Abstract:
The present disclosure is directed to a micromachined rotary actuator constructed of a central portion and an outer portion at least partially surrounding the central portion and separated from the central portion by an in-plane gap. A plurality of arms are each connected at one end to the central portion and at another end to the outer portion so as to span the in-plane gap. The arms are constructed of a plurality of horizontally stacked materials positioned to enable the arms to bend in-plane when heated. Conductors are positioned within the actuator for heating the arms. Because of the rules governing abstracts, this abstract should not be used to construe the claims.

Description:
BACKGROUND 
     The present disclosure is directed generally to micromachined devices, and in a particular embodiment, to a micromachined rotary actuator capable of being used to precisely position a transducer head within a disk drive. 
     Various micro-actuation techniques such as electrostatic, thermal, piezoelectric, or magnetic have been demonstrated. Some of the early electrothermal actuator designs are based on the bimorph effect, which relies on the difference of thermal expansion coefficients between two adjacent layers on the device. By heating these layers, a bending moment is created. However such actuators typically produce deflection in the direction normal to the substrate. 
     U.S. Patent Publication No. 2007/0103029 entitled Self-Assembling MEMS Devices Having Thermal Actuation is directed to a method for designing MEMS micro-movers, particularly suited for, but not limited to, CMOS fabrication techniques, that are capable of large lateral displacement for tuning capacitors, fabricating capacitors, self-assembly of small gaps in CMOS processes, fabricating latching structures, and other applications where lateral micro-positioning on the order of up to 10 μm, or greater, is desired. In self-assembly, motion is induced in specific beams by designing a lateral effective residual stress gradient within the beams. The lateral residual stress gradient arises from purposefully offsetting certain layers of one material versus another material. For example, lower metal layers may be side by side with dielectric layers, both of which are positioned beneath a top metal layer of a CMOS-MEMS beam. In electro-thermal actuation, motion is induced in specific beams by designing a lateral gradient of temperature coefficient of expansion (TCE) within the beams. The lateral TCE gradient is achieved in the same manner as with self-assembly, by purposefully offsetting the lower metal layers with layers of dielectric with respect to the top metal layer of a CMOS-MEMS beam. A heater resistor, usually made from a CMOS polysilicon layer, is embedded into the beam or into an adjacent assembly to heat the beam. When heated, the TCE gradient will cause a stress gradient in the beam, resulting in the electro-thermal actuation. 
     Turning now to a specific application, the servo system of a disk drive has two primary operations, namely track seek and track follow. Track seek is the operation of moving the head (containing the read transducer and the write transducer) from one data track to another, during which the voice coil motor (VCM) actuator may rotate through its full stroke of 20 to 30 degrees, if one track is at the inner diameter and the other track is at the outer diameter of the disk. After the completion of a track seek, the track follow operation maintains the read or write transducer close to the center of the data track. Challenges to keeping the transducer at the data track center include repeatable and non-repeatable runout of the data track, shock and vibration disturbances, windage disturbances (aerodynamic drag forces arising from laminar and turbulent air flow), and noise in the feedback measurements and electronics. 
     Head skew is the phenomenon where the longitudinal axis of a read/write head on a disk drive and the tangent of the data track, which the head is reading or writing, are not parallel. That is, the angle between the data track and the head axis is not zero. Head skew degrades the performance of recording in disk drives and is particularly troublesome for disk drives employing perpendicular recording technology, where long narrow poles are desired but cannot be used because they write tracks that are too wide when skewed. Due to curvature of the track the magnitude of the skew is generally less than one-half of the full stroke of the VCM, but can be on the order of 10 degrees. 
       FIG. 1  illustrates how head skew can be undesirable, particularly in perpendicular recording where head skew results in a wider track width than if the skew were always zero. Shown in  FIG. 1A  is the position of the head as it would occur at the inner diameter (ID) of the track. Shown in  FIG. 1B  is the position of the head as it would occur at the middle diameter (MD) of the track and in  FIG. 1C  as it would occur in the outer diameter (OD) of the track. The need exists for a method and apparatus for eliminating or reducing head skew in disk drives. 
