Valk Welding: 1999

The Belgian constructor Van Hool manufactures about 1500 coaches and busses each year and 5000 utility cars, such as trailers and tank containers. The company employs more than 800 welders (on a total of 4200 employees), and can boast on more than 18 years of experience with robot welding, and automatic welding. For some years now, the coach department is working with a welding FMS (Flexible Manufacturing System). During the day, the employees mount the parts on jigs. These are then being stored in an automated intermediate warehouse. Day and night, the robot system gets the jigs from the storage, the robot welds the parts and the jigs are replaced. The day shift unloads the finished pieces.

At this moment, the company is investing in welding robot installations for utility vehicles. For these, robot welding must be flexible, because of the smaller series. Six months ago, a new welding cell with four workstations for smaller parts was installed. Recently, a robot cell to weld large frames for tank containers was taken into use. In order to correct inaccuracies of weld positions, the robot has been fitted with a touch sensor to detect the correct position of the welds. Both installations were supplied by Valk Welding and equipped with Panasonic robots and possible off-line programming. Technical Management spoke with mister De Wilde of the department utility vehicles.

Welding of small parts
Utility vehicles also contain some smaller parts that are suitable for a welding robot installation. The department utility vehicles already had a few robot installations,  mostly installations with two workstations: one where the operator is putting the parts in the jig and one where the robot is welding. Each workstation is working with a jig adjusted to the part to be welded in which the parts of the workpiece are positioned with clamps. By mounting the jig always in the same position on the workstation (f.e. in a certain position on a turn table), the different parts of the working piece - and also the seams to be welded - can exactly be positioned in the same place for each product series (and if need be produced with days in between) and produced with the same robot programme. The robot can turn 180 degrees and go from one workstation to the other and finish the welding programme there. Experience has learnt that when using welding cells with two workstations, both robot and/or operator often do nothing: welding time and mounting time can be different, changing of jigs (when another type of part must be welded) can cause production in a cell to stop. Traditional welding robots are being programmed by "teach in" on the robot and on the correct welding place, meaning that for a welding cell with two workstations, the robot must be programmed twice. Al these aspects "typical for a traditional welding robot cell" demand quite a lot of (unrewarding) time for each welding position that cannot be used as production time, certainly when "high flexibility" (many small series of different parts) is required. It is obvious, that the investment does not offer maximum yield, unless such an installation is used for larger series of similar parts (and in fact this is not the purpose of a welding robot cell).

The new welding robot cell for the production of small parts was taken into use about six months ago,  to weld as many different (smaller) parts as possible in the most efficient way. In the meantime, more than 100 different parts are being produced on the cell. In order to guarantee flexibility, the new welding robot installation consists of four workstations and (as a result thereof) with a robot on a carriage to move horizontally. It is easier to adjust operator time and work content (external movements as bringing parts, monitoring and polishing of welding needles …) according to the welding time. When changing products, the jigs can be changed without having to stop the welding robot. In search of the suitable supplier, mister De Wilde came to the conclusion that most welding robots on the market do a fine job. The real difference is in the "intelligence" of the machine. However, this intelligence highly determines the final output, with programming options playing an important role. With traditional robot cells different cells also mean different programming for each workpiece,  because of the fact that there is a manipulator in each working cell, enabling the workpiece jigs to turn in order to offer the workpiece to the robot welding head in a better way (extra axle). On this "turn table" jigs are being mounted in a fixed position. Each turn table has its own height in horizontal position, each turn table has its own ground position. Alternatives were searched on the market, and Panasonic was chosen because of its off-line programming and the possibility to start working without too many modifications. Each working cell has a 'zero adjustment position' specific for the robot (moreover used after a collision to recalibrate the robot). All important position data (location of the table, horizontal level of the table) are entered once via robot measurements. This is used as a basis for off-line conversion of welding programmes that have been made via "teach in" on one robot cell, from one cell to the other three cells using the repositioning calculations present in the programme. In practice, an identical jig is made per cell, than in one of the workstations the programme is realised with "teach in" and that programme is taken over by a PC via cable in the work preparation and copied off-line by a CAD-programmer to the other cells. And production can start. With the Panasonic software, the welding programming can be done from a CAD-diagram.

