Measurement 46 (2013) 1257–1271

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Measurement

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Review

Tactile sensors for robotic applications

0263-2241/$ - see front matter � 2012 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.measurement.2012.11.015

⇑ Corresponding author. Tel.: +351 218 418 488; fax: +351 218 417 672.E-mail addresses: PEDRO.RAMOS@LX.IT.PT (P.S. Girão), pedro.m.ramos@lx.it.pt (P.M.P. Ramos), opostolache@lx.it.pt (O. Postolache), joseper@

(J. Miguel Dias Pereira).

Pedro Silva Girão a,⇑, Pedro Miguel Pinto Ramos a, Octavian Postolache b,c,José Miguel Dias Pereira b,c

a Instituto de Telecomunicações/IST/UTL, Av. Rovisco Pais, 1, 1049-001 Lisbon, Portugalb Instituto de Telecomunicações/ISCTE/IUL, Av. das Forças Armadas, 1649-026 Lisbon, Portugalc LabIM – EST/IPS Setúbal, Portugal

a r t i c l e i n f o

Article history:Received 4 July 2012Received in revised form 5 November 2012Accepted 9 November 2012Available online 3 December 2012

Keywords:Tactile sensorsTactile sensingRobotics

a b s t r a c t

In this paper, the authors look at the domain of tactile sensing in the context of Robotics.After a short introduction to support the interest of providing robots with touch, the basicaspects related with tactile sensors, including transduction techniques are revisited. Thebrief analysis of the state-of-the-art of tactile sensing techniques that follows providesindicators to conclude on the future of tactile sensing in the context of robotic applications.

� 2012 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12572. Tactile sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1258

2.1. Basic concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12582.2. Tactile sensors: transduction principle; classic implementations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12592.3. Tactile sensors: state-of-the-art . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1263

2.3.1. MEMS tactile sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12632.3.2. Nanotechnology tactile sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1263

2.4. Tactile sensors applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1264

2.4.1. Touch screens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12642.4.2. Medical devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12652.4.3. Industry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12652.4.4. Robots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1266

3. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1269References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1270

1. Introduction

It is perhaps difficult to agree on what a robot is, butmost people working in Robotics probably would say like

Joseph Engelberger, a pioneer in industrial robotics, that‘‘I can’t define a robot, but I know one when I see one’’.According to the International standard ISO 8373, a robotis ‘‘an automatically controlled, reprogrammable, multipur-

est.ips.pt

Fig. 1. Human tactile receptors [13].

1258 P.S. Girão et al. / Measurement 46 (2013) 1257–1271

pose, manipulator programmable in three or more axes, whichmay be either fixed in place or mobile for use in industrialautomation applications’’. This definition, not particularlyof our liking, is clearly more restrictive than for instancethe one of the Cambridge Advanced Learner’s Dictionarythat states that a robot is ‘‘a machine used to perform jobsautomatically, which is controlled by a computer’’. Thislast definition is in our opinion too poor because it doesnot imply something that, also in our opinion, is intrinsicto the robot concept: the inclusion of elements to gatherinformation of the robot’s environment, i.e., sensors.Potentially disputable is the need of a robot to have thepossibility of action choice based on sensing information.

It is well known that industry tends to implement theeasiest solutions because, in general, they are the more ro-bust and cheaper. To do this, the industrial environmenttends to be well structured and the robots used tend to fol-low a pre-defined program, leaving little or no room fordecision making. Without the aim of advancing a proposalfor a definition, we think that one can say that the charac-teristics of a robot or robotic system include the existenceof a computer, of sensors and of mechanical or electrome-chanical parts actuated by actuators the whole beingprogrammed to perform a particular job. Thus, robotscome in many different shape and sizes and aim manydifferent applications. From simple domestic devices tohuman replicas used at home, in industry or medicine,the abilities of robots are quite diverse, which means alsodifferent sensing requirements.

Taking the human being as a paradigm, science has longbeen trying to provide robots with the five basic humansensing capabilities: hearing, sight, smell, taste and touch.Each of these senses is important to a human and has do-mains in which it is fundamental. It is thus only natural toprovide a robot with an equivalent sensing ability when itis aimed at replacing a human in a task requiring or advis-ing the use of a particular sense.

In the following paragraphs, we will look into the senseof touch in general and into the sensors that have beendeveloped to provide robots with tactile perception, in par-ticular. We will give our vision about the future of tactilesensing in Robotics based on the past and present evolutionin the domain. We call the reader’s attention to the fact thatwe do not consider this paper to be a review paper. Fromthe several reviews and overviews on tactile sensors andRobotics [1,2] and to a lesser extent [3–6] do provide a goodinsight and are thus recommended for reading.

2. Tactile sensing

2.1. Basic concepts

There are several robotic applications requiring oradvising the emulation of human skin sensitivity to pres-sure. For robotic applications, the two main research areason tactile sensor development are related with object con-trolled lifting and grasping tasks and with the ability tocharacterize different surface textures.

