Source: http://www.google.com/patents/US5982721?dq=7,346,545
Timestamp: 2016-05-26 03:55:39
Document Index: 756730390

Matched Legal Cases: ['art 212', 'art 212', 'art 218', 'art 2', 'art 226', 'art 230', 'art 230', 'art 234', 'art 238', 'art 238', 'art 200', 'art 204']

Patent US5982721 - Optical disc drive comprising switching gains for forcing phase states to ... - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inPatentsAn optical disc storage system comprises a sliding mode controller for actuating an optical read head assembly over an optical disc during focus capture, focus tracking, track seeking and centerline tracking. The sliding mode controller is a non-linear control system which operates by switching between...http://www.google.com/patents/US5982721?utm_source=gb-gplus-sharePatent US5982721 - Optical disc drive comprising switching gains for forcing phase states to follow a sliding line trajectory in a servo systemAdvanced Patent SearchPublication numberUS5982721 APublication typeGrantApplication numberUS 08/625,462Publication dateNov 9, 1999Filing dateMar 29, 1996Priority dateMar 29, 1996Fee statusPaidAlso published asEP0798704A1Publication number08625462, 625462, US 5982721 A, US 5982721A, US-A-5982721, US5982721 A, US5982721AInventorsLouis Supino, Paul M. Romano, Francis H. ReiffOriginal AssigneeCirrus Logic, Inc.Export CitationBiBTeX, EndNote, RefManPatent Citations (25), Non-Patent Citations (24), Referenced by (21), Classifications (17), Legal Events (5) External Links: USPTO, USPTO Assignment, EspacenetOptical disc drive comprising switching gains for forcing phase states to follow a sliding line trajectory in a servo system
US 5982721 AAbstract
1. An optical disc drive storage system for recording digital data, comprising:(a) at least one rotating optical disc comprising a plurality of data tracks recorded thereon; (b) an optical read head positioned over the optical disc for reading the digital data from the optical disc; (c) an actuator connected to the read head for positioning the read head over the optical disk; (d) a motor connected to the actuator comprising an input for receiving a motor control signal, the motor for controlling the motion of the actuator; (e) a phase state generator for generating at least one phase state signal; and (f) a sliding mode controller, responsive to the at least one phase state signal, for generating the motor control signal;wherein: the optical disc drive storage system comprises at least two phase states; the sliding mode controller switches between a first and a second structure; the first structure causes the two phase states to chance relative to a phase plane to follow a first phase trajectory; the second structure causes the two phase states to chance relative to the phase plane to follow a second phase trajectory; the first and second phase trajectories intersect in opposite directions in at least part of the phase plane; and by switching between the first and second structures the sliding mode controller causes the two phase states to change relative to the phase plane to substantially follow a predetermined third phase trajectory. 2. The optical disc drive storage system as recited in claim 1, wherein the phase state generator comprises a state estimator.
3. The optical disc drive storage system as recited in claim 1, wherein the sliding mode controller operates in a seek mode to move the read head from a current track to a selected track.
4. The optical disc drive storage system as recited in claim 1, wherein the sliding mode controller operates in a tracking mode to maintain the read head over a centerline of a selected track.
5. The optical disc drive storage system as recited in claim 1, wherein the sliding mode controller operates in a focus mode to maintain the read head in a focus position over the disk.
6. The optical disc drive storage system as recited in claim 1, wherein the at least one phase state signal comprises an actuator position error signal proportional to a difference between a desired actuator position and an estimated actuator position.
7. The optical disc drive storage system as recited in claim 1, wherein the at least one phase state signal comprises an actuator position error velocity signal proportional to a derivative of an actuator position error.
8. The optical disc drive storage system as recited in claim 1, wherein the at least one phase state signal comprises an actuator velocity error signal proportional to a difference between a reference actuator velocity and an estimated actuator velocity.
9. The optical disc drive storage system as recited in claim 1, wherein the at least one phase state signal comprises an actuator acceleration signal proportional to a derivative of an actuator velocity.
10. The optical disc drive storage system as recited in claim 1, wherein:(a) the first structure comprises a first multiplier for multiplying a positive gain term by the at least one phase state signal; and (b) the second structure comprises a second multiplier for multiplying a negative gain term by the at least one phase state signal. 11. The optical disc drive storage system as recited in claim 10, wherein the positive gain term and the negative gain term are selected from the set consisting of 2n where n is an integer.
