Electrical generator or motor structure – Dynamoelectric – Linear
Reexamination Certificate
1999-11-24
2001-02-27
Enad, Elvin (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Linear
C310S012060
Reexamination Certificate
active
06194796
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to linear actuators, and particularly to a voice coil linear actuator for long stroke actuation in a tape drive unit.
BACKGROUND OF THE INVENTION
Linear voice coil motors in tape drive systems are known. For example, the 9840 tape drive system from Storage Technology uses magneto-restrictive (MR) elements in its read/write head and a voice coil linear actuator to position the head with respect to the tape. This type of tape head does not tolerate a large external magnetic field as it operates to sense magnetic fields on the surface of the tape. Specifically, it is desirable to keep the magnetic field in the vicinity of the read/write head below five Orsted.
One problem with the prior art voice coil linear actuator is it produces a relatively large fringing field (approximately forty Orsted) at the bottom of the head, even with magnetic shielding. Consequently, prior art voice coil linear actuator systems effectively permit only a short stroke to traverse a linear track of about 0.015 inch. While this is fine for tape drive systems that record in one tape band, with the head spread over the entire tape width; this short stroke is not sufficient for recording in two bands to span a distance of nearly 0.300 inch. Furthermore, systems that record in two tape bands require a much larger linear actuator in order to provide performance in linear torque over the entire distance. By contrast, the short stroke prior art actuators translate almost entirely within the actuator.
One object of the invention is thus to provide a long stroke voice coil linear actuator. Another object of the invention is to provide a voice coil linear actuator for the 9840 Eagle Drive system by Storage Technology, with reduced magnetic fringing from the actuator.
These and other objects will become apparent in the description that follows.
SUMMARY OF THE INVENTION
Certain background is useful in understanding the invention, and thus the following patents are incorporated herein by reference: U.S Pat. No. 5,793,574, U.S. Pat. No. 5,644,453; U.S. Pat. No. 5,627,702; U.S. Pat. No. 5,434,458; U.S. Pat. No. 5,146,122; U.S. Pat. No. 4,833,558; U.S. Pat. No. 4,747,004; U.S. Pat. No. 4,559,571; U.S. Pat. No. 4,602,175; U.S. Pat. No. 4,528,607; U.S. Pat. No. 4,393,425; U.S. Pat. No. 4,190,870; and U.S. Pat. No. 4,003,092.
The invention of one aspect provides a voice coil linear actuator that has reduced magnetic fringing and that has substantially the same form-fit shape as prior art actuators to maintain operable interface with other mechanical elements, such as the Head Position Servo, in the tape drive system. According to one aspect, leakage flux at the top of the actuator is reduced by an auxiliary magnet that substantially cancels magnetic fields at the read/write head (i.e., to less than about two Orsted). With reduced fringing, the main magnet size is reduced from conventional sizes and the voice coil is repositioned to achieve the forces needed and/or desired during translation.
In one aspect, the invention provides an improvement to a voice coil linear actuator of the type that includes a bobbin connected to a voice coil, a center pole piece, a main magnet, and an outer pole and housing. An auxiliary magnet is constructed and arranged for attachment to the center pole piece such that the center pole piece is sandwiched between the auxiliary magnet and the main magnet. The main magnet has a first magnetic polarization adjacent to the center pole piece at a first end of the main magnet; and the auxiliary magnet has a second magnetic polarization adjacent the center pole piece, such that fringing fields from the auxiliary magnet substantially cancel fringing fields from the main magnet.
In another aspect of the improvement, the first magnetic polarization has a North orientation and the second magnetic polarization has a North orientation.
In still another aspect of the improvement, a non-magnetic spacer is coupled to a second end of the main magnet to alter the force exerted by the bobbin as a function of current through the winding.
In another aspect, a voice coil actuator is provided, including:
a substantially cylindrical outer pole and housing having a first hollow interior along a force axis; a substantially cylindrical voice coil having a winding about a second hollow interior and axially disposed along the force axis in the first hollow interior; a substantially cylindrical main magnet axially disposed along both interiors, the main magnet having a first end with a first magnetic polarization and a second end with a second magnetic polarization; a substantially cylindrical center pole piece adjacent the first end of the main magnet and axially disposed along both interiors; a substantially cylindrical auxiliary magnet axially disposed along both interiors and adjacent to the center pole piece, wherein the pole piece separates the auxiliary magnet from the main magnet, a bobbin connected to the voice coil for translation with the voice coil along the force axis, wherein current through the voice coil generates a force on the bobbin along the force axis, magnetic fringing fields from the auxiliary magnet and the main magnet being substantially canceled external to the actuator.
Preferably, voice coils of the invention generate less than about five Orsted of magnetic flux external to the actuator at the point of a tape drive read/write head.
The invention also provides a method for attenuating external magnetic fringing fields of a voice coil linear actuator that moves a bobbin along a force axis, including the steps of:
shielding a main magnet within the actuator with an auxiliary magnet disposed between the bobbin and the main magnet; and extending an outer pole housing of the actuator to shield windings of the voice coil.
The invention is next described further in connection with preferred embodiments, and it will become apparent that various additions, subtractions, and modifications can be made by those skilled in the art without departing from the scope of the invention.
REFERENCES:
patent: 5008578 (1991-04-01), Pace
patent: 5267111 (1993-11-01), Nishimura et al.
patent: 5434458 (1995-07-01), Sturat et al.
Bailey Wayne P.
Duft, Graziano & Forest
Enad Elvin
Jones Judson H.
Storage Technology Corporation
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