EP3851678A1 - Method for controlling the speed of a centrifugal pump - Google Patents

Abstract

The present invention relates to a method for regulating the speed of a centrifugal pump, the controller of the pump control calculating a target speed of the pump drive taking into account a target and actual delivery head and the actual speed, the controller for determining the control value at least components of the affinity law is based on, in particular assumes a quadratic relationship between speed and delivery head for the calculation of the manipulated variable.

Description

The present invention relates to a method for regulating the speed of a centrifugal pump operated in a hydraulic circuit, the controller of the pump control determining a set speed of the pump drive taking into account a set and actual delivery head and the actual speed.

Today's speed-controlled centrifugal pumps of the prior art mainly use PI controllers to determine the setpoint speed. The P component can be used to determine how quickly the pump reaches its setpoint. The I component can be used to set how dynamic control deviations should be eliminated. With an I component of zero, there is always a permanent control deviation.

Since the configuration of both controller parameters influences the dynamics of the overall system, the controller parameters cannot be set separately, but only with a holistic view of the system dynamics. In practice, the correct setting of these parameters is therefore an enormous challenge. The classic setting rules for PI or PID controllers also relate to linear systems, otherwise a linearization must be carried out in an operating point beforehand. If the latter is the case, the controller parameters found are usually only optimally set in the vicinity of the selected operating point.

For the reasons mentioned above, the use of a PI controller for a speed-controlled centrifugal pump is not the optimal solution. On the one hand, pumps show a highly non-linear behavior, on the other hand, pumps must be able to be operated stably in different operating ranges. For example, the operating point when starting up the pump can be different from that during constant pump operation. The setting of the controller parameters of a PI or PID controller is therefore always based on a compromise between these different operating points of the pump.

Due to the problems described above, other control approaches have already been tested. One example are so-called affinity regulators, which work on the basis of the laws of affinity. These controller types are considered to be robust, especially in different operating situations, and make the previously discussed complex setting of the controller parameters obsolete. A disadvantageous limitation of these types of controllers, however, is that they have so far only been able to be used in closed hydraulic circuits. In the case of an open circle, in which a geodetic height may have to be overcome, the mathematical relationship between the variables mentioned changes and the control does not lead to a satisfactory result.

A suitable controller modification to solve the aforementioned problem is therefore being sought.

This object is achieved by the method with the features of claim 1. Advantageous embodiments of the method are the subject matter of the dependent claims.

According to the invention, a method for regulating the speed of a centrifugal pump operated in a hydraulic circuit is proposed. The basis of the method is a controller of the pump control, which calculates a target speed of the pump drive taking into account a target and actual delivery head as well as an actual speed. This controller is neither a PI nor a PID controller. The controller modification sees the expansion in order to provide at least one correction parameter for taking into account and compensating for a geodetic height to be managed by the pump. With the help of this correction parameter, the control approach can also be used for open hydraulic circuits.

It is particularly preferred if, by means of the correction parameter, a shift of the delivery head-speed curve, in particular a vertical shift of the delivery head-speed curve, is effected. This allows the geodetic height to be compensated for without any problems.

The proposed control approach uses the affinity law to determine the setpoint or manipulated variable; such a controller type is referred to below as an affinity controller. According to an advantageous embodiment, the control approach is based on a quadratic relationship between speed and delivery head for the calculation of the manipulated variable. This results in a parabolic control curve that is optionally shifted up or down by the correction parameter.

With such controller types, the quadratic relationship between the set speed and the set delivery head is also preferably set in relation to the square relationship between the actual speed and the actual delivery head for calculating the set speed. The target speed can be determined on the basis of this ratio. By inverting the quadratic relationship, the non-linear behavior of the pump can be compensated at the same time. This allows the pump to be stabilized like a linear system.

According to a further preferred embodiment of the invention, the parabola of the control curve defined by the quadratic relationship between speed and delivery head is shifted into the coordinate origin by the correction parameter, whereby a geodetic height can be compensated either on the pressure or suction side of the pump.

In practice, the correction value depends on the conditions of the entire hydraulic system. The geodetic height and thus the required value of the correction parameter can also change during ongoing pump operation. For this reason, it is desirable for the value of the correction parameter to be determined automatically by the pump control while the pump is in operation.

