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Unit 4: Computer Numerical Control Systemthe basics of the core of CNC machine tool-CNC unit:

functionsworking principleshardware configuration and

software structure

to view the main features of CNC unit in more general terms:

stressing why things are handled the way they are even more than the

specific techniques used with any one particular CNC machine toolbe able to easily zero in on any kind of CNC machine tool

two typesnumerical control (NC)

(have to read the program each time a part is runthey have no means of

editing existing programs)

computer numerical control (CNC)

(can store and allow editing of loaded programs)

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4.1.1 NC systemThe NC machine can be divided into three basic areas:

control unit + drive mechanisms + the machine

The most basic function of any NC machine is automaticprecise

and consistent motion control

All forms of NC equipment have two or more directions ofmotion--called axes

These axes can be precisely and automatically positioned along their

lengths of travel.

The two most common axis types are linear(driven along a straight path)

and rotary (driven along a circular path)

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conventional machine tools: motions are by manually turning

cranks and hand-wheels

NC machines: motions are actuated by servomotors under

control of the NCand guided by the part program

themotion type (rapid, linearand circular)

the axes to movetheamount of motion

and themotion rate (feed rate) are programmable

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An NC command tells the drive motor to rotate a precise amount of angle

The rotation of the drive motorin turn rotates the ball-screw

The ball-screw drives the linear axis

A feedback device at the opposite end of the ball-screw allows the control

to confirm that the commanded number of rotations has taken place

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numerical data :for controlling the relative motion between cutting tool and piece part

the component dimensions

( from engineering drawings--the geometric data),

to define feed rates,

spindle speedstool identification numbersand miscellaneous functions

(to perform such operations as tool or

work changingor coolant control--the technological data).

The combination ofall this numerical information

in a sequence is a part program

the process ofcreating the data in a correctly structuredformat is programming

Numerical controls use standard microelectronic modules that havebeendeveloped forcomputer hardware (special modules needed in servo-

control circuits )

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4.1.2 CNC system and functions of CNC unitControls today are soft-wiredwhich allows greater

flexibility in changing and upgrading the computer control

The modern CNC machine tool issoftware drivencomputercontrols are programmed instead of hard-wired

CNC system: is a special computer system that is equipped

with certain interface circuits and servo driversand can dopart or all the works an NC system do by running the software

stored in its memories

CNCs have expandable memories that can store

large numbers of programsplus subroutines

and correction dataextensive memory capacity : sophisticated graphic displays

and dynamic simulation

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CNCs comprises: CNC unit

feed motion servo subsystem

spindle servo subsystem

and some auxiliary control cells.

Operation panel

I/O DevicesCNC

Control Unit

PLC

Keyboard

Spindle servo cell

Machine tool I/O circuits and devices

Spindle driver

Feed motion

servo cells

Detect device

Feed drivers

Machine

tool

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CNCs comprises: CNC unitfeed motion servo subsystem

spindle servo subsystem

and some auxiliary control cells.

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CNC unit is the core of the whole CNC system

It is composed ofhardware and software

Hardware: is made up of microprocessors

electronic memory modules

I/O interfaces,

and position control modulesetc

software controls all functions of the machine,

manages all local programming,

and graphic simulation of the cutting

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hardware and the software work together to doall functions:

graphic display, HMI (Human-Machine Interface)

system diagnosis

implementing of various complex tool-path controlling,

compensation algorithms

intelligent controlling

online programming

and Web communicationsetc)

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main functions and features of CNC unitControlling function

the number of control axes and synchronously control axes:linear axes and rotate axes, basic axes and auxiliary axes

(The more the number of controlled axesespecially the number of axes

being controlled synchronouslythe more powerful functions of the CNC

unit, and the more complex structure of the CNC unitthe more difficulty

of programming.)

