EchoTube: Robust Touch Sensing along Flexible Tubes using ... · a robust, modular, simple, and inexpensive system for sensing ... need for fragile electronics that work only at short

EchoTube: Modular and Robust Press Sensing alongFlexible Tubes using Waveguided Ultrasound

Carlos E. TejadaUniversity of Copenhagen

Copenhagen, [email protected]

Jess McIntoshUniversity of Copenhagen


Klæs Alexander BergenUniversity of Copenhagen


Sebastian BoringUniversity of Copenhagen

Aalborg UniversityCopenhagen, Denmark

[email protected]

Daniel AshbrookUniversity of Copenhagen


Asier MarzoUniversidad Publica de

NavarraPamplona, Navarre, [email protected]

(a) (b) (c)Figure 1. EchoTube senses press events along flexible tubes using ultrasonic pulses. It (a) only requires simple, off-the-shelf hardware, (b) senses pressevents with up to 1.25 mm accuracy at up to 3 m, and (c) is robust against crushing, bending, water, and environmental noise.

ABSTRACTWhile pressing can enable a wide variety of interesting appli-cations, most press sensing techniques operate only at closedistances and rely on fragile electronics. We present EchoTube,a robust, modular, simple, and inexpensive system for sensinglow-resolution press events at a distance. EchoTube works byemitting ultrasonic pulses inside a flexible tube which acts asa waveguide and detecting reflections caused by deformationsin the tube. EchoTube is deployable in a wide variety of situa-tions: the flexibility of the tubes allows them to be wrappedaround and affixed to irregular objects. Because the electronicelements are located at one end of the tube, EchoTube is robust,able to withstand crushing, impacts, water, and other adverseconditions. In this paper, we detail the design, implementation,and theory behind EchoTube; characterize its performance un-

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der different configurations; and present a variety of exemplarapplications that illustrate its potential.

Author KeywordsSensors; ultrasound; sound; waveguide; tubes; press sensing

CCS Concepts•Human-centered computing→ Sound-based input / out-put;

INTRODUCTIONAlthough sensing press locations of presses can enable a va-riety of interesting applications such as buttons or collisiondetection, the ability to sense presses is often restricted by theneed for fragile electronics that work only at short distances orapproaches prone to occlusion or other environmental interfer-ence. Capacitive and resistive press sensing require exposingthe sensors to the environment, risking potential damage; com-puter vision is sensitive to occlusion and changes in light; andacoustic sensing methods can suffer from interference fromenvironmental noises.

In this paper, we present EchoTube, a system that utilizeswaveguided ultrasound to enable press sensing which is afford-able, quickly deployable, and robust to damage. EchoTube is

Paper Session 3: Ultrasonic Techniques and ISS’19, November 10–13, 2019, Daejeon, Republic of Korea

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comprised of a flexible tube attached to an ultrasonic trans-ducer which detects tube deformations caused by objects orfingers pressing against the tube. Our system is constructedfrom off-the-shelf components with simple modifications ap-plied; it is inexpensive, easy to set up and use. We characterisethe accuracy of EchoTube in multiple configurations, andintroduce additions to the basic technique, including bendsensing, limited multi-press sensing, and multiple tubes. Wepresent a series of applications that demonstrate the versatilityof EchoTube, showing how the properties of tubes give rise tomultiple useful capabilities:

Flexibility: Tubes can be wrapped around objects with ir-regular surfaces, enabling EchoTube to quickly add press-sensing capabilities to existing geometries.

Mechanical robustness: Tubes can endure physical stress,for instance being crushed by vehicles. The electronicsare placed at one end of the tube, isolated from the externalenvironment, so only the tube is subject to mechanical stress.The tube is also waterproof, easily enabling interactionin places where it was previously challenging to deploysensors.

Simple hardware: The sensor placed at the end of the tube iscompact and only two off-the-shelf hardware components(microcontroller and ultrasonic rangefinder) are required toassemble EchoTube.

Simple software: No training or calibration is required forEchoTube to determine press locations, making it fast todeploy.

Modularity: Tubes can be chained together on the fly to in-crease the interaction space or have multiple input devices.Multiple tubes may also be connected to a single transducer,enabling more precise multi-press detection.

Robust sensing: Because the tube contains and guides theultrasonic pulses emitted by the sensor, EchoTube is ro-bust against environmental interference such as extraneoussound.

