Linear actuator

A linear motor actuator particularly adaptable to position a magnetic transducer in a magnetic disk storage system including a modular stator assembly and a modular carriage assembly. The stator assembly includes a frame with a first pair of spaced parallel guide rails; configured to have two flat guide surfaces adjacent to the carriage assembly and running in a direction parallel to the stroke of the actuator. A magnetic flux return path is integrally connected with a guide shaft and positioned between the guide rails. The carriage assembly includes a self supporting cantilevered coil positioned about the return path and guide rail, and offset from the flat surface. The carriage assembly is supported by a first set of angled ball bearings which ride on the guide shaft and a second set of spaced ball bearings which ride on the guide rails to preload the carriage assembly and prevent rotation. A linear tachometer strip is attached to the carriage assembly and associated with a light emitting/light receiving assembly to produce electrical signals indicative of the carriage assembly position.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates generally to electromagnetic actuators. More 
particularly, the invention relates to an actuator that produces linear 
movement. Although not limited thereto, devices in accordance with the 
present invention are especially useful in positioning a magnetic 
transducer or print head relative to a disk in a disk memory system or the 
like. 
2. Prior Art 
The use of linear actuators or linear motors to position a transducer 
relative to a selected disk in a disk memory system is well known in the 
prior art. Prior art linear actuators consist of a frame to which a 
magnetic structure is attached. The magnetic structure generates a 
plurality of magnetic flux lines. A coil is positioned within the magnetic 
structure and is subjected to the magnetic flux line. As is well known in 
the art, by passing current through the coil, a force is created which 
propels the coil in its to and fro motion. Attached to the coil is a 
carriage assembly to which the head is attached so as to access (that is 
read or write) data on a selected disk in the disk pack. 
In order to restrict the motion of the coil and its carriage assembly to a 
linear path, an elongated precision rod is mounted to the frame and 
relative to the magnetic structure. The precision rod is generally aligned 
in precise parallelism with the desired path of movement of the magnetic 
transducer. Sleeve bearings are used singly or in combination with other 
forms of sliders to propel the carriage assembly along the precision rod. 
Invariably the rod is supported at its two extremeties only, which result 
in unusual flexing of the rod as the carriage is transported to and fro. A 
more detailed discussion of the structure of the prior art linear 
actuators may be found in U.S. Pat. Nos. 3,587,075 and 3,899,699. 
Although the prior art linear actuators work satisfactorily for its 
intended purpose, these actuators are plagued by several drawbacks. Before 
addressing the drawbacks of the prior art actuators, it is worthwhile 
noting that in order to control the accessing of data from a disk pack the 
carriage assembly is controlled by a servo loop. As is well known to those 
skilled in the art, the servo loop for a linear actuator is functional or 
effective over a given frequency range. The lower end of the frequency 
range is called the cross-over point. The frequency range in turn is 
related to the track density of the recorded data. The trend in present 
day disk storage systems is to provide high performance storage systems. 
High performance storage system means a storage system in which the track 
density is relatively high, for example, in the range of 100 tracks per 
inch. 
Since the functional frequency range for the controlling servo loop is 
inter-related with data density, the higher the data density the higher is 
the functional frequency range for the associated controlling servo. 
Alternately, a storage system in which the functional frequency range of 
the associated servo is narrow unnecessarily restricts the data density of 
the system. 
In view of the inter-relationship between data density and the functional 
frequency range of the controlling loop the optimum condition is to have a 
servo loop which is effective to control over a relatively wide frequency 
range. 
One factor which adversely affects the controlling servo (which may be 
closed loop) of a linear actuator is the resonant frequencies of the 
actuator. Particularly, the resonant frequency affects the functional 
frequency range of the servo. The resonance in the actuator is transferred 
to the transducer which rides on the carriage of the actuator. Because the 
transducer is in the controlling servo loop, an instability is introduced 
into the controlling servo loop. The net result is that the servo cannot 
control the movable assembly so that the transducer can faithfully follow 
a selected track on a target disk. 
Whenever the resonant frequency is relatively close to the functional 
frequency range of the controlling servo, a plurality of servo errors is 
generated. The errors adversely affect system throughput and system 
reliability. The resonant frequency is a direct result of mechanical 
vibration in the actuator. Although all actuators will vibrate at some 
frequency, the desirable approach is to design the actuator so that it 
will resonate at a relatively high frequency so that the resonant 
frequency of the actuator does not affect the functional frequency range 
of the controlling servo. 
