User Guide for FSR Sensor

Page 8 FSR Integration Guide and Evaluation Parts Catalog
with Suggested Electrical Interfaces
FSR Integration Notes
A Step-by-Step Guide to Optimal Use
For best results, follow these seven steps when beginning any new product design, proof-of-concept,
technology evaluation, or first prototype implementation:
1. Start with Reasonable Expectations (Know Your Sensor)
The FSR sensor is not a strain gauge, load cell or pressure transducer. While it can be used for dynamic
measurement, only qualitative results are generally obtainable. Force accuracy ranges from
approximately ± 5% to ± 25% depending on the consistency of the measurement and actuation system,
the repeatability tolerance held in manufacturing, and the use of part calibration.
Accuracy should not be confused with resolution. The force resolution of FSR devices is better than
± 0.5% of full use force.
2. Choose the Sensor that Best Fits the Geometry of Your Application
Usually sensor size and shape are the limiting parameters in FSR integration, so any evaluation part
should be chosen to fit the desired mechanical actuation system. In general, standard FSR products have
a common semiconductor make-up and only by varying actuation methods (e.g. overlays and actuator
areas) or electrical interfaces can different response characteristics be achieved.
3. Set-up a Repeatable and Reproducible Mechanical Actuation System
When designing the actuation mechanics, follow these guidelines to achieve the best force repeatability:
• Provide a consistent force distribution. FSR response is very sensitive to the distribution of the
applied force. In general, this precludes the use of dead weights for characterization since exact
duplication of the weight distribution is rarely repeatable cycle-to-cycle. A consistent weight (force)
distribution is more difficult to achieve than merely obtaining a consistent total applied weight
(force). As long as the distribution is the same cycle-to-cycle, then repeatability will be maintained.
The use of a thin elastomer between the applied force and the FSR can help absorb error from
inconsistent force distributions.
• Keep the actuator area, shape, and compliance constant. Charges in these parameters significantly
alter the response characteristic of a given sensor. Any test, mock-up, or evaluation conditions
should be closely matched to the final use conditions. The greater the cycle-to-cycle consistency of
these parameters, the greater the device repeatability. In human interface applications where a finger
is the mode of actuation, perfect control of these parameters is not generally possible. However,
human force sensing is somewhat inaccurate; it is rarely sensitive enough to detect differences of less
than ± 50%.
• Control actuator placement. In cases where the actuator is to be smaller than the FSR active area,
cycle-to-cycle consistency of actuator placement is necessary. (Caution: FSR layers are held
together by an adhesive that surrounds the electrically active areas. If force is applied over an area
which includes the adhesive, the resulting response characteristic will be drastically altered.) In an
extreme case (e.g., a large, flat, hard actuator that bridges the bordering adhesive), the adhesive can
present FSR actuation