PWM soldenoid driver

Discussion in 'General Electronics Chat' started by Gibson486, Jan 10, 2014.

  1. Gibson486

    Thread Starter Member

    Jul 20, 2012
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    I am making a PWM solenoid driver using a mosfet. I tried to insert a bias on the gate with a DAC. I am doing this so i can achieve full resolution from the PWM since you lose about 30% of the lower end of the resolution due to friction of the valve. In other words, I am tryring to use the bias to crack the valve.

    The issue I am coming up with is that the bias does not really do anything. Instead, it just chops the bottom half of the PWM signal and effectively makes it a 5v - Vdac signal. I thought this was what i wanted, but it turns out it does not really do much at all.

    Anyone have any other ideas on what I can do to crack the valve?
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    What kind of solenoid is it that you need PWM control?
    Normally they are single acting/double acting devices that are simple ON/OFF?
    Normally a DC coil requires full voltage to energize, retention is a different story.
    Max.
     
  3. Gibson486

    Thread Starter Member

    Jul 20, 2012
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  4. MaxHeadRoom

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    OK proportional valve, not a solenoid valve.
    I have just used small servo drivers for these.
    Max.
     
  5. Gibson486

    Thread Starter Member

    Jul 20, 2012
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    Isn't that the same thing as using PWM and a mosfet?
     
  6. MaxHeadRoom

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    The servo drive I used were PWM with ±10vdc analogue input.
    What are you controlling them from and do you have any feedback element?
    Max.
     
  7. MaxHeadRoom

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    I have also used the Nat semi LM759 to control these types of valves.
    Max.
     
  8. wayneh

    Expert

    Sep 9, 2010
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    Bias on the gate is not the same as bias on the solenoid itself, unless you have a feedback loop controlling the gate. The MOSFET has a narrow linear region and in this usage is ideally either on or off, not somewhere in between. Biasing the gate risks having a high internal resistance in the MOSFET, thereby burning it out.
     
  9. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Or you could use a valve in the same series with a larger orifice. This would give the same amount of flow at a lower duty cycle. This is done with electronic fuel injection for higher performance all the time.
     
  10. Gibson486

    Thread Starter Member

    Jul 20, 2012
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    So.... I would probably have to put "just enough" current through the valve to over come the internal valve resistance? I am guessing I would need to feed the valve with a bais via an opamp and a resistor? Sorry for the "GibsonCAD drawing". I know, it is ugly. And yes, I know I did not draw the freewheeling diode.
     
  11. Alec_t

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    Sep 17, 2013
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    Doesn't your valve provide a position feedback signal? If so, you should use that. The control loop will then automatically overcome the valve stiction.
     
  12. Gibson486

    Thread Starter Member

    Jul 20, 2012
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    no, it's the valve only. I am making the controller from scratch
     
  13. Alec_t

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    In that case I suggest you start by making/attaching something (e.g. a pot or linear variable differential transformer) which will give you a signal indicating the position of the valve.
     
  14. Gibson486

    Thread Starter Member

    Jul 20, 2012
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    signal feedback will not work. I am monitoring pressure, not flow, so the valve feedback will not be of any use.
     
  15. MaxHeadRoom

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    Normally with proportional valves they do not come with any feedback item, the application uses a type that is suitable for the medium or motion being measured, encoder, linear scale, pressure sensor, load cell etc.
    This is then fed to the PID controller.
    Max.
     
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