EP3463840B1 - Method for pressing a workpiece with a predetermined pressing force - Google Patents

Description

The invention relates to a method for pressing a workpiece with a predetermined pressing force.

Various methods for pressing a workpiece are known from the prior art. The methods known from the prior art relate in particular to production plants in which workpieces are to be pressed with a predetermined pressing force. The workpieces must be pressed with a predetermined pressing force, the actual amount of the pressing force having only a small tolerance range.

The DE 19606842 A1 discloses a method for pressing a workpiece, wherein a press ram is provided which is coupled to a drive motor by means of a gear. During the pressing process, the drive motor is first accelerated to a maximum speed and then braked to a reduced speed. When the drive motor is operated at the reduced speed, the electrical variables of the drive motor are monitored in order to detect the torque applied to the drive motor.

The methods known from the prior art have the disadvantage that, in order to achieve the required pressing force while observing the required tolerance range, the pressing speed must be selected to be correspondingly low in order to prevent dynamic effects which distort the pressing force and which occur due to the inertia of the individual components of the drive train .

The object of the present invention was to overcome the disadvantages of the prior art and to provide a method which has an increased process speed while at the same time maintaining process accuracy.

This object is achieved by an apparatus and a method according to claim 1.

• Beschleunigen des Elektromotors in Zudrehrichtung auf eine vorgegebene Maximaldrehzahl, wodurch das Umformwerkzeug auf das Werkstück zubewegt wird;
• Betreiben des Elektromotors in Maximaldrehzahl bis die Antriebswelle des Elektromotors eine vorgegebene Anzahl an Spindelumdrehungen absolviert hat oder das Umformwerkzeug eine vorgegebene Position erreicht hat, wobei während diesem Verfahrensschritt das Umformwerkzeug frei auf das Werkstück zubewegt wird ohne dieses zu berühren;

• Reduzieren der Drehzahl des Elektromotors auf eine vorbestimmte reduzierte Drehzahl;
• Betreiben des Elektromotors in reduzierter Drehzahl bis von einer dem Elektromotor nachgeschalteten Messeinheit ein Presskraftanstieg detektiert wird welcher einen vorbestimmten Schwellenwert überschreitet oder am Elektromotor ein Drehmomentenanstieg detektiert wird welcher einen vorbestimmten Schwellenwert überschreitet, wobei der Presskraftanstieg dann auftritt, wenn das Umformwerkzeug am umzuformenden Werkstück zum Anliegen kommt;
• Umformen des Werkstückes unter ständiger Erfassung der Presskraft mittels der Messeinheit oder des Drehmomentes am Elektromotor bis die vorbestimmte Presskraft erreicht ist.

According to the invention is a method for pressing a workpiece with a predetermined pressing force by means of a forming tool, which is connected to a screw drive with a Electric motor is coupled, provided. A screw drive converts the rotary movement of a drive shaft of the electric motor into a translatory movement of the forming tool. The electric motor is controlled by a regulation. The process comprises the following process steps:

• Accelerating the electric motor in the direction of rotation to a predetermined maximum speed, whereby the forming tool is moved towards the workpiece;
• Operating the electric motor at maximum speed until the drive shaft of the electric motor has completed a predetermined number of spindle revolutions or the forming tool has reached a predetermined position, the forming tool being freely moved towards the workpiece during this process step without touching it;

• Reducing the speed of the electric motor to a predetermined reduced speed;
• Operating the electric motor at a reduced speed until an increase in pressing force is detected by a measuring unit connected to the electric motor which exceeds a predetermined threshold value or an increase in torque is detected on the electric motor which exceeds a predetermined threshold value, the increasing pressing force occurring when the forming tool comes to rest on the workpiece to be formed ;
• Forming the workpiece while continuously recording the pressing force using the measuring unit or the torque on the electric motor until the predetermined pressing force is reached.

An advantage of the method according to the invention is that the method is divided into a wide variety of method steps, the electric motor having a different speed in the individual method steps. This measure ensures that the pressing time can be shortened as much as possible and at the same time the required pressing force can be achieved as precisely as possible. In particular, when the electric motor is operated at a predetermined maximum speed, the forming tool is delivered as quickly as possible. In this process step, the forming tool is moved in the direction of the workpiece, care being taken that the forming tool is freely moved towards the workpiece and the forming tool is not yet in contact with the workpiece. It is only in the subsequent process step in which the electric motor is operated at a reduced speed that the forming tool comes into contact with the workpiece to be machined. As an alternative to a spindle drive, another means can also be used which is suitable for converting the rotary movement of the electric motor into a translatory movement of the forming tool. The predefined position of the forming tool can be detected, for example, using a linear measuring unit. The increase in torque in the electric motor can be detected, for example, by detecting the motor current in the electric motor.

Furthermore, it can be expedient if, after the increase in the pressing force is detected, the electric motor is braked to a predetermined minimum speed. The advantage here is that excessive braking and thus the transfer of the pressing force can be prevented by braking the electric motor to minimum speed.

Furthermore, it can be provided that the electric motor is operated at a minimum speed for a predetermined or predeterminable period of time until vibrations which occur in the drive system from the reduced speed to the minimum speed due to the braking process have largely subsided. The advantage here is that by operating the electric motor at the minimum speed in a predetermined period of time it can be achieved that the drive system can swing out and thus there is no falsification of the measured pressing force on the measuring unit. In extreme cases, it may be necessary to select complete standstill as the minimum speed. The vibrations that have to decay arise due to the inertia or the inertial forces of the individual components in the drive system and due to the abrupt braking maneuver.

