Disk servo control

Abstract: A servo control method for controlling the head driver of a disk recording apparatus to move the head to a target position on the disk is disclosed. Servo information on the disk is periodically detected, corresponding to the position of the head in a current sampling period and is stored as a current position value. A moving distance value is calculated as the distance from the current position value to the target position and a current velocity value is calculated. A velocity command value is calculated by substituting the moving distance value into one of a pair of predetermined cubic expressions having predetermined factor values, the appropriate member of the group being determined from whether the moving distance value is higher or lower than a predetermined threshold. Finally, the current velocity value is subtracted form the velocity command value and the subtracted result converted into an analog signal and applied to the head driver.

Description: The present invention relates to a servo control apparatus and method for controlling the head driver of a disk recording apparatus to move the head to a target position on the disk, upon performance of a seek operation. In a general disk recording system, a magnetic disk used as a record medium is divided radially to provide a plurality of tracks. Each track is divided into a servo area which stores servo information, used to execute servo control and a data area for recording data. To read and write data at a target position, the head has to move to the target position. The movement of head is called a seek operation. After the head has reached the target track, an operation for accurately placing the head on a data line of the target position is executed, known as a track following operation. In a method for controlling the position of the head in a disk recording system, control of velocity is performed until the head reaches the target position, and control of position is performed at the target position, so that the head lies on-track. FIG. 1 is a block diagram illustrating servo control of a general disk recording system. In this configuration, a microprocessor 10 is connected to a programmable read only memory 20 (PROM) and a static random access memory 30 (SRAM). A head 40 performs a horizontal movement on a disk 50, and reads/writes data at a target position on the disk 50. A voice coil motor 60 (VCM) serves as an actuator and is connected to the head 40 to activate the head 40 in the horizontal direction on the disk 40. A VCM driver 70 is connected to the VCM 60 and controls the activation thereof in response to a predetermined input current. A DAC (digital/analog converter) 80 is connected between the microprocessor 10 and the VCM driver 70. The DAC 80 receives a digital control input signal u from the microprocessor 10 and converts the signal into an analog signal to provide an analog-converted signal to the VCM driver 70. A preamplifier 90 is connected to the head 40, amplifies a signal read by the head 40, and outputs the amplified signal. Additionally, the preamplifier 90 amplifies an input signal to be written and outputs the amplified input signal to the head 40. A reading decoding and writing encoding part 100 is connected to the microprocessor 10 and the preamplifier 90. Under the control of the microprocessor 10, the reading decoding and writing encoding part 100 receives write data from the microprocessor 10 and encodes the data to an analog flux conversion signal, thereby outputting the signal to the preamplifier 90. Also, the reading decoding and writing encoding part 100 decodes and outputs the analog detecting signal received from the preamplifier 90. An A/D converter (analog/digital converter) 110 is connected to the reading decoding and writing encoding part 100, from which the A/D converter 110 receives the analog detecting signal. The A/D converter converts the analog detecting signal to digital data, that is, a position error signal (PES), and outputs the PES to the microprocessor 10. In the construction as mentioned above, the microprocessor 10 loads an estimator algorithm and a predetermined control program and executes the overall control operation in the servo control apparatus of the disk recording system. FIG. 2 is a block diagram illustrating a prior art servo control configuration of a disk recording system. In the figure, a PROM 20 used as a velocity trajectory generator stores velocity command data obtained from the velocity command trajectory at discrete values of to the moving distance. FIG. 3 is a graph illustrating head moving velocity in an initial acceleration phase to the target position, upon the seek operation of the servo control apparatus. An explanation of the prior art servo control apparatus and method will now be described in detail with reference to FIGS. 2 and 3. Referring to FIG. 2, servo control of a head driving part in a disk recording system is performed using the following sequential operations. First, the microprocessor 10 detects an analog signal induced from the disk 50 in a plant 200 comprising the disk 50, a spindle motor (not shown), head 40, VCM driver 70, VCM 60, etc. Then, the microprocessor 10 receives the detected analog signal as PES servo information digitized the A/D converter 110, and extracts a current position x ∧ 1 and a current velocity x ∧ 2 of the head with the estimator algorithm. Then, a velocity command value V CMD produced from the current position x ∧ 1 is compared with the current velocity x ∧ 2 . A difference value obtained by the comparison is considered as an error velocity V ERR and compensated by a factor k. As a result, the microprocessor 10 finally outputs a control signal u for controlling the VCM driver 70. The velocity command value V CMD is defined as a velocity, in a velocity command trajectory which mathematically indicates how the velocity