METHOD FOR CONTROLLING VARIABLE SPEED FLUID PUMPS

Description

[0001] The present invention relates to a method for controlling variable-speed fluid pumps, as well as a computer program and a computing unit for carrying out the method. Background of the invention

[0002] In hydraulics, fluid pumps with a fixed displacement pump driven by a variable-speed drive (so-called variable-speed fixed-displacement pumps) are often used. During operation of such pumps, the flow rate and / or discharge pressure are typically regulated by adjusting the speed accordingly. Such pumps are used, for example, for hydraulic units, pump drives, or servo-hydraulic actuators, for example, in machine tools or in plastics processing, such as injection molding machines, blow molding machines, presses, etc., where high discharge pressures are required.

[0003] The control of such variable-speed pumps can be implemented, for example, by a cascaded controller structure in which a setpoint pressure is specified and includes a pressure control loop, a speed control loop, and a current control loop for the drive motor. However, the speed control loop is controlled by the load torque. M L This is disturbed, which is adjusted depending on the current pressure via the pump displacement. The pressure control loop is also disturbed by the pump's leakage flow. These disturbances impair the control quality of the drive system.

[0004] WO 2018 / 0207157 A2 discloses a method for controlling pressure and flow of hydraulic fluid through a hydraulic control unit, comprising receiving a pressure command from an electronic control unit. The received pressure command is processed to generate a pressure-based torque command and a shaft angle-based torque command or a volume-based torque command. The generated pressure-based torque command and the generated shaft angle-based torque command or the volume-based torque command are further processed to generate a duty cycle for a field-oriented controlled motor.

[0005] DE 10 2019 220 322 A1 relates to a method for operating a variable-speed variable displacement pump, in which a conveyor mechanism adjustable in a displacement volume per working cycle is driven by means of a variable-speed drive, wherein, within the scope of a control, at least one variable is controlled to a setpoint by specifying a speed setpoint for a speed of the drive and a setpoint for a parameter determining the displacement volume per working cycle, wherein the speed setpoint is adapted by means of a pre-control when the setpoint for the parameter determining the displacement volume per working cycle changes.

[0006] It is therefore desirable to improve the control of variable speed fluid pumps. Disclosure of the invention

[0007] According to the invention, a method for controlling a pump with a variable-speed drive, as well as a computer program and a computing unit for implementing the method, are proposed. Advantageous embodiments are the subject of the dependent claims and the following description.

[0008] In particular, a method is proposed in which a target pressure value is first input into a cascaded controller structure, wherein the controller structure comprises a pressure control loop, and wherein the controlled system of the pressure control loop comprises a speed control loop into which a target speed is input as a manipulated variable of a pressure controller of the pressure control loop. The controlled system of the speed control loop also comprises a drive control loop into which a target drive torque is input as a manipulated variable of a speed controller of the speed control loop. In addition, a pilot control value for the drive torque is formed as the result of a time differential equation of the target pressure, and this pilot control value is added to the target drive torque input into the controlled system of the speed control loop.

[0009] According to the invention, the time differential equation of the setpoint pressure comprises an inverse system model of the controlled system of the cascaded controller structure.

[0010] This inverse system model can, for example, be formed depending on a pressure dynamic in a load volume in which the pressure is controlled and a torque dynamic of the drive.

[0011] The pilot control value for the drive torque can be determined in exemplary embodiments by the following equation: M cmd ff = J eq 2 πC h V g p ¨ L d + 2 π V g J eq C l + d M C h p ˙ L d + d M 2 πC l V g + V g 2 π p L d

[0012] Included ist J eq an equivalent moment of inertia, C h a hydraulic capacity, V g a displacement volume of the pump, p L d< a target load pressure, C l a pressure-dependent leakage coefficient and d M a speed-related friction torque of the pump.

[0013] It is also possible to create a pre-control value for the speed, whereby the pre-control value for the speed is determined by: n cmd ff = C h V g p ˙ L d + C l V g ⋅ p L d

[0014] This is C h the hydraulic capacity, V g the pump’s displacement, p L d< the target load pressure and C l the pressure-dependent leakage coefficient.

[0015] This pilot control value can then be added to the target speed, which is output by the pressure controller as a control variable. This prevents the speed control loop from working against the torque pilot control.

[0016] Optionally, the target pressure value can be input into a target value filter which is configured to output a target trajectory for the target pressure value and the derivatives of the target pressure value, and this target trajectory can then be input into the differential equation for forming the pilot control value.

