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09/07/2004
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Steel wire wound onto a drum has to be guided so that it only forms single layers.
 Problem: This is normally achieved by some wire guiding mechanism, often working in conjunction with grooves on the drum. Nowhere is this more necessary than in a wire driven encoder, where position is inferred from the drum's angular position.
Solution: The Sick Stegmann BTF/PRF range of wire driven encoders uses a drum that runs on a shaft equipped with a screw thread, with the pitch of the thread similar to that of the diameter of the wire. As the drum winds in under coil spring force, it moves one thread pitch along for each revolution of the drum.
The drive mechanism is fitted with a scraper to keep the wire clean. Any remaining dirt and moisture is removed by a set of brushes and a grease chamber before it enters the wire drive.
The encoders are either incremental or absolute, and provide linear measurement up to 50m with a resolution of 25 microns. For incremental encoders, interface options include HTL or TTL signals. SSI, Parallel, Profibus-DP, DeviceNet or CANopen outputs are available for absolute encoders.
Applications: The basic idea is applicable to just about anything that is wound in, whether it be steel cable on a winding hoist drum, hose pipe or fishing line.
Uses for the encoders include: measuring height reach on fork lift trucks, lifting platforms, and cranes, and in drilling and excavator systems, presses, punches, plastic injection moulding machines, weir systems, locks, theatre stages, woodworking and stone processing machines and also medical equipment. They are especially suitable for any mechanism involving hydraulic, frictional or winch drives, where linear motion cannot be accurately inferred from measuring drive rotation.
The encoders are chunky and solid and fitted with strong stainless steel wires with a long life expectancy. They may be set to a nominal zero of 1,000 counts by pressing a button. TS
Erwin Sick
Email to Erwin Sick
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Author
Tom Shelley
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