thi gian di hn so vi h thng Q nn mc d tr an ton cho h thng P ln hn. P+L > L.

V d 6 : Tnh P v T Nhu cu mt loi hng c phn phi chun, vi gi tr trung bnh l 40 n v/ tun v lch chun ca nhu cu hng tun l 15 n v. Thi gian vn chuyn 3 tun. Doanh nghip hot ng 52 tun 1 nm v s dng h thng kim tra theo nh k (P). Xc nh P l bao nhiu chi ph xp x vi m hnh EOQ vi Q* = 400 n v ? Xc nh lng hng tn kho mc tiu bit rng mc dch v theo chu k l 80% ? Gii

Trc ht ta phi tnh nhu cu D sau mi tnh P D = 40x52 tun = 2080 n v. P = (400x52)/2080 = 10 tun. Nh vy chng ta kim tra 10 tun 1 ln. xc nh lch chun ca nhu cu trong sut thi gian (Pt + Lt) ta c : Pt + Lt = 10 + 3 = 13 tun. http://www.ebook.edu.vn 51

Pt : l khong cch thi gian gia hai ln kim tra

P + L = t Pt + Lt = 15 13P+L = 54 n v tnh T cn tnh Z, ta tra bng xc sut phn phi chun ca 0,8 th Z =0,84 T = D P + L + Z P + L T = 40 x 13 + 0,84 x 54 = 565 n v Vy mi ln cn t hng vi s lng hng l 565 n v. So snh nhng u im ca h thng P v Q : u im ca h thng P u im ca h thng Q 1. Khong cch thi gian b sung hng tn kho 1. C th xc nh r khong cch thi gian b c nh to thun li cho nh qun tr. sung n hng. 2. C th kt hp nhiu n hng vo cng mt 2. Ph hp vi n hng chit khu theo sn ngi cung ng. lng hoc c gii hn v cng sut. 3. H thng tn kho n nh (d tr an ton 3. D tr an ton t. nhiu)V. H THNG PHI HP (H THNG LAI) HYBRID SYSTEM 1. H thng b sung c la chn : (i khi cn c gi l h thng kim tra c la chn). L kim tra mc tn kho vo nhng thi im c nh, nu mc tn kho gim xung di mt mc xc nh trc th s t mt n hng bng c l hng thay i sao cho c th p ng c nhu cu mong i. n hng mi ln a mc tn kho ln n mc tn kho mc tiu. Tuy nhin cc n hng khng c t sau khi c s kim tra, tr khi mc tn kho IP gim xung mc ti thiu. Mc ti thiu coi nh im t hng li R trong h thng Q. Tc dng ca n m bo c s lng t hng hp l c t. Nu mc tn kho mc tiu l 100 v mc tn kho ti thiu l 60 th c t hng ti thiu l 40 = 100 60. H thng ny trnh vic kim tra thng xuyn nu chi ph kim tra v chi ph t hng ln. 2. H thng d tr gc (A base stock system) H thng ny c t hng b sung khi c vic xut hng ra khi kho. S lng hng c t tng ng s lng hng xut ra. H thng ny duy tr mc tn kho IP bng mc d tr gc, tng ng vi nhu cu mong i trong sut thi gian vn chuyn L + d tr an ton. Mc d tr gc, tng ng vi im t hng li R trong h thng Q. H thng ny m bo lng hng tn kho thp. 3. H thng trc quan (visual system) Cn c vo mc s dng qu kh xc nh n t hng h thng trc quan c p dng cho loi hng c gi tr thp v chi ph tn kho thp. Hai hnh thc c bn c dng l h thng mt thng v h thng hai thng : H thng mt thng : Mc tn kho ti a c nh du trn k hay trn thng hoc bng mt thanh o. Mc tn kho c tng n ch nh du mt cch nh k. V d : cc bn cha xng ti cy xng, v cc thng cha ph tng cc nh my ch to. Phng php ny l mt h thng P vi mc tn kho mc tiu c thit lp mt cch trc quan. H thng hai thng : hng tn kho c ct gi hai thng khc nhau. Thng th nht c dng trc v khi ht thng th nht s c bo hiu cn c mt n hng mi


52

CHNG XI PHNG PHP KIM TRA QU TRNH THNG K(Statistical Process Control Methods SPC) Khi nim SPC

