Significance and Use
4.1 An MFS has similar properties to a load cell or strain gauge. However, an MFS is not suitable for precision measurements.
4.2 MFS pressure versus resistance data can be calculated if the force probe is providing uniform pressure over a distributed area or if the sensor is exposed to measurable air or hydraulic pressure.
4.3 MFS force-resistance plotted results are not linear and results may change when exposed to repeated force cycles – Test Method F1578. It is useful to note that the force-resistance curve models closely to mathematical form of y = 1/x.
4.4 Static forces may contribute to drifting test results (also known as creep).
Scope
1.1 This test method covers the force versus resistance measurement of a membrane force sensor (MFS) where the electrical resistance decreases as the force on the sensor is increased.
1.2 An MFS may or may not be electrically open in its static state. This depends on the attributes required for the application. If the MFS has a measureable resistance in static state, it was most likely designed to be used as a variable resistor, not as a normally open switch. A high but measurable resistance, in static state, may still be considered an open switch if the resistance is above the closed resistance threshold recognized by the interface electronics.
1.3 Special printed conductive polymer inks or characteristics, or both, of the sensor design are used in MFS to achieve variable resistance when compressed. As force is applied to the MFS the resistance continues to decrease, but not linearly, until a point where additional force does not change the resistance appreciably. Ideally, when force is removed from the MFS the resistance will return to, or close to, its original value.
1.4 Materials other than conductive polymers can be used in an MFS and also exhibit reduced resistance with increasing force.
1.5 This test method should not be confused with Test Method F2592 for measuring the force-displacement characteristics of a membrane switch (MS) that is designed for momentary closure. Although the resistance of a MS does change during contact closure the change from high resistance to contact resistance is very sudden and additional force does not have a significant effect on the resistance; that is, an MS is not designed to be used as a variable resistor.
1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.