Patent Publication Number: US-8973886-B2

Title: Actuator including mechanism for converting rotary motion to linear motion

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 12/732,321, published as U.S. Published Pat. App. 2011-0233364-A1, entitled “Actuator Including Mechanism for Converting Rotary Motion to Linear Motion”, filed Mar. 26, 2010 by Breen et al., which is herein incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     Active vibration control systems have been employed to control vehicle seat vibration. For example, as a replacement for passive systems including springs and dampers which reduce seat response to vehicle vibration, active vibration control systems detect seat vibration and control the position of the seat to cancel detected motion and thereby isolate the seat from vehicle vibration. Such active vibration control systems may include a linear actuator controlled by a controller. The linear actuator is positioned below the seat to control seat position relative to the vehicle frame. For example, the linear actuator may include a linear electromagnetic motor, including an armature fixed at one end to the seat. The armature linearly extends and retracts relative to a stator based on control signals from the controller, thereby positioning the seat. 
     Controlled linear actuators have application to systems other than vehicle seat vibration control. For example, controlled linear actuators are also known to be used in vehicle wheel suspension systems and in engine valve control systems. 
     In many applications, a challenge associated with using such linear actuators to control object position includes providing a linear motor providing sufficient linear travel within a limited space, for example between the seat and the floor in an active seat vibration control system. Other challenges include known cost and maintenance issues associated with linear motors. 
     SUMMARY 
     In some aspects, an active vibration control device configured to control the position of a body includes at least one sensor configured to provide input signals corresponding to movement of the body in at least one direction, a rotary motor configured to control the position of the body, and a linkage including at least two pivotably-joined links connecting the rotary motor to the body. The linkage is configured to convert rotary motion output from the motor into a linear motion of the body. The device further includes a controller which, based on the input signals from the at least one sensor, provides control signals to the rotary motor which acts through the linkage to position the body in the at least one direction. 
     In another aspect of the invention, an actuator comprises a rotary motor including an output shaft and a motor housing; and a linkage connected to the output shaft of the rotary motor. The linkage includes the motor housing which has a housing pivot pin defining a first rotation axis and a first link fixed to the output shaft. The output shaft defines a second rotation axis, and the second rotation axis is parallel to and spaced apart from the first rotation axis. The first link includes a first link pivot pin disposed at a location spaced apart from the second rotation axis and defines a third rotation axis that is parallel to the first rotation axis. The linkage includes a second link pivotably connected at a first end to the first link pivot pin. The second link includes a second link pivot pin defining a fourth rotation axis that is parallel to the first rotation axis. The second link pivot pin is disposed between the first end of the second link and a predetermined point of the second link. The linkage further includes a third link pivotably connected at a first end to the housing pivot pin and pivotably connected at a second end to the second link pivot pin. During operation of the actuator, rotation of the output shaft results in a linear motion of the predetermined point relative to the housing. 
     The active vibration control device and actuator may include one or more of the following features: The torque generated by the motor at the body is substantially constant over a 100 degree angular rotation of the output shaft. The linkage is configured to convert the rotary motion of the output shaft to linear motion such that the motion of the body is substantially proportional to the angular displacement of the output shaft over a 180 degree rotation of the output shaft. The linkage is configured to convert the rotary motion of the output shaft to linear motion such that the torque is substantially constant over a range of displacement of the body of at least four inches. 
     The controller of the active vibration control device provides output signals to the rotary motor which acts through the linkage to position the body such that an attitude of the body controlled. The active vibration control device includes a second linkage, with one of said linkages connected to the output shaft of the motor on each of opposed sides of the motor. The device further includes a second rotary motor and a second linkage configured to control the position of the body, the first and second rotary motors arranged such that their respective rotor axis are parallel. The device further includes a second rotary motor and a second linkage configured to control the position of the body, the first and second rotary motors arranged such that their respective rotor axis are co-linear. 
     In certain implementations, the body includes a vehicle seat, for example disposed in a vehicle, with the rotary motor, fixed relative to a floor of the vehicle, being disposed between the floor and the seat. The linear travel of the body is at least 4 inches. The controller provides control signals to the rotary motor to position the body according to a motion that is opposed and opposite to the motion detected by the at least one sensor. 
     The actuator may further include one or more of the following features: The actuator includes a second linkage, and one of the linkages is connected to the output shaft of the motor on each of opposed sides of the motor. Each of the housing pivot pin and the first and second link pivot pins are supported on bearings, and the links are configured such that the bearings are substantially co-planar. 
     The active vibration control device and the actuator advantageously employ a rotary motor and include a mechanism to converts rotary motion of the motor to linear motion. The actuator has many applications, one of which is to control the position of an object along a linear path. The actuator, in which the rotary motor acts through a mechanical linkage to position the object, has several advantages over known positioning devices which employ linear motors. For example, rotary motors are much less expensive to fabricate and are more easily sealed than a linear motor. In addition, rotary motors, in combination with the mechanical linkage, are more compactly sized than a linear motor while providing equal or greater range of linear motion. This feature is important for example in vibration control of vehicle seats, where the spacing between the seat and floor, in which the control mechanism is disposed, is limited. 
     Moreover, when combined with a controller, the actuator can be used as a motion control device. For example, in some implementations, the actuator combined with a controller can be used to provide active control of valves in an internal combustion engine or a compressor. In some implementations, the actuator combined with a controller can be configured to act as a position source, a velocity source or a force source. In some implementations, the actuator combined with a controller can be used in an active vibration isolation control device. For example, the actuator and a controller can be used to control the position and/or the acceleration of a vehicle seat, as explained further below, or to control the position and/or acceleration of the sprung mass of a vehicle (i.e. the passenger compartment or an automotive vehicle). 