     SUMMARY 
     The present disclosure is directed to a micromachined rotary actuator comprising a central portion and an outer portion at least partially surrounding the central portion and separated from the central portion by an in-plane gap. A plurality of arms are each connected at one end to the central portion and at another end to the outer portion so as to span the in-plane gap. The arms are comprised of a plurality of horizontally stacked materials positioned to enable the arms to bend in-plane when heated. Conductors are positioned within the actuator for heating the arms. 
     The disclosed micromachined rotary actuator is suitable for rotating the head assembly of a hard drive about an axis parallel to the axis of the VCM to compensate for head skew. Both skew compensation and high bandwidth control of the position of a hard disk head assemble can be achieved. The stroke of the actuator in one embodiment is on the order of ±1 to 5 degrees, enough to compensate for the head to track skew in a disk drive. In its high bandwidth mode, the stroke is on the order of 100 nanometers operating at a bandwidth of several kilohertz. Those, and other advantages and benefits will become apparent from the description below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For the present disclosure to be easily understood and readily practiced, the present invention will now be described, for purposes of illustration and not limitation, in connection with the following figures, wherein: 
         FIGS. 1A ,  1 B, and  1 C illustrate how head skew can be undesirable, particularly in perpendicular recording where head skew results in a wider track width than if the skew were always zero; 
         FIG. 2  is a plan view of the primary internal components of a disk drive of the type in which the present invention may be employed; 
         FIG. 3  is a functional block diagram of a disk drive control system employing the teachings of the present disclosure; 
         FIG. 4  is a simplified block diagram illustrating an actuator constructed according to the teachings of the present disclosure located between the slider/head assembly and the flexure of an actuator arm; 
         FIG. 5A  is a plan view looking down and  FIG. 5B  is a cross-sectional view along the line B-B in  FIG. 5A  of one embodiment of a micromachined rotary actuator constructed according to the teachings of the present disclosure; 
         FIG. 6  and  FIG. 7  are simplified block diagrams illustrating two possible orientations for a rotary actuator constructed according to the teachings of the present disclosure; 
         FIG. 8A  is a plan view looking down on an arm of one embodiment of an actuator constructed according to the teachings of the present disclosure,  FIG. 8B  is a sectional view taken along the line B-B in  FIG. 8A , and  FIG. 8C  is a sectional view taken along the line C-C in  FIG. 8A ; 
         FIG. 9A  is a plan view looking down on one embodiment of an arm of an actuator constructed according to the teachings of the present disclosure before release and illustrating the forces within the arm,  FIG. 9B  illustrates the same arm after release; 
         FIGS. 10A and 10B  are plan views looking down on one embodiment of a single arm of an actuator constructed according to the teachings of the present disclosure before and after heating, respectively. The labeling in  FIG. 10A  indicates relative expansions that will happen upon heating to produce the deflection in  FIG. 10B ; 
         FIGS. 11A and 11B  are plan views looking down on one embodiment of a micromachined rotary actuator constructed according to the teachings of the present disclosure before and after heating of the plurality of arms, respectively; 
         FIG. 12  is a plan view looking down on one embodiment of a micromachined rotary actuator constructed according to the teachings of the present disclosure which is experiencing lateral displacement; 
         FIG. 13  is a cross-section view of one embodiment of a micromachined rotary actuator constructed according to the teachings of the present disclosure which is experiencing axial displacement; 
         FIG. 14  shows three orthogonal beam deflection modes that can be analyzed during the design of the arms of a micromachined rotary actuator constructed according to the teachings of the present disclosure; 
         FIG. 15  illustrates some typical stiffness numbers for a micromachined rotary actuator constructed according to one embodiment of the present disclosure; 
         FIG. 16  illustrates lateral displacement under a shock of 300 g vs. torsional stroke for a micromachined rotary actuator constructed according to one embodiment of the present disclosure. Also shown is the resonant frequency for lateral displacement for one embodiment; 
         FIG. 17  illustrates axial displacement under a one gram load vs. torsional stroke for a micromachined rotary actuator constructed according to one embodiment of the present disclosure; 
         FIG. 18  is a plan view of another embodiment of a micromachined rotary actuator constructed according to the teachings of the present disclosure and having radially aligned arms attached at an angle; 
         FIG. 19  is a plan view of another embodiment of a micromachined rotary actuator constructed according to the teachings of the present disclosure and having radially aligned arms attached perpendicularly; 
