Whining PWM motor control

Hello Robert,

It all looks as if the motor loses too much current between pulses which
may be due to my KNOWINGLY poor choice of freewheel diode. I'd like to
put in a BYV28-100 but my supplier is out of them. Any better ideas for
a few-amps fast recovery diode?
Click to expand...
As John said, Schottkys are very fast in recovery. Look at the ONSemi
MUR/MURS series depending on whether you'd like SMT or through-hole.

Regards, Joerg

http://www.analogconsultants.com
 
Hello Robert,

Whatever it is, it has to go into the inaudible range. These are small
motors, it should be doable.
Click to expand...
It should be feasible. If you use a fixed frequency oscillator as you
already do that takes the first hurdle. Now you have to make sure your
loop cannot fall into slight instabilities where it starts "pumping".
For example, if your pulses cut in and out that will modulate the drive
signal and become audible with the motor as the "sepaker". You could
look at the (filtered) feedback signal with a scope to see how that behaves.

Then there are noises inherent to the motor construction that depend
solely on speed. There is next to nothing you can do about those other
than selecting a different motor.

My sister's boyfriend's job used to be the acoustics of auto gears. He's
moved on to motor acoustics now. They're really picky about what a car
sound like on the inside.
Click to expand...
Once I got a ride in a Ford Model A. Big surprise: This old car is
surprisingly quiet on the outside but quite noisy inside. But it offers
a lot more legroom than many modern cars.

Regards, Joerg

http://www.analogconsultants.com
 
Hello Jim,

..., once in First Class ;-)
Click to expand...
If that was on American Airlines, did you get the caviar on blini?

Once I snoozed so soundly in a first class seat that I didn't feel the
landing in Europe and they had to wake me up. All the other passengers
were already gone. Pretty embarrassing.

Regards, Joerg

http://www.analogconsultants.com
 
On Fri, 19 Aug 2005 20:08:14 GMT, Joerg
<notthisjoergsch@removethispacbell.net> wrote:

Hello Jim,

..., once in First Class ;-)

If that was on American Airlines, did you get the caviar on blini?
Click to expand...
Yep. And it was American Airlines. And I drank so many (nearly
frozen) Vodka shots along with the blinis, that I awoke in the middle
of the night with an urge to barf. Fortunately I was able to sip
enough water to dilute the stuff ;-)

Once I snoozed so soundly in a first class seat that I didn't feel the
landing in Europe and they had to wake me up. All the other passengers
were already gone. Pretty embarrassing.

Regards, Joerg

http://www.analogconsultants.com
Click to expand...
Those 747 landings are pretty amazing.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
 
Hello Jim,

Those 747 landings are pretty amazing.
Click to expand...
Didn't know AA still uses those. My flights were usually on a 767.

Regards, Joerg

http://www.analogconsultants.com
 
On Fri, 19 Aug 2005 20:44:06 GMT, Joerg
<notthisjoergsch@removethispacbell.net> wrote:

Hello Jim,

Those 747 landings are pretty amazing.

Didn't know AA still uses those. My flights were usually on a 767.

Regards, Joerg

http://www.analogconsultants.com
Click to expand...
This was in '93-'94.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
 
Jim Thompson skrev:

On Thu, 18 Aug 2005 19:17:35 -0700, Tim Wescott <tim@seemywebsite.com
wrote:

Jim Thompson wrote:
[snip]

Use a clock, so that it runs constant frequency, then PWM.

The whine is probably the pole pieces flexing. I did a controller
design for Bosch/Mercedes and passing the noise spec was the hardest
part... The director of engineering stuck his head in the air duct,
listened, and decided yes or no ;-)

...Jim Thompson

I hope that when faced with a spec that says "whatever he likes" you
gave an estimate that said "however long I take".

Of course. Also made about a dozen trips to Germany in Business
Class, once in First Class ;-)

...Jim Thompson
Click to expand...
lol, I can't imagine even First Class making 10 hours across the
atlantic
something you would do for fun ;)

-Lasse
 
On 19 Aug 2005 14:03:31 -0700, langwadt@ieee.org wrote:

Jim Thompson skrev:

On Thu, 18 Aug 2005 19:17:35 -0700, Tim Wescott <tim@seemywebsite.com
wrote:

Jim Thompson wrote:
[snip]

Use a clock, so that it runs constant frequency, then PWM.

