I’ve now managed to get all the custom and long lead time parts in house for the first version of a quadruped based on the new actuators I’ve been designing.

That includes all the motors, custom brackets, and at least moderately working versions of all the custom PCBs.  Now I just have to get the local rework done, get the software into a semi-reasonable state, and put it all together!

Now that I have a semi-reliable actuator, I need to connect 4 of them together into a single quadruped robot.  Additionally, it needs to be able to mount a battery, the turret, and all the other miscellaneous pieces of a walking robot.

My draft design looks like this in CAD:

The four corners each are set to mount one leg to.  The central cavity will eventually house a battery compartment.  On the top is a mounting location for the turret, and the front has mounting studs for a power distribution PCB.  Each of the screw holes is designed to take a thermoplastic insert heat fit into place.

The full quadruped robot needs to both distribute power from the primary battery and RS485 serial network to all 12 servos.  To make the wiring of that easier, I’ve made up a junction board to provide power connectors, distribute the data network, and act as the IMU for when that is necessary.

The RS485 network is bridged between two halves of the robot.  One connection comes in from the controlling PC and two separate links go out, one for the left side and one for the right side.  This could eventually allow the controller on the junction board to take intelligent actions itself, such as querying the force applied on all 12 servos.  It could then return the result in a single RS485 transaction to the host computer.  I am expecting that will be necessary to achieve closed loop control approaching 1kHz.

All my testing to date on the improved actuators for SMMB have used 3d printed parts from Shapeways.  Both to have a faster iteration time, and to reduce costs, I’ve optimized all the parts to be printed on my Prusa MK3s.

Here’s all of the individual parts:

Them assembled into a leg with the other necessary hardware:

While wiring up the first 3 degree of freedom mammal actuator, I knew I was going to have a need to distribute power to each of the three motor controllers.  Thus, enter my simplest ever PCB.  It is just 4 holes for each of power and ground with traces connecting them.

It took an annoying amount of time to actually solder in all the necessary wires, but it was still better than the alternative of a bunch of ring terminals bolted together.

After my self-education on MLCC derating I spun yet another low-volume prototype run of the servo controller.  This one has more than double the effective capacitance by doubling the number of capacitors and by selecting capacitors that have less derating.  I also fixed an incorrect pad geometry for the 6 pin ZH connector, optimized the BOM count a bit and reselected parts that were no longer available.

After getting the first version of the mammal geometry leg working and jumping I worked on a second revision.  At a minimum, I wanted to fix all the problems that required hand machining, however I also decided it was trivial enough to add a reduction ratio to the tibia through the belt drive, that I should just go ahead and do it.  My inverse kinematics calculations showed that this would make a big difference in average power consumption.

It is great to see that Ben Katz was finally able to announce his work on the MIT Mini Cheetah!  I’m looking forward to reading their ICRA paper, if nothing else to figure out what motor selection they made and how to design a more compact gearing system.  My current prototypes rely exclusively on belts for reduction, as I decided that my current geared prototypes were too cumbersome.

UPDATE 2019-03-06: I now see that Ben’s thesis in his post is actually his master’s thesis, which fully describes the actuators and robots.  I had erroneously thought it was just his undergrad thesis.  Thanks for publishing so much!

While designing the improved actuators for SMMB I’ve given Shapeways a lot of business.  I can definitely recommend it, their selective laser sintering (SLS) parts are easy to order, their website gives plenty of control, and you can expedite things to your hearts content.

That said, with the amount of 3d printing I am doing, I could have already paid for a fused deposition modeling printer several times over.  Thus, I recently acquired a Prusa i3 MK3S.  It certainly can’t print everything that you can do with an SLS process, but with slightly tweaks to the models it can do a lot of it.  The biggest upsides of course are the lower per-part costs… something like 20-100x cheaper, and the faster turnaround time.  Nearly anything I care about I can have a draft of overnight.

The controllers for the improved actuators for SMMB have a moderate amount of power to deal with.  During jump maneuvers they can put 60 amps of phase current into the motor, and I’ve applied for very short intervals over 500W of power to a motor.  The FETs on the board are relatively high performance, but there is still a fair amount of heat that has to be dissipated.

When getting started, I knew I would likely have to do something to get heat out of the board and had a two stage plan.  The first was to heatsink the back of the controller board and second, if that wasn’t enough, heat sink the front of the board.