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Original question:

Using a high voltage solenoid as sensing coil

Monitor Plunger Movement in DC Solenoids using a LDC

Ian Stumpe
Prodigy 20 points

I want to verify the position of a 12Vdc 1amp solenoid valve.  Would it be possible to use a LDC to monitor the inductance change of the solenoids coil as the armature moves through it?  Based off the response in the related post this is possible but the LDC needs to be isolated from the power going through the coil.  How would this be done? Where do the high impedance inductors need to be placed?  Is there a referance design?  Regarding referance designs I did review the TIDA-00289 design where the back emf of the coil is used to determine the position, unfortanitly this will not work with this vavle due to very little back emf (no measurable current cusp).  A screen capture showing the voltage (yellow) and current (blue) when the valve is actuated is attacheched. 

  • 0
    TI__Mastermind 18545 points

    Hello, 

    We don't have any reference design for this type of implementation. Depending on your application needs, you may still be able to use an LDC device to monitor the valve position. I have a couple questions for you to help clarify the application so we can offer better guidance. 

    • What kind of resolution do you need for the valve position? Is it just an open/closed application or do you need more accuracy than that? 
    • Would you be able to place an LDC sensor coil outside the valve to monitor the valve position? If so, would it be perpendicular to the motion or parallel to it? 

    Thank you, 

    Justin Beigel

  • 0 in reply to Justin Beigel
    Prodigy 20 points

    Hi Justin thank you for the response. The solenoid is an on/off type so resolution shouldnt be an issue (stroke 0.070").  Useing an LCR meter I measured 75.88mH (off state) and 71.32mH (on state). Useing an LDC sensor coil outside of the vavle is not feasible (unless I am missing something).  The picture below shows the test manafold for the valve (testing 4 at once), green marks the flow path (nitrogen up to 4kpsi @ 100C).  I was origianly thinking about using an external senosr such as a proximity sensor but give the high pressure and temp no sutible sensor could be found.   

     

  • 0 in reply to Ian Stumpe
    TI__Mastermind 18545 points

    Hello, 

    If you can't place the sensors outside of the solenoid to monitor the motion, then it becomes a challenge of isolating the solenoid actuation from the LDC sensor. The LDC drives it's sensor coils at their resonant frequency based on the capacitor used in the design and uses that frequency to take a measurement. 

    Ideally, you would disconnect the solenoid from its drive signal, connect the LDC to the solenoid coil, take an LDC measurement, then disconnect the LDC and reconnect the solenoid drive signal to make sure there is complete isolation but that doesn't work so well when you want to measure when the solenoid is driven. 

    You mention the solenoid is 12Vdc. If your coil can support a sinusoidal signal superimposed onto the driving DC signal, then theoretically there could be a solution where you isolate the DC signal from the LDC. You would have to have something that can allow the LDC sine wave to pass through while blocking the 12Vdc signal and minimizing any impacts from when the solenoid switches on an off. Unfortunately, we don't have any reference designs or previous work for this type of implementation.  

    Best Regards, 

    Justin Beigel

  • 0 in reply to Justin Beigel
    Prodigy 20 points

    Thank you for the feedback, looks like I will need to do some prototyping to see if I can get it to work.  In theory a series cap on the input and output of the LDC should isolate it from the DC signal, but alowing the AC sinusoidal signal to pass through....  Thanks again for the help, if I figure out anything I will update my post.

  • 0 in reply to Ian Stumpe
    TI__Mastermind 18545 points

    Hello, 

    Sounds good. Let us know if you have any other questions as well. 

    Best Regards, 

    Justin Beigel