View source for Force-sensing resistor

{{Short description|Material whose resistance changes when a force is applied}}
{{Use American English|date = April 2019}}
{{Use mdy dates|date = April 2019}}

A '''force-sensing resistor''' is a material whose [[Electrical resistance|resistance]] changes when a [[force]], [[pressure]] or mechanical [[Stress (mechanics)|stress]] is applied.  They are also known as '''force-sensitive resistor''' and are sometimes referred to by the [[initialism]] '''FSR'''.<ref>[http://acronyms.thefreedictionary.com/FSR FSR Definitions]</ref>
[[File:LedDemonstration.webm|thumb|FSR usage]]

== History ==
The technology of force-sensing resistors was invented and patented in 1977 by Franklin Eventoff. In 1985, Eventoff founded [[Interlink Electronics]],<ref>{{cite web|title=Interlink Electronics|url=https://www.interlinkelectronics.com/}}</ref> a company based on his force-sensing-resistor (FSR). In 1987, Eventoff received the prestigious International IR 100 award for developing the FSR. In 2001, Eventoff founded a new company, Sensitronics,<ref>{{cite web|last1=Physics and Radio-Electronics|title=Force Sensitive Resistor|url=http://www.physics-and-radio-electronics.com/electronic-devices-and-circuits/passive-components/resistors/forcesensitiveresistor.html}}</ref> that he currently runs.<ref name=revised_reference>[http://www.sensitronics.com/about_us.htm Sensitronics]</ref>

== Properties ==
Force-sensing resistors consist of a [[conductive polymer]], which predictably changes resistance following applying force to its surface.<ref>{{Cite web |title=Tactile Sensors |url=https://web.archive.org/web/20010424021523/http://www.soton.ac.uk/~rmc1/robotics/artactile.htm|archive-date=April 24, 2001|url-status=dead |archive-url=https://web.archive.org/web/20010424021523/}}</ref>  They are normally supplied as a polymer sheet or [[ink]] that can be applied by [[screen printing]].  The sensing film consists of electrically conducting and non-conducting particles suspended in a matrix. The particles are sub-micrometre sizes formulated to reduce temperature dependence, improve mechanical properties and increase surface durability. Applying a force to the surface of the sensing film causes particles to touch the conducting electrodes, changing the film's resistance. As with all resistive-based sensors, force-sensing resistors require a relatively simple interface and can operate satisfactorily in moderately hostile environments. Compared to other force sensors, the advantages of FSRs are their size (thickness typically less than 0.5&nbsp;mm), low cost, and good [[shock resistance]]. A disadvantage is their low precision: measurement results may differ by 10% and more. [[Force-sensing capacitor]]s offer superior sensitivity and long-term stability, but require more complicated drive electronics.

