Patent Publication Number: US-10767669-B2

Title: Rotary piston type actuator with a central actuation assembly

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims the benefit of the priority to U.S. patent application Ser. No. 13/921,904, filed Jun. 19, 2013 which is a Continuation in Part of U.S. patent application Ser. No. 13/831,220, filed Mar. 14, 2013, now U.S. Pat. No. 9,163,648, which is a Continuation in Part of U.S. patent application Ser. No. 13/778,561, filed Feb. 27, 2013, now U.S. Pat. No. 9,234,535, the disclosures of which are incorporated by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     This invention relates to an actuator device and more particularly to a rotary piston type actuator device wherein the pistons of the rotor are moved by fluid under pressure and wherein the actuator device includes a central actuation assembly adapted for attachment to and external mounting feature on a member to be actuated. 
     BACKGROUND 
     Rotary hydraulic actuators of various forms are currently used in industrial mechanical power conversion applications. This industrial usage is commonly for applications where continuous inertial loading is desired without the need for load holding for long durations, e.g. hours, without the use of an external fluid power supply. Aircraft flight control applications generally implement loaded positional holding, for example, in a failure mitigation mode, using substantially only the blocked fluid column to hold position. 
     In certain applications, such as primary flight controls used for aircraft operation, positional accuracy in load holding by rotary actuators is desired. Positional accuracy can be improved by minimizing internal leakage characteristics inherent to the design of rotary actuators. However, it can be difficult to provide leak-free performance in typical rotary hydraulic actuators, e.g., rotary “vane” or rotary “piston” type configurations. 
     SUMMARY 
     In general, this document relates to rotary actuators. 
     In a first aspect, a rotary actuator includes a housing, a rotor assembly rotatably journaled in said housing and including a rotary output shaft, a central actuation assembly including a central mounting point formed in an external surface of the rotary output shaft, said central mounting point proximal to the longitudinal midpoint of the rotary output shaft, a mounting assembly adapted for attachment to an external mounting connector of a mounting surface of an aircraft structural member, and an actuation arm removably attached at a proximal end to the central mounting point, said actuation arm adapted at a distal end for attachment to an external mounting feature of an aircraft assembly to be actuated. 
     Various embodiments can include some, all, or none of the following features. The central actuation assembly can further include a radial recess formed in an external peripheral surface of the housing proximal to the central mounting point of the rotary output shaft, and wherein said actuation arm extends through the radial recess. The mounting assembly can include a radially projecting portion of the housing disposed at a midpoint of the housing, said mounting assembly disposed about 180 degrees from the radial recess of the central actuation assembly. The housing can define a first arcuate chamber including a first cavity, a first fluid port in fluid communication with the first cavity, and an open end, while the rotor assembly can further include a first rotor arm extending radially outward from the rotary output shaft, and the rotary actuator can further include an arcuate-shaped first piston disposed in said housing for reciprocal movement in the first arcuate chamber through the open end, wherein a first seal, the first cavity, and the first piston can define a first pressure chamber, and a first portion of the first piston can contact the first rotor arm. The housing can further define a second arcuate chamber comprising a second cavity, and a second fluid port in fluid communication with the second cavity, the rotor assembly can further include a second rotor arm, and the rotary actuator can further comprise an arcuate-shaped second piston disposed in said housing for reciprocal movement in the second arcuate chamber, wherein a second seal, the second cavity, and the second piston can define a second pressure chamber, and a first portion of the second piston can contact the second rotor arm. The central actuation assembly can further include a radial recess formed in an external peripheral surface of the housing proximal to the central mounting point of the rotary output shaft, and wherein said actuation arm can extend through the radial recess. The rotary actuator can further include a rotary actuator comprising a stator mounted to the housing and a rotor coupled to the rotary output shaft. The rotary actuator can be one of a rotary piston type actuator, a rotary vane type actuator, or a rotary fluid type actuator. The rotary actuator can be an electromechanical actuator. 
     In a second aspect, a rotary actuator includes a housing including a mounting assembly, a rotor assembly rotatably journaled in said housing and including a rotary output shaft, a central actuation assembly including a central mounting point formed in an external surface of the rotary output shaft, said central mounting point proximal to the longitudinal midpoint of the rotary output shaft, and an actuation arm removably attached at a proximal end to the central mounting point, said actuation arm adapted at a distal end for attachment to an external mounting feature of a member to be actuated. 
