Patent GB-1411944-A (GB)
Title: Apparatus for controlling a variable at each of a plurality of locations
IPC: G05B11/26
Filed: 1971-10-16
Abstract: 1411944 Automatic control of heat treatment GEORGE KENT Ltd 16 Oct 1972 [16 Oct 1971] 48242/71 Heading G3N [Also in Division G4]  In a textile yarn heat treatment machine the temperature at locations x 1 , ... x 160  is controlled by a heater provided at each location, the electrical power to each heater being regulated to maintain each temperature between required limits. Thermocouples at x 1 , ... x 160  are scanned in turn every 40msec (corresponding to two cycles of a 50Hz A.C. supply) and the analog readings are transmitted serially to a double-ramp A/D converter 7, the digital versions of the temperatures passing via a linearizer 11 to a comparator 15 where they are compared with pre-selected reference temperatures to generate a 4-bit difference signal (0 or 7 times a "unit" error). Each 4-bit word is input (one every 40msec) to a circulating memory 19, each word in the memory being stepped on every 10Á sec. The words are input by signals from a gate 27 obtained on coincidence between the outputs of a scanner address unit 23 (incremented every 40msec) and a repeating counter (incremented by a 100KHz clock 21). The circulating words are applied every 10Ásec to a decoder 29 to produce for any word (i.e. any error level) a "0" output on one of lines 0-15, Fig. 2 forming inputs to OR-gates 39. A counter 33, incremented every 40msec., steps a scanner decoder 31 to cause it to output sequential "0" signals on lines 0-15 connected via a diode matrix 40 to gates 39. For a 10Ásec "O" signal on any output of decoder 29, the corresponding OR-gate 39 to which this signal is input transmits a "1" or a "0" signal to an AND-gate 41 according as the second input of that gate 39 is connected by diodes 40 to a "1" or a "0" output from decoder 31. The output from 41 passes to D-type latches 43: that latch which corresponds to the locality whose measured temperature difference error level has just been input to memory 19 is clocked 1.6msec. later (and every 1.6msec. subsequently for that 40msec., since there are 25 'circulations' of 160 words in memory 19 during 40msec.) so that the value in that latch can be updated. A "1" output from a latch causes a corresponding transistor 47 to conduct and trigger a respective triac (not shown), which energizes the corresponding heater, the triac continuing to conduct for the remainder of the prevailing half-cycle of the mains supply. To avoid the simultaneous switching on or off of large numbers of heaters, a unit 35 repeatedly adds the values 0-15 from a counter 37 to the value in counter 33 so that the output from decoder 31 steps every 10Ásec. instead of every 40msec: the result is that only ten heaters can be switched together. A circuit 53 provides a visual or audible alarm whenever a temperature measurement exceeds pre-selected upper and lower limits. A gallium arsenide diode display 51 steps through the values 0-160 at a speed of either 40msec. (the scanner speed) or 6.4 sec. (time for a complete scan): at the faster speed, the indicator stops at the first location to change to an alarm condition.