Servo architecture to minimize access time in optical disk drive

Abstract

An apparatus and method are disclosed to minimize seek time for dual stage actuator from both theoretical and real application viewpoints in optical disk drive application, where 1) a head mounted on a sled is positioned by a sled actuator; 2) A lens is mounted on the head with spring connection and optically coupled to a photo-sensor. A tracking actuator positions the lens with respect to tracks on the disk. The algorithms and designs include: a) dual stage mechanical models description from the real application consideration for track following and seek modes, respectively. The dual stage mechanical models describe the motion of lens and head driven by tracking and sled actuator in each mode. Meanwhile a simplified dual stage mechanical model with reduced parameters is given to decouple the link between lens and head; b) simplified model in track following mode and LHCE estimator design in track following mode. The LHCE is defined as error between head and lens physical centers in the dual stage mechanical moving direction. The LHCE estimator designs are based on simplified mechanical models in order to make head center following lens center movement in track following mode; c) a control architecture to position lens and sled based on LHCE estimator designs in seek modes; d) an architecture to switch design rules between tracking mode and seek mode. The method can also be applied to those cases where optical writer mechanism does not have LHCE sensor.

Description

DRAWINGS

FIG. 1 illustrates a dual stage moving system in optical disk drive application field.

FIG. 2 illustrates a simplified dual stage mechanical model based on the description in FIG. 1

FIG. 3 illustrates simplified mechanical model in track following mode based on FIG. 2

FIG. 4 illustrates control block for track following mode where lens control block is used to estimate LHCE for head control block

FIG. 5 illustrates a decouple structure in seek mode, where head and lens can be positioned individually with minimal coupling effect

FIG. 6 illustrates a mode switch structure to switch from track mode to seek mode and from seek mode to track mode also.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be discussed with reference to an optical disc drive. One skilled in the art will recognize that the present invention may also be applied to other data storage device, such as a magneto-optical disk drive

1. Mechanical Behavior Description with LHCE Definition

A dual stage moving system is presented in FIG. 1 for the application in optical storage field. A disk 107 is rotated by the spindle motor 109 through a spindle motor axis 110. Photo diode 105 receive reflected laser beam from disk surface where data can be allocated through lens 102. Lens 102 mounted on head 101 is connected through springs 104, 103. Force Fp 108 is applied to lens center 115 through tracking driver to position lens 102 and Force Fs 112 from sled driver is applied to head center 116 to position head 101. Starting point 106 is a common reference for the measurement of lens 102 and head 101 position. Starting point could be any where as long as the reference number is not changed during lens and head position measurement. x1114 is distance from lens center 115 to starting point 106. x3113 is distance from head center 116 to starting point 106. LHCE 111 is defined as lens to head center error, i.e. LHCE=x1−x3.

Force Fp 108 applied to lens center 115 moves lens 102 to a position measured by track on a disk 107, another force Fs 112 applied to head moves head center 116 to a position in the dual stage mechanical movement. LHCE 111 will vary as lens and head are moving together. The variation will result spring force to react on lens and head, respectively. In order to achieve the smooth landing, the lens to head center error 111 should be kept minimal in all movement processes to avoid large bias force during lens and head settle. If head center 116 can always be aligned with lens center 115 during whole moving process, the long seek can be finished in one time with reliable settle on target track because the bias force to lens caused by LHCE is eliminated. The design target for the dual stage moving system is to position lens followed by head with a minimized LHCE.

2. Simplified Mechanical Model

According to descriptions above, a simplified mechanical model is presented in FIG. 2 for the dual stage mechanical system in FIG. 1. Since head mass is much larger than lens mass, the reaction force applied to head is neglected to simplify dynamical response analysis of the dual stage mechanical system. The consideration results in the decoupled analysis for lens and head system, respectively. It is noticed that head system controlled by SDO 201 generates a reaction force proportional to difference between x1114 and x3113, but the reaction force applied to head system is neglected. TDO 200 and SDO 201 are the control voltages applied to tracking actuator and sled actuator to generate control forces Fp 108 and Fs 112, respectively. The symbols are defined as follows (x1 and x3 refer to FIG. 1114 and 113)

• • x1: distance from lens center 115 to starting point 106.
  • x2: lens moving velocity
  • x3: distance from head center 116 to the starting point 106.
  • x4: head moving velocity
  • R1203 and R2210 are motor driver parameters.
  • Kb 202 and Kb2216 are Back Electromagnetic Field (BEMF) to tracking actuator and sled actuator.
  • Ks 208: Spring coefficient. The springs 103, 104 are used to connect head and lens.
  • m 205 and J 212: lens 102 and head 101 mechanical masses.
  • Kf 211: mechanical coefficient of tracking actuator.
  • Kg and Kt 211: mechanical coefficients of sled actuator 102.

