One of my goals with mjbots is to make building dynamic robots more accessible to researchers and enthusiasts everywhere. To make that more of a reality, I’m lowering the prices in a big way on the foundational components of brushless robotic systems, the moteus controller and qdd100 servo.
I’ve now got the last custom board from the quad A1 up in the mjbots store for sale, the mjbots pi3 hat for $129.
This board breaks out 4x 5Mbps CAN-FD ports, 1 low speed CAN port, a 1kHz IMU and a port for a nrf24l01. Despite its name, it works just fine with the Rasbperry Pi 4 in addition to the 3b+ I have tested with mostly to date. I also have a new user-space library for interfacing with it that I will document in some upcoming posts. That library makes it pretty easy to use in a variety of applications.
Finally, as is customary with these boards, I made a video “getting started” guide:
Now that the mjbots.com store has qdd100 quasi direct drive servos, moteus controllers, and the new power dist board, it is time to start getting some useful accessories in stock. While each of these components comes with mating connectors, sometimes you need more or find that a cable harness you built previously needs to be scrapped. Availability of Amass connectors isn’t that great outside of the Chinese market, so I’ve now got XT30U male and female solder cup connectors up in packs of 10. Each pack is just $6.
This board has convenient connectorization for powering sub-components of your robot, and also provides a smooth pre-charge sequence so that you can safely connect a large battery to high capacitance loads. I made a short video to show it off.
https://shop.mjbots.com is now https://mjbots.com (don’t worry, the old site redirects)! The functionality is largely the same, you can still get your qdd100 actuators or moteus controllers. The biggest differences are 1) it looks slightly nicer, and 2) shipping rates are improved, and international shipping rates drastically so. For instance, DHL “Express” 2 day shipping to some points in Europe is now under $35 USD, whereas previously 2 day shipping was over $300. That is often cheaper than even USPS International Priority — which is typically 2-4 weeks.
I’ll be adding some more products over the next couple of weeks, and I wanted to make them as accessible to a worldwide audience as possible!
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 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 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 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.