동시구동 및 순차센싱을 이용한 대형 정전용량 터치스크린용 ... · 2018. 10....
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Journal of The Institute of Electronics and Information Engineers Vol.52, NO.4, April 2015 http://dx.doi.org/10.5573/ieie.2015.52.4.062
ISSN 2287-5026(Print) / ISSN 2288-159X(Online)
논문 2015-52-4-9
동시구동 순차센싱을 이용한
형 정 용량 터치스크린용 고속 센싱 기법
( A Fast Sensing Method using Concurrent Driving and Sequential
Sensing for Large Capacitance Touch Screens )
모하메드 모하메드 가말 아흐메드*, 김 형 원**, 조 태 원***
(Mohamed G.A. Mohamed, HyungWon Kim, and Tae-Won Choⓒ )
요 약
최근 스마트폰의 발달과 더불어 형 TV, 의료용 장비 자 칠 에도 터치스크린의 수요가 증하고 있다. 스크린 사이
즈가 증가 할수록 고해상도를 하여 훨씬 더 많은 채 이 추가 되면서 한 임을 스캔하는데 긴 시간이 소요되어 터치감지
지연이 큰 문제가 되고 있다. 본 논문에서는 이러한 문제를 해결하기 하여 새로운 드라이빙 센싱 기법을 제안한다. 이
기법은 differential 드라이빙 방법으로 2 단계로 수행되어진다. 먼 고속 센싱 로세스를 통해 터치가 발생된 센싱 라인들을
우선 략 으로 도출해 낸 후 정확한 터치 치 스캔을 해서 터치된 라인에서만 감지가 수행되어 진다. 이 방법을 사용하
면 터치 패 의 frame refresh rate를 향상 시킬 수 있다. 제안된 구조는 FPGA와 개발된 AFE board로 구 되었으며, 23인치
상용 터치패 을 사용하여 테스트하 다. 이 기법은 기존 비 frame scan rate를 8.4배 향상시킨다.
Abstract
Recently the demand for projected capacitance touch screens is sharply growing especially for large screens for medical
devices, PC monitors and TVs. Large touch screens in general need a controller of higher complexity. They usually have
a larger number of driving and sensing lines, and hence it takes longer to scan one frame for touch detection leading to a
low frame scan rate. In this paper, a novel touch screen control technique is presented, which scans each frame in two
steps of simultaneous multi-channel driving. The first step is to drive all driving lines simultaneously and determine which
sensing lines have any touch. The second step is to sequentially rescan only the touched sensing lines, and determine
exact positions of the touches. This technique can substantially increase the frame scan rate. This technique has been
implemented using an FPGA and an AFE board, and tested using a commercial 23-inch touch screen panel. Experimental
results show that the proposed technique improves the frame scan rate by 8.4 times for the 23-inch touch screen panel
over conventional methods.
Keywords : Touch screen, Mutual-Capacitive, Touch Screen Controller, Multi-touch
* 학생회원, ** 정회원, 충북 학교 자공학과
(School of Electrical Engineering and Computer
Science, Chungbuk National University)ⓒ Corresponding Author(E-mail: [email protected])
※ 이 논문은 2013년도 충북 학교 학술연구지원사업
의 연구비 지원에 의하여 연구되었음.
Received ; February 4, 2015 Revised ; March 20, 2015Accepted ; March 26, 2015
Ⅰ. Introduction
Touch screen panels (TSPs) are used to detect
human touch or a special pen by changing one of
their physical properties. Nowadays, capacitive touch
screens are the most popular type among many TSP
types. There are different types of capacitive TSPs:
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surface capacitive and projected capacitive types. The
projected capacitive panels are further classified as
mutual capacitance, and self-capacitance types[1]. For
medium and small-size TSPs for mobile products,
mutual capacitive-type TSPs have superior visibility
and durability, and also provide multi-touch functions.
Nowadays projected mutual capacitance TSPs are
most widely used for medium to small-sized mobile
products[2-3], and are also increasingly adopted for
large PC monitors and TV screens[4]. They have
superior visibility and durability and also exhibit a
multi-touch function. In mutual capacitance TSPs,
touch detection is determined by measuring the
change in the mutual capacitance[5].