SPC Kim tra qu trnh thng k dng kim tra cht lng ton b qu trnh hot ng, hiu qu ca SPC l o lng c cht lng hin ti ca sn phm v pht hin nhng nguyn nhn lm thay i cht lng trong qu trnh.Trnh t tin hnh

-

Chn mt mu ngu nhin kim tra cht lng Lng ho c trng cht lng : nu o lng mu nm ngoi gii hn kim tra (UCL v LCL) th cn tm nguyn nhn gy ra nh hng . (V d : my mc h hng, nguyn vt liu km cht lng, nh qun tr khng c kinh nghim...)Gii hn kim tra (a) UCL (b) UCL

LCL

LCL

(a) Trng hp s o ca mu trong gii hn kim tra (b) Trng hp s o ca mu c xu hng bin i r rt.(c) UCL (d) UCL

LCL

LCL

(c) Trng hp s o ca mu c bin i t bin sau mt qu trnh n nh tuy cha vt ra khi gii hn nhng cn tm r nguyn nhn xy ra hin tng ny v nu tnh trng ny ko di s dn n sn phm km cht lng. C th do my mc h hng cn thay th t c gi tr o lng mu nm trong gii hn kim tra n nh, ng thi c th ko di tui th ca my. (d) Trng hp s o ca mu vt ra khi gii hn kim tra, cn kim tra ngng sn xut. im mu cht ca SPC l quan st nhng bin i s o ca mu so vi gii hn ca UCL v LCL c thit k theo tiu chun xc nh nhm pht hin nhng bin i tht thng tm ra nguyn nhn khc phc nhm m bo cht lng ca qu trnh hot ng.


53

BNG KIM TRA THUC TNH CHT LNG Bng P Khi nim : Bng P vi i lng c trng l t l ph phm i din. S dng rt thng thng cho cc thuc tnh cht lng. Trnh t tin hnh 1. Chn mt mu ngu nhin 2. Kim tra tng phn t trong mu xc nh s ph phm c trong mu ngu nhin . S ph phm trong mu 3. Tnh P = ----------------------------------------Tng s sn phm c trong mu

4. V ng biu din t l ph phm ln th v so snh vi gii hn kim tra xc nh qu trnh sn xut c nm ngoi gii hn khng ?V d : Cc thuc tnh cht lng c s dng trong Ngn Hng : Cc thuc tnh T l so vi 1. Tng s tin gi ngn hng 1. S ti khon sai c ghi chp 2. Tin gi NH khng c chuyn nhng 2. Tng s tin gi ngn hng 3. Khon b st 3. Tng s nhn xt 4. S giao dch ngng thanh ton sai 4. Tng s s giao dch ngng thanh ton 5. Khon b nhm 5. Tng sec v n 6. Nhng nhn xt c gi i 6. Tng s bn nhn xt 7. S iu chnh sai 7. Tng s bn nhn xt Cch tnh : - Do vic chn mu thuc tnh lin quan n vic xt s ph phm nm trong hay ngoi gii hn kim tra nn ngi ta p dng lut phn phi nh thc (binominal distribution). Tuy nhin nu kch thc mu ln th p dng lut phn phi chun s thch hp hn - lch chun

P(1 P) n Trong : n kch thc mu (s phn t trong 1 mu). P t l ph phm trung bnh c xc nh trong qu kh.

P =

-

Gii hn Gii hn

UCL = P + Z

P(1 P) n

P(1 P) n - Theo quy tc 3 sigma -> Z=3 c ngha 99,74% nhng bin i thuc tnh ca cht lng ri vo gii hn kim tra, nh vy xc sut rt nh cho cc trng hp bin i thuc tnh cht lng nm ngoi gii hn kim tra 0,26%. (C 400 trng hp th c 1 trng hp nm ngoi gii hn kim tra) Khi mt t l ph phm nm ngoi gii hn kim tra, chng ta cn thay i qu trnh sn xut m bo cht lng. LCL = P Z http://www.ebook.edu.vn 54

V d : Kch thc mu n = 2500, ngi ta tin hnh kho st 12 mu v c kt qu theo bng. S th t mu S sn phm hng trong mu T l P 1 15 0,0060 2 12 0,0048 3 19 0,0076 4 2 0,0008 5 19 0,0076 6 4 0,0016 7 24 0,0096 8 7 0,0028 9 10 0,0040 10 17 0,0008 11 15 0,0046 12 3 0,0012 147