     A still further advantage of the actuator is that at least some of the mechanical linkage is incorporated into the motor housing and rotor shaft, providing a actuator that is still more compact, less complex and requires fewer parts. Furthermore, the actuator is a direct drive device in which the rotor is connected to the object to be positioned via a single rigid link, and without any intervening gears, belts or other devices which introduce error and/or complexity into positioning control. 
     In a further aspect of the invention, a mechanism for converting rotary motion into linear motion comprises a plate including a plate pivot pin defining a first rotation axis, and a first link fixed to a shaft. The shaft is rotatably supported on the plate and defines a second rotation axis, the second rotation axis being parallel to and spaced apart from the first rotation axis. The first link includes a first link pivot pin disposed at a location spaced apart from the second rotation axis and defines a third rotation axis that is parallel to the first rotation axis. The mechanism includes a second link pivotably connected at a first end to the first link pivot pin. The second link includes a second link pivot pin defining a fourth rotation axis that is parallel to the first rotation axis, and the second link pivot pin is disposed between the first end of the second link and a predetermined point on the second link. The mechanism further includes a third link pivotably connected at a first end to the plate pivot pin and pivotably connected at a second end to the second link pivot pin. In the mechanism, rotation of the shaft results in a linear motion of the predetermined point relative to the plate. 
     The mechanism may include one or more of the following features: The predetermined point moves linearly for about a 180 degree rotation of the shaft. The mechanism includes a first bar length defined by the distance between the first link pivot pin and the shaft, a second bar length defined by the distance between the shaft and the plate pivot pin, a third bar length defined by the distance between the plate pivot pin and the second link pivot pin, and a fourth bar length defined by the distance between the first link pivot pin and the predetermined point, and the ratio of the first bar length to the second bar length to the third bar length to the fourth bar length is 1:2:2.5:5. Each of the plate pivot pin, first and second link pivot pins and the shaft are supported on bearings, and the bars are configured such that the bearings are substantially co-planar. The plate further comprises a stop member configured to limit rotation of the first link relative to the plate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an actuator for converting rotary motion to linear motion. 
         FIG. 2  is a side sectional view of the actuator as seen along section line  2 - 2  of  FIG. 1 . 
         FIG. 3  is a schematic representation of a Hoeken&#39;s linkage. 
         FIG. 4  is a perspective view of the actuator of  FIG. 1  illustrating the four bars of the linkage. 
         FIG. 5  is a graph of rotor shaft angular displacement (degrees) versus displacement (inches) of a predetermined point of the first link. 
         FIG. 6  is an end view of the actuator of  FIG. 1 . 
         FIG. 7 . is a graph of rotor shaft angular displacement (degrees) versus torque (Nm) output of the motor required to provide a constant 1100 N force at the predetermined point of the first link. 
         FIG. 8  is a graph of displacement (inches) of the second end of the first link versus torque (Nm) output of the motor required to provide a constant 1100 N force at the predetermined point of the first link. 
         FIG. 9  is a perspective view of a position control device employing two actuators shown in a retracted configuration. 
         FIG. 10  is a perspective view of the position control device of  FIG. 9  shown in an extended configuration. 
         FIG. 11  is a perspective view of an alternative implementation of a position control device employing two actuators. 
         FIG. 12  is a perspective view of another alternative implementation of a position control device employing two single-linkage actuators. 
         FIG. 13  is a schematic view of an active vibration control system for a vehicle seat. 
         FIG. 14  is a sectional view of a portion of a cylinder bank of an internal combustion engine in which the actuator of  FIG. 1  is directly connected to an engine valve. 
         FIG. 15  is a sectional view of a portion of a cylinder bank of an internal combustion engine in which the actuator of  FIG. 1  is indirectly connected to an engine valve. 
         FIG. 16  is a perspective view of an alternative implementation of an actuator. 
     
    
    
     DETAILED DESCRIPTION 
     As will be described in greater detail below, an actuator including a rotary driver combined with a linkage having particular mechanical characteristics provides conversion of rotary to linear motion in a manner that is well suited for applications in which the linear range of travel is maximized within a limited space. 
       FIGS. 1 ,  2 ,  4  and  6  show an actuator having this desired characteristic in the form of a rotary motor and a four-bar linkage. An alternative implementation of the actuator is shown in  FIG. 16 .  FIGS. 5 ,  7  and  8  illustrate the mechanical characteristics of the actuator including proportionality of the displacement of a predetermined point on the linkage to the angular displacement of the rotary motor, and a constant force at the predetermined point for both angular displacement as well as displacement of the predetermined point in the linear portion of the motion of the linkage  52  for a constant torque output of the rotary motor. Several implementations of such an actuator when used as a positioning device are shown in  FIGS. 9-12 . In particular, the actuators are connected to and control movement of a platform. An implementation of the actuator used in an active vibration control system is described with reference to  FIG. 13 . In addition, an implementation of the actuator used to control engine valves is described with reference to  FIGS. 14 and 15 . 
     Referring now to  FIGS. 1 and 2 , the actuator  50  for converting rotary motion to linear motion includes a rotary motor  60  supported by and disposed within a motor housing  62 . The actuator includes a first linkage  52  connected to, and driven by, one end of the motor  60 . The linkage  52  is arranged as a Hoeken&#39;s linkage, and can be used to position an object connected to the linkage along a linear path. The actuator  50  advantageously provides a compact approach to linearly positioning the object in space. 
