MICROFABRICATED VARIABLE CAPACITANCE FORCE SENSORS

C. Cristalli*, C.R. Edwards and M.R. Neuman
Case Western Reserve University, Cleveland, OH, USA
* Instrumentation Laboratory, Milano , Italy

INTRODUCTION Variable capacitance force sensors have been designed employing a compliant dielectric structure using thick- and thin- film technology. The resulting thin sensor can be used in many applications including electronic force measurement instrumentation and biotelemetry devices. Different geometries have been developed for a variety of applications utilizing single and multielement devices.

METHODS The sensing element consists of a parallel plate capacitor with a compliant, elastic dielectric material. As a force normal to the plates is applied, their separation decreases and the capacitance increases. The sensors consist of thin, gold films on flexible polyimide substrates that serve as the plates. These are separated by narrow strips of silicone elastomer deposited on top of each plate using thick-film techniques. Sensor capacitance is measured by sampling the voltage at the end of a fixed charge pulse applied to the structure at regular intervals. Stray capacitance effects are minimized by locating an isolation amplifier or multiplexer-amplifier on or near the sensor. Fabricated sensor structures include: (1) a single element 10 mm diameter sensor, (2) a linear array of eight 7 mm square elements, and (3) an 8x8 square array of 2 mm square elements. Sensors were tested and calibrated element by element using a special test fixture that applied weights of differing mass over the active surface area of that element. They were characterized in terms of sensitivity, stability, reproducibility, response time, hysteresis and temperature stability. Special circuits or algorithms were developed to minimize crosstalk in multielement arrays and to determine total force over the array.

RESULTS Single element sensors had a range of 0 - 10 N, and multielement sensors had a range of 0 - 5 N for each element. All sensor systems were linear over this range, although hysteresis was present. The sensitivity and baseline capacitance from one sensor to the next could vary because the thick-film process does not have tight tolerance on dielectric film thickness. Sensors have been applied to measure grasping force, contact force for limb length measurement, and to determine interfacial pressure distributions between a sphygmomanometer cuff and the arm.

DISCUSSION An advantage of using microfabrication film technology in constructing these sensors is that very thin structures can be built. This makes it possible to intersperse the sensor between a sphygmomanometer cuff and the arm or a finger tip and the object being grasped with only minimal effect on the system due to the presence of the sensor. The use of polyimide substrates allows the sensors to be flexible although sensor flexure results in a shift in baseline force. The use of microfabrication technology opens the possibility to economically batch fabricate these devices.