Patent GB-1341551-A (GB)
IPC: G05B15/02
Filed: 1971-04-19
Abstract: 1341551 Automatic control of temperature ROSEMOUNT ENG CO Ltd 19 April 1971 [2 March 1970] 9808/70 Heading G3R [Also in Division G4] A control system includes a common data processing unit for processing data in digital form and a plurality of separate control channels, each channel having a digital store for storing input information, output information and other data relevant to that channel, there being sequencing means for sequentially connecting the data processing unit to the channel stores, and an algorithm store for providing instructions to the data processing unit for appropriately processing the input data on the selected channel and feeding the processed input data to the channel store, connection of the algorithm store to the processing unit being controlled by a storage matrix with manual input and visible indication. Each channel store is a shift register. A clock-driven BCD channel number counter selects the shift registers in turn, the count also being decoded to address a manual diode plug-in pin-board which controls a scaler interposed between a console and the channels. It also feeds a comparator for comparison with a sampling interval counter driven by the channel number counter, to control the intervals between successive input samplings in the various channels. It permits manual selection, for each channel separately, of the processing algorithms (viz. whether the control loop for the channel is to include proportional, integral, derivative, sum, difference, product and/or ratio terms, lead-lag algorithms also being selectable), and conditions under which alarms will be given. Jumper wires can be used on the pin-board to connect a selected out-put of one channel as an input to another, thus fluid flow temperature can be sensed in one channel providing proportional plus integral control using a set-point defined by the operator to give the set-point for a second channel which also senses the temperature and provides proportional plus derivative control to produce an output signal to a heater for the fluid, the two sensing positions being downstream of the heater with the first channel's position being furthest from the heater. The algorithms are stored in two programme matrices addressed by a programme counter, and control the processing which occurs in an arithmetic unit having a serial adder, an accumulator register, two working registers and an input-output register. An analogue input value at a channel is converted to digital form in an A-D converter respective to the channel or shared between all the channels, the result being stored in the shift register. A digital output value from the arithmetic unit to a channel is stored in the shift register then converted to analogue form, the D-A conversion having a battery backed-up power supply and a capacitor hold store in its output, in case of failure in the main power supply &c. The shift registers also communicate in both directions with a digital interface for other digital equipment. D-A conversion is by having each bit in a particular stage of the shift register for a total time corresponding to the bit significance, and smoothing the output to produce a voltage which is converted into a current. A-D conversion uses a ladder network with switches controlled by the stages of a shift register to derive the digital result in the register bit-bybit, most significant bit first, by a series of trials and comparisons, the analogue input being a voltage derived from a current using a resistor and gated to the A-D converter using an FET. Console. Manual keys select a channel for monitoring on the console and for entry of control data from the console. Numerical visual displays show channel number, measured input, set-point, and output. The output display and an extra display show the control data in the channel's shift register. The extra display can show integral time constant, derivative time constant, proportional band or sampling interval, under control of keys. Keys can select automatic control, manual control or hold, for the selected channel, or normal, manual or hold operation for all channels. Fault indicators are provided for power supply failures, arithmetic check failure, and data transfer failure. Alarm levels may be keyed in for input and output signals, the alarms being lamps in a process flow diagram, or may be audible.