Re: [CAD_CAM_EDM_DRO] reducing the encoder line count

I sent a post on this problem a week or so ago. When you have a stepper
and an encoder operating together, the dead lock area in the stepper cycle
must correspond with the dead no transition area of the encoder. This is
called interlacing. There isn't an acceptable following error in a
positioning system, unlike an interpolating drive.

Therefore we interlace it. we adjust the dead band of the encoder to the
middle of the stepper lock area by using a meter or scope count display,
depending on the system and loosening the lock screws on the encoder. Then
turn the encoder in each direction until you get a change without driving
the stepper to step. Then split the difference and lock the stepper down.

If the system oscillates at any stopped position it means the interlace is
off due to :

1. The symmetry of the square waves on the encoder is failing. Both
square waves are no longer square. Detect this with a scope. Traverse the
axis rapid while synching on one square wave input and looking at both of
them. If they arent pretty near equal up and down and 90 degrees out of
phase you have lost symmetry and must repair the encoder. Follow
instructions for that machine or post a request for further instructions.

2. The interlacing is off. Follow the instructions for interlacing.

3. A connector between the ball screw and the encoder, the ball screw and
the stepper, or the ball screw and the encoder is loose or broken. Shear,
roll, and taper pins are often used to hold the shaft of the stepper to the
ball screw. They fail or elongate their holes. Worse yet, a set screw is
used. They fail easily. Sometimes a key and keyways are used, another
source of failure. All are repairable with some thought. Don't do any
repair without thinking it through and getting some advice from an old
machine repairman if you are not completely sure your fix is appropriate.

Wiping off the encoder disk will sometimes help, but if it does, it usually
means symmetry is failing and the cleaner disk let more light through
between a failing light source and a photosensor, or a failing photosensor
circuit.

Ted Robbins

>Scott Nelson wrote:
>
>> I think I need to toss a little bit in on the P-W Tape-O-Matic. I am an
>> engineering type out of the aerospace arena and we had lots of these

around.

>> I think the last ones I used were '62 models. They did indeed have the
>> encoder on the opposite end of the shaft. It was under a simple cover and
>> just a basic plastic wheel about 4" in diameter that ran between a reader
>> head. The machines worked very well for what they were designed to do. That
>> is the thing they were not mills but drilling machines. They positioned the
>> table and waited until the operator cycled the quill then moved on.
>
>Ahh, this is a totally different situation from a milling machine! This

is what

>is called a "positioning" drive, rather than an "interpolating" drive. The
>servo bandwidth requirements are much less for a positioning drive, and
>so the problem with oscillation is much less.

Jon