The major parameters are considered in FIG. 2. Some parameters with small contribution to the dual stage movement are neglected, such as the small reaction force applied to head 101 due to lens center 115 moving away from head center 116 is not considered in this model. Also the friction forces for lens and head movement are not counted. Based on the simplified mechanical model presented in FIG. 2, following formulas are obtained.

For Lens Model

$\begin{array}{cc}x1^{\prime }=x2& \left(2.1\right)\\ \begin{array}{c}x2^{\prime }=\left[\left(\mathrm{TDO}-\mathrm{Kb}*x2\right)*\mathrm{Kf}/R1-\mathrm{Ks}*\left(x1-x3\right)\right]/m\\ =-\mathrm{Kb}*\mathrm{Kf}*\left(1/R1\right)*\left(1/m\right)*x2-\mathrm{Ks}*\left(1/m\right)*\\ \left(x1-x3\right)+\mathrm{Kf}*\left(1/R1\right)*\left(1/m\right)*\mathrm{TDO}\\ =A*x2+B*\left(x1-x3\right)+C*\mathrm{TDO}\end{array}& \left(2.2\right)\end{array}$

Where x1′ is derivatives of x1, x2′ is derivatives of x2,

A=−Kb*Kf*(1/R1)*(1/m),

B=Ks*(1/m),

C=Kf*(1/R1)*(1/m)

For Head Model

$\begin{array}{cc}x3^{\prime }=x4& \left(2.3\right)\\ \begin{array}{c}x4^{\prime }=\left(\mathrm{SDO}-\mathrm{Kb}2*x4\right)*\mathrm{Kt}*\mathrm{Kg}*\left(1/R2\right)*\left(1/J\right)\\ =-\mathrm{Kb}2*\mathrm{Kt}*\mathrm{Kg}*\left(1/R2\right)*\left(1/J\right)*x4+\mathrm{Kt}*\mathrm{Kg}*\\ \left(1/R2\right)*\left(1/J\right)*\mathrm{SDO}\end{array}& \left(2.4\right)\end{array}$

Where, x3′ is derivatives of x3, x4′ is derivatives of x4,

D=Kb2*Kt*Kg*(1/R2)*(1/J),

E=Kt*Kg*(1/R2)*(1/J)

With Eq. (2.1), Eq. (2.2), Eq. (2.3) and Eq. (2.4), the following state equations are obtained:

$\begin{array}{cc}\begin{array}{c}{X}^{\prime }\left(t\right)=\left[\begin{array}{c}x1^{\prime }\\ x2^{\prime }\\ x3^{\prime }\\ x4^{\prime }\end{array}\right]\\ =\left[\begin{array}{cccc}0& 1& 0& 0\\ B& A& -B& 0\\ 0& 0& 0& 1\\ 0& 0& 0& D\end{array}\right]\left[\begin{array}{c}{x}_1\\ x_2\\ x_3\\ x_4\end{array}\right]+\left[\begin{array}{cc}0& 0\\ C& 0\\ 0& 0\\ 0& E\end{array}\right]\left[\begin{array}{c}\mathrm{TDO}\\ \mathrm{SDO}\end{array}\right]\\ =\Phi X\left(t\right)+\Gamma U\left(t\right)\end{array}& \left(2.5\right)\\ \begin{array}{c}y\left(t\right)=\left[\begin{array}{cccc}1& 0& 0& 0\end{array}\right]\left[\begin{array}{c}{x}_1\\ x_2\\ x_3\\ x_4\end{array}\right]\\ =\lambda X\left(t\right)\end{array}& \left(2.6\right)\end{array}$

All mechanical dynamical response can be derived from the 4^th order state equation Eq. (2.5) and Eq. (2.6), which will be used to do control design for the dual stage mechanical system.