Preparatory functionG function, describes the motion modes of CNC machine

The motion modes comprise instructions of basic motions

dwellplane selections,coordinate system settings

tool compensations

reference point return

fixed operation circles

and metric/inch unit transferetc

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Interpolation functioninterpolation by using software real-time calculations

Linear interpolationcircular interpolationhelical interpolation

and polar coordinate interpolation

Feed functionFeedrate designation

(machining centers only allow the feed rate to be specific in per-minute

format: inches or millimeters per minute;

Turning centerswhich have position encoders in their spindlesalso

allow feed rate to be specified in per-revolution format: inches ormillimeters per revolution)

Feed Rate Override:multiple position switchon control panel

allows the operator to change the programmed feed rate during cutting

(The switch is usually segmented in 10 percent increments

that range from 0 percent through 200 percent )

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Rapid motion

to minimize non-productive time during the machining cycle

to command motion at the machines fastest possible rate

Common uses for rapid motion: non-cutting motion

| include :

positioning the tool to and from cutting positions

moving to clear clamps and other obstructions

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Spindle function

specify the spindle speedOn operation panelthere is a button to turn the spindle on andoffas well as a rheostat to control spindle speed

Miscellaneous functionallow a variety of special functions

Miscellaneous functions aretypically used asprogrammable switches

(spindle on/offcoolant on/offand so on)

C f

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Compensation function

allow the CNC user to

allow for unpredictable conditions related to tooling

Tool length compensationcutter radius compensation

and tool nose radius compensationenable the CNC machine to adjust cutting tool to zero in the right

position

when wear occurs on cutter tool or changing of cutting tool

Technical parameters compensation,( fixture offset

NRZ(Non-return-to-Zero) of axis while counter-movingdistortion of machine tool

any unpredictable situations during programming

a form of compensation to deal with the problem

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Enhanced functiongraphic display function

CRT or LCD displayer: show programsparametersvarious

compensation datacoordinatesfault information

part graphs;

monitor dynamiccutter tool paths while machining

Self-diagnosis functionvarious diagnosis programs : prevent faults occurring

or going worse

to sho rten the t ime of brok en-down

Communication function

RS-232-C communications (serial) port personal computers

Some form of communications software allow transmissions

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communications networks------usually referred to asDNC systems

Some CNC units support for MAP(Manufacturing Automation Protocol)

connect with industry communication nets to

fulfill the needs of FMS and CIMS

P i f ti

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Programming function

Manual programmingall CNC programmers should have a good understanding ofmanual programming techniques regardless of whether or notthey are used

conversational (shop-floor) programming

created using graphic and menu-driven functionsvisual check

and CAM system programminghelps the programmer in three major areas

keeps the programmer from having to do math calculations

makes easy to prog ram di f ferent k inds o f machineswith thesame basic language

helps with certain basic machining pract ice func t ions

4 2 Functional interface

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4.2 Functional interface

between hardware and software in CNC unitCNC unit : hardware + software

work together to perform all functions of CNC unit

different characters:

Hardware higher speedmore expensive

software flexibleslow on processingproportionbetween hardware and software is determined

by cost-performance of the CNC unit

(In the earlier NC equipmentsall functions were implemented by hardware;computer was introduced into CNC system)

participations of computerdifferent in different CNC units

in different time

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four different functional interfaces between hardware and software:

Program

Input

Interpola

tion

Positio

n

Pretreatm

ent

Speedcon

trol

Servom

otor

DetectorHard

ware

Software Hardware

Hardware

Hardware

Software

Software

1

2

3

4 Software Hardware

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4.3HARDWARE ARCHITECTURE OF CNC UNITMicro-processors + electronic memory modules

+ I/O interfaces + position control modules

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Types of CNC Unitdetermined by the architecture of printed circuit

boards:

All-in-one board architecture and multi-functionalmodules architecture

determined by the number of microprocessors:single microprocessor and multi-microprocessors

determined by the manufacturing fashions:ordinary personal computer architectureand special purpose architecture

determined by the opening degree :close architecturePC-plug-in-NC architectureNC-plug-in-PC architecturesoftware based open architecture

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All-in-one board architectureComprises:

main boardposition control cardPLC carddisplayer card

and power supply

Main board is a big printed circuit board

other functional cards are small size

each is installed in a card rack on the main board

M lti f ti l d l hit t

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Multi-functional modules architecture

CNC unit is divided into several function modules:

CNC control moduleposition control card

PLC cardgraph display card

and communication card, etc

Hardware and software are designed withmodularization method each function module is made

of printed circuit of same size

customers can establish their own CNC units by

combining theirselected function modules into the

card racks of a motherboardconnected by industry

standard busIPC bus or STD bus

e g : a holonomic CNC lath system of this architecture

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e.g.: a holonomic CNC lath system of this architecture

modules can be connected together by industry standard bus: IPC (Industry PC) bus or STD bus

Si l i hit t

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Single microprocessor architecture

only one microprocessor (on duty of controlling and

occupying the system bus )which processes various

NC tasks by time-sharing in a centralized controlway

simple and can be implemented easily

centralized controllingits performances and abilities

are limited by the byte lengthdata widthaddress

abilityand calculating speed of microprocessor.

