Re: PWM Back Emf

It's actually very simple. A motor slows down with increasing load.
Increasing the motor voltage speeds it up. Sense the motor load and
increase the motor voltage just enough to compensate. Result is no
motor speed change as motor load goes from zero to max. Uphill will be
just as fast as downhill.:-)

Mariss

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Frank" <ftkalcevic@...> wrote:
>
> Thanks for that. Not the simple answer I was hoping for.
>
> I'm not sure that solution will entirely solve my problem. This is
> for a radio controlled toy train. I have a handset that can set the
> speed digitally from 0-63. I want to keep the speed constant at the
> set value regardless of load eg number of carriages or gradient. I
> think I still need the back-emf (or something) to determine the
> speed, and once there, your circuit will keep it there.
>
> I have tried a simple encoder feedback (a disk with 4 stripes) and
> had some success with it, but it was a bit bulky to fit in the small
> train.
>
> Thanks
> Frank
>
>
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Mariss Freimanis"
> <mariss92705@> wrote:
> >
> > 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
> > >
> >
>