         FIG. 20  is similar to  FIG. 19 , but the arms are trapezoidal in shape; 
         FIG. 21A  is plan view illustrating another embodiment of a micromachined rotary actuator constructed according to the teachings of the present disclosure and having segmented heaters to accomplish multiple motions shown in  FIGS. 21B  (before heating) and  21 C (after heating). The motion shown in  FIG. 21C  is in addition to rotary motion not shown in  FIG. 21C ; 
         FIG. 22  is a simplified diagram illustrating a possible interconnect routing for data and control signals; and 
         FIG. 23  is plan view illustrating another embodiment of a micromachined rotary actuator constructed according to the teachings of the present disclosure and having internal position and temperature sensing. 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG. 2 , a disk drive  100  is illustrated as one example of the type of disk drive in which an electrothermal actuator for compensating for head skew disclosed herein may be used. The disk drive  100  includes a base  102  to which various components of the disk drive  100  are mounted. A top cover  104 , shown partially cut away, cooperates with the base  102  to form an internal, sealed environment for the disk drive  100  in a conventional manner. The components include a spindle motor  106  which rotates one or more disks  108  at a constant high speed. Information is written to and read from tracks on the disks  108  through the use of an actuator assembly  110 , which rotates during a seek operation about a bearing shaft assembly  112  positioned adjacent the disks  108 . The actuator assembly  110  includes a plurality of actuator arms  114  which extend towards the disks  108 , with one or more flexures  116  extending from each of the actuator arms  114 . Mounted at the distal end of each of the flexures  116  is a transducer head  118  which includes an air bearing slider  117  enabling the head  118  to “fly” in close proximity above the corresponding surface of the associated disk  108 . 
     During a seek operation, the track position of the heads  118  is controlled through the use of a voice coil motor (VCM)  124 , which typically includes a coil  126  attached to the actuator assembly  110 , as well as one or more permanent magnets  128  which establish a magnetic field in which the coil  126  is immersed. The controlled application of current to the coil  126  causes magnetic interaction between the permanent magnets  128  and the coil  126  so that the coil  126  moves in accordance with the well known Lorentz relationship. As the coil  126  moves, the actuator assembly  110  pivots about the bearing shaft assembly  112 , and the heads  118  are caused to move across the surfaces of the disks  108 . The spindle motor  106  is typically de-energized when the disc drive  100  is not in use for extended periods of time. 
     A flex assembly  130  provides the requisite electrical connection paths for the actuator assembly  110  while allowing pivotal movement of the actuator assembly  110  during operation. The flex assembly  130  includes a preamplifier printed circuit board  132  to which head wires (not shown) are connected; the head wires being routed along the actuator arms  114  and the flexures  116  to the heads  118 . The printed circuit board  132  typically includes circuitry for controlling the write currents applied to the heads  118  during a write operation and a preamplifier for amplifying read signals generated by the heads  118  during a read operation. The flex assembly  130  terminates at a flex bracket  134  for communication through the base deck  102  to a disc drive printed circuit board (not shown) mounted to the bottom side of the disc drive  100 . 
     Referring now to  FIG. 3 , shown therein is a functional block diagram of the disc drive  100  of  FIG. 2 , generally showing the main functional circuits which are resident on the disc drive printed circuit board (not shown) and used to control the operation of the disc drive  100 . The disc drive  100  is operably connected to a host product  140  in a conventional manner. Control communication paths are provided between the host product  140  and a disc drive microprocessor  142 , the microprocessor  142  generally providing top level communication and control for the disc drive  100  in conjunction with programming for the microprocessor  142  stored in microprocessor memory  143 . The memory  143  can include random access memory (RAM), read only memory (ROM) and other sources of resident memory for the microprocessor  142 . 
     The disk(s)  108  are rotated at a constant high speed by the spindle motor  106  under control of a spindle motor control circuit  148 . During a seek operation, wherein the actuator  110  moves the head  118  between tracks, the position of the head  118  is controlled through the application of current to the coil  126  of the VCM  124 . A servo control circuit  150  provides such control. During a seek operation the microprocessor  142  receives information regarding the velocity of the head  118 , and uses that information in conjunction with a velocity profile stored in memory  143  to communicate with the servo control circuit  150 , which will apply a controlled amount of current to the VCM coil  126 , thereby causing the actuator assembly  110  to be pivoted so as to place head  118  in the desired position with respect to the disk  108 . 
