Maximizing responsiveness of touch sensing via charge multiplexing in touchscreen devices


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Maximizing responsiveness of touch sensing via charge multiplexing in touchscreen devices

  1. 1. Y. Park et al.: Maximizing Responsiveness of Touch Sensing via Charge Multiplexing in Touchscreen Devices 1905 Maximizing Responsiveness of Touch Sensing via Charge Multiplexing in Touchscreen Devices Yongsuk Park, Member, IEEE, Jinwoong Bae, Eungsoo Kim, and Taejoon Park, Member, IEEE Abstract — This paper presents a novel touch sensing capacitance, placing fingers on the screen makes thescheme for capacitive touchscreen devices that is capable of capacitance to be slightly increased at the points of touch.maximizing the responsiveness of touch sensing, thereby Although it requires fine-grained, costly detection of thisenabling the realization of high-speed/resolution touchscreens. change, above-mentioned benefits of capacitive sensing makeThe heart of our proposed scheme is to measure various it highly suitable for premium devices.combinations of channels of the touchscreen in a way to However, capacitive sensing has its own limitations (e.g.,preserve orthogonality among channels, and to detect the difficulty of handwriting recognition) arising from the slowpresence of touch by decomposing the measurements into sensing speed. This is in part due to the fact that it takes turnsorthogonal components. After designing the measurement and to process the channels one by one, implying each channel isdetection algorithms in detail, we demonstrate via simulation sensed intermittently rather than continuously. Moreover, eachthat our proposed scheme indeed works.1 channel sensing requires time-consuming measurement of the time duration taken to charge and discharge. Index Terms — capacitive touch sensing, charge multiplexing,charge transfer, touchscreen devices. Motivated by these limitations of existing capacitive sensing techniques, we propose a novel approach based on the I. INTRODUCTION concept of charge multiplexing (QM) to maximize the responsiveness of touch sensing in touchscreen-equipped With recent proliferation of touchscreen-equipped mobile devices. 2 To speed up sensing, we process all the channelsdevices typified by iPhones and Android phones, there has simultaneously instead of sequentially, by performing severalbeen rapid growth of interest in touch sensing techniques that rounds of measurements for the total charges (of all channels),enable a number of novel user interface (UI) possibilities, e.g., each after charging a subset of channels. The heart of ourgestural interactions, referred to as a touch UI [1][2][3][4]. approach is to select various combinations of channels to beWhile a traditional graphic UI relies on the sense of sight, the charged according to a rule derived from ‘orthogonal’ codetouch UI is based on the sense of touch to offer a user sequences. This way, we can detect the presence of finger(s)differentiated level of interactions with the device [5]. from charge measurements by utilizing the orthogonality Touch sensing methods for mobile devices, coupled with property to decompose the measurements. The maintouchscreen types, can be classified as either resistive or contributions of the paper are: the development of a novel QMcapacitive. In resistive sensing [4], a screen is covered with touch sensing method to achieve fast response time; thetwo thin, transparent, conductive layers, e.g., indium tin oxide performance evaluation via simulation to show its capability(ITO) films, separated by a narrow gap, and hence, pressing a to accurately locating the touch; and the implementation of ancertain point on the screen causes the two layers to be analog front end.connected at that point, which is then detected by the sensing The remainder of the paper is organized as follows. Sectioncircuit. Thanks to its low manufacturing cost and unrestricted II overviews the touchreen technologies and associated touchuse of pointing devices, resistive sensing was widely used sensing methods. Section III presents the proposed sensinguntil a few years ago despite its poor durability and inherent method, while Section IV describes the simulation results anddifficulty to detecting multiple touches at a time. implementation details. Finally, the paper concludes with Capacitive sensing [6], on the other hand, features multi- Section V.touch support allowing a user to interact with the device