GB1472294A - Strain measuring device - Google Patents

Abstract

1472294 Semi-conductor stress transducer WELWYN ELECTRIC Ltd 18 Dec 1974 [28 Jan 1974] 03760/74 Heading G1N [Also in Division H1] The diffused semi-conductor Piezo-resistive elements of a Wheatstone bridge for measuring mechanical stress are so formed that the bridge, when mounted under its intended operating conditions, has a temperature coefficient of resistance within Œ 1% of a value α and a temperature coefficient of strain sensitivity (defined in the Specification) within Œ 3% of a value # where α + # + α##T = 0, #T being the range of temperature over which the bridge is adapted to operate, whereby the electrical output of the bridge for a given mechanical stress changes by no more than Œ 0À03% per ‹C over the temperature range #T. All, or only two, of the four bridge resistors may be semi-conductor Piezo-resistors, formed either in individual semi-conductor chips or in a common chip 46 as shown. Conventional B or P diffusion into Si is used to form the Piezo-resistors, the preferred method being described. In the structure illustrated a gap is left between the ends of adjacent Piezo-resistors 37, 38 to permit an external resistor 44 to be inserted in series with the Piezo-resistor 37 to reduce the output voltage appearing across the bridge in the absence of applied stress. A further external resistor 45 across the Piezo-resistor 38 reduces the temperaturedependence of any residual zero-stress output voltage, and in an alternative arrangement for achieving the same end a variable resistor 19 Fig. 5 (not shown), is used with its wiper connected to an output terminal of the bridge and its ends connected across the bridge input terminals.