Motion Control & Drives


Drives save the day

August 2009 Motion Control & Drives

Elevator drives system stops maintenance costs going up.

When an elevator in a cheese drying system broke down at a Danish dairy the entire plant was brought to a standstill with a loss of some five tonnes of cheese per hour. The suppliers estimated that it would take a fortnight to get the parts to rectify the situation. Engineering staff were facing the possibility of having to remove stocks of unfinished cheese to warehouses elsewhere, with storage and transport costs escalating as time went on.

“We already used drives from Control Techniques elsewhere in the plant, so I contacted Jens Holst at the Control Techniques Drive Centre in Greve. He and his team replaced the existing software with their own elevator software and managed to get us up and running again later that same day,” says Per Hansen, chief electrical engineer at MD Foods’ dairy in Denmark.

“We had been having constant problems with the original programming,” explains Hansen. “It was written in such a way that we could not adjust the programming ourselves and we had already spent some £8000 trying to get it right. In one day, Control Techniques achieved what the manufacturers could not. But it was a temporary fix and we asked Control Techniques to give us a better permanent solution.”

Plug-in control

Some months later, Control Techniques Denmark carried out a complete rebuild of the elevator control system utilising Unidrive AC drives and 115 UMB Unimotor servo motors under the overall control of the existing PLC controller. Each of the Unidrives, working in servo mode, is fitted with the unique Control Techniques plug-in program module. In co-operation with the supplier of the PLC controller, Control Techniques software engineers completely rewrote the positioning software.

“The instruction comes from the PLC, but all of the positioning is now carried out by the software within the Unidrives’ own program modules, completely eliminating any further operational problems,” says Hansen.

The elevators feed 200 gm packs of cheese into, and remove them from, a 21 level cheese dryer. Packets of feta and mozzarella cheese are cooled and dried in long tunnels prior to being sent to the packing department. The PLC selects the level for each pack of cheese and then the Unidrive servo system takes over, moving the elevator adjacent to the chosen track where the pack is pneumatically pushed onto a slow moving conveyor.

Feedback is from shaft-mounted encoders on the Unimotor brushless servo motors.

Superior performance

“The system now responds faster, more accurately and above all, reliably. Reliability is my number one requirement in this production facility. Each week I have to answer for every minute’s lost production and what I am doing to eliminate weak points. That is why I have used Control Techniques drives ever since this plant opened in 1989,” continues Hansen.

“Unidrives are used in various locations, but we are not using the plug-in program modules yet, except on the drier,” he adds. “I plan to do so in the future because it will make maintenance so much faster and easier. If I have to change a drive, there is no reprogramming of parameters. Just plug in the module and off it goes. If it saves even five minutes, it is a good investment.”

The dairy has an annual output of around 35 000 tonnes of mozzarella and feta-type soft white cheese. “This is a very busy plant,” concludes Hansen, “and any stoppage has major cost implications. That is why I need complete confidence in my drives and in the local support that is available. With Control Techniques, I have both!”

For more information contact Bill Tedd, Control Techniques, +27 (0)11 462 1740, bill.tedd@emerson.com, www.controltechniques.com



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