Abstract:
An actuator system having a controlled element configured for rotary movement about a first axis relative to a reference structure; a linkage system between the element and reference structure; a first actuator and a second actuator configured to power a first degree of freedom and an independent second degree of freedom of the linkage system, respectively; the linkage system having a first link configured for rotary movement about a second axis not coincident to the first axis and second link configured for rotary movement about a third axis; the linkage system configured such that a first angle of rotation may be driven independently of a second angle of rotation between said first link and said reference structure; wherein one of the first or second actuators is configured and arranged to drive rotation of the element about the first axis when the other is operatively locked.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates generally to the field of actuator systems, and more particularly to an electromechanical redundant actuator. 
       BACKGROUND ART 
       [0002]    Redundant actuator systems are generally known. These systems typically arrange multiple actuators in a way in which their displacement is summed, or their torque is summed. 
       BRIEF SUMMARY OF THE INVENTION 
       [0003]    With parenthetical reference to the corresponding parts, portions or surfaces of the disclosed embodiment, merely for the purposes of illustration and not by way of limitation, the present invention provides an actuator system ( 100 ,  200 ) comprising: a controlled element ( 180 ,  280 ) configured for rotary movement about a first axis ( 105 ,  203 ) relative to a reference structure ( 110 ,  210 ); a linkage system ( 170 ,  270 ) connected to the element and the reference structure; a first actuator ( 120 ,  220 ) configured and arranged to power a first degree of freedom of the linkage system ( 123  relative to  122 ,  223  relative to  222 ); a second actuator ( 140 ,  240 ) configured and arranged to power a second degree of freedom of the linkage system ( 143  relative to  142 ,  243  relative to  242 ), the first degree of freedom and the second degree of freedom being independent degrees of freedom; the linkage system having a first link ( 123 ,  223 ) configured for rotary movement about a second axis ( 104 ,  204 ) relative to the reference structure; the first axis and the second axis not being coincident; the linkage system having a second link ( 143 ,  243 ) configured for rotary movement about a third axis ( 105 ,  205 ) relative to the reference structure; the first link and the second link coupled ( 160 ,  260 ) such that rotation of the first link about the second axis in a first direction ( 126 ,  146 ,  226 ,  246 ) causes rotation of the second link about the third axis in a second direction ( 126 ,  146 ,  226 ,  246 ); the linkage system configured and arranged such that a first angle of rotation ( 144 ,  244 ) between the element and the reference structure may be driven independently of a second angle of rotation ( 125 ,  225 ) between the first link and the reference structure; wherein one of the first or second actuators is configured and arranged to drive rotation of the element about the first axis when the other of the first or second actuator is operatively locked 
         [0004]    The first axis ( 105 ,  203 ) and the second axis ( 104 ,  204 ) may be substantially parallel and operatively offset a substantially constant distance. The first axis ( 203 ), the second axis ( 204 ) and the third axis ( 205 ) may be substantially parallel and operatively offset a substantially constant distance. The third axis may be substantially coincident with the second axis ( 104 ). The first link and the second link may be coupled with a coupling comprising a connecting link ( 160 ) having a first pivot ( 160   b ) and a second pivot ( 160   c ). The coupling may comprise a bar link ( 160   a ) between the first pivot and the second pivot. The first link and the second link may be coupled with a coupling comprising meshed gears. The first link and the second link may be coupled ( 160 ,  260 ) such that the first direction of rotation ( 126 ,  226 ) of the first link is opposite to the second direction of rotation ( 146 ,  246 ) of the second link. The first link and the second link may be coupled ( 260 ′) such that the first direction of rotation ( 226 ′) of the first link is the same as the second direction of rotation ( 226 ′) of the second link. The actuator system may further comprise: a third actuator ( 320 ) configured and arranged to power a third degree of freedom of the linkage system ( 323  relative to  322 ); a fourth actuator ( 340 ) configured and arranged to power a fourth degree of freedom of the linkage system ( 343  relative to  342 ), the third degree of freedom and the fourth degree of freedom being independent degrees of freedom; the linkage system having a third link ( 323 ) configured for rotary movement about a fourth axis ( 304 ) relative to the reference structure; the linkage system having a fourth link ( 343 ) configured for rotary movement about a fifth axis ( 305 ) relative to the reference structure; the fourth axis and the fifth axis not being coincident with each other or with the first axis or the second axis; the third link and the fourth link coupled ( 360 ) such that rotation of the third link about the fourth axis in a first direction causes rotation of the fourth link about the fifth axis in a second direction; wherein one of the third or fourth actuators is configured and arranged to drive rotation of the element about the first axis when the other of the third or fourth actuator is operatively locked. The first, second, third or fourth actuators may be configured and arranged to drive rotation of the element about the first axis when the others of the first, second, third and fourth actuators have failed open. The third link and the fourth link may be coupled ( 360 ) such that the first direction of rotation of the third link is opposite to the second direction of rotation of the fourth link. The third link and the fourth link may be coupled ( 260 ′) such that the first direction of rotation of the third link is the same as the second direction of rotation of the fourth link. Each of the actuators may be supported by a bearing ( 436 ). The first actuator may comprise a planetary gear stage ( 600 ). The linkage system may comprise at least five links. The linkage system may comprise a plurality of pivot joints between the links. The first actuator may comprise a rotary actuator. The first actuator may comprise a rotary motor, a hydraulic actuator, or an electric motor. The first link may comprise a stator and the second link may comprise a stator. Each of the actuators may comprise a brake. The actuator system may further comprise a brake ( 603 ) configured and arranged to hold one of the degrees of freedom of the linkage system constant. The actuator system may further comprise a spring ( 604 ) configured and arranged to bias one of the degrees of freedom of the linkage system. The spring may be selected from a group consisting of a torsional spring, a linear spring, and a flexure. The actuator system may further comprise a damper ( 605 ) configured and arranged to dampen rotation of at least one link in the linkage system. The damper may be selected from a group consisting of a linear damper and a rotary damper. The first actuator and the second actuator may comprise a stepper motor or a permanent magnet motor. The first actuator and the second actuator may comprise a magnetic clutch ( 607 ). The element may be selected from a group consisting of a shaft and an aircraft control surface. The element may be selected from a group consisting of a wing spoiler, a flap, a flaperon and an aileron. The reference structure may be selected from a group consisting of an actuator frame, an actuator housing and an airframe. 
         [0005]    In another aspect, an actuator system ( 100 ′,  200 ′) is provided comprising: a controlled element ( 180 ,  280 ) configured for rotary movement about a first axis ( 105 ,  203 ) relative to a reference structure ( 110 ,  210 ); a linkage system ( 170 ,  270 ) connected to the element and the reference structure; a first actuator ( 120 ,  220 ) configured and arranged to power a first degree of freedom of the linkage system ( 123  relative to  122 ,  223  relative to  222 ); a hold device ( 140 ′,  240 ′) configured and arranged to selectively lock a second degree of freedom of the linkage system ( 143 ′ relative to  142 ′,  243 ′ relative to  242 ′), the first degree of freedom and the second degree of freedom being independent degrees of freedom; the linkage system having a first link ( 123 ,  223 ) configured for rotary movement about a second axis ( 104 ,  204 ) relative to the reference structure; the first axis and the second axis not being coincident; the linkage system having a second link ( 143 ′,  243 ′) configured for rotary movement about a third axis ( 105 ,  205 ) relative to the reference structure; the first link and the second link coupled ( 160 ,  260 ) such that rotation of the first link about the second axis in a first direction ( 126 ,  146 ,  226 ,  246 ) causes rotation of the second link about the third axis in a second direction ( 126 ,  146 ,  226 ,  246 ); the linkage system configured and arranged such that a first angle of rotation ( 144 ,  244 ) between the element and the reference structure may be driven independently of a second angle of rotation ( 125 ,  225 ) between the first link and the reference structure; wherein the hold device is configured and arranged to lock the second degree of freedom when the first actuator is operational and to unlock the second degree of freedom when the first actuator is operatively locked. 
