sensors Article Static Tactile Sensing for a Robotic Electronic Skin via an Electromechanical Impedance-Based Approach Cheng Liu 1,*, Yitao Zhuang 2, Amir Nasrollahi 3 , Lingling Lu 4, Mohammad Faisal Haider 3 and Fu-Kuo Chang 3 1 Department of Mechanical Engineering, Stanford University, Stanford, CA 94305, USA 2 Zenith Aerospace, Redwood City, CA 94063, USA; [email protected] 3 Department of Aeronautics and Astronautics, Stanford University, Stanford, CA 94305, USA; [email protected] (A.N.); [email protected] (M.F.H.); [email protected] (F.-K.C.) 4 Key Laboratory for Mechanics in Fluid Solid Coupling Systems, Institute of Mechanics, Chinese Academy of Science, Beijing 100190, China; [email protected] * Correspondence: [email protected] Received: 31 March 2020; Accepted: 14 May 2020; Published: 16 May 2020 Abstract: Tactile sensing is paramount for robots operating in human-centered environments to help in understanding interaction with objects. To enable robots to have sophisticated tactile sensing capability, researchers have developed different kinds of electronic skins for robotic hands and arms in order to realize the ‘sense of touch’. Recently, Stanford Structures and Composites Laboratory developed a robotic electronic skin based on a network of multi-modal micro-sensors. This skin was able to identify temperature profiles and detect arm strikes through embedded sensors. However, sensing for the static pressure load is yet to be investigated. In this work, an electromechanical impedance-based method is proposed to investigate the response of piezoelectric sensors under static normal pressure loads. The smart skin sample was firstly fabricated by embedding a piezoelectric sensor into the soft silicone. Then, a series of static pressure tests to the skin were conducted. Test results showed that the first peak of the real part impedance signal was sensitive to static pressure load, and by using the proposed diagnostic method, this test setup could detect a resolution of 0.5 N force. Numerical simulation methods were then performed to validate the experimental results. The results of the numerical simulation prove the validity of the experiments, as well as the robustness of the proposed method in detecting static pressure loads using the smart skin. Keywords: robotic tactile sensing; electronic skin; piezoelectric sensors; static pressure load sensing; electromechanical impedance-based method 1. Introduction Tactile sensing in human and animal skins enables them to touch, sense temperature, etc. The haptic perception, if added to robots, can significantly enhance their performance through better human-robot and robot-environment interactions. In comparison, even the most sophisticated robots have at most a few dozen tactile sensors. Regardless of over thirty years of research, tactile sensing still falls behind progress in computer vision methods. The reason for this discrepancy is that compared to cameras, tactile sensors must be compliant, tough and flexible enough to coat the surfaces of robotic limbs and hands. In addition, as the number of sensors increases, wiring and signal transfer become a major issue [1–5]. To overcome the aforementioned challenges for robotic tactile sensing, Stanford Structures and Composites Laboratory (SACL) developed a smart skin shown in Figure1 by embedding a multi-modal stretchable sensor network [6] into a soft silicone. Guo [7] has documented in detail the fabrication and Sensors 2020, 20, 2830; doi:10.3390/s20102830 www.mdpi.com/journal/sensors SensorsSensors 20202020, ,2020, ,x 2830 FOR PEER REVIEW 22 of of 14 14 fabrication and material selection process of this skin. This artificial skin has been added to a robotic armmaterial for realizing selection autonomous process of this control skin.,This which artificial leverages skin advanced has beenadded sampling to a- roboticbased motion arm for planning realizing techniquesautonomous [7]. control, Utilizing which the signals leverages of the advanced multi-modal sampling-based sensors, which motion are embedded planning techniquesin the skin [in7]. stateUtilizing awareness the signals algorithms of the as multi-modal input parameters, sensors, the which robotic are arm embedded can sense in and the skinreact into stateenvironmental awareness changalgorithmses such as as input temperature parameters, variance the robotic and armlocal can dynamic sense and impacts react toonto environmental the skin [7]. changesHowever, such one as criticaltemperature aspect variancein tactile andsensing local has dynamic not been impacts well studied, onto the which skin [ 7is]. how However, to detect one static critical pressure aspect in in thetactile smart sensing skin using has not lead been zirconate well studied, titanate which (PZT) is howelements to detect, also static known pressure as piezoelectric in the smart sensors. skin using lead zirconate titanate (PZT) elements, also known as piezoelectric sensors. Figure 1. Robotic electronic skin (a.k.a. smart skin) with embedded multi-modal sensor network. Figure 1. Robotic electronic skin (a.k.a. smart skin) with embedded multi-modal sensor network. There exist different tactile sensors for measuring static pressure and contact conditions using differentThere transduction exist different mechanisms tactile sensors [8–11 for]. Resistive-basedmeasuring static tactile pressure sensors and cancontact reach conditions high sensitivity, using differentbut have transduction high power consumptionmechanisms [8 and–11] lack. Resistive the measurement-based tactile of sensors contact can forces reach [12 high]. Optical sensitivity, tactile butsensors have arehigh also power capable consumption of reaching and high lack sensitivity, the measurement although of they contact will force shows [ loss12]. ofOptical light duetactile to sensorsmicro bending are also chirping.capable of Meanwhile, reaching high power sensitivity, consumption although is also they abig will challenge show loss [12 of]. light Triboelectric due to microtactile bending sensors havechirping. the advantageMeanwhile, of power being self-powering,consumption is although also a big their challenge long-term [12]. unreliability Triboelectric is tactilestill an sensors issue [ 13have]. One the ofadvantage the most of common being self tactile-powering sensors, although are capacitive their long tactile-term sensors, unreliability which can is stillachieve an issue high [13 sensitivity]. One of [the4]. most However, common noises tactile coming sensors from are temperature capacitive tactile and humidity sensors, which variations, can achieveand even high electrical sensitivity noise [4] introduced. However, bynoises unshielded coming from power temperature supplies, may and significantlyhumidity variations decrease, and the evencapacitive electrical signal-to-noise noise introduced ratio. This by issue unshielded imposes power a signal supplies conditioning, may stage significantly in order to decrease obtain a highthe capacitivesignal-to-noise signal ratio,-to-noise which ratio. results This in issue complex imposes circuitry a signal [4]. In conditioning addition, under stage highly in order repetitive to obtain loads, a highthe capacitive signal-to-noise sensors ratio, are pronewhich toresults failure in due complex to mechanical circuitry fatigue,[4]. In addition, which undermines under highly the repetitive reliability loads,of the the method capacitive [14]. sensors are prone to failure due to mechanical fatigue, which undermines the reliabilityCompared of the method with capacitive [14]. sensors, PZT sensors are excellent in terms of mechanical robustness andCompared their simplicity with ofcapacitive use and noisesensors, resistance. PZT sensors Piezoelectricity are excellent in in PZT terms is a of well-known mechanical transduction robustness andmechanism their simplicity for dynamic of use forceand noise measurement resistance. byPiezoelectricity using the direct in PZT piezoelectric is a well-known effect. transduction However, a mechanismdifferent mechanism for dynamic is required force measurement to sense static by loads using using the PZTdirect sensors. piezoelectric One promising effect. However, method a is differentto use PZT mechanism sensors in is a required resonant to piezoelectric sense static sensingloads using mode. PZT Safour sensors. and One Bernard promising [15] demonstrated method is to usethat PZT the sensors measured in a electric resonant admittance piezoelectric spectrum sensing of mode. a PZT Safour sensor and can Bernard be correlated [15] demonstrated to the static forces that theapplied measured to the PZTelectric sensor. admittance Although spectrum their study of wasa PZT focused sensor on can the be applied correlated force into the the order static of forces 100 N, appliedwhich is to relatively the PZT high,sensor. and Although applied their directly study on thewas PZT focused sensor, on theirthe applied results showedforce in the possibilityorder of 100 of N,this which method is forrelatively tactile sensing.high, and In applied general, directly tactile sensing on the requires PZT sensor, a higher their resolution, results showed and sensors the possibilitymust be embedded of this method in a skin for material.tactile sensing. Suchinclusion In general, of tactile the sensors sensing inside requires skin materiala higher likeresolution silicone, andrubber sensors or fiberglass must be canembedded prevent in sensors a skin frommaterial. direct Suc contacth inclusion with externalof