Machine Design

Absolute Encoders Keep Paperboard Carrier Belts on Track

Chicago Electric,

Fraba Inc.,

Optical encoders working over a DeviceNet network let a paperboard mill automate the positioning and tensioning of conveyor belts.

At the Aurora, Ill. mill operated by Rock-Tenn Co., each of 10, 170-in.-wide wire mesh carrier belts ferry binder board through a 240°F dryer. The belts tend to wander toward the side of the conveyor frame and rub against the frame if they are not periodically recentered. To automate the positioning and tensioning of the belts, Chicago Electric, Carol Stream, Ill., put a Fraba Inc. (Hamilton, N.J.) Optocode (OCD) DeviceNet encoder and a half-horsepower Marathon ac motor on each of the 30 axes. The installation was economical partly because one DeviceNet card handled all of the high-resolution, absolute encoders. Control was via Control Techniques’ (Eden Prairie, Minn.) half-horsepower ac vector drives run from an Allen-Bradley Control/Logix PLC and a touchscreen.

Using DeviceNet encoders and relatively inexpensive drives, motors, and a PLC kept Chicago Electric’s quote at just under $200,000. A more-typical system with servo or stepper technology and separate interface cards for each of the 30 encoders would have cost as much as 50% more.

Each OCD absolute encoder uses an integrated Opto-ASIC that provides up to 30-bit multiturn resolution and can withstand the heat near the dryer. At Aurora, the fully automated closed-loop system for positioning the belts replaced a completely manual process. If a belt slackens, a worker taps a button on the touch screen to take up the slack.

© 2010 Penton Media, Inc.

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