[Behind the Lines] Ep. 3 with Ken Stafford on Motors and Gearboxes!

[Francis-134]

Join GameSense and FIRST for the third episode of Behind the Lines, LIVE tonight at 8 PM ET!

This week on Behind the Lines, we will be going over selecting motors and gearboxes for your robot. Motors are one of the, if not the most important component on an FRC robot, and using them to their fullest potential should be the goal of every team. You’ll learn why putting 6 CIMS and 4 mini-CIMS on your drivetrain might not be a great idea, how to select motors for a specific job, what gearboxes make sense in common FRC applications, and learn about some off the shelf solutions available to teams.

Evan and Ty from GameSense will be joined our expert, Prof. Ken Stafford from WPI and team 190. Ken has been an expert on DC motors since his days at graduate school, and has taught this material to thousands of high school and college students over his 15 years in academia and FIRST.

We want to hear YOUR questions about our topic (ever released some magic smoke and wondered why?). Email them to us at btl@frcgamesense.com, or tweet them with #FRCBTL before or during the show!

Whether you’re using the kit-bot, or are a master drivetrain designer, you don’t want to miss tonight’s episode. See you at frcgamesense.com, twitch.tv/FRCGameSense, or youtube.com/FRCGameSense at 8 PM ET tonight!

[Francis-134]

We're going live in 15 minutes. Check it out at youtube.com/FRCGameSense or twitch.tv/FRCGameSense.

[Mike Marandola]

Fantastic episode. I thoroughly enjoyed it.

[Francis-134]

Originally Posted by Mike Marandola

Fantastic episode. I thoroughly enjoyed it.

Glad that you enjoyed it!

For those who missed it, check out the recording at https://www.youtube.com/watch?v=WPX3z-qLSk0.

Thanks again to Ken Stafford for putting on a great show. To check out Ken's PowerPoint or his stats on all the motors from last year's kit, go to http://www.usfirst.org/roboticsprogr...hind-the-lines.

Our next episode is on 11/19 at 8 PM ET, where we will be discussing effective team communication with our guests Mike Corsetto from team 1678 and Andy Baker from team 3940. We're all really excited about this episode, so be sure to tune in!

[Ether]

Originally Posted by Francis-134

Thanks again to Ken Stafford for putting on a great show. To check out Ken's PowerPoint or his stats on all the motors from last year's kit, go to http://www.usfirst.org/roboticsprogr...hind-the-lines

Does Ken have an account here on CD?

[MooreteP]

Excellent tutorial on motors, transmission, chains, and belts, oh my.

A bit esoteric for students, great for mentors.

Thank you for your in depth explorations of FRC concepts.

[JesseK]

Originally Posted by Ether

Does Ken have an account here on CD?

Yep, but I don't know when the last post I saw from him was.

Here's other contact info:
http://www.wpi.edu/academics/facultydir/kzs.html

[Ether]

Originally Posted by JesseK

Yep

Thanks.

, but I don't know when the last post I saw from him was.

That would be about five and a half years ago.

Here's other contact info:
http://www.wpi.edu/academics/facultydir/kzs.html

I'll try that.

[Joe Johnson]

Some comments.

While I am glad to have more folks knowledgeable about motor selection and, in particular, how to pick the right motor and gear box combination for various tasks with respect to FIRST, there was a lot that I was not so happy about.

Motors:
Why all the discussion about van door motors? Are team using the van door motors that much often? They have not been included in the Kit for years. I know they are legal but I just don't see them being used very often since they are no longer shipped with the KOP crates. I love these motors but I just don't see them very often any more and in fact, with all the other motors in the kit, I am not sure they buy their way on board any more. They are harder to mount, harder to interface to, they are much more backdriveable than window motors. I love 'em, but I think there are almost always better options for FIRST teams with today's rules.

Fisher Price? I love these motors too but, they were not allowed last year and I don't suppose they'll be in this year either.

