EP1347846A1

EP1347846A1 - Method and device in a sheet metal working machine

Info

Publication number: EP1347846A1
Application number: EP01999444A
Authority: EP
Inventors: Jorma Taijonlahti; Kari Liinamaa
Original assignee: Lillbacka Jetair Oy
Current assignee: Lillbacka Jetair Oy
Priority date: 2000-12-04
Filing date: 2001-12-03
Publication date: 2003-10-01
Anticipated expiration: 2021-12-03
Also published as: DE60112797T2; DK1347846T3; EP1347846B1; FI20002653A0; ATE302075T1; ES2246354T3; WO2002045879A1; FI109410B; DE60112797D1; AU2002220771A1

Abstract

The invention relates to a method for transferring a carriage (11, 12) in a sheet metal working machine, wherein a carriage (11, 12) of a moving carriage arrangement in connection with said sheet metal working machine is transferred by means of at least two actuators (16a, 17a; 18a, 19a) parallel in the direction of movement of said carriage (11, 12) controlled by a control value (S) of the position given by a control centre (41), in order to transfer a sheet (14) to be machined and attached to the mounting means (13) of the carriage arrangement, substantially in the main plane of the sheet (14) to perform machining directed to the sheet (14). In accordance with the invention, to prevent twisting caused by asymmetric loading, individual separate measuring values (M16,M17) of the position are determined for the at least two parallel actuators (16a,17a;18a,19a) by means of separate positioning systems (16b,17b;18b,19b) attached in connection with each of said actuators, and each of said separate actuators (16a,17a;18a,19a) is controlled independently and separately by means of difference value between a control value given by the control centre (41) and the measuring value (M16,M17) of the position related to said individual actuator. The invention further relates to an apparatus implementing the method.

Description

Method and device in a sheet metal working machine

The invention relates to a method for moving a carriage that transfers a sheet to be machined in a sheet metal working machine according to the accompanying claim 1. The invention also relates to an apparatus according to the preamble of the appended claim 8 for implementing the aforementioned method.

The present invention relates to sheet metal working machines in which separate sheets are manipulated in order to machine them into desired form. As a general rule, the objects that are machined are metal sheets fabricated of various metal alloys and having a size of e.g. 1250 x 2500 mm or 1500 x 3000 mm. Typically, the thickness of the metal sheets is variable between 0.5 mm and 3.5 mm, wherein they are generally referred as so-called thin sheets and as thin sheet metal working machines, respectively. Typical measures performed for sheets using sheet metal working machines include e.g. punching, angular cutting, thread cutting or riveting.

US patent 4,658,682 discloses an automatic sheet metal working machine. This type of sheet metal working machine typically comprises a frame, a first carriage moving relative to the frame, and a second carriage mounted in the first carriage and moving in a direction perpendicular to the direction of motion of said first carriage. Said second carriage comprises mounting means to attach the sheet to be machined to the carriage. By means of a carriage arrangement composed of the first and second carriage the sheet to be machined can be moved in a plane X, Y that is substantially parallel with the main plane of the sheet relative to the machining device used, e.g. a punch or a cutter.

Modern sheet metal working machines are controlled by numerical computer control. In the memory of a control centre a machining program is stored, which the sheet metal working machine performs automatically controlled by the control centre. The control centre (or sub systems connected thereto) obtains information, such as location or position data necessary for the control of different actuators and drives, from various measuring systems and/or sensors connected in the sheet metal working machine. Devices related to automatic control of sheet metal working machines, as well as functions related thereto, are well known in the field as such, and they will therefore not be described in more detail in this context.

Automatic numerical control of sheet metal working machines enables increasing the operation speed of machining apparatuses, and therein increasing the productivity. The time per one sheet to be machined can be reduced, on one hand by accelerating the movement of sheet between and/or during the machining actions, and on the other hand by performing the machining actions faster. The present invention is focused on the former possibility in making the operation of sheet metal working machines more effective.

As mentioned earlier, the sheet to be machined is moved on the plane X, Y using the carriage arrangement composed of the first carriage and the second carriage connected thereto. Both carriages of the carriage arrangement are traditionally moved mechanically using a driving mechanism operating e.g. by means of a ball-race screw or toothed rack mechanism. Moving mechanisms operating by a chain belt or a toothed belt are also known. One problem with the above-described mechanical solutions in modern sheet metal working machines is how to build one moving mechanism with both high velocity of motion and good positioning precision. By combining e.g. a moving mechanism operating by a ball race to a separate positioning system indicating the position of a carriage and operating e.g. according to an optical principle, a good positioning precision can be achieved, wherein a good machining precision can be accomplished, but correspondingly the motion velocity will be compromised to a slower level. In a corresponding manner, better motion velocities can be achieved by toothed rack mechanisms, but in that case a good positioning precision will be more difficult to achieve.

