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Unread 14-10-2008, 15:39
Lancer Robotics Lancer Robotics is offline
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Motor Testing

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?
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Unread 14-10-2008, 15:43
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Re: Motor Testing

Well, if you attach an encoder to a motor, and use an adjustable 12V dc power supply, it should be very easy to measure RPM vs. Voltage.

After that, get a torque wrench (correctly sized) and you should be able to measure stall torque.

http://lancet.mit.edu/motors/motors3.html
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Unread 14-10-2008, 15:43
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Re: Motor Testing

I don't know what you would use but I would like to say that for any purposes in FIRST, the published data is usually close enough. I'll let more experienced members answer your question.

It would be an effort to do a self test and it would probably cost some money too.

-Vivek

edit: what tom said...
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Unread 14-10-2008, 16:48
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Re: Motor Testing

*chokes on drink* umm... I recommend just relying on the published data, unless your team has quite a surplus of money, batteries, and motors. While Tom's suggestion for getting no-load speed is easily doable, trying to measure stall torque is going to get rather expensive and hair-raising on some of the motors. Specifically, the small CIMs. If you're trying to measure characteristics at 12V, the CIMs have a stall current of 133A. Which is a lot. You'll be needing a power supply that won't sag much under this kind of load (possibly a bank of 5 FRC batteries), plus a clamp on ammeter that you trust more than the data sheets. Plus a torque wrench that you trust more than the data sheets that can actually measure torques in the range we're interested in. (21-22 in-lb for a CIM at 12V)

Now granted, you can decrease the voltage on the CIM to decrease the stall current and stall torque (mostly linearly), but then you'll have an even tougher time measuring the torque accurately enough.

And that's just the CIM. Our various other motors will all present their own problems, of course. Like the fisher-price motors being likely to melt if you stall them, and having ridiculously low stall torques anyways. Plus no-load speeds likely to give your encoder or tach fits. Or the window, van door, or globe motors, where the output is going to depend on the efficiency of particular motor's gear box.

So. It's possible to build a setup to measure all these different parameters, but it's going to be more difficult than a simple tach and torque wrench. And you're quite likely going to lose some of the motors you're testing. So, really, I think you and/or your mentors need to examine just how accurate they actually need these numbers and how many motors you're willing to sacrifice and/or beat up to get reliable numbers.

Oh, didn't I mention that? Chances are you're going to want to test multiple different samples of each kind of motor, to make sure the numbers you're running with aren't from a fluke motor that's extra bad or good. So atleast 3 samples, but 5 or more would get you better data. Oh. And the multiple runs on each motor in both directions to check for hysteresis. Plus, you'll be wanting to calibrate all your testing equipment to make sure it's giving you those super accurate readings you're looking for. Plus.... Well. 4+ paragraphs probably carries the point of how much work you'll likely need to do to get more reliable numbers that we've got as of right now.
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Unread 14-10-2008, 17:57
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Re: Motor Testing

Awww, come on Kevin. You're ruining the fun.

It's a fun engineering problem.

By getting datapoints in the ranges you can easily measure, you should be able to use best-fit lines to get a reasonable idea of what's going to happen.

Even free speed should be reasonably easy to figure out given you know at least 3 or 4 data points at lower voltages so you can fit a line to it.
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Unread 14-10-2008, 19:35
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Re: Motor Testing

We did something similar 10 years ago, with a car battery and a torque wrench. I think the most valuable thing we learned was which motors catastrophically fail when stalled. Note that back then, the motors were not nearly as powerful as they are now.
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Unread 14-10-2008, 20:13
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Re: Motor Testing

Free speed is the easiest condition to measure. Stall is the tough condition to measure, unless you don't like your motors. I note that McMaster has some high accuracy wrenches with top ends of 2,5,10, and 25 in-lbs, so you can probably decrease the voltage enough to avoid blowing the CIM and your power supply and get a few data points. The FP would be exceedingly interesting, as it tops out at 63 oz-in and 70A on 12V. 35A at 6V might be safe for it, as that's only 200W. So that's one data point around 32oz-in. And then you work down from there. The window lifts declare a stall of something like 100+ in-lb, but they've got such a huge error bar on the chart that I don't know why you'd bother even trying... You'd just be testing what makes worm gears more and less efficient.

Anyways, I suppose if you have a large amount of accurate torque wrenches laying about and a good 0-12V 0-50A adjustable power supply hanging about, then it'd be a doable project.... But if you have to buy all that stuff, it's going to be a multi-grand project. I'd just as soon go with the specs as written and de-rate them by whatever you like.
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Unread 14-10-2008, 20:25
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Re: Motor Testing

Quote:
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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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
Russ Beavis Russ Beavis is offline
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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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