Re: PWM Back Emf

That -V 'dip' is caused by the inductive energy stored in the motor.
You can read the motor back EMF only when this current decays to zero.
This negative pedestal will get wider as you apply an increasing load
to the motor.

This puts unacceptable restrictions on your circuit:

1) It only works at low speeds (low PWM duty cycle)
2) It only works at low PWM frequencies (<<1kHz).
3) It only works at light loads (<10%).

A much more effective technique is to use "IR" compensation. It
removes all the above restrictions. IR compensation adds positive
feedback by increasing the PWM duty cycle proportional to motor load
and can regulate your motor speed to within +/-2%. Do this:

1) Use a current-sense resistor to generate a voltage prop. to load.
2) Filter and amplify this voltage.
3) Use a trimpot to attenuate (2) to the exact value needed.
4) Sum (3) with the PWM command voltage using an op-amp.

Adjust the trimpot to where the motor speed doesn't change with load.
Too little and the motor will slow down, too much and the motor will
speed up with increasing load.

Mariss

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "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
>