     Various embodiments can include some, all, or none of the following features. The central actuation assembly can further include a radial recess formed in an external peripheral surface of the housing proximal to the central mounting point of the rotary output shaft, and wherein said actuation arm extends through the radial recess. The mounting assembly can comprise a radially projecting portion of the housing, said mounting assembly disposed about 180 degrees from the radial recess of the central actuation assembly, said mounting assembly adapted for attachment to an external mounting connector of a mounting surface. The radially projecting portion of the housing can be a radially projecting central portion of the housing. The housing can define a first arcuate chamber including a first cavity, a first fluid port in fluid communication with the first cavity, and an open end, the rotor assembly can further include a first rotor arm extending radially outward from the rotary output shaft, and the rotary actuator can further include an arcuate-shaped first piston disposed in said housing for reciprocal movement in the first arcuate chamber through the open end, wherein a first seal, the first cavity, and the first piston can define a first pressure chamber, and a first portion of the first piston can contact the first rotor arm. The housing can further defines a second arcuate chamber comprising a second cavity, and a second fluid port in fluid communication with the second cavity, the rotor assembly can further comprise a second rotor arm, and the rotary actuator can further comprise an arcuate-shaped second piston disposed in said housing for reciprocal movement in the second arcuate chamber, wherein a second seal, the second cavity, and the second piston can define a second pressure chamber, and a first portion of the second piston can contact the second rotor arm. The central actuation assembly can further include a radial recess formed in an external peripheral surface of the housing proximal to the central mounting point of the rotary output shaft, and wherein said actuation arm extends through the radial recess. The rotary actuator can further include a linear actuator mounted at a first end to the housing, and a second end mounted to a first rotor arm extending radially outward from the rotary output shaft. The rotary actuator can further include a rotary actuator comprising a stator mounted to the housing and a rotor coupled to the rotary output shaft. The rotary actuator can be one of a rotary piston type actuator, a rotary vane type actuator, or a rotary fluid type actuator. The rotary actuator can be an electromechanical actuator. The rotary actuator can include a linear actuator and a linear-to-rotary motion conversion assembly coupled to the rotor. The housing can be formed as a one-piece housing. The external mounting feature can be attached to one of an aircraft structural member or an external mounting connector of an external surface, and the mounting assembly can be attached to the other of the aircraft structural member or the external mounting connector. 
     In a third aspect, a method of rotary actuation includes providing a rotary actuator including a housing, a rotor assembly rotatably journaled in said housing and including a rotary output shaft, a central actuation assembly including a central mounting point formed in an external surface of the rotary output shaft, said central mounting point proximal to the longitudinal midpoint of the rotary output shaft, and an actuation arm removably attached at a proximal end to the central mounting point, said actuation arm adapted at a distal end for attachment to an external mounting feature of a member to be actuated, energizing the rotor assembly, urging rotation of the rotary output shaft, urging rotation of the actuation arm, urging motion of the member to be actuated. 
     Various embodiments can include some, all, or none of the following features. The central actuation assembly can further include a radial recess formed in an external peripheral surface of the housing proximal to the central mounting point of the rotor shaft, and wherein said actuation arm extends through the radial recess. The mounting assembly can further comprise a radially projecting portion of the housing, said mounting assembly disposed about 180 degrees from the radial recess of the central actuation assembly, said mounting assembly adapted for attachment to an external mounting connector of a mounting surface. The radially projecting portion of the housing can be a radially projecting central portion of the housing. The central actuation assembly can further include a radial recess formed in an external peripheral surface of the housing proximal to the central mounting point of the rotary output shaft, and wherein said actuation arm extends through the radial recess. The rotary actuator can include a stator mounted to the housing and a rotor coupled to the rotary output shaft. The rotary actuator can be one of a rotary piston type actuator, a rotary vane type actuator, or a rotary fluid type actuator. The rotary actuator can be an electromechanical actuator. The rotary actuator can include a linear actuator and a linear-to-rotary motion conversion assembly coupled to the rotor. 