Fig. 1 shows a part of projected mutual
capacitance touch screen panel electrodes driven by a
sequence of pulses which results in a staircase signal
after integration of the output signal[6]. This
technique is the conventional touch screen controller
system. Touch screen panel structure used in Fig. 1
uses a cross-line structure with large plates to
magnify the value of the mutual capacitance to get
better touch detection results. Different ways of
driving[7] and sensing[8] TSPs have been examined
through previous research efforts in order to improve
signal to noise ratio (SNR) and so avoid incorrect
touch detection result. Mutual capacitance touch
ADC
TX switches
RX switches
Integrator
Input & output of the system
Measured by ADC
Pulse generator
Self‐cap
Mutual‐cap
그림 1. 단일 센싱 기법의 터치 스크린 컨트롤러 시스
템. (신호 경로는 오 지색으로 강조 표시됨)
Fig. 1. Touch screen system controller using driving
one TX line and sensing one RX line. (Note:
signal path is highlighted in orange color.)
screen panels are susceptible to noise, so much of
research has been focused on how to reduce the
noise and increase the touch signal strength[9-10].
While there are software algorithm approaches[11-15],
many papers proposed circuit hardware approaches to
address the noise reduction problem.
In this paper, we propose a new approach based on
both hardware and software using concept of
differential driving for faster touch detection. Most of
previous research efforts have been focused on a
small touch screen controllers. As the size of TSPs
grows, in general, the number of driving (TX) lines
and sensing (RX) lines tends to increase to keep high
resolution of touch detection. It becomes therefore,
difficult to control a large TSP because of frame
scan rate is much lower than required. Our solution
also provides an efficient method of concurrent
driving and selective sensing, which allows frame
scan rate improvement and effective noise
cancellation.
One of the methods to cancel supply noise is using
differential driving technique. In our method, two
adjacent cells are driven with opposite polarity
signals to cancel out supply noise. This idea is based
on the high probability of affecting two adjacent cells
with opposite noise level.
This paper is organized as follows. Section II
introduces the proposed touch screen controller
system. Section III presents simulation results of the
controller system. Finally conclusions are presented in
section IV.
Ⅱ. Touch Screen Controller System
Various scanning techniques of touch screen
controllers have been published to improve their noise
immunity and increase the SNR. However few
scanning techniques have been reported in literature
to efficiently improve the frame scan rate of large
TSPs.
Large touch screen panels usually have a large
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number of driving (TX) and sensing (RX) lines in
order to improve touch detection resolution. This
imposes high burden on touch screen controller
design. The time required to scan one frame grows
along with the number of TX and RX lines in most
touch screen control methods. However most touch
screen controller systems have a target frame scan
rate that they must satisfy to ensure touch detection
quality which is usually in the range of 100Hz~
200Hz.
The proposed touch screen controller provides an
effective solution with a very high scan rate for large
touch screen panels. It consists of three components
driving TX lines, sensing RX lines, and running
detection algorithm software on the sensed data and
send touch position data to PC . A novel driving and
scanning technique is introduced, which reduces the
frame scan time. The controller can be used for
touch screen panels of different sizes, fabricated with
various materials and various patterns. Fig. 2 shows
our system implementation to drive a commercial 23"
TSP with 44 TX lines and 78 RX lines.
그림 2. 구 된 터치 스크린 컨트롤러를 용한 23인치
터치 스크린 검증.
Fig. 2. Touch screen panel controller system to drive
23” touch screen panel using our technique
implemented in Zynq FPGA.
1. Differential driving and differential sensing
Fig. 3(a) shows a differential driving[4] controller
architecture and their output signal output, while Fig.
3(b) illustrates a differential sensing[5] controller case.
Differential sensing can be conducted by applying a
series of pulses to one TX line and taking the
(a)
(b)
그림 3. (a) 차동 분기를 이용한 기존의 차동 센싱 기
법. (b) 반 되는 극성의 구형 를 사용한 차동
구동 기법.
