Hardware Reference
In-Depth Information
Therefore, though these methods have been used successfully in other applica-
tions, they have never been found appropriate for HDD.
Some of the sensorless methods used for detecting rotor position in BLDC
are
1. direct back-EMF detection [93],
2. back-EMF integration [12] [131],
3. integration of back-EMF third harmonic [146],
4. conducting state of the free-wheeling diodes of the inverter [153], and
5. phase current sensing [129].
These sensorless methods for detecting rotor position of the BLDC motor
are less computationally intensive and the software complexity can be greatly
reduced using suitable hardware.
According to the categorization of the PM AC motors, the spindle motors
used in HDDs belong to the PMSMs because they have sinusoidal back-EMF.
However, as the concerns like cost and reliability, BLDC drive mode is still
used in HDDs to drive the spindle motors. This is not an optimal way to
drive the spindle motor from the points of EM torque generation and motor
efficiency (it will be discussed in section 4.4.4), but it can be implemented with
less cost and less dependence on high-power DSPs and yet with satisfactory
performance.
From above introduction, two states are included in the BLDC drive mode:
energized state and silent state as shown in Figure 4.76. The drive current
goes through the winding in the energized state to produce EM torque. In the
silent state, there is no current in the winding, and the phase back-EMF can
thus be obtained by measuring the phase voltage in the silent state.
4.4.2.2 Different Modes of Operation for BLDC Drives
Three BLDC drive modes are widely used: constant current BLDC mode
(CC-BLDC mode), constant voltage BLDC mode (CV-BLDC mode) and pulse-
width modulation BLDC mode (PWM-BLDC mode). In the CV-BLDC mode,
the line voltage between the two terminals of the motor is kept constant in the
120
◦
energizing state. If the drive voltage needs to be adjusted, a DC regulator
is used to change the DC link voltage while keeping the two transistors in the
“full-on” status [102]. Figure 4.77 illustrates the inverter gate signals in the
CV-BLDC mode.
Similarly, in the CC-BLDC mode, the current in one phase winding is kept
constant in its 120
◦
energizing state [100]. Current references for different
loads are used to compare with the measured current and a hysteresis current
controller is used to adjust the drive current to meet the reference value.
