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High freq + dither vs req freq alone

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SJ Industries:
Hello, new to the world of Guide, I started to learn about 6 months ago, it has been a very steep learning curve! I am a diesel fitter by trade so have never required this sort of thing in my working life and you could say I have bitten off more than I can chew.

I have a project which requires control of a high pressure relief valve, I have tried various settings and combinations of High freq plus dither (config 5) freq (config 4) also initially used a stand alone amplifier which used high freq plus dither. 

The results with setting 5 are not as good as I would like. Is there a way on config 4 to control req freq so I can continuously change the freq inline with my desired current. I have written a program to do this, it compiles and works, but when I look at the current, duty and Hz on a scope I can see the control must switch the output off momentarily when it changes the frequency, it looks even worse if it is also changing the current output at the same time. I have changed the program to change current and freq separately and it has smoothed it out a little but I don't think it would be very good once live on the hydraulic system. It would have to ramp very slowly.

Thank you, Ben

FluidPowerTom:
Good morning.  It sounds like you've got things pretty well figured out - especially for a beginner.  In my experience setting the PWM frequency (config 4) or dither frequency (config 5) at an ideal value is important, but I've never seen a situation where it's necessary to change it at different current values.  I'll venture that you'd have to be measuring performance very closely to see an effect.  How much are you needing to change the frequency?  My experience is that 10-20 Hz doesn't make a significant difference in most valves. 

My initial feeling is that if the system performance needs are barely being met to the degree that you're having to optimize dither frequency for different currents then you might just need to use a better valve or make changes to the hydraulic system.  Many times I've been asked to use tricks with the electronics to make up for a subpar hydraulic and/or mechanical system, and those situations typically become a long trip down the rabbit hole.

SJ Industries:
Hi Tom, thankyou for your reply. It appears I may already be down the rabbit hole. I have hundreds of data logged tests on this system using a standard amplifier and the results were ok, tjere would sometimes be differences that i have struggled to determine the cause. We changed to an MC050 controller to gain more functionality on our test rig to automate some of the tests and to gain more repeatability.
If I were using a standard buffered guage or display to read the results i may never have gotten this deep. Our recordings have about 36k data points for each test and the freq issue really starts to show.
In my opinion high freq plus dither may not be the best option at all as it creates a high level of "harshness" i suppose you would call it. On some systems it is not as noticeable on on others it makes a huge difference.
I have come up against a problem with the 4000Hz setting that when the pump rpm and its frequency lines up with the frequencies of the control it sends my relief completely unstable. I have to try freq alone next time as i ran out of time to keep changing setting.
I will write up more test results once i am on a computer and not a phone.
I fknd it interesting that a lot of info out there is pushing the high freq plus dither as the best option. It has led me on a big adventure that is for sure.
On the varying frequency while running, i started at 100Hz and moved to 350Hz at 50% duty cycle, on a scope it looks really good electrically except for the contrl turning on and off the output during step changes. I think if it were possible to do in a live system it would give better results than both the current options, my opinion only.

FluidPowerTom:
What valve are you using?  What function is this proportional relief valve performing?  Is the intention for it to just jump between a few setpoints, or is it's setting varying proportionally with some system parameter?  Is it being used in a closed loop (pressure transmitter as feedback and PI loop)?

The pump RPM having anything to do with the signal frequency to the valve seems highly suspect.  What type of pump is it?  I'm guessing it's a gear, vane, or piston pump.  I'm not a pump expert, but I know some pumps do produce small pressure ripples as they turn.  However, those ripples aren't necessarily occurring at the pump's RPM because, I think, in one pump shaft rotation will expel flow from several vanes/pistons/gear teeth.

oiltronic:
If open-loop, have you tried creating a non-linear profile (using the profile function) to translate the desired relief setting to PWM output current?  How was the relief response not correct when using a fixed frequency?

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