Patent GB-586229-A (GB)
Title: Improvements in or relating to equipment for use with electrical resistance strain gauges
IPC: G01L1/22
Filed: 1943-11-29
Abstract: 586,229. Resistance measurements ; determining dimensions of materials. AUGHTIE, F. Nov. 29, 1943, No. 19865. [Class 37] In equipment for use with electrical strain gauges which incorporates a sensitive indicating or recording instrument having a D.C. movement a circuit breaker is provided, either in the circuit of the instrument or of the circuit supplying power, which is of electronic type or is arranged to operate before the instrument circuit is completed elsewhere, the breaker in either case automatically opening before the instrument can be damaged when a condition exists which would seriously overload the instrument. The circuit breaker may be operated by the aid of a valve amplifier or may itself be a thermionic valve or gas discharge tube. Fig. 1 shows an arrangement for protecting a galvanometer from excessive voltage due to an open circuit in the gauge itself such as may occur if the specimen or structure is tested to destruction by applying a progressively increasing load. The main gauge M and temperature compensating gauge C form two arms of a bridge, the other two arms comprising resistances R1, R2. The bridge is energised from a battery B1 through a valve V2 and an additional resistance R3. A second valve V1 is also supplied through a resistance R4 from the same source. The grid of the valve V2 is connected to the junction of two resistances R5, R6 connected between the anode of the valve V1 and the grid bias battery B2. The grid of the valve V1 is similarly connected to the anode of the valve V2 with resistances R7, R8. The trigger circuit so formed has two stable states, in one of which the valve V2 is conducting whilst the valve V1 is non-conducting owing to its grid being at a considerable negative potential and in the other of which the reverse condition obtains. The change from one condition to the other can be effected by an impulsive voltage applied at some point in the circuit. A small condenser C<1> in parallel with the resistance R5 increases the rapidity of change from valve V2 conducting to valve V2 non-conducting. In the working condition the valve V1 is non- conducting and the ratio of the resistances R5, R6 is adjusted so that the'valve V2 passes the desired bridge current, the ratio of the resistances R7, R8 being made such that the valve V1 is only just non-conducting. The grid of the valve V1 is connected through a condenser C2 to the junction of the gauges M, C. If an open circuit develops in the gauge M the junction C, M becomes more positive and causes the valve V1 to become conducting. The potential of the anode falls and is communicated to the grid of the valve V2, thereby cutting off the current through this valve and the bridge. The anode potential of the valve V2 then rises and this, through the resistances R7, R8, keeps the potential of the valve V1 such that it continues to conduct grid current to the valve V1 is reduced by a resistance R9. The galvanometer G may be protected against momentary high voltage by a condenser in shunt and an inductance in series. To reset the circuit by bringing the valve V2 back to the conducting condition the connection from its anode to the grid of the valve V1 is broken by depressing a key K1. Alternatively, the anode cathode or screen grid connection to the valve VI can be broken or the resistance R8 may be short circuited. When using a single source of power such as rectified A.C. to replace the batteries B1, B2, cold cathode gas discharge tubes may be used as voltage regulators to provide anode, screen grid, and grid bias potentials. Fig. 3 shows an arrangement in which two separate bridges are supplied from the same circuit, and both galvanometers are protected. Valves V4, V6 correspond to the valve V2, Fig. 1, and have their grids connected in parallel. Valves V3, V5 correspond to the valve V1 and have their anodes in parallel. Alternatively, each of the valves V3, V5 may have its own anode resistance, the anodes being coupled through condensers. To permit individual adjustment of the grid potential of these valves, each has an assembly of resistances corresponding to R7, R8, R9, Fig. 1. Resistances R12, R13 may'be included in the cathode circuits of the valves V4, V6 to permit individual adjustment of the current in the separate bridge circuits. Cold cathode gas discharge tubes T5, T6, T7, T8, T9 provide the necessary valve potentials. The trigger circuit may be constituted by an additional valve in conjunction with the valves V3, V5, the grid of this valve being