Patent GB-1342722-A (GB)
Title: Control circuits for use in control systems
IPC: G05D23/24
Filed: 1970-01-16
Abstract: 1342722 Automatic control of temperature HONEYWELL Ltd 13 Jan 1971 [16 Jan 1970] 2335/70 Heading G3R In a retroactive control system Fig.1, e.g. for automatically controlling temperature of a building, in the event of a prolonged error signal a constant voltage is produced at an amplifier output terminals 10 for a period proportional to the input error signal change and subsequently a pulse voltage appears thereacross (i.e. output voltage alternating between two constant levels one of which may be zero) having an amplitude equal to the constant voltage and a mark/space ratio such that the mean output voltage increases or decreases depending on whether the error signal increases in a positive or negative sense. Thereby, firstly proportional control and then substantially integral control is effected. In Fig. 1, the actual room temperature sensed by a resistor 3 in a bridge circuit, is compared with a desired temperature preset on R9. The output of a controller 6 comprising an amplifier with a feedback network including an RC circuit, is applied to the energizing circuit of a constant speed motor via one of two triacs (not illustrated). The motor drives a valve 13 to control the flow of warmed air through a duct 2 and reduce the error to zero. The controller amplifier 18 Fig.3 and Fig.2 (not shown) is such that with zero voltage across its input terminals (error signal), equal positive potentials appear at outputs 18a, 18b. The output at terminals 10 to the valve actuator circuit is therefore zero. When the temp. sensed by sensor 3a deviates from the desired temp. preset on R9a a proportional error signal is applied to the amplifier and depending on its sense either the potential at 18a or that at 18b falls by a fixed amount so that the actuator is energized to open or close the valve by a proportionate amount. Assuming the potential at 18a falls, then a diode 61, is forward biased and the charge on C67 slowly decreases via R77, R76, diode 61 and amplifier 18. Hence the potential at the gates of FET's 68, 69 slowly decreases. Thereby FET 69 slowly becomes more conductive and FET 68 less conductive, with the result that the potential at the junction of R78 and R79 slowly falls. This potential is fed back via R49 to the amplifier 18 and eventually balances the initial error signal the time required to rebalance depending upon the amplitude of the error signal. This is the proportional action. In the rebalanced state the potential at the junction of the gates of FET's 68, 69 and C67 is lower than that at the junction of R63, R64 by an amount dependent on the magnitude of the original error signal, since the charge on C67 was decreased to balance the original error signal. Once balance is established again, C67 begins to charge through R63, R65, R66 causing the gate potentials of the FET's to rise whereupon their conductivity changes differentially and an increasing feedback signal is produced which disturbs the balance of the amplifier (when it exceeds the threshold level thereof). Once balance is upset the amplifier produces an output at 10 to the valve actuator and the charge on C67 is again altered to rebalance the amplifier. The procedure is repeated so long as the error signal is present, the resulting pulsed output of the amplifier causing the valve actuator to effect intermittent correction at a mean rate which is the greater, the greater the change in the error signal. In the embodiment Fig.2 (not shown) the feedback networks includes voltage dependent resistance (diodes), whereby once balance has been re-established, the charge in a capacitor is caused to change in a manner such that the length of the period before balance is disturbed is inversely dependent on the input signal change, and thereby the mean value of the amplifier output is greater for a greater change in input signal.