In the most recent sheet metal working machines, linear servo motors are used, which enable a high motion velocity and, at the same time, good positioning precision when connected to be used with a separate, precise, e.g. optical positioning system. Maximum motion velocities that are obtainable by linear servos are of the order 3 to 5 m/s and the maximum accelerations are of the order 20 to 40 m/s².

However, practical embodiments have demonstrated that even if linear servo motors as such enable moving the carriages and the sheet to be machined in a carriage arrangement rapidly, particularly between machining phases, deformations are caused in the structures of the moving carriage arrangement by high accelerations and decelerations of the moving masses, which limits the velocities usable upon moving. For example in a situation when the second carriage of the carriage arrangement, together with a sheet secured on the mounting means thereof, substantially on the side of the carriage, is transferred by moving the first carriage fast from one position to another, such eccentric load will cause in the carriage arrangement structures significant forces that will strain and bend/twist the structure. When stopping the motion of the carriage arrangement suddenly, vibration is caused in the structures, which will impair the high positioning precision necessary in sheet machining.

Undesirable bending/twisting in the carriage arrangement structures caused by the aforementioned eccentric load is present also in other solutions than those utilizing linear servos when the aim is to significantly increase motion velocities and trajectories of the carriage arrangement.

A technique is known in prior art for diminishing problems caused by twisting/bending of the above-mentioned structures, wherein one carriage of the carriage arrangement is moved by using two actuators, instead of one actuator, parallel relative to the direction of motion of the carriage, e.g. by using two parallel ball-row actuators. Thus, e.g. the second carriage of the carriage arrangement is arranged to move relative to the first carriage supported by two parallel actuators, the actuators being controlled using positioning data given by one position sensor. By above-described duplexing of the actuators necessary for moving the carriage, said actuators direct, when the carriage is accelerated or decelerated, forces to two locations of the carriage, thus cutting the moment forces directed to the carriage. This solution diminishes to some extent the twisting tendency of structures. In such a prior art solution, both actuators are controlled in a mutually identical manner using information given by a mutual position sensor, i.e. both actuators are controlled using one control unit, which control unit inputs the same control to both actuators. When using ball screw actuators e.g. the power of one drive motor is transmitted mechanically to two parallel ball screws, which ball screws rotate with a mutually same speed.

It can be considered to eliminate the problem caused by deformation of structures by sufficient reinforcement and stiffening of the structures belonging to the carriage arrangement, but e.g. if the structures of the second carriage moving together with the first carriage are reinforced, by simultaneously increasing their mass significantly, this will on the other hand complicate the situation as the moving masses are growing. Additionally, this will bring about an increase in the manufacturing costs of sheet metal working machines because more stock material is needed for manufacturing more robust structures and, in a corresponding manner, moving a more heavier carriage arrangement requires for more powerful drive motors, heavier linear conductors etc.

It is the primary object of the present invention to provide a new method for moving a carriage in a carriage arrangement transferring a sheet to be machined, wherein the above-described problems can be diminished to a significant degree, when moving said carriage fast when high accelerations and decelerations are present. To attain this purpose, the method according to the invention is primarily characterized in what will be presented in the characterizing part of the independent claim 1.

It is also an aim of the invention to provide an apparatus implementing the aforementioned method. The apparatus according to the invention, in turn, is primarily characterized in what will be presented in the characterizing part of the independent claim 8. The invention is essentially based on the idea that a carriage belonging to the carriage arrangement of the sheet metal working machine is moved by means of two or a plurality of parallel and, separately controlled actuators in a manner that each actuator is controlled in accordance with the measurement data on the position provided by its own separate positioning system. This enables detection and active compensation of a positioning error in the position of said carriage, i.e. twisting that takes place in a plane that is substantially parallel with the direction of motion of the carriage. The compensation of the twisting takes place in a manner that separate actuators mounted on the carriage each react individually to the position error, which is the difference between the measuring value of the real position given by the positioning system separate for an individual actuator, and the control value of a position given by the control centre of the sheet metal working machine.

