Three joint arm

[JVN]

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

[AdamHeard]

Originally Posted by PAR_WIG1350

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.

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

[IKE]

Originally Posted by JVN

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?
5. How do the "initial configuration" and Maximum dimension requirements effect what I would normally choose as ideal?

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.

[Taylor]

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.

[Nick Lawrence]

Originally Posted by IKE

I put "need" in quotes because of the 2011 1503 machine that seemed to defy common logic on what was needed to be excellent.

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

[pfreivald]

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! )

[Kyler Hagler]

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.

[Peter Matteson]

Originally Posted by pfreivald

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! )

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.

[yarden.saa]

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

[Brandon Holley]

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

[pfreivald]

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.

[Ekcrbe]

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.

[MichaelBick]

Originally Posted by Ekcrbe

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.

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.

[Kyler Hagler]

Originally Posted by MICHAELABICK

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.

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.

[bardd]

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...