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Better by Design: Hollow-shaft encoder

Dec. 1, 2011
Noncontact magnetic rotary encoders are hard to beat when it comes to measuring rotary motion in machinery, as they are relatively small, strong, and

Noncontact magnetic rotary encoders are hard to beat when it comes to measuring rotary motion in machinery, as they are relatively small, strong, and inexpensive devices compared to traditional potentiometers. Simple versions are used in high-volume applications, such as vehicle antilock braking systems, while more sophisticated encoders are used for demanding motion control tasks, such as industrial automation systems and medical equipment.

Basic incremental encoders send out a pulse each time the shaft moves a certain amount. Yet for many motion control applications, this isn't enough information: The control must know the exact position of each drive shaft at all times, including an accurate measurement of angular shaft rotation plus a count of complete rotations. With a simple incremental encoder, the control system must assume responsibility for keeping track of shaft position during equipment operation. This can lead to problems during a power outage or sudden shutdown, requiring time-consuming system restarts. Even worse, serious safety issues can arise from lost or corrupted position data.

Magnetocode (MCD) absolute rotary encoders from FRABA Inc.'s POSITAL unit, Hamilton, N.J., employ dual technologies for super-accurate positional feedback — a two-axis Hall effect sensor that measures change in the magnetic field and reports angular measurement to the control system, and a rotation counter based on the Wiegand effect. Measurements occur without the need for external power, so that changes in absolute shaft position (number of turns plus angular displacement) are registered and reported correctly to the control system, even if power is lost.

One potential limitation of the basic MCD design is that key measurement components (namely the permanent magnet) must be mounted at the device centerline, preventing a true hollow-shaft encoder design. The new MCD configuration places the encoder unit beside the shaft: A hollow-centered bushing fits over the machinery shaft — like a ring around a finger — and sits beside the encoder, with the two components connected by gears inside a rectangular case.

VITAL STATS

On display: Hollow-shaft Magnetocode (MCD) absolute rotary encoder

Key features: Precise measurement of rotating shaft position using a Hall effect sensor to detect angular shaft rotation and a self-powered Wiegand wire to count complete rotations; hollow-shaft version includes a bushing to fit over machine shafts.

What it means to you: Accurate, absolute shaft position information, even if power loss occurs; hollow-shaft version enables easy mounting on shafts to 20 mm in diameter.

What else: Analog (voltage/current) and digital electrical interfaces available; digital outputs include serial (SSI), CANopen, and DeviceNet; rated to IP69K.

Innovator: POSITAL unit of FRABA Inc. • Hamilton, N.J.
(609) 750-8705 • fraba.com

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