In addition, it can be provided that when the workpiece is formed, the control of the electric motor is predetermined by the control on the basis of the pressing force measured in the measuring unit. After this predetermined period of time in which the sensor signal is falsified, it can be switched over to pressing force control in order to be able to achieve the required pressing force.

Also advantageous is a configuration according to which it can be provided that the reduced speed is between 0.1% and 100%, in particular between 0.5% and 99%, preferably between 50% and 80% of the maximum speed. The advantage here is that when the electric motor is operating at a reduced speed, a pressing force that exceeds a predetermined threshold value can be detected and, because of the reduced speed, there is then sufficient time to further reduce the speed and to set the required pressing force.

According to a further development, it is possible for the further activation of the electric motor to be predetermined by the control on the basis of the pressing force directly after the detection of the pressing force increase, the electric motor being braked to a predetermined minimum speed after the detection of the pressing force increase and in an initial period during the Braking process, the pressing force recorded in the measuring unit is overlaid by a pressing force based on a model calculation, and after the initial period the pressing force detected by the measuring unit serves as an input variable for the control. This alternative variant has the advantage that the process time can be further shortened and optimized. By blending the recorded pressing force with a pressing force based on a minor calculation, the measurement error that occurs due to the oscillation of the system after the braking process of the electric motor can be compensated for.

Furthermore, it may be expedient if the inertia and / or the spring stiffness and / or the damping and the angular or linear accelerations of the individual components installed in the drive train are taken into account in the model calculation. The advantage here is that the dynamic behavior of the drive train can be calculated precisely on the basis of these values or on the basis of these state variables, and thereby a falsification of the measured pressing force when braking or accelerating the electric motor can be compensated for.

In addition, it can be provided that the model calculation is adapted on the basis of the respective previous cycles in an iterative learning process, the temporal course of the measured value of the torque in the measuring unit, as well as the motor torque and the associated angle of rotation of the drive shaft in the electric motor being used to adapt the model calculation becomes. The advantage here is that the drive method can be adapted and improved during operation, which on the one hand can increase the accuracy to achieve the pressing force and can further shorten the process time.

Furthermore, it can be provided that a disturbance variable observer, in particular a Kalman filter, is used for the model calculation, which is also regulated in the first step and is only blended to the force detected in the measuring unit from a certain point in time. The advantage here is that such a disturbance variable observer can estimate the actual force actually present on the basis of the manipulated variable and the sensor signals, and the estimated external force can be predefined for the control system, which can improve the accuracy when the predetermined pressing force is reached.

Provision can also be made to expand the control loop by a pilot control for force and / or inertia compensation if the dynamics of the subordinate controllers are not sufficient. The feedforward controls can be derived using the mathematical models. It may be sufficient to use a highly simplified model, such as a pure rigid body system, which only takes the moments of inertia and no dynamic elements into account. Alternatively, a dynamic system as described in this document can be used for mathematical modeling.

Furthermore, it can be provided that a piezo sensor is used as the measuring unit, which is arranged in the region of the forming tool for detecting the pressing force. The advantage here is that a piezo sensor on the one hand has a high measurement accuracy and, in addition, has a very fast response behavior.

Furthermore, it can be provided that directly after the detection of the increase in the pressing force, the further control of the electric motor is predetermined by the control on the basis of a target trajectory of the pressing force value, the speed curve being calculated in a feedforward control from the target trajectory of the pressing force value. If a disturbance variable observer is used, the force actually acting can be estimated. Disruptions can be hidden by fading to this estimated force.

Furthermore, it can be provided that in a first phase after the detection of the pressing force increase, the pressing force value is estimated by a disturbance variable observer and that in a second phase after the detection of the pressing force increase, the pressing force is detected directly by the measuring unit and serves as an input variable for the control. By specifying the pressing force value by means of the disturbance variable observer, vibrations or disturbances in the system can be filtered so that there is no oscillation in the control. After the vibrations have subsided, the torque actually measured on the measuring unit can then serve as an input value for the control.

Furthermore, it can be provided that the transition between different speeds of the individual method steps is specified in such a way that no sudden increases in the Acceleration occur. By avoiding sudden increases in acceleration, the jerk that acts on the individual components of the press can be reduced and the longevity of the press can be increased.

The maximum speed to which the electric motor is accelerated does not necessarily have to correspond to the maximum possible speed of the electric motor. Rather, it is also possible that the maximum speed results from the process parameters and is a calculated value. The specified maximum speed can vary from one pressing process to the next.

gewählt wird.It can further be provided that a third-order low-pass filter of the form as a controller $R_F\left(s\right)=\frac{{\mathrm{<figure-callout\; id="1"\; label="k\; FP"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">k\; </figure-callout>}}_{\mathit{FP}}}{{\left(1+\frac{s}{{\omega }_{\mathit{Fg}}}\right)}^{\mathrm{3rd}}}$

is chosen.

Furthermore, it can be provided that controller parameters can be set using a loop shaping method.

The threshold value of the pressing force or the torque which are detected can be a predetermined or individually predeterminable absolute value of the pressing force, for example in N, or the torque, for example in Nm.