usingmore than one finger, as well as resistance to moisture or II. BACKGROUNDsurface contaminations. These desirable properties areachieved by building a screen as a collection of capacitors, This section covers an overview of recent advances in thecalled channels, formed with two ITO layers and an insulation touchscreen technologies and touch sensing methods.layer in between; since a finger also possesses small A. Touchscreen Technologies 1 A touchscreen refers to an electronic display that can detect This work was supported by 2008 Korea Aerospace University FacultyResearch Grant. the presence of a touch within the display area. It enables one Yongsuk Park is with Samsung DMC R&D, Suwon, Korea. to interact directly with what is displayed, rather than Jinwoong Bae, Eungsoo Kim and Taejoon Park are with the School of indirectly with a cursor controlled by a mouse. Two majorElectronics, Telecommunication and Computer Engineering, Korea AerospaceUniversity, Gyeonggi-do, Korea. 2 Taejoon Park is a corresponding author (e-mail: Note Q is the standard notation for electrical “charge.”Contributed PaperManuscript received 07/14/10Current version published 09/23/10Electronic version published 09/30/10. 0098 3063/10/$20.00 © 2010 IEEE
  2. 2. 1906 IEEE Transactions on Consumer Electronics, Vol. 56, No. 3, August 2010touchscreen technologies, namely, resistive and capacitive then repeated many times gradually increasing the voltagetouchscreens, have widely been used in mobile and consumer across the integrating capacitor, which is then measuredelectronics (CE) devices, while other methods, e.g., based on against a reference voltage or read using an A/D converter.infrared (IR) and surface acoustic waves (SAW) were mainly When the reference voltage is reached, the time or the numbertargeted at large displays [4]. of steps taken is measured. QT is known to be quite stable and Resistive touchscreens consist of a panel coated with two maintain a comparatively high signal to noise ratio.electrically-conductive, resistive layers made with ITO films Other capacitive sensing techniques include sigma-deltaand separated by invisible spacers [4]. Accordingly, touch measurement [10] and relaxation oscillator-based sensing [11].sensing in resistive touchscreens hinges on the fact that a But most of existing capacitive sensing techniques are basedtouch creates contact between the two layers causing the panel on charging and discharging the channels of the touchscreento behave as a pair of voltage dividers. to induce variations in the voltage level or in the oscillation Capacitive touchscreens are made up of two conductive frequency depending on whether or not a touch is made; formetal (e.g., ITO) electrodes filled with an insulation layer to example, QT repeatedly charges a channel to make multipleform a capacitor [4][6]. Whenever these electrodes get closer measurements of the channel, thereby achieving robustness toto each other, their electric fields interact to form capacitance. the environmental changes as well as noise/interference.Since human finger is also an electrical conductor, placing afinger on the touchscreen creates additional tiny capacitance III. PROPOSED APPROACHbetween the finger and the electrodes. As such, capacitive We present the motivation for, and key idea of, ourtouch sensing detects this difference to tell if there’s a touch. proposed approach, and detail how it works. Table 1 compares the resistive and capacitive touchscreen A. Key Ideatechnologies. It shows the capacitive method has a number of Existing capacitive sensing techniques inevitably sufferdesirable features than the resistive counterpart. from slow sensing speed for the following reasons. First, they TABLE I take turns to scan all the channels one by one, delaying the TOUCHSCREEN TECHNOLOGIES response to a touch as the number of channels grow. Second, Resistive Capacitive sensing each channel requires the measurement of time Transparency Fair, 75~85% Good, >92% duration taken to charge/discharge, which is a rather time- Resolution Good Good consuming operation. Cost Low High Surface Unaffected by surface Resistant to moisture or other To minimize the response time, we propose a charge- Containments containments surface contaminations multiplexing (QM) scheme that simultaneously reads the Polyester top sheet, capacitance values of all channels at the same time. To this, Sensor Substrate Glass substrate with ITO Glass with ITO coating we utilize the desirable properties