2.3 Design Considerations
2.3.1 Magnet Selection
The magnet selected for testing was
, an N42 Grade diametric ring magnet. The magnet is specified
with an inner diameter of 4.75 mm (0.1875 in) and an outer diameter of 12.7 mm (0.5 in). The magnet thickness
is 6.35 mm (0.25 in).
Figure 2-2. Diametric Ring Magnet
2.3.2 Magnet Shape
A ring magnet format was selected for its ability for easily mount to a rotary shaft. Solid cylindrical magnets
without a bore hole in the center, must be mounted using a secondary fixture to ensure proper alignment and
centering
2.3.3 Magnet Rotation Speed
As the speed of the motorized system increases, this impacts a fixed-phase delay error from the sensor due
to the actual propagation delay in the system. The ability of the controller to manage this delay impacts device
settings and end performance.
For instance,
offers averaging modes which can help improve the observed input referred noise.
Minimizing this noise helps achieve the highest accuracy result, but proportionally increases the overall
integration time. This increase in integration time determines the resulting phase-related error.
AVERAGING MODE
SAMPLING PATTERN
TOTAL CONVERSION TIME
1 ×
XY
75 μs
32 ×
XY
1.625 ms
1 ×
XYZZYX
175 μs
32 ×
XYZZYX
4.825 ms
This integration delay can cause speed limitations for a sensor. In the case of the 32 × sampling using the
XYZZYX conversion pattern, the total integration time is 4.825 ms. At a motor speed of about 35 RPM, the
System Overview
Absolute Angle Encoder Reference Design With Hall-Effect Sensors for
Precise Motor Position Control
5
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