P=147:(12 x 2500) = 0,0049UCL = 0,0049 3 0,0049(1 0,0049) = 0,00909 0,00071 2500

Qua bng trn ta thy mu 7, t l ph phm vt ra khi gii hn kim tra, cn phi tm ra nguyn nhn hiu chnh nhm lm cho qu trnh sn xut n nh hn. Sau khi loi b mu 7 ta c T UCL = 0,00847 LCL = 0,0047 P= 147 24 = 0,00447 11x 2500

XC NH T L KHUYT TT CA MI N V SN PHM Khi nim : Bng C xc nh t l khuyt tt ca mi n v sn phm Phm vi p dng : p dng trong trng hp khng xc nh c t l ph phm nh bng P. (VD: khng th c tnh t l nhng vt try xc cho mt nhm my ht bi). Phn phi p dng L phn phi Poisson, vi trung bnh phn phi l C v lch chun l cn bc hai ca C gii hn trn UCL = C + 2 c LCL = C - 2 c

nn

V Z = 2 theo quy tc 2 sigma V d : S dng bng C kim tra li cho sn phm ca cng ty giy W. Khi thnh phm qua my o cht lng th ngi ta thy trung bnh c 20 li/01 cun giy. C ngha C=20. - Tp hp 1 bng kim tra tm s li mi cun giy - S dng quy tc 2 sigma ta c : UCL = 20 + 2 20 LCL = 20 -2 20 - Nu cc cun giy kim tra ch c 27 li th qu trnh sn xut vn nm trong gii hn kim tra. Nu cc cun giy kim tra ch c 5 li th qu trnh sn xut nm ngoi gii hn kim tra, chng ta cn xc nh r nguyn nhn hon thin qu trnh sn xut hn na bng cch iu chnh tr s li trung bnh xung http://www.ebook.edu.vn 55

BNG KIM TRA S BIN THIN CA CHT LNG Khi nim - Hai bng kim tra bin thin ca cht lng gm bng R v bng X - Hai bng ny a ra nhng gii hn bin thin m chng ta c th kim tra c thng qua 2 i lng : Gi tr trung bnh Phm vi dao ng mu i vi c trng cht lng no - Hai bng R v X c xc nh trc khi kim tra qu trnh sn xut Bng R (Range chart) bng gii hn dao ng a- iu kin thit k : Yu cu kh nng thay i qu trnh phi nm trong gii hn kim tra th bng R xy dng mi m bo tnh hp l v chnh xc, ng thi ko theo mc chnh xc ca bng X. b- Gii hn kim tra ca bng R LCLR = D3.R UCLR = D4.R Trong : R (gi tr trung bnh ca nhng gi tr R trong qu kh) D3, D4 (l nhng h s cung cp t gii hn lch chun theo quy tc 3 sigma vi n cho trc) Cc h s tnh ton vi gii hn 3 i vi bng X v bng R Kch Cc h s tnh UCL v Cc h s tnh ton Cc h s tnh ton LCL i LCL i vi bng R vi bng R (D4) thc mu LCL ca bng X (A2) (D3) 2 1,88 0 3,267 3 1,023 0 2,575 4 0,729 0 2,282 5 0,577 0 2,115 6 0,483 0 2,004 7 0,419 0,076 1,924 8 0,373 0,136 1,864 9 0,337 0,184 1,186 10 0,308 0,223 1,777 BNG X (Bng trung bnh)

a- iu kin thit k : Yu cu kh nng thay i qu trnh phi nm trong phm vi kim tra th bng X mi m bo tnh hp l v chnh xc. b- Gii hn kim tra bng X LCLX = X A2R UCL = X + A RX 2