12. The optical disc drive storage system as recited in claim 1, wherein:(a) the third phase trajectory comprises a first substantially parabolic segment having an originating end and a terminating end connected to an originating end of a first substantially linear segment; (b) the first substantially linear segment having a terminating end connected to an originating end of a second substantially parabolic segment; (c) the second substantially parabolic segment having a terminating end connected to an originating end of a second substantially linear segment; and (d) a part of the second substantially linear segment is near the origin of the phase plane. 13. The optical disc drive storage system as recited in claim 12, wherein:(a) the servo controller operates in a seek mode to move the read head from a current track to a selected track and in a tracking mode to keep the read head substantially aligned over a centerline of the selected track while reading the data recorded on the disc; (b) when the servo controller switches to seek mode in order to move the read head to the selected track, the sliding mode controller switches between the first and second structure repeatedly according to a first switching algorithm so that the two phase states follow the first substantially parabolic segment, thereby accelerating the read head toward the selected track; (c) when the two phase states substantially reach the originating end of the first substantially linear segment, the sliding mode controller switches between the first and second structures repeatedly according to a second switching algorithm so that the two phase states follow the first substantially linear segment, thereby moving the read head toward the selected track at a substantially constant velocity; (d) when the two phase states substantially reach the originating end of the second substantially parabolic segment, the sliding mode controller switches between the first and second structures repeatedly according to a third switching algorithm so that the two phase states follow the second substantially parabolic segment, thereby decelerating the read head toward the selected track; (e) when the two phase states substantially reach the originating end of the second substantially linear segment, the sliding mode controller switches between the first and second structures repeatedly according to a fourth switching algorithm so that the two phase states follow the second substantially linear segment, thereby decelerating the read head toward the selected track; and (f) when the two phase states are within a predetermined minimum distance from the part of the second substantially linear segment near the origin of the phase plane, the servo controller switches to the tracking mode and the sliding mode controller continues switching between the two structures repeatedly in order to keep the two phase states near the origin of the phase plane, thereby keeping the read head near the centerline of the selected track. 14. The optical disc drive storage system as recited in claim 1, further comprising switching logic, responsive to the at least one phase state signal, for controlling the switching between the first and second structures.
15. An optical disc drive storage system for recording digital data, comprising:(a) at least one rotating optical disc comprising a plurality of data tracks recorded thereon; (b) an optical read head positioned over the optical disc for reading the digital data from the optical disc; (c) an actuator connected to the read head for positioning the read head over the optical disk during a seek mode by moving the read head radially over the optical disc toward a target data track; (d) a motor connected to the actuator comprising an input for receiving a motor control signal, the motor for controlling the motion of the actuator; (e) a phase state generator for generating at least one phase state signal; and (f) a sliding mode controller, responsive to the at least one phase state signal, for generating the motor control signal, comprising:(i) a first switching gain block having a first and second gain value connected to receive a first actuator phase state signal; (ii) a second switching gain block having a third and fourth gain value connected to receive an actuator position error signal X1; and (iii) a means for attenuating the effect of the second switching gain block during at least part of the seek mode. Description
Conventional optical disc drives use a head assembly comprised of a laser diode for generating the laser beam which is focused onto the surface of the optical disc through an objective lens. FIG. 1A illustrates a typical three beam optical head assembly, the operation of which is well known by those skilled in the art. A laser diode 1 produces a light beam which passes through a diffraction grating (not shown) splitting the main beam into three separate beams 2 (a center beam and two side beams); the three beams 2 then pass through a polarization beam splitter 3 and a collimator lens (not shown). The light beams 2 are then reflected by a prism 4, through an object lens (OL) 5, and onto the surface of the optical disc (not shown). The beams 2 reflect off the optical disc, again pass through the OL 5, and then reflect off the prism 4 back toward the polarization prism 3. The polarization prism 3 deflects the center beam onto a quadrant photodetector 6, and deflects the two side beams onto two tracking photodiodes (7A, 7B). The quadrant photodetector 6 generates a focus error signal (FES) for focusing the OL 5, and it generates an RF read signal for reading the recorded data. The tracking photodiodes (7A,7B) generate a tracking error signal (TES) used to maintain the position of the OL 5 over the centerline of a selected track while reading data from the disc.
