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LDC0851: Design implementation

Carlo
Expert 7715 points
Part Number: LDC0851
Other Parts Discussed in Thread: LDC3114

Hello E2E team,

Our customer is interested in using the LDC0851 differential inductive switch and we need your expertise in implementing the device to their design. Here is the full query:

I am a design engineer and have an application in a new design (which we hope to have pilot production run by the end of the year) in which I would like to use your LDC0851 - Differential Inductive Switch - for inductance sensing for metal (steel). I have limited space on the PC board. The board is standard epoxy-glass 0.062 thick with 1 oz. copper traces. This is a double-sided board and cannot be multi layers. Along with top and bottom traces, there is a ground plane as much as possible. The sense coil and reference coil (identacle) will be side by side with a small space between them - not stacked. The area for the sense coil is about 0.24" (6mm) wide and can be up to 1.4" (35mm) long. The metal to sense will approach from the bottom side of the board and will only go under the sense coil - not the reference coil. Sensing distance is from no metal to 0.24" (6mm) so therefore would like switching to be at approx 7mm or 8mm. Only want to sense - is the metal present (larger than the sense coil) or not. According to my calculations - a circular coil of 0.24" (6mm) does not provide sufficient inductance to sense metal at a distance of  6, 7, 8mm since the sense coil diameter should be 2 to 3 times the sensing distance.

QUESTIONS:
1) - Since I have a rectangular area available - can I make a circular coil of 0.24" (6mm) (10 mil trace and 10 mil spacing - giving about 6 turns)
and put say 3 or 4 or even 5 of these coils in that rectangular space and connect them in series since series inductance adds, even if i need to
put jumpers on the board. This could give me 3 to 4 to 5 times the inductance.

2) - Your info shows that the top coil should be wound clockwise and the bottom coil should be counter clockwise. I am presuming that this info is looking through the board. As the board is laid out top traces (red) would be CW and bottom traces (blue ) would be CCW.
One of the responses to a question -was that the unit did not function because the coils winding direction was reversed.
Since I am sensing metal from the bottom of the board - should my coils be reversed giving top side CCW and bottom side CW?
Why do the coils need to be wound in opposite direction?
When calculating the coil - is this only for the coil on one side of the board or is it for the series of top and bottom coils?

3) - Your data also states that the inductance goes to approx half when in the presence of metal and to make sure that the oscillator frequency
remains within the specifications.

4) - Data also states that the more inductance, the lower the frequency (obviously) but the greater the sense distance.

5) - Under these conditions (assuming I can series the coils) can I lay out multi coils to give sufficient inductance to sense steel at the needed distance?

6)- If so - what is your suggestion of quantity of turns ( I calculate it to be about 6) As per data - start with the first turn to be approx 30% of the total diameter.

This is not running on battery - so current consumption is not a determining factor. Your calculator spreed sheet is set up for LDC3114 - not LDC0851 Your "Design with LDC0851" is only for 4 layers and will not let me use anything else. One of the apps uses resistors and does not use coils. Should this be a consideration?I anticipate operating in "basic mode"

Thank you for your guidance.

Regards,
Carlo

  • 0
    TI__Genius 17660 points

    Hello Carlo, 

    Here is some information for the questions: 

    1. With the space available, you can consider using a racetrack/oval design which will increase the inductance while still keeping the 6mm constraint in one direction. This still doesn't change that the 6mm dimension would be considered the coils diameter for determining sensing range. Having the inductors in series could help in a similar manner but each coils coupling to the target will still be driven by the 6mm diameter. 
    2. For coil winding direction, you want the current flowing through the sensor to form a magnetic field in the same direction between the top and bottom coils. You can swap your coil directions so that the bottom is CW but the most important factor is making sure the two layers are opposite. 
    3. The shift in inductance depends on how close the target is to the sensor and the sensor design. For a target distance that is the same as the coil diameter, there won't be much change in the sensed inductance. Another factor is the target material. You mention steel as the target which has a medium conductivity. Metals like aluminum and copper have a better response. 
    4. The main impact on sensing distance is the coil diameter. This is because the magnetic field of the coil is larger with larger coils. The https://training.ti.com/ti-precision-labs-inductive-sensing-overview-inductive-sensing-technology TIPL talks about this relationship some more. 
    5. Sensing steel at this distance with only a 6mm coil diameter is a tough application. The 2-3 times coil diameter recommendation is for best resolution so you could still see a shift in the sensors inductance for your target motion, but it may not be enough to have an output on the LDC0851. 
    6. Based on the 6mm diameter, 4 turns is a good value to keep the coil fill ratio above 20%. 

    The LDC Calculator Spreadsheet has a drop down menu for the LDC device to be selected. There is also an LDC0851 tab that you can just input the inductance and see more information about the device operation. I am not sure what you mean by using resistors instead of coils. This device is designed for inductive coils and would not perform normally otherwise. 

    Best Regards, 

    Justin Beigel