CA2225065C - Process for regulating or controlling an injection moulding machine - Google Patents

Abstract

In a regulation- or control process speed steps are predetermined for the displacement of a movement unit and a reference-speed-curve determined out of it. Means for calculation detect out of it a reference-curve of the path (s) over time for determination of a position reference variable for the speed regulation or -control. At the time (t l) determined for the modification position (s i) for switching between contiguous speed steps, the speed of the drive motor (44) is maintained until the comparison means determines, that the movement unit has reached the modification position. When the modification position (s i) has been reached, the reference position value curve is triggered (Fig. 9) at the switching time.

Description

The invention concerns a method for regulating or controlling of an injection moulding machine for processing plastifiable materials, Such a regulation is known in the case of a position reference variable as a regulation for the injection procedure, the filling phase and the pressure dwell in USA-A 5,342,559. Thereby, the speed or the pressure reference values, which should be attained in specified positions of the feed screw, are preset. Ramps are provided at the transition points between contiguous speed steps. The feed screw which is designed as a feeding means is then tracked to this reference curve in the framework of a regulation, an attempt being made, by means of corresponding adjusting values on the motor, to reduce the deviation between reference and actual values as far as possible.
Therefore, the objective is an as complete as possible closed-loop-regulation in order to suppress any disturbance variables occurring. Though it is necessary for this purpose that continuous regulation is conducted.
Furthermore an injection regulation is known from EP-B 264 453. Here injection speeds are set and a reference speed curve is constituted therefrom, the transitions between contiguous speed steps being provided with ramps, which represent either linear or any interpolation between the transition points. The feed screw formed as feeding means is then tracked to this desired curve within the scope of a regulation, either speed sensors being provided or path values serving as actual values being put into the control by a servomotor provided with incremental-path-measuring. Both embodiments have the disadvantage, that on the one hand concerning the equipment the speed sensors or servomotors are expensive, on the other hand no direct comparison of comparable measured values is made, so that a time intensive algorithm is necessary, in order to control the machine.
From EP-B 167 631 a regulation is kno:vn as injection regulation, in which the motor torque of a servomotor is used as regulation variable in conjunction with the injection pressure. In addition this solution is costly and time consuming. The same is true for the back pressure regulation according to EP-B 245 522 and the pressure regulation according to EP-B
260 328.
Therefore the state of the art mentioned to date uses servomotors continuously for the axes connected with the injection movement, but also for the movement of other axes of a plastic injection moulding machine.
Along these lines EP-A 576 925 also develops the prior art, electrical servomotors which are cooled by liquid being provided there for all axes in one injection moulding unit. Though it is also suggested there, in claim 16, to assign to the drives of the different subassemblies linear potentiometers as an analog-path-measuring-system for the regulation of the position and speed of the electrical servomotors which are cooled by liquid.
From DE-A 44 46 857 a method for adjusting automatically an injection moulding speed condition is known. For this the reference pressure is set as a reference pressure curve and compared with an actual pressure in the mould cavity. If the pressure detected deviates from the reference value, the movement of the piston of the feeding means is adjusted correspondingly.
As a result an over- and undershooting around the reference pressure curve is produced, however a tracking of the curve at the time at which the respective pressure is achieved, is not made.
In EP-B 331 735 static sensors for detection of the injection forces arising during an injection cycle in the bearing region of the feeding means, as for example a feed screw, are provided. The measured values obtained in this way are usually put into a force- or pressure-dependent injection control, provided that measured values above the internal mould pressure cannot be detected sufficiently or due to the material used can no longer be detected reliably from a determined time on in the injection cycle.
From EP-B 436 732 it is furthermore known to arrange a pressure sensor for the same purpose in the area of the feed screw itself.
The known devices, however, have the problem, that the injection force in an injection moulding machine during the actual injection ranges over a very high area of mostly several tons. During the dosing process, i.e. during the material preparation on the other hand the resolution, however, should only be within the kilogram range. Static force transducers, however, can now measure continuously starting from a zero-point up to the high forces without losing their zero point, but they do not proceed, particularly in .the lower area, above the desired resolution or, if they do proceed above the resolution, they are not suited to measure the high forces.
Based on this state of the art, the object underlying the invention is to develop a method of the kind mentioned at the beginning in such a way that a quick injection regulation or -control is achieved in a simple and cheap way.
This object is solved by the features of the claims 1, 9 or 11.
The speed regulation or pressure- or force-regulation according to these claims is attributed to a position regulation. Each programmed reference speed- or reference-pressure-value is integrated in a position reference variable. The thus suggested reference-position-value-regulation or -control has the advantage, that the regulator itself, in a relatively simple manner, can carry out a comparison between the actual value and the pre-calculated reference variable, without having to run through an extensive regulation algorithm.
Dynamic influences such as for example mass differences or accelerating or braking times can thereby be taken into account without problem by means of triggering.
