A move to AC drives negates the need for a separate PLC

31 May 2011

A manufacturer of extrusion, winding and coiling equipment for the metals, wire/cable and plastics industries has eliminated the need for a separate PLC and reduced the need for maintenance of its semi-automatic equipment by switching from DC to an intelligent AC drives solution.

Meltech Engineering has taken a fresh look at the control system for its semi-automated plastics coiling machines, which can coil a variety of small plastic profiles such as the fine plastic tubes used in aerosols.

The solution chosen to simplify and improve reliability has been to switch to Control Techniques’ Unidrive SP AC drives, four in total for each coiler, each drive features a programmable on board plug-in SM Applications module - a PLC in its own right. This has eliminated the need for a separate controlling PLC and removed a considerable amount of wiring between the PLC and the drives, cutting both build time and an area of potential weakness.

The Meltech semi-automatic plastic coiler features two reels mounted on a transfer disc. When the active winder is full, the disc is rotated 180° and the extrusion is automatically transferred and attached onto the empty reel to allow continuous uninterrupted operation of the extrusion line. Depending on the product to be wound, the machine can operate at line speeds of up to 300m/min on a variety of drum and coil sizes.

The winding drives are both 1.1kW Unidrive SP AC drives in speed control, the transfer disc is controlled by a 1.5kW Unidrive SP in open-loop vector mode and the traverse is controlled by a 0.37kW Unidrive SP in servo mode twinned with a Control Techniques Unimotor FM servo motor.

All four drives communicate with Control Techniques' CT-Net network, facilitated by the SM-Applications Plus modules in each drive. The entire machine application control program is on just one of these units.

"We were looking for ways to cut manufacturing costs while maintaining or improving reliability and decided to switch from DC to AC," said Meltech's engineering manager, Andrew Procter.

“The feed for the winder drives is by slip rings. We now only need six slip-rings for the three phases on each motor - we needed eight before, which has cut build cost, improved reliability and cut some maintenance. In our development phase we considered whether we still needed encoder feedback for accurate speed control - but found that the Unidrive SP's open-loop vector mode performance enabled us to eliminate the encoders, again saving costs and improving reliability. All users will understand that AC means less maintenance than DC, so there's another saving here.

"However, perhaps the biggest saving has been in the elimination of the PLC. The programmable SM-Apps modules have enough intelligence to run the machine. We found them very straightforward to programme, any one who is familiar with regular PLC programming will have no problems in adapting this approach. All of the drives are an identical platform, so we don't need to mix and match different drives and servo-drives."

Meltech engineers used CT-Soft, Control Techniques' software configuration tool for setting up and to parameterise the drives and SyptPro for programming the PLC machine control program.

The Unidrive SP 'solutions platform' AC variable speed drive range spans 0.37kW right up to 1.9MW. It is configurable into five operating modes - open and closed loop, vector, servo and regenerating modes. With a range of plug-in module options, its on-board PLC can be supplemented with programmable and specialist feedback and communication modules. The SM Applications Plus module provides programmable automation from simple start/stop sequencing of a single drive to more complex machine and motion control applications. The module gives high-speed drive-to-drive communications via CT-Net and extremely fast I/O.

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