Patent GB-1328260-A (GB)
Title: Methods and apparatus for use in control by computer
IPC: G05B15/00
Filed: 1970-12-11
Abstract: 1328260 Programmed process control MARATHON OIL CO 11 Dec 1970 [2 Oct 1970] 58990/70 Heading G3N [Also in Division G4] An interface between a digital computer and a controlled process consists of a number of addressable control stations each having a memory for storing a digital representation of the parameter controlled by that station, the arrangement being such that in operation the computer addresses a selected remote station, resets to zero the contents of the memory in that station, and reads a new digital representation into the memory. Computer: The computer may be any suitable programmed digital machine which receives measurements of process parameters and calculates optimum values for those parameters. In the specific embodiment the process is a reaction supplied with two fluids, e.g. in an oil refinery, the parameters being rate of flow of one fluid. pressure of the other fluid, and temperature in the reaction chamber. Details: Having calculated a parameter the computer addresses the appropriate control station by supplying an address in parallel over a common line to all the stations. A NOR gate decoder in the selected station responds to the address and a signal from the computer to clear its memory. In a modification serial address data may be supplied to a shift register which supplies parallel data to the decoder. The new parameter is then fed into the memory of the selected control station in parallel over a line common to all the stations. The parameter stored in the memory is then converted into analogue form by a number of transistors connected to respective outputs of the memory, each transistor having a different valued output resistor, Fig. 3 (not shown). The analogue signal is passed through circuitry Fig. 4 (not shown) which limits the rate of change of the analogue signal and the upper and lower limits of that signal. The rate of change limitation is provided to give "stephen" transfers when a new parameter is supplied by the computer. The signal is then converted into AC so that it may be passed through a transformer which connects each process controller to its associated control station. The signal is rectified and utilized to control the process. When the system is switched from local (manual) to computer control it operates in a 'scale-up' mode in order to prevent a large change in the stored digital parameter. When the memory, which consists of a number of cascade connected flip-flops, has been cleared it is connected to an oscillator and supplied with clock pulses whereupon it acts as a counter. The output of the D.A. converter connected to the memory is continuously compared with a signal stored on a capacitor in the associated process controller which represents the value of the previous parameter. When equality is detected the counting pulses are stopped. The Specification describing the circuit components utilized in some detail. Briefly, the decoder consists of a number of NOR gates connected to a predetermined combination of input terminals, and the comparator consists of a differential amplifier connected via scaling registers. One of a number of lamps is lit in the computer when a decoder recognizes an address and a fault indicating lamp is lit in response to faults occurring in the system. It is stated that the address signals may be supplied over mark and space leads and a redundancy check performed.