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'Motor controling question'
1999\01\06@120203 by

Hi,

I4m thinking of measuring the speed of a Motor by evaluate the
current-spikes
I4ll get while the motor commutate (don4t know if it4s the right term)
well - I mean the short moment where a brush connects to two "coils".

My problem is how to filter this information from noise and the
current impulse caused by the PWM for driving the motor. Due to the
characteristic
of the Motor I 4ve to use a very low PWM frequency of about 150Hz.
Motor turns max 1000rpm and should run from approx 30 rpm controled up
to 1000rpm.

Especially how to filter when the frequency of commutation and the pwm
frequency are
nearly the same ?

I know it would be much more easy to use a reflex-coupler to get the
speed but
I4m somewhat sticking with this Idea because I think it should be doable
without
having th right idea on how-to.

The other question would be if a 16Cxx(x) Pic will have enough numeric
power
for the required math.

Kind regards

Stefan
sswoikossw.de

Stefan,

I came across an application that measured the back EMF from a DC motor.
The motor drive was PWM and during the 'off' times the system read the EMF
voltage from the motor into a sample & hold circuit. Using this the
rotational
speed could be calculated (the higher the voltage on the Sample & hold
output
the faster the motor is running).

Paul Fletcher.

Stefan Sczekalla-Waldschmidt wrote:

{Quote hidden}

>Especially how to filter when the frequency of commutation and the pwm
>frequency are
>nearly the same ?

You can know exactly when the PWM frequency is occurring by tapping into
the FET driver.

>I know it would be much more easy to use a reflex-coupler to get the
>speed but
>I4m somewhat sticking with this Idea because I think it should be doable
>without
>having th right idea on how-to.

Look at the FET gate current as well.  You need a shunt in the line feeding
the FET.

>The other question would be if a 16Cxx(x) Pic will have enough numeric
>power
>for the required math.

Very easy for a PIC.

Andy

==================================================================
Andy Kunz - Statistical Research, Inc. - Westfield, New Jersey USA
==================================================================

Hi Paul,

Paul Fletcher wrote:
>
> Stefan,
>
> I came across an application that measured the back EMF from a DC motor.
> The motor drive was PWM and during the 'off' times the system read the EMF
> voltage from the motor into a sample & hold circuit. Using this the
> rotational
> speed could be calculated (the higher the voltage on the Sample & hold
> output
> the faster the motor is running).
>
>     Paul Fletcher.

This is the common way.  I look for something different because the
common
way is not applicable with my motor. (series wound motor).

Kind regards,

Stefan

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