Miniature Reflective Encoder

A new miniature three-channel reflective encoder from Avago Technologies features built-in interpolation for high-resolution measurement that is ideal for use in closed-loop stepper motors, miniature motors, printers, copiers, card readers, insulin pumps and other types of industrial, consumer and medical equipment.

"The integrated third-channel, compact size and high-resolution encoding performance of the Avago AEDR-850x modules is truly revolutionary, enabling smaller motor encoding systems than ever before possible," says Hassan Hussain, general manager of the Motion Control Products Division at Avago. "These modules cut design costs and save PCB board space for our customers, and expand the reach of our reflective encoder offerings into the industrial automation and medical segments."

A standard reflective encoder module uses two digital output channels for direction sensing, and another module for indexing purposes. For motion feedback applications requiring high-resolution measurement, an external interpolation device has previously been necessary. The new reflective encoders integrate an LED light source, photo detection and interpolator circuitry, and the three channels in a single package have a 3.95 mm by 3.40 mm footprint. With high resolution encoding of 304 lines per inch (12 lines per mm), the compact, highly-integrated modules enable a new level of miniaturization for applications where size and space are primary concerns.

"There are two major areas of uniqueness with this encoder," says Swee-Hing Young, technical marketing engineer for Avago. "It is the world's first LED-based miniature reflective encoder, and the design actually accommodates A, B and index using a single track for the three channels."

"In a conventional optical encoder, typically there are separate tracks for the A and B channels, and a separate optical track for the index. With this new encoder, it allows us to add our existing two-channel reflective disk into an area that is less than four millimeters square," Young says. "The single track helps make the device achieve both miniature and low cost design objectives, along with the use of LED technology."

By comparison, laser-based three-channel encoders on the market are more expensive and require much higher power consumption than LED encoders. Those solutions are also much larger, as much as seven times larger than the reflective encoder.

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The third index channel which is typically used for homing is also useful. Since the devices are incremental encoders, using the index (I) channel provides the homing position (or zero reference). With only one index signal in one mechanical revolution, the system knows its position from the index point onwards.

The device also offers power consumption savings, primarily because of its LED light source. Because the design uses TTL signals, even the interpolator that provides the TTL or frequency multiplication is logic-based and customers that are already using TTL can interface the encoder in a straightforward way.

The AEDR-850x encoders offer interpolation of up to 4X and enable various resolution designs by changing the code wheel size. The optical-based modules are significantly less susceptible to EMI compared with Hall Effect devices. With an absolute operating temperature range of -20 to 85C, the rugged encoders are suitable for commercial and industrial operation environments. The encoders are priced at $6.75 each in 100 piece quantities.