27-01-2009 21:00

Dave McLaughlin

Re: pic: Skunkswerve Module Cross Section

This is the final design for the swerve modules we are using on this year's robot. I would be happy to answer any questions.

27-01-2009 22:00

Nuttyman54

Re: pic: Skunkswerve Module Cross Section

Looks good. How much does it weigh, and can you put up an isometric view? I'm having a hard time visualizing the bearing mounts for the sprocket-bevel gear shaft.

27-01-2009 22:14

Lesman

Re: pic: Skunkswerve Module Cross Section

Our design is similar, I'm just curious as to how you guys built it. Our modules are milled and assembled from various pieces of square tubing.

27-01-2009 22:17

Dave McLaughlin

Re: pic: Skunkswerve Module Cross Section

There is a bearing on each side that is press fit. We some left material in the bottom of the hole for the bearing that keeps them from being pressed through. We drilled and tapped the shafts so that we can secure them in the opposite direction with a washer and a bolt. I'm uploading an Iso view right now.

27-01-2009 22:21

Dave McLaughlin

Re: pic: Skunkswerve Module Cross Section

We used 4x4 aluminum extrusion (1/4 W) and 1/4 plate welded to the top. Then our new machining sponsor CNCed out all the holes and add the curve at the bottom that can be seen in an Iso view (Another pic is uploading as I post this).

27-01-2009 22:35

Dave McLaughlin

Re: pic: Skunkswerve Module Cross Section

The Isometric view can be seen here: http://www.chiefdelphi.com/forums/sh...threadid=73090

30-01-2009 21:43

5n1p3r

Re: pic: Skunkswerve Module Cross Section

ok now this is cool, i haven't seen much people put the split view to work so i gotta give you that.

02-03-2009 16:59

big1boom

Re: pic: Skunkswerve Module Cross Section

looks fairly similar to 2022's swerve modules. Except that we CNCed the parts out of 1/8" aluminum plate and welded them together. So our modules are bigger.

02-03-2009 18:01

Dave McLaughlin

Edit

02-03-2009 18:03

Dave McLaughlin

Re: pic: Skunkswerve Module Cross Section

I'm interested as to how you ensured that the holes for the drive shaft, and other holes that need to be precise, lined up after welding. With most of our welded parts was eperience at least some warping, which is why we chose to have the "cans" welded and then machined.

I was also wondering how large your modules are and how much they weigh for comparison. Thanks in advance.

02-03-2009 18:13

big1boom

Re: pic: Skunkswerve Module Cross Section

http://www.chiefdelphi.com/forums/sh...ad.php?t=75407


Each wheelbox has this lazy susan on top and bottom
http://www.mcmaster.com/library/20080826/1544T100L.GIF

I don't have the exact specs but I am guessing that they are each 8 or 9 inches tall, then +2 inches for the turning chain and drive chain. So the entire drivebase is less than 1 foot tall.

02-03-2009 18:40

Dave McLaughlin

Re: pic: Skunkswerve Module Cross Section

I envy how easy it must be to get your hands inside the module for maintenance. The top of our drive base frame is also about 1' off the floor, with steering and power chains on top. Do you employ differential power between the left and right side? Our drive does and this allows us to change from "half-swerve" to "crab" to "tank" all without any mechanical changes.

02-03-2009 19:06

big1boom

Re: pic: Skunkswerve Module Cross Section

No differential. All wheels are powered the same amount. But we are able to implement a type of "Warthog" steering where to the front wheels turn opposite of the back wheels.

It is fairly easy maintence on the swerve moduals.

We have around a third of an inch of ground clearance.

02-03-2009 19:19

Dave McLaughlin

Re: pic: Skunkswerve Module Cross Section

We use the same type of 4x4x4 or "warthog" drive when we are swerving. We are using a three axis joystick and use x and y to translate or "crab" relative to the front of the robot. When we use the z axis, yawing on the joystick, the modules are oriented "warthog" style AND receive left-right differential power. We have found that this is quite effective.

02-03-2009 19:21

big1boom

Re: pic: Skunkswerve Module Cross Section

We also had at 3 axis controller. X and Y for crab steering, twist for warthog.

02-03-2009 19:21

keehun

Re: pic: Skunkswerve Module Cross Section

Excuse my lack of knowledge on Crab Drive, you only needed one CIM because all the wheels move in the same "direction" but the wheel base themselves turn?

02-03-2009 19:26

big1boom

Re: pic: Skunkswerve Module Cross Section

Under normal traction circumstances you need at least 2 CIM's, but with the Regolith 1 CIM is adequate for our team at least.

02-03-2009 19:26

Dave McLaughlin

Re: pic: Skunkswerve Module Cross Section

With a crab drive all of the wheels are oriented in the same direction relative to the field, and all wheels are powered so that the robot moves in that direction. With a pure crab drive there is no way to change the orientation of the robot relative to the field, so most teams implement a turreted manipulator (Team 118 in 2007 and 2008 are good examples). A pure swerve is where all wheels are powered independently AND steered independently so that each wheel can be facing a different direction relative to the field. Our "half swerve" has the front and rear wheels steered together, and the left and right wheels powered together.

02-03-2009 19:29

big1boom

Re: pic: Skunkswerve Module Cross Section

Just wondering, did your team do any prototype in the off season? Or were you like us and decide that tank will not work this year in the middle of week 2.

02-03-2009 19:33

Dave McLaughlin

Re: pic: Skunkswerve Module Cross Section

We had kicked around a swerve last year until we went to CMP and saw how much machining it took to make a proper one. This year necessity drove our design like it always seems to do

02-03-2009 19:38

big1boom

Re: pic: Skunkswerve Module Cross Section

We decided that tank would be inadequate this year, so we CADed up parts and CNCed them on our 20+ year old machine.

