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AD7866 Scheda tecnica(PDF) 8 Page - Analog Devices |
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AD7866 Scheda tecnica(HTML) 8 Page - Analog Devices |
8 / 13 page 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 Rev. 0 | Page 8 of 13 |
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