Questions about crab/swerve drive.

[Team1710]

Anyone know of a good supplier of pots and what type would be needed? Since pots can only go a fixed distance, what would a setup be like?

[AdamHeard]

I'm seeing an awful lot of people jump on the swerve guru bandwagon recently, and I'd to say something about that.

Before you hand out advice that someone may take as 100% perfect, think for a moment, do I REALLY know what I'm talking about? Can someone take my words, act on them, and be satisfied with the results?

Successfully using a swerve this year was a challenge, but also one much less so than on carpet. Most swerve designs I have seen this year would probably result in severe damage to the modules/turning shafts if they had been used on carpet.

Since I doubt we'll be getting anything other than carpet, that's what a new crab should probably be designed around.

[Steve_Alaniz]

Originally Posted by AdamHeard

I'm seeing an awful lot of people jump on the swerve guru bandwagon recently, and I'd to say something about that.

Before you hand out advice that someone may take as 100% perfect, think for a moment, do I REALLY know what I'm talking about? Can someone take my words, act on them, and be satisfied with the results?

Successfully using a swerve this year was a challenge, but also one much less so than on carpet. Most swerve designs I have seen this year would probably result in severe damage to the modules/turning shafts if they had been used on carpet.

Since I doubt we'll be getting anything other than carpet, that's what a new crab should probably be designed around.

Could you give some examples please? Any drive that might stall will have problems but that is pretty much ...any drive. I think you are referring to a transmission problem that might occur under stress but I wouldn't want to put words in your mouth. You are correct that the slick surface allowed for easy turning but our drive actually worked better on carpet, so it was actually a challenge for the drivers to adjust to the regolith.
Our shafts were 1/2 in steel so they were not going to break but we did worry a bit about the transmission so we used the banebot 16:1. I am under the illusion we were pretty bullet proof. At least we plan to build on what we learned this year.

[AdamHeard]

Originally Posted by Steve_Alaniz

Could you give some examples please? Any drive that might stall will have problems but that is pretty much ...any drive. I think you are referring to a transmission problem that might occur under stress but I wouldn't want to put words in your mouth. You are correct that the slick surface allowed for easy turning but our drive actually worked better on carpet, so it was actually a challenge for the drivers to adjust to the regolith.
Our shafts were 1/2 in steel so they were not going to break but we did worry a bit about the transmission so we used the banebot 16:1. I am under the illusion we were pretty bullet proof. At least we plan to build on what we learned this year.

I'm actually referring to the loads the module itself has to deal with, the sideloads on the module can be massive if you turn while moving, or are pushed, or just about any situation really.

Many modules I saw this year made me think, "wow, if that was on carpet, it would just collapse when you turn".

Working isn't good enough, it has to "Work" while turning at 20fps and getting it from another robot, or being pushed sideways by two robots. If your modules can't survive those loads, it'll be a rough season.

[Steve_Alaniz]

Originally Posted by AdamHeard

I'm actually referring to the loads the module itself has to deal with, the sideloads on the module can be massive if you turn while moving, or are pushed, or just about any situation really.

Many modules I saw this year made me think, "wow, if that was on carpet, it would just collapse when you turn".

Working isn't good enough, it has to "Work" while turning at 20fps and getting it from another robot, or being pushed sideways by two robots. If your modules can't survive those loads, it'll be a rough season.

Good point! So I guess you mean the turning bearing on the module is a critical spot. I'd have to agree with you. (A Lazy Susan probably wouldn't cut it. )
Thanks for clearing that up.

Steve

[Steve_Alaniz]

Originally Posted by Team1710

Anyone know of a good supplier of pots and what type would be needed? Since pots can only go a fixed distance, what would a setup be like?

What application do you mean? If you mean speed encoders, the banebot encoders that fit on the transmissions is a very easy solution (http://banebots.com/pc/ELECTRONICS/EN-G0561-KT).
If you mean a position sensor, USDigital has a great absolute encoder that we used to link our left and right side drives together so we didn't have a chain across the robot. (http://www.usdigital.com/products/en...ary/shaft/ma3/)
USDigital also has rotary encoders that you can use with a wheel for floor speed if you are looking to do some traction control.
Ok USDigital IS pricey but I can't argue with how easy they were to use (a simple analog signal) and reliability. I think they are worth a look.

