First look at Dynamixel

I received the Dynamixel I had ordered as a sample of what they can do. So far I'm impressed.

Accessories

The servo itself is a boring thing to receive. It's small plastic box. But I also, wisely, bought all the accessories that I needed to make it go.

First, I needed a way to give it instructions. Instead of spending my time trying to get my Arduino board to control it (which sounds like a real struggle), I bought a USB2Dynamixel adapter that allows me to control it from my computer. My final product will still be using a computer as its brain -- rather than simple robots that can be offloaded onto a little processor board -- so I suspect I might still be using this adapter in the final product.

The USB2Dynamixel is a chunky thing, as it provides an old-school serial port (I'm sure they have a technical name), a 3-pin plug for TTL Dynamixels, a 4-pin plug for RS-485 Dynamixels, and of course the USB plug for the computer. There's a selection switch on the side to activate one of the outputs.

Second, I needed power for the servo. Despite there being a power cable as part of the RS-485 connector, the USB standard doesn't supply enough juice to make the servo go. So I bought an external 12V power supply, like the kind you plug into a laptop (and, in retrospect, I probably just should have looked around for an old 12V supply that I'm no longer using).

So far everything sounds very organized and easy. But the people at Robotis really dropped the ball in one area: how you deliver the external power to the Dynamixel. There is no connector for it. No board. No dongle. No adapter. I had to build my own. Robotis knows this is necessary because they provide a quick drawing of what you need to do on their website. That's nice of them, as I wouldn't have known what to do otherwise, but they should just provide an appropriate adapter as part of the USB2Dynamixel package.

To get the power hooked up, I had to get out my soldering iron. Thankfully I found it, and it still worked. I had to cut one of the wires in a RS-485 connector, and attach the positive power supply wire to that (so that power is connected to the Dynamixel, but not the USB2Dynamixel). Then I had to strip a little bit of the wrapping on another wire, and splice in the ground wire from the power supply (so that ground is connected to both the Dynamixel and the USB2Dynamixel). Being a clumsy amateur, that splice was the ugly one and took me a while to get right. But in the end, my connections seem to work, and I didn't burn myself.

Here's a picture of my doctored RS-485 connector. I'm waiting for some electrical tape to arrive to make it look pretty and prevent it from electrocuting me. Note to others: get some shrink wrap sleeves to cover this mess instead of electrical tape.

Here's a picture of the USB2Dynamixel with the connector and the power supply and the Dynamixel. This is the whole setup.

Software

Robotis provides a free download of the software to manage and test your Dynamixel. Sadly it is for Windows only, but I have a Windows machine that still works. I believe there is a Linux SDK that I will have to investigate if I want to use the USB2Dynamixel in the final product, but Robotis says that the Windows software is necessary for configuration and firmware updates.

You have to select the correct COM port; the one that the USB2Dynamixel is being presented as. Then it has to search for your Dynamixel over the cable. I think it is just sending out "are you there?" messages to all the possible receiving addresses until it gets a response. Get a response it did.

The software then presents all of the status and settings for the device. There's actually a fairly long list of things there. Things like limits to how far/fast/hot it considers acceptable. Things like the accelerating and decelerating at the beginning and end of a servo move to make it smooth. Things like the current voltage, load, speed, position. And -- most importantly -- the current goal position.

If you change that goal position, the servo moves. They have a cute little software dial, and clicking your mouse on the dial makes the servo rotate to that position. Much more glamorous than my push-button Arduino controller.

Results

The speed of the Dynamixel seems about right. It was rated at 0.079 seconds per 60 degrees. This is the fastest of the Dynamixels (aside from one that is designed to be a wheel, and doesn't have much power). It rotates from -150 degrees to +150 degrees -- that's something to keep in mind for my arm design.

Strength is difficult to measure, as I don't have any attachments for the servo. I have the servo horn it comes with, which provides a way to screw it to your robot, but I don't have a robot yet. I'm attaching a piece of boxboard just so that I can see it rotate better. But until I find some material to make my arm out of, I don't have anything that can test the strength or measure its speed under load. But the specs say it's supposed to be 360 oz-inches (compare that to the 21 oz-inches for the cheap servo in my Arduino post).

Here's a video of it turning the cardboard. Whee.

So that's it for now. My first experience with Dynamixel has been a good one, and I plan to design the robot to use them.