Another (perhaps old fashiond) way to measure DC permanent magnet motor speed is
 to measure the back EMF from the motor when the voltage to the motor is momenta
rally off.  If you are doing PWM, then when the current is off, if you read the
back EMF voltage from the motor, it is directly related to speed.  I have used t
his and the speed control is quite accurate -- You are using the motor as a DC g
enerator for an instant.

This method takes more parts than measururing ON current, usually needs an op-am
p circuit, but way cheaper than a tach.  This is probably to complex a method fo
r an H bridge driver, but its quite easy for a PWM single direction of rotation
application.
  Regards,
     Ron Fial

At 12:56 PM 6/22/98 +0200, you wrote:
>Hi all,
>
>As a newcomer to the list, I am interested in speed regulation of a
>small DC brush motor (e.g. as used in model trains) by using a PIC. Most


Regards,
  Ron Fial