Servo - October 2013

10 SERVO 10.2013

Q. I want to make a walking robot. I’ve looked at hobby servos and they don’t seem to be a good match for this project.

I want some kind of feedback so that I can balance while walking, and I don’t want a huge bundle of wires. Is this possible? Are there servos that can give me that information?

— Bobby via email

A. Bobby, I like where you are going with this, and yes, there are servos that can offer a great deal of feedback on not only their position, but current draw, heating, and other useful data. Let’s look at a few! (There are more, but the ones I’ll discuss are the servos that I found information about.)

First off, let’s look at hobby servos and see what they offer. Hmm, My research also shows that only one of the major (and minor) makers of servos offers a contiguous bus configuration to minimize wiring. That one manufacturer is Futaba which has its S-bus receiver and servo system. The S-bus system allows S-bus servos to be programmed to know their location in the system.

Further, the servos reside along a single three-wire cable connected to the receiver. There are also S-bus translators that allow non-S-bus servos to be used in an S-bus system.

That’s nice, but these servos are expensive and still don’t provide position or force feedback.

Now, we’ll move on to intelligent networked servos that you don’t have to continually send pulses to in order for them to continue to function.

I have an admission to make. I know of three bus-networkable robot actuator systems. There may be more, They are the Robotis Dynamixel, RoboBuilder wCK, and Dongbu HerkuleX. The Dynamixel, HerkuleX, and wCK servos come in a variety of strengths and speeds. You can create a microcontroller interface to any of them (they all use standard UART protocols; either straight digital or RS485-style links.)

Before we go into any kind of details, here is where you can find programming information on all of these servos: Dynamixel AX- 12 www.trossenrobotics.com/images/ productdownloads/AX-12% 28 English% 29.pdf

Dongbu HerkuleX www.robotshop.com/content/PDF/ manual-drs-0101.pdf

RoboBuilder wCK http://robosavvy.com/RoboSavvy Pages/Robobuilder/robobuilder-creator-users-manual.pdf Table 1 shows these servo actuator systems and their overall notable abilities. Each of these servo actuators have many more settings that you can configure, including the ability to get multiple servos to act at the same time, set motion limits, torque limits, and error conditions. It is a good evening’s time in the arm chair of your choice to read through all of the possibilities.

It seems like every week there is breaking news from a Japanese or Korean lab dealing with walking bipedal robots. Awesome! In the US, there is news from "leg labs" from time to time, and more details about military robot "horses" that carry loads and sound like large leaf-blowers chasing you.

(Is that a prelude to some kind of movie or perhaps nightmare?) If you’ve come to the conclusion that I'm a bit biased towards more mundane uses for robots, you'd be right. Robot assistants for people who have lost the ability to walk or use their arms, or could use a little help around the house seem far more worthy endeavors than new, improved ways to kill people. However, just to prove I'm not completely consistent, I love the new RoboOne competitions with robot Sumo, wrestling, and boxing matches! Our question this month is from a reader who appears to be thinking along these same lines ...

by Dennis Clark

Our resident expert on all things robotic is merely an email away.

roboto@servomagazine.com

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