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Original question:

LDC0851: Design of Target wheel and inductive sensors PCB required for three phase output

LDC0851: Maximum RPM calculation and sensor accuracy using LDC0851

Ravindra Gaikwad
Intellectual 480 points
Part Number: LDC0851

Dear All,

I am working on project of three phase hall effect sensor replace to three LDC0851 IC with three signal output H1,H2 and H3.

The main application is to calculate motor speed. In our case we are using target disk contains 5 conductive regions,

each covering 36° and separated by gaps of 36°.

For calculation speed we are place three sensor on circular PCB with 120 degree each other.

Below are our observations.

1. We are not getting 50% duty cycle. Its around 60% or varies.

2. 120 degree phase shift is not achieved. The phase shift is vary 110 to 140 degree.

Please confirm can we achieve below point using LDC0851 IC.

1. Maximum RPM speed approximate 7500 to 8000 rpm.

2. Sensor duty cycle accuracy : 50%, + / - 1%.

3. Phase shift accuracy : 120 degree electric, + / - 1 degree.

4. Sensor period accuracy : + / - of coil diameter(in degree)

  • 0
    TI__Genius 17660 points

    Hello, 

    Achieving an RPM of 8000 is possible with this device depending on the sensor frequency and oversampling applied to the application. For an oversampling factor of 3 and 5 gear teeth, you would need at least a 17.3MHz frequency coil to achieve this RPM. 

    The sensor duty cycle and phase shift accuracy depend on the oversampling applied and the mechanical layout of the sensor and target. Layouts following the design procedures in LDC0851 Incremental Rotary Encoder Design Guide should result in the best accuracy. 

    Additionally, the switching hysteresis will play a factor in the accuracy. 

    How much inductance shift you see from your target moving will change the hysteresis. The more change, the less hysteresis will be a factor on the mechanical movement. 

    Are you able to share any details on your sensor layout and target design for this application? 

    If you decrease the RPM and increase the oversampling, are you achieving the desired phase accuracy? 

    Best Regards, 
    Justin Beigel