Yet another problem associated with optical disc servo systems is the optical coupling or feed through phenomena that occurs between the focus tracking and centerline tracking loops. U.S. Pat. No. 5,367,513 discloses one solution to the optical feedthrough problem, but it has disadvantages which are overcome by the present invention--mainly, cost of implementation.
An optical disc storage system comprises a sliding mode controller for actuating an optical read head assembly over an optical disc during focus capture, focus tracking, track seeking and centerline tracking. The sliding mode controller is a non-linear control system which operates by switching between positive and negative feedback in order to force certain phase states (such as the read head's position error and velocity) to follow a predetermined phase state trajectory. The positive and negative feedback gains need only be within a predetermined range, thereby allowing gain values of 2n which significantly reduces the complexity and cost of the gain multipliers.
FIG. 1G is a plot of the tracking error signal (TES) versus the mistracking position of the objective lens relative to the centerline of a selected track.
FIG. 2A and 2B are exemplary block diagrams of the disc drive control systems according to the present invention.
FIG. 10 shows an integrator for integrating sgn (σ) in order to smooth the motor control signal.
FIG. 13 shows a nominal focus reference which was injected into the simulated focus servo loop of FIG. 12 for demonstrating performance of sliding mode control of the present invention as compared to conventional linear control.
FIG. 14B shows, for the nominal reference of FIG. 13, the response of the sliding mode controller of the present invention to the focus capture transient.
FIG. 15B is a resulting histogram of the focus error signal during focus tracking when simulating the focus servo loop using the nominal reference signal of FIG. 13 and the sliding mode controller of the present invention.
FIG. 16B is a resulting histogram of the focus error signal during focus tracking when simulating the focus servo loop using the worst case reference signal of FIG. 16A and the sliding mode controller of the present invention.
FIGS. 20A and 20B show the frequency and phase response of a typical centerline servo tracking system in an optical disc drive used to simulate operation of the present invention.
FIG. 21 shows a nominal radial reference which was injected into the simulated centerline servo tracking system of FIG. 20 for demonstrating the performance of sliding mode control of the present invention as compared to conventional linear control.
FIG. 22B is a resulting histogram of the tracking error signal during centerline tracking when simulating the tracking servo loop using the nominal reference signal of FIG. 21 and the sliding mode controller of the present invention.
FIG. 2A is an overview of the optical disc drive servo control system of the present invention which implements the coarse seeking operation. A spin motor 13 spins an optical disc 14 with computer data recorded thereon over an optical read head assembly 8. The read head assembly 8, as described above with reference to FIG. 1A, comprises an objective lens (OL) 5 for focusing a laser beam onto the disc. During a coarse seek operation, the read head assembly 8 slides radially under the disc 14 along a lead screw 9 until positioned under a selected track. A DC motor (DCM) 15 rotates the lead screw 9 in order to effectuate the coarse seek operation. A track counter 16, connected to receive an RF read signal 17 from the read head 8, detects track crossings and generates an estimated read head track position 18A. Other well known methods for detecting the read head track position, such as shaft encoders or Hall sensors, may be used in place of the track counter 16. A state estimator 19 processes the estimated track position 18A and the DCM motor control signal 24 to generate a more accurate estimated position (Est. POS) 18B. The Est. POS 18B is subtracted from a reference position (Ref. POS) 20 and the OL carriage unit VCM control signal 26 (after low pass filtering 25) at adder 21 to generate a position error signal X1 22. The reference position (Ref. POS) 20 indicates the selected track from which data is to be read. A sliding mode controller 23, responsive to the position error X1 22, computes a motor control signal U 24 applied to the DCM 15, thereby sliding the read head 8 to the selected track.
Track counters 16 for use in optical disc storage systems are well known by those skilled in the art, an example of such is disclosed in U.S. Pat. No. 5,406,535 the disclosure of which is hereby incorporated by reference.
The state estimator 19 filters out errors in the track position information caused by noise in the recording channel and errors generated by the track position transducer (track counter 16). In the embodiment of the sliding mode controller shown in FIG. 6, the state estimator 19 can also replace the differentiator 102 in order to generate the position error velocity phase state X2. State estimators are well known by those skilled in the art, with one example described in U.S. Pat. No. 4,679,103, the disclosure of which is hereby incorporated by reference. In addition to state estimators, other well known techniques may be used for generating the actuator phase states without departing from the scope of the present invention.
