Differential?

[Darunada]

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

[AdamHeard]

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

[=Martin=Taylor=]

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...

[AndyB]

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.

[neutrino15]

and it turned out to be a royal pain in the butt.

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."

[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....

[Spylake]

Originally Posted by Darunada

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

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

[artdutra04]

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:

// We assume that our existing calculations determined the outer radius to be 124 inches, and the inner to be 100 inches. opInterfaceSpeed is how fast the driver is telling us to drive the robot
outerRadius = 124;
innerRadius = 100;
opInterfaceSpeed = 255;

// First, we shift the opInterfaceSpeed by 127, so that -127 is reverse and 127 is forward. 
robotDriveSpeed = opInterfaceSpeed - 127;

// Find our circumference of both circles
outerCir = ((outerRadius * 100) * 2 * 314) / 100;
innerCir = ((innerRadius * 100) * 2 * 314) / 100;

// Now since we want the robot to complete both inner circles and outer circles at the same time, we find the ratio between the two. The diameterRatio should an integer between 0 and 100
diameterRatio = (innerCir * 100) / (outerCir);

// Now we find out how much we want to slow down the inner wheel. The output will be between 0 and 255, the PWM signal we want to send to the inside motor.
innerSpeed = 127 + ((robotDriveSpeed * diameterRatio) / 100);

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!

[caraddicted101]

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.