     Although object positioning can be achieved using a single linkage  52 , in the illustrated implementation, the actuator  50  further includes a second linkage  252  connected to, and driven by, a second end of the motor  60 . The second linkage  252  is a minor image of the first linkage  52 , and is configured to move synchronously and in concert with the first linkage  52 , as discussed further below. Elements common to both linkages  52 ,  252  are identified by the same reference number. Thus, the configuration of each linkage will be described with reference to first linkage  52  only. 
     The rotary motor  60  includes a stator  72  fixed to the housing  62 , and a rotor  80  disposed coaxially within the stator  72  so as to be rotatable about a rotor axis  82 . The rotor  80  is a hollow cylindrical body having opposed first and second ends  84 ,  85  rotatably supported on the housing  62 . The rotary motor  60  may be a conventional frameless kit motor such as model K127300 made by Bayside® Motion Group, of Port Washington, N.Y. 
     The housing  62  includes closed sidewalls  63  capped at each end by housing end plates  64 . Each end plate  64  includes a plate pivot pin  68  that extends outward in a direction parallel to the rotor axis  82 , supports a bearing  194 , and defines a first rotational axis  76  of the linkage  52 . In the illustrated implementation, the plate pivot pin  68 , and thus the first rotational axis  76 , overlies and is substantially vertically aligned with the rotor axis  82 . 
     An end cap  100  is fixed to a first end  84  of the rotor  80 . The end cap  100  is a hollow cylindrical body having a closed first end  101 . The end cap  100  is rotatably supported in an opening  66  formed in an end plate  64  of the housing  60  so that the outer surface  102  lies generally within the plane of the end plate  64 . Adjacent to the first end  101 , an outer periphery of the end cap  100  is supported by a rotor bearing  89  mounted in the housing end plate  64 . The rotor bearing  89  may be a thin section bearing such as a Silverthin™ model SB035 angular contact bearing sold by Mechatronics Corporation of Preston, Wash. 
     The end cap  100  extends inward from the outer surface  102 , and terminates at an open second end  103 . The outer diameter of the end cap  100  is reduced at the second end  103 , forming an annular protrusion  128  sized to be press fit within an inner surface of the rotor  80 . Relative rotation of the end cap  100  with respect to the rotor  80  is prevented by securing the end cap  100  to the rotor. This can be achieved, for example, by providing screws (not shown) in mutually aligned screw holes  86 ,  130  formed in the rotor  80  and annular protrusion  128 , respectively. Thus, the end cap  100  rotates with the rotor  80  and serves as an output shaft of the motor  60 . The rotational center  132  of the end cap  100  is coaxial with the rotor axis  82 , which corresponds to a second rotational axis of the linkage  52 . 
     The outer surface  102  of the end cap  100  includes a protruding step portion  104  formed at the periphery of the end cap  100  in the shape of a segment of a circle, in which the chord defining a side of the segment is not a diameter of the end cap  100 . A shoulder  106  is formed which joins the step portion  104  to the remainder of the outer surface  102 . An end cap pin  108  is provided in the step portion  104  adjacent to the periphery of the end cap  100 . The end cap pin  108  protrudes outwardly from the step portion  104 , supports a bearing  158 , and defines a third rotational axis  110  of the linkage  52  that extends in parallel to the rotor axis  82 . 
     The motor  60  includes an external optical encoder  120  to determine the angular position of the rotor  80 . In this implementation, an encoder shaft  118  protrudes from an outer surface  102  of the first end  101  coaxially with the rotor axis  82 . The encoder shaft  118  is connected to the input shaft  122  of the encoder using a flexible coupling  124 , permitting accurate determination of the angular position of the rotor  80 . However, the actuator  50  is not limited to this configuration. For example, the motor  60  may be provided with an internal encoder. 
     A second end cap  200  is fixed to a second end  85  of the rotor  80 . The second end cap  200  is substantially similar in form and function to that of the first end cap  100 , and like elements of the second end cap  200  are identified with the same reference numbers. For this reason, a detailed description of the second end cap  200  will be omitted except to point out the following differences relative to the first end cap  100 : The end cap  200  does not include an encoder shaft  118 . The end cap  200  is provided with a through hole  202  that is coaxially aligned with the rotor axis  82 . The through hole  202  provides access to the interior of rotary motor  60 , which is advantageous during assembly and disassembly of the actuator  50 . 
     As stated above, the linkage  52  is arranged as a Hoeken&#39;s linkage. A Hoeken&#39;s linkage is a four-bar linkage that converts rotational motion to approximate straight line motion. With reference to  FIG. 3 , the Hoeken&#39;s linkage includes a rotating first bar I, a fixed second bar II which joins the first bar Ito a fourth bar IV, a third bar III driven at one end by the first bar I, and the fourth bar IV which supports a mid portion of the third bar III. Due to the rotation of the first bar I, the point P of the third bar III moves along the closed-loop path  53  indicated by the dashed line. As seen in the figure, the path includes a substantially linear portion  55 . 
     Referring to  FIG. 4 , the four bars of the linkage  52  are defined as follows: 
     The first bar  116  of the linkage  52  is provided by the end cap  200 . More specifically, the first bar  116  includes the portion of the end cap  200  extending between the rotational center  132  of the end cap  200  and the end cap pin  108 . The first bar  116  rotates relative to the housing  62  about the second rotational axis  82  in a plane corresponding to outer surface  102  of the end cap  200 . 
