Magnetic head servo system with steppermotor for head positioning

Abstract: A device for positioning a magnetic head to an operative position relative to a selected track of a multi-track magnetic tape in a magnetic tape write/read device. A magnetic head is disposed at one end of a magnetic head carrier while the opposite end of the head carrier is axially movable along a shaft fixed to the tape recorder housing frame. An inchworm-type servo system is provided which has two clamping devices arranged stacked axially along the shaft and a positioning device connected between the two clamping devices which can be altered in length to move the clamping devices toward or away from each other. By alternatingly clamping the two clamping devices to the shaft and altering the distance between the two clamping devices in coordination, the servo system can crawl up and down the shaft to position the magnetic head. In another mode of operation, the clamp which mounts the head carrier is left unclamped with respect to the shaft, the opposite clamping device is clamped with respect to the shaft, and the extending device is allowed to expand or contract over its operable range to minutely position the head with respect to the tape. In the preferred embodiment, the clamping devices and the extending devices are piezoelectric elements which receive a voltage potential to correspondingly expand or contract. By contracting the clamping devices are actuated to clamp tightly to the shaft to prevent axial movement.

Description: BACKGROUND OF THE INVENTION It is necessary to position the write/read head or heads of a magnetic write/read unit very precisely relative to the closely-packed data tracks of the magnetic tape, to insure that data is written onto, or read from a desired, selected track. Since the data and racks run parallel to the length of the tape (i.e., parallel to the transport direction), the write/read head must be moved in both directions perpendicular to the transport direction. A device for positioning a magnetic head relative to tracks of a magnetic tape in a magnetic tape write/read unit is described in German Published Patent Application No. 31 12 895.5, filed March 31, 1981 and corresponding to U.S. Patent No. 4,491,890. In EP-A 0,026,524 a device with positioning systems which are provided with piezoelectric elements is described. A first positioning system consists of piezoelectric elements that extend substantially perpendicular to the surface of a magnetic tape, a second positioning system is supplied for pivoting the magnetic head about a pivotal axis. This document is used for the delimitation of appended claims 1 and 8. From U.S. Patent No. 4,606,959 an arrangement is known, by which the positioning of a magnetic head occurs by the employment of a worm drive and a screw drive connected to a drive motor which can be a DC motor or stepping motor. In U.S. Patent No. 4,609,959 a magnetic head disposed on a magnetic head carrier is positionable by means of a worm drive means and a screw drive means wherein the worm drive is formed of a worm spindle gear driven by a drive motor and associated with a worm wheel gear, and the screw drive is formed by an inside thread of the worm wheel and an associated outside thread on a shaft. The shaft is mounted in a fixed manner to the housing frame of the tape recorder. The magnetic head carrier is arranged so as to be displaceably disposed on the shaft and non-positively connected to the worm wheel, thus securing the magnetic head carrier against rotation during the rotation of the worm wheel but allowing the head carrier to follow the axial motion of the worm wheel. Piezoelectric actuators are known for translating a shaft along a piezoelectric motor. Piezoelectric materials have the property that when an electric potential is applied to them, they undergo a physical deformation. Piezoelectric materials can be crystals or ceramics. The piezoelectric materials can be formed into a wide variety of shapes and sizes. Once subjected to an electrical potential of a defined polarity, the piezoelectric element undergoes a dimensional change. A piezoelectric motor arrangement is commercially available from Burleigh Instruments, Inc., known as an "inchworm" motor, wherein three piezoelectric actuators are utilized in synchrony to incrementally move the shaft along the motor. Upper and lower (or left and right) piezoelectric elements act as clamps and are arranged on either side of a central piezoelectric element which acts as an extender. By clamping one of the piezoelectric clamps, and extending the piezoelectric extender, the opposite clamp (in unclamped condition) is progressed with respect to the shaft. By then clamping this opposite clamp and releasing the first clamp and thereafter shrinking the extender, the first clamp progresses along the shaft in the same direction that the opposite clamp had progressed previously. The process can thereafter be repeated to incrementally creep or inchworm the shaft along the motor. In the known Burleigh inchworm motor the piezoelectric actuator is fixedly mounted and the stepping or creeping along the shaft actually moves the shaft with respect to the mounting rather than translate the piezoelectric elements with respect to a fixed shaft. Piezoelectric actuators and such "inchworm motors" have heretofore not been employed in seeking to solve the problem of positioning a magnetic head relative to tracks on a magnetic tape in a magnetic tape write/read unit. SUMMARY OF THE INVENTION It is an object of the present invention