of orthogonal codes like coating Can use any pointing Walsh codes as follows. For each measurement, we Touch Method Human touch only 1. select a varying set of channels according to a rule device derived from the orthogonal codes; 2. charge the selected channels by connecting them to a B. Capacitive Touch Sensing voltage source; As mentioned, the heart of capacitive sensing is a set of 3. share all charges of those channels with an integratingconductors interacting with electric fields and the conductive capacitor, to cause the voltage level across the capacitorproperty of a human finger [6]. Placing a finger near the to rise proportionally to the accumulated charge; andelectric fields creates additional charge storage capacity 4. measure the voltage level of the integrating capacitorknown as finger capacitance, cf, whereas the capacitance using an A/D converter.without a finger presence is denoted as parasitic capacitance, This way, we have a collection of measurements representingcp. an aggregation of charge behaviors of all individual channels Capacitive sensing requires an accurate measurement of that are coupled together in a way to preserve orthogonalitycapacitance leading to the robust resolution of a change in among them. The touch detection is then carried out bycapacitance due to a touch. However, sensing a very small decomposing the measurements into individual components(e.g., less than 0.5pF) change under the constraints of using the orthogonal codes, analogously to the process ofresponse time and power consumption is challenging, CDMA (Code Division Multiple Access) communicationespecially in the presence of environmental changes, systems.measurement errors and electromagnetic interference. The most well-known capacitive sensing technique is the B. Proposed Touch Sensingcharge-transfer (QT) sensing [7][8][9]. QT first connects one Fig. 1 illustrates the architecture of an analog front end thatelectrode of the panel to a voltage source to accumulate realizes the proposed QM sensing. In general, a touch panelcharge on its capacitor, and then have it share the stored has a total of (M+N) channels, where M and N correspond tocharge with the larger integrating capacitor. This process is the number of vertically- and horizontally-formed channels,
  3. 3. Y. Park et al.: Maximizing Responsiveness of Touch Sensing via Charge Multiplexing in Touchscreen Devices 1907respectively. These vertical and horizontal channels are Accordingly, we can (1) fully charge the channel, (2) preserveresponsible for detecting y and x components of the relative the charge trapped in the channel, and (3) share the chargelocation being touched. Hereafter, we consider a simplified with the measurement capacitor, by setting Chk_ChargeCtrltouch panel where M = 5 and N = 3 for ease of explanation. to 1, high-Z and 0, respectively. Employing the capacitive touch panel, each channel can be Moreover, all (M+N) tri-state switches are attached to anviewed as a capacitor with its own capacitance value, i.e., the analog bus, or the on/off switch, and hence, can be connectedcapacitance of kth channel in the panel (k = 1, …, 8 in Fig. 1) is to the same measurement capacitor through the control ofgiven by cp,k when there’s no finger on top of the channel. If ChargeMuxCtrl. This means, after charging a multitude oftouched, it is slightly increased to cp,k + cf where cf is the finger’s channels, we can have them share their accumulated chargescapacitance. In Fig. 1, a touch took place at the northwest side with the measurement capacitor by simultaneously controllingof the panel corresponding to channels 2 and 8. Conventional all Chk_ChargeCtrl’s and ChargeMuxCtrl to 0’s and on,technique detects this event by continuously measuring the respectively. This causes the voltage level at the input of thecapacitance of each and every channel, one after another, and A/D converter to be raised to a certain value proportional tofiguring out if there’s a noticeable increase in the capacitance the aggregated charge.value. That is, it decides that channel 2 has been touched if thecapacitance measurement of that channel was cp,2 at time i, andcp,2 + cf at time i+1, and computes the location of the touch alongthe y axis from the relative vertical location of channel 2. Fig. 2. Timing diagram for generating switch control signals. As shown in Fig. 2, we divide a sensing interval (to measure all of the channels once) into 23 sub-intervals (K = 3), and judiciously generate a