Long sensing range: The simplest setup of EchoTube sup-ports sensing along a two-meter tube, with a modified setupwe are able to sense through tubes of up to five meters.

Inexpensive: Ordinary tubes are available at low prices (e.g.,US$0.06 per meter), and the ultrasonic sensors cost lessthan US$15.

The contributions of our research include:

• EchoTube, a method for using ultrasound to sense presslocations along a flexible tube;

• Characterisation of the performance of EchoTube undervarious conditions;

• Demonstrations of the benefits of EchoTube via exampleapplications; and

• Preliminary investigations into additional features ofEchoTube including bend sensing, multi-press and multipletubes.

RELATED WORKAcoustic SensingAnalysing how the acoustics varies when users touch, press,slide or tap their fingers on different objects is a well-studiedinteraction technique. The work can be generally dividedinto passive and active techniques. We distinguish betweentouching and pressing, as pressing also implies some force; inour case, enough force to deform a flexible tube.

Passive Acoustic SensingPassive techniques use the sound generated by the user’s inter-action with an object. ScratchInput uses audio to detect ges-tures made on a surface with a fingernail [4]. Lopes et al. [14]and Harrison et al. [5] demonstrated the ability to differen-tiate whether a user is tapping on a surface with a fingertip,fingernail, or knuckle. Passive techniques have also been usedto localise the user’s interaction; for example, Toffee usesmultiple microphones to determine the direction of a tap on asurface around a mobile device [28]. This is the closest fromthe passive techniques to our work, however the detection oftap distance is still limited and the surface must be flat andcontinuous.

Fabricated objects can be engineered to make particular soundswhen a user interacts with them. Acoustic Barcodes usedunique patterns of grooves to create distinctive sounds when auser slides a finger or tool over them [6], Lamello and Tickersand Talkers use 3D-printed objects with structures that makedistinctive sounds when users “pluck” them [22]. AcousticVoxels [13] and Blowhole [24] use cavities of varying sizesthat make distinct sounds when users blow into them.

A microphone can capture the air generated when bellows andtubes are squeezed to detect how strongly they are pressed orto differentiate twists from stretches or bends [23]. Similarly,SqueezaPulse [7] captures the sound emitted by bellows whensqueezed to detect the press strength or characteristics of thetubes. These techniques are not suited for the detection ofpress distance.

Active Acoustic SensingPassive methods require the user to impart enough force toan object to create a detectable sound, limiting the possibleinteractions. By including a sound-generating element, activesensing systems enable more-subtle interactions by detectinghow the user’s action change the acoustic characteristics of theobject. A piezoelectric speaker and a microphone attached toan object can detect how it is grasped [18] and the strength ofthe grip [19] by analysing the spectral response of the object.This technique can be extended to detect hand motion aboveobjects [26]. Acoustruments [12] uses a similar technique: byplacing a short tube between the speaker and microphone ofa mobile phone, it can detect modifications on the tube via3D-printed knobs, sliders, and other tangibles. UTAP emitsultrasonic sweeps through solid waveguides, detecting userinteraction via the change in the response spectrum [21]. Allthese methods frequency-domain features, and require thetraining of a classifier; our time-domain approach, on the otherhand, operates without this necessity and permits to start usingEchoTube without calibration even if the tubes are wrapped orextended on the fly.


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Contrary to the previous work, Echotube uses only one fre-quency both for emitting and receiving. It is more efficientto receive and emit at a single frequency with high resonanttransducers than having wide-band elements.

Pulse-echo through waveguidesOutside of HCI, ultrasonic pulse-echo through a waveguideis used to detect cracks [15] or obstructions [2] along pipes.It has also been used to detect the displacement of a magnetalong the exterior wall of a tube [1, 17] via a metal sphereinserted in the tube. This principle is similar to EchoTube, butrequires a more-complex setup and is less robust.

Non-acoustic techniquesNon-acoustic techniques for touch and press sensing along adistance have been demonstrated in the literature. Here, wecompare these examples with EchoTube.

Wimmer and Baudisch demonstrated using time-domain re-flectrometry to touch-enable conductive wires [27]. Similar toEchoTube, this method analyses the reflection of a signal alongthe wire, but uses an electric rather than an acoustic pulse. Al-though offering accuracy superior to EchoTube, this methodis limited to detecting touches on conductive objects, requiresmore expensive and bulky equipment, and is susceptible toenvironmental interference from radio sources.

