GB1537154A - Electret sensing devices for temperature and stress variations - Google Patents

Abstract

1537154 Measuring stress and temperature electrically MINNESOTA MINING & MFG CO 3 Feb 1976 04227/76 Heading G1N A temperature or stress sensor comprises two electret layers and is arranged so as to compensate for stress or temperature variations respectively. Temperature sensor Fig. 2 comprises two oppositely poled electret layers 2, 3, having both pyroelectric and Piezo-electric properties, in contact with equal area conducting layers 7, 4, 8 layer 4 being relatively stiff with respect to layers 7, 8. A sensing circuit 10 such as a voltmeter or ammeter is connected as shown to the layer 4 and grounded layers 7, 8. When the temperature of the sensor is varied uniformly a uniform distribution of charge as shown appears on the surfaces of the electrets which is detected by circuitry 10. If any bending of the layers occurs conductive layer 4 serves as a point of zero compression or tension and layer 2 is compressed and layer 3 extended. Due to the Piezo-electric properties of the electrets positive and negative charges on the outer and inner surfaces of layer 3 and negative and positive charges on the outer and inner surfaces of layer 2 appear which cancel out. In an alternative embodiment Fig. 4 (not shown) the two electret layers are poled in the same direction and the sensing circuit is connected across the outer layers 7, 8. Stress Sensor Figs. 5 to 7 (not shown). In one embodiment the arrangement is the same as in Fig. 2 except that the electrets are poled in the same direction. The distribution of charges due to temperature changes cancel out whereas bending of the sensor produces a potential across the sensing circuitry. In the arrangemement of (Fig. 7) the two electrets are poled oppositely and the sensing circuitry connected across the two outer layers.