     The systems and techniques described herein may provide one or more of the following advantages. First, the system can provide an actuator that is mounted and/or actuated at a midpoint of the actuator. Second, the system can provide rotary actuation in a compact space. Third, the system can provide the aforementioned rotary actuation with reduced deformation between the mounting point of the rotary actuator and the assembly to be actuated. Fourth, the system can provide the aforementioned advantages as an actuator that is implemented in an aircraft wing application, including aircraft wings made of composite materials. 
     The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view of an example rotary piston-type actuator. 
         FIG. 2  is a perspective view of an example rotary piston assembly. 
         FIG. 3  is a perspective cross-sectional view of an example rotary piston-type actuator. 
         FIG. 4  is a perspective view of another example rotary piston-type actuator. 
         FIGS. 5 and 6  are cross-sectional views of an example rotary piston-type actuator. 
         FIG. 7  is a perspective view of another embodiment of a rotary piston-type actuator. 
         FIG. 8  is a perspective view of another example of a rotary piston-type actuator. 
         FIGS. 9 and 10  show and example rotary piston-type actuator in example extended and retracted configurations. 
         FIG. 11  is a perspective view of another example of a rotary piston-type actuator. 
         FIGS. 12-14  are perspective and cross-sectional views of another example rotary piston-type actuator. 
         FIGS. 15 and 16  are perspective and cross-sectional views of another example rotary piston-type actuator that includes another example rotary piston assembly. 
         FIGS. 17 and 18  are perspective and cross-sectional views of another example rotary piston-type actuator that includes another example rotary piston assembly. 
         FIGS. 19 and 20  are perspective and cross-sectional views of another example rotary piston-type actuator. 
         FIGS. 21A-21C  are cross-sectional and perspective views of an example rotary piston. 
         FIGS. 22 and 23  illustrate a comparison of two example rotor shaft embodiments. 
         FIG. 24  is a perspective view of another example rotary piston. 
         FIG. 25  is a flow diagram of an example process for performing rotary actuation. 
         FIG. 26  is a perspective view of another example rotary piston-type actuator. 
         FIG. 27  is a cross-sectional view of another example rotary piston assembly. 
         FIG. 28  is a perspective cross-sectional view of another example rotary piston-type actuator. 
         FIG. 29A  is a perspective view from above of an example rotary-piston type actuator with a central actuation assembly. 
         FIG. 29B  is a top view of the actuator of  FIG. 29A . 
         FIG. 29C  is a perspective view from the right side and above illustrating the actuator of  FIG. 29A  with a portion of the central actuation assembly removed for illustration purposes. 
         FIG. 29D  is a lateral cross section view taken at section AA of the actuator of  FIG. 29B . 
         FIG. 29E  is a partial perspective view from cross section AA of  FIG. 29B . 
         FIG. 30A  is a perspective view from above of an example rotary actuator with a central actuation assembly. 
         FIG. 30B  is another perspective view from above of the example rotary actuator of  FIG. 30A . 
         FIG. 30C  is a top view of the example rotary actuator of  FIG. 30A . 
         FIG. 30D  is an end view of the example rotary actuator of  FIG. 30A . 
         FIG. 30E  is a partial perspective view from cross section AA of  FIG. 30C . 
         FIG. 31A  is a perspective view from above of another example rotary actuator with a central actuation assembly. 
         FIG. 31B  is another perspective view from above of the example rotary actuator of  FIG. 31A . 
         FIG. 31C  is a top view of the example rotary actuator of  FIG. 31A . 
         FIG. 31D  is an end view of the example rotary actuator of  FIG. 31A . 
         FIG. 31E  is a partial perspective view from cross section AA of  FIG. 31C . 
     
    
    
     DETAILED DESCRIPTION 
     This document describes devices for producing rotary motion. In particular, this document describes devices that can convert fluid displacement into rotary motion through the use of components more commonly used for producing linear motion, e.g., hydraulic or pneumatic linear cylinders. Vane-type rotary actuators are relatively compact devices used to convert fluid motion into rotary motion. Rotary vane actuators (RVA), however, generally use seals and component configurations that exhibit cross-vane leakage of the driving fluid. Such leakage can affect the range of applications in which such designs can be used. Some applications may require a rotary actuator to hold a rotational load in a selected position for a predetermined length of time, substantially without rotational movement, when the actuator&#39;s fluid ports are blocked. For example, some aircraft applications may require that an actuator hold a flap or other control surface that is under load (e.g., through wind resistance, gravity or g-forces) at a selected position when the actuator&#39;s fluid ports are blocked. Cross-vane leakage, however, can allow movement from the selected position. 
