Abstract
Force-sensing resistors (FSRs) are inexpensive alternatives to load cells. They are suitable for applications where noninvasive devices are needed to measure force, stress, or pressure. However, they have been proved to be hysteresis prone and offer nonrepeatable readings due to their highly voltage-dependent electrical resistance. A piezocapacitive effect has been found as an alternative phenomenon that is able to offer force-dependent readings of capacitance with less hysteresis error. Also, this capacitance is not dependent on voltage, which also improves repeatability in force measurements. Since measuring capacitance is more expensive than resistance, the least costly conditioning circuitry is desired. An inexpensive alternative using an LM555 that oscillates depending on capacitance is here presented. Hysteresis and repeatability errors have been reduced for a widespread-used force-sensing resistor brand.
Highlights
Conductive polymer composites (CPCs), known as quantum tunneling composites (QTCs), are manufactured by randomly dispersing conductive particles within an insulating polymer matrix
A model of the effective conductive area at unloading forces has not been proposed, but the data presented in this paper suggests that the effective conductive area presents additional hysteresis, which boosts the total hysteresis error (HE)
Piezoresistive effect has been useful for force sensing due to sensitivity, low invasive profile, and low cost
Summary
Conductive polymer composites (CPCs), known as quantum tunneling composites (QTCs), are manufactured by randomly dispersing conductive particles within an insulating polymer matrix. A straight linear path is established form s0 to sf as the FSR is loaded by a timeincreasing stress, but the way the interparticle separation recovers to its nonloaded state is clearly nonlinear This hysteresis phenomenon affects the electric behavior of the FSR since its electrical resistance depends on the instantaneous value of s [11]. A model of current density J considering s and u, which E mainly depends on, was developed by Simmons [11] considering the multiple quantum tunneling effects occurring in a CPC This approach was moved forward for FSRs in [31], where (11) and (12) are taken into account.
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