Climb Stopping Mechanism

FIRST General Forum
#52

I’ve been mulling over whether or not it’s legal cut the rope to release the robot. It’s legal for your robot to damage team provided ropes (G15) as long as it doesn’t leave debris on the field, and once the rope has been removed from the Davit it’s no longer considered part of the field (R08 blue box).

As far as I can determine, it would not be legal because up to the point where the rope is cut and the robot is released, it’s technically a person damaging part of the field (even though that part of the field is legally allowed to be damaged by a robot. Just not anything else). But it was an interesting read through the rules.

Apex Robotics is not planning to release our robot in this fashion. But it did make me curious if it’s a quick fix for teams who haven’t planned around this

#53

We’re using a flex-head ratcheting wrench, i.e. just an ordinary ratcheting wrench except there’s a pivot between the head and the handle. In order to lower the robot, we simply undo the velcro strap holding the wrench handle in a constrained position (i.e. between two bolts) and pivot it out of the way. The wrench rotates with the winch as it unspools.

#54

Wow. I was at Lowe’s shopping for ratchet wrenches this evening, and thought of essentially the same thing when I looked at the flex-head wrenges. I was thinking of using a non-flex head, non-reversible wrench, wedging it in a piece of aluminum channel (similar to your two bolts), and lifting it out parallel to the shaft. If you do this, be sure to mark the ratchet wrench as to WHICH SIDE FACES OUT, or you’ll have a useless climber for a match or three.

I concur that with the rules as currently written, it would not be legal (Drive team damage vs robot damage). However, given the preamble to the answer to Q299, I suspect that a question on this topic would get a finding of legal even if it meant a rule change.

#55

The nice thing about the flex-head wrench is that we can constrain the head in-place to avoid that exact problem (though we probably ought to mark it anyway).

For those who are interested, this is the wrench in question:

#56

From my experience, that is much easier said than done (with the non-flexible non-reversing). The wrench always wedged itself pretty tightly against the stop and we still needed to lift or rotate the robot slightly to get it out. There needs to be pretty much no load on the wrench to be able to slide it along the shaft to get it off (or far enough along the shaft to not be impeded). Granted, I’ve always done it using two bolts/screws as the stop, never channel.

I’ll second the comment about marking which side of the wrench needs to be facing out, or which way the switch needs to be flipped if you’re using a reversible wrench. Drill it into your pit crew’s and drive team’s heads. You can never double-check too many times.

#57

Have you tried this after crashing into the top and holding full stall?

Depending on where it is in the gear reduction and the spool diameter it’ll either be completely locked and tough to remove or trivial to remove by hand.

#58

The wrench is right on the spool shaft, and the spool diameter is 1.25’’. It’s got a nice long handle, and I seriously doubt it will be a problem though we have not tried “wedging” the robot like that yet (we will be current-limiting the climbing motor to prevent that, at any rate).

#59

Might as well test “catastrophic failure mode” while you get a chance to do so in a more friendly environment.

#60

After reading all this, maybe it would just be better to climb early and let the robot backdrive as time expires… This seems tricky otherwise.

#61

Hmm… I wonder if you could build a slip knot into the Retaining Feature of the rope. It holds under tension, but after a match, make sure the robot is supported and release an inch or two of slack. It’d be all in the rope, so even the teams that show up without the slightest consideration of this issue can implement a solution.

#62

a prudent approach:

Decouple the winch and the sensor trigger system. Winch system needs to be powerful, sensor trigger system does not.

  1. Stop the climb well short of the sensor disk. Do this via encoder or careful operator control.
    a ratcheting device, such as a wrench, ensures that when power is killed the climber drum doesn’t unwind. Robot is now hanging a few inches below sensor.

  2. Activate a low power device (pneumatic piston or a cam on a small motor with its own ratchet) that extends up and holds pressure on the sensor disk through the end of match. Or do it passively with a compression spring.

  3. After match lift robot up. As long as the trigger device on robot is low power two people should be able to overcome it easily and thus easily take tension off the winch ratchet system.

This type of solution has likely been covered in another thread but it seems worthwhile to emphasize in this thread that triggering the sensor with your winch system, and the trouble that can ensue, is unnecessary.

The forces involved that can damage the field, your robot, and possibly you, are of such magnitude that added complexity in the form of the additional trigger subsystem would likely pass the cost/benefit test.

#63

Out team will be using a ratcheting winch that will shut off once our Vex bumper switch is pressed. The shut off is on a delay so our robot will be able to push the pad more than an inch before we stop climbing.

#64

Hmmmm… Not sure if I understand your why here.

As the robot climbs the rope the weight of the robot causes tension on the rope equal to the weight of the robot … Agreed

But once the robot encounters the hard stop at the Davit, it will give an upward force on the davit equal to the torque of the winch. Since every action produces an equal and opposite reaction the Davit will push down on the robot equal to the torque of the winch. This will be added to the force of gravity on the robot, thus the rope will see full force of the winch + weight of the robot.

Please show me where I am wrong.

#65

We just tested a few ideas today, and settled on a process. So we put a standard socket wrench (locked in place) on the end of the climber hex shaft. Our climber is very close to the bottom of the bot. Two drive team members than lifted the bot enough so that tension of the rope was taken off the hex shaft, and since the climber is close to the bottom of the bot, the person on that side of the bot, was able to flick the lever easily with one hand, while holding that side of the bot with the other hand, and then both drive team members guided the bot slowly to the ground, and the rope just unspooled.

#66

I found this in Q&A and it may help a few teams. While it it legal for a PILOT to assist in removing the ROPE from the DAVIT, the laws of physics likely wouldn’t allow it in the ‘clamping to the field’ scenarios above. https://frc-qa.firstinspires.org/qa/381

We’re going to pneumatically actuate a vice grip into a brake off the gearbox of our winch. We’ll probably add some cushioning to our contacts to make sure the touchpad stays engaged if we drop a few tenths of an inch. Anyone know alternative names for them?

#67

Did you do this after crashing into the top and holding full stall for 1 seconds?

#68

Oh… nope. Ya now that you mention it, I can see how that may be a problem, lol. sigh back to the drawing board

#69

The force on the davit is equal to the torque of the winch minus the weight of the robot - opposite forces cancel. The davit will push down on the robot equal to that force - thus: weight of robot (due to gravity) + winch force - weight of robot = winch force.

#70

Based on tests today of our first iteration climber, it seems reasonable to (a) get the climb scored but not stall into the touchpad, (b) relieve the load on a switchable ratcheting wrench by lifting the robot further into the touchpad thereby allowing you to switch the ratchet, © remove the rope from the field thereby releasing the robot, and (d) remove the rope from the robot in the pit. Our tests proved that this is difficult but feasible with a manual climb, but would be much more repeatable and faster with an automatic climb. The key is to not finish your climb with full power. I would warn folks that with the high reductions many climbing gearboxes have, it could be difficult to remove rope by backdriving the winches by hand.

#71

We tried this today. We climbed with 130lbs attached to our climber’s production frame using production motors & gearing. Rope touch to Touchpad was about 4 seconds, so it wasn’t a ‘crash’ into the top - but we did intentionally stall the motors at the top for a second or two. The ratchet wrench was held in place by one 1/4-20 bolt - the threaded end, no less. While under load we popped it loose with a large flathead screwdriver very easily. During further tests and refinement, we added a string to pop it out at the end of the tests. It also worked very well. We climbed about 30 times, and it worked every time. We’ll put a 3/8" steel stud around that bolt in the future.