GB854215A - Improvements in and relating to electric control systems - Google Patents

Abstract

854,215. Automatic - control systems. ENGLISH ELECTRIC CO. Ltd. Sept. 26, 1957 [Sept. 28, 1956], No. 29675/56. Class 38 (4). An electric control system for controlling a physical characteristic of a continuously moving material comprises : (i) regulating means to vary the physical characteristic ; (ii) detecting means positioned beyond (i) in the direction of motion and giving a signal proportional to the physical characteristic; (iii) reference means producing an adjustable reference signal; (iv) comparison means giving an output proportional to the difference between (ii) and (iii) and having an input time constant equal to the time for travelling from (i) to (ii); (v) actuating means, responsive to the signal from (iv), which adjusts the regulating means; (vi) interrupter means to intermittently operate (v) at a predetermined frequency. Two embodiments are described, the physical characteristic in one being the weight of material of a conveyer and, in the other, being the thickness of steel strip emerging from a rolling mill. Material 11 is supplied from hopper 12 and variable speed conveyer 10 to a constant speed-weighing conveyer 13. Deflection of arm 27, restrained by spring 26, varies the output of differential transformer 28. This voltage output is rectified and applied in series opposition to an adjustable reference voltage from potentiometer 33, the difference error voltage charging capacitor 36. The charging circuit time constant is long to prevent response to random fluctuations. Magnetic amplifier 38 is switched on periodically by cam-operated contacts 40a related to the speed of conveyer 10 and its output controls reversible motor 39. This adjusts rheostat 23 in the speed reference circuit for conveyer drive motor 14, this circuit comprising tacho-generator 21 with its variable field, a potentiometer 22, control rheostat 23, magnetic amplifier 20 and motor control field 14c. The speed of motor 14 is thereby varied to deliver the correct weight per unit length of conveyer. The motors 14 and 15 are supplied by generator 16 and any change of the generator voltage by means of its field control 17 is compensated in the comparison circuit by potentiometer 34 and in the speed reference circuit for potentiometer 22. Furthermore, any desired speed for conveyer 10 may be set into the system by motor field regulator 19 likewise ganged for compensation, in the speed reference circuit to field shunt resistance 24 and in the error comparison circuit to potentiometer 33.