Why LowDrift FSR?

Emerging "smart shelf" technologies simplify inventory management, help detect theft, and improve customer experience. Force Sensing Resistors (FSRs) are often considered for use in these applications because of their low cost, high dynamic range, and ability to precisely identify product footprints, but drift can pose a significant challenge.

All Force Sensing Resistors drift, or become more conductive over time, under a fixed load. This effect can be very significant. The conductance of some FSRs will increase by more than 30% in 48 hours. Such high rates of drift complicate the use of FSRs in scale-type applications, requiring sophisticated sensor models and heuristics. Sensitronics' LowDrift FSR ink aims to make FSRs more applicable to these types of applications by reducing drift to levels that are easily managed, or in some cases even ignored.

The charts below provide a snapshot of our LowDrift FSR technology. At present there are several families of LowDrift FSR ink. We're highlighting those that exhibit excellent drift characteristics at low pressures, such as those likely to be observed on a smart shelf. The first chart relates the logarithmic drift rate to pressure for the selected LowDrift FSR inks as well as our most mature FSR inks, collectively identified as Standard FSR:

It should be noted that there are two basic types of FSR assembly represented here: The LowDrift #2 print is assembled over etched copper conductors on a rigid FR4 PCB while the others are assembled over screen-printed silver conductors on PET. It has been observed that drift rates are generally somewhat lower with etched copper than with screen-printed silver. We are presently testing our LowDrift FSR inks with other conductor types as well as various thicknesses of conductor to determine the optimum combinations for both flexible and rigid sensors.

For a given period of time, the total drift can be estimated using the following formula, where driftlog is the logarithmic drift rate and t is the duration in seconds:

drifttotal = driftlog * log10(t)

The next chart plots this total drift over 48 hours for each of the selected LowDrift FSR inks and Standard FSR. It can be seen that under a constant pressure of 2 PSI, our Standard FSR drifts almost 12%. LowDrift #2 drifts only about 1.2%, or 90% less than Standard FSR:

Note that these drift percentages describe a change in the conductance, not the resistance, of the sensor. FSR characteristics are commonly described in terms of resistance, however the resistance of an FSR has a logarithmic relationship with the applied force. This logarithmic relationship is one of several reasons why circuits incorporating FSRs, and especially those used for collecting data from FSR matrixes, employ transimpedance amplifiers to instead measure the current passing through the FSR, and hence the FSR's conductance. Consequently drift is also observed as a change in conductance. The conductance of an FSR generally has a much more linear relationship with applied force.

FSRs come in a variety of types, chiefly ShuntMode, Hybrid, and ThruMode. ShuntMode and Hybrid parts are generally the most cost-effective and the easiest to integrate, owing to construction that requires only a single circuit layer. The graphs above were generated using data gathered from these types of FSRs. ThruMode, which has two opposing circuit layers, tends to require higher quantities of conductive inks but is particularly well suited to high-density matrixes (sensors can be as small as the intersection of two circuit traces). In our testing, ThruMode parts benefited less from LowDrift inks, showing improvements of approximately 50% (total drift over time) versus 75%-90% for their ShuntMode and Hybrid counterparts. Given these factors, we would suggest evaluating ShuntMode and Hybrid FSRs first unless your project requires especially high sensor densities.

We welcome the opportunity to answer any questions you might have. This is a significant and ongoing R&D effort within Sensitronics, and we'll provide additional updates as the work continues.

Pictured above: Evaluation kits will soon be available for LowDrift FSR. These will include a 1x1ft rigid FSR matrix with approximately 1000 sensing points (bottom), and a larger 48x21in flexible FSR matrix with approximately 4000 sensing points (top). Electronics and software for Microsoft Windows will be included. For pricing and availability, email info@sensitronics.com.