Dynamic Braking Precisely Portions Cheese

Integral dynamic braking precisely portions blocks of cheese weighing up to 85 kg (187 lb), dramatically cutting the time for the task and giving accurate and consistently repeatable results. Control and accuracy of moving the blocks is attributed to motor drives on the custom cheese-cutting machine from Northwood Food Machinery of Stoke-on-Trent in the U.

06/01/2006


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Integral dynamic braking precisely portions blocks of cheese weighing up to 85 kg (187 lb), dramatically cutting the time for the task and giving accurate and consistently repeatable results. Control and accuracy of moving the blocks is attributed to motor drives on the custom cheese-cutting machine from Northwood Food Machinery of Stoke-on-Trent in the U.K.

Tippagral SA 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% of our throughput is grated, but many customers—particularly wholesalers, manufacturers of pizza, and sandwich/baguette producers—require cheese to be cut into specific sizes and shapes (for example, 9 x 9-cm, 7.2 x 7-cm, and 12 x 5-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 can cut the blocks in two-dimensional axes. Tippagral bought a Northwood cheese-cutting machine following the successful use of a similar machine at its parent company, 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."

Dynamic stopping crucial

According to Northwood Food Machinery director Mike Southwick, a previous solution was less than ideal. "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 (CT), a division of Emerson Industrial Automation. CT was "confident that it could give the degree of position control required using the smallest of its general industrial drives, Commander SK," says Southwick. "On first trial, these drives stopped the motors on target—and without any additional cost of braking resistors."

The cheese-cutting 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 cheeses as they move through the machine. The second conveyor section moves the cheese into the machine and, when the way is clear, over to the third section. This conveyor has optical sensors with position feedback to the programmable logic controller (PLC), which initiates a rapid stop exactly between two pneumatically controlled pusher-plates. This ensures that the cheese is exactly centered on the conveyor.

After centering, the cheese is moved by the next drive onto the lift section of the conveyor, and then is stopped at the required position. A scissor-lift elevates the block to the 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.

Commander SK ac drives are available in the 0.25 to 132 kW power range. According to Control Techniques, they require as little as half the footprint space and volume compared to other drives of the same power rating. The line of drives features 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 PID control, 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 with off-the-shelf motors. The drives on Northwood's cheese-cutting machine are hard-wired into the PLC, providing a fast digital link that gives smooth acceleration and instant stopping as needed.

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.






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