step angle question??

[kywoodwrkr]

What are the pros and the cons of step angle value?
For instance how does a step motor with 0.9 degree step angle compare with one which is 1.8 degrees step angle all else being equal.
Is the 0.9 more accurate. Is it just slower.
Or is it slower?

Thanks.
DaveP

[chuckknigh]

It means that a step (a single pulse from the computer) will move .9 degrees, or 1.8 degrees when instructed to.

All else being equal, including the number of steps per second generated by the computer, the .9 degree will rotate at half the speed of the 1.8 degree. The .9 will divide up the motion into smaller bits though, so the results with a sufficiently precise machine, would be more accurate.

Another way to think of these is 200 steps per revolution, and 400 steps per revolution. So, with a fine pitched screw, let's say 1/4" x 20tpi, the distance moved per step would be .00025" for the 1.8 degree motor, and .000125" for the .9 degree motor.

I'll let you decide if the difference is worth it.

-- Chuck Knight

[ger21]

Chuck, that's .00025/ step for 1.8° with 1/4-20 screws ( 1/(20*200) ), .000125 for .9°.

And what you said about all being equal is confusing. As long as the computer can output steps fast enough for the .9° motor, if you do a G1 X5 F30, it's going to move the same speed with either one.

[jeffs555]

On a stepper, the torque drops off as the stepping rate increases. For the same rpm the 0.9 deg needs twice the stepping rate of the 1.8 deg, so on equivalent motors the torque will drop off at a lower rpm on the 0.9 deg stepper.

[chuckknigh]

Originally Posted by ger21 Chuck, that's .00025/ step for 1.8° with 1/4-20 screws ( 1/(20*200) ), .000125 for .9°.

You are, of course, correct. I don't know what I was thinking when I posted that...but looking at the time I posted it, the mistake is not surprising.

And what you said about all being equal is confusing. As long as the computer can output steps fast enough for the .9° motor, if you do a G1 X5 F30, it's going to move the same speed with either one.

How is that confusing? Given the same step rate, the same pitch screw, the same electronics, etc...the speed of rotation of the 1.8 degree stepper will be twice as fast as the speed of rotation of the .9 degree stepper.

If you double the step rate, then you're not comparing the motors.

It would be like saying that a Yugo can go faster than a Ferrari...it can, if the Ferrari is driven in a school zone during rush hour, and the Yugo is plummeting off a cliff. It's a meaningless comparison. For a comparison like this to be meaningful only the single variable should be, well, variable...all else should be equal.

-- Chuck Knight

[kywoodwrkr]

Thank you all for taking time to answer.
I found the VEXTA website which displays the Torque Vs Pulse Speed and see where the .9 motor torque drops more(at about 60 RPM) than the 1.8 motor which dips similarily at about 120 rpm. Both of these drops in torque are for a very short duration on the RPM scale. In other words it is a negative spike. The .9 drops to about 1/3 it's max rated torque and the 1.8 drops to about 1/2 it's max rated torque.
I will now assume that the actual speed(number of discreet signals) will be dependent upon either the controller chips(their cycle time) or the cycle time of the driving processor. Or does the controller even have a 'cycle' time?
Is this a correct assumption?
Do any of the controller manufacturers disclose their signal rates?
If so how is it stated?
Thanks.
DaveP

[ger21]

By controller I think you're referring to motor drivers. Controller generally refers to the control software that actually sends the step signals. The software will be the limiting factor for step rates. TurboCNC can output 25,000 steps per second, maybe more. This can vary a lot from cumputer to computer. Mach2 can output 45,000 steps per second, with ~ 1Ghz or faster PC. DeskCNC can output 125,000 steps per second by using an external pulse generator.

I looked at the charts from Vexta as well, and like Jeff mentioned above, above a few hundred rpm, the .9° motors will only have about 1/2 the torque of the 1.8° motors. I'd say that unless the .9° are a lot cheaper, I'd go with the 1.8's.

[kywoodwrkr]

Gerry,
Thanks for the information.
In my logic a controller is always an outboard computer, whether it be a switch or a full blown computer so to speak.
Software are drivers or application programs.
Guess difference is perspective-or knowledge of the industry! :-|
The important thing is you are explaing things in a meaningful manner to everyone no matter what perspective they are looking at these items from.
I have a handfull of stepper motors out of some copiers.
They are rated at 118 In-Oz so even at 50%, will be about 60 in-oz.
I'm just working on a little device so I don't think the lack of power will be terribly detrimental.
Actually speed is not my thing on this project.
If I get to the implememntation stage and they are undersized, then getting bigger ones will not be a problem.
Again, thanks for your understandable answers.
DaveP