Saturday, January 24, 2009

Dr. Claw: Hand of Destruction

I wanted to build something really cool using purely Meccano/Erector components. It's tough to integrate electronics with Meccano, but i wanted to get something robotic. I wanted to make sure also that it was only Meccano. I also wanted it to look really good. Not just stuck together with bubble gum and duct tape. So here's what you get:

... I mean even better looking than this...

So here's what I made:

It's a robotic arm. I modeled it off an industrial robotic arm with 5 degrees of motion. It has base rotation, shoulder, elbow, and wrist rotate as well as wrist tilt. Also has a sweet lookin' claw attached to the end.

Here's a shot of the claw in full vertical extension. You can see that I'm using dual servos at each joint. This will allow me to have plenty of strength to move the (heavy) metal Meccano pieces as well as whatever's in the claw at the time.

Here's a horizontal extension:

Build Details:
The above picture is the rotational base. It is purely Meccano, and rotates quite well. I set it up so that I'll be able to drive it with a stepper motor later on via the gear on the left. The stepper will allow me to drive it to a specific angle for precise movement. I really had to put on my thinking cap to come up with the design above.
Here's the stepper I'll be using:

In order to actuate the arm, I had to attach the servo horns somehow. I made servo links from some steel wire i picked up at ACE. Holy crap that stuff was hard to bend. And I wore out a pair of Diagonal Cutters too. My hand is going to be sore tomorrow.

Getting the wire to couple with the erector set was a challenge too. but i found a way to wrap the wire into some stock meccano "ring terminals." I had to crimp the terminals down and wrap the wire around the post for stability, but it turned out great. I also used some hefty HS-645MG metal gear servos from Hi-Tec. These servos have phenomenal torque.

Probably the most difficult part in designing the arm was figuring out how I'd get the wrist to actuate. The shoulder and elbow were tough, but nothing comparted to the wrist. I must have built 10 different claw configurations, and each one wouldn't work for a different reason. Some weren't stable enough, some couldn't hook up to the servos, some were too heavy, too weak, etc. Finally I came up with the design you see here. I think it's perfect, and it balances well with the rear end of the wrist.

You can see that I used the stock Meccano rubber pads for gripping on the claw.

It's important when working with hobby servos, not to stress the servo horn radially. This can damage the axle of the servo, particularly with the wimpy HS-325HB servos I would be using at the wrist. These servos have "karbonite" inner gears. These gears are tougher than plastic, but don't have the durablilty to withstand much abuse. They're kind of like a nylon material. I decided I'd mount the wrist tilt using dual servos facing inward toward the wrist. This would ensure that there was only a low radial load.

Adding the wrist rotation was even more difficult. I wanted to get at least 520 degrees of rotation for manipulating small objects, but servos only have about 190 degrees at most. I used some stock Meccano gearing and built a servo bracket to mount a drive gear vertically: The smaller gear at the bottom is on a shaft that the claw is bracketed to, making the shaft and claw rotate perfectly. (Sounds easy when it's right in front of you, but this took days to figure out)

Below a shot of the pinchers i tried to use. (Grabber attempt #7). I used Shapelock, which is a great product for prototyping. Unfortunately they turned out to be too flexible for my liking. Plus they went against my goal of a pure meccano build.

Shapelock is a low temperature thermoplastic that can be reheated and reshaped endlessly. It's like modeling clay, but better! Given the correct thickness, they would have been less flexible, but I didn't want to worry about the weight etc.
I stayed pretty true to my goal of using only erector set components, in fact, the only non-meccano parts in my build were:
Linkage Rods.
Home-made Servo Brackets
6 brass collars
Plexiglass Base (seen in indoor shots above)
make controller
So there it is: DR CLAW. A pure meccano build that's sturdy, functional, and fairly attractive.
Feast your eyes.
I also built an external power supply for driving all these crazy servos. The next step will be to hook this sucker up to my make controller and write the software to control it. Fortunately, thought, i think that will be the easy part. I'll try to take some videos during testing.


Aaron, Vicki Tunell said...

This looks really good honey. Can't wait to see the finished product. Lucky me, I live with you so I get first glance!


Anonymous said...

Very cool stuff!

Anonymous said...

Finally, something new!! I faithfully chck back but for SOOOOO LOOOONG it didn't change. These are very cool and reward the wait.

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