pic: Coaxial Swerve Drive Module with 2-speed Ball Drive and Nitrile Tread

[pwnageNick]

Originally Posted by Lyler1

Hi!

My team has been designing drivetrains this past off-season and I was very intrigued by your guys' design. I was wondering if you could tell me the name of the student who designed it and perhaps a way to speak with him/her, because my team is very curious and may want to ask some questions. Thanks a ton!

Originally Posted by Abhishek R

It's possible that the responses are high quality, and that anything wrong could be confirmed/dispelled by the OP.

Although, you could probably also reach out to 'Kevin Ainsworth' via PM.

I'm not sure how often Kevin checks CD so I'll let him know there are some people interested in some more information. I do know that the team has been working on this design for a couple months now, since IRI ended.

I would say feel free to ask any questions you have here and I'm sure there will be some informed CD community that will probably know the answer. I tend to check daily and can either hop on and correct anything said or provide information or get the answer you're looking for.

-Nick

[asid61]

Originally Posted by asid61

Crazy! This is definitely the most advanced/ best swerve I have ever seen. Dual speed at a weight of 7bs per module rivals that of WCD; it's like having torquey holonomic motion at the cost of 4-5 lbs.

I have a couple of application questions:
1. Why did you not flip the cim? It look like you have the room to do so. Flipping it and adding a belt drive to the first stage of the gearbox would add half a pound or so, but would alloy you to add another cim or a minicim to the drivetrain if you wished.
2. Why the small banebots motors over something powerful like the RS-775 18v?
3. Why did you choost to have a seperate pair of gears for the absolute encoder instead of simply having an encoder on the versaplanetary output?
4. Tons and tons of machined parts (the most notable to me being the miter gears). What do you expect the turnaround time to be for these?
5. How are you planning on fixing anything if a module breaks? It seems very compact and complex and hard to repair.

Again, very nice swerve drive. It's a bit beyond my team's capabilities, but I hope that you can use this this year and improve this further next year.

Sorry, but I wasn't sure of you saw my questions, and I do want to see how you got this right.

Also, are those standoffs going to be okay? They seem quite thin.

[nathannfm]

Originally Posted by asid61

I have a couple of application questions:

While I had nothing to do with this (awesome) design I think I can take a few of these.

Originally Posted by asid61

1. Why did you not flip the cim? It look like you have the room to do so. Flipping it and adding a belt drive to the first stage of the gearbox would add half a pound or so, but would alloy you to add another cim or a minicim to the drivetrain if you wished.

To me it actually doesn't look like there is room to flip it without increasing the bellypan footprint, which may have been a greater design concern than COG or expandability for them.

Originally Posted by asid61

2. Why the small banebots motors over something powerful like the RS-775 18v?

There is simply no need for such a powerful motor in this application, 1640 has been using the even less powerfull RS-540s in this application for years with no problem. Personally I would use the AM-0912 (which has power between a 540 and 550) because you can use 4 of them and that frees you up to use any of the BB motors on the rest of your robot for things that may require the power of an RS-775

Originally Posted by asid61

3. Why did you choost to have a seperate pair of gears for the absolute encoder instead of simply having an encoder on the versaplanetary output?

Not sure on this one, it certainly looks like there is enough room for an idler encoder gear down there https://imgur.com/IwbQ4JL maybe mounting issues, gonna have to wait for the OP on this one.

Originally Posted by asid61

4. Tons and tons of machined parts (the most notable to me being the miter gears). What do you expect the turnaround time to be for these?

I would have actually said most notably literally everything else lol, the miter gears can be purchased from VexPro but there are certainly some other complicated parts on here.

Originally Posted by asid61

5. How are you planning on fixing anything if a module breaks? It seems very compact and complex and hard to repair. I hope that you can use this this year and improve this further next year.

I know most teams make at least 1 spare module (if not 4) that way they simply swap the entire module out if anything happens to a competition module. The problem module can then be repaired at home under controlled conditions


Now I have a few questions:

Why the hollow drive axle?

According to the CAD the shifter gears are steel but reference aluminium VexPro gears, what's up with that?

Amazing looking drive, great work, I hope I get to check it out in person at champs!

