Static Tactile Sensing for a Robotic Electronic Skin via an Electromechanical Impedance-Based Approach

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: electromechanical impedance-based method; electronic skin; piezoelectric sensors; robotic tactile sensing; static pressure load sensing.

Figure 1

Robotic electronic skin (a.k.a. smart skin) with embedded multi-modal sensor network.

Figure 2

The framework of using the EMI-based method to detect static pressure loads for application on smart skin with an embedded PZT sensor.

Figure 3

(a) Schematic of smart skin sample with a PZT sensor embedded in soft silicone; (b) the real sample with wiring and connectors.

Figure 4

Schematic of the experimental test setup.

Figure 5

The electromechanical impedance behavior from the PZT sensor at zero static pressure load.

Figure 6

(a) Real part of the impedance response under different static pressure load; (b) maximum impedance amplitude change at each static load with respect to the value at 0.5 N.

Figure 7

(a) Real part of the impedance response under different static pressure loads, including baseline data; (b) tactile index at different static loads.

Figure 8

The FEM model and mesh of the smart skin sample with an embedded PZT sensor.

Figure 9

Nonlinear stress-strain relationship for Smooth-On Ecoflex 00-30 under static compressive load.

Figure 10

Simulation result of the nodal electric charge at one node at the top surface of the PZT sensor.

Figure 11

Comparison of the experiment and simulation results on the impedance behavior of a PZT sensor embedded in the silicone rubber: (a) real part; (b) imaginary part.

Figure 12

(a) Simulation results of the static normal pressure load effect to the smart skin sample with a PZT sensor embedded in the silicone rubber; (b) simulated maximum amplitude change at each static load, with respect to the value at 0.5 N.

Figure 13

Electro-mechanical coupling between the PZT sensor and the structures.

Figure 14

Displacement distribution of a PZT sensor in thickness direction under a static pressure load of 6.29 kPa. (U and U3 are displacement in m.).