Tech – Using Renishaw Absolute Encoder

The search for a LBB

As already mentioned in Tech – First Hardware/Software Test and Tech – The Telescope Mount we plan to upgrade the telescope mount with absolute encoders to increase pointing precision and to get rid of aligning the telescope every now and then, and after every shutdown.

As also mentioned in Tech – Telescope Control, we are planning to use the SiTech (Sidereal Technology) Servo II controller. In this document we found a reference to using absolute encoders with 26 bit encoder resolution. However this reference was more or less a sub clause in the document. It also mentioned a “LBB”, a little black box which is needed to interface with the encoders.

After some research we didn’t find out what this LBB is and if absolute encoders really work with the SiTech Servo II. We had our doubts, since there are no dedicated connection ports for absolute encoders and only this document mentioned them. You even do not find the Box in the SiTech online shop.

Our last hope then was the registration in the SiTech support forum and asking our questions there. Btw., we should have done this earlier, since the SiTech guys are really responsive and the forum in general is really accommodating.

So after a short chat there, we got the info that they do have this ominous LBB and they also sell it.

This box is designed to work with absolute encoders supporting the BiSS-C protocol. However SiTech recommends encoders from the British company Renishaw. On the SiTech controller side it will plug into the RS232 interface of the controller. The LBB is also prepared to directly work with the plugs used by Renishaw on their encoder systems.

The Renishaw encoder system

Now that we knew SiTech indeed supports absolute encoders and we also now knew the encoders to go with, we now started the evaluation on the encoder system we really need.

The main problem there was the fact, that our mount does not have “real” axis as described in the post Tech – The Telescope Mount. So the first important point was found: if we would like to use absolute encoders, we have to go with a system which has ring encoders — other designs won’t work on our mount.

The second important point was also figured out quite fast: the minimum inner diameter of the ring encoders. On the DEC axis we have to use one with minimum of 14cm of inner diameter. The problem is the RA axis, since the ring encoder has to fit around the quite thick neck of the fork part of the mount. Here we, at least need an inner diameter of 38cm. Another important point was the supported temperature range of the encoder rings and read heads.

We found, that the Renishaw Resolute measuring system, in combination with their RESA scale rings, will fulfill all of our needs:

  • It is compatible to the LBB (natively → plug and play)
  • It supports the required 26bit encoder resolution
  • It will support our need of an extended temperature range (-40°C to 80°C, the important value is the -40°C since we can reach temperatures well below -30° at our observation site) with their ETR read heads
  • The scale rings are available in sizes we need
  • The scale rings and read heads are small enough to fit with our mount.
The Renishaw absolute encoder system showing two RESA scale rings and the corresponding Resolute read head. (Image by Renishaw)

RA and DEC Mounting

The last issue we had to solve when we use absolute encoders, is the mounting of the encoder on the axes.

On RA it was quite easy, apart from using a really big scale ring, since there is enough space. The other thing is, due to its bad shape, we have to replace the RA friction wheel at all. This makes it now possible to lathe the encoder mounting directly to the RA wheel during remake.

Here you see the current RA friction wheel with the neck towards the fork. The RA wheel will be replaced by a new one including the mounting ring for the encoder scale ring. (Upper blue part = fork, lower gray part = pole block, marked section = RA friction wheel + neck) (Photo by Erich Meyer, 1999)

For DEC it is a little bit more difficult, since the free space is quite narrow. But also in DEC we plan to redo the friction wheel and the bearing cover plate. The bearing cover plate is then an ideal point to mount the encoder scale ring. For this purpose, the cover has to be remade including the mounting ring.

The silver disk is the current bearing cover which will be remade a little bit bigger and with the encoder mounting. (Photo by Erich Meyer, 1999)




4 thoughts on “Tech – Using Renishaw Absolute Encoder”

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