[0017] Such a setpoint filter may, for example, comprise a second-order state variable binomial filter.

[0018] Overall, the flatness-based feedforward control proposed here dynamically compensates for load torques and disturbance volume flows. This improves the system's control behavior and stability margin. The drive controller can be designed for noise immunity, as the follow-up behavior is significantly improved by the feedforward control. This reduces the load on the speed and pressure controllers. At the same time, the feedforward control is insensitive to sensor noise.

[0019] A computing unit according to the invention, e.g. a control unit of a hydraulic system, is configured, in particular in terms of programming, to carry out a method according to the invention.

[0020] Implementing a method according to the invention in the form of a computer program or computer program product with program code for performing all method steps is also advantageous, as this entails particularly low costs, especially if an executing control unit is also used for additional tasks and is therefore already present. Suitable data storage devices for providing the computer program include, in particular, magnetic, optical, and electrical storage devices, such as hard disks, flash memories, EEPROMs, DVDs, and others. Downloading a program via computer networks (Internet, intranet, etc.) is also possible.

[0021] Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawings.

[0022] It is understood that the features mentioned above and those to be explained below can be used not only in the combination specified in each case, but also in other combinations or on their own, without departing from the scope of the present invention.

[0023] The invention is illustrated schematically in the drawing using exemplary embodiments and is described in detail below with reference to the drawing. Character description

[0024] Figur 1 shows schematically a fluid pump with a variable speed drive; and Figur 2 shows an exemplary controller structure according to a possible embodiment. Detailed description of the drawing

[0025] Figur 1 schematically shows components of a variable-speed fluid pump for which the methods described here can be applied. In this example, the pump 10 has a pumping mechanism 20 that can pump in two directions, a variable-speed drive 30, and a computing unit 40 for operating the pump.

[0026] The pumping mechanism 20 can, for example, be designed as a fixed-displacement pump that achieves a specific delivery volume per stroke or per revolution. The drive 30 is designed here as a standard motor comprising an asynchronous motor 31 and a frequency converter 32. Alternatively, synchronous servomotors or other drive units can also be used. The speed n of the asynchronous motor 31 is variable, with the speed being adjusted, for example, by the current applied to the drive motor 31.

[0027] The computing unit 40 can be used to control the pump 10, and can be supplied with at least a target delivery pressure and, optionally, a target volume flow. The computing unit 40 can comprise a drive controller implemented in hardware or software, which determines the required manipulated variables and transmits them to the motor 30. Optionally, the actual pressure can be determined via a sensor. The computing unit 40 can further determine, e.g., by monitoring the electrical current and the rotational speed, which drive torque is currently being delivered by the drive, and can then transmit, for example, a target rotational speed to the drive unit 30 as a control signal.Alternatively, it is also possible for the drive unit 30 itself to comprise certain control or regulation elements and thus for the computing unit 40, for example, to specify only a target torque for controlling the drive and thus the pump and to transmit this torque to the drive 30 as a control signal.

[0028] It is understood that this pump drive system is described only as an example, and the controller variants described below are not limited to this system. In particular, the present invention can also be applied to any other pumps and is not limited to fixed-displacement pumps. Likewise, it is possible for certain parts of the cascaded controller structure to be implemented in several separate processing units, for certain sensors to be absent in other systems, or for additional parameters to be measured via sensors, or for the drive unit to be designed differently than described here.

[0029] Figur 2 shows a controller structure for controlling a variable speed fluid pump according to a possible embodiment.

[0030] A cascaded controller structure is provided, which nests a pressure control loop, a speed control loop, and a drive control loop, such as a current control loop. The current control loop 230 for the pump drive is shown here only in a simplified and exemplary manner and can be implemented in any way, e.g., as a PI controller; for a different drive variant, a different control loop can also be used at this point to control the drive torque of a pump drive. The current control loop 230 receives a target drive torque. M R< cmd as a reference variable and feeds the actual value 232 of the drive torque M act This ensures a controlled drive torque M M receive.

[0031] The target drive torque M R< cmd in turn is the control variable of a speed control loop. The speed control loop receives a setpoint speed n cmd of the pump as a reference variable for a speed controller 220 and performs an actual speed n act back 222, whereby the current control circuit 230 together with the moment of inertia element 224, which consists of the drive motor ment M M with the moment of inertia of pump and motor the controlled speed n outputs, forms the controlled system of the speed control loop.