A significant advantage of the invention, as compared to prior art, is that it enables implementation of the sheet metal working centre, together with its carriage arrangement, in a lighter structure than before, but yet makes it possible to achieve an excellent positioning precision, wherein a high-quality result is obtained in machining. By using lighter structures, considerably cost savings are achieved in the manufacture of sheet metal working machines.

Since the carriage arrangement has lighter structures, the moving of the sheet to be machined can be further accelerated between and/or during machining phases, wherein the capacity and productivity of the sheet metal working machine can be increased in production.

Furthermore, the solution according to the invention makes it possible to efficiently compensate such location and/or position error of the carriage, which error is caused to one motion axis of the carriage arrangement by the movement according to a second motion axis of the carriage arrangement. E.g. a fast movement of the first carriage can cause an error in the position of the second carriage mounted therein, due to the twisting of the second carriage (and the sheet to be machined attached thereto), even if the control value of the position of the second carriage is kept as default during said movement, i.e. the aim is not to move the carriage. However, in accordance with the invention, the twisting in the position of the second carriage can be actively compensated during the movement of the first carriage.

Advantageously, the parallel and separately controlled actuators of the carriage are, in the solution of the invention, implemented by linear servo motors. An advantage of the linear servo motors is their speed, which enables fast reaction when the incipient twisting of the carriage is detected, wherein the turning can be effectively compensation at an early stage before it has grown to considerable dimensions.

The following, a more detailed description of the invention with examples will more clearly illustrate, for anyone skilled in the art, advantageous embodiments of the invention as well as advantages to be achieved with the invention in relation to prior art.

in the following, the invention will be described in more detail with reference to the appended drawings, in which

Fig. 1 illustrates in principle an embodiment of the invention in a top view,

Fig. 2 illustrates in principle forces affecting the carriage in a top view,

Fig. 3 illustrates in a reduced side view the essential parts of the linear servo motor,

Fig. 4 illustrates a control system implementing the method according to the invention, and

Fig. 5 illustrates in principle another embodiment of the invention in a top view. It is obvious that the embodiments of the invention are not restricted solely to the examples presented hereafter, but they may vary within the inventive aspects of the claims to be presented hereinbelow.

Fig. 1 illustrates in principle a preferred embodiment of the invention in a top view. Fig. 1 shows in a reduced view those essential parts of the carriage arrangement of the sheet metal working machine which enable moving of the sheet to be machined relative to the machining device of the sheet metal working machine.

The sheet metal working machine of Fig. 1 comprises, mounted on a frame 10, typically on the lower part of the frame, a first carriage 11 arranged to move along guides or the like in the frame 10 in a direction Y relative to the frame. A second carriage 12 is arranged to move along the guides or the like of the first carriage 11 in a direction X, this direction being perpendicular against the direction of motion of the first carriage 11. In the second carriage 12 there are secured mounting means 13 to attach the edge part of the sheet to be machined 14 to said carriage 12. The sheet 14 to be machined can be moved by means of the carriage arrangement composed of the first carriage 11 and the second carriage 12 in the plane X, Y along the main plane of the sheet 14 relative to the machining device 15. The machining device 15 can be e.g. a punch, an angular cutter, a screwing or a riveting machine. Typically the sheet 14 to be machined is arranged to be supported from underneath on a working table or the like (not shown in Fig. 1), along the surface of which working table the sheet 14 is moving by means of the above-described carriage arrangement.

In Fig. 1 the first carriage 11 is, in accordance with the invention, arranged to be moved by two parallel and independently controlled actuators 16a, 17a, both of which actuators are controlled on the basis of an individual positioning data obtained from a positioning system 16b, 17b, which is separate for each actuator. In other words, the actuator 16a is controlled by positioning data given by the positioning system 16b, and the actuator 17a, in a corresponding manner, is controlled by positioning data given by the positioning system 17b. For clarity, Fig. 1 does not show the separate guides or the like possibly used for supporting the carriage 11.

In Fig. 1 the second carriage 12 is arranged to be moved along the guides or the like of the first carriage 11 (not shown in Fig. 1) by utilizing an appropriate prior art, in other words, e.g. by means of one actuator 18a that is controlled by using the positioning system 18b. It is of course obvious for anyone skilled in the art, that the method of the invention can be used also for moving the second carriage 12, but since relatively minor forces that create twisting in the carriage 12 are directed to said second carriage 12 in the carriage arrangement of Fig. 1 , the benefit achieved by the invention is also less significant in this situation.