[asid61]

Originally Posted by nathannfm

While I had nothing to do with this (awesome) design I think I can take a few of these.


To me it actually doesn't look like there is room to flip it without increasing the bellypan footprint, which may have been a greater design concern than COG or expandability for them.


There is simply no need for such a powerful motor in this application, 1640 has been using the even less powerfull RS-540s in this application for years with no problem. Personally I would use the AM-0912 (which has power between a 540 and 550) because you can use 4 of them and that frees you up to use any of the BB motors on the rest of your robot for things that may require the power of an RS-775


Not sure on this one, it certainly looks like there is enough room for an idler encoder gear down there https://imgur.com/IwbQ4JL maybe mounting issues, gonna have to wait for the OP on this one.



I would have actually said most notably literally everything else lol, the miter gears can be purchased from VexPro but there are certainly some other complicated parts on here.


I know most teams make at least 1 spare module (if not 4) that way they simply swap the entire module out if anything happens to a competition module. The problem module can then be repaired at home under controlled conditions


Now I have a few questions:

Why the hollow drive axle?

According to the CAD the shifter gears are steel but reference aluminium VexPro gears, what's up with that?

Amazing looking drive, great work, I hope I get to check it out in person at champs!

Ah, that makes sense. Although I believe the current motor rules allow 4 banebots style motors, regardless of whether they are RS-550 or RS-775. I will double check that.

Also, I mentioned the bevel gears because although they are direct copies of the Vexpro bevel gears, they are machined out of 7075 in the cad.
Also, another question: Why are the standoffs so thin? Will they be able to support the weight?

[AdamHeard]

Originally Posted by asid61

Ah, that makes sense. Although I believe the current motor rules allow 4 banebots style motors, regardless of whether they are RS-550 or RS-775. I will double check that.

Also, I mentioned the bevel gears because although they are direct copies of the Vexpro bevel gears, they are machined out of 7075 in the cad.
Also, another question: Why are the standoffs so thin? Will they be able to support the weight?

The standoffs aren't taking the robot weight just the gearing. They're plenty strong for that.

[Kevin Ainsworth]

Thanks for all the great feedback, some of these questions were already answered so sorry if I repeat them. Also, sorry for the long posts but we would like to make sure the students questions are given an educational answer. I would like to thank Aren Hill for his inspiration for our "In Wheel" swerve as well as this unit.

Originally Posted by Ether

Where did you get the "actual" numbers from?

We have been recording our theoretical to actual numbers for a few years now and 82% is our average actual speed to theoretical. So the working of "actual" should be changed to "expected".

Originally Posted by asid61

Crazy! This is definitely the most advanced/ best swerve I have ever seen. Dual speed at a weight of 7bs per module rivals that of WCD; it's like having torquey holonomic motion at the cost of 4-5 lbs.

I have a couple of application questions:
1. Why did you not flip the cim? It look like you have the room to do so. Flipping it and adding a belt drive to the first stage of the gearbox would add half a pound or so, but would alloy you to add another cim or a minicim to the drivetrain if you wished.
2. Why the small banebots motors over something powerful like the RS-775 18v?
3. Why did you choost to have a seperate pair of gears for the absolute encoder instead of simply having an encoder on the versaplanetary output?
4. Tons and tons of machined parts (the most notable to me being the miter gears). What do you expect the turnaround time to be for these?
5. How are you planning on fixing anything if a module breaks? It seems very compact and complex and hard to repair.

Again, very nice swerve drive. It's a bit beyond my team's capabilities, but I hope that you can use this this year and improve this further next year.

1) We considered flipping the CIM due to reasons already explained and a guess that since the field has been flat for the last few years we are expecting a field obstruction this year. We can always reconfigure later. A second motor on each wheel was considered also, see our previous discussion on using 8 MiniCIM's. The 3" OD wheels would probably be overpowered in a pushing match but the extra motors would be helpful for acceleration. You might just see dual motors from us in the future if it fits the game and if something like current monitoring is effective enough against blown fuses.

2) Already answered, but no need for a RS775. Aren Hill used a RS395 on the first "In Wheel" swerve. We have been wondering what happens when our 2014 swerve bot gets pushed up on two wheels and the wheels are not pointed in the direction of the push. Do the steering motors have enough power to change direction? It seems like the edges of the wheels could be digging into the carpet and requiring extra force to rotate. On our off-season list of tests to perform.

