A Temperature-based Touch-sensor for Non-Emissive Textile ......Tic-tac-toe table cloth Figure 8: Tic-tac-toe game implemented on a table cloth (a) System set up with 9 peltier elements

A Temperature-based Touch-sensorfor Non-Emissive Textile Displays

Roshan Lalintha PeirisNGS, National University ofSingaporeMIT Media Lab (FluidInterfaces),Massachusetts Institute [email protected]

Ryohei NakatsuIDMI, National University [email protected]

Copyright is held by the author/owner(s).CHI 2013 Extended Abstracts, April 27–May 2, 2013, Paris,France.

ACM 978-1-4503-1952-2/13/04.

AbstractNon-light-emissive textile displays have become a popularresearch area that allows subtle and ambient animationson textiles. This work explores one such existingtechnology which uses thermochromic inks and peltiersemiconductors to implement a touch sensitivenon-light-emissive textile display. To achieve this weinvestigate a new method that detects temperaturetransients (caused by a touch) in order to detect a touchwhich allows interactive touch sensitive textile displayswithout any changes to the existing hardware.

Author KeywordsGuides, instructions, author’s kit, conference publicationsTouch sensor, organic interface, temperature,thermochromic, peltier, textile, fabric, interface

ACM Classification KeywordsH.5.2 [Information interfaces and presentation (e.g.,HCI)]: User Interfaces.

IntroductionInteractive textiles explore various ways and means inwhich textiles can become a medium of communicationand expression. As such, textile displays are a commonlyinvestigated topic in this field of research. They have beenexplored as emerging display technologies for organic

Work-in-Progress: Tactile CHI 2013: Changing Perspectives, Paris, France

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interfaces where textile displays provide more fluid andflexible customized display devices [1].

This paper explores the making of an interactive textiledisplay based on one such widely used non-light-emissivetechnology, thermochromic inks. Most suchthermochromic ink textile display systems accompanytemperature control systems as thermochromic inks arethermally actuated. Some of the commonly used thermalactuators for such control systems are conductive yarn [3]or peltier semiconductor elements [2]. However, as seen inrecent thermochromic ink related works, the textiledisplay is not directly interactive. That is, the technologyfunctions only as a display which may be programmedanimations [2], or is passively interactive through externaltriggers such as body heat. Therefore this paper exploresthe implementation of a temperature based touch sensorbased on existing technologies of thermochromic inks andpeltier semiconductor elements.

Our main motivation is to have the display and thesensing through the same textile interface similar to a lowfidelity touch screen. To achieve this functionality, touchinput is recognized by analysing temperature variationsand conducting a transient analysis on such variations todetect the touch. This way, the existing temperaturecontrollers can be modified for thermochromic ink basedtextile displays to make them touch sensitive forinteractivity without the use of external touch sensorssuch as pressure sensors allowing the sensing and thedisplay through the same textile interface.

System DescriptionThe setup of a single peltier element with thethermochromic inks is as seen in Figure 1. For the textiledisplay we use the existing technologies of thermochromic

inks and peltier semiconductor elements. For the purposeof implementing the textile display, we use the structureof AmbiKraf [2] with minor changes to achieve thenon-light-emissive display.

Figure 1: System setup of a single peltier pixel

Peltier semiconductors (2.5cm X 2.5cm) are used, as itallows rapid temperature changes [2] which is suitable forour work. The peltier semiconductors’ ability to actuatewithin a wide range of temperatures is another mainreason for its use. Due to the temperature sensitivenature of this work, we evaluate our work with atemperature range from 160C to 400C. With theseevaluations, we can extend our work to other thermalactuators such as conductive yarn or flexible heater padswhich we will discuss in the Discussion section.