One possibility for automatic determination of the correction parameter is to initially assign a definable initial value to the correction parameter when the pump is started up. A suitable initial value is, for example, the value zero. The required value of the correction parameter to compensate for the geodetic height can then be determined from the control error that occurs during operation, because both the target delivery head and the actual delivery head are known to the pump control. The value of the correction parameter can then be adjusted until the target delivery head is reached.

beschreiben. Der Ausdruck err charakterisiert hier den Fehlerwert, der sich zwischen Soll-Förderhöhe und sich einstellender Ist-Förderhöhe einstellt. Durch Erfassen der Differenz zwischen Soll- und sich einstellender Ist-Förderhöhe ist der Pumpensteuerung demzufolge der aktuelle Fehlerwert bekannt und die Pumpensteuerung kann auf Grundlage der obigen Gleichung den Korrekturwert k berechnen.Mathematically, the correction parameter k can be determined using the following equation $$k=err\cdot {\left(\frac{H_{\mathrm{Should}}}{H_{\mathrm{I.}st}}-1\right)}^{-1}$$

describe. The expression err here characterizes the error value that is set between the target delivery head and the actual delivery head that is established. By detecting the difference between the setpoint and the actual delivery head that is set, the pump control system therefore knows the current error value and the pump control system can calculate the correction value k on the basis of the above equation.

It is particularly preferred if the determination of the correction parameter takes place regularly, particularly preferably repeated at periodic intervals. This is particularly useful if the geodetic height changes while the pump is running can change. It is also useful to determine the correction parameter immediately after commissioning. Alternatively, it is possible to determine the correction value at random, indefinite times.

In addition to the method according to the invention, the present invention also relates to a centrifugal pump with a pump control for carrying out the method according to the invention. Accordingly, the same advantages and properties result for the centrifugal pump as were already discussed in detail above with reference to the method according to the invention. For this reason, a repetitive description is dispensed with.

Figur 1:

eine Drehzahl-Förderhöhenkennlinie im geschlossenen hydraulischen Kreis;

Figur 2:

eine Drehzahl-Förderhöhenkennlinie im offenen hydraulischen Kreis;

Figur 3:

ein Zeit-Förderhöhediagramm zur Verdeutlichung der Regelungsqualität der erfindungsgemäßen Regelung gegenüber konventionellen Regeltechniken.

Further details and advantages of the invention are to be explained in more detail below with the aid of several figure representations. Show it:

Figure 1:

a speed-delivery head characteristic in a closed hydraulic circuit;

Figure 2:

a speed-delivery head characteristic in an open hydraulic circuit;

Figure 3:

a time-delivery head diagram to illustrate the control quality of the control according to the invention compared to conventional control techniques.

The core idea of the present invention consists in the use of a new type of regulator for regulating the speed of a centrifugal pump. In contrast to what is proposed in the prior art, a PI or PID controller is not used, but instead a so-called affinity controller, which uses the affinity laws for setting / setpoint determination and, consequently, a quadratic relationship between the speed and the resultant Head of the centrifugal pump goes out.

beschreiben lässt. Zudem ist in der Figurendarstellung der Figur 1 exemplarisch eine Ist-Drehzahl n_ist sowie eine Soll-Drehzahl n_soll angedeutet. Aufgrund des quadratischen Zusammenhangs ist das Verhältnis zwischen den Soll- und Istwerten gemäß der folgenden Gleichung festgelegt: $$\frac{H_{\mathit{soll}}}{H_{\mathit{ist}}}=\frac{n_{\mathit{soll}}^2}{n_{\mathit{ist}}^2}$$

For a functional description of this affinity regulator, refer to the Figure 1 referenced. In the diagram, the delivery head is plotted against the set pump speed. The diagram shows in detail a quadratic relationship between the delivery head H and the speed n, which is expressed by the equation $$H\sim {\mathrm{<figure-callout\; id="2"\; label="n"\; filenames="imgf0003.png"\; state="\{\{state\}\}">n</figure-callout>}}^2$$

can be described. In addition, the figure shows the Figure 1 an actual speed n _ist and a target speed n _{soll are} indicated by way of example. Due to the quadratic relationship, the relationship between the setpoint and actual values is determined according to the following equation: $$\frac{H_{\mathit{should}}}{H_{\mathit{is}}}=\frac{n_{\mathit{should}}^2}{n_{\mathit{is}}^2}$$

The target and actual delivery head are always known during operation. The current actual speed is also known. The target speed (control value) is calculated by the affinity controller according to Eq. 3 as follows: $$n_{\mathit{should}}=\sqrt{\frac{H_{\mathit{should}}}{H_{\mathit{is}}}}\cdot n_{\mathit{is}}$$

In this way, the controller permanently sets the correct target delivery head. By inverting the quadratic relationship between delivery head and speed, the non-linear behavior of the pump is compensated and the pump can be stabilized like a linear system. The controller is robust in different operating situations and there is no need to set the controller parameters.

