Servo - November 2013

The famous American anthropologist Margaret Mead once said:

“Never doubt that a small group of thoughtful, committed people can change the world. Indeed, it is the only thing that ever has. “ I started this biped project in 2006. I felt like I was climbing the highest mountain in the world on my own. This year, Chris and Girts joined up and we became a team. Now, I can see the top. This month’s article is a team effort. Each member is working on different parts of the biped, so it is divided into the three sections: mechanical changes, the biped systems, and the Gazebo simulation.

Mechanical Changes — Daniel

Finding servos that are high torque that don’t require you to take a second mortgage on your house is not an easy task. Our original servos claimed over 400 oz/in of torque, but they just could not deliver those numbers. We have just placed an order with a company in China that has a giant size R/C servo for under $100 with a claimed 850 oz/in of torque.

In the meantime, we have made some significant changes in the biped. Using the Gazebo simulator, we finally came up with some hard torque numbers. Strong servos are crucial for two joints in particular. The hip in/out and the knee servos need about 1,600 and 1,200, respectively, to lift and hold the upper body weight.

The first change we made was to replace one of our original servos that could not hold the hip in place with a super strong albeit heavy 3,200 oz/in Torxis servo from Invenscience. Unlike hobby servos, I was not able to back-drive these. This works out nice when the biped is standing still or when that joint doesn’t need to move.

These are powerful servos built with strong output shafts. I can simply shut them down and they hold their positions. At under $300 each, I would have used them for