Three joint arm
 

Hey, I'm the new build team leader of our team (#2212 The Spikes) and I decided to have the crew bulid soething we've neve tried before. We decided on building a multiple-joint arm, preferrably more than two, just to make things a little more interesting.

Did anyone ever see (or build) such a thing and direct me to someone who can explain it to me, or atleast a team? Pictures would be very helpful too.

Thanks in advance,
Bar, #2212

Re: Three joint arm
 

Wildstang's 2007 robot comes to mind (3rd joint pivoted along arm's axis, allowing them to reach to the side). If you do this, you should also devise a way to control such an arm reliably, there is no point in having a fancy arm if the driver can't use it effectively.

http://www.chiefdelphi.com/media/photos/27788
http://www.chiefdelphi.com/media/photos/27192

Re: Three joint arm
 

I actually meant arms with all joints on the same plane ( --O--O--O-- ). And you're right, for compatition purposes it's pointless to have a fancy and uncontrollable arm, but in this case the goal is to have a big fancy arm that has no purpose (maybe Isould have mentioned that earlier...).

But thank you very much :)

Re: Three joint arm
 

Team 33 2005 was a double jointed arm, and 2007 was a triple (shoulder, elbow, and rotating wrist). There is a lot of good documentation on the 2007 arm in the Behind the Design book from 2007.

Most FRC games I have seen can effectively be played with the 2 joint arm. The third joint adds more complexity than it would likel be worth for many games, but it would be a good learning exercise.

I think it is wise to protype an arm as the likelihood for a pick and place game is pretty high this next year. Also important to any arm design is the object manipulator. Instead of doing a 3rd joint, I would focus some effort on trying to pick up an odd shaped piece like a cone, cylinder, or some other non-ball shaped piece (inner-tube, tetra, box...).

Re: Three joint arm
 

Our 2001 and 2007 robots have several joints.

Another picture of the '07

The CAD will be posted to FRC-Designs.com soon.

Re: Three joint arm
 

Team 2056's 2011 robot is another example of a successful 3 jointed arm.

Re: Three joint arm
 

2056 was only a two joint. Shoulder and wrist.

Team 1519's 2007 robot was a triple joint.

Re: Three joint arm
 

How about 25's 2004 robot's arm?

http://www.chiefdelphi.com/media/photos/17115

Re: Three joint arm
 

2056's 2011 robot was only a 2 joint arm. But it had a "jaw" like tube manipulator. :P

EDIT: BrendanB beat me to it :P

Re: Three joint arm
 

Our too-slow, wonky-clawed two joint arm could pick up from the floor and place objects up to 12 feet off the ground. Why would you need a third?

http://www.youtube.com/watch?v=m3TE2FCp9A8

Re: Three joint arm
 

For the 2011 year we built a three jointed arm: shoulder, elbow and wrist. The picture below shows them fairly well -
http://team1912.com/photos/mediapage...ach11/_h21.png

From that year we learned 3-jointed arms are probably not the best idea. (although i dont doubt that effective ones can be beautifully made). We could have built something with less degrees of freedom that would have weighed less, been easier to control, and operated in faster and safer mode. It was a hard learned lesson and I hope we don't make similar mistake any time soon.

Re: Three joint arm
 

My suggestion would be to do everything you can to make the segments farthest from the attachment point of the arm as light as possible. and those closest to the attachment point as strong as possible. Flexing and torque are your enemy. I suggest very thin wall high-strength aluminum round tubing for the last section or two, 1/16th for the second segment, and 1/8th for the first. I also recommend that all segments except for the last one be constructed from two parallel sections. this makes the load distribution more symmetrical and results in less twisting. Your first joint will need either a strong motor, or large bore pneumatic cylinders. to reduce the torque caused by the large cantilever, place the motors at the beginning of the segment before the one that it moves. Also, the first segment should be balanced with surgical tubing so that it is stable with the arm folded. To get an idea of the kind of load the arm will be under, bolt three pieces of aluminum together and bolt one end down to something sturdy so that it pivots at that end. Then try to lift it from the pivot end.

Re: Three joint arm
 

Here's a short clip of our 3-jointed arm from 2007. The controls were fairly simple, because for that game most of the joints could be slaved to the position of a master joint. So the arm driver just specified where the tip needed to go and the rest of the arm followed.
There was a tightly constraining 80" cylinder written into the rules that year, so the software managed all the joints to keep the whole arm within that imaginary cylinder.

