Re: PWM Back Emf

That doesn't really help me understand. It sounds like if I try to
modify the speed by changing the PWM, it won't; it will return to
preset speed. I think I am going to have to put a circuit together
and try and understand it.

Thanks for trying to explain it.



--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Mariss Freimanis"
<mariss92705@...> wrote:

>
> 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
> > > > > >
> > > > >
> > > >
> > >
> >
>