In order to maintain system reliability, if the linear actuator has a 
relatively low resonance frequency, then the functional frequency range 
for the controlling servo is invariably forced to be lower than the 
resonant frequency of the system. In view of the above discussion, this 
condition implies a low density storage disk system, an undesirable 
result. 
Returning now to problems affecting prior art actuators, and in particular, 
actuators for use with flexible disk storage systems perhaps the most 
pressing problem is that these actuators have a relatively low natural 
resonant frequency; typically from 100 to 500 hertz. Due to the low 
resonant frequency response, the previously described defects which are 
associated with linear actuators having low resonant frequency response 
are attributes of the prior art actuators. This being the case, the prior 
art actuators are unsuitable for use in high density flexible disk storage 
systems. 
One of the contributing factors for the low frequency response of the prior 
art actuator is the fact that the precision rod which guides the carriage 
assembly is susceptible to unusual flexing. As stated previously, the 
precision rod is only supported at its two ends with no support along its 
length. 
Another factor stems from the fact that the slider side which rides against 
the precision rod creates an unusual amount of frictional resistance to 
motion. 
Another problem which affects the prior art actuator is that the actuators 
do not lend themselves to modular design. One important characteristic of 
a modular design is that functional elements (e.g., the carriage assembly, 
etc.) hereafter called Field Replaceable Unit (FRU), can be changed in an 
actuator without interrupting the actuator's alignment with its associated 
disk storage system. 
The non-modularity characteristic of prior art actuators stems from the 
fact that the design philosophy in these actuators requires the center of 
thrust or center of motive force (supplied by the coil) must coincide with 
the center of mass of the carriage assembly. This design philosophy 
requires a more complicated design which does not lend itself to 
modularity. 
Associated with the non-modularity defects of the prior art is the further 
defect that the prior art actuators cannot be satisfactorily arranged so 
that a plurality of these actuators access a common disk storage system. 
One of the restraints which is necessary for plural accessing is that the 
separation be minimum between the carriage assembly, including the head 
arm with transducer thereon. To satisfy this restraint, it is necessary 
that at least one side of the actuator, preferably next to the carriage 
assembly and along the direction of actuator stroke, be a reference side, 
preferably flat. This appears to be the most auspicious method of 
designing the actuator so that a second actuator with a characteristic 
side similar to the previously described side can be placed adjacent to 
each other without unnecessary interference with one another. However, due 
to the complicated design of prior art actuators, the minimum separation 
requirement cannot be realized and, therefore, said actuators cannot be 
used for plural accessing. 
Still another problem which is associated with the prior art actuators is 
that the coil which produces the motive force for moving the carriage 
assembly is not self supporting. A self supporting coil is one which does 
not require a bobbin to support it when it is used in a linear actuator. 
Almost invariably, the coil used in prior art actuators are wound on a 
coil supporting member generally called a bobbin. The bobbin and coil are 
then positioned within the air gap formed by the actuator's structure and 
are used to position the head assembly. Several undesirable results are 
associated with these coil bobbin assemblies. Firstly, the mass of the 
moveable assembly is increased. With more mass, more current is needed for 
driving the actuator. More current increases the cooling requirements and 
cost of the actuator. Probably more important is the fact that the bobbin 
tends to reduce the natural resonance frequency of the actuator and, as 
stated previously, adversely affects the overall operation of the 
actuator. 
SUMMARY OF THE INVENTION 
The above mentioned prior art problems are solved by the linear actuator of 
the present invention in which the natural resonant frequency is 
substantially higher than was heretofore been possible, a flat reference 
side which significantly minimizes the separation between the head arm 
when two actuators are used to access one storage file, a suspension 
system with lower frictional resistance than was heretofore possible for 
moving the carriage assembly, and a structure that can be easily removed 
and replaced. 
In one feature of the invention the linear actuator includes an elongated 
stator assembly and a movable armature assembly suspended for motion 
within said stator assembly. 
The stator assembly includes a frame having two end sections and two 
L-shaped side sections. Each of the end sections is attached by mounting 
means to one end of an integral elongated magnetic flux return path and 
precision guide rod combination. The spacing between the two end sections 
defines the stroke of the actuator. Positioned on each side of the 
combined flux return path and in spaced alignment therewith, is a pair of 
elongated permanent magnets. The magnets are connected by mounting means 
to the L-shaped side sections. Two spacer means are positioned between the 
elongated magnets and its associated L-shaped side section. The L-shaped 
side sections and the spacer means are connected to the end sections to 
form a uniform and sturdy structure. The L-shaped side sections are 
positioned so that one surface of the shortest dimension of the L operates 
as a guide rail for the carriage assembly while the other surface extends 
into a common plane to define a flat reference side or reference surface 
for the actuator. The coil assembly which propels the actuator carriage 
assembly is offset from the flat reference surface. 
