SERVO SYSTEM OF POSITION FOR PLAY-BACK HEAD OF MAGNETIC DISKS HAS HIGH DENSITY OF TRACKS

This invention relates generally to the position servo devices and more particularly to dual control devices for use in association with the files to magnetic disk of high density tracks.

In systems random access memory using spinning discs to magnetizable surfaces, discrete areas of these surfaces are magnetized along concentric tracks in accordance with signals representing the information or data to be stored. The data signals are transmitted with the disks or taken from them by transducers which move radially, on command, to select particular tracks. The transducers are generally mounted on a moving element whose movement is controlled by a control device or actuator.

By increasing the number of tracks per unit length of stain radius to increase the memory capacity of information disk stores, it is necessary to improve the accuracy of the positioning of the transducers relative to data tracks. A which is join to achieve such accuracy enhancement is to use each track for storing both and of servo information transducer.

A method of incorporating the servo information of position of the transducer along a data track is allocating sectors of the disc to such information, the servo information located thus picked up at a frequency dependent upon the number of sectors and the rotational speed of the disc. Therefore, the transducer bed alternately control information the position of the transducer relative to the track, and the stored data. In one such method of track following, an error signal to periodic sampling whose magnitude and polarity represent the degree and direction of the misalignment or tracking error is preserved by a circuit

•stores and is processed by the servo position of the transducer to reduce the position error of the transducer. It is known that in certain systems échantiHonneurs -and-hold circuit s, single can be maintained the last value of the error, in which case the system is said zero order. For periodic sampling higher order error, the error indicated depends on the value of two or more previous error signals. The mounting storing zero order has been found to provide the best stability ' for the control device presented herein, and it is the easier to implement; however, is lost of the surface which, otherwise, could be used for data, and there is inherently a certain response delay due to the fact that the error signals are received at discrete intervals.

Various methods have been proposed and used to enhance the stability and the response characteristics of the systems above track following. One of the methods is to fill the voids of the error signal sampled by a position signal for stain restore the lost information between sampling time. However, that method by "filling" or interpolation cannot be optimum in the sense of a complete rendering of the error signal is given the inaccessibility of the actual position signal between samplings. Another method consists in imposing to the error to be zero at a particular frequency of interest by providing compensators adapted to assume into account the phase delay and the gain difference between the input and output of the continuous system. Such a solution, which can be called "technique-compensated matched filter", also has disadvantages in that the open loop system tends to be unstable.

The second fundamental method for embedding data and servo positioned along the same track by superimposing such signals recorded, for example, on a coercive field discs and double-layer as described in the United States Patent no. 3,614 756. The data signals and *; position are thus read both continuously by the transducer. Although the response and the stability of position servo-control continuously working surpass those periodic sampling systems and that the entire surface of the disk is allocated to the data, the discs double layer are expensive and the composite output signal of the transducer is to be processed by detection circuitry suitable to separate the data and information servo position.

In some systems storage existing disks, positioning the transducer is rendered more precise and faster by first applying a positioning signal approximate to move a carriage or moving equipment carrying the transducer to bring it within a range of predetermined tolerance corresponding to a radial position in which the transducer is located in active relation relative to the track addressed. After operation in the track access mode, an operation fine positioning commences and continues to control the position of the transducer in a track following mode. The control method reproduced split obtain a considerable reduction in the overall track width due to the accuracy of the track following mode, but the access time, the acquisition time track and the bandwidth are limited.

A further improvement resides in the use of a device wherein a dual actuator carriage-or main moving element, driven by a main actuator, supports in mounting telescopic stepped a smaller carriage moving element or secondary low mass movable relative to the main carriage by a secondary actuator, the transducer is coupled to the secondary carriage, and it is provided that as many secondary carriages of transducers in the file. 1' main actuator is capable of providing a large force and movement so as to be able to quickly move the moving assembly to dual trolley to access the entire range of tracks. Each secondary carriage, whose movement is normally limited to a width of individual track, fluctuation occurs to follow the radius of the track. The actuator assembly and secondary carriage, whose mass is very small in comparison with that of the main carriage, is capable of providing a fast response. The response characteristic larger bandwidth of the secondary carriage assembly to the components at the higher frequencies the fluctuation radial and some vibration components to be tracked with very high accuracy.

