3 Measurement Non-Linearity

Beyond the material properties of the magnet that can influence the strength of the field, mechanical misalignments and tolerances can result with non-linearity in angle calculations.

At a high level, the end error results that can be observed fall into four main categories. These each have a direct impact on the linearity of the measurement and need to be managed appropriately to minimize final angle error. These are:

• Amplitude Mismatch
• Offset
• Phase Error
• Distortion

Amplitude Mismatch

Amplitude mismatch is the dominant factor which influences accuracy. When plotting the separate inputs against each other, it is possible to compare against the ideal unit circle.

Figure 3-1 Amplitude Mismatch Non-Linearity

Notice that at the axis crossings, the error is zero. At points in between, the error can become quite significant. This is typically managed by applying a normalization factor to either of the two input sources. Either the amplitude of the larger input can be attenuated, or the amplitude of the smaller input can require an increase in gain. This function is integrated into both TMAG5170 and TMAG5273 to assist with system calibration.

In cases where dual or redundant sensors are required, amplitude matching is a particular concern as a result of sensor placement. TMAG5170D-Q1 simplifies the sensor placement challenge by stacking each sensor vertically. This reduces mechanical placement errors that produces varying amounts of amplitude mismatch.

Offset

Input referred offset results with inputs not centered about zero. This imbalance results with the unit circle not being properly centered.

Figure 3-2 Offset Non-Linearity

Phase Error

Phase error between the two inputs used for arctangent calculations results from the inputs being out of phase by more or less than 90 degrees. Combined together, the resulting plot of these inputs appears similar to the amplitude mismatch case, but is not skewed along either axis.

Figure 3-3 Phase Error Non-Linearity

Distortion

Distortion is the result of the magnetic inputs being sensed not being purely sinusoidal. This can result from various factors such as inconsistent range to the magnet or irregularities in magnet shape. For instance, rotating a non-circular magnet or varying the distance to the sensor can produce inputs with a more complex non-linearity.

Figure 3-4 Input Distortion Non-Linearity