Controlling a technical variable using a control cascade

Abstract

The invention relates to a method for controlling a technical variable, comprising the steps of: providing a control cascade having at least one current controller in the innermost region of the control cascade; and engaging a torque pilot control at the input of the current controller, wherein an acceleration torque component and / or a friction torque component can be pilot-controlled and a determined external torque component is pilot-controlled, wherein the external torque component pilot-controls at least one process-related cyclic disturbance.

Description

[0001] 202412308 Foreign version Fair copy

[0002] 1

[0003] Description

[0004] Control of a technical quantity using a control cascade

[0005] The invention relates to a method for controlling a technical quantity with a control cascade having at least one current controller in the innermost part of the control cascade, and an associated control unit.

[0006] It is an essential requirement for production machines to operate drives with high accuracy in order to manufacture high-quality products.

[0007] Machine drives must therefore operate with high precision to maintain the desired positions, speeds, and accelerations. Inverter-driven motors are used to achieve high levels of accuracy. Even with high speeds and the associated increase in productivity, maintaining the necessary precision remains essential. For this purpose, cascaded control systems such as current controllers, speed controllers, and position controllers are employed in the drive controllers. However, deviations between the target and actual position can occur, particularly during highly dynamic movements and under the influence of external disturbances, such as a cutting tool entering the material or mechanical shocks. These deviations negatively impact product quality and the efficiency of the production process.

[0008] Since control systems are always reactive, additional feedforward controls such as friction torque feedforward and acceleration torque feedforward are often integrated. These known disturbances can thus be compensated for in advance, relieving the controller of this burden and allowing it to concentrate on unforeseen disturbances.

[0009] US Patent US 5,105,135 A discloses a feedback controller for NC-controlled machine tools in which the position and speed of a workpiece are detected and compared to the desired position. This comparison is used to calculate the working speed and compare it to the current speed, which determines the torque of a drive motor. A torque compensation signal generator, which receives the current speed, generates a compensation signal according to a predefined speed / torque function. 202412308 Foreign version Fair copy

[0010] 2

[0011] The publication LIANG MANAN ET AL: "A Nonlinear Friction Identification Method Combining Separable Least Squares Approach and Kinematic Orthogonal Property", INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING, KOREAN SOCIETY FOR PRECISION ENGINEERING, SPRINGER, Vol. 23, No. 2, January 12, 2022 (2022-01-12), presents a method for identifying nonlinear friction parameters in servo-driven mechanisms that utilizes the separable least squares (SLS) method to optimize the Stribeck model with low computational effort. The additional application of the kinematic orthogonality (KOP) property eliminates the need to estimate inertial forces and accelerations, thereby minimizing measurement noise and enabling open-loop operation.

[0012] Against this background, an object of the present invention is to provide a method and a control unit that minimize deviations between the target and actual positions in a driven machine, both under varying operating conditions and in the presence of external disturbances. This object is achieved by the features of the independent claims. Advantageous embodiments are specified in the dependent claims.

[0013] Regardless of the grammatical gender of a particular term, persons with male, female or other gender identities are included.

[0014] The invention relates to a method for controlling a technical quantity, comprising the following steps:

[0015] - Providing a control cascade with at least one current controller at the center of the control cascade,

[0016] - Intervention of a torque feedforward control at the input of the current controller, wherein an acceleration torque component and / or a friction torque component can be feedforward controlled and a determined external torque component is feedforward controlled, wherein the external torque component feedforward at least one process-related, cyclic disturbance.

[0017] Control variables such as position or speed are converted into a target torque for an electric drive. This torque is then directly proportional to the drive's required current. In addition to a drive-specific current, which maintains the electric field, the rotating electromagnetic field is determined by a voltage and frequency. With a suitable predefined value at any given time, 202412308 Foreign version Fair copy

[0018] 3

[0019] The target torque ensures that the motor is moved at the correct position and speed at all times.

[0020] However, torque is subject to various influencing factors such as acceleration, friction and external effects.

[0021] External effects arising from a process in which the controlled technical variable is used, and which occur cyclically, constitute a disturbance. This data is not available to a drive controller, such as a PLC. Furthermore, these effects have specific dependencies on the process. For example, a change in process speed also affects the controlled technical variable.

[0022] By taking into account the external effects, which are process-specific and cyclical, the technical quantity is advantageously operated at the correct operating point at all times, particularly by means of the current controller and a provided drive.

[0023] Depending on the application, only the external torque component, or the external torque component plus the acceleration torque component and / or a friction torque component, can be pre-controlled. In applications where a user utilizes friction torque pre-control in another unit, for example, in a drive control system that may be in addition to a higher-level control unit, it is important to ensure that this signal is not double-pre-controlled.