     Data is transferred between the host product  140  and the disc drive  100  by way of an interface  144 , which typically includes a buffer to facilitate high speed data transfer between the host product  140  and the disc drive  100 . Data to be written to the disc drive  100  is thus passed from the host computer to the interface  144  and then to a read/write channel  146 , which encodes and serializes the data and provides the requisite write current signals to the heads  118 . To retrieve data that has been previously stored by the disc drive  100 , read signals are generated by the heads  118  and provided to the read/write channel  146 , which performs decoding and error detection and correction operations and outputs the retrieved data to the interface  144  for subsequent transfer to the host product  140 . Such operations of the disc drive  100  are well known in the art. The remainder of the signals shown in  FIG. 3  are discussed below in conjunction with  FIG. 23 . 
     In perpendicular recording, write operations are performed with the face of the pole in “contact” with the disk. As the head moves from the inside to the outside of the disk  108 , the amount of the face in contact with the disk increases thus making the tracks wider and creating the skew problem The disclosed microactuator solves that problem. 
       FIG. 4  is a simplified block diagram illustrating an actuator  154  constructed according to the teachings of the present disclosure located between the slider  117 /transducer head  118  and the flexure  116  of an actuator arm (not shown in  FIG. 4 ) to enable the position of the transducer head  118  to be positioned relative to the flexure  116 . In  FIG. 4 , the arrow  155  indicates skew, the arrow  156  indicates fly height, the arrow  157  indicates motion in a cross-track direction, and the arrow  158  indicates motion in an in-track direction. The actuator  154  enables the slider  117 /transducer head  118  to be rotated with respect to the flexure  116  to compensate for the skew indicated by arrow  155 . The combination of the actuator  154 , slider  117 , and transducer head  118  may be referred to as a head assembly  160 . 
     Because of the size constraints on the actuator  154 , the actuator  154  may be constructed using CMOS MEMS silicon micromachining techniques such as those pioneered by Fedder et al, at Carnegie Mellon University. See U.S. Publication No. 2007/0103029 entitled Self-Assembling MEMS Devices Having Thermal Actuation, the entirety of which is hereby incorporated by reference for all purposes. 
       FIG. 5A  is a plan view looking down and  FIG. 5B  is a cross-sectional view along the lines B-B in  FIG. 5A  of one embodiment of a micromachined rotary actuator  162  constructed according to the teachings of the present disclosure. The rotary actuator  162  is comprised of a central portion  164 . An outer portion  166  surrounds the central portion  164  and is separated from the central portion  164  by an in-plane gap  168 , seen best in  FIG. 5B . Although in  FIG. 5A  the outer portion  166  is shown completely surrounding the central portion  164 , that need not be the case. Additionally, the central portion  164  need not be circular as shown in  FIG. 5A . 
     A plurality of arms  170  is provided with each arm connected at one end to the central portion  164  and connected at another end to the outer portion  166 . In that way, each of the arms  170  spans the in-plane gap  168 . In the embodiment of  FIG. 5 , the arms  170  have a length L, a width W, and a thickness t. The distance from the midline of one arm  170  to the midline of an adjacent arm  170  is S where S is greater than W at the perimeter of the central portion  164 . Each of the arms  170  is electrothermally actuated as described below. 
       FIGS. 6 and 7  are simplified block diagrams illustrating two possible orientations within a head assembly for the rotary actuator  154 . In  FIG. 6 , the central portion  164  is bonded to the flexure  116  while the outer portion  166  is bonded to the slider  117  using conventionally known bonding techniques. In  FIG. 7 , the rotary actuator  154  is connected in the opposite manner. More specifically, the outer portion  166  is bonded to the flexure  116  and the central portion  164  is bonded to the slider  117 . The reader will recognize that in  FIG. 6 , the central portion  164  remains fixed, while the outer portion  166  moves in response to heating or cooling of the plurality of arms  170 . In  FIG. 7 , it is the outer portion  166  which remains fixed, and the central portion  164  which moves in response to the heating and cooling of the plurality of arms  170 . Thus, either the central portion  164  or the outer portion  166  may function as a stationary portion with the other portion functioning as the rotary portion. 