As an alternative to this, it is also possible that an absolute value of the pressing force or the torque is not specified as the threshold value, but that a predetermined or individually predeterminable increase in the pressing force per travel path of the forming tool (the travel path can be measured directly on the forming tool, or via the Number of revolutions of the drive motor can be calculated) or torque increase per angle of rotation unit of the motor is specified. The threshold value of the press force increase can be defined approximately in N per mm of travel or in N per ° angle of rotation of the electric motor. The threshold value of the torque increase can be defined in Nm per ° angle of rotation.

In yet another embodiment variant, it is conceivable for the threshold value to be a maximum change in the increase in pressing force per travel path of the forming tool (the Travel distance can be measured directly on the forming tool, or can be calculated from the number of revolutions of the drive motor) or the torque increase per angle of rotation unit of the motor. The maximum change in the press force increase per unit of rotation angle can be calculated, for example, by first deriving the function of the press force increase per unit of travel of the forming tool. This threshold value for the change in the torque increase can be defined approximately in ΔN per Δmm travel. The maximum change in the torque increase per unit of rotation angle can be calculated, for example, by the first derivative of the function of the torque increase per unit of rotation of the motor. This threshold value of the change in the torque increase can be defined approximately in ΔNm per Δ ° angle of rotation.

A regulation in the sense of this document can be understood as a two-degree of freedom force regulation with a subordinate motor regulation, whereby a control circuit with this regulation can also have additional pilot controls.

It can further be provided that a speed curve is calculated on the basis of the load characteristic and a desired target trajectory for the external pressing force. This speed picks up at the reduced speed and is brought to a standstill. This speed profile ensures that the external pressing force follows the desired target trajectory sufficiently well. This makes it possible to compensate for the remaining control deviation with a linear controller R _F. If a disturbance variable observer is used, then the estimated signal is controlled and, at the end of the trajectory, faded over to the measurement signal. If the disturbance variable observer is not available because the quality of the measurement signal is sufficiently good, then the measurement signal is controlled directly and therefore no cross-fading is carried out.

For a better understanding of the invention, this will be explained in more detail with reference to the following figures.

Fig. 1

eine schematische Darstellung eines möglichen Aufbaues einer Presse;

Fig. 2

ein Ablaufdiagramm einer ersten Regelungsstrategie zum Pressen eines Werkstückes;

Fig. 3

ein Strukturschaltbild des mechanischen Modells der Presse;

Fig. 4

eine Kraftverlaufsdarstellung der Presse;

Fig. 5

ein Strukturschaltbild eines Regelkreises für die Kraftregelung;

Fig. 6

eine exemplarische Regelstrecke einer Kraftregelung;

Fig. 7

ein Ablaufdiagramm einer weiteren Regelungsstrategie zum Pressen eines Werkstückes;

Fig. 8

ein Strukturschaltbild eines Regelkreises mit Störgrößenbeobachter und Lastvorsteuerung, Kraftvorsteuerung sowie Trägheitskompensation;

Fig. 9

ein Strukturschaltbild eines Regelkreises mit Störgrößenbeobachter und Kraftvorsteuerung sowie Trägheitskompensation;

Fig. 10

ein Strukturschaltbild eines Regelkreises mit Störgrößenbeobachter und Kraftvorsteuerung;

Fig. 11

ein Strukturschaltbild eines Regelkreises mit Störgrößenbeobachter und Lastvorsteuerung sowie Kraftvorsteuerung;

Fig. 12

ein Strukturschaltbild eines Regelkreises mit Lastvorsteuerung, Kraftvorsteuerung sowie Trägheitskompensation;

Fig. 13

ein Strukturschaltbild eines Regelkreises mit Kraftvorsteuerung sowie Trägheitskompensation;

Fig. 14

ein Strukturschaltbild eines Regelkreises mit Kraftvorsteuerung;

Fig. 15

ein Strukturschaltbild eines Regelkreises mit Lastvorsteuerung sowie Kraftvorsteuerung.

Each show in a highly simplified, schematic representation:

Fig. 1

a schematic representation of a possible structure of a press;

Fig. 2

a flowchart of a first control strategy for pressing a workpiece;

Fig. 3

a structural diagram of the mechanical model of the press;

Fig. 4

a force history representation of the press;

Fig. 5

a structural diagram of a control circuit for force control;

Fig. 6

an exemplary controlled system of a force control;

Fig. 7

a flowchart of a further control strategy for pressing a workpiece;

Fig. 8

a structural diagram of a control circuit with disturbance variable observer and load precontrol, force precontrol and inertia compensation;

Fig. 9

a structural diagram of a control circuit with disturbance variable observer and force feedforward control and inertia compensation;

Fig. 10

a structural diagram of a control circuit with disturbance variable observer and force feedforward control;

Fig. 11

a structural diagram of a control loop with disturbance variable observer and load precontrol and force precontrol;

Fig. 12

a structural diagram of a control circuit with load feedforward control, force feedforward control and inertia compensation;

Fig. 13

a structural diagram of a control circuit with force feedforward control and inertia compensation;

Fig. 14

a structural diagram of a control circuit with force feedforward control;

Fig. 15

a structural diagram of a control circuit with load feedforward control and force feedforward control.

In the introduction, it should be noted that in the differently described embodiments, the same parts are provided with the same reference numerals or the same component names, the disclosures contained in the entire description being able to be applied analogously to the same parts with the same reference numerals or the same component names. The location information selected in the description, e.g. above, below, to the side, etc., referring to the figure described and illustrated immediately, and if the position is changed, these are to be applied accordingly to the new position.

