Positioning Method and Apparatus Therefor

The present invention relates to a positioning method and apparatus, and more particularly to a positioning method and apparatus for precisely positioning a magnetic head to a record medium in a magnetic disc device or a flexible disc device.

The magnetic disc device or the flexible disc device usually has a head positioning mechanism for precisely positioning the magnetic head to a track on a record medium. The head positioning mechanism positions the magnetic head to a hard disc by, for example, linearly moving or rotating the magnetic head by a voice coil motor. A positioning mechanism for positioning a magnetic head to a flexible disc record medium has a stepping motor for rotational movement, a steel belt wrapped around a capstan of the stepping motor, a carriage having the opposite ends of the steel belt fixed thereto and a guide rail for guiding the carriage radially of the flexible disc. The positioning mechanism converts the rotational movement of the stepping motor to a linear movement by the steel belt to position the magnetic head mounted on the carriage, which is guided by the guide rail, to the track on the flexible disc. A positioning mechanism of the aforementioned type is disclosed in, for example, U.S. Pat. Nos. 4,170,146 and 4,161,004, and the positioning mechanism which uses a lead screw is disclosed in, for example, U.S. Pat. Nos. 3,678,481 and 3,879,757.

However, the magnetic head positioning mechanism which uses the voice coil motor requires a large size voice coil motor to generate a motive force and hence the positioning mechanism is of large size and expensive. With a positioning mechanism which uses the stepping motor and steel belt, it is difficult to position the magnetic head on the track on the high density recorded disc because a rotational angular precision of the stepping motor includes a positioning error of .+-.3%.about..+-.5% per step. Further, due to a hysteressis effect of the stepping motor, a positioning of the magnetic head by a clockwise rotation of the motor and positioning by a counterclockwise rotation do not coincide, a precise positioning of the magnetic head is difficult to attain.

It is an object of the present invention to provide a positioning method and apparatus for precisely positioning a workpiece such as a magnetic head by a simple and light construction.

Another object of the present invention resides in providing a positioning method and apparatus for precisely positioning a workpiece while eliminating a positioning error due to a rotation angle precision of a stepping motor.

Yet another object of the present invention resides in providing a positioning method and apparatus capable of eliminating a positioning error due to the direction of rotation of the stepping motor.

In order to achieve the above objects, in accordance with the positioning method and apparatus of the present invention, the stepping motor is driven by that number of stepping motor driving current pulses which is determined by multiplying the number of pitches required for the workpiece to be positioned to a desired position with the number of stationary pole windings of the stepping motor.

The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments of the invention taken in connection with the accompanying drawings, in which:

FIG. 1 is an overall partially schematic perspective view of a flexible disc device to which the present invention is applied;

FIG. 2 is a perspective view of a portion of the magnetic head positioning mechanism of FIG. 1;

FIG. 3a is a cross sectional view of a stepping order taken along the line IIIa--IIIa in FIG. 2;

FIG. 3b is a cross-sectional view of the stepping motor taken along the line IIIb--IIIb of FIG. 2;

FIG. 4 illustrates a positioning method in which a magnetic head is moved one track for every one step of the stepping motor shown in FIGS. 3a and 3b;

FIG. 5 illustrates a positioning method in which the magnetic head is moved one track for every four steps of a stepping motor in accordance with one embodiment of the present invention;

FIG. 6 is a drive circuit in accordance with one embodiment of the present invention for driving the stepping motor shown in FIG. 5;

FIG. 7 is a flow chart of the drive operation of the control circuit of FIG. 6;

FIG. 8 illustrates a positioning precision in the present invention;

FIG. 9 illustrates a relationship between a rotation step angle of a stepping motor and a displacement of an angular error due to an angular displacement;

FIG. 10 illustrates a relationship between a rotation step angle of a stepping motor and a displacement of an angular error for explaining an positioning method of one embodiment of the present invention;

FIG. 11 is a block diagram for implementing the positioning method of the embodiment of FIG. 10; and

FIG. 12 is a flow chart of the operation of a control circuit used in the embodiment of FIG. 11.

Referring now to the drawings wherein like reference numerals are used throughout the various views to designate like parts and, more particularly, to FIG. 1, according to this figure, a flexible disc device comprises a spindle 50 rotated by a motor (not shown), a collet 12 for mounting a flexible disc 4 to the spindle 50, a head positioning mechanism 20, a stepping motor 1 for driving the positioning mechanism 20 radially of the disc 4, a rotary member 14 for loading and unloading a magnetic head 19 of the positioning mechanism to and from the flexible disc 4, a rotary solenoid 13 for rotating the rotary member 14 around a shaft 15, and a control circuit 11 for controlling the respective electrical elements.

The stepping motor 1 drives the head positioning mechanism 20 while the flexible disc 4 is mounted to the spindle 50 by the collet 12 so that the magnetic head 19 is positioned, and the rotary solenoid 13 rotates the rotary member 14 to load and unload the magnetic head 19 mounted on the positioning mechanism 20.

As shown most clearly in FIG. 2, the positioning mechanism 20 comprises a carriage block 60 which carries the magnetic head 19 and is movable along the guide arms 16a and 16b, a steel belt 3 wrapped around the guide arm 16a on the carriage block 60, with opposite ends thereof fixed in tension, and a capstan 2 around which the steel belt 3 is wrapped and which is rotated in synchronism with the shaft of the step motor 1.

As the stepping motor 1 is rotated and the steel belt 3 wrapped around the capstan 2 of the stepping motor 1 is wound and unwound, the rotational movement of the stepping motor 1 is translated to a linear movement of the carriage block 60 along the guide arms 16a and 16b. By the linear movement of the carriage block 60, the magnetic head 19 mounted at one end of the block 60 is positioned to a desired track 6 on the flexible disc 4.

As shown in FIGS. 3a and 3b, the stepping motor 1 comprises a rotary pole 9, stationary poles 7 and stationary pole windings 8 arranged around the rotary pole 9 at an interval of 90 degrees, a rotary bearing 13 for rotatably supporting a motor shaft 12 of the rotary pole 9 and a block 100 which surroundingly holds the poles 7, 9 and the bearing 13. By selectively exciting the stationary pole windings 8, the rotary pole 9 is rotated so that the shaft 12 effects the rotational movement. When a four-phase stepping motor having four stationary poles 7 and four stationary pole windings 8 is driven in a two-phase excitation system, the same stationary pole winding 8 is excited for every four steps of the stationary poles 7 of the stepping motor 1.

However, because of a circumferential arrangement angular error of the stationary poles 7, a centering error of the stationary poles 7 and the rotary pole 9, and nonuniformity of the exciting current of each of the stationary pole windings 8, the angular position of each step of the stepping motor 1 includes an angular error.

As shown in FIG. 4, when the stepping motor 1 is rotated by one step (3.6 degrees) from a reference angle (0 degree shown by point P(0), a rotation angular error is approximately +0.1 degree as shown by a point P(1), and when the stepping motor 1 is further rotated by one step (3.6 degrees), the error is approximately +0.02 degree as shown by a point P(2). As the stepping motor 1 is rotated stepwise, the error changes to approximately -0.06 degree and -0.01 degree as shown by points P(3) and P(4). The error of the stepping motor 1 varies in accordance with a period of a four-step rotation.

While it was contemplated to move the magnetic head 19 by one track width of the flexible disc 4 for every one-step rotation of the stepping motor 1, it was determined that the stepping motor 1 produced an angular error of approximately .+-.0.1 degree as shown by an error range 200. This error corresponds to an error of approximately .+-.3% in the stepping motor 1 having one-step angle of approximately 3.6 degrees. Thus, it is not suitable for the high precision positioning of the magnetic head 19.

It has also been determined that the substantially same positioning error occurs for every one revolution of the stepping motor 1. Consequently it is necessary for the magnetic head positioning method and apparatus to have a very small positioning error which may be accomplished by moving the magnetic head 19 by one track width for every four-step drive of a stepping motor 1 having four stationary poles 7.

As shown in FIG. 5, a point P(1), which corresponds to the angle (3.6 degrees) after one-step rotation of the stepping motor 1, is taken as a reference point, and the magnetic head 19 is positioned at every four-step rotation point from the reference point P(1) such as points P(5), P(9), P(13) and P(17). The magnetic head 19 traces a path shown by a broken line in FIG. 5 but it is positioned at the points marked with dots (.multidot.). Thus, the magnetic head 19 is apparently moved along a solid line and the positioning angular error 201 is suppressed to be approximately 0.02 degree or less.

