Good afternoon, I am currently working on the climber for our team and apparently, once the match ends, the power will be cut from our robots and the robots who climb will need a way to keep hanging in the air for 5 seconds. Unfortunately, we don’t really know how we would go about doing that right now. We are planning to use the thrifty telescoping tube as our climbing mechanism. Thanks in advance.
I’d start with putting the motor in brake mode and see if that’s enough to hold it up
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The WCP friction brake can do this as well so take a look to see if it suits your purpose
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Are you expecting to raise then lower your climbing apparatus one cycle, or do you intend to go higher than the mid rung?
If you’re reaching for the mid rung, you could have a ratchet in your gearbox. Start up with the climber winch wound completely the other way, then as you turn it, the line will be let out and the apparatus will extend. Keep going in the same direction and it will pull back in.
This is a relatively easy solution to implement - it only costs one ratcheting wrench. The drawback is it does not allow for multiple extensions/retractions within the course of a single match.
In 2020 we had a similar issue. Easy to implement solution was to add a simple mechanical, spring loaded latch that grabs the telescope once it is fully contracted.
Downside is there is no way to extend again once engaged.
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Play match almost contracted but about 1" short
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extend
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full contract to pull-up to bar and latch clicks in place
Think of the latches on extension ladder
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This is what we used on our Thrifty Bot climber a pancake piston and a machined out 1/4 inch 6061 aluminum slider to engage the shaft. The square hole is used to place a nut from threaded of to the piston to attach the mechanism. We custom 3d Printed the bracket to attach the bearing plate. Warning though don’t engage until you stop climbing to avoid damaging the aluminum brake. It worked very well for us at this weekends competition. The nice thing is that it is light weight and is easy to engage and disengage at any height as desired this eliminated a one way only ratchet mechanism and allowed us to make adjustments as needed to the climber height.
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there’s many things you can try:
put the motor controller in brake mode (i prefer to do this in the code itself rather than phoenix tuner that way its done every time your robot enables)
try a higher gear ratio (a higher ratio will require more force to backdrive)
use a friction brake like mentioned above on your winch shaft
use a ratchet, ratcheting wrenches work great for this but vex also sells them to add into a versaplanetary (this one is risky as you dont get any second chances climbing, if you go too far and start driving your arm downwards before grabbing the bar you’ve missed your chance. hence you would want software control to have the arm extend to max height then stop before going back down)
to piggy bak on the ratchet you could make the ratchet actuatable by a small piston or otherwise (team 319 did this on their elevator winch in 2018 to hold position without having to stall out their 775’s, I’m sure there are other examples this is just what came to mind first)
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To clarify power isn’t cut to your robot, it’s simply disabled. This means that motor controllers set to brake mode will still be engaged. I highly recommend trying brake mode before using any of the more complex solutions suggested in this thread
We used this all year and had a consistent climb. You can see that we would drift down after the match, but we were well above the carpet after the 5 seconds elapsed. I believe it took about 18 seconds to completely sink to the floor.
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For an anecdotal data point (since backdriving a climber seems to be a bit of black magic), can you please share your setup? What ratio are you running, and which motors?
Yes the gear ratio and motor(s) used in very important in determining the effectiveness of brake mode. The amount of negative torque generated by a motor depends on its rpm. As rpm approaches zero so does the braking force. The motor of course plays a part since a higher power motor has the potential for greater braking effect than a lower power motor.
This is what we do for our climb as well.
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Two Falcons on two WCP GreyT Telescopes. We had problems with the 8T falcon pinion devouring the 54 tooth aluminum gear and binding on the 1st stage so we swapped it out.
Our final setup per telescope was:
11T falcon pinion : 52T aluminum gear
28T aluminum gear : 44T aluminum gear
For a final reduction of 7.4:1 driving a 7/8’’ OD roller and 1/8’’ rope
We were pretty light (93lb at inspection) and climbed lightning fast with this setup. Keep in mind we were lifting ourselves with TWO gearboxes.
I would recommend anyone trying to reproduce this setup to use a more aggressive reduction (12:1 or 15:1 per telescope) as we pulled quite a bit current and the milliseconds we saved weren’t worth it. Even so, the back drive was plenty slow.
Thanks for sharing.
The brushless motors allowed you to get away with minimal backdrive with an agressive ratio.
For anyone wondering, on a 3/4" Spool, with an 81:1 reduction on two 775 pros we drop ~2" in 5 seconds.
Hopefully together we can help teams come up with a “good enough for low backdrive solution.”
We use a Servo on a versa ratchet slice.
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We have a modified WCP dog shifter shaft with a pancake cylinder that pushes the dog into a plate with the dog pattern machined into it. Fun little gearbox.
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That’s not the case – robots are disabled; power is NOT cut. The distinction is important. Our robot’s winch will backdrive pretty quickly if the power is cut, but not if it’s just disabled (as long as it’s in ‘brake’ mode before being disabled.)
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Anything that jams your gears controlled with a servo would do the work.
We are using a window motor. It has a built in pin preventing rotations when not powered.
Kind of slow motor but it has the torque and hold without power…
I posted this same post on another thread this morning, but seems that it fits here as well:
Depending on how your climber is driven and geared, running the motors in brake mode can be the easiest and best solution. Your bot slowly lowers, yes, but if done correctly, you can still easily be above the ground at the 5 second mark.
The idea behind brake mode is that the leads of the motors are shorted, and as the robot lowers, it runs the motor in reverse as a generator. You can calculate the rate of falling from the basic power equation:
mgv = EI = E^2/R
where m = mass, g = gravitational field, v = falling speed, E = emf of motor, I = generated current and R = motor resistance.
The emf generated by the motor is proportional to the rotor speed and can be estimated to a very close approximation by realizing that 12 V is generated at the 12 V free speed. The motor resistance can be found by dividing 12 V by the 12 V stall current.
Doing all the math gives the falling speed (I use all MKS units):
v = (mgr^2R)/(G^2k^2n)
where r = radius of drive (winch drum, etc.), G = gear ratio, k = 12 V/motor free speed, n = number of motors
We drove our climber with 2 Falcons on a 40:1 reduction with a 3" diameter drum:
R = 0.044 ohms
k = 0.018 volt seconds/radian
calculated falling speed = 3.4 cm/s
So as long as we lift more than, say, 20 cm off the carpet (7 or 8 inches), we are good. Note that the above assumes no friction, so the calculated value is very conservative. Falling speeds in reality can be slower than the calculated value by quite a bit.
So we just try to make sure that we gear and climb so that brake mode works. We find that easier than a mechanical solution.
We have used pancake cylinders against gear teeth to great effect many times, but you’ve got to make sure to trigger the brake before the robot is disabled.
A single solenoid plumbed right can engage the cylinder by default. When the robot becomes disabled the brake would engage.