Re: PWM and dither

On 10 Mar 2008 02:14:17 GMT, Pasquale <desmomito@xxxxxxxxxxxx> wrote:

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

In article <vr0Bj.4811$e52.3303@trndny01>, Martin Riddle says...

"Pasquale" <desmomito@xxxxxxxxxxxx> wrote in message
news:47d48b6a$0$16039$5fc30a8@xxxxxxxxxxxxxxxxxx
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
.

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