Patent GB-1012062-A (GB)
IPC: G01R17/10
Abstract: 1, 012, 062. Wheatstone bridge circuits; measuring electrically. STATHAM INSTRUMENTS, Inc. Feb. 7, 1963 [July 2,1962], No. 4987/63. Headings G1N and G1U. In a Wheatstone bridge circuit a current source in addition to that used for supplying the bridge is connected through a resistance greater than the bridge impedance to at least part of at least one arm of the bridge so that the current normally flowing in the bridge in the unbalanced condition is modified, it being arranged that the modifying current is a function of the bridge unbalance. The bridge may take the form of a resistance strain gauge pressure transducer (see below), the output of which is normally in millivolts. The purpose of the present invention is to deliver an output measurable in volts, whilst at the same time improving the operating stability of the bridge. In one embodiment (Fig. 2) the two opposite polarity outputs from the bridge corners 2 and 3 are applied to two amplifiers 11, 11<SP>1</SP> arranged in push-pull, the outputs from the amplifiers being fed back to opposite bridge corners through the resistors 16,16<SP>1</SP>. The current fed back acts so as to restore the bridge balance, where unbalance existed, so that the difference in outputs from the amplifiers, measured at 17,18 gives a measure of the original bridge unbalance. A similar arrangement using a single D.C. amplifier may be used (Fig. 6 not shown), while instead of returning the amplifier outputs to the bridge output terminals, one of the amplifier outputs may be applied at a power input terminal of the bridge, (Fig. 4 not shown). In a further embodiment somewhat similar to Fig. 2 the outputs from the amplifiers 11, 11<SP>1</SP> are applied to two small fixed resistors added to the bridge at the corner 4 and in series with the variable resistors 7 and 8 respectively (Fig. 3 not shown). Such an arrangement enables temperature effects to be compensated. Instead of such closed loop systems, the bridge may be balanced by applying current from a second battery source 30, 31, Fig. 5 across additional resistors in the bridge as just mentioned. The balance is found in this case by adjusting the potentiometers 27, 28, but alternatively these may be controlled by a servomotor actuated by the bridge unbalance output (Fig. 7 not shown). A mixed bridge wherein two of the arms are fixed inductances and two are variable resistors is described (Fig. 8 not shown), the A.C. output from the bridge being amplified and demodulated, and this demodulated signal then being applied in the required current-assist relation to the resistive arms of the bridge. The resistance strain gauge pressure transducer (Figs. 9,11) consists of a frame mounted in a case 110 and having an armature 106 attached to it by means of leaf springs 108. Sapphire pins are mounted on the armature and corresponding pins are mounted on the frame, with metallic resistance strain wires wound in tension between the corresponding pins to form the Wheatstone bridge. The four corners of the bridge are connected to the output pins 121 while at the other end of the casing a diaphragm 109 is connected via a rod 107 to the armature, so that changes in fluid pressure may be detected.