Optical fibre sensing has been used to measure bend and loca-tion. ShapeTape, a commercial product using fibre optics, wasdemonstrated for graphical curve manipulation [3]. However,optical fibres are expensive, fragile, hard to couple, and singlefibres cannot report bend location unless marked and measuredwith expensive industrial processes [9].

Efforts like [10] employ resistive sensing techniques to sensepressing interactions. Although resistive techniques success-fully enable press sensing to smaller objects, they cannot detectpress locations at longer ranges. In comparison, EchoTubecan sense press events up to five meters from its electroniccomponents.

ECHOTUBEEchoTube is based on the principle of acoustic time-domainreflectrometry: a transducer emits an ultrasonic pulse, whichtravels through the air and reflects on the objects that it encoun-ters. These reflections travel back towards the transducer. Bymeasuring the amount of time ∆t between emitting the pulseand receiving its reflection, the distance d of the reflectingobject can be calculated according to 2d = c∆t, where c is thespeed of sound.

The commercial ultrasound ranging modules we use inEchoTube use this principle to measure distance. However,because they are usually used in robotics and other uncon-strained sensing applications, these devices are most oftenused for sensing in open air. We make two key modificationsto enable EchoTube.

First, we constrain the sensing volume of the module to anarrow path by enclosing the transducer in one end of a flexibletube. Doing so, prevents the ultrasound from being reflectedby objects in the environment and guides it along the tube. In

this tube-based configuration, the ultrasonic pulse propagatesuntil it is reflected by discontinuities in the tube such as a hole,or, in our application, a deformation formed by an externalobject.

Second, rather than interfacing with the ultrasonic rangingmodules’ digital output, which gives only the calculated rangeto the first reflection, we directly analyse the raw output fromthe sensor itself, allowing calculation not only of the distanceto the first reflecting object, but further objects as well. Inaddition, by examining the amplitude of the reflected pulse,we can determine the degree of deformation caused by anexternal object. This modification is key, since otherwise theranging module would always report a distance of zero due tothe first echo produced on the coupling with the tube.

Our prototype system is comprised of several components.On the hardware side, we connect a commercial ultrasonicranging module to a flexible silicone tube. We use a micro-controller to directly measure the analogue waveform outputof the transducer. On the software side, the system processesthe received ultrasonic signal to detect peaks that representthe echoes from deformations in the tube, calculates the dis-tances and magnitude of the deformations, and takes actionscorresponding to the intended application.

In the remainder of this Section, we will describe EchoTube’shardware and software components, and characterise the sys-tem’s capabilities under different configurations and condi-tions.

HardwareEchoTube’s hardware is comprised of three components: amicrocontroller, an ultrasonic transducer, and a tube coupledto the transducer.

MicrocontrollerMost commercially available ultrasonic transducers operateat 40 kHz; we must therefore sample the transducer at a mini-mum of 80 kHz to fully capture its output. We implementedEchoTube on several microcontrollers: the AdaFruit Trin-ket M01, the PJRC Teensy 3.52, and the Feather WICED3. Thelatter MCU has better Analog to Digital Converters (ADCs)in terms of sampling rate and bit-depth, which will lead toimproved performance. Subsequently, our experiments wereconducted with this microcontroller.

Ultrasonic TransducersWe connect an ultrasonic range-finding transducer to the micro-controller. Ultrasonic rangefinders come in two types: single-and dual-transducer. In the single-transducer configuration, thetransducer acts first as emitter and then as receiver. The inter-nal piezoelectric element is first excited by a microcontroller togenerate the emitted pulse; then, the microcontroller switchesinto receiving mode, with the read pulse routed through anoperational amplifier (opamp) into the microcontroller’s ADC.The single-transducer setup is compact and easy to coupleto a tube; however, it can suffer from “ringing,” where the

1https://www.adafruit.com/product/35002https://www.pjrc.com/store/teensy35.html3https://www.adafruit.com/product/3056


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https://www.adafruit.com/product/3500

https://www.pjrc.com/store/teensy35.html

https://www.adafruit.com/product/3056

Figure 2. Sampled pin from a MaxBotix MB1040 single-transducer mod-ule to obtain the raw signal captured by the transducer.

