Re: ATM Stepper Motor Rates

[Bob Segrest <bobslan@erols.com>]

Sorry, I messed up and sent this to an individual instead of the mailing list the first time around...

Greetings,

I have been working with similar step and reduction numbers for the past month or so...  Mel has responded to a couple of my queries and I'll attempt to repay the courtesy by trying to explain what I think I have learned...  Mel, if I get it wrong bail me out.

The glory of the stepper motor is its relative angular precision.  The weak points are reduced torque and speed.  Ten turns per second (RPS or 600 RPM) is fairly fast for the kind of stepping motors we are talking about.  To achieve this you have to ramp up the speed and full step the motors.  Another point to remember is that when you full step (or even half step) one of these motors, you get fast (nothing is instantaneous) snapping movement to the next position.  If you hold the end of the motor shaft and single step it you will notice that there is nothing smooth about the movement.

Mel has done a pretty good job of balancing these factors into his design concept.  First of all he minimized the jerky full or half step movement of the motor by micro stepping.  This provides a smooth turning action and increases the effective resolution of a common 200 step motor to 2000 steps.

Now if we start with a motor that has effectively 2000 steps per turn and a target step resolution of 0.25 arc seconds, we can figure out the reduction required.  For example:

360 degrees X 60 minutes X 60 seconds = 1296000 arc seconds per rev

1296000 seconds / 0.25 arc sec target resolution = 5184000 step per rev

5184000 / 2000 motor steps = 2592 additional gear reduction required

Mel's design uses two additional reduction steps.  First a gear reduction is used...

2592 / 60:1 (example worm gear reduction) = 43.2 required final drive ratio

Mel's design uses a simple roller on the end of the drive shaft and gravity to drive the azimuth or elevation bearing circles.  For example...

43.2 * 0.625 diameter roller = 27 inch elevation circle

All this gets you a single micro step resolution of 0.25 arc seconds.  Now for the good part...  To calculate the maximum slew rate do the following...

10 micro steps X 0.25 arc sec resolution = 2.5 arc sec per full step

(2.5 arc sec X 200 steps per rev X 10 RPS) / 3600 arc per deg = 1.39 deg/sec

or about 65 seconds to slew 90 degrees....

My first attempt at redesigning Mel's solution was to single step using a 500 step motor and gear the rest down in the secondary and final drive.  I only had to hold the motor shaft during one or two single steps and calculate my maximum 90 degree slew rate at about eight and a half minutes to know I had missed the mark...

When all of this material is carefully digested we learn a couple of key concepts:

> Too much secondary and/or final reduction will result in a very
> slow maximum slew rate even when full stepping...
>
> Full or half stepping is going to give you a rough and jittery movement
> unless we shock isolate the stepper motor.. Which will probably  result
> in greater angular errors.. Micro stepping is the solution...

Well, did I get it right ???

Bob Segrest