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Cylindrical capacitive sensor

khalid umer68
Prodigy 30 points
Other Parts Discussed in Thread: FDC1004

Hi David and all 

 I have used cyindrical capacitive sensor with FDC1004 .

Construction : An alluminium rod is passing from the centre of a  aluminum cylinder or pipe. Rod is connected to CIN1 and outer pipe is grounded.

Method: I have not used refrence sensor . instead , I have taken reading in an empty tank and considered it at zero level then take a reading with full tank and save the reading as 100% . supposing  probe a linear sensor , 

level % ={ count running  -   count (empty) } / count(max) - count (min)   x  100

reading I got is excellent but counts are not stable. counts are jumping till the 9th and 10th bit.  

I have used it in generator tanks . and facing problem that the reading is drifting. Specially during day times it increases and at night it decreases. It seems to be happening like this b/c of temperature. 

Also when the generator starts it heats up sensor and reading changes. 

Some times without any reason. error is from 1 to 5 % . 

Is the active shielding can fix the problem ??? where do I make the active shielding?? over the rod which is passing the centre?? or should I put another cylender over the outer pipe and connect it to shield output. ???

or anyother solution please give . 

I have used temp. sensor to compensate in software but the problem is that the response of sensor is much faster that the output of the probe. it worsen the overall response .  

IS it possible to impliment OOP on this type of probe sensor. ?? b/c u said in application note that symettry is the key for both parts of sensor. 

Kindly give your suggestion to make the sensor accurate and rubust . 

With best regards

khalid 

  • 0
    TI__Expert 6480 points
    Hi Khalid,

    Can you describe how the generator heats up the sensor and what sort of temperature change you're getting?

    Active shielding won't help with temperature drift, instead, we would recommend using a reference sensor. This way, if you take a ratiometric measurement, the temperature drift will cancel out (assuming that they both drift in the same direction).

    Thanks,
    Rachel
  • 0 in reply to Rachel Liao
    Prodigy 30 points

    Dear rachel

     Thanks for your reply. 

    When the generator starts the heat from the engine increases the overall temperature of all parts of generator and specially the tank which is just below the engine usually. our sensor is mounted in the fuel tank. 

    In case of cylindrical sensor I am not able to understand that how the reference sensor can be inserted along with the actual level sensor. It will take much more space and how to do that ..?

    What should be the material of level electrode which should be prone to temperature change and environmental effects. 

    simple Aluminum pipes will do ?? or need some aluminum alloy ?? or any thing else ??

    What are the factors other than temperature that can contribute to inaccuracy and drift. 

     

    With best regards

    khalid

     

  • 0 in reply to khalid umer68
    Prodigy 30 points

    Dear Racheil 

     I am still waiting for your reply. 

    regards

    khalid

  • 0 in reply to khalid umer68
    TI__Expert 6480 points

    Hi Khalid,

    Sorry for the delay, I've been on travel.

    I see what you mean, is there any way you could do something similar to the method shown in this app note?

    We typically like to use copper as the material for our sensors. Other than temperature, there could be oscillator drift or drift caused from noisy external interferences.

    Thanks,

    Rachel

  • 0 in reply to khalid umer68
    Prodigy 10 points

    If this could help.

    I'm usign the FDC1004 for a level sensor project. For the probe I'm using a plactic tubing (1" dia) closed at one end. (the electrodes never touch the water and stays dry)

    For the electrodes are made of copper tape for shileds and level but I did not implemented references electrodes and It works good in my environnement.

    Adding references electrodes could help to be immune to temperature changes.

    I put copper tape inside the tubing that stays dry and it gives good results. With tape 6mm wide you can measure up to 1 meter of liquid and more.

    Michel