Patent Publication Number: US-2016226349-A1

Title: Electromechanical linear actuator

Description:
BACKGROUND 
     The present disclosure relates generally to actuators, and more specifically to electromechanical linear actuators. 
     Linear actuators are motor assemblies that create movement along a single axis or straight line. To accomplish this, the actuators contain motors that produce a force to drive a member along a single axis. Many motors have been used to create linear motions. For example, an electric direct current motor may be used to rotate a threaded rod inside of a threaded channel, where the rod will extend linearly as it is rotated by the motor. Another example is to pump hydraulic fluid into and out of a chamber, resulting in linear extension of a rod due to hydraulic pressures. Another example is to use an electromagnetic stator to drive a permanent magnetic rod to extend from the stator. 
     Linear actuators are applied to many applications from industrial machines, to amusement park rides, to HVAC valves, to aircraft flight control mechanisms. In certain applications, such as in aircraft flight control, redundancy may be required along with component safety factors of 1×10 −9 , and high levels of accuracy. In applications having these requirements, a solution is necessary that reduces error and increases redundancy while minimizing costs normally associated with meeting these criteria. 
     SUMMARY 
     In one embodiment, a linear actuator includes an actuator housing. The actuator housing includes a plurality of motors providing linear movement along a motor drive axis wherein the motor drive axes are parallel, and wherein the motors are within the housing. Each motor includes a stator for applying an electromagnetic force and a rod movable within the stator. The electromagnetic force from the stator drives the rod to extend from and retract into the stator along the motor drive axis of that motor. 
     In another embodiment, a method (for driving a linear actuator having a plurality of motors arranged within a common housing so that motor drive axes of the motors are parallel) includes receiving a command signal for each motor. The method also includes using a plurality of controllers, where each controller is associated with a different one of the motors, to produce a drive signal to be sent to the associated motor of each controller based on the command signal for that motor. The method also includes sending the drive signal to a stator of each motor from its associated controller. The method further includes driving a rod of each motor along one of the motor drive axis of that motor based on electromagnetic force produced by the stator in response to the drive signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are side and end perspective views of a linear actuator. 
         FIGS. 2A and 2B  are cross-sectional views along  2 A- 2 A and  2 B- 2 B, respectively, of the linear actuator of  FIG. 1 . 
         FIG. 3  is a block diagram of a control system of the linear actuator of  FIGS. 1A, 1B, and 2 . 
         FIG. 4  is a block diagram of another embodiment of a control system of the linear actuator of  FIGS. 1A, 1B, and 2 . 
     
    
    
     DETAILED DESCRIPTION 
     According to the techniques described this disclosure, a linear actuator may include multiple electromagnetic stators. The stators may work to provide linear motion to their forcer rods which together drive a common surface. To provide efficient operation and to prevent jamming or binding of the rods, the actuator can include linear variable differential transformers (LVDTs), which provide position feedback of the rods. The actuator can also include, between the actuator housing and the stator, force sensing elements, such as multi-directional load cells, to determine the force exerted on the individual rods by the stators. This force and position measurements may be used to discover inconsistencies and malfunctions in operation of the individual stator assemblies, upon detection of which, corrections can be made to the individual stators to drive the rods in unison effectively and efficiently. 
       FIGS. 1A and 1B  are perspective views of linear actuator  10  are discussed concurrently. Linear actuator  10  includes housing  12 . Within and attached to housing  12  are rods  14   a - 14   c , stators  16   a - 16   c  (shown in  FIGS. 2A and 2B ), armatures  18   a - 18   c , plate  20 , access plates  22 , access fasteners  24 , housing mounts  26 , mount fasteners  28 , guide rod  30 , rod fasteners  32   a - 32   c , armature fasteners  34   a - 34   c , and electronics connectors  36   a - 36   c . Also displayed are ends E 1  and E 2 , and sides S 1 , S 2 , S 3 , and S 4  of linear actuator  10 . 
