Winch problem

[16flippinb]

Hello all, we are having a problem with our winch not holding tension. When we stop pulling the boxes off of the ground, and reach the desired height, the winch begins to unwind very slowly. The more totes and cans we have, the faster it unwinds, and vice versa. We are using a CIM motor to a Vexpro base versaplanetary gearbox with a 1/2" HEX output. The gearbox is attached to a flywheel that winds and unwinds the winch. Is this a problem with our programming, the gearbox, or something else? Any advice would help! Thanks -Team 5216

[Jon Stratis]

When you tell a motor to stop, all it does is supply 0V across the leads. This means that the motor can backdrive, which is what you're seeing. You have two options... Tell the motor to go "up" very slowly in order to hold its position, or implement some sort of mechanical brake. Keep in mind, stalling a motor (telling it to move while not letting it move) will cause heat build up. Depending on the situation, it's possible to burn out a motor by stalling it too long.

[DarkRune]

^This guy beat me to it, but yeah sounds like the motor is back driving, that can be fixed with a small upwards rotation, or with a clamping break like on a bike. What's the gear ratio in the versaplanetary?

[16flippinb]

the gear ratio is 70:1

[Anthony Galea]

When you tell a motor to stop, all it does is supply 0V across the leads. This means that the motor can backdrive, which is what you're seeing. You have two options... Tell the motor to go "up" very slowly in order to hold its position, or implement some sort of mechanical brake. Keep in mind, stalling a motor (telling it to move while not letting it move) will cause heat build up. Depending on the situation, it's possible to burn out a motor by stalling it too long.

Would another option be to have the motor move forward then backward in small, short, yet equal amounts?

[Mike Marandola]

Would another option be to have the motor move forward then backward in small, short, yet equal amounts?

Depending on the weight and friction in the system, the amounts may not be able to be equal. It might just drop slower.


For OP, here is some more information about mechanical brakes:
http://www.chiefdelphi.com/forums/showthread.php?t=133272
http://www.chiefdelphi.com/forums/showthread.php?t=132825
http://www.chiefdelphi.com/forums/showthread.php?t=132158

[Jon Stratis]

Would another option be to have the motor move forward then backward in small, short, yet equal amounts?

You can pulse the motor like that, yes. Depending on the deisgn and what you need it to do, such a motion may not be practical. Implementing a mechanical brake is a much better Design, IMO.

[jimwick]

Do you have your motor controller set to "brake" instead of "coast?" That can help some.

We have the same problem and are planning to give the CIM a little voltage to hold the system in place. That will eventually burn out the CIM, but they are pretty rugged and will probably survive. We plan to bring some extra CIMS!

[Chris is me]

Would another option be to have the motor move forward then backward in small, short, yet equal amounts?

It wouldn't be much better, really, especially for CIM style motors.

You can safely stall a CIM style motor at a fairly low power percentage for a decent amount of time. I wouldn't worry too much about it. Just add the feed-forward.

[Gregor]

Attached is an image of our mechanical lock (disk brake) to alleviate this issue.

[Chinske4296]

Dry an anti back drive system, via ratchets or some other means. Last year we used a ball shifter, instead of a second stage we took the gears out acting as a neutral gear. I see no reason you couldn't use a similar function with a ratchet.

[Chinske4296]

We had a ratchet on the shaft allowing the ratchet to turn with out moving the shafts connected to the motor, thus preventing backdrive.

[rookierunner]

Although the software fix described above will work and cims are very durable... there is another non-software option. As worm gear will not allow the motor to back drive. I wouldn't recommend a window motor for the task, but they are an example of a gearbox with a worm gear.
Good luck to you this season.

[richwong]

Attached is an image of our mechanical lock (disk brake) to alleviate this issue.

What do you have "pulling" the brake cable?

[GortGortGort]

One solution not mentioned is the use of a two drum winch design with one cable from one drum pulls up while the other cable pulls down .

A spring or latex tubing attached to your lift with continuous upward force properly adjusted to your lifting weight will work fine to control motor/ gearbox back-drive. It really makes your winch work to pull the lifting arm down first
and less force to pull weight up.

http://info.vulcanspring.com/first-robotics-competition

Check out voucher from Vulcan Spring . You could add this quickly to your bots
lift to stop the back-drive with out changing your design as long as your winch
is not relying on a gravity return lift arm . Are you using a single cable drum ? Photo of your winch might be useful for us to help you solve the problem.:]

http://www.chiefdelphi.com/media/photos/41279?

[Neima]

What do you have "pulling" the brake cable?

My team is also using a disc break. We are engaging it with a window motor. I have also seen a few teams use pneumatics to engage it.

[Aur0r4]

Are there any good worm gear transmissions for CIM's? That solves the problem right there.

We built two worm gear transmissions for CIM motors last year. They are pretty simple and as long as your machinists can hold reasonable tolerances they work very well. Far simpler and a bit less fussy than planetary or even spur gear transmissions.

[Lil' Lavery]

the gear ratio is 70:1

In the Versa Planetary, or is this additional reduction afterwards? 70:1 is above the recommended reduction for using a CIM motor with Versa Planetary gearboxes, per their user guide.

[Gregor]

What do you have "pulling" the brake cable?

Sorry, didn't see this until now.

See the attached image.

[JamesBrown]

Are there any good worm gear transmissions for CIM's? That solves the problem right there.

We built two worm gear transmissions for CIM motors last year. They are pretty simple and as long as your machinists can hold reasonable tolerances they work very well. Far simpler and a bit less fussy than planetary or even spur gear transmissions.

I suppose good is open to interpretation, but we are using one of these http://www.harborfreight.com/2000-lb-capacity-geared-winch-5798.html lovejoy style flexible coupling between a cim and the worm gear.