     The second bar  88  of the linkage  52  is provided by the housing  62 . More specifically, the second bar  88  includes a portion of the end plate  64  and extends between the plate pivot pin  68  and the rotational center  132  of the end cap  200 . The second bar  88  is a fixed bar relative to the housing  62 , and defines the orientation of the linear motion produced by the linkage  52 . 
     The third bar  151  of the linkage  52  is provided by the first link  150 . The first link  150  is an elongate rigid bar of rectangular cross section, and includes a first end  152 , and a second end  154  opposed to the first end  152 . The first and second ends  152 ,  154 , and the mid point  156  between the first and second ends  152 ,  154  are provided with through holes  165  that extend between opposed broad faces  166 ,  168  of the first link  150 . The bearings  158 ,  160 , (midpoint bearing not shown) are press fit into the respective through holes  165  and are sized and shaped to receive a pivot pin. For example, the bearing  158  at the first end  152  of the first link  150  receives the end cap pin  108 , and permits rotation of the first link  150  about the end cap pin  108  (and third rotational axis  110 ) relative to the housing  62  and the end plate  100 . The bearing disposed at the mid point  156  supports a link pin  162 . The link pin  162  protrudes outwardly from both broad faces  166 ,  168  of the first link  150 , and defines a fourth rotational axis  164  of the linkage  52  that extends in parallel to the rotor axis  82 . Constituted by the first link  150 , the third bar  151  of the linkage  52  extends between the end cap pivot pin  108  and the centerline of the bearing  160  (which coincides with point P in  FIG. 4 ). 
     The fourth bar  181  of the linkage  52  is provided by the second link  180 . The second link  180  is an elongate rigid bar of rectangular cross section, and includes a first end  182 , and a second end  184  opposed to the first end  182 . The first end  182  is provided with a through hole  195  that extends between opposed broad faces  196 ,  198  of the second link  180 . A bearing  194  is press fit into the through hole  195  and is sized and shaped to receive the plate pin  68 . Thus, the first end  182  of the second link  180  rotates about the plate pin  68  (and first rotational axis  76 ) relative to the housing  62 . The second end  184  of the second link  180  is bifurcated so that the distance between the broad faces  196 ,  198  at the second end  184  is greater that that at the first end  182 , and so that the second end  184  forms a yoke including spaced yoke arms  186 ,  188  which straddle mid portion of the first link  150  and engage the link pin  162 . Thus, the second end  184  of the second link  180  rotates about the link pin  162  (and the fourth rotational axis) relative to the housing  62  and the first link  150 . Constituted by the second link  180 , the fourth bar  181  of the linkage  52  extends between the plate pivot pin  68  and the link pin  162 . 
     By providing the second link  180  with yoke arms  186 ,  188 , the first end  182  of the second link  180  can be arranged to be in the same plane as the first link  150 . In addition, by providing the end cap  100  with the step portion  104 , and by locating the end cap pin  108  on the step portion  104 , a space is provided between the main link  150  and the housing  64  which can accommodate the inner yoke arm  188 . In combination, these features advantageously permit the pivot pin bearings  158 ,  160  and  194 , which are conventional radial ball bearings, to be arranged within a single plane, whereby twisting loads on the links are avoided when in use. However, the linkage  52  is not limited to this configuration, and in some embodiments, the second link  180  may be formed without a yoke and may instead be formed having an offset portion or having a linear configuration. 
     The linkage  52  is used to convert the rotary motion of the rotor  80  into a linear motion at a predetermined point P on the first link  150 . In the illustrated implementation, the center of the bearing  160  at the second end  154  of the first link  150  is defined as the predetermined point P at which linear motion is generated. By adjusting the relative lengths of the respective first through fourth bars  116 ,  88 ,  150 ,  180 , the motion of the point P can be specified. In the actuator  50 , the first bar length is defined by the distance between the end cap pin  108  and the rotational center  132  of the end cap  200 , the second bar length is defined by the distance between the rotational center  132  of the end cap  200  and the plate pin  68 , the third bar length is defined by the distance between the end cap pin  108  and the point P, and the fourth bar length is defined by the distance between the link pin  162  and the plate pin  68 . In the illustrated implementation, the bar lengths are as follows: The first bar  116  is 1 inch, the second bar  88  is 2 inches, the third bar  151  is 5 inches and the fourth bar  181  is 2.5 inches. The range of linear travel which is achieved with this configuration is about 4 inches. Of course, an increased range of linear travel can be obtained by proportionally increasing the size of the bars of the linkage. For example, for respective first through fourth bar lengths of 1.25 inches, 2.5 inches, 6.25 inches and 3.125 inches, the range of linear travel which is achieved is about 5 inches. Conversely, for applications in which a smaller range of linear travel is required, the mechanism can be scaled down, resulting in an even more compact device. 
     In the linkage  52 , the ratio of the first bar length to the second bar length to the third bar length to the fourth bar length is 1:2:5:2.5. By using these proportions, at least the following several advantages are realized: 
     The linear portion of the motion of the point P occurs along a line that is parallel to the fixed second bar  88 . In the illustrated implementation, the fixed second bar  88  is oriented vertically, and thus the linear portion of the motion of the point P also has a vertical motion. 