     Housing  12  contains or attaches to the components of actuator  10 . Within housing  12  are rods  14   a - 14   c , which extend from housing  12  at end E 2  and make contact with the inside of plate  20 , which is shown as being of a round shape (but may be of any shape), at the proximate end of rods  14   a - 14   c . The other ends of rods  14   a - 14   c  terminate within stators  16   a - 16   b  and are not connected to anything. Rods  14   a - 14   c  are attached and secured to plate  20  by rod fasteners  32   a - 32   c , which can be a bolt, screw, or any other permanent or non-permanent fastening device. Armatures  18   a - 18   c  also reside within housing  12 . Armatures  18   a - 18   c  also extend from housing  12  at end E 2 , make contact with the inside of plate  20 , and are secured to plate  20  by armature fasteners  34   a - 34   c . Connected to the outside of plate  20  is guide rod  30 , which is affixed to access plate  22  and extends axially away from side S 3 . 
     Attached to housing  12  are access plates  22 . Access plates  22  attach to side S 1  of housing  12  and are secured to side S 1  of housing  12  by access fasteners  24 . Access plates  22  can be attached to housing  12  by access fasteners  24  at any of Sides S 1  through S 4 . Also attached to housing  12  are housing mounts  26 . Housing mounts  26  are shown attached to sides S 1  and S 3 , but can attach to any of the sides of housing  12 , depending on the desired mounting configuration. Passing through housing mounts  26  are mount fasteners  28 , which may be bolts or other fastening devices. Mount fasteners  28  connect housing mounts  26  to the desired mounting surface (not shown), such as a set of brackets configured to receive housing mounts  26  and mount fasteners  28 . Located at end E 1  of housing  12  are electronics connectors  36   a - 36   c  which are physically connected to housing  12  and electrically connect to the internal electrical components of actuator  10 . 
     Rods  14   a - 14 C move into and extend from actuator housing  12 , which causes plate  20  to extend away from and draw towards end E 2 . When fully recessed into housing  12 , rods  14   a - 14 C will draw plate  20  into end E 2  so that the outside of plate  20  is flush with E 2 . The movement of plate  20  caused by rods  14   a - 14 C results in the movement of armature  18  which, like rods  14   a - 14 C, extend from and retract into actuator housing  12 . Armatures  18   a - 18   c  are extendable rods of linear variable differential transformers LVDTa-LVDTc, which reside within housing  12 , but are not fully shown in  FIG. 1A or 1B . 
     Removal or unfastening of cover fasteners  24  allows for access plate  22  to be removed from side S 1  of housing  12 . Upon removal of access plates  22 , the components residing in housing  12  are readily accessible from outside of housing  12 . Specifically, removal of access plates  22  can allow for access to the electrical components (such as controllers  50   a - 50   c  shown in later FIGS.) within linear actuator  10 . 
       FIG. 2A  shows the interior of linear actuator  10  along section  2 A- 2 A of  FIG. 1B . Linear actuator  10  includes housing  12 . Within and attached to housing  12  are rods  14   a - 14   b , stators  16   a - 16   b , armatures  18   a - 18   b , plate  20 , mount fasteners  28 , guide rod  30 , rod fasteners  32   a - 32   b , armature fasteners  34   a - 34   b , and electronics connectors  36 . Also included in linear actuator  10  are plate recess  38 , stator retainers  40   a - 40   b , retainer cavities  42   a - 42   b , load cells  44   a - 44   b , stator mounts  46   a - 46   b , and load cell mounts  48   a - 48   b . Also displayed are ends E 1  and E 2 , and sides S 1  and S 3  of linear actuator  10 . Also within housing  12 , but not displayed are rod  14   c , stator  16   c , armature  18   c , rod fastener  32   c , armature fastener  34   c , electronics connector  36   c , stator retainer  40   c , retainer cavity  42   c , load cell  44   c , stator mount  46   c , and load cell mount  48   c . These elements are shown in  FIG. 2B . 
     The connections of the components in  FIG. 2A  are consistent with those shown in  FIGS. 1A and 1B ; however, additional detail is provided in  FIGS. 2A and 2B . Stators  16   a - 16   b  of linear actuator  10  are hollow cylindrical electromagnetic motors, but may be of any other geometry allowing stators  16   a - 16   b  to operate, that are mounted within housing  12 . Rods  14   a - 14   b  are movable within stators  16   a - 16   b . Stators  16   a - 16   b  terminate near end E 2  of housing  12 , but stop short of end E 2 . This creates plate recess  38 , into which plate  20  may recede. 
