Mecanum Drive overdraw
 

Hello everyone,

Description of issue:
-first year seriously considering mecanum drive on a competition bot, played with it last year
-after adding robot level weight to our mecanum (using old dead batteries), we began to experience dramatic drop in Battery Charge. In simulating a match (2:30 sec), driving around at 100% speed, and some pneumatic operation, the battery charge would drop from 130% to 65-75%
-After each cycle, the cims, battery lead, and talons would be fairly hot
-After testing the bot in tank mode, the battery dropped only a difference of 10%

Theories regarding issue roots:
-wheels aren't perfectly aligned, with a difference of about 1 in L vs W.
-amperage issue? we aren't currently monitoring amperage pull per cim and my controls people don't seem to have any clue as to how. I've heard the number 15 for amps, but I don't know how much we are pulling.
-hardware failure? control board or other major board is failing?

ruled out:
-not a singular battery issue
-likely not a battery beak issue

Any ideas or comments would be greatly appreciated. We really don't want the purchase of wheels to go to waste and interfere with out strategy.

Thanks,
Luke

Re: Mecanum Drive overdraw
 

I have coached teams that have used some version of mecanum drives for over 5 years. From my experience, what you are describing does not seem unreasonable. The physics of mecanum drives mean you loose half of your force vector just driving forward. Strafing is VERY inefficient as well. If you tack on a compressor, that charge loss is expected.

P.S.: Not to down play mecanum. It is an excellent drive.

P.P.S.: make sure you have set your wheel in the "X" formation.

Re: Mecanum Drive overdraw
 

Could you explain what you mean by this?

Re: Mecanum Drive overdraw
 

sorry for the quick answer, but i've been up late with a 7 month old with a bad case of RSV for the past two nights. The ratio of 1:1 of the forward and perpendicular vectors was a mistake stemming from a long held assumption (and wrong I might add) that mecanum works the same way an omni drive does. I believe you proved this assumption wrong with this quick paper back in 2010.

I would ask the OP what gear ratio/FPS they are running with.

Re: Mecanum Drive overdraw
 

What is the gear ratio you are using? If you are geared too fast it will make mecanum draw a lot of power (we learned this from our 2008 robot).

Re: Mecanum Drive overdraw
 

A few other things to add:

Is your tank drive also a 4-motor drive train or only two.

Was the electrical setup similar for both drivetrains: speed controllers, lengths of leads, wire gauge?

Re: Mecanum Drive overdraw
 

Transmissions: cimple box

http://www.andymark.com/CIMplebox-p/am-0734.htm

Sorry. When I said tank drive I meant that we were running the mecanum base with tank drive style code, so there was no "mecanum" driving going on.

Re: Mecanum Drive overdraw
 

Is there another reduction in the chain or belt, also what is the wheel diameter?
FYI, if you do not have another reduction in the chain or belt it is probably geared to fast.

Re: Mecanum Drive overdraw
 

No other reductions that I know of. I'm starting to think this may be the issue. 6 in wheels. currently switching from andymark to vex

Re: Mecanum Drive overdraw
 

you can probably switch to 4 inch wheels or you can do a reduction in the chain/belt (a much cheaper option). At around 24-25 ft/s, strafing will definitely eat your battery.

Re: Mecanum Drive overdraw
 

What would a good overall reduction be for a Mecanum setup with 6" wheels?

Re: Mecanum Drive overdraw
 

You can check with Alex, he's darned good at this sort of thing, but a final speed of 12-14 ft/s would be reasonable. If memory serves, we used Banebots 12:1 when we set this up when I ran 2106.

Edit: Alex corrected me. a 12:1 would give you something just shy of 10 ft/s.

Re: Mecanum Drive overdraw
 

With 6 inch wheels my team has found a 8.45:1 works well (we used the tough box nano). It gives you a good speed of around 14 ft/s, and allows you to strafe at about the same speed.