Technical field.

This invention relates to a method and apparatus for achieving good transformer welding capacity without decreasing the accessibility at spotwelding robots and without the need of integrating the trans¬ former into the design of the robot (including secondary current supply) or integrating the transformer into the welding gun.

This invention even relates to a method and apparatus for keeping the working area of the robot without the need to integrate

This invention even relates to a method and apparatus for increasing the transformer welding capacity by a standard equipment which can be used for many different robot and gun suppliers.

This invention even relates to a method and apparatus for increasing the transformer welding capaci'ty by a standard equipment which can be supplied for different types of welding guns and which,together with gun, can be exchanged to another with a different type of gun during the production cycle.

Background art.

During the last 20 years production of spotwelds, mainly in car in- dustries, has been performed by industrial programable robots. The capital cost at such production is very high and the produced numbers of spots/year are of main importance for the economy. Many improve¬ ments has been done to increase the economy such as speed, mainte¬ nance need and accessibility. However nowadays the performance often is limited by the capacity of the welding equipment, mainly by the capacity of the transformer.

An industrial robot is normally designed to perform three movements over what here will be called axis to reach a point and another three movements over what here will be called axis to achieve a desired direction in a three-dimensional space.

The system of axis for the desired direction has for many robot sup¬ pliers become very similar and one example is shown in Fig. 1 where D,E and P show the axis. The axe D and P can be set parallel but not always in line as shown in the figure. The axis E is often but not always perpendicular towards axis D. The moving of the axis closest to the gun (P) has traditionally the most limited capacity and/or most sensitive design.

For the welding a transformer is used which normally supply 8-15 kA at the secondary to the welding gun. Experience has shown much main¬ tenance problems if the secondary current is supplied to the gun by traditional cables passing several of the robot axis.

One solution for this problem has been to integrate the transformer into the gun design. Another has been to integrate the transformer into the robot design.

The transformer integrated in the guns has in practice a capacity limit because of the limited size and cooling of the transformer. In practice it sometimes happens the last spots in a sequence is not supplied with the proper current. More details of the transformer demands is given in international standard ISO-5826 where even the influence of the limited cooling ( ^" thermal time constant of the secondary winding") is specified in detail. The limited capacity of the transformer can even cause a longer welding time (= less spots/ year) because welding time is often increased if current is limited.

Another disadvantage by the transformer integrated in the gun is the centre of gravity upon the gun is moved more far from the axis for achieving desired direction decreasing the speed and/or handling capability of the robot. (The limited weight handling capacity gives limited electric capacity upon the transformer described above.)

Still another disadvantage by the transformer integrated in the gun is the increase of distance between the centre of the axis for achieving desired direction and the electrode tips upon the gun which

in practice decrease the access of the robot and by that the possible number of spots which can be set by one robot upon one detail.

Still another disadvantage by the transformer integrated in the gun is the size of the transformer affecting the access of the gun to the detail.

If the transformer not is integrated into the gun the transformer has been mounted upon the robot behind more than the last direction axis or separate over the robot.

The solutions with transformer with secondary current supply inte¬ grated into the robot design has shown to work properly but has be¬ come very expensive. The feed of the secondary current over moving axis has in one case been solved by contacts that open when the axis move and close before the current is supplied. Other systems with contact even during movement are known.

Exchange system for gun with integrated transformer exist . This has many time increased the accessibility for the robot. For robots with integrated transformer the problem with gun exchange is to keep the surface of the contacts as clean as needed for the high currency (8- 15 kA) in combination with the low voltage (7-10 V).

Disclosure of the invention.

In view hereof the principal object of the invention is to improve the transformer capacity without secondary current feed over more than one robot axis and to keep the robot working range without inte¬ grate the transformer into the gun design with mentioned disadvan¬ tages and by this to improve the economy of robotized welding.

A particular object of the invention is to make possible a gun ex- change system without integration of the transformer into the gun and without the problem to keep the contact surfaces clean on the secon-

dary si de.

Another particular object of the invention is to make possible to have very clean border of responsibility between the robot supplier, the supplier of invented unit, and gun supplier making it possible for the end user to choose the most preferred combination.

The possibility for this is given primary by this new invented mount¬ ing of the transformer in a way that not interfere with other axis and not more far from the gun than secondary current can be feed only over one moving axe.

By the close mounting of the transformer towards the gun one can keep the impedance losses on the secondary current low, one advantage often claimed as an advantage of the transformer integrated into the gun.

It is understood the deliver of invented unit may include more in his responsibility, such as needed valves and even such things as thyris- tor may be suitable to include.

The invention use the type of axis for the desired direction shown in Fig. 1 for fixing the transformer towards the mechanical part where two axis for direction has been working and towards where the third direction axe closest to the gun for finally moving the gun is working. By this the axe with the tradi¬ tionally most limited capacity or most sensitive design is not used for moving the transformer.

This invention means the transformer by the mounting principle upon design shown in fig 1 will not hit any part of the robot when any axe is moving. By this there is only one movement between the transformer and the gun over which the secondary current shall be supplied. It is understood electrical contact between transformer and gun shall be performed by use of any of the known methods available on the market. ,By this mounting the centre of gravity upon the gun is not moved

more far from the axis for achieving desired direction with described disadvantages.

By this mounting there is even no increase of distance between the centre of the axis for achieving desired direction and the electrode tips upon the gun which described disadvantages.

By this invention there is possibility to have a gun exchange system which does not have the disadvantage of the transformer integrated in the gun or the problem to keep the surfaces for secondary current contact clean if the transformer not is exchanged together with the gun.

Such a gun-transformer exchange system use components similar to those used for exchange of transformer guns but here there is two units instead of one:

1. One unit who fix the transformer towards the mechanical part including service of primary current, cooling water and others where two axis for direction has been working and towards where the third direction axe closest to the gun for finally moving the gun is work¬ ing.

2. A second unit which fix the gun and those parts of the equip- ment who are moving together with the gun. Service such as cooling water and air may even become supplied over this unit.

Brief description of the Drawings.

Fig 1 shows a typical set-up of axis to achieve the desired direction an a typical welding gun (1). The movements are performed around the axis D, E and P. When D is rotated even E and P are rotated around the axis D, when E is rotated even P is rotated around the axis E.

Fig 2 shows the spaces (2 and 3) where anything can be mounted, not

moving together with axe P but moved by axis D and E without inter¬ fering with any mechanical part or the gun during the movement.

Fig 3 shows the mounting of the transformer (4) mounted upon part (7) in one area shown in fig 2 which is moved by part (8) around the axis E. The gun (1) and the contact unit (6) is moved around the axis P relative to the part (7). The secondary current supply (5) connect the transformer to the contact unit (6) for feeding the sec¬ ondary current.

Fig 4 shows same as fig 3 but with the robot disconnected when an exchange system for the unit transformer, secondary current supply and gun is used. (Details of exchange system not shown. This is known technology upon the market).