Re: PWM Back Emf

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
> > > >
> > >
> >
>