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'[EE]: DC motor control'
2002\08\26@021022 by Justin Grimm

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Does anyone know the best way to control the speed of a 12v DC motor without
losing too much torque? Would pwm work or just varying the voltage?

Thanks
Justin

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2002\08\26@085726 by Scott Touchton

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To the best of my knowledge, the only way to control the speed of the motor
and retain its most of the torque at very slow rpm's is via a PWM scheme
based on actual shaft feedback.

I did this real simply once with an opamp and a smaller dc motor acting as a
generator.  I used another dc motor as a reference.  As I spun the reference
motor, the controlled one would duplicate.  Adding a comparator and an H
bridge allowed my to also control direction.  (this became the heart of a
model boat when I was a kid.... dead slow to full throttle).
{Original Message removed}

2002\08\26@093122 by Spehro Pefhany

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At 07:03 AM 8/26/02 +0100, you wrote:
>Does anyone know the best way to control the speed of a 12v DC motor without
>losing too much torque? Would pwm work or just varying the voltage?

Open-loop PWM works pretty well on PM DC motors (or DC motors with a
separately energized field winding that you leave at full voltage).
Depends what you need, of course. No-load RPM will be approximately
proportional to the average voltage, IOW  Vin* (ton/(ton+toff)).
You can't compare it to AC motors under similar circumstances (much better).

If you *do* need feedback, there are a couple of sensorless ways of doing
it (each with its own limitations and advantages), or you could just
hang a tacho off the motor and code up a PI controller.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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2002\08\26@094149 by Justin Grimm

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Its for a plotter Im making. I'm going to use an encoder for feedback
(position) and maybe a PI loop. Its pretty heavy duty, Id use steppers but I
cant find any powerful enough. I want to slow the dc motor down as it
approaches the encoder position I want. I was thinking of using power window
winder motors.

Justin

{Original Message removed}

2002\08\26@095641 by Spehro Pefhany

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At 02:36 PM 8/26/02 +0100, you wrote:
>Its for a plotter Im making.

Ah, the plot inspissates.

>I'm going to use an encoder for feedback
>(position) and maybe a PI loop. Its pretty heavy duty, Id use steppers but I
>cant find any powerful enough. I want to slow the dc motor down as it
>approaches the encoder position I want. I was thinking of using power window
>winder motors.

You might want to look for surplus servomotors. They generally have the encoder
and are made for continuous duty service. I've seen them for very reasonable
prices, like $10 (US).

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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2002\08\26@110343 by Scott M. Thomas

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I bit banged a PWM routine (several bits of sample code available on
piclist.com under I/O routines) then immediately before turning on the
output transistor I measured the voltage at the motor using the A/D module.
I used this measured value to adjust the duty cycle up or down.  Worked
pretty well.  All I did for a feedback circuit was a 10k resistor from the
drain of the transistor to the RA0 input with a 6.8k to ground to divide the
voltage down and a .33uF cap to ground to smooth out the waveform.  I was
running at about 150 Hz so I had lots of time to measure and do
calculations.

{Original Message removed}

2002\08\26@131725 by David Minkler

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Justin,

Without feedback you will not be able to control motor speed without
losing torque.  What Scott is describing is using the EMF from the motor
(operating as a generator / tachometer) at the end of the PWM off time
as his feedback signal.  With good filtering, sampling at the right
time, and not attempting to operate at very low speeds this technique
works well.

Regards,
Dave

"Scott M. Thomas" wrote:
{Quote hidden}

> {Original Message removed}

2002\08\26@135002 by Nelson Hochberg

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I vote for the surplus servo motors.  The backlash on the power window
motors will probably drive you nuts.  I got two servo motors from
http://www.meci.com/ to build a tug to move my 3200 pound Piper Aztec around
(didn't need the encoder outputs). I bet those babies would have enough
power for your app.  Don't know if Meci still has those.

Nelson
nelsonspamKILLspamnosuffering.com
http://www.nosuffering.com

{Original Message removed}

2002\08\26@143930 by Donovan Parks

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Hello Spehro and PICsters,

I'd be very interested in these sensorless ways of obtaining feedback.  Can
you elobrate or point me to some resources?

Donovan,

> If you *do* need feedback, there are a couple of sensorless ways of doing
> it (each with its own limitations and advantages), or you could just
> hang a tacho off the motor and code up a PI controller.
>
> Best regards,
>
> Spehro Pefhany --"it's the network..."            "The Journey is the
reward"
> .....speffKILLspamspam.....interlog.com             Info for manufacturers:
http://www.trexon.com
> Embedded software/hardware/analog  Info for designers:
http://www.speff.com
> 9/11 United we Stand
>
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>
>
>

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2002\08\26@145413 by Peter L. Peres

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On Mon, 26 Aug 2002, Donovan Parks wrote:

>I'd be very interested in these sensorless ways of obtaining feedback.  Can
>you elobrate or point me to some resources?

The motor is also a generator and if you turn off the drive momentarily
and measure the voltage you have a pretty accurate assessment of rpm. This
happens too fast for the motor or the load to notice. It is also possible
to do this without interrupting the voltage at all in a linear driver.
This is harder to explain but it works on the same principle (except it
also needs to know the dynamic internal resistance of the motor). The
speed regulated cassette tape (walkman) motors almost all use this
principle (excepting high end units which have tachos and/or are
synchronously driven by a mcu).

