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Climb Stopping Mechanism
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?
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Honestly the best option seems to be driver control. It's a huge light. If they can see the rope well enough to grab it, they can see the touchpad light.
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Our plan so far is to use a gyro to detect when the robot is angled up at more than, say, 5 degrees. Then we'll use an encoder on our climber winch to find our height, and stop when we hit the pad. The issue we've come up with is that the rope won't always wrap in the same path, so the distance you climb on each revolution will vary slightly. We're planning on using a velcro strip though, so this shouldn't be too bad. If it does end up being intolerable, we'll probably put a limit switch on the front of the robot to detect when we hit the plate and use the encoder to then climb another inch or two.
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There is a 1 second delay between triggering the plate and the light activating.
that is plenty of time to break components at stall or burn up motors. i don't have any testing to support it but i assume that with the can talons it will be super easy to detect the current spike once you hit the plate and then shut down the motors. it appears like a simple solution to me until testing proves otherwise |
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I think you are looking at it wrong. Each team has a clear view of one of the davits. Teams just need to climb the rope that corresponds to their view. ![]() |
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Don't, if you gear a CIM motor for optimal climb speed you should be able to stall for at least 30 seconds. The pressure plate has a metal U channel that should prevent you from damaging it anyway and while you may worry about it I'm sure FIRST knows plenty of other teams will test its limit. No need to over complicate things.
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The light doesn't turn on until after you've pressed it for one second.
Going to just have analog manual control of the speed of the climber, slow down as you get to the top, press it down firmly, not really worried. This shouldn't be hard. |
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A suggestion someone on my team had was to measure the current draw from the climber motor, and when it spikes past a certain point, you know you've hit the top of the rope and are pressing the button down.
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You got to make a stop mechanism so you don't slide back down after time is stopped. One the match is done the rope points are calculated, so you got to keep up until the points are GIVEN to the team.
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Step 1: If your climber is a winch, get rid of all of your sensors and use the PDB's built-in current sensors. When Current / Applied Voltage > a ratio (we're using 60A & 12V, so ratio of 5) then disable the motor from going in that direction for N milliseconds.
Step 2: Test Step 3: Test Some More. Current through the motor is responsive and reliable to read and react upon. Getting it from the PDB is one of the best things added to the control system in recent years. |
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Depending on what T=0 means, you don't NEED to hold your robot on the button after power is cut, just as power is cut, provided you have already held the button for >1 second. If your mechanism can't do that, it's not like prior years where you can't get the points at all, it just requires more commitment in advance and eliminates last-second scoring.
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Our team is using a hex shaft, so we will include a 1/2" ratcheting wrench to help us climb. This was seen to be effective from Ri3D 1.0 and our team is worried that our bot might fall after the buzzer (or at least glide down, which still isn't safe).
I've changed my opinion since the reveal. I thought that it would be stronger to use a single grabbing motion (like the 2013 pyramid climbers), but I've noticed that you would need to dedicate a huge portion of your bot to that. Our team is using a type of winch. It's not an original idea, but our testing shows its effectiveness. We are quite excited to be climbing this year. Good luck all! |
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According to the 2015 FRC motor information sheet, a CIM motor pulls 133 amps at stall. Well above the allowed 40 amp breaker allowed per motor under rule R56 Table 8-3.
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thanks! We were going to just slow down at the top, and stop after we visibly moved the touch pad up as much as it takes to get the light to go on (practice day, you know). But the current thing looks like a fun way to keep a few students busy for a few more days |
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We have an encoder integrated into the climber. We plan on monitoring the current to detect when the climb starts, and set a stop distance based on that start point. The operator will have the ability to jog higher if we stop too soon, but hoping this approach leads to automatic climbing.
Still untested theory at this point. Fingers crossed. Would like to have working repeatable solution by week 1, we are going to need it. |
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I think the easiest solution is to simply use the motor controllers to sense when the motor is stall. This is what 6101 is doing. The Davit has a C-channel piece to protect it in brief high-stress situation and the 40A breakers wouldn't pop immediately. I don't see any reason why this wouldn't work.
To prevent gliding, we are using a ratchet head on the winch similar to Ri3D 1.0. |
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This shouldnt be too hard.
Once you see the light go on, dont drive the lift anymore. We plan to use a simple ratchet to prevent it from backdriving. |
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If not, what keeps you from effectively clamping onto the field when waiting the 1 second? |
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Hope your knot holds. |
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That's probably a decent amount of force (at least 300 lbs) |
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Now imagine everything in the system is a spring, you're going to compress/stretch parts of the robot/field and load everything up. Now there is a ratchet on there preventing you from backdriving (and you can't go forward any more as you're already loaded pretty firm into the field). Now that's how you lock yourself onto the field. |
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The current measuring method works very well. We've used it in other instances in the past and plan on using it this year for the climber mechanism.
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Imagine trying to release a ratchet strap that has been ratcheted to significant tension. If you try to pull the release with your fingers without taking up some of the load with the handle, you will never get it undone. You have to release tension off the release bar in order to move it. This is exactly what will happen in the scenario outlined above. |
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It is legal to remove the rope from the field without removing the robot from the rope. So couldn't you have two team members support the robot, while another team member releases the rope from the davit?
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I do recall your 2010 robot and didnt realize you had those issues also. |
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Or we can also do the other suggestion earlier with the current draw. Lots of time left to figure this out since our climber already made and it works. |
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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.) |
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Wait... if you're using a ratchet wrench, could you not just reverse the direction the wrench is ratcheting?
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If you use a ratchet wrench you could clamp the non-ratchet end with a bicycle quick-release lever that can slide in a slot to drop below the wrench when the lever is opened.
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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* |
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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.
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For those who are interested, this is the wrench in question: https://www.amazon.com/TEKTON-WRN570...words=WRN57010 |
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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. |
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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. |
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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.
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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.
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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. |
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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.
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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. |
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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.
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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? |
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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.
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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. |
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