Theory of Device Operation

4.7.5 Spindle motor control

Hall-less three-phase twelve-pole motor is used for the spindle motor, and the PWM type 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 including the serial data 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 the serial data to the SVC to charge the charge pump capacitor of the SVC.

b)When the charge pump capacitor is charged enough, the MPU sets the SVC to the motor start mode.

It moves to 3) or 5) step depending on the condition of spindle motor. It is treaded as the stop condition, it moves to the step 3). Whereas, it is treated as the free-wheeling condition, it moves to the step 5).

c)The phase of the current flowed in the motor is changed 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).

The above operations mean the generation of rotational magnetic field.

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

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 the phase switching to the SVC. The SVC starts a phase switching by itself based on the back 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 waits 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 SVC builds the PLL circuit into, and to become the rotational speed of the target, controls a stable rotation with hardware.

The firmware calculates time of one rotation from PHASE signal. PHASE signal is outputted from the SVC. And the firmware observes an abnormal rotation.

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