Peter

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2002\08\26@150537 by Spehro Pefhany

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At 11:37 AM 8/26/02 -0700, you wrote:
>Hello Spehro and PICsters,
>
>I'd be very interested in these sensorless ways of obtaining feedback.  Can
>you elobrate or point me to some resources?

There are two different principles, both of which depend on back-EMF
being proportional to RPM. Keep in mind that the armature current
is proportional to the torque (including windage and friction). These
apply to PM and parallel wound motors with separate field energization.

1)      Sample back-EMF (no-load armature voltage) after the inductive
        spike dies down and before re-applying voltage to the armature.
        PWM frequency has to be relatively low (more noise and losses)
        and the reading must be synchronized. There's only  limited
        speed range possible this way.

2)      Measure the armature current and increase the voltage applied
        (through increasing the PWM on-time) to just compensate for the
        internal resistance (if the internal resistance was 0, the motor
        would maintain a constant speed regardless of torque). The amount
        of compensation varies from motor to motor, and within the motor's
        range of operation.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
EraseMEspeffspam_OUTspamTakeThisOuTinterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
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2002\08\26@160420 by Olin Lathrop

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> Does anyone know the best way to control the speed of a 12v DC motor
without
> losing too much torque? Would pwm work or just varying the voltage?

Both would work, but PWM allows for much better efficiency, and can be
directly produced by a PIC.


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2002\08\26@162212 by Digiled dot com

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Hello Justin,
JG> Does anyone know the best way to control the speed of a 12v DC motor without
JG> losing too much torque? Would pwm work or just varying the voltage?
JG> Thanks
JG> Justin

   It depends on the motor type.
   If it's a 4-wire motor you could use independent field excitation.
   If it's a permanent magnet motor, PWM is the choice (prefered with
   feedback closed loop)

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2002\08\27@031822 by Justin Grimm

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First of all thanks everyone for your replies. I think when I get access to
the internet I'll check out the servo motors. In the meantime though this
method of pwm then sampling the generated voltage intrigues me.
I want to deliberately control the speed, so if I'm generating say a 50%
duty cycle pwm, and I want to slow the motor down, I just shorten the duty
cycle then read off the inferred speed (generated voltage) and if its not
slow enough shorten the pwm again. Is this correct?

Is there a easy way of knowing what the generated voltage would be?

Thanks
Justin

{Original Message removed}

2002\08\27@073234 by Olin Lathrop

face picon face
> First of all thanks everyone for your replies. I think when I get access
to
> the internet I'll check out the servo motors. In the meantime though this
> method of pwm then sampling the generated voltage intrigues me.
> I want to deliberately control the speed, so if I'm generating say a 50%
> duty cycle pwm, and I want to slow the motor down, I just shorten the duty
> cycle then read off the inferred speed (generated voltage) and if its not
> slow enough shorten the pwm again. Is this correct?

I think so, if "shorten the PWM" means to decrease the duty cycle but leave
the period the same.  Otherwise it's not PWM (Pulse Width Modulation),
although other schemes can be used too.

However, there are some gotchas with this scheme of detecting the speed of a
DC motor:

1  -  It doesn't work at high PWM duty cycles.  The motor has significant
inductance, and current will continue to flow for a while thru the flyback
diode after the drive voltage is removed.  This flyback voltage is large
noise on the signal you are trying to measure, which is the open circuit
back EMF created by the motor running as a generator.  One way to get around
this is to use a higher voltage than the maximum average voltage you ever
want to apply to the motor.  That guarantees a maximum PWM duty cycle, but
also alows for nasty consequences in case of software bug or other failure.

2  -  The back EMF of a DC generator is not a nice flat DC signal.  Each
winding by itself produces an AC signal, which are selectively switched to
the output by the brushes at the right time so that the winding with the
maximum positive peak is always selected (this is a bit simplified, but a
workable model in this context).  Point sampling this waveform is a bad idea
because the AC component is quite significant.  It would have to be averaged
over at least one motor pole.  This is usually not practical or possible.


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2002\08\27@092718 by Scott M. Thomas

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Yes, shorten the duty cycle to slow down, lengthen to speed up.

As far as knowing the generated voltage, it all depends on how precise you
need to be.  I did not need to be that precise, just maintain high torque at
low speed so I *guessed*.  My motor was rated 5000 rpm at 12V so I *assumed*
that the generated EMF would be 12V at 5000 rpm.  I also assumed that the
generated voltage was linear (i.e. 6V = 2500 rpm).

Now, please note all the assumptions.  IF YOU NEED HIGH PRECISION DON'T DO
THIS!  You might be able to spin the motor at various speeds and measure the
voltage to make a more accurate determination.

The description of my feedback circuit is attached, if you want more details
or a picture I can dig up a schematic.  I found the RC low pass filter
described to be adequite for my needs.  The waveform I observed looked like
a full wave rectified sine wave so the capacitor smoothed that out nicely.

I was originally skeptical of this approach myself, but I tried it and it
worked!

Thanks
Scott

{Original Message removed}

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