== Operation principle of FSRs ==
There are two major operation principles in force-sensing resistors: [[percolation]] and [[quantum tunneling]]. Although both phenomena co-occur in the conductive polymer, one phenomenon dominates over the other depending on particle concentration.<ref name=stassi2014>{{cite journal|last1=Stassi|first1=S|last2=Cauda|first2=V|last3=Canavese|first3=G|last4=Pirri|first4=C|title=Flexible Tactile Sensing Based on Piezoresistive Composites: A Review|journal=Sensors|date=14 March 2014|volume=14|issue=3|pages=5296–5332|doi=10.3390/s140305296|pmid=24638126|pmc=4003994|bibcode=2014Senso..14.5296S|doi-access=free}}</ref> Particle concentration is also referred in the literature as the filler volume fraction <math>\phi</math>.<ref>{{cite journal|last1=Bloor|first1=D|last2=Donnelly|first2=K|last3=Hands|first3=P|last4=Laughlin|first4=P|last5=Lussey|first5=D|title=A metal-polymer composite with unusual properties|journal=Journal of Physics D|volume=38|issue=16|pages=2851|date=5 August 2005|doi=10.1088/0022-3727/38/16/018|bibcode=2005JPhD...38.2851B|url=https://www.pure.ed.ac.uk/ws/files/4945595/2005_JPhysD_preprint_Metal_polymer_composite_with_unusual_properties.pdf|hdl=20.500.11820/53811f2f-2093-43a7-9f35-854338273c94|s2cid=84833095|hdl-access=free}}</ref> More recently, new mechanistic explanations have been established to explain the performance of force-sensing resistors; these are based on the property of [[contact resistance]] <math>R_C</math> occurring between the sensor electrodes and the conductive polymer. Specifically the force induced transition from [[Ballistic conduction#Importance|Sharvin contacts]] to conventional [[Electrical contacts#Electrical contact theory|Holm contacts]].<ref>{{cite journal|last1=Mikrajuddin|first1=A|last2=Shi|first2=F|last3=Kim|first3=H|last4=Okuyama|first4=K|title=Size-dependent electrical constriction resistance for contacts of arbitrary size: from Sharvin to Holm limits|journal=Materials Science in Semiconductor Processing|volume=2|issue=4|pages=321–327|date=24 April 2000|doi=10.1016/S1369-8001(99)00036-0}}</ref> The [[contact resistance]], <math>R_C</math>, plays an important role in the current conduction of force-sensing resistors in a twofold manner. First, for a given applied [[Stress (mechanics)|stress]] <math>\sigma</math>, or [[force]] <math>F</math>, a plastic deformation occurs between the sensor electrodes and the polymer particles thus reducing the [[contact resistance]].<ref name=kalantari/><ref name=paredes_static/> Second, the uneven polymer surface is flattened when subjected to incremental forces, and therefore, more contact paths are created; this causes an increment in the effective Area for current conduction <math>A</math>.<ref name=paredes_static/> At a macroscopic scale, the polymer surface is smooth. However, under a [[scanning electron microscope]], the conductive polymer is irregular due to agglomerations of the polymeric binder.<ref name=luheng/>

To date, no comprehensive model is capable of predicting all the non-linearities observed in force-sensing resistors. The multiple phenomena occurring in the conductive polymer turn out to be too complex such to embrace them all simultaneously; this condition is typical of systems encompassed within [[condensed matter physics]]. However, in most cases, the experimental behavior of force-sensing resistors can be grossly approximated to either the [[percolation theory]] or to the equations governing quantum tunneling through a [[rectangular potential barrier]].

=== Percolation in FSRs ===
The percolation phenomenon dominates in the conductive polymer when the particle concentration is above the [[percolation threshold]] <math>\phi_c</math>. A force-sensing resistor operating based on percolation exhibits a positive coefficient of pressure, and therefore, an increment in the applied pressure causes an increment in the [[electrical resistance]] <math>R</math>,<ref>{{cite journal|last1=Knite|first1=M|last2=Teteris|first2=V|last3=Kiploka|first3=A|last4=Kaupuzs|first4=J|title=Polyisoprene-carbon black nanocomposites as tensile strain and pressure sensor materials|journal=Sensors and Actuators A: Physical|volume=110|issue=1–3|pages=142–149|date=15 August 2003|doi=10.1016/j.sna.2003.08.006}}</ref><ref>{{cite journal|last1=Yi|first1=H|last2=Dongrui|first2=W|last3=Xiao-Man|first3=Z|last4=Hang|first4=Z|last5=Jun-Wei|first5=Z|last6=Zhi-Min|first6=D|title=Positive piezoresistive behavior of electrically conductive alkyl-functionalized graphene/polydimethylsilicone nanocomposites|journal= J. Mater. Chem. C|volume=1|issue=3|pages=515–521|date=24 October 2012|doi=10.1039/C2TC00114D}}</ref> For a given applied stress <math>\sigma</math>, the electrical resistivity <math>\rho</math> of the conductive polymer can be computed from:<ref>{{cite journal|last1=Basta|first1=M|last2=Picciarelli|first2=V|last3=Stella|first3=R|title=An introduction to percolation|journal=European Journal of Physics|volume=15|issue=3|pages=97–101|date=1 October 1993|doi=10.1088/0143-0807/15/3/001|bibcode=1994EJPh...15...97B|s2cid=250782773}}</ref>
:<math>\rho=\rho_0(\phi-\phi_c)^{-x}</math>
where <math>\rho_0</math> matches for a prefactor depending on the transport properties of the conductive polymer, and <math>x</math> is the critical conductivity exponent.<ref>{{cite journal|last1=Zhou|first1=J|last2=Song|first2=Y|last3=Zheng|first3=Q|last4=Wu|first4=Q|last5=Zhang|first5=M|title=Percolation transition and hydrostatic piezoresistance for carbon black filled poly(methylvinylsilioaxne) vulcanizates|journal=Carbon|date=2 February 2008|volume=46|issue=4|pages=679–691|doi=10.1016/j.carbon.2008.01.028|bibcode=2008Carbo..46..679Z}}</ref> Under percolation regime, the particles are separated from each other when mechanical stress is applied; this causes a net increment in the device's resistance.