to provide a device for positioning a magnetic head relative to various tracks of a magnetic tape in a magnetic recording or reading device which exhibits a particularly low overall height or profile while still providing an extremely accurate positioning of the magnetic head without the drawbacks of a rotational drive system using gears and motors. In accordance with the principles of the present invention, the above object is achieve in an arrangement of three actuators, used to progress a magnetic head across a magnetic tape in a direction perpendicular to the tape transport direction, selectively to read or write various tracks of the tape. The three actuators can preferably be of the piezoelectric type. The first of the three actuators is formed as a clamp surrounding a shaft, the shaft arranged parallel with a width of the tape that is to be read or written on. The second of the actuators also surrounds the shaft below the first actuator. The third actuator is arranged between the first and second actuator and functions mechanically to spread the first and second actuators from each other along the shaft, or to retract the first and second actuator toward each other on the shaft. By fixing one of the first and second actuators and expanding or retracting the third actuator and then fixing the second actuator with respect to the shaft and releasing the first actuator, and repeating these steps, the assembly can be progressed or "inchwormed" upwardly or downwardly along the shaft. By mounting a magnetic head carrier to either the first or second actuator a magnetic head can therefore be carried up and down along the shaft with the arrangement. Alternately, especially for use in closed loop track servo applications, the lower actuator element can remain clamped with the upper actuator element remaining unclamped. In this application, the head carrier is mounted to the upper actuator element. By actuating the third actuator element, such as when the third actuator element is a piezoelectric element and selective voltages are applied thereto, the first actuator element and thus the head can now move quickly up and down continuously with the expanding or contracting third actuator element and the head can follow a prerecorded servo track on the tape. By using the aforementioned described arrangement of three actuators such as piezoelectric actuators, an improved positioning arrangement can be achieved. The arrangement comprises fewer parts and is simply manufactured. Step motors and gears, and the problems associated therewith, are eliminated. The arrangement is suitable for coarse movement and at the same time can also be used for fine servo movement. The system provides excellent azimuth/zenith stability control. The system provides very good control of the clearance between the shaft and the bearings such as the piezo clamping elements. Azimuth/zenith stability control is a problem with low profile systems because of the reduced distance between upper and lower bearings. The aforementioned system provides for a low profile product requiring small space. Unlike many mechanical systems, the actuator system, especially using piezoelectric elements, provides for a long life with very little wear. With piezoelectric elements, the conversion of the electrical energy applied thereto into mechanical motion occurs without a generation of any significant magnetic field. With regard to piezoelectrics, these materials can be operated for millions of cycles without wear or deterioration. With regard to the piezoelectric application, resolution can be very fine and accurate in the nanometer range if needed. Therefore, utilizing piezoelectrics at the actuators for this arrangement provides sufficient accuracy anticipating products far into the future. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a plan view of a servo system of the present invention applied to a magnetic tape read/write apparatus with a portion of a cover removed for clarity; Figure 2 is a sectional view of the apparatus of Figure 1 taken generally along line II-II; Figures 3A-3C are partial views taken from Figure 2 demonstrating progressively the operation of the servo apparatus; Figure 4 is a voltage/time graph showing the synchronized applied voltages over time of the three piezo actuators of the present invention; and Figure 5 is an alternate embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 shows a magnetic write/read apparatus generally at 10 which receives thereinto a tape cartridge 14 which holds a magnetic tape 16 spooled therein as is known. The recording device 10 further provides a magnetic head 20 which fronts the magnetic tape 16 to either read or write information from or onto the tape 16. The magnetic head 20 is mounted on a head carrier 24. A servo system is shown generally at 30 comprising a first element or actuator 34, a second element or actuator 36, and a third element or actuator 38. The third actuator 38 is mounted to the first actuator 30 via a first arm 40; and the third actuator 38 is mounted to the second actuator 36 via a second arm 42. In a preferred embodiment, the first actuator 30, the second actuator 36 and the third actuator 38 can comprise piezoelectric elements. However, any or all of these elements could be plunger-type solenoids or other devices which can be electrically activated to perform the below described tasks. The first and second actuators 34, 36 are mounted axially onto a shaft 50. The first and second actuators are selectively movable along the shaft 50. The first element 