동시구동 및 순차센싱을 이용한 대형 정전용량 터치스크린용 ... · 2018. 10....

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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2015년 4월 전자공학회 논문지 제52권 제4호 63

Journal of The Institute of Electronics and Information Engineers Vol.52, NO.4, April 2015

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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64 동시구동 및 순차센싱을 이용한 대형 정전용량 터치스크린용 고속 센싱 기법 모하메드 모하메드 가말 아흐메드 외

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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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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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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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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Transcript of 동시구동 및 순차센싱을 이용한 대형 정전용량 터치스크린용 ... · 2018. 10....