Tactile sensors, i.e., devices that respond to contactforces, are used for touch, tactile and slip sensing. Contrary

to force and torque sensors used to measure the totalforces being applied to an object, tactile sensors are devicesable to measure the parameters that define the contact be-tween the sensor and an object, i.e. a localized interaction.Touch sensing consists in the detection and/or measure-ment of a point contact force. Tactile sensing consists notonly in the detection and/or measurement of the spatialdistribution of forces perpendicular to an area but also onthe interpretation of the corresponding information. Tac-tile sensing implies, thus, an array of a coordinated groupof touch sensors. Slip sensing involves the detection and/or measurement of the movement of an object relative tothe sensor. There are sensors specially designed for thispurpose but the information can also be obtained by theinterpretation of the data from a tactile sensor. When thetactile information is displayed as a colored picture (e.g.fingerprint identification) the sensor is usually named tac-tile image sensor.

Robotic applications do often require more than tactileperception. Object shape and surface roughness recogni-tion or grasping and sliding information are only possibleif both sensory and motor systems are used so as to pro-vide haptic perception. Haptic perception is not only morehardware demanding but also much more software depen-dent than tactile perception: the performance of a hapticsystem is highly determined by signal processing and sen-sors’ data fusion implemented algorithms. This importantaspect is beyond the scope of this paper. Enlighteninginformation on what has been recently done can be foundin [7–12].

The human touch, distributed all over the body, is sup-ported on four kinds of tactile receptors (mechanorecep-tors) distributed in layers in the skin as shown in Fig. 1taken from [13]: the Meissner corpuscles, the Merkel cells,the Ruffini endings, and the Pacinian corpuscles.

Meissner corpuscles are a type of mechanoreceptorresponsible for sensitivity to light touch. They are distrib-uted throughout the skin, but concentrated in areas espe-cially sensitive to light touch (e.g. the fingertips). A littledeeper in the skin one can find Merkel cells. They are asso-ciated with sensory nerve endings, when they are known

P.S. Girão et al. / Measurement 46 (2013) 1257–1271 1259

as Merkel nerve endings. They are the most sensitive of thefour main types of mechanoreceptors to vibrations at lowfrequencies, and because of their sustained response topressure, Merkel nerve endings are classified as slowlyadapting. Ruffini endings are also a class of slowly adaptingmechanoreceptor sensitive to skin stretch, and contributeto the kinaesthetic sense and control of finger positionand movement. Like Ruffini endings, Pacinian corpusclesare found in deep subcutaneous tissue, and detect deeppressure touch and high frequency vibration. They are rap-idly adapting receptors, which means that they respondwhen the skin is rapidly indented but not when the pres-sure is steady.

Even if some attempts have been made recently to mi-mic the behavior of human touch [13,14], robotic applica-tions rarely use tactile sensing and when they do, thesensors used tend to be as basic as possible. This is partic-ularly true in industrial environment, since it is wellknown that simple, reliable, and durable solutions are bet-ter and cost less than complex solutions. Thus, tactile sen-sors for robotic applications have been generally designedas an array of touch sensitive sites, each usually called tac-tel, taxtel, texel or sensel in analogy to a pixel (picture ele-ment) in an image sensing array. It is based on the contactforces measured by the sensor that the large amount ofinformation required to determine the state of a grip is ob-tained. Texture, slip, impact and other contact conditionsgenerate force and position information that can be usedto assess the state of a manipulation.

In what concerns the transduction principle used toimplement the taxel, the number of implemented solu-tions is high. In tactile sensing, we are in the domain offorce (or pressure) measurement and so most of thosesolutions are common to normal force transduction. It isimportant to mention here that, (a) in most cases, theimplementation of a tactile sensor was and still is veryapplication oriented, which also accounts for the reducednumber of industrial and general purpose robots incorpo-rating them, (b) that the operation of a touch or tactile sen-sor is very dependent on the material of the object beinggripped, and (c) that, generally speaking, the sensing solu-tions that are very briefly discussed next may work quitewell with rigid objects but often require modifications toadapt them to operate with non-rigid materials.

2.2. Tactile sensors: transduction principle; classicimplementations

Tactile sensors are based but not confined to the follow-ing transduction principles:

Fig. 2. Resistive tactile sensors [15].

1. Mechanically based sensors. Each taxel is a mechanicalmicro-switch that is in one of two states, on or off.The force required to switch it on depends on theswitch actuating characteristics and on externalconstraints. Some mechanical based sensors use asecond device, such as a potentiometer or a LVDT toprovide a voltage proportional to the local force(pressure).

