Patent GB-1218185-A (GB)
Title: Process control apparatus
Inventor: CRAIG JOHN HENRY STUART
IPC: G05B1/03
Filed: 1967-05-16
Abstract: 1,218,185. Automatic temperature control systems. SERCK R & D Ltd. May 6, 1968 [May 16, 1967J, No.22666/67. Heading G3R. In a digital control system as described in the Parent Specification 1,106, 651 whereinthe output of a measured value e.g. temperature controlled pulse generator is compared in a counter with that from a reference pulse generator and the resultant used to control for example a fluid control valve, and wherein to produce a component proportional to the rate of change of a parameter the measured value generator feeds pulses of frequency f 1  into a counter for afirst interval t 1 , and a reference generator feeds pulses of a frequency f 0  for an instant t 2 , the measured value generator then subtracting pulses at a frequency f 2  for an interval (t 1  + t 2 ) from which the rate of change of the parameter may be determined, the frequency of the reference generator is adjustable to allow selection of one of a range of frequencies, and a third means is provided producing another range of selectable frequencies, which may be derived from the reference generator, and means whereby signals from the third means are admitted to the counter during a further stage of the cycle of operation to cause to the count state of the counter to approach its predetermined value. The reference pulses may be derived from a common constant frequency oscillator through a binary dividing counter having a selection means in the form of a switch 14, 15. The binary counter may also be employed to determine the length of various stages of the cycle of operations. Two counters 10, 11 are employed of which initially counter 10 receives pulses from a reference oscillation generator 12 and counter 11 receives pulses from a measuring oscillation generator 16, these counters being cross connected so that at some later stage generator 12 feeds into counter 11 and the output from counter 11 is passed into counter 10 through a switch 14 which may select a lower frequency. Each cycle of operations has several stages and commences with both counters empty and termination of the previous cycle changes a bi-stable circuit B2 being one of a number of connected bistable circuits B2-B7 to "1" as shown, permitting a desired value count to be admitted into counter 10 from a circuit (not shown). Generator 16 then feeds into counter 11 and generator 12 into counter 10, the latter reducing at a constant rate until counter 11 is full i.e. reverts to zero. When the counter 11 reaches zero there will be a remainder in counter 10. If the parameter is on one side of a measured value the remainder in 10 will be positive but if the parameter is on the other side the counter 10 will reach zero before counter 11 hence a bi-stable circuit B9 will be reversed and counter 10 will begin to count up until counter 11 reaches zero. The circuit then changes through the logic elements provided so that oscillator 12 then feeds into counter 11, and pulses from counter 11 are also passed into counter 10 through a switch 14 providing a selectable frequency division. During this stage the count in 10 will be applied to either of terminals 17 or 18 depending on the state of B9 to adjust the control element appropriately. When counter 10 reaches zero a circuit N9 resets B1 to reset counter 11 to zero. Circuit B9 is reset if necessary. In the next stage pulses from generator 16 again pass to counter 11 and generator 12 into counter 10 at a frequency dependent on the position of a switch 15. When counter 11 is agin at zero the count direction of counter 10 is reversed. Counter 11 is again counted to zero, counter 10 still receiving pulses from generator 12 and if there has been a change in frequency from generator 16 there will be a residue in counter 10 of magnitude and sign corresponding to the change sign being determined by circuits B9 as before. The count on 10 is again led out over terminals 17 or 18. When counter 10 has emptied circuit B2 is again driven into the "1" state as shown and the cycle recommences. The frequencies of generators 12, 16 may be adjusted in concert to give a short or long cycle time, the mean frequency of generator 16 preferably being adjusted so as to be proportional to that of generator 12 for a given value of the parameter. The arrangement may be modified to control a plurality of devices in a time division mode of operation, as described in the parent Specification.