Disk brakes

[Lil' Lavery]

Originally Posted by Joel J.

My post was a follow up to M. Krass's.

As for what you are saying about brake mode: it is not that strong. It does provide some resistance to motion, but not alot. Brake mode seems good for allowing the robot to stop on a dime, and also for helping to keep an arm stationary. Break mode may not help much in a pushing match.

The 2nd half of my post was a generic address to the topic as well, not specifically at you (just the initial traction comment). But the brake mode works very well from what I have heard (and determined playing against it) during pushing matches. I don't believe that 116 has ever implemented it though, so perhaps there are quirks that I didn't know.
Regardless, a vast majority of pushing in FIRST does occur rotationally though.

[Jay TenBrink]

If you have potentiometers or a similar devices to measure the position of your wheels, you can program in a reactive braking feature that we refer to as “dynamic brakes”. When the robot is at rest and you want to stay put, the dynamic brakes are activated. The controller senses when a wheel is beginning to turn and applies just enough current to the drive motors for that wheel to keep it stationary. This can work on each side of the robot in opposing directions to keep you from being spun around.

These dynamic brakes only work when you have power, so they won’t keep you on an inclined ramp at the end of a match. They are, however, lightweight (only software) and effective in keeping the wheels from turning. The limit to the staying power is the traction between the tires and carpet.

Jay

[Billfred]

Originally Posted by Jay TenBrink

If you have potentiometers or a similar devices to measure the position of your wheels, you can program in a reactive braking feature that we refer to as “dynamic brakes”. When the robot is at rest and you want to stay put, the dynamic brakes are activated. The controller senses when a wheel is beginning to turn and applies just enough current to the drive motors for that wheel to keep it stationary. This can work on each side of the robot in opposing directions to keep you from being spun around.

These dynamic brakes only work when you have power, so they won’t keep you on an inclined ramp at the end of a match. They are, however, lightweight (only software) and effective in keeping the wheels from turning. The limit to the staying power is the traction between the tires and carpet.

Jay

Would these potentiometers be made for continuous rotation, or am I missing something?

[Joel J]

Originally Posted by Billfred

Would these potentiometers be made for continuous rotation, or am I missing something?

Yea, I would think continuous rotation. Your gut is correct, you probably won't find many recommendations for using potentiometers where encoders are more appropriate.

You don't need to measure position (absolute) of your wheel, you can get a software based dynamic brake by using a PID-based velocity control algorithm, and setting the desired velocity to zero (this would be a side effect of the driver letting up on the joystick). Eh, I think this is what he was talking about anyway.

[Andy A.]

To elaborate on the victors dynamic breaking feature.

The key to remember is that it is 'dynamic'. The resistance is proportional to the speed that the motor shaft is turning. So, at rest, there is no resistance to movement. As the shaft begins to turn resistance to movement rises. Movement will either accelerate or decelerate until a balance between resistance and the force causing motion is reached. Then acceleration stops but motion will continue.

Because the braking power is linked to shaft speed it can never completely halt motion. If the resistance where enough to completely stop movement, then the brake can no longer function (no shaft movement means no current generated). Watching a drooping arm with a victor braking it you can sometimes see a little herkyjerky behavior- thats the arm accelerating till it is stopped and then accelerating again over and over again. Other times it just looks like it's falling in slow motion.

A Victors dynamic braking alone will not hold a robot in place and will not hold an arm horizontal. It will slow movement down, but the arm will droop and the robot will be pushed. I wouldn't depend on it to hold a robot in place, because it can't. A brake, either in the drive train or an anchor, is preferable if you absolutely have to stay in place.

-Andy A.

[Tom Bottiglieri]

I really like the way 25's brakes worked.

They drilled a bunch of concentric holes in their gears, then used a servo to shove a dowel pin in. With their big knobby tires, they were almost impossible to push.

If you watch Newton Finals, you'll see 25 push Wildstang across the field, then hit the brakes. 111 tries to push them for about 15 seconds getting nothing done. 111 backs off to get some momentum and 25 breaks for the goal. Perfect driving.