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LDC1000 inductive sensor use to measure distance between two metallic object moving towards each other??

sudhanshu bajpai
Prodigy 175 points
Other Parts Discussed in Thread: LDC1612, LDC1614, LDC1312, LDC1314, LDC1101, LDC1312EVM

Hello All, 

Currently i am working on some PoC where we need to measure the distance between two metallic object (Assume metallic door) moving towards each other, we identified the LDCxxx Series is capable of measuring the distance, however our requirement is for 0.5 mm to 10 mm detecting distance, to be more precise we need to measure the metallic door gap of 4 mm. Please suggest will this sensor would be able to serve the purpose?

If, Yes then what would be the suitable place to mount this sensor module?

we thought of mounting it on the metallic part of the door, however we are not sure whether mounting the sensor on metallic door will create any problem in sensing the distance??

  • 0
    TI__Genius 13650 points
    Hello Sudhanshu,
    Have you considered using one of our newer devices for this application? They offer many advantages over the LDC1000:
    - The high-resolution multi-channel LDCs (LDC1612, LDC1614) are L-only devices which offer greatly improved measurement accuracy, better part-to-part variation, lower power consumption, easier design integration, and multiple channels.
    - The mid-resolution multi-channel LDCs (LDC1312, LDC1314) are L-only devices which offer better part-to-part variation, lower power consumption, easier design integration, and multiple channels.
    - The high-resolution, high-speed LDC1101 supports both L- and RP-measurements, greatly improved measurement accuracy, better part-to-part variation, lower power consumption, and easier design integration.

    Can you provide a drawing of your mechanics? Without it, it is difficult to suggest any component placement.

    Having a second (interfering) metal nearby is ok as long as it is at a fixed distance. It will cause a permanent offset, which is usually not a concern. However, it will also cause a reduction in dynamic range; the closer the interfering metal is, the smaller the dynamic range (and therefore the lower the accuracy). This blog shows how to deal with interfering metal: e2e.ti.com/.../inductive-sensing-how-to-shield-from-metal-interference
  • 0 in reply to Ben Kasemsadeh
    Prodigy 175 points
    Hello Ben,

    -- Thanks for your clarification, I would like to explain the use case where we want to use this L to D converter.
    we have the metallic Door made of Stainless Steel, both side of the door will be sliding each other and we need to identify the air gap between both the door (slide door) at the closed condition. Ideally the air gap of the door should be 4 mm.

    -- The sensor part we need to mount on one side of sliding door (stainless steel) and the target metal (made of Copper) shall be mounted on the other side sliding door.

    -- Basically in our case the Interfere metal is stainless steel, on which our sensor will be mounted and which is intended to detect the target metal Copper which is mounted on the other side of door on the stainless steel, so in this case how our sensor will be affected by the interferer.

    -- After converting the inductance to digital, we will get the digital count, is there any example algorithm which can be used to convert into the distance? (any formula which has been used)
  • 0 in reply to sudhanshu bajpai
    TI__Genius 13650 points
    Hello Sudhanshu,
    An LDC1312 is probably a good choice for your application. I recommend a coil diameter of 2-3x the sensing range for best results. For example, if the maximum target distance is 4mm, then for prototyping you may try the LDC1312 EVM sensor or coil M from the LDC coil reference board www.ti.com/.../ldccoilevm . I recommend a distance of >1 coil diameter between the stainless steel interfering metal and the sensor. If this is not possible, sensitivity is affected. In case the drop of sensitivity due to the interfering metal is too much to meet system spec, you may need to shield with a ferrite sheet as described in the blog.
  • 0 in reply to Ben Kasemsadeh
    Prodigy 175 points
    Hello Ben,
    Thank you for confirming the part number for our application, however we need little more clarification on below mentioned points as :

    1. The Algorithm we need to apply to convert the the count to get the output in distance (in mm), do we have any algorithm which we can use?
    2. As in my block diagram, i mentioned that the Target would be the "Copper" metal, which will be mounted on the Stainless steel metallic door and the sensor also would be mounted on the metallic part of the door and sensor and the target metal will be moving towards each other in that case how our measurement will be affected??
    -- Mounting the copper metal on stainless steel can be the problem??
    -- do we need to isolate the target from stainless steel as well as sensor by Ferrite?
  • 0 in reply to sudhanshu bajpai
    TI__Intellectual 2725 points

    Hello Sudhanshu,

    I have addressed your queries below:



    1. The Algorithm we need to apply to convert the the count to get the output in distance (in mm), do we have any algorithm which we can use?

    -> The algorithm would be specific to your sensing system. The easiest way to do this would be by performing a  system level characterization and generating a look up table correlating LDC output code and target distance. You could use a second or third order correction in the look up table based on the performance needs.

    2.  As in my block diagram, i mentioned that the Target would be the "Copper" metal, which will be mounted on the Stainless steel metallic door and the sensor also would be mounted on the metallic part of the door and sensor and the target metal will be moving towards each other in that case how our measurement will be affected??
    -- Mounting the copper metal on stainless steel can be the problem??
    -- do we need to isolate the target from stainless steel as well as sensor by Ferrite?

    ->The copper metal would provide better sensitivity than the stainless steel door. As long as the copper target thickness is significantly greater than the skin depth (at least 3x) you should not need a ferrite separating the target from the stainless steel door. Operating the LC tank at higher frequencies reduces the skin depth. A skin depth calculator along with a bunch of additional tools can be found here.

    In addition if you will be using one of the  multi-channel devices you should consider implementing a differential measurement. 

    Please let us know if you have any other questions. Thank you for considering LDC devices for your application.