         [0006]    The first axis ( 105 ,  203 ) and the second axis ( 104 ,  204 ) may be substantially parallel and operatively offset a substantially constant distance. The first axis ( 203 ), the second axis ( 204 ) and the third axis ( 205 ) may be substantially parallel and operatively offset a substantially constant distance. The third axis may be substantially coincident with the second axis ( 104 ). The first link and the second link may be coupled ( 160 ,  260 ) such that the first direction of rotation ( 126 ,  226 ) of the first link is opposite to the second direction of rotation ( 146 ,  246 ) of the second link. The first link and the second link may be coupled ( 260 ′) such that the first direction of rotation ( 226 ′) of the first link is the same as the second direction of rotation ( 226 ′) of the second link. The actuator system may further comprising: a second actuator ( 320 ) configured and arranged to power a third degree of freedom of the linkage system ( 323  relative to  322 ); a second hold device ( 340 ′) configured and arranged to selectively lock a fourth degree of freedom of the linkage system ( 343 ′ relative to  342 ′), the third degree of freedom and the fourth degree of freedom being independent degrees of freedom; the linkage system having a third link ( 323 ) configured for rotary movement about a fourth axis ( 304 ) relative to the reference structure; the linkage system having a fourth link ( 343 ′) configured for rotary movement about a fifth axis ( 305 ) relative to the reference structure; the fourth axis and the fifth axis not being coincident with each other or with the first axis or the second axis; the third link and the fourth link coupled ( 360 ) such that rotation of the third link about the fourth axis in a first direction causes rotation of the fourth link about the fifth axis in a second direction; wherein the second hold device is configured and arranged to lock the fourth degree of freedom when the second actuator is operational and to unlock the fourth degree of freedom when the second actuator is operatively locked. The first actuator may comprise a rotary actuator. The first link may comprise a stator. The element may be selected from a group consisting of a shaft and an aircraft control surface. The reference structure may be selected from a group consisting of an actuator frame, an actuator housing and an airframe. 
         [0007]    In another aspect, an actuator system ( 100 ,  200 ) is provided comprising: a controlled element ( 180 ,  280 ) configured for rotary movement about a first axis ( 105 ,  203 ) relative to a reference structure ( 110 ,  210 ); a plurality of actuator units, each of the actuator units comprising: a linkage system ( 170 ,  270 ) connected to the element and the reference structure; a first actuator ( 120 ,  220 ) configured and arranged to power a first degree of freedom of the linkage system ( 123  relative to  122 ,  223  relative to  222 ); a second actuator ( 140 ,  240 ) configured and arranged to power a second degree of freedom of the linkage system ( 143  relative to  142 ,  243  relative to  242 ), the first degree of freedom and the second degree of freedom being independent degrees of freedom; the linkage system having a first link ( 123 ,  223 ) configured for rotary movement about a second axis ( 104 ,  204 ) relative to the reference structure; the first axis and the second axis not being coincident; the linkage system having a second link ( 143 ,  243 ) configured for rotary movement about a third axis ( 105 ,  205 ) relative to the reference structure; the first link and the second link coupled ( 160 ,  260 ) such that rotation of the first link about the second axis in a first direction ( 126 ,  146 ,  226 ,  246 ) causes rotation of the second link about the third axis in a second direction ( 126 ,  146 ,  226 ,  246 ); the linkage system configured and arranged such that a first angle of rotation ( 144 ,  244 ) between the element and the reference structure may be driven independently of a second angle of rotation ( 125 ,  225 ) between the first link and the reference structure; wherein one of the first or second actuators is configured and arranged to drive rotation of the element about the first axis when the other of the first or second actuator is operatively locked. 
         [0008]    In another aspect, an actuator system ( 100 ,  200 ) is provided comprising: a controlled element ( 180 ,  280 ) configured for rotary movement about a first axis ( 105 ,  203 ) relative to a reference structure ( 110 ,  210 ); a plurality of actuator units, each of the actuator units comprising: a first actuator ( 120 ,  220 ) configured and arranged to power a first degree of freedom of the linkage system ( 123  relative to  122 ,  223  relative to  222 ); a hold device ( 140 ′,  240 ′) configured and arranged to selectively lock a second degree of freedom of the linkage system ( 143 ′ relative to  142 ′,  243 ′ relative to  242 ′), the first degree of freedom and the second degree of freedom being independent degrees of freedom; the linkage system having a first link ( 123 ,  223 ) configured for rotary movement about a second axis ( 104 ,  204 ) relative to the reference structure; the first axis and the second axis not being coincident; the linkage system having a second link ( 143 ′,  243 ′) configured for rotary movement about a third axis ( 105 ,  205 ) relative to the reference structure; the first link and the second link coupled ( 160 ,  260 ) such that rotation of the first link about the second axis in a first direction ( 126 ,  146 ,  226 ,  246 ) causes rotation of the second link about the third axis in a second direction ( 126 ,  146 ,  226 ,  246 ); the linkage system configured and arranged such that a first angle of rotation ( 144 ,  244 ) between the element and the reference structure may be driven independently of a second angle of rotation ( 125 ,  225 ) between the first link and the reference structure; wherein the hold device is configured and arranged to lock the second degree of freedom when the first actuator is operational and to unlock the second degree of freedom when the first actuator is operatively locked. 
         [0009]    In another aspect, an actuator system is provided comprising: a reference structure; an output member rotatably coupled to the reference structure for rotation about a first axis; a first actuator having a first member and a second member, the second member configured to rotate relative to the reference structure, the first member configured to rotate relative to the reference structure about a second axis, the first member configured to rotate relative to the reference structure independent of the rotation of the second member relative to the reference structure; a second actuator having a first member and a second member, the second member configured to rotate relative to the reference structure, the first member configured to rotate relative to the reference structure about a third axis, the first member configured to rotate relative to the reference structure independent of the rotation of the second member relative to the reference structure; a first link pivotally connected between the first member of the first actuator and the first member of the second actuator, the first member of the first actuator and the first member of the second actuator coupled such that rotation of the first member of the first actuator in a first direction causes rotation of the first member of the second actuator in a second direction; a second link pivotally connected between the second member of the first actuator and the output member. 
         [0010]    The actuator system may further comprise a third link pivotally connected between the second member of the second actuator and the output member. The first direction and the second direction may be the same. The first direction and the second direction may be opposite. The first axis and the third axis may be coincident. The first member may be a stator. The second member may be a rotor. 
         [0011]    In another aspect, an actuator system is provided comprising: a reference structure; an output member rotatably coupled to the reference structure for rotation about a first axis; a first actuator having a first member and a second member, the second member configured to rotate relative to the reference structure, the first member configured to rotate relative to the reference structure about a second axis, the first member configured to rotate relative to the reference structure independent of the rotation of the second member relative to the reference structure; a holding device having a first member and a second member, the second member configured to rotate relative to the reference structure, the holding device configured to alternate between a first configuration where the rotational position of the first member relative to the second member is locked and a second position where the first and second members are free to rotate relative to each other; a first link pivotally connected between the first member of the first actuator and the first member of the holding device, the first member of the first actuator and the first member of the holding device coupled such that rotation of the first member of the first actuator in a first direction causes rotation of the first member of the holding device in a second direction; a second link pivotally connected between the second member of the first actuator and the output member; wherein the holding device moves from the first configuration to the second configuration when the first actuator is operatively locked. 
         [0012]    The actuator system may further comprise a third link pivotally connected between the second member of the holding device and the output member. The holding device may be a magnetic clutch. The first direction and the second direction may be the same. The first direction and the second direction may be opposite. The first axis and the third axis may be coincident. The first member may be a stator. The second member may be a rotor. 
         [0013]    In another aspect, a method of controlling an actuator system is provided comprising the steps of: providing an actuator system comprising: a controlled element configured for rotary movement about a first axis relative to a reference structure; a linkage system connected to the element and the reference structure; a first actuator configured and arranged to power a first degree of freedom of the linkage system; a second actuator configured and arranged to power a second degree of freedom of the linkage system, the first degree of freedom and the second degree of freedom being independent degrees of freedom; the linkage system having a first link configured for rotary movement about a second axis relative to the reference structure; the first axis and the second axis not being coincident; the linkage system having a second link configured for rotary movement about a third axis relative to the reference structure; the first link and the second link coupled such that rotation of the first link about the second axis in a first direction causes rotation of the second link about the third axis in a second direction; the linkage system configured and arranged such that a first angle of rotation between the element and the reference structure may be driven independently of a second angle of rotation between the first link and the reference structure; wherein one of the first or second actuators is configured and arranged to drive rotation of the element about the first axis when the other of the first or second actuator is operatively locked; and providing power to the first actuator and the second actuator simultaneously such that the controlled element is rotated about the second axis and an angular position of the first link is held constant about the first axis. 
         [0014]    In another aspect, a method of controlling an actuator system is provided comprising the steps of: providing an actuator system comprising: a controlled element configured for rotary movement about a first axis relative to a reference structure; a linkage system connected to the element and the reference structure; a first actuator configured and arranged to power a first degree of freedom of the linkage system; a hold device configured and arranged to selectively lock a second degree of freedom of the linkage system, the first degree of freedom and the second degree of freedom being independent degrees of freedom; the linkage system having a first link configured for rotary movement about a second axis relative to the reference structure; the first axis and the second axis not being coincident; the linkage system having a second link configured for rotary movement about a third axis relative to the reference structure; the first link and the second link coupled such that rotation of the first link about the second axis in a first direction causes rotation of the second link about the third axis in a second direction; the linkage system configured and arranged such that a first angle of rotation between the element and the reference structure may be driven independently of a second angle of rotation between the first link and the reference structure; wherein the hold device is configured and arranged to lock the second degree of freedom when the first actuator is operational and to unlock the second degree of freedom when the first actuator is operatively locked; and providing power to the first actuator and the hold device simultaneously such that the hold device link locks the second degree of freedom of the linkage system, and the first actuator applies a desired torque to the controlled element 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is an isometric view of a first actuator system. 
           [0016]      FIG. 2  is an isometric view of an alternate embodiment of the first actuator system. 
           [0017]      FIG. 3  is a perspective view of a second actuator system. 
           [0018]      FIG. 4  is a perspective view of an alternate embodiment of the second actuator system. 
           [0019]      FIG. 5  is a perspective view of a third actuator system. 
           [0020]      FIG. 6  is an alternate embodiment of the third actuator system. 
           [0021]      FIG. 7  is an alternate embodiment of the second actuator system. 
           [0022]      FIG. 8  is a front perspective view of a fourth actuator system. 
           [0023]      FIG. 9  is a rear perspective view of the fourth actuator system. 
           [0024]      FIG. 10  is a front elevational view of the fourth actuator system. 
           [0025]      FIG. 11  is a side elevational view of the fourth actuator system. 
           [0026]      FIG. 12  is a sectional view taken along lines  12 - 12  of  FIG. 11 . 
           [0027]      FIG. 13  is a rear elevational view of the fourth actuator system. 
           [0028]      FIG. 14  is a partially exploded front perspective view of the fourth actuator system. 
           [0029]      FIG. 15  is a partially exploded rear perspective view of the fourth actuator system. 
           [0030]      FIG. 16  is a front perspective view of the fourth actuator system with a reference structure removed for clarity. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0031]    At the outset, it should be clearly understood that like reference numerals are intended to identify the same structural elements, portions or surfaces consistently throughout the several drawing figures, as such elements, portions or surfaces may be further described or explained by the entire written specification, of which this detailed description is an integral part. Unless otherwise indicated, the drawings are intended to be read (e.g., cross-hatching, arrangement of parts, proportion, debris, etc.) together with the specification, and are to be considered a portion of the entire written description of this invention. As used in the following description, the terms “horizontal”, “vertical”, “left”, “right”, “up” and “down”, as well as adjectival and adverbial derivatives thereof, (e.g., “horizontally”, “rightwardly”, “upwardly”, etc.), simply refer to the orientation of the illustrated structure as the particular drawing figure faces the reader. Similarly, the terms “inwardly” and “outwardly” generally refer to the orientation of a surface relative to its axis of elongation, or of rotation, as appropriate. 
         [0032]    Referring now to the drawings, and initially to  FIG. 1  thereof, this invention provides an improved actuator system, of which a first embodiment is generally indicated at  100 . Reference structure  110  may comprise a rigid material. Reference structure  110  has a first portion  110 A and a second portion  110 B, which are rigidly connected to each other through a third portion  110 C. First portion  110 A holds two couplings  112  and  113 , which are connected to shaft  121  and shaft  141  respectively. Coupling  112  holds shaft  121  in rotary engagement for rotation relative to reference structure  110  about axis  104 . Similarly, coupling  113  holds shaft  141  in rotary engagement for rotation relative to reference structure  110  about axis  105 . Axes  104  and  105  are generally parallel to each other and separated by a fixed distance. 
         [0033]    First rotary actuator  120  has a first member  123  and a second member  122  which are configured and arranged for relative rotary motion to each other about axis  104 . Rotary actuator  120  is an electric motor, however other actuator types such as, but not limited to, hydraulic actuators, pneumatic actuators, or other similar actuators may also be used. First member  123  may be referred to as a stator and second member  122  may be referred to as a rotor, however, it should be noted that neither stator  123  nor rotor  122  are stationary relative to reference structure  110 . 
         [0034]    Rotor  122  is rigidly coupled to shaft  121 . Stator  123  is specifically not rigidly mounted to reference structure  110 . More concretely, stator  123  is able to rotate relative to reference structure  110  about axis  104  independent of the rotation of rotor  122  relative to reference structure  110 . Stated another way, first rotary actuator  120  has two degrees of freedom relative to reference structure  110 . A first degree of freedom can be defined as angle of rotation  124  of rotor  122  relative to reference structure  110 . A second degree of freedom can be defined as angle of rotation  125  of stator  123  relative to reference structure  110 . 
         [0035]    Second rotary actuator member  140  has first member  143  and second member  142  which are configured and arranged for relative rotary motion to each other about axis  105 . Rotary actuator  140  is an electric motor, however other actuator types such as, but not limited to hydraulic actuators, pneumatic actuators, or other similar actuators may also be used. First member  143  may be referred to as a stator and second member  142  may be referred to as a rotor. However, it should be noted that neither stator  143  nor rotor  142  are stationary relative to reference structure  110 . 
         [0036]    Rotor  142  is rigidly coupled to shaft  141 . Stator  143  is specifically not rigidly mounted to reference structure  110 . More concretely, stator  143  is able to rotate relative to reference structure  110  about axis  105  independent the rotation of rotor  142  relative to the reference structure  110 . Stated another way, second actuator  140  has two degrees of freedom relative to reference structure  110 . A first degree of freedom can be defined as angle of rotation  144  of rotor  142  relative to reference structure  110 . A second degree of freedom can be defined as angle of rotation  145  of stator  123  relative to reference structure  110 . 
         [0037]    Output member  180  is rigidly coupled to rotor  142 . Therefore, output member rotates together with rotor  142  relative to reference structure  110  about axis  105 . Second portion  110 B of reference structure  110  has couplings  115  and  116  which respectively provide additional support in holding output member  180  and rotor  122  in rotary engagement with reference structure  110 . Output member  180  may be coupled to an object to be driven, such as an aircraft control surface. 
         [0038]    Stator  123  and stator  143  are rotationally coupled together through coupling  160 . Coupling  160  causes stator  123  to rotate relative to reference structure  110  by an angle opposite to the rotation of stator  143  relative to reference structure  110 . More specifically, coupling  160  causes any change in angle  125  to cause an equal and opposite change in angle  145 . In other words, a degree of freedom between rotary actuator  120  and reference structure  110  is caused to be shared with one degree of freedom between rotary actuator  140  and reference structure  110  by coupling  160 . Coupling  160  is a link pivotally connected to stator  123  and pivotally connected to stator  143 . Drive arm portion  123   a  is disposed on stator  123 , and drive arm portion  143   a  is disposed on stator  143 . Link  160   a  is pivotally connected between drive arm portions  123   a  and  143   a . However, coupling  160  my alternatively be a gear coupling, a belt coupling, or other similar coupling. 
         [0039]    Rotor  122  and rotor  142  are also coupled together through coupling  190 . Coupling  190  causes rotor  122  to rotate relative to reference structure  110  by an angular direction equal to how rotor  142  rotates relative to reference structure  110 . More specifically, coupling  190  causes any change in angle  124  to equal a change in angle  144 . In other words, a degree of freedom between rotary actuator  120  and reference structure  110  is caused to be shared with one degree of freedom between rotary actuator  140  and reference structure  110  by coupling  190 . As shown in  FIG. 1 , coupling  190  is a link  190   a  pivotally connected to drive arm portion  122 A of member  122  and pivotally connected to drive arm portion  142 A of member  142 , however, coupling  190  my alternatively be a gear coupling, a belt coupling, or other similar coupling. 
         [0040]    While coupling  160  causes stator  123  and stator  143  to rotate in opposite directions relative to reference structure  110 , coupling  190  causes rotor  122  and rotor  142  to rotate in equivalent directions relative to reference structure  110 . 
         [0041]    Linkage  170  is a set of rigid links and joints between reference member  110  and output member  180 . More specifically, linkage  170  comprises couplings  160  and  190 , and members  121 ,  122 ,  123 ,  141 ,  142 , and  143 . Linkage  170  has two degrees of freedom relative to reference  110 . In other words, the state of linkage  170  relative to reference  110  can be described by two independent variables. For example, knowing angle  144  (which represents the angle of rotor  142  to reference structure  110 ) and angle  124  (the angle of shaft  121  relative to reference structure  110 ) specifically define the state of linkage  170  since no member (link) within linkage  170  can be moved without adjusting angles  144  or  124 . In this view, angle  124  and angle  144  represent two independent degrees of freedom of linkage  170 . Alternatively, the two degrees of freedom of linkage  170  can be defined as angle  125  and angle  144 . No linkage  170  member can be moved relative to linkage  110  without changing angle  125  or angle  144 . 
         [0042]    Rotary actuator  100  is generally operated by powering first actuator  120  and second actuator  140  together at the same time to cause output member  180  to move relative to reference structure  110  in a desired manner. For example, if a user desires to cause output member  180  to rotate clockwise relative to reference structure  110 , in other words, if angle  144  is to be decreased, actuator  120  and actuator  140  would be actuated at the same time, actuator  120  providing a torque of equal and opposite magnitude as actuator  140 . More specifically, actuator  120  is actuated so as to apply a torque urging rotor  122  to rotate clockwise relative to stator  123 . At the same time, actuator  140  is actuated so as to apply a torque urging rotor  142  to rotate clockwise relative to stator  143 . Under this scenario, counteracting torques from actuator  120  and actuator  140  act against each other through coupling  160 . When actuator  120  applies a torque to rotor  122  in the clockwise direction, an equal and opposite torque is applied to coupling  160 , urging coupling  160  to rotate counterclockwise. The torque applied by actuator  120  onto coupling  160  manifests as a downward rightwards force on coupling  160 . When actuator  140  applies a torque to rotor  142  in the clockwise direction, an equal and opposite torque is applied to coupling  160 . The torque applied by actuator  140  onto coupling  160  manifests as an upwards-leftwards force applied on coupling  160  by actuator  140 . The force applied by actuator  120  onto coupling  160  is generally equal and opposite the force applied by actuator  140  onto coupling  160 . This generally results in stators  123  and  143  remaining stationary while rotors  122  and  142  rotate clockwise. Coupling  190  causes the angles of rotation  124 ,  144  of rotors  122  and  142  relative to reference structure  110  to remain equivalent. 
         [0043]    In order to cause output member  180  to rotate counter clockwise relative to reference structure  110 , rotary actuators  120  and  140  are actuated in the reverse direction compared to when causing output member  180  to rotate clockwise. 
         [0044]    Actuator  100  has the advantageous characteristic that if either actuator  120  or actuator  140  lock up (such as an electromechanical jam, or hydraulic valve lock), output member  180  will continue to be actuated in the desired direction of rotation by the non-failing actuator. This is because the locked up actuator will still be able to provide a counteracting torque to the other actuator through coupling  160 . For example, consider a user desiring to rotate output member  180  clockwise relative to reference structure  110  (decreasing angle  144 ) when actuator  120  inadvertently rotationally locks stator  123  relative to rotor  122 . Because stator  123  is rotationally locked to rotor  122 , any change in angle  124  between rotor  122  and reference structure  110  will necessary equal any change in angle  125  between stator  123  and reference structure  110 . Note that stator  123  and rotor  122  may still rotate together as a unit relative to reference structure  110 . When actuator  140  applies a clockwise torque to rotor  142 , the equal and opposite torque on stator  143  is distributed through coupling  160  as an upwards and leftwards force on coupling  160 . This upwards and leftwards force on coupling  160  results in a clockwise torque applied to stator  123  which is transmitted through the locked up actuator as a clockwise torque onto rotor  122 . Coupling  190  causes the rotation of rotors  122  and  142  to be equalized, while output member  180  is rotated clockwise as desired through the jam. 
