Patent GB-606899-A (GB)
Title: Improvements in limit stop apparatus for controlling a controlled body
IPC: G05D3/14
Filed: 1945-08-02
Abstract: 606,899. Control of motors. SPERRY GYROSCOPE CO., Inc. Aug. 2, 1945, Nos. 19935 and 19936. Convention dates, Aug. 2, 1944, and Aug. 22, 1944. [Class 38 (iii)] An electric motor is adapted to be driven, when within a zone of normal control, in a sense and at a rate dependent upon a control signal governing the supply of current to the motor, and a limit switch, which operates when the motor reaches the extremes of said zone, effects such changes in the motorsupply circuit that the motor is rapidly braked and brought to a standstill, while the control signal is operative, if it reverses, to energize the motor and drive it out of its limit position. A control signal is applied to an amplifier 13 which, if the control signal is of alternating voltage, applies balanced unidirectional voltage to the grids of amplifying valves 18 and 19. The amplified control signal is applied to the grids of power amplifier valves 33 and 34, the anodes of which are connected, through opposing field windings 38 and 39 of a motor 40, to a positive potential source 41, so that the effective motor field depends upon the control signal. The motor, the armature of which is connected to a constant source, drives a rack and pinion through a shaft 44. Limit switches 51 or 52 are closed by the rack to energize relays 55 or 56 which operate contacts 57 or 58 respectively. The operation of switch 57, for example, disconnects the valve 33 from the valve 18, and the valve 33 is cut-off to deenergize the field winding 38 and remove the motor driving-torque. The valve 18 passes current through a resistor 61 to apply a positive impulse to the grid of the valve 34 through a condenser 62, momentarily increasing the current passed by that valve to the field winding 39, and producing a reverse torque which is just sufficient to stop the motor. A similar operation obtains when the limit switch 52 is actuated. The voltage pulse applied through the condenser 62 can be made proportionate to the control signal by suitably arranging the potential of the point 61 in relation to other circuit constants. The characteristics of the system may be so arranged as to make the reverse torque proportionate to the motor speed. In a modification the motor is supplied from a constant-speed generator which is provided with opposing field windings adapted to be energized in accordance with a control signal, through a system similar to that described above. In this arrangement the relays, similar to relays 55 and 56, are each provided with two contacts. Actuation of a limit switch energizes one of these relays to cut-off the power amplifier valve in series with the operative generator field winding by disconnecting its cathode from a bias resistor and connecting it directly to a source of positive potential, and, at the same time, a positive voltage impulse is applied to the control grid of the other amplifier valve, which is normally connected through a resistor to a negative potential source, but is now connected to a resistor-condenser network which is normally short-circuited through a relay-contact, the condenser becoming charged on the actuation of said contact. To stabilize the motor operation a damping system is provided wherein the control signal voltage is opposed by a voltage proportional to the motor speed which is derived from a motor-driven tachometer generator. During normal operation the effective motor field depends upon the difference of currents in the opposed field windings as determined by the current passed by power amplifier valves the grid potentials of which are in accordance with the difference between the control and velocity-damping signals. Specification 446,665 is referred to in this connection. In the arrangement shown in Fig. 4, a control signal is applied to an amplifier 289 having a balanced output through the leads 290 and 291 which apply the amplified control signal differentially across resistors 292 and 293 connecting the control grids of valves 296 and 297 respectively to earth. The cathodes of valves 296 and 297 are connected through a common resistor 300 to earth and the anodes are connected through resistors 303 and 304, switches 327 and 328, and opposing motor field windings 305 and 306 to a source of positive potential 308. Protective networks 314 and 315 are connected across the field coils. When a rack 320 opens a limit switch 327 a relay coil 325 is energized to close a contact 323 and short-circuit the motor armature. In the case of a transmission or followup control signal the short-circuit of the motor armature will retard the motor and increase the control signal thereby increasing the field current and rendering the dynamic braking more effective. Similar operation obtains when the load 320 reaches its lower limit. In a modification of this arrangement the motor is supplied from a generator, the field windings of which are controlled in a manner similar to that described in connection with the field windings 305 and 306. In this case relaycontacts are arranged to open the generatorarmature circuit as well as short-circuiting the motor armature. Specification 594,397 also is referred to.