Hi guys, I've been tearing apart a Brother PT-1000 label printer with the eventual goal being to connect it to a desktop PC to print labels for my various component storage boxes. I've figured out how the print head communicates with the controller board, which just leaves the motor drive circuitry.
Naturally, being a low cost device (£15), the PT-1000 doesn't use anything remotely stepper-motor-like for the label feed. Instead, it uses a cheap Mabuchi DC pancake motor (an RF-300C-11440, for which I have yet to find a datasheet) and a speed reduction gearbox. The speed control is performed by a ROHM BA6220 chip.
Ideally I'd like to eliminate the ROHM chip (seeing as it's basically unobtainium) and use a PIC of some description (probably a 12F675) to do PWM speed control of the motor. But first I'd like to understand how the ROHM chip manages to do what it does.
I found a datasheet for the '6220 here:
.. but like most ROHM datasheets, it doesn't say much about the chip, other than that it uses back-EMF sensing and how to determine one of the two external resistor values.
What I don't get is that there's no obvious way for the chip to sense back-EMF. Everything I've been able to find about bEMF sensing suggests that it's normally used with PWM control -- the motor is powered up for a short period of time, then in the off period the voltage across the motor is sampled and used to (roughly) determine the motor speed. Unless it's sensing current, but if it is, the "application circuit" (BA6220 datasheet, Fig. 2, page 2) doesn't look like any current sensing circuit I've ever seen. In fact, it looks like a voltage comparator, but the polarity of the voltage reference doesn't look quite right...
I've hooked the scope up to the driver IC's pins (and the motor itself) and didn't see anything that suggested the driver IC was switching the power on and off. In fact, the voltage remained more or less constant, excepting the ~50mV sine wave (plus one ~200mV spike per cycle) modulation that I suspect is being caused by the motion of the commutator relative to the brushes.
Can anyone shed some light on this?
Like I said, I'm getting rid of this thing anyway, but I'd rather like to understand how the existing circuit works first, if at all possible...