During operation of the split control device, after a track access mode in which the two carriages are moved as a whole under the control of a signal for approximate positioning, the main carriage is locked in position, its actuator is rendered insensitive to position error signals and the secondary carriage is then used to make positioning corrections end. Generally, using two feedback control loops, in other words a loop coarse-tuning and a loop of fine adjustment, which act independently and in succession.

The present invention substantially improves the stability and the frequency response of the device to dual trolley described above, to provide a significant reduction of the positioning tolerance transducer and thereby enable much more track densities. For example, whereas files disk currently available have a radial density of the order of 120 tracks per centimeter, the present have the density values that exceed 400 tracks per centimeter.

Although the development of the invention has been performed in the context of a system for servo positioning sector using a servo signal sampled current position, and the invention is therefore applied in a particular way, the provisions characterizing it are equally applicable to the systems servo position error signal permanent, in which case the performance of such systems are significantly improved.

Overall, the invention provides a novel technique for the quasi-optimal control of a follower track sampled data. The device provides positioning on a relatively large radial distances while providing positioning accuracy and response speeds own transducer to allow the head to follow the small fluctuations of radial position of an information track recorded which take place in the rotation of a magnetic disk about a central axis at a rate of about 2000 to 4000 t/m.

A head positioning device according to the invention essentially comprises a carriage or main moving element that is transversely movable over the entire range of written tracks extending lengthwise along a surface information storage, a carriage moving element or secondary low-mass mounted transversely to position the transducer across short distances of the order of the width of a track with respect to the main carriage ., and primary and secondary actuators mounted to position the primary and secondary carriages in response to respective control signals of main actuator and secondary actuator. During operation according to a track following mode, an error detector for detecting the position error is coupled to receive the position information and assigned position and effective to provide a position error signal permanently present at in response thereto. The position error signal is compensated to generate tin command signal excitation. As long as the actual position differs by at least a track width of the assigned position, the device operates in a track access mode wherein the excitation command signal is maintained at a constant maximum amplitude and, its polarity is dependent on the direction of the error. A main actuator control circuit generates a control signal of main actuator in response to the excitation command signal to position the main carriage so as to reduce the position error, and an actuator control circuit generates a secondary ■ actuator control signal in response to the secondary excitation information of the main carriage, may be formed by means of a main-carriage speed and acceleration information by a main slide which can be provided by the excitation command signal.

The secondary carriage may be subjected to a return bias to a displaced position relative to the main ehariot zero by means of a mechanical spring, or a spring tie electronically simulated which can be accomplished by introducing a signal of relative movement of carriage secondary component of the secondary actuator control signal.

The features and advantages of the invention shall become apparent also of the detailed description given hereinafter by way of example with reference to the attached drawing, on which:

figure 1 represents in a block diagram a follower driver of tracks to high density according to the invention, and

figure 2 represents in a block diagram an alternative embodiment of a follower driver of tracks to high density according to the invention.

In the provisions as shown by the Figure 1, a follower driver of tracks to high density 10 according to the invention comprises a magnetic disk 12 rotating about a central axis 14, a transducer head, conventional disk 16 and a control assembly 18 provided to assume the radial positioning of a transducer head 16 relative to the disc 12. In information is recorded on the magnetic disk 12 on a plurality of concentric tracks, one of which is schematized in 20. The concentric tracks extend in a transverse direction through a range of radii between a track 22 a innermost and outermost track 24 adjacent to the outer periphery of the disc 12.

Since known per se, each track 20 is divided into several sectors for the addressing of the information recorded on the disk 12. A number of these sectors, which are represented as indicative by the sectors 26, contain the stored position information that is read by the head 16 to indicate the track 20 on which is located the head transducer 16 and the position of the transducer head 16 from the center of a track particular 20. Recording These arrangements are conventional, and it will not be further examined herein. In other provisions conventional recording, the actual track position information is recorded in a continuous way along each track 20 superimposed on the data information. Although the method actually used for detecting and indicating the radii defining the actual position of the transducer head 16 relative to the disc 12 is not important for the invention, is assumed for the purpose of this description that the position information is recorded on more sectors 26 angularly spaced.

The control assembly 18 includes a readout circuit 28, a control circuit 50 of track following, an access control circuit 32, a subset 34 control-< the main carriage, a subset control 36 secondary carriage 38 and a positioning mechanism that positions the transducer head radially 16, under the control of the control subassemblies main slide 34et of secondary carriage 36.

The readout circuit 28 receives and amplifies the information supplied by the transducer head 16 in response to the reading of information recorded on the magnetic disk 12. Suitable After amplification, the information provided by a transducer head is separated into position information and data information, the data information being transmitted to a data processing circuit (not shown). The position information is processed by the read circuit 28 to provide position signals are delivered periodically each time one of the sectors 26 position information is passed on to the line with the transducer head 16. This location information is used by the read circuit 28 to generate a current position signal which indicates the position of a transducer head 16 relative to the center of the tracks 20 and a position signal representative of the approximate global position of the head 16 relative to the tracks 20.

The circuit 30 tracking control comprises an error detector 40, an amplifier 42 having a gain Eg and a compensation circuit or correcting circuit 44. The error detector 40 receives information current position and the position information assigned, and it comprises a circuit able to provide a position error signal representative of the position deviation of the head 16 from the center of a target track. The error detector 40 also includes a sample- Lloqueur maintaining constant the position error signal between the times of updating error. The position error signal is updated in response to the periodic current position information from the read circuit 28 and in response to new orders positioning. Orders positioning may be made by conventional circuitry disk storage in accordance with prescriptions system-inherent requiring that such particular track 20 of the disc 12 is called.

Suitable After amplification by the amplifier 42, the position error signal from the error detector 40 is transmitted to the correcting circuit 44. Although the call can be generally to any species of conventional correcting to improve the frequency response of the system, it proved beneficial in the present example using a correction to advance with a pole and a zero located symmetrically about a gain cutoff frequency desired unit.

The access control circuit 32 receives a signal position command and a position signal from the approximate ' readout circuit 28. The signal position command is to be at least sufficient to indicate a desired track 20. The approximate position is to be sufficient to allow the access control circuit determine the track perpendicular to which is positioned the transducing head 16. For example, the access control circuit could include a register receiving and storing a sequence number positioning indicates a position of desired track, a counter which is incremented or decremented in response an approximate position signal to indicate the track perpendicular to which is effectively positioned the head 16, and a comparator to indicate whether respective states of the register and the counter may differ or not from an account greater than unity. If the difference is equal to or greater than two, with the result that the delivery of a signal access control, constant amplitude and high, whose polarity is chosen such that the head 16 is moved in a direction for reducing the error, and the delivery of a mode control signal that indicates a track access mode. If the error is less than two, the access control signal is terminated and a mode control signal that indicates a track following mode is generated. A mode control switch 52 acts in response to the control signal mose from the access control circuit 32 to provide a control signal for excitation equal to the position error signal for a track following mode and equal to the access control signal from the access control circuit 32 while operating in track access mode.

Suitable After amplification by a main power amplifier 54-, the excitation control signal corrected, which is proportional to the acceleration of a carriage or moving element main 4-8, is used to excite a main actuator 50 for positioning of the main carriage 4-8. The excitation control signal is also transmitted through an amplifier 56 providing suitable amplification to one of the input terminals of an adder 58. The amplifier 56 is adapted to be saturate in response to the magnitude of excitation control signal track access mode and thus providing an output signal having a maximum amplitude is not likely to damage the control assembly 18. A

speed sensor 60 is connected to provide a O

speed signal in response to movement of the main carriage 4-8 that is part of the mechanism 38 head positioning. The main carriage 46 is moved by 1' main actuator 50 to provide a displacement X ^ relative to a full reference 62 which is fixed with respect to the central axis 14 of the disk 12. Movement X ^ thus is representative of the radial displacement of the main carriage 48 relative to the disc

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12. The speed signal represents the derivative X.

