Method for reducing the rotation speed of an automatic ball balancing system of an optical disk reading device

A method for reducing the stable rotation speed of an automatic ball balancing system is used with an automatic ball balancing system that has a rotor that has an annular track, with at least one ball provided for rotation inside the track. First, an optimum rotation speed ws that is between the unstable critical rotation speed wc of a ball and a working rotation speed w is first determined. In the next step, the rotor is accelerated to the optimum rotation speed ws. Next, the acceleration of the rotor is reduced until the balls are stationary relative to the track, and then the rotor is accelerated to the working rotation speed w.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical disk reading device, and in particular, to a control method for reducing the stable rotation speed of an automatic ball balancing system. In particular, the present invention pertains to a method that is applied to the rotor mechanism of an optical disk reading device to reduce the amount of vibration before the rotor mechanism reaches the working rotation speed, so as to effectively improve the service life of the bearing of the rotor.

2. Description of the Prior Art

General optical disk reading devices, such as CD-ROM, DVD-ROM, CD-RW, DVD-RAM, and other optical data reproducing or recording devices, have been widely used in multimedia computer systems and have become an important component among the peripheral devices of computer systems.

The data reading or storing speed of a conventional optical disk drive is expressed as multiplication speed (i.e., 1 multiplication speed=150 kbyte/sec). The magnitude of the multiplication speed is directly related to the spindle motor that drives the optical disk inside the optical disk drive. The faster the spindle motor rotates, the higher the multiplication speed for the reading or storing operation in the optical disk. However, when the spindle motor rotates at high speeds, the centrifugal deviation force generated by unbalance of the optical disk is also increased, which will lead to vibration, noise, and other problems. Also, excessive vibration will lead to an out-of-focus optical reading head and other unstable situations. Consequently, in order to effectively suppress vibration to ensure that the data stored in the optical disk can be read correctly by the optical disk drive, optical disk drive manufacturers have developed a type of automatic ball balancing system that functions to reduce vibration. As described in greater detail below, the automatic ball balancing system has a track on which one or more balls are disposed for movement.

The theory for the balls of the above-mentioned automatic ball balancing system to reach the desired balanced positions is based on the theory of rotor dynamics.FIGS. 1A–1Cillustrate three possible conditions for the balls2in the automatic ball balancing system. First, when the stable rotation speed of the spindle motor is lower than the unstable critical rotation speed (called natural frequency of the suspending system), the unbalance amount of the balls2and the imbalance center of mass of the disk10of the system are in the same phase state (as shown inFIG. 1A). Second, when the stable rotation speed of the spindle motor is equal to the unstable critical rotation speed, there is a phase difference of 90° between the unbalance amount of the balls2and the imbalance center of mass of the disk10of the system (as shown inFIG. 1B). The numeral20inFIG. 1Billustrates the position of the balls2during this condition. Third, when the stable rotation speed of the spindle motor is higher than the unstable critical rotation speed, there is a phase difference of 180° between the unbalance amount of the balls2and the imbalance center of mass of the disk10of the system (as shown inFIG. 1C). Again, the numeral20inFIG. 1Cillustrates the position of the balls2during this condition.

Here, the unstable critical rotation speed wcis the rotation speed at which the balls2are stable. As shown inFIG. 2, when the spindle motor rotates under a uniform acceleration (that is, the initial acceleration of α1) and the speed increases above the unstable critical rotation speed wc, the center of mass of the balls2moves to the position with the lowest potential energy. When the spindle motor reaches a stable rotation speed ws, reducing the acceleration of the motor can stabilize the balls2within the shortest period of time. After the balls2are stabilized, the balls2will have no relative movement with respect to the track. At that time, the spindle motor can be accelerated again to working rotation speed w. Consequently, when the spindle motor reaches the stable rotation speed ws, the rotor mechanism of the optical disk drive can be balanced automatically.

In the application of the automatic ball balancing system to the rotor mechanism, if the spindle motor is operated under a uniform acceleration to reach the working rotation speed w directly, it is still possible to reduce the unbalance amount of the rotor. However, under this condition, the rotation speed of the motor will be very high when the balls2are balanced. This will eventually result in the rotor being accelerated slowly and the force exerted on the rotation shaft being increased, thereby reducing the service life of the rotation shaft and the bearing.

Thus, there still remains a need to reduce the vibration in the automatic ball balancing system of an optical disk drive before the rotor mechanism reaches the working rotation speed so as to effectively improve the service life of the rotation shaft and the bearing.