34 is preferably a piezoelectric disk having electric leads 34a, 34b. When a voltage of selected polarity and magnitude is applied across these leads, the piezo disk 34 constricts at its center aperture to clamp the shaft 50. The second element 36 is also preferably a piezoelectric disk and has electric leads 36a, 36b. When a voltage of selected polarity and magnitude is applied across these leads, the second piezo disk 36 constricts at its aperture to clamp the shaft 50. The third element 38 is preferably a piezo element and has electric leads 38a, 38b and when a voltage of selected polarity and magnitude is applied thereto the third piezo element 38 expands in a vertical direction with respect to Figure 2 to separate further the first element 34 from the second element 36. Figures 3A-3C show how this servo arrangement 30 can be used to translate the head 20 vertically with respect to the tape 16. Figure 3A shows the first element 34 in a released condition with regard to the shaft 50 and the second element 36 in a clamped condition with regard to the shaft 50. In Figure 3B the third element 38 has a voltage applied to it and has expanded to translate the first element 34 upwardly with respect to the second element 36. Because the second element 36 is fixed by clamping with respect to the shaft 50, the first element 34 has translated upwardly with respect to the shaft thus carrying the head carrier 24 and the head 20 upwardly with it. Figure 3C shows the further step of now applying the voltage to the first element 34 to thus clamp the first element 34 to the shaft 50, the second element 36 has been released from the shaft 50 by removing the voltage potential from the second element 36. By removing the potential from the third element 38, the third element 38 has contracted and by so doing has lifted the second element 36 upwardly with respect to the first element 34, and because the first element 34 is clamped with respect to the shaft 50, the second element 36 has been translated upwardly with respect to the shaft 50. Figure 4 shows in graphical representation, with voltage as the ordinate and time as the abscissa, the synchronized applied voltages to the three elements 34, 36 and 38 to effect the "inchworm" translation of the servo system 30 up and down with respect to the shaft 50. One cycle is displayed, but it is readily apparent that the cycle can be multiplied and/or reversed. At time T1 the second element has a voltage applied to it and thus clamps the shaft. At time T2 the first element is released from its clamping voltage. Between time T3 and time T4 a voltage is applied in increasing fashion to the third element 38 to expand the third element in length. At time T5 the first element has a voltage applied to it to clamp the first element to the shaft. Between time T4 and T7 a constant voltage is applied to the third element 38. At time T5 a voltage is applied to the first element 34 to clamp the first element 34 to the shaft. At time T6 the voltage is removed from the second element 36 to release its clamping action on the shaft 50. Between time T7 and T8 the voltage is ramped down from the third element 38 to contract the third element 38. Thus, at this point the first element is clamped to the shaft and the third element 38 has shrunk lifting the second element 36 upwardly with it. The cycle can be repeated to affect this inchworm climbing of the servo system 30. Figure 5 shows an alternate embodiment of the structure of Figure 2, wherein the third element 38 is replaced by an alternate third element 138 which is located axially between the first and second elements 34, 36. Thus, the first arm 40 and the second arm 42 are deleted as the third element 138 expands or contracts to directly translate the unclamped respective one of the first element 34 and the second element 36. The head carrier 24 has been replaced with an alternately configured head carrier 124. In this embodiment, the first element 34, the second element 36, and the third element 138 behave in similar synchronous fashion to that described in Figures 3A-3C and Figure 4. Alternately, for rapid minute positioning of the head 20 with respect to the tape 16, the second element 36 can remain clamped and the positioning of the head 20 up and down can be achieved solely through the expansion and contraction of the third element 138. The range R would thus be limited to the maximum expansion or contraction ot the third element 138 depending on the voltage applied thereto. In an embodiment for one-inch format drives, wherein a profile height W is 25.4 mm, the range R would be 0-10µm. This is particularly useful in a closed loop track servo wherein the head must move quickly up and down continuously, so that the head can follow a prerecorded servo track on the tape. The disclosed system, however, can be used to advantage in any size drive format. The center line of the tape 16 is indicated as CL1. Figure 5 shows in dashed lines the upper translated position of the head 20 indicated as 20'. In this position the center line of the head 20 is located at CL2. The range difference a between CL1 and CL2 is 3.5 mm in the preferred embodiment. Also shown in dashed lines is the lowest range position of the head 20 indicated as 20". The center line of the head 20 at this lowest position is indicated as CL3. The range difference b between CL1 and CL3 is also 3.5 mm. In any one cycle of extension by the third element 138, the unclamped element, either element 34 or element 36 can move up approximately 10µm in the expected embodiment.