A FES generator 29, responsive to the focus error signals 27 from the quadrant photodetector 6, generates a focus error signal 30 applied to a sliding mode controller 31 which generates a motor control signal 32 applied to the OL VCMs (10A, 10B). As described above, the RF read signal is computed as (A+B+C+D) and the focus error signal (FES) 30 is computed as (A+C)-(B+D). A TES generator 33, responsive to the tracking error signals (28A,28B), generates a centerline tracking error signal (TES) 34 which, as described above, is computed as (F-E).
For fine seeking operations, where the seek distance is less than 200 tracks, the OL carriage unit 11 can perform the entire seek without having to move the entire sled assembly 8. The track counter 16 and state estimator 19 operate the same as in a coarse seek operation, and the position error X1 35 is generated by subtracting the Est. POS 18B and the TES 34 from the reference position (Ref. POS) 20 at adder 36. A sliding mode controller 37, responsive to the position error X1 35, generates a motor control signal 26 applied to the OL VCMs (10A,10B) in order to rotate the OL carriage unit 11 in the direction of the selected track. At the end of a fine seek operation, the OL VCM control signal 26 is applied to the sled servo control system of FIG. 2A at adder 21, thereby moving the sled assembly 8 toward the selected track until the OL carriage unit 11 is again in its center position.
The switching operation is understood with reference to FIG. 3C where the predetermined third phase trajectory is shown as a linear segment 60. When a new track is selected, the initial head position error is at point A, and the control system is initially switched to select the positive gain (i.e., negative feedback). As the head begins to accelerate toward the selected track, the phase states follow the arc trajectory 64 of the negative feedback mode. When the phase states reach the beginning of the third phase trajectory 60 at the intersection point B, the sliding mode controller switches to the negative gain and the phase states begin to follow the hyperbola trajectory 66 of the positive feedback mode. When the phase states cross the third phase trajectory 60 at point C, the controller switches back to the positive gain to drive the phase states along arc 68 back toward the third phase trajectory 60. This switching action is repeated so that the phase states slide along the linear segment 60 toward the origin of the phase plane. When the phase states are within a predetermined minimum distance from the origin of the phase plane, the system switches to a tracking mode where the sliding mode controller repeatedly switches between positive and negative feedback in order to keep the phase states near the origin of the phase plane, thereby keeping the read head 8 aligned over the centerline of the selected track.
X1(t)=X1(t1)e-C (t-t1)                                (8)
The overall response of the system is made faster by increasing the slope of the sliding line (i.e., increasing C). However, an important limitation in sliding mode control is that the third phase trajectory must be constrained to a region in the phase plane where the positive and negative feedback phase trajectories intersect in opposite directions. From FIG. 3C it follows that the slope of the sliding line must be constrained to 0<C<√K. A further relationship derived from this constraint is: ##EQU7## Equation (11) is known as the existence equation and it is used to determine values for the positive and negative gains.
&#963;3=X2+C2�X1; where:
X21=a predetermined constant position error velocity.
The first linear segment σoi 72 represents an acceleration of the actuator 8, the second linear segment σ2 74 represents a constant velocity of the actuator 8, and the third segment σ3 76 represents a deceleration of the actuator 8 toward the selected track.
As described above, C2 is constrained to 0<C2<√k, but all three segments are also constrained by the maximum acceleration, constant velocity, and deceleration limits of the actuator. Once the phase trajectory is selected to be within the physical limitations of the actuator, the controller operates substantially independent of parameter variations and external load disturbances.
The optimum phase plane trajectory, and the preferred embodiment of the present invention, is illustrated in FIG. 4. This trajectory comprises a substantially parabolic acceleration segment (σ1 80, a linear constant velocity segment σ2 82, a second substantially parabolic deceleration segment (σ3 84, and a linear deceleration segment σ4 86:
C3=a predetermined slope of the linear deceleration segment. The linear deceleration segment σ4 86 is necessary because the slope of the parabolic deceleration segment σ3 84 becomes too steep near the origin to support sliding mode (i.e., the deceleration becomes too large). The linear constant velocity segment σ2 82 is not necessary if the inter-track seek distance is sufficiently short (i.e., the phase states will transition from σ1 directly to σ3 if the initial position error is less than a predetermined threshold).
