Patent GB-538761-A (GB)
Title: Improvements in and relating to control, indicating or recording systems incorporating electrically controlled servo motors
IPC: G08C19/00
Filed: 1940-02-09
Abstract: 538,761. Electric control systems. DAVIS, N. R., and SUN-VIC CONTROLS, Ltd. Feb. 9, 1940, No. 2564. [Class 40 (i)] [Also in Group XXXV] A reversible servomotor is electrically controlled by a transmitter which periodically varies the input to a relay having a movable system oscillating about a mean position due to the periodic change of input to effect forward and reverse drive of the motor, the average input from the transmitter to the relay being changed by a controlling quantity to change the mean position of the movable system of the relay. The motor may restore the mean position of the movable system and simultaneously position a device in correspondence with the controlling quantity or the motor may maintain the quantity constant or within certain limits. The controlling quantity may be positional, electrical, thermal or fluid pressure. As shown, the circuits at the transmitter and of the relay comprise contacts controlled by thermal devices, but the periodic change at the transmitter may be effected by charging and discharging of a condenser and contacts may be replaced by capacitative, inductive or magnetic coupling. The motor may also control the transmitter or relay by auxilary windings on the thermal devices or by auxiliary thermal devices. Piezo-electric crystals may be used in place of thermal devices. In the arrangement shown in Fig. 1, the transmitter comprises a member 4 moved by the controlling quantity and a bi-metal strip 5, and the relay comprises a bi-metal strip 10 controlling the reversible motor 8. The windings of the strips 5, 10 are connected in series,and the strips bend downwardly on heating so that the circuit is periodically opened at contacts 3, the movements of strip 5 being followed by strip 10 which has substantially the same time constant. The motor, in addition to controlling a load or the original variable resets the strip 10 to the normal mean position by means of screwed shaft 18 and lever 11. Alternatively Fig. 2 (not shown), the motor may control a variable resistance in the heating circuit to restore the setting. In another modification, Fig. 3 (not shown) the position of the transmitting strip 5 is controlled by the variable, whereas in a further arrangement, Fig. 6 (not shown) the contact engaged by strip 5 is fixed so that the make and break ratio of the contacts 3 is constant. The power input circuit to the relay 10 is an independent circuit the input being regulated by the controlling variable but the strip 10 is caused to follow the movements of strip 5 by an auxiliary relay controlled by contacts 3 and intermittently opening the circuit of relay 10. In other modifications, Figs. 4 and 5 (not shown), the relay 10 is constituted by two bimetal strips one controlling the forward drive and the other the reverse drive of the motor through snap-action switches such as those described in Specification 419,917, [Group XXXVII]. When the arrangements are used for automatic control of a variable, additional compensating movement may be applied to the transmitter as described in Specification 538,118, [Group XXXVII].