Thus with regard to the technical process it is guaranteed, with the simplest calculation algorithms that as is common nowadays, the material flow at constantly identical filling degrees of the mould is the same independent of any disturbing variables in the former time course. Since, it is not necessary furthermore that the real drive is in "closed loop", a rough adjusting compatibility of the process is achieved, independently from the "quality" or "type" or "rigidnessn of the drive.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 the plan view on a schematically represented injection moulding machine, Fig. la a schematic representation of the injection moulding machine with control unit, Fig. 2 an enlarged section from Fig. I in the area of the injection moulding unit, Fig. 3 a section according to line 3-3 of Fig. 2, Fig. 4 a representation according to Fig. 3 in a further embodiment, Fig. 5 a programmed reference-speed-profile, Fig. 5a a programmed reference-force-profile, Fig. 6 a reference-speed-profile internally-controlled provided with ramps, Fig. 7 the course of a position reference variable over the time, Fig. 8 the actual course of the path s over the time t, Fig. 9 a flow diagram for the speed regulation or optionally also the force regulation over the path or over the time, Fig. 10 a wiring diagram for the wiring of the sensors, Fig. 11 the force course during the injection process.
In Figure 1 an injection moulding machine for processing plastifiable materials such as for example plastics, pulverised or ceramic masses, especially an injection moulding machine for processing plastic materials, is represented schematically, showing on the left side a mould closing unit F and on the right side an injection unit S. In this case this is an electro-mechanically driven injection moulding unit, in which for example the injection moulding unit is moved via spindles 34. A closing mechanism 31, which here is also moved via spindles, is assigned to the mould closing unit, of course, also other types of electro-mechanical drive mechanisms as well as other types of drives such as for example hydraulic, pneumatic or the like being able to be provided on the mould closing side as well as on the injection moulding side. Absolute-path-measuring-systems are assigned to both sides of the injection moulding machine. On the mould closing side, absolute-path-measuring-systems 19, 20 are provided in the form of linear potentiometers, the values of which are conveyed via distance potential transformers to a superset control unit C. On the injection moulding side, a linear potentiometer as an absolute-path-measuring-system 22 with an associated distance potential transformer for attaching the nozzle to the stationary mould earner 41 and an absolute-path-measuring system 21 in the form of a linear potentiometer with an associated distance potential transformer for the movement of the feed screw are provided. In the present case a feed screw designed as a feeding means 10 is moved via a rotational drive 18. A torque transformer 33 is assigned to the rotational drive 18.
The measured values detected by the sensory mechanism can be used in the scope of a regulation or control of the displacement of a movement unit of any movement axis of the injection moulding machine for processing plastics. In this case, on the injection moulding side for example the injection process, the movement of the injection unit for attachment to the mould M or on the mould closing side the mould closing movements can be considered. Generally speeds or pressures are regulated therefore via path s and time t. In the following, however, this regulation or control will be explained in greater detail based on an injection regulation or -control for the feeding means 10 during the injection of plastifiable material into the mould cavity 43 of the mould M.
Here a drive motor 44 serves for moving the feeding means 10 axially during the injection. According to Fig. la, adjusting means 46 for adjusting a speed v;, or a force F; and thus a pressure for a limited number of speed-, force- or pressure steps and also for adjusting respectively one modification position s; of the feeding means 10 are provided. The modification of the speed or of the pressure between contiguous speed- or pressure steps is realised at the modification position s;. The set values are stored in storing means 47 of a data processing unit D. Means 48 for calculating a reference speed curve or a reference pressure curve and also comparison means 49 for comparison of reference values to actual values are provided, so that a set value for the drive motor 44 for tracking the injection moulding unit S
corresponding to the reference curve can always be achieved by the motor regulator S0. The following embodiments are now continued based on a speed regulation. In order to achieve an analog pressure regulation or force regulation the term speed should merely be substituted in all cases by pressure or force.
The speed regulation is basically attributed to a position regulation. This is of special advantage, if absolute-path-measuring systems and no speed sensing elements are provided at the axis for the actual-position-value-detection. Each programmed reference-speed-value is integrated in a position reference variable. At the same time maximum admissible accelerations of the system are considered. Protracting errors which possibly might occur at the actual-path-curve set (t), caused by exterior influences such as torque limitation, a cascade pressure limitation regulation or limited, available drive forces, are eliminated at the latest at the next switching point of the speed profile by triggering the reference-switching-point to the actual-switching-point (c.f. below). The calculation of the position reference variable is effected target controlled, i.e. looking ahead to the next speed switching point, position ramps are pre-calculated with the admissible acceleration/brake-force position ramps arid co-ordinated with the real actual-position-values, in order to obtain an approximation to the reference-speed-curve. As soon as the unit hence reaches the position sm the ramps are re-calculated on the basis of the then reached time ;., or merely an actual-value-transfer takes place keeping the originally calculated ramps time-delayed by the difference of t,'-t~. In certain circumstances, for example when ejection units are applied to which there are no path-measuring-systems assigned, only the time can also be preset.

