Dynamixel ax-12+

Hi, I’m in the process of designing a biped, and I would like to us a Pololu mini serial servo controller.
I need to figure out what servos to use, but i am mainly looking at the dynamixel ax-12+, and the hitec hsr-8498. I know
that the hitec is compatible with the controller, bu i can find nothing about the dynamixel. I’m plan on using an arduino to control the servo controller.

The Dynamixel servos don’t use the hobby servo PWM protocol, and would not work with any of the Pololu servo controllers.

Not that I hold that against them, however, they are pretty darn cool. Each AX-12 is a high-end digital servo based on an AVR microcontroller! You can think of it as a servo with the servo-controller built in. It performs its own position sensing and control, torque sensing, and a bunch of other features, and lets you read this data back. You can command position or continuous rotational speed, holding torque (“compliance”), position limits, etc.

To use multiple servos you program each one with an identifying number, then string them together on a one-wire (half-duplex) TTL serial bus. You can command these servos with a microcontroller, or from a computer using a USB to TTL level serial adapter. Since the bus is only one wire, you need to transmit and receive bytes on the same serial line. Some AVR microcontrollers have half-duplex serial hardware built in (any one with “USI” hardware), or you can use a regular UART (like the one the Arduino has) and some diodes, and maybe a pull-up resistor. Alternatively I bet there’s a library to set up a bit-bang half-duplex serial port on any I/o Pin.

You would need to set up the same diode/resistor circuit to use a regular USB to Serial adapter (like Pololu’s), or you could just pick up the official USB2Dynamixel adapter. I haven’t used it myself, but I hear from friends that if you’re controlling these from a computer they have a really easy and feature-rich API for the C-type languages, with functions that do things like scan all serial ports and open the one that the Dynamixel servos are connected to!

Anyway, I do like Hitec servos too, and use them much more often. I’m not trying to be a Dynamixel salesman here, I just think they’re cool. They’re especially convenient if you have a servo application where you need compliance or force-feedback.

So, have you made your decision?


Yes, I’ve made my decision. It seems I will be using the hitecs, if only they weren’t so expensive. I’m on quite a budget (hopefully less than $400 for all servos. Pushy?), and I need ten robot-grade servos.
I know this is off topic, but are there any other cheap, good quality servos out there?

If by “robot-grade” you mean you want daisy-chainable servos with direct computer serial control built in, torque and position feedback, and a freewheeling bearing side, then no, you can’t go much cheaper.

If you just want an analog servo the HS-645MG has very similar specs for $20 less (and is one of my favorites). Wow, you can get a lot of features for $20 more dollars!