     The suspension of the microactuator  154  must be stiff enough to transmit a 10 milli-Newton (mN) force from the flexure  116  to the slider  117  without excessive vertical displacement. Even though the micromachined actuator  154  is shown in two different orientations in  FIGS. 6 and 7 , the design constraints with respect to the transmission of the 100 mN force remains the same. 
       FIG. 8A  is a plan view looking down on one of the arms  170 . Arms  170  are commonly referred to as “beams” in the art. Looking at  FIGS. 8A ,  8 B, and  8 C, it is seen that arm  170  is constructed of silicon dioxide  178  having layers of aluminum  176  embedded therein. Materials other than silicon dioxide and aluminum may be used so long as the desired difference in the temperature coefficients of expansion between the two materials is obtained. To the left of a dividing line  180 , the layers of embedded aluminum  176  are on the right side of arm  170  as seen in  FIG. 8B . To the right of dividing line  180  in  FIG. 8A , the layers of embedded aluminum  176  are on the left side of the arm  170  as shown in  FIG. 8C . By placing the layers of embedded aluminum  176  on one side of the arm  170  for half of the arm  170 , and then placing the layers of embedded aluminum on the other side of the arm  170  for the other half of the arm  170 , unequal forces can be built into the arm  170  to cause the arm  170  to move. 
     The arm  170  is referred to as consisting of two bimorphs in series. Typically, these bimorphs have static deflection after release and before actuation. This static deflection is due to the fact that the metal within the beams typically has a tensile stress relative to the oxide, causing the metal side of the beam to contract more than the oxide side. The two bimorphs of opposite sign in series help to simulate a guided end condition in one of the ends of the arm. That situation is shown  FIGS. 9A and 9B  which illustrate a single arm  170  before microstructure release ( FIG. 9A ) and after release ( FIG. 9B ). 
       FIGS. 10A and 10B  illustrate the result of electrothermal actuation of a single arm  170 .  FIG. 10A  shows the arm just after heating but before motion while  FIG. 10B  shows the same arm  170  after heating and after motion. Passing electric current through heating conductors (not shown) embedded in the arm or adjacent to the arm heats the arm. This current is typically delivered via the metal interconnect in the arm, which themselves are not sufficiently resistive to cause heating. As mentioned, the two sides of the bimorph (metal and oxide) have different coefficients of thermal expansion. The different coefficients of thermal expansion cause differential expansion of the two sides of the bimorph, resulting in a bending moment. As the bimorphs bend, the two ends of the arm displace laterally with respect to each other. If the two series bimorphs are the same length, there is no relative rotation of the two ends of the beam, resulting in the so called “guided end condition.” If the two bimorphs are not the same length, then there will be some relative rotation of the two ends of the beam as the beams move laterally. Such relative rotation may be desirable to accommodate the rotation of the movable portion of the microactuator. 
       FIGS. 11A and 11B  correspond to  FIGS. 10A and 10B , respectively.  FIG. 11B  shows how the collective movement of all of the arms  170  results in rotation of the outer portion  166  relative to the central portion  164 . In this case the arms  170  comprising the suspension attach to the central portion  164  and to the outer portion  170  at an angle. As the arms  170  bend, they move laterally and they straighten, resulting in the movement of the two attachment points for each arm away from each other. 
     Electrothermal operation allows relatively low voltage driving circuitry (not shown), but dissipates more power than electrostatic methods. The speed of rotation depends on how quickly the bimorphs heat up and cool down. Low thermal conductivity in the arms will lead to quick heating and fast positive rotation with low power, but slower cooling and slow negative rotation (slow return to the starting point). Conversely, high thermal conductivity in the arms will lead to slow heating and slow positive rotation with higher power, but fast cooling and fast negative rotation. The thermal design of the arms is important for fast response with reasonable power requirements. Additionally, control of the temperature of the arms enables the movement to be controlled so that a range of rotations can be achieved. 
     In the skew compensation mode, the rotational slew rate must be sufficient to move the rotary actuator  154  through its full range of motion in the time needed for the VCM  124  to move the head assembly  160  from the inside diameter to the outside diameter of the disk  108  in  FIG. 2 . This time is on the order of 5 ms in currently available high performance commercial disk drives. Rotary motion of the rotary actuator  154  on the order of ±2 degrees will provide enough movement to be useful for skew compensation. This ±2 degree specification corresponds to a lateral displacement of the ends of the arms  170 ±1.74 microns for an actuator with a central portion having a diameter of 50 microns. Currently, there is a continuously available analog signal indicating the position of the head on the disk. That signal could be used to implement known, fixed amounts of skew compensation, in discrete steps or continuously changing skew compensation adjustments. Alternatively, skew could be measured and either discrete or continuous amounts of skew compensation implemented in response to the measured amount of skew. 