Fig. 1 shows a schematic representation of a process press 1. The process press 1 comprises an electric motor 2, and a forming tool 3 coupled to the electric motor 2. The forming tool 3 can act on a workpiece 4 in order to be able to deform it. Such a deformation can be an embossing, for example. Furthermore, it is also conceivable that the workpiece 4 is bent, for example by means of the forming tool 3. The forming process of the workpiece 4 can be automated. The forming tool 3 can have a wide variety of shapes.

It can also be provided that the electric motor 2 is designed as a servo motor. Such a servo motor can be a synchronous motor, for example. It can also be provided that the electric motor 2 is connected to a control 5. Furthermore, it can be provided that a frequency converter is designed which interacts with the electric motor 2 and specifies the speed of the electric motor 2.

How out Fig. 1 it can also be seen that a spindle drive 6 is coupled to the electric motor 2. Such a spindle drive 6 can be designed, for example, as a screw drive, preferably as a ball screw drive. A ball screw has the advantage that it has little play. As a result, high accuracy of the process press 1 can be achieved.

The rotary drive 6 of the electric motor 2 can be converted into a translatory movement of the forming tool 3 by means of the spindle drive 6.

Furthermore, it can optionally also be provided that a gear 7 is arranged between the spindle drive 6 and the electric motor 2, by means of which the speed of the drive shaft 8 of the electric motor 2 can be reduced.

If a transmission 7 is provided in the drive train, then a spindle 9 of the spindle drive 6 is coupled to a transmission output shaft 11 arranged on the transmission output 10 and has the same rotational speed as this.

If no transmission 7 is provided in the drive train, then the spindle 9 of the spindle drive 6 is coupled to the drive shaft 8 of the electric motor 2 and has the same rotational speed as this.

Furthermore, it is provided that a measuring unit 12 is arranged between the spindle drive 6 and the forming tool 3 and is designed to detect the pressing force applied to the forming tool 3. The measuring unit 12 can be designed in particular as a force sensor or as a load cell. It can preferably be provided that the measuring unit 12 is designed as a piezo sensor. The measuring unit 12 is coupled to the control 5.

Furthermore, it can be provided that a coupling 13 is provided for connecting the electric motor 2 and the gear 6 or for connecting the gear 7 and the spindle drive 6. The couplings 13 are used in particular for torque transmission between the individual components and are therefore arranged between the individual components.

It can further be provided that the spindle 9 of the spindle drive 6 is mounted on a bearing 14 which serves to absorb the axial forces and radial forces introduced into the spindle 9. Furthermore, it can be provided that the spindle drive 6 comprises a threaded nut 15, which is coupled to the spindle 9 and converts the rotational movement of the spindle 9 into a translatory movement of the threaded nut 15.

A slide 16 can be coupled to the threaded nut 15, which can serve to receive the forming tool 3. In particular, it can be provided that the measuring unit 12 is arranged between the slide 16 and the forming tool 3.

In an embodiment variant, not shown, it can be provided that the measuring unit 12 is integrated in the carriage 16.

It can preferably be provided that the forming tool 3 is detachably coupled to the slide 16. It can thereby be achieved that different forming tools 3 can be coupled to the slide 16 for different application requirements.

Furthermore, it can be provided that the carriage 16 is guided on a guide rail 17.

Based on Fig. 1 the general functioning of the process press 1 will now be explained.

The forming tool 3 is moved towards the workpiece 4 by means of the spindle drive 6, the spindle drive 6 being driven by the electric motor 2. In a first method step, the forming tool 3 is freely moved towards the workpiece 4, it being ensured that the forming tool 3 does not touch the workpiece 4. In other words, one can also speak of a delivery process.

At the end of this infeed process, a pressing surface 18 of the forming tool 3 comes into contact with the workpiece 4, as a result of which the force acting on the forming tool 3 increases suddenly. The forming tool 3 is then pressed into the workpiece 4, as a result of which the workpiece 4 is deformed by means of the forming tool 3.

It can be said that the pressing process is divided into two stages. The first stage is an infeed process in which the forming tool 3 is freely moved towards the workpiece 4 without touching it.

The second stage is a forming stage, in which the pressing surface 18 of the forming tool 3 bears against the workpiece 4 and the workpiece 4 is deformed by means of the forming tool 3 is, an increased torque must be applied to the drive shaft 8 of the electric motor 2.

During the infeed process, it can be provided that the electric motor 2 is superimposed, speed-controlled, until a predefined pressing force is exceeded or the impact of the forming tool 3 on the workpiece 4 is detected using a gradient method. In the forming stage, provision can be made for the electric motor 2 to be torque-controlled, with the measured pressing force being used to regulate the electric motor 2.

In the forming stage, a predefined pressing force can be set using a cascaded two degree of freedom control. This cascaded control consists of an internal speed control, a superimposed torque control or force control and a corresponding model-based pilot control.

With the help of the model-based pre-control, the pressing force that occurs is compensated for according to the load and the inertia of the drive. If the mechanical coupling between the electric motor 2 and the forming tool 3 is sufficiently stiff, the pressing force recorded on the measuring unit 12 can be used as a direct feedback variable for the torque control or force control.

The difficulty with the control is to keep the process speed high and to keep the pressing force within predetermined limits. If an ideal, trouble-free route is assumed, an engine speed curve can be found, which makes it possible to set a desired pressing force. In real use, however, faults and measurement noise occurring in the measuring unit 12 are to be expected.

In order to achieve a defined pressing force and to keep the process speed as high as possible, the control strategies according to the invention were developed.

