JPH0731554B2 - Flow controller - Google Patents

Description

TECHNICAL FIELD The present invention relates to a flow rate control device applied to, for example, a pipeline for fuel or air supplied to a heating furnace.

(Prior Art) Conventionally, as shown in FIG. 3, this type of flow rate control device performs a direct comparison between a flow rate measurement value and a required flow rate setting value,
It is usual to perform control so as to eliminate the deviation.

The configuration can be roughly divided into a measurement control target process, a measurement system, and a control system, and these are connected in series in a loop to form a so-called feedback control loop.

That is, in the example shown in FIG. 3, the fixed throttle mechanism 12 such as an orifice provided in the pipeline 11 to be controlled, and the fixed throttle mechanism
A measurement system configured by combining a differential pressure sensor 13 that detects the pressure difference between before and after 12, and a signal from the differential pressure sensor 13, usually a signal proportional to the square of the flow rate and the required flow rate value as the required differential pressure value. A subtractor 14 that inputs the converted set value and outputs a deviation between the two, a so-called PID controller 15 that has a function of performing a proportional operation, an integral operation, and a derivative operation as necessary based on the deviation. The control system is configured by combining a control valve 17 having a drive mechanism 16 that operates the valve opening degree according to the output signal of the PID controller 15.

(Problems to be Solved by the Invention) The PID control adopted in the above-mentioned device has high versatility and is widely used in many fields. On the other hand, the linearity of each element in a series of feedback control loops Due to characteristics such as responsiveness and its combination, it is often difficult to make stable adjustments to control and maintain stability, and it is necessary to make multiple adjustments to stabilize control at the start of operation, as well as changes in process status during operation. However, there is a drawback in that frequent adjustments are required to stabilize the control corresponding to.

In addition, many of these adjustments must be performed by trial and error, which makes handling difficult and time-consuming.

In a feedback control system using PID control, the element characteristics that deteriorate the control stability can be broadly classified into nonlinear characteristics and response delay characteristics.

As described above, the non-linear characteristic element in the flow rate measurement control system using the fixed throttle mechanism is that the differential pressure of the fixed throttle mechanism is proportional to the square of the flow rate as described above, which changes the feedback control loop gain depending on the flow rate level. Therefore, a control error may occur due to insufficient gain, or a vibration control state (so-called cycling or hunting) may occur due to excessive gain.

As a solution to this, a method of fixing the loop gain by using a differential pressure sensor equipped with a square root calculation mechanism, or a so-called linearization by adding a square root calculator is used in some cases. In particular, since the measurement accuracy is significantly reduced in the low flow rate region, it is unsuitable for those that require high measurement control accuracy even in the low flow rate region, such as combustion control and mixing control, and naturally expensive due to the addition of functions and equipment. Becomes

Other non-linear characteristic elements include mechanical frictional force and backlash of the control valve and its drive mechanism, and reaction force due to the fluid of the control valve.

A dead zone element with a non-linear characteristic is artificially added in order to reduce wear (shortening of the life) of the control valve and its drive mechanism due to the vibration control state of the flow rate control loop or valve opening control loop. However, it is necessary to keep in mind that if the dead zone is increased carelessly, the control accuracy will deteriorate.

As the response delay characteristic element of the flow rate measurement control system, since the response speed of the normal measurement control target process, that is, the fluid pipe is relatively fast, the response delay mainly relating to the valve drive becomes a problem.

The response delay characteristics related to valve drive differ depending on the valve drive system, such as electric type, pneumatic type, hydraulic type, etc., but basically all are high-order delays of 2nd or higher, and vibration when combined with PID control. A control state is likely to occur.

Note that the pressure and differential pressure sensor and its pressure guiding path may have a considerably slow response depending on the method and structure.

In addition to PID control, generally, the more non-linear characteristic elements and response delay characteristic elements in the feedback control loop (higher order response delay), the more unstable the control becomes, and the vibration control state also occurs in the flow rate measurement control. In many cases, the life of the control valve and its drive mechanism is shortened, the life of the control valve is shortened, operation and adjustment are made difficult, and the production accuracy, product quality, and yield are deteriorated due to reduced control accuracy. is there.

The present invention has been made to solve the conventional problems, and an object of the present invention is to provide a flow rate control device capable of improving control stability and accuracy.