schedule for joint charging and sharing throughout the entire sensing interval using orthogonal Walsh code sequences. Denoting wk as the kth Walsh code, 8 Walsh codes are given by:Fig. 1. Architecture of an analog front end according to the proposedQM touch sensing technique. w1 = [ 1, 1, 1, 1, 1, 1, 1, 1] w2 = [ 1, − 1, 1, − 1, 1, − 1, 1, −1 ] As opposed to the conventional techniques relying on w3 = [ 1, 1, − 1, − 1, 1, 1, − 1, −1 ]sequential scanning of channels, our proposed approach w4 = [ 1, − 1, − 1, 1, 1, − 1, − 1, 1]simultaneously senses 2K channels through the concept of (1) w5 = [ 1, 1, 1, 1, − 1, − 1, − 1, −1 ]charge multiplexing (QM), where K is the smallest integer w6 = [ 1, − 1, 1, − 1, − 1, 1, − 1, 1]satisfying 2K ≥ (M+N), e.g., K = 3 in Fig. 1. With QM sensing, w7 = [ 1, 1, − 1, − 1, − 1, − 1, 1, 1]each channel, say channel k, is connected to a tri-state switch w8 = [ 1, − 1, − 1, 1, − 1, 1, 1, −1 ]controlled by Chi_ChargeCtrl. This control signal offers 3modes of operation as follows: if Chk_ChargeCtrl is The charging/sharing schedule of channel k is governed by 1, it makes a connection between channel k and VCC; wk in that 8 elements of wk determine whether or not to 0, it connects channel k with an on/off switch controlled charge/share in the consecutive sub-intervals. That is, channel by ChargeMuxCtrl, which, in turn, attaches the channel k is involved in charging/sharing process in the jth sub-interval to the measurement capacitor if the switch is on on; and if jth element of wk is 1; otherwise, remains disconnected from at high-Z state, the channel is disconnected from both the bus. For example, channel 2 has to be charged and shared VCC and the ChargeMuxCtrl switch. in the 1st, 3rd, 5th, 7th sub-intervals with reference to w2. Note
  4. 4. 1908 IEEE Transactions on Consumer Electronics, Vol. 56, No. 3, August 2010this agrees with the timing diagram in Fig. 2. The key benefit where (c m − c p ) is the average value. Fig. 4 plots both cm − cpof using Walsh codes is that we can easily decompose themultiplexed signals into pieces thanks to its orthogonality. and cf.Fig. 3. Block diagram for digital processing in the charge multiplexedsensing technique. Fig. 3 presents the block diagram of the digital processing Fig. 4. Multiplexed signal samples after the level adjustment when channels 2 and 8 have been touched.part in our proposed approach. The A/D conversion blockdigitizes the voltage level of measurement capacitor (after The decision block finally makes a decision of whichcompleting the charge sharing among capacitors) to generate channels have been touched by making best use of themeasurement samples, one per sub-interval (with at least 12- orthogonality of Walsh codes, which can be expressed asbit resolution). Let cm,j denote the sampled value in the jth sub- follows:interval, then a measurement vector cm is given by: ⎧ 0, i ≠ j w i • wT = ⎨ [ c m = cm,1 cm, 2 cm,8 ] (2) j ⎩ 8, i = j (6)At the end of the sensing interval, the A/D converter produces Notice that cf can be expressed as a linear combination of wk’sthe measurement vector cm and feeds it to the panel (with increase capacitance values) because it was obtained bycapacitance estimation and level adjustment blocks. ‘multiplexing’ charges of channels, e.g., when a touch took The panel capacitance estimation block stores a vector cp place over the crossing point of channels 2 and 8,that equals cm when no touch has been made. Note that cp isbuilt solely from panel’s capacitance values, cp,k, k = 1, …, 8 φas follows: cf = (w 2 + w 8 ) (7) 2 ⎡ 8 ⎤ Therefore, the decision block can detect if there’s a finger on ∑ c p = ⎢ c p, k ⎢ k =1 ∑ c p, k ∑ c p, k ⎥ k =1, 4, 6, 7 ⎥ (3) top of the kth channel by (1) computing an inner product of cf ⎣ k =1,3,5, 7 ⎦ and wk as The level adjustment block receives cm and cp and performs ⎧ 0, k = 1, 3, 4, 5, 6, 7 c f • wT = ⎨ (8) ⎩ 4φ , k = 2, 8appropriate level adjustment to produce an adjusted vector cf. kTo achieve this, it first subtracts cp from cm to extractcapacitance components of the finger. Let’s consider the and (2) determining if the computed value exceeds a certainscenario of Fig. 1, in which channels 2 and 8 were touched. threshold, say φ. Accordingly, it decides channels 2 and 8Since cf = φ is added to the measurement sample whenever have been touched in the above example.channels 2 and/or 8 are charged, we obtain: This decision algorithm clearly shows that we can easily extract information from the multiplexed signal by exploiting c m − c p = [ 2φ , 0, φ , φ , φ , φ , 2φ , 0 ] (4) ‘orthogonality’ of the charging/sharing schedule. C. DiscussionWe perform further adjustment to make it zero-mean, i.e., we Placing a finger on a certain channel not only increases thecompute: capacitance of that channel but also affects other nearby channels, since the capacitance is inversely proportional to c f = c m − c p − (c m − c p ) distance and the finger covers an area rather than a point. For (5) example, touching channel 2 increases cp,2 by cf (in channel 2 = [ + φ , − φ , 0, 0, 0, 0, + φ , − φ ] itself), cp,1 and cp,3 by α1 × cf (δ meters apart), cp,4 by α2 × cf (2δ