As long as the forming tool 3 is freely moved towards the workpiece 4 and is not in contact with it, no significant increase in the pressing force actually applied to the forming tool 3 is to be expected. It is therefore advisable to specify an engine speed profile directly in this forming stage without additional torque control or force control. Only when the pressing surface 18 of the forming tool 3 contacts the workpiece 4, there is a rapid increase in the pressing force applied to the forming tool 3 and the torque control or force control becomes active. In the forming stage, an engine speed profile is specified in which different speed levels are continuously connected to one another. This ensures that the mechanical components of the process press 1 are not unnecessarily stressed and the excitation of vibrations in the system is kept low.

The aim of the regulation is to regulate the pressing force actually applied to the forming tool in such a way that it reaches a defined value, also referred to as a predetermined pressing force.

The pressing force actually applied to the forming tool 3 should be measured with the aid of the measuring unit 12 and serve as a feedback variable in the control. However, it should be mentioned that the pressing force measured in the measuring unit 12 only corresponds to the pressing force actually applied to the forming tool 3 if the forming tool 3 is not being accelerated or braked and therefore no dynamic effects occur due to the inertia of the individual components. In other words, the pressing force actually applied to the forming tool 3 can be measured precisely by the measuring unit 12 when the forming tool 3 is stationary or moving at a constant feed speed, this state also having to last for a certain period of time, so that vibrations have already subsided.

Fig. 2 shows a flow diagram of a schematic sequence of a first control strategy for pressing the workpiece 4.

There is a plus (+) on the decision paths for condition is fulfilled. A minus (-) stands for condition is not met.

In method step 1, the drive shaft 8 of the electric motor 2 is accelerated to the maximum speed. In order to accelerate the electric motor 2 to the maximum rotational speed, a certain temporal course of the angular velocity or a certain acceleration ramp can be specified on the basis of which the electric motor 2 is accelerated. In the query A is queried as to whether the drive shaft 8 of the electric motor 2 has already completed a predetermined number of spindle revolutions or, as a result, how far the forming tool 3 has already been moved in its linear movement by means of the spindle drive 6.

The electric motor 2 is operated at maximum speed until, in query A, reaching the predetermined number of spindle revolutions or reaching the predetermined feed path of the forming tool 3 leads to the condition being met. The number of spindle revolutions which serves as a trigger for changing to method step 2 is chosen as high as possible, but chosen so low that in all conceivable cases due to the tolerances it is ensured that the pressing surface 18 of the forming tool 3 is not on the workpiece during this method step 4 concerns. During method step 1 it can be provided that the pressing force measured on the measuring unit 12 is not queried or at least not included in the engine control.

The electric motor 2 is then operated at reduced speed in method step 2. The reduced speed serves to ensure that there is sufficient time in the measuring unit 12 when an increase in the pressing force is detected in order to reduce the motor speed or to switch over to a force control. The speed of rotation in the reduced speed depends on how quickly the electric motor 2 can be braked and which travel path the forming tool 3 can be moved even further after it has been placed on the workpiece 4. This maximum travel distance is also called the offset. If the intended insertion depth is very large, for example, the reduced speed can have a high value and, for example, be approximately the same size as the maximum speed.

The transition from maximum speed to reduced speed can also take place according to a predetermined temporal profile of the angular speed. During operation of the electric motor 2 at a reduced speed, the measuring unit 12 is activated in order to be able to detect when the pressing surface 18 of the forming tool 3 comes into contact with the workpiece 4, which leads to a sudden increase in the pressing force detected in the measuring unit 12. Query B determines whether the pressing force detected in the measuring unit 12 has reached a certain predefined threshold value and method step 3 is initiated when the threshold value is reached.

Force control is then carried out in method step 3, as shown in the structural diagram of the control loop in Fig. 5 with the controlled system in Fig. 6 is shown activated. The electric motor 2 is controlled by means of the force control in such a way that the predetermined pressing force is reached.

Fig. 3 shows a structural diagram of the mechanical model of the process press 1. This serves as the basis for the modeling of the process press 1. The input variable of the model represents the engine torque M _m , which counteracts the friction torque M _{rm of} the drive. The motor moment of inertia is determined by θ _m . The clutch 13 is modeled as a linear spring-mass damper element. This is characterized by the spring constant c _k , the damping constant d _k and the moment of inertia θ _k , half of which is taken into account on the input and output sides. The moment after the clutch 13, which acts as the drive torque of the spindle 9, is referred to as M _sp . The friction losses are taken into account with the moment M _rs . With θ _sp the moment of inertia of the spindle 9 is indicated. The ball screw transforms the rotational movement of the spindle 9 into a translatory movement of the slide 16. The transmission ratio of this transformation is denoted by i _g . The measuring unit 12, which connects the slide 16 with the mass m ₁ and the forming tool 3 with the mass m ₂ , is modeled with a linear spring-damper model with the spring constant c _s and the damping constant ds. The position of the carriage 16 is indicated by s ₁ and the position of the forming tool 3 by s ₂ . The transformed spindle torque causes the force F _a , which acts on the slide 16. The force F _s indicates the measured value of the measuring unit 12 and F _ext the external force occurring during the pressing.

Fig. 4 shows an exemplary course of the external force over the course of the position of the forming tool 3. The exemplary course of the external force can be determined by a test. This exemplary course is also referred to as the load characteristic.

