Trying to find absolute to incremental encoder converter


Thread Starter

Jonathan Manton

I'm working with a company that recently purchased two linear motion control stages for a precision application. The stages have linear motors and built-in encoders that output a quadrature RS-422 signal for feedback control.

For one of the stages, I would like to be able to use an interferometer for position feedback rather than the integrated encoder. The interferometer provides a 32-bit parallel absolute position, rather than the incremental RS-422 quadrature input expected by the motor controller we have. The vendor of the controller does not offer an option to do feedback control using an absolute encoder (at least not with the product we've already purchased...).

What I need (I assume) is a device that will convert from absolute position to quadrature pulses. E.g., in each time interval, it measures the difference in the absolute position, and sends the requisite number of pulses to the motor controller (with the understanding that the position reported to the controller could be delayed if the position changes too fast).

Now, I could build such a device using a microcontroller and a couple of RS-422 drivers. But the engineering time to build something custom would cost several thousand dollars (at a minimum), and wouldn't be nearly as robust as a commercial product.

Does anyone here know of any devices out there that would perform this function? The only thing I could find using Google was the Allen-Bradley AEC Absolute Encoder Converter, but it appears to be a product that is either no longer made or has been changed sufficiently that I cannot find it in their catalog any more (or maybe I just don't know where to look).

Any help would be greatly appreciated!
BEI makes an EnDat encoder that puts out both absolute and quadrature.

To convert from absolute to quadrature would just give you the worst from both worlds. Slower response time and no positive positioning. If you can only use control that accepts incremental I would stay with the quadrature that is already mounted.

Jonathan Manton

The built-in encoders do not have the resolution or repeatability required for this application. I'm in the unfortunate position of coming on to the project as a consultant months after all the expensive equipment was procured. I can't tell them to throw away the $25k interferometer, I need to find a way to make it work...

We are using an ultra-precise interferometer that is usually used for other types of applications, and doesn't have an option for an incremental output. What it does have is a very fast (220 ns latency) absolute output. The controller has a 10kHz servo loop, so I can still make this work without adding too much latency with a relatively simple converter.

Just need to know if there is a $1k device out there I can buy, rather than spending several times that to build a custom solution.