The positioning angular error can be suppressed to be so small because the positioning is accomplished by using the stationary pole windings 8 of the same condition. Even when the positioning is effected for every four steps 1 as shown in FIG. 5, the positioning error 201 is included; however, this is mostly due to a variation of a tooth shape tolerance of the rotary pole 9 and is so small that it sufficiently satisfies the allowance (.+-.1%) of the positioning error.

As shown in FIG. 6, the positioning drive unit comprises a control circuit 111 for controlling a drive circuit 110 in accordance with the input number of track pitches to be traversed, with the drive circuit 110 driving the stepping motor 1 by a drive or excitation current under the control of a control circuit 111. A driven object 200, for example, the magnetic head is driven by the rotation of the stepping motor 1 through the positioning mechanism 20. As shown in a flow chart of FIG. 7, the control circuit 111 multiplies the number of pitches N.sub.p by the number of stationary pole windings n and activates the drive circuit 110 until the product or the number of stepping pulses N.sub.s reaches zero so that the driven object 200 is positioned.

In the magnetic head positioning mechanism of the present invention, the diameter of the capstan 2 is small because the stepping motor 1 is rotated at a higher rotational speed than the prior art.

FIG. 8 provides an illustration of a relationship between the positions of the magnetic head 19 positioned by the present invention and the prior art mechanism and the positioning error. More particularly, a broken line A represents the prior art positioning which includes a positioning error width 202 of approximately 13.7 .mu.m for a positioning pitch t.sub.3 of approximately 265 .mu.m , while a line B represents the positioning by the present invention which includes an error width 201 of approximately 2.1 .mu.m, approximately one seventh of that of the prior art.

The stepping motor has a feed pitch of 3.6 degrees for one step although the present invention is not limited thereto. For example, a stepping motor 1 having a minimum feed pitch of 0.9 degree may be used and the positioning may be at every four-step rotation. In this case, the diameter of the capstan 3 of the stepping motor 1 is same as that of the prior art mechanism. While the magnetic head 19 is positioned at every four steps in the above embodiment, the positioning may be effected at every integral multiple of the number of stationary pole windings 8 of the stepping motor 1 such as, for example, at every eight steps.

According to the embodiments described above, the positioning error of the magnetic head can be reduced.

The inventors constructed a flexible disc device in accordance with the above embodiments and to measure the positioning precision of the magnetic head a flexible disc drive was constructed and it was determined that when the magnetic head 19 was positioned by rotating the stepping motor 1 in one direction, the positioning error was be reduced however, a problem arose when the stepping motor 1 was rotated in the opposite direction and then rotated in the first direction to position the magnetic head 19 to the original position since the magnetic head was not positioned at the same position. Thus, it was determined that the position of the magnetic head 19 positioned by the forward rotation of the stepping motor did not coincide with the position of the magnetic head 19 positioned by the reverse rotation of the stepping motor 1. The cause of the above-mentioned problem regarding the positioning of the magnetic head 19 can best be understood by considering the relationship illustrated in FIG. 9. More particularly, if the stepping motor 1 is rotated one step to a reference point P (1) and then sequentially rotated from the reference point P (1) and the positioning is effected at every four-step points, namely, P(5), P(9), P(13), P(17) and P(21), the positioning angular error of the stepping motor 1 is maintained at approximately 0.1 degree as shown by the arrows E. When the stepping motor 1 is rotated in a reverse direction from the point P(21) and the positioning is effected at every four steps, the angular error is maintained at approximately zero degrees as shown by arrows F emanating from points .theta.(25), .theta.(29), .theta.(33), .theta.(37) and .theta.(41), because of the hysteresis of the stepping motor 1. Thus, the position of the magnetic head 19 positioned by the forward rotation of the stepping motor 1 differs from that positioned by the reverse rotation of the stepping motor 1. For example, when it is desired to position the magnetic head 19 to a position of step angle of 46.8 degrees corresponding to the fourth track, the angular error at the point P(13,) when the positioning is effected by a forward rotation of the stepping motor 1 from the point P(1) is approximately 0.11 degrees, while the angular error at the point .theta.(29) when the positioning is effected by a reverse rotation of the stepping motor 1 is approximately 0.2 degree. Thus, there is an error of approximately 0.9 degree therebetween leading to a degradation of the reproducibility of the magnetic head positioning.

