Dither for PWM circuit driving a proportional solenoid

Hi guys,

First of all I'd like to say that I have seen the same or similar questions asked in many forums but nobody has ever come up with a concrete solution or posted anything about the results of trying out the various suggestions given to them. So here I am still trying to figure things out so please help me out. And now to my problem

I am planning to proportionally drive a solenoid of the following specs: 24VDC,1.19A 28.5W resistance 20.2ohms +-5% at 20C

I'm planning to use the arduino to accomplish this. I'm aware of the whole concept of dither which is necessary to avoid stiction in the solenoid valve. As most of you will be aware of, we cannot freely choose the pwm frequency in the arduino but are restricted to a few discrete frequency values of which only 30.5Hz (Pin 3,9,10,11), 61Hz (pin 5,6) and 122Hz (Pin 3,9,10,11) are below 200Hz. I found optimal dither at 122Hz which is okay. ( A tiny part of the project is taken care of)

But I can't use the the dither signal alone to drive the solenoid due to obvious reasons as it is not possible for the dither signal to form a constant amplitude since the PWM frequency has to be continuously adjusted as a function of the stipulated desired current value and in the process the amplitude and the frequency of the dither will signal change accordingly. In other words i need the dither to be a constant while i'll still be able to change the average current in the solenoid to manipulate the spool proportionally.

Then I stumbled upon a concept of superimposing the dither ( low frequency signal ) on a high frequency PWM signal.



But i do not know how to do this. Some smart guy by the name of Drazzy suggested in an old arduino forum :"You could put source of a small MOSFET on one pin, and gate on the other....". If i got him right he means using one MOSFET at a lower frequency to switch another MOSFET at a higher frequency (or the other way around? i'm confused:| ).

I also came across a website that talked about combining PWM outputs. http://www.openmusiclabs.com/learning/digital/pwm-dac/dual-pwm-circuits/
Is there anyway that i can use this in this application in context?

Either way could someone shed some light on this. will this work practically? what would be the circuit like? Any useful information is welcome. Any other ways to achieve the same are also welcome.

Help a friend and thank you in advance

 

The main frequency would be the default on the pins, I believe that about 490 Hz on some and about 970 Hz on others. I guess 490 Hz will have to do

 

Ahh, i wasnt aware of it, thanks for the info

 

modulate a PWM signal with another? -> yes, if that's what it takes to get a control signal of that nature.

how do I AND 2 PWM Signals ? could you explain in brief

by feeding it back into a couple of input ports and 'doing the business' - > are you talking about a feedback and some PID algorithm?

 

sort of read it before half way and left it thinking it would be of no good to me in this context. But now that you brought it up i guess i'll read it completely and lets see what ideas spring up

 

Thanks Bob. I will post the results soon.

 

Hey Kiwi, thanks for your reply, I'm not exactly working on a proportional valve, I'm trying to get it to be proportional

 

for those who have com here in the mean time, it will be until October that i post the results.

 

it's not completely formatted but you'd still get an idea. check the projects section on my linkedin page.
Electronic control of hydraulic systems
https://www.linkedin.com/in/joe-david