GB1515301A - Control of variables - Google Patents

Abstract

1515301 Automatic control systems TECHSYSTEMS Ltd 16 May 1975 [11 March 1974] 10767/74 Heading G3R [Also in Division D2] Errors between actual and desired values of a variable are corrected by means which adjust the variable for a time period which is determined by a digital circuit and depends on the error signal, the time period being less than that required to correct the error. The object is to gradually approach the set value without overshoot. Fig. 5 illustrates the invention applied to control fluid-flow by means of a valve; when the measured value (top curve) is a long way from the set point the valve is adjusted by a large amount (70%) for a time Tl. After a sampling time delay for the effect to work through to the measuring point a further correction, proportional to the new, smaller, error, is applied for time T2, and so on. In this instance adjustment takes place at a constant speed for a variable time, but alternatively the adjustment rate also might be varied. A "backlash compensation" may be applied, i.e. the initial adjustment is increased when the sign of the error changes. The final steps also may be made larger than proportional, to overcome mechanical stiction at small errors. In a digital embodiment Fig. 2A (not shown) the error is represented as a number stored in a counter which is periodically counted down to zero by clockpulses from an oscillator, the control signal being supplied for the duration of this count down. The departure from proportionality referred to above is achieved if required by varying the frequency of the clockpulses in response to the error. In an analogue alternative Fig. 6 (not shown) the valve motor is operated for a time determined by a uni-junction transistor controlled by an RC circuit. The resistance is varied by relay operated switches in response to the error signal in digital (binary) form and a suitable capacitance selected by manual switching to match the parameters of the process. In Fig. 7 the invention is applied to paper making. A desired "bone dry weight" of product is set at 114 and periodically compared at 112 with the measured value of this quantity from 102. The result is switched at 122 to adjust the set point 110 for a system which controls the flow of fibre/liquid feedstock. For this purpose, switches 106 and 122 are periodically changed over so that a valve is adjusted by controller 116 in response to the difference between measured value 100 and the adjusted set point 110. If the error exceeds a prescribed value a fault is assumed, the valve is held at its instant position by "freeze" 126 and a warning is given.