M lti i hit t

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Multi-microprocessors architecture

three kinds of multi-microprocessor architectures:

distributed systemmaster-and-servant systembus based multi CPU system

distributed system

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distributed system

a communication network route between microprocessors

Interactions within microprocessors and shared resourcesvia network communication technology

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Master-and-servant systema master microprocessor: control the bus;

access the resources attached with the bus;

control and monitor servant microprocessors;

coordinate the operations of servant microprocessors;

(others) servant microprocessors:do what the master microprocessor command;

cannot do decision-making and scheduling;

all independent systemsThe communication:

by interactive responses through I/O interface

or by duplex RAM

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Bus based multi CPU system

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Bus based multi CPU system

each CPU can directly access all the system resources,

can use all of theirown resources freely and independently

CUPs compete to occupy the parallel bus ??

bus-intermediatoris used

to assign and manage priority of each CPU

a parallel main bus links several microprocessor systems

Microprocessors cannot be divided into master and servant:

at one timeonly one CPU with higher prioritycan use the parallel bus

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IPC based CNC system

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IPC based CNC system

CNC unit : base on industry computer (IPC) platform

built by plugging in certain control cards and NC software

IPC platform has made CNC unit cheaper and cheaper

and easily be substituted when fault occurs

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NC plug in PC

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NC-plug-in-PC

IPC+ PMAC (Programmable Multi-Axis Control Card )

|

plugged in the card rack of IPC

( On PMAC cardhigh speed DSP is used as CPU

high performances on motion control and PLC control )

PMAC card(itself is a NC system)can be used independently:

Its opened prog rams l ibrary package

can be used on Windows p lat form

to establish con trol systemas users purpo se.

open architecture motion control

4 4 CNC SYSTEM SOFTWARE

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4.4 CNC SYSTEM SOFTWAREIn CNC system: software codes fall into two groups

user software and system software

User software: part program

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User software: part program

It is compiled into a program with various G-codes and

M-codes

according to the process sequence

System software: an operating system

It is specifically designed for the type of machine that itis destined to control.

be sorted into two packages

managing sof twareand con tro l sof tw are

To execute the part program:

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To execute the part program:

CNC unit compiles the codes into the format

does preparatory and interpolation functions

(control software)

all functions of control software are implemented

under the supervision and coordination of

(managing software)

system software

previously written in Assemble Language

achieve higher executing speed

take more time in the course of development;

now written in advanced languagessuch as C orC++etc

4.4.2 Control Software

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4.4.2 Control Software(main functions of a CNC system)

does the following work by correspondingsubroutines

(1) Compiling the part codes inputted by userspart codes: in ISO orEIA formatCompiler: translatetrim and store

decode and do decimal-binary transitions

calculate tools center path considering tool radius offset

pre-calculate some constants used in interpolation andspeed control process

Objective of pre-process:save time for real-time interpolation calculations

( can be done before machining

or in the idle intervals while machining)

(2) Interpolation calculations

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(2) Interpolation calculations

to assign electrical pulses for axes

a strict real time program:

demands as less as possible number of instruction codes

to shorten the time for performing interpolation calculations

time is determinative

combine rough interpolation and fine interpolation:

software : rough interpolation

that interpolates a tiny line each time

Hardware: fine interpolation

by turning the tiny line into a series of

single pulses

and outputting the pulses

(3) Speed control

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(3) Speed control

aims at controlling the speed of pulses assigning(controlling the frequency of interpolation calculation)

to guarantee the preset feed velocity

speed control subroutine: automatically speed up or slowdown the velocity to avoid pace-loss

implemented by:software method (software t imermethod)

hardware means (velocity code controlling an osci l latorthen by in terrupts or quer iesto guarantee the feed velocity)

Further more:

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Further more:

software combining with speed integrator hardware

Software: processing the speed control data

+ Speed Integrator Hardware

constant compound velocity controllingcan be realized

the feed velocitycan be improved greatly

(4) Position control

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(4) Position control

position loop of servo system

done by either software or hardware

Position control software:

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Position control software:

comparesinterpolation-calculated position

with real measured position

in each sampling cycle

control the motor using the difference

adjustthe magnification of the position loop circuit

compensatethe error of screw pitch

the non-return-to-zero when reverse the motion

location precision be improved

Position control implements functions on calculations:

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p

4.4.3 Managing Software

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Managing software is composed of :

part program I/O,

display (part programtool position

system parametersrunning status,

alarm informationstatic and dynamic tool path)

diagnosis codes (by which CNC unit can carry out

self- diagnosis and spot the faults)

two groupsdepends on hardware: manages and drives peripheral equipment

performs real time processes

independent of hardware: manages timetasksmemory,synchronizes various procedures

the capability of parallel processing:

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p y p p g

many function procedures (e.g.: preparatory functions

and interpolation process)

need to be implemented in parallel

Input

Display Control

Diagnosis I/O

Encoding

Compensation

Speed process

Interpolation

Position

processing

Preparatory functions

the capability of real time processing:

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p y p g

tasks ofhigher priority will be implemented in time

tasks with lower priority will wait in queue

(For examplefrom the point of view ofurgency

collision detecting procedure should havehigher priority

than information display procedure)

CNC system can react to the event (s) in time

system will keep in a safe status

Parallel process means:

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p

computer implements two or more tasks at the same time

or within the same time interval

improve the processing speed greatly

methods to realize parallel process:time overlap: based on flow line techniquestaggers the processing time of

procedures

makes many procedures at the same

time.

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resource-sharing: based on time-sharing principle

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g g p p

many procedures occupy CPU processing time in turn

Real time processmeans:

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p

CNC system react in time to

the requests and events of system

methods to realize real time process:

Interruption technologymany types of interruption

external interruption: external monitor interruption,

input interruption of keyboard / operation panel;

internal timer Interruption: interpolation loop interruption,

position sampling interruption;

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hardware fault interruption:memory errortimer error

timeout during interpolation calculation

hardware fauIt (sent out by hardware fauItdetectors when fault occurs)

software interruption:the alarm interruption when there is exception

during software execution

e.g. : various overflow error and reset

4.4.4 Architecture of CNC System Software

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y

organizing and managing modeof CNC system software

two different architectures

tandem architecture

interruption architecture

tandem architecture

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CNC system software is divided into two groups

foreground application:a real time interruption service program

Realize: the real time functions(interpolation calculationposition controllogical control of machine tool, etc.)

background application: a loop running program

Realize: the input of part program

some preparatory processes

(encodingtool compensation calculationspeed calculation)

manages tasks of procedures

work cooperatively

The system program carried out in the courses

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firstsystem startup

secondinitializing process

then entering the loop executing of background application

During executing of background application

real time interruption procedures intervene every now and then

to implement real time controls

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Executed by

interrupting

Foregroundapplication

Fault process

Position control

Interpolation

calculation

...

Executed in

loop

Backgroundapplication

Encoding

Display

Tool compensation

Speed calculation

Input/Output

Interruption architecture

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no distinction of background and foreground applications

initializing procedure+ interruption service procedures

with different priorities

a large multilevel interruption system

I i i li i

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Initialization

Interruption manage system (hardware+software)

0leve

linterruptserv

ice

1leve

linterruptserv

ice

2leve

linterruptserv

ice

Nl

evelinterruptservice

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e.g. Tasks div is ion and their pr ior i t ies

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in HuaZhong I system softw are

eight types of tasks with different priorities (from high down to low): Position control (4 ms)

Interpolation calculation (8 ms)

Data collection (12 ms)

PLC task (16 ms)

Tool compensation task (condition (buffer) drivable)

Program codes interpreting (condition (buffer) drivable)

Dynamic display (96 ms)

HMI: Human-Machine-Interface (menu managedloop continuously);

Position control task has the highest priority:hence has the shortest invoking time interval 4 ms

HMI task has the lowest priority:

is an infinite loop procedure that starts only one time

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Real Time Operating System -----RTOS

CNC

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,

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,

TCB

CNC

CNCCNCPCDOS

WINDOWSCNC

DOS WINDOWS