(Means for Solving the Problems) In order to solve the above problems, the present invention is provided in a pipe that is a flow rate control target, the flow rate is adjustable, and the relationship between the opening degree and the flow rate coefficient is known. A variable throttle mechanism, an opening sensor that detects the opening degree of the variable throttle mechanism, a pressure sensor that can detect pressure in the inlet side conduit of the variable throttle mechanism, and an opening degree of the variable throttle mechanism in advance. And the relationship between the outlet side pressure of the variable throttle mechanism and the flow rate are stored, and based on this relationship and the signal from the pressure sensor and the flow rate set value separately input, the variable throttle mechanism is stored. An arithmetic unit that calculates the opening required every moment, and a controller that outputs a control signal for eliminating the deviation between the two based on the signal from the arithmetic unit and the signal from the opening sensor, In response to this control signal, the variable diaphragm The mechanism is a variable aperture drive mechanism for opening the mechanism to a predetermined opening.

(Embodiment) Next, an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 shows a flow rate control device according to the present invention, in which a variable flow rate control mechanism 2 in which flow rate can be adjusted and a relationship of flow rate coefficient with respect to an opening is known in a pipeline 1 which is a flow rate control target, and the variable throttle mechanism 2. The pressure sensor 3 enables pressure detection at the entrance side of the throttle mechanism 2.
Are provided, and the output signal of the pressure sensor 3 is input to the arithmetic unit 4 using a microprocessor or the like.

In this computing unit 4, the flow coefficient with respect to the opening degree of the variable throttle mechanism 2 and the relationship between the outlet side pressure of the variable throttle mechanism 2 and the flow rate are stored in the form of a formula or a table. Based on the relationship and the signal from the pressure sensor 3, and the flow rate setting value that is separately determined and is input to the calculator 4, the opening degree required for the variable throttle mechanism 2 is calculated every moment.

On the other hand, the variable throttle mechanism 2 is provided with an opening sensor 5 capable of detecting the opening. The output signal of the opening sensor 5, that is, the opening at the present time and the opening calculated by the computing unit 4 are displayed. The signal is input to the controller 6, where the deviation between the two opening signals is calculated,
A control signal for controlling the opening of the variable aperture mechanism 2 is output to the variable aperture drive mechanism 7 so as to eliminate this deviation, and the variable aperture drive mechanism 7 is formed to set the variable aperture mechanism 2 to a predetermined aperture. There is.

As a result, the flow rate in the pipeline 1 becomes a predetermined flow rate.

Here, the first important point of the present invention is that the required opening of the variable throttle mechanism 2 is expressed by the relational expression between the opening and the flow coefficient, the relationship between the outlet pressure and the flow, that is, the flow coefficient, A method of taking out the measurement system out of the feedback control loop by directly calculating from the side pressure, that is, the original pressure of the fluid to be controlled and the required flow rate setting value will be described in detail.

By the way, as a precondition for this method, the difference between the value of the opening pressure at least at that point in time and the value when the required opening is calculated and controlled based on these values is small enough to be ignored. , And the variation of the flow coefficient on the outlet side of the variable throttle mechanism must be small enough to meet the required accuracy, but these conditions are used for combustion control, mixing control, etc. where many types of flow rate measurement control devices of this type are used. In general, they are easily obtained, so these are assumed below.

Differential pressure P ₁ and the pressure P ₂ of the exit side of the variable throttle mechanism 2 is proportional to the square of the flow rate Q, a source pressure P ₀ The addition of both.

These relationships can be expressed as follows using the flow rate coefficient V of the variable throttle mechanism 2 and the flow rate coefficient N on the exit side.

P ₀ = P ₁ + P ₂ (1) P ₁ = (Q / V) ² (2) P ₂ = (Q / N) ² (3) ∴P ₀ = Q ² (1 / V ² + 1 / N ² ) (1a) The flow rate coefficient N on the outlet side of the variable throttle mechanism 2 is known and
Since P ₀ is obtained as a measured value, the required flow rate Qs can be rearranged by substituting it into the equation (1a) to obtain the following equation (4), that is, the required flow rate coefficient Vs of the variable throttle mechanism 2.

Relationship F between the flow coefficient V of the variable throttle mechanism and its opening S
Since (v) is also known, the required opening degree Ss can be obtained by the equation (5) as follows.

Ss = F (Vs) (5) The relationship between the flow coefficient V of the variable throttle mechanism 2 and its opening S does not necessarily have to be a function (mathematical expression), and is calculated in the form of, for example, a table. It may be stored in the unit 4 and the opening degree S corresponding to the flow rate coefficient V may be obtained by interpolation calculation.

