Climber - Getting Down

The team has built a pretty successful climbing
device using a chain driven drum.

We are using a Vex 775 motor with a planetary gearbox
rations 10:1, 10:1, 5:1. The drum is chain driven using
2:1 gears for a total of 1000:1. The drum moves at a perfect
pace and with plenty of torque.

They are employing a ratchet to insure the robot does
not slide down when the power is cut.

Our problem is we cannot back spin the motor even
after removing the ratchet.

Our question is will FIRST let you power the motor
after the match to get down or must we come up
with some other method. Perhaps lowering the
gearbox ratios?

Thanks in advance

They won’t let you power the robot up, but there are ways to get it down. If you are already using a ratchet wrench you can lift the robot and flip the switch on the wrench to let it unwind. If there is too much tension in your winch system you’ll have to find a different solution though

If you plan on using your own rope, you can always just take the rope with the robot still attached to it off the field and then take it off then if that makes sense.

You cannot power up the bot to release the rope. You have a few options:

  1. You can lift the robot removing the tension off the rope and have your pilot remove the rope from the davit. The rules state you can remove the bot with the rope still attached to the bot. I’m not a fan of this as I have a concern students in the pilot role have a chance to get their fingers/hands pinched (or even worse) by the rope if the bot is moved before they are ready.

  2. Cut the rope. Not a fan of this technique as you will need a new rope every climb. You also risk injury as students will need to be ready when the bot is ready to ‘drop’.

  3. Different gearing - this I would recommend. Most teams are able to climb utilizing 50:1 - 100:1 ratios. You will probably also find your climb is faster. Do some of your own testing, but removing one of the 10:1’s in the versaplanetary is a good start.

Do not assume you will be able to do this. In order to depress the touchpad, your robot will be clamping against the field, and unless you have a specific way to relieve that tension, it will be extremely difficult if not impossible to “flip” the ratchet by hand. I do not think this is going to “just work”.

On the bright side, is there really any reason you need to hold position without power this year? Obviously if you can do it, it’s preferred, since you can climb with less time left over on the clock, but the Q&A has specifically stated that a robot that descends after T=0 will score the bonus as long as it was pressing the touchpad for >1 second before T=0.

If you use a disc brake (like on a bicycle), activated by a pneumatic cylinder, when you release pressure at the end of the match, you can easily take it down.

Why such a massive reduction? That’s WAY more torque than the VersaPlanetaries can handle, according to the Load Ratings Table. Also, what’s your drum diameter?

I had thought about that, but it depends on the setup of your winch. On my teams bot we are able to do it, but I can see how you would be stuck against the touchpad with no slack to take up.

The spiffy thing to do is to make your own spring loaded touchpad, with stiffer springs than the field touchpad, and put a switch in it so that when it starts do deflect, the winch stops. Then you can wind the winch “up” just enough to release the ratchet.

but with the backdrive situation…our robot will just not quite backdrive from it’s own weight, we need to help it down. So I think we might just remove our ratchet.

Have you done this against an actual touchpad, or something similar? It’s the unique combination of the touchpad you have to deflect which moves past a rigid stop you press against that makes this so difficult - what works on a team built hanging rope setup may not at all work on the real field. Just be careful.

Yes

The drum diameter is about 2.25 inches

What we’re doing is using a ratchet that flips 90 degrees. At the end of the match, it can freely rotate and won’t get caught on the piece preventing the ratchet from turning with the shaft.

As Chris has mentioned, because it doesnt have to be pressing the touchpad at T=0, you can press for greater than 1 second and then let it backdrive and still get the bonus points.
Our ratio is 40:1 with a single CIM. As we cut power, it backdrives a little but either very slow or not at all from its own weight. We are then able to spin by hand to release.

In order to score you must hold the plate for at least 1 second AND be holding it at T=0.

Whether that second starts before T=0 or ends after T=0 is not important as long as you are holding the plate at T=0 AND you hold the plate for at least 1 second.

I’m having trouble visualizing how your ratchet can freely rotate after the match but not during the match. Care to share a picture? :slight_smile:

Actually I read that early on and then got caught up in this thread and forgot. Thanks for pointing it out.
If it was only after T=0 that it would have to be held for 1 second, then a ratchet or piston that is able to release it somehow seem like the only options once power is cut out.
Glad the rule allows before also.

Consider a team that cannot hold position with either some sort of catch (ratchet) or brake system. They will be forced to try to time their climb perfectly or continue to drive and release the climber. I predicted early on in the season that we will see quite a few of these teams that will suffer catastrophic damage to their robot, or damage the touchpad due to climbing past the lower end of the davit.

This is incorrect. The >1 second timer must be finished before T=0. If you have held the touchpad for 0.999 seconds at T=0, you won’t get points.

Many climbing systems can hold position by feeding a fairly small voltage to the motor - in effect stalling it, but at such low power that damage is not an issue. A ratchet is not a prerequisite for a system that holds position when powered, not at all.

This is clearly incorrect as indicated in the manual. If you are pressing for <1 second, then T=0 happens, if you finish the rest of that second after the match ends, you will get points. See the graph in the manual.

Now, if your robot isn’t capable of staying in contact with the touchpad after T=0, that’s different.