Patent GB-887004-A (GB)
Title: Apparatus for controlling industrial processes
IPC: G05B11/01
Filed: 1958-12-08
Abstract: 887,004. Automatic control systems. FOXBORO CO. Dec. 8, 1958 [Dec. 9, 1957], No. 39536/58. Class 38 (4). In a system for automatically maintaining a variable at a desired value, a D.C. signal dependent on the difference between the magnitude of the variable and a reference value is opposed by a D.C. feedback signal, and the resultant signal converted into an A.C. signal, amplified and reconverted to a D.C. signal which serves to correct the variable and provide the D.C. feedback signal through two variable reactance circuits introducing differential and integral components. In Fig. 3B the difference indicated by a meter 10 between a D.C. signal dependent on the magnitude of a process variable, e.g. rate of flow or temperature, and an adjustable D.C. reference signal is applied between terminals 142, 243a and chopped by an A.C. vibratory relay 144. The resulting pulses are amplified by a four-stage thermionic valve amplifier 160, 194 provided with both positive and negative feedback as well as signal-limiting rectifiers 196, 220, 222. The sine wave output of the amplifier, developed in a tuned circuit 228, is applied to a rectifier 240 which produces a variable magnitude D.C. output signal of one polarity or the other according to whether the amplifier output is greater or less than an A.C. reference signal derived from a mains transformer winding 236. Finally, the D.C. output from rectifier 240 corresponding to zero amplifier output is backed off by a D.C. reference signal derived from a rectifier 346. With a switch 250 in the " automatic " position shown, the D.C. error signal is applied to a magnetic amplifier 296 which produces a signal, indicated by a meter 16, to operate a valve (not shown) correcting the variable. A stabilizing negative feedback signal derived from rectifier circuit 240 is applied through an adjustable resistance-capacity integrating and differentiating network 254 to further input terminals 146, 243a associated with vibrating chopper relay 144. The integrating and differentiating network 254 may be eliminated from the feedback circuit by operation of a switch 276. The amount of negative feedback is adjustable simultaneously with the amount of positive feedback by ganging a potentiometer 278 and a potential divider 243. If it is desired to change to manual control, switch 250 is first moved to its intermediate position so that a manually-operated potentiometer controller 270 may be first adjusted to give an output equal to that produced by rectifier circuit 240 as indicated by a null reading on meter 10 which has been reconnected by switch 250. The output of potentiometer 270 may then be substituted for the output of rectifier circuit 240 without shock by moving switch 250 to its uppermost position at which meter 10 again indicates the value of the variable to be controlled. During manual control the integrating and differentiating network 254 is disabled by switch 250 but condenser 262 is maintained charged by the feedback in readiness for reversion to automatic control without shock. In a modification, errors of two variables of a process are detected by duplicating the apparatus enclosed within the dotted line 380, and rectifiers are arranged so that the variable which has the greater error is, at any instant, effective to control the process through the magnetic amplifier. Specifications 836,609 and 887,005 are referred to.