Detailed Study of Amplitude Nonlinearity in Piezoresistive Force Sensors

Abstract

This article upgrades the RC linear model presented for piezoresistive force sensors. Amplitude nonlinearity is found in sensor conductance, and a characteristic equation is formulated for modeling its response under DC-driving voltages below 1 V. The feasibility of such equation is tested on four FlexiForce model A201-100 piezoresistive sensors by varying the sourcing voltage and the applied forces. Since the characteristic equation proves to be valid, a method is presented for obtaining a specific sensitivity in sensor response by calculating the appropriate sourcing voltage and feedback resistor in the driving circuit; this provides plug-and-play capabilities to the device and reduces the start-up time of new applications where piezoresistive devices are to be used. Finally, a method for bypassing the amplitude nonlinearity is presented with the aim of reading sensor capacitance.

A Novel and Inexpensive Approach for Force Sensing Based on FSR Piezocapacitance Aimed at Hysteresis Error Reduction

“…Confronted particles are able to accumulate electrical load and to establish an electrical field as shown in Figure 6. The usage of FSRs' based on piezocapacitive effect for several applications has been reported by [13][14][15], and strong evidence of capacitance existence in FSRs has been published in [12] where phase shifting between voltage and current was observed when the FSR was fed with AC power. The combined piezocapacitive effect with quantum tunneling conduction result in the R-C circuit is depicted in Figure 7.…”

“…Thus, expressions for the resistance per unit area as well as for the capacitance per unit area are finally obtained in the set from (16) through (18) and in (15), respectively. It is worth to recall that (16) through (18) are the result of dividing u by (10) through (12). Hence, (16) yields in a voltage-independent expression, only valid when u is small.…”

“…Thus, a total resistance of the FSR can be obtained but its calculation as a circuit is not straightforward. Resistance per unit area is the result of dividing the voltage in the CPC, u, by the current density J from (10) through (12). This yields an expression that depends on s(σ) and as a consequence of quantum tunneling, on u.…”

“…Journal of Sensors where e is the electron's charge, m its mass, φ 0 is the height of the potential barrier, and the constant ℏ = h/2π, being h the Planck's constant. Equations (10) through (12) show two aspects to be assessed. The dependence on s(σ) and on u are as follows:…”

“…Previous works [12][13][14][15][16] have demonstrated that FSRs based on CPCs also embrace a capacitive effect. Their capacitance can be measured and it varies when the CPC is subjected to mechanical stress.…”