=== Quantum tunneling in FSRs ===
[[Quantum tunneling]] is the most common operation mode of force-sensing resistors. A conductive polymer operating on the basis of quantum tunneling exhibits a resistance decrement for incremental values of stress <math>\sigma</math>. Commercial FSRs such as the FlexiForce,<ref>{{cite web|last1=Tekscan, Inc|title=FlexiForce, Standard Force \& Load Sensors Model A201. Datasheet|url=https://www.tekscan.com/sites/default/files/resources/FLX-A201-A.pdf}}</ref> Interlink <ref>{{cite web|last1=Interlink Electronics|title=FSR400 Series Datasheet|url=http://www.interlinkelectronics.com/datasheets/Datasheet_FSR.pdf}}</ref> and Peratech <ref>{{cite web|last1=Peratech, Inc|title=QTC SP200 Series Datasheet. Single Point Sensors|url=https://www.peratech.com/assets/uploads/datasheets/Peratech-QTC-DataSheet-SP200-Series-Nov15.pdf}}</ref> sensors operate based on quantum tunneling. The Peratech sensors are also referred to in the literature as [[quantum tunnelling composite]].

The quantum tunneling operation implies that the average inter-particle separation <math>s</math> is reduced when the conductive polymer is subjected to mechanical stress; such a reduction in <math>s</math> causes a probability increment for particle transmission according to the equations for a [[rectangular potential barrier]].<ref>{{cite journal|last1=Canavese|first1=G|last2=Stassi|first2=S|last3=Fallauto|first3=C|last4=Corbellini|first4=S|last5=Cauda|first5=V|title=Piezoresistive flexible composite for robotic tactile applications|journal=Sensors and Actuators A: Physical|volume=208|pages=1–9|date=23 June 2013|doi=10.1016/j.sna.2013.11.018|s2cid=109604106|url=http://porto.polito.it/2524092/}}</ref> Similarly, the contact resistance <math>R_C</math> is reduced amid larger applied forces. To operate based on quantum tunneling, particle concentration in the conductive polymer must be held below the percolation threshold <math>\phi_c</math>.<ref name=stassi2014/>

