Patent GB-489271-A (GB)
Title: Improvements in servomotor systems for effecting position control
IPC: G05D3/12
Filed: 1936-10-22
Abstract: 489,271. Electric control systems ; relays. NISBET, R. H., and SPERRY GYROSCOPE CO., Ltd. Oct. 22, 1936, No. 17251. [Class 40 (i)] [Also in Group XXXIX] In a position control system employing a servomotor for driving a secondary object in accordance with the variations of a primary quantity, the servomotor is operated by a control quantity that is the sum of two terms, viz. : (1) a term which is a function of the error in position of the secondary object and (2) a term which involves the first and second derivatives of the desired displacement of the secondary object ; alternatively, the control quantity may consist of the sum of a function of the error and its derivatives and also a derivative of the desirec displacement. In one form, the derivates of the desired displacement are derived from variations of the primary quantity, whilst in another form they are derived from the movements of the secondary object. Fig. 1 shows a system for controlling the movements of a searchlight 27 by a remotely located telescope 1. A selsyn type system 28, 32 links the searchlight to the telescope, the searchlight being driven synchronously with movement of the telescope by a motor 15 which is operated in accordance with a value corresponding to the positional error and derived from the selsyn system over a transformer 34 and valves 36, 37 and a further value representative of the desired values, velocity and acceleration of the searchlight that are necessary to ensure synchronous operation with the telescope. The latter value is derived from a tachometer generator 9 driven from the sight operating handle 2 and producing a current proportional to velocity and consequently voltage drops across resistance R<1> and inductance L<1>, which by adjustment of the electrical characteristics of the circuit, may be made proportional to the desired velocity and acceleration of the searchlight. The outputs from the two value deriving circuits supply corresponding parallelly-connected coils 12a, 112a ; 12b, 112b of two saturable-core reactors which act as magnetic amplifiers. The reactors are provided with biassing windings 13, 113 supplied from a D.C. source 14 and with windings 11, 111 which are connected additively in series with transformer windings 20, 21. The other windings 18, 19 of the transformers are supplied from an A.C. source which also supplies continuous excitation to one winding of a two-phase motor 15. The second winding 24 is connected across diametrical points of the network 11, 111 and carries no current when exact synchronism between the searchlight and telescope exists owing to the balanced condition of the reactors. The reactors are connected in such a manner that the parallelly-connected coils have opposite magnetic effects on the reactors and consequently when the balance is upset the winding 24 is energized to drive the motor in one or other direction. The network 11, 111 may include tuning condensers in order to increase the response. Damping control terms may be introduced into the system by a tachometer generator 109 which is driven by the searchlight motor and supplies an inductance-resistance circuit M, R<2>, the inductance being in the form of an auto-transformer arranged to produce an artificial neutralization of the inertia. The derived current supplies additional windings 12c, 112c on the reactors acting in opposition to the other control windings. In a modification, Fig. 4 (not shown), wherein saturable reactors are dispensed with, control values representing displacement error and values of velocity and acceleration derived therefrom in the manner disclosed in Specification 480,115 are applied to gas-discharge valves for controlling the motor. The armature current of the motor produces two voltage drops, viz. : (1) a drop across the armature and (2) a drop across a series resistance, which are functions of the load velocity and acceleration respectively. These voltages are fed back through filters and phase shifters to the input circuits of the valves. Instead of using tachometer generators, governor - controlled variable resistances may be employed ; moreover, the control system may be of the Ward- Leonard type, the control values being used to vary the generator field. Each magnetic amplifier comprises a saturable core 10 having a central limb carrying a D.C. biassing winding 13 and one or more operating windings 12 and outer limbs which carry A.C. windings 11 which produce flux in opposite directions in the central limb and in the same direction in the outer limbs. In a modification, a single winding 11 is arranged on the central limb which is provided with a window to form two limbs for the windings 12, 13. Specifications 368,281, [Group XXXIII], 473,753, and 483,635 also are referred to.