Kv=Coefficient of Viscous Damping;
Ks=Coefficient of Coulomb Friction; and
The position error phase state X1 66 is observed at the output of adder 68, and the position error velocity phase state X2 is observed as the negative of the DCM/VCM velocity 92. Alternatively, the position error velocity phase state X2 can be generated by differentiating the position error signal X1 66 or generated by the state estimator 19. From FIG. 5, the phase state equations can be written as: ##EQU8## With U=�K�X1, the phase state equations are similar to equations (1) and (5), and the phase plots are similar to those shown in FIG. 3B.
FIG. 6 shows one embodiment for the sliding mode controllers of the present invention. The position error X1 22 is input into the sliding mode controller, and a differentiator 102 differentiates the position error X1 22 to generate the position error velocity signal X2 100. In an alternative embodiment not shown, the state estimator 19 generates the position error velocity X2 100. Two switching gain circuits 104 and 106 multiply the position error ˜X1 130 and error velocity ˜X2 132 control signals, respectively. Multipliers 108 and 110, responsive to the phase states ˜X1 and X2 and the current trajectory segment σi, control the switching operation of the gain circuits. The sign of the resulting multiplication determines the state of the switch so as to drive the phase states X1 and X2 toward the predetermined sliding line trajectory shown in FIG. 4. A σ processing block 112, responsive to the phase states X1 and X2, implements the trajectory segment switching logic to determine which segment σi of the phase trajectory the phase states are to follow. The operation of the σ processing block 112, the integrator 116, the reference error velocity generator 114, and multiplexors 118, 120, and 122, are discussed in detail bellow.
The gain values αi, βi, γi and ζi in switching gain blocks 104 and 106 are programmably set to appropriate values according to the current trajectory segment being followed by the phase states. Also, the gain values are programmed to predetermined values depending on whether the controller is executing a forward or reverse seek. Using the existence equation (11) and the phase trajectory equations (13), (14), (15) and (16), appropriate constraints for the gain values for each segment of the phase trajectory shown in FIG. 4 can now be computed to ensure sliding mode.
For σ=σ1 (seek accelerate), differentiating equation (13) with respect to time and multiplying by equation (13) obtains: ##EQU9## From equation (17) and factoring σ1�X2 obtains: ##EQU10## From FIGS. 5 and 6, and ignoring the ψ3 term as insignificantly small during seeking: ##EQU11## From equations (18) and (19), and ignoring term ##EQU12## as insignificantly small: ##EQU13## In order to satisfy existence equation (11) (i.e., equation (22) is negative for any X1 and X2), the gain constants must satisfy the following inequalities: ##EQU14## For σ=σ2 (seek at constant velocity), differentiating equation (14) with respect to time and multiplying by equation (14) obtains: ##EQU15## From equation (17): ##EQU16## From equation (19): ##EQU17## In order to satisfy existence equation (11) (i.e., equation (23) is negative for any X1 and X2), the gain constants must satisfy the following inequalities: ##EQU18## For σ=σ3 (seek decelerate), differentiating equation (15) with respect to time and multiplying by equation (15) obtains: ##EQU19## From equation (17) and factoring σ1�X2 obtains: ##EQU20## From equations (18) and (19), and ignoring term ##EQU21## as insignificantly small: ##EQU22## In order to satisfy existence equation (11) (i.e., equation (25) is negative for any X1 and X2), the gain constants must satisfy the following inequalities: ##EQU23##
Assuming the servo control system is initially in the tracking mode 200 of FIG. 7A, the read head 8 tracks 204 the currently selected track until a seek forward or seek reverse command is received. When a forward seek is initiated, SEEK? 206 is YES and the head reference position, Ref. POS 20, is updated to a newly selected track. The initial head position error X1 22 at the output of adder 21 of FIG. 2A is the difference between the current track 18B output from state estimator 19 and the newly selected track (i.e., Ref. POS 20). This initial position error is also shown in FIG. 4 as the beginning of trajectory segment σ=σ1 80 at X1r. Segment σ=σ1 80 is a parabolic trajectory that defines the desired acceleration of the read head 8 toward the selected track.