Also, Ken seemed to have a bias against CIMs in favor of open frame, fanned motors (500 and 700 series motors). In my view, he misses the point. First, full marks for his discussion about ratios and picking them well so that motors don't get that hot. BUT the biggest difference between the CIM and the 500 & 700 series motors when you are pushing them to their limit, the fan helps you very little because motor speeds are likely very low (i.e. near stall) AND heat only kill motors via TEMPERATURE. Size and thermal mass help CIMs survive when a 500 series motor would be on fire. Size is the much bigger factor. The rise in temperature for a given amount of heat needed to dissipate is proportional to area. Those CIMs have much more area and so run cooler.

Chain.
120 degree wrap. This is good advice for chain without tensioners but with tight chain you can get away with much less. That said, it is generally good advice if you can manage it.

As to his comment about larger than 70 some odd teeth, I have made sprockets with 100s of teeth that worked very well. I just have never had a problem with sprockets being too large. Have I been lucky or what?

Gearboxes:
I am a gear expert. I love gears. I design gears for a living. There is basically no reason for me to build my own gearboxes in for FIRST any more. There are many off the shelf options that are just too good to and too easy. If you want to build your own and you feel it is inspiring to your team, Feel Free. But honestly, I think you are better off spending your time designing & building the rest of your robot than you are rolling your own gearbox.

Worm gears:
There is almost NEVER a good reason to use a worm gear (other than inside an existing motor/gearbox like a window motor). If you think you need it, think again. There is almost always a better way. Really. Worm gears are hard to get right.

Powered Anti-backdrive:
While you can get into trouble doing this (and a lot of teams do), it is my considered opinion that he's just wrong. You need to design the gearbox and ratio for this condition but I have build many many robots that used very backdriveable motor/gearboxes to hold very large arms in a set position. You should think very hard about ways to get as much counter balance as you can in the systems. In addition if you cannot provide enough counter balance you are going to need to design the motor to be running WAY to the left on the speed-torque curve so that it will take only a small torque at the motor armature to resist backdriving, but saying that you must use a brake or a one way clutch is just too strong of a limitation.

FYI, the main arm pivot on this robot (CD5 - ah, I loved that robot) had almost no counter balance, a HUGE long arm and was easily backdriveable and used the motor to hold it in place. YMMV but it definitely can be done.

By the way, I am not talking about holding winches like last year in those cases, by all means, get the load off the motor.

Multi Motor Drives:
This is a much more complex situation that his answer suggested. There are acceleration concerns and there are current draw concerns (circuit breakers & battery amperes limits). For most teams 4 CIMs (2 on a side) are generally the right answer but there are other solutions that can make sense. My suggestion is that you know what you are doing before you consider other options. There is a high bar to get over before you go to another answer.

Belt Vs. Chain:
Good answer. I really like chain and can't see a good reason go to belt. For all the reason they said. Preserving the option to change designs late in the process is a huge advantage.

Final note:
I would have liked to have some discussion around what the motors curves do when we run at different voltages. This is important because some times the motor specs are given at 18V or 10V or some other voltage but it also help people think about what happens when the motors are put into PID loops or other situations where less than 12V are applied to the motors (i.e. almost every FIRST application).


So... ...I like the show in general. Still, I can wish for better.

Joe J.

[Jared]

Originally Posted by Joe Johnson

Some comments.
Worm gears:
There is almost NEVER a good reason to use a worm gear (other than inside an existing motor/gearbox like a window motor). If you think you need it, think again. There is almost always a better way. Really. Worm gears are hard to get right.

Out of curiosity, why do you say this? I agree that they aren't the easiest gearbox to get right, but I don't think they're impossible. You can get some really huge reductions. We had success with putting a worm gear reduction in a 2" x 2" aluminum square tube with .125" wall. We drove the thing with three CIM motors, so I don't think the typical FRC planetaries would have worked in that situation.

I agree with all your other comments, especially the note about the CIM motors. They're big and heavy, but when you consider how long some teams stall them for on their drives, it's very impressive that they last as long as they do.

[Joe Johnson]

Originally Posted by Jared

Out of curiosity, why do you say this? I agree that they aren't the easiest gearbox to get right, but I don't think they're impossible. You can get some really huge reductions. We had success with putting a worm gear reduction in a 2" x 2" aluminum square tube with .125" wall. We drove the thing with three CIM motors, so I don't think the typical FRC planetaries would have worked in that situation.

