Patent GB-550385-A (GB)
Title: Improvements in or relating to devices for indicating strain
IPC: G01M13/00
Filed: 1940-08-21
Abstract: 550,385. Strain gauges. SPERRY PRODUCTS, Inc. Aug. 21, 1940, No. 13307. Convention date, Sept. 9, 1939. [Class 40 (i)] [Also in Group XIX] A strain gauge, e.g. for the supporting members of a rolling mill, comprises a differential transformer which is unbalanced in response to the strain to be measured, the unbalance currents being automatically balanced out by a potentiometer driven by a motor which at the same time drives an indicator and a recording pen. To avoid errors due to temperature changes, it is arranged that the above action takes place only in response to large unbalance signals, smaller ones being automatically balanced out by a second motor-driven potentiometer without any movement of the indicator and recording pen. Construction of sending device.-A bar 16, welded to a suitable part of the supporting structure of the mill, carries an adjustable pin 31 which is held in contact by means of a spring 33 with a hardened portion 32 of a pivoted member 23 forming part of the magnetic circuit of a transformer with a single primary winding 28 and differential secondary windings 29, 30. Changes in strain cause the member 23 to be tilted and so change the amplitudes of the voltages from the secondary windings in opposite directions. Circuit arrangements ; control of indicator and recorder.-The unbalance voltages after amplification by a valve 85 are combined in the input circuit of an amplifier 39 with voltages derived from a balancing potentiometer 56 and a compensating potentiometer 78. A sudden large increase in the unbalance voltage due to the entry of a sheet between the mill rolls causes the operation of a relay 42 in the anode circuit of a valve 86 coupled to the amplifier 39 and the output circuit of this amplifier is thus connected to a shaded pole induction motor 44 which drives the balancing potentiometer 56 until the signal voltages in the input circuit of 39 are balanced out and at the same time drives an indicator and a recording pen. A limit switch stops the motor in the case of excessive deflections. An offnormal contact 64 biases the valve 86 so as to keep 42 energized when the strain is relieved and the phase of the resultant current from 39 is therefore reversed. The motor then drives the indicator and recording pen to zero, the last part of the movement being assisted by a spring-pressed lever acting with a slot in one of the gear wheels. Small unbalance voltages do not operate 42 and are automatically balanced out by the compensating potentiometer 78, which is driven by a motor 76, without affecting the indicator &c. When the apparatus is first switched on, the total change in condition since the last operation may be considerable and to prevent the operation of the indicator in response to this change it is arranged that valve 86 is biased to its inoperative condition during the charging time (say 30 seconds) of a condenser 101. During this period, 42 cannot operate and the unbalance signal is balanced out by the compensating potentiometer. In a modification, Fig. 9 (not shown), the balancing voltage is derived from a variable transformer similar to that in the gauge sending device, but having the pivoted magnetic member rocked by the balancing motor. In another modification, Figs. 10 and 11 (not shown), the function of relay 42 is performed by contacts controlled by the slider of the balancing potentiometer which is straight. When a large signal is received, this slider in moving off normal shunts out the compensator motor and removes a shunt resistance from one of its own windings, this shunt being provided to make it unresponsive to small signals. An additional spring contact is operated near to the end of the return journey to shunt one of the windings of the motor momentarily and give it a kick towards the zero position. This modification also comprises a relay whereby the compensating arrangement may be disabled to assist in the initial adjustment of the mill rolls, the relay preferably being operated at the same time as the motor for screwing down the rolls.