In addition, in the embodiment shown in FIG. 1, since the feedback control loop via the pressure sensor 3 and the computing unit 4 is configured, it looks like the conventional method at first glance. It is an auxiliary minor loop for flow control, and as described above as a prerequisite,
Since the fluctuation of the source pressure of the measurement control target fluid is usually very small (the feedback control loop gain is very small) as compared with the fluctuation of the required flow rate set value, the control operation is hardly disturbed. In other words, this loop is just a slight correction.

Incidentally, when the flow rate error due to the fluctuation of the source pressure can be ignored, the pressure sensor 3 can be omitted and the loop can be eliminated by storing the value of the source pressure as a constant in the calculator 4.

If a differential pressure sensor 8 for measuring the differential pressure of the variable throttle mechanism 2 and / or a pressure sensor 9 for measuring the pressure on the outlet side are added as shown in FIG. 2, the above formula (2) or / and ( By giving these measured values, that is, the differential pressure P ₁ of the variable throttle mechanism 2 and / or the output side pressure P ₂ to the equation 3),
Since the flow rate coefficients V and N are known, the flow rate measurement value Q can be calculated and taken out as the output signal 28 to be used for display / recording.

Further, it is also possible to equip an automatic calibration system for the characteristics of the variable throttle mechanism 2 and the flow coefficient on the output side by utilizing these sensors and the like.

Next, the second point of the present invention is to enhance the function and performance of the opening control of the variable throttle mechanism 2, that is, the variable throttle at a speed proportional to the square root of the deviation between the required opening and the opening at that time. Mechanism 2
Of opening the variable throttle mechanism 2 at a speed that is proportional to the deviation between the required opening and the opening at that time or the square root thereof, and a speed that is proportional to the changing speed of the required opening. The method of controlling the degree will be described in detail.

The mechanical inertia of the movable portion of the variable aperture mechanism 2 and its variable aperture drive mechanism 7 is one factor of the response delay, and the control error due to the delay at the time of acceleration of the movable portion is also a problem, but there is a further problem. , Overshoot due to inertia in the process from deceleration to stop.

This is one of the main causes of the vibration control state.The flow control in which the response delay related to the drive of the variable throttle mechanism accounts for the major part of the delay of the feedback control loop, especially the measurement system is taken out of the feedback control loop and used as a feedback control loop. Has a large weight in the apparatus according to the present invention which is limited to the opening control of the variable throttle mechanism.

Since digital control by a microprocessor or the like is intermittent control, such overshoot may be promoted.

Further, if the gain of the feedback control loop is reduced to such an extent that overshooting does not occur even when the deviation is maximum, the gain becomes insufficient as the deviation becomes smaller, which causes a problem that the control error due to the tracking delay increases.

In order to prevent such an overshoot due to mechanical inertia of the movable part, the opening control of the variable aperture mechanism of the present invention (that is, the controller 6, the variable aperture drive mechanism 7, the variable aperture mechanism 2, the aperture sensor 5). The main feedback control loop consisting of (1) is provided with a minor feedback control loop for controlling the opening / closing speed, and the speed setting value is set to a value proportional to the square root of the deviation between the required opening and the opening at that time.

The control operation will be described below in accordance with how to obtain the proportional constant in this case.

The acceleration of the movable part is almost determined by the force obtained by subtracting the frictional force from the force generated by the drive source during acceleration, the force obtained by adding the frictional force to the force generated by the drive source during deceleration, and the inertia of the movable part. Since the value is the same during acceleration and deceleration, the maximum acceleration is larger during deceleration.

Here, since the problem is the process from deceleration to stop, this process will be described assuming that the maximum acceleration during deceleration is αm.

The time t from decelerating from an arbitrary speed u at a constant acceleration αm until stopping and the opening amount x that moves during that time can be obtained from the known equation of motion as follows.

t = u / αm (6) x = u · t / 2 = u ² / (2 · αm) (7) Here, deceleration is performed at the maximum acceleration αm from the point of response speed, that is, it is required from speed u to stop. Assuming that the time t is the shortest, the opening amount x that moves during this time is also the minimum, and is uniquely determined by the speed u.

Therefore, rearranging the equation (7) into the following equation (7a) for obtaining the speed u, and letting x be the deviation between the required opening and the opening at that time, u is the opening control. The proportional constant G, which is obtained as the required speed and is multiplied by the square root of the deviation x between the required opening and the opening at that time, is also determined by the following equations (7a) and (8).

