Some time ago I put in orders for all the long lead time items on a second version of the moteus servo.  This is primarily aimed at improving the manufacturability and reliability, along with some minor performance improvements.  I’ve now got at least samples of all the long lead time parts in house!

While not perfect, now that I have flux braking in place, I have now succesfully pronked around for a while without faulting!  There are a number of outstanding problems that still need to be addressed:

• Sometimes the landing phase is erroneously cut short
• There is occasionally a grinding like noise that sounds like some controller is unstable
• I think the lateral movement is not working correctly
• The gait needs to be smarter about moving the legs past the center point when in mid-flight, and changing the gait period to achieve different speeds
• And probably a bunch of other problems I haven’t even identified yet

That said, it is still fun to watch it romp around!

In the last post in this series, I conducted a fit test on the new chassis.  After my ignominious belly-flop, I now had a more urgent need to complete the switch.

The chassis cracked in the corner, completely separating.  Doing anything more with this chassis was likely to result in many more things breaking very quickly.

Build process

So, here are the photos as I put everything together.

When I was trying my first pronking, I kept having over-voltage issues when the servos were trying to dump power back onto the DC bus, no matter how high I set the voltage limit.  During that test, I was running with a nearly full battery, so my working theory is that the battery protection circuit was disconnecting the battery either because of too high a charging current, or too high a system voltage.  When the battery was disconnected, and the servos were still putting power onto the bus, that resulted in the voltage spiking arbitrarily high, followed by a total loss of power when they all faulted and then nothing was powering the bus at all.

Now that I can do controlled jumps, and have a UI which allows me to use the joystick, I finally started actually working towards pronking gaits.  Too bad for you, this post doesn’t have any details, merely a video snapshot of my learning and debugging process…

Now that I had a controlled jump with the quad A0, I wanted to chain those jumps together into a pronking gait.  The first part of that was creating a mechanism by which I could actually command varying motion commands.  For the previous full rate experiments, all I had built was a CLI that allowed you to type commands.  That sufficed for initiating a single jump, but not really for moving around in space with a dynamic gait.  Something with a joystick would be necessary.

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.

After CADing up the second revision of the chassis, I set to work with the 3d printer and printed up all the pieces.

There were a few minor post-modifications I had to make, which were all much faster than printing the pieces again.  All the holes for M3 bolts were slightly undersized, so I drilled them out.  The battery holder had a channel to let the power wires out, which inexplicably terminated before reaching the edge of the holder.  I also had to install all the heat set inserts.

As described in my roadmap, the chassis for the quad A0 was on the verge of failing, or causing the shoulder motors themselves to fail, after only a few hours of walking around.  Also, it was nigh impossible to assemble, disassemble, or change anything about it.  Thus, the chassis v2!

More than one piece

The old chassis was a single monolithic print that took about 35 hours of print time.  Because of its monolithic nature, there were lots of interference problems during assembly.  For instance, the shoulder motors could only have 4 of the 6 possible bolts installed, and 2 more of the bolts extended beyond the chassis entirely.  I decided to break it up into multiple pieces, which uses a lot more inserts and bolts, but should allow for a feasible order of assembly and manageable repair.

Now that I have a full rate inverse kinematics and dynamics solution, I can begin to do more interesting things.  A while ago I did the first jump on the quad A0 – in that video I used a limited technique just to verify that the platform was indeed capable of jumping.  The joints were commanded in an open loop fashion, and really only at the transition points of the jump sequence, relying on the control loops in the servo to actually achieve each stage of the jump cycle.  That resulted in the jump only being minimally controlled… tracking errors would result in the robot taking off from a not-level position and the timing was not super reliable to boot.