A Miniature Electrooptical Force Transducer

We have developed a 5 × 6 × 3 mm electrooptical force-sensing transducer from a metal U-shaped spring. Force compresses the open end of the spring so that it decreases light passing from a light-emitting diode to a light-sensing photodiode. Parts cost is $2. The operating range is from 0 to 50 N, but can be changed by changing the spring metal and dimensions. Nonlinearity is less than ±7 percent. Hysteresis is less than 3 percent. Random error is about 6 percent peak-to-peak. Temperature related zero drift is less than 1 percent/°C uncompensated and much lower compensated. When embedded in a rubber insole, this thin transducer has measured force on the sole of the foot during normal human gait.