Depending on the speed control method or the acceleration control method in the speed control loop, it may not always be possible to decelerate at the maximum acceleration, but in that case, depending on the application, generally it will stop from deceleration even if the response speed is somewhat sacrificed. It is a good idea to avoid the overshoot by setting the proportional constant G according to the maximum acceleration that can always be taken in the process of.

Also, when the opening control (including speed control, etc.) is performed digitally, intermittent control is used, but depending on the interval, the minimum value of the opening deviation, that is, the minimum time from the speed corresponding to the opening deviation to the stop Since the value is regulated, it may be necessary to take measures to prevent overshoot due to a dead zone or the like.

By the way, the method of controlling the opening / closing speed of the variable throttle mechanism to a speed proportional to the square root of the deviation between the required opening and the opening at that time does not consider the changing speed of the required opening at all, so to speak, it is static. However, this is sufficient when the speed of change of the required opening is very small compared to the opening / closing speed.However, flow rate control usually has a large speed of change of the required opening, so error due to tracking delay Often occurs.

In order to compensate for this, the opening and closing is performed at a speed that is the speed for achieving the required opening plus the change speed of the required opening, that is, the speed set value signal for speed control of the variable throttle mechanism 2 is set to the required opening and the opening at that time. The speed value is the speed proportional to the square root of the deviation from the degree and the change speed of the required opening degree.

(Effects of the Invention) As is apparent from the above description, according to the present invention, the flow path is provided, the flow rate can be adjusted, and the relationship between the opening degree and the flow coefficient is known. A certain variable throttle mechanism, and an opening sensor that detects the opening degree of this variable throttle mechanism,
A pressure sensor provided in the inlet side of the variable throttle mechanism to detect pressure, a flow coefficient with respect to the opening of the variable throttle mechanism, and a relationship between the outlet pressure and flow rate of the variable throttle mechanism are stored in advance. An arithmetic unit that momentarily calculates the opening required for the variable throttle mechanism based on this relationship and the signal from the pressure sensor and a flow rate setting value that is separately input, and a signal from this arithmetic unit. A controller that outputs a control signal for eliminating a deviation between the two based on a signal from the opening sensor; and a variable diaphragm drive mechanism that receives the control signal and sets the variable diaphragm mechanism to a predetermined opening. It is formed from.

For this reason, most of the elements related to flow rate measurement control are clarified theoretically or logically, the combinations are organized and integrated, and their characteristic values are quantitatively determined and built into the device at the design stage or manufacturing stage. As a result, it is possible to stabilize the control and improve the accuracy, and it is possible to significantly reduce adjustment points, particularly trial-and-error adjustment at the time of operation start or during operation, which is often inconvenient. .

[Brief description of drawings]

FIG. 1 is a system diagram showing a flow rate control device according to the present invention, and FIG.
FIG. 3 is a system diagram showing a flow rate control device according to another embodiment of the present invention, and FIG. 3 is a system diagram showing a conventional flow rate control device. 1 ... Pipe line, 2 ... Variable throttle mechanism, 3 ... Pressure sensor,
4 ... calculator, 5 ... opening sensor, 6 ... controller, 7 ...
… Variable aperture drive mechanism.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Akira Nakayasu 2-4-7 Kyomachibori, Nishi-ku, Osaka City, Osaka Prefecture Chugai Prox Co., Ltd. (72) Hideaki Narahara 2-4-4 Kyomachibori, Nishi-ku, Osaka City, Osaka Prefecture No. 7 in Chugai Prox Co., Ltd. (56) Reference JP-A-57-120624 (JP, A) JP-A-59-103116 (JP, A) JP-A-59-62920 (JP, A) JP-A-62- 229311 (JP, A) Actually opened 63-130806 (JP, U)

Claims (1)

[Claims] 1. A variable throttling mechanism which is provided in a pipe which is a flow rate control target, whose flow rate is adjustable, and whose relationship between the opening degree and the flow coefficient is known, and the opening degree of this variable throttling mechanism are detected. Opening sensor, a pressure sensor provided on the inlet side of the variable throttle mechanism to detect pressure, a flow coefficient with respect to the opening degree of the variable throttle mechanism, and the pressure and flow rate on the outlet side of the variable throttle mechanism. And a calculator for calculating momentarily the opening required for the variable throttle mechanism based on this relationship, the signal from the pressure sensor, and the flow rate set value separately input. Based on the signal from the calculator and the signal from the opening sensor, a controller that outputs a control signal to eliminate the deviation between the two, and the variable throttle mechanism to a predetermined opening by receiving this control signal. Variable aperture drive mechanism Flow control device according to claim.