2. Resistive based sensors. The basic principle of this typeof sensor is the change of electrical resistance withpressure of a material placed between two electrodesor in touch with two electrodes placed at one side ofthe material (Fig. 2 taken from [15]). One solution toimplement this pressure sensitive resistor is using aconductive elastomer or foam or elastomer cords laidin a grid pattern, with the resistance measurementsbeing taken at the points of intersection. The conduc-tive elastomer or foam based sensor is relatively sim-ple but (a) has a long nonlinear time constant. Inaddition the time constant of the elastomer, whenforce is applied, is different from the time constantwhen the applied force is removed; (b) the force-resis-tance characteristic of elastomer based sensors ishighly nonlinear; (c) they have poor medium andlong-term stability because of the migration of theresistive medium of the elastomer and to its perma-nent deformation and fatigue. In spite of these limita-tions, elastomer-based resistive sensors have beenquite popular because of the simplicity of their designand interface to a robotic system.

Resistive taxels can also be made using conductivepolymers and thin semi-conductive coating (ink). In thefirst case, the polymer is made piezoresistive by screenprinting it with a film of conductive and non-conductivemicron particles. Ink-based sensors developed by Tekscan,Inc. [16] consists of two thin, flexible polyester sheetswhich have electrically conductive electrodes depositedtypically in row–column pattern separated down to about0.5 mm (Fig. 3 taken from [16]). Before assembly, thepatented, thin semi-conductive ink is applied as an inter-mediate layer between the electrical contacts. When thetwo polyester sheets are placed on top of each other, a gridpattern is formed, creating a sensing location at eachintersection.

The conditioning circuits of resistive-based tactile sen-sors are fairly simple, which is one of their advantages.Fig. 4 shows a 3 � 3 array of resistive taxels and the cir-cuitry that can be used to implement a tactile transducer.

3. Capacitive based sensors. A capacitive taxel is a capacitorwhose capacitance changes with the applied force. Theforce can produce either the change in the distancebetween capacitor plates or its area.

A good design of a high resolution sensor implies: (a)small dimension taxels; (b) the maximization of thecapacitance and the change of its value when force is ap-plied, which advises using high permittivity dielectricssuch as polymeric based materials; (c) high sensitivity.Generally speaking, if the capacitor plates area to distance

Fig. 3. Tekscan resistive sensor [16].

−+ +

−

Vcc

f11 f12 f13

f21 f22 f23

f31 f32 f33

R11 R12 R13

R21 R22 R23

R31 R32 R33

Row 1

Row 2

Row 3

Column 1 Column 2 Column 3

Rf

Decoder Multiplexer

V0

Fig. 4. Resistive tactile transducer.

1260 P.S. Girão et al. / Measurement 46 (2013) 1257–1271

ratio is smaller than one, the change of plates area is better,which recommends a coaxial cylindrical capacitor (Fig. 5a).

Nevertheless, and even other solutions have been pro-posed [17], parallel plate capacitors are easier to fabricatethan cylindrical ones, which justify their popularity evennowadays (e.g. [18]).

To measure the change in capacitance, several condi-tioning circuits can be used depending also on the type

of the desired output signal. A few examples for a dc out-put voltage: (a) a current generator that charges the capac-itor (taxel) for a fixed time interval, the capacitor voltagebeing then proportional to the inverse of the capacitance;(b) an oscillator whose running frequency depends onthe capacitance value [19] followed by a frequency-to-voltage converter; (c) an AC Wheatstone bridge followedby an instrumentation amplifier and a peak detector. The

Mul

tipl

exer

~

F

Δh

h

l

h0

w Electrodes Dielectric

Multiplexer

~vS

Ci j

RdCd vd

+

−

vS

vd

Capacitor moving plate

Elastomer

Applied force

Capacitor fixed plate

(a) (b)Fig. 5. (a) Capacitive taxel based on a cylindrical capacitor; (b) voltage-based conditioning circuit and its equivalent electric circuit: Cij – taxel (i, j)capacitance; Rd and Cd – equivalent resistance and capacitance of the circuit yielding vd.

Elastic Membrane

Emitter Detector

Fig. 6. Optical obstruction-type taxels.

P.S. Girão et al. / Measurement 46 (2013) 1257–1271 1261

circuit of Fig. 5b yields an ac voltage, vd whose value de-pends on the capacitance of each taxel, Cij.

Like other capacitive sensors, tactile ones are prone tocapacitive coupled interference namely when they areplace close to robot metallic parts (stray capacitances).

Optical Source

Optical Detector

Reflectivsurface

Spring

(a)Fig. 7. (a) Left: optical reflective-type taxel; (b) right: light intensity, I, versus d

This means that when quantitative, accurate informationis required – which is not the case for instance of tactilesensors for touchpads, good circuit layout and mechanicaldesign of the touch sensor is needed to minimize the prob-lem [20]. Also, careful and dedicated conditioning circuitry

e

Operating range

distance x

Inte

nsity

, I

(b)istance between the reflective surface and the light emitter–receiver, x.

Elastic Layer

d

θ

h0 hi

Emitter-Receiver Fiber pair

Fig. 8. Optical fiber based reflective-type taxel.

1262 P.S. Girão et al. / Measurement 46 (2013) 1257–1271

is required to take advantage of the excellent sensitivityand repeatability achieved by some implementations ofcapacitive sensors [21].

4. Optical Sensors. The operating principles of optical-based sensors may be included in one of two types:intrinsic, when the intensity, phase, or polarization ofthe transmitted light is modulated by the applied forcewithout interrupting the optical path, and extrinsicwhen the applied force interacts with the light externalto the primary light path. In robotic tactile sensing, theextrinsic sensor based on intensity measurement is themost widely used due to its simplicity of construction,signal conditioning and information processing. Twoexamples of optical transduction based taxels:� Modulating the intensity of light by moving an

obstruction into the light path. The force sensitivityis determined by a spring or elastomer (Fig. 6).

� Modulating the intensity of light by moving a reflec-tive surface into the light path (Figs. 7a and 8). Theintensity of the received light is a function of dis-tance (Fig. 7b), and hence of the applied force.

Among the positive characteristics of optical sensors forrobotic applications are their low susceptibility to electro-magnetic interference, intrinsically safety and low electri-cal wire demand.

Electrodes Piezoelectric Material

1

2

3

A

F

h

Fig. 9. Piezoelectric based taxel

5. Optical fiber based sensors. Apart from the uses just men-tioned where optical fibers are used for light transmis-sion only, an optical fiber can be used as the sensoritself. The underlying idea is the light attenuation inthe core of a fiber when a mechanical bend or perturba-tion (of the order of few microns) is applied to the outersurface of the fiber. The attenuation depends not onlyon the radius of curvature and spatial wavelength ofthe bend but also on the fiber parameters. A recentexample using also nanoparticles is described in [22].

6. Piezoelectric sensors. Some materials, like quartz, ceram-ics and polymers have piezoelectric properties and canthus be used for tactile sensing. Polymer polyvinylidenefluoride (PVDF) and ceramic lead zirconium titanate(PZT) are perhaps the materials more widely used.The taxel is made by applying a thin layer of metalliza-tion to both sides of the piezoelectric material, consti-tuting the whole a parallel plate capacitor (Fig. 9).

The conditioning circuit of a piezoelectric sensor isbased on ultra-high input impedance amplifiers and thebandwidth of the circuit does not go down to dc, whichmeans that, as it is well known, piezoelectric transducersare not adequate for static force transduction. This prob-lem can be overcome by vibrating the sensor and detectingthe difference in the vibration frequency due to the appliedforce [23].

V +

−

S

C

V0

Cf

−

+−+

and conditioning circuit.

P.S. Girão et al. / Measurement 46 (2013) 1257–1271 1263

7. Magnetic based sensor. The changes of the magnetic fluxdensity, of the magnetic induction of an inductor or themagnetic coupling between circuits are the most usedprinciples in magnetic tactile sensing. One way ofmeasuring the magnetic flux is using a device whosemagnetic properties are force dependent (e.g. magneto-resistance). Using a magnetoelastic material for the coreof an inductor its electrical parameters change whenforce is applied to the core (e.g. [24]). The same typeof material used in the core of a transformer leads tomagnetic coupling changes between transformerwindings under the same conditions, that is, when forceis applied to the core.

Magnetorestrictive or magnetoelastic based tactilesensors may have some positive characteristics, namelyhigh sensitivity and physical robustness. Nevertheless,they still fail to be a valuable alternative to the types oftactile sensors above-mentioned.

8. Deformation based sensors. The fact that the surface of amaterial changes in length when it is subjected toexternal forces can be used for tactile sensing. Thisdeformation is then converted to the electrical domainby means of strain gauges made either from resistiveelements or from semiconducting material and bondedto the stressed material. The conditioning circuit morecommonly used includes a Wheatstone bridge followedby an amplification stage.

2.3. Tactile sensors: state-of-the-art

In recent years, tactile sensors have been under partic-ular attention due mainly to two domains of applicationsnot exactly related to robotics: (1) human machine inter-faces (touch screen displays), namely for communicationdevices and computers [25–27]. Touch screen technologyhas been used since 1980–1990s, but it is the last genera-tions of mobile devices that fostered the research in thelast 5 years reflected in a large quantity of patents; (2)medicine, namely minimally invasive and remote surgeryand therapy and tissue characterization [28–32]. Virtualreality applications are also playing a part on haptic sens-ing development [33]. Nonetheless, and with the exceptionof the two-above mentioned cases, the development of tac-tile and touch sensing has evolved but not in a particularlywell oriented way for several reasons. It is true that con-trary namely to sight, touch does not produce well quanti-fied signals, which means that a lot of work has beennecessary just to deal with the basics of collecting the mostrelevant data, but perhaps one important reason has to dowith the lack of objective and thus of detailed requiredspecifications. Particularly in the domain of tactile sensors,specifications do naturally change with the application butthe definition of some general purpose values wouldclearly provide a sounder base for sensor development.Several authors (e.g. [34–37]) did contribute to the clarifi-cation of the major requirements of tactile sensors for ro-botic applications, but in no decisive way. In the absenceof such specifications the development related with touchsensing has been either driven by applications [38–45] or

as a product or by-product of micro- or nanotechnologiesthat use the transduction principles mentioned in Sec-tion 2.2, namely silicon or polymer based micro-electro-mechanical-systems (MEMSs) and nanotechnologies.

As in other domains, micro- and nano-technologies areparticularly attractive to tactile sensing implementationbecause they can produce not only high density arrays ofsensors but also devices incorporating both the sensors,the required conditioning electronic circuits and even thehardware for signal acquisition, digital signal processingand transmission (embedded devices, smart sensors, i.e.,sensors with namely self-diagnostics, calibration and test-ing capabilities). The devices can and should also incorpo-rate temperature and humidity transducers, quantitiesthat influence the tactile sensing performance, but thatare also important by themselves because they convey tac-tile information.

2.3.1. MEMS tactile sensorsThe large majority of tactile sensors recently developed

use, more or less intensively MEMS technology. They aremainly either polymer with organic material substratebased or silicon based sensors.

Polymer-based sensors [46–51] usually use piezoresis-tive rubber as force sensing element. Polymer-based sen-sors are more suitable for wide area tactile sensors thansilicon sensors because of their lower fabrication cost perunit area but have the disadvantages of low spatial resolu-tion (around 2 mm) and upper limitation of the number oftaxels due to wiring. The organic-FET switching matrixsolution used in [49] is an improvement but long term reli-ability in this type of application remains to be evidencedand the integration density of organic-FET is much lowerthan the current silicon technology.

Reports of silicon-MEMS tactile sensors abound in theliterature (e.g. [52–57]). Silicon micromachined sensorstake advantage of silicon high tensile strength, reducedmechanical hysteresis and low thermal coefficient ofexpansion. Most of silicon-MEMS tactile sensors are basedon piezoresistive or capacitive taxels, and through the inte-gration of a switching matrix fabricated using CMOS tech-nology, the number of wires can be reduced. The CMOStechnology allows also the integration of the taxels arrayconditioning circuits. Silicon micromachining tactile sen-sors allow higher spatial resolution than polymer-basedtactile sensors but it is difficult to realize flexible sensorsurface. In [57] a solution to this problem is presented.By their nature, MEMS tactile devices are prone to mechan-ical damage. To overcome this problem and also to providea flexible skin like coating over the sensors poly-dimethylsiloxane (PDMS) can be used. It is a waterproof,chemically inert, and non-toxic silicon-based organic poly-mer supplied as a two part mix, a monomer and hardener,which are combined at a weight ratio of 10:1, commonlyused for embedding or encapsulating electroniccomponents.

2.3.2. Nanotechnology tactile sensorsIn the present context, nanotechnologies encompass

technologies that work on the nanometer scale indepen-dently of their output, material, device or system.

1264 P.S. Girão et al. / Measurement 46 (2013) 1257–1271

To our knowledge, no fully nanoscience based tactilesensor has yet been produced. There are however reportsof tactile sensors using nanomaterials (nanotubes, nano-particles and nanowires) (e.g. [58–60]), some of them pat-ented protected [61]), and of nanomaterials that can beused for very low force sensing [62] and for touch screens[63,64]. Nevertheless, perhaps the most interesting imple-mentation of a tactile sensor using nanomaterials is re-ported in [65]. A 100-nm-thick film is built on anelectrode-coated glass backing. On top of the glass are fivealternating layers of gold and cadmium sulfite nanoparti-cles, separated from each other by polymer sheets. The de-vice is topped off with an electrode-coated, flexible plasticsheet. When the plastic that covers the sensor is pressed,the nanoparticle layers become closer and an electricalcurrent flows. When electrons bounce between the nano-particle layers, the cadmium sulfite nanoparticles glowand the light can be picked up on the other side of theglass. Both the sensor’s output electrical current and lightare proportional to the pressure on the sensor. It wasreported that when recording the received light with acamera, the nanofilm can take about 5–10 readings persecond, while recording the electrical current can raisethose figures to about 20–50.

The sensor is reported as having a resolution of 40 lmhorizontally and about 5 lm vertically. When pressedagainst a textured object, the film creates a topographicalmap of the surface, by sending out both an electrical signaland a visual signal that can be read with a small camera.The spatial resolution of these ‘‘maps’’ is as good as that

Fig. 10. Surface acoustic wave touch screen. (a) Constitution; (b) non-distur

achieved by human touch. The sensor is aimed at to im-prove minimally invasive surgeries but its potential inhelping robots grip sensitive objects is very attractive evenif the sensor cannot inform on the direction of pressure. Itremains to see if nanoparticle layers can sense this kind oftactile information that is required for dexterous manipu-lation of sensitive objects.