The whine is probably the pole pieces flexing. I did a controller
design for Bosch/Mercedes and passing the noise spec was the hardest
part... The director of engineering stuck his head in the air duct,
listened, and decided yes or no ;-)

...Jim Thompson

I hope that when faced with a spec that says "whatever he likes" you
gave an estimate that said "however long I take".

Of course. Also made about a dozen trips to Germany in Business
Class, once in First Class ;-)

...Jim Thompson

lol, I can't imagine even First Class making 10 hours across the
atlantic
something you would do for fun ;)

-Lasse
Click to expand...
The food was quite good, and most of it was "night".

My standard trick to Germany was to leave late afternoon from Phoenix,
arrive Frankfurt at 8:30AM, then drive straight to Buhlertal and work
all day.

After dinner I was definitely ready to sleep on a German schedule.

Returning, I did just the opposite, arrive Phoenix just in time for
dinner, eat, drink, go to bed, back to Phoenix time in one cycle ;-)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
 
Jim Thompson skrev:

On 19 Aug 2005 14:03:31 -0700, langwadt@ieee.org wrote:

snip

lol, I can't imagine even First Class making 10 hours across the
atlantic
something you would do for fun ;)

-Lasse

The food was quite good, and most of it was "night".
Click to expand...
amazing, the few time I've been flying with AA across the atlantic they
served something I wouldn't give to a dog :))
Lufthansa's generally has pretty good food

My standard trick to Germany was to leave late afternoon from Phoenix,
arrive Frankfurt at 8:30AM, then drive straight to Buhlertal and work
all day.
Click to expand...
I've been on that Frankfurt-Phoenix flight quite a few times, they used
to have a 747 everyday , but now it is no more, so now it's usually via
Denver:(

-Lasse
 
On 19 Aug 2005 16:14:07 -0700, langwadt@ieee.org wrote:

Jim Thompson skrev:

On 19 Aug 2005 14:03:31 -0700, langwadt@ieee.org wrote:

snip

lol, I can't imagine even First Class making 10 hours across the
atlantic
something you would do for fun ;)

-Lasse

The food was quite good, and most of it was "night".

amazing, the few time I've been flying with AA across the atlantic they
served something I wouldn't give to a dog :))
Lufthansa's generally has pretty good food


My standard trick to Germany was to leave late afternoon from Phoenix,
arrive Frankfurt at 8:30AM, then drive straight to Buhlertal and work
all day.

I've been on that Frankfurt-Phoenix flight quite a few times, they used
to have a 747 everyday , but now it is no more, so now it's usually via
Denver:(

-Lasse
Click to expand...
In '93-'94 when I was flying to Germany, it was Phoenix to Dallas,
then Dallas to Frankfurt.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
 
Robert Latest wrote:
On Fri, 19 Aug 2005 00:24:22 GMT,
Joerg <notthisjoergsch@removethispacbell.net> wrote
in Msg. <WE9Ne.554$zD3.534@newssvr29.news.prodigy.net

If you want absolutly constant frequency you can build that oscillator
and follow with a one-shot that can also be made from a Schmitt gate.
Then quench that one-shot instead. There are six in a chip so you have
plenty.


Hmm, that sound exactly like what I'm doing already: I'm using a
constant-frequency oscillator to periodically set a flip flop to "on"
(the oscillator clocks a D-FF with D tied to High), and the comparator
resets the FF when the sense current exceeds the limit.
Click to expand...
That's not what's happening. The current limit is not resetting the FF
during each 50us period, so the clock just re-clocks the '1' and the
MOSFET stays on continuously until the current limit is reached. All you
are doing is granulizing time into 50us intervals at which you enable
the MOSFET to make an off-to-on transition. This is not PWM, it is some
kind of quasi PFM half-hysteretic thing...

