Why is PWM needed for proportional control solenoids? 4

[hydroman247]

Probably a very simple question but I can't find an answer. For current controlled proportional solenoids such as those used on hydraulic directional control valves, why is a PWM signal needed? Why can't I use a variable resistor in series?

 

[VEBill]

PWM has the advantage of avoiding power/heat losses in your proposed resistor.

If it was a big solenoid, then the variable resistor might have to be huge to dissipate the power safely.

 

[hydroman247]

Some numbers.

12VDC
Coil resistance 3.1 Ohms
Current from 500mA to 2500mA.

Does that seem like it would cause heat issues?

 

[VEBill]

A rule of thumb is that max power occurs when (in this case) your variable resistor matches the solenoid coil resistance.

Beware that the "12VDC" is actually likely to be a vehicle voltage that might be more like 14 volts. That is enough of a difference that it's worth using 14 instead of 12.

So 7 volts (one half of 14) over the resistor set to 3.1 Ohms is about 16 watts. 20 watts is the nearest common power rating, but a bit too close for my liking. So you might need to use a 50 watt variable resistor, if you can't find a 30 watt.

I'm not sure how easy it would be to find a variable power resistor with these sorts of values for resistance and power.

Also, you might find the taper of the resistance curve to be unfeasible. In other words, all the action might end up happening within a very small range crammed into one end of the total range of motion.

Also, one must consider that the power dissipation in the worst case might also be confined to a small section of the total variable resistor. In other words, 16 watts of heat crammed into one small end of the resistor.

One wouldn't want to simply install a variable resistor without carefully analyzing it at all the critical settings along the adjustment range.

You can probably start to see why PWM might actually be easier.

 

[waross]

Given the precise position control required to throttle and control the flow through a valve, combined with the often non-linear current/position response of a solenoid plunger, the rapid open/closed cycle of PWM does a much better job of controlling flow.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[hydroman247]

Ok, thanks for the help. PWM seems much easier now.

 

[Skogsgurra]

One more aspect: the dithering that PWM causes helps reduce stiction in the valve.



Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[hydroman247]

One more aspect: the dithering that PWM causes helps reduce stiction in the valve.

Yes, there is a distinct vibration when I put my screwdriver in the middle of the coil.

 

[jacksopher]

In a proportional solenoid the current needs to be controlled. Applying a varying DC is possible (like what you were suggesting) but it is also less efficient and the Power Dissipation is much greater than with pulse width modulation. If the coil current required is constantly varying by very small amounts then it cannot be manually controlled. Furthermore copper is expensive and in order to make a solenoid with a low current requirement, the coil resistance needs to be sufficient for the applied voltage (e.g. 12V at 1 Amp = 12 Ohms and also Power Dissipation of 12 Watts). This requires many turns of small AWG wire and with 12 Watts this can get hot.

Now with pulse width modulation there is something else in the background: Current control. Fewer turns with heavier gage wire so it does not get hot, and more current giving the same Force to the Solenoid. The current is controlled and the Power Dissipation is much lower. As the pulse is applied current in the coil rises and with pulse width modulation when the pulse is turned off the current falls but only by a small amount before the next pulse comes along. Thus applying an average current of whatever the solenoid requires to maintain its position with some amount of ripple. A transistor is used as a switch to turn the current on and off and the Power Dissipation in it is very small. This transistor acts like the variable resistor in series with the coil.