In another feature of the invention a third elongated permanent magnet is 
attached to a spacer means and a third side section. The combination is 
then mounted to the end sections of the actuator with the third elongated 
permanent magnet positioned in spaced alignment with the third side of the 
return path. 
In still another feature of the invention, limiting means are mounted to 
the end sections of the actuator and function to stop the backward and 
forward motion of the armature assembly. 
The armature assembly includes a head support platform movably connected to 
an offset self supporting coil. The coil is positioned in the space 
defined by the return path and elongated magnets. This space is called the 
air gap. The support platform is supported for motion along the precision 
rod and guide rails by a six ball bearing suspension system. Four of the 
bearings ride against the precision rod while the other two bearings 
preload the armature assembly so as to prevent rotation and ride against 
the guide rails. 
In one feature of the invention one of the preload bearings is rigidly 
fixed to the armature assembly while the other preload bearing is mounted 
on a flexible member which is displaced from the support platform and 
creates the preloading. 
In still another feature of the invention a linear tachometer strip is 
mounted to the armature assembly and is transported past a light 
emitting/light receiving source from which positional information is 
obtained. 
In still another feature, electrical connectors are connected and operable 
to deliver electrical energy to the coil. 
The foregoing and other features and advantages of the invention will be 
apparent from the following more particular description of preferred 
embodiments of the invention, as illustrated in the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings, the preferred embodiment of the Linear 
Actuator or Linear Motor is shown in FIG. 1. The linear actuator comprises 
two basic parts: an elongated stator assembly 10 (FIGS. 1 and 2) and a 
movable armature assembly 12 (shown in FIGS. 1 and 3). The armature 
assembly is slideably movable along the length of and relative to the 
stator assembly 10. In order to simplify the detailed description of the 
linear actuator, like members in the drawings will be identified by common 
numeral. 
Referring now to FIGS. 1 and 2, the elongated stator assembly 10 includes 
two end plates, 14 and 17 respectively, only one of which is shown 
completely in the drawings and identified as end plate 14. Since the other 
end plate 17 is substantially identical to end plate 14, the described 
characteristics of end plate 14 should be attributed to the other end 
plate 17. End plate 14 has a substantially rectangular shape with four 
sides, 16, 18, 20 and 22 respectively. The end plate has two surfaces, 
only one of which is shown in the drawings and identified as 24. The other 
surface is opposite to surface 24. The two end plates are positioned in 
space alignment so that at least one of the flat surfaces faces the other. 
The distance between the flat surfaces of the end plates define the stroke 
of the linear actuator. Although it is within the skill of the art to use 
the teaching disclosed herein and design actuators having various strokes 
without departing from the spirit and scope of this invention, in the 
preferred embodiment of this invention the stroke of the actuator is 
approximately four inches. Stated another way, the effective distance 
beteen the facing surfaces of the two end plates are substantially 
equivalent to six inches. Semi-circular notches 26 and 25 respectively, 
are machined into sides 18 and 15 of the end plates. An elongated rod 28 
is seated in the semi-circular notch. A notched detent 27, hereinafter 
called latching means, is fabricated on the precision rod. The latching 
means co-acts with the ball bearing to lock the carriage assembly. Stated 
another way, when current is not applied to the bobbinless offset coil 
120, a pair of the ball bearing suspension rests within the latching 
means. The coaction between the ball bearings and the latching means 
prevents the carriage assembly from moving. The carriage remains in the 
rest position until current is applied to the coil. 
In addition to the latching function, the latching means identifies the 
beginning of the stroke of the actuator. The rod is seated in the 
semi-circular notch so that one of its curved surfaces, 30, protrudes 
slightly above sides 18 and 15 respectively. The rod need not be 
completely circular. It may be a semi-circle, flat or other geometric 
shape. However, in the preferred embodiment of the invention the surface 
which protrudes above sides 18 and 15 respectively is curved and smooth. 
As will be explained subsequently, the elongated rod 28 operates to 
restrict the motion of the movable armature assembly in a linear path. In 
the preferred embodiment of the present invention, the elongated rod is 
fabricated from a nonmagnetic material, preferably stainless steel, with a 
smooth finish on curved surface 30. With a smooth surface finish, the 
frictional resistance which is offered to the rolling mechanism riding on 
the curved surface is minimal. Also, by using nonmagnetic material to 
fabricate the guide rod contamination, due to magnetically attracted 
particles, is reduced. 