I agree with all your other comments, especially the note about the CIM motors. They're big and heavy, but when you consider how long some teams stall them for on their drives, it's very impressive that they last as long as they do.

Worm gears are hard to get right. If you know what you're doing, sure, you can make it work and you will get some benefit. I think that the percentage of teams that will net out to a positive is under 1%.

Perhaps my thoughts will evolve on this. I used to be dead set against bevel gears as well, which are nearly as hard, but which also provide a very nice benefit for certain kind of swerve implementations, so if you are careful and you really want that particular swerve, I think you can come out ahead. But, worm gears are harder and have other negatives so I just don't see benefits outweighing the costs and risks.

YMMV.

Joe J..

[Ether]

Originally Posted by Joe Johnson

Final note:
I would have liked to have some discussion around what the motors curves do when we run at different voltages. This is important because some times the motor specs are given at 18V or 10V or some other voltage but it also help people think about what happens when the motors are put into PID loops or other situations where less than 12V are applied to the motors (i.e. almost every FIRST application).

In the presentation, he discussed his motor performance spreadsheet. You can download it here. It shows how he downrates (and uprates) the motor curves for applied voltages which differ from the spec voltage.

[Joe Johnson]

Originally Posted by Ether

In the presentation, he discussed his motor performance spreadsheet. You can download it here. It shows how he downrates (and uprates) the motor curves for applied voltages which differ from the spec voltage.

Yeah, I caught that. I think the idea is important enough that I would have made it explicit. I think explaining how the Speed-Torque curves shift with voltage provides incite that leads to deeper understanding. But... ...this is more of my personal biases showing.


Of course, this was not my presentation. Everyone has different points of view. I am coming off much harsher than I intended. It was a good talk. I may disagree with this or that point but in general it's fine. I shouldn't throw the baby out with bath water.

This is especially true since I gave a talk on Friday in which I told about 30 FIRST mentors that one key to success is to have at least one person on the the team understand the Speed-Torque Curves for motors. If teams go with the information in this Behind The Lines talk, they'll be a lot better off than just using tribal knowledge and tradition to pick motors and ratios.

My criticisms should have been more temperate.

Joe J.

[Richard Wallace]

Originally Posted by Joe Johnson

My criticisms should have been more temperate.

Joe J.

Dr. Joe, I think your criticisms were, as they have always been here on CD, directly on point. I hope if you find the opportunity to offer criticism of my contributions you will be just as direct and on point. Please keep calling 'em as you see 'em. BTW, I recall your presentation of similar material (at the first Novi remote kickoff, IIRC) being a great inspiration to me.

My only minor addition to Prof. Stafford's excellent presentation would be a reminder that motor power is also limited by the motor's internal temperature rise, specifically the temperature of its armature windings and commutator. As copper's temperature rises, so also does its electrical resistivity. An armature winding that is 100 Celsius degrees above the temperature at which the motor's performance characterstics were measured will reduce that motor's peak output by about 30%. I posted some test results for CIM motors a while ago that confirm this.

Rep points to the first poster who can provide a link to that data, and to the first poster who can provide a calculation based on theory that explains the 30% reduction mentioned above. [Thank Ether for inspiring the rep reward.]

To support Prof. Stafford's comments on fan cooled motors, it has been my observation that such motors exhibit much less armature temperature rise than enclosed motors, provided that they are operated on the correct (high speed) side of their power curve peaks. However, as Dr. Joe pointed out, a stalled motor gets no benefit from its fan, so in applications such as drive trains where stalling is a foreseeable situation, a larger enclosed motor (e.g., CIM) should be preferred over a smaller fan cooled one.

[Jared]

Originally Posted by Richard Wallace

Rep points to the first poster who can provide a link to that data, and to the first poster who can provide a calculation based on theory that explains the 30% reduction mentioned above. [Thank Ether for inspiring the rep reward.]

It's funny you should bring this up today, as I was just looking at this last week.

Here's the data:
http://www.chiefdelphi.com/forums/sh...13#post1215913