Force sensors are piezoelectric transducers that convert input mechanical forces into electrical output signals. They can measure gravitational force (weight), compression, strain, and magnetic forces. These sensors fall into two main categories: load cells and force sensing resistors (FSRs). Load cells return a signal proportional to the mechanical force applied to the system. They primarily utilize strain gauges to measure force, but can also use the deflection of springs or hydraulic elements for the same purpose. Load cells are found all over us, in hospital patient hoists, printing machines, conveyor scales, and automotive testing systems. Unfortunately, load cells are bulky and cannot fit into compact spaces and lack the location accuracy of other pressure sensors. This is why force sensing resistors are useful. FSRs are flexible and measure compression forces directly, rather than through calculating the strain from the force applied.
At the core of a force sensing resistor is a conductive polymer composite that changes resistance uniformly according to the force applied. The conductive polymer is sandwiched between two metal electrode films. The metal films are conventionally made of printed, piezoresistive silver and carbon ink. In the case of quantum tunneling, particle concentration in the polymer foam is below the percolation threshold, and so applying a force to the surface of the film will create more conduction paths. The two layers of conductive ink press together, and electrical resistance decreases. However, when particle concentration is above the percolation threshold, the resistance rises with more applied stress. A charge converter then converts the resulting change of current into an output voltage signal.
In Depth: The Mechanisms of Conduction
- i) Quantum Tunneling – The tunneling effect drives conduction in a composite wherein conductive fillers are dispersed inside a polymeric matrix. When pressure is applied, the distance between conductive particles inside the composite changes, varying the overall material
- ii) Percolation – Percolation is in effect when particle concentration surpasses a threshold between an insulating and conductive state. The threshold is dependent upon the applied pressure since compression modifies the contact between particles, which creates or destroys direct conductive paths inside the matrix. The resistance of the composite material is measured through the use of electrodes that are in contact with the polymeric composite.
Each layer of a force sensing resistor is thin and flexible. Because of the sub-micrometer dimensions of the polymer particles and lightweight metal inks, the entire sensor need be only a few millimeters thick. These sensors have exceptionally broad measuring ranges, with a force range of about 0 to 100 Newtons and resistance range from infinite (no pressure) to 100kΩ. An FSR’s metal electrodes are highly versatile as they can be glued to various materials – including elastics, fabrics, and even orthopedic implants –, placed in contact with the material, or embedded inside during fabrication. When an FSR is integrated within a circuit, it serves to measure the applied force and relay that information to specified target electronics. Moreover, the inks used in the FSR can be customized for different functions simply by reshaping and rearranging the ink pattern.
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Ariadna is a junior at Cornell University studying Materials Science and Engineering. She is a staff writer for The Cornell Daily Sun and a member of the Nanoscale Materials for Energy Lab.