dither frequency with pwm to drive proportional valve

Thread Starter

mah

Joined Mar 15, 2010
393
i am planning to drive proportional valve using pwm by atmega. i will do functions of ramp up and down in c code . dither frequency is needed to avoid stiction of the valve and to smooth the pwm signal . this dither frequency is ranging from 70–350 Hz and amplitude 0–10% of maximum current . the question here is how to do the dither in software programming?
 

Thread Starter

mah

Joined Mar 15, 2010
393
for now i will do it without feedback for simplification

any help?
 
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MaxHeadRoom

Joined Jul 18, 2013
23,084
I am still not sure how you are going to position without some kind of feedback indication?
Especially if the Hydraulic load varies?
Max.
 

jpanhalt

Joined Jan 18, 2008
11,088
:confused:Surely adding dither will introduce additional modulation components, which is the opposite of a smoothing effect?
As I understand it, dither is used to counteract the effects of catch and release ("stiction"). Saying it smooths the PWM was a small misstatement. It smooths the movement produced by the PWM control signal (http://www.qualityhydraulics.com/blog/what-dither-versus-pwm/). I know nothing about how to generate that dither, but my current project -- if it gets that far -- will face the same problem.

Here's an ST Micro reference on generating dither: http://www.st.com/st-web-ui/static/...ical/document/application_note/DM00119042.pdf

In a practical sense, I wondered whether bit banging the control variables in PWM might work:
upload_2015-4-30_6-52-35.png
Thus, alternating between different PR2 or CCPxCON<5:4> values might produce cycle-by-cycle dither. One problem is that dither produced like that would be at the same amplitude as the main PWM.

Again, I can't add much, but am certainly interested in the answer.

John
 
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