Stepper motor for antenna rotation

[VK5PJ]

Hello,
over time I have been working towards a small EME station, one aspect has always presented a hurdle, how to move the antennas slowly enough to track the moon without over shoot of direction control. As normal motors need time to spin up and spin down there is generally some degree of over or under shoot of the mark.

I have a rather large and unique gearbox /rotator that came from a VK3 back in June, it was advertised on VKHAM and is a made by a US company called: Houston Fearless

Will post a photo or a link to it soon.

This thing stands 1.2 meters tall and has 900 x 900 mm footprint, it uses a very large worm drive to create rotation and had a 100V AC motor to provide the grunt (reverse the field connection to go in reverse). While testing its operation it became apparent the speed was a bit fast for my liking and even though it used a magnetic brake to reduce over run it still was hard to be accurate with it like this.

I have always had stepper motors in the back of my mind to be the motor for a rotator on my EME system as they can be pulsed to provide small known steps in rotation and hence provide small increments of azimuth BUT now comes the crunch, have others out here in the blogeshpere had any real world experience with these beast as to how much torque they can generate or are they gutless beasts not suitable to drive a gearbox?
Regards,
Peter, vk5pj

[VK3PP]

Hi peter

Slightly off topic but have you considered a variable speed drive (variable frequency drive)?

The ones i have used have been totally adjustable in many respects, one also incorporated dc breaking.

just a thought.

Matt

[VK5PJ]

Thanks Matt,

Hi peter
Slightly off topic but have you considered a variable speed drive (variable frequency drive)?
The ones i have used have been totally adjustable in many respects, one also incorporated dc breaking.
just a thought.
Matt

I do remember you mentioning them when we last met but that had not come back to me, how SLOW can they run? any ideas on how to find out more info on them? I am guessing this is an AC motor with some form of pulse width modulation, will have to add that to my searching.

EDIT: the technology to do variable speed drives looks like it might be a LARGE source of RFI, which I need to avoid having near the antennas.

Peter

[VK3PP]

Hi peter

Yes they can be large source of RFI witch i had forgotten about butt the newer ones involve ways to reduce/ eliminate it.
when we install VSDs in the workplace we run shielded cable form the VSD to the motor and try keep it as short as possible, we also do some extra earthing. i have not had anything affected by a VSD RFI wise but i might take the 817 into the plant room and have a scan around and see.

At trade school we ran an ac motor down to 15hz! very very slow not recommended but it defiantly worked. The other advantage with a VSD is you wont loose torque unlike other reduced voltage (ac) starters.

Not sure exactly on what they do to the waveform but they reduce the sin-wave by chopping it similar to pwm. I believe something like this http://s.hswstatic.com/gif/dimmer-switch-diagram-6.gif

There are other options with ac motors or even dc motors just food for thought

Hope i have helped in some way

Matt

[VK5ZD]

What's the beamwidth of the antenna? Unless it's very sharp (i.e. >25db gain) a bit of overshoot probably won't matter.

[VK3WRE]

Hi Peter, I use a stepper motor drive system on my 2.5 m dish. I replaced the original motor in a very sick KR400 rotator with a stepper
from a large printer. The gearing in the rotator is enough that the stepper is not stressed to much and takes about a minute for 180 deg
of rotation. The original idea came from Russel VK3ZQB in a Gippstech presentation some years ago. food for thought.
Ralph VK3WRE.

[VK5PJ]

things evolve,
have been playing with other ideas for powering the Houston Fearless worm drive, on the list of things to try is a old or maybe even a new roller door opener. They appear to have good torque ability and are already geared down to a usable speed. more as it happens, film at the 11pm news broadcast as the classic TV presenters used to say .

Still want to try the stepper motors but the recent weeks have been a bit on the silly side here with floods and many LONG power outages.

Peter.

[VK2ZIW]

Hi,
With the Raspberry Pi being so so available, look at this:

PiRotator
URL:
jkry.org/ouluhack/PiRotator
Ville Jussila OH8ETB
wilh(at)jkry(dot)org

I've emailed and have his code.

Solution for EMR, absolute encoders, worm gearboxes that don't slip back
and, turn off the stepper controllers once position is attained.

Alan VK2ZIW