Pushing the Bridge down

[Arjun]

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!

[EricH]

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.

[MrForbes]

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.

[Arjun]

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

[MrForbes]

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.

[eddie12390]

Originally Posted by squirrel

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.

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.

[MrForbes]

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

[commonsense]

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

[lcoreyl]

Originally Posted by eddie12390

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.

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.

[RRLedford]

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

[apalrd]

Originally Posted by RRLedford

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

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

[waialua359]

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

[IndySam]

Originally Posted by eddie12390

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.

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.

[MrForbes]

Unless you use a clever linkage.

Look at the legs on a folding table.

[IndySam]

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