Hi,
I need to control a solenoid valve using PWM but I have also some 
specifications about "the dither" in order to reduce friction etc.. I now 
what the dither is able to do in theory. I now how to built an electronic 
circuit concerning PWM (I'd do that using a micro plus a MOSFET etc.) but 
I don't have any idea about "the dither". Should I sum another PWM with 
lower frequency to the higher frequency PWM which control the valve 
position ? Do you have any example or links in order to clarify this 
aspect ?

Thank you in advance,
Pasquale 

"Pasquale" <d...@home.invalid> wrote in message news:47d48b6a$0$16039$5...@news.tiscali.it...
> Hi,
> I need to control a solenoid valve using PWM but I have also some
> specifications about "the dither" in order to reduce friction etc.. I now
> what the dither is able to do in theory. I now how to built an electronic
> circuit concerning PWM (I'd do that using a micro plus a MOSFET etc.) but
> I don't have any idea about "the dither". Should I sum another PWM with
> lower frequency to the higher frequency PWM which control the valve
> position ? Do you have any example or links in order to clarify this
> aspect ?
>
> Thank you in advance,
> Pasquale

A 'dither' technique typically increases the PWM reolution by one or two bits depending upon your tolerance for dither 
ripple.
What is your dither spec and is it the same?

Cheers

On Mon, 10 Mar 2008 01:48:11 +0000, Martin Riddle wrote:

> "Pasquale" <d...@home.invalid> wrote in message
> news:47d48b6a$0$16039$5...@news.tiscali.it...
>> Hi,
>> I need to control a solenoid valve using PWM but I have also some
>> specifications about "the dither" in order to reduce friction etc.. I
>> now what the dither is able to do in theory. I now how to built an
>> electronic circuit concerning PWM (I'd do that using a micro plus a
>> MOSFET etc.) but I don't have any idea about "the dither". Should I sum
>> another PWM with lower frequency to the higher frequency PWM which
>> control the valve position ? Do you have any example or links in order
>> to clarify this aspect ?
>>
>> Thank you in advance,
>> Pasquale
> 
> A 'dither' technique typically increases the PWM reolution by one or two
> bits depending upon your tolerance for dither ripple.
> What is your dither spec and is it the same?
> 
> Cheers

My specs about the dither are the frequency (lower than the Pulse Width 
Modulation), the duty cycle fixed on 50% and the spec on the amplitude 
current (which still I need to understand). I'm sorry but I could not 
catch what you meant.

Thanks,
Pasquale.

On Sun, 09 Mar 2008 22:10:51 -0500, Robert Adsett wrote:

> In article <vr0Bj.4811$e52.3303@trndny01>, Martin Riddle says...
>> 
>> "Pasquale" <d...@home.invalid> wrote in message
>> news:47d48b6a$0$16039$5...@news.tiscali.it...
>> > Hi,
>> > I need to control a solenoid valve using PWM but I have also some
>> > specifications about "the dither" in order to reduce friction etc.. I
>> > now what the dither is able to do in theory. I now how to built an
>> > electronic circuit concerning PWM (I'd do that using a micro plus a
>> > MOSFET etc.) but I don't have any idea about "the dither". Should I
>> > sum another PWM with lower frequency to the higher frequency PWM
>> > which control the valve position ? Do you have any example or links
>> > in order to clarify this aspect ?
>> >
>> > Thank you in advance,
>> > Pasquale
>> 
>> A 'dither' technique typically increases the PWM reolution by one or
>> two bits depending upon your tolerance for dither ripple.
> 
> I think he's suggesting an actual dithering of the valve positionYes,.  
IE
> the valve actually vibrates around its setpoint. Possibly to overcome
> stiction?
> 
> Robert

Yes, that's correct. I need a PWM to control the valve current plus 
anothes signal called "dither" to overcome the stiction. I should 
implement it but actually I don't know how to consider the dither.

Thanks,
Pasquale.

On 10 Mar 2008 02:14:17 GMT, Pasquale <d...@home.invalid> wrote:

>On Sun, 09 Mar 2008 22:10:51 -0500, Robert Adsett wrote:
>
>> In article <vr0Bj.4811$e52.3303@trndny01>, Martin Riddle says...
>>> 
>>> "Pasquale" <d...@home.invalid> wrote in message
>>> news:47d48b6a$0$16039$5...@news.tiscali.it...
>>> > Hi,
>>> > I need to control a solenoid valve using PWM but I have also some
>>> > specifications about "the dither" in order to reduce friction etc.. I
>>> > now what the dither is able to do in theory. I now how to built an
>>> > electronic circuit concerning PWM (I'd do that using a micro plus a
>>> > MOSFET etc.) but I don't have any idea about "the dither". Should I
>>> > sum another PWM with lower frequency to the higher frequency PWM
>>> > which control the valve position ? Do you have any example or links
>>> > in order to clarify this aspect ?
>>> >
>>> > Thank you in advance,
>>> > Pasquale
>>> 
>>> A 'dither' technique typically increases the PWM reolution by one or
>>> two bits depending upon your tolerance for dither ripple.
>> 
>> I think he's suggesting an actual dithering of the valve positionYes,.  
>IE
>> the valve actually vibrates around its setpoint. Possibly to overcome
>> stiction?
>> 
>> Robert
>
>Yes, that's correct. I need a PWM to control the valve current plus 
>anothes signal called "dither" to overcome the stiction. I should 
>implement it but actually I don't know how to consider the dither.

