[pixpop]

I'm designing an axis with a linear motor, and am trying to decide what kind of feedback device to get. I've narrowed it down to 2 choices: A US Digital linear encoder, or a sealed encoder such as this one: http://cgi.ebay.com/digital-readout-...item27ae9e487b

Which looks similar to a Heidenhain sealed unit.

I understand how to mount the US Digital version, it's just a plastic strip and a small encoder module.

The other one seems to be made for a DRO, and I'm just guessing it's suitable for motor feedback in a servo system. I'm not sure how this mounts, whether it has its own linear bearing inside, how difficult it will be to align with my axis, etc.

I guess these are the same problems encountered when adding a DRO to an existing machine.

Anyway, you can see I have no clue about these devices. Any info would be greatly appreciated. My inclination is to go with the US Digital device, because it just seems simple and easy for me to understand, but the other looks more robust, and offers greater precision.

Thanks,

Neil

[Al_The_Man]

The link you show is a standard looking etched glass quadrature linear scale, there are quite a few manufacturers out there.
They are essentially the same design as a rotary encoder except being linear, it is important when mounting that the head and scale that they remain aligned for the total length of the scale. i.e. rigidly mounted.
They usually come with mounting instructions.
There are a couple of manuf where the reading method is inductive, Sony and Newall.
These are more impervious to contamination of coolant etc.
If you pick up a Heidenhain scale you need to check if they are 5v TTL as some are 10µa or 1Mv sine wave output.
Al.

[pixpop]

Thanks, Al.

Do these devices tend to have linear bearings inside, that keep the reader correctly positioned relative to the scale? Or does the alignment need to be provided by the system it is attached to?

With the US Digital encoder, there's no mechanical contact between the reader and the scale. When you install it, you have to guarantee spacing of .015 to .030 inch along the length of the scale. This seems like a generous tolerance, but then the scale is flexible, so that might make it more difficult.

Of course, being open means it's easier for contamination to become a problem. I don't have coolant to worry about, but I will have smoke. (This is a laser machine).

I'd like to go with the enclosed type, but my fear is that if there are bearings inside, the alignment requirements might be much more stringent, much like trying to align two linear rails with each other.

Neil

[Mike Everman]

The sealed units are made for this. They usually have a little bearing carriage inside that keeps the read head the right distance from the glass scale. There is a flexure coupling to the flange you bolt to the moving element, so no, it does not need to be perfectly aligned, but aligned within reason. Grizzly and other Chinese distributors have some really cheap sets. US digital has a length limit for the mylar type, but yeah it's really cheap and not so good with contamination.

[RICHARD ZASTROW]

The Newall linear encoders use a tube sliding through a reader "block", great for dirty environment and where vibration may be a problem.

You might want to "google" them. Interesting method.

Dick Z

[mactec54]

I have used the Newall linear encoders for many years & they are the best to get for what you want, they cost more than the cheap encoders but it depends on what you are trying to do,The cheep ones may be all you need.

I have the Newall microsyn which has a resolution of .0001/.00005 but you can have what ever accuracy you want/need for what ever you are wanting to use it for