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Quote:

Originally Posted by Gary Dillard

You would think so, but unfortunately I believe that's not the case with this particular worm drive gear; I seem to remember using it for a lift one time, only to find out it would backdrive when the power was cut - it had an oddball pitch angle so that it could be backdriven (a safety feature since it's used for closing van doors). I know they've changed the particular van door motor we get in the kit since a few years back - can anybody confirm if this motor can be backdriven?

I can tell you that the van door motor will back drive. Plus you need to gear this motor better than you have it.

Here is a picture of us hanging
http://www.valleytech.k12.ma.us/robo...s/roboti87.jpg

That motor was being gear for a 24t to 48t. So if you think that buy just putting a drum on it would work let me save you some time.

The tyco motors into a gear box would be the best bet for this. 15,000 rpm reduced 200:1 would give you a good starting motor and if you put two of them together you would have a fast strong winch. you could say you could climb up at a rate of 6 pi = around 18" per second. which is 1 1/2 foot per second within 6 seconds you will be at 9 feet.

Make sure your winch can pull you and one other robot. by using the pass competitions someone might be holding you down

The van door motors Can be backdriven it is just verv difficult

how difficult? will it support the 130 lbs when the power is cut? That's really all I need to know..... :)

The van door motor can be backdriven rather easily. I was able to do it with my bare hands, I doubt it will be able to hold up a 130lb robot. Another thing to look into is an electronic brake. I'm not completely familiar with the electronics, but I'm pretty sure the speed controllers have a jumper on them that will enable a "brake" of sorts. It electronically prevents the motor from being back-driven. I can't say if it will be able to hold 130lbs, but the two combined (van door motor + electronic brake) might be able to do the job.

[edit] The electronic brake remains enabled as long as the robot has power, so it will work after the robot is disabled at the end of the match.

a pneumatic arm w/ winch attached to that could be an idea....

the Van door motor and the speed control brake will not hold 130lbs This has been tested many times.

There is a easy fixs to all that and it is not hard to do.

Quote:

Originally Posted by Mike Norton

the Van door motor and the speed control brake will not hold 130lbs This has been tested many times.

There is a easy fixs to all that and it is not hard to do.

We've established that. If anyone actually wants to discuss how to build a winch, continue. If you're just going to post about how "the Van Door motors won't hold a robot" like the last 8 or so posters have, please don't, or I'll lock this thread.

C'mon Jeff - discussing how to design / build a winch includes discussing how NOT to do it, especially if it's backed up by experience. If you want to know how a winch works, got to howstuffworks.com; if you're looking for more details on designing and building one within the constraints of the FIRST competition (including which motors to use) you look in these threads. I made the point that the van door might not be the best choice but I didn't have any recent experience or solid memory, so some people chimed in who did. A forum is just that - a forum for people to share ideas and experience. I hardly think the last few replies were beating a dead horse.

If you want a way to use a winch try this

Take your drivetrain and put 2 motors per side to run it. When you are done moving use the same power to lift your robot.

A nice fast strong lifter

Have fun

Quote:

Originally Posted by Gary Dillard

Gary, I very much agree that how NOT to build something is a good element of discussion. I did, however, feel it was starting to get off track, because people were starting with the "chiming in" replies that we're trying to prevent. It was intended to just be a friendly reminder to keep the discussion moving, so I don't have to lock a good thread because 10 more people decide they need to mention that a van door motor alone can't hold a robot. Anyways.... it's moving along again, so carry on.

Quote:

Originally Posted by Mike Norton

If you want a way to use a winch try this
Take your drivetrain and put 2 motors per side to run it. When you are done moving use the same power to lift your robot.

This doesn't quite make sense to me... are you saying power the winch via the same motors that are driving the main wheels?

Tytus made a good point that this is not the place to discuss methodology of attaching a winch to a surface. However, note: The bar is 9' from the platform and probably 5' from the average robot's winch "launch" point.
Vmax = 10 f/s
or
Vmax = 5 f / .5 s

I don't think it will be too much of a problem to "launch" a latch by a vertical means in over .5 seconds. Yes, I understand that a vertical launch is different from an arc'd launch, but you get the idea. 10 f/s is pretty darn fast.