The arm was modeled and controlled in Vex first to test the control logic.

Re: Three joint arm
 

My personal design philosophy is single-joint or bust. Simpler to build, easier to control, easier to program, fewer points of failure, and require fewer motors (and speed controllers). There are plenty of other options to increase an arms "reach" other than adding more joints.

I do appreciate the desire and lessons that can be learned from taking this on as an off-season project, especially in regards to the control perspectives.

Re: Three joint arm
 

68's robot from 2011 had 3 joints (well 2 and 1/2; we had a shoulder, a elbow, and could flick our "wrist".)


Skip to 2:57 for the arm, sorry I couldn't find any better pictures. They're all at school.
http://www.youtube.com/watch?v=QWKMNXKR7Ns

Re: Three joint arm
 

My Arm Design Methodology:
1. What orientation is the scoring object in when I pick it up?
2. What orientation is the optimum orientation of the object for scoring?
3. What elevation changes do I need to make to the object?
4. What is the simplest mechanism I can use to accomplish the above orientation and elevation changes?

Now, #4 is a bit tricky since "simplest" is somewhat subjective. How do I define "simplest"? My personal concept:

• The less degrees of freedom the better.
• The less actuators required, the better. (I give bonus points for pneumatics over DC motors, since they are simpler control wise).
• Easy to control -- I want to be able to drive it with default code. I want my drivers to be able to maneuver it almost intuitively.
• Low power -- I don't want to need to use a lot of motor power to actuate it. I like any arm I can use passive assistance on (i.e. surgical tubing) to do the majority of the work.

IMHO, multiple jointed arms rarely result from this methodology...

-John

Re: Three joint arm
 

The entire structure could easily be 1/16". 1/8" is very inefficient in comparison.

Re: Three joint arm
 

I added one to John's list that has often required the "need" for an additional joint even if the joint was temporary. In my opinion, many of the great arms in FRC have required a "stowage" joint in order to meet the first 4 objectives and still fit in the starting configuration box. The "stowage" joint would sometimes be a non-articulating joint that would have a release and lock position (Team 67-2008). While great designs can come from out of the box thinking, the rules often require that you at least start in the box and grow from there.

I put "need" in quotes because of the 2011 1503 machine that seemed to defy common logic on what was needed to be excellent. I would put team 25's "2012 bridge manipulator" in the same category.

Re: Three joint arm
 

I know it's not FRC, but I thought of NI's Scorbot ... if your school offers Computer Integrated Manufacturing or something like it, you've likely got one of these in-house.
The joint controlling motors are housed near the shoulder joint; the motor at the hand operates the end effector. This design also turrets.

Re: Three joint arm
 

That robot was optimized to score the top row, making tube reorientation unneccesary, as the tube was always in the same plane as the arm. The plan from the get-go was to take care of the top row and let more complicated robots worry about the bottom two. We discussed having a wrist on the end of the arm even after we had abandoned fighting for tubes on the floor, and deemed it still unneccesary for our strategy that year.

-Nick

Re: Three joint arm
 

That's why our 2011 robot had a two-joint arm: reaching the floor and the top peg while fitting within the starting configuration. There were of course other ways to do that -- single-joint arm with an extender or forklift-type mechanism, but I don't know that they were in fact any easier or more elegant than what we came up with. (Some were faster, which mattered far, far too much! :) )

Re: Three joint arm
 

Here's a look at out 2 jointed arm. The Shoulder is chain driven by the cim on the top half of the robot and the second stage of the arm is driven by a window motor. The claw was controlled by a pneumatic piston that opens and closes. The metal on the arm was .060 i believe, and had aluminum circle tubing that spanned to each side that we riveted into it. This would be something to look at. Message me if you want some more pictures. You could use this for some ideas. :)

Re: Three joint arm
 

In 2011 we had both an extender to aid in picking up off the floor and a forklift style lift to reach the upper row. We thought this enabled the best and simplest driver control.

The driver drove the arm into a hard stop to go to the down position, to pickup he extended the claw out, as soon as he had the tube he released the extension and could rotate the arm up and raise the list to position at the same time without haveing to adjust to rotational joints. The extension had multiple effects, it kept the arm as short as possible so it wouldn't sway bounce or move as far during adjustments, it gave us a quick move to steal tubes if was were going at the same one as an opponent, protected the claw by keeping it inside the robot perimeter most of the time, and allowed us to hang middle row without raising the lift.

