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Measuring capacitance using cylindrical sensor.

kudiarasu
Intellectual 285 points
Other Parts Discussed in Thread: FDC1004

Hi all,

I am trying to make proto board to measure the diesel level through cylindrical sensor using FDC1004 eval board,

I have connected the cylindrical sensor sensing electrode to the FDC 1004CIN1 , Ground electrode to FDC1004 GND and  cable shield to the FDC1004 SHLD1 pin (used in between sensor and FDC1004 chip)

Our sensor capacitance range is 183 pF to 167 pF for full and empty diesel levels ,

I want to measure the same through FDC1004 Eval board,

Anyone can guide me how to configure the GUI tool to measure the sensor level capacitance of 183pF to 167pF.

Regards

kudiarasu.m

  • 0
    TI__Expert 7400 points
    Hi Kudiarasu:

    The FDC1004 has an input capacitance range of +/-15pF, but with the CAPDAC, you can extend that range to 115pF. The CAPDAC has the ability to offset the total capacitance up to 100pF. For 167-183pF, it is not possible with the current configuration. One thing that you would try is to put a capacitor in series with the cap sensor to divide the capacitance down to around 50pF or so, then you can use the CAPDAC to offset it. The only issue with this is that since you are dividing down the capacitance, your sensitivity will be reduced (because you are basically diluting your signal).

    Our next generation part will be able to handle up to 10's of nF's of range, but will not be released until June/July timeframe. Please be on the look out for that since this option will fit your application better since you have a high input capacitance.

    Thanks
    -David Wang
    Capacitive Sensing Applications
  • 0 in reply to David Wang11
    Intellectual 285 points

    Hi David,

    Thanks for your reply,

    As you said, I have tried with series capacitance and expectedly sensitivity reduced to 0 to 5 pf range, Also I have notice another issue , our capacitance sensor range measured as 167 to 183 in calibrated LCR bridge , but i am able to measure through EVM by setting CAPDAC 96.875pf and  the cap output will between -1 to 15 pF (Even it is in out of range ).

    Then I have tried with standard known value (10pf, 15pF,22pF,33pF)of capacitance to verify the measurement of EVM board . Please find the results below

    Fixed (Known )capacitance (pF)

    CAPDAC (pF)

    Measure in EVM GUI (pF)

    10

    6.250

    13.00

    10

    9.375

    9.8

    10

    12.5

    6.7

    From my understanding, EVM GUI measure the capacitance (6.250+13.00) 18.250pF instead of 10pf ?

    Fixed (Known )capacitance (pF)

    CAPDAC (pF)

    Measure in EVM GUI (pF)

    15

    12.5

    13.25

    15

    15.625

    10.17

    15

    21.875

    21.875

    Similarly 25.7pF instead of 15pF

    Fixed (Known )capacitance( pF)

    CAPDAC (pF)

    Measure in EVM GUI (pF)

    22

    21.875

    10.11

    22

    25

    7.04

    22

    28.125

    2.04

    ~ 32pF instead of 22 pF

    Fixed (Known )capacitance(pF)

    CAPDAC (pF)

    Measure in EVM GUI(pF)

    33

    28.125

    9.4

    33

    31.25

    6.4

    33

    34.325

    3.3

    ~37pF instead of 33pF.

    Except the 33pF capacitor other 3 will measured 10pf higher than expected, also tested the all standard capacitance's through LCR and error within the range if +-10%,

    if the error rate is even  we can offset it,but here 33pF offset is different from others, kindly suggest me  cause for the same.

    Is it every chip required calibration?

    Kindly share your view on the above readings and measured capacitance with high error rate.

    Regards

    Kudiarasu.M

  • 0 in reply to kudiarasu
    TI__Expert 7400 points

    Can you tell me how you have it setup? Did you break the sensor portion off and just connecting one side of the cap to CIN and the other to GND?

    Are you using the same channel for the measurement? Also can you give me a screenshot of the configuration registers and bench setup?

    Thanks

    -David Wang

    Capacitive Sensing Applications

  • 0 in reply to David Wang11
    Intellectual 285 points

    Hi David,

    No didn’t break the sensor board, But I have removed( cut) the track between the sensor and board, I have enclosed the picture for reference,

    Yes I have connected capacitor across CIN1 and GND. I have used only CIN1 channel for measurement,

    Please find the attached configuration snapshot.

