Tag Archives: qdd100

Ground truth torque testing for qdd100

First, a limited number of qdd100 servos are available for sale to beta testers!  Check them out at shop.mjbots.com.

After building up the first set of qdd100 servos, I wanted to empirically measure their performance parameters.  Some astute commenters uncovered in my terrible juggling video, that I didn’t actually have any ground truth measure of torque with these actuators.  Given that the ultimate torque is a pretty useful performance metric, it’s a good thing to have a solid understanding of.

To measure this, I built a simple test fixture (which is also the qdd100 beta development kit), consisting of two brackets.  The first lets the servo be bolted to a table, and the second mounts to the output and has set screws to hold a 1″ diameter pipe.  I used this to insert a 1 meter pipe which then can press against a digital scale.

1m is pretty long for my workshop!

Then I created a simple C++ application which emitted torque commands in response to joystick input and reported back telemetry from the servo: qdd100_test

Using these I was able to generate a plot of actual torque vs motor phase current:

qdd100 torque vs phase current

There are a couple of interesting things here, one is that the torque constant at low phase currents is slightly lower than I had estimated based on the motor’s Kv rating.  Second, the torque constant drops off faster at higher currents than I had anticipated, and third, the motor Kv rating was lower than I had predicted.  Those things combined result in a peak torque of between 12.5 and 15Nm depending upon the servo.  That’s still enough torque to do some serious jumping, but exploring those discrepancies is now on my backlog.

Here’s a video showing how this testing (and max speed testing) was done:

Simple walking gait on the quad A1

After I restructured my control laws to take advantage of high rate force feedback for the pronking experiments, I haven’t actually managed to port the walking gait yet.  Now that I have a brand new robot, it seemed like a good time!

This gait is basically the same thing as I ran on the quad A0 in principle.  The opposing feet are picked up according to a rigid schedule, and moved to a point opposite their “idle” position based on the current movement speed.  Any feet that are completely placed on the ground just move with the inverse of the robot’s velocity.

What differs now is that the leg positions and forces are controlled in 3D at a high rate, 400Hz for now.  At each time step, the position and velocity of all 12 joints is measured.  The gait algorithm calculates a desired 3D position, velocity, and force.  Feedforward force is currently only used to control the weight supporting legs.  Then, those 3D parameters are transformed into a joint position, velocity, and force based on the current joint position, and the command is sent out.

While not conceptually too different, just controlling the system in 3D at a high rate gives significantly improved results for a range of walking parameters.  There is still a lot left to do, but it is a good start!

(also, a limited number of the qdd100 servos are now open for sale to beta testers at shop.mjbots.comhttps://shop.mjbots.com/product/qdd100-beta-developer-kit/)