liner motor control help

Discussion in 'General Electronics Chat' started by onlyaman, Apr 18, 2008.

  1. onlyaman

    Thread Starter New Member

    Apr 18, 2008
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    I am looking for a way to control a valve using a liner motor. The valve will set a level in a tank. I was thinking of using two Potentiometers. One for tank level and one for valve position. Then some how be able to read the voltage diffrence to turn on relays. > would turn on a relay until voltage was same on both Potentiometers and < for the other. I need a very fine level control for this project. My motor is 12 vdc. I sure someone out there can help. Thanks
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Do you mean linear motor?

    Don't have quite enough information yet.
    Do you mean to control the level using a single valve; ie: fill only or drain only?
    Or do you mean to control the level in a certain range, using drain and fill valves?
    Or is this a single valve that has both fill and drain capabilities?

    Anyway, it really sounds more like a logic problem, something like:

    Code ( (Unknown Language)):
    1.  
    2. Check the level.
    3. IF the level is too LOW (pot reading is low) THEN
    4.    IF the FILL valve is closed (pot reading is low) THEN
    5.        Do the OPEN FILL valve routine (run the motor one way until valve is open)
    6.       Wait for level to rise to within limits
    7.       Do the CLOSE FILL valve routine (run the motor the other way until valve is closed)
    8.    END IF
    9. ELSE IF the level is too HIGH (pot reading is low)
    10.    If the DRAIN valve is closed THEN
    11.       Do the OPEN DRAIN valve routine
    12.       Wait for level to drop to within limits
    13.       Do the CLOSE DRAIN valve routine
    14.    END IF
    15. END IF
    16. Loop back to check level again
    17.  
    Something like that. Of course, that's simplified.
     
  3. onlyaman

    Thread Starter New Member

    Apr 18, 2008
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    Sorry about spelling been a long day. This is one valve draining the tank, but I want the valve position to change with the tank level. Not just an open or closed position . As the level changes so will the valve. So thats why I was thinking the potentiometers to read positions of tank level and valve. I am trying to keep this level with in .125 In. There ia a pump that will fill the tank at the same time the other line is draining.
    Mabe I am going about this the wrong way, not sure. Thanks for you reply









    [​IMG]
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    Sensing the liquid level to any arbitrary degree of accuracy is quite feasible. The variable that can make this difficult is the volume of the tank - the greater the volume, the longer the lag between sensing a displacement in the liquid level and having the corrective action at the valve show up.

    Can you supply some details about the nature of the valve? Is it mechanical - gate, ball, or stopcock? If solenoid, does it proportion? What is the control mechanism that adjusts the valve?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    As a tangent, but a couple of possible simple solutions:

    1) If the flow coming in to the tank is reasonably steady, could you simply use the open end of a tube at the desired level, similar to the overflow tube in a toilets' tank?

    2) If the flow coming in to the tank varies considerably, could you cut a wide horizontal slot in the side of the tank, much like a dams' emergency spillover?

    Depending upon the surface tension and viscosity of the liquid, these ideas may or may not work very well. But, you would eliminate lots of electromechanical parts.
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    In keeping with the above, there is also a passive device called a tantalus siphon. It won't maintain an accurate level, but can keep the tank from overfilling.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    There is also the Archimedes Screw; with a variable speed motor drive, it could move a lot of fluid if necessary - and might simplify your plumbing.
     
  8. onlyaman

    Thread Starter New Member

    Apr 18, 2008
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    Thanks for the ideas, but the return line must stay completely under water because I don't want to introduce air into the system. The the valve I am using is a ball valve with a linear motor attach to it for opening and closing. I have tried all the mechanical solutions I can find that's why I am exploring the route.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    That's a pretty fine level to maintain.

    But anyway, a ball valve generally has 90° of travel from fully open to fully closed.
    My understanding of your situation is that you wish to maintain the liquid level "L" from "L" to L+0.125".

    This could be tough to do if the liquid's surface area is large. Even small vibrations from machinery, passing vehicles, trains etc. may cause ripples on the surface of the liquid, which may cause the valve controls to fluctuate quite a bit. A baffle (like a perforated enclosure) would help to integrate (average out) the fluctuations, but will also slow response time.

    What is the diameter of the tank, or the surface area, or if rectangular, what are the length and width?
     
  10. onlyaman

    Thread Starter New Member

    Apr 18, 2008
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    The tank is 6' x 4' x 4' tall. I have this worked out as far as keeping the water level stable with the use of baffels. My problem is still how to interface the level control with the valve motor and what is the best to use for level control. Iam only interested in keeping the in the tank it self at 0.125 and your right that is a tight tolerance to keep. If I have too I can lower my expectations but at this time its what I am trying for. Response time is not much of a concern because the flow rate is fairly stable and there is extra capacity in the tank if needed. I have a strong back ground in the mechanical side but I will admit I am weak in the electronic side. I am trying to use this to help expand my knowledge while I accomplish my goal. Once again thanks for your interest and help.
     
  11. onlyaman

    Thread Starter New Member

    Apr 18, 2008
    6
    0
    PS
    I don't know if this if this is permitted on these forms, but if anyone is interested in talking to me in person about this project just let me know and I will e mail you my phone number.
     
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