KEYED PID CONTROL APPARATUS

Abstract: Where the output of a sensor (11) falls below a preset value K to release the key lock, a comparator circuit (19) produces an output which sets a flip-flop (21), whereby the gate of an AND circuit (22) is opened. Next, as the output of the sensor (11) falls below a preset value R for reset, a comparator circuit (20) produces an output, whereby a reset signal is sent to an integrated value holding unit (16) via the AND circuit (22), the flip-flop (21) is reset, and then the integrating operation is resumed by the integrated value holding unit (16).

Description: The present invention relates to a PID process control apparatus which executes, e.g., temperature control of a heating furnace. In a heating furnace, for example, when heating is started in order to set the temperature of the furnace at a predetermined heating temperature, the furnace temperature is gradually increased up to the predetermined heating temperature from an ambient temperature. In this case, the furnace temperature is continuously monitored by a temperature measuring element. When the furnace temperature reaches the preset heating temperature, heating is stopped, and thereafter, heat supply to the furnace is controlled so as to maintain the furnace at this temperature. The heating temperature increase step will be considered in more detail. Heating is continued until a furnace temperature reaches the preset temperature, and when the furnace temperature has reached the preset temperature, heating is stopped. However, when the furnace temperature is to be increased quickly, even if heating is stopped when the furnace temperature has reached the preset temperature, the actual furnace temperature exceeds the preset temperature, and temperature control is performed in a state wherein the furnace temperature greatly varies with respect to a preset central temperature. When temperature control of the furnace is performed by PID control, the temperature is increased from a low temperature to a preset central temperature when control is started, and an integral action is taken so as to correspond to an error with respect to the central temperature which is indicated by a temperature increase curve. In a state wherein the temperature increase curve exceeds the preset central temperature, the integrated value for the exceeded portion is subtracted from that corresponding to the temperature increase curve up to the central temperature. When the temperature is controlled so that the subtraction result becomes zero, the temperature is set to be the preset central temperature. However, when such control is executed, the integrated value becomes very large while the temperature is increased from the low temperature to the preset central temperature. For this reason, even if the integrated value, when the temperature exceeds the central temperature, is subtracted from this integrated value, the integrated value cannot be effectively decreased, resulting in an overshoot state in which the temperature greatly exceeds the central temperature. Thus, convergence control characteristics with respect to the central temperature are degraded, and the temperature control characteristics become poor. In order to overcome such drawbacks, it is proposed to set a PID constant while the temperature is increased to the central temperature and another PID constant after the temperature exceeds the central temperature, and to selectively use one PID constant in accordance with a temperature. However, with such a means, two PID constants must be set, and an integral control element (I) must be set high. Therefore, it is difficult to control the temperature quickly. Alternatively, a method is proposed wherein a plurality of points are set until a central value is reached, and the rate of increase is decreased at each preset point so as to polygonally control the temperature. With this method, overshooting can be effectively prevented. However, it takes a long period of time to increase the temperature from a low to a preset central temperature, and any disturbance cannot be overcome. The same problems occur when the temperature greatly changes near the central temperature, also resulting in an overshoot state. From US-A-3 798 426 is known a pattern evaluation method and apparatus for adaptive control for automatic adaptation of process controller parameters according to the requirements of a non-linear process using a reset gain change for unspecified purposes. The present invention has been made in consideration of the above situation, and has as its object to provide a PID process control apparatus for, e.g. heating control, wherein an actual temperature can be quickly and smoothly convergence-controlled with respect to a preset central temperature without causing overshooting when the temperature is increased up to the preset central temperature. The solution of this object is achieved by a PID process control apparatus which comprises: First means for setting a central value to which a controlled variable detected by a sensor is to approach and connected to a control unit; second means for setting a proportional band (P) in which said central value is centered and connected to said control unit; first circuit for setting a first pair of preset values (K) equally separated from said central value within said proportional band; second circuit for setting a second pair of preset values (R) closer to the central value than the first pair of preset values