In my experience the easier it is for the driver to understand the movements the better they drive. 2 rotational joints allow multiple ways to get to the same position and creates many possible combinations. of how to get there. Which violates the very solid design rules that JVN outlined above.

Re: Three joint arm
 

Three joints arm is hard to the driver to control. my team had in 2011 1 joint and a telescope which is much simplier and more effective
https://picasaweb.google.com/1136626...48580818847090

https://picasaweb.google.com/1136626...48660390128850

Re: Three joint arm
 

Elaborating on JVN and Ike's point...

If additional joints and DOF are necessary, simplifying their execution is key. Our 2011 arm utilized a rotation "elbow" and a very simple "wrist". The wrist only allowed us to fit inside the starting envelope at the beginning of the match and hold the roller claw parallel to the floor. Small pneumatic cylinder with a 1/4-20 bolt as the pivot point. Super simple, super effective.

-Brando

Re: Three joint arm
 

Yup, ease of use should always be high on the design priority list. The reason we went with screw drives (in restrospect at least partially a mistake) is because it was easy to make push-button controls for each necessary height -- there was a driver override just in case something went wrong with the encoders, but otherwise all the positions were pre-set and slaved to specific buttons.

Re: Three joint arm
 

A few people have mentioned surgical tubing to help out the motors. Definitely look into this or maybe air shocks, which we used last year on our arm.

Re: Three joint arm
 

While air shock look nicer than surgical tubing, the nice thing about surgical tubing is that you can tune it to the right strength. Unless you are trying to do something with a lot of power(ie. lifting your robot off the ground), usually surgical tubing is fine.

Re: Three joint arm
 

We actually used a gas strut to help raise the arm in our robot back last year and used one this year for our ball intake/bridge pull down. I would recommend looking into using one.

Re: Three joint arm
 

Thanks alot guys, I found your replies really helpful :)
We might actually go for two joints after all, but it's still a great discussion so no reason to stop here...

Re: Three joint arm
 

Two joints is a very good compromise of simplicity (although it isn't that simple), usefulness, and still providing a good programming challenge.

Re: Three joint arm
 

The good thing about 2 joints over 3 is that with 2 you can get away with default code and a pair of joysticks if you need to. In my experience, it gets a lot worse when you add the third joint (especially if it's a third motor-powered joint with complete freedom) and you need to start thinking about more sophisticated controls. Of course, if you can do it with one joint, your controls are even simpler still...

Re: Three joint arm
 

Team 1538's arm in 2011 was really neat, using a single joint powered by a motor and then gas shocks on the roller claw that moved the claw into the correct position without the need to power the 2nd joint directly.

Re: Three joint arm
 

Credit goes to 67's 2007 robot for the idea. :)

Re: Three joint arm
 

1731's arm in 2007 was nothing short of epic, both in how it was built and in how it was controlled. It had a z-rotating (aka yaw) turret at the base, a x-axis rotation shoulder (aka pitch), a x-axis rotating elbow (pitch), a y-axis rotating wrist (aka roll), and some sort of x-axis rotating grasping mechanism.

It was all controlled by a near-identical arm made of potentiometers on the drivers station.

Was it 100% necessary? No. But it DID work well, which was benefit 1. Benefit 2 was the incredible amount of unique materials used compared to the competition at the time. For a tech company, Benefit 1 pays the short-term bills, and Benefit 2 is what brings in licensing royalties long after other companies "innovate" in the tech space (Xerox is a great example).

Re: Three joint arm
 

While you can't tune the gas shock itself, you can certainly tune the system by adjusting the mounting points.

Re: Three joint arm
 

Although more expensive, you can also purchase adjustable gas shocks. Just be careful to do the force calculations correctly and over compensate, then release gas as needed.

http://www.mcmaster.com/#gas-shocks/=hmecz4

edit: go to reducible force springs

Re: Three joint arm
 

You change the mounting points, but this requires extra machining, along with extra calculations(surgical tubing requires none of this).

Of course, you can buy adjustable gas shocks as Akash said, this is more expensive. The nice thing about surgical tubing is that it is very cheap. But if you are willing and have the budget to spend the money, then it is worth it.

Re: Three joint arm
 

It should be noted that you get what you pay for (you may find yourself replacing sugical tubing, but the odds of having to replace gas shocks are very low).

Changing the mounting points isn't that hard. If you know what you need/what you have, take a drill and make more holes (your robot probably needs them anyway). The calculations shouldn't be that onerous, and I would guess that you would actually want to do the calculations for surgical tubing as well.