Precision welding for car alternators

Consistent precision positioning was the key design requirement when Forward Precision Equipment set out to design an automated machine for resistance welding of contacts on automotive alternators.

Consistent precision positioning was the key design requirement when Forward Precision Equipment (FPE) in Brighton set out to design an automated machine for the resistance welding of contacts on automotive alternators.

Previously the alternators were manually soldered, but this was slow, expensive and - a cardinal sin in the automotive industry - of variable quality.

To achieve this the designers used a three-axis motion system based on servo driven Hoerbiger-Origa linear actuators with encoder feedback.

"The first stage in this project was to prove that the joints could be welded", says FPE's Managing Director Andy Ward.

"Our company was founded 33 years ago to focus on exactly this technology, so it did not take us long to identify that resistance spot welding would provide us with the optimum joint in this case".

It was decided to build the machine so that the welding head is stationary and the workpiece is moved about beneath it.

The requirement was for 24 welds on one side of the alternator and three on the other, with the cycle time of 100 seconds being determined by the fact that machine was to form part of a four-process manufacturing cell.

"A hundred seconds is plenty of time for our operation so we decided we'd weld two workpieces in that time, the upper side of one and the lower side of the other, 27 joints in total.

In fact we complete the cycle in 78 seconds and have a short dwell time between operations.

However we knew that we'd need a high performance motion system to make sure each weld position was accurate", says Ward.

FPE called in pneumatic and electric machine actuation specialists Hoerbiger-Origa, with whom they had worked successfully in the past.

The design Hoerbiger's engineer suggested was based on its belt driven electric actuators.

Two of these are used to create the x-axis and one on the y-axis.

The y-axis and one of the x-axis actuators are driven by CT servomotors, acting through epicyclic gearheads, an arrangement used by Hoerbiger-Origa on several previous jobs.

The second x-axis actuator is not directly driven, but slaved to the first.

A rotational axis was also required; this like the linear axes uses an encoder to provide real time position feedback to the motion control software, which is programmed to take each weld location in sequence to the welding head.

The motion software communicates, again in real time, with the FPE's own weld monitoring programme.

Thus if a substandard weld is made, the machine automatically alerts the operator, giving precise details of the problem, so that he can decide between reworking and rejection.

If the former is chosen, intelligence built into the machine's control system means that only the required reworking is carried out and perfect joints are not reworked.

The control architecture is such that the motion profiles are managed from the drives' onboard intelligence, and these communicate with a relatively small (64 I/O) PLC which acts in a supervisory role to provide overall machine control.

FPE has built self-teaching into the control system, so that the machine can easily adapt to product variants and new versions of the alternator in the future.

"We generally prefer PLCs to PCs", comments Ward, "because they are so robust and therefore well suited to industrial requirements.

The mounting of the welding head was subject to considerable design effort, as it was required to meet several objectives.

It had to move out of the way to allow the operator clear access to the workpieces at the beginning and end of each cycle, which is achieved with a pivot-action pneumatic cylinder.

Pneumatic cylinders and slides were used to give the weld head a self-centring floating characteristic and so that it can change its position and thus achieve a perfect weld for even the most awkward of joints.

And finally, it is designed to be flexible and adaptable for future changes and reconfigurations.

Andy Ward sums up: "Many of our machines are one-offs, but we always learn something new and innovative from each job which can be carried forward into future work.

For instance this machine is fitted with light curtains to provide operator safety, an innovation a couple of years ago but now virtually standard on all jobs.

This time I think that with Hoerbiger-Origa's help we have probably perfected our knowledge of machine actuation and motion control".

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