Figure 3. Actual-size cross-sections of tubes tested in our experiments.The inscribed number is the inner diameter of each tube; all tubes had1 mm-thick walls except for the 20 mm with 2 mm-thick walls.

transducer continues to vibrate for some time after the pulsehas been sent. This effect forces the system to insert a delaybetween sending the pulse and listening for the reflection, thisleads to a larger minimum range within which the echo can-not be detected. Figure 1a illustrates the setup with a singletransducer.

Most rangefinders calculate distance via their on-board hard-ware and return a distance measurement when queried. How-ever, in order to enable limited multi-press capabilities, weneed the raw received signal (i.e., the echo). To acquire thissignal, we probed the range finder circuit board to find the am-plified received signal; we attached the ADC of our microcon-troller to this output, bypassing the onboard range calculations.Figure 2a illustrates this modification on a MaxBotix MB1040rangefinder.

TubeThe final piece of hardware is the tube itself. Depending onthe desired application, a wide range of tube materials canbe used, but the stiffness of the tube affects the properties ofthe system. A tube that is stiffer can resist flattening whenit is routed around corners, which will prevent the ultrasonicsignal from bouncing back and causing a false measurement.However, stiffer tubes are harder for a person to press. Weexperimented with tubes of different materials, and selectedsoft silicone tubing, as its high degree of flexibility offeredversatility for human interaction. Silicon tubing is also ro-bust against mechanical stress and can be easily cleaned, itis widely used in medical equipment and peristaltic pumps.Experimental results regarding system performance with dif-fering tube diameters and lengths are provided in section 4.Figure 3 illustrates the actual size cross-sections of the testedtubes.

SoftwareThe code running on the microcontroller triggers the range-finder measurement pulse and records the response. The mi-

crocontroller’s ADC samples the signal at approximately 312kHz and transmits the raw digitised signal wirelessly overUDP. We are able to send 6000 samples at 20 fps. The re-ceiving end is written in C# using the .NET framework. Thelaptop used for data recording and in our example applicationshad 16 GB RAM and an Intel i5 Processor with 3.1 GHz.

Filter Pipeline: On each incoming signal, EchoTube uses aset of filters to allow for later peak detection. Because thesignal is not centered around zero, but follows a logarithm-likecurve (which makes later rectification difficult), we run a lowpass filter (bound to 100 Hz) to get the actual baseline of thesignal. We then subtract that baseline from the raw signal tolevel the signal at zero. Subsequently, we rectify the signal(i.e., negative values are flipped along the zero line). Then, werun an envelope filter (with α = 0.05) over the rectified signalto find the peaks in that signal, which correspond to detectedpresses. Finally, we use an amplification along the signal,which has an increasing factor as the distance to the transducerincreases, this is done to compensate for the attenuation of theecho as its traveling distance increases.

Background signal: The transducer’s ringing and reflectionsfrom the end of the tube can hinder measurements. To preventthis, we subtract the background signal from each incomingsignal. This signal is obtained when the system starts up,and can be taken again, if required, while the system is run-ning. The raw background signals undergo the same filteringpipeline as described above. The background (and its ampli-fied envelope respectively) is then removed from the amplifiedenvelope of each incoming signal.

Finding Peaks: Before processing the enveloped signal, weset a minimum distance threshold where peaks can be detected.This is again due to the ringing of our transducer. Naively,one could then go over the filtered signal and find the highestpoint. This indeed often corresponds with a peak that wasproduced by a finger pressing the tube. Using this approach,however, has two limitations: (1) the detected highest pointmight be a small reflection, which our amplification then turnsinto a high value; and (2) this approach makes it difficult tofind more than one peak (which is essential for our limitedmulti-press capabilities). Thus, we scan the enveloped signalfor intersections with a threshold above the baseline. Everyintersection has a peak before and a valley thereafter (or viceversa). As the envelope still has some noise, intersections veryclose to each other are merged into one.

We use the remaining intersections to find peaks and valleysof larger widths (so that they do not constitute a spike of afew sampling points). For each peak and valley, we obtain themaximum (and minimum) values. Finally, we check whetherthere is a valley between two neighboring peaks, which sig-nificantly drops in amplitude (for example, the low point of avalley in between two peaks should be lower than half the peakamplitude). This is necessary because two neighboring peakswithout a substantial valley between them may be produced bya finger not fully pressing on the tube. The remaining peak(s)are those produced by fingers pressing the tube.


