On stepper motors

I have never designed a system with stepper motors in them, but I have used such systems in the past. After my first experiments with the SUNDAR, it became apparent that the best solution would be to replace the RC servor with a stepper. This I did using a small stepper and control board from Pololu. Both were simple to use and I had a somewhat working system in short order.

But it had warts. The motor did not seem to have much torque , and it would skip steps if I tried to make it spin faster than a crawl. I had been using full steps assuming I would get the fastest speed in this way, but it turns out the answer is more complicated. When using full steps, the coil current is either -100%, 0%, or 100%. Changing from one configuration to another is a large shock to the system. Its like getting hit with a hammer. The inertia of the rotor may prevent it from moving to the next step. But if you micro step, you are gradually ramping up one coil current as to decrease the other. The resulting torque is applied much smoother and the rotor can keep up.

So by decreasing the time between steps but using micro stepping I was able to at least double the motor speed without any lost steps. As a test, I ran the motor for one hour rotating a revolution one direction and then back the other. After the hour, the motor was still perfectly alignent. Now it is possible that I lost as many steps as I gained and had a zero sum gain, but I suspect that the motor simply did not miss any steps.

During this same test I also monitored the temperature of the motor. Rather surprisingly, the temperature went from ambient of 67F to about 130F! This is very different than a DC motor with little load. But a stepper always has the sum of its coils at 100% current and hence consuming 6 watts of power. 6 watts into a small chunk of metal will get hot. Consulting the datasheet the temperature rise was well within spec.

Since these initial tests, I have also done some mechanical work on the SUNDAR. I built a new platform to hold the motor and a new platform for the sensor. I am now working on a way to run the three needed wires from the base to the rotating sensor platform without getting tangled. I am also adding an optical beam break sensor to index the sensor platform on startup.

But at the same time, I am working with a group of elementary to middle school students on their FIRST LEGO League (FLL) research project. Been teaching them how to program Arduinos and the basics of electricity. Since they will be coming over to use my shop next weekend, I have been spending much of my time cleaning up to make room for them. Then the two weekends after that, I am a judge for FLL. Then after that we get close to Thanksgiving and after that Christmas. I do not see a way to get everything I need to complete done by June of next year for the 2014 AVC. Perhaps 2015?

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