Developing the moteus brushless servo controller has been a verylongjourney, and while it isn’t over yet I have a reached a significant milestone. The first batch of production moteus controllers are now available for general purchase at shop.mjbots.com and shipment worldwide for $119 USD each!
First, a limited number of qdd100 servos are available for sale to beta testers! Check them out at shop.mjbots.com.
After building up the first set of qdd100 servos, I wanted to empirically measure their performance parameters. Some astute commenters uncovered in my terrible juggling video, that I didn’t actually have any ground truth measure of torque with these actuators. Given that the ultimate torque is a pretty useful performance metric, it’s a good thing to have a solid understanding of.
To measure this, I built a simple test fixture (which is also the qdd100 beta development kit), consisting of two brackets. The first lets the servo be bolted to a table, and the second mounts to the output and has set screws to hold a 1″ diameter pipe. I used this to insert a 1 meter pipe which then can press against a digital scale.
Then I created a simple C++ application which emitted torque commands in response to joystick input and reported back telemetry from the servo: qdd100_test
Using these I was able to generate a plot of actual torque vs motor phase current:
There are a couple of interesting things here, one is that the torque constant at low phase currents is slightly lower than I had estimated based on the motor’s Kv rating. Second, the torque constant drops off faster at higher currents than I had anticipated, and third, the motor Kv rating was lower than I had predicted. Those things combined result in a peak torque of between 12.5 and 15Nm depending upon the servo. That’s still enough torque to do some serious jumping, but exploring those discrepancies is now on my backlog.
Here’s a video showing how this testing (and max speed testing) was done:
I’ve received my first production run of the fdcanusb CAN-FD USB adapters and they are up for sale at shop.mjbots.com!
While this is necessary for interacting with the moteus controller, it is also a fine general purpose CAN-FD adapter. At the moment, the USB interface is a platform independent line based serial one (Windows, Linux, MacOS). It doesn’t yet interoperate with SocketCAN on linux, but hopefully that will be resolved in the not too distant future.
In my quest to create a more dynamic quadruped, I’ve started accumulating a lot of parts from bulk buys that could be reasonably useful to other hobbyists and experimenters. To maybe make life easier for everyone, I’ve started up what may be the worlds ugliest online shop where you can buy some of these components. For now, I have some bearings and custom gears that are useful when building servos for dynamic robots.