Fig. 3. (a) A common differential sensing scheme using
a differential integrator. (b) A differential driving
scheme using pulse signals of two opposite
polarities.
difference between two adjacent RX lines in order to
sense two adjacent cells in the same raw. On the
other hand, differential driving can be conducted by
applying opposite polarity pulses to two adjacent TX
lines and sensing one RX line. Hence two adjacent
cells in the same column are sensed at the time.
Differential sensing removes the noise affecting
two adjacent cells in the same row (TX line) which
is ambient noise. Differential driving often reduces
the frame scan period.
2. Proposed concurrent driving technique
The proposed touch screen sensing scheme
consists of two steps to scan one frame of touch
screen panel. In the first step, all TX lines (rows) are
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2015년 4월 전자공학회 논문지 제52권 제4호 65
Journal of The Institute of Electronics and Information Engineers Vol.52, NO.4, April 2015
concurrently driven by opposite polarity pulses, which
is described by Fig. 4(a). This step determines the
touched RX lines by observing the output of each RX
lines.
The idea behind this technique is to cancel the
common mode signal while leaving only the
differential signal by applying opposite excitation
pulses to every adjacent cells in the same RX line.
This concurrent driving results in zero output in case
of no touch while giving non-zero output if any cell
(a)
(b)
그림 4. 제안하는 2단계 구동 기법: (a) 1단계: 동시다발
으로 모든 TX 라인 구동 후 RX 라인 센싱.
(b) 2단계: 연속 으로 2개의 TX 라인 구동 후
RX 라인 센싱.
Fig. 4. The proposed driving technique works in 2
steps: (a) The 1st step: drive all TX lines
concurrently and sense RX lines sequentially. (b)
The 2nd step: sequentially drive each pair of
two adjacent TX lines with opposite polarity
pulses, and determine the touched positions.
on the RX line is touched. Fig. 5 shows the output of
RX line in the touched and untouched cases. The
second step is to sequentially scan each pair of two
adjacent cells in only the touched RX lines as shown
in Fig. 4(b).
The proposed scheme eliminates the needs for
scanning all cells every frame which is the major
drawback of conventional schemes, and so it can
improve the frame scan rate.
For an example touch screen of TX x RX = 44 x
78 lines, a conventional sensing schemes require
(3432 Δt) seconds where (Δt) is the time for scanning
one cell. The proposed scheme, on the other hand,
needs only (78Δt+ n×22×Δt) where (n) is the number
of RX lines that are sensed as touched lines.
Here, the 1st term 78Δt is the time consumed by
the 1st step which scans all RX lines to find touched
lines. The 2nd term n×22×Δt is the time consumed by
the 2nd step which scans all cells for each of the
touched RX lines.
Consider an example where five fingers are
touching different RX lines. Assuming that one touch
point affects 3 RX lines on average, the number of
affected RX lines is 15. Then the proposed scheme
has a total scanning time of 78Δt + 15×22×Δt = 408
Δt. This is an improvement of 8.4 times compared
with the conventional scheme.
For the proposed scheme, a generalized form of the
frame scan rate (FSR) is given by:
(1)
while the frame scan rate for the conventional
scheme is given by:
(2)
The proposed scheme can employ either a
differential sensing circuit or a single-ended sensing
circuit. In this paper, we assume that the sensing
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그림 5. 제안하는 센싱 기법들의 출력 결과.
Fig. 5. Different outputs of the proposed scanning
technique.
circuit is a single-ended one in order to focus on the
description of the differential driving scheme.
The sensing circuit has a single ended integrator
that senses the output of each RX line (the entire
column in Fig. 4) and results in a stair case output
as shown in Fig. 5.
The proposed scheme can substantially speed up
the overall sensing process allowing a very fast
frame scan rate. It, therefore, is well suited for large
touch screens.
3. Touch detection algorithm
The proposed scheme is implemented by a touch
detection algorithm as illustrated by the flow diagram
in Fig. 6. In this algorithm, all RX lines are scanned
sequentially with all TX line driven concurrently
through the first step. It then drives each pair of TX
lines sequentially by opposite pulses until all touched
RX lines are scanned.