The whole speed regulation thus can be realised in two precision steps. The first step consists of the position reference variable regulation via reference variable triggering by comparison with the actual value from the external absolute-path-measuring-system and the following "open loop" drive system at the axis or moving axis.
The second step can be realised by a "closed loopp drive at the axis, by a servo drive, a so-called path-fine-interpolation with position regulation via an absolute-path-measuring-system or an incremental transmitter being carried out automatically.
Although the method is used in a preferred way in conjunction with electromechanical drives, a corresponding operation in conjunction with known hydraulic drives is also possible. Different drives at the injection moulding machine for processing plastics are distinguished from a technical-control point of view only by the corresponding "rigidness" of the axes, i.e. by the admissible accelerations and forces.
According to Fig. 5 the user can feed in different speed steps. This input is made for example via the adjusting means 46. Speeds m, v2,...v, are fed in between different path points so, s~, s2, s;. Fig. 5a shows a corresponding reference force curve or reference pressure curve, which in the following, however, would be dealt with in the same manner as indicated in the Figs.
6-8.
According to Fig. 6 this reference-speed-profile is provided internally-controlled with ramps, in order not to generate infinite accelerations or brake effects. These ramps can have a linear or spline-like character, i.e.
they can be interpolated in any desired manner between the transition points. The means 4$ for calculation detect from the speed course a reference-curve of the path s over time t. This calculation is made for reasons of simplicity without consideration of dynamic influences, such as for example mass differences. Soft transitions in the reference curve of the g path over time result from the internally-controlled located ramps. The path points s,', s2' and s;' mark the transitions, at which the engagements of the ramps end. From these points on, the axis has reached its respective reference speed and linear character. The ramps can have any courses and start or end at the modification positions or be calculated beyond these.
This course serves as position reference variable s~,rer (t).
In Fig. 8 this reference course of the path s over time t now can be found again in sections as a continuous line. The dotted curve reflects the actual path course sa~~ (t). Since the dynamic influences have not been considered, the dotted curve initially deviates from the ideal line. At the time t, detected for example for the modification position s, the modification position still has not been reached. For this reason the set value for the drive motor 44 and thus the linear speed are maintained until the modification position s~
is reached, i.e. until the comparison means 49 show that the feeding means has reached the modification position s1. When now the modification.
position is reached, it is already time t~' and a triggering will be realised for the further reference course by equating for the further reference curve the time t~ detected for the modification position with the actual time t,'. The same takes place at the spots s2 at time ta' and at the modification position s, at time t;'.
The use of the position reference variable has the advantage, especially when absolute-path-measuring-systems 19, 20, 21 and 22 are applied, that a direct comparison of the actual value detected by the absolute-path-measuring-system with the reference value can be made and the regulation algorithm, which has to run very fast, becomes extremely simple. The calculation of the reference variable, on the other hand, can be effected essentially "more slowly", in order to obtain a machine which operates precisely. The application of absolute-path-measuring--systems in the form of linear potentiometers saves an expensive reference running of the individual axes of the injection moulding machine after voltage loss or when work starts, since the absolute-path-system always knows immediately, at which spot the injection moulding unit is located.