  5. 5. Y. Park et al.: Maximizing Responsiveness of Touch Sensing via Charge Multiplexing in Touchscreen Devices 1909meters apart), and cp,5 by α3 × cf (3δ meters apart) where α3 < and 8, while decision values for other channels are attenuatedα2 < α1 < 1. proportionally to parameters, α1, α2, and α3. QM sensing is very robust to this inter-channel interferenceas it can nevertheless detect the location of touch. That is, inthe presence of inter-channel interference, (7) is changed to: φ 200 cf = [w2 + w8 +α1(w1 + w3 + w7 ) +α2 (w4 + w6 ) +α3w5 ] (8) 2 150So, the inner product value is computed as: 100 ⎧ 4φ ⋅ α1 , k =1 50 ⎪ 4φ , k =2 100 ⎪ 0 80 ⎪ 4φ ⋅ α1 , k =3 1 2 3 60 ⎪ 4 40 4φ ⋅ α 2 , 5 T ⎪ k =4 Channel 6 7 20 Iteration c f • wk = ⎨ (9) 8 ⎪ 4φ ⋅ α 3 , k =5 ⎪ 4φ ⋅ α 2 , k = 6 Fig. 5. Touch sensing in the presence of inter-channel interference. ⎪ ⎪ 4φ ⋅ α1 , k = 7 ⎪ B. Implementation ⎩ 4φ , k =8 As shown in Fig. 6, we implemented the analog front end of QM scheme as H/W prototype with its touch panel consistingObviously, the values for channels 2 and 8 are the largest of 21 channels. We used a total of 44 switching ICs in thesince α3 < α2 < α1 < 1. Thus we can successfully determine analog front end, i.e., 21×2 for charging and bus connection, 1the correct location of touch. for charge sharing, and the rest for discharging. A challenge in Finally, we can apply the same algorithm when the total developing the prototype was that each switching IC has itsnumber of channels, (M+N) is strictly less than 2K, i.e., by own capacitance that is comparable to or even greater than thesimply setting the capacitance of the missing channel to zero. finger’s capacitance, which means the sensitivity of touchThis works because we only consider the finger’s capacitance detection is seriously degraded. To deal with this problem, wein the calculation of decision parameters, completely adopted switching ICs with smallest capacitance values, i.e.,removing the effects of channels’ capacitance.3 9.5pF and 5pF when it’s on and off, respectively, and also optimized the switching control logic. IV. PERFORMANCE EVALUATION A. Simulation Results We would like to confirm that our QM scheme is capable ofaccurately detecting the touch location in the presence ofinter-channel interference. So, we simulated the QM scheme under the followingsimulation environment. The touch panel has 8 channels, eachwith capacitance-equivalent values, 919, 954, 842, 895, 871,881, 931 and 983. Note that these values were measured usingan analog front end circuit that produces count valuesproportional to the channel’s capacitance. Touch is made on Fig. 6. Screenshot of the touch sensing prototype implementing the proposed technique.the crossing of channels 2 and 8. α1, α2, and α3 are set to 0.5,0.25 and 0.167, respectively. The signal-to-noise ratio (SNR)is 20dB throughout the simulation where the noise is V. CONCLUSIONgenerated according to the Gaussian distribution with variance In this paper, we have considered a problem of maximizing10 times smaller than the magnitude of cf. the responsiveness of the capacitive touch sensing, and have By applying (2) through (7), we obtain per-channel (inner proposed a touch sensing scheme based on the concept ofproduct) values for touch detection. Fig. 5 plots the results of charge multiplexing that simultaneously processes all100 simulation runs. Clearly it shows two peaks at channels 2 channels and detects the presence of touch by utilizing the desirable properties of orthogonal Walsh codes. Moreover, we 3 This is analogous to the demodulation process of communication systems have demonstrated our proposed scheme accurately locates thethat removes the carrier and feeds the signal component (corrupted with noiseand interference) to the detector. touch even in the presence of inter-channel interference.