The flow diagram of Fig. 6 starts by driving all
TX lines concurrently with opposite polarity pulses
and reads one RX line at a time. Through this step,
each RX line RXn is marked as touched or
untouched. Once all the touched RX lines are
determined, the second step starts by selecting the
first touched RX line RX0, and drive the first TX
pair (TX0, TX1). Each cell on the selected RX line is
marked based on the integrator’s output. A positive
그림 6. 제안하는 센싱 알고리즘; N: RX 라인의 개수,
K: TX 라인 의 개수, n: 선택된 RX 라인의
지표, m: 터치된 RX 라인의 개수, k: 선택된 TX
라인 의 지표.
Fig. 6. Flow diagram of the proposed scan algorithm
where N is the number of RX lines, K is the
number of TX pairs, n is the index of selected
RX line, m is the number of touched RX lines,
and k is the index of selected pair of TX lines.
integrator output indicates that the first cell TX0 of
the TX pair is touched, while a negative integrator
output indicates that the second cell TX1 is touched.
The above process is repeated for all TX pairs to
scan all touched RX lines. Once all the touched RX
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Journal of The Institute of Electronics and Information Engineers Vol.52, NO.4, April 2015
lines are scanned with all TX pairs concurrent
driven, all touched cells (touch positions) are
extracted.
The ambient noise on TSP, however, often makes
the integrator output deviate from the ideal V and V-
values. In the evaluation of the integrator outputs, we
compare the outputs with certain thresholds to
effectively determine each cell as touched or
untouched under practical ambient noise signals.
Ⅲ. Results and Discussions
The results presented in this section is measured
from our system implementation with FPGA and
AFE board as shown in Fig. 2. We have also
implemented the proposed scheme in a silicon chip,
which was fabricated in a TSMC CMOS 0.18um
process in a Multi-Project Wafer MPW. We plan to
present the result of this chip, when the chip is
available.
1. Scan rate improvement
As described above, the proposed touch detection
technique employs a fast scan algorithm , unlike a
conventional method which repeatedly scans all cells
exhaustively. The conventional method needs a total
time of (no. of RX × no. of TX)Δt seconds to finish a
scan of one frame, where Δt represents the time
required to scan one cell and it is determined by the
frequency of driving signals and the number of
integrations required to get a target output. On the
other hand the proposed technique takes (no. of RX)Δt
seconds to finish the first scan step and (n/2 × no. of
TX)Δt seconds to finish the second scan step, where n
is the number of touched RX lines found.
As shown in Fig. 7 the frame scan rate for the
proposed technique is substantially improved by 8.4
times for 23 inches touch screen panel and 3.62 times
for 10 inches touch screen panel compared with a
conventional method assuming that one touch point
affects three adjacent RX lines. In this experiment, Δt
(a)
(b)
그림 7. (a) 다양한 터치스크린 크기에 따른 임 스
캔 시간. (b) 터치 포인트 개수에 따른 제안된
기법의 임 스캔 시간.
Fig. 7. (a) Frame scan time in milliseconds for different
sizes of touch screens. (b) Frame scan time for
the proposed scheme with various number of
touch points.
(the time required to scan one cell) is configured as
1.6us. We also configured the input signal frequency
as 5MHz. The number of integrations of the sensing
circuit is configured as 8 integrations to achieve
practical detection performance.
2. Output voltage difference
In ideal case, differential driving and differential
sensing give an integration output of 0V when there
is no touch event over the two adjacent cells. In real
TSPs, it, however, has a non-zero integration output.
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(a)
(b)
(c)
그림 8. 2개의 TX 라인에 차동 구동을 용한 경우. (a)
2 개의 TX 라인의 차이를 보여주는 터치스크
린 패 의 모델 (차이는 빨간색으로 강조 표시
됨). (b) 같은 진폭을 갖는 차동 구동 신호의
분기 출력 결과. (c) 보정된 진폭을 갖는 차동
구동 신호의 분기 출력 결과.
Fig. 8. Differential driving of two TX lines with (a)
Schematic model of a touch screen panel
showing the difference between the two signal
paths. (b) Integrator output with 2 differential TX
inputs having the same amplitude. (c) Integrator
output with 2 differential TX input with
calibration.
