Tactile sensor using metal-powder rubber compound

This paper describes a new tactile sensor that uses a metal-powder rubber compound. In the sensor probe, the compound comes into contact with an eddy-current coil supplied with alternating current of 300kHz. When contact forces act on the compound, the volume of the compound decreases in proportion to the magnitude of the forces. The density of the metal-powder increases in proportion to the decrease in the compound volume. Thus, the output signal is proportional to the contact forces acting on the compound, because the eddy-current loss increases with the density of the metal-powder in the compound. We developed two prototypes sensors. The first was a common type using a compound composed of 20% Nickel-iron magnetic alloy and 80% Silicon rubber. We examined the sensitivity, response, and temperature drift characteristics of the common type. Our tests verified that the sensor output was roughly proportional to the contact forces. The gain was constant, and the phase delay was practically non-existent in the DC to 300Hz frequency range. The sensor had a hysteresis because of the visco-elasticity of the Silicon rubber. But the main drawback of the common type was a large temperature drift resulting from changes in the impedance of the eddy-current coil. To reduce the temperature drift, we produced a second prototype we refer to as a differential type. The differential type was identical to the common type in all characteristics except the temperature drift. The temperature drift of the differential type was negligible. We demonstrated experimentally that the sensor can be used to maintain constant contact force between a gripper and a brittle structure such as a glass beaker. Compared with the conductive sensor, new sensor has no trouble with the contact resistance between the compound and the lead wires.