     Near end E 1  are stator retainers  40   a - 40   b  of stators  16   a - 16   b . Stator retainers  40   a - 40   b  have a profile configured to fit within retainer cavities  42   a - 42   b  of housing  12 . Stator retainers  40   a - 40   b  are attached and connected to load cells  44   a - 44   b  at stator mounts  46   a - 46   b . Stator mounts  46   a - 46   b  can be comprised of any fastening device that allow for stator mounts  46   a - 46   b  to secure load cells  44   a - 44   b  to stators  16   a - 16   b . On the side of load cells  44   a - 44   b  nearest to side E 1  are load cell mounts  48   a - 48   b , which are configured to mount and secure load cells  44   a - 44   b  to housing  12  at end E 1 . Covering load cell mounts  48   a - 48   b  are electronics connectors  36   a - 36   b , which also connect the electronic components shown, such as stators  16   a - 16   c , linear variable differential transformers LVDTa-LVDTc (shown in later FIGS.), load cells  44   a - 44   c , all of the electronics not shown (such as a controller, wiring, a drive circuit, and other electronic components required to operate linear actuator  10 ) to flight control computers. Electronics connectors  36   a - 36   b  also distribute power to the components requiring electrical power. Electronics connectors  36   a - 36   b  may by MIL-DTL-38999 style connectors, but may be any style of connector allowing for electrical communication and power to be received and distributed. 
     Discussed below is the functionality of stator  16   a  and rod  14   a  and the components of linear actuator  10  with which stator  16   a  and rod  14   a  interact. Stators  16   b - 16   c  and rods  14   b - 14   c  operate and interact with other components of linear actuator  10  consistent with the description of stator  16   a  and rod  14   a  below. 
     Stator  16   a , which can be a coil of electrically conductive wire (such as copper), receives electrical power and creates electromagnetic force, which drives rod  14   a , that is made of a magnetic material, to extend out of housing  12 , or to retract into housing  12 . The result of the movement causes plate  20  to extend from or draw near end E 2  of linear actuator  10 . The movement of plate  20  will typically drive an external component to move. However, in place of plate  20 , rods  14   a - 14   c  could be mounted directly to the surface or object being moved by plate  20 . 
     Because armature  18   a  is also attached to plate  20 , armature  18   a  extends and retracts in unison with rod  14   a . This results in the creation of a position feedback signal by each linear variable differential transformers LVDTa-LVDTc based on the position of armature  18   a  relative to housing  12 . 
     The force applied to rod  14   a  by stator  16   a  creates a reaction force in stators  16   a , which is applied to load cell  44   a  through stator retainer  40   a , and is detected by load cell  44   a . Load cell  44   a  creates a force feedback signal based on the force detected by load cell  44   a . Load cell  44   a  is able to receive and sense the forces applied by stator  16   a , because load cell  44   a  is mounted to housing  12  as well as stator retainer  40   a . This mounting configuration prevents load cell  44   a  from moving relative to stator  16   a . When a compressive reaction force is transferred from stators  16   a  to load cell  44   a , load cell  44   a  is compressed between end E 1  of housing  12  and stator retainer  40   a  of stator  16   a . Similarly, a bidirectional load cell  44   a  may also detect a tensile force when the reaction force applied by stator  16   a  pulls load cells  44   a  (via stator mount  46 ) away from end E 1  and towards end E 2 . In this case, load cell  44   a  is able to detect the tensile force, because it is fastened by load cell mount  48   a  preventing load cell  44   a  from moving relative to stator  16   a.    
     Stator mount  46   a  is the primary attachment of load cell  44   a  to stator retainer  40   a  securing stator  16   a  to housing  12 . Stator retainer  40   a , in conjunction with retainer cavity  42   a , operates as a secondary retention of stator  16   a  in case the attachment at stator mount  46   a  fails. Stator retainer  40   a  has a profile that fits securely within retainer cavity  42   a , which thereby resists movement of stator  16   a . To accomplish this, the surfaces of stator retainer  40   a  will contact the portion of housing  12  that forms retainer cavity  42   a . This contact prevents stator  16   a  from moving when a reaction force is transferred to stator  16   a.    