The amazing thing is, that we really only started assembling at the end of week 3 I think

02-03-2009 19:44

Dave McLaughlin

Re: pic: Skunkswerve Module Cross Section

We decided in our initial brainstorm that swerve would be the way to go. After that we CADed our brains out and sent out parts to be water jet cut and welded for our frame in week one (Thank you Omax). The modules 4"x4" (1/4 W) was cut into segments and sent to another welder along with the 1/4" al. plate to be welded into the modules initial shape. From there they were taken to a new machining sponsor (Puget Sound Precision) and were machined on their 9 axis CNC mills. We got them back and assembled them during week three and were driving at the end of week 4

02-03-2009 19:52

big1boom

Re: pic: Skunkswerve Module Cross Section

Nice. We had the plates CNCed in our small little shop out of 1/8" plate. It took a long time.

02-03-2009 20:33

Aren_Hill

Re: pic: Skunkswerve Module Cross Section

wait...tank doesnt work? did i miss a memo...

02-03-2009 20:37

big1boom

Re: pic: Skunkswerve Module Cross Section

Tank works, we just decided that this year would be a good year to start swerve, and that it would help a lot with the trailer.

02-03-2009 20:44

Swttrt224

Re: pic: Skunkswerve Module Cross Section

Big1Boom....their modified swerve is what we call a twerve....nice job guys, that's a very creative idea. i had a similar idea last year but have never been able to actually implement it. If i may ask, how is that working out for you perfomance wise?

02-03-2009 21:06

Sam N.

Re: pic: Skunkswerve Module Cross Section

Surely you are running closed-loop position control for your steering motors. What type of sensor are you using on them?

02-03-2009 23:12

Dillon Carey

Re: pic: Skunkswerve Module Cross Section

we had the same problem with our first swerve drives, one somewhat simple thing you can do is to move the chain and sprockets connecting the wheel axle to the upper axle to the outside of the module. As long as this doesn't cause your module to become wider then the diameter of the wheel it shouldn't hit anything. this makes tensioning and replacement of this chain much easier, also it opens up more room on the inside of the module.

http://www.chiefdelphi.com/forums/sh...=swerve+module

This is our teams last swerve design, one other thing we did as far as maintenance was use hex axles and snap rings to make everything very easy to change. Our '08 bot,which used these modules,never broke in competition but it was way better than '07 when we had to get 4 people and sit for an hour to get the wheels out and replace the tread, now it only takes 1 person 30 minutes or less to replace all 4 wheels.

03-03-2009 00:21

Sam N.

Re: pic: Skunkswerve Module Cross Section

I think all of the wheel modules I've seen online have had their main housing either welded together or machined from solid.

Our modules are bolted together using aircraft-grade 6-32 machine screws.

http://www.chiefdelphi.com/media/photos/32573

Don't get me wrong though, I'm a big fan of welding. It just seems to me that it would be easier to do service work on a given wheel module if it could be taken apart. Using screws to fasten the parts together does offer a serviceability advantage IMO.

I also believe that the weight of bolted housing is comparable to the weight of a welded housing - our modules weigh less than 4lbs each.

Still, the best wheel module is one that's so reliable it doesn't need to be serviced very often, or even at all!

03-03-2009 01:37

Dave McLaughlin

Re: pic: Skunkswerve Module Cross Section

Performance is quite satisfying, we are able to use crab drive and "warthog" swerve and transition between the two seamlessly. BUT, we can and do also use simple tank differential drive for rotating in place as it seems to be the best for that.

We are using seperate PID control for both front and back steering. As for the sensor, we are using an unlimited turn potentiometer. While this does have a 20-degree dead band, we oriented it in such a way that the dead band is facing the rear of the robot. When we near the dead band, the swerve module flips orientation and reverses power so that we can move in that direction. The "flop" is extremely fast and is nearly seamless. While we would prefer to have the truly seamless feedback that an absolute optical encoder would provide, the two eight dollar a pop pots were MUCH more affordable than the 350 dollar optical encoders were. Next year we plan to utilize two pots per wheel and align them in opposition to effectively eliminate the dead band.

After this year I think running the chain on the outside of the module will taken into consideration with high necessity.


We have had ZERO problems with the modules themselves, the steering chain runs have been the problem children. However, thankfully we were able to identify and solve this problem after 3 days on our practice field prior to ship. PORTLAND HERE WE COME!

03-03-2009 02:00

AdamHeard

Re: pic: Skunkswerve Module Cross Section

$350 eh?

We used those on our KingKrab, and so far have had no issues. 111 and some teams have reported some possible issues with static discharge however.

03-03-2009 02:02

Sam N.

Re: pic: Skunkswerve Module Cross Section

We used rotary encoders (non-absolute) to close a position loop around our steering motors. Our modules can twist in any one direction indefinitely.

To align our wheels upon robot enable (to tell software where "forward" is so we can measure displacement from zero) we use a pair of IR emitters/optical sensors. The wheels automatically snap forward (it takes .5 seconds) and then normal operation begins.

It's not the best solution... two separate sensors + wiring and a pre-operation software routine, but it works.

03-03-2009 02:02

Aren_Hill

Re: pic: Skunkswerve Module Cross Section

they've been used on every 1625 swerve, we started also replacing pots elsewhere with them. Never had the static issue 111 had with them though

so another vote for ma3's

03-03-2009 02:03

Dave McLaughlin

Re: pic: Skunkswerve Module Cross Section

I stand corrected thanks for the tip, I assure you that it will not fall upon deaf ears...

03-03-2009 15:50

Ken Zaballos

Re: pic: Skunkswerve Module Cross Section

You are right, Dave! We will use this in the future. Thanks Adam.