in parallel with the grids of the valves V4, V6. More than two bridge circuits may be employed, but if beyond a certain number are used, a cathode follower may be inserted between the resistance network and the parallel connection of the grids of the circuit-breaker valves. Instead of having a separate tripping valve for each circuitbreaker valve a pair of trigger valves may be coupled to the circuit-breaker valves through a cathode follower, the switching impulses from the bridge circuits being transmitted to the trigger pair through buffer valves. When it is not desired to make individual adjustments of the current through the various bridge circuits a single circuit-breaker valve may supply a number of bridge circuits in series or parallel. Where both gauges are mounted on the specimen or structure, as in measuring bending or shear, it is necessary to protect the galvanometer against failure of either gauge. This may be effected by inserting between the junction of the gauges C, M and the input to the trigger circuit a device which transmits a pulse of the correct polarity irrespective of the sign of the change of potential of the junction. One such arrangement is shown in Fig. 8 where a valve V29 has resistances in both anode and cathode circuits and both anode and cathode are connected through condensers C6, C7 and rectifiers V30, V31 to the input to the trigger circuit. Another arrangement is shown in Fig. 9 where a second condenser C9 and an inductance L2 are added to the circuit of Fig. 1. The values of the inductance L2 and condensers C8, C9 are such that the circuit is oscillatory when the damping is reduced by the open circuiting of the gauge C. On failure of the gauge M the circuit operates as that of Fig. 1. On failure of the gauge C the potential of the C, M junction falls, making the grid of the valve V32 still more negative. This is without effect as this valve is already biassed to cut off. Theimpulse, however, sets the circuit L2, C8, C9, R1 R2, M in oscillation and the second half-cycle of its oscillation carries the grid of the valve V32 positive relative to its initial condition so that the trigger circuit is tripped and cuts off the current through the valve V33. Gas discharge triodes may be used either as trigger valves or as circuit-breaker valves. As shown in Fig. 11, a trigger pair of valves V36, V37 is supplied separately by a battery B4. Two gas discharge triodes V38, V39 operate as a full-wave rectifier the output of which is smoothed by a choke L3 and condenser C9. Connection to the trigger valve V36 is made through a condenser C10. To prevent stray induced voltages from causing premature tripping a filter may be added. Fig. 14 shows an arrangement in which faults existing initially are also protected against. A key K3 has, besides the central position shown, a locking and a non-locking position, in the former of which a resistance R38 is shortcircuited and in the latter the key operates as the key K1, Fig. 1, to set the circuit. The resistance R38 is so chosen that with either of the gauges C, M short-circuited or opencircuited the galvanometer is not damaged. The condenser C14 is connected directly to the galvanometer lead so that the circuit is immediately tripped if the key is thrown to the locking position whilst the main gauge is opencircuited. A second key of the doublelocking type may be so connected that in the central position the sensitivity of the galvanometer G is reduced for preliminary adjustment, whilst the two locking positions give full sensitivity with reverse connections of the galvanometer. The resistances R7, R8, Fig. 1, are replaced by three resistances R30, R31, R32, of which R31 has a sliding contact for adjusting the grid potential of the valve V45. The resistances R5, R6 are similarly replaced by three resistances R34, R35, R36. The ratio arms R1, R2 are replaced by a series connection of resistances R22, R23, R24, the resistance R23 having a sliding contact for initially balancing the bridge within a certain range, of the ratio of the gauges M, C. The resistance R23 is shunted by two resistances R25, R26 in series. Instead of separate galvanometers being connected to the bridge circuits, a single galvanometer may be connected to each circuit in turn. In this case the switch may be fitted with an additional exploring contact arm placed ahead of the main contact arm so that abnormal potentials are communicated to it before the main contact arm reaches the corresponding stud. This exploring arm can be connected to a relay which keeps open the circuit of either the galvanometer or the bridge supply. Specifications 477,392 and 586,268 are referred to.