3. Control Architectures and Estimator in Track Following Mode

In the section, a further simplification on model is presented in FIG. 3 for track following mode. Dynamical responses of head model 305 and lens model 302 are presented in a simpler form by neglecting the contribution BEMF Kb 202 due to the slow movement in track following mode. The couple effect between lens 302 and head 305 is introduced through summing function 303 and spring coefficient 304. Control architecture in FIG. 4 is presented for head and lens movement. Target position 405 generated from track center is an input signal to lens position system. The target center 405 is increased linearly in radial displacement as disk 107 spirals by spindle motor rotation. The input 405 is compared with lens current position x1114 and tracking error signal (TE) 404 is derived. The tracking actuator compensator 400 compensates the lens and tracking driver to be a stable system. And its output TDO 200 positions the lens location. All the function blocks related to lens movement are defined as lens control block 406 with 2 inputs and one output. Input counts on Target position 405 and head current location x3113, output is TDO 200 Head control block 407 has 2 inputs and one output also. One input is from lens control block output TDO 200 and another one is target LHCE 408 that is proportional to desired difference between lens center 115 and head center 116 for the dual stage system movement. The target LHCE 408 is set to zero in normal practice.

Since lens movement is really slow and constant, Velocity x2 and acceleration x2′ of lens movement are closed to zero. From the reasonable assumption

x2=x2′=0   (3.1)

and Eq. (2.2), the following relationship can be derived

B*(x1−x3)+C*TDO=0

B*LHCE=−C*TDO   (3.2)

The mathematical analysis can be explained as that the feedback voltage through Kb 202 is not significant since the velocity and acceleration are very small during spiral mode (track following). In order to spiral smoothly (acceleration=0), the force F1218 should be very close F2 (spring force 202) with proper gain setup to meet the assumption Eq. (3.1). While F2 spring force 202 is proportional to LHCE 111, the control voltage (TDO 200) for lens system in track following mode should be proportional to LHCE also. The statement has been proved in Eq. (3.2). In another word, a stabilized lens closed loop system can be viewed as LHCE estimation system with the TDO as the estimation system output during track following mode. The TDO (estimated LHCE) is used as feedback signal for head closed loop control. Therefore, the two inputs and one output system (dual stage system) can be separated as two individual control systems, lens control block 406 and head control block 407. Lens control block is used as LHCE 111 estimation (TDO) and head control block is used to minimize the difference between target LHCE 408 and estimated LHCE. In another word, head is controlled to follow the lens center with target LHCE while lens center follows the track center. A sled actuator compensator 401 can stabilize the head control block and output SDO. If LHCE is zero, i.e. the same centers for lens and head during track following movement, TDO should be closed to zero.

4. Lens and Head with Tracking and Sled Actuator Control Architecture in Seek Mode

FIG. 5 shows the control block diagram for the lens 102 and head 101 movements in seek mode. The block diagram contains following blocks:

Simplified dual stage moving system model block 505.This block for dual stage mechanical system is different from model in track following by considering the BEMF contribution Kb 202 due to fast moving speed.

Lens position signal generation block 506. Lens position signal counted in track crossing are generated in this block. Lens center location on disk x1114 is modulated to track crossing (TZC) and mirror signals. A counter with quarter track resolution is developed to count lens center position on disk. The counter can also figure out the positive track and minas track depending on lens center movement direction. The counter output is defined as Lens position (LP) and inputs to the lens distance to go calculation block 501. The detailed description to generate current position signal is given in another invention.

Lens distance to go calculation block 501. Target lens position TLP is compared with current LP from block 506 to generate track position to go (TPTG) signal. TPTG is input to block 502.

Lens velocity control block 502. Target lens velocity profile generator 507 can generate target velocity in the function of TPTG. There is many way to do the profile design, such as table search or formula form or other ways. The most important thing for the velocity profile design 507 is to consider the implementation availability in real application environment. Too complex design will be unpractical in real implementation, but too simple design will also result a bad resolution to lead the lens moving speed to target track. Lens velocity detector 508 is used to calculate the lens moving speed in quarter track resolution. There are still many way to estimate or calculate lens velocity feedback. Lens velocity error comes from the difference between target lens velocity and estimate lens velocity and is used as input to gain with saturation block 509. The gain is saturated on both bottom and top to insure the control effort TDO 200 within limit. The saturated gain outputs to block 503. The detailed descriptions for profile design and lens center velocity detection are given in another invention

