Sunday 3 June 2007

Less speed more torque

This weekend I built a speed controller for my milling spindle motor, AKA Minicraft drill, after attending and recovering from the Stockport Beer & Cider Festival . I used one of the MSP430F2013 micros I described in the previous post. I built it on Veroboard, but hopefully all my future boards will be milled PCBs.

I never plan my Veroboard layouts in advance, I just make it up as I go along. In fact this circuit was so simple I didn't even draw a schematic. I did come a cropper this time though. I started on a board that was too small!

I only realised this after I had put on a regulator, LED and connector, so I had to desolder them and start again. The LED did not survive the ordeal. In my experience, they are one of the most fragile electronic components. They certainly don't like stress on their legs while being soldered.

The circuit is very simple, I will publish a schematic and the software when I complete it, assuming it works, which I have no reason to think it won't.

You may think "how is that puny little micro controlling a 40W motor"? What you can't see in the picture above are two surface mount FETs on the underside.

The big one is a BTS134D "smart low side power switch" with over voltage, over current, thermal and ESD protection. It has an on resistance of only 50mΩ so I can get away without a heatsink. With devices like these I don't know why anybody uses Darlingtons for switching nowadays. Darlingtons have a fundamental flaw in that they cannot be saturated so the on voltage is over a volt leading to significant power dissipation, and hence a large heatsink, for these sorts of currents.

The tiny FET next to it simply boosts the gate drive from the 3V output of the micro up to 12V to ensure the big FET delivers the smallest possible on resistance.

The picture below shows the board being tested. It is connected to a 39W PSU, the drill motor and a scope.

I have verified that it can turn the motor on and off OK. It just needs some software now. The micro has a timer with a PWM facility so controlling the speed of the motor should be pretty simple. The large resistor at the bottom left is for sensing the motor current. The micro has an ADC so I should be able to measure the speed to allow some feedback and also shut it off if it stalls. The unpopulated connector is the I²C link which will go to HydraRaptor's main controller.

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