and equally separated from said central value within said first pair of preset values; said control unit also being connected to the sensor and to an integrating means and being for obtaining error data representing a difference value between said target value and the controlled variable and for calculating a control value; said integrating means being for integrating said error data, which include means for holding the integrated value and means for commencing the integrating means when the controlled variable reaches within said proportional band; setting means connected to the first circuit and the control unit and for setting said integrating means in a reset enable state when said error data reaches one of the first pair (K) of preset values from outside of a range defined by the first pair of preset values; resetting means connected to said second circuit and said control unit and for resetting said integrating means when said error data reaches one of the second pair (R) of preset values when the reset enable state is set; said control unit also being for starting an integral action of said integrating means when an integrated value is reset by said resetting means; and output means connected to said control unit and for outputting said control value. According to the present invention, during PID process control, even when a measurement state considerably different from the central value is present, convergence control action can be quickly and smoothly executed with respect to the central value by a very simple arrangement, thereby effectively eliminating occurrence of an overshoot control state. Fig. 1 is a block diagram for explaining a keyed PID control apparatus according to an embodiment of the present invention; Fig. 2 is a graph for explaining the operation state of the embodiment; Fig. 3 is a flow chart for explaining a function of the embodiment; and Figs. 4 and 5 are respectively graphs for explaining the operation state of the embodiment. An embodiment of the present invention will be described with reference to the accompanying drawings. Fig. 1 shows an arrangement for the temperature control of a heating furnace 1. A temperature signal supplied from a temperature sensor 11 comprising a thermocouple or the like provided in the heating furnace 1 is supplied to a control unit 13 using a microprocessor through an amplifier 12. The heating furnace 1 is arranged so that air heated by a burner 2 is introduced to the furnace's main body 4 through a duct 3. A belt conveyor 5 is arranged in the furnace's main body 4, and a heating object is conveyed from an entrance 4a to an exit 4b. The temperature sensor 11 is arranged in a boundary portion between the duct 3 and the furnace's main body 4. A volume of the heated air supplied from the burner 2 to the furnace's main body 4 is controlled by opening/closing a control valve 7 provided adjacent to the temperature sensor 11 on the upstream side thereof. The opening of the control valve 7 is adjusted by an actuator 8 driven in response to a control output supplied from the control unit 13 through an output unit 10, and a signal indicating the opening is supplied from a position sensor 9 to the control unit 13. The control unit 13 receives a signal corresponding to a central temperature C from a central value setting circuit 14 for setting a target temperature, i.e., the central temperature C, and +P and -P signals, generated from a proportional band setting circuit 15, for designating a band of the central value C for PID control. The control unit 13 samples the temperature signal from the sensor 11 at specified intervals so as to obtain the error E between the detection value and the central value C. The error is supplied to an integrated value holding unit 16 which sequentially adds the error and integrates a value indicated by a curve representing a change in temperature so as to hold the integrated value. In this case, when the temperature signal from the sensor 11 is out of a preset proportional band ±P, discrimination of the temperature signal with respect to the central value C is not performed. Therefore, only the error E with respect to the central value C of the temperature signal within the preset proportional band ±P is supplied to the integrated value holding unit 16, thus being subjected to integral action. In this apparatus, first and second setting circuits 17 and 18 are provided. The first setting circuit 17 sets key-lock release lines ±K sufficiently separated from the central value C within the proportional band ±P within which an original integral action is expected. The second setting circuit 18 sets reset lines ±R for setting limit values of the integral action. Output signals K and R from the first and second setting circuits 17 and 18 are respectively supplied to comparators 19 and 20. A signal corresponding to the error E with respect to the error C from the control unit 13 is supplied to the comparators 19 and 20 as a comparison signal. When the preset value K or R is small with respect to the error signal, i.e., when the error of the detection temperature with respect to the central temperature is smaller than K or R, an output signal is generated from the comparator 19 or 20. The output signal from the comparator 19 sets a flip-flop 21, and the output signal from the comparator 20 resets the flip-flop 21. In this case, the comparator 19 preferably generates a pulse output signal when the error crosses ±K, and the flip-flop 21 is set when the