       FIG. 2B  shows the interior of linear actuator  10  along section  2 B- 2 B of  FIG. 1B . Linear actuator  10  includes housing  12 . Within and attached to housing  12  are rod  14   c , stator  16   c , core  17   c , armature  18   c , coils  19   c , plate  20 , mount fastener  28 , guide rod  30 , rod fastener  32   c , armature fasteners  34   c , and electronics connector  36   c . Also included in linear actuator  10  are plate recess  38 , stator retainers  40   c , retainer cavity  42   c , load cell  44   c , stator mount  46   c , and load cell mount  48   c . Also displayed are ends E 1  and E 2 , and sides S 1  and S 3  of linear actuator  10 . 
     The connections of the components in  FIG. 2B  are consistent with those shown in  FIGS. 1A and 1B ; however, additional detail is provided in  FIG. 2B . Stator  16   c  of linear actuator  10  is a hollow cylindrical electromagnetic motor. Rod  14   c  is movable within stator  16   c.    
     Near end E 1  is stator retainer  40   c  of stator  16   c . Stator retainer  40   c  has a profile configured to fit within retainer cavity  42   c  of housing  12 . Stator retainer  40   c  is attached and connected to load cell  44   c  at stator mount  46   c . On the side of load cell  44   c  nearest to end E 1  is load cell mount  48   c , which is configured to mount and secure load cell  44   c  to housing  12  at end E 1 . Covering load cell mounts  48   c  is electronics connector  36   c , which also connect to electronic components as described above. 
     Residing within housing  12  is core  17   c , which connects to armature  18   c . Armature  18   c  may connect to, pass through, or terminate in core  17   c . Core  17   c  is a permanent magnet that is surrounded by, but not contacted by, coils  19   c , which are integrated into housing  12 . However, coils  19   c  may not be integral to housing  12 , and may instead be integrated into a linear variable differential transformer LVDTc that resides within housing  12 . Regardless, coils  19   c  are fixed within housing  12 , and are therefore not free to move relative to core  17   c . Three of coils  19   c  are shown; however, more or less of coils  19   c  may be used. Coils  19   c  are windings or coils of wire, typically copper. 
     Discussed below is the functionality of core  17   c , armature  18   c , coils  19   c , and the components of linear actuator  10  with which these components interact. Cores  17   a - 17   b , armatures  18   a - 18   b , and coils  19   a - 19   b  operate and interact with components of linear actuator  10  consistent with the description below. 
     Stator  16   c  drives rod  14   c  as described above, which moves plate  20 . Because armature  18   c  is also attached to plate  20 , armature  18   c  extends and retracts in unison with rod  14   c . Because armature  18   c  is connected to core  17   c , core  17   c  is moved by armature  18   c  within housing  12 . Coils  19   c , which are positionally fixed relative to core  17   c , can produce a voltage signal based on movement of core  17   c . This is accomplished by one of coils  19   c  producing a voltage that passes through one of coils  19   c , which, through core  17   c , causes a voltage to be induced in the other of coils  19   c . The induced voltage signals change as core  17   c  moves relative to coils  19   c , resulting in the creation of a position feedback signal by linear variable differential transformers LVDTc based on the position of armature  18   c  relative to housing  12 . 
       FIG. 3  is a block diagram of control system  110   a  of linear actuator  10  of  FIGS. 1, 2A and 2B . Control system  110   a  includes motors Ma-Mc (which include rods  14   a - 14   c  and stators  16   a - 16   c ). Control system  110   a  also includes load cells  44   a - 44   c , controllers  50   a - 50   c , flight control computers  52   a - 52   c , linear variable differential transformers LVDTa-LVDTc, and cross-channel communication network  56 . Also illustrated in  FIG. 3  are force balancing algorithms A 1   a -A 1   c , position algorithms A 2   a -A 2   c , and current algorithms A 3   a -A 3   c.    
     Flight controllers  52   a - 52   c  receive position input signals from an external source, such as pilot demand inputs (such as sticks, knobs, or buttons) or airframe sensors (such as speed or location sensors). Flight control computers  52   a - 52   c  communicate their received input to all of the other flight control computers through cross channel communication network  56 . Each flight control computer  52   a - 52   c  performs the same position demand algorithm, an algorithm which decides how much to move rods  14   a - 14   c . The result of this algorithm is a position demand signal to be sent to controllers  50   a - 50   c . Flight control computers  52   a - 52   c  compare algorithm outputs and decide, based on a voting system, what position demand signal will be sent by each control computer. Thereafter, flight control computers  52   a - 52   c  send position demand signals to controllers  50   a - 50   c . For example flight control computer  52   a  sends a position demand signal to controller  50   a.    