SUMMARY OF THE DISCLOSURE

It is an object of the present invention to provide a control method for reducing the stable rotation speed of an automatic ball balancing system.

It is another objective of the present invention to reduce the stable rotation speed of the automatic ball balancing system to reduce the vibration before the spindle motor reaches the working rotation speed so as to effectively improve the service life of the balls.

In order to accomplish the objects of the present invention, the present invention provides a control method for reducing the stable rotation speed of an automatic ball balancing system. The automatic ball balancing system has a rotor that has an annular track, with at least one ball provided for rotation inside the track. According to the method of the present invention, an optimum rotation speed wsthat is between the unstable critical rotation speed wcof a ball and a working rotation speed w is first determined. In the next step, the rotor is accelerated to the optimum rotation speed ws. Next, the acceleration of the rotor is reduced until the balls are stationary relative to the track, and then the rotor is accelerated to the working rotation speed w.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Although the principles of the present invention are described below in connection with a pull-in type of compact disk player, the present invention can be applied to all optical disk reading devices, including but not limited to CD drives, DVD drives, CD/DVD drives, DVD/RW combo drives, car audio drives, etc.

The present invention pertains to a control method for reducing the stable rotation speed wsof an automatic ball balancing system.FIG. 3illustrates an automatic ball balancing system according to the present invention, which is applied to the rotor mechanism installed in the optical disk drive to provide rotating movement. The rotor mechanism includes a spindle motor16and a rotor11driven by the spindle motor16. The rotor11is connected to the rotation shaft15that is carried by the spindle motor16. The automatic ball balancing system is shown as being installed above the spindle motor16, although it is also possible to install the automatic ball balancing system below the spindle motor16. The rotor11is shaped like a plate with at least one concentric annular track12facing upwardly. The track12is formed at the top of the rotor11, and at least one ball13is placed on the track12. The balls13can move freely along the track12. When the optical disk drive is operated, the spindle motor16drives an optical disk (not shown inFIG. 3but is above the rotor11) to rotate via the rotor11. During the rotation movement, the balls13on the track12are pressed against the outer side wall of the track12under centrifugal force. When the rotation speed of the rotor11is between the natural frequency wnand the unstable critical rotation speed wc, the system phase angle of the balls13will fluctuate in a stable interval, and the overall vibration of the system will decrease significantly. As a result, the balls13will move to the balanced positions.

FIG. 4shows the theoretical model of the automatic ball balancing system according to the present invention. The following equations of motion can be derived from this theoretical model.
M{umlaut over (X)}+C{dot over (X)}+KX=Mre({umlaut over (β)} sin β+{dot over (β)}2cos β)+mR└({umlaut over (β)}+{umlaut over (φ)})sin(β+φ)+({dot over (β)}+{dot over (φ)})2cos(β+φ)┘
M{umlaut over (X)}+C{dot over (X)}+KX=Mre({umlaut over (β)} cos ββ+{dot over (β)}2sin β)+mR└({umlaut over (β)}+{umlaut over (φ)})cos(β+φ)+({dot over (β)}+{dot over (φ)})2sin(β+φ)┘
{umlaut over (X)}sin(β+φ)−Ÿcos(β+φ)−R{umlaut over (φ)}=R{umlaut over (β)}±μ(R{dot over (φ)}2+2R{dot over (φ)}{dot over (β)}−r{dot over (β)}2)
where, μ is the movement resistance of the ball.

The aforementioned equations of motion are converted into the polar coordinate form. Also, it is assumed that the angle of the ball with respect to the unbalance amount is π when the ball reaches the stable state, that is, φ=π. When this is substituted into the aforementioned equations under the aforementioned assumption, the following can be obtained:

(μ+(Mr·e-m·R)M·r⁢sin⁢⁢ψ⁢⁢cos⁢⁢ψ)⁢(β.ωn)2=(Rr+(m·R-Mr·e)M·r⁢sin2⁢ψ)⁢(β¨ωn2)+2·χ.·ξr·ωn⁢sin⁢⁢ψ+χr⁢sin⁢⁢ψEquation⁢⁢(1)
where, 0≦ψ≦π, and {umlaut over (β)} is the rotating acceleration and {dot over (β)} is the rotation speed,

It is also possible to derive the condition for movement of the balls13at a constant rotation speed with respect to the track12.

