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Thursday, December 17, 2009

Mirror Mirror

When I first got it into my head that I wanted to build a humanoid robot, I spent quite a lot of time researching various types of servos I could use. They needed to be high-torque, not too big and above all cheap - other features were just gravy.

This led me to consider a number of things, such as using unbranded RC hobby servos, buying unbelievably cheap micro servos and upgrading the motors, and even building my own.

These are not bad options, per se, but what I failed to consider were just how useful those extra features that high quality servos include can be.

Fortunately, I was saved from myself by the timely arrival of my birthday (which I have written about previously).

Though the wCK servos have many additional features, the one that I am finding particularly useful right now is the ability to query a specific servo for it's current position. Although it doesn't seem like much on it's own, it has enabled me to do cool little tricks like this:

The video simply shows me moving the robot's left arm, and having the program mirror the actions on the right arm.

Non-robologists don't seem to find this very impressive, but hopefully the implications of this are apparent if you are in the robot game. Effectively (although crudely), the robot now has a sense of touch.

Don't have a distance sensor? No problem - just set your robot into zombie posture and wait until one of his arms is bumped by a wall.

Worried that you'll damage a servo? Just check to see if it's stuck and turn it off to avoid stripping a gear.

Oh, and before you ask - yes, that is Donatello in the background holding the power and serial plugs.

Here's another video of him with his arms attached (the robot, not the turtle).

The servos are placed in passive mode, which is another special feature of the wCKs, which powers them up but turns off the PID control. Because the servos use an RS485 type bus, they can then be queried by the RBC and then take control of the bus to report their current position.

Although it is a minor achievement, this is another important step on the road to my grand plan. What plan, you ask? You'll just have to wait and find out.

The code is a bit sketchy at the moment, so give me a chance to clean it up before I upload it. Watch this space.

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