Re: Brakes
 

What is a Worm Drive? Can someoen Explain?

Re: Brakes
 

A drive system which utilizes worm gears.

Worm gears are nearly impossible to backdrive, so when you're not driving them with your motors, anything powered by a wormgear won't move. Therefore in a drive system, the wheels don't turn when you don't want them to. Worm gears are also terribly inneficient, and you can potentially lose 30% or more of your power in a system using them.

Re: Brakes
 

If anyone cares, 772 (Sandwich SS from LaSalle, ON) used a worm drive on each side of their drivetrain (with two small CIMs in total).

The trick with worm gears is that as their efficiency increases, the ability to backdrive them also increases—it's that inefficiency (sliding contact, mostly) that's common in so many of them that permits them to resist being backdriven. Sandwich, if I remember correctly, used custom two-start worms, which were supposed to be reasonably efficient, given the substantial reduction.

Re: Brakes
 

After our two regionals (and after Jack Jones called us out for being easily turnable) we realized that we needed to get better at holding our position. During the fix-it window after WI we added code to dynamically control the brake mode on the drive motor speed controllers. Using the encoders & code we already had on the robot for our our autonomous driving, we regularly poll our custom circuit for the number of ticks each encoder moved since the last request. If both encoder readings tell us that the wheels are "turning slow" we put all four speed controllers in brake mode. All this happens without the driver's input. When the robot is shooting, the driver puts the the robot into low gear. While this doesn't make us unmovable like 25, it does make us very difficult to move or turn if we're locked up against the ramp wall.

Re: Brakes
 

Worm gears are terribly inefficient, that's correct. We used them on our robot, and struggled getting enough power to push and climb the ramp. I don't think we'll be using them next year as they didn't work out as much of an asset this year. But at least we can say we tried! And as far as backdrive, you could still push our robot relativly easily even with the wormgears, they did have some give to them, which kind of defeated the purpose of having them on there in the first place.