Voltage divider for hydraulic valve

Discussion in 'The Projects Forum' started by chrisaus2009, Mar 29, 2009.

  1. chrisaus2009

    Thread Starter New Member

    Mar 29, 2009
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    I want to be able to control a hydraullic valve I have where I have two sources, one is from my PLC (variable voltage -10 to +10V), one is a constant +24V DC supply from a switch mode PSU I have. The hydraullic valve sits at rest with +12V, spins one way at +7V and the other at +17V, I have made this circuit http://web.aanet.com.au/chrisaus/cct.JPG

    the circuit does exactly what I want, the only problem is I want to be able to isolate the PLC from the +24V supply so either terminal (common/supply) on the PLC doesn't see more than 10V, otherwise it will cook the PLC. Is there any way to do this without using transistors and without using a POT (even though there's one in there for simulation purposes)? So that the circuit is ONLY controlled by the variable PLC voltage +/-10V.

    Thanks
     
  2. BillB3857

    Senior Member

    Feb 28, 2009
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    Have you checked to see if the valve has a mechanical balance adjustment? If so, you could offset the zero flow point to be +5v and with max flow at 0 volts one direction and 10 volts for the opposite direction. Also, how many coils for the valve? Many hydraulic valves have two coils and can be set up in either series for push/pull operation or parallel for single ended amplifier operation.
     
  3. chrisaus2009

    Thread Starter New Member

    Mar 29, 2009
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    I don't know the answers to that in regards to the coil, I'll have to check with the mech engineer who's asked me to do this for him but he's on leave for a little bit.. assuming its a single coil with no offset adjustments for now, is it possible to do what i want to do?
     
  4. BillB3857

    Senior Member

    Feb 28, 2009
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    If you could bias one end of the coil at negative 7 volts, you should be able to get zero flow with +5 volts out of your PLC, full flow in one direction with 0 volts from PLC and full flow in the other direction with +10 volts out of the PLC. A regulated power supply set for 7 volts should work. You would need to insure the power supply is capable of the current demand of the valve. ON EDIT---- Does your PLC have two variable outputs? If so you could set one at negative 7 volts and leave it there and let the other swing from 0 to +10 to control flow with +5 being no flow.
     
  5. chrisaus2009

    Thread Starter New Member

    Mar 29, 2009
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    That's definately an option, but one that would have to wait for the mechanical eng to come back. I've come up with this:

    http:\\web.aanet.com.au\chrisaus\cct1.JPG

    Just using the PLC to shift the circuits reference voltage from -10 to +10 to give V8 +7 to +17V.

    That shouldn't harm the PLC will it? As the entire source voltage (+24V) will be dissipated over the 3 resistors.
     
  6. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
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    Do you have the spec sheet for the PLC analog output module you will use? It should tell what the maximum current draw is allowed. Are the PLC analog outputs truly bi-polar, ie. they swing both positive and negative under program control? You then need to measure the resistance of the valve coil to make sure that 10 volts across the coil will not result in more current than the PLC can deliver safely. I'm really not sure where the +24 volt supply comes into play. What kind of system is this being used in. If a full blown position servo system with accurate positioning required, you also need to consider dither for the valve. Dither is a low level A/C signal applied to the valve to "keep it alive" and ready to move with even very small signals. Usually it is adjusted so that the moving member just starts to respond to the dither, then backed off until it can't be felt any more. I've seen 60 hz to 400 hz dither on various valves. Frequency usually is lower with large valves and higher with small valves. It seems to be an inertia thing with the moving members of the valve.
     
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