LHCE estimation block 503. In order to know the difference between lens center and head center, LHCE 111, the center error estimation 510 is implemented. The estimator 510 has 3 inputs consisting of TDO, SDO and LP, and one output estimated LHCE. The estimator can be designed in many different ways, such as open loop estimator or closed loop estimator or reduced order closed loop estimator or other form. Input and output signal number can be vary differently depending on the implementation way. The estimators are useful for those cases where no sensor is available to measure LHCE 111 with a proper resolution. The detailed description for LHCE estimator is given in another invention Head motion control block 504. The output signal estimated LHCE from estimator 510 is compared with target LHCE where is normally set to zero. The difference after the comparison is amplified with saturation gain. TDO 200 amplified by Kfd 511 works with LHCE error (LHCEE) together to drive SDO 201. By set different sled gain 512 and Kfd 511, the head centers 116 can follows the lens center 115 movement with minimal LHCEE or LHCE if target LHCE is set to zero.

5. Switching Structure between Track Following and Seek Mode

There are 2 modes for the dual stage mechanical movement as stated above. The switch structure between the modes is presented in FIG. 6. Switcher 605 is used to operate mode switching function according to different criteria. The criteria could be preset before a seek start and adjust dynamically depending on application. The switcher can set 2 modes, seek mode and track following mode and be used in many place in FIG. 6. If the switcher is connected to track following mode, the seek mode will not operate and all functions are the same as those described in FIG. 4. If the switcher 605 is set to seek mode, the track following mode will not operate and all function in the case will be the same as those described in FIG. 5.

In this way, the invention gives the dual stage mechanical control structure. Since the LHCE estimation is introduced, the control scheme is systemized based on the simplified mechanical model in different modes. This results a uniform control rule for any seek length and makes one time seek be practical. Therefore, the access time for dual stage mechanism movement is reduced.

The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the invention. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed; obviously many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims and their equivalents, rather than by the example given.

Claims

1. Simplified dual stage mechanical structure drawing (see FIG. 1) to position optical lens center for optical disk drive. Lens is mounted on head with spring connection and positioned by tracking actuator (normally voice coil motor but not limited). Head is mounted on sled and positioned by a sled actuator (normally DC motor or step motor but not limited). Force Fp is applied to lens mass or physical center through tracking actuator in dual stage mechanical moving direction. Force Fs is applied to head mass or physical center through sled motor in dual stage mechanical moving direction.

2. Definition of lens position measurement (see FIG. 1) including x1=distance from lens center to starting pointx3=distance from sled center to starting pointLHCE=distance from lens center to head center errorFree starting point and said lens and head center could be mass center or physical center but not limited to other centers those forces are applied to.Force applied to lens moves lens to a position measured by track on a disk, another force applied to head moves head to a position in the dual stage mechanical movement stated in claim 1.a. LHCE is minimized to zero or a target value through controlling said two different forces in the dual stage mechanical movement in claim 1.a.

3. Block diagram (see FIG. 2) for simplified dual stage mechanical connection drawing including a set of parameters as follows Tracking actuator parameter includes motor coil resistant R1, Back Electromagnetic Field (BEMF) Kb1, mechanical inertial m and mechanical coefficient Kf.Sled actuator parameter includes motor coil resistant R2, Back Electromagnetic Field (BEMF) Kb2, mechanical inertial J, torque constant Kt and Gear gain Kg.Ks: Spring coefficient to apply force on lens and sled

4. Reaction force applied to head from lens during the dual stage mechanical movement is neglected.

5. Use state variables to describe dynamic movement in claim 3. 4 state variables (but not limited to 4) are defined as following x1: distance from lens center to starting pointx2: said lens moving velocity and is derivatives of x1x3: distance from head center to starting pointx4: said head moving velocity and is derivatives of x3

6. Voltage or current driver circuits to generate voltage or current, Tracking driver (voltage or current driver) output (TDO) is applied to tracking actuatorSled driver (voltage or current driver) output SDO is applied to sled actuator

7. Observe variable y resulted from state variables combination in claim 5.

8. Using multiple dimensional state equations and observing equation structure to describe the dual stage mechanical movement claim 3 with state variables in claim 5, control variables in claim 6 and observe variable in claim 7.

