It depends on what you want to keep constant. If you want constant RPM the current does not really give you any valuable information for this loop. It would give you information about the torque though.
Some small grinders, like those made by proxxon that have adjustable speed, have a current feedback speed control. Basically they notice a higher current means more load so it will slow down so they apply more voltage to keep the speed up. This is done because it is much cheaper to sense current than to sense RPM, but you can get a better system if you really measure RPM.
One of my power drills has a magnetic sensor on the shaft, that is really a good one, stable RPM and lotsa torque even if you set it to the slowest speed and load it badly like for driving in screws. If you try that on a cheap drill without the sensor feedback you will get far too many RPM with no load and it will stall if loaded.
ST
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On Wed, 03 May 2006 21:10:45 +0200, lcdpublishing <lcdpublishing@...> wrote:
Ah but this is a great chance to learn a lot for me too!
While at first glance, you wouldn't think a PCB dril would cause a
load on a spindle motor, however, when that motor is little, the
load is relative to the motor, a small drill and a small motor
should make for a good experiment!
In reality, other than for knowing if the motor actually stopped
turning (Stalls), it isn't needed at all for the little machine.
But for working out many of the details of that process, this very
well could be a great test bed.
Hmmm, thinking about this a bit more, what if I also bring into the
loop the current draw of the motor? If the current is near peak,
then I know the motor is actually stalling.
Having only stepper motors to compare against, I don't know about
this at all. When using PWM for motor speed control, the voltage is
constant, but the current is switched on and off. Would I still be
able to use a current sensor in such a circuit like this?
Hmm, this is getting more and more interesting by the minute!
Balancing the feedback of current and speed against what the motor
should be doing - I like!
Chris