         [0045]    In order to operate in a dual tandem mode, each actuator  120 ,  140  is provided with a braking mechanism which may be internal or external and a controller. These brakes will allow the actuator system  100  to continue working if one of the actuators fails in an open state (e.g. an actuator loses power allowing the stator and rotor free rotation relative to each other). The brake is configured within each actuator to lock rotation between the actuators stator and rotor relative to each other. The brake may be a fail-safe brake which does not need power in order to brake. In this dual tandem configuration, when one of the actuators  120 ,  140  fails in an open state, the brake in that failing actuator is engaged. This allows the remaining actuator  120 ,  140  to still cause actuation of output member  180 . However, during such a failure the speed that output member  180  rotates relative to the working actuator will be half the speed that the output member  180  rotates at when both actuators are working. 
         [0046]    Turning to  FIG. 2 , which shows an alternate embodiment  100 ′, actuator  120  is paired with a holding device  140 ′ which includes, but is not limited to, a brake, a magnetic clutch, a toroid motor, or the like. Under normal operation, holding device  140 ′ locks the rotational position between member  143 ′ and member  142 ′. If the actuator  120  jams then the holding device  140 ′ releases the lock between member  143 ′ and member  142 ′ which effectively releases any effect actuator  120  has on output member  180 . This allows output member  180  to be driven by another actuator (not shown). This arrangement is a simplex configuration because it includes one actuator  120  and one holding device  140 ′ and if the actuator  120  fails, the unit drops out of the network as will be described in greater detail below. In yet another alternate simplex configuration, two actuators may be provided without any brakes on either actuator, where one actuator is configured to only hold its rotor and stator position, while the other actuator is used to drive output member  180  through linkage system  170 . 
         [0047]    In  FIG. 3 , a second actuator system is generally indicated at  200 . Reference structure  210  comprises a rigid material. Reference structure  210  has a first portion  210 A and a second portion  210 B, which are fixed. First portion  210 A holds two couplings  212  and  213 , which are connected to shaft  221  and shaft  241  respectively. Coupling  212  holds shaft  221  in rotary engagement for rotation relative to reference structure  210  about axis  204 . Similarly, coupling  213  holds shaft  241  in rotary engagement for rotation relative to reference structure  210  about axis  205 . Axes  204  and  205  are generally parallel to each other and separated by a fixed distance. 
         [0048]    First rotary actuator  220  has a first member  223  and a second member  222  which are configured and arranged for relative rotary motion to each other about axis  204 . Rotary actuator  220  is an electric motor, however other actuator types such as, but not limited to, hydraulic actuators, pneumatic actuators, or other similar actuators may also be used. First member  223  may be referred to as a stator and second member  222  may be referred to as a rotor, however, it should be noted that neither stator  223  nor rotor  222  are stationary relative to reference structure  210 . 
         [0049]    Rotor  222  is rigidly coupled to shaft  221 . Stator  223  is specifically not rigidly mounted to reference structure  210 . More concretely, stator  223  is able to rotate relative to reference structure  210  about axis  204  independent of the rotation of rotor  222  relative to reference structure  210 . Stated another way, first rotary actuator  220  has two degrees of freedom relative to reference structure  210 . A first degree of freedom can be defined as angle of rotation  224  of rotor  122  relative to reference structure  210 . A second degree of freedom can be defined as angle of rotation  225  of stator  223  relative to reference structure  210 . 
         [0050]    Second rotary actuator member  240  has first member  243  and second member  242  which are configured and arranged for relative rotary motion to each other about axis  205 . Rotary actuator  240  is an electric motor, however other actuator types such as, but not limited to hydraulic actuators, pneumatic actuators, or other similar actuators may also be used. First member  243  may be referred to as a stator and second member  242  may be referred to as a rotor. However, it should be noted that neither stator  243  nor rotor  242  are stationary relative to reference structure  210 . 
         [0051]    Rotor  242  is rigidly coupled to shaft  241 . Stator  243  is specifically not rigidly mounted to reference structure  210 . More concretely, stator  243  is able to rotate relative to reference structure  210  about axis  205  independent the rotation of rotor  242  relative to the reference structure  210 . Stated another way, second actuator  240  has two degrees of freedom relative to reference structure  210 . A first degree of freedom can be defined as angle of rotation  244  of rotor  242  relative to reference structure  210 . A second degree of freedom can be defined as angle of rotation  245  of stator  223  relative to reference structure  210 . 
         [0052]    Output member  280  is coupled to rotors  222 ,  242 . Therefore, output member  280  rotates together with rotors  222 ,  242  relative to reference structure  210 . Second portion of  210 B reference structure  210  has couplings  215  and  216  which respectively provide additional support in holding rotors  222 ,  242  in rotary engagement with reference structure  210 . Couplings  214 ,  219  hold output member  280  in rotary engagement for rotation relative to reference structure  210 . Output member  280  may be coupled to an object to be driven, such as an aircraft control surface. 
         [0053]    Stator  223  and stator  243  are rotationally coupled together through coupling  260 . Coupling  260  causes stator  223  to rotate relative to reference structure  210  by an angle opposite to the rotation of stator  243  relative to reference structure  210 . More specifically, coupling  260  causes any change in angle  225  to cause an equal and opposite change in angle  245 . In other words, a degree of freedom between rotary actuator  220  and reference structure  210  is caused to be shared with one degree of freedom between rotary actuator  240  and reference structure  210  by coupling  260 . Coupling  260  is a link  260   a  pivotally connected to drive arm portion  223   a  of stator  223  and pivotally connected to drive arm portion  243   a  of stator  243 . However, coupling  260  may alternatively be a gear coupling, a belt coupling, or other similar coupling. 
         [0054]    Rotor  222  and rotor  242  are both coupled to output member  280  through coupling  270 . Coupling  270  causes the rotation of both rotors  222  and  242  to be transmitted to the output member  280  such that the output member  280  rotates in the same direction as the rotors  222 ,  242  relative to the reference structure  210 . More specifically, coupling  270  causes the rotation of the rotors  222 ,  242  to be summed together at the output member  280 . Coupling  270  comprises a pair of links  270   a  and  270   b . Link  270   a  is pivotally connected between drive arm portion  222 A of member  222  and drive arm portion  280   a  of output member  280 . Link  270   b  is pivotally connected between drive arm portion  242 A of member  242  and drive arm portion  280   b  of output member  280 . However, coupling  270  may alternatively be a gear coupling, a belt coupling, or other similar coupling. 
         [0055]    While coupling  260  causes stator  223  and stator  243  to rotate in opposite directions relative to reference structure  210 , coupling  270  causes rotor  122  and rotor  142  to rotate in equivalent directions relative to reference structure  210 . 
         [0056]    Linkage  290  is a set of rigid links and joints between reference member  210  and output member  280 . More specifically, linkage  290  comprises couplings  260  and  270 , and members  221 ,  222 ,  223 ,  241 ,  242 , and  243 . Linkage  290  has two degrees of freedom relative to reference  210 . In other words, the state of linkage  290  relative to reference  210  can be described by two independent variables. For example, knowing angle  244  (which represents the angle of rotor  242  to reference structure  210 ) and angle  224  (the angle of shaft  221  relative to reference structure  210 ) specifically define the state of linkage  290  since no member (link) within linkage  290  can be moved without adjusting angles  244  or  224 . In this view, angle  224  and angle  244  represent two independent degrees of freedom of linkage  290 . Alternatively, the two degrees of freedom of linkage  290  can be defined as angle  225  and angle  244 . No linkage  290  member can be moved relative to linkage  210  without changing angle  225  or angle  244 . 