[0024] For example, the control concept provides that the actual torque is output as a feedforward control variable.

[0025] Advantageously, no intervention in the control structure takes place. Therefore, the integration of the described feedforward control does not cause any instability of the drive and enables easy commissioning.

[0026] According to one embodiment, at least one process-related, cyclical disturbance is caused by a punching process, a cutting process, a pressing process, a printing process, or a transfer or gripping process. 202412308 Foreign version Fair copy

[0027] 4

[0028] Examples of external effects include, in cutting / punching applications, immersion in the material; in printing presses, the printing cylinder with the raised areas of the printed image; in presses, the placement of the press tools; and in conveyor belts, the automated placement of products onto a conveyor belt or processing station.

[0029] For example, a change in the process speed when cutting or punching material results in a different external cutting force. Similarly, a different material also results in a different external cutting force. Such process-specific influences are advantageously taken into account.

[0030] In one embodiment, the control cascade includes a higher-level speed controller in addition to the current controller. This forms a typical cascaded controller. The current controller time is, in particular, less than or equal to that of the speed controller.

[0031] According to one embodiment, the control cascade additionally includes a higher-level position controller. This controller is cascaded, whereby, in particular, the position controller time is greater than or equal to the speed controller, and the speed controller in turn is greater than or equal to the current controller.

[0032] In one embodiment, the acceleration torque component is pre-controlled. This advantageously takes into account the influence of acceleration on the required torque. Therefore, the torque is advantageously pre-controlled with respect to both external disturbances and acceleration dependency, thus minimizing the control error.

[0033] According to one embodiment, the acceleration torque component is determined as a function of mass inertia and angular acceleration. The higher the acceleration, the more torque is required to accelerate the same mass. For example, if the drive unit needs to travel from point A to point B at a specific speed, it must accelerate to this speed and decelerate upon reaching the target position. The required acceleration torque can be easily calculated using the mass inertia and the angular acceleration.

[0034] According to one design, the frictional torque component is pre-controlled. The frictional torque is speed-dependent. The faster the motor rotates, the more friction is generated. For the 202412308 foreign version fair copy

[0035] 5

[0036] The friction torque component is either calculated using a target speed or by measuring an actual speed and using torque feedforward control.

[0037] According to one embodiment, the frictional torque component is determined as a function of speed, in particular by measuring it during a test drive at several speed levels. The frictional torque changes depending on the speed. For example, the frictional torque is measured at several speed levels during the commissioning of a drive and then linearized between the speed measurement points.

[0038] According to one embodiment, an actual torque is supplied to the torque feedforward control. This advantageously utilizes a relatively easy-to-determine actual torque for feedforward control. The input occurs primarily during runtime. For example, a program block in a control program, such as one used in a PLC, outputs the actual torque. This actual torque might consist of acceleration torque and / or friction torque, as well as a torque resulting from external effects. Subtracting the friction torque and the acceleration torque leaves the torque resulting from external effects, which is then fed into the feedforward control in the next cycle. If, for example, a moment of inertia is specified, the acceleration torque component is subtracted. If a friction characteristic curve is specified, this is also subtracted, and only the remaining value is then used for feedforward control of the external torque component.

[0039] According to one implementation, the actual torque is measured in a first cycle and fed into the torque feedforward control in a subsequent cycle. This allows for rapid adjustment of the control system in response to a detected external disturbance without interfering with the control concept or adjusting the controller parameters.

[0040] According to one embodiment, a derived torque, calculated using a moving average over several successive cycles, is supplied to the torque feedforward control. This advantageously allows outliers or non-controllable external effects, which manifest themselves in the measurement of the 202412308 foreign version fair copy, to be eliminated.

[0041] 6

[0042] The torque becomes noticeable and is reduced without achieving maximum penetration in the next cycle.

[0043] According to one embodiment, the cycle is based on a continuously increasing value as a reference for cycle specification. A cycle can include a period of stationary position of the axis being controlled, while still requiring different torque to be applied at the same position, for example, during transient processes or back pressure during a standstill. Due to the variance of the process speed up to standstill, a continuously increasing value is used as the reference for cycle specification instead of a cycle with a defined time base. Such a reference value can be, for example, the master axis, but also an abstracted or imaginary quantity, such as a counter.

[0044] According to one embodiment, a maximum torque is defined as the maximum torque that can be controlled by the torque feedforward. This allows the user to limit the maximum torque that can be fed forward, and reduces the negative effects of errors in the feedforward value output due to measurement errors or other feedforward disturbances.

[0045] According to one embodiment, the torque feedforward control of the external torque component, in particular the torque feedforward control of all feedforward-controllable components, is activated, and in particular, switching on or off is implemented with a rising or falling ramp. This avoids jerky interventions.

