Patent GB-1339258-A (GB)
Title: Proportional-integral electric regulating means
IPC: G05B11/42
Filed: 1970-12-17
Abstract: 1339258 Automatic control of temperature DANFOSS AS 17 Dec 1970 [17 Dec 1969] 60092/70 Heading G3R In an ON/OFF type regulating system having an adjustable proportionality range wherein proportionality obtains between an error signal or deviation and the control rate of a control element -e.g. for adjusting by means of servomotor a valve which controls the flow of a hot medium for heating a room in dependence upon the deviation in actual temperature sensed, from a desired value, - a voltage X w  proportional to the deviation is applied at a connection E to the input of an amplifier 1 Fig.1, via a resistor R 1  in opposition to a voltage occurring in an R-C element R 4 C 1  Means including a diode D 1 , are provided to limit the charging voltage of the capacitor and the time constant of R 4 C 1  is adjustable and hence also the ON/OFF periods of the servomotor, independently of this limit value. The output of the amplifier 1 is applied to a relay unit 2 and depending upon the polarity of the output a relay coil K 1  or K 2  is energized to close switches K' 1  and K 11  in synchronism or K' 2  and K 22 , whereby a servo motor M will turn in one direction or the other depending upon the deviation sense, to reduce the deviation to zero. Voltage dividers R 7 , R 8  and R 10  R 11  are connected across a D.C. supply the mid-point of which is a reference potential (earth). The potentials at points e and f determine the limiting voltage to which C is charged and the width of the proportionality range (P-range) and can be selected by adjustment of R 12 . In operation, (considering deviation in one direction only) with no deviation K 1 ' and K 11  are open and the motor M at standstill. If a deviation occurs which is sufficiently large to cause actuation of relay K 1  then K' 1  and K 11  will close and the motor will turn, and at the same time C 1  charges. The voltage across C 1  sends current through R 3  and the deviation voltage X w  sends current through R 1 , which currents oppose at point F, their signs being opposite. This corresponds to opposing the two voltages in the ratio of R 1  to R 3 . If the capacitor voltage has risen such that the sum of the currents at F is zero or below a value at which the amplifier output can actuate the relay K 1  then K' 1  and K 11  will open and the motor will stop. C 1  then discharges through R 4  until the sum of currents at F is such that K 1  is energized again. Each time the motor is switched ON it reduces the deviation value XW until eventually the deviation is substantially zero. The ratio of the ON period of the motor to its OFF period and thus its mean control rate V depends, within the P-range, both on the magnitude of deviation-X w  and on the charging rate of C 1  The greater R 4  the smaller the control rate V and the greater X w  the greater is the control rate. If the deviation voltage X w  is greater than the voltage across R 8  then C 1  would charge to a value higher than the voltage across R 8 . This is prevented by a diode D 1  which conducts so that the voltage across C 1  is limited to the voltage across R 8 . As long as the deviation is above this limit value the motor will turn at maximum speed. By selecting the value of R 4  the control rate with respect to deviation X w  can be predetermined as illustrated in Fig. 2 (not shown).