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.
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.
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.
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.
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.)
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.
Will it? Disabling robots doesn’t change solenoid states, so a spring-out air-retract cylinder would stay retracted.
Unless I’m missing something. We’ve never actually used single solenoids on our robots, always double.
Our climber is using a NEO 1650 on a 36:1 gear box. Once power is cut, we just let it drift down. Takes about 18 seconds to hit the floor.
A single solenoid uses power to shift from one valve state to the other. When power is cut (or robot disabled in this case) the valve shifts automatically to the default state. If you plumbed your cylinder correctly, when the robot disabled, the valve would return to the default state and the cylinder would engage the brake.
Single solenoids can be used with air out/air in cylinders.
I had to help another team this weekend that was climbing on two 775s, and were drifting down each match. They had no ability to add functionality like a brake or ratchet, but there was one thing that helped.
I fully admit it’s a bad solution, but I had them tighten down their bearing blocks to add friction to the system, and it worked.
Since my team is using a climber with un-actuated hooks to hold higher bars, we hook on with the un-actuated hooks to hold weight when disabled. It does take a bit longer, and we still have physical stops that won’t let us fall all the way if we don’t get them latched (per question 134 in the q&a, if you extend past the height limit after the match, no penalty is assessed, so a physical stop on your telescoping arms is a very viable solution. The un-actuated hooks solution works on the mid bar; the telescoping arm solution works on higher bars.
Neat! Good to know, thanks!
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