     Flight control computers  52   a - 52   c  also receive force feedback signals from load cells  44   a - 44   c , respectively, and communicate their received force feedback signals to all of the other flight control computers through cross channel communication network  56 . Flight control computers  52   a - 52   c  each perform force balancing algorithm A 1   a -A 1   c  to determine the force compensation signal to be sent to controllers  50   a - 50   c . The force balancing algorithm considers how much force is to be applied to rods  14   a - 14   c , how much force was to be applied to rods  14   a - 14   c  and how much force was actually applied. The algorithm then determines a compensation value (contained in a compensation signal) that should be applied to the new position demand signal. Then, flight control computers  52   a - 52   c  compare algorithm outputs and decide, based on a voting system, what force compensation signal will be sent to controllers  50   a - 50   c  and then send the force compensation signals to controllers  50   a - 50   c.    
     Discussed below is the connectivity and functionality of controller  50   a ; however, controllers  50   b - 50   c  operate in accordance with the description and explanation of controller  50   a.    
     The position demand signal and force compensation signal sent by flight control computer  52   a  are received by controller  50   a . Also received by controller  50   a  is a position feedback signal provided by linear variable differential transformer LVDTa. In position algorithm A 2   a , controller  50   a  considers the position of the rod to be moved (position demand signal), the position the rod is currently in or has moved (the position feedback signal), and the adjustment to be made on this position based on the force compensation value (force compensation signal). Controller  50   a  then performs position algorithm A 2   a  based on the position demand signal, the position feedback signal, and the force compensation signal, which results in a current demand signal that is passed internally in controller  50   a . The current demand signal carries information packets containing how much current should be applied to motor Ma to achieve the position of rod  14   a  determined by position algorithm A 2   a.    
     Controller  50   a  also receives a current feedback signal from motor Ma. Controller  50   a  uses the current demand signal from position algorithm A 2   a  and the current feedback signal from motor  58   a  as inputs to current algorithm A 3   a . Current algorithm A 3   a  considers the amount of current that is desired to be supplied to motor Ma (current demand signal), and the amount of current that is being or was supplied to motor Ma. The result of current algorithm A 3   a  is a drive signal, which is sent to motor Ma. The drive signal commands motor Ma to move rod  14   a  in a particular direction with a specified amount of power. The result is movement of rod  14   a  relative to stator  16   a  and housing  12 . 
     As described above, the movement of rod  14   a  results in the movement of plate  20  and therefore movement of armature  18   a . The movement of armature  18   a  is detected by linear variable differential transformer LVDTa. Based on this movement of armature  18   a , linear variable differential transformer LVDTa creates position feedback signal to be sent to position algorithm A 2   a  within controller  50   a , thus creating a position feedback loop. Similarly, the force applied on rod  14   a  created by stator  16   a , as described above, results in a force sensed by load cell  44   a . Based on the sensed force, load cell  44   a  creates a force feedback signal, which is sent to force balancing algorithm A 1   a  within controller  50   a , creating a force feedback loop. Another feedback loop is created by a current transducer, or other current sensing device located on motor Mc, which senses the amount of current sent to motor Mc. This sensor creates a current feedback signal based on the sensed current, which is sent to current algorithm A 3   a  within controller  50   a , creating a current feedback loop. 
       FIG. 4  is a block diagram of control system  110   b  of linear actuator  10  of  FIGS. 1, 2A, and 2B . Control system  110   b  includes motors Ma-Mc, which include rods  14   a - 14   c  and stators  16   a - 16   c . Control system  110   a  also includes load cells  44   a - 44   c , controllers  50   a - 50   c , flight control computers  52   a - 52   c , linear variable differential transformers LVDTa-LVDTc, and cross-channel communication networks  56   a  and  56   b . Also illustrated in  FIG. 4  are force-balancing algorithms A 1   a -A 1   c , position algorithms A 2   a -A 2   c , current algorithms A 3   a -A 3   c , and position algorithms A 4   a -A 4   c.    
