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.
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).
Might as well test “catastrophic failure mode” while you get a chance to do so in a more friendly environment.
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.
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.
a prudent approach:
Decouple the winch and the sensor trigger system. Winch system needs to be powerful, sensor trigger system does not.
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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. -
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.
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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.
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.
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.
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.
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?
Did you do this after crashing into the top and holding full stall for 1 seconds?
Oh… nope. Ya now that you mention it, I can see how that may be a problem, lol. sigh back to the drawing board
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.
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.
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.
We ran a test today using around 150lbs and a flex-head wrench held in place in some 1x1 c-channel about halfway down the wrench. Worked like a charm; wrench folded up with no problems!
1/2 inch ratcheting flatwrench for retaining. Use the onboard current sensing of the PDP to kill the motor when it reaches a load higher than just lifting, aka semi-stall instance.
tig567899’s reply above states it pretty well.
However, as an illustration to help think through it, consider a robot that weighs 150 pounds ('bot + battery + bumpers), and a winch that can pull with 151 pounds of force.
If the 150-pound robot with a 151-pound-pulling winch were halfway up the rope, and a bystander put a hand out and pushed down with just one pound of force, the robot would stop ascending the rope, and be able to go no higher. It would only take 1 pound of downforce to do that.
The situation is no different with the davit – only 1 pound of additional downforce is needed to stop the 151-pound-pulling winch from hoisting the 150-pound robot.
Depending on the geometry, this is likely to run afoul of the perimeter rules. You’d need to be hanging at an angle such that this extension doesn’t reach past the bumpers. (Or have a smaller-than-maximum-size robot.)