Patent GB-652299-A (GB)
Title: Improvements relating to electric bridge measuring instruments
IPC: G01R27/02
Filed: 1948-02-28
Abstract: 652,299. Dynamo-electric machines. RICHARDSON, R. D. Feb. 28, 1948, No. 6278. Convention date, Jan. 24, 1947. [Class 35] [Also in Groups XXXVI, XXXVIII and XL (c)] In an instrument for indicating and recording a variable such as pressure, draught, flow, temperature, gas composition or the like, an A.C. fed Wheatstone bridge containing an impedance responsive to the variable has variable impedances connected in parallel with its legs to form a normally balanced network which is unbalanced by changes in the variable to develop a signal dependent in amplitude and phase sense on the magnitude and sense of a change in the variable, which is amplified to drive a reversible A.C. motor to adjust two of the parallel impedances to rebalance the network and operate a recorderindicator device. One or more of the resistance arms 27, 28, 29, 30 of a Wheatstone - bridge 26 (Fig. 1), may be varied by a device (not shown), responsive to the variable to be measured, and the bridge is energized through leads 24, 25 from a winding of a mains transformer 18, which also supplies the amplifier 16. The arms 27, 28 are shunted by the resistances 32, 36 in series with the potentiometer 33 the slider of which is returned to the junction of the arms 27, 28, and the arms 29, 30 are shunted by the resistances 41, 44 in series with the potentiometer 46, whose slider is returned to the junction of the arms through the variable resistance 47 and resistances 50, 51, 52, 53. The bridge is normally balanced at the zero of the variable to be measured by adjustment of the potentiometer 33, and an unbalance signal across leads 55, 57 tapped off between the junction of bridge arms 27, 28 and the junction of resistances 51, 52 is dependent in amplitude and phase on the magnitude and sense of the deviation of the variable to be measured from the valve at which the bridge is balanced. The signal is applied to a 3-tube amplifier (Fig. 2) in which the heater of the first tube 92 is fed with a reduced voltage from transformer 18 through the series resistance 97 to stabilize the phase shift in the first stage due to heater voltage fluctuations. The amplifier comprises a coupling transformer 101 tuned to accept only signals at the bridge frequency, and the output tube 102 is cathode coupled to the winding 66 of a two-phase drag cup A.C. motor 64 whose other winding 65 is energized from the mains. The motor (Fig. 4) comprises windings 65, 66, section wound on alternate armature poles, within which a cupshaped rotor 83 of non-magnetic metal rotates on a shaft 84 driving the slider of potentiometer 42. Within the rotor is a concentrically fixed magnetic centre-piece 85 having a bore surrounding the shaft, and when the unbalance signal appearing at the amplifier output energizes coil 66 in an out-of-phase relation with the energization of coil 65, a rotating flux is set up which reacts with the rotating eddy current flux induced in the rotor to rotate it in the appropriate direction to rebalance the bridge and reduce the unbalance signal to zero. The motor 64 is geared to drive a shaft which re-adjusts the tapping of potentiometer 42 (in which the slide-wire is carried by a movable drum and the slider is stationary), and rotates a cam against a bell-crank to actuate an indicating pointer and a recording pen, which moves over a rotary chart (Figs. 11, 12, 13, not shown). The cam is cut to correct for non-linearity of response in the instrument. The variable resistance 47 is adjustable to correct the scale calibration, and the range of measurement is variable by adjustment of the values of resistances 52, 53 or 50, 51. The latter adjustments may be dependent on e.g. pressure, temperature, to give automatic compensation for calibration changes. The device may be unit constructed for interconnection by multiple connectors, and each transmitter unit may be individually precalibrated. The motor may control other apparatus.