     Linear pistons use relatively mature sealing technology that exhibits well-understood dynamic operation and leakage characteristics that are generally better than rotary vane actuator type seals. Linear pistons, however, require additional mechanical components in order to adapt their linear motions to rotary motions. Such linear-to-rotary mechanisms are generally larger and heavier than rotary vane actuators that are capable of providing similar rotational actions, e.g., occupying a larger work envelope. Such linear-to-rotary mechanisms may also generally be installed in an orientation that is different from that of the load they are intended to drive, and therefore may provide their torque output indirectly, e.g., installed to push or pull a lever arm that is at a generally right angle to the axis of the axis of rotation of the lever arm. Such linear-to-rotary mechanisms may therefore become too large or heavy for use in some applications, such as aircraft control where space and weight constraints may make such mechanisms impractical for use. 
     In general, rotary piston assemblies use curved pressure chambers and curved pistons to controllably push and pull the rotor arms of a rotor assembly about an axis. In use, certain embodiments of the rotary piston assemblies described herein can provide the positional holding characteristics generally associated with linear piston-type fluid actuators, to rotary applications, and can do so using the relatively more compact and lightweight envelopes generally associated with rotary vane actuators. 
       FIGS. 1-3  show various views of the components of an example rotary piston-type actuator  100 . Referring to  FIG. 1 , a perspective view of the example rotary piston-type actuator  100  is shown. The actuator  100  includes a rotary piston assembly  200  and a pressure chamber assembly  300 . The actuator  100  includes a first actuation section  110  and a second actuation section  120 . In the example of actuator  100 , the first actuation section  110  is configured to rotate the rotary piston assembly  200  in a first direction, e.g., counter-clockwise, and the second actuation section  120  is configured to rotate the rotary piston assembly  200  in a second direction substantially opposite the first direction, e.g., clockwise. 
     Referring now to  FIG. 2 , a perspective view of the example rotary piston assembly  200  is shown apart from the pressure chamber assembly  300 . The rotary piston assembly  200  includes a rotor shaft  210 . A plurality of rotor arms  212  extend radially from the rotor shaft  210 , the distal end of each rotor arm  212  including a bore (not shown) substantially aligned with the axis of the rotor shaft  210  and sized to accommodate one of the collection of connector pins  214 . 
     As shown in  FIG. 2 , the first actuation section  110  includes a pair of rotary pistons  250 , and the second actuation section  120  includes a pair of rotary pistons  260 . While the example actuator  100  includes two pairs of the rotary pistons  250 ,  260 , other embodiments can include greater and/or lesser numbers of cooperative and opposing rotary pistons. Examples of other such embodiments will be discussed below, for example, in the descriptions of  FIGS. 4-25 . 
     In the example rotary piston assembly shown in  FIG. 2 , each of the rotary pistons  250 ,  260  includes a piston end  252  and one or more connector arms  254 . The piston end  252  is formed to have a generally semi-circular body having a substantially smooth surface. Each of the connector arms  254  includes a bore  256  substantially aligned with the axis of the semi-circular body of the piston end  252  and sized to accommodate one of the connector pins  214 . 
     The rotary pistons  260  in the example assembly of  FIG. 2  are oriented substantially opposite each other in the same rotational direction. The rotary pistons  250  are oriented substantially opposite each other in the same rotational direction, but opposite that of the rotary pistons  260 . In some embodiments, the actuator  100  can rotate the rotor shaft  210  about 60 degrees total. 
     Each of the rotary pistons  250 ,  260  of the example assembly of  FIG. 2  may be assembled to the rotor shaft  210  by aligning the connector arms  254  with the rotor arms  212  such that the bores (not shown) of the rotor arms  212  align with the bores  265 . The connector pins  214  may then be inserted through the aligned bores to create hinged connections between the pistons  250 ,  260  and the rotor shaft  210 . Each connector pin  214  is slightly longer than the aligned bores. In the example assembly, about the circumferential periphery of each end of each connector pin  214  that extends beyond the aligned bores is a circumferential recess (not shown) that can accommodate a retaining fastener (not shown), e.g., a snap ring or spiral ring. 
