Stepper Motor Selection

[dberndt]

I have a project I'm in the planning stages of, that involves using a largish threaded rod, ballscrew, etc. The axis runs in the vertical, and it carries a grabber, that all told probably will weigh something like 5-10lbs. The distance between the limits on this axis will be somewhere in the 4 to 6 foot range and I'm looking for a solution to traverse the area quickly.

I've never worked with stepper motors before so I don't really know how I should judge my needs, what realistic stepping speeds (rotations per second), etc are. But my rough calculations are as follows:

rod has 10 turns per inch
1.8 degree stepper motor=200 steps per rotation
something like the salemcontrols single axis controller will do about 1000 steps/second (limit impossed by their programming interface)
assuming I can get a full 1000 steps/second thats 5 turns per second, or in other words, .5" per second.

In this scenario it seems like the stepping motor is going relatively fast in terms of rpm and steps per second, so some sort of a gear system to increase the ratio of rod rotation to motor rotation would seem like a good idea. But then there is the question of how much I can get away with, how do I calculate my requirements?

For example, my initial thinking goes something like this, friction will be small, so we will leave that out for now, and look simply at the weights involved and the gear ratios. So a 10tooth per inch thread acts like a 10:1 ramp or 1:10 depending on how you want to look at it. Throw in a gear box at the motor that is 1:10 putting the motor turns per inch at 1 to 5. So 48 inches in ~10 seconds which is much more to my liking. But the question of how much torque will the motor actually need to move that 5lb-10lb object straight up still remains.

It's 2:30 in the morning so thats about as far as I will carry this for now. I will encourage anyone to come in here and give me ballpark figures for what I want or if anyone has any equations to show the actual requirements that would be nice. Please point out any errors in my plan, I'm new to this and might have some wrong ideas.

Edit, forgot to mention, My goal is something like repeatable positioning along the axis to something like +/- 1/20th of an inch. I dont really need any more precision then that.

[arvidb]

Read the PDF file here: What motor, screw and gearing should I choose?

... for equations for torque and speed needed.

Arvid

[ger21]

You probably don't want to run those in full step mode. Use at least 1/4 step mode. The reason is that microstepping reduces resonances that may cause lost steps.

You can run that board with TurboCNC, and easily get 10,000 steps/second probably over 20,000. (depends on your computer). Using Mach2, and a 1Ghz computer, you should be able to get 35,000, maybe even 45,000.

A bigger problem with steppers, is that as the motor spins faster, torque can drop significantly. By gearing the motor to spin faster, you may not see any gains, and can actually lose torque. THe best way to prevent this is to supply higher voltages to the motor. Usually, doubling the voltage will double your high speed toque. Since the SalemControl board is limited to 35V (You probably shouldn't use over 30V), that may be a factor Using Gecko drives you can use up to 70V. And up to 7a as well. This will allow you to use bigger motors.

After reading your post again, you said 48" in 10 seconds. Most machines speeds are given in inches per minute, so your speed above would be about 288ipm. The salemControl board is relatively new, and I haven't read much info on it. But it is basically the same as a Xylotex (same chip, same ratings), and typical speeds with a Xylotex and 1/2-10acme are probably 30-60ipm. This may be with a little more torque than you need, so you may be able to go a little faster. I don't know about 300ipm, though. You'll be limited to about a 250oz-in stepper with those boards, as well.

Use the link Arvid gave to calculate speeds and torque required, and then go from there.

[dberndt]

Thanks for the replies guys. That stuff was really informative. I Plugged around a bit with the formulas somewhat and it seems to be based on the weight to be lifted against gravity, friction, etc, as per the formulas, something relatively low in terms of torque might cut it, as little as 50ozin maybe, 10lbs @ .005 rotation and a .4 efficiency on an acme ball screw results in somethng like ~.12Nm required, or roughly 16oz in. Now ofcourse this means I need enough torque to be able to accelerate to the 2000steps per second. This may seem low to others but please remember that this is more of a pick and place machine then anything like machining. I dont need to force a tool into a chunk of metal.

So, Microstepping might be nice, and I do plan on using it if I get a capable controller. Question, is microstepping as effective in preventing resonance and stalling problems on a 1.8 degree motor as something with bigger steps, such as a 7.5 degree motor. 1.8 degrees seems very standard but I could go for larger steps if there arent any side effects that I dont know about.

[ger21]

Originally Posted by dberndt So, Microstepping might be nice, and I do plan on using it if I get a capable controller. Question, is microstepping as effective in preventing resonance and stalling problems on a 1.8 degree motor as something with bigger steps, such as a 7.5 degree motor. 1.8 degrees seems very standard but I could go for larger steps if there arent any side effects that I dont know about.

I haven't seen any info on people using motors with bigger steps. 1.8° is very common, or sometimes 0.9°. But yes, microstepping works very well (is almost a necessity, some would say) with 1.8° motors.

[dberndt]

Might be getting a bit offtopic for this forum. But is it reasonable to consider replacing this setup with a servo system? Why is it that steppers are so often used? Whats the major advantage? cost?

[ger21]

Cost, ease of setup, usually less hardware required. Steppers are usually directly connected to screws, servos usually need to be geared down. I think in either this forum or the servo forum thaie is a thread with a good comparison. It was posted within the last month.

[ger21]

Servos or stepper