X : l gi tr trung bnh ca gi tr trung bnh cc mu kim tra trong qu kh; cng c th l gi tr trung bnh ca mc tiu thit lp cho qu trnh. A2 : h s ly t gii hn lch chun theo quy tc 3 sigma c tra bng trn. V d : Cng ty West Allis sn xut c vt c bit cung cp cho khch hng ln nht ca h. ng knh c vt ang c vn trong qu kh X=0,500 v R=0,18. D liu ca 4 mu kim tra c cho theo bng sau : S liu S trung Mc dao S o c ca 4 mu mu bnh ca 4 ng 1 2 3 4 mu 1 0,51 0,63 0,39 0,35 0,47 0,28 2 0,50 0,56 0,42 0,64 0,53 0,22 3 0,68 0,49 0,53 0,62 0,58 0,19 4 0,45 0,33 0,47 0,55 0,45 0,22 5 0,70 0,58 0,64 0,68 0,65 0,12 http://www.ebook.edu.vn 56

Hy tnh UCLR v LCLR ?Gii : UCLR = D4. R = 2,282*0,18 = 0,411 LCLR = D3. R = 0*0,18 = 0 Nh vy nu ta em biu din cc gi tr ca mu kim tra ln th v nhn thy rng : khng c mu no c dao ng nm ngoi gii hn cho php. Nu bt k mt dao ng ca mu nm ngoi gii hn cho php th chng ta phi tm ra nguyn nhn s bin thin ca ng knh c vt. Cng ty li tip tc xc nh :

UCLX

v

LCLX

UCLX = X + A2 . R = 0,500 + 0,729*0,18 = 0,631 LCLX = X A2 . R = 0,500 0,729*0,18 = 0,369 Kt qu ng knh c vt ca cng ty West Allis nm ngoi gii hn kim tra mu s 5. Lc ny chng ta cn tm r nguyn nhn c th xc nh hiu chnh qu trnh sn xut.0,5 0,4 0,3 0,2 0,1 R = 0,18 LCL = 0CC VN CN QUAN TM TRONG QU TRNH KIM TRA THNG K

Phm vi dao ng

UCLR = 0,411

Hai vn trung tm trong qu trnh kim tra thng k cht lng l : La chn kch thc mu Gii hn phm vi kim tra 1. Kch thc mu

Vic kim tra cht lng sn phm hoc dch v khng th tin hnh ton b m ty thuc vo : Tnh cht thit k k thut Chi ph kim tra Yu cu hy b sn phm sau kim tra m chng ta chn mt c mu kim tra thch hp cho ton b l hng 1.1 Bng P v bng X i lp nhau v kch thc mu Bng P (bng kim tra thuc tnh cht lng sn phm) : Yu cu mt kch thc mu ln pht hin t nht 1 ph phm xc nh t l tng i P. V d : T l dch v khng t yu cu ca ngnh ngn hng l 0,005 i hi mt c mu Nmin = 200 Bng X ch 4 5 sn phm l c th xc nh nhng bin i so vi gi tr trung bnh. So snh gia bng P v X th N ca P ln hn rt nhiu so vi N ca X, do kim tra thuc tnh cht lng sn phm th bao gi c mu cng ln hn v tn km hn nhiu. 1.2 Mc kim tra. Kch thc mu s nh hng n mc kim tra hay kh nng pht trin s thay i cht lng ca qu trnh sn xut. bng P : cng thc xc nh phm vi kim tra c N nm mu s nu xem bng cc h s ta thy rng N cng tng gi tr A2 cng gim. iu c ngha khi N tng gii hn kim tra s thu hp (tin gn v ng trung bnh) nh vy d pht hin s thay i cht lng ca qu trnh sn xut. http://www.ebook.edu.vn 57

2. Gii hn kim tra Gii hn kim tra thng dao ng 3 lch chun so vi ng trung bnh chng ta c th la chn gii hn phm vi kim tra khc 2 lch chun s lm tng xc sut pht hin bin i v cht lng v nhng nguyn nhn bin i cht lng ca qu trnh. Vic la chn gi tr Z hay la chn gii hn phm vi kim tra ph thuc vo chi ph tm ra nhng nguyn nhn bin i Z cng nh, chi ph kim tra cng t. 3. Chn mu chp nhn a. Chn mu chp nhn kim tra cht lng ton b l hng l mt vic cn thit. Mu chp nhn cn hi 2 iu kin nh sau : Mt mu ngu nhin c chn l mt l hng cn kim tra. Mt mu ngu nhin qua c qu trnh kim nghim s c khch hng chp nhn, ngc li s khng c khch hng chp nhn. b. Chn mu chp nhn thc cht l cung cp 2 thng tin. Cung cp cho nh sn xut nhng nguyn tc quyt nh gii hn ri ro khi giao sn phm n tay khch hng. Cung cp cho khch hng nhng nguyn tc quyt nh gii hn ri ro khi chp nhn l hng t pha nh sn xut. c. Mu chp nhn gm 2 ch tiu nh sau : 1. Mc cht lng chp nhn : AQL (Acceptable Quality Level) AQL Mc cht lng chp nhn c trnh by trong hp ng hay n t hng. Khch hng s cn c vo AQL i chiu vi t l ph phm qua kim tra thc t quyt nh chp nhn hay t chi l hng. Thng thng AQL = 0,005% 5% Vic t chi l hng khi so vi AQL c gi l ri ro ca nh cung cp. Mc ri ro ca ngi cung cp gi l . Mc ri ro khi tr li l hng u gy bt li cho c 2 pha : c nh cung cp v khch hng, v s xy ra cc vn nh sau : To ra mi quan h khng thin cm gia nh cung cp v khch hng. Ph v qu trnh sn xut ca khch hng v gia tng thi gian lu kho nguyn vt liu. Ko di mt cch v ch i vi nhng sn phm hon tt. 2. Mc cht lng thp nht LTPD (The lot tolerance proportion defective) LTPD Mc cht lng thp nht m khch hng s chp nhn l hng hay cn gi l t l ph phm c php dao ng ca l hng. Thng thng LTPD = 0,1% 10% Xc sut chp nhn l hng vi LTPD cho trc s l mc ri ro ca khch hng i vi l hng s phi nhn t pha nh cung cp. Mc ri ro gi l . Tt c khch hng u mong mun cng nh gim bt vic nhn nhng l hng c cht lng xu c th xy ra. Ni dung Chi ph kim tra tip nhn 1 l hng l ht sc cn thit. Chn mu hp l khi kim tra tip nhn l hng l rt hiu qu, c bit khi kim tra phi ph b sn phm. 2. o lng thuc tnh i lp vi o lng thuc tnh nhm quyt nh chp nhn hay t o lng bin i cht lng chi 1 l hng da vo c trng cht lng no . o lng bin i cht lng nhm lng ho lch ca c trng cht lng so vi gii hn hay kim tra thng k. 3. AQL Mc cht lng chp Nhng thng s ny lng ho ri ro xy ra vi nh sn xut v nhn l ri ro ca nh sn xut khch hng. Chng ta cn c nhng nguyn tc c bn xc gi l nh mc ri ro , ti u cho vic chn mu. LTPD T l ph phm thp nht l ri ro ca khch hng. d. Quyt nh chn mu chp nhn Quyt nh 1. Chn mu lun i lp vi kim tra ton b


58

4. Chn mu n Phng php ny gip ta quyt nh chp nhn hay t chi l hng da trn mt mu ngu nhin c chn theo cc thng s nh sau : n Kch thc mu c chn t l hng cn kim tra c s ph phm c th chp nhn trong mu d s ph phm thc t tm thy trong mu Nu d c th khch hng chp nhn l hng Nu d > c th khch hng t chi l hng. 4.1 ng cong c trng a. Khi nim ng cong c trng OC l ng cong m t bng th cch chn mu theo mi quan h gia xc sut chp nhn l hng vi t l ph phm dao ng, ta gi l ng cong c trng tc nghip. b. Cch v ng cong c trng OC iu kin cho trc n v c (kch thc mu v s ph phm c th chp nhn) Phn phi nh thc c p dng v mi sn phm kim tra ch 2 bin c xy ra : hoc l ph phm hoc t yu cu.

Pa =

c

p =0

n! P (1 P) n p p!(n p)!

