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Original question:

LDC1312: LDC1312 Coil Design Questions.

LDC1614: LDC1614 Coil Design Questions.

Vidit Katlana
Prodigy 60 points
Part Number: LDC1614
Other Parts Discussed in Thread: LDC2114, LDC0851
Hi Everyone,
I have been working on evaluating LDC1614 on theoretical grounds.
Three things are main drivers of design in our application
1. Size of Coil - Can be Max 6 mm dia.
2. Power consumption - At 1 or 2 Sample per Second I need average current to be less than 4uA
3. Distance of Detection Target - Target is at around 5 mm
Our application is basically to detect rotation of a half metal and half plastic dial (Water Meter Dial - Photo Attached), for this we are want to put three coils in triangle to detect clockwise or anti-clockwise rotation.
I have used the excel sheet provided on TI website (Sheet attached) while following the LDC Sensor Design Application to calculate IC config, Power Consumption etc. The power consumption is very high, and the frequencies seems to be going out of range of IC ratings.

So I need some guidance in tweaking the frequencies and inductance to achieve above mentioned targets.

Thanks !

LDC_Tools-ext48.xlsx

  • 0
    TI__Genius 17440 points
    Hi Vidit,

    Thanks for attaching the LDC_Tools file and the picture. They are very helpful! Unfortunately, I think the LDC1614's current consumption will be too high for your application. Using the minimum conversion time, I was only able to reduce the average current to about 6 uA. If you could sample at 0.5 SPS it might work, but this may not work depending on the accuracy requirements of your system.

    We do have a lower power LDC device (the LDC2114), but it functions differently from the LDC1614. If you clarify a few things about your application we can decide if the LDC2114 would work for you. Do you need to detect the absolute position of your dial, or just the direction of rotation? Also, how fast is the dial rotating, and does the speed vary? Finally, how close can the coils be to the metal on the dial?

    Regards,
  • 0 in reply to Kristin Jones93
    Prodigy 60 points

    Hi Kristin,

    Thanks a lot for reply., I will evaluate LDC2114 also

    In my application I just need to detect direction of rotation and count the number of rotation, and I have to make sure I don't miss any rotation.
    Yes Speed will keep varying,
    5-4mm will be the distance between coil and Metal Body.

    Also that whole dial has a diameter of 12mm so my sensor col can only be 5-6mm

    Thanks for your hard work,

    Cheers !

    With Regards
    Vidit Katlana
    Engineer - Hardware & Firmware
    SmarterHomes Technologies Pvt. Ltd.
  • 0 in reply to Kristin Jones93
    Prodigy 60 points
    Also, what do you think about LDC0851 ? Can this IC be used for our application ?
  • 0 in reply to Vidit Katlana
    TI__Genius 17440 points
    Hi Vidit,

    It sounds like the dial could be rotating fairly slowly, so I would not recommend using the LDC2114. It has baseline tracking, which is especially helpful for monitoring sudden target shifts (like button pushes) while compensating for slow environmental changes (like temperature change). If 1 SPS is enough data to monitor the rotation, I worry that the rotations could be slow enough that the baseline tracking could mask them.

    However, the LDC0851 could work for you. Unlike other LDC devices, it is an inductive switch, so it does not give you raw data from sensor measurements. Instead, the device outputs a logic high or low based on a comparison between the inductances of a sensor coil and a reference coil. It is frequently used for event counting. I would recommend using the battery life calculator in the LDC0851 section of the LDC Tools calculator.

    If the current consumption is low enough, I believe that you could implement a sensing solution using only two coils. You would definitely be able to keep track of the rotations. However, you might need to change the shape of the metal on the dial in order to detect the rotation direction. Is this possible? If so, I have some ideas. If not, the only other way would be to use two LDC0851s, which may draw too much current.

    Regards,
  • 0 in reply to Kristin Jones93
    Prodigy 60 points

    HI Kristin,

    Thanks a lot for your reply.
    Interestingly that's the IC from which we started our research :D

    Yes you are right, the dial is slow moving, so LDC2114 is out of question then.
    I can't change the shape of dial, this is made by different vendor and is standard in metering industry

    But I am very curious what is your idea, can you please tell me ?

    About LDC Calculator, how can I design coil for LDC0851? I need coil to be within 5-6mm diameter, maybe I have to use Solenoid with soft Iron Core ?

    Thanks, Cheers !

    Regards
    Vidit Katlana




  • 0 in reply to Vidit Katlana
    Prodigy 60 points
    For coil design I am only able to find PCB Trace coil guide ..
  • 0 in reply to Vidit Katlana
    TI__Genius 17440 points

    Hi Vidit,

    The LDC0851 requires the two sensor coils to be very closely matched for proper operation. For this reason, I strongly recommend using PCB coils instead of wire-wound coils. The PCB coils will be matched much better. You should be able to design PCB coils that suit your 5-6mm size requirement. The major limiting factor for sensor size is the minimum inductance. If the coil's inductance is too low, the LDC devices will not be able to drive the sensor with enough current to keep it properly oscillating. 

    The LDC0851 is essentially an inductance comparator. It will output a logic high or low depending on which coil (either the reference or the sensor) has a lower inductance. If it were possible to change the target shape, you could use two metal sectors of different arc lengths to create a unique output signature. The coils would be placed next to each other so that the rotating dial could cover them equally. For example, if the output stayed high when the plastic part of the dial was over the coils, the output could be briefly pulled low by the small metal sector, then pulled high by a gap between the metal sectors, then pulled low for a longer duration as the larger metal sector rotated past the coils, and finally returning high as the plastic once again covers the coils. You could detect the direction of rotation using the order of the logic low pulses (either short->long or long->short) and could use either pulse duration to detect each individual rotation. 

    It may be possible to achieve a similar effect by varying the shape of the coils, but as I mentioned above the LDC0851 requires very closely matched coils, so this would be very challenging. 

    Regards,

  • 0 in reply to Kristin Jones93
    Prodigy 60 points

    HI Kristin,

    LDC0851 is having challenges in switching on distance, power consumption, form factor etc.

    My question is, is there any other alternative than LDC ?
    Anything that solves the purpose, I am happy with it.