Re: A question for Don


Dan Hummel
 

Thanks Don and Russell for clarifying the absolute encoder issue. You save me a lot of grief, anguish and confusion. I'm a little embarrassed that I hadn't worked this out myself. If your a shoot from the hip engineer, you can end up shooting yourself in the foot. Looks like I'm committed to a 1:1 encoder drive. With the limitations of the axis shaft diameter, bolt head clearance and bolt head access I have little wiggle room, but I thank I can make this work.

This brings up a new question. We are using a Renishaw on the RA axis now. Without checking the Using Absolute Encoders (plural) check box are we actually taking advantage its resolution? I was under the impression that you needed absolute encoders on both axis to have this checked.

Russell, here are a few images you requested.

What started out as a drag and track scope is now a remotely operated goto. Being a split ring drive, the declination axis can't be directly driven. We use two 50:1 stacked harmonic drives to get the needed reduction. Two matched diameter drums, one mounted on the drive output and one one the dec axis, are linked with .010 inch by .5 inch stainless steel feeler stock tensioned with modified guitar tuners. The RA drive is a single 150:1 harmonic drive. The 2 inch drive wheels against the 36 inch split ring adds another 18:1 to reach the needed reduction. It had to be mounted outboard and back from the original drive to make clearance for the large harmonic drive. The original dec encoder was low resolution and was driven with plastic gears. I'm trying to mount a pulley and cable system to drive the Renishaw encoder in the gap between the dec axis clamp and the dec bearing housing where the original gear was mounted. These just are a few of the many modifications we've had to made over the years.


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