3) The ratio between the Vex Versa gear isn't 1:1 to the modules steering gear so a 360 degree absolute encoder can't be used. The two plastic gears are 1:1 so a 360 degree absolute encoder can be used. An incremental encoder could be used but adds another place for human error. We like the robot to not depend on a human to set the wheels at the beginning of a match.

4) Miter gears are purchased from Vex and are 4140.

5) If designed properly the units should last a full season without repair.
If repair is needed they will be bolted onto the chassis and can be changed in ten minutes. We bring at least one extra unit to competition in case a swap is needed. Last year our mechanic Jose and I changed out a steering box on our robot between rounds in finals on Curie. We had previously swapped in a Banebots gearbox from our practice bot at Chicago. During our debugging of our first swerve drive ever we burned up about a dozen steering motors. Mainly due to the rotation stops we used so as not to pull CIM motor wires out. We reused the pinion gears and this particular pinion had been installed/removed one too many times. The motor shaft spun inside the pinion and we lost one wheel of steering. Took about ten minutes for Jose to install a new one while I ran around grabbing the replacement parts. We had students dedicated to our swerves from the very beginning of last year to clean, inspect, repair, etc.

Originally Posted by page2067

This looks beautiful. PWNAGE has been advancing swerve to new levels of elegance.

I do not see the steering motor in the views I have seen (or am blind) - I see the steering encoder, pneumatic cylinder and CIM - what are you using for steering motors/transmissions?

Also are you planning to be field-centric steering next year (of course with game dependency disclaimers)?

Steering box question was already answered correctly, hidden in view shown.
We had field centric software installed by IRI last year with the successful integration of the Kauai Labs gyro. There was a great thread on this a little while ago. "Best gyro for frc." Big props to our students Bennett and Duffy. Bennett for coding the entire swerve software, about 10 iterations, and to Duffy for spending the better part of the season fine tuning our teams first gyro ever used.

Originally Posted by Jared Russell

If 2451 truly gets about 82% of the theoretical free speed out of these transmissions, that's in the efficiency ballpark of most FRC skid-steer drive trains. Some teams can hit the low 90s, but that is the exception rather than the rule.

Also, the intuition of "what's too fast for high gear" is somewhat different for an independently steered/driven swerve than for a skid-steer drive train. Your wheels never need to fight each other, so you are asking less of your drive motors than in a 6/8WD where you need to force wheels to slip sideways in order to turn.

Great point, I never thought of it that way. I think that being able to overdrive the outside wheel is important because in a high speed strafing turn the fastest wheel is the limiting factor. If just one wheel is traveling the outside radius it will limit overall speed. The other three wheels can power you through the turn.

Originally Posted by Bryce2471

I really love this design overall. I've wanted to design a swerve with a single reduction ball-shifter for quite some time. I do have some questions and concerns though.

1. It looks like there is no thrust bearing above the vertical miter gear. This is not a big deal by its self, (We used thrust washers last year) but it will increase the speed of deterioration on the gears, and decrease efficiency in that gear-set. It also looks like you're planning on using the VEX Pro aluminum Miter gears. This is also not a big deal, lots of team use them, but I suspect the fact that they are aluminum will increase wear speed. With those two things combined, your drive performance may not suffer, but you will probably find yourselves replacing the miter gears pretty often. What is the process for replacing a miter gear? How long will it take?
2. How many man-hours will you spend to machine all the parts? How about to assemble the modules? I feel like you could have designed a module with the same performance, that would have been much less resource demanding to build.
3. What kind of bearing are you using to move the robot's weight from the red base plate to the top of the castor? In the cross section, it looks very thin.
4. Not sure if it is a problem or not, but I couldn't help but notice that your pneumatic cylinder is mounted to a plate, that has standoffs to a plate, that has standoffs to a plate, that is stood off from a plate, that has standoffs to a plate, that is mounted to your frame. Not sure why, but that makes me cringe a little.
5. What gears are you using on the ball-shifter shaft, and as the CIM pinions?