It is preferred that the actuator 16a and the actuator 17a are each implemented in Fig. 1 by a separate linear servo motor, the operation of which is described in more detail in the text hereinbelow. It is naturally obvious that the invention is not limited solely to the usage of linear servo motors, but the actuator 16a and actuator 17a can be implemented also by using any other manner obvious for anyone skilled in the art. These include e.g. ball-race screw, toothed rack, chain or toothed belt operated mechanisms.

It is advantageous that the positioning system 16b and positioning system 17b are implemented by a sensor operated on an optical basis, a so-called optical measuring beam. An excellent measuring and positioning precision is achieved using such an optical measuring beam, which as such is very well know in the field. Nevertheless, the invention is not, in this respect, limited solely to the use of positioning systems/sensors operative on optical principles, but any other solution obvious to anyone skilled in the art can be used for this purpose.

In the following, the invention is further described in more detail with reference made to Figs. 1 and 2.

In Fig. 1 , the sheet 14 to be machined can move within a rectangular movement range moved by the carriage arrangement composed of the first carriage 11 and the second carriage 12, the extreme positions of the sheet 14 in the crosswise angles of said movement range being marked as reference letters A and B in Fig. 1. The sheet 14 in the position B, the first carriage 11 and the second carriage 12 are illustrated in Fig. 1 with broken lines. Positions C and D are also marked in Fig. 1 , these corresponding to the two other corners of the movement area mentioned above. For clarity reasons, the sheet 14 in the position C or D are not separately drawn in Fig. 1.

As the second carriage 12 moves in Fig. 1 from the position A along the carriage 11 towards the position C, the mass of said second carriage 12 and the sheet 14 attached thereto, which mass is eccentric relative to the carriage 11 and its support, causes twisting forces in the support between the first carriage 11 and the frame 10. These twisting forces tend to twist the first carriage 11 in relation to the frame 10.

Fig. 2 illustrates in more detail the effect of the aforementioned turning forces on the actuators 16a and 17a of the first carriage 11.

In Fig. 2 the magnitude and direction of the twisting forces 21 , 22 twisting the first carriage in the plane X, Y relative to the frame and the actuators 16a, 17a depend, among other things, on the following circumstances:

- the distance between the second carriage 12 and the support point of the carriage 11 on the frame 10, in other words, the length of "the torque arm" developed in this way,

- the state of movement of the second carriage 12, in other words, whether the movement of the second carriage 12 is accelerating, decelerating or smooth,

- the combined mass of the second carriage 12 and the sheet 14 attached thereto, and the location of its centre of mass relative to the longitudinal axis of the carriage 11.

In accordance with the invention, the twisting forces 21 , 22 twisting the carriage 11 can now be compensated as follows. It is first considered a situation, in which the control value of the position given by the numeric control centre of the sheet metal working machine to the actuators 16a and 17a is default, in other words the aim is to keep the position of the first carriage 11 at default/unchanged. In this case, however, when the twisting forces 21, 22 caused by the movement of the second carriage 12 tend to twist the first carriage 11 relative to the frame 10, the positioning system 16b of the actuator 16a detects in the measuring point 16c, and, in a corresponding manner, the positioning system 17b of the actuator 17a in the measuring point 17c, that the real position values differ from the above-mentioned control values of the control centre. As a result, the individual and from each other separate control units of the actuators 16a and 17a, each tend to eliminate the difference between the measured, real difference between the position values and the above-mentioned control value of the position. This causes that the actuators 16a and 17a are controlled in a manner that the actuators direct counterforces 23, 24 relative to the frame 10 in order to compensate the effect of the twisting forces 21 , 22.

In a situation, in which the first carriage 11 is transferred in the direction Y, the numerical control centre of the sheet metal working machine inputs constantly changing control value of the position to the actuators 16a, 17a. Thus, the independent control units of the actuators 16a and 17a tend to eliminate the difference between said control value and the position values measured by means of the positioning systems 16b and 17b, wherein the first carriage 11 moves by using the actuators 16a, 17a in a manner corresponding to the change in the position value. In a manner corresponding to the one described above, in this case also the control units of the actuators 16a and 17a can react to the twisting of the first carriage 11 , which is caused e.g. by the movement of the second carriage 12 in view of the first carriage 11. The above-described situation occurs e.g. when moving in Fig. 1 diagonally from the position A to the position B, wherein the both carriages of the carriage arrangement move simultaneously.