       FIG. 3  is a perspective cross-sectional view of the example rotary piston-type actuator  100 . The illustrated example shows the rotary pistons  260  inserted into a corresponding pressure chamber  310  formed as an arcuate cavity in the pressure chamber assembly  300 . The rotary pistons  250  are also inserted into corresponding pressure chambers  310 , not visible in this view. 
     In the example actuator  100 , each pressure chamber  310  includes a seal assembly  320  about the interior surface of the pressure chamber  310  at an open end  330 . In some implementations, the seal assembly  320  can be a circular or semi-circular sealing geometry retained on all sides in a standard seal groove. In some implementations, commercially available reciprocating piston or cylinder type seals can be used. For example, commercially available seal types that may already be in use for linear hydraulic actuators flying on current aircraft may demonstrate sufficient capability for linear load and position holding applications. In some implementations, the sealing complexity of the actuator  100  may be reduced by using a standard, e.g., commercially available, semi-circular, unidirectional seal designs generally used in linear hydraulic actuators. In some embodiments, the seal assembly  320  can be a one-piece seal. 
     In some embodiments of the example actuator  100 , the seal assembly  320  may be included as part of the rotary pistons  250 ,  260 . For example, the seal assembly  320  may be located near the piston end  252 , opposite the connector arm  254 , and slide along the interior surface of the pressure chamber  310  to form a fluidic seal as the rotary piston  250 ,  260  moves in and out of the pressure chamber  310 . An example actuator that uses such piston-mounted seal assemblies will be discussed in the descriptions of  FIGS. 26-28 . In some embodiments, the seal  310  can act as a bearing. For example, the seal assembly  320  may provide support for the piston  250 ,  260  as it moves in and out of the pressure chamber  310 . 
     In some embodiments, the actuator  100  may include a wear member between the piston  250 ,  260  and the pressure chamber  310 . For example, a wear ring may be included in proximity to the seal assembly  320 . The wear ring may act as a pilot for the piston  250 ,  260 , and/or act as a bearing providing support for the piston  250 ,  260 . 
     In the example actuator  100 , when the rotary pistons  250 ,  260  are inserted through the open ends  330 , each of the seal assemblies  320  contacts the interior surface of the pressure chamber  310  and the substantially smooth surface of the piston end  252  to form a substantially pressure-sealed region within the pressure chamber  310 . Each of the pressure chambers  310  may include a fluid port  312  formed through the pressure chamber assembly  300 , through with pressurized fluid may flow. Upon introduction of pressurized fluid, e.g., hydraulic oil, water, air, gas, into the pressure chambers  310 , the pressure differential between the interior of the pressure chambers  310  and the ambient conditions outside the pressure chambers  310  causes the piston ends  252  to be urged outward from the pressure chambers  310 . As the piston ends  252  are urged outward, the pistons  250 ,  260  urge the rotary piston assembly  200  to rotate. 
     In the example of the actuator  100 , cooperative pressure chambers may be fluidically connected by internal or external fluid ports. For example, the pressure chambers  310  of the first actuation section  110  may be fluidically interconnected to balance the pressure between the pressure chambers  310 . Similarly the pressure chambers  310  of the second actuation section  120  may be fluidically interconnected to provide similar pressure balancing. In some embodiments, the pressure chambers  310  may be fluidically isolated from each other. For example, the pressure chambers  310  may each be fed by an independent supply of pressurized fluid. 
     In the example of the actuator  100 , the use of the alternating arcuate, e.g., curved, rotary pistons  250 ,  260  arranged substantially opposing each other operates to translate the rotor arms in an arc-shaped path about the axis of the rotary piston assembly  200 , thereby rotating the rotor shaft  210  clockwise and counter-clockwise in a substantially torque balanced arrangement. Each cooperative pair of pressure chambers  310  operates uni-directionally in pushing the respective rotary piston  250  outward, e.g., extension, to drive the rotor shaft  210  in the specific direction. To reverse direction, the opposing cylinder section&#39;s  110  pressure chambers  260  are pressurized to extend their corresponding rotary pistons  260  outward. 