     Furthermore, as shown in  FIG. 5 , the motion of the point P is substantially proportional to the angular displacement of the end cap pin  108  over a 180 degree rotation of the rotor  80 . That is, the point P moves approximately linearly within the range of rotational motion of the rotor  80  indicated by reference lines A and B, corresponding to an approximate range of 180 degrees. 
     In the actuator  50 , two external links  150 ,  180  are provided which respectively serve as the third III and fourth IV bars of the Hoeken&#39;s four-bar linkage. The remaining two bars (the first and second bars I, II) are provided by the components of the motor  60  and motor housing  62 . Specifically, the second end cap  200  which incorporates the first bar  116  serves as the rotating first bar I of the Hoeken&#39;s linkage, and the motor housing  64  which incorporates the second bar  88  serves as the fixed second bar II of the Hoeken&#39;s linkage. This configuration, in which the first and second bars  116 ,  88  are not formed as external links but instead are incorporated into the motor assembly itself, reduces the number of components required to achieve the desired motion, and results in a compact actuator assembly. 
     Referring now to  FIG. 6 , the actuator  50  is configured so that the second end  154  of the first link  150  is constrained to move back and forth within the linear motion range identified between A and B of  FIG. 5 . In some implementations, a controller  14  ( FIG. 13 ) connected to the motor  60  prevents the rotor  80 , and thus the end cap  100 , from rotating beyond the 180 degree range. In addition, a stop member  90  is provided to mechanically interfere with the shoulder  106  of the outer surface  102 , whereby rotation beyond the linear range is prevented. The stop member  90  is fixed to the housing  62 , and extends radially inward to overlie a portion the opening  66  in the end plate  64 . When the linkage is in a fully extended configuration corresponding to one end of the linear range (shown in solid lines in  FIG. 4 ), a first portion the shoulder  106  abuts a first stop surface  96  of the stop member  90 . In this position, the first end  152  of the first link  150  is positioned on a horizontal line passing through the rotor axis  82  at a location to the left of the rotor axis  82  as viewed in the figure. In addition, as viewed in the figure, the point P is located at a position that is lateral to, and above an upper side of, the housing  62 . When the end cap  100  rotates counterclockwise, the linkage  52  moves downward, and the point P travels downward along a linear path L. When the linkage  52  is in a retracted position corresponding to the opposed end of the linear range (shown in dashed lines in  FIG. 4 ), a second portion of the shoulder  106  abuts a second stop surface  98  of the stop member  90 . In this position, the first end of the first link  150  has rotated through a 180 degree arc, and is now positioned on the horizontal line passing through the rotor axis at a location to the right of the rotor axis  82  as viewed in the figure. In addition, the point P is now located at a position that is lateral to, and below an upper side of, the housing  62 . 
     Further advantageously, in one embodiment as shown in  FIG. 7 , the four-bar linkage  52  is configured to convert the rotary motion of the rotor  80  to linear motion such that the torque output of the motor  60  required to provide a constant 1100N force at the point P is substantially constant over most of the angular displacement range of the motor associated with the linear travel range of the point P. The torque output of the motor  60  is substantially constant within the range of rotational motion of the rotor  80  indicated by reference lines C and D, corresponding to a range of about 100 degrees. 
     In addition, in one embodiment as shown in  FIG. 8 , the four-bar linkage  52  is configured to convert the rotary motion of the rotor  80  to linear motion such that the torque output of the motor  60  required to provide a constant 1100N force at the point P is substantially constant over most of the linear range of motion of point P. The torque output of the motor  60  is substantially constant over the majority of the range of tip linear displacement indicated by reference lined E and F, corresponding to about 4 inches. 
     Referring to  FIGS. 9 and 10 , the position control device  350  is an implementation of the actuator  50 , in which the device  350  includes two actuators  50 ,  50 ′ arranged such that the rotor axes  82 ,  82 ′ of the respective rotary motors  60 ,  60 ′ are substantially coaxial. In addition, the linkages  52 ,  252  of the first actuator  50  are configured to rotate in opposition to the linkages  52 ′,  252 ′ of the second actuator  50 ′. In the illustrated implementation, the position control device  350  is used to control the vertical position of a platform  16  relative to a base  22 , and is connected to the platform  16  through several downwardly extending legs  18 . In particular, each leg  18  includes a pivot pin  20  which is rotatably supported by the bearing  160  at the second end  154  of the respective first link  150  of each linkage  52 ,  52 ′,  250 ,  250 ′, a location corresponding to point P. In  FIG. 9 , the position control device  350  is shown in a first, retracted configuration in which the vertical distance between the platform  16  and the base  22  is a distance d 1 . In this implementation, there is substantially no vertical spacing between the platform  16  and the housings  62 ,  62 ′, whereby the retracted configuration is very compact. In addition, the platform  16  is substantially centered over the position control device  350 . In  FIG. 10 , the position control device  350  is shown in a second, extended configuration in which the vertical distance between the platform  16  and the base  22  is a distance d 2 , where d 2  is greater than d 1 . The platform  16  remains centered over the position control device  350  during the transition between retracted and extended configurations, and while in the extended configuration. Although the illustrated implementation shows the actuators  50 ,  50 ′ as being axially spaced a distance s 1 , this configuration is non-limiting. For example, the two actuators  50 ,  50 ′ may be spaced apart a distance which is greater or less than s 1 . 
