Hi Phil, On Mon, Sep 1, 2008 at 8:11 AM, Philip Pemberton wrote: > But surely a normal voltage regulator would work as well -- the current > through the motor increases, so the output of the regulator increases to keep > the voltage constant. Of course, in practice, it seems "constant voltage in = > constant speed out" isn't always true... But it isn't simply regulating the voltage. A good, simple mathematical model of a motor is a generator in series with a resistor. The motor torque is equal to Kt*I where Kt is the torque constant and I is the current. The generator produces a voltage of Kv*RPM with Kv being the voltage constant. This "generator voltage" is what is known as the "back EMF". The voltage on the output terminals equals Kv*RPM+R*I. Note that it is NOT exactly equal to the back EMF, there is also a resistance voltage drop proportional to current (torque). This is why, when you apply a constant voltage to a motor's input terminals, the speed decreases when you apply a load to the motor. 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. 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. > > I think the best plan might be to put a tachometer (laser diode + LED as > photosensor + reflective tile) on the output gear of the motor, see what speed > the ROHM chip has been set for, then get a PIC-based PWM controller (with > back-EMF sensing in the off period) to hold the same speed. I've found a motor > of the exact same type in an old CD-ROM drive, so I've got something to play > with on the breadboard at least. > > I did notice a lot of spikes on the motor's power lines -- these seem to be > due to the rotation of the commutator relative to the brushes. It looks like > multiplying by 60 and dividing by 3 gives a rough estimation of motor speed in > RPM. Of course, a certain Roman Black seems to have already discovered this > before me: 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. Sean > > Hmm. More thinking required, I think. > > Thanks, > -- > Phil. > piclist@philpem.me.uk > http://www.philpem.me.uk/ > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist