ScissorsLift vs. DoubleJointed Arm.

[Joe_Widen]

What would you say the Pros and Cons are for both. Also, please vote in the poll. And can you post picture, or links to pictures of past double jointed arms or scissor lifts. Thank you.

[DonRotolo]

Poll?

Scissors Lift suffers fron robustness problems, kinda floppy and each joint is critical. DJ Arm is often heavier and technically more challenging, but stable, strong and potentially more robust

Don

[eshteyn]

Both can be used for their own purposes. However i like the scissor lift better because if you build it to support all 4 corners it will be a lot more stable and safer. the most important part is to have cross bars and the joints must be made very well

[Lil' Lavery]

Scissor lifts are (typically) more difficult to manufacture, more complex, heavier, and often slower. Additionally, they cannot articulate as accurately, or create any horizontal displacement or manipulation.

[Rick TYler]

It's a lot harder to build a good scissors lift than an arm. I assume the OP was talking about a device to lift tubes, right? I wouldn't use a scissors lift unless I was picking up something heavy. You could build a pretty light arm for the tubes, although it's tempting to build a heavier one that could right the robot if it tips.

[kjohnson]

From an engineering standpoint (even though I'm not an engineer by any means) a scissor lift is not meant to work without springs and/or pneumatics. To build a scissor lift that is robust enough to not sway would take considerable weight, and you may not have enough weight left over for a full pneumatics system. Also, I believe springs are still outlawed in the rules.

I guess this makes a double jointed arm your best choice.

[Guru]

Originally Posted by nukemknight

Also, I believe springs are still outlawed in the rules.

I believe energy storage through springs is perfectly legal, although i cant find the rule right at this moment.

[Cody Carey]

For the application of lifting a robot (or two) 12 inches, none of the sway problems stated above would really apply to the situation, and since you would only need a single joint, neither are the "every joint being critical" arguments. The single joint would allow you to Brace the lift in multiple places.(Shown by red circles in the below picture)

Now, in addition to keeping the scissor lift from swaying, the braces would also keep it from closing all the way, as is pictured below... but that is still adequate for the amount of space we have available to us.

A scissor lift could be a wonderful Idea for this years competition, It just depends which application you are using it for

For a scoring arm, however this would be an entirely different story...

[DonRotolo]

Originally Posted by Cody C

For the application of lifting a robot (or two) 12 inches, none of the sway problems stated above would really apply to the situation,


For a scoring arm, however this would be an entirely different story...

I think the choice of arm or lift was meant for a scoring system, not a robot lift, but I admit it's completely ambiguous given this year's game... Ain't it fun?

Don

[ChrisH]

Either approach can be made to work but there are a few things you need to understand.

Scissor Lift
These require tight joints that only move the way intended. Sideways slop in your pivot joints will make the structure completely unmanageable. The members of the lift must also be pretty stiff, they cannot deflect much under load. Again deflections can make the lift unmanageable.

Another thing to consider is that the lower the scissors lift gets, the more force is required to raise it. The force required can be several times the weight of the lift and the object you are lifting combined. That is why springs connecting the joints help so much, they apply force in the direction needed.

However you only have one thing to control and it should be fairly easy to get precise positioning if you use a ball screw or something similar to activate the lift. This can be a real advantage if you are trying to acheive a precise height.

Double Jointed Arm
The double jointed arm has many fewer parts. This will make it structurally much easier to build than a scissors. But it will be much more difficult to position precisely. This is because a small variation in the angle of the arm at the pivot will result in a large displacement at the end of the arm. Now add the uncertainty of the second joint to that. You will get a fairly large uncertainty in your vertical position. You will also have a similar uncertainty in your horizontal position

You might have trouble resolving the joint angle sufficiently acurately to achieve precise position control. Then you will have to rely on your arm driver to compensate.

The higest stress on the arm will be when it is fully extended horizontally. Be sure that the arm will move upwards from this position while carrying a payload. Your operator may know that he or she should not exceed a certain limit to prevent arm failure. But I can guarantee that sooner or later they will make a mistake and the dreaded full extension will occur.

[KarenH]

Originally Posted by Guru

I believe energy storage through springs is perfectly legal, although i cant find the rule right at this moment.

Robot Rules, 8.3.1 General Design & Safety Rules
Under <R02> (about energy used on the robots--one of 4 permitted energy sources):
"Storage achieved by deformation of ROBOT parts. Teams must be very careful when incorporating springs or other items to store energy on their ROBOT by means of part or material deformation. A ROBOT may be rejected at inspection if, in the judgment of the inspector, such items are unsafe."

In summary, SAFE springs are legal. The definition of a spring seems pretty broad. I believe a spring is safe if it is:
1) Not likely to break
2) Well secured at both ends
3) A suitable strength for the application.

Humorous illustration: There was a fiction piece in the December 2006 Boys Life magazine about a boy who designed a revolving Christmas tree using a wind-up mechanism powered by an inner tube. He did not have adequate restraints on his device, as he learned when he released the catch...

[Heretic121]

no talk of elavators!?! =O

[DB_UPS]

Originally Posted by eshteyn

Both can be used for their own purposes. However i like the scissor lift better because if you build it to support all 4 corners it will be a lot more stable and safer. the most important part is to have cross bars and the joints must be made very well

Thank you, thats is the same points i brought up.

[Donut]

Uh, neither? Our team has had bad experiences with both; a scissor lift in 2002 had trouble supporting any kind of weight at all (similarly, team 898 from next door to us tried it in 2004 and had to toss it when it couldn't support weight), and a double jointed arm in 2003 could not even support its own weight and destroyed itself (part of this was due to a miscalculation in the strain on sprockets, but regardless it still wouldn't have gone well).

We're convinced the way to go is a combination elevator/arm like we used in 2005; we've already decided we'll be doing it for this year's bot, it's just a question of exactly how they are given power to move that is up now.

[Joe_Widen]

My team and I were having a debate, and for some reason there was a red flag shooting up in my mind whenever I hear scissor lift. I'm just trying to figure out what the red flag is....?