We used black color thermochromic inks of actuationtemperatures 310C for our experiments andprototypes(when heated beyond the actuationtemperature the ink changes color and regains the originalcolor when cooled). Thermochromic inks were combinedwith textile binder and screen printed on to the textile.Double-sided copper adhesive tapes are used to attachsurfaces and components on to the peltier element foroptimal heat transfer. The temperature of the peltier


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elements is controlled by a closed loop PID (proportional,integral, derivative) controller. A temperature sensorplaced on each peltier pixel gives a feedback of thetemperature to the controller.

Touch Sensing

Figure 2: Overall operation principle (SP-set point,T-Temperature, t-time)

Touch sensing is achieved making use of this PIDtemperature controller. Closed loop PID temperaturecontrollers have the ability to achieve and maintain adesired temperature with relatively high accuracy. Thus,once the system reaches this steady state, it wouldmaintain the temperature within a certain thresholdunless effected by an external thermal source. Thisexternal source can be considered as an impulse input tothe system. As a result to this ‘impulse’, it would onceagain adjust itself to reach the steady state after a certaintime. Figure 2 shows this principle where a steady statetemperature is affected by an external source.

In this work, we use this principle where we consider thefinger to be an external temperature source. Thus, asindicated in Figure 2 the touching of a temperaturecontrolled surface could induce a temperature change onthe surface due to the temperature of the finger.Therefore, by monitoring such impulsive changes in thetemperature, it is possible to sense the touch on thesurface of the textile display.

Figure 3: Algorithm of the controller

Figure 3 depicts the firmware algorithm which implementsthe above concept. We monitor the touch input only inthe steady state. Therefore we avoid the transient statewhere the temperature changes from the current value tothe set value (state ‘a’ of Figure 2). During the transient,either the temperature change rate

∣∣dTdt

∣∣ is greater thanzero, or, at the peaks/valleys the rate could be zero asseen in Figure 2. However, at the peaks/valleys, generallythe difference between set temperature and the currenttemperature is significant. Next, we define Tthreshold


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based on the accuracy and the maximum error of thecontroller (Figure 2). We define Rthreshold as theminimum temperature change rate required to detect atouch. Thus, at the steady state, if the

∣∣dTdt

∣∣ is closer tozero or less than Rthreshold within the Tthreshold value,we can assure that the system has passed the transientstate and is in the steady state.

Thereby the presence of an external source can bedetected in the steady state (Figure 2), where, when thetouch occurs, temperature change rate is significantlyhigher than that of the steady state.

Results

Figure 4: Normalised temperature changes caused by thefinger for different steady state temperatures

A preliminary study was conducted to identify the impulsepatterns created by a single touch. We used ourtemperature controller at steady state for differenttemperatures and logged the changes that were caused bya single touch. The average finger temperature was31.270C and the ambient temperature was 220C. Thetouch was performed for approximately 1s. Figure 4

indicates the impulses caused by a single touch fordifferent temperatures. The temperatures have beennormalized for ease of reference with 1 being the desiredset point(SP).

Figure 5: Temperature change rates caused by a touch atdifferent steady state temperatures

Figure 5 indicates the∣∣dTdt

∣∣ for the touched state. Throughobservation it can be concluded that the temperaturechange rates are higher when the difference between thesteady state temperature and finger temperature is higher.In addition, the change of temperature at higher steadystate temperatures are generally low but the rate ofchange is higher as observed by Figures 4 and 5.

Touch sensing resultsFigure 6 depicts the usage of this system where the colorchanging pixels operates as a switch to turn the color onor off. As seen, as the user touches the textile, the systemtriggers the color change from color to colorless and viceversa. The resulting temperature change for colorless tocolor state (360C to 240C) is as seen in Figure 7. Thisindicates that the touch can be detected in less than 0.6s.


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Figure 6: Sequence of images showing a peltier pixel workingas a touch-sensed switch to switch between color and colorlessstates

Figure 7: Transient detection and trigger by the system

ApplicationsWe implemented two table cloth based prototypes toidentify potential applications of the above sensingtechnique. These prototypes are used to demonstrate thenew interactivity of the system in comparison to mostprevious works which were fixed animations. For thispurpose we use different arrangements of peltier modulesand black color thermochromic ink of 310C actuationtemperature screen printed on to a yellow color silk textile.