Referring now to FIG. 7B, at the beginning of SEEK ACCELERATE (σ=σ1) 208 the sliding mode controller 23 initializes various parameters 210. The gain constants in blocks 104 and 106 of FIG. 6 are updated to the values corresponding to the acceleration trajectory σ=σ1 80. In order to reduce switching noise during a seek operation, the position error phase state X1 22 is switched out of the sliding mode control. The σ processing block 112 selects, over line 126, the ground plane as the output of multiplexor 122. As a result, ˜X1 130 is set to zero in order to disable the switching action of multiplier 110 and to remove the contribution of ψi from the computation of the motor command U 24 at the output of adder 103. Because the position error phase state ˜X1 130 is disabled, the velocity phase state ˜X2 132 is initialized to a predetermined value to ensure the actuator begins moving in the desired direction (i.e., moving in reverse toward the selected track). To accomplish this, the σ processing block 112 selects, over line 124, X2Ref 114 as the output of multiplexor 120. The σ processing block 112 also selects, over line 126, the predetermined constant C 134 as the output of multiplexor 118 (the third input ψ3 109 to adder 103). The function of the predetermined constant C 134 and the integrator 116 are discussed in further detail bellow.
After the control parameters have been initialized for the acceleration trajectory σ=σ1 80, the sliding mode controller 23 continuously computes and outputs the motor command signal U 24 at the output of adder 103. Referring to flow chart 212, σ1 is updated according to equation (13) and σi 128 is assigned to σ1. Multiplier 108 (which can be implemented as a simple XOR of the operand sign bits) multiplies σi by X2 and switches gain block 104 to γi if the result is positive and to ζi if the result is negative. Gain block 104 multiplies ˜X2 132 (X2Ref 114) by the selected gain to generate ψ2. Adder 103 adds ψ1, ψ2, and ψ3 to generate the motor command U 24. Since ψ1 is zero during acceleration and ψ3 is insignificantly small, the motor command signal U 24 is predominately equal to ψ2.
As the read head 8 begins to accelerate in reverse toward the selected track, the track counter 16 updates the current track position. The state estimator 19 processes the track count 18A and the current motor command 24 to update the estimated position signal 18B. Adder 21 outputs the new position error X1 22, and differentiator 102 computes the new velocity phase state X2 100 as X1 (N)-X1 (N-1).
The σ processing block 112 continuously checks to determine when the velocity of the read head reaches a predetermined value. If X2≦X2Ref? 214 is NO, then the sliding mode controller loops around and computes the next motor command U 24 according to flow chart 212. If X2≦X2Ref? 214 is YES, then the σ processing block 112 selects, over line 124, X2 100 as the output of multiplexor 120 (setting ˜X2=X2 216). In other words, once the velocity of the read head 8 (X2 100) reaches a predetermined speed (X2Ref 114), the sliding mode controller 23 generates the motor command signal U 24 in flow chart 218 as a function of the velocity phase state X2 100.
The σ processing block 112 continuously checks the location of the phase states with respect to the acceleration trajectory σ1 80 to determine when to switch to the next trajectory segment. The next trajectory segment will either be the constant velocity segment σ2 82 or, if the seek distance is sufficiently short, the deceleration segment σ3 84. By comparing the σ values, the σ processing block 112 determines when to switch to the next trajectory. If σ1≦σ3? 220 is YES, then the σ processing block 112 switches to the deceleration trajectory σ3 84. Else if σ1≦σ2? 222 is YES, then the σ processing block 112 switches to the constant velocity trajectory σ2 82. If both 220 and 222 are NO, then the sliding mode controller 23 loops around and computes the next motor command U 24 according to flow chart 2.
Referring now to the constant velocity flow chart 226 shown in FIG. 7C, first the gain constants for switching gain blocks 104 and 106 are updated 228 to values corresponding to the constant velocity trajectory σ2 82 of FIG. 4. Then, in flow chart 230, the σ processing block 112 updates σ2 and σ3 according to equations (14) and (15), respectively. The output σi 128 of σ processing block 112 is assigned to σ2. Again, in response to σi and X2, multiplier 108 sets the state of switching gain block 104 in order to drive X1 and X2 toward the σ2 82 phase trajectory. The next command U 24 is generated and the read head 8 continues to move toward the selected track.