should be changed in accordance with the variation of the distance from a target position in order that the head rapidly and stably moves to the target position, considering mechanical and circuitry conditions for the purpose of controlling the disk recording system. An algorithm block producing the velocity command value V CMD is called a velocity trajectory generator. Upon performance of a seek operation, the servo control construction of the prior art servo control apparatus can be indicated by the following expressions. In these expressions, the following nomenclature has been adopted: X(ts) the position of the head at sampling time ts relative to the target position, X ˙(ts ) the velocity of the head at sampling time ts K T torque constant of the head driver, K E back EMF factor of the head driver, J moment of inertia of the head driver, and R total resistance of the head driver. The expressions (1) and (2) must be pre-calculated with a computer since it is difficult to compute them in real time with a general microprocessor 10 used in the disk recording system. Thus, the calculated values are stored as a table and in the PROM 20. The PROM 20 storing the predetermined number of velocity command data obtained from the velocity command trajectory generator at discrete values of the moving distance. However, the prior art servo control method and apparatus have a problem that since a ROM capable of storing at least 2K byte to 4K byte is necessary for storing the afore-mentioned calculated values, the cost of production of the system becomes high. Further, for performing a servo control operation by the above table, only velocity values corresponding to discrete moving distances of the head are stored, thereby deteriorating resolution of the velocity command. Therefore, an object of the present invention is to provide a servo control method of a disk recording system using a velocity value with a high resolution, upon performance of a seek operation. Another object of the present invention is to provide a servo control apparatus of a disk recording system which can have a velocity value with a high resolution, upon performance of a seek operation and be constructed in a simple manner. Accordingly, the present invention provides a servo control method for controlling the head driver of a disk recording apparatus to move the head to a target position on the disk, the method comprising the steps of: periodically detecting servo information on said disk where said head is positioned in a current sampling period and outputting said servo information as a current position value; calculating a moving distance value from said current position value to said target position and a current velocity value and outputting the calculated values; deriving a velocity command value for movement of said head in the next sampling period from said moving distance value; and subtracting said current velocity value from said velocity command value, converting the subtracted result into an analog signal, and applying the analog converted signal to said head driver; characterised in that said velocity command value is calculated by substituting said moving distance value into a pre-determined second or higher degree polynomial expression having predetermined factor values. The predetermined polynomial expression may be a predetermined cubic function of said moving distance value. The present invention also extends to a servo control apparatus for controlling the head driver of a disk recording apparatus to move the head to a target position on the disk, the apparatus comprising: servo information detecting means for periodically detecting servo information on said disk where said head is positioned in a current sampling period and outputting said servo information as a current position value; estimating means for estimating a current position value and a current velocity value from said servo information data received from said detecting means; calculating means for calculating a moving distance value from said current position value to said target position and for deriving a velocity command value for movement of said head in the next sampling period; subtracting means for receiving said velocity command value and said current velocity value and outputting a control signal corresponding to the difference between the received values; and output means for outputting said control signal to said head driver; characterised in that said calculating means derives said velocity command value by substituting said moving distance value into a predetermined second or higher degree polynomial expression having predetermined factor values. Preferably, said servo information detecting means includes first converting means for converting the detected analogue servo information to digital and for outputting the digital converted servo information as said current position data, and said output means includes second converting means for converting said digital control signal into an analog control signal to be output to said head driver. Preferably, to allow a greater degree of accuracy in determining velocity control values without increasing computational complexity, the velocity command value is calculated by substituting said moving distance value into one of a group of pre-determined second or higher degree polynomial expression having predetermined factor values, the appropriate member of the group being determined from said moving distance value. The said group may comprise two such polynomial expressions, the first being used to calculate said velocity command value when said moving distance value is higher than a predetermined threshold and the second being used to calculate said velocity command value when said moving distance value is lower than said predetermined threshold. The present invention will now be described by way of example with reference