Practice learns that quite simple parts, of which frequent series of some ten pieces have to be produced, can be welded with a robot cell in an efficient way: the gain of time by robot work is sufficient to justify the making of a jig (from some days to weeks of work) and the investment in the software. During preparation, CNC-programmed folding and cutting units and CNC-programmed drilling machines are being used, to ensure the production accuracy of the parts without having to take other measures in the welding robot installation. Changing jigs can be done in less than one hour and because there are four workstations, without having to stop the welding robot. For very small pieces, different jigs are used per turn table, resulting in more workpieces being mounted and welded in one workstation. The success of the investment and the quantity of parts to be produced is thus high, that it was decided to install a second welding robot with again four workstations.

Welding of large pieces with positioning sensor
The new Panasonic robot installation to weld frames, supporting structures of tank containers with sizes of 2.5 x 3 m is quite unique. Each day 8 of these containers are produced and since there are two frames required for each container, the production volume is sufficiently large to invest in a specific welding robot installation, aiming at 2 shifts for the robot. Compared with manual welding, the welding time was strongly reduced.

This is a welding robot installation that could not have been made with the technology of five years ago. The installation includes (at this moment,) four workstations and a welding robot mounted in hanging position and fitted with a carriage. This carriage can move horizontally on a longitudinal guide of 23 meter. The useful working range of the robot installation is about 112 square meter over a length of 27.25 meter. In the future, the installation will be made double this length, with three extra workstations and two robot carriages next to each other on the same longitudinal guide. This installation too has a turning manipulator in each workstation (suitable to manipulate weights of 2 tons). Between the two drives of the turning manipulator there is a mould to mount the jig for a specific frame type. With the tipping table,  the frame can be turned for top and bottom welding without manual manipulations. The mould between the two turning points of the manipulator is thus high in horizontal position that the operator is able to clamp the frame parts in an ergonomical way. It turns eccentric and hence there is not need for a floor pit-hole to turn the frame 180 degrees, rather the required space for the workstation has been increased. For this cell too, off-line programming can be used to convert a programme (for which a few hours to some days of work are required for a large variety of frames) from teach mode in one workstation to another welding programme for other workstations. This will save a lot of time and will increase the production time.
In this application, the welding robot has been provided with a "weld detection system" to compensate size inaccuracies of the parts. A container frame consists of a number of cast and pre-welded constructions. These parts are not completely stable in size: they can deviate some millimetres, which is not much for such parts, but a problem for the welding robot . Welds that have shifted millimetres, cannot be correctly welded with a fixed programme, meaning that the various welds will be bad: one part might be burnt, the other might have a loose weld…Problems indeed.

For each seam, the weld shifting is scanned before welding using this detection system. Once all seam edges have been detected and their position determined, the welding programme will calculate the shifting and the adjusted welding position, then the robot can start welding. This recalculation can be done with the Panasonic robot control. At first, the installation was supplied with the well-known traditional induction system. The detection of the product was sometimes disturbed by a contaminated torch due to the coated products and critical welding parameters selected by Van Hool (difficult welding positions with guarantee of a quality weld, an absolute requirement in view of the purpose of the end products). In order to solve this problem, Valk Welding developed a specific electronic control, that replaces the induction system. The stable sensor is maintained (the reliability of this sensor is proven on Panasonic-installations) and an electronic system is added to the control. This system has the advantage that the large contamination of the gas head has less influence on the "touch sensing". The sensitivity of this system can also be set 'to measure' in order to guarantee optimal operation. This setting cannot be done with a traditional induction system. There is electricity in each workspace, with the required supply lines for welding (CO2, argon, mixing gasses…), for manual welding (corrections, non-robot welding…) without people having to provide lines, or move welding appliances…
The cells with the workstations are being protected during welding with diode light curtains, enabling an unlimited view of the cell with security switching off when an employee should enter the cell during welding. A cell must be released by the operator before the robot can enter it and start welding. There is no territory guarding, since this is impossible with a moving robot in the workspace. The robot can be put into "safety mode" with reduced speed for maintenance works, whenever people need to enter the robot cell to activate the robot. The installation was completely supplied by Valk Welding, together with all CE-labels.

(Source: Technisch Management - February 1999)