measured temperature exceeds the preset value K. The flip-flop 21 generates an enable signal for resetting the holding operation of integrated value data in the integrated value holding unit 16. The reset enable signal from the flip-flop 21 in a set mode is supplied to an AND gate 22. The AND gate 22 also receives the output signal from the comparator 20, and the output signal therefrom is supplied to the integrated value holding unit 16 as an integrated value holding reset instruction. Thus, the integrated value data is reset and the holding operation of the integrated value is restarted. The integrated value data Id held in the integrated value holding unit 16 whose integrated value holding operation is controlled in the above manner is supplied to the control unit 13, and the integrated value data is supplied to the output unit 10 from the control unit 13 and is then supplied to the actuator 8 as a control signal for the valve 7 in the duct 3 of the heating furnace 1, thus controlling the temperature of the heating furnace 1 to approach the preset central value C. The operation state of the apparatus with the above arrangement will be explained with reference to Figs. 2 and 3. As described above, the proportional band ±P is bet to include the central value C therein from the central value setting circuit 14. The first preset values ±K to be the key-lock release lines are set within the proportional band ±P by the setting circuit 17, and the second preset values ±R to be the reset lines are set within the range defined by the first preset values ±K. A case will be examined wherein the first and second preset values K and R shown in the embodiment are not set. When heating is started from a heating furnace stop state, the temperature of the furnace is increased from a temperature lower than -P to the preset central temperature C, as shown in Fig. 2. When the temperature exceeds -P, the integral action is started, and the integrated values are added and held in the integrated value holding unit 16. In this case, the temperature is detected in response to a sampling signal at specified intervals, as indicated by broken arrows in Fig. 2. The length of the arrow corresponds to the integrated value in a sampling mode. The integral action is repeated so as to correspond to the temperature increase state. When the temperature exceeds the preset central value C, a direction of the broken arrows is inverted in the sampling mode, so that the integrated value data held in the integrated value holding unit 16 is subjected to subtraction. In other words, when the integral action is executed within the proportional band, the errors within the range of a time period A are added during a time period a . Therefore, since the integrated value data is subtracted during the time period a , the temperature is controlled in an overshoot state. In contrast to this, as described in the above embodiment, when the first and second preset values K and R are set, an input value of the sensor 11 is measured in step S1 of Fig. 3. The input value is supplied to the control unit 13, and in step S2, the difference between the input value and the central value C is obtained as the error E. When the measured temperature is increased and exceeds -P, the integral action is started. However, if it is detected in step S3 that the temperature exceeds the first preset value K, the flip-flop 21 is set in response to the output signal from the comparator 19. In step S4, a key flag is set, thus setting the reset enable state. When the temperature is further increased and reaches the second preset value R, this state is detected in step S5, and the output signal is generated from the comparator 20. It is determined in step S6 that the key flag is set. The integrated value holding reset instruction is supplied from the AND gate 22 to the integrated value holding unit 16, and the previous integrated value data are cancelled in step S7, thus restarting the integrated value adding/holding operation. In this case, the flip-flop 21 is reset in response to the output signal from the comparator 20, and the key flag is also reset in step S8. After the reset signal is supplied to the holding unit 16, the flip-flop 21 is no longer set until the output signal is generated from the comparator 19, i.e., until the measured temperature crosses the first preset value K. In this state, the flow returns to step S1. However, since the error is equal to or less than R, the temperature is converged to the central value C through steps S5 and S9. A reset enable flag is set at a point k when the measured temperature passes the key-lock release line sufficiently separated from the central value C, and the current integral action is detected as an abnormal operation, i.e, an excessive integral action. The integrated values are cleared at a point r when the temperature passes the reset lines ±R, and the integral action is performed for the time period B from the point r. Therefore, according to the present invention, as indicated by a solid curve, the integrated values are subtracted during a time period b much shorter than the time period a . That is, PID control having only a small overshoot amount is executed. Positions of the lines defined by the first and second preset values are set as optimum values for a control object, e.g., the heating furnace. For example, when ±50% of the proportional band P was experimentally set for the key-lock release lines and ±25% thereof was set for the reset