At 11:37 AM 8/26/02 -0700, you wrote:
>Hello Spehro and PICsters,
>
>I'd be very interested in these sensorless ways of obtaining feedback.  Can
>you elobrate or point me to some resources?

There are two different principles, both of which depend on back-EMF
being proportional to RPM. Keep in mind that the armature current
is proportional to the torque (including windage and friction). These
apply to PM and parallel wound motors with separate field energization.

1)      Sample back-EMF (no-load armature voltage) after the inductive
         spike dies down and before re-applying voltage to the armature.
         PWM frequency has to be relatively low (more noise and losses)
         and the reading must be synchronized. There's only  limited
         speed range possible this way.

2)      Measure the armature current and increase the voltage applied
         (through increasing the PWM on-time) to just compensate for the
         internal resistance (if the internal resistance was 0, the motor
         would maintain a constant speed regardless of torque). The amount
         of compensation varies from motor to motor, and within the motor's
         range of operation.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
9/11 United we Stand

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