     The pressure chamber assembly  300 , as shown, includes a collection of openings  350 . In general, the openings  350  provide space in which the rotor arms  212  can move when the rotor shaft  210  is partly rotated. In some implementations, the openings  350  can be formed to remove material from the pressure chamber assembly  300 , e.g., to reduce the mass of the pressure chamber assembly  300 . In some implementations, the openings  350  can be used during the process of assembly of the actuator  100 . For example, the actuator  100  can be assembled by inserting the rotary pistons  250 ,  260  through the openings  350  such that the piston ends  252  are inserted into the pressure chambers  310 . With the rotary pistons  250 ,  260  substantially fully inserted into the pressure chambers  310 , the rotor shaft  210  can be assembled to the actuator  100  by aligning the rotor shaft  210  with an axial bore  360  formed along the axis of the pressure chamber assembly  300 , and by aligning the rotor arms  212  with a collection of keyways  362  formed along the axis of the pressure chamber assembly  300 . The rotor shaft  210  can then be inserted into the pressure chamber assembly  300 . The rotary pistons  250 ,  260  can be partly extracted from the pressure chambers  310  to substantially align the bores  256  with the bores of the rotor arms  212 . The connector pins  214  can then be passed through the keyways  362  and the aligned bores to connect the rotary pistons  250 ,  260  to the rotor shaft  210 . The connector pins  214  can be secured longitudinally by inserting retaining fasteners through the openings  350  and about the ends of the connector pins  214 . The rotor shaft  210  can be connected to an external mechanism as an output shaft in order to transfer the rotary motion of the actuator  100  to other mechanisms. A bushing or bearing  362  is fitted between the rotor shaft  210  and the axial bore  360  at each end of the pressure chamber assembly  300 . 
     In some embodiments, the rotary pistons  250 ,  260  may urge rotation of the rotor shaft  210  by contacting the rotor arms  212 . For example, the piston ends  252  may not be coupled to the rotor arms  212 . Instead, the piston ends  252  may contact the rotor arms  212  to urge rotation of the rotor shaft as the rotary pistons  250 ,  260  are urged outward from the pressure chambers  310 . Conversely, the rotor arms  212  may contact the piston ends  252  to urge the rotary pistons  250 ,  260  back into the pressure chambers  310 . 
     In some embodiments, a rotary position sensor assembly (not shown) may be included in the actuator  100 . For example, an encoder may be used to sense the rotational position of the rotor shaft  210  relative to the pressure chamber assembly or another feature that remains substantially stationary relative to the rotation of the shaft  210 . In some implementations, the rotary position sensor may provide signals that indicate the position of the rotor shaft  210  to other electronic or mechanical modules, e.g., a position controller. 
     In use, pressurized fluid in the example actuator  100  can be applied to the pressure chambers  310  of the second actuation section  120  through the fluid ports  312 . The fluid pressure urges the rotary pistons  260  out of the pressure chambers  310 . This movement urges the rotary piston assembly  200  to rotate clockwise. Pressurized fluid can be applied to the pressure chambers  310  of the first actuation section  110  through the fluid ports  312 . The fluid pressure urges the rotary pistons  250  out of the pressure chambers  310 . This movement urges the rotary piston assembly  200  to rotate counter-clockwise. The fluid conduits can also be blocked fluidically to cause the rotary piston assembly  200  to substantially maintain its rotary position relative to the pressure chamber assembly  300 . 
     In some embodiments of the example actuator  100 , the pressure chamber assembly  300  can be formed from a single piece of material. For example, the pressure chambers  310 , the openings  350 , the fluid ports  312 , the keyways  362 , and the axial bore  360  may be formed by molding, machining, or otherwise forming a unitary piece of material. 
       FIG. 4  is a perspective view of another example rotary piston-type actuator  400 . In general, the actuator  400  is similar to the actuator  100 , but instead of using opposing pairs of rotary pistons  250 ,  260 , each acting uni-directionally to provide clockwise and counter-clockwise rotation, the actuator  400  uses a pair of bidirectional rotary pistons. 
     As shown in  FIG. 4 , the actuator  400  includes a rotary piston assembly that includes a rotor shaft  412  and a pair of rotary pistons  414 . The rotor shaft  412  and the rotary pistons  414  are connected by a pair of connector pins  416 . 
     The example actuator shown in  FIG. 4  includes a pressure chamber assembly  420 . The pressure chamber assembly  420  includes a pair of pressure chambers  422  formed as arcuate cavities in the pressure chamber assembly  420 . Each pressure chamber  422  includes a seal assembly  424  about the interior surface of the pressure chamber  422  at an open end  426 . The seal assemblies  424  contact the inner walls of the pressure chambers  422  and the rotary pistons  414  to form fluidic seals between the interiors of the pressure chambers  422  and the space outside. A pair of fluid ports  428  is in fluidic communication with the pressure chambers  422 . In use, pressurized fluid can be applied to the fluid ports  428  to urge the rotary pistons  414  partly out of the pressure chambers  422 , and to urge the rotor shaft  412  to rotate in a first direction, e.g., clockwise in this example. 
     The pressure chamber assembly  420  and the rotor shaft  412  and rotary pistons  414  of the rotary piston assembly may be structurally similar to corresponding components found in to the second actuation section  120  of the actuator  100 . In use, the example actuator  400  also functions substantially similarly to the actuator  100  when rotating in a first direction when the rotary pistons  414  are being urged outward from the pressure chambers  422 . e.g., clockwise in this example. As will be discussed next, the actuator  400  differs from the actuator  100  in the way that the rotor shaft  412  is made to rotate in a second direction, e.g., counter-clockwise in this example. 
     To provide actuation in the second direction, the example actuator  400  includes an outer housing  450  with a bore  452 . The pressure chamber assembly  420  is formed to fit within the bore  452 . The bore  452  is fluidically sealed by a pair of end caps (not shown). With the end caps in place, the bore  452  becomes a pressurizable chamber. Pressurized fluid can flow to and from the bore  452  through a fluid port  454 . Pressurized fluid in the bore  452  is separated from fluid in the pressure chambers  422  by the seals  426 . 
     Referring now to  FIG. 5 , the example actuator  400  is shown in a first configuration in which the rotor shaft  412  has been rotated in a first direction, e.g., clockwise, as indicated by the arrows  501 . The rotor shaft  412  can be rotated in the first direction by flowing pressurized fluid into the pressure chambers  422  through the fluid ports  428 , as indicated by the arrows  502 . The pressure within the pressure chambers  422  urges the rotary pistons  414  partly outward from the pressure chambers  422  and into the bore  452 . Fluid within the bore  452 , separated from the fluid within the pressure chambers  422  by the seals  424  and displaced by the movement of the rotary pistons  414 , is urged to flow out the fluid port  454 , as indicated by the arrow  503 . 
     Referring now to  FIG. 6 , the example actuator  400  is shown in a second configuration in which the rotor shaft  412  has been rotated in a second direction, e.g., counter-clockwise, as indicated by the arrows  601 . The rotor shaft  412  can be rotated in the second direction by flowing pressurized fluid into the bore  452  through the fluid port  454 , as indicated by the arrow  602 . The pressure within the bore  452  urges the rotary pistons  414  partly into the pressure chambers  422  from the bore  452 . Fluid within the pressure chambers  422 , separated from the fluid within the bore  452  by the seals  424  and displaced by the movement of the rotary pistons  414 , is urged to flow out the fluid ports  428 , as indicated by the arrows  603 . In some embodiments, one or more of the fluid ports  428  and  454  can be oriented radially relative to the axis of the actuator  400 , as illustrated in  FIGS. 4-6 , however in some embodiments one or more of the fluid ports  428  and  454  can be oriented parallel to the axis of the actuator  400  or in any other appropriate orientation. 
       FIG. 7  is a perspective view of another embodiment of a rotary piston assembly  700 . In the example actuator  100  of  FIG. 1 , two opposing pairs of rotary pistons were used, but in other embodiments other numbers and configurations of rotary pistons and pressure chambers can be used. In the example of the assembly  700 , a first actuation section  710  includes four rotary pistons  712  cooperatively operable to urge a rotor shaft  701  in a first direction. A second actuation section  720  includes four rotary pistons  722  cooperatively operable to urge the rotor shaft  701  in a second direction. 
