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  #16   Spotlight this post!  
Unread 15-10-2008, 10:45
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Re: Motor Testing

I'm not holding out that it can't be done by FIRST students. I'm simply declaring that it's somewhat impractical and uneconomical... At least doing it in any of the straight-forward obvious ways the were first discussed here. I think Ken's suggestion of using known rotary inertias plus a large amount of math and testing is the most practical solution. Given the datalogging capabilities of the cRIO, you could probably pull it off with the right instrumentation and careful calibration. It wouldn't be quite as simple as he's hoping, I think. The motor would be accelerating the known inertia of your load, plus the unknown inertia of the motor's rotor. In fact, there's a fair few parameters you'd have to account for to model the motor better than a first order approximation. And if you only care to a first order approximation, why aren't you using the spec sheets? Anyways, here's the parameters I can recall/think of:

Motor torque constant
Motor emf constant (should ideally be the inverse of the above, not necessarily independent)
Motor armature resistance (can be tested at low voltages with locked rotor)
Motor armature inductance (ditto)
Motor rotor inertia
Motor static friction
Motor dynamic friction

I know there's possibly external magnetic field losses in there somewhere, but I think those might be negligible in the motors we work with. Anyways, ignoring the resistance and inductance as separate problems, that gives you 4 unknowns, so you'd want 4 independent data points at a minimum. I'm pretty sure it could be done with at least two different load inertias, and two different voltages applied as step functions to the system. That plus some educated initial guesses based on no-load speed, etc., and you could probably get some decent values. Once you've worked out all the non-linear regression stuff you'd be needing. But it could probably be done. It'd certainly make a heck of a capstone project for any FIRSTer that needed one.
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Unread 15-10-2008, 10:54
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Re: Motor Testing

Quote:
Originally Posted by Kevin Sevcik View Post
And if you only care to a first order approximation, why aren't you using the spec sheets?
Sorry. I became confused. First order approximation does not mean a first order system.There becomes a certain point where engineers just do not care and have to chop the problem down to size. The first order approximation is probably the best model to work because it still is a third order system. Any higher modeling would mean that you would get a forth,fifth,sixth order system which is painfully hard to work with. I forgot what my engineering professor said tends to be the cutoff point in which you actually need to make the model simpler.
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Last edited by Adam Y. : 15-10-2008 at 11:05.
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Unread 15-10-2008, 11:25
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Re: Motor Testing

Quote:
Originally Posted by Kevin Sevcik View Post
I'm not holding out that it can't be done by FIRST students. I'm simply declaring that it's somewhat impractical and uneconomical... At least doing it in any of the straight-forward obvious ways the were first discussed here. I think Ken's suggestion of using known rotary inertias plus a large amount of math and testing is the most practical solution. Given the datalogging capabilities of the cRIO, you could probably pull it off with the right instrumentation and careful calibration. It wouldn't be quite as simple as he's hoping, I think. The motor would be accelerating the known inertia of your load, plus the unknown inertia of the motor's rotor. In fact, there's a fair few parameters you'd have to account for to model the motor better than a first order approximation. And if you only care to a first order approximation, why aren't you using the spec sheets? Anyways, here's the parameters I can recall/think of:

Motor torque constant
Motor emf constant (should ideally be the inverse of the above, not necessarily independent)
Motor armature resistance (can be tested at low voltages with locked rotor)
Motor armature inductance (ditto)
Motor rotor inertia
Motor static friction
Motor dynamic friction

I know there's possibly external magnetic field losses in there somewhere, but I think those might be negligible in the motors we work with. Anyways, ignoring the resistance and inductance as separate problems, that gives you 4 unknowns, so you'd want 4 independent data points at a minimum. I'm pretty sure it could be done with at least two different load inertias, and two different voltages applied as step functions to the system. That plus some educated initial guesses based on no-load speed, etc., and you could probably get some decent values. Once you've worked out all the non-linear regression stuff you'd be needing. But it could probably be done. It'd certainly make a heck of a capstone project for any FIRSTer that needed one.
By using a large enough flywheel error due to the rotor inertia can be neglected if you're willing to accept a small % of error.

Assuming you're willing to accept 5-10% error, you can neglect many of the losses (like friction) that you mention above anyway. Afterall, this is more about the process and the approximation than getting the exact industry answer.
  #19   Spotlight this post!  
Unread 15-10-2008, 11:56
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Re: Motor Testing

Another interesting option with respect to software tools might be -

http://sine.ni.com/nips/cds/view/p/lang/en/nid/13853

You might be able to get an eval copy. I'm not sure whether this is fully supported by cRIO.

For educational purposes, it might make sense to brute force this without using such high-end tools.

