Archives: 2020-06

Cartesian leg PD controller

As I am working to improve the gaits of the mjbots quad A1, one aspect I’ve wanted to tackle for a long time is improving the compliance characteristics of the whole robot. Here’s a small step in that direction.

Existing compliance strategy

The quad A1 uses qdd100 servos for each of its joints. The “qdd” in qdd100 stands for “quasi direct drive”. In a quasi direct drive actuator, a low gearing ratio is used, typically less than 10 to 1, which minimizes the amount of backlash and reflected inertia as observed at the output. Then, high rate electronic control of torque in the servo based on current and position feedback allows for dynamic manipulation of the spring and dampening of the resulting system.

New product Monday: mjbots power dist r3.1

I’ve displayed versions of this numerous times in the past (May 2019, Feb 2020, March 2020), but now I’m proud to announce that I have a productized version of the power dist and precharge board available in the mjbots store for $79.

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.

Improved swing trajectory

Now that I finally have tplot2 working sufficiently to diagnose problems in 3D, it is time to start actually fixing those problems. The first obvious thing I noticed when watching data replay was that the legs scooted around a lot after making contact with the ground. Absent 3D visualization, I knew something was wrong, but couldn’t easily tell what.

Diagnosing the first problem

Once I was able to plot the commanded position and velocity trajectory, I could clearly see a number of problems. For one, the trajectory was not terribly achievable. The velocity jumped in a discontinuous manner between different phases of the swing cycle, which resulted in large tracking errors when moving the physical legs:

Primitive derived fields in tplot2

One of the features that I wanted to get working in the newer tplot2 is some facility for rendering values which are calculated from the things in the log, even if not directly logged there. Straightforward simple cases would be things like the lengths of vectors, unit conversions, or quaternion to euler angles. You could imagine needing arbitrarily complex values plotted after the fact.

In past systems I’ve designed, I built in a generic scripting interface to allow arbitrary things to be plotted. I’d like to do that here as well eventually, but in the short term I had a need to plot the total normal force exerted on the ground by all stance legs. And I didn’t want to spend a lot of time designing a generic mechanism. Thus, I rigged up a very primitive C++ only mechanism, where a function can be registered which returns an arbitrary serializable structure. That is then rendered in the tplot2 tree view in a dedicated area, and has a pretty “hacky” way of getting its values on the plot if necessary.

Video and telemetry synchronization (diagnostics part 8)

This is part of a continuing series on updated diagnostic tools for the mjbots quad A1 robot.  Previous editions are in 1, 2, 3, 4, 5, 6, and 7.  Here I’ll be looking at one of the last pieces of the puzzle, synchronizing the video with the rest of the telemetry.

As mentioned previously, recording video of a robot running is an easy, cheap, and fast way to provide ground truth information on all of the sensors and actuators.  However, it is only truly useful if it can be accurately synchronized in time to the other telemetry streams for the robot.

3D rendering in tplot (diagnostics part 7)

In previous posts of this series, I covered some diagnostics improvements I’ve made to help work on more advanced gaits for the mjbots quad A1 (1, 2, 3, 4, 5, 6).  This post will cover the last major new piece of diagnostics I added to tplot2, 3d rendering of telemetry data.

3D rendering

While it should be obvious, I’ll give a little exposition.  tplot2 in its state prior to this could show a “tree view” of all data logged in numeric form.  It had a “plot view” which let you plot any single floating point scalar vs time.  As of recently, it could also render video associated with a given point in time in the log.  However, as anyone who has ever tried to debug a 3d dimensional software application, much less a 3d dimensional robot, can attest, debugging with scalar numbers and time plots is only productive for a very limited range of problems.

New mjbots.com

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.