         [0057]    Rotary actuator  200  is generally operated by powering first actuator  220  and second actuator  240  together at the same time to cause output member  280  to move relative to reference structure  210  in a desired manner. For example, if a user desires to cause output member  280  to rotate clockwise relative to reference structure  210  (as shown in the apparatus orientation in  FIG. 2 ), actuator  220  and actuator  240  would be actuated at the same time, actuator  220  providing a torque of equal and opposite magnitude as actuator  240 . More specifically, actuator  220  is actuated so as to apply a torque urging rotor  222  to rotate clockwise relative to stator  223 . At the same time, actuator  240  is actuated so as to apply a torque urging rotor  242  to rotate clockwise relative to stator  243 . Under this scenario, counteracting torques from actuator  220  and actuator  240  act against each other through coupling  260 . More specifically, when actuator  220  applies a torque to rotor  222  in the clockwise direction, an equal and opposite torque is applied to coupling  260 , urging coupling  260  to rotate counterclockwise. The torque applied by actuator  220  onto coupling  260  manifests as a downward rightwards force on coupling  260 . When actuator  240  applies a torque to rotor  242  in the clockwise direction, an equal and opposite torque is applied to coupling  260 . The torque applied by actuator  240  onto coupling  260  manifests as an upwards-leftwards force applied on coupling  260  by actuator  240 . The force applied by actuator  220  onto coupling  260  is generally equal and opposite the force applied by actuator  240  onto coupling  260 . This generally results in stators  223  and  243  remaining stationary while rotors  222  and  242  rotate clockwise. Coupling  270  causes the angles of rotation  224 ,  244  of rotors  222  and  242  relative to reference structure  210  to remain equivalent. 
         [0058]    In order to cause output member  280  to rotate counter clockwise relative to reference structure  210 , rotary actuators  220  and  240  are actuated in reverse compared to when causing output member  280  to rotate clockwise. 
         [0059]    In order to operate in a dual tandem mode, each actuator  220 ,  240  is provide with a brake that may be internal or external and a controller. If one of the actuators  220 ,  240  loses power then the brake in the failing unit will be applied, allowing the remaining actuator  220 ,  240  to move the output member at one half normal speed. Actuator  200  also has the advantageous characteristic that if either actuator  220  or actuator  240  lock up (such as an electromechanical jam, or hydraulic valve lock), output member  280  will continue to be actuated in the desired direction of rotation by the non-failing actuator. This is because the locked up actuator will still be able to provide a counteracting torque to the other actuator through coupling  260 . For example, consider a user desiring to rotate output member  280  clockwise relative to reference structure  210  when actuator  220  inadvertently rotationally locks stator  223  relative to rotor  222 . Because stator  223  is rotationally locked to rotor  222 , any change in angle  224  between rotor  222  and reference structure  210  will necessary equal any change in angle  225  between stator  223  and reference structure  210 . Note that stator  223  and rotor  222  may still rotate together as a unit relative to reference structure  210 . When actuator  240  applies a clockwise torque to rotor  242 , the equal and opposite torque on stator  243  is distributed through coupling  260  as an upwards and leftwards force on coupling  260 . This upwards and leftwards force on coupling  260  results in a clockwise torque applied to stator  223  which is transmitted through the locked up actuator as a clockwise torque onto rotor  2122 . Coupling  270  causes the rotation of rotors  222  and  242  to be equalized, while output member  280  is rotated clockwise as desired through the jam. 
         [0060]    Turning to  FIG. 4 , the actuator  220  is paired with holding device  240 ′ which includes, but is not limited to, a brake, a magnetic clutch, a toroid motor or the like. Under normal operation, holding device  240 ′ locks member  243 ′ and rotor  242 ′ relative to each other. If the actuator  220  jams or loses power then the holding device  240 ′ releases the rotor  242 ′ and the actuator  220  and hold device  240 ′ go into a bypass mode and rotate freely under the power of another actuator in the network. In yet another simplex configuration, actuators  220  and  240  of  FIG. 3  may be provided without any brakes. 
         [0061]    Turning to  FIG. 5 , a system with dual tandem actuators  220  and  240  is paired with dual tandem actuators  320  and  340  to form a third actuator system generally indicated at  300 . Reference structure  310  comprises a rigid material. Reference structure  310  has a first portion  310 A and a second portion  310 B, which are fixed. First portion  310 A holds two couplings  312  and  313 , which are connected to shaft  321  and shaft  341  respectively. Coupling  312  holds shaft  321  in rotary engagement for rotation relative to reference structure  310  about axis  304 . Similarly, coupling  313  holds shaft  341  in rotary engagement for rotation relative to reference structure  310  about axis  305 . Axes  304  and  305  are generally parallel to each other and separated by a fixed distance. 
         [0062]    First rotary actuator  320  has a first member  323  and a second member  322  which are configured and arranged for relative rotary motion to each other about axis  304 . Rotary actuator  320  is an electric motor, however other actuator types such as, but not limited to, hydraulic actuators, pneumatic actuators, or other similar actuators may also be used. First member  323  may be referred to as a stator and second member  322  may be referred to as a rotor, however, it should be noted that neither stator  323  nor rotor  322  are stationary relative to reference structure  310 . 
         [0063]    Rotor  322  is rigidly coupled to shaft  321 . Stator  323  is specifically not rigidly mounted to reference structure  310 . More concretely, stator  323  is able to rotate relative to reference structure  310  about axis  304  independent of the rotation of rotor  322  relative to reference structure  310 . Stated another way, first rotary actuator  320  has two degrees of freedom relative to reference structure  310 . A first degree of freedom can be defined as angle of rotation  324  of rotor  322  relative to reference structure  310 . A second degree of freedom can be defined as angle of rotation  325  of stator  323  relative to reference structure  310 . 
         [0064]    Second rotary actuator member  340  has first member  343  and second member  342  which are configured and arranged for relative rotary motion to each other about axis  305 . Rotary actuator  340  is an electric motor, however other actuator types such as, but not limited to hydraulic actuators, pneumatic actuators, or other similar actuators may also be used. First member  343  may be referred to as a stator and second member  342  may be referred to as a rotor. However, it should be noted that neither stator  343  nor rotor  342  are stationary relative to reference structure  310 . 
         [0065]    Rotor  342  is rigidly coupled to shaft  341 . Stator  343  is specifically not rigidly mounted to reference structure  310 . More concretely, stator  343  is able to rotate relative to reference structure  310  about axis  305  independent the rotation of rotor  342  relative to the reference structure  310 . Stated another way, second actuator  340  has two degrees of freedom relative to reference structure  310 . A first degree of freedom can be defined as angle of rotation  344  of rotor  342  relative to reference structure  310 . A second degree of freedom can be defined as angle of rotation  345  of stator  323  relative to reference structure  310 . 
         [0066]    Output member  280  is coupled to rotors  322 ,  342 . Therefore, output member  280  rotates together with rotors  322 ,  342  relative to reference structure  310 . Second portion of  310 B reference structure  310  has couplings  315  and  316  which respectively provide additional support in holding rotors  322 ,  342  in rotary engagement with reference structure  310 . Output member  280  may be coupled to an object to be driven, such as an aircraft control surface. 
