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Questions about Target design 1 degree dial

Melanie Loy
Prodigy 80 points
Other Parts Discussed in Thread: LDC1614

Dear TI Team,

inside tidu-953.pdf on page 21 Figure 18 shows two equivalent target designs.

I also found the target shape calculator which only contains the calculation for the target shape on the left side of Figure 18.

Concerning the target shape for this kind of application I would like to have the answers to my following questions please:

1. When do I need to choose triangular shape and when the other ?

2. Why did you choose the left form in your EVM ?

3. How would the calculation look like for the triangular target ?

4. Which things I have to consider in the target design for a triangular target ?

5. Is it possible to simply exchange the target which is inside the EVM with the triangle one that is similar ?

I already know that for angular sensing the distance to the coil should be constant all the time and as small as possible.

I also know that I need to have a low frequency , and metal at leas 3 * times of the skin depth for my selected frequency of the LC Tank to get good resolution. Further I know that the width of the metal is the largest in the middle where the centre point for the rotation will be. Also that at starting position we have the largest metal area covering the coil and then during the movement the area that covers the sensor will be getting smaller.

Thanks for your answers in advance.

Best Regards,
Melanie

  • 0
    TI__Genius 13650 points
    Hi Melanie,
    1. Which two shapes are you referring to? Figure 18 shows how a rhombus shape can be wrapped around a center point to create the crescent moon-like shape that is recommended for rotary applications.
    2. It allows full 360 degree detection of the position with two coil pairs.
    3. A triangular target would not allow full 360 degree detection with this approach. It is only recommended in applications in which less than 360 degree detection is required. If 360 degree detection is required, I recommend the target as shown in tidu-953.pdf, which can produce a system that is insensitive to z-axis variation and temperature.
    4. What is the reason for considering a triangular target for this design instead of the rotated rhombus shape? The rhombus shape is better for full 360 degree rotation and z-axis compensation.
  • 0 in reply to Ben Kasemsadeh
    Prodigy 80 points

    Hi Ben,

    thanks for your fast reply. I was referring to the two target shapes you see in the picture below. It's clear now that the one on the left side is for 360 degree measurements that are insensitive to z axis variations and temperature. In case that I would have to measure angles within +/-50 degree for example how would I start to get an triangular target then ? The rotation point is in the middle of the triangle where we have the thickest metal and the largest distance to the apex of the triangle. I know also that I have to make sure that distance in z-axis is constant. So how do I decide which width for the metal in the middle I use and how to get from there to get the shape I need. One thing that is also not yet so clear for me concerning the triangular target is how much area of the coil the target should cover at 0 position.  Thanks already for making things again more clear for me.

    Best Regards,

    Melanie

  • 0 in reply to Melanie Loy
    TI__Expert 6400 points
    Hi Melanie,

    Thank you for informing the angular range for your measuerment.
    While, we would like to know what you expect for triangular shap.
    In my understanding, the output data with triangular shap should be more complex to caluculate the angule than crescent moon shap of figure 18 even if angular range is within +/-50 degree.
    Could you please draw the example of triangular shap(equilateral , obtuse, etc? ) in order for us to understand your expectation properly?

    Best regards,
    Iwata Etsuji
    Application Engineer
    Texas Instruments
  • 0 in reply to Iwata Etsuji
    Prodigy 80 points
    Hi Iwata,

    thanks for your reply. At the moment we want to try first measurements with an triangular target and an LDC1614 EVM Revision B. The idea is to get an idea how later on we can get an measurement with high accuracy within a certain range e.g. the +/- 50 ° .

    Our idea currently for first tests with an triangular shape is to make the rotational point of the triangle into the perforation between the two coils of the EVM. This means 1.6 cm to the middle of each coil. Coil diameter is 13.9 mm meaning 1.39 cm.
    We want to understand the pro and cons between the two target forms. And more important we want to understand how the target design also for a triangle shape works .

