A force-sensing resistor is a material whose resistance changes when a force, pressure or mechanical stress is applied.
[4] Force-sensing resistors consist of a conductive polymer, which predictably changes resistance following applying force to its surface.
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.
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 mm), low cost, and good shock resistance.
Force-sensing capacitors offer superior sensitivity and long-term stability, but require more complicated drive electronics.
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.
[7] 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
, plays an important role in the current conduction of force-sensing resistors in a twofold manner.
, a plastic deformation occurs between the sensor electrodes and the polymer particles thus reducing the contact resistance.
However, under a scanning electron microscope, the conductive polymer is irregular due to agglomerations of the polymeric binder.
[11] 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.
[15] 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.
A conductive polymer operating on the basis of quantum tunneling exhibits a resistance decrement for incremental values of stress
Commercial FSRs such as the FlexiForce,[16] Interlink [17] and Peratech [18] sensors operate based on quantum tunneling.
To operate based on quantum tunneling, particle concentration in the conductive polymer must be held below the percolation threshold
, 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.
The most widely accepted model for tunneling conduction has been proposed by Zhang et al.[23] based on such equation.
When subjected to dynamic loading, some force-sensing resistors exhibit degradation in sensitivity.
[24][25] 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.
are experimentally determined factors that depend on the interface material between the conductive polymer and the electrode.
This formulation accounts for the increment in the number of conduction paths with stress: Although the above model [10] 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.
[26] Another approach to reduce drift is to employ Non-aligned electrodes to minimize the effects of polymer creep.
[27] 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,[26] changing the electrode configuration to minimize drift and/or hysteresis,[27] investigating on new materials type such as carbon nanotubes,[28] or solutions combining the aforesaid methods.
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, foot pronation systems, and portable electronics.