In Fig. 1 , the twisting of the first carriage 11 relative to the frame 10 can also be caused by a situation, in which only the first carriage 11 is moved relative to the frame 10 without the second carriage 12 moving relative to the first carriage. Such a situation is caused e.g. when moving in Fig. 1 from the position A to the position D, wherein the second carriage 12 and the sheet 14 attached thereto are located eccentrically in relation to the frame 10.

In Fig. 2, the actuators 16a and 17a are illustrated to be implemented by means of linear servo motors. Fig. 3 shows in a reduced view and in principle the essential parts for the operation of a linear servo motor. The linear servo motor is a brushless electric motor, in which a rotor 31 is arranged to move by means of a magnetic field generated by an electric current directed to coils included in the rotor 31 supported by guides (not shown in Fig. 3) along a track formed by permanent magnets 32. In between the rotor 31 and the permanent magnets 32 there is a small air gap, typically of the order 1 mm, which remains constant while the above-mentioned guides support the rotor 31 on a determined distance from the track formed by the permanent magnets 32.

In the usage according to the invention, an advantage of linear servo motors is their speed, which enables fast reaction when an incipient twisting of the carriage is detected, wherein the twisting can be effectively compensated at an early stage before it has grown to considerable dimensions.

Fig. 4 illustrates in principle a control system applicable for controlling the actuators 16a, 17a by using the method according to the invention.

The control system is composed of a control centre 41 of the sheet metal working centre and individual, separate control units 16 and 17 of the actuators 16a, 17a. As the control centre 41 gives a control value S of the position, the control units 16, 17 input to the actuators 16a, 17a, advantageously to the linear servo motors separate controls D₁₆,D₁₇, by influence of which controls the actuators 16a, 17a move in a manner, that the aim is to minimize the differences S— M₁₆ = Δ₁₆ and S— -M₁₇ = Δ₁₇ between the measurement values of the position M₁₆,M₁₇ and the control value S. When the carriage 11 ends up in a desired position S, the differences Δ₁₆ and Δ₁₇ can both get the value zero. As a result, the controls D₁₆,Dι₇ also get the value zero. When the difference values Δ_i6 and Δ-|₇ are zero, the control centre 41 knows that the carriage 11 is in the desired location.

In a situation, in which the control value S is constant, i.e., the position of the carriage 11 is kept constant, but forces are directed to the carriage 11 twisting it in a plane parallel with the direction of movement of the carriage 11 , this is detected as a change in which this difference value Δ-ι_β or Δ₁₇ or both is different from zero. Thus, to compensate the twisting of the carriage 11, the control units 16 or 17, or both tend independently, by using an appropriate control D ₆,D₁₇ to return the difference values to zero.

When the carriage 11 is moved from one position to another, the control centre 41 inputs continuously changing control value S of the position to the control units 16, 17. Thus, when the difference values Δ₁₆ and Δ₁₇ continuously differ from zero, the controls D₁₆,D₁₇ of the actuators 16a, 17a are also given a value different from zero to move the carriage 11 towards a desired direction. In this situation, the twisting of the carriage 11 can be detected according to the fact that the difference values Δ₁₆ and Δ₁₇ are unequal with each other, i.e. Δ = Δ₁₆ - Δ₁₇≠ 0. As a result, the actuators 16a and 17a obtain controls D-ι₆ and D₁₇ which are unequal with each other, wherein the twisting of the carriage 11 is compensated.

By monitoring the difference Δ = Δ₁₆ - Δ ₇ of the difference values the control centre 41 can detect the magnitude of the twisting of the carriage 11. The difference A of the difference values can also be given a maximum value, and when it is exceeded, the control centre 41 can cut down the rate of change of the control value S of the position, in other words, the acceleration or deceleration of the carriage 11, wherein the stress caused to the structures of the carriage arrangement is cut down in a corresponding manner. This enables e.g. that, when the quantity or location of the mass attached to the carriage 1 (the location of the second carriage 12 and the mass/size of the sheet 14 secured thereto) is such that the twisting tendency of the carriage 11 is small, it is possible to move the carriage 11 by higher accelerations and decelerations compared to a situation in which forces which, due to the quantity and location of the mass to be moved, cause major twisting are directed to the carriage 11.