Referring to FIG. 2 for a moment, the stator assembly further includes an 
elongated bar 32. A channel 34 is fabricated within the elongated bar. The 
elongated bar is then securely attached along the elongated channel to 
elongated rod 28. The two ends of the elongated bar, only one of which is 
shown in the figure and identified as end 36, are rigidly attached to the 
facing surface of end plates 14 and 17 respectively. The attached 
elongated bar serves two purposes. Firstly, the bar serves as a continuous 
support and minimizes flexing of the elongated rod. By reducing the 
flexing the resonance frequency of the actuator is increased. Stated 
another way, by supporting the elongated bar along the entire length 
increases the stiffness of the bar and hence increases the natural 
frequency at which the actuator resonates. Secondly, the bar acts as the 
magnetic flux return path of the magnetic circuit of the actuator. In 
order for the bar to operate as a magnetic flux return path, it is 
manufactured from a ferro-magnetic material with relatively high flux 
passing characteristics, for example, soft iron. A thin layer or film of 
copper is plated onto the surface of the elongated bar. The layer is 
referred to in the art as a shorted turn. The layer of copper allows for a 
quicker current rise in the coil. The result is that the response 
characteristic of the actuator is improved. It is worthwhile noting that 
although one means for supporting the elongated rod is disclosed herein, 
it is within the skill of the art to fabricate other types of support 
means extending along the length of the elongated rod without departing 
from the scope of the present invention. 
Still referring to FIG. 2, magnetic flux generating means 38, 40 and 42 are 
positioned in proximity to elongated bar 32. In the preferred embodiment 
of this invention the flux generating means are a plurality of elongated 
rectangular slab magnets. The magnets are positioned in spaced 
relationship with the elongated bar 32 so as to define air gap 44 between 
the bar magnets and the elongated bar 32. As will be explained 
subsequently, a coil assembly is positioned within the air gap. By 
supplying current to the coil and the coil co-acting with the flux line 
generated from the elongated magnets a force is created which propels the 
armature assembly between end plates 14 and 17 of the linear actuator. 
Referring again to FIG. 1 for the moment, end plate 14 is fastened to one 
end of elongated bar 32 (FIG. 2) by fastening means 46 and 48 
respectively. A similar pair of fastening means (not shown) attaches end 
plate 17 to the other end of the elongated bar. In the preferred 
embodiment of this invention screws are used as the fastening means. By 
torquing the screws in a clockwise direction, the end plates are securely 
attached to the elongated bar and by torquing the screws in a 
counterclockwise direction the end plates are loosened and allow the end 
plates to be removed from the elongated bar. 
From the above description it can be seen that the magnetic structure 
circuit for the actuator includes the flux generating means 38, 40 and 42, 
the end plates 14 and 17, the spacer means 58, 60, and 62 and possibly the 
L-shaped side plates 75 and 76. 
In order to stop the movable armature assembly, stopping means 50 is 
securely attached by fastening means 52 to one end of elongated rod 28. A 
similar stopping means 51 is attached to the other end of rod 28. Of 
course, the stopping means may be attached to the end plates. The stopping 
means is so positioned that it extends slightly above curved surface 30. 
As will be explained subsequently, as the armature assembly traverses the 
elongated rod in a to and fro motion, a protruding notch on the bottom 
surface of armature assembly coacts with the stopping means to stop the 
movable assembly at the end or beginning of the stroke of the actuator. In 
the preferred embodiment of this invention, the stopping means is 
fabricated from a laminate comprised of a resilient member, for example, 
hard rubber 54 and a relatively hard backing member, for example, steel 
backing 56. The stopping means is mounted so that the hard rubber portion 
interfaces the steel backing and the end plates or elongated rod. By 
mounting the stopping means in this manner the hard rubber acts as a shock 
absorber when the movable assembly hits the stopping means. In the 
preferred embodiment of this invention, a hole is bored in the laminated 
stopping means and the end plate and a screw is used for mounting the 
stopping means. 
Referring again to FIGS. 1 and 2, the flux generating means 38, 40 and 42 
are mounted to the elongated spacer means 58, 60 and 62 respectively. In 
the preferred embodiment of this invention, the spacer means are elongated 
rectangular pieces of magnetic material, for example, steel. Since the 
relationship between the flux generating means and the elongated spacer 
means are the same for each laminate, only one of these laminates, for 
example flux generating means 42 and elongated spacer means 62, will be 
described. 