[0046] The invention further relates to a control unit for regulating a technical quantity, designed and configured to carry out the method according to one of the embodiments described above. The program for carrying out the method runs, for example, on a PC, a PLC, an industrial PC, a drive controller, etc.

[0047] The invention further relates to a computer program comprising instructions which, when the program is executed by a computer, cause the computer to execute the method according to one of the embodiments described above, wherein the computer program is executed, in particular, on a virtual controller. 202412308 Foreign version Fair copy

[0048] 7

[0049] The invention is explained in more detail below with reference to exemplary embodiments and the figures. The figures show:

[0050] Figure 1 is a diagram showing recorded positions, speeds and positional deviations of a cutting blade to illustrate the effects of process-related, cyclic disturbances;

[0051] Figure 2 is a schematic representation of a control diagram according to an exemplary embodiment.

[0052] In the figures, functionally equivalent elements are provided with the same reference symbols, unless otherwise specified.

[0053] Figure 1 shows a diagram where, in the top section x, the recorded actual position x_t of a rotating cutting tool is traced over time t as it periodically plunges into the material being cut. Figure 1 illustrates the effects of process-related, cyclic disturbances in application setups according to the prior art. For example, paper, film, or metal is cut using this method. The corresponding actual velocity v_t is shown in the trace in the middle section v. The bottom section D shows the deviation between the target and actual positions. It can be seen that this deviation is not always zero. In areas of high acceleration, shown at points D_a1 and D_a2, the actual position deviates from the target position. Furthermore, external disturbances, such as the plunge of the blade into the material, also cause cyclic disturbances that lead to positional deviations.In the present example, immersion in and exit from the material manifest themselves as positional deviations D_e1 and D_e2.

[0054] Figure 2 shows a control concept according to an embodiment of the invention. The mechanics of an underlying machine, for example a cutting machine for cutting webs of paper, fabric, or similar materials, are described by the moment of inertia of the motor J_Mot driving the machine and the moment of inertia of the load J_Load, which in the model are coupled by means of a spring with stiffness c and damping constant d.

[0055] A position controller x-ctrl is provided, which determines the motor's position based on a setpoint x_set and an actual position x_act, which is determined by means of a sensor. 202412308 Foreign version Fair copy

[0056] The position is controlled by the position controller x-ctrl. A speed controller n-ctrl is subordinate to the position controller x-ctrl, receiving the speed value n_act, derived from the actual position value x_act using a differentiator Diff, as its input. A torque is then pre-controlled for the current controller i-ctrl in the innermost control cascade. For this purpose, a torque pre-control value M_pre is added to the output of the speed controller n-ctrl. Using an M / i converter M / i, a suitable current is supplied to the motor from the pre-controlled torque value and a fed-back current value i_act.

[0057] The torque and a suitable torque feedforward can be categorized according to three influencing factors:

[0058] 1. Acceleration: If a drive needs to travel from A to B at a specific speed, it must accelerate to this speed and decelerate upon reaching the target position. The required acceleration torque can be calculated using the inertia and the angular acceleration. A feedforward control system that adjusts the acceleration torque component has an effect on the irregularities D_a1 and D_a2 in the trace shown in Figure 1.

[0059] 2. Friction: The frictional torque increases depending on the speed. During the commissioning of a drive, the frictional torque can be measured at several speed levels and then linearized between the speed measurement points. A feedforward control system that adjusts the frictional torque component is already active in the trace shown in Figure 1.

[0060] 3. External Effects of Process-Related, Cyclic Disturbances: External effects can be identified in Figure 1 by the irregularities D_e1 and D_e2. The corresponding data describing these external effects is not available to a PLC or a drive controller. Therefore, the described feedforward control, which also feeds forward the external torque component, is advantageously used here. For example, in a printing press application, the position of the printhead experiences deviations due to mechanical shocks caused by protrusions on the printing cylinder.

[0061] Numerous alternative or additional process-related, cyclical disturbances can be taken into account by the feedforward control. In cutting or punching 202412308 Foreign version Fair copy

[0062] 9

[0063] In certain applications, the insertion of a cutting tool into the material generates a sudden force that affects the position of the drive. With gripper arms used to pick up objects, the initial approach to or contact with a product to be picked up can trigger a reaction in the drive.

[0064] The torque feedforward control M_prectrl is designed so that the individual torque components M_Acc, M_ext, and M_Fr can be switched on and off via parameterization. For example, only the external torque component M_ext caused by process-related, cyclic disturbances is fed into the control, or combinations of the external torque component with the acceleration torque component and / or the friction torque component are fed into the control. When all three influencing factors are activated, the individual torque components M_Acc, M_ext, and M_Fr are summed, and the resulting value M_pre is output by the torque feedforward control M_prectrl as the torque feedforward value.