One limitation of the affinity regulator is that in the previous configuration it can only be used in closed hydraulic circuits. In the case of an open circle, in which a geodetic height has to be overcome, the H / n curve shifts out Figure 1 and the mathematical context changes.

The idea of the present invention is to modify the affinity regulator in such a way that it also leads to acceptable results within an open hydraulic circuit. According to the invention, this is achieved by adding a parameter to the affinity controller to describe the geodetic height.

The curve in Figure 2 shows the relationship between delivery head and speed under the assumption that a suction-side geodetic head prevails. Due to the geodetic height, the parabolic curve no longer runs through the origin of the coordinates, but is shifted downwards by the value k. $$H\sim n^2-k$$

If a pressure-side geodetic height were to prevail, the curve would be shifted upwards. If the affinity regulator is used in its previous form, the target delivery head would not be reached, but a delivery head that is shifted by an error value (err).

This error (err) can be corrected by using the relationship from Eq. 1 and Eq. 2 is extended by the parameter k: $$\frac{H_{\mathit{should}}+k}{H_{\mathit{is}}+k}=\frac{n_{\mathit{should}}^2}{n_{\mathit{is}}^2}$$

In this way, the parabola is shifted back to the origin and the target speed is calculated according to Eq. 6: $$n_{\mathit{should}}=\sqrt{\frac{H_{\mathit{should}}+k}{H_{\mathit{is}}+k}}\cdot n_{\mathit{is}}$$

One challenge is that the geodetic delivery head and thus the parameter k required for the controller may not be known. Therefore, within the scope of this idea, it is proposed to determine the parameter k during operation. Therefor k is initially assumed to be zero when the controller is switched on. As in Figure 2 shown, the actual delivery head is consequently missed by an error value (err). The error value (err) is known by the pump control recording the difference between the setpoint and the actual delivery head. By equating Eq. 2 and Eq. 5, the correction value k can be determined on the basis of the error value. $$k=\mathit{err}{\left(\frac{H_{\mathit{should}}}{H_{\mathit{is}}}-1\right)}^{-1}$$

The determination of k takes place regularly in pump operation, since the geodetic delivery head can change during operation.

Figure 3 shows a test result with three different controller types. The controlled system tested is a pump that has to overcome a geodetic delivery head. A PI controller, a conventional affinity controller and an affinity controller with the addition of the correction parameter according to the invention are tested as controllers. The desired delivery head for all tested controller types is 5 m.

Figure 3 shows a time diagram of the actual delivery head set by the individual controller types. Curve 2 of the conventional affinity controller without correction of the geodetic height initially shows a very strong overshoot, but due to the iterative correction of the control deviation, the target delivery head is still achieved. The PI controller with curve 3 also reaches its setpoint, but this result requires a lot of effort when setting the controller parameters correctly. Curve 1 of the affinity regulator, taking into account the geodetic height, shows the best result. There is no overshoot, no permanent control deviation and the target delivery head is reached quickly. In addition, it is not necessary to set controller parameters. This ensures a high level of stability of the controller even when the operating behavior changes.

Claims (10)

Method for regulating the speed of a centrifugal pump, the controller of the pump control determining a target speed of the pump drive taking into account a target and actual delivery head and the actual speed,
characterized in that
the controller is based on at least components of the affinity law for determining the manipulated variable, in particular assumes a quadratic relationship between speed and delivery head for the manipulated variable calculation. Method according to Claim 1, characterized in that the controller determines the target speed from the ratio of the quadratic relationship between the target speed and the target delivery head and the quadratic relationship between the actual speed and the actual delivery head. Method according to Claim 1 or 2, characterized in that the controller takes into account a correction parameter for describing the geodetic height for calculating the setpoint speed. Method according to 3, characterized in that the parabola defined by the quadratic relationship is shifted into the coordinate origin by the correction parameter. Method according to one of Claims 3 or 4, characterized in that the value of the correction parameter is determined while the pump is running. Method according to Claim 5, characterized in that the correction parameter is assigned a known initial value when the pump is started up, in particular assumes the value zero. Method according to Claim 5 or 6, characterized in that the correction parameter is derived from the control error, in particular the difference between the setpoint and actual delivery head, while the pump is in operation. Method according to claim 7, characterized in that the correction parameter by means of the equation $k=\mathit{err}\cdot {\left(\frac{H_{\mathit{should}}}{H_{\mathit{is}}}-1\right)}^{-1}$

is calculated. Method according to one of the preceding claims 5 to 7, characterized in that the determination of the correction parameter takes place during the pump operation during initial start-up and / or regularly at periodic intervals and / or randomly. Centrifugal pump with a pump control for carrying out the method according to one of the preceding claims.

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