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to time of the displacement X ^. The speed signal X ^ the main trolley is amplified by an amplifier 64 appropriate gain Ky and is used to drive a second input of the adder 58" The direction of orientation of the co-ordinates and polari -

tees signals are chosen such that a trolley speed

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main X ^ having a direction which tends to reduce the position error signal tends also to reduce the magnitude of an excitation signal compensated as a function of the speed which is outputted from the adder 58 in response to the summation of the command signal and speed signal of the main carriage.

The excitation signal to velocity compensation the leading one of the inputs of a second adder 66 having an output to provide a secondary actuator control signal to excite a secondary actuator 68 suitable after amplification by a power amplifier 70 secondary.

A second input, inverting, of the adder 66, is responsive to a position signal X₂ it indicates the displacement of a carriage moving element or secondary 72 with respect to the main carriage 48. A position detector 74 relates to the relative position of the secondary carriage 72 to generate the secondary carriage position signal X₂ , which is suitably amplified by an amplifier 76 having a gain and provided with a correction to appropriate delay before being transmitted to the inverting input of the adder 66. The relative displacement X₂ has the effect of influencing the control of 1' secondary actuator 68 so as to tend to zero the relative displacement X₂ *

The main actuator 50 is mounted so as to ensure the positioning of the main carriage 48, and it can be realised by a conventional rotary engine having-d ' suitable transmission members, of a linear motor such as a voice coil motor, or even a hydraulic or pneumatic actuator. The main actuator 50 provides the positioning of a transducer head 16 over the entire width of the radial range between the innermost track 22 and the outermost track 24. The main actuator 50 should be able to move the main carriage 48 through a stroke of several centimeters in response to the drive control signals amplified from the switch 52 and the amplifier 54.

The main carriage 48 is designed so as to form the support structure essential of a transducer head 16, the secondary carriage 72 and 68 of the secondary actuator 16 when the transducing head is moved radially to be positioned above the various tracks 20 that are available for recording. The main carriage 48, by including 1¹ secondary actuator 68, may have a mass of about 14 to 36 grams, for example, and its mass is substantially higher than that of the secondary carriage 72 .

The secondary actuator 68 is mounted on the main carriage 48 to provide fast positioning with a low deflection of the transducing head 16 with respect to the main carriage 48, this clearance being for example of the order of -13 microns. The secondary actuator 68 may be a type piezoelectric actuator bar longitudinal expansion or bimorph type cantilever mounted and working in bending. Alternatively, the secondary actuator 68 could be constituted by a small voice coil actuator supported on a membrane such as those commonly used in the loudspeakers for high frequencies. The actuator 68 may be able to exert a force of about 50 grams over the course of its relatively low displacement path.

The secondary carriage 72 is relatively lightweight in comparison of the main carriage 48 and its mass, by including that of the transducer head 16, is of the order of 5 to 10% of that of the main carriage 48. The secondary carriage 72 is positioned, under the control of the actuator 68, so as to be subject to a displacement P^ar respect to the position of the carriage, main 48. Since the transducing head 16 is mounted directly on the secondary carriage 72, the total displacement X ^, of a transducer head 16 with respect to the reference plane 62 is equal to + X₂ . ^ has relatively low mass of the secondary carriage 72, transduetrice of head 16, with respect to that of the main carriage 48 provides a fast response of the control device 10 to variations in the position error signal while also allowing the secondary carriage 72 and to the head 16 transduetrice of undergoing high accelerations without imparting significant vibratory motions to the main carriage 48 under the effect of the reaction forces of the opposite direction, which are communicated to the main carriage 48 by the secondary actuator 68 under the effect of the accelerations of the secondary carriage 72 transduetrice 16 and head.