Fig. 9 shows a flow diagram for the speed regulation or alternatively or optionally also a force regulation according to Fig. 5a or pressure regulation. The regulation is effected via the path s or the time t. This diagram with the steps 91 - 9? is run through periodically in each schedule-time from for example 10 ms by the superset control unit C. . As the switching point, the point at which determined positions s. are reached comes into action. The force regulation can be carried out optionally as a superset pressure limiting regulation or even as a real superimposed pressure regulation. In the simplest case it can be also omitted entirely;
then the force (pressure) at the axis is determined by the torque, which the drive can generate. If in a pressure limiting regulation a preset force limiting value is reached, the position reference variable is weakened, because the force limiting regulation acts or intervenes as superimposed regulator, if necessary even operating along with the previously described speed regulation.
In step 91, as explained in Fig. 5, a reference-speed-profile is preset and calculated via the adjusting means 46. A determined time t or path s is assigned to each speed. Alternatively or supplementarily a force- or pressure profile can be calculated in step 92. The profiles obtained in this way are provided with ramps (step 93) by a ramp generator as shown in Fig.
6. Then, during the injection in step 94 the comparison means 49 comes into action. As a result the actual value sa~~ of the absolute-path-measuring-system 21 is comes into play. If the switching point is not reached, the motor adjuster or motor regulator continues receiving a signal according to step 96. According to step 95 it is possible to apply optionally or alternatively a force limiting regulator, which receives its measured value from the force transducer 23. If the switching point has been reached, the control unit jumps back to step 91 and, by setting equal i = i + 1 continues the algorithm with the next speed support point.
If no feed back about the actual position comes from the incremental transmitter in the motor or from the absolute-path-measuring-system, then 1~
just a control also is possible. For example only the starting and the end point can be preset. As soon as the moving axis reaches the goal, without testing in the interim what the drive is doing, the new speed is then preset.
Thus the step-like intermediate steps indicated in Fig. 7 would be omitted.
Fig. 3 in an enlarged section now shows the bearing region for the feeding means 10. In this case on the left side a locking mechanism V can be recognised, which is in connection with the shaft of the rotational drive 18 via fixing means 35. With respect to the exact development of this locking mechanism reference is made to DE-C 42 36 282.
In a bore hole 16a of the injection bridge 16 now this shaft is supported at bearings 36, 37, the rotational motor 18 being arranged on the side opposite to the stationary mould carrier 41 of the injection bridge 16. At the bottom of the bore hole 16a a recess 16b is provided, which in Fig. 3 receives a force measuring ring 15. In the force measuring ring there are two sensors, the measured values detected by these sensors being transmitted to the control unit C via the force potential transformers assigned to the force transducers 26, 27. Such a force measuring ring 15 is above all useful, when a static sensor is provided in connection with a dynamic sensor, since in this case they can be easily integrated in the force measuring ring. The first static sensor 11 serves for determination of the low forces arising during the dosing and the material preparation, whereas the dynamic sensor 12 operates in the remaining force range, which is dealt with further on in more detail. The force measuring ring 15 is arranged between the axial pressure bearing 17 and the forming 16c of the injection bridge 16 in such a way, that it encloses the drive shaft 18a of the rotational drive 18.
Alternatively a development according to Fig. 4 is possible. In this connection a first static sensor 11 is provided, which is spring-loaded by a spring 14. The sensor is, Iike the second static sensor 13, screwed to the injection bridge by thread sections l la, 13a. Deformations of the injection bridge I6 produce values measured by the sensors. The spring force of the spring 14 decreases and is coordinated to a pre-determined limit value Fl in such a way, that if this limit value is exceeded, the first static sensor is decoupled from the force flux by the spring, that the spring prevents the transmission of further deformations and thus forces acting on the first static sensor 11. As a result, destruction of the sensor at maximum injection force is avoided the spring 14 acting like a bypass. The first static sensor 11 thus takes over in a range of lower forces from for example 1% -25% of the maximum forces, preferably a force range which usually is not exceeded during the dosing. In the higher force range a further sensor then works as second static sensor 13. The first static sensor 11 transmits a signal via the force transducer 28 and the second static sensor 13 a signal via the force transducer 29 to the control unit. The sensors can be formed as measuring-extension-strips. In both cases the sensors can be adjusted or pretensioned correspondingly, in order to be able to measure tensile forces as well as pressure forces. Consequently, it is guaranteed that a supervision or regulation or control is possible during the entire injection cycle.
The sensors determine the forces arising at the feeding means 10 of the injection moulding unit S, which are characteristic for a pressure which is exerted by the material to be processed onto the feeding means 10, for example for use in the scope of an injection regulation. At least one first static sensor 11 has a high resolution and serves in essence for determination of the forces arising during the material preparation in the form of a f"first measured value. A further force sensor in the form of a second static sensor 13 or a dynamic sensor 12 is assigned to this first static sensor 11, said sensors 12, 13 essentially determining the forces arising during the injection in the form of a second measured value. A
switch over or transfer is effected at a transfer point P (Fig. 11), as soon as the first measured value reaches a pre-determined limit value Fl. At the transfer point P the first static sensor 11 transfers for example at time to the further detection of the force to a further sensor, the force being determined at this point, so that at the same time the further sensor can be calibrated or zero-set. Thus especially for a dynamic sensor 12 a starting-point results, so that in each injection cycle a reset of the dynamic sensor is realised and a drifting away of the measured value or sensor signal is avoided. The dynamic sensor 12 or the second static sensor I3 is then adapted to detect the injection forces which are high compared with those arising during the material preparation.
Fig. 10 shows the function of the application of two sensors as shown in Figs. 3 and 4. The first static sensor 11 supplies a measured value via the force transducer 26, which is fed to the comparator 39 and there is compared with a reference value, which corresponds to a pre-determined limit value Fl. According to the result of this comparison the second measured value of the dynamic sensor 12 is still not interconnected, in this case there results a position of the switches S1, S2 according the diagram.
Thus only the first measured value detected by the first static sensor 11 is fed into the summarizer 40. If the pre-determined limit value is reached, the control means 38 transmits a signal to the switches S 1, S2. Thus switch S 1 is opened and the force transducer 27 leaves the reset condition. From now on the dynamic sensor 12 also delivers a second measured value via the force transducer 27. In the summariser 40 two measured values are now available, which are added one to the other, so that even above the pre-determined limit value Fl up to the maximum injection force Fx a continuous measuring result from zero to Fx is achieved. This leads to the required range apportioning. In the example of Fig. 3 this evaluation logic is located in the force measuring ring, so that the control is relieved of this decision. In Fig. 4 the evaluation logic is located for example in the switching cabinet, which is not represented graphically.
From Fig. 11 a corresponding force measuring range division can be learned. In the measuring range B 1 of the force F from zero to F I the first static sensor 11 operates, whereas the further sensor operates above that in the measuring range above the pre-determined limit value F1 up to the maximum injection force Fx. The transfer is effected at the transfer point P
at time t~,, and in the entire following range until time to the force in essence is detected by the further sensor in the form of the force-time-course Fy~t~. After the time to the "work" is passed again to the first static sensor 11.
It is in itself understood that this specification can be subjected to the most varied modifications, changes and adjustments, which range within the area of equivalents of the enclosed claims.

