Patent GB-997200-A (GB)
Title: Temperature compensated transducer
IPC: G01L19/04
Filed: 1961-09-28
Abstract: 997,200. Measuring electrically; dynamometers. S. Y. LEE, and Y. T. LI. Sept. 28, 1961 [Dec. 21, 1960], No. 34937/61. Headings G1N and G1W. The sensitivity of a force sensing transducer is maintained constant by temperature compensating means coupled to a deflectable member, to which the transducer input may be coupled, so as to provide it with a stiffness with respect to deflection which varies as a function of temperature. In Fig. 9, deflectable member 18 carries resistance strain gauges R1 to R4 and the deflection of its free end is opposed by a spring force from temperature compensating means K c . K c  comprises a bimetallic spring 12 with its free ends joined by a deformable member comprising rigid links 14, hinged to the spring and to each other. Spring 12 applies a temperature dependent force to links 14 which transmit the force through link 16 to member 18. It is shown mathematically that by suitable choice of the initial spring force P o  at a reference temperature, temperature-induced changes in the sensitivity of the transducer may be eliminated. It is also shown that by adjusting the length of link 16 temperature-induced zero-drift of the transducer may be eliminated. In Fig. 10 the hinged links 14 are replaced by a flexure bar 20, having reduced portions 28 acting as frictionless hinges. The initial spring force is adjusted by nuts 24, 26, and the length of link 38 is adjusted by nuts 42, 44. Having carried out these adjustments in this order a bias spring 46 is adjusted by screw 48 to bring the transducer output to zero at zero input. In a modification (Fig. 11 not shown) the strain gauges are mounted directly on the flexure bar, zero compensation being provided by a pair of V-shaped wires (such as 91 in Fig. 13) mounted on the flexure, and arranged for bending out of the plane of the flexure to give it an initial deflection. Zero output is then restored by a bias spring. The temperature-compensating means need not comprise an actual spring, and in Fig. 12 (not shown) an armature connected to the deflectable member is arranged between unlike poles of two magnets whose opposite poles are adjacent parts of a cantilever-supported bimetallic member. Variations in temperature alter the air-gaps between the bimetallic member and the magnets to alter the force on the armature. The initial "spring" force is set by adjusting the position of the bimetallic member relative to the magnets. In another arrangement (Fig. 13) a differential transformer type of transducer is temperature compensated. In this arrangement the bimetallic member is also dispensed with, the ends of the link 86 being secured to members 87, 88 which have a coefficient of linear expansion which differs substantially from that of link 86. The initial force is set by adjusting the bowing of members 87, 88 using nuts 90. Zero shift is compensated by bending wires 91 as before. Measuring pressure. When used for measuring high or low pressures, requiring the use of diaphragms much stiffer or weaker than the transducer, a "matching coupling" is provided. For high pressure measurements (Fig. 16) diaphragm D is much stiffer than member 18 so the apparent stiffness is reduced by magnifying the movements of the diaphragm. The arrangement is reversed for low pressures (Fig. 17 not shown). In a high fluid pressure measuring transducer (Fig. 18) fluid is admitted to chamber 112 and acts on bellows 116. The force is transmitted through flexure 118, lever 120, flexure 131, lever 132 and rod 150 to deflectable member 152 carrying the strain gauges. Movement of lever 132 is opposed by flexure 142 and the spring rate of this member is automatically adjusted by bimetallic member 158. Zero-shift is eliminated by adjustment of nuts 151, 153. The elongated deformable member (e.g. 20 in Fig. 10) may be replaced by a disc-shaped member, with the temperature responsible means (e.g. spring 12) applying a radial force at a plurality of points, or completely round the disc. Zero-shift compensation may be provided by an adjustable bimetallic strip acting on the deflectable member. It is stated that the transducer can be adapted to measure any variable which can be converted into a force, for example displacement or acceleration.