Fig. 5 illustrates yet in principle in a top view the invention applied in a sheet metal working machine having a different type of carriage arrangement. The sheet metal working machine of Fig. 5 comprises, mounted on the frame 10, on the upper part of the frame, a first carriage 11 arranged to move along guides or the like in the frame 10 in a direction X in relation to the frame 10. A second carriage 12 is arranged to move along the guides or the like of the first carriage 11 in a direction Y. In the second carriage 12 there are secured mounting means 13 to attach the sheet to be machined 14 to said carriage 12.

In a manner corresponding the sheet metal working machine shown in Fig. 1 , the sheet 14 machined in Fig. 5 can move within a perpendicular movement range moved by the carriage arrangement composed of the first carriage 11 and the second carriage 12, in order to machine the sheet by means of a machining device 15. Reference letters A to D in Fig. 5 mark the extreme positions of the sheet 14 in the different corners of said movement range.

The method according to the invention can be applied in the sheet metal working machine of the type shown in Fig. 5, for moving both the first carriage 11 and the second carriage 12. In Fig. 5, for moving the first carriage, two independently controlled actuators 16a and 17a are used, which are controlled using the measurement data obtained from the positioning systems 16b and 17b. In a corresponding manner, for moving the second carriage, two independently controlled actuators 18a and 19a are used, which are controlled using the measurement data obtained from the positioning systems 18b and 19b.

The method according to the invention is preferably applicable to be used in the situation of Fig. 5, particularly for moving the second carriages 12 (actuators 18a and 19a), because the structure of the second carriage and the guides supporting the carriage should be advantageously as light-weighted as possible when moving together with the first carriage 11. To support the first carriage 11 , the frame 10 and the guides or the like attached thereto to support the carriage can, if necessary, be built very rigid (and thus heavy), because they are not moving parts of the sheet metal working machine, and their weight does not directly effect the moving of the carriage arrangement.

It is of course obvious, that one carriage can also include more than two parallel actuators. Thus, to inhibit/compensate the twisting of the carriage, at least two of these actuators should be controlled in accordance with the invention, but preferably all the actuators of the carriage are controlled, according to the invention, independently and separately with control units of their own.

It is of course obvious to anyone skilled in the art that the present invention is not limited solely to aforementioned machining of metal sheets, but it can be applied also in the machining of sheets manufactured of other materials, such as in machining of plastic and bakelite sheets.

Furthermore, it is of course obvious to anyone skilled in the art that by combining in different manners the methods, modes of operation and apparatus structures presented above in connection with different embodiments of the invention, it is possible to provide various embodiments of the invention in accordance with the spirit of the invention. Therefore the above-presented examples must not be interpreted as restrictive to the invention, but the embodiments of the invention can be freely varied within the scope of the inventive features presented in the claims hereinbelow.

Claims

Claims:

1. A method in a sheet metal working machine, wherein a carriage (11 , 12) of a moving carriage arrangement connected to said sheet metal working machine is transferred by means of at least two actuators (16a, 17a; 18a, 19a) parallel with the direction of movement of said carriage (11 , 12) controlled by a control value (S) of a position given by a control centre (41) in order to transfer a sheet (14) to be machined, which is attached to mounting means (13) of the carriage arrangement, substantially in the main plane of the sheet (14) to perform machining directed to the sheet (14), characterized in that

— individual, separate measuring values (M₁₆,M₁₇) of the position are determined for the at least two parallel actuators (16a, 17a; 18a, 19a) of the carriage (11 , 12) by means of separate positioning systems (16b, 17b; 18b, 19b) connected with each of said actuators, and

— each of said separate actuators (16a, 17a; 18a, 19a) of the carriage (11 ,12) is controlled independently and separately by means of a difference value obtained as a difference between the control value (S) obtained from the control centre (41) and the measurement value (M₁₆,Mι₇) of the position relating to said individual actuator.

2. The method as set forth in claim 1 , characterized in that each of said least two separate actuators (16a,17a;18a,19a) of the carriage (11 , 12) are controlled independently to minimize the difference value related to the actuator in question.

3. The method as set forth in claim 1 or 2, characterized in that each of said at least two separate actuators (16a, 17a; 18a, 19a) of the carriage (11 , 12) are controlled independently to keep the difference value related to the actuator in question equal with the corresponding difference values of the other actuators (16a, 17a; 18a, 19a) of the carriage (11 , 12).

4. The method as set forth in any of the preceding claims, characterized in that for the difference values of said actuators (16a, 17a; 18a, 19a) the greatest allowable value is determined for any of said individual difference values, i.e., a so-called first limiting value is determined.

