Richard,

A quadrature encoder with index is not an absolute encoder. At power up time you have no idea of the position of the "thing" that drives the encoder. It is necessary to go through a homing sequence to find index and then you supposedly will know where you are as long as you ensure that you can maintain reliable monitoring of the quadrature signal pair.

The real absolute encoder provides a direct readout of the encoder position that will stay proper even through a power cycle.

By the way, the number of steps per revolution, 512 or 2500, has no direct bearing on whether an encoder may be relative or absolute. There is of course a cost tradeoff of resolution versus the number of tracks on the encoder with sensors to read out an absolute position.

It is indeed fairly common for cost effective absolute encoders to be of lower number of steps per rotation than an equivalent cost relative encoder.

Long linear encoders that are used along the slide axes of things like precision X/Y tables are almost always going to be relative types and will not even have much use for an Index channel. Sheer length at some good resolution makes a binary or gray coding absolute output rather impractical. Say you have a slide axis that is 22 inches long with a 2500 steps resolution per inch quadrature output. That would be a total of 55000 steps end to end and if you equipped the slide with an absolute linear encoder you would require one with 16 read-out channels to get enough range to encode all positions.

There are products made that are absolute devices that can be used on tables and slides. Many of these will have some digital electronics right at the slide instead of feeding all the readout channels back to the system controller. Below is a link to an example of one such product that is available with up to 8 inches travel at a resolution of 1000 counts per inch. At 8000 total counts that implies that the device is using 13 channels of position pickup. These are typically cabled back to the controller system via a small cable that carries a serial protocol to fetch out the digital reading. The link also shows some the manufacturers other linear products as well.

http://www.mitutoyo.co.jp/eng/produ...0Scale.pdf

Michael Karas