In order to enable a wide range of press applications and to ensure the simplicity of the model adaptation, the load model of the specific use cases is empirical determined. The aim is to record a characteristic curve that indicates the relationship between the external force F _ext and the position of the forming tool 3 s ₂ . For this purpose, the forming tool 3 is moved to the workpiece 4 at a constant speed, depending on the application, until a defined limit force is reached. The connection between force and path thus determined is in Fig. 4 shown and corresponds to that of a non-linear spring of the form F _ext (s ₂ ) = k (s ₂ ) * s ₂ with the position-dependent spring stiffness k (s ₂ ). The characteristic curve is divided into two areas. While the forming tool 3 is moving freely, there is no significant increase in force. F _ext = 0N is assumed for this process. Only when the forming tool 3 strikes the workpiece 4 does a noticeable increase in force occur. If this increase in force F _ext ≈ F _s > F _{trigger is} detected, the forming step begins. The associated slide position is designated with s _trigger .

Fig. 5 shows a structure diagram of a control circuit for force control, the force controller is designed for the forming stage and is active in this.

With some pressing processes it can happen that the curve of the pressing force has a very steep increase. In other words, the pressing force rises steeply with only a slight movement of the forming tool 3. It may therefore be necessary for the forming tool 3 to be brought to a standstill within a short distance in order to be able to achieve a predetermined value of the pressing force. However, due to the inertia of the system or due to the inertia of a conventional regulation of the electric motor 2, the dynamics of the underlying speed controller of the electric motor 2 may not be sufficient for this braking maneuver.

To avoid this problem, not only a force feedforward control but also a motor speed feedforward control is used for inertia and load compensation. This extended control loop is in Fig. 5 shown. Due to the high rigidity in the relevant frequency range, Vor _m = ϕ _sp = s ₂ / i _{g is} assumed for the pilot control and motor speed controller design, where ϕ _m for the motor angular position of the electric motor 2 and ϕ _sp for the spindle angular position of the spindle 9 of the spindle drive 6 stands. First, the total translational mass, ie m _t = m ₁ + m ₂ corresponding to the transmission ratio i _g with the moment of inertia of the drive train, becomes a total moment of inertia θ = θ _m + θ _k + θ _sp + m _t i _g ² added, where θ _{m represents} the moment of inertia of the electric motor 2, θ _{k represents} the moment of inertia of the clutch 13 and θ _{sp represents} the moment of inertia of the spindle 9. This results in the simplified design model ${\theta }_{{\omega }_m^{\ast }}=M_m^{\ast }-F_{\mathit{ext}}{i}_G.$

, so folgt die Motor-Drehzahlvorsteuerung zu $M_{\mathit{FF}}^{\ast }=M_{\omega ,\mathit{FF}}^{\ast }+$

$M_{\mathit{ext},\mathit{FF}}^{\ast }.$

Mit dem Vorsteueranteil $M_{\omega ,\mathit{FF}}^{\ast }$

kann in weiterer Folge der Einfluss der Trägheitsmomente und -massen der Presse während der Beschleunigungsphasen kompensiert werden. Der Vorsteueranteil zur Kompensation der externen Kraft F_ext lautet $M_{\mathit{ext},\mathit{FF}}^{\ast }=\mathrm{F}*{\mathrm{i}}_{\mathrm{g}}.$

Ist die Annahme der hohen Steifigkeit nicht gerechtfertigt, gilt dieses vereinfachte System nicht und die Vorsteueranteile müssen anhand des Systems in Fig. 3 berechnet werden.With F * = F _ext and $M_{\mathit{FF}}^{\ast }=M_m^{\ast },$

, the engine speed precontrol follows $M_{\mathit{FF}}^{\ast }=M_{\omega ,\mathit{FF}}^{\ast }+$

$M_{\mathit{ext},\mathit{FF}}^{\ast }.$

With the input tax share $M_{\omega ,\mathit{FF}}^{\ast }$

the influence of the moments and masses of inertia of the press can be compensated during the acceleration phases. The input tax component for compensation of the external force F _ext is $M_{\mathit{ext},\mathit{FF}}^{\ast }=\mathrm{F}*{\mathrm{i}}_{\mathrm{G}}.$

If the assumption of high rigidity is not justified, this simplified system does not apply and the input tax components must be based on the system in Fig. 3 be calculated.

ist in Fig. 6 dargestellt. Die Übertragungsfunktion G _wm (s) mit dem Motormoment M_m als Eingang und der Motordrehzahl ω_m als Ausgang bildet die Ausgangsrückführung ω_m für den unterlagerten Drehzahlregelkreis T_ω (s) = $\frac{R_{\omega }\left(s\right)}{1+R_{\omega }\left(s\right)G_{\mathit{\omega m}}\left(s\right)}$

dieser bilde gemeinsam mit der Übertragungsfunktion G_Fs (s) mit dem Motormoment M_m als Eingang und der Sensorkraft F_s als Ausgang, die gesamte Regelstrecke $G_{{\omega }_m^{\ast },F_s}\left(s\right)=\frac{F_s}{{\omega }_m^{\ast }}=T_{\omega }\left(s\right)G_{\mathit{Fs}}\left(s\right)$

von der Solldrehzahl ${\omega }_m^{\ast }$

als Eingang zur Sensorkraft Fs als Ausgang ab. Als Regler wird ein Tiefpassfilter dritter Ordnung der Form $R_F\left(s\right)=\frac{k_{\mathit{FP}}}{{\left(1+\frac{s}{{\omega }_{\mathit{Fg}}}\right)}^3}$