In FIG. 1 elongated spacer means 62 is attached at its two ends to end 
plates 14 and 17 respectively. Flux generating means 42 (FIG. 2) is 
shorter than the elongated spacer means and is fitted against the central 
section of the spacer means so that the ends 64 and 66, respectively, do 
not align with the end of the spacer means. Stated another way, a space or 
void 68 and 70 respectively exist around the end of elongated bar 32 (FIG. 
2) between surfaces 72 and 74, respectively, of the end plates and the 
ends of the flux generating means 38, 40 and 42 respectively. As will be 
explained subsequently, the stopping means stops the movable assembly so 
that the coil aligns with the ends of the flux generating source when the 
movable assembly is stopped by either of the two stopping means. 
Still referring to FIGS. 1 and 2, side plates 75 and 76, respectively, are 
attached via a plurality of fastening means 78, 80, 82 and 84 respectively 
to spacer means 60. A plurality of similar fastening means (not shown) are 
used to attach side plate 76. As is evident from the drawings, side plate 
75 and side plates 76 are identical and, therefore, only side plate 75 
will be discussed in detail. The side plate is machined from a common 
piece of metal into a L shaped member. A plurality of recessed holes 86, 
88, 90 and 92, respectively, are machined along the sides of the member. 
The holes may be positioned so that recess hole 78 and 84 aligns with the 
end plates. With this alignment, when screws 78 and 84 are fitted into 
holes 86 and 92 respectively, the side plate is securely fastened to the 
end plates to form a unified structure. In FIG. 1 holes 78 and 84 do align 
with the end plates. The inner recess surface of recess holes 86, 88, 90 
and 92 fits against the under surface of screw heads 78, 80, 82 and 84 
respectively and binds the side plates into a unified structure. A similar 
set of holes and screws are machined into side plate 76 and are operable 
to bond side plates 76 into the end plates and spacer means 62 (FIG. 2). 
The L shaped members 75 and 76 respectively are mounted to the end plates 
of the actuator so that flat surfaces 98 and 100 respectively which are 
positioned on the small sides of the L shaped members 75 and 76, 
respectively, extend into a common plane beyond curve surface 30 at 
elongated rod 28. Stated another way, the L shaped side members are 
mounted to the sides of the end plate so as to be symmetrical to the 
elongated rod 28. The sides having the shortest dimension of the L shaped 
members 75 and 76, respectively, project into a common plane which is 
equidistance from the curved surface 30 along the length of the elongated 
rod 28. As will be explained subsequently, the sides of the L shaped side 
members having the shortest dimension herein referred to as rails 94 and 
96 operates with the curved surface of the elongated rod to form a linear 
track along which the movable armature assembly suspended on a plurality 
of ball bearings is propelled. 
Still referring to FIGS. 1 and 2, elongated hole 102, with stepped central 
portion 104 (FIG. 2) is machined into the side of side plate 75. If 
desired, a similar hole can be fabricated into side plate 76. Since both 
holes are positioned in the same manner and serve the same function, only 
hole 102 will be described. The hole 102 is so positioned that it aligns 
with the space generated between guide rails 94 and 96, respectively, and 
curved surface 30 on the guide rod. 
A light receiving means (not shown) is attached to side 31 of holder 
assembly 29. The light receiving means may be a photosensitive transistor. 
The holder assembly is mounted via fastening means 108 and 112 to the side 
plate. Likewise, a light emitting means (not shown) is mounted to side 
110. The light emitting means may be a light emitting diode (LED). The 
light emitting means is positioned in optical alignment with the light 
receiving means. The alignment between the light emitting and light 
receiving means is such that the beam which is radiated from the light 
emitting source falls on the light receiving means. However, when linear 
tachometer strip 116 on which a plurality of light and dark windows are 
positioned passes between the light receiving and light emitting source, a 
plurality of pulses are outputted which can be used to servo control the 
movable assembly. Since the use of an optical disk is well known in the 
art, a detailed description of the strip and its interaction with the 
light emitting/light receiving package will not be described any further. 
Referring now to FIGS. 1, 3 and 4, movable armature assembly 12 is shown. 
In FIG. 1 top surface 118 of the carriage assembly is shown while in FIG. 
3 the underside of the actuator which interfaces curved surface 30 when 
the actuator is assembled is shown. As was stated previously, the function 
of the movable armature assembly is to position a head/arm (not shown) 
which supports a magnetic transducer (not shown) relative to a selected 
disk within a disk pack. The movable armature assembly includes bobbinless 
coil 120, head support platform 122 and a suspension system which includes 
ball bearings 124, 126, 128, 130, 132 and 134. 