to the accompanying drawings, in which like numbers indicate the same or similar elements: FIG. 1 is a block diagram illustrating a servo control configuration of a general disk recording system; FIG. 2 is a block diagram illustrating a prior art servo control configuration for a disk recording system; FIG. 3 is a graph illustrating a head moving velocity in an initial acceleration phase to the target position, upon the seek operation of servo control apparatus; FIG. 4 is a block diagram illustrating a servo control configuration of a disk recording system according to the present invention; and FIG. 5 is a graph illustrating a velocity command state in an initial acceleration phase to a moving distance to the target position, upon the seek operation of servo control apparatus. Referring to FIG. 4, a block diagram illustrates a servo control configuration of a disk recording system according to the present invention. A microprocessor 400 includes an estimator 412 for estimating and outputting a current position x ∧ 1 and a current velocity x ∧ 2 by receiving PES servo information from an A/D converter 110. A velocity trajectory generator 418 is provided to calculate a velocity command value from a cubic expression algorithm including predetermined factor values. Different cubic expressions are used depending on the moving distance phase, i.e the value of the moving distance. The velocity trajectory generator substituting the moving distance into the appropriate cubic expression algorithm to calculate the velocity command value. A subtracter 414 outputs an error velocity V ERR by subtracting the current velocity x ∧ 2 from the velocity command value. The microprocessor 400 outputs a control signal u for controlling the VCM driver 70 by compensating the error velocity V ERR by a control constant k. These various components are included in the microprocessor 200 by a suitable programmed algorithm. FIG. 5 is a graph illustrating velocity command state in an initial acceleration phase against moving distance to the target position, upon the seek operation of the servo control apparatus. In the seek operation of the disk recording system, as shown in FIG. 3, it can be appreciated that the velocity command state of the target velocity is described in parabolic form by the servo control expressions (1) and (2). Accordingly, based on the graph illustrating the velocity command state by the prior art servo control expressions (1) and (2), if a target distance phase is established where the variable rate of factor values in the cubic expressions of the graph of FIG. 3 is in a minimum state, as shown in FIG. 5, and if the target distance is subdivided into two at the point K where the target distance phase is divided, it will be understood that phase X is a part of a parabola of a first cubic equation A1 - A2 and phase Y is a part of a parabola of a second cubic equation B1 - B2. These can be indicated by the following equations (3-1) and (3-2). A1 - A2 : velocity command value = a 1 x 3 + a 2 x 2 + a 3 x + a 4 B1 - B2 : velocity command value = b 1 x 3 + b 2 x 2 + b 3 x + b 4 The velocity command value can be directly calculated by using the equations (3-1) and (3-2) without calculation of the expressions (1) and (2). In the equations (3-1) and (3-2), factor values a 1 , a 2 , a 3 , a 4 , b 1 , b 2 , b 3 , and b 4 can be previously defined by calculation by the computer. For example, the factor values may be used with the following values. a 1 = 1.5492548*10 -7 b 1 = 1.0609415*10 -5 a 2 = -1.6584892*10 -3 b 2 = -1.9482181*10 -2 a 3 = 1.0792343*10 b 3 = 2.2278887*10 a 4 = 5.2510289*10 3 b 4 = 2.4743881*10 3 Since the afore-mentioned factor values are unchangeable constant values, if the microprocessor 400 varies only the scale of the factor values for the convenience of calculation, 8 constants are to be stored. In addition to the preferred embodiments of the present invention, the velocity command phase can in the alternative be divided by using quadratic equations or indeed any polynomial expression of order two or more. The PES servo information is received from the plant 200 through the A/D converter 110 and the microprocessor 400 extracts the current position x ∧ 1 and the current velocity x ∧ 2 of the head, and calculates the moving distance from the current position x ∧ 1 to the target position. Thereafter, the microprocessor 400 calculates a corresponding moving distance phase from the moving distance and the velocity command value V CMD by substituting the moving distance into whichever of the expressions (3-1) and (3-2) includes the factor values of the corresponding moving distance phase. Then, the difference between the velocity command value V CMD and the current velocity x ∧ 2 is considered as the error velocity V ERR , which is compensated by the control constant k. As a result, the microprocessor 400 outputs the control signal u for controlling the VCM driver 70. Thereafter, the D/A converter 80 receives the control signal u and converts the control signal u into an analog signal. The analog converted signal is applied to the VCM driver 70, thereby finally executing servo control. As previously set forth, the present invention provides a servo control method, in which only factor values for a polynomial expression having at least two or more factors are stored without having a target velocity table. The expressions are used to compute a target velocity value upon performance of a seek operation, thus allowing the construction of the servo control to be more simple and reducing the cost of production. In addition, the present invention provides a servo control method, in which the target velocity value is obtained with the functional calculation from a real moving distance calculated from a current position of the head, thereby producing a velocity value with a high resolution upon performance of the seek operation.

