ScissorsLift vs. DoubleJointed Arm.

[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

[rees2001]

Originally Posted by Cody C

These pictures are for you ... They are all "effective" scissor lifts. All that is needed to reveal a wealth of effective Ideas is a look into the past.
Some of what has been being said in this thread is nothing but a bunch of rash generalizations. If you look into Industry... you will see numerous instances of scissor lifts being used, because they are good at what they do. some mechanisms are better than others at certain tasks, It all depends on the application.
You can't just say: "Scissor lifts aren't good because they haven't been made to work yet. They will always be inferior. " It doesn't work that way.

Taken from HERE

Taken From HERE

Funny you chose this picture... I was going to post how our team used a scissor lift in our rookie year and we found it easy to build and easy to use. It was fairly light & worked well enough. We connected across the joints with surgical tubing to give it that "spring" you talk about. The lift is still together but not attached to the base. Arms can be a challange.

[GaryVoshol]

Originally Posted by Cody C

These pictures are for you ... They are all "effective" scissor lifts. All that is needed to reveal a wealth of effective Ideas is a look into the past.
Some of what has been being said in this thread is nothing but a bunch of rash generalizations. If you look into Industry... you will see numerous instances of scissor lifts being used, because they are good at what they do. some mechanisms are better than others at certain tasks, It all depends on the application.
You can't just say: "Scissor lifts aren't good because they haven't been made to work yet. They will always be inferior. " It doesn't work that way.

//pictures snipped for space//

Taken From HERE

Wow, they let those go without pinch-point protection?

[USAF Capt. Sean]

From what i can tell is that scissor lifts are more stable and can handle more weight. Plus the scissor lift can colapse down to under 10" and extend to over 90". by far this is way better then a double jointed arm.

[ggoldman]

The debate of arm vs scissor has two views:

Scissor Lift:

It is an inherantly more complex *Mechanism* since there are multiple sets of moving parts. If one small roller fails, the whole mechanism could be broken. That being said, the actuation of a scissor is simple in that it just takes a pneumatic cylinder or screw drive. Control is easier since it is a single input, but speed could be slower depending on actuation method.

Articulated Arm:

This is a simple *Mechanism* since there are only one or two moving parts involved. The control of an articulated arm poses a greater problem since you have to learn how to control different sections of the arm to create different motions. Sometimes, there are multiple positions of the arm that can get to the same point, so you need to plan on how you are going to move it.


In terms of strategy, simplicity, and usefullness, I would put my vote towards an articulated arm.

If our team has an arm, we will most likely do articulated.

[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.