Patent GB-1192745-A (GB)
Title: Process Control Systems
Inventor: ROBERTS MALDWYN HUGH
IPC: G05B5/01
Filed: 1967-02-25
Abstract: 1,192,745. Automatic moisture control systems. COTTON SILK & MAN MADE FIBRES RESEARCH ASSOCIATION. Feb. 21, 1968 [Feb. 25, 1967; June 7, 1967], Nos. 9092/67 and 26253/67. Heading G3R. A control system for a process having a substantial dead time, i.e. the time for material being processed to travel from the processing point to the point at which the result of the processing is measured, comprises a model of the process which defines the relationship between the controlled variable and a manipulated variable and which produces output signals in response to the application to the model of input signals corresponding respectively to desired and actual values of the controlled variable, the output signals in turn corresponding to the desired and actual values of the manipulated variable and being used to derive a difference signal for intermittently correcting the manipulated variable so that the effect of such correction is detected before a subsequent correction. The application described relates to the moisture control of cloth strip 10 passing through a dryer 12 by variation of the speed of a motor 22 driving the strip therethrough. An actual value of cloth moisture is obtained from an electrode 14 in contact with the strip emerging from the dryer 12 and the signal from electrode 14 is then passed through a filter and time constant circuit 16 to obtain an average actual moisture signal for application to a process controller 18. This member also receives a desired moisture signal from a setting device 20 and a correction - initiating signal from an integrating and pulse generating circuit 26, which produces a pulse each time a predetermined length of cloth has passed through the dryer, this length being chosen to enable the effect of a correcting action by controller 18 to be detected and also to take account of the time constant of the circuit 16. The controller 18 acts upon a speed controller 28 for the motor 22 and the circuit 26 is supplied by a tachogenerator 24 also supplying its signal to the controller 28. Alternatively the pulse generator may be controlled by a displacement transducer, e.g. a potentiometer, whose output signal depends on the speed of the motor. In another embodiment, Fig. 2 (not shown), correction - initiating signals are obtained by using a switch controlled by a cam driven by the motor. Mention is made of generating pulses by magnetic, photo-electric or capacitive transducers and counting such pulses to produce a correctioninitiating signal when a predetermined length of cloth has passed. Such an arrangement using a magnetic transducer is described, Fig. 3 (not shown). Figure 10 shows the layout of the process controller 18, which includes a model relating the moisture C to the speed M and comprising networks of resistors and/or diodes and/or amplifiers such as are used in analogue computing equipment. Change-over switches before and after the model are synchronized so that the application of the desired moisture value C B  to themodel causes the corresponding model output signal M B  to be applied to an inverter and store for subsequent comparison with the model output signal M A  obtained by operating the change-over switches to apply the actual moisture signal C A  to the model input. The difference signal (M B -M A ) may then be applied to the speed controller 28. However, to ensure that a change in desired moisture value (caused by manual adjustment of the set point signal C B ) produces a rapid change in the correcting signal to the controller 28, the signal M B  is combined with the difference signal (M B  -M A ) and this combined signal is used to control the motor speed. An embodiment relating to this modification, Fig. 11 (not shown) applies the difference signal to an analogue computer type of circuit known as a "memory pair", which circuit includes changeover switches synchronized with those of Fig. 10 so that a difference signal applied to the "memory pair" is maintained and recirculated indefinitely even though the original signal falls to zero. The output of the "memory pair" is then combined with the main signal M B  for application to the controller 28. This arrangement is stated to ensure that error in the model is corrected. The Specification also discusses linear and non-linear relationships between C and M and factors determining the characteristics of the model, Figs 4 to 8 (not shown).