2.4. Tactile sensors applications

Throughout the past paragraphs we have mentioneddomains where tactile sensors have been used. We elabo-rate here about the most common applications.

2.4.1. Touch screensAlthough very interesting as man–machine interfacing

devices, basic touch screens are quite simple from thepoint of view of the tactile sensing hardware componentbut they require a controller with processing capabilityto run the software that determines the point or pointsof contact. The best quality touch screen monitors arebased on surface acoustic waves (SAWs) sensing [66].Two transducers (one receiving and one sending) areplaced along the two axes (x,y) of the monitor’s glass plate(Fig. 10a). Also placed on the glass are reflectors that reflectthe electrical signal sent from one transducer to the other(Fig. 10b). When the user touches the glass surface, theuser’s finger absorbs some of the energy of the acousticwave and the receiving transducer is able to detect it andcan locate it accordingly (Fig. 10c). The screen has no

bed acoustic waves pattern; (c) acoustic wave pattern when touched.

P.S. Girão et al. / Measurement 46 (2013) 1257–1271 1265

metallic layers, allowing for 100% light throughput andthus perfect image clarity, which makes this type of touchscreens the best for displaying detailed graphics. Because itis glass constructed, SAW touch screens are durable, canoperate even if scratched, and some of their disadvantages,namely the possibility of false touches due to moving liq-uids or condensation, of creation of non-touch areas dueto solid contaminants, and the vulnerability to bad usecan be overcome by special design. SAW screen must bestimulated by finger, gloved hand, or soft tip stylus.

SAW touch screens are not efficient for drawing anddragging and do not allow multi-touch detection, i.e.,simultaneous detection of touches in different points ofthe screen. Multi-touch is increasingly necessary whenoperating smart devices running applications, such asweb browsers, requiring using more than one finger (orany object) to stretch, rotate, or shrink an object, and scrollthrough menus. Capacitive-based sensors using In-PlaneSwitching (IPS) technology like those used in Apple iPadand in late versions of the iPhone are then a commonsolution.

2.4.2. Medical devicesArrays of tactile sensors have been used for breast can-

cer detection as an alternative to ultrasound based sys-tems, mammography, and other complex systems. Anarray of 12 � 16 tactile sensors can be more sensitive thanthe human sense of touch and the detection of lesions assmall as 5 mm under the skin’s surface is possible. Oneexample of a capacitive-based palpation imaging systemfor clinical breast examination is described in [67,68].The sensing probe has 200 sensors and the system is underexploitation with success [69].

Arrays of tactile sensors of different sizes and configura-tions have been used also for a wide variety of medicalapplications, namely: minimal invasive surgery, manage-ment of diabetic foot, orthotic assessment, optimizationof the seating and positioning of the neurologically com-promised people, prosthesis and brace fitting, orthopaedicjoint research, and dental prosthesis (e.g., tooth contactand occlusal force balance) [70]. In [71] some of these

Fig. 11. (a) da Vinci surgical system (Courtesy of Intuitive Surgical, Inc.). From leof the patient car; (c) detail of the positioning of a sensing module for tool-tissuesurgical tool [73].

applications are discussed and framed in the developmentof tactile sensing.

After non-invasive surgery, minimal invasive surgery(MIS) is the more aimed procedure because of the less neg-ative impact it has on patients and also because it shortenshospital stays, recovery time and often costs. There areseveral techniques that can be classified and minimallyinvasive. Most of them are carried out through the skinor through a body cavity or anatomical opening and recurto laparoscopic devices for indirect observation of the sur-gical field and to remote-control manipulation of instru-ments. Fig. 11a shows the commercial available robot-assisted da Vinci MIS [72]. It consists of three major com-ponents, an input device (surgeon’s console), a digitalinterface (vision system), and an output device (manipula-tor, patient-side cart). Currently, the basic system does notincorporate tactile or haptic sensing capabilities but sev-eral works have been carried out in that direction bothfor surgery (e.g. [73,74]) and training purposes (e.g. suturetraining [75]).

Gait analysis can also benefit from tactile sensors use.Fig. 12 shows an in-shoe sensor developed by TekscanInc. to provide information regarding the symmetry in footfunction during gait. Asymmetry in foot function duringgait can generate undesired torque and stress componentsthat, over-time, place wear and tear on body tissues andcan potentially cause symptoms of discomfort and pain.

Tactile sensing has been commercially introduced indental implants. The patented technology [76] includedin the Tactile Technologies, Inc., ILS system [77] allowsobtaining the mechanical image of the maxillary bone con-tour without removing any gum tissue. The sensor uses amatrix of micro-needles that are inserted through thegum tissue until contact with bone is attained. The needlesused are ultra-thin with specially designed geometry toensure negligible trauma. Their insertion is measuredusing miniature position encoders and digital signal pro-cessing electronics [78].

