Spindle Motor Control - Hard Disk Drive: Mechatronics and Control - page 302

related with drive mode used. In other words, the acoustic noise of the spindle
motor caused by the UMP is independent to the drive mode.
Cogging torque cannot produce effective drive torque, but causes torque
ripples in the motor operation. It was discussed in section 4.2.4 that cogging
torque is caused by the EM structure of the spindle motor. As the order of the
cogging torque is much higher than the fundamental torque, and the position
signals obtained with sensorless method are limited, it is difficult to use drive
current to create an opposite torque to compensate for the cogging torque
in the sensorless BLDC driving. Therefore, the cogging torque can only be
minimized by proper design of the motor. This is an important requirement
for the design of spindle motors for application in HDD.
As the drive mode can affect the EM torque generated in the motor opera-
tion, it can certainly affect the acoustic noise generated in the motor operation.
4.4.5.1 Effects of EM Torque Ripple on Acoustic Noise
It was explained in section 4.4.4 that the operation torque in BLDC mode
consists of intrinsic torque ripples which is caused by the BLDC mode itself.
In using drive circuit to drive the motor, current commutations induce 'jumps'
in the current waveform (illustrated in Figure 4.99). As a result, the current
waveform becomes rich in harmonic components. These harmonics generates
torque ripples and contribute to acoustic noise in the motor operation. This
kind of torque ripples is known as extrinsic BLDC torque ripples. Now the
question is, caused in the EM sources causing the acoustic noise, what are the
roles of the intrinsic and extrinsic torque ripples?
A special CV-BLDC drive mode, presented in [130], can eliminate the in-
trinsic torque ripple. However, the measurements of acoustic noise in spindle
motors show that, eliminating the intrinsic torque ripple has very little effect
on the level of acoustic noise, as shown in Figure 4.100 and Figure 4.101.
When the spindle motor is driven using sinusoidal current, i.e., the PMSM
drive mode, both the intrinsic and the extrinsic torque ripples are eliminated.
Acoustic measurements on spindle motors using this drive mode show signi fi -
cant reduction in noise [131].
Figure 4.102 and Figure 4.103 show the experimental results from the test
on acoustic noise in a FDB spindle motor driven by normal CV-BLDC drive
and PMSM drive, respectively. It is obvious from these results that the PMSM
drivemodecanreducetheacousticnoiselevelatdifferent speeds of operation.
Since the PMSM drive mode induces neither the intrinsic torque ripple nor
the extrinsic torque ripple, and since the intrinsic torque ripple contributes
very little to the acoustic noise, it can be concluded that the extrinsic torque
ripple is the major EM source of acoustic noise. Therefore, in order to reduce
acoustic noise, the extrinsic torque ripples caused by suddent jumps in current
waveform must be kept low, i.e., the current jumps in commutation must be
small.