The coil in conventional actuators is wound on a bobbin and the bobbin and 
coil become part of the final assembly of the actuator. In 
contradistinction to the prior art coil assembly, the coil 120 which is 
used in the actuator, according to the present invention, does not have a 
bobbin. The term bobbinless coil, when used in this application, means 
that the coil is self supporting and does not require a bobbin when placed 
in the actuator according to the present invention. The coil may be formed 
in any desired cross section, for example, triangular, circular, etc. In 
the preferred embodiment of this invention, the coil is formed into a 
rectangular cross section. Not withstanding, the cross sectional 
configuration of the coil, it is necessary that the coil be designed with 
at least one straight side. As will be explained subsequently, the plane 
which is defined by the straight side attaches the coil to the head 
support platform so that the coil is cantilevered or offset from the 
straight side or the head support platform. Due to the cantilevered manner 
of attaching the coil to the head support platform, two of the actuators 
can be arranged in a back-to-back manner for dual accessing of a disk 
pack. Among the beneficial results which eminate from the cantilevered 
attaching of the coil to the head support platform is that the center of 
force which is generated by the coil for driving the head support platform 
and its attachment, for example, the head/arm and magnetic transducer, is 
offset from the center of mass of the head support platform. This allows 
for a modular design and, of course, the previously mentioned back-to-back 
attachment of dual actuators. 
Referring now to FIGS. 3 and 4, coil 120 is attached to coil holder 136. 
Although a plurality of coil holders may be used in the preferred 
embodiment of this invention, the coil holder is fabricated from a 
relatively light weight non magnetic material with a flat surface 138 and 
two raised rectangular end sections 145 and 147 respectively. Each of the 
raised rectangular end sections has a flat surface which projects into a 
common plane above flat surface 138. Channel 144 is fabricated on the 
underside of the coil holder. In fabricating the coil, the channel is 
fitted over a mandrel or some other coil form (not shown). The coil is 
then wound over the holder and mandrel. The coil is then saturated with 
epoxy. The mandrel is then withdrawn and the coil itself is a structural 
member affording self support. It is worthwhile noting that although a 
particular method is disclosed for fabricating the coil, it is within the 
skill of the art to devise other methods without departing from the scope 
of the invention. 
An alternative way of fabricating the coil is to wind the coil on the 
mandrel, saturate the coil with epoxy or some other material and then 
attach the coil to a coil holder. The coil holder, with the offset or 
cantilevered coil, is then fastened at the flat surface to head support 
platform 122. In the preferred embodiment of this invention, receiving 
holes 146, 148 and 150 are drilled into the raised end sections 145 and 
147 of the coil holder. Mounting screws 152, 154, and 156 (FIG. 1) are 
used to attach the coil holder to the head support means. 
In FIG. 1 the head support platform, as viewed from the top surface 118, is 
shown removed from the fixed section (that is the stator) of the actuator. 
In FIG. 3 the undersurface of the head support platform is shown. With 
reference to FIGS. 1 and 3, the head support platform is fabricated from a 
substantially elongated member. The top surface 118 of the head support 
platform is in the shape of a truncated cone with T shaped surface 158 
machined in one of the sloping surfaces of the cone. The T shaped surface 
is used for attaching a head arm (not shown) to the head support platform. 
The underside of the head support platform shown in FIG. 3 is 
substantially U shaped with base 160 and sides 162 to 164 respectively. 
Ball bearings support means 166 and 168 are fabricated on base 160 of the 
head support platform. The ball bearing support means are positioned at 
each end of the head support platform and project above the base of said 
platform. The ball bearing support means are fabricated towards the center 
of the base of the head support platform. The ball bearing support means 
are positioned in spaced alignment and are separated by a predetermined 
distance taken along the length of the head support platform. The distance 
is equivalent to the length of the coil holder measured in a direction 
along arrow 170 (FIG. 4). With this relationship as shown in FIG. 3, the 
raised end portion 145 and 147 of the coil holder fits snugly against ball 
bearing support means 166 and 168 respectively. 
Still referring to FIGS. 1 and 3, strip holder 172 is fastened to sides 164 
of the head support platform. As was stated previously, linear tachometer 
strip 116 is mounted to the strip holder. As the head supported platform 
is transported in a path parallel to arrow 174, it intercepts the light 
beam between the light emitting source and the light receiving source and, 
as a result, controlled pulses are outputted. The pulses are used for 
servoing (that is to position the head relative to a track on a selected 
disk). Although a plurality of means may be used for mounting the strip 
holder to the head support platform, in the preferred embodiment of this 
invention a plurality of pins, for example, 176 and 178, are fabricated on 
surface 180 on side 172. The pins are positioned so as to project above 
surface 180. The strip holder is then attached to the pins. 
The ball bearing support means are further characterized by a truncated 
trapezoidal shape. The sloping sides of the trapezoid (for example, sides 
182, 184, 186 and 188) are inclined (that is angled) to base 160 while 
flat surfaces 140 and 142, respectively, interface the curved surface 30. 
It is worthwhile noting that base 160 is parallel to a plane which runs on 
the apex of curved surface 30. This being the case, and as will be 
explained subsequently, the ball bearings which ride against curved 
surface 30 are at an angle to said surface. Stated another way, the ball 
bearings are angled relative to elongated rod 28. Four pins are rigidly 
mounted to the four sloping sides of the ball bearing support means, 
respectively. In FIG. 3, two of the rigidly mounting pins 190 and 192, 
respectively, are shown. Four of the ball bearings are mounted to the 
mounting pins. 
Referring now to FIGS. 3 and 6, the suspension system which propels the 
movable assembly of the tachometer along curved surface 30 and guide rails 
94 and 96 are shown relative to the stator of the actuator. In normal 
operation, the movable assembly travels in a linear path perpendicular to 
the plane of the paper. As was stated previously, the suspension includes 
six ball bearings. Four of the ball bearings, 124, 126, 130 and 132, 
respectively, are mounted via rigid mounting pins to ball bearing support 
means 166 and 168 respectively. The four ball bearings are angled to the 
curved surface of the elongated rod 28 and rides against said surface. 
Ball bearings 128 and 134 ride against rails 94 and 96 respectively, and 
prevent the movable assembly from rotational movement. Stated another way, 
the movable assembly of the actuator is preloaded against rail 94 and 96 
by ball bearing 128 and 134, respectively. Ball bearing 128 is mounted to 
side 164 of the head support platform by a rigid pin 194. Ball bearing 134 
is mounted on flexible member 198, hereinafter called cantilever beam 198. 
The cantilever beam is fastened into side 162 of the head support 
platform, in spaced alignment with steel pin 194 and ball bearing 128. The 
preloading of the carriage assembly is achieved by the cantilevered beam 
and ball bearing 134. The pre-load bearing is positioned just about the 
mid point of the elongated head support form. 
Current to coil 120 is supplied by electrical conductor 200 (FIG. 1). The 
conductor is connected to terminal 202. Hole 204 is fabricated in the head 
support platform and the conductors are threaded through said hole to 
attach to the coil. This completes the description of the movable 
assembly. 
As was stated previously, by designing the coil to have at least one flat 
surface which is mounted to the carriage assembly and by designing the 
side plates so that the actuator has at least one flat surface displaced 
from the plane of the precision rod, two of the actuators designed 
according to the above teaching can be mounted in a back-to-back fashion 
to access data from a common disk pack. In FIG. 5 a configuration which 
allows for back-to-back operation of two transducers is shown. As was 
stated previously, one requirement is that the separation distance 206 be 
minimum so that the head/arms 208 and 210 can be relatively close. In FIG. 
5, actuator 212 and actuator 214 are mounted on a support means 216. 
Mounted to the carriage assemblies are head/arms 208 and 210. Magnetic 
transducers 222 and 224 are attached to the head/arms. Magnetic storage 
system 226 is positioned on support means 228 in alignment with the 
actuators. The magnetic storage system includes backing plates 230 and 232 
respectively. Separator means 234 divides the storage system into two 
compartments. Each compartment has a plurality of disks and are aligned so 
that either of the two magnetic transducers can access a selected track on 
a selected disk as the magnetic storage system rotates about an axis 
parallel to line 236. By designing a system using two actuators, the 
reliability, availability and service-ability of the overall system is 
improved. This is so because if one of the actuators is defective then 
data can still be obtained from the storage system with the non defective 
transducer. 
Referring now to FIG. 7, an alternate embodiment of a linear actuator, 
according to the teaching of the present invention, is shown. The linear 
actuator includes an elongated stator assembly 302 and a movable assembly 
304. The movable assembly coacts with the stator assembly in a 
conventional manner to effectuate to and fro motion. 
The stator assembly includes a frame, 306, fabricated from a U-shaped 
member. The frame is fabricated from a casting. End plate 308 is fastened 
at the open side of the U-shaped member by fastening means 310. An 
elongated magnetic structure, including a plurality of elongated magnets 
(not shown) but substantially similar to the magnetic structure previously 
described, is connected to the inner surfaces of the frame. Precision rod 
28 is fitted to elongated bar 32 and the assembly, (i.e., the elongated 
bar and the precision rod) is mounted to the frame in spaced alignment 
with the permanent magnets. The assembly functions as a flux return path 
to return flux generated by the permanent magnets. The relationship 
between the permanent magnets and the flux return path is such that an air 
gap (not shown) is defined therebetween. Stop crash means 312 is mounted 
to the frame and functions to limit the stroke of the actuator in its to 
and fro motion. 