     Flight controllers  52   a - 52   c  receive position input signals from an external source, such as pilot demand inputs (such as sticks, knobs, or buttons) or airframe sensors (such as speed or location sensors). Flight control computers  52   a - 52   c  communicate their received input to all of the other flight control computers through cross channel communication network  56   a . Each flight control computer  52   a - 52   c  performs the same position demand algorithm, an algorithm which decides how much to move rods  14   a - 14   c . The result of this algorithm is a position demand signal to be sent to controllers  50   a - 50   c . Flight control computers  52   a - 52   c  compare algorithm outputs and decide, based on a voting system, what position demand signal will be sent by each control computer. Thereafter, flight control computers  52   a - 52   c  send position demand signals to controllers  50   a - 50   c . For example, flight control computer  52   a  sends a position demand signal to controller  50   a.    
     Discussed below is the connectivity and functionality of controller  50   a ; however, controllers  50   b - 50   c  operate in accordance with the description and explanation of controller  50   a.    
     The position demand signal sent by flight control computer  52   a  is received by controller  50   a . Also received by controller  50   a  is a position feedback signal provided by linear variable differential transformer LVDTa. In position algorithm A 4   a , controller  50   a  considers the position of the rod to be moved (position demand signal) and the position the rod is currently in or has moved (the position feedback signal). Controller  50   a  then performs position algorithm A 4   a  based on the position demand signal, and the position feedback signal, which results in a force demand signal that is passed internally in controller  50   a  to force demand algorithm A 2   a . The force demand signal carries information packets containing how much force should be applied to rod  14   a  of motor Ma based on the position determined by position algorithm A 4   a.    
     Controller  50   a  also receives a force feedback signal from load cell  44   a  and communicates the received force feedback signals to controllers  50   b - 50   c  through cross channel communication network  56   b . Controllers  52   a - 52   c  each perform force balancing algorithms A 1   a -A 1   c  to determine the force compensation signal to be sent to controllers  50   a - 50   c . The force balancing algorithm considers how much force is to be applied to rods  14   a - 14   c , how much force was to be applied to rods  14   a - 14   c  and how much force was actually applied. The algorithm then determines a compensation value (contained in a compensation signal) that should be applied to the new position demand signal. Then, controllers  50   a - 50   c  compare algorithm outputs and decide, based on a voting system, what force compensation signal will be sent used by controllers  50   a - 50   c  and then send the force compensation signals internally. 
     The force compensation signal is sent within controller  50   a  to force demand algorithm A 2   a . In force demand algorithm A 2   a , controller  50   a  considers the force to be applied to rod  14   a  (force demand signal), and the adjustment to be made on this force based on the force compensation value (force compensation signal). Controller  50   a  then performs position algorithm A 2   a  based on the force demand signal and the force compensation signal, which results in a current demand signal that is passed internally in controller  50   a . The current demand signal carries information packets containing how much current should be applied to motor Ma, based on the position determined by demand algorithm A 2   a.    
     Controller  50   a  also receives a current feedback signal from motor Ma. Controller  50   a  uses the current demand signal from position algorithm A 2   a  and the current feedback signal from motor Ma as inputs to current algorithm A 3   a . Current algorithm A 3   a  considers the amount of current that is desired to be supplied to motor Ma (current demand signal), and the amount of current that is being or was supplied to motor Ma. The result of current algorithm A 3   a  is a drive signal, which is sent to motor Ma. The drive signal commands motor Ma to move rod  14   a  in a particular direction with a specified amount of power. The result is movement of rod  14   a  relative to stator  16   a  and housing  12 . As in control system  110   a  described in  FIG. 3 , control system  110   b  creates a position feedback loop, a force feedback loop, and a current feedback loop. 
     Though the systems described herein contain three motors, it is to be understood that the techniques of this disclosure would apply to a linear actuator system containing any number of motors. For example, a linear actuator containing two or four motors could employ the techniques of this disclosure. 
     A major benefit of linear actuator  10 , in any of the embodiments discussed, is the redundancy provided through the use of multiple motors. In aviation, actuators must be very accurate. In addition to the accuracy requirements, some applications of linear actuators require high safety factors, such as a safety factor of 1×10 −9 . One of the few ways to achieve a safety factor of this magnitude is to make use of redundant components, so that if there is a failure, another component having identical specifications can perform the same function as the failed component. It is common in aviation, where redundancy and accuracy are needed, to use multiple actuators to actuate a surface or object. 