Claims (13)

1. Method for regulating or controlling of an injection moulding machine for processing plastifiable materials comprising .cndot. a drive motor (44) for displacing a movement unit, .cndot. an adjusting means (46) for adjusting respectively one speed (v i) for a limited number of speed steps and for adjusting respectively one modification position (s i), at which the modification of the speed (v i) of the movement unit between contiguous speed steps should be effected, .cndot. storing means (47) for storing the values adjusted by the adjusting means, .cndot. means (48) for calculation of a reference-speed-curve based on the values adjusted by the adjusting means (46) by production of transition ramps between the contiguous speed steps in the area of the modification position (s i), wherein the means for calculation (48) generate a reference curve of the path (s) over the time (t) for determination of a position reference variable (s i,ref(t)), for the speed regulation or control, .cndot. position detecting means (21) for detecting the actual position of the movement unit, .cndot.time detecting means (45) for detection of times during the displacement of the movement unit, .cndot. comparison means (49) for comparison of reference values with actual values for detection of a set value for tracking the movement unit to the reference-speed-curve, characterised in that at the time (t i) detected for the modification position (s i), the set value which determines the speed of the drive motor (44) is maintained until the comparison means (49) determine that the movement unit has reached the modification position (s i), and in that when the modification position (s i) is reached, the actual-switching-time (t i') is equated for the further reference curve to the time (t i) detected for the modification position and the adjusting value is adjusted.

2. Method according to claim 1, characterised in that the movement unit is a movement axis of the mould closing unit (F).

3. Method according to claim 1, characterised in that the movement unit is a movement axis of the injection moulding unit.

