TIDUF97 September   2024

 

  1.   1
  2.   Description
  3.   Resources
  4.   Features
  5.   Applications
  6.   6
  7. 1System Description
    1. 1.1 Key System Specifications
  8. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Design Considerations
    3. 2.3 Highlighted Products
      1. 2.3.1 TMAG6180-Q1
      2. 2.3.2 MSPM0G3507
      3. 2.3.3 THVD1454
  9. 3System Design Theory
    1. 3.1 Hardware Design
      1. 3.1.1 Angle Sensor Schematic Design
      2. 3.1.2 MSPM0G3507 Schematic Design
      3. 3.1.3 RS485 Transceiver Schematic Design
      4. 3.1.4 Power Supply and Reference Voltage
    2. 3.2 Software Design
      1. 3.2.1 Angle Calculation Timing
      2. 3.2.2 Rotary Angle Calculation
      3. 3.2.3 Rotary Angle Error Sources and Compensation
      4. 3.2.4 Encoder Communication Interface
  10. 4Hardware, Software, Testing Requirements, and Test Results
    1. 4.1 Hardware Requirements
      1. 4.1.1 PCB Overview
      2. 4.1.2 Encoder and JTAG Interface
      3. 4.1.3 Software Requirements
    2. 4.2 Test Setup
    3. 4.3 Test Results
      1. 4.3.1 AMR Sensor Sin and Cos Outputs Measurement
      2. 4.3.2 Static Angle Noise Measurement
      3. 4.3.3 Rotary Angle Accuracy Measurement
        1. 4.3.3.1 Impact of Airgap on Noise, Harmonics, and Total Angle Accuracy
      4. 4.3.4 RS485 Interface and Signal Integrity
  11. 5Design and Documentation Support
    1. 5.1 Design Files
      1. 5.1.1 Schematics
      2. 5.1.2 BOM
      3. 5.1.3 PCB Layout
      4. 5.1.4 Altium Project Files
      5. 5.1.5 Gerber Files
      6. 5.1.6 Assembly Drawings
    2. 5.2 Tools and Software
    3. 5.3 Documentation Support
    4. 5.4 Support Resources
    5. 5.5 Trademarks
  12. 6About the Authors

3.2.3 Rotary Angle Error Sources and Compensation

For accurate angle measurement, the center of the magnet need to be aligned to the center of the sensor with acceptable tolerances. Follow these steps to calibrate the sensor for best accuracy:

  • Set the reference angle based on the magnet alignment to the sensor. This error can be saved in the microcontroller for run time absolute position calculation. This error is also known as angle offset in a system.
  • Electrical offset calibration, see Calibration of AMR Angle Sensors for the offset calibration procedure. If the sensor cannot be rotated across the full range, then the electrical offsets cannot be calibrated.
  • Amplitude mismatch calibration, see Calibration of AMR Angle Sensors for the amplitude mismatch calibration procedure. If the sensor cannot be rotated across the full range, then the amplitude mismatch cannot be calibrated.

Further error sources include non-linearity of the sensor signal chain such as the 3rd harmonics, and a mechanical error through coupling a reference angle encoder to the shaft of the absolute magnetic encoder. Figure 3-7 through Figure 3-10 outline the error source and the impact to the angular error to understand and compensate these types of errors.

TIDA-010947 Shaft Coupling Mechanical DisplacementFigure 3-7 Shaft Coupling Mechanical Displacement
TIDA-010947 AMR Sensor DisplacementFigure 3-9 AMR Sensor Displacement
TIDA-010947 Angle Error Example due to Shaft Coupling DisplacementFigure 3-8 Angle Error Example due to Shaft Coupling Displacement
TIDA-010947 Angle Error Example due to AMR Sensor DisplacementFigure 3-10 Angle Error Example due to AMR Sensor Displacement

Electrical offset, gain-mismatch and non linearity (3rd harmonics) of the sensor signal chain impact the angle error, examples are shown in Figure 3-11 to Figure 3-13.

TIDA-010947 Angle Error Example due to AMR Sin/Cos Signal Chain Offset (0.1%)Figure 3-11 Angle Error Example due to AMR Sin/Cos Signal Chain Offset (0.1%)
TIDA-010947 Angle Error Example due to AMR Sin/Cos Signal Chain Non-Linearity (-0.1%)Figure 3-13 Angle Error Example due to AMR Sin/Cos Signal Chain Non-Linearity (-0.1%)
TIDA-010947 Angle Error Example due to AMR Sin/Cos Signal Gain Mismatch (0.1%)Figure 3-12 Angle Error Example due to AMR Sin/Cos Signal Gain Mismatch (0.1%)

Table 3-1 summaries the impact on the angular error pattern.

Table 3-1 Errors Sources and Impact on Angle Error Harmonics
Error sources Shaft coupling displacement AMR sensor displacement AMR sensor signal chain offset AMR sensor signal chain gain mismatch AMR sensor signal chain nonlinearity (3rd harmonic)
Angular error harmonic over 360 degree 1st 2nd 2nd 4th 8th

For more information on angle position calculation algorithms refer to Achieving Highest System Angle Sensing Accuracy, application note.