Maxwell – Robot Servo Linearity Testing

When I visited Akihabara last Saturday with Inoue-san, I picked up several different types of LEDs and phototransistors to experiment with. The initial challenge was to hack together a way for Maxwell to test the linearity of his servo motors. Later the knowledge gained will be useful in developing an encoder strategy and for other applications.

The first approach used a discrete LED and a photodiode. It was workable, but maintaining the physical positioning of the components and aligning them with the servo wheel marking turned out to be the big challenge. The second approach used a packaged LED/phototransistor pair.A small opaque 'flag' was fabricated from tape and attached to the side of the wheel, then trimmed so that it passed through the center of the detector.

A simple circuit was breadboarded on Maxwell's BOE -


Note: D1 and Q1 share the same packaging with a small slot for the flag to pass between them interrupting the optical path.

The circuit output signal was connected to IN3 on the Basic Stamp controller. A test program was written and debugged to cycle through a range of PULSOUT values measuring the number of commands required to rotate the servo motor 360 degrees. Since the flag position was unknown at the beginning of each cycle, the program had to reposition it automatically. An unexpected benefit of this approach was that the servo was already in motion at the start of each measurement. During the previous tests the servo was always at rest at the start of each test cycle. This meant that the servo had to overcome some initial inertia and accelerate to the test speed during the test itself. The impact would be minor for low rotational speeds, but would rapidly impact the overall measurements as the speed increased. Having the servo already rotating at the speed under test is a more accurate test.

At the completion of each cycle, the program outputs the measurements via the DEBUG console where they can be copied and moved to Excel for analysis. The analysis process can be simplified quite a bit if the program outputs the data in a pre-processed form. For example, if it writes the data to the DEBUG console in comma delimited format, it's easy to just cut and paste into Excel without having to reformat and/or edit the data.

This chart shows the test results for Maxwell's right servo motor when tested with PULSOUT values ranging from 550 to 740 in increments of 5 -


The automated testing provides much more consistent results as well as allowing for improved measurement granularity. For example, the earlier tests were run manually with test increments of 10. The initial automated test increment was 5, but could be as low as 1. One concern is that for fast rotational speeds the number of PULSOUT commands sent is relatively small - in the range of 70 or so per 360 degree rotation. To improve the measurement granularity the program can be modified to measure the number of commands required for mulitple rotations.

The biggest single advantage? Simple - I can start the testing, then go do something else. I don't have to babysit it. And, it allows me to run longer, more extensive testing than the manual approach.


One thought on “Maxwell – Robot Servo Linearity Testing

  1. Wow, I just found your site and it’s great!
    I will be adding a link back to you from my site (focuses on news and developments in robotics).
    good luck with your robot!

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