Calculating Required Torque

[Macavenger]

I have been working on designing a small robot, and would like to use a pair of stepper motors to drive and steer the robot, using diferential stering. My problem is that I have been unable to figure out how to calculate the required torque for the motors. I am an electrical engineering student, so while I can figure out how to wire everything together and program the controller, my knowledge of how to deal with torque and the like is somewhat lacking.

My robot base is approximately 6" square, so I need the motors to be as small as possible. The single heavest component of the robot, asside from the motors themselves, is a solenoid which weighs slightly less than 10 oz. The remaining components are all fairly light, so I would guess a total we
ight of about 15-20 Oz. Since the motors will be driving the robot, I do not need high speeds - 1.5 RPS should be more than sufficient. If anyone can help me with figuring out how much torque I will need from the stepper motors, I would greatly appreciate it. Thanks.

[JesseD]

Hello Macavenger,

It sounds to me like you are needing assistance determining the required torque of your system so you can select which motors you should use. A tutorial on Selecting the Proper Size Stepper Motor contains information that should help you out. This tutorial is actually an excerpt from a book that is available for purchase.

Regards,

Jesse D.
Applications Engineer
National Instruments

[Macavenger]

Thanks for the response. As stated in my original question, determining the required torque of my system so I can select what motors to use is exaclty what I need assistance with. I have read the mentioned article, and it did not help. Part of the procedure it went through for determining torque was naturally, determing the required force, which, in turn, requires knowledge of acceleration. And therin lies the rub. I couldn't care less what kind of acceleration I get, as long as it is reasonable. If it takes a second or two to get up to full speed, that is fine. I can't, however, just throw in any number I feel like for acceleration, because then I could get any number I like for torque, and I already know that I can easily not have enough. The stepper moto
rs I salvaged from some 5 1/4" floppy drives for the last version, for instance, did not have enough torque to adequately drive the robot. What I need is assitance with determining how much torque I actually need to adequatly drive the aformentioned load. Can anyone help me with this? even if just by giving me some random, yet reasonable, number to use for the acceleration? Thanks.

[JesseD]

Another very useful resource is "The Smart Motion Cheat Sheet" which is provided by AutomationSolutions. This document is very comprehensive and can be found on AutomationSolutions' website by clicking on the link to the "Free Motion Control CheatSheet." Again, this document contains many of the same equations but leaves it up to you to determine the critical move parameters (such as acceleration and velocity) specific to your application.

My only other suggestion would be to directly contact the motor manufacturers for additional assistance. Since they design and sell the motors, they should be able to give you a better idea of what you might need for your application.

Best Regards,

Jesse D.
Applicat
ions Engineer
National Instruments

[Macavenger]

Thanks again. That document was indeed interesting. It has quite a bit of usefull information in it. Although as you said it still leaves it up to me to determine acceleration, which, As I mentioned, is neither something that matters for my application, nor is it something where I can just make up a number, since I don't know what a reasonable value would be. Contacting the manufacturers seems like a good idea, but I have been trying that with every manufacturer and distributer I can find for the last month, and my e-mails have been stubornly ignored by everybody 😞 Oh well, I guess I'll just keep asking around until I can find someone who can at least give me a reasonable value to use for acceleration. Thanks again for trying. It really do
es make me feel better to at least get someone responding and trying to help. I only wish the manufacturers would try as well 😛