     Accuracy requirements are much more difficult to comply with when multiple motors are used, because the multiple motors will frequently contain inconsistencies from manufacturing, resulting in differences in performance. The inconsistencies often will not matter when operating a single motor, because the inconsistencies may be easily accounted for. However, when multiple motors are used, the inconsistencies cause unevenly distributed forces and differences in actuation distance. These problems can lead to component failure and operational failure. The industry&#39;s solution has been to use multiple actuators, physically separated from one another, to drive a common surface or object. Then, the inconsistencies in force and actuation distance can be spread out over the maximum dimensions of the object being driven. For example, in the case of a wing flap, three actuators may be spread out over the span of the entire flap, providing the accuracy and redundancy required to actuate a flap, while the flap may offer the flexibility required to compensate for motor inconsistencies in load and distance of the actuators. While this solution is effective, it is expensive, because three separate actuators having their own attachment points must be individually connected and wired. This solution is also inefficient, because multiple actuators having their own housings, mounting and wiring will weigh more than a single, multi-motor actuator. 
     One benefit of the present invention is the ability to use redundant motors in a single housing. The present invention overcomes the issues above in many ways. First, non-binding electromagnetic linear motors, for example motor Ma, are used. Motor Ma includes an electromagnetic stator  16   a  and a permanent magnet rod  14   a  that is forcibly driven by stator  16   a . There is minimal contact between stator  16   a  and the rod  14   a , drastically reducing binding and jamming occurrences that other types of linear actuators may experience. 
     Second, linear variable differential transformers LVDTa-LVDTc are paired with motors Ma-Mc to determine the position or travel of rods  14   a - 14   c  relative to stators  16   a - 16   c  and housing  12 . This allows for control system  110   a  or  110   b  to detect any differences in the driven distance of rods  14   a - 14   c , perform a calculation, and provide an updated signal to motors Ma-Mc to account for any positional differences between rods  14   a - 14   c  operating in unison. This allows for rods  14   a - 14   c  to drive a common surface or object at a high level of accuracy. 
     Third, bi-directional load cells  44   a - 44   c  are used to detect the forces exerted on rods  14   a - 14   c . This provides a control system, flight control computer  52   a  for example, with knowledge of how much force is being applied to rods  14   a - 14   c , allowing flight control computer to determine if rods  14   a - 14   c  are working against each other, or fighting, to maintain position. That is, stator  16   a  may be forcing rod  14   a  to extend, while stator  16   b  is forcing rod  14   b  to retract. The result may be two evenly positioned rods; however, the forces applied by stators  16   a - 16   b  may be the maximum force capable of being produced by stators  16   a - 16   b , but in opposing directions. More simply, there may be force or load differences applied by stators  16   a - 16   b  that, because of plate  20  interconnecting rods  14   a - 14   b , are restricted and difficult to detect. Both of the conditions are problematic, because they reduce component life. The incorporation of load cells  44   a - 44   c  allows this condition to be detected. Upon detection of a force imbalance by load cells  44   a - 44   c , flight control computers  52   a - 52   c  can make adjustments to reduce imbalance or fighting. The result of using electromagnetic, non-binding, linear actuators, linear variable differential transformers, load cells, and a control system is an accurate, redundant, and efficient linear actuator in a compact housing. This makes linear actuator  10  a good solution for safety critical or mission critical applications. 
     Another major benefit of this system is true redundancy. With rotary motors having gear trains to convert rotational movement into linear movement, there are many potential points for a jam failure, making the redundancy difficult. This is not a problem with the present invention, because stators  16   a - 16   c  do not physically restrict the movement of rods  14   a - 14   c . When, for example, stators  16   a  and  16   c  do not receive power, plate  20  may be driven by stator  16   b  and rod  14   b . In this case, the movement of plate  20  will also result in the movement of the unpowered rods  14   a  and  14   c . In effect, any of stators  16   a - 16   c  working alone can drive their rod to move plate  20 . A benefit of this system is that the common surface being driven can be completely rigid. Because rods  14   a - 14   c  are capable of being driven in near perfect unison, the surface or object being driven does not need to account for inconsistencies in driving rods  14   a - 14   c.    
