JPS6229802B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a process control device that eliminates startup problems, particularly in processes where startups and stops are frequent, and enables optimal process control.

Generally, when a process is started, there is a large difference between the process signal and the setting signal, and the process signal itself changes rapidly. Therefore, if only normal proportional-integral (PI) control is used, a large overshoot will occur and it will usually take a long time for the system to stabilize.

Furthermore, if a deviation between the setting signal and the process signal continues for a long time, the output signal of the regulator will generally reach a saturation value due to the integral operation. Therefore, even if the process returns to a controllable state (a state in which the difference between the process signal and the setting signal has become smaller), the output signal of the regulator will not change until the process signal reaches the setting signal. Rapid changes in process signals cannot be controlled, resulting in extremely poor controllability and overshoot.

Conventionally, as a method of preventing this overshoot, a regulator with an anti-reset wind-up circuit has been used.

This method is simple because the regulator output signal changes before the process signal reaches the set signal.
Although it provides better control than a PI regulator,
This was insufficient in systems involving steep changes in process signals. Incidentally, FIG. 3 is a characteristic diagram showing an outline of the control operation by such a regulator with an anti-reset wind-up circuit. In the figure, E ₀ indicates the output signal E _P of the regulator, and the process signal.

In addition, the control characteristics of the regulator change depending on its integral time constant (by changing the integral time constant, the point at which the regulator output changes changes), but the optimum starting characteristics that reduce overshoot at startup are The major weakness of this method is that the integral time constant required for optimal control in the steady state after startup is not necessarily the same as that required for optimal control in the steady state after startup. It is virtually impossible to set the integration time constant to obtain
Currently, the integration time constant has been set with emphasis on steady state control characteristics.

Particularly in recent thermal power plants, as the frequency of starting and stopping thermal power units increases, the front valves of pressure control valves are frequently opened and closed in auxiliary steam, soot blowers, ejector steam, heavy oil pressure control systems, etc. It can be done. Therefore, at startup, the process is disturbed and the safety valve is activated, making stable process control impossible and placing a large burden on the equipment. There has been a strong desire for a control device that can prevent such disturbances in the control system and provide highly efficient operation.

In view of the above, the present invention provides a control device that prevents overshoot that occurs during startup, smoothly switches from startup operation to normal operation, and has excellent controllability both during startup and steady operation. It is intended to. That is, by comparing and extracting the output of a ramp function generator whose time constant can be arbitrarily set and the output of the regulator using a signal selector, and applying external feedback to the regulator using the output of the signal selector,
At start-up, the signal of the ramp function generator is selected and controlled by that signal, and when the set signal and process signal are close, a smooth switchover to the regulator is automatically performed.

Also, at startup, the proportional gain and integral time constant of the regulator are clamped to constant values, and the output of the ramp function generator is used as the control output, and after startup, when the outputs of the regulator and ramp function generator are equal, The output of the regulator is used as the control output, and if the difference between the process signal and the set signal falls within a preset value, the clamp is released and the time constant of the ramp function generator and the proportional gain/integration time of the regulator are changed. This makes it possible to realize optimal control that is uniquely determined by constants both during start-up operation and during steady-state operation.

The present invention will be described below with reference to the embodiment shown in FIG. In the figure, _M1 is a differential amplifier, one output terminal of which receives a process signal E _P from a converter (not shown), and the other input terminal of which receives a setting signal E _S via a resistor. R _P and r ₁ are connected to the differential amplifier M ₁ through a proportional gain setting variable resistor and a feedback resistor. Differential amplifier M ₁
The output of is passed through a parallel circuit of a capacitor C, which forms an integrating circuit, and a variable resistor R _I for setting the integration time constant, to a differential amplifier M ₂ (operational amplifier, which forms an integrating circuit).
is supplied to one input terminal of Differential amplifier _M2
The other input terminal of the amplifier is connected to common and
The output of _M2 is supplied to a signal selector 7.

Reference numeral 5 denotes a ramp function generator whose time constant can be arbitrarily set and which has a hold function, and on its input side there is an input terminal 8 for a start signal to start it.
and an input terminal 9 for a hold signal to hold the output thereof, and an input terminal 10 for a target value signal to set a target value of the ramp function generator, and the output of the ramp function generator is connected to the signal selector 7.
is connected to the other input terminal of

The signal selector 7 is a low signal selector in the illustrated embodiment, which combines the output of the differential amplifier _M2 and the ramp function generator 5.
The lower signal is selected and sent to the control valve 11.