The σ processing block 112 continuously checks the location of the phase states with respect to the constant velocity trajectory σ2 82 to determine when to switch to the deceleration trajectory segment σ3 84. If σ2≦σ3? 232 is YES, then the σ processing block 112 switches to the deceleration trajectory σ3 84. Otherwise, the sliding mode controller 23 loops around and computes the next command U 24 according to flow chart 230.
Continuing now to the deceleration flow chart 234, first the gain constants for switching gain blocks 104 and 106 are updated 236 to values corresponding to the deceleration trajectory σ3 84 of FIG. 4. Then, in flow chart 238, the σ processing block 112 updates σ3 and σ4 according to equations (15) and (16), respectively. The output σi 128 of σ processing block 112 is assigned to σ3. Again, in response to σi and X2, multiplier 108 sets the state of switching gain block 104 in order to drive X1 and X2 toward the σ3 84 phase trajectory. The next command U 24 is generated to decelerate the read head 8 toward the selected track.
The σ processing block 112 continuously checks the location of the phase states with respect to the deceleration trajectory σ3 84 to determine when to switch to the tracking trajectory segment σ4 86. If 94≦σ3? 240 is YES, then the σ processing block 112 switches to the tracking trajectory σ4 86. Otherwise, the sliding mode controller 23 loops around and computes the next command U 24 according to flow chart 238.
Referring again to flow chart 200 of FIG. 7A, at the beginning of a tracking operation the gain constants for switching gain blocks 104 and 106 are updated 202 to values corresponding to the tracking trajectory σ4 86 of FIG. 4. The 6 processing block 112 selects via line 126 the output of integrator 116 as the output of multiplexor 118 (i.e., ψ3). The σ processing block 112 also switches the position error phase state X1 22 back into the sliding mode computation, by selecting via line 126, as the output of multiplexor 122, the position error phase state X1 22 as the input to multiplier 110. Again, the position error phase state X1 22 is not used during seeks in order to reduce switching noise.
Referring now to flow chart 204, the σ processing block 112 updates σ4 according to equations (16). The output σi 128 of σ processing block 112 is assigned to σ4. In response to σi, X1, and X2, multipliers 108 and 110 set the state of switching gain blocks 104 and 108, respectively, in order to drive X1 and X2 toward the σ4 86 phase trajectory. The next command U 26 is generated and applied to the OL VCMs (10A,10B) to continue tracking the centerline of the selected track.
For reverse seeks, the sliding mode controller (23,37) operates as described in the flow charts of FIGS. 7A, 7B, and 7C except that the inequalities are reversed. The σ processing block 112 can also adjust the slope of the linear phase trajectory segment as shown in FIG. 3D. An alternative embodiment of the σ processing block 112 would be to compare the position error and velocity phase states to values stored in a look up table where the stored values represent the phase plane trajectory shown in FIG. 4.
During seek accelerate and seek decelerate, a reference velocity Vref is generated as a function of the position error X1 corresponding to the velocity profiles σ1 80 and σ3 84 shown in FIG. 4. The reference velocity generator can be implemented with a lookup table or with polynomial equations. An actuator velocity error phase state Xv is generated by subtracting an estimated actuator velocity -X2 from the reference velocity Vref. An actuator acceleration phase state Xα is generated by taking the second derivative of the position error X1. Phase states Xv and Xα are multiplied by respective switching gain blocks to generate control signals ψ2 and ψ4. As discussed with reference to FIG. 6 and 7, control signal ψ1 is disabled during seeks and ψ3 is insignificantly small. Therefore, the motor control signal U is a function of ψ2 and ψ4 during seek accelerate and seek decelerate. During seek at constant velocity (σ=σ2) and tracking (σ=σ4), Vref is set to zero such that Xv=X2, and ψ4 is disabled by setting the gains δ and θ in the switching gain block to zero. In this manner, the sliding mode controller of FIG. 8 operates as described in FIG. 6 and 7 during seek at constant velocity and tracking.