         [0067]    Stator  323  and stator  343  are rotationally coupled together through coupling  360 . Coupling  360  causes stator  323  to rotate relative to reference structure  310  by an angle opposite to the rotation of stator  343  relative to reference structure  310 . More specifically, coupling  360  causes any change in angle  325  to cause an equal and opposite change in angle  345 . In other words, a degree of freedom between rotary actuator  320  and reference structure  310  is caused to be shared with one degree of freedom between rotary actuator  340  and reference structure  310  by coupling  360 . Coupling  360  is a link  360   a  pivotally connected to drive arm portion  323   a  of stator  323  and pivotally connected to drive arm portion  343   a  of stator  343 . However, coupling  360  may alternatively be a gear coupling, a belt coupling, or other similar coupling. 
         [0068]    Rotor  322  and rotor  342  are both coupled to output member  280  through coupling  370 . Coupling  370  causes the rotation of both rotors  322  and  342  to be transmitted to the output member  280  such that the output member  280  rotates in the same direction as the rotors  322 ,  342  relative to the reference structure  310 . More specifically, coupling  370  causes the rotation of the rotors  322 ,  342  to be summed together at the output member  280 . Coupling  370  comprises a pair of links  370   a  and  370   b . Link  370   a  is pivotally connected between drive arm portion  322 A of member  322  and drive arm portion  280   a  of output member  280 . Link  370   b  is pivotally connected between drive arm portion  342   a  of member  342  and drive arm portion  280   b  of output member  280 . However, coupling  370  may alternatively be a gear coupling, a belt coupling, or other similar coupling. 
         [0069]    While coupling  360  causes stator  323  and stator  343  to rotate in opposite directions relative to reference structure  310 , coupling  370  causes rotor  322  and rotor  342  to rotate in equivalent directions relative to reference structure  310 . 
         [0070]    Linkage  390  is a set of rigid links and joints between reference member  310  and output member  280 . More specifically, linkage  390  comprises couplings  360  and  370 , and members  321 ,  322 ,  323 ,  341 ,  342 , and  343 . Linkage  390  has two degrees of freedom relative to reference  310 . In other words, the state of linkage  390  relative to reference  310  can be described by two independent variables. For example, knowing angle  344  (which represents the angle of rotor  342  to reference structure  310 ) and angle  324  (the angle of shaft  321  relative to reference structure  310 ) specifically define the state of linkage  390  since no member (link) within linkage  390  can be moved without adjusting angles  344  or  324 . In this view, angle  324  and angle  344  represent two independent degrees of freedom of linkage  390 . Alternatively, the two degrees of freedom of linkage  390  can be defined as angle  325  and angle  344 . No linkage  390  member can be moved relative to linkage  310  without changing angle  325  or angle  344 . 
         [0071]    Rotary actuator  300  is generally operated by powering first actuator  320  and second actuator  340  together at the same time to cause output member  380  to move relative to reference structure  310  in a desired manner. For example, if a user desires to cause output member  280  to rotate clockwise relative to reference structure  310  (as shown in the apparatus orientation in  FIG. 5 ), actuator  320  and actuator  340  would be actuated at the same time, actuator  320  providing a torque of equal and opposite magnitude as actuator  340 . More specifically, actuator  320  is actuated so as to apply a torque urging rotor  322  to rotate clockwise relative to stator  323 . At the same time, actuator  340  is actuated so as to apply a torque urging rotor  342  to rotate clockwise relative to stator  343 . Under this scenario, counteracting torques from actuator  320  and actuator  340  act against each other through coupling  360 . More specifically, when actuator  320  applies a torque to rotor  322  in the clockwise direction, an equal and opposite torque is applied to coupling  360 , urging coupling  360  to rotate counterclockwise. The torque applied by actuator  320  onto coupling  360  manifests as a downward rightwards force on coupling  360 . When actuator  340  applies a torque to rotor  342  in the clockwise direction, an equal and opposite torque is applied to coupling  360 . The torque applied by actuator  340  onto coupling  360  manifests as an upwards-leftwards force applied on coupling  360  by actuator  340 . The force applied by actuator  320  onto coupling  360  is generally equal and opposite the force applied by actuator  340  onto coupling  360 . This generally results in stators  323  and  343  remaining stationary while rotors  322  and  342  rotate clockwise. Coupling  370  causes the angles of rotation  324 ,  344  of rotors  322  and  342  relative to reference structure  310  to remain equivalent. 
         [0072]    In order to cause output member  280  to rotate counter clockwise relative to reference structure  310 , rotary actuators  320  and  340  are actuated in reverse compared to when causing output member  280  to rotate clockwise. 
         [0073]    In order to operate in a dual tandem mode, each actuator  220 ,  240 ,  320 ,  340  is provided with a brake that may be internal or external and a controller. If one or more of the actuators  220 ,  240 ,  320 ,  340  lose power then one of the remaining actuators  220 ,  240 ,  320 ,  340  can move the output member  280 . The third actuator system  300  also has the advantageous characteristic that if any of the actuators  220 ,  240 ,  320 ,  340  lock up (such as an electromechanical jam, or hydraulic valve lock), output member  280  will continue to be actuated in the desired direction of rotation by at least one of the non-failing actuators. This is because, in the case of failure of actuator  220  or  240 , the locked up actuator will still be able to provide a counteracting torque to the other actuator through coupling  260 . For example, consider a user desiring to rotate output member  280  clockwise relative to reference structure  210  when actuator  220  inadvertently rotationally locks stator  223  relative to rotor  222 . Because stator  223  is rotationally locked to rotor  222 , any change in angle  224  between rotor  222  and reference structure  210  will necessary equal any change in angle  225  between stator  223  and reference structure  210 . Note that stator  223  and rotor  222  may still rotate together as a unit relative to reference structure  210 . When actuator  240  applies a clockwise torque to rotor  242 , the equal and opposite torque on stator  243  is distributed through coupling  260  as an upwards and leftwards force on coupling  260 . This upwards and leftwards force on coupling  260  results in a clockwise torque applied to stator  223  which is transmitted through the locked up actuator as a clockwise torque onto rotor  2122 . Coupling  270  causes the rotation of rotors  222  and  242  to be equalized, while output member  280  is rotated clockwise as desired through the jam. Also, rotors  320 ,  340  continue to rotate output member  280  in the clockwise direction. 
         [0074]    Turning to  FIG. 6 , each of the actuators  220 ,  320  is paired with a holding device  240 ′ and  340 ′ for simplex unit operation as described above in connection with  FIGS. 2 and 4 . 
         [0075]    In  FIG. 7 , coupling  260 ′ includes a link  260   a ′ that pivotally connects between drive arm portion  243   a  of stator  243  and drive arm portion  223   a  of stator  223  at pivot points  290 ,  292 . In contrast to the arrangement of coupling  260 , the link  260   a ′ does not cross a line  261 ′ between the centers of axes  204  and  205 . The coupling  270 ′ includes a link  270   a ′ pivotally connected between drive arm portion  242   a  and drive portion  280   a  of output member  280  at pivot points  294 ,  295  and a link  270   b ′ pivotally connected between drive arm portion  222   a  and drive portion  280   b  of output member  280  at pivot points  296 ,  297 . The link  270   b ′ crosses over to the opposite side of axis  205 . 
