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

4.3.2 Static Angle Noise Measurement

TIDA-010947 Static Angle at 224.8 deg Over 2000 Samples at 32kHz Sample RateFigure 4-10 Static Angle at 224.8 deg Over 2000 Samples at 32kHz Sample Rate
TIDA-010947 Histogram of Angle at 224.8 deg, 2000 Samples 1LSB at 17-bit bin WidthFigure 4-11 Histogram of Angle at 224.8 deg, 2000 Samples 1LSB at 17-bit bin Width

Keep the motor shaft at fixed 224.8° mechanical angle. MSPM0 calculates angle at 32kHz. Get 2000 angle samples, plot the time-domain figure and histogram in Figure 4-10 and Figure 4-11.

The corresponding standard deviation and ENOB versus full-scale position measurement range are shown in Table 4-5. The RMS of angle noise is 0.0033°, which means 94.7dB SNR versus 360° full scale range and 15.4 ENOB.

Table 4-5 Standard Deviation, SNR and ENOB at Static Angle 224.8° for TMAG6180
ParameterValueComment
Standard deviation [°]0.0033RMS
Full-scale range [°]0-360
SNR [dB]94.7SNR=20×log10(±180 deg/STDEV)
ENOB [bit]15.4ENOB=(SNR-1.76)/6.02

Assuming uncorrelated noise, the theoretical total resolution of the absolute mechanical angle can increase by 2-bit versus the effective number of bits (ENOB) of the AMR sensors sine and cosine signal chain including the 12-bit ADC with 64-times oversampling. The theoretical resolution of the sin/cos interpolated angle over one electrical AMR sensor period equals the sine and cosine subsystem’s ENOB + 1 bit. With this design there are two electrical periods per mechanical revolution, resulting in one additional bit.

Change the motor shaft angle with 22.5° interval. Get 2000 samples at each angle, the corresponding RMS and peak to peak value is listed in Table 4-6 and Table 4-7. The peak to peak static angle noise is around 0.02° and maximum value occurs at 225°.

Table 4-6 Static Angle Noise Over First Half Revolution (0 Approximately 180°)
Mechanical angle [°]022.54567.590112.5135157.5
Standard deviation[°]0.00250.00200.00280.00270.00260.00220.00280.0028
Peak to peak [°]0.01850.01600.02270.01850.01840.01730.01820.0192
Table 4-7 Static Angle Noise Over Second Half Revolution (180° Approximately 360°)
Mechanical angle [°]180202.5225247.5270292.5315337.5
Standard deviation [°]0.00290.00240.00310.00320.00260.00270.00190.0027
Peak to peak [°]0.01820.01820.02250.02450.01840.01820.01510.0206

Figure 4-12 shows the peak to peak angle noise with black color with reference to the y-axis on the left over one mechanical revolution. The angle standard deviation is shown in red color with reference to x-axis in the right.

TIDA-010947 Static Angle Noise Over One RevolutionFigure 4-12 Static Angle Noise Over One Revolution
Table 4-8 Static Angle Noise vs Airgap
Mechanical angle [°]04590135180225270315
Standard deviation at 0.8mm airgap [°]0.00310.00250.00200.00290.00270.00330.00320.0031
Standard deviation at 2.3mm airgap [°]0.00380.00240.00350.00310.00350.00340.00340.0033

Change the effective air gap to 2.3mm, measure the static angle noise at different mechanical angle, the comparison test results are shown in Table 4-8. At a larger airgap, the maximum noise RMS is 0.0038° while it’s 0.0033° under 0.8mm airgap. A larger airgap can slightly increase the noise but the impact on the SNR and ENOB can be small.

TIDA-010947 Static Angle Noise vs AirgapFigure 4-13 Static Angle Noise vs Airgap