According to the magnetic head positioning method of the present invention, when the positioning is to be effected by a forward rotation of the stepping motor 1, the stepping motor 1 is overrun by one step from a desired position and rotated in reverse by one step so that a position correction is accomplished by utilizing position shifts in forward and reverse directions due to the hysteresis. In this manner, the magnetic head 19 is positioned at substantially the same position in both the forward and reverse rotation of the stepping motor 1, a control circuit 111 drives the stepping motor 1 which, in turn, drives the positioning mechanism 20 such that it is overrun by one step and then rotated reversely by one step.

In FIGS. 10 and 11, it is assumed that the stepping motor 1 is rotated in a forward direction such that the magnetic head 19 is moved from a first track of the flexible disc 4 to fourth and sixth tracks and then the stepping motor 1 is reversed such that the magnetic head 19 is again positioned to the fourth track.

When the magnetic head 19 is moved from the first track of the flexible disc, i.e., 3.6 degrees position P(1), to the fourth track, the stepping motor 1 is rotated by twelve steps to a position P(13) along a broken line. An error from the desired rotation angle of 46.8 degrees at the point P(13) is approximately +1.1 degrees. The stepping motor 1 is rotated in a reverse direction to again position the magnetic head 19 to the fourth track. With the present invention, the stepping motor is rotated in a reverse direction by steps from the point P(21) to a point Q(30) along the chain line and then the stepping motor 1 is rotated in the forward direction by one step to the point P(13). Thus, with the present invention, the stepping motor 1 is overrun by one step from the destination rotation angle and then reversely rotated by one step.

As a result, the angular error included when the stepping motor 1 is rotated in the forward direction from a smaller step angle and the angular error included when the stepping motor 1 is rotated in the reverse rotation from a larger step angle are substantially equal and the reproduceability of the magnetic head positioning is enhanced. While the magnetic head is overrun by one step and then reversed by one step, it is understood that the present invention is not limited thereto. The stepping motor 1 may be excited to overrun past the magnetic head 19 and then be oppositely excited before the magnetic head 19 is positioned so as to increase the positioning speed of the magnetic head 19.

As shown in FIG. 11, the shift direction and the number of shift pitches are externally supplied to the control circuit 112, which, in turn, supplies a signal to the drive circuit 110 as shown most clearly in FIG. 12. The drive circuit 110 switches an excitation current supplied to the stepping motor 1 to rotate the stepping motor shaft, and the rotational movement of the shaft is translated to a linear movement by the positioning mechanism 20 to move a driven object 300 such as, for example, a magnetic head.

In accordance with a positioning pitch ratio m and the number of oversteps k under a given condition, the required number of step pulses N.sub.p and the number of oversteps k are produced without conflicting the logic shown in FIG. 12. The movement directions IN and OUT may be exchanged in the flow chart of FIG. 12, with positioning pitch ratio m being defined by positioning pitches P/stepping motor feed pitches P.

While the present invention has been described in connection with the disc device shown in FIG. 1, the present invention is also applicable to a positioning mechanism having a lead screw which utilizes a stepping motor as a drive source. A positioning mechanism having a lead screw comprises a lead screw which rotates in syncronism with a stepping motor, with a cylindrical outer periphery of the lead screw extending radially of a magnetic disc and having a groove or projection of a helical shape on the cylindrical outer periphery, a carriage supports the magnetic head 19 and has a projection or groove to be engaged with the groove or projection provided on the lead screw. A guide rail is provided for guiding the carriage radially of the magnetic disc, whereby, upon a rotation of the lead screw, driven by the step motor, the carriage, the projection or groove of which engages with the groove or projection provided on the lead screw, moves along the guide rail radially of the magnetic disc. The basic structure of such positioning mechanism is described in above noted U.S. Pat. No. 3,678,481, and the present invention may be implemented by driving the step motor of this structure such that the same stationary pole winding is excited for positioning as in the case of the invention described hereinabove, or, the overrunning and backing may be utilized when exciting the same stationary pole winding for positioning.

The present invention is also applicable to a positioning mechanism having a looped steel belt utilizing a step motor as a source of driving, or to a positioning mechanism of a rotary access type such as described in U.S. Pat. No. 3,946,439 or No. 4,161,004, and the present invention is implemented by driving step motors utilized in these mechanisms in the same manner as described hereinabove.

In the head positioning method and apparatus of the present invention, the magnetic head 19 is positioned at every four steps of the stepping motor 1. In a positioning mechanism which positions the magnetic head 19 by one track width at every one step, the stepping motor 1 may be overrun by one step and then rotated in a reverse direction by one step thereby enabling a relatively high precision positioning of the magnetic head 19.