&#963;1=[Xv-C1�X&#945;];
&#963;3=-[Xv-C2�X&#945;];
The sliding mode controller of the present invention provides further improvements in chatter reduction by defining a boundary layer around the sliding trajectory σ. This is illustrated in FIG. 9 which shows a linear segment for the sliding trajectory σ having a boundary layer defined as an offset �ε added to σ. The boundary layer reduces chatter by reducing the amount of switching in the system. Without the boundary layer, the switching gain blocks 104 and 106 of FIG. 6 will switch every time the phase states cross the sliding line (i.e., every time σ changes sign). The boundary layer results in hysteresis which causes the gain blocks to switch only after the phase states cross over the boundary line.
The boundary layer offset �ε added to the sliding line σ is a predetermined constant until the phase states reach a predetermined value (X1c,X2c) at which time the offset �ε is computed as the sum of the phase states X1 and X2 so that the boundary layer converges to the origin of the phase plane, as shown in FIG. 9, in order to prevent oscillations around the origin. The σ processing block 112 of FIG. 6 computes σi as follows:
if switching gain blocks (104,106) are set to select gains (γi,αi) then
else if switching gain blocks (104,106) are set to select gains (ζi,βi) then
&#963;i=&#963;-&#949;; where:
&#949;=constant for X1&gt;X1c and X2&gt;X2c ; and
&#949;=|X1|+|X2|for X1&#8806;X1c and X2&#8806;X2c.
In an alternative embodiment, rather than compute ε as the sum of X1 and X2 when X1≦X1c and X2≦X2c, the slope of the sliding line is changed (i.e., σ=X2+C1 �X1; or σ=X2+C2 �X1; depending on the current state of the switching gain blocks (104,106)).
The sliding mode controller of the present invention achieves still better chatter reduction by generating the control signal U proportional to an integral of sgn (σ). In effect, the control signal is smoothed to attenuate the high frequency components that can generate electromagnetic and/or acoustic emissions.
In an embodiment of the present invention, as shown in FIG. 10, σi from σ processing block 112 is input into an integrating block 101 which computes the following function: ##EQU28## The output 128 of the integrating block 101 controls the state of switching gain blocks (104,106) and is also input into an absolute value function 111. The control signal at the output of adder 103 is then attenuated by the absolute value of the integrated sgn(σi) through a multiplier 113 to generate the smoothed control signal U.
The σ processing block 112 of the present invention can be implemented using a lookup table rather than switching between trajectory segments. As mentioned previously, the phase states can be used as an index into a lookup table in order to implement the phase state trajectory σ. To reduce the size of the table, the phase state trajectory is redefined according to the following derivation:
X1-X1I=1/2�Acc�t2 ; where:          (27)
During seeks, the phase state trajectory σ is defined as:
&#963;=X2+(2�(X1-X1I)�Acc)1/2 =0(30)
where X2 (Ideal) in equation (29) is the ideal actuator velocity and X2 in equation (30) is the estimated actuator velocity. The ideal velocity of equation (29) can be computed using a lookup table indexed by one phase state, X1, thereby reducing the size of the lookup table and the overall cost of the sliding mode controller. FIG. 11 illustrates the lookup table implementation of the σ processing block 112.
In order to verify operability, the present invention was simulated according to a computer generated model of the optical disc drive and sliding mode controller. For the focus aspect, the optical transport mechanical transfer function was characterized with the frequency and phase response shown in FIG. 12A and 12B. The focus control loop was then simulated with a nominal focus reference shown in FIG. 13. FIG. 14B illustrates the response of the sliding mode controller of the present invention to the capture transient which is much better than for a typical linear controller as shown in FIG. 14A. Also, FIG. 15B shows a histogram of the focus error signal (FES) during focus tracking and, as compared to the conventional linear controller of FIG. 15A, sliding mode control operates within the required 5 micron deviation. Further, however, sliding mode control better compensates for parametric variations and external load disturbances. Even under a worst case focus reference condition as shown in FIG. 16A, an optical disc drive system employing the present invention remained in focus as shown in FIG. 16B without plant modifications; whereas a system employing conventional linear control could not maintain the required 5 micron deviation.
Many changes in form and detail could be made without departing from the scope of the present invention; the particular embodiments disclosed are not intended to be limiting. For instance, the sliding mode controller may be implemented in hardware or software and in continuous or discrete time, and higher order phase states could be used in place of, or in addition to, the particular phase states disclosed. These and other modifications derived from the disclosed embodiment are within the intended scope of the present invention as properly construed from the following claims.
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