[0032] The target speed n cmd forms the control variable of a pressure control circuit, whose pressure controller 210 as a reference variable a Soll-Lastdruck p L d< receives and an actual -Wert p L of the load pressure 212. The control system of the pressure control loop thus comprises the speed control loop 220, 222, 224, the current control loop 230, 232 and further control elements. The further control elements of the system comprise a proportional element 214 to control the controlled speed obtained from the speed control loop n by taking into account the specified swallowing volume V g the pump per revolution into a volume flow Q p Then, using a PI element 216, the volume flow Q p by taking into account the equivalent compression module E' Öl und of the load volume V L , in which the pressure is regulated, the regulated load pressure p L The actual pressure p act is in turn fed back via the pressure control circuit 212 to the input of the pressure regulator 210.

[0033] The speed control loop is controlled by a load torque M L disturbed, which results from the existing load pressure p L and the swallowing volume V g the pump can be represented as a proportional element 240. The resulting load torque M L acts on the drive torque M M in contrast to.

[0034] In addition, the pressure control circuit is controlled by the leakage volume flow Q L the pump is disturbed, which results from the load pressure p L and the leakage coefficient C l and can be modeled as proportional element 242. The leakage volume flow Q L affects the volume flow Q p towards the pump.

[0035] In order to compensate for these disturbances, a pilot control 250 can now be provided according to the invention, which adjusts the commanded drive torque M R< cmd for the pump drive, which is output by the speed controller 220 as a manipulated variable and enters the current control circuit 230 as a reference variable, by a pre-control value M ff< cmd corrected. The actual value that is thus input into the current control loop or the control system of the speed control loop is therefore the sum of the drive torque M R< cmd and the input tax value M ff< cmd .

[0036] The pre-control value can be calculated using an inverse system model of the controlled system. This system model can be configured according to the Figur 2 shown controller structure can be derived as follows: The pressure dynamics in the load volume, ie the change p L of the load pressure is p ˙ L = 1 C h ⋅ V g 2 π ⋅ ω M − C l ⋅ p L where C h = V L / E' Öl the hydraulic capacity with the replacement compression module E ' Öl and the load volume V L is , V g the displacement of the pump is ω M is the torque of the drive, and C l is the pressure-dependent leakage coefficient.

[0037] By differentiating with respect to time, we obtain p ¨ L = 1 C h ⋅ V g 2 π ⋅ ω ˙ M − C l ⋅ p ˙ L

[0038] The torque dynamics on the motor-pump shaft are: ω ˙ M = 1 J eq M M − V g 2 π p L − d M ω M where J eq is the equivalent moment of inertia, which results from the moments of inertia J Pumpe the pump and J Motor of the drive motor, J eq = J Pumpe + J Motor ; where M M is the drive torque; where V g is the displacement of the pump, and where d M a speed-related friction torque of the pump.

[0039] Inserting equation (3) into equation (2) results in: p ¨ L = 1 C h ⋅ V g 2 π ⋅ 1 J eq M M − V g 2 π p L − d M ω M − C l ⋅ p ˙ L

[0040] This yields a second-order differential equation that represents the system model. The inverse system model can now be used as a flatness-based feedforward control 250 of an additive torque for the trajectory sequence of a required pressure dynamic. p L , p ˙ L , p ¨ L can be used. For this purpose, equation (4) can be rearranged according to the drive torque using equation (1), so that the pre-control law results in an inverse system model: M cmd ff = J eq 2 πC h V g p ¨ L d + 2 π V g J eq C l + d M C h p ˙ L d + d M 2 πC l V g + V g 2 π p L d

[0041] Since this is a cascaded control loop, the superimposed speed control loop can optionally be additionally supplied with the pilot control signal from which the torque pilot control is derived. This prevents the speed control loop from working against the torque pilot control. The corresponding pilot control signal can be calculated using equation (1) by dividing the equation according to the rotational frequency. ω cmd ff or is resolved according to the speed: n cmd ff = C h V g p ˙ L d + C l V g ⋅ p L d

[0042] This pre-control value can then be added to the input of the speed control loop.