2.4.3. IndustryIndustrial applications of commercial tactile sensors are

particular numerous in the car industry [70]: brake pad de-

ft to right: surgeon console, vision system, and patient-side cart; (b) detailforce interaction measurement to fit between a da Vinci arm and da Vinci

Fig. 12. Pre- and post-orthotic pressure (scale raises from blue to red). Hardware: Tekscan F-Scan system. Number of sensing elements: 960/foot; spatialresolution: 4 sensors/cm2; size of sensor: trimmable from men’s size 14 USA; sampling rate: 165 Hz; pressure range: 1–150 psi; sensor thinness: 0.15 mm(Courtesy of Tekscan Inc.). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 13. Left: car seat with Tekscan; Inc. tactile sensors; right: pressure distribution of the driver before and when he prepares to apply the brakes. (Courtesyof Tekscan, Inc.).

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sign and performance assessment based on the measure-ment of the dynamic pressure distribution between brakepads during actual braking; assessment of door mountingquality trough monitoring of pressure distribution inhinges and door latches; assistance in windshield wiperdesign through analysis of the force distribution betweenthe windshield and the blade at various positions alongblade length; tire to wheel interface assessment by mea-suring the bead seat/seal pressure profile; tire tread designperformance evaluation and suspension testing analysis;and design of automotive seats (Fig. 13).

2.4.4. RobotsScientific and technical literature includes several dif-

ferent tactile sensing implementations for robotic applica-tions, some of which are referred throughout this article.The goal is to provide robot manipulators (hands/fingers)with as accurate information as possible about the objectsto grab, hold and handle. How to achieve these and other

objectives is detailed in [79] where the authors present arobot tactile sensing classification worth mentioning.

Fig. 14 shows an example using commercial products, arobotic hand from Barrett Technologies, Inc. [80] and tac-tile sensors from Pressure Profile Systems, Inc. (PPS) [21].

An implementation of tactile sensing in a commercialWillow Garage PR2 robotic platform using capacitive sen-sors also manufactured by Pressure Profile Systems, Inc.is described in [81]. Each robot gripper fingertip is instru-mented with a pressure sensor array consisting of 22 indi-vidual cells. The 22 cells are divided between a five bythree array on the parallel gripping surface itself, two sen-sor elements on the end of the fingertip, two elements oneach side of the fingertip, and one on the back (seeFig. 15). The sensors measure the perpendicular compres-sive force applied in each sensed region. The sensing sur-face is covered by a protective layer of silicone rubberthat provides the compliance and friction needed for suc-cessful grasping. Equipped with such tactile sensing, PR2is capable of picking up and setting down delicately, but

Fig. 14. Barrett hand. Top: basic hand; bottom left: hand with PPS sensors; bottom right: close-up of PPS sensors on the finger. (Courtesy of BarrettTechnologies, Inc. and Pressure Profile Systems, Inc.).

Fig. 15. The PR2 robot gripper. The pressure sensors are attached to therobot’s fingertips under the silicone rubber coating [81].

Fig. 16. Stanford University BDML tactile sensors in a Robotiq AdaptativeGripper.

P.S. Girão et al. / Measurement 46 (2013) 1257–1271 1267

firmly, real-world objects without crushing or droppingthem.

Also capacitive pressure sensors [82] have been devel-oped by the Biomimetics & Dextrous Manipulation Labora-tory (BDML) of Stanford University, led by Professor MarkCutkosky namely to improve grasping with an under-actu-ated hand. Fig. 16 exemplifies the installation of such sen-sors on a Robotiq Adaptative Gripper [83].

Optical sensors have also been used to convey robotarms tactile and haptic perception. Fig. 17 shows a robotarm equipped with an optical three-axis tactile sensor toimprove sensitization quality in robotic hand system

[84]. The tactile information is used as feedback for thearm actuators and is obtained by digital processing imagedata. The tactile sensor is based on the optical waveguidetransduction method [85]. As described by the authors,the sensor consists of an acrylic hemispherical dome, anarray of 41 pieces of sensing elements made from siliconrubber, a light source, and an optical fiber scope connected

Fig. 17. Robot arm mounted with optical three-axis tactile sensors.

Fig. 18. SynTouch BioTac tactile sensor.

Fig. 19. Barrett hand (left) and Shadow hand (right) with BioTac equipped with SynTouch BioTac tactile sensors.

1268 P.S. Girão et al. / Measurement 46 (2013) 1257–1271

P.S. Girão et al. / Measurement 46 (2013) 1257–1271 1269

to a CCD camera. The sensing element has of one columnarfeeler and eight conical feelers that remain in contact withthe dome surface and optical fibers conduct the light emit-ted by the light source towards the edge of the dome.When an object contacts the columnar feelers they col-lapse due the contact pressure and light is diffusely re-flected out of the reverse surface of the acrylic surfacebecause the rubber has a higher reflective index. It is theimage with bright spots caused by the collapse of the feel-ers that is processed to provide tactile information.