4 is an external feedback circuit to the regulator 1 in which a feedback capacitor (integrating capacitor) C _I is connected between the output of the signal selector 7 and the input of _the differential amplifier _M2 ; amplifier _m2
and the variable resistor R _I form an integrating circuit.

2 and 3 are contacts that clamp the proportional gain and integral time constant of the controller 1 at startup to a constant value, respectively. Contact 2 is connected in parallel with the variable resistor _RP for setting the proportional gain, and contact 3 is connected in parallel with the variable resistor _RI . is connected to via a resistor r.

6 closes the contacts 2 and 3 at startup to clamp the proportional gain and integral time constant of controller 1 to a constant value, and after completion of startup, the deviation between the set signal Es and the process signal is smaller than the preset value. This alarm has the function of opening the contacts 2 and 3 to release the clamp when the temperature is low.

In addition, in the figure, contacts 2 and 3, ramp function generator 5,
The alarm device 6 and signal selector 7 are newly provided according to the present invention, and include operational amplifiers M ₁ and M ₂ and a resistor R.
The portion surrounded by the dashed line consisting of _{the P} and R _I capacitors C constitutes a well-known P _I operation regulator.

Next, the operation of the above embodiment will be explained with reference to FIG. First, the outputs of the regulator 1, that is, the operational amplifier M ₂ and the ramp function generator 5 are set as E ₀₁ and E ₀₂ respectively, and the output of the low signal selector 7 is set as E 01 and E 02 respectively.
E ₀ , the values of capacitors C and C _I are equal,
Let that value be C _I. Further, let e be the potential at the connection point between the output side of the differential amplifier M ₁ and the variable resistor _RP , and let r be the value of the resistor connected in series with the contact 3.

When starting a process as well as stopping it, it is generally E
Since the state is _S > E _P , the output of regulator 1
Since the amplifiers M ₁ and M ₂ have high gains, E ₀₁ has reached the saturation value on the Pupus side.

Therefore, E ₀₁ >E ₀₂ and the output E ₀ of the low signal selector 7 is equal to the output E ₀₂ of the ramp function generator and E ₀
=E ₀₂ . Here, when the ramp function generator 5 is started at time to by applying a start signal to the start signal terminal 8, its output E ₀₂
(=E ₀ ) gradually increases as shown in FIG. 2, and the control valve 11 is driven. As a result, the process signal E
When _P gradually increases and approaches the setting signal E _S , feedback is applied to the regulator 1 through the external feedback circuit 4, and the output E ₀₁ of the regulator 1 decreases.
E ₀₁ = E ₀₂ , and after time t' it switches to the regulator circuit, and after switching, the output E ₀ of the low signal selector 7 becomes the output E ₀₁ of the regulator 1, and the output from the regulator 1 until the clamp is released. Determined by a clamped time constant
PI control is performed.

Incidentally, the time t' at which the regulator 1 is switched can be adjusted by the rate of change of the output signal of the ramp function generator 5 or by the capacitors C and C _I and the resistor r, as will be described later.

In this case, the ramp function generator 5 is set to a sufficiently large value by the signal supplied to the target value setting terminal 10, and continues to rise to the set target value after time t'.

On the other hand, if the alarm device 6 is set in advance to the set value E _S close to the target control deviation value, the control deviation will be large at the time of startup, and the alarm 6 will be in an alarm state, and the contacts 2 and 3 will be closed and the proportional gain of the controller will be increased. , the integration time constant is clamped to a constant value. After startup, it is controlled by the output E ₀₂ of the ramp function generator 5, and from time t' onwards, the PI control is determined by the time constant clamped by the regulator 1, and the process signal E _P approaches the set signal E _S. If the control deviation between the two falls below the preset value △, the alarm 6 becomes a non-alarm state, contacts 2 and 3 are opened, and the proportional gain and integral time constant of the controller are set to the optimal control during steady operation. It returns to constant and startup is completed.

Therefore, E ₀₁ = E ₀₂ and if the time constants for clamping from the time t' when switching to the regulator and the start completion time t'' are set appropriately, the optimum PI control for the process from start-up can be achieved without overshoot. can be transitioned to with a smooth start-up.

The conditions for switching to control using the output E ₀₂ of the ramp signal generator 5 and the output E ₀₁ from the regulator 1 will be described.

