Author Topic: PWM Signal Issue  (Read 8259 times)

Offline LBee11

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PWM Signal Issue
« on: May 03, 2017, 05:34:53 PM »
Hi All,

I'm experiencing an issue that has stumped me and colleagues regarding sending a PWM signal to a solenoid on a valve.

The valve we're trying to control is currently fed a PWM signal from an old Parker box that states is a 12vDC 33hz signal. Theres a pot on the front to control the pulse width and I've checked with a scope that this is working as stated. Apart from only getting 27hz from the box, the details are correct.

I've simulated the PWM signal from a 24pin controller and whilst I get the same square wave on the scope, the valve does not behave in the same way at all. Instead of getting a nice smooth ramp control from the pot, I get either flat out or nothing. The pulse width responds in exactly the same way as the current Parker box that is being used and I'm struggling to understand why the Plus+1 PWM signal is not working the same way, considering a scope meter shows exactly the same square wave (if a little less noisy).

Has anyone ever seen anything like this before?


Offline FluidPowerTom

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Re: PWM Signal Issue
« Reply #1 on: May 03, 2017, 06:59:35 PM »
I'd recommend measuring the current output.  Can you put an ammeter or multimeter in there and see what the current is doing?  With a PWM frequency that slow you might see a little bouncing in the current read out, but that's where I'd start.

I suspect that the Danfoss controller has trouble at such low PWM frequencies.  Looking at an MC050-110 program I see that the controller can control PWM frequency between 30 and 4,000 Hz.  So, I think that's the answer.  When you're pot is turned below the position such that it's commanding under 30 Hz then it won't work.

That's an extremely low PWM frequency.  Most valves I've seen prefer 100-250 Hz.
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Hydra-Power Systems

Offline jashom1

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Re: PWM Signal Issue
« Reply #2 on: May 15, 2017, 05:24:56 AM »

I wonder if the statement of 33Hz was referring to the dither frequency for the valve?  As Tom said 33Hz is very low for a solenoid, unless the solenoid in question already has some driver circuit on it?

I would normally use the 1000Hz PWM and play with dither frequency (which could be around that 33Hz area) which is designed to prevent the valve from sticking and helps with hysteresis.