Educational robot arm
Phil Pilgrim (PhiPi)
Posts: 23,514
in Robotics
This coming semester, I've volunteered again to teach a robotics course at the local high school. The class of 20 students will be studying CAD and programming, beginning with ActivityBot programming in Spin. I will have 10 students on Tuesdays for an hour and a half, and the alternate ten on Thursdays for the same amount of time.
The course objective this semester is to design and program a robot arm capable of solving the Towers of Hanoi problem. (Thanks, erco, for the idea!) The CAD side will design the mechanical parts in Rhino, to be cut out on the school's laser cutter. The programmers will have to learn how to control the robot and figure out the puzzle algorithm.
I've never built or programmed a robot arm before. In order to stay a step or two ahead of the kids, I needed to do that -- pronto! So I went online and found a laser-cuttable arm design (the uArm) that uses standard servos and is free to duplicate under a Creative Commons license. In order accommodate the cheap 2.7mm meranti doorskin that I wanted to use, my own bearings and bushings, the Activity Board, and my own gripper design, I needed to make a considerable number of mods to the uArm patterns. Here's a short video of the result:
[video]
I like the uArm design, since it incorporates parallelograms that keep the gripper relatively level. That way a lot can be done without needing a wrist motor. It also has the advantage of keeping all three motion servos affixed to the base, so the arm does not have to lift them.
The video demos my first program, and the motions were sketched in totally by trial and error. I need to do a thorough kinematics and reverse-kinematics analysis yet, in order to produce predictable results. I'm thinking of redesigning some of the parts to incorporate protractor elements on the joints. That would make it easier to calibrate in terms of pulse-width vs. arm angle.
Anyway, I'm hoping to get the Towers of Hanoi program operating in time for the first class day. I can then invite a volunteer to try to beat the robot, using a separate of set of spindles and disks.
-Phil
The course objective this semester is to design and program a robot arm capable of solving the Towers of Hanoi problem. (Thanks, erco, for the idea!) The CAD side will design the mechanical parts in Rhino, to be cut out on the school's laser cutter. The programmers will have to learn how to control the robot and figure out the puzzle algorithm.
I've never built or programmed a robot arm before. In order to stay a step or two ahead of the kids, I needed to do that -- pronto! So I went online and found a laser-cuttable arm design (the uArm) that uses standard servos and is free to duplicate under a Creative Commons license. In order accommodate the cheap 2.7mm meranti doorskin that I wanted to use, my own bearings and bushings, the Activity Board, and my own gripper design, I needed to make a considerable number of mods to the uArm patterns. Here's a short video of the result:
[video]
I like the uArm design, since it incorporates parallelograms that keep the gripper relatively level. That way a lot can be done without needing a wrist motor. It also has the advantage of keeping all three motion servos affixed to the base, so the arm does not have to lift them.
The video demos my first program, and the motions were sketched in totally by trial and error. I need to do a thorough kinematics and reverse-kinematics analysis yet, in order to produce predictable results. I'm thinking of redesigning some of the parts to incorporate protractor elements on the joints. That would make it easier to calibrate in terms of pulse-width vs. arm angle.
Anyway, I'm hoping to get the Towers of Hanoi program operating in time for the first class day. I can then invite a volunteer to try to beat the robot, using a separate of set of spindles and disks.
-Phil
Comments
I also like that Vimeo opens with one click!
Edit: I can see the video if I use http instead of https.
John Abshier
And my original BS2 arm:
Ken, what do you think? Let's get Matt laser cutting arylic or better yet plywood arm parts ASAP!
BTW, I'd definitely vote for plywood in the form of meranti doorskin. It's cheap! Even from Edensaw, it's only $17 for a 4x8 panel. (Okay, one side is good; the other needs a bit of sanding. And sometimes there is filler in the inner plies that the laser cutter won't touch; but that's rare enough not to discount the advantages.)
Small, mechanically-critical parts, like the gripper arms and servo splines, are easily laser-cut from Delrin or an equivalent, such as acetal copolymer.
-Phil
[video]
School starts next Tuesday. Am I nervous? As always with a new class! But I think it will be fun. The emphasis this semester will be on "mechanisms": gears, levers, pulleys, etc. The robot arm is just one example of the interaction of a micro with the mechanical world.
-Phil
The video seems to be missing.
Jim
Are you logged in under HTTP or HTTPS ?
vimeo.com/138045291
-Phil
(over-achiever)
Love that gripper-friendly pickup flange on each disk.
move(how_many, from_pin, to_pin) if how_many == 1 Just do the move. else move(how_many - 1, from_pin, other_pin) move(1, from_pin, to_pin) move(how_many - 1, other_pin, to_pin)'Don't even have to think about the low-level sequence: it works seemingly like magic!
-Phil
Let us know how the class does too and how it is received. I wish I had had a teacher like you in robotics/programming when I had been in High School - oh wait - all the computers tat existed during those years were a bit larger and more expensive. ;-)
1972-1976 for High School - by the way... We did have more affordable choices as I left college in 1982.
Now, I will have to make or get me a robot arm too.
My biggest development issue was the positioning accuracy. Despite the tapered spindles and conical holes in the bottoms of the discs, the arm would still miss once in awhile, leaving a disc hanging on the top of a spindle, and it was driving me nuts. I was beginning to blame the cheap HobbyKing servos I was using. But then I realized that I had not yet glued the spindles into the base and that they were a bit wobbly. Aligning them and securing them with wood glue solved the problem.
As a test, I programmed the bot to move the spindles from 1 to 3, then back to 1, continuously. After running for a couple hours, it had not missed once, so I feel better about presenting it to my class as a demo. Still, though, just moving it from my shop to the school might be enough to cause a disturbance in the Force ...
-Phil
My money is on you Phil!
BTW from my experience, most people actually like to see a robot fail occasionally. IMO it makes them think they might have a fighting chance when the bots become self aware and Judgement Day starts. Just to be safe, make sure the ToH arms your students build can't hold a gun or throw knives. AND NO BUILT-IN SELF DESTRUCT SEQUENCES!
Jim
I was watching one of my walking sticks eat yesterday. They usually feed at night, so this was kind of rare, and I videoed it. She slowly rotates her head around, gnawing a perfect arc out of the leaf. Like my arm, Mother nature does not support rectilinear motion!
Smooth semi-circles. That walking stick is up for the figure-8 challenge!
As long as I can "leave" a trail of edible bits in a figure 8 pattern. At that point, it's more of a line following (gobbling) challenge.
Yes, I'm planning to do that once I've cleaned up and annotated my Corel drawing. I also need to create a bill of materials for it. The design is based on -- well, a close copy of -- the uArm. The gripper is my own design.
-Phil
Congratulation for your project! As always your project is fantastic! The disk's design is amazing and makes the arm's functionality much more easier!
I decided to follow the same design and copy your project in order to demonstrate it to my students!
Here is my attempt in order to make the disks! I made them as a set of two parts each one .
https://dropbox.com/s/o42kd3tmgoybr2h/Hanoi_tower_disks.jpg?dl=0
Now is time for the arm. there is a newer version (update) of uarm, the Lite Arm i2. What do you think about it? Is it better than the old version?
1) No peg is required, each disk stacks on the previous disk's knob.
2) Each disk's knob is the same size, so the gripper opening never changes.
Hard to appreciate these little niceties until you've made and programmed an arm.
@Nikos: I love your optimism of 9 stacking disks. Each disk you add more than doubles the time & number of moves required.
erco,
9 disks are not too much.
Acording to the algorithm we need 511 steps .... I hope that Phil's arm makes the 1 step in 1sec..... so 9 disks are 8,5 min