In Figure 1, the differential amplifier M ₂ at the beginning of startup
Since a feedback circuit is not formed, the voltage ε = 0 at the negative input terminal does not hold, and E ₀₁
is completely unselected. Therefore, it is necessary that ε=0 at the time when switching occurs. The following formula holds true for the current flowing into the negative input terminal of differential amplifier _M2 .

i ₁ + i ₂ + i ₃ = 0 ... (1) where i ₁ is the current flowing through capacitor C, i ₂ is the current flowing through resistors RI and r, and i ₃ is capacitor C
is the current flowing through _I. That is, i ₁ = Cd/dt(e-ε), i ₂ = e-ε/r+e-ε
/R, i ₃ =C _I d/dt(E ₀ −ε). Since ε=0 and r<<RI, the following relationship exists immediately before switching.

Cd/dte+e/r+C _I d/dtE ₀ =0 rCd/dte+e+rC _I d/dtE ₀ =0...(2) On the other hand, the differential amplifier _M1 with contacts 2 and 3 clamped (closed) When the gain is K ₀ , e=K ₀ (E _P −E _S ) (3). Substituting equation (3) into equation (2) and transforming it, we get E _P −E _S =−rC _I /K ₀ d/dtE ₀ −rC/K ₀ d/dt(E _P −E _S ) ……( 4) The voltage E ₀₂ is of opposite polarity to the voltage (E _P -E _S ), and (E _P -E _S ) is a decreasing change value.

The value of the second term in equation (4) is d/dt(E _P
-E _S ) is small and can be made smaller than the first term. In such a case, the first term is decisive, but switching occurs when equation (4) holds. r is adjustable, and when r is small, the control deviation (E _P −E
_S ) is small. If r is set to a large value, switching will occur earlier while leaving a slight control deviation. Even if the second term cannot be ignored, switching can still be performed at a time suitable for the process by adjusting r. In either case, since the switching is performed in a state where the control deviation (E _P -E _S ) is small, the subsequent automatic control is stable.

Note that the output voltage E ₀₁ of the differential amplifier M ₂ changes immediately before equation (4) is established, but it can change rapidly in a state where no feedback circuit is formed. When E ₀₁ passes through the selector 7 (switching to automatic control), a feedback circuit is also formed and the differential amplifier M ₂ enters into operation as a feedback amplifier.

As described above, according to the present invention, it is possible to select the optimum control value for the proportional gain and integral time constant of the regulator by considering only the control characteristics during steady operation of the process, and to keep the optimum control value selected. By selecting the rate of change of the output signal of the ramp function generator, it is possible to shift to the optimum P _I control for the process with a smooth rise without overshoot.

Therefore, even in processes under harsh conditions where the process signal fluctuates sharply at start-up and the start-up and stop are severe, it is possible to perform optimal control over both start-up and steady-state operations without overshooting. can.

In addition, if the output of the ramp function generator can be held at a desired value by applying a hold signal as in the example, it will be possible to easily wash the piping at startup, which will uniquely improve the optimal startup characteristics. can be programmed.

In the example, the output of the ramp function generator is gradually increased, and the lower of this output and the output of the regulator is selected by the low signal selector to be used as the control signal. It is also possible to use the signal selector 7 as a high signal selector, to gradually decrease the output of the ramp function after activation, and to use the higher of the output of the ramp function and the output of the regulator as the control signal.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a characteristic diagram for explaining the operation of FIG. 1, and FIG. 3 is a characteristic diagram for explaining the conventional regulator with an anti-reset wind-up circuit. It is a diagram. 1: Controller, M ₁ , M ₂ : Differential amplifier, R _P : Variable resistor for proportional gain setting, r, R _I : Resistor, C,
C ₂ : Capacitor, 2: Contact for proportional gain clamp, 3: Contact for integral time constant clamp, 4: External feedback circuit, 5: Ramp function generator, 6:
Alarm, 7: Signal selector, 11: Control valve.

Claims (1)

[Claims] 1. A controller that performs PI calculation control by comparing a process signal and a set signal, comprising: a mechanism for clamping the PI constant of the controller to a predetermined value; a ramp function generator; a signal selector that compares the output signal with the output signal of the ramp function generator and selects one output as the control output; and an operational amplifier that constitutes the output side of the signal selector and an integration circuit of the regulator. An external feedback circuit with an integrating capacitor connected to the input side and an alarm that releases the clamp on the PI constant of the controller after the deviation between the process signal and the setting signal reaches a predetermined value are installed. At startup, when the deviation between the signal and the set signal is large, the PI constant of the regulator is clamped and the output of the ramp function generator is used as the control output, and after startup, the output of the regulator and the output signal of the ramp function generator are The overshoot prevention function is characterized in that the output of the regulator is set as a control output at the time when A control device having: