ScissorsLift vs. DoubleJointed Arm.
 

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.

Re: ScissorsLift vs. DoubleJointed Arm.
 

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

Re: ScissorsLift vs. DoubleJointed Arm.
 

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

Re: ScissorsLift vs. DoubleJointed Arm.
 

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.

Re: ScissorsLift vs. DoubleJointed Arm.
 

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.

Re: ScissorsLift vs. DoubleJointed Arm.
 

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.

Re: ScissorsLift vs. DoubleJointed Arm.
 

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

Re: ScissorsLift vs. DoubleJointed Arm.
 

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.

Re: ScissorsLift vs. DoubleJointed Arm.
 

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....?

Re: ScissorsLift vs. DoubleJointed Arm.
 

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

Re: ScissorsLift vs. DoubleJointed Arm.
 

you are right whenever I hear scissor lift i see red flags in my head, they are unstable, uncontrollable, and not durable, oh and really really heavy. There really isn't and comparison between scissor lift and a double jointed arm, a properly designed double jointed arm wins. If created properly they can be very controllable and far more flexible than a scissor lift. If you are looking for a vertical moving arm look at an forklift (elevator) type system (111 in 2005 for example). I have only ever seen one, yes one, effective scissor lift.. and too many non-effective ones.

Re: ScissorsLift vs. DoubleJointed Arm.
 

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 :D

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

Re: ScissorsLift vs. DoubleJointed Arm.
 

Maybe its that the best scissor lifts (beings used as an "arm") still ends up being garbage, and is still inferior to a mediocrely designed jointed arm.

Re: ScissorsLift vs. DoubleJointed Arm.
 

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

Re: ScissorsLift vs. DoubleJointed Arm.
 

Yeh, the scissors lift would be primarily an arm. I think that Mr. Norris nailed it on the head. Maybe I just think that those red flags were more evident. Anyways, feel free to keep posting, I find this quite interesting.

Re: ScissorsLift vs. DoubleJointed Arm.
 

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... :ahh:

Re: ScissorsLift vs. DoubleJointed Arm.
 

no talk of elavators!?! =O

Re: ScissorsLift vs. DoubleJointed Arm.
 

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.

Re: ScissorsLift vs. DoubleJointed Arm.
 

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

Re: ScissorsLift vs. DoubleJointed Arm.
 

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.:yikes:

Re: ScissorsLift vs. DoubleJointed Arm.
 

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.

Re: ScissorsLift vs. DoubleJointed Arm.
 

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

Re: ScissorsLift vs. DoubleJointed Arm.
 

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.