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Figure 4. Peak detection algorithm. The blue line is the raw signal, theenvelope of which can be seen in red (rectified, low-pass filtered) andpeak detection applied to it (green).

Distance Calculation: Following the previously mentionedformula of 2d = c∆t, we can use a peak’s position p in thesignal to calculate the physical distance to the sensor, where∆t = p/SampleRate. This assumes, however, that the signalis reflected directly by a press without changing its angle.Whether or not the signal is reflected perpendicularly dependson both the tube’s diameter as well as its hardness. For in-stance, using our 20mm tube (which is relatively soft), wefound that a press does not bend the tube perpendicularly(i.e., 90◦), but only with an angle of 60◦. The signal is notreflected directly on the opposite direction and thus it doesnot travel in parallel with the tube, but at an angle of 30◦. Forour 20mm tube, this means that the distance the signal travelsback increases by 30% (or 15% overall, as the signal travelledcorrectly towards the press). This needs to be factored in whencalculating the distance.

Figure 5. Echo signal received when a finger presses the tube at 15cm (a),30cm (b), and both at the same time (c). Horizontal axis is time, verticalis amplitude.

TECHNICAL EVALUATION

Single-Press Distance Estimate AccuracyWe tested EchoTube’s ability to locate a single press. Accuracyis how close the detected press position is to the actual pressposition. We gathered data from tubes of multiple diameters,and at multiple distances from the transducer. For each tube,we made marks 5 cm apart. We laid out the tube straight on atable with the transducer coupled to one end and the other endopen. In the cases where the tube inner diameter would notalign with the transducer’s, we coupled them using a laser-cutadapter. In order to get the best-case performance, we fullycompressed the tube using the index finger. We pressed the

Figure 6. Error of detected press positions along tubes of different innerdiameters. Each dot represents the predicted distance of one press on thetube, while the green line illustrates the median value for each location.

tube at each mark and recorded the estimated distance at thatpoint, repeating the procedure ten times for each distance oneach tube.

Figure 6 depicts the results of our accuracy testing. We ploteach individual press as a dot, then plot the median as a greenline. The best performance was obtained with the eight- andten-millimetre tubes, the 10 mm tube presents the long rangeand smallest error. The 10 mm tube had better than 1 mmaccuracy (std=.84 mm) between 85 and 300 cm, and showsbetter than 5 mm accuracy over nearly its entire range, 15–330 cm; the 20 mm tube gives a mean accuracy of 3.8 mm(std=6.1 mm) in the range of 50–390 cm.

While our experiments indicates that the performance ofEchoTube is unaffected by tube curvature—as long as thetube is not bent enough to deform and cause echoes—we veri-fied the accuracy of the 10 mm tube under high curvature. Wemarked the tube at 5 mm intervals, then spiraled the tube intothree nested loops about 40 cm in maximum diameter. Wethen performed the same procedure as with the straight tubes,pressing each location ten times. The result can be seen in Fig-ure 7b: as compared with the straight tube, we get equivalentaccuracy.

Our results show that the detectable range of our system sig-nificantly decreases when using tubes of small diameters. Wetheorise this is caused by sub-optimal couplings between thetransducer and the tube, not taking advantage of the full signalpower.

Repetition rateRepetition rate or sampling rate is how fast the sensor canreport the measurement. In EchoTube, this is determined bythe length of the tube and the speed of sound through themedium (340m/s in air at 20 degrees C). The ultrasonic pulsehas to travel twice the length of tube. For example, if thedesired sensing length is 50cm, the maximum repetition rate is


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(a)

(b)Figure 7. (a) A 10 mm tube curved into a spiral to test accuracy aroundcurves. (b) Error of detected press positions along the 10 mm tube, bothstraight and curved into a spiral.

340Hz, if the sensing length is 1.5m then the repetition rate is113Hz. The repetition rate can be increased beyond this limitif it is not necessary to detect the position of the deformationbut just if the tube was pressed or not.

The repetition rate is higher in EchoTube partially because it isnot necessary to sweep through multiple frequencies. Also, thepeak detection algorithm is faster than the FFT computationused in previous work to obtain the spectrum. In contrast toprevious work [12, 21] where the repetition rate ranges from10 to 20 Hz, EchoTube can achieve rates faster than 3 kHz,provided that the position of the interaction is not of interest.