Claims: A servo control method for controlling the head driver of a disk recording apparatus to move the head to a target position on the disk, the method comprising the steps of: periodically detecting servo information on said disk where said head is positioned in a current sampling period and outputting said servo information as a current position value; calculating a moving distance value from said current position value to said target position and a current velocity value and outputting the calculated values; deriving a velocity command value for movement of said head in the next sampling period from said moving distance value; and subtracting said current velocity value from said velocity command value, converting the subtracted result into an analog signal, and applying the analog converted signal to said head driver; characterised in that said velocity command value is calculated by substituting said moving distance value into a pre-determined second or higher degree polynomial expression having predetermined factor values. The servo control method as defined in claim 1, in which said predetermined polynomial expression is a predetermined cubic function of said moving distance value. The servo control method as defined in claim 1 or claim 2, in which the velocity command value is calculated by substituting said moving distance value into one of a group of predetermined second or higher degree polynomial expression having predetermined factor values, the appropriate member of the group being determined from said moving distance value. The servo control method as defined in claim 3, in which the said group comprises two such polynomial expressions, the first being used to calculate said velocity command value when said moving distance value is higher than a predetermined threshold and the second being used to calculate said velocity command value when said moving distance value is lower than said predetermined threshold. A servo control apparatus for controlling the head driver of a disk recording apparatus to move the head to a target position on the disk, the apparatus comprising: servo information detecting means for periodically detecting servo information on said disk where said head is positioned in a current sampling period and outputting said servo information as a current position value; estimating means (412) for estimating a current position value and a current velocity value from said servo information data received from said detecting means; calculating means (418) for calculating a moving distance value from said current position value to said target position and for deriving a velocity command value for movement of said head in the next sampling period; subtracting means (414) for receiving said velocity command value and said current velocity value and outputting a control signal corresponding to the difference between the received values; and output means (80) for outputting said control signal to said head driver; characterised in that said calculating means (418) derives said velocity command value by substituting said moving distance value into a predetermined second or higher degree polynomial expression having predetermined factor values. A servo control apparatus as defined in claim 5, in which: said servo information detecting means includes first converting means (110) for converting the detected analogue servo information to digital and for outputting the digital converted servo information as said current position data; and said output means includes second converting means (80) for converting said digital control signal into an analog control signal to be output to said head driver. The servo control apparatus as defined in claim 5 or claim 6, in which said predetermined polynomial expression is a predetermined cubic function of said moving distance value. The servo control apparatus as defined in any one of claims 5-7, in which said calculating means (418) comprises means for substituting said moving distance value into one of a group of pre-determined second or higher degree polynomial expression having predetermined factor values, the appropriate member of the group being determined from said moving distance value, to calculate said velocity command value. The servo control apparatus as defined in claim 8, in which the said group comprises two such polynomial expressions, and said calculating means uses the first to calculate said velocity command value when said moving distance value is higher than a predetermined threshold and the second to calculate said velocity command value when said moving distance value is lower than said predetermined threshold.

Inventors: KIM GWON-IL, KIM, GWON-IL
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO., LTD.
Application year: 1995
Main classification: G11B5
IPC codes: G11B21