Tic-tac-toe table cloth

Figure 8: Tic-tac-toe game implemented on a table cloth (a)System set up with 9 peltier elements and temperature sensors,(b) Table cloth with the tic-tac-toe game (c) touching to selectsquares (d) final stage of the game with four black squaresthree yellowish squares and two unselected light black squares

To identify an application for this concept, weimplemented a simple game on a ‘table cloth’. This gamewas based on the popular tic-tac-toe game where twoparticipants compete to select squares to try to form threein a straight line to win the game. The system setup isseen in Figure 8(a). At the start of the game thetemperature of all nine squares are maintained at 280Cwhich turns the ink into a faint black color. Next as eachuser alternatively selects squares by touching them, thesystem alternatively turn each square to a ‘colored’(black) or ‘colorless’(yellow) square as seen in Figure 8(c).Figure 8(d) indicates the final status of the game.


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Drawing-pad table cloth

Figure 9: Interacting with the Drawing-pad application todraw a heart shape on the textile

In this application, we used a 6x10 array of peltierelements to implement a drawing pad on a table cloth. Atthe beginning all the squares would be maintained at the‘colored’ state at 240C. With the user’s touch, they areactuated to become colorless at 360C. If the user needs to‘erase’ a certain actuated pixel, she can do so by touchingthat pixel again which will turn it back to the ‘color’ state(as was depicted in Figure 6). Figure 9 shows the usage ofthis prototype.

DiscussionThis work presents a methodology for the implementationof a low fidelity touch sensor for thermally actuated textiledisplays. As observed, a touch can be detected within 0.6swhich is of satisfactory levels for the current use contexts.In addition, since each peltier pixel can be temperaturecontrolled individually this system could also detectmultiple touches at the same time. Tic-tac-toe anddrawing-pad prototypes were some examples of how asimple table cloth can become subtly interactive with thistechnology. These examples indicate the potential of thiswork being used in large scales such as wall papers whichcan easily adopt these technologies and become adhoctouch sensitive displays similar to a white board. Whilenot in use, the wall paper could display subtle animations,where upon touch, it could become a white board likeinteractive display or an interactive gaming platform.

As future works, we aim to investigate the expandabilityof this work to other similar thermochromic actuationmechanisms such as conductive yarn. In addition, we hopeto conduct user evaluations with proper user contexts ofsuch display to identify the strengths, weaknesses, andusability of the system.

ConclusionTo conclude, this paper presents an in-progress method tomodify existing temperature controllers to become touchsensitive for non-light-emissive textile displays. Here wepresent the sensing principle of analysing impulsetemperature changes in the steady state of the controllerto identify touch inputs. The paper detailsimplementation methodology and presents results. Thus,through this technology we present a novel way to usetemperature as a touch sensing mechanism to convertexisting temperature controllers to touch sensors.

References[1] Co, E., and Pashenkov, N. Emerging display

technologies for organic user interfaces. Commun.ACM 51, 6 (June 2008), 45–47.

[2] Peiris, R. L., Tharakan, M. J., Fernando, N., andChrok, A. D. Ambikraf: A nonemissive fabric displayfor fast changing textile animation. In Embedded andUbiquitous Computing (EUC), 2011 IFIP 9thInternational Conference on (oct. 2011), 221 –228.

[3] Wakita, A., and Shibutani, M. Mosaic textile:wearable ambient display with non-emissivecolor-changing modules. In ACE ’06: Proceedings ofthe 2006 ACM SIGCHI international conference onAdvances in computer entertainment technology,ACM (New York, NY, USA, 2006), 48.


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A Temperature-based Touch-sensor for Non-Emissive Textile ......Tic-tac-toe table cloth Figure 8: Tic-tac-toe game implemented on a table cloth (a) System set up with 9 peltier elements