Diameter of the tubeThe diameter of the tube attached to EchoTube affects theenabled interactions. A large diameter tube provides moreinput space since it can be deformed along a larger depth, how-ever it is more cumbersome and hence hard to route. Fromthe theoretical point of view of waveguided ultrasound, thediameter of the tube significantly affects wave propagation.Several papers have studied the propagation through waveg-uides, from Helmholtz [8], Kirchhoff [11] and Rayleigh [20],to modern methods [25]. In general, tubes with a diameterlarger than half-wavelength will conduct complex modes ofsound producing echoes with multiple peaks (as it can be seein Figure 3). On the other hand, narrow tubes will introducelosses due to thermo-viscous effects.

Coupling with tubesThe coupling between the transducer and the tube is importantsince we want maximum transfer of acoustic power from thetransducer into the tube. A simple laser-cut or 3D-printedpiece can couple the transducers to the tube, Figure 8. If thetube is narrower than the transducer, multiple tubes can beconnected to the same sensor (Figure 8.d) although it is not

Figure 8. Coupling a 16mm transducer to a 16mm (a), 10mm (b), 6mm(c), and multiple 8mm tubes (d).

possible to differentiate the tube being pressed. The alreadyavailable sensors (i.e., MaxBotic MB1040 and HC SR-04) aremounted with 16mm diameter transducers, it is possible todesolder and replace them with 10mm transducer to directlycouple narrower tubes.

Length of the tubeAs the deformation is produced further away, the reflected echobecomes less powerful (i.e., smaller amplitude) and eventuallybecomes undetectable.

MiscellaneousThere is a minimum distance after which the tube can beginto sense pressure, because the emitted pulse interferes withthe receiver due to ringing of the initial pulse. The minimumdistance is 3cm for the two-transducer setup and 6cm forthe single-transducer arrangement. The micro-controller withthe Bluetooth module and a two-transducer element occupies5x2x3cm.

MULTI-PRESSTo a limited extent, EchoTube can detect two simultaneouspresses. It can do so in situations where the first press (closer tothe transducer) does not fully close the tube, but allows somepassage for the ultrasonic waves to reflect off of a seconddeformation.

Single TubeIn exploratory experiments, we found that more than twopresses attenuated the signal too much to allow detection ofthe deformations. Figure 5c shows the signal for two presses.

Multi-tubeAs noted in the tube coupling section, we can attach multipletubes to the same transducer. By doing so, we can detectpresses in both tubes simultaneously without a press in onetube affecting the other tube. The limitation to this technique isthat for areas in which the tubes “overlap,” the tubes cannot bedifferentiated; that is, with tubes of lengths 10 and 20 cm, onlythe last 10 cm of the longer tube will be identifiable as comingfrom a particular tube. We take advantage of this property inour glove example application to detect two different fingersbending with a single transducer.

We tested the accuracies of detection with two tubes connectedto one transducer. We connected two 4 mm tubes to a singletransducer and tested combinations of pressing on both tubes


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Figure 9. Position detected for two points pressed at the same time. Gridintersections at every 5 cm mark where the two points were pressed. Thedistances estimations show their covariances around them.

from 30–50 cm. The results are illustrated in Figure 9 andshow high detection accuracy.

APPLICATIONSEchoTube’s properties of being robust, flexible, modular, ex-pressive, usable at a variety of ranges, and simple to deployenable it to be used in a multitude of application domains.Here, we present a number of examples of how EchoTube canbe used, each of which illustrate some of these properties.

EchoTube is simple to set up: to deploy each example, a userattaches a tube to the sensor, places it as desired (securing it,for example, with tape or hot glue), trims it to length and thenperforms a short configuration procedure which assigns pressposition to desired functionalities.

Because EchoTube’s vulnerable electronic components arelocated at one end of a tube, they can be positioned to remainprotected while the tube extends to provide sensing in harshenvironments. This ability allows robust position sensingto be quickly deployed in places where other sensors mightbreak. We illustrate these capabilities with several exemplarapplications.

Figure 10. EchoTube used as a bicycle tracker, demonstrating its abilityto operate at long range and remain undamaged by impact or compres-sion.

Figure 12. EchoTube as a smart home light controller. Left: a single 3D-printed button with a hole for the tube controls a single light. Middle:the user can add a second button by attaching a new segment of tube.Right: a third button added in the same way.

