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PWM circuit to control hydraulic proportional valve

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NiHaoMike:

--- Quote from: IDEngineer on August 03, 2020, 12:34:25 am ---Also, for a bidirectional motor application, there are two such proportional valves with two separate solenoids. You don't have a midpoint (such as 50% PWM duty cycle) and drive it up or down, you drive one coil from 0-100% for clockwise rotation and the other coil 0-100% for counterclockwise rotation. The two coils are never actuated at the same time.

--- End quote ---
For that case, you can connect the coils in series with diodes in parallel with each one. Then it can be driven with a H bridge. Probably doesn't save on component cost, but it does cut down on the number of wires required.

IDEngineer:
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.

David Hess:

--- Quote from: IDEngineer on August 03, 2020, 12:34:25 am ---
--- Quote from: David Hess on August 02, 2020, 11:30:50 pm ---An LC filter can be added between the switching circuit and solenoid so that it is driven with DC rather than low frequency PWM.  This combines the efficiency of PWM with a linear drive signal.
--- End quote ---

Interestingly, the pulsed nature of the PWM signal is an advantage with proportional valves. This is why I said "frequency set low enough to act as dither to improve hysteresis response" above. The valves tend to have a certain amount of "stiction" if held in a single position for a while, so the manufacturers recommend that the PWM signal be of a low enough frequency to induce dither - thus causing the valve to mechanically move a bit. This improves response to small changes. Thus we don't want to use an LC filter, we actually want the actuator to experience the "AC" of the PWM signal. The nonzero mass of the actuator obviously prevents it from responding to the electrical rise/fall time.

Also, for a bidirectional motor application, there are two such proportional valves with two separate solenoids. You don't have a midpoint (such as 50% PWM duty cycle) and drive it up or down, you drive one coil from 0-100% for clockwise rotation and the other coil 0-100% for counterclockwise rotation. The two coils are never actuated at the same time.
--- End quote ---

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.

I have had problems in the past with hysteresis losses in steel cores when the switching frequency was too high.

IDEngineer:

--- 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.

artag:
You may find you get better control by using two loops : a fast one controlling the valve current / PWM fraction against pressure, and a slower one controlling pressure demand against RPM.

Also, think carefully about what you're actually controlling. In many cases, mechanical control is not direct but there is an implied integration or differentiation in the mechanical loop. Ignoring this mucks up your stability criteria.

The valves will likely have a long dead zone, a fairly fast change from fully on to fully off, and then a long dead zone at the other end. Make sure these dead zones don't cause integrator windup or other saturation effects. Don't rely only on the integrator to get them into the working zone.

I've done this with both solenoid valves (Rexroth - clutch control of a rally car)  and Moogs (F1) and they both worked (and won races). The Moogs are better but more expensive and very fussy about contamination. The valves are horribly nonlinear but you can get around that with software.

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