A low-power capacitive proximity sensor has been developed as a prototype of wheel-contact sensors for a small robotic vehicle. The sensor is integrated into a wheel and consists of only a few components that add very little mass to the wheel. The output of the sensor serves as an indication of contact with (or proximity of) the ground; this output can be used as feedback for a vehicle control system. An important aspect of the circuit design is the use of capacitive (instead of slip-ring) coupling between the sensor circuit and associated external circuitry; in comparison with a slip ring, a capacitive coupler is more reliable, less electrically noisy, lighter in weight, and less mechanically complex.
The figure schematically depicts the sensor and the associated external circuitry. An application-specific integrated circuit (ASIC) generates a 1/2-duty-cycle square wave with a repetition frequency of 12.5 MHz. The square-wave signal is low-pass filtered to obtain a 12.5-MHz pseudo-sine wave for excitation of the sensor.
The sensor includes an inner aluminum foil ring covered by a layer of quartz glass fibers, covered in turn by an outer aluminum foil ring, over which are wrapped a ring of polyimide sheet and then a narrower ring of metallized polyimide sheet. The innermost aluminum foil ring acts as electrical ground. The outer aluminum foil ring acts as a driven shield, which is excited in phase with the sensor electrode described next. The metal layer on the metallized polyimide sheet serves as the capacitive sensor electrode, which is part of an inductor/capacitor/resistor circuit that resonates at a frequency of 11 MHz in the absence of nearby objects.
Proximity of the ground or an obstacle manifests itself through the effect of the ground or obstacle material on the capacitance of the sensor electrode. Typically, contact with the ground or an obstacle causes the capacitance to change by an amount between 0.3 and 1.3 pF. The change in capacitance causes a change in the resonance frequency and thus a change in signal amplitude along the slope of a voltage-vs.-frequency resonance curve at a suitable measurement point in the sensor circuit. The voltage is sampled at the measurement point and converted to a dc output voltage, which is then digitized for further processing by the vehicle control system.
This work was done by R. Scott Cozy, Brian Wilcox, and Mike Newell of Caltech and John Vranish of Goddard Space Flight Center for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Electronics & Computers category.