5. The method as set forth in claim 4, characterized in that as the difference value of one the separate actuators (16a, 17a; 18a, 19a) of the carriage exceeds said pre-determined first limiting value, the rate of change of the control values of the position is reduced, i.e., the acceleration or deceleration of the carriage is reduced.

6. The method as set forth in any of the preceding claims, characterized in that for the difference between the difference values of said actuators (16a, 17a; 18a, 19a) the greatest allowable value is determined, i.e., a so-called second limiting value is determined.

7. The method as set forth in claim 6, characterized in that as a difference between the difference values of two of separate actuators (16a, 17a; 18a, 19a) of the carriage (11 , 12) exceeds said predetermined second limiting value, the rate of change of the control value (S) of the position is reduced, i.e., the acceleration or deceleration of the carriage (11 , 12) will be reduced.

8. An actuator apparatus for transferring a carriage (11 , 12) in a sheet metal working machine, wherein a carriage (11 , 12) of a moving carriage arrangement in connection with said sheet metal working machine is arranged to be transferred by means of at least two actuators (16a, 17a; 18a, 19a) parallel in the direction of motion of the carriage (11 , 12) controlled by a control value (S) of the position given by a control centre (41) in order to transfer a sheet (14) to be machined which is attached to the mounting means (13) of the carriage arrangement, substantially in the main plane of the sheet (14) to perform machining directed to the sheet (14), characterized in that the actuator apparatus of the carriage (11 , 12) comprises at least — separate positioning systems (16b, 17b; 18b, 19b) attached in connection with at least two parallel actuators (16a, 17a; 18a, 19a) of the carriage (11 , 12) in order to determine measuring values (M₁₆,M₁₇) of the position separately for each of said actuator (16a, 17a; 18a, 19a), and

— individual separate control units (16, 17) for each of said at least two parallel actuators (16a, 17a; 18a, 19a) of the carriage (11 , 12) for controlling each of said actuator (16a, 17a; 18a, 19a) independently and separately by means of a difference value obtained as a difference between the control value (S) given by the control centre (41) and the measuring value (M₁₆,M₁₇) related to the actuator (16a,17a;18a,19a) in question.

9. The actuator apparatus as set forth in claim 8, characterized in that the control units (16, 17) are arranged to control said at least two individual actuators (16a,17a;18a,19a) of the carriage (11 , 12), each of them independently, to minimize the difference value related to the actuator in question.

10. The actuator apparatus as set forth in claim 8 or 9, characterized in the control units (16, 17) are arranged to control said at least two individual actuators (16a,17a;18a,19a) of the carriage (11 , 12), each of them independently, to keep the difference value related to the actuator in question equal with the corresponding difference values of other actuators (16a,17a;18a,19a) of the carriage (11 , 12).

11. The actuator apparatus as set forth in any of the preceding claims 8 to 10, characterized in that the actuator apparatus comprises means (41) for determining the greatest allowable value for the individual difference values related to said actuators (16a,17a;18a,19a), i.e., a so-called first limiting value.

12. The actuator apparatus as set forth in claim 11 , characterized in that when the difference value of any individual actuator (16a,17a;18a,19a) of the carriage (11 , 12) exceeds said predetermined first limiting value, the control unit (41) is arranged to reduce the rate of change of the control value (S) of the position, i.e., to reduce the acceleration or the deceleration of the carriage (11 , 12).

13. The actuator apparatus as set forth in any of the preceding claims 8 to 12, characterized in that the actuator apparatus comprises means (41) for determining the greatest allowable value, a so-called second limiting value, for the difference between the difference values of said actuators (16a, 17a; 18a, 19a).

14. The actuator apparatus as set forth in claim 13, characterized in that when the difference between the difference values of two individual actuators (16a,17a;18a,19a) of the carriage (11 , 12) exceeds said predetermined second limiting value, the control unit (41) is arranged to reduce the rate of change of the control value (S) of the position, i.e., to reduce the acceleration or the deceleration of the carriage (11 , 12).

15. The actuator apparatus as set forth in any of the preceding claims 8 to 14, characterized in that the positioning systems (16b, 17b; 18b, 19b) are measuring beams or the like operating on an optical principle.

16. The actuator apparatus as set forth in any of the preceding claims 8 to 15, characterized in that the actuators (16a, 17a; 18a, 19a) are linear servo motors/actuators (31 , 32).