4.7 Servo Control. Fujitsu C141-E090-02EN

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4.7 Servo Control. Fujitsu C141-E090-02EN | Manualzz

4.6.4

Synthesizer circuit

The drive uses constant density recording to increase total capacity. This is different from the conventional method of recording data with a fixed data transfer rate at all data area. In the constant density recording method, data area is divided into zones by radius and the data transfer rate is set so that the recording density of the inner cylinder of each zone is nearly constant. The drive divides data area into 15 zones to set the data transfer rate. Table 4.2

describes the data transfer rate and recording density (BPI) of each zone.

Zone

Cylinder

Transfer rate

[MB/s]

Table 4.2

Write clock frequency and transfer rate of each zone

0

0 to

1230

1

1231 to

2560

2

2561 to

3820

3

3821 to

5610

4

5611 to

6510

5

6511 to

6940

6

6941 to

8640

7

8641 to

9530

40.69

40.69

39.67

38.08

37.21

36.78

34.90

33.89

Zone

Cylinder

Transfer rate

[MB/s]

8

9531 to

10870

9

10871 to

11530

10

11531 to

12200

11

12201 to

13300

12

13301 to

14030

13

14031 to

15160

14

15161 to

15870

32.29

31.42

30.58

29.10

28.09

26.49

24.54

The MPU transfers the data transfer rate setup data to the RDC that includes synthesizer circuit to change the data transfer rate.

4.7

Servo Control

The actuator motor and the spindle motor are submitted to servo control. The actuator motor is controlled for moving and positioning the head to the track containing the desired data. To turn the disk at a constant velocity, the actuator motor is controlled according to the servo data that is written on the data side beforehand.

4 - 12 C141-E090-01EN

4.7.1

Servo control circuit

Figure 4.4 is the block diagram of the servo control circuit. The following describes the functions of the blocks:

Head

(2)

Servo burst capture

(3)

ADC

Position Sense

MPU

(1)

DSP unit

(4)

DAC

SVC

(5)

P.

Amp.

VCM current

CSR

VCM

(6)

Spindle motor control

(7)

Driver

Spindle motor

CSR: Current Sense Resistor

VCM: Voice Coil Motor

Figure 4.4

Block diagram of servo control circuit

(1) Microprocessor unit (MPU)

The MPU includes DSP unit, etc., and the MPU starts the spindle motor, moves the heads to the reference cylinders, seeks the specified cylinder, and executes calibration according to the internal operations of the MPU.

The major internal operations are listed below.

a. Spindle motor start

Starts the spindle motor and accelerates it to normal speed when power is applied.

b. Move head to reference cylinder

Drives the VCM to position the head at the any cylinder in the data area. The logical initial cylinder is at the outermost circumference (cylinder 0).

C141-E090-01EN 4 - 13

c. Seek to specified cylinder

Drives the VCM to position the head to the specified cylinder.

d. Calibration

Senses and stores the thermal offset between heads and the mechanical forces on the actuator, and stores the calibration value.

Servo frame

(72 servo frames per revolution)

4 - 14

Figure 4.5

Physical sector servo configuration on disk surface

C141-E090-01EN

(2) Servo burst capture circuit

The four servo signals can be synchronously detected by the STROB signal, full-wave rectified integrated.

(3) A/D converter (ADC)

The A/D converter (ADC) receives the servo signals are integrated, converts them to digital, and transfers the digital signal to the DSP unit.

(4) D/A converter (DAC)

The D/A converter (DAC) converts the VCM drive current value (digital value) calculated by the DSP unit into analog values and transfers them to the power amplifier.

(5) Power amplifier

The power amplifier feeds currents, corresponding to the DAC output signal voltage to the

VCM.

(6) Spindle motor control circuit

The spindle motor control circuit controls the sensor-less spindle motor. This circuit detects number of revolution of the motor by the interrupt generated periodically, compares with the target revolution speed, then flows the current into the motor coil according to the differentiation (aberration).

(7) Driver circuit

The driver circuit is a power amplitude circuit that receives signals from the spindle motor control circuit and feeds currents to the spindle motor.

(8) VCM current sense resistor (CSR)

This resistor controls current at the power amplifier by converting the VCM current into voltage and feeding back.

C141-E090-01EN 4 - 15

4.7.2

Data-surface servo format

Figure 4.5 describes the physical layout of the servo frame. The three areas indicated by (1) to

(3) in Figure 4.6 are described below.

(1) Inner guard band

The head is in contact with the disk in this space when the spindle starts turning or stops, and the rotational speed of the spindle can be controlled on this cylinder area for head moving.

(2) Data area

This area is used as the user data area and SA area.

(3) Outer guard band

This area is located at outer position of the user data area, and the rotational speed of the spindle can be controlled on this cylinder area for head moving.

4.7.3

Servo frame format

As the servo information, the drive uses the two-phase servo generated from the gray code and

Pos A to D. This servo information is used for positioning operation of radius direction and position detection of circumstance direction.

The servo frame consists of 5 blocks; write/read recovery, servo mark, gray code, Pos A to D and PAD. Figure 4.6 shows the servo frame format.

4 - 16 C141-E090-01EN

1.64

µ s

0.24

µ s

1.16

µ s

0.18

µ s

0.84

µ s

0.92

µ s 0.84

µ s 0.84

µ s

0.84

µ s

0.48

µ s

PA

ASM

W/R Recovery Field

7.98

µ s

SSM

SCD

POS

A

POS

B

POS

C

POS

D

PAD

Servo

Frame

DATA

Servo

Frame

DATA

115.7

µ s

Figure 4.6

72 servo frames in each track

(1) Write/read recovery

This area is used to absorb the write/read transient and to stabilize the AGC.