p

Pa l xc sut chp nhn l hng vi n v c cho trc theo t l ph phm p. Nu n > 20 v p = 0,05 ta c th p dng phn phi Poisson khng b rng buc bi khi lng tnh ton. Trung bnh phn phi Poisson l n.p , gi tr Pa s c tra trong bng ph lc. v ng OC ta cn tnh Pa cho mt khong gi tr p theo cc thng s n v c cho trc. - Nhn n.p - Xc nh gi tr n.p theo trc honh - Tra bng i vi ng cong vi gi tr c thch hp - Di chuyn theo trc honh v pha tri vi n.p v c thch hp xc nh Pa trc tung.c. V d Nh my Noiseking lp t h thng gim m cht lng cao. Ngi qun tr khi va nhn mt l hng 1000 n v sn phm, vn t ra l phi kim tra trc khi a n kho lu tr. Cho trc n = 50 v c = 1. Theo mu chn hp ng vi nh sn xut c ghi AQL = 1% v LTPD = 6%. Yu cu : V ng cong OC cho mu chn Xc nh mc ri ro i vi nh sn xut v ri ro i vi khch hng. Gii Ly p = 1% = 0,01 Ta c n = 50, AQL = p = 1% n.p = 50*1% = 0,5 vi n.p = 0,5 v c = 1 - Tra ph lc th Pa = 0,91 - Tng t nh vy ta tm Pa cho cc gi tr p - Khi c cc gi tr p, Pa ta v ng cong OC - Tm c = 9% v = 20% c 2 gi tr ny u cao hn mc bnh thng. d. Thay i n v c theo yu cu qun tr Nn thay i mu chn trnh 2 trng hp khng tt xy ra : Trnh t chi l hng tt Trnh chp nhn l hng xu Mun lm iu ny th nn thay i 2 gi tr n v c nh th no ? Gi c/n l t l chp nhn Trong v d trn c1/n1 = 1/50 = 0,02 Nu ta cho thay i n = 100 v c = 2 th c2/n2 = 2/100=0,02 c ngha t l chp nhn vn khng thay i. http://www.ebook.edu.vn 59

CHNG XII H THNG VA NG LC 1. Gii thiu Nm 1978 cng vic kinh doanh xe my ca Cng ty Harley-Davidson i vo b tc khi m d c gng nhng Cng Ty vn tht bi chng minh vi To rng cc i th Nht Bn ang ph gi, a sn phm vo th trng vi gi thp hn gi thnh chim th phn. Ban iu Hnh ca cng ty Harley-Davidson tht ngc nhin khi thy cc cng ty Nht t c gi thnh thp hn mnh ti 30%. Do thy r cc hot ng ca Cng Ty khng hiu qu nn Ban iu Hnh Harley quyt nh t chc thc hin 3 phng thc thc hnh kiu Nht m h cho l cc yu t thnh cng ca cc Cng Ty Nht : Chu trnh cht lng (Quality circle) Kim tra qu trnh bng thng k (Statistical process control) Ch to va ng lc (Just in time manufacturing)

Mc d con ng dn ti thnh cng tng lc c nhiu chng gai nhng k t khi thc hin H Thng Va ng lc (JIT) vo nm 1981 Cng Ty Harley gim 40% hng tn kho, tng nng sut ti 129% - tnh theo s xe c ch to/1 cng nhn, gim cc c s cung ng ti 30% v li nhun tng t 1983. Cch thay i trn gip Ban iu Hnh Harley san bng khong cch gia Harley vi cc i th cnh tranh v gia tng th phn. 2. Yu t cu thnh h thng JIT H thng JIT c thit k sn xut v phc v theo yu cu s dng tn kho ti thiu. JIT l mt quan nim trit l c mc ch gim thi gian khng hiu qu v khng sn xut trong qu trnh sn xut. Trong h thng JIT ta thy c p dng : Cc quan nim v ci tin lin tc, kim tra cht lng ton b cng nh s tham gia ca CBCNV v gim tn kho. JIT ng nhin l mt h thng c bit rt nhiu di cc tn khc nhau, v d nh : Tn kho bng 0 (Hewlett Packark) Sn xut khng c hng tn kho (Harley Davidson) Cung cp vt liu khi cn (Harley Davidson) Sn xut theo dy chuyn (IBM) Trong phn ny chng ti s cp ti cc yu t sau y ca h thng JIT. Phng php b tr dng vt liu Kch thc l hng nh Thi gian chun b t. K hoch sn xut chnh ng b Tiu chun ho cc cu kin v phng php lm vic. Cht lng cao n nh S rng buc cht ch vi nh cung cp. Lc lng lao ng a nng. Hng vo sn phm. Bo tr d phng. Ci tin lin tc. 2.1 Phng php b tr dng vt liu C 2 phng php chnh, y l Phng Php Ko v Phng Php y. phn bit c s khc nhau gia 2 phng php, chng ta hy cng nhau xem xt h thng sn xut nh hng MC Donald. Hy tp trung vo mt sn phm Bnh cn Anh Mt ngi c trch nhim sn xut bnh ny. Nhn tht phi c chin, bnh phi c nng, sau n cho thm vo bnh st cay, nc st chua, st du trng mayonnaise, rau dip, ph mt, sau khi nhn tht c kp vo bnh th bnh c ln khay. Mt ngi khc mang khay i v vo thng tn tr. Mc tn kho phi c gi cng thp cng tt v bnh tn tr qu 7 pht phi c ph hy. http://www.ebook.edu.vn 60