[0043] To implement the pilot control 250, the time derivatives of the set pressure p L p ˙ L p ¨ L by a setpoint filter 260. For this purpose, a second-order state variable binomial filter can be used, for example. As an example, the setpoint trajectory can be determined using the following differential equation: p ¨ L d = 1 τ 2 p cmd − 2 τ p ˙ L d − 1 τ 2 p ¨ L where τ is the time constant of the setpoint filter. In principle, the setpoint pressure values ​​can also be determined differently. p cmd the commanded target pressure value.

[0044] The target trajectory obtained from equation (7) can now be inserted into the pre-control law, equation (5), and the pre-control moment M calculated with it ff< cmd , can be calculated additively on the input of the current control loop 230. Likewise, the desired trajectories can be inserted into the pre-control law (6) for the speed.

[0045] A simpler alternative to the pre-control for the drive controller is to only control the load torque M L , 240, so that the pre-control value for the drive torque in this variant is M ff< cmd = ( V g / 2π ) p cmd from the commanded setpoint pressure value. A setpoint filter is not required in this case. The pilot control value is then added to the output value of the speed controller 220 or the input value of the current control loop 230, as in the previous embodiment.

[0046] By applying the model-based feedforward control according to the invention, the control behavior of the control is improved, since the actuator (ie the drive motor of the pump) reacts immediately and in a highly dynamic manner, even if no large control deviation occurs.

[0047] The follow-up behavior of the drive control (i.e., the current control loop) is also significantly improved by the feedforward control. This reduces the load on the speed and pressure controllers. At the same time, the feedforward control is insensitive to sensor noise.

[0048] A control system as described here can be used for all variable-speed pumps, especially for pumps used to control hydraulic units or hydraulic actuators. The described method with pilot control can also be applied to the force control of servo-hydraulic actuators; in this case, a pressure is also controlled via a motor-pump unit, which can then be converted into a force value based on the cylinder surfaces.

Claims

1. Method for controlling a pump with a variable-speed drive, comprising: inputting a target pressure value (pcmd) into a cascaded controller structure, wherein the controller structure comprises a pressure control loop (210, 212, 214, 216), wherein the controlled system of the pressure control loop comprises a speed control loop (220, 222, 224), into which a target speed (ncmd) is input as the manipulated variable of a pressure controller (210) of the pressure control loop; wherein the controlled system of the speed control loop comprises a drive control loop (230, 232), into which a target drive torque (MRcmd) is input as the manipulated variable of a speed controller (220) of the speed control loop; forming (250) a pilot control value (Mffcmd) for the drive torque as the result of a temporal differential equation of the target pressure; and adding the pilot control value to the target drive torque that is input into the controlled system of the speed control loop, wherein the temporal differential equation of the target pressure comprises an inverse system model of the controlled system of the cascaded controller structure.

2. Method according to Claim 1, wherein the inverse system model is formed on the basis of pressure dynamics in a load volume in which the pressure is controlled, and torque dynamics of the drive.

3. Method according to one of the preceding claims, wherein the pilot control value for the drive torque is determined by: M cmd ff = J eq 2 πC h V g p ¨ L d + 2 π V g J eq C l + d M C h p ˙ L d + d M 2 πC l V g + V g 2 π p L d where Ch is the hydraulic capacity, Vg is the displacement volume of the pump, Cl is the pressure-dependent leakage coefficient, Jeq is the equivalent moment of inertia,dM is a speed-related friction torque of the pump, and pLd is the target load pressure.

4. Method according to one of the preceding claims, furthermore comprising: forming a pilot control value (nffcmd) for the speed, wherein the pilot control value for the speed is determined by: n cmd ff = C h V g p ˙ L d + C l V g ⋅ p L d where Ch is the hydraulic capacity, Vg is the displacement volume of the pump, Cl is the pressure-dependent leakage coefficient and pLd is the target load pressure, and adding the pilot control value to the target speed output by the pressure controller (210) as the manipulated variable.

5. Method according to one of the preceding claims, furthermore comprising: inputting the target pressure value into a target value filter (260) which is configured to output a target trajectory for the target pressure value and the derivatives of the target pressure value, and inputting the target trajectory into the differential equation in order to form the pilot control value.

6. Method according to Claim 5, wherein the target value filter comprises a second-order state variable binomial filter.

7. Computing unit (40) comprising a processor configured to perform the method according to one of the preceding claims.

8. Computer program comprising instructions which, when the program is executed by a computer, cause the latter to perform the method according to Claims 1 to 6.

9. Computer-readable data carrier on which the computer program according to Claim 8 is stored.