     Referring to  FIG. 11 , position control device  450  is an alternative implementation of the actuator  50  Like the previous position control device  350 , the position control device  450  includes two actuators  50 ,  50 ′, the second actuator  50 ′ being identical to the first actuator  50 . In the position control device  450 , the actuators  50 ,  50 ′ are arranged such that the rotor axes  82 ,  82 ′ of the respective rotary motors  60 ,  60 ′ are parallel and spaced apart. In addition, the linkages  52 ,  252  of the first actuator  50  are configured to rotate in opposition to the linkages  52 ′,  252 ′ of the second actuator  50 ′. In the illustrated implementation, the position control device  450  is used to control the vertical position of a platform  16  relative to a base  22 , and is connected to the platform  16  through several legs  18 . In particular, each leg  18  includes a pivot pin  20  which is rotatably supported by the bearing  160  at the second end  154  of the respective first link  150  of each linkage  52 ,  52 ′,  250 ,  250 ′, a location corresponding to point P. Although the illustrated implementation shows the axes  82 ,  82 ′ of the actuators  50 ,  50 ′ as being spaced a distance s 2 , this configuration is non-limiting. For example, the axes  82 ,  82 ′ may be spaced apart a distance which is greater than s 2 . In addition, although the illustrated implementation shows the actuators  50 ,  50 ′ as being co-planar, the actuators can instead be offset to lie in different planes while maintaining parallel axes  82 ,  82 ′. 
     Referring to  FIG. 12 , position control device  550  is another alternative implementation of the actuator  50 . The position control device  550  includes two single-linkage actuators  250 ,  250 ′. In particular, each actuator  250 ,  250 ′ is provided with a single linkage  52 ,  52 ′. In the position control device  550 , the actuators  250 ,  250 ′ are arranged such that the rotor axes  82 ,  82 ′ of the respective rotary motors  60 ,  60 ′ are coaxial. In addition, the linkage  52  of the first actuator  250  is configured to rotate in opposition to the linkage  52 ′ of the second actuator  250 ′. In the illustrated implementation, the position control device  550  is used to control the vertical position a platform  16  relative to a base  22 , and is connected to the platform  16  through several legs  18 . In particular, each leg  18  includes a pivot pin  20  which is rotatably supported by the bearing  160  at the second end  154  of the respective first link  150  of each linkage  52 ,  52 ′, a location corresponding to point P. The position control device  550  operates similarly to the position control device  350 , but is less complex, requires fewer bearings, and is more compact in the axial direction than the position control device  350 . Although the illustrated implementation shows the two actuators  250 ,  250 ′ as being axially abutting, this configuration is non-limiting, whereby the actuators  250 ,  250 ′ may be axially spaced apart. 
     In each of the above-described position control devices  350 ,  450 ,  550 , by using linkage mechanisms arranged on opposing sides, when the actuators  50  move in unison, the respective reaction torques at the base  22  due to the load significantly reduced. In addition, by using two rotary motors  60 ,  60 ′ to position platform  16  rather than a single rotary motor  60 , each of the two rotary motors  60 ,  60 ′ can be reduced in size, resulting in a mechanism that is even more compact. In addition, in some implementations the respective linkages  52 ,  252 ,  52 ′,  252 ′ can be mechanically tied together so that the platform  16  can remain level in the event of failure of one of the rotary motors  60 ,  60 ′. 
     Referring to  FIG. 13 , the position control device  350  may be used in an active vibration control system  5  used in a vehicle  2  to mitigate or eliminate vehicle seat  8  vibration resulting from vibration of the vehicle frame  4 . The vehicle seat  8  is fixed to a rigid seat base  10 , and supports at least one sensor  12 . For example, the sensor  12  may include an accelerometer for detecting motion of the seat relative to the ground g. The seat  8  and base  10  rest on and are supported above the vehicle frame  4  by the position control device  350 . The position control device  350  may be attached indirectly to the vehicle frame via ancillary seat support structures, or attached directly to the frame itself, whereby the position control device  350  is fixed relative to the vehicle frame  4 . The position control device  350  serves to position the base  10 , and thus the seat  8 , relative to the vehicle frame  4  based on control signals received from a controller  14 . The controller  14  receives signals including seat movement data from the sensor  12 , and encoder signals indicating rotor position relative to the housing  62 ,  62 ′. Based on these signals, the controller  14  outputs control signals to the rotary motors  60 ,  60 ′ of the position control device  350  such that the position of the vehicle seat  8  is controlled relative to the vehicle frame. Although the illustrated implementation employs the position control device  350 , this is non-limiting. For example, any of the disclosed position control devices  450 ,  550  may be substituted for device  350 . Moreover, a single actuator  50 ,  250  may be used in combination with supplementary seat support structure to form an active vibration control system. 
     In some implementations, the respective rotary motors  60 ,  60 ′ are controlled to position the base  10 , and thus the seat  8 , so as to cancel the detected seat motions in order to isolate the seat  8  from vehicle vibration. In some implementations, the respective rotary motors  60 ,  60 ′ are controlled to act in concert. For example, the distance of the second end  154  of the first link  150  of both actuators  50 ,  50 ′ from the vehicle frame  4  is controlled to be the same. In other implementations, the rotary motor  60  of the first actuator  50  may be controlled independently of the rotary motor  60 ′ of the second actuator  50 ′, whereby the attitude of the seat base  10  relative to the vehicle frame  4  may be controlled. In such an implementation, at least one additional degree of freedom would be required between the linkages  52 ,  252 ,  52 ′,  252 ′ and the seat base  10  to permit relative motion between these components. This can be accomplished, for example, by providing an additional pivot point at a location G. 
