Differential?
 

Hello all FIRST participants,

We're considering using a differential on our robot this year. Does anyone know of a good supplier for a small yet sturdy one that would be suitable for, say, a robot drive train?

Either a supplier or plans to make one would be great, we have the capability to make something but don't want to spend the time designing it.

Thanks a lot,
Dave

Re: Differential?
 

Have you tried looking at go kart parts? I'm sure there is something suitable (although probably a wee bit heavy) there.

Re: Differential?
 

This place always has cool transmission parts (including differentials). They say they need a "hydrostatic pump" to operate. Might be a problem...

SDP-SI also carries some smaller differentials (look under "Miscellaneous"). They might serve your purpose, provided you arn't going to put tons of force through them...

Re: Differential?
 

Hey Darunada, hope everything at Arrowhead is going well.

Regarding your inquiry about differentials, I just want to heed you fair warning, our team (269) tried doing a differential in 2003 and it turned out to be a royal pain in the butt. Please proceed with caution and make sure you use as much precision as possible.

Re: Differential?
 

Can you elaborate on exactly what you found challenging? We were planning on using a diff in our robot this year to handle turning the corners well. From my extremely limited knowledge of diffs, i thought they were relatively "plug and play."

Re: Differential?
 

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

Re: Differential?
 

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

Re: Differential?
 

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.......

Re: Differential?
 

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.

Re: Differential?
 

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

Re: Differential?
 

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. :cool:

Re: Differential?
 

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....

Re: Differential?
 

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

Re: Differential?
 

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.

Re: Differential?
 

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

Re: Differential?
 

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

Sorry bout that

Re: Differential?
 

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....

Re: Differential?
 

http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_35768_35768

You would have to cut the axles and I think the housing is about 4.5 inches in diam.

Weight may be a problem. price is $80

Re: Differential?
 

Depending on your application, you might be able to save a bit of weight if each of your rear wheels is independently driven, with a software differential.

To make a software differential, we break the robot down into simpler parts. As we turn the steerable wheels in the front (Ackermann-steering would have an advantage here), we can use the angle that the wheels have turned from center to gauge how much of a nominal radius the center of the robot will turn at.

For this, you can do calculations and create a lookup table (variable array) to save processor time on the playing field, and map out the nominal radius for a given turn angle. Once we know the nominal radius to the center of the robot, we subtract half the width of the robot for the inside wheel and add half the width of the robot for the outer wheel.

Next, we read the operator interface values for how fast we want the robot to be going, as we set this to the outer wheel. (Unless the robot is going perfectly straight, then both wheels). Here's the [simplified] calculations involved:

Now this example is very simplified. Right now, this example will work just like a real car; it will only make your robot turn if you "give it gas". Also, there are no checks in here for frictional losses, or attempting to turn in radaii so small the innerSpeed value is so small it cannot overcome the friction of the drive train.

Some additional tweaks to the code can solve this, but that's something for all of you to experiment with and figure out. What are you waiting for?! Go get a Vex robot and an Advanced Gear Kit from VexLabs, and start testing code! :)

Re: Differential?
 

I saw a team use a differential a few years back. They steered the robot using a rack and pinion design. Since you are asking this question for a game like this it makes me want to wonder, Drifting!? It would be a hard task to accomplish seeing as the floor is carpet, but it's a thought.

Re: Differential?
 

QC Elite last year used a software/electronic differential for our coordinatated steering system. We found the two arcs that would pass through the wheels on each side, and then used a ratio between the two sides to figure out the different speeds the wheels needed to turn. Worked real well.
Malhon

Re: Differential?
 

((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.))

I agree completely but suggest that both outputs from the differential transmit 'equal' torque. By that I mean that both wheels get the same amount of torque, but, because the wheel in the air only requires say .2 ft/lbs to turn it due to lack of resistance, that same .2 ft/lbs of torque at the other wheel is not enough to cause that wheel to rotate. So when the free spinning wheel gets brakes applied, the amount of torque required to turn it increases (at both wheels) until the wheel with good traction is easier to rotate than the now stopped wheel, and the vehicle moves.
Same effect but just a different explanation.

Re: Differential?
 

Drifting sounds like a good way to completely destroy FIRST's carpets...:D :ahh:

We've used a software differential for the last few years. Usually with encoders. Though last season we got one working using only a gyro.

Works well enough in either case. You don't even need to do the proper trig - all you want to do is reduce the slip on the inside wheel to an acceptable level.

Re: Differential?
 

by reducing the slip you decrease the RPM's of the inner wheel?

Re: Differential?
 

We have decided against a differential in hardware, and probably in software as well. I hope this thread can help others though!

Thanks!

Re: Differential?
 

I have personally dealt with open diff's and locking diff's in my vehicle. My truck had the stock rearend with the open gears in it, it behaved by doing great in traffic but off-road it sucked if you got one rear wheel in dirt/gravel/air that wheel would spin and the vehicle would go backwards (I forgot to mention I have a 2wd). So I bought a powertrack locking diff and it kicks butt because it "ratchets" around corners using plates to unlock the rear wheels together however it is temperamental and you can’t WOT around a corner or else it will lock up. I can go into a corner and floor it (WOT) the locking diff will lock causing the wheels to chirp and the truck's turning is greatly decreased. When it receives alot of torque from the motor and one wheel slips it will engage and lock the two rear wheels together this makes it great offroad and when doing burnouts (no more pegged leg burnouts ;P).
I doubt that there would be a situation in this years game where you’re robot would have ½ of the drive wheels on one side of the roboto off the ground and thus, an open diff would be a good mechanical means of keeping the wheels nice and smooth through turns.
And here’s a great article on how open diff’s work
http://auto.howstuffworks.com/differential2.htm
I like the idea of programming a “diff” though.

Re: Differential?
 

if you want to build a diff, look at a lego diff, it's just a dumbed down version. i would however add in a fourth gear to make it work more like a real diff

Re: Differential?
 

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.