Re: PWM Back Emf

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
> >
>