Programmed drive cuts motor-plant cycle time

When a vehicle drop section at Nissan's Sunderland factory became a potential bottleneck, the company turned to Control Techniques for help in its redesign.

When a vehicle drop section at Nissan's Sunderland factory became a potential bottleneck, with the possibility of damage to the cars on the production line, the company turned to Control Techniques for help in its redesign.

The result has been a substantial improvement in the control of the process, allowing the introduction of a longer car onto the line while maintaining the cycle time and eliminating any chance of collision during the dropping of the car carrier.

"The line takes several models including the new Micra and Almera", explains Nissan's Senior Facilities Engineer, Peter Jones.

"The problem arose with the addition of the sedan version of the Almera, which has a longer wheelbase, since this reduced the margin for error to almost zero".

"We have a total cycle time for the whole process of just 60s, which meant that the first part of the operation had a 'window' of just 5s".

"The pneumatic actuator which controlled the initial 'grab', bringing the car carrier jig into position for lowering, didn't have a fine enough control and there was every chance of the longer wheelbase vehicle colliding with the car in front - clearly not an acceptable option".

"So we had to find a simple and cost-effective solution - and that was where Control Techniques came in".

The U-shaped mixed vehicle line starts with a painted car body shell and, just 165min later, it emerges off the end of the line as a completed vehicle.

The vehicle drop section takes virtually completed vehicles, each suspended on a carrier, from an overhead conveyor and carefully drops each into wheel cups on a moving floor conveyor.

"We needed to trim at least 4.5s off the cycle time", says Jones, "and to be able to programme in a minimum dynamic gap to miss the car in front during the lowering of the carrier".

"This proved impossible with our existing system".

Control Techniques was heavily involved in the design of the new system from day one.

"We were very pleased that the whole of the programming could be carried out on the drive itself", adds Jones, "which meant the whole process was very much simpler, leaving the PLC programming virtually as it was".

The new system starts with a completely new "booster" - the mechanism that locks onto the carrier and pulls it off the overhead conveyor.

This comprises a booster carriage, which incorporates a positive locking mechanism, and this is moved along the new aluminium booster frame by a toothed drive belt.

Control Techniques provided the gearbox and 1.5kW AC motor (from sister company Leroy-Somer) as well as the 1.5kW Unidrive, complete with plug-in UD75 program module, which also provides CT-Net communications, and additional second encoder module.

A CT-Net controlled Beckhoff I/O unit was also needed to cope with the additional signals from proximity detectors and limit switches.

Communication between the drive and the I/O module as well as with the PLC has been simplified by use of the high-speed two-wire CT-Net networking protocol.

This also simplified installation as well as giving clear and flexible communication that can be changed easily if required in future.

The programmed cycle comprises the movement to the end of the booster, to locate the carrier, which is locked into place on the booster carriage, a steep S-ramp acceleration and deceleration to the point at which the drop can begin.

The second processor module within the drive uses encoder inputs from both the floor conveyor and the booster motor to calculate the real time position of the carrier, and is currently programmed to give a minimum dynamic gap to the car in front of just 50mm.

The drive itself initiates the start of the lowering process, which is now also under the control of a 15kW Control Techniques Unidrive, replacing the 10kW DC motor and drive previously used.

"This gives a much smoother transfer", says Jones.

"The old DC drive was struggling with the extra weight of the new Almera".

Once the car has been dropped into place on the floor conveyor and the carrier is released, it is rapidly raised up to a higher level and the carrier is ejected, before moving back down to start the process all over again.

"Even this process has been improved", says Jones.

"The booster carriage inches up to the carrier and locates it before accelerating to eject it onto the chain conveyor".

"This prevent shock loads and reduces wear and tear on the carriers".

The result? "The cycle is completed well within the 60 second 'window'", concludes Jones, "and it has been working exceptionally well, with virtually no down time".

"We are very careful how we introduce new technology and new designs - we can't afford to have breakdowns".

In a factory that prides itself on achieving 98.5% overall efficiency, this allows less than 8min downtime a day for the whole shop.

It is a measure of Nissan's satisfaction that other vehicle drop sections are now to be converted to this new design and many other Unidrive SP applications are confidently predicted.

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