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Cothron Theiss 04-02-2017 03:39

Re: Climb Stopping Mechanism
 
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.

James3245 04-02-2017 08:48

Re: Climb Stopping Mechanism
 
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.

AveryLevin. 04-02-2017 11:25

Re: Climb Stopping Mechanism
 
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.

Daniel_LaFleur 04-02-2017 19:39

Re: Climb Stopping Mechanism
 
Quote:

Originally Posted by Ken Streeter (Post 1640060)
Just FYI, I agree with your point that teams need to have a solution to the problem.

However, in this circumstance, the force applied on the rope after hitting the stop at the top isn't "the full power of the winch + the weight of the robot pulling on the rope..."

Rather, it's the maximum of the two. (Which is likely to be the "at stall" torque from the winch.)

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.

dirtbikerxz 04-02-2017 20:23

Re: Climb Stopping Mechanism
 
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.

JesseK 04-02-2017 20:48

Re: Climb Stopping Mechanism
 
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?

AdamHeard 04-02-2017 22:27

Re: Climb Stopping Mechanism
 
Quote:

Originally Posted by dirtbikerxz (Post 1640515)
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.

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

dirtbikerxz 04-02-2017 22:39

Re: Climb Stopping Mechanism
 
Quote:

Originally Posted by AdamHeard (Post 1640573)
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

tig567899 04-02-2017 23:07

Re: Climb Stopping Mechanism
 
Quote:

Originally Posted by Daniel_LaFleur (Post 1640493)

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.

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.

Basel A 05-02-2017 18:21

Re: Climb Stopping Mechanism
 
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, (c) 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.

JesseK 05-02-2017 18:30

Re: Climb Stopping Mechanism
 
Quote:

Originally Posted by Oblarg (Post 1640147)
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.

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.

Karibou 05-02-2017 18:54

Re: Climb Stopping Mechanism
 
Quote:

Originally Posted by JesseK (Post 1640783)
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!

electronicsdude 05-02-2017 21:07

Re: Climb Stopping Mechanism
 
Quote:

Originally Posted by sauhaarda (Post 1636914)
What strategies are you using to stop climbing the rope after the button is pressed at the proper pressure? Are you planning on a manual stop? A pressure plate stop? A stop based on stall torque? It seems to me that there is a very small window to stop the robot from applying to much pressure on the plate. How are you teams trying to address this issue?

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.

Ken Streeter 05-02-2017 22:52

Re: Climb Stopping Mechanism
 
Quote:

Originally Posted by Daniel_LaFleur (Post 1640493)
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.

Quote:

Originally Posted by tig567899 (Post 1640587)
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.

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.

BethMo 05-02-2017 23:04

Re: Climb Stopping Mechanism
 
Quote:

Originally Posted by James3245 (Post 1640232)
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.

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.)


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