     The active vibration control system  5 , which employs the actuators  50  to convert rotary motion output from the rotary motor  60 ,  60 ′ into a linear motion, has several advantages relative to a control system employing a linear motor. For example, rotary motors are much less expensive to fabricate and are more easily sealed than a linear motor. In addition, rotary motors, in combination with the mechanical linkage, are more compactly sized than a linear motor while providing equal or greater range of linear motion. This feature is important for example in vibration control of vehicle seats, where the spacing between the seat and floor, in which the control mechanism is disposed, is limited. A still further advantage of the actuator is that at least some of the mechanical linkage is incorporated into the motor housing and rotor shaft, thereby providing a actuator that is even more compact, less complex and requires fewer parts. 
     Furthermore, in some implementations, the actuator  50  can be a direct drive device in which the rotor is connected to the object to be positioned via a single rigid link, and without any intervening gears, belts or other devices which introduce error and/or complexity into positioning control. 
     In addition, a further advantage to using the position control device  350  in the active vibration control device  5  lies in the fact that rotary motors are inherently more efficient than linear motors. For example, there are 3 different armature/stator relationships which can be useful in a linear motor: 1) An under hung relationship in which the coils and poles of the stator extend beyond the length of the armature magnets, so that as the magnets move back and forth, the armature for at least some range of travel remains within the stator poles. The design may be such that at maximum excursion the armature still stays within the coils, or it may begin to extend past them at some point. 2) An even hung relationship in which the armature magnets are the same length as the stator poles. In this design, as soon as the armature begins to move, some magnets move outside of the stator poles. 3) An over hung relationship in which the armature magnets exceed the length of the stator poles. In the over hung design, movement of the armature does not change the amount of magnet residing within the stator poles, over at least some excursion range. In this design, the whole excursion range can be used, or just some part. 
     In any of the above described relationships, a trade off is made between efficiency and cost. For example, as soon as some magnets move outside of the stator poles, their contribution to force output is reduced rapidly. Due to the relatively high expense of the magnets, is desirable to make full use of the magnets all the time. 
     When used in limited space conditions as found in the active seat vibration control application, and for example, when using an under hung design, it is possible to make full use of the magnets. However, the amount of force produced over the majority of the excursion range for a fixed input current will be less than if more magnets were used. Thus, the efficiency of the linear motor is reduced, where efficiency is defined as output mechanical power divided by input electrical power. An even hung design trades off between these factors. 
     The advantage in using a rotary motor rather than a linear motor is that it is inherent in the rotary design that all the magnets see the poles of the stator for all angles of rotation. This is an optimum condition for trading off efficiency and cost. For this reason it is advantageous to use a rotary motor and a mechanism for converting rotary motion to linear motion, rather than a linear actuator, for linear positioning applications. 
     Although the illustrated implementation shows the actuator  50  for converting rotary motion to linear motion used to actively control vibration of a vehicle seat, the actuator  50  is not limited to this application. For example, the actuator  50  is also suitable for use in other aspects of vehicle vibration control including wheel suspension systems and engine vibration control systems. Moreover, the actuator  50  is not limited to vibration control, and has general application to object position control. For example, the actuator can be used to control engine valve motion, whereby engine efficiency can be improved. 
     Referring now to  FIG. 14 , the actuator  50  may be used in an internal combustion engine  700  to control engine valve position, replacing traditional cam-shaft driven valve trains. The engine  700  has a plurality of cylinders  712  (only one cylinder is shown) disposed in a cylinder block  718  arranged in a V configuration to form cylinder banks  714  with the upper ends of the cylinders  712  being closed by cylinder heads  716 . A pair of inlet valves  740  (only one of which is shown) are longitudinally aligned on the inner side of the cylinder  712  and its associated combustion chamber  744 , and an exhaust valve  762  is located on the outer side of the cylinder  712 . An igniter in the form of a spark plug  766  or similar device is also disposed in the combustion chamber  744  of each cylinder  712 . 
     An actuator  50  is provided for each inlet and exhaust valve  740 ,  762 , and predetermined point P of the first link  150  is pivotably connected to the corresponding valve stem. The actuator  50  serves to position the inlet and exhaust valve  740 ,  762  relative to the cylinder block  718  based on control signals received from a controller (not shown). The controller receives signals including valve movement data from encoder signals indicating rotor position relative to the housing  62 , and crankshaft position data. Based on these signals, the controller outputs control signals to the rotary motor  60  of the actuator  50  such that the position of the valve  740 ,  762  is controlled relative to the cylinder block  718 . 
     Referring to  FIG. 15 , in other implementations, the actuator  50  may be indirectly connected to the respective inlet and exhaust valves  740 ,  762 . For example, the predetermined point P of the first link  150  can be connected to an inlet push rod  730 . The push rod  730  actuates an inlet rocker arm  732  that rocks on a pivot axis  734 . Rocker arm  732  includes a pair of actuating arms  736  each of which preferably carries a hydraulic lash adjuster  738 . The lash adjusters  738  engage the pair of inlet valves  740 . Although hidden in this view by the actuator  50  and the inlet push rod  730 , another actuator  50  is connected to an exhaust push rod for actuating a primary rocker arm  754  which is pivotable on the same pivot axis  734  as the inlet rocker arm  732 . The primary rocker arm  754  in turn engages a secondary push rod  756  which engages with a secondary rocker arm  758 . An actuating arm of rocker arm  758  directly engages the exhaust valve  762 . Like the previous implementation, the actuator  50  serves to position the inlet and exhaust valve  740 ,  762  relative to the cylinder block  718  based on control signals received from a controller (not shown). The controller receives signals including valve movement data from encoder signals indicating rotor position relative to the housing  62 , and crankshaft position data. Based on these signals, the controller outputs control signals to the rotary motor  60  of the actuator  50  such that the position of the valve  740 ,  762  is controlled relative to the cylinder block  718 . 
