March 4, 2019

Hello There, Mini Cheetah

Now that this is officially out I can finally put up something about what I've been doing for the past 2 years.

Photo Credit: Bayley Wang



This project has been a continuation of the Hobbyking Cheetah project, but for research rather than out of my pocket.  Here's my thesis on the actuator and robot design.  I designed and built all the all the hardware, both mechanical and electronics (with some design and fabrication help from Alex), and Jared wrote pretty much all the software and high-level control running the robot, including the Convex Model Predictive Control for locomotion.

The design principles behind the robot are very similar to Cheetah 3:  High torque electric density motors, low gear ratio, efficient transmissions to minimize friction and reflected inertia, lightweight legs with motors placed to minimize leg inertia.  A few things are different, though.  Mini Cheetah has 12 identical modular actuators with built-in motor control, gearbox, and support structure.   The electric motors are off-the-shelf and very cheap, unlike Cheetah 3's custom designed motors.  Mini Cheetah has even more range of motion than Cheetah 3 at the hip, so it's able to point it's legs completely sideways.  This means that it should be possible to make the robot land feet-first regardless of what orientation it falls in.  

I finished the hardware for the robot last May, but the publication cycle is kind of slow, so I wasn't  able to put up any info about it before.  There's going to be a paper about the robot in ICRA 2019 in Montreal, so the robot and I will both be there.

-------------

Jared and I (mostly Jared) did the backflips for our final project for Underactuated Robotics last spring.  There are some more details about how it works in the thesis, but basically we did a nonlinear trajectory optimization offline, and just played back the resulting joint torques on the motors, with a little bit of joint position control to the optimized trajectory on top.  Our simulation of the dynamics was accurate enough that it worked on the first try in the hardware.



Here are the videos from back in May.  At the time we weren't set up for backflips completely untethered and wireless.  Since then we've adjusted the flip trajectory a bit, made the landing less bouncy and more reliable, and gone fully wireless.






February 9, 2019

Big Dyno Up And Running

A long overdue progress update on the big dyno.

The torque sensor got new electronics Bayley designed, which have a built in power supply, A/D, and micro, to convert the signal to digital as quickly as possible.  I made a nice box to house all the electronics:


The torque sensor mounted, with the absorber on the right and the motor under test on the left:


To power and control the test motor, there's a bank of terminals for DC input and motor phase output, if you want to drive the test motor with the Prius inverter built into the dyno.  And quick disconnect fittings for water cooling the test motor:


Speaking of inverters, here's a look under the sheet metal covers:  Underneath are two Prius "bricks" mounted to a big waterblock.  Although the Prius power modules have two 3-phase inverters each, we had to use two of them so we could separate out the DC link current measurement for the inverter powering the test motor.


The dyno has an internal Mac Mini with linux on it, runing a fork of my dyno software.  Thanks to the cross-platform-ness of Python and Qt, all I had to do was install all the libraries and change the "COM" serial ports to "/dev/ttyUSB" and it just worked.  Yes, that's the "Hybrid Synergy Drive" plaque from the Prius power module covering up the Apple logo:


To go with the computer, it has a built-in monitor which can be raised and lowered for storage:


The absorber and all the sensors work, and we've helped the FSAE team do a little bit of testing on their new motors.  Next up is populating the second half of the control electronics so the dyno can control the test motor.  Then we'll be able to do a full optimization of the Sonata HSG and get even more power out of the big kart.