You have some error signal controlling the PWM output. You can add a
dither signal of the appropriate amplitude and frequency to that error
signal, such that the valve is kept in constant motion.
-- 
John

> You have some error signal controlling the PWM output. You can add a
> dither signal of the appropriate amplitude and frequency to that error
> signal, such that the valve is kept in constant motion.

Do you means directly by programming the PWM output of the 
microcontroller ?

> Lets assume you had an interrupt handler for each starting ON duty.
>   and the timer value of lets say 500 is to much and 499 is not enough
> to get the width you need for the precise value. What you would do in
> this case is to have a variable in the background that would give you a
> counter or a bit image to use to oscillate the value between 499 and
> 500.
>    if you were to alternate these values, you would get half way between
> the values. etc..
>    Your filtering would dampen this out and give you an average signal.

Ok I will have an average signal because the load will damp it out. For 
this reason I'll have some error in my feedback control that i cannot 
correct but it's not what I want. Or better I should have a lower 
frequency PWM and use your way to do in order to make the valve moves 
around its setpoint value. 

In article <vr0Bj.4811$e52.3303@trndny01>, Martin Riddle says...
> 
> "Pasquale" <d...@home.invalid> wrote in message
news:47d48b6a$0$16039$5...@news.tiscali.it...
> > Hi,
> > I need to control a solenoid valve using PWM but I have also some
> > specifications about "the dither" in order to reduce friction etc.. I now
> > what the dither is able to do in theory. I now how to built an electronic
> > circuit concerning PWM (I'd do that using a micro plus a MOSFET etc.) but
> > I don't have any idea about "the dither". Should I sum another PWM with
> > lower frequency to the higher frequency PWM which control the valve
> > position ? Do you have any example or links in order to clarify this
> > aspect ?
> >
> > Thank you in advance,
> > Pasquale
> 
> A 'dither' technique typically increases the PWM reolution by one or two bits depending upon your tolerance for
dither 
> ripple.

I think he's suggesting an actual dithering of the valve position.  IE 
the valve actually vibrates around its setpoint. Possibly to overcome 
stiction?

Robert

-- 
Posted via a free Usenet account from http://www.teranews.com

Ciao Pasquale,
a word of caution about using dithering, this is based on my experiences of
eons ago. Dithering is used to overcome the effect of "stick-slip", a highly
non-linear behavior that can cause instability in mechanical closed loop
position
systems. To be effective dithering has to actually vibrate the mechanical
elements that are part of the control loop. The question is: how much has to
be the mechanical displacement and at what frequency? Both these questions
have a great relevance on the life (wear characteristic) of the mechanical
elements and their performance as far as the long-term stability of the
loop. For the injection of the dithering parameters, I leave it to the
others that have replied to your posting. But said that, if your PWM is
based on a fixed frequency, why not "kill two birds with one stone"? In
other word why not use the PWM frequency as the dithering frequency? Can be
done?
Buona fortuna con il tuo progetto.
Gene

Pasquale wrote:

> On Mon, 10 Mar 2008 01:48:11 +0000, Martin Riddle wrote:
> 
> 
>>"Pasquale" <d...@home.invalid> wrote in message
>>news:47d48b6a$0$16039$5...@news.tiscali.it...
>>
>>>Hi,
>>>I need to control a solenoid valve using PWM but I have also some
>>>specifications about "the dither" in order to reduce friction etc.. I
>>>now what the dither is able to do in theory. I now how to built an
>>>electronic circuit concerning PWM (I'd do that using a micro plus a
>>>MOSFET etc.) but I don't have any idea about "the dither". Should I sum
>>>another PWM with lower frequency to the higher frequency PWM which
>>>control the valve position ? Do you have any example or links in order
>>>to clarify this aspect ?
>>>
>>>Thank you in advance,
>>>Pasquale
>>
>>A 'dither' technique typically increases the PWM reolution by one or two
>>bits depending upon your tolerance for dither ripple.
>>What is your dither spec and is it the same?
>>
>>Cheers
> 
> 
> My specs about the dither are the frequency (lower than the Pulse Width 
> Modulation), the duty cycle fixed on 50% and the spec on the amplitude 
> current (which still I need to understand). I'm sorry but I could not 
> catch what you meant.
> 
> Thanks,
> Pasquale.
Lets assume you had an interrupt handler for each starting ON duty.
  and the timer value of lets say 500 is to much and 499 is not enough
to get the width you need for the precise value. What you would do
in this case is to have a variable in the background that would give
you a counter or a bit image to use to oscillate the value between
499 and 500.
   if you were to alternate these values, you would get half way between
the values. etc..
   Your filtering would dampen this out and give you an average signal.


-- 
http://webpages.charter.net/jamie_5";