Quote:

Originally Posted by Gadget470

I would not launch anything, I would use a arm to deploy the cable. then bring the arm back in.

But back to the motors. You can tie in the drive motors to run the winch. this way you save on motors and get to have a very powerful winch. This can be done in many ways. I was just showing poeple ideas on what motors they could use to pull there robot up and anybody elses robot that wants a free ride up.

Less motors move power you can't go wrong.

Quote:

Originally Posted by SarahB

Actually I think he's using the van door motor. Notice the gearbox at the end. This is better than using the CIM because the van door motor uses a worm gear, which means it cannot be backdriven(at least theoretically) so you don't have to worry too much about it falling after power is cut at the end of the match.

The van door can lift a robot .... we've actually built a "winch" that can pull up a 130 lb robot...and trust me it CAN be backdriven!

Sounds like time for a little Physics lesson.

Power = work/time = torque*rotational speed = volts*amps

work = force*distance

We know the force we want, 130lbs minimum. If you're worried about another robot climbing on top or otherwise holding you down then double it.

How far do you want to go? if lifting from the floor you need to go at least a foot. To be safe I'd say two feet. So the work that needs to be done is 260 ft/lbs or 351 joules. Your robot is so feature packed that the only motor available is the lowly Globe. According to Dr Joe, the Globe puts out a maximum of 50 watts with the gear box.

To find out the minimum time it will take, we rearange the power equation above:

time = work/power or 351 J/50 W, 1 W = 1 J/s

so the time to lift 2' at maximum power is approximately 7 seconds. That is for 2 feet, one foot will take half that and more than do the job in most cases and you decide that is acceptable. The next step is to decide how big you winch drum needs to be.

Dr Joe also lists the stall torque for the globe at 19 Newton Meters. A weight of 130 lbs corresponds to 578 N. Now because of the way electric motors work, you probably don't want to run at anywhere near stall. I like to design things to run at 1/4 stall torque for a number of reasons I won't go into. To make things easy let's assume 4.75 Nm as the usable torque from the Globe.

The maximum drum radius (in meters) is then simply the usable torque divided by the force we want in Newtons. In this case 4.75 Nm/ 578 N = 0.0082m or 8.2 mm. A little further conversion gives a drum diameter of 0.647 inches. To get it to a standard size let's use 5/8" or 0.625". Since the drum is a little smaller we get a little more force which gives some margin. 0.625" = 0.0158 m and the force available is 600 N

Now to back check. At 1/4 stall torque the poor little Globe is going to be running at about 75rpm or 1.25 rev/sec. In 7 seconds it will go 8.75 revolutions. Each revolution pulls in pi*0.625" of drum so the amount of cable pulled in is

L = 8.75*pi*0.625" = 17.2"=0.44m

Checking the current draw:

600 N * 0.44 m/ 7s = 38 W

Amps = power/volts = 38 W/ 12 v = 3.2 A

Not quite the 2' we wanted, but we did reduce the shaft diameter somewhat which will reduce the speed at which the cable comes in. Since we are operating far below stall torque, there is probably plenty of excess to ensure this actually works. Also, the motor will be operating well below its maximum output. The current required is pretty low as well which will keep your electrical guys happy.

You could repeat the calculations with whatever motor your heart desires and the only things that will change are the time to lift and the drum size. It doesn't matter how much torque you have, just how long you're willing to take to get there. I picked the Globe to be a little on the rediculous side, but looking at the numbers, I might have to go redesign our winch. It'll probably save us a couple of pounds. Of course there's the cost of a few extra seconds, the strategy guys will have to figure out if we can afford that....

BTW whether a worm drive can be backdriven or not depends on the pitch angle of the worm. If it is >5deg then it will backdrive. Unfortunately this is the case with the van door motor, as I learned through hard experience.

This is a really cheap design, but I made this in about 5 minutes in 3D studio max, just a real basic design, when I have time, i might make it more detailed.