Sean Breheny wrote:
> As you apply a load, the motor
> draws more current because of the required torque, which in turn
> causes more voltage drop across the internal resistance, so that the
> speed must slow to the point where the "generator" back EMF reduces
> and makes the back EMF+resistor drop = input voltage again.

And this would be another one of those times where I completely forgot to 
account for series resistance in a circuit...

I get the idea now -- more load = increased current, V=IR states that as 
current increases, the voltage drop also increases...

> This chip you mentioned is effectively a voltage regulator which
> purposely INCREASES its output voltage in proportion to the amount of
> current drawn from it. Its output voltage equals something like
> Vo+F*I. F has units of ohms but it has the opposite effect (voltage
> increase instead of voltage drop). You adjust F to be close to the
> internal resistance of the motor and you get very little speed
> variation with torque.

In other words, measure the resistance of the motor while it's at rest (so 
back-EMF has little or ideally no effect), and then use Ohm's Law to figure 
out how much voltage to add on in order to negate the voltage drop.

Thanks for the explanation, Sean. I think the gears have finally clicked into 
place now :)

> Well, all of this depends upon how accurate you need the speed control
> to be. Anything which supplies a known number of pulses per revolution
> will allow you to be much more accurate than back EMF measurement,
> since the Kv value of the motor varies from motor to motor and with
> temperature and other variables.

I doubt it needs to be particularly accurate. All this motor is doing is 
feeding the label tape past the print head -- as long as it maintains its 
speed fairly well when the motor load changes. I suspect any load change will 
be very slight -- mostly due to the tape resisting if it gets stuck inside the 
cartridge.

Seeing as I can't get to the innards of the printer mechanism without ripping 
it apart, I'm going to attach my "spare" motor to the controller board, glue a 
piece of tinfoil to the output gear and see if I can get a decent speed 
reading to use as a reference.

Thanks,
-- 
Phil.
piclist@philpem.me.uk
http://www.philpem.me.uk/
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