Still referring to FIG. 7, guide rails 314 and 316, respectively, are 
attached to the frame. As was explained previously, two of the ball 
bearings (only one of which is shown in the figure and is identified as 
element 318) which forms the suspension mechanism for the movable assembly 
roll against the rails and prevents the movable assembly from rotating. 
Guide rail 316 is attached to the frame by fastening means 320 and 322. A 
light emitting/light receiving assembly 324 is mounted to the frame. The 
assembly 324 is fitted with a slot through which linear tachometer strip 
326 is transported. Guide rail 314 is resiliently attached to the frame. 
By resiliently attaching the guide rail to the frame, the anti-rotational 
characteristic of the assembly which is achieved by the coaction between 
the guide rails and the rolling ball bearings is enhanced. 
Although it is within the skill of the art to devise a plurality of ways 
for resiliently attaching guide rail 314 to the frame in the preferred 
embodiment of this invention, the rail is spring loaded relative to the 
frame. Guide rail 314 is attached at both ends to support members 328 and 
330 respectively. The support members are mounted to the frame so that the 
guide rail is offset relative to one of the long sides of the U-shaped 
frame. A pair of holes, 332 and 334, respectively, are bored into the 
guide rails. A pair of thread screws, 336 and 338, are fitted with 
cylindrical springs 340 and 342 respectively. The combination (i.e., 
spring and screw), is fitted into holes 332 and 334 respectively. The fact 
that the rail is cantilevered relative to the frame and the fact that the 
screws are spring loaded creates the preload for the carriage assembly. By 
torquing the screws, the preload on the carriage assembly is adjustable. 
Still referring to FIG. 7, the movable assembly includes a support platform 
344. Coil assembly 346 is wound onto the platform. The coil assembly is 
cantilevered from the support platform into the air gap defined by the 
permanent magnets and the flux return path. The platform and coil assembly 
is supported by a ball bearing suspension system. The suspension system 
includes six ball bearings. Four of the six ball bearings are 
substantially similar to those previously described. The four ball 
bearings are mounted to the support platform in groups of two, which are 
arranged at an angle relative to the precision rod 28. The other two ball 
bearings are mounted to the support platform and prevent the assembly from 
rotating. Since the two ball bearings, which limit rotation of the 
assembly are identical, only one of said ball bearings identified as 
element 318 will be described. A pin, 348, is fitted to the support 
platform and the ball bearing is mounted on the pin. The ball bearing then 
rolls against rail 314 to preload the assembly. Linear tachometer strip 
326 is fitted to the carriage and is transported therewith. 
OPERATION 
When the linear actuator, according to the present invention, is assembled, 
for example as shown in FIG. 6, the permanent magnets 38, 40 and 42 
radiate magnetic flux which crosses the air gap between elongated bar 32 
and the magnets through coil 120 and into the elongated bar. The magnetic 
flux then exits the bar at its two ends then travels to end plates 14 and 
15 respectively through the spacer means and back into the permanent 
magnet to complete the closed magnetic loop. In the meantime, electrical 
current is supplied via terminal 202 through conductor 200 to the coil. 
The interaction between the magnetic field created by the permanent 
magnets and the field created by the current carrying coil generates a 
force which propels the movable assembly along its linear path defined by 
elongated rod 28 and guide rails 94 and 96 respectively. Stopping means 50 
interacts with ball bearing support means 166 and 168 respectively to stop 
the movable assembly at the end of the actuator stroke. 
When a linear actuator is fabricated in accordance with the teaching 
disclosed herein, the following advantages are observed: 
The physical arrangement allows for back to back operation of two actuators 
in close proximity. This allows two actuators to access a single disk file 
simultaneously. 
The natural frequency at which the actuator resonates is relatively high. 
The fact that the coil assembly is offset or cantilevered allows for the 
head-arm-carriage assembly to be easily removed for servicing while 
maintaining the alignment of the actuator with a disk file storage system. 
While the invention has been particularly shown and described with 
reference to the preferred embodiments thereof, it will be understood by 
those skilled in the art the various changes in form and detail in 
addition and/or subtraction to various embodiments specifically mentioned 
herein, may be made without departing from the spirit and scope of the 
invention.