Stepper motor

Jonathan Allison

Hey guys / gals,

On the parallax site, Accessories Portal -> Motor Control there is a description of this stepper motor:

"The unipolar stepper motor is ideal for precision control, and may be easily operated in forward and reverse directions at varying speeds. This four-phase motor has a step angle of 3.6 degrees and requires 12 VDC for operation"

When I click the link to view more details about the product:

"This four-phase motor has a step angle of 7.5 degrees and requires 12 VDC for operation."

I wonder which it really is? I've been thinking of using this motor for my "Wire winder" as the guide motor.

Any math gurus around? What I need to figure out is how much travel this thing has per step.
Does this formula makes sense:
Travel = ((Diameter * PI) / 360) * Step Angle
so, if the diameter is, just guessing, 8 MM, and the step angle is 3.6 we get this
X = ((8 * 3.14) / 360) * 3.6
X = (25.13 / 360) * 3.6
X = (0.06) * 3.6
X = .2513
So the travel per step on a 3.6 degree step would be .2513 MM ?

If that is true and the motor is a 3.6 then this will do, if not I will need something more percise.

Thanks

Johnny

Bean

I looked at the mfg datasheet and it says 7.5 degrees. But you could use "half-steps" and get 3.75 degrees per half-step.
The pdf file shows how to do "half-steps".
The 3.6 degree motor was originally sold, but now they sell the 7.5 degree motor.

Bean.

Jon Williams

I think Bean is right -- we may have typos due to a change in motors. I'll investigate. In the meantime, download the data sheet from that page for the best specs.

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Jon Williams
Applications Engineer, Parallax
Dallas Office

LoopyByteloose

You can buy other stepper motors that are 1.8 degrees and half step them for smaller increments of 0.9 degree [noparse][[/noparse]but the holding torque is less consistent than full steps].

The prime concept in most steppers is that they divide a circle by 100 parts or multiples or fractions there of (so 100 parts = 3.6 degrees, 200 parts = 1.8 degrees, 50 parts = 7.2 degrees) I am not sure where these 7.5 degree steppers came from or if they are really 7.2 degrees.

What really begs the question is how do you convert the trigonometry to 100 unit circles?

I would suggest that you consider adding a couple of gears (10 to 18 ratio) to even out the math into degrees rather than try to do it all in software. And, obviously, the diameter of your final 'winder' can exactly define the travel.

If and when you do buy gears - buy a matched pair as it is nearly impossible to measure the ones on the steppers for an accurate fit.

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G. Herzog in Taiwan

Post Edited (Kramer) : 1/3/2005 7:58:51 AM GMT

kelvin james

As kramer says, finding a higher rez motor is not a problem, but you could go with a 7.5,half-step it,and use gear reduction, say a 1:4 ratio , that would give you 8 times the rez.
I think the gear reduction is a given anyway to improve accuracy and eliminate some of the play/backlash in the gearing. Also you will want a fine gear setup, like a 48 p.d. and a 20 degree pressure angle. And keep in mind most stepper motor shafts are metric, so check the specs before you order any gears. Most gear suppliers sell matched rack and pinion combos. As far as translating a rotational measurement to linear, i have not had to do this myself, but this is the way i see it. First determine the circumferance of the drive gear ( eg. .500 inch dia x Pi 3.14 ) gives you 1.57 inches of travel for every 360 degree rotation. Divide the 1.57 by the step angle rez , say .9 degrees ( 360/.09 )= 400 steps per rev--- 1.57/ 400= .004 inch linear travel per step. This is providing the gearing is at a 1:1 ratio. Anyone can correct me if i am wrong here. Anyway, gives you an idea of the kind of accuracy you can get.
I would also recommend a controller for the motor, there are numerous ones you can get, i use the lilstepu available here. Uses serial commands from the stamp, can be set to full, half or wave steps, keeps track of position, has ramping, speed and markable positions. And has the feature to power down the coils after movement to prevent the motor from heating up, which unfortunately stepper motors do being held under current, drawing a lot of power.
It also has a dual power input on board, one for logic and one for the motor
You can still run a stepper through basic programming, i just like the easy way.
Anyway, hopes this helps a bit.

kelvin

bobclark

www.jameco.com has the 3.6 degree step motor you are looking for.