Force sensor (alternatively called a force-sensing resistor or simply an FSR) has a variable resistance as a function of applied pressure. In this sense, the term “force-sensitive” is misleading – a more appropriate one would be “pressure-sensitive”, since the sensor's output is dependent on the area on the sensor's surface to which force is applied.
Force sensors are fabricated with elastic material in four layers, consisting of:
A layer of electrically insulating plastic;
An active area consisting of a pattern of conductors, which is connected to the leads on the tail to be charged with an electrical voltage;
A plastic spacer, which includes an opening aligned with the active area, as well as an air vent through the tail;
A flexible substrate coated with a thick polymer conductive film, aligned with the active area. This polymer is very often replaced by a layer of FSR ink.
When external force is applied to the sensor, the resistive element is deformed against the substrate. Air from the spacer opening is pushed through the air vent in the tail, and the conductive material on the substrate comes into contact with parts of the active area. The more of the active area that touches the conductive element, the lower the resistance becomes. All FSRs exhibit a “switchlike response”, meaning some amount of force is necessary to break the sensor's resistance at rest and push it into the measurement range