The displacement signal X₂ creates a restoring force which tends to return the secondary carriage 72 to a position of zero motion with respect to the main carriage 48. Therefore, when the head 16 transduetrice achieved almost in the center of a desired track position, the amplitude of the excitation control signal to velocity compensation available at the summer 66 falls below the amplitude of the displacement signal X₂ soirte that the secondary carriage 72 is driven in a direction tending to zero the displacement X₂ with respect to the main carriage 48. The relative movement of the secondary carriage 72 can actually tend to increase slightly the amplitude of the position error signal. However, the main actuator -50 continues to react to the excitation control signal, which depends on the position error signal, and it moves the main carriage 48 in the direction of the track center position while the relative movement of the secondary carriage 72 is returned to the value zero. The gain of 1.' amplifier 76 is chosen such that the displacement signal X₂ is, not capable of inducing errors track position of sufficient magnitude to disrupt the read and write operations of the disk. In other words, the maximum amplitude of the signal X₂ applied to the summer 66 does not exceed the maximum value of the excitation control signal to velocity compensation applied to the summer 66 lorsoue transd. uctiu.ce the head 16 is sufficiently close to the center of a target track to render possible reading and writing. The secondary carriage 72 is automatically centered relative to the main ehariot 48 to ensure in the shortest possible period 1¹ setting, a optimum response in monitoring the small fluctuations the radius of a track during rotation of the disc 12 below the transducer head 16 without creating track position error amplitude sufficient santqjéour obstructing the read and write operations.

Suppose example that the error detector 32 receives a control command causes the head 16 to move radially inwardly of at least some track positions. The access control circuit 32 generates a signal access control large amplitude in response to which the secondary control circuits 36 act on the secondary actuator 68 so as to provide the secondary carriage 72 to inward radial displacement maximum amplitude relative to the position of the main carriage 48. Simultaneously, the control signal resulting excitation acts on the subset main control 30 to excite the main actuator 50 to starting to move radially inwardly the carriage

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main 48 at a rate rapid advance X ^ which is substantially less than the maximum speed of the secondary carriage 72. During the travel of the head 16 in the direction of the desired track position, the position signals and effective coarse position are periodically updated and, when the distance from the head 16 to the position of target track becomes adjacent about one track position, the access control circuit results in a transition from a mode access of track to track a tracking mode wherein the amplitude of the excitation control signal decreases sufficiently to initiate the main actuator 50 to decelerate the carriage prinei. pal 48. When the position of the head 16 becomes sufficiently near the target track to enable operations to read or write begin, the position error signal becomes very low, and it is further reduced O

by the speed signal which is transmitted to the summer 58 to reduce the excitation control signal compensated speed to a smaller amplitude than the displacement signal X₂ which is applied to the summer 66 by the amplifier 76. The secondary actuator 68 is therefore controlled to subjecting to start a relative radial displacement outwardly at secondary carriage and to return the latter to a position of zero relative movement..

However, since the main actuator 50 continues to receive a positive position error signal, the main carriage 48 continues to move radially inwardly at a speed which is greater than that of the radial outward displacement of the secondary carriage 72 with respect to the main carriage, whereby the head 16 continues to move toward the center of the track and that the value of the position error signal continues to decrease.

When the head 16 they near the center than desired track and relative motion X^ the secondary carriage 72 becomes close to zero, the movement of the main carriage 48 takes substantially end. However, the secondary carriage 72 continues to meet the relatively low instantaneous variation of the position error signal to allow the head 16 follow the small fluctuations of the radius of the track service during the rotation of the disk 12 about the axis 14.

In a variant embodiment of the invention which is shown in Figure 2, the electronic circuits for returning the secondary carriage 72 zero to a displaced position with respect to the main carriage 48 are replaced by a spring 80 connecting the secondary carriage to the main carriage. The spring 80 is mounted such that the position of zero deflection of the spring 80 is the position of zero relative movement of the secondary carriage 72. For the above values of the respective masses of the main carriage 48 and the secondary carriage 72, the spring 80 must have a spring constant K of the order of 520 H/mm or more. The return to the position of zero motion of the secondary carriage 72 being produced by the spring 80, the secondary power amplifier 70 responds to the error signal from the compensated speed sonmateur 58.

Although the above description has been based on the magnetic disk files, the field of applications of the invention extends to other recording devices high accuracy such as magnetic tape drives and the processors magnetic card. Therefore, although has shown and described above particular embodiments of controllers double carriage designed to achieve the positioning of a transducer head of a magnetic disk storage, thereby allow a technician skill average constructing, or operating the invention, it is to be invention born are not limited to such embodiments, and that various modifications and variants may be made to the arrangements described and illustrated without departing from to the scope of the invention as defined by the scope of the claims herein appended.