  6. 6. 1910 IEEE Transactions on Consumer Electronics, Vol. 56, No. 3, August 2010 REFERENCES Jinwoong Bae received the B.S. degree in electronics engineering from Korea Polytechnic University, Gyeonggi-[1] K. Hinckley and M. Sinclair, “Touch-sensing input devices,” Proc. do, Korea in 2008. Since 2009, he has been with Korea ACM Conf. on Human Factors in Computing Systems, 1999. Aerospace University, Gyeonggi-do, Korea, working[2] P. Hanrahan, “Technical perspective: a graphical sense of touch,” towards the M.S. degree in information and Communications of the ACM, vol. 52, no. 12, December 2009. telecommunication engineering. His research interests[3] IEEE Software Staff, “Touch screens now offer compelling uses,” IEEE include touch sensing and digital circuit design. Software, vol. 8, no. 2, pp. 93-94, March 1991[4] Wikipedia, “Touchscreen,” Wikipedia, The Free Encyclopedia, Eungsoo Kim received the M.S. degree in electronics[5] F. Golshani, “TUI or GUI -- its a matter of somatics,” IEEE Computer engineering from Yonsei University, Seoul, Korea in 2003, Society, 2007. and the B.S. degree in electronics engineering from[6] L. K. Baxter, “Capacitive sensors – design and applications,” IEEE Press, Myongji University, Gyeonggi-do, Korea in 1994. Since 1997. 2009, he has been with Korea Aerospace University,[7] H. Philipp, “Charge transfer sensing,” Sensor Review, vol. 19, no. 2, pp. Gyeonggi-do, Korea, working towards the Ph.D. degree in 96-105, 1999. information and telecommunication engineering. His[8] H. Philipp, “Charge transfer capacitive position sensor,” U.S. Patent research interests include touch sensing and analog circuit design. 7,148,704, issued December 12, 2006.[9] W. Westerman and J. G. Elias, “Touch sensing with a compliant conductor,” U.S. Patent Application 2007/0268275, filed July 30, 2007.. Taejoon Park (S’04-M’05) received the Ph.D. degree in[10] K. Hargreaves, J. K. Reynolds, D. Ely and J. Haines, “Methods and electrical engineering and computer science from systems for detecting a capacitance using sigma-delta measurement University of Michigan, Ann Arbor, MI, USA, the M.S. techniques,” U.S. Patent 7,301,350, issued November 27, 2007. degree in electrical engineering from Korea Advanced[11] T. Peng and Z Qin, “Apparatus and method for recognizing a tap gesture Institute of Science and Technology (KAIST), Taejon, on a touch sensing,” U.S. Patent Application 2007/0229466, filed March Korea, and the B.S. degree (summa cum laude) from 30, 2006. Hongik University, Seoul, Korea. Dr. Park is currently an assistant professor at Korea Aerospace University, Gyeonggi-do, Korea. BIOGRAPHIES Earlier, he was LG Electronics as an engineer from 1994 to 1999, then as a senior engineer in 2000, and with Samsung Advanced Institute of Technology, Yongsuk Park received the Ph.D. and M.S. degrees both in Korea, as a principal engineer from 2005 to 2008, where he conducted computer science from Polytechnic Institute of New York research and development of various digital consumer electronics products as University, New York, USA and the B.S. degree in well as led projects on mobile handsets security. He published a number of computer science from Sogang University, Seoul, Korea. Dr. technical papers in international journals, book chapters, and conference Park is a head researcher at SAMSUNG Electronic Co, Ltd., proceedings. He also invented or co-invented numerous international patents Korea where he is focusing on Wireless Networks and and is an inventor of essential patents for the 4C DVD licensing pool. His Mobile Device Security. Earlier, he was with AT&T current research interests are in mobile, ubiquitous computing and networkingLaboratories at Middletown, NJ, USA as a senior technical staff member from with emphasis on secure, service-centric, autonomous and bio-inspired1999 to 2003, where he conducted various research and development about computing. He is a member of optimization and management for data network and IP service. Since1995, from time to time, he was also with the City University of New York,New York, USA as a research and teaching staff working on wirelessnetworks and teaching CS courses. He served as Editor for several journalsand conferences such as Journal of SAMSUNG and published a number ofpapers and patents. His current research interests include broadband wirelesscellular network and utility computing with emphasis on secure, seamless,cognitive aspects. He is a member of IEEE.