[0065] The actual torque M_act is determined, for example, from the current value i_act before the motor using an i / M converter i / M and supplied as an input to the torque feedforward control M_prectrl at runtime.

[0066] Torque measurements used for feedforward control are subject to inaccuracies. According to the exemplary embodiment, filters are provided in the module that implements the feedforward control in the motor controller. For example, outliers and non-feedforward-controllable, non-cyclic external disturbances in the torque measurement are possible, which should not lead to maximum torque in the next cycle. Therefore, an averaged torque, calculated using a moving average over several periods, is used as the feedforward variable.

[0067] In this module, users can specify the maximum torque that may be pre-controlled. This ensures that a maximum limit can always be set, regardless of system measurements.

[0068] The module advantageously features additional modes besides pure feedforward control, allowing the user to assess data quality before the actual feedforward control process. The following modes are implemented according to the exemplary embodiment:

[0069] 1. A mode for measuring the input control variable,

[0070] 2. A mode for learning input tax (measurement and input tax) 202412308 Foreign version Fair copy

[0071] 10

[0072] 3. A manual mode in which the user can specify the variable to be controlled.

[0073] A quantity that can be easily pre-controlled by calculation does not need to be measured in a time-consuming manner. Therefore, it is advantageous to calculate and specify a setpoint value in the module for acceleration torque pre-control.

[0074] In many applications, it is desirable not to provide 100% of the torque as feedforward control. Therefore, a scaling factor is necessary to allow the feedforward variable to be adjusted relatively, for example, between 0% and 200%. This adjustment must be dynamic and possible at any given time, for instance, to ensure a breakaway torque of the drive—the maximum torque required to break an existing static connection between the stator and rotor components. The switching on and off of the feedforward control, or of individual components of the torque feedforward control (M_prectrl), is implemented with a rising or falling ramp. This prevents abrupt engagement.

Claims

202412308 Foreign version Fair copy 11 Patent claims 1. A method for controlling a technical quantity, comprising the following steps: - Providing a control cascade with at least one current controller (i-ctr) at the center of the control cascade, - Intervention of a torque feedforward control (M_prectrl) at the input of the current controller (i- ctrl), wherein an acceleration torque component (M_Acc) and / or a friction torque component (M_Fr) can be feedforward controlled and a determined external torque component (M_ext) is feedforward controlled, wherein the external torque component (M_ext) feedforward controls at least one process-related, cyclic disturbance, and an actual torque (M_act) is supplied to the torque feedforward control (M_prectrl).

2. Method according to claim 1, wherein the at least one process-related cyclic disturbance is caused by a punching process, a cutting process, a pressing process, a printing process or a transfer or gripping process.

3. Method according to claim 1 or 2, wherein the control cascade comprises a higher-level speed controller (n-ctrl) in addition to the current controller (i-ctrl).

4. Method according to one of the preceding claims, wherein the control cascade additionally comprises a superior position controller (x-ctrl).

5. Method according to one of the preceding claims, wherein the acceleration torque component (M_Acc) is pre-controlled.

6. Method according to claim 5, wherein the acceleration torque component (M_Acc) is determined depending on a mass inertia and an angular acceleration.

7. Method according to one of the preceding claims, wherein the frictional torque component (M_Fr) is pre-controlled.

8. Method according to claim 7, wherein the frictional torque component (M_Fr) is determined as a function of speed, in particular by measuring it in a test drive at several speed levels. 202412308 Foreign version Fair copy 12 9. Method according to claim 8, wherein the actual torque (M_act) is measured in a first cycle and supplied to the torque feedforward control (M_prectrl) in a subsequent cycle.

10. Method according to claim 8 or 9, wherein the torque feedforward control (M_prectrl) is supplied with a derived torque which is formed by means of a moving average value over several successive cycles.

11. Method according to claim 9 or 10, wherein the cycle is formed based on a continuously increasing size as a reference for a cycle specification.

12. Method according to one of the preceding claims, wherein a maximum torque is defined as the maximum torque that can be pre-controlled by the torque feedforward control (M_prectrl).

13. Method according to one of the preceding claims, wherein the torque feedforward control of the external torque component, in particular the torque feedforward control of all feedforward-controllable components, is switched on and in particular switching on or off with a rising or falling ramp is implemented.

14. Control unit for regulating a technical quantity, designed and configured to carry out the method according to one of the preceding claims.

15. Computer program comprising instructions which, when the program is executed by a computer, cause it to execute the method according to any one of claims 1 to 13, wherein the computer program is executed in particular on a virtual controller.

Images