Figure 11. EchoTube deployed as a temporary waterproof music con-troller, demonstrating its ability to withstand environmental factors.(Note that for illustration purposes the laptop is close to the sink, but thelength of the tube allows it to be placed safely out of reach of splashes ifdesired.)

Bicycle TrackerSimilar to commercial pneumatic road tubes which use airpressure to count traffic [16], EchoTube can be used to sensethe passage of vehicles. We a built a system to monitor bicycletraffic (Figure 10) by laying a tube in a U-shape; a bike cross-ing the tube causes two sensing events per wheel, allowingus to estimate both its speed and direction. This applica-tion demonstrates not only EchoTube’s resistance to physicalstress, but also its ability to work at long ranges—here, we usea four-meter-long 10 mm tube.

Waterproof Music ControllerEchoTube is not only robust to impact, but to environmentalfactors as well. Figure 11 illustrates its waterproof naturevia a temporary sink music controller. Deployed by simplydropping a 10 mm tube into the sink and pushing it to theedges, the controller allows users to control music via theusual functions: play, pause, next, previous, volume up, andvolume down, with the sensitive electronics safely out of reachof the water.

Modular Light ControllerEchoTube is modular: tubes can be quickly joined togetherto extend the interaction area. We illustrate this principlevia a simple smart home light controller, shown in Figure 12.The controller is made with passive 3D-printed buttons, the4 mm tube passes through them. When the button is pressed,EchoTube senses the interaction and turns the light on or off.


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Figure 13. A quickly prototyped EchoTube pinch glove with two tubesconnected to a single sensor. To distinguish between the index andthumb, we lengthen the index tube to place the signal peak resultingfrom the index bend further away than the peak related to the thumb.The inset images show the detected posture.

When the user adds more lights, more buttons can be addedsimply by joining a new segment of tube to the prior segments.

GloveFigure 13 demonstrates a control glove quickly prototypedwith EchoTube, showing that bends in the tube are detectedin the same way as presses. Here we use two 4 mm tubesconnected to a single sensor. To detect the index and thumbseparately, we must ensure that the sensed location of the bendfor the index is further away from the location of the bend ofthe thumb. We do so by extending the length of the thumbtube by the length of the index tube; in this way, the sensordetects index and thumb movement in two different regions ofthe echo signal. In this prototype, only movements from theinstrumented fingers are detected by our system, illustratingits robustness against external interference.

LIMITATIONS AND FUTURE WORKWhile EchoTube demonstrates high performance in sensingpress interaction with tubes, it is limited in several ways. First,there is a tradeoff between tube diameter and sensing precision.Smaller tubes are more convenient for many applications asthey are smaller and more-easily routed, but their accuracyquickly degrades. However, as we illustrated with the gloveand modular light controller example applications, even short-range sensing has its uses.

Another limitation of our technique relates to its mechanism ofoperation. Because we rely on the speed of sound to estimatethe locations of the interactions, changes in the environmentcan result in added errors to EchoTube’s performance. Duringour experiments, we assume a constant temperature of 25C,however for outdoor applications temperatures of up to 40Ccould be expected, varying the speed of sound by 7%, thusintroducing errors to our calculations. This issue could bemitigated by adding a humidity and temperature sensor to oursetup and calculating the speed of sound based on the currentconditions.

Finally, because EchoTube relies on the passage of acousticsignals through a tube in order to sense deformation events, de-tecting many presses is not possible. Once the signal is blockedor sufficiently attenuated by a deformation, any presses be-yond that point cannot be detected. One possible solution is touse multiple tubes bundled together so that a single press does

not impact the sound traveling through the other tubes; how-ever, this method would increase the complexity of deployingEchoTube.

CONCLUSIONIn this paper, we presented EchoTube: a robust, inexpensivesystem that utilizes wave-guided ultrasound to enable quicklydeployable press sensing. EchoTube enables modular sensorswhich are robust to adverse physical events such as crushingor getting wet. We presented our algorithm for determiningthe location of presses on tubes, characterized EchoTube’sperformance for multiple tube sizes and lengths, discussedenabling limited multi-press, and presented several exemplarapplications that demonstrate EchoTube’s versatility and easeof use.

ACKNOWLEDGEMENTSThis project has received funding from the European ResearchCouncil (ERC) under the European Union’s Horizon 2020research and innovation program (grant agreement 648785).

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