gewählt. Die Grenzfrequenz ω_FG und die Verstärkung k_FP werden so angepasst, dass sich ein stabiles Verhalten für den geschlossenen Regelkreis einstellt. Die Reglerparameter können mithilfe eines Loop-Shaping-Verfahrens eingestellt werden.A replacement model for the controlled system $G_{{\omega }_m^{\ast },F_s}\left(s\right)$

is in Fig. 6 shown. The transfer function G _wm ( s ) with the engine torque M _m as the input and the engine speed ω _m as the output forms the output feedback ω _m for the subordinate speed control loop T _ω ( s ) = $\frac{{\mathrm{<figure-callout\; id="1"\; label="R\; \omega \; s"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">R\; </figure-callout>}}_{\omega }\left(s\right)\left(\right)}{1+R_{\omega }\left(s\right)G_{\mathit{\omega m}}\left(s\right)}$

together with the transfer function G _Fs ( s ) with the motor torque M _m as input and the sensor force F _s as output, this forms the entire controlled system $G_{{\omega }_m^{\ast },F_s}\left(s\right)=\frac{F_s}{{\omega }_m^{\ast }}=T_{\omega }\left(s\right)G_{\mathit{Fs}}\left(s\right)$

from the target speed ${\omega }_m^{\ast }$

as an input to the sensor force Fs as an output. A third-order low-pass filter of the form is used as the controller $R_F\left(s\right)=\frac{{\mathrm{<figure-callout\; id="1"\; label="k\; FP"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">k\; </figure-callout>}}_{\mathit{FP}}}{{\left(1+\frac{s}{{\omega }_{\mathit{Fg}}}\right)}^{\mathrm{3rd}}}$

chosen. The cut-off frequency ω _FG and the gain k _FP are adjusted so that a stable behavior is achieved for the closed control loop. The controller parameters can be set using a loop shaping process.

Fig. 7 shows a flow diagram of a schematic sequence of a further control strategy for pressing the workpiece 4, the first two method steps being the same as in the flow diagram below Fig. 2 are.

In method step 3, the electric motor 2 is operated at a minimum speed. The minimum speed can vary from process to process and is specified based on the current process parameters. In extreme cases, it may even be necessary for the minimum speed to be zero or close to zero. Braking from reduced Speed in minimum speed should go as quickly or abruptly as possible within the strength values of process press 1. In method step 3, the electric motor 2 is operated at minimum speed until the vibrations occurring in the drive train due to the abrupt braking maneuver have subsided. For this purpose, a pre-calculated period of time for the oscillations to subside is queried in query C.

In an alternative variant, it can also be provided that the time required to decay the vibrations is not calculated on the basis of a model, but rather that it is adapted in an iterative process or that the vibrations decay by detecting the motor torque in the electric motor 2 in comparison with the measured one Torque is determined in the measuring unit 12.

Force control is then carried out in method step 4, as shown in the structural diagram of the control loop in Fig. 4 or in the controlled system in Fig. 5 is shown activated. The electric motor 2 is controlled by means of the force control in such a way that the predetermined pressing force is reached.

In the Figures 8 to 14 Various structural diagrams of possible control loops for force control are shown. In order to avoid unnecessary repetitions, the Figure 5 or the previous figures.

In the embodiment according to Fig. 8 serves as the input variable for the force regulator R _F not the sensor signal Fs, as shown in Fig. 5 the case is, but an estimated force F̂ _{ext is} provided as an input variable for the force regulator R _F by a disturbance variable observer 19. Furthermore, a force feedforward control V _F , a load feedforward control Vext and an inertia compensation V _{ω are} provided.

In the embodiment according to Fig. 9 serves as an input variable for the force regulator R _F, a force F̂ _ext estimated by the disturbance variable observer 19 . Force feedforward control V _F and inertia compensation V _{ω are also} provided.

In the embodiment according to Fig. 10 serves as an input variable for the force regulator R _F, a force F̂ _ext estimated by the disturbance variable observer 19 . Force feedforward control V _{F is also} provided.

In the embodiment according to Fig. 11 serves as an input variable for the force regulator R _F, a force F̂ _ext estimated by the disturbance variable observer 19 . A force feedforward control V _F and a load feedforward control Vext are also provided.

In the embodiment according to Fig. 12 serves as an input variable for the force controller R _F, the sensor signal Fs. Furthermore, a force feedforward control V _F , a load feedforward control V _ext and an inertia compensation V _{ω are} provided.

In the embodiment according to Fig. 13 serves as an input variable for the force controller R _F, the sensor signal Fs. Furthermore, a force precontrol V _F and an inertia compensation V _{ω are} provided.

In the embodiment according to Fig. 14 serves as an input variable for the force regulator R _F, the sensor signal Fs. Furthermore, a force feedforward control V _{F is} provided.

In the embodiment according to Fig. 15 serves as an input variable for the force controller R _F, the sensor signal Fs. Furthermore, a force feedforward control V _F and a load feedforward control V _{ext are} provided.

The exemplary embodiments show possible design variants, it being noted at this point that the invention is not restricted to the specially illustrated design variants of the same, but rather also various combinations of the individual design variants with one another are possible and this variation possibility is based on the teaching of technical action through the present invention Ability of the specialist working in this technical field.

The scope of protection is determined by the claims. However, the description and drawings are to be used to interpret the claims. Individual characteristics or combinations of characteristics from the different exemplary embodiments shown and described can represent independent inventive solutions. The object on which the independent inventive solutions are based can be found in the description.