1) Vex 4140 pre-hard steel miter gears
Not sure I fully agree with the needle bearing vs radial bearing arguement
The radial bearing can take over 100lbs of thrust force.
I get nervous when the balls are too small in diameter
2) Easier said than done, especially with the two speed design
3) Silverthin 4 point contact bearing with 1100lbs of dynamic thrust load
Only 1/4" cross section but should hold up, we are supporting completely.
We were going to use a cross roller bearing but Aren Hill informed us of these lower cost bearings
4) Lower standoffs support plates, upper standoffs support shifter. Non support the weight of the robot
5) 12:60 (low) or 28:44 (high) CIM to ball shifter, with 12:30 for 2nd stage, to 1:1 miter to wheel

Originally Posted by magnets

This is a fantastic design. There have been many designs of two speed swerves posted over the years, but nothing has come close to what you've put together.
I know from looking at your swerve design from last year that you guys shouldn't have trouble machining the parts in this design, but I've got to ask, how do you guys even have time to make these parts and finish the robot on time? What sort of tools do you have, and how many people do you have working?

There was a tear in my eye last year when on a Saturday we had two lathes, two CNC's and six Bridgeport Prototrak's (2D CNC) going at the same time. All students except for the CNC machines. Our program is unique in that we teach machining to all students that want to learn and we have enough machines for kids to use at the same time.

Originally Posted by Tyler2517

I can confirm what Bryce2471 is saying we saw reduced life and over time reduced efficiency due to us not having the thrust bearings between the mitter gear and the radial bearing.

Radial bearing will not hold up as long as a thrust bearing, true.
The OD of a needle thrust bearing being too large to fit is the reason we went with a radial bearing. Sometimes you have to make sacrifices.

Originally Posted by nathannfm

Now I have a few questions:

Why the hollow drive axle?

According to the CAD the shifter gears are steel but reference aluminium VexPro gears, what's up with that?

Amazing looking drive, great work, I hope I get to check it out in person at champs!

The axle is hollow for weight. It needs to be steel since the axle runs through needle bearings (with thrust bearings). An aluminum axle on needle bearings would wear too much.

The two ball shifter gears are aluminum in our model. The two CIM gears are steel to reduce wear. We like to run a steel against an aluminum gear, we keep the smaller gear steel for weight. This greatly reduces the wear and therefore efficiency loss of the gears over the season. We've found the aluminum on aluminum ceramic coated gears will destroy each other over time.

[Ether]

Originally Posted by Kevin Ainsworth

We have been recording our theoretical to actual numbers for a few years now and 82% is our average actual speed to theoretical. So the working of "actual" should be changed to "expected".

Thanks for the clarification.

Couple of follow-up questions:

1) You are using the same 82% factor for both hi and lo speeds. I would have expected them to be somewhat different. Does your data show any correlation between gear ratio and actual-to-free ratio?

2) Do you have any actual data on swerve drives, or is your 82% number based on all non-swerve drives?

[JesseK]

Originally Posted by Kevin Ainsworth

We have been recording our theoretical to actual numbers for a few years now and 82% is our average actual speed to theoretical. So the working of "actual" should be changed to "expected".

Average top speed after "full" acceleration? Or after how many seconds from a dead stop?

Sorry for the nitpick, there's a reason for such a specific question, and either a forthcoming thread or a paper for a 'better rule of thumb'.

[Bryce2471]

2) Easier said than done, especially with the two speed design.

Challenge excepted! lol

In all seriousness, I've considered CADing a light weight ball shifting swerve module for quite a while now, so I'll try my hand at it, and see what I come up with.

I still curious about one more thing:
Have you done a BOM for this design? If so, what was the cost per module?

[Kevin Ainsworth]

Originally Posted by Ether

Thanks for the clarification.

Couple of follow-up questions:

1) You are using the same 82% factor for both hi and lo speeds. I would have expected them to be somewhat different. Does your data show any correlation between gear ratio and actual-to-free ratio?

2) Do you have any actual data on swerve drives, or is your 82% number based on all non-swerve drives?

