Patent GB-782523-A (GB)
Title: Dry grinding control
IPC: G05D5/06
Filed: 1955-02-17
Abstract: 782,523. Automatic control systems for grinding mitts. WESTON, D. Feb. 17, 1955 [April 30, 1954], No. 4820/55. Class 38 (4)'. A dry grinding mill is operated at a desired point on its output versus charge grinding curve A by controlling its feed in order to maintain the sound level of the mill or the power input to the mill motor at a datum value corresponding to the desired point, the quantity selected for control being that which has the greater rate of change at this desired point. For operation at a point such as P on curve A, where the power versus charge curve B has a greater slope for the value of charge corresponding to point P than the inverse sound versus charge curve,C, a power signal would be selected for comparison with a datum equal to the ordinate QS, the resulting difference signal being applied to the mill feeder .for proportionally regulating, the mill charge to restore the mill to its desired operating condition. For operation at a point such as X, however, a sound level signal would be selected for com-  parison with a sound datum equal to the ordinate WY, the resulting difference signal being used for feed control. The sound difference signal may be monitored by a power signal to prevent mill motor overload ; and the power differenee signal may be monitored by a sound signal to compensate for power variations at a constant charge due to changes in. the charge constituents.  Circuit details.-A dynamic microphone detects the sound or vibration, of the mill, and may be associated with a filter for passing desired frequencies (sonic frequencies above 2000,c.p.s. are said to be well representative of mill conditions). An electronic wattmeter detects the mill motor power input. The outputs of the detecting circuits may be rectified. pulses of magnitude proportional to the detected quantity and may be compared with potentiometer-derived references for application to the first stage of a conventional amplifier. Alternatively, the comparison may be made in a bridge network. The amplified difference signal may be applied to the control winding of a saturable reactor controlling thyratrons associated with the feeder, as described in Specification 729,925. This type of control requires the inversion of the sound signal, as indicated by curve C in the Figure, since the sound level increases with increase in the charge. The feeder may be of the electromagnetic pulse type in which the feed is regulated by the amplitude of vibration of a feeder plate, or, alternatively, a variable speed conveyer. The feeder further comprises either a single unit feeding a mixture of fixed proportions or a multiple unit feeding separate constituents simultaneously in fixed proportion. A circuit is described, Fig. 6 (not shown), for monitoring the sound level control signal in accordance with the motor power input, in which the control signal is compared with the datum on a potentiometer, the difference signal being applied to one grid of a double triode to vary its output to the grid of a second triode, whereby the output of the latter to the control winding of a saturable reactor controlling feed regulating thyratrons is also varied to regulate the feed in a sense as to restore the sound level to the datum value. The wattmeter output representing the motor power is compared with a power reference on a second potentiometer, the difference voltage being applied to the second grid of the double triode, whereby, if the power exceeds the reference value, the other half of the triode conducts to modify the output to the second triode and hence limit the feed control. Specification 632,532, [Group II], also is referred to.