Several authors have developed theoretical models for the quantum tunneling conduction of FSRs,<ref>{{cite journal|last1=Li|first1=C|last2=Thostenson|first2=E|last3=Chou|first3=T-W|title=Dominant role of tunneling resistance in the electrical conductivity of carbon nanotube–based composites|journal=Applied Physics Letters|volume=91|issue=22|pages=223114|date=29 November 2007|doi=10.1063/1.2819690|bibcode=2007ApPhL..91v3114L}}</ref><ref>{{cite journal|last1=Lantada|first1=A|last2=Lafont|first2=P|last3=Muñoz|first3=J|last4=Munoz-Guijosa|first4=J|last5=Echavarri|first5=J|title=Quantum tunnelling composites: Characterisation and modelling to promote their applications as sensors|journal=Sensors and Actuators A: Physical|volume=164|issue=1–2|pages=46–57|date=16 September 2010|doi=10.1016/j.sna.2010.09.002}}</ref> some of the models rely upon the equations for particle transmission across a [[rectangular potential barrier]]. However, the practical usage of such equations is limited because they are stated in terms of electron energy, <math>E</math>, that follows a Fermi Dirac probability distribution, i.e., electron energy is not a priori determined or can not be set by the final user. The analytical derivation of the equations for a [[rectangular potential barrier]] including the Fermi Dirac distribution was found in the 60`s by Simmons.<ref name=Simmons>{{cite journal|last1=Simmons|first1=J|title=Electrical tunnel effect between dissimilar electrodes separated by a thin insulating Film|journal=Journal of Applied Physics|volume=34|issue=9|pages=2581–2590|doi=10.1063/1.1729774|bibcode=1963JAP....34.2581S|year=1963}}</ref> Such equations relate the [[current density]] <math>J</math> with the external applied voltage across the sensor <math>U</math>. However, <math>J</math> is not straightforward measurable in practice, so the transformation <math>I=JA</math> is usually applied in literature when dealing with FSRs.

Just as in the equations for a [[rectangular potential barrier]], the Simmons' equations are piecewise regarding the magnitude of <math>U</math>, i.e., different expressions are stated depending on  <math>U</math> and the height of the rectangular potential barrier <math>V_a</math>. The simplest Simmons' equation <ref name=Simmons/> relates <math>I</math> with <math>U</math>,<math>s</math> when <math>U\approx0</math> as next:

:<math>I(U,s)=\frac{3A\sqrt{2mV_a}}{2s}(\frac{e}{h})^{2}U\exp(-\frac{4{\pi}s}{h}\sqrt{2mV_a})</math>

where <math>V_a</math> is in units of electron volt, <math>m</math>, <math>e</math> are the electron's mass and charge respectively, and <math>h</math> is the [[Planck constant]].
The low voltage equation of the Simmons' model <ref name=Simmons/> is fundamental for modeling the current conduction of FSRs. The most widely accepted model for tunneling conduction has been proposed by Zhang et al.<ref name=Zhang>{{cite journal|last1=Xiang-Wu|first1=Z|last2=Yi|first2=P|last3=Qiang|first3=Z|last4=Xiao-Su|first4=Y|title=Time dependence of piezoresistance for the conductor-filled polymer composites|journal= Journal of Polymer Science Part B: Polymer Physics|volume=38|issue=21|pages=2739–2749|date=8 September 2000|doi=10.1002/1099-0488(20001101)38:21<2739::AID-POLB40>3.0.CO;2-O|bibcode=2000JPoSB..38.2739Z}}</ref> based on such equation. By re-arranging the equation above, it is possible to obtain an expression for the conductive polymer resistance <math>R_{Pol}</math>, where <math>R_{Pol}</math> is given by the quotient <math>U/I</math> according to the [[Ohm's law]]:

:<math>R_{\it{Pol}}=\frac{s}{A\sqrt{2mV_a}}(\frac{h}{e})^{2}\exp(\frac{4{\pi}s}{h}\sqrt{2mV_a})</math>

When the conductive polymer is fully unloaded, the following relationship can be stated between the inter-particle separation at rest state <math>s_0</math>,the filler volume fraction <math>\phi</math> and particle diameter <math>D</math>:

:<math>s_0=D\Big[\Big(\frac{\pi}{6\phi}\Big)^{\frac{1}{3}}-1\Big]</math>

Similarly, the following relationship can be stated between the inter-particle separation <math>s</math> and stress <math>\sigma</math>

:<math>s=s_0(1-\frac{\sigma}{M})</math>

where <math>M</math> is the  [[Young's modulus]] of the conductive polymer. Finally, by combining all the equations above, the Zhang's model <ref name=Zhang/> is obtained as next:

:<math>R_{\it{Pol}}=\frac{D\Big[\Big(\frac{\pi}{6\phi}\Big)^{\frac{1}{3}}-1\Big](1-\frac{\sigma}{M})}{A\sqrt{2mV_a}}\big(\frac{h}{e}\big)^{2}\exp\Big(\frac{4{\pi}D}{h}\Big[\Big(\frac{\pi}{6\phi}\Big)^{\frac{1}{3}}-1\Big](1-\frac{\sigma}{M})\sqrt{2mV_a}\Big)</math>

Although the model from Zhang et al. has been widely accepted by many authors,<ref name=luheng>{{cite journal|last1=Wang|first1=L|last2=Ding|first2=T|last3=Wang|first3=P|title=Influence of carbon black concentration on piezoresistivity for carbon-black-filled silicone rubber composite|journal=Carbon|date=30 June 2009|volume=47|issue=14|pages=3151–3157|doi=10.1016/j.carbon.2009.06.050|bibcode=2009Carbo..47.3151L}}</ref><ref name=kalantari>{{cite journal|last1=Kalantari|first1=M|last2=Dargahi|first2=J|last3=Kovecses|first3=J|last4=Mardasi|first4=M|last5=Nouri|first5=S|title=A New Approach for Modeling Piezoresistive Force Sensors Based on Semiconductive Polymer Composites|journal=IEEE/ASME Transactions on Mechatronics|volume=17|issue=3|pages=572–581|doi=10.1109/TMECH.2011.2108664|year=2012|s2cid=44667583|url=https://spectrum.library.concordia.ca/974523/1/Masoud_Kalantari_Final_Version.pdf}}</ref> it has been unable to predict some experimental observations reported in force-sensing resistors. Probably, the most challenging phenomenon to predict is sensitivity degradation. When subjected to dynamic loading, some force-sensing resistors exhibit degradation in sensitivity.<ref>{{cite journal|last1=Lebosse|first1=C|last2=Renaud|first2=P|last3=Bayle|first3=B|last4=Mathelin|first4=M|title= Modeling and Evaluation of Low-Cost Force Sensors|journal=IEEE Transactions on Robotics|volume=27|issue=4|pages=815–822|doi=10.1109/TRO.2011.2119850|year=2011|s2cid=14491353}}</ref><ref>{{cite journal|last1=Lin|first1=L|last2=Liu|first2=S|last3=Zhang|first3=Q|last4=Li|first4=X|last5=Ji|first5=M|last6=Deng|first6=H|last7=Fu|first7=Q|title= Towards Tunable Sensitivity of Electrical Property to Strain for Conductive Polymer Composites Based on Thermoplastic Elastomer|journal=ACS Applied Materials & Interfaces|volume=5|issue=12|pages=5815–5824|doi=10.1021/am401402x|year=2013|pmid=23713404}}</ref> Up to date, a physical explanation for such a phenomenon has not been provided, but experimental observations and more complex modeling from some authors have demonstrated that sensitivity degradation is a voltage-related phenomenon that can be avoided by choosing an appropriate driving voltage in the experimental set-up.<ref name=Paredes_dynamic>{{cite journal|last1=Paredes-Madrid|first1=L|last2=Matute|first2=A|last3=Bareño|first3=J|last4=Parra|first4=C|last5=Gutierrez|first5=E|title=Underlying Physics of Conductive Polymer Composites and Force Sensing Resistors (FSRs). A Study on Creep Response and Dynamic Loading|journal=Materials|volume=10|issue=11|pages=1334|date=21 November 2017|doi=10.3390/ma10111334|pmid=29160834|bibcode=2017Mate...10.1334P|pmc=5706281|doi-access=free}}</ref>