1) We haven't tested low speed since 2012, we really only cared about top speed and time to travel a distance up until now so that we can get an approximate speed. I therefore can't give an accurate real world relationship between gear ratio and free speed to theoretical speed. From a standing start to 20 feet our 2012 was 69% efficient in low and 50% efficient in high. These numbers are misleading because they aren't max speed they include acceleration time. We haven't been very scientific about our testing and I can only give an average or a specific value for a given drive train on an unknown date in an unknown condition. You have got me thinking and tire wear, gear wear, bearing wear, etc can all effect the efficiency. I really don't even want to relate my data to efficiency for that reason, I think there are too many variables that can effect this calculation based on how you got to your theoretical value.

2) Our 2014 swerve bot started with 5" OD tires and they are 4.6" OD now, that is a 88% to 81% efficiency variation. I am not sure what diameter they were when we tested the speed so I can only say the 2014 swerve bot was somewhere between 81% and 88%. I also can't prove we were at a constant speed the entire 20 feet.

Does a smaller wheel allow for a higher speed due to less torque required to drive the wheel (torque required at a given radius) so do they offset each other?

Does gear wear help at first to reduce friction and then hurt later when the gears start to wear out?

What effect does the gear ratio have like you asked?

What is the real world loss when an extra stage of gearing is added?
Should be around 4% per theory.

Our 2013 8WD had 40mm OD/28mm ID bearings has the least rolling resistance I can imagine (unless we attempt air bearings someday). But I can't test efficiency accurately because the aluminum Vex gears are half gone. It was only 70% efficient after testing it against the 2014 swerve bot but I know that it was much, much faster when it was brand new. We also calculate speed with nitrile tread at a compression factor. A nitrile tread that measures 4.25" at the OD is probably more like 4" at the base of the tread so
who knows who takes that into account in their calculations.

I guess in the end I can't trust a stated efficiency, there are too many variables that someone can make a mistake on when calculating their percentage. I care about real world performance. I just want to be able to predict the top speeds and time to travel a distance accurately.

I know that there are spreadsheets that have been developed for this and I think they are good tools to use.
I trust real world times over theoretical calculations.

Originally Posted by JesseK

Average top speed after "full" acceleration? Or after how many seconds from a dead stop?

Sorry for the nitpick, there's a reason for such a specific question, and either a forthcoming thread or a paper for a 'better rule of thumb'.

I hope you can gather this because I would be very interested in the data.
Maybe a standing start to 20 and 40 feet.
This would show acceleration and usable times to cover almost the entire field.

For our 2014 swerve bot: (free speed calculated = 13.3FPS)
2 seconds to accelerate from a standstill to 20 feet=10 FPS
1.7 seconds to cover 20 feet already at full speed=11.76 FPS
So it only added around .3 seconds to cover 20 feet from a standstill.

For our 2013 8WD dual CIM: (free speed calculated = 17.69FPS)
2.2 seconds to accellerate from a standstill to 20 feet=9.09 FPS
1.6 seconds to cover 20 feet (not sure if it hit full speed) =12.5 FPS
This bot is worn out, has bent chassis rails (scrubbing due to angles wheels).
Acceleration suffers but time to cover the distance is reduced.
I wish I had numbers for this one when it was new.

These tests were an average of three runs with stopwatches.
I would like to use the encoders and data logging to get more accurate results.

Originally Posted by Bryce2471

Challenge excepted! lol

In all seriousness, I've considered CADing a light weight ball shifting swerve module for quite a while now, so I'll try my hand at it, and see what I come up with.

I still curious about one more thing:
Have you done a BOM for this design? If so, what was the cost per module?

Bench racing is fun! And there is always a different way to do something.
Cost per module is probably pretty high, no data is available yet.
Looks like only half the parts have costs associated to them.

[pandamonium]

so swerve question from someone who has never done swerve. Would having a 2 inch or 3 inch wide wheel be more beneficial? Our team has done west coast drive with 3 inch wheels in the past. One thought is that it would be harder to turn, the other is that it would be better for defense.

[AdamHeard]

Originally Posted by pandamonium

so swerve question from someone who has never done swerve. Would having a 2 inch or 3 inch wide wheel be more beneficial? Our team has done west coast drive with 3 inch wheels in the past. One thought is that it would be harder to turn, the other is that it would be better for defense.

This is correct(ish).

You get a traction gain, but at a maneuverability loss. It likely makes more sense to keep the maneuverability to play better positional defense rather than pushing.