FUJITSU MHT2060BH - Manual (Page 73)

4.7 Servo Control
C141-E203-01EN 4-19
(3) Stable rotation mode
The SVC calculates a time for one revolution of the spindle motor based on the
PHASE signal. The MPU takes a difference between the current time and a time
for one revolution at 4,200 rpm that the MPU already recognized. Then, the MPU
keeps the rotational speed to 4,200 rpm by charging or discharging the charge
pump for the different time. For example, when the actual rotational speed is
4,000 rpm, the time for one revolution is 15.000 ms. And the time for one
revolution at 4,200 rpm is 14.286 ms. Therefore, the MPU charges the charge
pump for 0.714 ms
×
k (k: constant value). This makes the flowed current into
the motor higher and the rotational speed up. When the actual rotational speed is
faster than 4,200 rpm, the MPU discharges the pump the other way. This control
(charging/discharging) is performed every 1 revolution.