lines, satisfactory PID control could be performed. The apparatus is not only operated for temperature increase control when the operation is started. Even if the measured value is abruptly changed, when the measured value changes so as to exceed the first preset value, the integrated value holding reset operation is executed, and convergence control with respect to the central value is quickly performed. Assume that temperature control is performed on the reset line -R lower than the central value C, as shown in Fig. 4. In this case, although the output is generated from the comparator 20, the flip-flop 21 is kept reset, and therefore, the integrated value holding unit 16 is not reset. As a result, since PID control is conducted while the integrated value holding unit 16 holds the integrated values, a malfunction such as divergence will not occur. In this state, assume that the error E is increased to exceed k1, as shown in Fig. 4. If the error E is lowered below the key-lock release line -K at a point of k2 immediately after k1, the key flag is set again, and the flip-flop 21 is reset, thus resetting the integrated value holding unit 16. Assume that the furnace 1 shown in Fig. 1 is inoperative when the furnace 1 is satisfactorily controlled to be maintained at the central value C. In this case, the temperature of the furnace 1 is further lowered below a crossing point k-I with the key-lock release line -K, as shown in Fig. 5. If the operation of the furnace 1 is restarted after, e.g., 20 minutes, the temperature of the furnace 1 exceeds the key-lock release line -K at a crossing point k-II, and the flip-flop 21 is set. Then, the temperature crosses the reset line -R at a reset point r-I, and the integrated value holding unit 16 is reset. In this manner, according to the present invention, if process control is restarted after a standby period of 20 minutes, initial setting need not be performed again. When the embodiment was applied to dry a metal plate subjected to color off-set printing on its surface as the heating object 6, a very good result was obtained. When a color-printed metal plate used for manufacturing aluminum cans for soft drinks is dried, it is known that printed color tones highly depend on temperature. Conventionally, color tones are degraded by heating due to excessive, integral action. However, in this embodiment, since excessive integral action can be completely avoided, the temperature of the furnace 1 can be stabilized, thus completely preventing a change in color tones.

Claims: A PID process control apparatus comprising: first means (14) for setting a central value (C) to which a controlled variable detected by a sensor (11) is to approach, and connected to a control unit (13); second means (15) for setting a proportional band (P) in which said central value is centered, and connected to said control unit (13); first circuit (17) for setting a first pair of preset values (K) equally separated from said central value within said proportional band; second circuit (18) for setting a second pair of preset values (R) closer to the central value than the first pair of preset values and equally separated from said central value within said first pair of preset values; said control unit (13) also being connected to the sensor (11) and to an integrating means (16) and being for obtaining error data representing a difference value between said central value and the controlled variable and for calculating a control value; said integrating means (16) being for integrating said error data, which include means for holding the integrated value and means for commencing the integrating means when the controlled variable reaches within said proportional band; setting means (19, 21, 22) connected to the first circuit (17) and the control unit (13) and for setting said integrating means in a reset enable state when said error data reaches one of the first pair (K) of preset values from outside of a range defined by the first pair of preset values; resetting means (20, 21, 22) connected to said second circuit and said control unit (13) and for resetting said integrating means when said error data reaches one of the second pair (R) of preset values when the reset enable state is set; said control unit (13) also being for starting an integral action of said integrating means when an integrated value is reset by said resetting means; and output means (10) connected to said control unit (13) and for outputting said control value. A process control apparatus according to claim 1, characterized in that said reset enable state setting means includes a first comparator (19) for generating an output when the error data is decreased below the first preset value, a flip-flop (21) for generating an output upon reception of the output from said first comparator, and an AND gate (22) enabled in response to a set output from said flip-flop. A process control apparatus according to claim 2, characterized in that said resetting means includes a second comparator (20) for generating an output when the error data is decreased below the second preset value, means for supplying the output from said second comparator to a reset terminal of said flip-flop (21) and to an input terminal of said AND gate (22), said integrated value holding circuit (16) being reset by the output from said AND gate.

Inventors: JINNO, MASATOSHI, YOSHIZAWA, YASUO
Applicants: YOSHIKI INDUSTRIAL CO. LTD
Application year: 1985
Main classification: G05B11
IPC codes: G05B7, G05B11