     After arriving at a data track, the system switches to track following mode. The rotary actuator  154  maintains a nominal angular displacement to compensate for skew, and then rotates very slightly around that nominal angular displacement to produce the cross-track lateral displacements necessary to keep the head on the data track. The rotation of the head will be very small in this mode. Typical cross-track displacements during track following will be less than 100 nm. If the center of rotation is 250 μm from the transducer head  118 , the rotation of the transducer head  118  will only be 200 μradians or 1.2 mdeg. This small rotation will not affect the skew compensation. In track following mode it is desirable that the rotation rate be sufficient to move the head at a bandwidth of several kilohertz over this 100 nm range of motion. 
     It is desirable to make the disclosed rotary actuator  154  stiff enough to resist two undesired modes of displacement while making it compliant enough in torsion to allow sufficient stroke. These two undesirable displacement modes are lateral displacement as shown in  FIG. 12  and axial displacement shown in  FIG. 13 . The lateral displacement in  FIG. 12  will typically be in response to a shock event and would cause either down track or cross track motion of the head assembly. The axial displacement (normal to the disk) in  FIG. 13  will principally occur in response to the static load of 10 mN from the head suspension. 
       FIG. 14  shows the three orthogonal arm deflection modes that can be analyzed to accomplish the desired design tradeoff. These three different arm deflections correspond to the three different modes of displacement, i.e., the rotary displacement shown in  FIG. 11B , the lateral displacement shown in  FIG. 12 , and the axial displacement shown in  FIG. 13 . A simple set of design trade-offs can be seen from the following set of steps to size the arms. 
     1) Identify minimum beam width W and maximum beam thickness t as set by the selected CMOS MEMS process. In the examples shown below, these are 2.4 μm and 7 μm CMOS processes, respectively. 
     2) Select a desired rotational stroke in degrees. 
     3) Select the length L of the arms needed to enable the desired rotational stroke. 
     4) Assess the lateral stiffness of the arms, i.e., resistance to the lateral displacement shown in  FIG. 12 . For this calculation it is assumed that only the subset of the arms that are in tension (circled in the  FIG. 12 ) contribute to this stiffness. The arms on the other side of the central portion are in compression, and not assumed to contribute. These arms deflect in the direction labeled “lateral displacement” in  FIG. 12 . This compliance also allows the calculation of a resonant frequency for lateral vibration based on the slider&#39;s mass. In the figures that follow, a total mass of 10 mg is assumed, which is an overestimate of the mass even including the actuator mass. Additionally, this stiffness allows the response to shock to be assessed, as well. In the discussion that follows, a shock of 300 g is assumed. If the arms are attached at a small angle as indicated in  FIG. 11A , then the stiffness in tension and compression would be less, but there would be no buckling of the arms and more arms would contribute to the stiffness. A detailed design will account for these trade-offs. 
     5) Assess the axial stiffness for axial displacements, i.e., the resistance to the axial displacement shown in  FIG. 13 . For this calculation it is assumed that all arms contribute equally, and the deflection of interest is the deflection under 10 mN of load. 
       FIG. 15  shows the rotational, lateral, and axial stiffness for typical microactuator dimensions, indicated in the sub-table labeled assumptions, and schematic top view. In this case the arms are 140 μm long (L). The resulting rotational stiffness permits a rotational displacement of ±2 degrees for a 140 degree Celsius temperature rise in the bimorph temperature. Assuming slider mass of 10 mg (which is significantly greater than current sliders), the lateral stiffness for this set of parameters gives a displacement of 0.2 μm in response to a 300 g shock and a resonant frequency for lateral vibration of 11 kHz. It also allows an axial displacement under a static load of 1 g of approximately 5 μm. 
     The trade-off in torsional stroke (at a ΔT of 140 C) with the lateral stiffness is shown in  FIG. 16 , while a similar tradeoff for axial displacement is shown in  FIG. 17 . For  FIG. 16 , the slider mass is again assumed to be 10 mg, and the shock is assumed to be 300 g with an infinitely long impulse (worst case). 