Nh vy cng vic c m t gm 2 trm : trm ngi lm bnh v ngi thu gom bnh. Dng vt liu i t ngi lm bnh n ngi thu gom bnh. Mt trong cc cch qun l l PHNG PHP Y. Vi phng php ny ngi iu hnh t chc lch nhn nguyn liu (nh tht, bnh v gia v) sn sng sn xut trc khi nhn yu cu lm bnh. Ngi lm bnh bt u lm 24 ci ( cho v nng bnh) v khi lm xong y qua cho ngi gom bnh, bnh c th phi ch ngi ny lm xong vic khc ri mi lm n, v ngi ny c th ang phi gom nhiu sn phm khc. Phng php th hai qun l, cng vic gia ngi lm bnh v ngi gom bnh l PHNG PHP KO. y ng l phng php s dng bi h thng JIT. Vi phng php ko, ngi gom bnh kim tra s lng tn khi thy gn ht th t thm 6 ci. Ngi lm bnh lm 6 ci v chuyn khay bnh cho ngi gom bnh, ngi y hon thnh, v vo ni ch bn. So snh hai phng php, Phng Php Ko thun li hn cho vic sn xut bnh. Hai ngi th phi phi hp cht ch m bo tn kho thp, ch yu l v gii hn thi gian s dng ch c 7 pht. Ngi gom bnh ch t hng theo nhu cu. Vic sn xut bnh mang tnh lp li cao, thi gian chun b v ch to ngn, dng vt liu c xc nh r, khng cn phi lm sm hn vi pht. Trong lnh vc ch to c 2 phng php trn cng u ang thnh hnh. Nhiu hng khc ang s dng Phng Php y nh Hoch nh nhu cu vt t (Material Requirement Planning, MRP). Ca hng sn xut sn phm s lng t vi tnh lp li thp thng dng Phng Php y. Trong iu kin ny khch hng c ha s giao hng vo mt ngy no trong tng lai. Sn xut bt u t trm u v y sang trm k giao hng ng hn. MRP p dng tt trong trng hp ny.2.2 Kch thc l hng nh. Ngi s dng JIT gi tng l tn kho cng nh cng tt bi v 3 l do sau : Trc ht l hng nh lm gim chu k tn kho. Nu Q l c l hng th chu k tn kho l Q/2. Nu Q nh hn th chu k tn kho cng nh hn. Hnh bn di cho ta thy s thay i chu k tn kho. Khi ta rt kch c cc l t 100 xung cn 50, gi s c nhu cu 10 n v/1 gi, chu k tn kho gim xung mt na.

100 75 Tn kho 50

Tn kho bnh qun

O=10

25 0 5 10 15 Thi gian 20 25

O=50

L do th hai l thi gian giao hng c rt ngn, vic ny li lm gim nguyn liu cn tn trn dy chuyn v thi gian sn xut l ln trn dy chuyn s di hn. Ngoi ra mt l ln thng phi ch lu hn trc khi c ch to v trm sn xut ang sn xut 1 l ln khc. Hn na sa cha l ln th cng cn thi gian ln hn. http://www.ebook.edu.vn 61

Sau cng l cc l c nh gip b tr cng vic iu ho trong khi l ln c vn v thi gian v v th c th lm ngng tr hoch nh thi gian, cc l nh c th c xoay tr d hn v th gip ngi hoch nh thi gian s dng nng lc sn xut hiu qu hn. Ngoi ra, cc trm sn xut c th p dng sn xut a h (sn xut nhiu hn 1 sn phm) bng cch gim thi gian xp hng ch sn xut. Chng ta s tr li im ny khi chng ta tho lun v k hoch sn xut chnh ng b.2.3 Thi

bai giang quan tri sx dv

Documents