Pushing the Bridge down
 

Hey guys,

Our team is going with a mecanum drive train that only goes over the bridge. However, when we were looking at mechanisms to push the the bridge down, we ran into a few issues. First off, it's been calculated that we need about 16-18 pounds of force to push the bridge down. We thought about using a servo-arm to do the job similar to this:
http://www.youtube.com/watch?v=HfLjg...eature=related

Does anyone have any suggestions on how to go about this or which servos to use?

Thanks for the help!

Re: Pushing the Bridge down
 

Use a motor, like a window motor or van door motor. Servo torque is measured in oz-in; motor in lb-ft. If you need lb out away from the robot, don't use a servo.

Re: Pushing the Bridge down
 

You might be able to use a window motor or the AM planetary gearmotor, you can do a quick check on the torque available using the motor data sheet linked at the bottom of this page

http://www.usfirst.org/roboticsprogr...rts-electrical

and then add a potentiometer to make a feedback loop, and with some programming you can make your own heavy duty servo.

Re: Pushing the Bridge down
 

Thanks! Is it safe to assume many teams are prototyping similar arm mechanisms...

Re: Pushing the Bridge down
 

I think so. We're planning on using pneumatics since we probably will have a pneumatic part in our ball handling stuff, but we haven't done more than talk about it yet.

Re: Pushing the Bridge down
 

How do you plan on compensating for the fact that you will be driving toward the bridge with the pneumatic extended? Our team had a very similar idea and the general consensus was that a wheel placed at the end would be sufficient but I am having trouble believing that.

Re: Pushing the Bridge down
 

We haven't designed it yet...but I expect it will be an arm, and the pneumatic activator might have an overcenter design that will "lock" the arm in the extended position, without putting much load on the cylinder

Re: Pushing the Bridge down
 

You could always have free rolling treads leading up to your mechnem wheels that will ramp you up the barrier.

Re: Pushing the Bridge down
 

You are correct. if you use an arm to push the bridge down, then the wheel would aid in the fact it is sliding on the surface, but the other problem is the length of the arm needed will change as you drive forward. you either need to rotate the arm back up, or retract it along it's length as you drive forward. If you don't, the front drive wheels lift off the ground and/or your arm breaks.

Re: Pushing the Bridge down
 

Use a wheel-tipped swing arm with an adjustable torque limiting friction disk slip sprocket to drive the arm. As you drive up the bridge, you can keep applying torque but the arm can still bend up for the angle rise of the bridge from the slip of the torque limiting clutch-sprocket.
Not so good for a tilt fight at center bridge in autonomous though, to see which way the balls there will get tilted
-RRLedford

Re: Pushing the Bridge down
 

..Or use a motor that can backdrive, and backdrive it back up as you approach the ramp?

Re: Pushing the Bridge down
 

I'd imagine this would be the most common way teams will get the ramp down.

Re: Pushing the Bridge down
 

A 3/4" cylinder will have more than enough power to push the bridge down but not enough to lift your robot wheels off of the ground. It will retract as you climb the ramp.

Re: Pushing the Bridge down
 

Unless you use a clever linkage.

Look at the legs on a folding table.

Re: Pushing the Bridge down
 

But you want it to retract, why would you want it to lock?

Re: Pushing the Bridge down
 

My team has been prototyping and I am working on a final CAD for a non ramp solution that forces the bridge down using a combination of passive power from the drive-train, surgical tubing, and a single window motor. We looked at other methods and decided that instead of figuring out a method of generating tons of torque on an arm (both heavy and unreliable depending on implementation) we would instead focus on using our drive-train to push the ramp down, I would definitely recommend something similar.

Re: Pushing the Bridge down
 

It does't need to lock to use that linkage principle..

Re: Pushing the Bridge down
 

Why is it that we always have similar ideas on one item each year?

Re: Pushing the Bridge down
 

Great minds think alike.
ps I believe we are thinking of something similar as well.