Re: [CAD_CAM_EDM_DRO] PWM Back Emf

If you use a closed servo loop set up for constant velocity, the error
signal is essentially a manifestation of the back emf (and thus the load).

Run the PWM much faster -- at least 20 k -- and filter and rescale the error
signal for a read of the motor's load.

On 1/13/07, Frank <ftkalcevic@...> wrote:
>
> This is slightly off topic, but it is motor control...
>
> I am building a small dc motor controller to drive toy brushed dc
> motors. This is all low voltage (<5v) and low current (150mA max).
> I want to measure the back emf between PWM pulses to provide a
> fairly constant motor speed. To measure the back emf, I have two
> resistors acting as a divider (10k each) between the motor
> terminals. The PWM only pulses the High side FETs, so when they
> turn off, the motor terminal runs through the divider and to
> ground. The divider gives me 1/2 the back emf.
>
> This picture shows the voltage divider trace -
> http://home.people.net.au/~frankt/pwm_bemf.png
>
> After the PWM pulse is switched off, the voltage divider shows a
> negative voltage, about -0.4V for about half the off pulse. After
> that, it pops back up and shows the back emf which I can measure.
> This unfortunately is causing me grief, because I have to wait until
> just before the next pulse to measure the back emf. What's even
> worse, is that period of negative voltage is fixed - if I double the
> PWM frequency I will not get any back emf. The sample shown in the
> capture is at a noise 4kHz - I'd like to get well above that.
>
> So, the questions, what is causing that small negative dip? Is it
> normal? Is there a way I can remove it? Or, with an increased
> frequency, do I have to stop PWMing until the back emf signal
> stabilises to read it?
>
> Thanks,
> Frank
>
>


[Non-text portions of this message have been removed]