ASTM F 3291 : 2017

1.1This 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.

1.2An 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.

1.3Special 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.

1.4Materials other than conductive polymers can be used in an MFS and also exhibit reduced resistance with increasing force.

1.5This 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.

1.6This 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.