Claims: A read/write apparatus (10) containing a device for positioning a magnetic head (20) relative to a selected one of a plurality of tracks of a magnetic tape (20), said tracks extending parallel to a tape transport direction in said device, comprising: a magnetic tape recorder housing frame; a position actuator (38, 138); a magnetic head carrier (24, 124) mounted to said position actuator; a magnetic head (20) secured to said magnetic head carrier (24, 124) ; characterized in that said magnetic tape recorder housing frame comprises a shaft (50) and means for mounting the shaft in a fixed manner perpendicular to said tape transport direction; said position actuator (38, 138) comprises an element having a portion thereof which can be altered in length between a first end and a second end thereof perpendicular to said tape transport direction by electrical stimulation thereof; said portion of said position actuator (38, 138) is connected at said first end to a first clamp actuator (34) and connected at said second end to a second clamp actuator (36); said magnetic head carrier (24, 124) is mounted to said first clamp actuator (34); said portion of said position actuator (38, 138) is electrically stimulatable to selectively extend or retract said magnetic head carrier (24, 124) in a direction perpendicular to said tape transport direction. A device as defined in claim 1, characterized in that said first clamp actuator (34) is at least partially surrounding said shaft (50) and slidable along said shaft (50) and electrically stimulatable to grip said shaft (50); said second clamp actuator (36) is at least partially surrounding said shaft (50) and slidable along said shaft (50) and electrically stimulatable to grip said shaft (50) and spaced from said first clamp actuator (34); and further characterized by means for repeatedly alternatingly electrically stimulating said first and second clamp actuators (34, 36) for causing one of said clamp actuators to grip said shaft (50) while the other of the clamp actuator is free to slide along said shaft (50); and means for repeatedly electrically stimulating said portion of said position actuator (38, 138), coordinated with the stimulation of said first and second clamp actuators (34, 36), for altering said length of said portion for advancing, in a selected direction, said clamp which is free to slide along said shaft (50) to progressively move said magnetic head (20) to a selected position. A device as defined in claim 1 or 2, wherein said first clamp actuator (34) and said second clamp actuator (36) and said position actuator (38, 138) comprise piezoelectric elements. A device as defined in claim 1 or 2, wherein said first clamp actuator (34) and said second clamp actuator (36) and said position actuator (38, 138) comprise plunger-type solenoid elements. A device as defined in claims 1 to 4, wherein said first clamp actuator (34) and said second clamp actuator (36) are ring-shaped elements slidably mounted on said shaft (50). A device as defined in claims 1 to 5, wherein said position actuator (38) comprises a first arm (40) connecting said first end of said portion to said first clamp actuator (34); and a second arm (42) connecting said second end of said portion to said second clamp actuator (36). A device as defined in claims 1 to 6, wherein said first clamp actuator (34), said second clamp actuator (36) and said position actuator (38) comprise cylindrical shapes, and are stacked axially on said shaft (50). A method for positioning a magnetic head (20) relative to a select one of a plurality of tracks of magnetic tape (16) in a magnetic tape write/read device, said tracks extending parallel to a tape transport direction in said device, comprising the steps of: providing a magnetic head (20) facing said magnetic tape (10) and mounted to a servo system (30) characterized by providing a shaft (50) arranged generally parallel to a width of the magnetic tape (16) to be read, said servo system being slidable on said shaft; providing the servo system with a clamping and extending mechanism, said extending mechanism being stimulatable to alter in length; and sequentially actuating said clamping and extending mechanism to move said servo system and said magnetic head progressively along said shaft in a selected direction. The method according to claim 8, characterized in that said clamping and extending mechanism comprises a first clamp actuator (34) mounted slidably on said shaft; a second clamp actuator (36) mounted slidably on said shaft and spaced from said first clamp actuator; a position actuator (38) having a portion thereof which can be expanded or contracted in length, said portion connected at a first end of said first clamp actuator (34) and connected to a second edge of said second clamp actuator (36); said first clamp actuator (34) and said second clamp actuator (36) being selectively electrically stimulatable to grip said shaft (50) to prevent axial movement of the respective clamp actuator with respect to said shaft (50); said portion of said position actuator (38) electrically stimulatable to selectively extend retract from said first clamp actuator (34) with respect to said second clamp actuator (36); wherein said sequentially actuating comprises the steps of: stimulating said second clamp actuator (36) to clamp said shaft (50); stimulating said position actuator (38) to extend said portion; stimulating said first clamp actuator (34) to grip the shaft (50) ; destimulating said second clamp actuator (36) to release said second clamp actuator (36) from said shaft (50); destimulating said position actuator (38) to retract said portion; and repeating these latter five steps, as a cycle, a selective number of times.

Inventors: GUTTORM RUDI, GUTTORM, RUDI
Applicants: TANDBERG DATA, TANDBERG DATA ASA
Application year: 1992
Main classification: G11B5
IPC codes: G11B5