As their name implies, force sensing resistors use the electrical property of resistance to measure the force (or pressure) applied to a sensor. A force sensing resistor is made up of two parts. The first is a resistive material applied to a film. The second is a set of digitating contacts applied to another film. Figure 5 shows this configuration. The resistive material serves to make an electrical path between the two sets of conductors on the other film. When a force is applied to this sensor, a better connection is made between the contacts, hence the conductivity is increased. Over a wide range of forces, it turns out that the conductivity is approximately a linear function of force ( , ). Figure 6 shows the resistance of the sensor as a function of force. It is important to note the three regions of operation of the sensor. The first is the abrupt transition which occurs somewhere in the vicinity of 10 grams of force. In this region the resistance changes very rapidly. This behavior is useful when one is designing switches using FSRs. Above this region, the force is approximately proportional to until a saturation region is reached. When forces reach this magnitude, additional forces do not decrease the resistance substantially.
Figure 7 shows a plot of conductance versus force for a typical FSR sensor. Notice that the x-axis is now a linear axis, and that above the break-point, conductance is approximately linear with force. It is important to make note of the fact that FSRs are not appropriate for accurate measurements of force due to the fact that parts might exhibit as much as 15% to 25% variation between each other.