Are stepper motors right for this project?

Hi all,

I have embarked on a project to build an antenna rotator (azimuth and elevation ). I chose stepper motors in the (possibly mistaken) belief that they could provide me with the pointing accuracy I was looking for, which is 1 degree.

The motors I have are a common 1.8deg per step 200 step per rev motor. In microstepping mode they are of course 1600 steps per rev which equates to 0.225 deg per step.

So here my problems start with trying to point at say 10 degrees, which is 44.44 [recurring] steps (10 \ 0.225)- which I'm assuming a stepper cannot handle. So 44 steps is probably close enough in the grand scheme of things, but then I am thinking that I will start running into rounding errors and the accuracy will get further and further off. At the end of each tracking session I will reset my steppers to a known "0" point using switches, so I have a known start point for each session.

So... have I

a) Gone the wrong way with using steppers b) Got the math wrong c) Approached the calculation for stepping wrong d) Got too worried about the math and accuracy e) None of the above/all of the above ??

Any and all thoughts appreciated.

Colin.

I would vote for d)

One degree is mighty fine even for the highest of gain antenna. The main problem is that you are going to have a lot of mass, that means momentum so you are going to have to have big motors if you don't want to be skipping pulses. You also don't say what speed you want to track, I assume it is a ham satellite which means about 10 mins horizon to horizon.

If a stepping motor can go to 0.001mm on a CNC it can manage a degree of pointing. However I would suggest that you look into gearing your motors.

(73's G8HBR)

Well I would have you take a step back and think about what you are proposing.

Are you are assuming that you need a stepper motor with a step resolution equal to or better then 1 degree of antenna rotation you require?

That would be true if the stepper motor was directly coupled to the antenna rotation shaft. I don't think you will find a stepper motor large enough to handle direct rotation duty, holding torque would have to be very large to keep it from slipping cogs in high winds.

I don't recall ever seeing an antenna rotator motor, steeper or conventional motor, that was directly coupled to a rotation axis, they always used some internal or external gearing down. So assuming you will have to have a gear reduction drive, then the step size of the stepper motor becomes less sever does it not? You just have to know how many motor steps equals one degree of output gear ratio rotation. "Let me count the ways" ( A little valentine joke) :D

Lefty

Hi Mike and Lefty,

Thanks for the feedback. I’m looking at a light weight Arrow type sat antenna for the ham sats. I intended to have it well balanced so as little torque as possible was required to move it (at least in elevation).

Tracking speed as you rightly point out will be pretty slow. I’m using WXtrack to output the Az/El commands and so far have Arduino displaying those numbers on an LCD. My next step was to get the math right for pointing. Update speed from the PC is anything from 1/sec to 1/min. I was thinking along the lines of every 5 secs, which is still a few degrees change on a high angle pass.

I was trying to avoid the complication of gearing the motors, but it looks like in reality I may have to. There are a bunch of DC geared motors around that are cheap and will have heaps of torque, so maybe I should head down that route. Back to the mechanical drawing board 8)

(73’s ZL2ARL)