The model proposed by Paredes-Madrid et al.<ref name=paredes_static>{{cite journal|last1=Paredes-Madrid|first1=L|last2=Palacio|first2=C|last3=Matute|first3=A|last4=Parra|first4=C|title=Underlying Physics of Conductive Polymer Composites and Force Sensing Resistors (FSRs) under Static Loading Conditions|journal=Sensors|volume=17|issue=9|pages=2108|date=14 September 2017|doi=10.3390/s17092108|pmid=28906467|pmc=5621037|bibcode=2017Senso..17.2108P|doi-access=free}}</ref> uses the entire set of Simmons' equations <ref name=Simmons/> and embraces the contact resistance within the model; this implies that the externally applied voltage to the sensor <math>V_{FSR}</math> is split between the tunneling voltage <math>V_{bulk}</math> and the voltage drop across the contact resistance <math>V_{Rc}</math> as next:

:<math>V_{FSR}=2V_{RC}+V_{bulk}</math>

By replacing sensor current <math>I</math> in the above expression, <math>V_{bulk}</math> can be stated as a function of the contact resistance <math>Rc</math> and <math>I</math> as next:

:<math>V_{bulk}=V_{FSR}-2RcI</math>

and the contact resistance <math>R_C</math> is given by:

:<math>R_C=R_{\it{par}}+\frac{R_C^0}{\sigma^k}</math>

where <math>R_{par}</math> is the resistance of the conductive nano-particles and <math>R_C^0</math>, <math>k</math> are experimentally determined factors that depend on the interface material between the conductive polymer and the electrode. Finally the expressions relating sensor current <math>I</math> with <math>V_{FSR}</math> are piecewise functions just as the Simmons equations <ref name=Simmons/> are:

When <math>V_{bulk}\approx0</math>
:<math>R_{\it{bulk}}=\frac{s_0(1-\frac{\sigma}{M})}{(A_0+A_1\sigma^{A_2})\sqrt{2mV_a}}(\frac{h}{e})^{2}\exp(\frac{4{\pi}s_0(1-\frac{\sigma}{M})}{h}\sqrt{2mV_a})</math>

When <math>V_{bulk}<V_a/e</math>
:<math>I=\frac{(A_0+A_1\sigma^{A_2})e}{2{\pi}hs_0^{2}(1-\frac{\sigma}{M})^{2}}\Bigg\{(V_a-\frac{V_{bulk}}{2})\exp\Bigg[-\frac{4{\pi}}{h}s_0(1-\frac{\sigma}{M})\sqrt{2m(V_a-\frac{eV_{bulk}}{2})}\Bigg]-(V_a+\frac{V_{bulk}}{2})\exp\Bigg[-\frac{4{\pi}}{h}s_0(1-\frac{\sigma}{M})\sqrt{2m(V_a+\frac{eV_{bulk}}{2})}\Bigg]\Bigg\}</math>

When <math>V_{bulk}>V_a/e</math>
:<math>I=\frac{2.2e^{3}V_{bulk}^{2}(A_0+A_1\sigma^{A_2})}{8{\pi}hV_as_0^{2}(1-\frac{\sigma}{M})^{2}}\Bigg\{\exp\Bigg[-\frac{8{\pi}s_0(1-\frac{\sigma}{M})}{2.96heV_{bulk}^2}\sqrt{2mV_a^3}\Bigg]-(1+\frac{2eV_{bulk}}{V_a})\exp\Bigg[-\frac{8{\pi}s_0(1-\frac{\sigma}{M})}{2.96heV_{bulk}}\sqrt{2mV_a^3(1+\frac{2eV_{bulk}}{V_a})}\Bigg]\Bigg\}</math>

In the equations above, the effective area for tunneling conduction <math>A</math> is stated as an increasing function dependent on the applied stress <math>\sigma</math>, and on coefficients <math>A_0</math>, <math>A_1</math>, <math>A_2</math> to be experimentally determined. This formulation accounts for the increment in the number of conduction paths with stress:

:<math>A=A_0+A_1\sigma^{A_2}</math>

=== Current research trends in FSRs ===
Although the above model <ref name=paredes_static/> is unable to describe the undesired phenomenon of sensitivity degradation, the inclusion of [[rheological]] models has predicted that drift can be reduced by choosing an appropriate sourcing voltage; experimental observations have supported this statement.<ref name=Paredes_dynamic/> Another approach to reduce drift is to employ Non-aligned electrodes to minimize the effects of polymer creep.<ref name=wang_electrode>{{cite journal|last1=Wang|first1=L|last2=Han|first2=Y|last3=Wu|first3=C|last4=Huang|first4=Y|title=A solution to reduce the time dependence of the output resistance of a viscoelastic and piezoresistive element|journal=Smart Materials and Structures|volume=22|issue=7|pages=075021|date=7 June 2013|doi=10.1088/0964-1726/22/7/075021|bibcode=2013SMaS...22g5021W|s2cid=108446573}}</ref> There is currently a great effort placed on improving the performance of FSRs with multiple different approaches: in-depth modeling of such devices in order to choose the most adequate driving circuit,<ref name=Paredes_dynamic/> changing the electrode configuration to minimize drift and/or hysteresis,<ref name=wang_electrode/> investigating on new materials type such as [[carbon nanotubes]],<ref>{{cite journal|last1=Cao|first1=X|last2=Wei|first2=X|last3=Li|first3=G|last4=Hu|first4=C|last5=Dai|first5=K|title=Strain sensing behaviors of epoxy nanocomposites with carbon nanotubes under cyclic deformation|journal=Polymer|volume=112|pages=1–9|date=10 March 2017|doi=10.1016/j.polymer.2017.01.068}}</ref> or solutions combining the aforesaid methods.

== Uses ==
Force-sensing resistors are commonly used to create pressure-sensing "buttons" and have applications in many fields, including [[musical instruments]] (such as the [[Sensel]] Morph), car occupancy sensors, artificial limbs, [[Pronation of the foot|foot pronation]] systems, and [[portable electronics]]. They are also used in [[Mixed reality|mixed]] or [[augmented reality]] systems<ref>{{cite conference | last1=Issartel | first1=Paul | last2=Besancon | first2=Lonni | last3=Isenberg | first3=Tobias | last4=Ammi | first4=Mehdi | title=2016 IEEE International Symposium on Mixed and Augmented Reality (ISMAR-Adjunct) | chapter=A Tangible Volume for Portable 3D Interaction | publisher=IEEE | year=2016 | pages=215–220 | isbn=978-1-5090-3740-7 | doi=10.1109/ismar-adjunct.2016.0079 | chapter-url=https://hal.inria.fr/hal-01423533/document| arxiv=1603.02642 }}</ref> as well as to enhance mobile interaction.<ref>{{cite conference | last1=Besançon | first1=Lonni | last2=Ammi | first2=Mehdi | last3=Isenberg | first3=Tobias | title=Proceedings of the 2017 CHI Conference on Human Factors in Computing Systems | chapter=Pressure-Based Gain Factor Control for Mobile 3D Interaction using Locally-Coupled Devices | publisher=ACM Press | location=New York, New York, USA | year=2017 | pages=1831–1842 | isbn=978-1-4503-4655-9 | doi=10.1145/3025453.3025890 | chapter-url=https://hal.inria.fr/hal-01436172/document}}</ref><ref>{{cite conference | last1=McLachlan | first1=Ross | last2=Brewster | first2=Stephen | title=Proceedings of the 17th International Conference on Human-Computer Interaction with Mobile Devices and Services | chapter=Bimanual Input for Tablet Devices with Pressure and Multi-Touch Gestures | publisher=ACM Press | location=New York, New York, USA | year=2015 | pages=547–556 | isbn=978-1-4503-3652-9 | doi=10.1145/2785830.2785878 }}</ref>

==See also==
{{Commonscat|Force sensing resistors|Force-sensing resistors}}
* [[Velostat]] – used to make hobbyist sensors
{{clear}}

== References ==
{{reflist}}

[[Category:Resistive components]]
[[Category:Sensors]]