Scissort Lift vs. Crane vs. 4-Bar Lift vs. Gas Springs

[AdamC]

Greetings fellow mech-maniacs.
Our team is in somewhat of a pickle this year, since we can't quite agree on what would work best. I've presented the concept of a 4-bar lift (used in some heavy industrial forklifts), which would be capable of lifting the ball to overpass level via worm-gear or lead-screw, and then releasing it to complete the hurdle.
We are also debating the use of a scissors-lift, although I am not very fond of them because of the large amounts of force necessary to lift them from compacted positions, and starting them from semi-open positions means they are taller.
We were thinking about using an angled crane which sloped back over the robot, potentially mounted on a scissors lift to carry it the extra few feet to the overpass, but I think that might be rather complex, and we've had bad experiences with complex machinery.
Finally, we were thinking of a combination of a 4-bar lift and a gas-spring. The lift would center the ball over a motor-wound gas-spring, which would then fire and kick the ball over. I'm skeptical about it, because our '06 tests showed that pneumatics and the like were barely capable of firing a poof-ball, let alone these new trackballs.
Any comments to help settle the predicament would be VERY welcome!

[MrForbes]

Can you build some quick mockups to test some of the different ideas?

[Hiteak]

I would not recommend doing a scissor lift. We did that for last year and it was really slow.

[dtengineering]

Not all scissors lifts need be slow. Team 639 built a scissors lift that could extend to grab "the bar" in about one second back in 2004. http://www.team639.org/index.php?page=r2004

What was more impressive was that it did this from a fully collapsed state. Anyone from 639 care to give some specs on how you did it?

Jason

[hipsterjr]

I have been curious as to exactly how much force it would take to launch a ball 10 ft at a 60 degree angle. ( for about 7ft high and between 5 and 7 ft horizontal from overpass.) I think arms and lifts are going to be too slow.

[NoSkaOnTheRadio]

The biggest thing I can advise you keep an eye on is where your designs put your bot's CG... too high and you're in danger of tipping, and unless your manipulator is also able to reorient your bot, your match will be a disaster. The inner tubes from last year weren't all that heavy, but the trackballs this year are pretty big; the difference between the CG of an unladen robot with its manipulator at full vertical extension and the CG ofa laden one in the same configuration is probably going to be substantial.

Anyway, your team has probably already thought of most of what I'm about to say, but just in case...

The scissor lift is good if you want your bot to have a low profile without a trackball; you can get a lot of vertical height out of an extended scissor lift for relatively little height in its starting configuration. The main drawbacks are power and speed, though. It takes a lot of power to get the lift going, and you'll probably have to gear down a motor (maybe a worm gear or something similarly drastic) or use a lead screw, both of which sacrifice a lot of speed. It's also a pretty tough task to build a robust scissor lift that is also mass-efficient.

The four bar linkage sounds viable, but depending on where it sits on your bot it could wreak havoc on its balance. Hold the trackball too far out in front of the bot, and you're all but asking it to fall forward. On the plus side, this sounds much easier to implement than the scissor lift.

The crane seems like a good idea balance-wise, but it would need to be robust. Team 1757 used a crane-like system last year with a claw on the end; we'd definitely be able to lift an inner tube over the overpass, but I don't think it's tough enough to lift a trackball.

My last word of advice is not to combine lifters! Not only does that fly completely in the face of K.I.S.S., I would bet you'd see a significant rise in man-hours required both for design and fabrication, and then for integration. It could certainly be done, but I think you'd need to be pretty confident...

[NoSkaOnTheRadio]

I forgot to mention the gas spring idea, but that's because it's beyond my ability to even think about designing something like that. I can say that one of the teams at the 2006 Boston Regional had a pneumatic poof ball launcher that could score from about half court, so I feel like you could get some pop out of one.

[dtengineering]

Originally Posted by hipsterjr

I have been curious as to exactly how much force it would take to launch a ball 10 ft at a 60 degree angle. ( for about 7ft high and between 5 and 7 ft horizontal from overpass.) I think arms and lifts are going to be too slow.

Take a look in your physics textbook for kinematics formulas. The math to solve these types of problems is quite simple and is a common part of the Physics 11 curriculum here in BC. My estimates are that to launch the ball six and a half feet (roughly 2m) requires an accelleration of 2g over a distance of 1m.

In other words if you have 20lbs of force behind the ball for about 40" you should be able to get it to go 6 1/2' straight up from the last point you contacted it. To account for your desired 60 degree angle, take a look at breaking a vector down into orthogonal vectors (ie, how much "up" and how much "over" you will need). These are actually quite useful formula for many types of questions and are not that difficult to use and understand.


Jason

[Cory]

Never use scissors lifts is a cardinal rule on our team.

[thefro526]

we used a scissor lift in our 04 year that was plenty fast but it wasn't strong

[Peter Matteson]

Originally Posted by Cory

Never use scissors lifts is a cardinal rule on our team.

For our team too.

[Dan Richardson]

4 Bars With Gas Springs have been used before, and quite effectively. However I feel like not in the way your describing.

http://www.chiefdelphi.com/media/photos/26796

http://www.chiefdelphi.com/media/photos/21477

http://www.chiefdelphi.com/media/photos/27982

Thats 1902, 1369, 217 in 2005, 2056 in 2007 just a few examples that I can think of.

[Cartwright]

I'm interested why those are your four options. Why not some sort of grabber? A scissor lift is quite difficult and a crane seems quite unwieldy. I don't know...we've not decided on our design, but we haven't really considered lift systems much.

[Greg Needel]

Originally Posted by Peter Matteson

For ours team too.

I agree, never use a scissors lift.

[seanl]

Originally Posted by Greg Needel

I agree, never use a scissors lift.

we had to learn that the hard way although it was extremely powerful, enough to lift a 150lb person. but it was slow and heavy..
you can look at my avatar thats the bot, it does look cool though.