     For  FIG. 17 , the gram load from the suspension is assumed to be 1 g (10 mN). From these two figures it is significant to note that the achievable stiffnesses and torsional strokes with this design are quite promising. The only stiffness that is marginal is the axial stiffness, but this could be increased substantially with only moderately thicker beams. Additionally, if a bearing point were used this might eliminate the need for significant axial stiffness. Advanced CMOS processes, which have a stack height of over 13 microns, can provide thicker arms with greater axial stiffness. A custom process can also be developed. In addition, other materials with high temperature coefficients and high coefficients of thermal expansion (e.g., nickel) can be implemented that will boost actuator efficiency. 
     The micromachined actuator  154  disclosed herein need not be limited to the construction shown, for example, in  FIG. 5A . It has already been mentioned that the central portion  164  need not be circular. Similarly, it has been mentioned that the outer portion  166  need not completely surround the central portion  164 . Additional embodiments include attachment angles other than the perpendicular attachment angle illustrated in  FIG. 5A  between the arms  170  and the central portion  164  and outer portion  166 . Other embodiments also include arms  160  which are of cross sectional shapes other than rectangular. Some examples follow. 
       FIG. 18  is a plan view of another embodiment of a micromachined rotary actuator constructed according to the teachings of the present disclosure in which the arms are radially aligned and attached to the central portion  164  and the outer portion  166  at an angle. 
       FIG. 19  is a plan view of another embodiment of a micromachined rotary actuator constructed according to the teachings of the present disclosure and having radially aligned arms  170  attached perpendicularly to the central portion  164  and outer portion  166 . 
       FIG. 20  is a plan view of another embodiment of a micromachined rotary actuator constructed according to the teachings of the present disclosure and having radially aligned, trapizoidally shaped arms, attached perpendicularly to the central portion  164  and outer portion  166 . 
       FIG. 21A  is a plan view illustrating another embodiment of a micromachined rotary actuator constructed according to the teachings of the present disclosure. In the embodiment shown in  FIG. 21A , the arms  170  from the twelve o&#39;clock position to the six o&#39;clock position can be heated independently of the arms from the six o&#39;clock position to the twelve o&#39;clock position. By making the arms independently actuable, various types of movement can be obtained. For example, as shown in  FIGS. 21B and 21C , a small amount of rotation θ can be obtained with respect to an axis  184 . 
       FIG. 22  illustrates a possible interconnect routing. Those of ordinary skill in the art will recognize that data signals representing data read from disk  108  or data to be written to disk  108  could pass through actuator  154  and avoid the use of a flexible connection (not shown) which bypasses the actuator  154 . Additionally, control signals, perhaps in the form of currents sent to heating conductors positioned in one or more arms, must be delivered to the actuator  154 .  FIG. 22  illustrates one possible embodiment for accomplishing the delivery of those signals. 
     In  FIG. 21 , a flex circuit  190  is connected to the flexure  116  to facilitate the relative motion between the flexure  116  and the actuator  154 . Solder connections  192  connect conducting traces  194  in the flex circuit  190  to internal metal conductors  196  formed in the actuator  154 . The internal metal conductors  196  are connected to pads  200  on the slider  117  through solder joints  198 . 
     Finally,  FIG. 23  illustrates a micromachined rotary actuator carrying a capacitive type of position detector  202  and a resistor  204  having a resistance that varies as a function of temperature. The position detector  202  and resistor  204  produce signals which may, after conversion to digital form, be input to the microprocessor  142  of  FIG. 3 . The microprocessor  142 , in response to the position and temperature information, may produce control signals for generating, via digital to analog conversion circuits, the currents applied to the heating conductors  206 . The D/A and A/D circuits may be discrete components or fabricated as part of the actuator  154 . The control signals generated by the microprocessor  142  may produce either rotary motion in discrete steps or continuous motion. Heating conductors could additionally or alternatively be located in the central portion  164  and/or the outer portion  166 . 
     While the present disclosure has been described in connection with preferred embodiments thereof, those of ordinary skill in the art will recognize that many modifications and variations are possible. The present invention is intended to be limited only by the following claims and not by the foregoing description which is intended to set forth the presently preferred embodiments.