Control Engineering Online Update for May 12, 2006


May 12, 2006


Sponsored by
Contemporary Controls

Faced with stopping these huge blocks of cheese instantly to ensure accurate cutting, Northwood Food Machinery sought AC drives that could stop conveyor motors on target. Tippagral S.A. in Dijon, France, expects the solution to streamline operations and cut labor costs.


Integral dynamic braking precisely portions cheese

Precise conveyor position control is needed to put blocks of cheese in the right position for cutting. Source: Control Techniques

Integral dynamic braking precisely cuts blocks of cheese weighing up to 85 kg. (187 lbs), dramatically cutting the time for the task and giving accurate and repeatable results. The control and accuracy of movement of the blocks is attributed to the drives on the custom cheese-cutting machine from Northwood Food Machinery of Stoke-on-Trent in the United Kingdom.

Tippagral S.A. in Dijon, France is a leading supplier of cheeses for the wholesale and food manufacturing markets in France and central Europe. Neil McAuley, director of Tippagral SA, says, "We portion some 7,000 [metric tons] of cheese per year, mainly emmental, but also cheddar, gouda, edam and mozzarella. Some 75 percent of our throughput is grated, but many customers—particularly wholesalers, manufacturers of pizza, and sandwich / baguette producers—require the cheese to be cut into specific sizes and shapes: e.g. 9x9-cm, 7.2x7-cm and 12x5-cm."

Tippagral needed a machine that could provide precise portion control. Northwood Food Machinery is an established family-run business with considerable expertise in the dairy industry. It has developed a range of specialty cheese cutters, from simple single-stage cutters to multi-stage machines that give the operator precise portion control, and which are able to cut the blocks in two axes.

Tippagral bought a Northwood cheese cutting machine following the success of a similar machine in use at its parent company, the Tipperary Co-op in Ireland. McAuley says Tippagral expects the new machine "will dramatically streamline this aspect of our operation and cut our labor costs."

According to Northwood Food Machinery director Mike Southwick, a previous solution was less than ideal. "A lack of friction in the gearboxes meant the blocks were over-running the target position by some 100 mm. We were faced with the task of stopping these huge blocks of cheese instantly."

Northwood brought in Control Techniques, a division of Emerson Industrial Automation. They "were confident that they could give the degree of position control required using the smallest of their general industrial drives, the Commander SK," says Southwick. "On the first trial, these drives stopped the motors on target—and without any additional cost of braking resistors."

Commander SK AC drives stopped the motors on target and without any additional cost of braking resistors. Source: Control Techniques

The machine is divided into four conveyor sections, each driven by a 0.37 kW Commander SK AC drive. The incoming conveyor's role is to ensure a fixed pitch between the cheeses as they move through the machine. The second conveyor section moves the cheese into the machine and, when the way is clear, on into the third section. This conveyor has optical sensors that feed back to the controlling PLC, which initiates a rapid stop exactly between two pneumatically controlled pusher-plates. This is to ensure that the cheese is absolutely centered on the conveyor.

After being centered, the cheese is moved by the next drive on to the lift section of the conveyor, and then is stopped at the required position. The block is elevated by scissor-lift to in front of the cutter section, where a heavy-duty pneumatic ram pushes the cheese through a grill of cutting blades. A guillotine at the reverse side cuts the cheese to the required length.

The Commander SK AC drives are available in .25 to 132 kW units. According to Control Techniques, they require up to half the footprint space and volume compared with other drives of the same power rating. The Commander SK line of AC drives provides automatic, no-spin autotune for optimal performance, open loop vector control, speed or torque control, and adjustable switching frequency for quiet motor operation. Drives also include PIC control, both linear and S-type acceleration and deceleration ramps, DC injection braking and an onboard EMC filter.

With integral dynamic braking as standard, a Commander SK drive can absorb the energy created by fast stops while giving smooth, precise control of off-the-shelf motors. The drives on the Northwood cheese-cutting machine are hard-wired into the PLC, providing a fast digital link that gives smooth acceleration and instant stopping when necessary.

Control Techniques helped Northwood Food Machinery set up the drives and machine, and provided the company with the programming. "We are delighted with the results," says Northwood's Southwick. And with the new machine, Tippagral is producing its precisely portioned pieces of cheese faster than ever before.

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