So I'm in fact running a constant frequency. The motor takes several
cycles to get up from zero to 1A of course (f=20kHz L=5mH V=27V), but I
had expected it to basically stay there via the freewheel diode and just
take an additional sip of current each cycle. All I can see on my old
analog scope is multiple-clock rising ramps all over the screen; I can't
see if the PWM drive takes longer breaks between ramps.
Click to expand...
You could if set triggering to DC coupled, NORMAL mode, negative
transition, on the current sense ramp channel, adjust timebase to
something like 500us/DIV or more, and then decalibrate the timebase
until you get a steady display.

It all looks as if the motor loses too much current between pulses which
may be due to my KNOWINGLY poor choice of freewheel diode.
Click to expand...
Yep- sounds like that's what's happening.
 
I read in sci.electronics.design that Joerg
<notthisjoergsch@removethispacbell.net> wrote (in
<mVqNe.3211$Z87.1091@newssvr14.news.prodigy.com>) about 'Whining PWM
motor control', on Fri, 19 Aug 2005:

Once I got a ride in a Ford Model A. Big surprise: This old car is
surprisingly quiet on the outside but quite noisy inside.
Click to expand...
Because it's a tin box with no damping on the panels.

But it offers a lot more legroom than many modern cars.
Click to expand...
.... which have 75 mm or so of insulator/absorber on every surface.

When we were students, my friend and I quietened our Austin 7 van quite
a bit with internal insulation. Kept us a bit warmer, too.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
In article <430695B3.1080101@nospam.com>,
Fred Bloggs <nospam@nospam.com> wrote:

That's not what's happening. The current limit is not resetting
the FF during each 50us period, so the clock just re-clocks the
'1' and the MOSFET stays on continuously until the current limit
is reached. All you are doing is granulizing time into 50us
intervals at which you enable the MOSFET to make an off-to-on
transition. This is not PWM, it is some kind of quasi PFM
half-hysteretic thing...
Click to expand...
Yes there is no sync between ON and OFF, so everything
is random. The OFF time can be anywhere between 0 and
50uS, making the drop in motor current during flyback
anywhere from 0 to 0.25A.... and that makes the ON time
vary from 0 to 2mS (roughly).

--
Tony Williams.
 
On Sat, 20 Aug 2005 02:30:14 GMT,
Fred Bloggs <nospam@nospam.com> wrote
in Msg. <430695B3.1080101@nospam.com>

That's not what's happening. The current limit is not resetting the FF
during each 50us period, so the clock just re-clocks the '1' and the
MOSFET stays on continuously until the current limit is reached. All you
are doing is granulizing time into 50us intervals at which you enable
the MOSFET to make an off-to-on transition. This is not PWM, it is some
kind of quasi PFM half-hysteretic thing...
Click to expand...
yeah. It drives the woman who has to work around the device
half-hystertic as well. I'll now go for a somewhat less direct approach;
i.e., generate the PWM with a sawtooth oscillator and control duty cycle
via low-passed current sense voltage. In other words, I'll throw a few
compoents in the general direction of an SG3524 and see what happens.

Many thanks for all the suggestions everybody. I'll be back if something
goes wrong.

robert
 
Robert Latest wrote:

On Thu, 18 Aug 2005 11:21:11 +0200,
Rene Tschaggelar <none@none.net> wrote
in Msg. <4304533d$0$1164$5402220f@news.sunrise.ch

Apparently the current takes longer to rise than the though 50us.
When the voltage is low the risetime to the current is longer.

Initially of course, but once the current is up it shouldn't drop so
much. The motor has 5mH, supply voltage is 27V (which means it takes 4
PWM periods to take the motor from 0 to 1 A).

Do you have a diode over the motor ?

I do, and as much as I hate to admit it, it's a 4001. Can this alone
kill the freewheel current in 50 us? The diode is going to be replaced
with an appropriate one as soon as I place my next order for parts.

Thanks,
robert
Click to expand...
You could probably get a suitable diode out of the secondary rectifier of a
PC power supply, if you have a few of those in your junk box.

To me it looks like your current comparator has some hysteresis, or perhaps
the gate drive current of the MOSFET or the reverse recovery current pulse
of the 1N4001 is affecting the current sense signal.

Is the FET turning on for a few nanoseconds every cycle, or is it really
staying off for several cycles at a time? Probe the gate voltage with a
fast scope.

Also make sure there is no deliberate positive feedback (hysteresis) in your
comparator.

Chris
 
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