χ¨β.2>μ·rsin⁢⁢ϕEquation⁢⁢(2)
If it is given that

G⁡(β.)=M·χ¨Mr·e·β.2
is the frequency response of the system, Equation (2) can be converted into the following:

G⁡(β.)M·Mr·e>μ·rsin⁢⁢ϕ
Therefore, {dot over (β)} can be changed so that

When Equation (3) holds, the balls are stable with respect to the track12. The minimum rotation speed that satisfies Equation (3) is called “the unstable critical rotation speed” wcof the ball. If {umlaut over (β)} is 0, then {dot over (β)} can be decreased by looking at Equation (1). Also, as can be seen from Equations (1) and (3), as long as Equation (3) is valid, it is possible to decrease the stable rotation speed of the balls13by reducing {dot over (β)}.

Based on the above, the objective of the present invention is to cause the balls13of an automatic ball balancing system to be stationary with respect to the track12even when the rotor mechanism is accelerated. In other words, the objective is to balance the balls13inside the track12so that there is no more vibration.FIG. 5is a diagram illustrating the acceleration curve of the rotor mechanism used in the present invention, and should be reviewed in conjunction withFIG. 6, which is a flow chart illustrating the control method of the present invention for reducing the stable rotation speed of the automatic ball balancing system.

The control method of the present invention has the following steps. In step100, the method determines a rotation speed wsthat is between the unstable critical rotation speed wcof a ball13and a working rotation speed w (that is, wc<ws<w). The rotation speed wsis the desired rotation speed that the present invention would like to obtain. The unstable critical rotation speed wcof the ball is determined by Equation (3) and is larger than the critical rotation speed wn(also called the natural frequency of the suspending system) defined by the rotor dynamics. The value of the critical rotation speed wnis derived from experiments and depends on the characteristics of the system. The working rotation speed w is the maximum rotation speed of the spindle motor16.

In step200, the rotor mechanism (spindle motor16) is accelerated to the rotation speed wsafter passing the unstable critical rotation speed wcof the ball. SeeFIG. 5. The acceleration used in this step is the initial acceleration α1, which is higher than the conventional initial acceleration α1′ shown inFIG. 2. At this rotation speed wsthe phase angle difference will be 180 degrees, but the balls13rotate at a constant speed with respect to the rotor11.

In step300, the acceleration of the rotor mechanism is reduced to allow the balls13to reach a stable position (i.e., B″, the acceleration, is reduced to 0). When the acceleration is reduced to zero in this step, the rotation speed wscan be used as the stable rotation speed at which the balls13reach the stable position. SeeFIG. 5. In this stable position, the balls13will not experience movement relative to the track12(i.e., the balls13are balanced). This stable rotation speed wsis also smaller than the conventional stable rotation speed ws′ shown inFIG. 2. If the acceleration is a very small acceleration α2(that is, the acceleration is almost zero),FIG. 5shows that it is also possible to obtain a stable rotation speed close to the rotation speed ws, and this stable rotation speed wsis still smaller than the conventional stable rotation speed ws′ shown inFIG. 2.

Since there is no longer any imbalance (i.e., the balls13do not experience movement relative to the track12), in the next step, step400, the rotor mechanism is accelerated again to the working rotation speed w. SeeFIG. 5. During this acceleration, the balls13will not experience movement relative to the track12. The acceleration α3used in this step is smaller than the initial acceleration α1of the spindle motor.

FIGS. 7 and 8are obtained from simulation experiments of the acceleration curves A and B shown inFIGS. 2 and 5, respectively. As can be seen fromFIG. 7, with the conventional acceleration curve A, even at 9,500 rpm, the ball13has not reached the balanced position. On the other hand, as shown inFIG. 8, with the acceleration curve of the present invention, the ball13reaches the balanced position by 6,600 rpm.

As described above, the present invention provides a new control method that can be applied to single-track or multi-track automatic balancing systems. The present invention provides an effective, stable, and reliable method that can be used to overcome the problems experienced by a conventional automatic ball balancing system. In other words, the method of the present invention can reduce the stable rotation speed at which the balls are balanced so as to correctly read data at high speed and to carry out a smooth operation. More specifically, the control method provided by the present invention for reducing the stable rotation speed of an automatic ball balancing system can effectively reduce the stable rotation speed of the automatic ball balancing system to reduce vibration before the spindle motor reaches the working rotation speed, thereby realizing the objective of effectively improving the service life of the bearing.