         [0076]    Referring generally to  FIGS. 8-11  and initially to  FIG. 8 , a fourth actuator system  400  includes six actuators  403 ,  406 ,  409 ,  412 ,  415  ( FIG. 9 ), and  418  ( FIG. 9 ). The actuators are arranged with moment cancellation and are all mechanically coupled to a common output member  421  ( FIG. 12 ) as described in detail below. The actuators may be arranged in pairs that may be dual tandem or simplex pairs. In the case of simplex pair units, one of the actuators in each pair is substituted with a holding device as described above. In the case of loss of power or a jam for an actuator paired with a holding device, the unit drops out of the network and freely rotates. In the case of a dual tandem unit, each actuator is provided with a brake that may be internal or external and a controller such that loss of power for one actuator of the pair results in the other actuator of the pair moving the tandem unit together. 
         [0077]    Output member  421  is configured to engage with a shaft  424 . As shown, the shaft  424  is a spline shaft, however, it will be evident to those of ordinary skill in the art based on this disclosure that other mechanical means for transmitting rotation from the output member  421  may also be used. Reference structure  427  and reference structure  430  are rigid members. A link  433  is fixedly attached to the reference structures  427 ,  430 . Reference structure  430  includes bearing  431 . 
         [0078]    Starting at the bottom  FIG. 8 , and working counterclockwise, a moment canceling arm  478  is connected to stator  439  of first actuator  403 . The moment canceling arm  478  rotates with the stator  439  about axis  481 . A link  484  is pivotally attached to arm  478  at one end and is pivotally attached to a moment canceling arm  487  connected to the stator  442  of actuator  406  which forms a pair with actuator  403 . Continuing counterclockwise, moment canceling arm  493  is connected to the stator  445  of actuator  409 . The link that connects arm  493  to its neighboring arm  496  has been removed for clarity. Moment canceling arm  496  is connected to stator  448  of actuator  412 . Moment canceling arm  499  is connected to stator  451  of actuator  415 . A link  502  is pivotally attached to arm  499  at one end and is pivotally attached to arm  505  at the opposite end. Arm  505  is connected to stator  454  of actuator  418 . The rotors  457 ,  460 ,  463  etc. are disposed between reference structures  427  and  430  and rotate relative to their respective stators. The rotors are coupled to the output member  421  as described in detail below. 
         [0079]    Reference structure  427  includes bearings  436  (best shown in  FIG. 15 ) for holding Stators  439 ,  442 ,  445 ,  448 ,  451 , and  454  ( FIG. 13 ) in rotary engagement for rotation relative to reference structure  427 . 
         [0080]    Turning to  FIG. 12  rotors  457 ,  460 ,  463 ,  466 ,  469 ,  472  are configured and arranged for rotary motion relative to their respective stators. The rotors  457 ,  460 ,  463 ,  466 ,  469 ,  472  are mechanically coupled to the output member  421 . Starting at the bottom right hand side of  FIG. 12  and moving counterclockwise, drive arm portion  511  of rotor  457  rotates with the rotor  457  about axis  514  normal to the page. A link  517  is pivotally connected to drive arm portion  511  at one end and is pivotally connected to a crank  520  at the opposite end. The crank  520  is fixedly attached to the output member  421 . Drive arm portion  523  of rotor  460  rotates with rotor  460  about axis  526  normal to the page. A link  529  is pivotally connected to drive arm portion  523  at a first end and is pivotally connected to a crank  532  at a second end. The crank  532  is fixedly attached to the output member  421  and is positioned below crank  520  with respect to the orientation of  FIG. 12 . Drive arm portion  535  of rotor  463  rotates with rotor  463  about axis  538  normal to the page. A link  541  is pivotally connected to drive arm portion  535  at a first end and is pivotally connected to crank  520  at the opposite end. Drive arm portion  544  of rotor  466  rotates with rotor  466  about axis  547  normal to the page. A link  550  is pivotally connected to drive arm portion  544  at a first end and is pivotally connected to crank  532  at the opposite end. Drive arm portion  553  of rotor  469  rotates with rotor  469  about axis  556  normal to the page. A link  557  is pivotally connected to drive arm  553  at a first end and is pivotally connected to crank  520  at the opposite end. Drive arm  559  of rotor  472  rotates with rotor  472  about axis  562  normal to the page. A link  565  is pivotally connected to the drive arm  559  at a first end and is pivotally connected to crank  532  at the opposite end. 
         [0081]    Turning to  FIGS. 14 and 15 , exploded perspective views show an actuator  415 . The actuator  415  includes a stator  451  which includes a torque tube  452  connected to the moment canceling arm  499 . All of the parts of the stator  451  are arranged for rotary motion relative to the reference structures  427 ,  430  and are configured for relative rotation with its rotor  469 . Rotor  469  has a drive arm portion  553  that is connected to the output member  421  as described above in connection with  FIG. 12 . Moment canceling arm  499  of stator  451  is connected to the moment canceling arm  505  of an adjacent actuator  418  by means of link  502 . Arms  499  and  505  are coupled together such that their moment is canceled. The remaining pairs of moment canceling arms  478  and  487  and  493  and  496  are configured the same way to form a network of three actuator units with each unit comprising two actuators connected in the same manner. 
         [0082]    In  FIG. 16 , the fourth actuator system  400  is shown with reference structure  430  removed for clarity. At the left side of the figure, the connection of the moment canceling drive arms  499  and  505  by means of link  502  is shown. The link  502  is pivotally attached at a first end to drive arm  499  at pivot point  506  and is pivotally attached to drive arm  505  at the opposite end at pivot point  507 . 
         [0083]    The output member  421  has a splined bore  422  for receiving a splined shaft  424  ( FIG. 8 ). The output member  421  may be provided with cranks  520  and  532  ( FIG. 12 ) that are coupled to the output member  421  such that forces on the cranks  520  and  532  cause the output member  421  to rotate. Rotor  463  is connected to the crank  520  via a connecting rod or link  541  that is pivotally attached to the drive arm portion  535  of rotor  463  at a first end at pivot point  536  and is pivotally attached to the crank  520  at the opposite end at pivot point  537 . The crank  532  is below or to the right in the axial direction with respect to the axis  550  of rotation of the output member  421 . The crank  520  may be provided with a generally triangular shape for connection to three of the rotors and crank  532  may also be provided with a generally triangular shape for connection to the three other rotors. 
         [0084]    Several modifications can be made to the disclosed embodiments. For example, position sensors, resolvers, and/or encoders may be added to actuators and/or any other linkage joint in order to provide useful feedback to a controller. Additionally, torque sensors, and/or tachometers may additionally be added to each actuator output and/or any other link joint in the linkage system to provide further feedback. In dual tandem configurations, one motor in a pair may be of a different type than its corresponding motor. For example, one motor may be a high torque, high velocity motor, whereas the other motor may be a low velocity, high efficiency, high torque motor. Additionally in configurations in which multiple dual tandem pairs are used, brakes may be safely removed since open actuator failures are not a major concern when a second pair of actuators is available to control the output member in the event of an open failure. 
         [0085]    The disclosed embodiments resulted in several significant advantages. The multiply redundant nature of the disclosed configurations provide high fail-safe statistical levels, especially in triplex configurations. Because there is an additional degree of freedom in each actuator pair, a self test may be safely conducted during use in which one actuator moves relative to another actuator without changing the position of the output member. The hexagonal arrangement of the fourth system provides a highly space efficient configuration which allows for arrangement in tightly constrained vehicle frames such as in aircraft airframes. 
         [0086]    Several actuator systems have been shown and described, and several modifications and alternatives have been discussed. Therefore, persons skilled in this art will readily appreciate that various additional changes and modifications may be made without departing from the spirit of the invention, as defined and differentiated by the following claims.