Tuesday, January 24, 2012

MKI: First steps

Yesterday three MKI boards came in the mail and today two motors were driven by one MKI board. I'm very pleased with this controller so far because whenever it doesn't do what I want it to, it turns out it's my fault and not its; which is great! Let me explain...



MKI was easy to solder together, despite having no solder mask. The trickiest part was actually preparing XPWMShield. To accommodate a motor controller upstairs, the 5V regulator was just too tall and had to be bent down; which also meant putting the battery input terminal on the bottom. In addition, two of the servo pwm inputs were flipped (to eventually incorporate servos). All power is passed through a temporary splitter that supply the boards individually. When TOBL2 comes together I'll make a nicer connector.


Shield mode

Once it was all together I ran into a slight problem with motor A. With no solder mask I shorted a small gap by one of the diodes which meant the motor would run fine and make the L298 burning hot in the other. After a few tests runs - being oblivious to this problem - it still works fine so the stock heat sinking does a great job, which was a concern given there is no space on the board to add heat sinking. Here are the motors running through a quick forward and reverse ramp program:

In getting to this stage I learned some subtleties about the L298. For example, I forgot to mention why the motors took their first steps today and not yesterday. In the datasheet ST says "The sense resistor, not of a wire wound type, must be grounded near the negative pole of Vs that must be near the GND pin of the I.C." Maybe it's just me but this doesn't get across the same message as "The current sense pins must be connected in some way to ground for the motors to work!" -which I found digging though forums. Until I get small enough resistors there is a wire in their place to keep the controller happy.

It's also important to note that for now at least, the regulator I had planned to use is bypassed by a wire, supplying 7.4 uncensored volts. It was hard to find a regulator with a low enough differential between input and output voltages to be efficient. Essentially with the current 2S pack I'd need a 4V regulator to support the voltage overhead and voltage loss during use. This would mean dissipating (read: wasting) a stupid amount of power. Combined and unloaded the motors draw 10mA times 3.4V loses only 34mW. However, during loading and more extreme conditions 2A is feasible which would be a 6.8W loss; which is quite significant! We'll see how it works without a regulator for now...glad I was talked out of it.

Next steps are to test the current sensing and implement my iPhone controller setup. Classes start tomorrow. Boooo.

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