The qdd100 servo uses a planetary geartrain as the transmission reducer. This consists of an outer ring gear, an inner sun gear connected to the rotor as the input, and 3 planets connected to the output. The tolerances of these gears directly impacts the performance of the servo, namely the backlash and noise.
To date, I’ve been hand-binning these and testing each servo for noise at the end of production. To make that process a bit more deterministic, and with less fallout, I’ve built up a series of manual and semi-automated gear metrology fixtures to measure various properties of the gears.
Some of the simple ones are just tools to hold micrometers in convenient locations relative to gears or meshing gears, like this one to measure the OD of the ring gears at various points:
Or this one to measure the meshing of the sun gear with a rack gear:
Or this one to measure the meshing of a ring gear with a reference sun gear:
Semi-automated tools
As I went to use these techniques for production, manually measuring the gears both was tedious, and still not as useful as it could be. It wasn’t feasible to do more than record a minimum and maximum when measuring a gear by hand, and for some parameters, measuring it at many points around the circumference is helpful. Thus, I’ve started on some automated gear testing fixtures.
The first is one that tests sun gears against a reference planet:
This has a few pieces. The motion platform is a moteus devkit motor with a reference planet gear attached. This spins a “test” sun gear which rests on a linear rail. Then a dial indicator is positioned to record the position of the carriage. An arduino connects to the SPC data port on the dial indicator to programmatically read the position. I used a technique similar to this forum post, except that my iGaging dial indicator runs off about 3V, so I didn’t bother with a separate pull down transistor and just toggled the REQ pin between input and output low to initiate readouts. That meant I could just plug the 4 wires directly into the Arduino.
When this runs, the reference planet is spun through small increments and the micrometer reading is captured at each point. This measures the “double flank” mesh distance of the gear pair. Here, the indicator spring applies a pressure to the test gear, forcing it to mesh with the master gear.
To make this work, I characterized the reference planet gear by running a reference sun gear (which is a 20 tooth M0.5 gear), at all 20 different phases relative to the planet. Then I took the median of the distance across all the runs as the “reference curve” for this planet.
Then test gears are measured relative to that reference curve. That shows the delta between the center distance at each point and the reference distance, so should be relatively well calibrated for the fact that my master gear is not perfect, nor mounted perfectly concentric. Here is a plot from the same gear taken four times at different phases, shifted laterally to compensate for the phase difference and shows that it is relatively consistent and repeatable.
The process is unfortunately slow, primarily because the dial indicator SPC port only emits data at 2Hz, and it takes about 2 readings to settle after each motion. I was using 8 points per planet tooth for the above plot, which works out to 320 total samples per evaluation. At 1.5s per sample, that is around 7 minutes per gear! Fewer points still give reliable results, at a corresponding reduction in fine resolution.
Forgiving the slow speed, this does seem to give profiles that are repeatable to within about 10 microns, which is good enough for the binning I am doing now.
is the input current, 
is the motor torque constant, 
, 
and 
are three constants that I fit to measured torque data. With some approximations, this can be calculated relatively efficiently on the STM32G4 that drives the moteus controller, adding only a microsecond to the overall loop time to go in both directions.
A Modicum of Fun 