    From figure 18 of the application not for the 1° dial it's only clear that the width of the metal changes with the distance and that the thickness of the triangle depends on the material we use and the frequency of the LC tank.
    So open questions we have currently are :

    1. How much area of the coils should be free of metal at 0 position ?
    0 position meaning that the triangle is in the middle of the coil (largest possible metal area covering the coil)
    2. Is there a rule which says which value the width of the metal at rotational points needs to be ?
    3. Is there a kind of formula to calculate the width of the metal dependent on the distance to the rotational point ?
    4. Is there a kind of standard procedure we can follow for target design for the two target shapes ?

    I am familiar with Ben's posts about linear position sensing part 1 and 2. Further I am familiar with application notes TIDU367, TIDU953 and TIDUBG4. I also know the target shape calculator .xlsx for the 1° dial target.

    Thanks for your further help.

    BR,
    Melanie
  • 0 in reply to Melanie Loy
    TI__Genius 13650 points
    Hi Melanie,
    I think there may be a misunderstanding about the purpose of different target designs.

    Linear position sensing:
    Triangular targets, such as the one described in www.ti.com/.../snoa931.pdf , are used for linear (lateral) position sensing. The best resolution is achieved when at the minimum position, the sensor has 0% target coverage, and at the maximum position, the sensor has 100% target coverage. This is the reason why the target in figure 3 of SNOA931 extends past the triangle by 30mm. This target can be scaled in the y dimension to different sensor sizes, and in the x dimension to different travel ranges.

    Angular position sensing:
    While it is possible to use a target such as the one described above and wrap it around a center point, it is usually not the preferred method because a system is more challenging to build to compensate for z-axis variation and full 360 degree rotation.
    The crescent moon shape target, which is actually a rhombus which is wrapped around a center point, is preferred for accurate dials. It is possible to implement this with 2 sensors (90 degree spaced apart), but for a better robustness to z-axis variation and temperature, I recommend the 4-sensor solution as described in TIDU953. Are you able to scale the solution in TIDU953 to your desired dimensions, or do you encounter any limitations?
  • 0 in reply to Ben Kasemsadeh
    Prodigy 80 points

    Hi Ben,

    first of all I am sorry for my late reply.

    Thanks for explaining the difference between Linear position sensing and angular position sensing.

    Dimensions will be an issue in our application . I am sorry that I am not allowed to tell you the exact dimensions.

    What is your recommendation for coil design and target design for very small dimensions because of your personal experience ?

    Your answers so far gave us the basic understanding of the difference between the two target forms which is already very helpful.

    Thanks for your further support .

    Have a nice day.

    BR,

    Melanie

  • 0 in reply to Melanie Loy
    TI__Genius 13650 points
    Hello Melanie,
    you can scale down the solution in the application note to your dimensions. As you decrease the solution in size, accuracy may decrease and keeping mechanical tolerances low becomes more important.
    Since the coils need to have a minimum inductance as specified in the datasheets, there is a practical limitation to how small the system can be. For circular coils, this is approximately 4-5 mm diameter for each coil (depending on PCB properties such as trace spacing and number of layers). You can use the Sprial Inductor Designer tab on the LDC tools spreadsheet to calculate coil inductance: www.ti.com/.../getliterature.tsp .
    You can reduce the coil size a little bit further by using trapezoidal coils instead of spiral coils because they provide more inductance for the PCB area, as explained in www.ti.com/.../snoa930.pdf. System design becomes challenging if the diameter of the overall sensor area is less than 10mm.
  • 0 in reply to Ben Kasemsadeh
    Prodigy 80 points

    Hi Ben,

    thanks for your help. This gives us the basic idea for now we wanted to have.

    Best regards,

    Melanie

  • 0 in reply to Melanie Loy
    TI__Genius 13650 points
    Hi Melanie,
    I'm glad I answered your questions. Thank you for considering the LDC for your application.