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PWM circuit to control hydraulic proportional valve
IDEngineer:
Excellent points, thank you!
I do expect things to be "less than linear" especially at the extremes. Another reason for the truly closed loop control, where we're measuring the actual results (RPM) and not presuming everything "just works" in the middle. A canned solution wouldn't work for this application anyway, but a lot of them just presume if they control the current that everything will just work out. Trouble is, there's a whole real world out there that often has other ideas. {grin}
Andreas:
Hello,
I am wondering if you are using current feedback to the control loop like in AD8203 datasheet.
https://www.analog.com/media/en/technical-documentation/data-sheets/AD8203.pdf
Usually this gives you a average value for the position of the valve.
If you have many power stages you might want to have a dedicated power stage controller with current sensing for hysteresis controlled PWM like the MC33816AE.
https://www.nxp.com/docs/en/data-sheet/MC33816.pdf
with best regards
Andreas
David Hess:
--- Quote from: IDEngineer on August 03, 2020, 03:17:59 pm ---
--- Quote from: David Hess on August 03, 2020, 01:34:39 pm ---An LC filter can make the output DC without preventing controlled variation. Think of an class-D audio power amplifier with has an output bandwidth of 20 kHz so the driving waveform can be precisely controlled.
--- End quote ---
I do understand the point you're making. And I guess we could tune LC values to "round off" the corners of the PWM signal if desired. My point is that it's unnecessary and could even be counterproductive. The valve wants to "oscillate" (mechanically dither) a bit around its set point to overcome the static friction inherent in such devices. Quoting the Comatrol specs: "An advantage of a PWM signal is that the dither it provides significantly reduces hysteresis. Comatrol recommends using a 100-200 Hz dither for best performance."
Other manufacturers openly state that if you are using a constant current, you should impose a small AC component to impart dither. For PWM systems you get that "AC" component for "free" by not electrically filtering it. That's why they recommend such a low PWM frequency, because much above that range the mass of the valve can no longer mechanically respond and you lose the dither effect.
--- End quote ---
You asked for discussion so I provided some.
As you point out, the class-D implementation would have to have the dither signal applied while the PWM implementation gets it for free, but it could also be adjusted in frequency and amplitude and even shape.
What might make a class-D implementation competitive is that integrated class-d audio amplifiers are available.
NiHaoMike:
--- Quote from: IDEngineer on August 03, 2020, 06:57:13 am ---An H-bridge would require four active components. Just driving the two solenoids individually requires only two. And the two coils each have either flying leads or integral two pin connectors so there will be four wires no matter how they're connected.
So far it's sounding like a traditional FET plus diode is the simplest and most straightforward circuit. Two FET's and two diodes, done.
--- End quote ---
H bridges are commonly available integrated into a single chip, including level translation required to work with 5V and 3.3V logic. Also, if the valve assembly is located in a separate module from the control system, going from 3 wires in the cable to 2 can be quite significant.
IDEngineer:
--- Quote from: David Hess on August 03, 2020, 10:10:44 pm ---What might make a class-D implementation competitive is that integrated class-d audio amplifiers are available.
--- End quote ---
True. I'll check into that. Hard to beat the simplicity and low parts count of an NFET and diode. Possibly also a smaller NFET to improve gate drive on the larger transistor, though at these frequencies one of the beefier MCU I/O pins may still yield decent edges.
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