4. Method according to claim 1, characterised in that the movement unit is a feeding means (10) for injecting the plastifiable material into the mould cavity (43) of a casting mould (M), said material being moved by the drive motor (44) during the injection process, in that the speed (v i) is the injection speed of the feeding means (10) and the modification position (s i) the respective modification position of the feeding means and in that the time detecting means (45) detect times starting from a time (t o), at which the injection process begins, until at least a time (t i), at which the injection process is finished.

5. Method according to claim 1, characterised in that the means (48) for calculation of the reference-speed-curve calculate the transition ramps by referring to pre-determined speed- or retardation-values looking ahead to the next modification position (s i) and, as soon as the movement unit has reached this modification position (s i), calculate them again on the basis of the time (t i') for approximation to the reference-speed-curve or set up the formerly calculated, still remaining reference-speed-curve.

6. Method according to claim 1, characterised in that the position reference variable (s i,ref(t)) is compared in the regulating cycle with a measured value for the actual position (s act), which is detected by absolute-path-measuring-systems.

7. Method according to claim 1, characterised in that an electrical drive motor is provided as drive motor (44) and in that a feed screw rotated by a rotational drive (18) is provided as feeding means (10).

8. Method according to claim 1, characterised in that a force limiting regulation is superimposed upon the speed regulation.

9. Method for regulating or controlling of an injection moulding unit for processing plastifiable materials comprising .cndot. a drive motor (44) for displacing a movement unit, an adjusting means (46) for adjusting respectively one pressure for a limited number of pressure steps and for adjusting respectively one modification position (s i) at which the modification of the pressure between contiguous pressure steps should be effected, .cndot. storing means (47) for storing the values adjusted by the adjusting means, .cndot. means (48) for calculation of a reference-pressure-curve based on the values adjusted by the adjusting means (46) by production of transition ramps between the contiguous pressure steps in the area of the modification position (s i), .cndot. pressure detecting means (23, 26) for detecting the actual pressure, .cndot. time detecting means (45) for detecting times during the displacement of the movement unit, .cndot. comparison means (49) for comparison of reference values with actual values for determination of an adjusting value for tracking the movement unit to the reference-pressure-curve, characterised in that the means for calculation (48) generate a reference curve of the pressure over the time (t) for determination of a pressure reference variable for the pressure regulation or -control, in that at the time (t i) detected for the modification position (s i), the set value which determines the pressure is maintained until the comparison means (49) determine that the movement unit has reached the modification position (s), and in that when the modification position (s i) is reached, the actual-switching-time (t i') is equated for the further reference curve to the time (t i) detected for the modification position and the set value is adjusted.

10. Method according to claim 9, characterised in that an electrical drive motor is provided as drive motor (44) and in that a feed screw, which is rotated by a rotational drive (18), is provided as a movement unit.

11. Method for regulating or controlling an injection moulding machine for processing plastifiable materials comprising .cndot. a drive motor (44) for displacing a movement unit, .cndot. an adjusting means (46) for adjusting respectively one force for a limited number of force steps and for adjusting respectively one modification position (s;) at which the modification of the force between contiguous force steps should be effected, .cndot. storing means (47) for storing the values adjusted by the adjusting means, .cndot. means (48) for calculation of a reference-force-curve based on the values adjusted by the adjusting means (46) by production of transition ramps between the contiguous force steps in the area of the modification position (s i), .cndot.force detecting means (23, 26) for detecting the actual force, .cndot. time detection means (45) for detecting times during the displacement of the movement unit, .cndot. comparison means (49) for comparison of the reference values with the actual values for determination of a set value for tracking the movement unit to the reference-force curare, characterised in that the means for calculation (48) generate a reference curve of the force (F) over the time (t) for determination of a force reference variable for the force regulation or -control, in that at the time (t i) detected for the modification position (s i), the sec value which determines the force is maintained until the comparison means (49) determine that the movement unit has reached the modification position (s i) and in that when the modification position (s i) has been reached the actual-switching-time (t i') is equated for the further reference curve to the time (t i) detected for the modification position and the adjusting value is adjusted.

12. Method according to claim 11, characterised in that an electrical drive motor is provided as drive motor (44) and in that a feed screw, which is rotated by a rotational drive (18), is provided as a movement unit.

13. Method according to claim 8, characterised in that the force limiting regulation is realised according to claim 11.