All information on value ranges in the objective description should be understood to include any and all sub-areas, e.g. the information 1 to 10 is to be understood in such a way that all sub-areas, starting from the lower limit 1 and the upper limit 10, are included, i.e. all sections start with a lower limit of 1 or greater and end with an upper limit of 10 or less, e.g. 1 to 1.7, or 3.2 to 8.1, or 5.5 to 10.

For the sake of order, it should finally be pointed out that, for a better understanding of the structure, elements have sometimes been shown to scale and / or enlarged and / or reduced.

List of reference symbols

1

Process press

2nd

Electric motor

3rd

Forming tool

4th

workpiece

5

regulation

6

Spindle drive

7

transmission

8th

drive shaft

9

spindle

10th

Gearbox output

11

Transmission output shaft

12th

Unit of measurement

13

coupling

14

storage

15

Threaded nut

16

Sledge

17th

Guide rail

18th

Press area

19th

Disturbance observer

Claims (14)

1. A method for pressing a workpiece (4) with a predetermined pressing force, by means of a forming tool (3), which is coupled by way of a gear mechanism, in particular a spindle drive (6), with an electric motor (2), wherein the gear mechanism converts the rotational movement of a drive shaft (8) of the electric motor (2) into a translational movement of the forming tool (3), and wherein the electric motor (2) is controlled by a closed loop controller (5), characterized in that the method comprises the following method steps:

   - accelerating the electric motor (2) in the clockwise direction of rotation, to a predetermined maximum rotational speed, whereby the forming tool (3) is moved toward the workpiece (4);

   - operating the electric motor (2) at the maximum rotational speed until the drive shaft (8) of the electric motor (2) has completed a predetermined number of revolutions or the forming tool (3) has reached a predetermined position, wherein during this method step, the forming tool (3) is freely moved toward the workpiece (4), without touching it;

   - reducing the rotational speed of the electric motor (2) to a predetermined reduced rotational speed;

   - operating the electric motor (2) at the reduced rotational speed until a pressing force increase is detected by a measuring unit (12) that follows the electric motor (2), which increase exceeds a predetermined threshold value, wherein the pressing force increase occurs when the forming tool (3) comes to lie against the workpiece (4) to be formed;

   - forming the workpiece (4) with constant detection of the pressing force by means of the measuring unit (12) until the predetermined pressing force has been reached.
2. The method according to claim 1, characterized in that the electric motor (2) is braked to a predetermined minimum rotational speed after the detection of the pressing force increase.
3. The method according to claim 2, characterized in that the electric motor (2) is operated at minimum rotational speed for a predetermined or predeterminable time period, until vibrations that occur in the drive system due to the process of braking from the reduced rotational speed to the minimum rotational speed have largely died away.
4. The method according to one of the preceding claims, characterized in that during forming of the workpiece (4), control of the electric motor (2) is set by the closed loop controller (5) on the basis of the pressing force measured in the measuring unit (12).
5. The method according to one of the preceding claims, characterized in that the reduced rotational speed amounts to between 0.1% and 100%, in particular between 0.5% and 99%, preferably between 50% and 80% of the maximum rotational speed.
6. The method according to one of the preceding claims, characterized in that directly after detection of the pressing force increase, further control of the electric motor (2) is set by the closed loop controller (5) on the basis of the pressing force, wherein after detection of the pressing force increase, the electric motor (2) is braked to a predetermined minimum rotational speed, and in an initial period during the braking process, a pressing force based on a model calculation is superimposed on the pressing force detected in the measuring unit (12), and after the initial period, the pressing force detected by the measuring unit (12) serves as an input variable for the closed loop controller (5).
7. The method according to claim 6, characterized in that in the model calculation, the mass inertia and/or the spring stiffness and/or the damping and the angular and/ or linear accelerations of the individual components built into the drive train are taken into consideration.
8. The method according to claim 6 or 7, characterized in that the model calculation is adapted on the basis of the respective preceding cycles in an iterative learning process, wherein for adaptation of the model calculation, the Time Progression of the measured value of the pressing force in the measuring unit (12), as well as of the motor torque and of the associated angle of rotation of the drive shaft (8) in the electric motor (2) are used.
9. The method according to one of claims 6 to 8, characterized in that an interference variable observer (19), in particular a Kalman filter, is used for superimposition of model calculation and pressing force detected in the measuring unit (12).
10. The method according to claim 9, characterized in that superimposition between the actual force estimated in the interference variable observer (19) and the force detected in the measuring unit (14) is carried out.
11. The method according to one of the preceding claims, characterized in that a Piezo sensor is used as a measuring unit (12), which sensor is disposed in the region of the forming tool, so as to detect the pressing force.
12. The method according to one of the preceding claims, characterized in that, directly after the detection of the pressing force increase, the further activation of the electric motor (2) is set by the closed loop controller (5) on the basis of a target trajectory of the pressing force value, wherein the rotational speed variation is calculated from the target trajectory of the pressing force value in a precontroller.
13. The method according to claim 12, characterized in that, in a first phase after the detection of the pressing force increase, the pressing force value is estimated by means of an interference variable observer (19) and, in a second phase after detection of the pressing force increase, the pressing force value is detected directly by the measuring unit (12) and used as the input variable for the closed loop controller (5).
14. The method according to one of the preceding claims, characterized in that the transition between different rotational speeds of the individual method steps is specified in such a way that no sudden increases in acceleration occur.

Images