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Unread 14-10-2008, 20:25
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Ken Patton Ken Patton is offline
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Re: Motor Testing

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Originally Posted by Tom Line View Post
It's a fun engineering problem.
I agree with Tom that this could be a cool engineering problem for teams to solve.

For example, those of you who have had physics and who are familiar with constant acceleration problems... could you come up with a way to test the motor with just some inertia attached to the motor as well as an encoder attached to the output shaft?

Those of you who have a spare IFI controller lying around.... could you come up with a routine to operate the test instrumentation?

This would make a good topic for a high school or college physics/engineering project. It would make a great topic for a presentation at FRC technical seminars like the one in Atlanta avery year.
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Unread 14-10-2008, 21:49
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Re: Motor Testing

Heck - using a flywheel to provide the inertia for each motor, you could easily rig up the encoder to labview and have it to the charting.

The, as he said above, you could back your way into the torque curves pretty easily...
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Unread 14-10-2008, 22:02
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Re: Motor Testing

A dynamometer is the device you want. Building an accurate one is a challenge (possibly beyond most FRC team's resources), buying one is expensive (again, but only money this time), so it can be solved...but is it necessary? (That is - is it a valuable use of resources?)
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Unread 14-10-2008, 23:26
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Re: Motor Testing

For anything FIRST related the published data is fine. Even if the the published figures were 20% off (which i doubt they are), you are still limited to using the motors in the KOP. In FIRST there is generally a clear best motor option for a particular mechanism. I doubt that you could even use better data. Its not like we have motors whose specs are within 1% of one another and we are trying to figure out which one to use. I cannot possibly think of any way that you would need more accurate data.

Another issue to consider is that a FIRST team has limited money and manpower resources. Your time would be better invested elsewhere.

If you are doing this to build more competitive robots i think your time/money would be better spent elsewhere. If you are doing this to learn about motors and instrumentation then please proceed. You will learn things in the process even if they are not what you originally set out to learn.
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Unread 14-10-2008, 23:39
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Re: Motor Testing

At the risk of sounding like a hack, I am sure that the good FIRST students can figure out some good enough way to produce a given torque and a good enough way to compensate for supply sag. This isn't rocket surgery, and the first order linear model of a brushed DC motor is good enough for anything we do here.

Claiming that this is too difficult for a FIRST student is simply begging to be proven wrong. I'm betting it can be done entirely with KoP items and some creativity.

A brushed DC motor is a resistor, an inductor, and a voltage source proportional to speed, all in series. Current through the motor is proportional to torque. The voltage and current constants are equal to each other.

If you can measure current, voltage, speed and torque in several conditions AND do a small bit of math, you can find these constants.
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Unread 15-10-2008, 00:41
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Greg Marra Greg Marra is offline
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Re: Motor Testing

Quote:
Originally Posted by EricVanWyk View Post
At the risk of sounding like a hack, I am sure that the good FIRST students can figure out some good enough way to produce a given torque and a good enough way to compensate for supply sag. This isn't rocket surgery, and the first order linear model of a brushed DC motor is good enough for anything we do here.

Claiming that this is too difficult for a FIRST student is simply begging to be proven wrong. I'm betting it can be done entirely with KoP items and some creativity.

A brushed DC motor is a resistor, an inductor, and a voltage source proportional to speed, all in series. Current through the motor is proportional to torque. The voltage and current constants are equal to each other.

If you can measure current, voltage, speed and torque in several conditions AND do a small bit of math, you can find these constants.
This sounds familiar...
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Unread 15-10-2008, 09:26
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Re: Motor Testing

Quote:
Originally Posted by EricVanWyk View Post
Claiming that this is too difficult for a FIRST student is simply begging to be proven wrong. I'm betting it can be done entirely with KoP items and some creativity.
I've tried motor testing before. I use the word try because the electronics were apt to blowing up while the motor was running. The underlying process is fairly simple but can't be done with only KoP items. You need a generator and a huge load bank that can handle large amounts of power. Another motor will not work (Actually tried that first) because of the fact that a generator and a motor are wound differently. The load bank is also a problem because you have to dissipate the power produced by the motor in a manner that is safe (I was working with 120V heater elements). As for measuring torque we used a torquemeter:
http://www.himmelstein.com/
Aligning such a device between the generator and the motor was a pain and it typically made ears bleed as we did not have the best ability to align the three things together. The noise was horrendous. Measuring motor speed was the easy part since it was a brushless motor. We just measured the magnet ticks off the back. I would say that most FIRST teams would not have the resources to do this. A generator motor costs a couple hundred of dollars. The torquemeter has to be fairly expensive. We were using Labview and a DAQ card to obtain the data. The load isn't necessairly a problem you just would need a hundred lightbulbs and their corresponding sockets which I guess is a hundred to two hundred dollars.
Quote:
This isn't rocket surgery, and the first order linear model of a brushed DC motor is good enough for anything we do here.
I don't think it's first order though. I think it's actually third order.
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Last edited by Adam Y. : 15-10-2008 at 09:34.
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Unread 15-10-2008, 10:21
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Re: Motor Testing

I would strongly encourage teams to try testing their motors. This isn't necessarily a practical thing to do and I wouldn't expect that you'll be able to pick-and-choose motors from a sample set. HOWEVER, this can be a highly effective educational tool.

I've attached a motor model with "everything you need to know". With a little bit of differential equations and math, a nice simulation can be created using only a handful of parameters. It's interesting to note that their are really only 4 parameters of interest when specifying a motor (other than size and maximum power/current/voltage) - winding resistance, winding inductance, "motor constant" and motor rotor inertia.

Enjoy!
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File Type: pdf motor model.pdf (14.1 KB, 100 views)
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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.
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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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