     Although linear actuator  10  has been described as being a component of an aircraft system, linear actuator  10  may be applied anywhere a high accuracy, redundant, linear actuator is required, such as in the operation of rides at amusement parks, in industrial processes, or as actuators for valves. 
     Though load cells  44   a - 44   c  were described as being bi-directional, they may also sense force in a single direction. Further, other means of determining force, such as a strain gauge or torque sensor may be used in place of a load cell. Similarly, another sensor for detecting displacement could be used in place of linear variable differential transformers LVDTa-LVDTc. For example, an optical proximity sensor could be used. Further, additional sensors could be implemented to further increase the accuracy of linear actuator  10 , such as additional position sensors, or different sensors entirely, such as acceleration sensors. 
     Although stators  16   a - 16   c  are shown as being arranged in a triangle, they may be arranged in any shape where the axes of motors Ma-Mc are parallel to each other. For example, they may be arranged in a straight line, which reduces the profile of actuator  10  in one direction. Motors Ma-Mc may also be arranged in configurations where their axes are not parallel, when required by the application. 
     Discussion of Possible Embodiments 
     The following are non-exclusive descriptions of possible embodiments of the present invention. 
     A linear actuator includes an actuator housing. The actuator housing includes a plurality of motors providing linear movement along a motor drive axis wherein the motor drive axes are parallel, and wherein the motors are within the housing. Each motor includes a stator for applying an electromagnetic force and a rod movable within the stator. The electromagnetic force from the stator drives the rod to extend from and retract into the stator along the motor drive axis of that motor. 
     The linear actuator of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components. 
     The linear actuator can further include a plurality of displacement sensors, wherein each displacement sensor can be associated with a different one of the rods, and wherein each displacement sensor can produce a displacement signal that can be a function of a position of its associated rod relative to the actuator housing. 
     The displacement sensors can be variable differential transformers. 
     The linear actuator can include a plurality of controllers, where each controller can be associated with a different one of the motors, a different one of the rods, and a different one of the displacement sensors, wherein each controller can provide a drive signal to its associated motor as a function of the displacement signal from its associated displacement sensor and a command signal. 
     The controllers can be connected to communicate between each other to synchronize movement of their associated rods. 
     The linear actuator can include a plurality of force sensors, wherein each force sensor can be associated with a different one of the stators, and wherein each force sensor can produce a force signal that is a function of the electromagnetic force applied by its associated stator. 
     The force sensors can be bi-directional load cells. 
     The bi-directional load cells can be disposed between an end of the actuator housing and its associated stator. 
     The linear actuator can include a plurality of controllers, where each controller can be associated with a different one of the motors, a different one of the rods, and a different one of the force sensors, wherein each controller can provide a drive signal to its associated motor as a function of the force signal from its associated force sensor and a command signal. 
     The plurality of controllers can be connected to communicate between each other to synchronize movement of their associated rods. 
     The linear actuator can include a plurality of displacement sensors, wherein each displacement sensor can be associated with a different one of the rods, and wherein each displacement sensor can produce a displacement signal that is a function of a position of its associated rod relative to the actuator housing. 
     The linear actuator can include a plurality of controllers, where each controller can be associated with a different one of the motors, a different one of the rods, a different one of the stator, a different one of the force sensors, and a different one of the displacement sensors, wherein each controller can provide a drive signal to its associated motor as a function of its force signal from its associated force sensor, its displacement signal from its associated displacement sensor, and a command signal. 
     The plurality of controllers can be connected to communicate between each other to synchronize movement of their associated rods 
     A method (for driving a linear actuator having a plurality of motors arranged within a common housing so that motor drive axes of the motors are parallel) includes receiving a command signal for each motor. The method also includes using a plurality of controllers, where each controller is associated with a different one of the motors, to produce a drive signal to be sent to the associated motor of each controller based on the command signal for that motor. The method also includes sending the drive signal to a stator of each motor from its associated controller. The method further includes driving a rod of each motor along one of the motor drive axis of that motor based on electromagnetic force produced by the stator in response to the drive signal. 
     The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components or steps. 
     The method can include receiving a displacement signal that can be a function of position of each rod relative to the linear actuator. The method can also include receiving a load signal that can be a function of the electromagnetic force applied to each rod by its stator, and producing the drive signal that can be based on the command signal, the displacement signal, and the load signal. 
     While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.