What is a Printed Force Sensitive Resistor?
A force sensor, or force sensitive resistor is a variable resistor in that its resistance varies with the amount of pressure placed upon it. When pressure is applied to the force sensor, the resistance changes inversely with the force applied, i.e., an increase in pressure applied causes a decrease in resistance and an increase in conductivity, while releasing the applied pressure causes the resistance to return to its native state.
There are various configurations that can be used to make a force sensing resistor, and they all use a special carbon based ink and a conductive silver ink printed on PET separated by a spacer. When pressure is applied to the active area of the force sensing resistor assembly, the carbon based and conductive silver inks make contact and complete the circuit. A special ink system is used to achieve the desired resistance values on the force sensor pads.
Cubbison Company can design and manufacture the following types of printed force sensitive resistors:
1. Discrete Force Sensing Resistor Array
2. Single Point Force Sensing Resistors – Shorting Pad Type and Across Type
3. X-Y Array – Shorting Pad Type and Across Type
A printed discrete force sensing resistor array is a series of force sensor pads that are screen printed on a heat stabilized polyester substrate. These force sensor array pads can be printed as a standalone circuit to interconnect directly with PCB electronics underneath the pads, or they can be printed in conjunction with, and used to activate printed conductive circuitry placed below them as part of a membrane switch type circuit.
Single Point Shorting Pad force sensing resistors consist of a top circuit printed sub-surface with a solid ink pad that is separated by a spacer from a bottom circuit printed top surface with an interdigitated silver conductive pad.
Single Point Across force sensing resistors consist of a top circuit printed sub-surface with conductive silver that is overprinted with a solid force sensing ink pad, and is separated by a spacer from a bottom circuit printed top surface with conductive silver and overprinted with a solid force sensing ink pad.
The X-Y Shorting Pad Type Array consists a top circuit with solid force sensing ink pad printed sub-surface and separated by a spacer and printed dielectric from a bottom circuit consisting of an array of interdigitated pads connected by jumpers and linear traces in an X-Y fashion. This type of sensor allows simultaneous measurement of pressure on each pad location.
The X-Y Across Type Array consists of a sub-surface printed top circuit consisting of linear, Y direction conductive silver traces overprinted with force sensing ink, that are separated by a spacer and printed dielectric from a top surface printed bottom circuit consisting of with linear, X direction silver conductive traces overprinted with force sensing ink. This type of force sensor allows simultaneous measurement of pressure on each pad location.
How Does a Printed Force Sensitive Sensor Work?
The force sensitive resistor and Conductive Silver pads on the top and bottom circuits of each type face one another and conduct when pressed together. As more pressure is applied, the resistance decreases and the conductivity increases.
Other Design Considerations
The force sensor assembly is completed by affixing a linered adhesive to the top and bottom of the force sensitive resistor.
The circuit tail of the force sensing resistor can be terminated using a 2.54mm housing, 1mm or 2.54mm bare tail ZIF, or 2.54mm male solder tabs.
Each type of force sensor can be manufactured using various shapes and sizes, and is custom designed for your application.
Cubbison’s in-house Engineers will design each resistor specific to your application. Contact us for additional information.
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