As shown in Fig. 8(b), the integration output is
larger than zero. This is due to the fact that the
signals propagate through two different paths as
illustrated by Fig. 8(a). While both paths are quite
similar, there is an additional RC circuit stage in the
path from TX0 compared to the path from TX1. This
additional RC circuit causes a non-zero difference in
voltage as shown in Fig. 8.
Fig. 9 shows the integrator output with all TX lines
driven with opposite polarity pulses in touch and untouch
events. In the case of driving all TX lines at the
same time, the non-zero outputs caused by the
(a)
(b)
그림 9. 모든 TX 라인에 한 차동 구동 신호의 분기
출력 결과 (a) 보정되지 않은 경우. (b) 보정된
경우.
Fig. 9. Integrator output with all TX lines driven with
opposite polarity pulses in touch and untouch
events (a) without calibration. (b) with calibration.
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difference of every two adjacent paths, tend to
accumulate and result in a high output voltage as
shown in Fig. 9 (a).
The proposed calibration method is to apply
different levels of input signals to compensate the
difference in signal paths. In this way, the signals
that propagate in longer paths are assigned with
larger amplitude. Hence the proposed scheme ensures
that the integration output is approximately zero in
untouched case as shown in Fig. 9 (b).
IV. Conclusions
We have presented a new driving and sensing
scheme for projected mutual capacitance touch
screens. We proposed a two-step algorithm to reduce
the time required to finish one frame scan. Using
simulation experiments with realistic TSPs of various
size, we have shown that the proposed scheme can
improve the frame scan rate by 8.4 times for 23“
TSP compared to a conventional scheme. The
proposed scheme tends to provide a faster frame scan
rate as the TSP size grows.
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70 동시구동 및 순차센싱을 이용한 대형 정전용량 터치스크린용 고속 센싱 기법 모하메드 모하메드 가말 아흐메드 외
자 소 개
Mohamed G. A. Mohamed
(학생회원)
2006, B.S in Communications
and Electronics Engineering,
Minia University, Egypt
2009, M.S in Communications
and Electronics Engineering,
Minia University, Egypt
<주 심분야 : Mixed signal SoC design, Low
power circuits, Memristive circuits and
systems, Single electron systems>
조 태 원 (평생회원)
1973, B.S degree in Electronics
Engineering, Seoul
National University
1986, M.S degree in Electronics
Engineering, University of
Louisville, US
1992, Ph.D degree in Electronics Engineering,
University of Kentucky, US
1992~Current, Professor, Chungbuk National
University
2010~Current, CEO, Youbicom, Korea
<주 심분야 : SoC design, Energy Management
Systems, Home Area Networks, Low power
circuits>
김 형 원 (평생회원)
1991, B.S degree in Electrical
Engineering, KAIST
1993, M.S degree in Electrical
Engineering, KAIST
1999, Ph.D degree in Electrical
Engineering, University of
Michigan, Ann Arbor
1998~1998, Intel, Hillsboro, Oregon, US
1999~2000, Synopsys, Mountain View,
California, US
2001~2005, Broadcom, San Jose, California, US
2005~2013, Founder & CEO, Xronet, Korea
2013~Current, Assistant Professor, Chungbuk
National University
<주 심분야 : Wireless sensor networks,
Wireless vehicular communications, Mixed
signal SoC designs for low power sensors and
bio-medical sensors>
“Distributed Architecture of Touch Screen
Controller SoC for Large Touch Screen Panels,”
in Proc. of International SoC Design Conference,
Jeju, Korea, Nov. 2014.
[14] I. Seo, T. W. Cho, H. W. Kim, H. G. Jang, S.
W. Lee, “Frequency Domain Concurrent Sensing
Technique for Large Touch Screen Panels", in
Proc. of IEEK Fall Conference 2013, Seoul,
Korea, pp.55-58, Korea Univ, Nov. 2013.
[15] U. Y. Jang, H. W. Kim, T. W. Cho, H. G. Jang,
S. W. Lee, “Architecture of Multi Purpose
Touch Screen Controller with Self Calibration
Scheme”, in Proc. of IEEK Fall Conference 2013,
Seoul, Korea, pp.162-166, Nov. 2013.
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