Differential?

[dtengineering]

Are you sure you NEED a diff?

Why not just drive each wheel from a separate motor/gearbox? If you are concerned about the wheels spinning at different RPMs, just use an old FLL trick and connect the two axles together with some tubing. The friction of the tube will help equalize the speed of the motors... it's in an FLL handbook/guidebook somewhere. I've got doubts, however that it will be a huge problem for you.

Jason

[Rob]

In 2001 we wanted a differential but did not have the space & weight to implement it or means to manufacture one. A differential is a complex device that can be tough to properly use in FIRST environment.

In 2001 we solved our problem using s software differential and driving each wheel with a different motor. The software read the angle of steering and scales the PWM signal to the wheels based on how extreme the angle was. Another great way to implement a software differential would be to use encoders on your wheels if they are independently driven. Inputs to determine the "differential" speed could be from a potentiometer on your steering array or a command signal from the operator interface based on what your driver is doing.

Good luck,

RAZ

[BaldGuy]

Adamherd suggested looking for GoKart differentials-
Go Karts use solid axles- They are explicitly forbidden under all the kart racing sanctioning bodies.
Go karts actually jack up on three wheels and require the inside rear tire to "unload" to turn effectively.

Now,... that being said you can go to GoKartParts.com and spend $162 dollars for a self contained diff (probably for a golf kart or a garden tractor or something-
Good luck on the your oval racer- Don't forget to check your stagger...

Oh no... here comes another left turn.......

[MrForbes]

but riding lawn mowers usually have differentials.....

Is it really necessary? what happens if you just drive both rear wheels with separate motors, with the same pwm signal to the speed controllers for each, won't there be enough "give" to let you go around corners ok? I am guessing there would be, based on our experience with 2WD/omni in 2006.

[Andrewpbhs]

Originally Posted by BaldGuy

Oh no... here comes another left turn.......

nice symbolism

Best bet would be to build one imo

look up the gears, shafts and build a housing.Model it after one thats pre-existing, such as the ones at https://sdp-si.com/

application of any idea is only hard if you lack faith in the design

[Ed Sparks]

I happen to write software for drivetrain systems for a living .......

One thing you need to be aware of when it comes to "Open Differentials" is that torque is applied to the wheel with the least traction.

- So -

If you were to build a drivetrain that utilizes an "Open Differential", and if you were to lift one of the wheels off of the carpet, the wheel off of the carpet would receive all of the torque and it would spin. The other wheel (on the carpet) would receive no torque and would not move.

Automotive manufacturers get arround this by using the braking system to detect the slip. The braking system will then apply brakes to the side that is spinning (off of the ground). Torque then gets transfered through the differential to the other side (on the ground).

Just be aware that if something were to pick up one side of your robot, you will effectively be stranded unless of course you have a means to brake the wheel that is off of the ground.

[MrForbes]

For the most part, automakers get around the limited traction problem of open differentials by inadvertently making the cars heavy and with suspension...it almost always takes care of the problem....

[dtengineering]

Originally Posted by squirrel

For the most part, automakers get around the limited traction problem of open differentials by inadvertently making the cars heavy and with suspension...it almost always takes care of the problem....

Until you get onto ice... it's not uncommon to see a new driver trying to get unstuck in the snow spinning one wheel while the other sits there unmoving. What the driver probably doesn't know is that while the speedo says "50km/hr", the wheel with traction is doing zero and the wheel on the ice is doing 100km/hr.

But hey... if you want to use a differential, go ahead and try one. Learning about what they are and how they work is a useful thing, even if they don't turn out to be the best solution for your robot. I can't help but wonder how many people learned a whole heck of a lot about planetary gearboxes while re-building their banebot trannies last year. :-)

Jason

[MrForbes]

Might also look into a viscous coupling, I don't know what's commercially available, but I think some of the riding mowers use them.

[King Duke]

couldn't use viscous coupling because of the hydraulic fluid used, however it was fun to look it up and see how it works.

[MrForbes]

oh....duh.....!

Sorry bout that

[Darunada]

Thanks Hachiban, that SDP-SI link looks promising.

The more I think about it, I'm thinking driving the weels seperately may be a good decision. That way, if we NEED to, we could implement a software differential. What are methods of doing that? Would we need to create a lookup table to map the speeds? Use encoders? We had terrible luck with the banebots encoders last year as our first attempt (Just couldn't get them calibrated!), and I think we're going to be hesitant to mess with them again....

[cobrawanabe1699]

A diff is overkill with robots this weight, and a Kart diff is not only heavy, but its also HUGE. I'm building a 2WD Radio Flyer with one of my friends, and we were shocked by the price (about $185) of one that wouldn't snap under any type of real tenion. This would not lose weight over dual CIMs. It would gain some.