Re: PWM Back Emf

That's where the trimpot comes in. Too much and the motor speeds up
with increasing load. Too little and it slows down. Just right and the
speed doesn't change at all. Kind of like Goldilocks, the 3 bears and
the beds, the chairs, the porridge or whatever.:-)

Mariss

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Frank" <ftkalcevic@...> wrote:
>
> I understand the theory you are describing - detecting a change in
> current implies a change in load and a change in speed, so change
> the PWM voltage to compensate.
>
> However, when I start the motor the current hits about 100mA before
> the motor starts spinning. When it spins the current drops to
> 80mA. Once spinning I can reduce the PWM duty cycle so the current
> is about 30mA and the motor is still spinning slowly. Don't I need
> to be at a known speed/load balance point before applying the
> compensations? I can't just say 50mA represents 10% speed because
> my motor will not have started spinning.
>
> Of course all my logic could be wrong because I am testing with an
> unloaded motor.
>
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Mariss Freimanis"
> <mariss92705@> wrote:
> >
> > 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
> > > > >
> > > >
> > >
> >
>