[amsat-bb] Optical shaft encoders

Daniel Cussen zg3410 at gmail.com
Fri Mar 18 10:29:52 UTC 2016

>for those with Alfa Radio HR model rotators, maybe you have same/similar issues?

I do not have a HR model, but I do have a similar system and we are
seeing similar problems.

First off there is no end stop safety switches. This means if the
control box becomes confused it can damage coax cables and move
elevation to positions to damage antennas etc.

Second, there is no absolute position sent, meaning if it gets
confused it cannot reset itself as only the amount of movement is
sent, not the actual current position. Over time this means errors
accumulate and grow.

The basic Yaesu G5500 has both safety protections meaning it is
unlikely a confused control box will result in damaged antennas or

>So first off, Bob is basically asking if anyone has built a custom optical shaft encoder to >replace the magnetic hall effect sensors in the High Resolution Big-Ras rotators.  >Machining, circuit design, performance.....?

I do not know, however I have nearly added safety switches to a
similar model to turn off the motor to protect the coax cables. This
is also very important if the relays stick in the control box.

Others have removed or replaced position sensors with more accurate
absolute positions sensors. In particular the HH-12 is used by a lot
of large EME stations as it is both accurate, cheap and absolute
position. If you connect it using a rubber hose it will protect itself
from damage if you try turn it too much.

Mounting it to the Big-Raz is unknown, but here is a similar project:
Here is a complete controller with two sensors:

>The noise voltage was 1 or 2 volts peak to peak when measuring the lines with an o->scope.

This is not good.

> The shields of the cables are connected together at the connector on the rotator (8 pin >MIC connector) and at the connector on the MD-01 control box.  The shield is also >jumpered to a good station ground at the control box.

To reduce noise the recommendation it to only connect the shield AT
ONE END, and not both ends. Normally only at the shack end. I think
this prevents ground loops.

>So that's about it in a 'nutshell.'

So in my case we have a similar sensor. All the HAMTV ground stations
in Europe (6) are using Prosistel Az/El with uses hall magnetic
sensors with thousands of pulses per second. We too are seeing
positions change with the motor stop, so much so thousands of pulses
must be read while stopped.

The solutions we have used so far:
1) Multiple screened cables as you suggest grounded at one end
2) Adding filters to the motor wiring to reduce motor noise/cross talk
3) I experimented with stepping up rotation feedback from 0/5V to 0/20V
4) I am working on a replacement control box, where we can modify the code
5) I am adding safety switches to protect the coax.

The real solution would be for the manufacturer to use absolute
position feedback. There is complete controllers available, if you can
manage to connect their sensors to your existing system using
belts/gears/cogs etc.

Another option I think you should consider is using your own control
box. We found that the Prosistel supplied control box was flawed in
some ways. The open source control box is already designed to take
pulse inputs, and seems to work with thousands of pulses per second.
You can even just hook the inputs in parallel to see if the problem is
the control box or the sensor outputs. All you need is an arduino
(mega preferably) and the correct version of the code. It will display
a second opinion of the position, so you can determine if the control
box has issues too. We found our control box misses some pulses, we
think it is busy updating the LCD or talking to the computer and
misses pulses.

Other features of this is a master/slave option, meaning the
controller can be mounted at the antenna, meaning only short cable
runs to the position sensor.


In the long term absolute position sensors combined with safety
switches are the only real  solution. For HF even a bog standard
potentiometer would probably work better. Some suppliers use 3 or 10
turn potentiometers to allow 360 degrees or 1.5 turns rotation.

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