Patent GB-760738-A (GB)
Abstract: 760,738. Electrical indicating systems. DAIMLER-BENZ AKT.-GES. March 26, 1954 [March 26, 1953], No. 8958/54. Class 40 (1). [Also in Group XXXVI] An electrical bridge measuring apparatus, responsive to a range of physical values and phenomena comprises a bridge network energized with an alternating current at carrier frequency and containing a measuring element variable in response to the unknown to be determined so that the bridge unbalance output consists of a signal at carrier frequency which is modulated in accordance with the bridge unbalance and is demodulated to energize an unbalance indicating instrument; the bridge network containing the measuring element being physically interchangeable as a unit with other bridge networks containing measuring elements responsive to other unknowns. In Fig. 1 an oscillator 1 is transformer coupled over a phaseshifting network 2 to energzie an interchangeable measuring bridge shown diagrammatically at B, whose unbalance signal is coupled through a harmonic filter 5 and a variable gain control 6 to tandem pentode amplifiers 4 whose outputs are transformer coupled to a rectifier ring demodulator 10 together with a steady signal from the bridge energization transformer to produce a unidirectional unbalance signal energizing an oscillograph 0 (or a moving-coil instrument or switching relay) through a modulation frequency filter 11. An indicator tube 7 of the " magic eye " type is energized from the screen grid of the record amplifier pentode for initial bridge balancing adjustments, and the phase of the demodulator input signal is compared with that of a winding of the bridge input transformer across a rectifier bridge 15 supplying a moving- coil instrument 16, whereby any phase shift due to the measuring bridge currently in use may be compensated by adjustment of phase shift network 2. Plural alternative measuring bridges may be connected at B by plugs and sockets as required by the quantity to be measured, and plural extensometer resistance elements may be switchably connected in a single exchangeable bridge unit (Fig. 9, not shown). Fig. 2 shows a bridge circuit for measuring a variable inductance 17 which is connected iu one arm in series with phase-balancing variable impedances 18a, 18b, while an adjacent arm comprises magnitude balancing variable impedances 19a, 19b, and the bridge is completed by a capacitance arm across which further impedance may be connected by switch 20, and an arm into which a calibrated resistance box (Fig. 8, not shown) may be connected over terminals 21, 22 to establish initial bridge unbalance to any desired extent. Inductance 17 and capacitance 26 may be interchanged for variable capacitance measurement. Fig. 3 shows a bridge circuit for determination of reciprocally variable inductance, capacitance, or resistance measuring elements 27a, 27b which are connected in adjacent arms of a bridge completed by balancing resistances 28a, 28b, a phasebalancing variable capacitance 29, and a capaci- , tance 30 of half the maximum value of capacitance 29, so that phase balance is attained normally at half the working range of capacitance 29. A range of capacitances 30a to 30x may be selectively inserted across either measuring arm by switch 31 for phase balancing, and the calibrating resistance box may be inserted as before over terminals 21, 22. Fig. 4 shows a circuit for determination of the output voltage of a temperature-measuring thermocouple 38 wherein a ring modulator 32 comprising resistances as well as rectifiers, is normally balanced for magnitude and phase by resistance 33 and variable capacitance 34 in series with a fixed capacitance across the vertical diagonal, and having the capacitance junction connected to the left-hand junction of the horizontal diagonal. The modulator is energized over terminals 35, 36, 37 from a winding of the oscillator output transformer, and the thermocouple voltage is connected over switch 43 between the centre point 36 of the energization winding and that of the primary of the transformer 40 energizing filters 5 and amplifiers 4 (Fig. 1) with an alternating unbalance signal modulated by the thermocouple voltage. Switch 43 enables a known unidirectional calibrating voltage from standardized potentiometer 47 to be substituted for the thermocouple voltage. The indicating instrument 0 (Fig. 1) may be calibrated in degrees centigrade of temperature. Fig. 5 shows a bridge circuit for measuring fluid flow velocities wherein a hot-wire resistance 50 is subjected to the fluid stream is connected in bridge with a magnitude balancing resistance 51, a phase balancing variable resistance 52 shunted by a capacitance 58, and a capacitance 59 in series with which a calibrated resistance box may be inserted over terminals 21, 22. The bridge is energized from the oscillator over a  matching transformer 64 and blocking capacitance 60, and an adjustable unidirectional heating current is applied to the wire resistance 50 over chokes 55, moving-coil instrument 56, and variable resistance 57. The unbalance signal from the bridge is transformer coupled to the filters 5 and amplifiers 4, and the temperature of the hot wire is set to any required value by resistance 57 which may be calibrated in degrees centigrade, after which the bridge is balanced for zero flow. The passage of fluid past the hot wire reduces its temperature (Fig. 7, not shown) and varies its resistance to unbalance the bridge, as a measure of the flow rate. Alternatively balance may be restored by adjustment of the bridge rebalancing resistances, or of the resistance 57 to return the wire to its original temperature, and in each case the readjustment is a measure of the flow rate. Fig. 6 shows a bridge for measurement of resistance strain gauges comprising in adjacent arms a measuring strain gauge 65 and advancing temperaturecompensating strain gauge 66 (replaceable by a fixed resistance for constant temperature measurements) and completely by ganged variable resistance 67 for coarse initial adjustment in series with ganged variable resistances 69 and calibrated switchable potentiometer 68 for fine adjustment. The bridge is excited from the oscillator output transformer and its unbalance signal is applied to the filters 5 and amplifiers 4 (Fig. 1). Initial balance is obtained by adjustment of 67, and after application of strain it is restored by adjustment of 69, 68 which may be directly calibrated in stress values. Dynamic measurement may be carried out by adjustment of 69, 68 to displace the oscillograph trace to a fiducial position. The strain gauges may be inductive or capacitative, with appropriate impedances in the remaining bridge arms. Plural bridge circuits comprising distinct strain gauges may be sequentially connected by switching means. A portable assembly (Fig. 10, not shown) may comprise duplicate interchangeable bridges, oscillation generators, amplifiers, demodulators, and indicators with a calibration resistance box mounted in a common housing, together with a power pack (Fig. 11, not shown) comprising a magnetic mains voltage stabilizer, with barretters and stabilizer tubes for the unidirectional supplies from transformer coupled rectifiers. It is stated that the apparatus is applicable to the measurement of expansion, torque, torsional oscillation, vibration, acceleration, pressure, temperature, and velocity.