Hand Hygiene

One of the main features of the new puck design is to incorporate a force sensitive resistor to bypass the use of spring (which requires calibration/tuning). Ideally, we would like to use a component that:

• is available in a wide variety of shapes and sizes
• is force-dependent rather than pressure-dependent
• has a linear relationship between resistance and applied force
• has a dynamic range appropriate to our application
• is relatively low-cost

An FSR that is force-dependent rather than pressure-dependent implies that the behavior of the component is not influenced by the footprint of the force acting on it. This means that the component could be used in a wider variety of applications, as we do not need to be concerned with how we are actuating it.

An FSR that provides a linear relationship between applied force and resistance may ease signal processing. This is speculative on my behalf, because I don’t know enough about the signal processing that would be required to essentially turn it into a reliable programmable switch.

The FSR’s produced by Interlink have a response characteristic that is described in this datasheet. To paraphrase the datasheet, the high end of the dynamic range is marked by a ’saturation point’ at which the decrease in resistance with respect to increased force (more specifically, pressure) tends to zero (in other words, how much pressure is applied before the sensor quits sensing). Therefore, the saturation point must be at least equal to the maximum pressure we would apply in application, but too much, and we lose resolution. The Interlink datasheet quotes a saturation point on the order of 100-200psi. I have roughly estimated the force to actuate the Avant/Avagard sanitizer bottles to be somewhere around 39-59N (8.8-13.3lbf). If we assume that a nurse in a hurry might apply up to 25lbf to the pump, the minimum sensor size to avoid over-saturation is 0.25in^2. Interlink offers three premade FSR’s: 4mm diameter (~0.01in^2), 0.5″ diameter (~0.79in^2), and 4×4cm (~2.5in^2).

Davie had much more luck with sourcing than I did. Initially, I only found “good” technical information from Interlink, but outlined in the blog post below are other sources that Davie found:

http://static1.shopify.com/s/files/1/0010/4352/files/Electrotap-t303c.pdf

http://sensitronics.com/characteristics.htm

http://www.tekscan.com/flexiforce/specs-flexiforce.html

It seems that, for the most part, the FSRs available from these manufacturers have comparable size/shapes (4-12mm diameter round, 0.5-1.5″ square), response ranges (1-150psi), and prices.