SynTouch, LLC has been working of tactile sensors aim-ing at emulating, as close as possible, the fingers sense oftouch. The most impressive result is the so-called biomi-metic BioTac sensor depicted in Fig. 18, which integratestemperature, force and vibration sensing capabilities usinga thermistor, a set of impedance sensing electrodes and ahydrophone, respectively. The current available version ofthe BioTac sensor, which is an upgrade of the work re-ported in [14], is well described in [86]. BioTac sensorsare available not only as an evaluation kit but also as kitsfor the Barrett hand and the Shadow hand [87] (Fig. 19).The potential of the BioTac sensor is considerable andclaims of achieving better performance than humans forspecial tasks have already been reported [88].

3. Conclusions

From the previous paragraphs, one can conclude thatthe basic tactile sensing transduction principles are wellestablished (emphasis on resistive, capacitive, piezoelec-tric and optical) and that micro- and nanotechnologiesprovide the means to implement increasingly complexsensors. However, it should be mention here that even ifthe number of manufacturers producing general purposeor custom tactile sensors is small, they do exist (e.g. Tek-scan, Inc. Pressure Profile Systems, Inc., Tactex Controls)some with designs that aim human-like compliance androbustness (e.g. SynTouch LLC). For selection purposes,the specifications of the devices include dimensionalparameters (width, length, and thickness), pressure rangeand allowable over-range, and sensing area and spatial res-olution. Additional considerations include flexibility, satu-ration force, linearity error, drift, repeatability, hysteresis,response time, and operating temperature. In the case oftactile imaging sensors, the type of electrical output maybe also an alternative: analog current (4–20 mA), analogvoltage, non-modulated or modulated analog voltage, anddigital output (e.g. RS232, RS422, and RS485, IEEE 488).

The number of people involved in research and devel-opment of tactile and haptic sensing and the number of re-ported works has increased particularly in the last coupleof years but the penetration of tactile sensors in Robotics,particularly in industrial robotics is still extremely low.Why? We think that basically there is no real market-oriented driving force boosting the tactile sensing domain:industrial automation aims efficiency at low cost. This gen-erally means using well established reliable and as simpleas possible technologies. Robots with tactile sensing arenot at that stage and some applications that could profitfrom them are implemented by forcing a structured

environment and using simpler sensing devices like prox-imity sensors; other domains like medicine, particularlysurgery, and service robotics are starting to play that roleonly now. To this we must add two other considerations:(1) tactile and particularly haptic sensing is quite demand-ing not only in terms of hardware but also of software. Theextraction of information from tactile sensors may requirethe implementation of complicated algorithms; (2) thehardware and software available, even at an experimentallevel are still not adequate for some already defined needs.

Our vision of the future in what tactile sensing is con-cerned is optimistic but only moderately. Assuming thatthe industry will not change very much its production stylein the near future, we think that it will be up to scientistsand engineers to go on developing new sensors suitable forother domains of applications. Robots to operate inunstructured environments will benefit from the incorpo-ration of the functionalities provided by tactile and hapticsensing. Medicine is clearly another example of presentand future market for tactile sensors because there is al-ready demand for solutions for instance to help surgeonsin minimally invasive surgery.

Like it happened in the past in other domains, we believethat it is up to research to produce sensing devices and sys-tems that later will interest industry. We expect and believethat the technology will be able to overcome some of thecurrent limitations of tactile sensing such as taxel dimen-sion (resolution) and arrangement (array organized sensorssuffer from crosstalk, i.e. several taxels can be excited by avery localized force), and integration of all components re-quired to output tactile sensation (sensors, conditioningcircuits, processing units, etc.). Nanosciences and nano-technologies will probably provide answers to these prob-lems but no one can assure if the solutions will have amajor impact on Robotics and when they will be available.

It seems relatively undisputable that tactile and hapticsensing finds a favorable ground in applications where ro-bots operate in unstructured environments or equivalentlywhen robots change of working environment. Medical,agricultural, livestock, food, prosthetic, and other nicheindustries are already demanding and using tactile sensingnot always incorporated in a robot [16,89]. The number ofrobots with dexterous grippers and manipulators requiringtactile and haptic sensing is expected to increase but it isalmost impossible to foresee up to what extent.

A subject that has been rarely addressed is wireless tac-tile sensors. It is clear that whenever the sensors need to beapplied for instance on a rotating surface the connectingwires become a major problem. Optical solutions have al-ready been proposed (e.g. [90]) but the alternative of usingRF based wireless sensors is increasingly more interestingwith the results obtained at the University of California,namely within the framework of Pister’s Smart Dust pro-ject [91,92], and of other research groups [93]. We believethat robots with tactile and haptic sensing will be able tobenefit from all the work that has been developed in thecontext of embedded wireless distributed systems. In thenear future, it is plausible to imagine the production ofsmart wireless taxels or smart wireless arrays of taxelspossibly incorporating other enriching sensing capabilities,such as temperature.

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