    Regards

    Kudiarasu. M

     

    This is the configuration file i have used for testing the  22pF cap.

  • 0 in reply to kudiarasu
    TI__Expert 7400 points
    When you are measuring the cap with your LCR bridge, do you have that long wire connected also? If not, then that is why your measurements are not consistent because that wire is showing up as a parasitic capacitance (affecting your total capacitance). Redo your measurements to verify that.

    Thanks
    -David Wang
    Capacitive Sensing Applications
  • 0 in reply to David Wang11
    Intellectual 285 points
    Hi David,
    Great ,
    As you said , i have introduced the same cable in LCR sensor measurement and i found that the reading almost close to reading measured through EVM, Also will try to connect sensor as close as possible to sensor and redo the same and let you know results.
  • 0 in reply to kudiarasu
    Intellectual 285 points
    Hi,
    After a week, i have resume the testing and connected the sensor at closest point , and now ican measured the capacitance between 0 to 15 pf with CAPDAC 62.500pF,

    Now i need to develop my own board for fuel measurement using FDC1004, before get into design, i want to understand the system fully,

    is it essential to connect the reference sensor ? what is the impact if i have not connected the reference sensor in level measurement.

    regards

    kudiarasu.M
  • 0 in reply to kudiarasu
    TI__Expert 7400 points

    Hi Kudiarasu:

    Yes, the reference sensor is needed to be able to calculate the level. if there is no reference sensor, you will only have the LEVEL sensor. they only way to calculate the level is if you have a look-up table that maps capacitance values to specific level intervals, and then interpolate the data for the capacitance in between. Typically, capacitance increases proportionally as the liquid height increases, but you will see that near empty and near full is where it will be slightly non-linear due to fringing effects.

    Using a lookup table is not bad, but it's not accuracy because absolute capacitance values can change due to environmental factors or system factors. That is why we use a reference sensor so that we are looking at a change in capacitance between the LEVEL and the REF and not the absolute values.


    Thanks

    -David Wang

    Capacitive Sensing Applications

  • 0 in reply to David Wang11
    Intellectual 285 points
    Hi,
    We have got sample chip and we made a proto board for the level measurement are trying to read data through i2c interface.
    We have connected the fixed value capacitance (22pf ), actually it is very close to FDC1004 channel,

    we have configured the registers with below values
    Measurement Configuration Register : 0x1080 (CIN1, single ended, CAPDAC 12.5pf )
    FDC Configuration Register : 0x580 ( repeat mode 100 S/s)

    While reading we have received some erratic value for example

    for 22pF and CAPDAC 12.500 pF , we have received the 24 bit raw data as 0x1b0653 ,
    computing the capacitance using the formula given in the data sheet
    Capacitance(pf)=((Two's Complement(measurement [23:0]))/2^19)+C_offset

    we got 40 pF as a result instead of 22pf.

    we are looking forward for your assistance.

    regards
    kudiarasu.M
  • 0 in reply to kudiarasu
    TI__Expert 7400 points

    Can you give me a picture of your setup?

    If you have a 22pF cap connected between CIN1 and GND, and you set the CAPDAC to 12.5pF, you should be seeing the capacitance (without the offset added back in) as 9.5pF. When I calculate it, 0x1b0653 --> 1771091 in decimal. 1771091/(2^19) = 3.378pF.

    Can you do a few tests on a few different fixed known capacitors?

    Thanks

    -David Wang

    Capacitive Sensing Applications

  • 0 in reply to David Wang11
    Intellectual 285 points
    Hi,
    Thanks for your reply.
    I have misunderstood the formula and did the 2s complement again for the 24 bit raw value that's the reason for the error,
    Regards
    kudiarasu.
  • 0 in reply to David Wang11
    Intellectual 285 points
    Hi David,

    I hope we can measure up ( +/- 15pF) in full scale reading, Is there any way to extend the full scale measurement range beyond +/-15.
    If this is possible, will help us to measure more distance.

    Regards
    Kudiarasu
  • 0 in reply to kudiarasu
    TI__Expert 7400 points

    Unfortunately, there is no way to increase the input range of the device. You have 30pF for the input capacitance range. 
    Thanks
    -David Wang
    Capacitive Sensing Applications