Patent GB-1003411-A (GB)
Title: Mechanical-to-electrical signal transducer and electrical measuring and control systems comprising the same
IPC: G01R17/02
Filed: 1961-08-22
Abstract: 1,003,411. Measuring electrically; potentiometric voltage measurement. HONEYWELL Inc. Aug. 22, 1961 [Aug. 23, 1960 (2)], No. 30310/61. Headings G1N and G1U. [Also in Division G3] A mechanical-to-electrical signal transducer comprises a motion reducing transmission mechanism for transmitting a motor drive to an angularly movable member which has connected to it at least one resistance wire strain gauge. As described, the transducer is used for positional feedback purposes in a servo system is preference to a conventional potentiometer. In the self-balancing measuring and recording system shown in Fig.2, an input voltage applied at terminals 5, 6 is compared with a balancing voltage derived from a bridge circuit 1, the arms 9, 10, 11,12 of which comprise strain-sensitive resistors, the bridge being energized with A. C. or D. C. as appropriate from a power supply 2. The comparison may be effected by applying the input and balancing voltages in series opposition to the input terminals of an amplifier 4 or, as shown, the voltages to be compared may be connected alternately via an A. C. energised chopper 36 to the amplifier 4 via a capacitor 46. In either case, the amplified error signal is supplied to a servo-motor 3 which operates to balance the system by varying the output of the bridge circuit 1, this is effected by varying the degree of strain in resistors 9 ...12,the strain being increased in two oposite arms of the bridge and decreased in the other two arms. Servomotor 3 also drives the pointer 29 of an indicator 30 and a pen 31 co-operating with a recording chart 32. The span of the apparatus may be adjusted by potentiometer 22, and zeroising is effected by adjustment of potentiometer 33. Transducer construction. In Fig. 3, strain gauge resistors 9 ... 12 are bonded to a cantiliver 47 near its mounting 48, and the rotation of shaft 49 of motor 3 is geared down by pulleys 50 54 and cable 51 and applied via a resilient cable 56 and linkage 59, 61 to deflect cantilever 47 thus varying the tension of the resistors 9 and 12 in the opposite sense to that of resistors 10 and 11. Fig. 4 shows an alternative transducer (see also Fig. 6) in which strain gauge resistors 9 ... 12 form U-shaped loops stretched between a cylindrical member 65 and the base of a rigid column 62 of generally H cross-section. The member 65 is mounted on a cruciform shaft 66 which is rigidly connected to pillars 63, 64 at the top of column 62, and is subjected to a torque in accordance with the rotation of an input shaft 88 via a sector 84, a flexible band 86, and a second sector 81 secured to member 65. Electrical connection to the strain gauges is made via terminals 73 ... 79 on an insulating terminal plate 80. The input shaft 88 is driven by the motor shaft 49 through gearing and a pulleys and cable arrangement. The gears and pulleys being mounted on a housing containing the transducer and the motor. (Fig. 5 not shown). Temperature measurement and control. Fig. 6. The output of a measuring thermocouple 99 is compared with the output of the balancing bridge circuit, the error signal being passed via a filtering and damping circuit 101 to amplifier 102 whose output supplies the control winding 108 of a two-phase servomotor 3. The feedback linkage 7 from the motor 3 operates control apparatus comprising a potentiometer 148 and two relays 155, 156 each powered through a manually adjustable potentiometer contact 150 or 151, and a further potentiometer contact 149 controlled by motor 3. When the temperature sensed approaches the limit represented by the setting of contact 150 or 151 the input to the related relay falls and at the predetermined value of temperature the relay operates to control heating or cooling apparatus to maintain the temperature within the set limits or to operate an alarm. A resistor 159 is shunted across the thermocouple so that, on failure of the latter, a high error signal drive the indicator pointer to its upper limit. The thermocouple voltage is compensated for variation of cold junction temperature by device 100 shown in detail in Fig. 7, which comprises a temperature sensitive resistor 121 wound on thermally connected, electrically insulated terminals 5, 6 which, being relatively massive and made of material of good thermal conductivity form a heat sink for the suppression of temperature transients. A regulated constant current flows through resistor 121. Voltage regulation and temperature compensation. Power supply circuit, Fig. 9, comprises an input comprises an input transformer 228, diode rectifiers 232, 233, and a filter capacitor 234, which produce a full-wave rectified D.C.between conductors 248, 249. Zener diodes 235 and 245 (Fig. 6) co-operate with resistors 238, 239 to form a cascaded two-stage voltage regulator, the output of which is fed via cascaded emitter-follower transistors 236, 237 to the bridge 1. Temperature variation in the transistors 236, 237 is compensated by using one NPN transistor and one PNP transistor. Zener diode 245 is kept in good thermal relation with the bridge 1 to cause the output of the power supply circuit so to vary with ambient temperature that the bridge energising current is kept constant; the tendency of 245 to over-correct is compensated by a resistor 112 of selected positive temperature coefficient of resistance which is also maintained at the temperature of bridge 1. Modifications. In Fig. 6, any desired suppression (i.e. the number of measuring units by which the electrical zero is displaced from the lowest point of the indicator) may be obtained by selection of the value of resistor 117. Figs. 11 ... 14 (not shown) depict modified circuits for measuring the resistance of a resistor, e.g. a resistance thermometer element, connected across the signal input terminals, by comparing the voltage drop across the resistor with the balancing voltage from the bridge. Voltage regulation is not required in this case since both bridge and resistor are supplied from the same source. Measuring the output of a D. C. tachogenerator is mentioned.