Steve

[Al Skierkiewicz]

Why not have a wheel encoder tied to a non-driving wheel. Use two at ninety degrees to each other using omni wheels and then you don't have to worry about running wires down inside the modules.
Please keep in mind that bevel gears (those normally used on coaxial drives) are relatively inefficient in use for driving and require some pretty meaty bearings to keep them aligned under load and stress.
Slip rings can be made by using circular, machined conductive surfaces, insulated from the shaft by PVC, polyeurathane or delrin. Brushes can be bought anywhere, even at Ace or TruValue. All you need to do is keep them aligned in the assembly and have some form of attachment. Using brass allows you to solder wires directly to the rings.

[Steve_Alaniz]

Originally Posted by Al Skierkiewicz

Why not have a wheel encoder tied to a non-driving wheel. Use two at ninety degrees to each other using omni wheels and then you don't have to worry about running wires down inside the modules.
Please keep in mind that bevel gears (those normally used on coaxial drives) are relatively inefficient in use for driving and require some pretty meaty bearings to keep them aligned under load and stress.
Slip rings can be made by using circular, machined conductive surfaces, insulated from the shaft by PVC, polyeurathane or delrin. Brushes can be bought anywhere, even at Ace or TruValue. All you need to do is keep them aligned in the assembly and have some form of attachment. Using brass allows you to solder wires directly to the rings.

Hey Al! Nice to see you... Really? You'd use slip rings? Ok but I would be concerned about the electrical noise they would make... One is bad enough but four? I guess that's why I never tried it. Still Id like to hear your thoughts.

Steve

[Al Skierkiewicz]

Steve,
You asked an engineering question, so I gave you an answer. I didn't say it was a good one. There is no possible way to not have some noise in a slip ring, particularly one that is carrying a lot of current. However, with large brushes or multiple brushes, you should be able to transfer enough current to be useful. Slip rings for sensor use should be fine since the current is fairly low, and the noise should also be low. Please be sure that power supply for sensors are filtered near the sensor.
There are some mercury filled rotary electric connections out there. I am against using them but I do believe the GDC answered (In a Q&A) that they could be used this year. In the hard hits, and misalignments I see on robots during inspection, I believe the chance for a mercury spill is pretty high. For that reason, I advise against their use.

[Jared Russell]

Originally Posted by Al Skierkiewicz

Steve,
You asked an engineering question, so I gave you an answer. I didn't say it was a good one. There is no possible way to not have some noise in a slip ring, particularly one that is carrying a lot of current. However, with large brushes or multiple brushes, you should be able to transfer enough current to be useful. Slip rings for sensor use should be fine since the current is fairly low, and the noise should also be low. Please be sure that power supply for sensors are filtered near the sensor.
There are some mercury filled rotary electric connections out there. I am against using them but I do believe the GDC answered (In a Q&A) that they could be used this year. In the hard hits, and misalignments I see on robots during inspection, I believe the chance for a mercury spill is pretty high. For that reason, I advise against their use.

Regarding mercury-filled electrical connectors, my team has some sitting on our shelf (in 2006 we toyed with the idea of a continuously rotating turret). We didn't end up using them in competition, but you should read the product documentation CAREFULLY if you want to. Currently, no Mercotac electrical contact (Mercotac was the only manufacturer of these as of a couple years ago) is rated for more than 30A of continuous current. This all but rules out these devices for a high-traction swerve drive.

One other warning: slip rings (especially home made ones) always add a bit of noise to a signal. Be careful when using analog sensors with slip rings! (I know from experience!)

[artdutra04]

190 built a custom high-power slip ring for their 2008 robot, along with a commercial slip ring inside of it for all of the sensors. Here's a photo of their [nearly completed] slip ring during the build season:

[Alan Anderson]

Originally Posted by Jared341

Currently, no Mercotac electrical contact (Mercotac was the only manufacturer of these as of a couple years ago) is rated for more than 30A of continuous current. This all but rules out these devices for a high-traction swerve drive.

That's 30 amps per contact, right? The one we used on the 2008 TechnoKats robot's continuous-rotation arm has eight contacts. Use four of them for a pair of 30A supply and return lines, and you still have four left over for an encoder if you want.

(The list price of such things might be higher than the single-component cost limit, but many suppliers have an educational discount that they offer to any team who asks for it.)

[Team1710]

To determine the position of the wheels on a swerve drive, would one want an absolute rotary encoder or a relative rotary encoder?

[Aren_Hill]

We've always used the Ma3 abolute analog encoder which kept its orientation so we didnt have to line up the wheels before we turned the bot on

[R.C.]

Originally Posted by Aren_Hill

We've always used the Ma3 abolute analog encoder which kept its orientation so we didnt have to line up the wheels before we turned the bot on

Aren,

A link would be very handy, we kinda wanted to buy some.

Thanks,

-RC