UG-739
ADA4571R-EBZ User Guide
Angular Error (Linearity)
There are two different motors that may be supplied with your
demo board: a brushless dc motor or a brushed dc motor. The
brushed dc motor will introduce extra errors into the system
due to cogging of the motor during rotation. To fully evaluate
the performance of the sensor, an externally applied magnetic
stimulus that has a smoother rotational velocity should be used
with the supplied LabVIEW GUI.
The third set of figures are labeled
Angular Error (Degrees)
(see Figure 6). This figure shows the linearity of the ArcTangent
plot discussed in the Arc Tangent2 (Magnetic Angle) section.
There is no encoder attached to the magnetic stimulus on the
board and therefore the program cannot compare the calculated
angle to the actual position of the magnet. Therefore, this plot is
calculating the linearity of both the
ADA4571
and the brushless
dc motor providing the magnetic stimulus.
Due to the nonidealities of the brushless dc motor providing
the magnetic stimulus, the reported angular error will always
be higher than the actual error of the
ADA4571
. The motor
provides a more constant rate of speed at lower rotational
velocities and therefore the reported linearity will be lower
at slower speeds. These extra errors are not indicative of the
bandwidth of the
ADA4571
as the part can handle motor
rotational speeds as high as 50,000 RPM.
Figure 9 shows the
Uncorrected Angular Error (Degrees)
plot.
When powered by the host PC USB supply this waveform
reports higher error than when powered by an external supply.
This increased error is because the internal 2.5 V reference is
subtracted in hardware from each ADC channel resulting in a
higher inherent offset of the sensor if the supply voltage is not
exactly 5 V. When powered by an external power source the
ADA4571
supply is regulated to 5 V and, therefore, the ADC
introduces no extra offset or errors to the system.
Figure 9. Uncorrected Linearity Error Plot
Figure 10 shows the
Offset Corrected Error (Degrees)
plot.
Large spikes appear in this plot due to the nonidealities of the
magnetic stimulus. When the motor commutes, there is a kick
from the excitation coils resulting in a higher reported linearity
error at these points. The sensor measures the position of the
magnetic stimulus and so these kicks during commuting will
show up as an error from an ideal linear plot. This nonideality
from the magnetic stimulus is more prevalent at higher speeds
but is not a result of the sensor. The included motor is very
smooth at lower speeds and, therefore, these spikes will not
appear in the error plot.
Figure 10. Offset Corrected Linearity Error Plot
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