    Regards,

    Varn Khanna,

    Applications Engineer, Sensor Signal Path,

    Silicon Valley Analog,
    Texas Instruments.

  • 0 in reply to Varn Khanna
    Prodigy 175 points

    Hello Van, 

    I was looking into the LDC1312EVM user guide and i understood that by this EVM and the GUI software i will be able to get the change in inductance value based on the Target moving or going away from the sensor (or based on the distance between the Target and the sensor), however for my application as of now i want to know the distance between the my target and the sensor.

    How i can get that data?

    The look up table part is little little unclear to me, please help me in that matter..

  • 0 in reply to sudhanshu bajpai
    TI__Intellectual 2725 points
    Hello Sudhanshu,
    I think you are really close to getting to a working solution.

    The EVM GUI will provide you with inductance values which will change as the target moves away. During prototyping, you will need to measure the target distance manually and grab the EVM GUI inductance output at multiple distances, once you have enough data points you can interpolate the data to correlate distance to the inductance/raw output code. As a simplification think of a two point line equation(y = mx+b, a first order polynomial calculated using two points on the line). Once you have the equation of the line you can calculate the y value given x, similarly by creating a look up table (4th, 5th, or 6th order polynomial) you should be able to calculate distance given an inductance/raw output code value. Excel charts are great tool for generating these higher order polynomials.

    You will only need to measure the target distance manually once for coming up with the look up table, after that you should be able to tell the target distance just by looking at the inductance value.

    The inductance response as a function of distance varies with sensor and target configurations, it would be unfeasible for the GUI to account for the infinite setup combination and hence the GUI does not output a distance value. A system characterization of inductance response as a function of distance is the best way to accomplish your goal.

    I hope this helps.

    Regards,

    Varn.
  • 0 in reply to sudhanshu bajpai
    TI__Genius 13650 points

    Hi Sudhanshu,

    it depends on many factors such as mechanical and coil design; therefore, there is no single formula for it.

    To relate inductance change to absolute distance, I recommend characterizing your system and then either use curve fitting to find a suitable polynomial, or use a look-up-table with interpolation in the microcontroller. 

  • 0 in reply to Ben Kasemsadeh
    Prodigy 175 points

    Hello, 

    I have received my LDC1312EVM module, i was referring the User manual for this EVM, where they have explained the old GUI 0.0.1.1 which is different than the new one which i downloaded V1.8.8

    I am not able to proceed with the measurement, please help me in this so that i can start understanding the functionality of this module.

  • 0 in reply to sudhanshu bajpai
    TI__Intellectual 2725 points
    Hello Sudhanshu,
    The EVM guide is in the process of being updated but the new Sensing Solutions EVM GUI is quite intuitive.

    Could you please provide me with additional specifics as to what aspect of the measurement were you struggling with? Once the Sensing Solutions GUI starts it should automatically detect the LDC1312 EVM, you can check this by looking at the bottom left corner of the GUI.

    Once that is done, even with the default settings you should be able to stream inductance values through the streaming page. The streaming page along with the configuration page appear if you hover towards the left edge of the GUI window.

    Regards,
    Varn.
  • 0 in reply to sudhanshu bajpai
    Prodigy 175 points

    Hello,

    I started with the EVM  "LDC1312EVM"  for the understanding where i followed the below mentioned steps:

    1. Installed the GUI V1.8.8

    2. Connected the EVM to PC, GUI detected the EVM module.

    3. I directly went to Data streaming section and started the streaming.

    4. i used coin to start with, so when i bringing coin near to Sensor the Raw code values are changing, however i am not able to see any changes in the Inductance values.

    5. Note that i have not  changed register & configuration section in the GUI (do i need to do any changes??)

    6. can you provide steps to use this module properly?? (i just want to make sure that i m progressing correctly)

  • 0 in reply to sudhanshu bajpai
    TI__Genius 13650 points

    Hi Sudhanshu,

    the EVM should work with the default register configuration. If you can see a change in raw code, then the Inductance should also change; the GUI calculates the inductance from the raw code and the capacitor value that you specify in the GUI. Is it possible that the GUI is configured to a very wide inductance window and the change is not noticeable? I recommend trying the autoscale function as shown in the picture.

  • 0 in reply to sudhanshu bajpai
    Prodigy 175 points

    Hello, 

    We are working on LDC1312 and we need to fill some data to compare this Sensor with others in terms of 

    Accuracy

    Maximum Measuring Range

    Resolution (in terms of mm)

    Sensitivity

    In datasheet i didnt get the accuracy and other details

    Can you please help to  derive these points??

  • 0 in reply to sudhanshu bajpai
    TI__Genius 13650 points
    Hi Sudhanshu,
    the resolution of the device depends on the device itself, its configuration, and its sensor frequency. This application note describes the resolution of the LDC1612: www.ti.com/.../snoa944.pdf . Note that the LDC1312 outputs 12 out of 16 bits of the LDC1612 (see also e2e.ti.com/.../inductive-sensing-improve-the-enob-of-a-multichannel-ldc-by-4-bits-in-3-simple-steps).

    In general, I recommend starting to build a system with the high-resolution LDC1612, and then swap it out for the less expensive and pin identical LDC1312 if system accuracy requirements can still be met.

    The system accuracy depends mainly on mechanical design, target distance, target choice, sensor design, and any compensation techniques such as differential measurements. Figure 66 in the LDC1612 datasheet (www.ti.com/.../ldc1612.pdf) show that the EVM coil can sense an Al target with an rms noise of less than 100nm. In almost all cases, the limitation is in the mechanical design and not in the measurement precision of the LDC.