(2) Servo mark (SMK1, SMK2)

This area generates a timing for demodulating the gray code and position-demodulating Pos A to D by detecting the servo mark.

C141-E090-01EN 4 - 17

(3) Preamble

This area is used to synchronize with the PLL, which is used to search the SSM by detecting the ASM.

(4) Gray code (including index bit)

This area is used as cylinder address. The data in this area is converted into the binary data by the gray code demodulation circuit.

(5) Pos A, Pos B, Pos C, Pos D

This area is used as position signals between tracks, and the device control at on-track so that

Pos A level equals to Pos B level.

(6) PAD

This area is used as a gap between servo and data.

4.7.4

Actuator motor control

The voice coil motor (VCM) is controlled by feeding back the servo data recorded on the data surface. The MPU fetches the position sense data on the servo frame at a constant interval of sampling time, executes calculation, and updates the VCM drive current.

The servo control of the actuator includes the operation to move the head to the reference cylinder, the seek operation to move the head to the target cylinder to read or write data, and the track-following operation to position the head onto the target track.

(1) Operation to move the head to the reference cylinder

The MPU moves the head to the reference cylinder when the power is turned. The reference cylinder is in the data area.

When power is applied the heads are moved from the inner circumference shunt zone to the normal servo data zone in the following sequence: a) Micro current is fed to the VCM to press the head against the inner circumference.

b) A current is fed to the VCM to move the head toward the outer circumference.

c) When the servo mark is detected the head is moved slowly toward the outer circumference at a constant speed.

d) If the head is stopped at the reference cylinder from there. Track following control starts.

4 - 18 C141-E090-01EN

(2) Seek operation

Upon a data read/write request from the host, the MPU confirms the necessity of access to the disk. If a read or instruction is issued, the MPU seeks the desired track.

The MPU feeds the VCM current via the D/A converter and power amplifier to move the head. The MPU calculates the difference (speed error) between the specified target position and the current position for each sampling timing during head moving. The MPU then feeds the VCM drive current by setting the calculated result into the D/A converter. The calculation is digitally executed by the firmware. When the head arrives at the target cylinder, the track is followed.

(3) Track following operation

Except during head movement to the reference cylinder and seek operation under the spindle rotates in steady speed, the MPU does track following control. To position the head at the center of a track, the DSP drives the VCM by feeding micro current. For each sampling time, the VCM drive current is determined by filtering the position difference between the target position and the position clarified by the detected position sense data. The filtering includes servo compensation. These are digitally controlled by the firmware.

4.7.5

Spindle motor control

Hall-less three-phase eight-pole motor is used for the spindle motor, and the 3-phase full/halfwave analog current control circuit is used as the spindle motor driver (called SVC hereafter).

The firmware operates on the MPU manufactured by Fujitsu. The spindle motor is controlled by sending several signals from the MPU to the SVC. There are three modes for the spindle control; start mode, acceleration mode, and stable rotation mode.

(1) Start mode

When power is supplied, the spindle motor is started in the following sequence: a) After the power is turned on, the MPU sends a signal to the SVC to charge the change pump capacitor of the SVC. The charged amount defines the current that flows in the spindle motor.

b) When the charge pump capacitor is charged enough, the MPU sets the SVC to the motor start mode. Then, a current (approx. 2.1 A) flows into the spindle motor.

c) The SVC generates a phase switching signal by itself, and changes the phase of the current flowed in the motor in the order of (V-phase to U-phase), (W-phase to U-phase), (W-phase to V-phase), (U-phase to V-phase), (U-phase to W-phase), and (V-phase to W-phase) (after that, repeating this order).

d) During phase switching, the spindle motor starts rotating in low speed, and generates a counter electromotive force. The SVC detects this counter electromotive force and reports to the MPU using a PHASE signal for speed detection.

C141-E090-01EN 4 - 19

e) The MPU is waiting for a PHASE signal. When no phase signal is sent for a specific period, the MPU resets the SVC and starts from the beginning. When a PHASE signal is sent, the SVC enters the acceleration mode.

(2) Acceleration mode

In this mode, the MPU stops to send the phase switching signal to the SVC. The SVC starts a phase switching by itself based on the counter electromotive force. Then, rotation of the spindle motor accelerates. The MPU calculates a rotational speed of the spindle motor based on the PHASE signal from the SVC, and accelerates till the rotational speed reaches 7,200 rpm. When the rotational speed reaches 7,200 rpm, the SVC enters the stable rotation mode.

(3) Stable rotation mode

The MPU calculates a time for one revolution of the spindle motor based on the PHASE signal from the SVC. The MPU takes a difference between the current time and a time for one revolution at 7,200 rpm that the MPU already recognized. Then, the MPU keeps the rotational speed to 7,200 rpm by charging or discharging the charge pump for the different time. For example, when the actual rotational speed is 7,400 rpm, the time for one revolution is 8.108

ms. And, the time for one revolution at 7,200 rpm is 8.333 ms. Therefore, the MPU discharges the charge pump for 0.225 ms

×

k (k: constant value). This makes the flowed current into the motor lower and the rotational speed down. When the actual rotational speed is later than 7,200 rpm, the MPU charges the pump the other way. This control

(charging/discharging) is performed every 1/6 revolution.

4 - 20 C141-E090-01EN

CHAPTER 5 INTERFACE

5.1

Physical Interface

5.2

Logical Interface

5.3

Host Commands

5.4

Command Protocol

5.5

Ultra DMA feature set

5.6

Timing

C141-E090-01EN 5 - 1

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