engleski

A pulse driven low-power capacitive force transducer

A low power, two layer capacitive transducers for measurement of vertical force during gait are developed. The foamy rubber is used as a dielectric in the capacitive sensors because of its good linearity and low hysteresis. Three such sensors are placed under the shoe sole on each leg. To reduce the power consumption constant current short pulses with low duty cycle (1:40) were applied instead of an A.C. bridge with continuous sine wave power supply. That resulted in reduced current consumption for approximately twelve times (for 6-channel unit from 14mA to 1.2mA). Also, instead of using two 9V batteries for symmetrical supply, asymmetrical supply with one 9V battery was used. During the pulse, a ramp voltage, as a consequence of the sensor� s capacitance charging with a constant current, rises depending on the applied force. Close to the end of the pulse, a sample, inversely proportional to the applied force, is taken and kept by a sample&hold circuit. Micro-power operational amplifiers with very low current consumption were used for signal conditioning, so the output voltage of the transducer is calibrated and proportional to the applied force.

gait; vertical force; energy saving; capacitive sensor; current pulses; low-duty cycle

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