9. Derived matrix coefficients for claim 8 based on claim 4A=−Kb*Kf*(1/R1)*(1/m),B=Ks*(1/m),C=Kf*(1/R1)*(1/m),D=Kb2*Kt*Kg*(1/R2)*(1/J),E=Kt*Kg*(1/R2)*(1/J)

10. Simplified dual stage mechanical structure in track following mode (see FIG. 3) base on claim 8

11. Decouple the dual stage mechanical system in track following mode, where moving velocity x2 and acceleration x2′ for lens are very small and can be neglected. LHCE is proportional to x1-x3 stated based on claim 8. The proportional relationship is described as followings but not limit to that B*(x1−x3)+C*TDO=0,Which results LHCE is proportional to x1−x3=−TDO*C/B

12. Decouple closed loop control structure presented in FIG. 4. Lens control block structure design. The lens distance with respect to track displacement is coupled to the photo sensor through lens. A feedback signal defined as x1 is obtained from photo sensor mounted on head. A target position signal is changed gradually as lens moves to disk out diameter (OD), spirally. The feedback signal x1 is compared with the target position and a tracking error (TE) signal is generated. The tracking actuator compensator applied by its input signal TE generates TDO to control tracing actuator for lens movement.Head control block structure design. TDO signal is compared with target LHCE to generate the error signal (LHCEE). The said error signal is applied to sled actuator compensator to control the head movement (x3). The position difference between head center and lens center (x1-x3) generate estimated LHCE. The estimated LHCE is proportional to a spring force applied to lens for lens movement. The spring force applied to head from LHCE is neglect reasonably.Lens control block design is used as an estimator of LHCE in the stable decouples closed loop control structure. The estimator's inputs are head center position and target position. Its output is tracking actuator compensator output TDO.Estimated LHCE is proportional to tracking actuator compensator output TDO and used as feedback signal for head moving system.The closed loop dual stage system is stable if and only if TE and LHCEE signal are a constant or zero in all time

13. Decouple closed loop control structure in seek mode presented in FIG. 5.

14. Lens distance to go calculation block in seek mode. Target track number as input to this block compares with lens position with respect to tracks on disk input from lens position signal generator block. A scaled calculation is implemented and the scaled position to go in seeks mode (TES) outputs to the lens velocity control block.

15. Lens velocity control block. Lens velocity is detected and lens target velocity profile is generated based on TES. Lens velocity error (LVE) is obtained by comparing estimated lens velocity and target lens velocity from lens velocity profile generator. TDO is generated with gain limit control by saturating very large number on the amplified LVE. TDO outputs to head motion control block to control sled actuator with estimated LHCE. Also, TDO outputs to tracking actuator in the simplified mechanical block to control lens moving velocity.

16. Head motion control block includes 4 inputs: TDO, SDO, lens position and target LHCE; one output SDO. Two functions are achieved in the block. One function is to estimate LHCE and another function is to control sled actuator, described as follows LHCE estimator design used to estimate LHCE can be achieved in open loop and closed loop forms with different estimator order. TDO, SDO and lens position signals works together with estimator design to generate estimated LHCEThe estimated LHCE is compared with target LHCE as one part of control effort on sled motion. TDO signal amplified by Kfd is used as another control effort on sled movement. The 2 effort summation is applied to sled actuator for head positionLens is controlled by tracking actuator according to target profile which is a function of lens position. Head follows the lens movement by sled actuator control. The basic control rule during the dual stage movement is to minimize the error LHCEE between target LHCE and estimated LHCE.

17. Lens position generator block structure. Track cross on disk is optically coupled to photo sensor through lens. The track cross generate track crossing (TZC) and mirror signals. TZC and mirror signals are plus and minus 90 degree phase shift depend on lens moving direction. A track crossing signal with quartered track resolution is generated. The quartered track signal is applied to lens distance to go calculation block.

18. One time seek for lens moving any distance. A seek is defined as moving lens from current position to target position, where position is location referred to a reference starting point. Traditional, the lens movement control is classified to 2 steps: long seek (search) and fine seek (search) in optical storage field depending on seek (search) length. Long seek (search) and fine seek (search) employ different control methods, respectively. The control structure in claim 4 is available for any seek (search) length.

19. Mode switching structure in FIG. 6Switchers between track following mode and seek modeOne seek (search) process includes mode switches from track following mode to seek mode and seek mode back to track following mode. The switchers are controlled during the said seek (search) process.Set switcher to track following mode is the basic structure for decouple closed loop control in track following mode in claim 3Set switcher to seek mode is the basic structure for decouple closed loop control in seek mode in claim 4