Russ
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Unread 15-10-2008, 13:00
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Re: Motor Testing

Quote:
Originally Posted by Lancer Robotics View Post
Some of our mentors have requested that we do our own testing of motors next season. they did not like relying on the published data (and I think they had not checked out CD) So do any of you have an easy device that you have used to get useful data on motors that we could copy and give them the data they want?
One important question is, “Why do your mentors not trust the given data?”

If they would like you to do this as a “Good stuff to learn project”, I would agree it is good stuff to learn.
If they want you to do this because they feel they are getting out of spec motors or because things they have calculated should work aren’t working, there could be a larger problem here. Often teams will design systems around stall torques neglecting to remember things like friction, efficiencies…. Then those motors you are using to power a winch that should lift the arm in 2 seconds doesn’t seem to have enough torque to get it moving. I know hitting the overpass at 10 fps added a lot of stiction (it really should be a word) to our elevator.

If it is as a “Good stuff to learn project”, Ken P is right, inertia dynos are relatively simple and accurate. National Instruments used to have a $25 data Acq. System (I think it may have even been free to High Schoolers) that can be modified into a decent data system. Otherwise a tac, and a stopwatch can give you good data.
  #21   Spotlight this post!  
Unread 15-10-2008, 13:47
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Re: Motor Testing

Quote:
Originally Posted by IKE View Post
I know hitting the overpass at 10 fps added a lot of stiction (it really should be a word) to our elevator.
Rejoice! Stiction might not be in Webster's, but it's a fairly common term in MechE papers.

Anyways, the Labview System ID Toolkit would definitely do most of the heavy lifting for you, but you should be careful in applying it. Waving about mathematical tools you don't have a good understanding of can leave you looking much like the Sorcerer's Apprentice.

Also, as to ignoring friction effects and swamping the rotor inertia with a larger load inertia..... Again, if you don't care about a 5-10% error, why don't you trust the datasheets? If this is a learning project as IKE proposes, then simply hooking up motors to loads and monitoring them would be fine for exploring the dynamics of how motors work, without worrying about the headaches or expense of trying to exactly model the motors.
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  #22   Spotlight this post!  
Unread 15-10-2008, 14:51
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Re: Motor Testing

What about the case where we don't have the datasheet?

Last time I checked we didn't have datasheets in the KoP. I believe that the datasheets for 2008 were not available until a week into the season. If you have a way to determine to determine the specs for 'the new FP motor' (even +/- 10%) faster than that then it puts you at an engineering advantage.

Oh, this motor's got twice the wattage of last year’s -> drivetrain.

Yeah, there are risks with doing testing on the motors that you need to use, but figuring it out before you need it allows you to truly understand the risks and make appropriate choices. I applaud teams that take on activities like this in the fall.
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  #23   Spotlight this post!  
Unread 15-10-2008, 23:53
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Re: Motor Testing

Of course the spec sheets will work fine (thats what we use... ). But it is a cool problem. And it would make a great school project for someone who wants to solve it with the materials on hand.

I hope the engineers talking about how hard it is don't scare away some student(s) from taking this on. I would lobby for the best solution (one that works and can be easily replicated by others) to be asked to give a presentation at the Championship FRC Conference next April. And then I'll make sure our hybrid drive people know who you are so they can get you in for a job interview. So now, not only do you have a cool problem to solve, but you have a little incentive... .

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Unread 18-10-2008, 10:03
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Re: Motor Testing

Turns out it wasn't a National Instruments product:

http://www.dataq.com/products/startkit/di194rs.htm

This is a $25 data acquisition system that could be good for a project of this scope, or for other projects. They even have a link with Science Fair Ideas.

Could even be free:
"Science Fair Project Submission Form
DATAQ Instruments, Inc. is proud to offer free DI-194RS Data Acquisition Starter Kits to qualified students grades four through twelve for use in their innovative and exciting science fair projects. DATAQ Instruments will give away one free DI-194RS per month (on average) to students for use in their science fair project. All we ask is that you send us a summary and photo of you and your project at the science fair that we can place on this website. Fill out the application form below for consideration (all fields required and will be validated)."


Here is a link for a frequency to voltage unit. With a little EE work, you have a Tach to connect to the Data Logger above:
http://www.national.com/mpf/LM/LM291...;20Description

NOTE: Don't use a hotmail account to sign up for a "free" sample as they charge several dollars for shipping and handling.

Last edited by IKE : 18-10-2008 at 11:47. Reason: more info
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Unread 22-10-2008, 18:16
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Re: Motor Testing

Thank you all for your input. Our mentors wanted the info so they had some information upon which they could match up the motor to the task. This past season we used motors based upon the best information available and recommendations. I think some of our gear ratios might have let us down but the motors seemed to be fine where we used them. I will pass on your recommendations.

thanks for he help
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