     Although the engine valve position control implementations illustrated here provide an actuator  50  for each engine valve  740 ,  762  of the cylinder  712 , this is non-limiting. For example, a single actuator  50  could be used to control multiple valves. For example, a single actuator could simultaneously actuate multiple input valves coupled to a single combustion chamber. 
     Using the actuator  50  to control valve operation advantageously allows motion of the valves to be decoupled from rotation of the engine crankshaft. In addition, a fully controllable valve allows complete control of timing and lift, over the entire range of engine speeds. This allows valve operation to be optimized over all operating conditions. It also allows variation with operation, enabling operation in an engine efficiency mode, or in maximum power delivery mode. It makes engine cylinder de-activation easy, and allows more complex de-activation schemes. For example, rather than de-activating an entire cylinder bank as is current practice, a portion of a cylinder bank or an individual cylinder can be deactivated. In addition, use of actuator  50  to control valve operation allows allow an engine to be self started, without the need for a separate starter to rotate the crankshaft. 
     Using the engine valve control system described herein, including the actuator  50  provides conversion of rotary to linear motion in a manner that is well suited for this application in which the linear range of travel is maximized within a limited space. For example, unlike a linear actuator which must be arranged in line with the valve shaft and extend upwards from the valve stem, the actuator  50  can control a valve lift profile at will from a location to one side of the valve, and thus does not add height to the valve train. Location of the actuator  50  to the side of the valve can provide linear displacement of the valve without requiring a lever or rocker arm, which can significantly reduce friction losses and wear of the valve guides. 
     This feature, in combination with the compact size of the actuator  50 , permits packaging of the actuators so that when multiple valves per cylinder are employed, multiple actuators can be fit around the cylinder or positioned remotely about the periphery of the cylinder while still providing full control of each valve. 
     Because the actuator  50  employs a rotary motor  60  which acts through a linkage to control valve position, the actuator  50  can be located away from the cylinder head  716 . This is advantageous since this permits the actuator  50  and sensors to avoid high temperatures associated with cylinder exhaust valves and manifold. This increases the amount of power than can be dissipated in the coils of the actuator before thermal demagnetizaton temperatures are reached. In addition, since the actuator motors are located away from the valves themselves, design of cooling devices is simplified. For example, cooling jackets can be provided that surround all the actuator motors without interfering with other structures. 
     In the actuator  50 , a rotary encoder  120  is used to sense position. This sensor is located with the rotary motor  60 , away from the location of the valve. The rotary encoder  120  can be much less expensive and more reliable than linear position and velocity sensors. It also can be located in a position where it sees lower temperatures. Because the actuator  50  employs a rotary motor  60 , design and manufacture of reliable sensors to detect valve position and velocity is relatively straightforward. 
     Because the actuator  50  employs a rotary motor  60 , this device is well suited for use in the engine  700  since substantial peak power at high engine speeds is required to overcome cylinder cracking pressure and open the exhaust valves. This requirement puts extreme demands on the power electronics of the system, and also drives a need for maximum efficiency in the actuator. For the reasons discussed above, a rotary motor is inherently more efficient than, for example, an actuator employing linear motor. The relative efficiency of the rotary motor can be used to possibly downsize the motor itself, or to reduce the electrical power requirements, or both. 
     From a packaging perspective, the actuator  50  including the rotary motor  60  and linkage  52  has much lower profile than a linear motor, and due to the linkage connection between the motor and the valve, the actuator can be integrated with the valve train such that the rotary motors does not sit directly over the valves. For example, the actuator  50  can be disposed between cylinder banks of V engines. Moreover, a separate linkage (if necessary) can connect the point P of the linkage  52  to the valves. By locating the rotary motor of the actuator away from the valves, it becomes much easier to package an actively controlled multiple valve per cylinder system. 
     Precise control is needed to avoid having the valve collide with the valve seat. The particular relationship between torque and position obtained by the actuator  50  simplifies the control of the engine valve. 
     Referring to  FIG. 16 , actuator  650  is an alternative implementation of the actuator  50 . The actuator  650  is substantially similar to the actuator  50 , except that the end caps  100 ,  200  of the actuator  50  are modified to improve ease of assembly. In particular, the modified end caps (not shown) are formed having a reduced outer diameter, and the large diameter rotor bearings  89  which support the end caps  100 ,  200  are replaced with similar bearings of smaller diameter. Due to the reduced diameter of the modified end caps, a third link  280  is provided which corresponds to the first bar  116  of the four-bar linkage and is dimensioned accordingly. 
     Although the illustrated implementation is described as using specific motor and bearings, the present invention is not limited to these components and it is understood that the motor and bearings are selected based on the requirements of the specific application. 
     A selected illustrative embodiment of the mechanism for converting rotary motion to linear motion is described above in some detail. However, it should be understood that only structures considered necessary for clarifying the present invention have been described herein. Other conventional structures, and those of ancillary and auxiliary components of the system, are assumed to be known and understood by those skilled in the art. Moreover, while a working example of the present invention has been described above, the present invention is not limited to the working example described above, but various design alterations may be carried out without departing from the present invention as set forth in the claims.