Ultra-thin force sensors - the essentials

Whether measuring force-feedback mechanically by machine, or manually, printed force sensors are an excellent method to embed force-sensing capabilities without sacrificing size or adding excess weight.

While the applications are extensive, these are four of the most common ways to use printed force sensors in an embedded application:
- Detect and Measure Relative Change in Force or Applied Load
- Detect and Measure Rate of Change in Force or Applied Load
- Detect Contact and/or Touch
- Identify Force Thresholds to Trigger an Action

Even applications in hot and humid environments are solvable with printed force sensors. Tekscan has developed advanced formulas for our conductive materials that will optimise sensor performance for extreme temperature and humidity applications. These formats are available as standard, off-the-shelf sensors, or are available for your custom sensor design.

Despite being thin and flexible, printed force sensor technologies withstand and maintain performance in some of the most rigorous operating environments. Flexibility in circuitry selection, and adjustable sensitivity, are two of the key benefits of using printed force sensors in an embedded design.

Adjustable sensitivity can be implemented by using a potentiometer, or a Digital-to-Analog Converter (DAC) to adjust the reference voltage. For a given sensor, you can increase or decrease the measurement range by adjusting reference voltage and/or feedback resistance.

FlexiForce sensors have such a wide dynamic range that they can capture much greater forces simply by changing the reference voltage and/or adjusting the gain of the circuit. Any circuit that presents a change in resistance may be applicable - Tekscan recommends an inverting op-amp, dual source circuit.

While there are very precise embedded force-sensing technologies available, not all applications necessarily require a high-level of precision, especially when direct and indirect costs are taken into consideration.

Printed force sensors – otherwise known as force-sensitive resistors, or FSRs – are available in two formats: thru mode and shunt mode. Tekscan manufactures thru-mode sensors because of the operational properties and benefits they offer.

Thru Mode Sensors

Ultra-thin and flexible printed circuits consist of two flexible substrates (polyester film). Silver circles with traces are positioned above and below a pressure-sensitive layer. Conductive polymer is applied on each layer, followed by a pressure-sensitive layer. Adhesive is used to laminate the two layers of substrate.

Shunt Mode Sensors

Polymer thick-film devices consisting of two membranes separated by a thin air gap. Membrane 1 has two sets of interlaced traces electronically isolated from one another. Membrane 2 is coated with a special textured, resistive ink.

In both of these sensor types, when force is applied, there is a drop in resistance that is inversely proportional to the applied force. They have high resistance (around 2Mohm) that drops when loaded. Conductance (the inverse of resistance) as a function of force is linear within the sensor’s designated force range. The fact that they are passive, have linear conductance responses, and have a very large dynamic range of resistance, allows the design engineer to use simpler electronics that do not require much filtering.