I’m working to improve the walking gait of the mjbots quad A1. In this iteration, I wanted to tackle an incremental step towards a more fully dynamic gait, but one that will still greatly increase the capability of the machine. As mentioned last time, the current walking gait cycles between all four legs, and then alternative opposing corner legs in order to move laterally. I’d like to keep that same basic structure, but be a bit smarter about what happens during the swing phase.

Now I’ve got a machine, the mjbots quad A1, which is capable of dynamic motions, but the only gait which takes advantage of these capabilities is the pronking one. That gait has the benefit that the dynamics are very simple. The entire time that that robot is in contact with the ground, it is in contact with all 4 legs, so in that regime it is fully controllable. Since it is fully controllable up to the point of lift-off, we can ensure that there is basically zero rotational rate while the machine is mid-flight, which means that it lands with all four legs largely at the same time. Of course, pronking isn’t a very fast or efficient way of getting anywhere, so I wanted to make the first steps… I guess pun intended, towards improving the more general walking algorithm to make the machine move faster in a more robust manner.

The first feet I built for the quad A0 lasted for maybe an hour of walking before snapping off. The current design, has been much more robust - completing a lot of intensive walking and jumping. However, all things must fail:

Looking at the failure, I was surprised I used so little material in the region in question. For now, I just made it 4x thicker and we’ll see how long that lasts, although ultimately it may need to be a different design or machined instead of 3d printed.

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.

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.

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:

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.

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.

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.

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.