A review of our main research on producing optimal thin sensors design that can handle any force or pressure input over any surface.
An introduction with contactors
A circuit composed of a pwer source and for instance a Led or another transducer often also integrates an interrupter which simply consists in opening (no contact) or closing (contact) the circuit to let the power source shine the led.
Metal contact: digital Information
When the materials of the connectors are metals, a single tiny contact will be sufficient to let a maximum current intensity through the circuit and maximal power. The led either shines intensively or it does not: a binary solution.
Resistive contact: analogue information
When the materials of the connectors are resistive (e.g. resistive papers), the intensity of the current will be function of the material resistivity and of the contact force or surface between the 2 parts of the interrupter electrons flow. The led either shines intensively or it does not: a binary solution.
Possible Layers Design
Force Sensing Resistors (Aka FSR) were invented by Franklin Eventoff in the 1980's who is still running the company Sensitronics. FSR has become a brand name of another company producing these components Interlink. These 2 companies use PET or PVC and Electrically resistive synthetic plastics as substrates for their products. Additional connectors, lines or surfaces are added onto the substrates to build the circuit and electrodes facingthe resistive layer (usually printed conductive silver inks)
Shunt Mode Design
This design is branded as FSR and is produced by both companies discussed above and consist in a "dual enterlacing comb design printed on a layer facing a resistive layer with a space in between that prevents contacts by default ("infinite" resistance). Each of the comb is a connector and electrode and any contact between the metal combs and the resistive layer will output an electrical resistance.
The stronger the contact, the lower the resistance.
It requires printing equipments to reproduce and is not the best c choice for custom and large design in terms of size and shapes. It also requires a lot of metal inks zhich can become significantly cost-effective.
Thru Mode Design
This design is much easier to conceptualise and involves to have a resistive layer sandwiched between 2 electrodes layers and each electrode has a connector. So here, the current goes through the assembly thickness. Here also:
The stronger the contact, the lower the resistance.
This design is easier and more interesting to reproduce especially for small sensors like those produced by Sensitronics or Tekscan. However, and even more than for the Shunt mode design, it requires a massive amount of metals inks (usually silver), which does not look appropriate for sustainable developments that can not ensure 100% recyclability.
Thru Mode Design with Paper Electrodes (TMDPE)
Silver is not the only alternative to produce electrodes but it is among the best coductors and oxydates moderately. Other possible solutions are alternative metals, metal oxydes and semi-metals, conductive polymers or else a huge variety of conductive layers generated by surface deposition like ITO or Graphene.
These may be less conductive than silver or more noble metals like gold, but they will do the job as long as their resistance is in our case negligible regarding the resistance of resistive layer and the produced sensor.
Carbon, graphite and derivates likes carbon fibers and graphene also find more and more interest for sustainable alternatives to non-renewable metals.
But high conductivity surfaces gets harder to produce and it may not be useful to make them fully conductive if the resistive layer has a much higher resistivity, and/or if the connections design is made well enough to enable to production large sensors with consistent response of local contact surface vs their location.
Single Complex structure (no air interface)
A great design would be to directly print electrodes on each faces of the resistive layer thus producing the sensor on a single sheet. If this is theoretically easy to imagine a perfectly elastic and piezoresistive material under compression, it is harder to implement such a design as the response range is much marrower than when having an air interface between 2 sheets.
We then chose to focuse on the TMDPE design and the ways it can be optimised for DIY to industrial production.
Load VS Resistance Model for TMDPE
The paper surface is non-uniform and non-homogeneous locally. It consists in a superposition of fibers that are relatively oriented and homogeneous at a macroscopic scale. It is a relatively plane surface with an average roughness of around 2 to 10 microns for standard printing papers.
Assuming for now the material is homogeneous rergarding our contact surface sizes, we can start modelling the load impact over a TMDPE sensor and try to find a relation between Load applied and output resistance.
Discarding the impact of the electrodes resistance as negligible, we assume the sensor should output the same resistance for a given load.
We also assume for now that there is only contact between the layers at the location of the load applied. The load is spread over a contact surface Sc over the force sensor, and the sensor will output this value when connected to an ohmmeter.
If the paper has a uniform volume resistivity at our scale named Ru...
Tob e coninued...
Influence of the contact surface for a given Force
Influence of the Connections location
Influence of the Contact surface
Influence of the Number of contacts