     Although examples using four rotary pistons, e.g., actuator  100 , and eight rotary pistons, e.g., assembly  700 , have been described, other configurations may exist. In some embodiments, any appropriate number of rotary pistons may be used in cooperation and/or opposition. In some embodiments, opposing rotary pistons may not be segregated into separate actuation sections, e.g., the actuation sections  710  and  720 . While cooperative pairs of rotary pistons are used in the examples of actuators  100 ,  400 , and assembly  700 , other embodiments exist. For example, clusters of two, three, four, or more cooperative or oppositional rotary pistons and pressure chambers may be arranged radially about a section of a rotor shaft. As will be discussed in the descriptions of  FIGS. 8-10 , a single rotary piston may be located at a section of a rotor shaft. In some embodiments, cooperative rotary pistons may be interspersed alternatingly with opposing rotary pistons. For example, the rotary pistons  712  may alternate with the rotary pistons  722  along the rotor shaft  701 . 
       FIG. 8  is a perspective view of another example of a rotary piston-type actuator  800 . The actuator  800  differs from the example actuators  100  and  400 , and the example assembly  700  in that instead of implementing cooperative pairs of rotary pistons along a rotor shaft, e.g., two of the rotary pistons  250  are located radially about the rotor shaft  210 , individual rotary pistons are located along a rotor shaft. 
     The example actuator  800  includes a rotor shaft  810  and a pressure chamber assembly  820 . The actuator  800  includes a first actuation section  801  and a second actuation section  802 . In the example actuator  800 , the first actuation section  801  is configured to rotate the rotor shaft  810  in a first direction, e.g., clockwise, and the second actuation section  802  is configured to rotate the rotor shaft  810  in a second direction substantially opposite the first direction, e.g., counter-clockwise. 
     The first actuation section  801  of example actuator  800  includes a rotary piston  812 , and the second actuation section  802  includes a rotary piston  822 . By implementing a single rotary piston  812 ,  822  at a given longitudinal position along the rotor shaft  810 , a relatively greater range of rotary travel may be achieved compared to actuators that use pairs of rotary pistons at a given longitudinal position along the rotary piston assembly, e.g., the actuator  100 . In some embodiments, the actuator  800  can rotate the rotor shaft  810  about 145 degrees total. 
     In some embodiments, the use of multiple rotary pistons  812 ,  822  along the rotor shaft  810  can reduce distortion of the pressure chamber assembly  820 , e.g., reduce bowing out under high pressure. In some embodiments, the use of multiple rotary pistons  812 ,  822  along the rotor shaft  810  can provide additional degrees of freedom for each piston  812 ,  822 . In some embodiments, the use of multiple rotary pistons  812 ,  822  along the rotor shaft  810  can reduce alignment issues encountered during assembly or operation. In some embodiments, the use of multiple rotary pistons  812 ,  822  along the rotor shaft  810  can reduce the effects of side loading of the rotor shaft  810 . 
       FIG. 9  shows the example actuator  800  with the rotary piston  812  in a substantially extended configuration. A pressurized fluid is applied to a fluid port  830  to pressurize an arcuate pressure chamber  840  formed in the pressure chamber assembly  820 . Pressure in the pressure chamber  840  urges the rotary piston  812  partly outward, urging the rotor shaft  810  to rotate in a first direction, e.g., clockwise. 
       FIG. 10  shows the example actuator  800  with the rotary piston  812  in a substantially retracted configuration. Mechanical rotation of the rotor shaft  810 , e.g., pressurization of the actuation section  820 , urges the rotary piston  812  partly inward, e.g., clockwise. Fluid in the pressure chamber  840  displaced by the rotary piston  812  flows out through the fluid port  830 . 
     The example actuator  800  can be assembled by inserting the rotary piston  812  into the pressure chamber  840 . Then the rotor shaft  810  can be inserted longitudinally through a bore  850  and a keyway  851 . The rotary piston  812  is connected to the rotor shaft  810  by a connecting pin  852 . 
       FIG. 11  is a perspective view of another example of a rotary piston-type actuator  1100 . In general, the actuator  1100  is similar to the example actuator  800 , except multiple rotary pistons are used in each actuation section. 
     The example actuator  1100  includes a rotary piston assembly  1110  and a pressure chamber assembly  1120 . The actuator  1100  includes a first actuation section  1101  and a second actuation section  1102 . In the example of actuator  1100 , the first actuation section  1101  is configured to rotate the rotary piston assembly  1110  in a first direction, e.g., clockwise, and the second actuation section  1102  is configured to rotate the rotary piston assembly  1110  in a second direction substantially opposite the first direction, e.g., counter-clockwise. 
     The first actuation section  1101  of example actuator  1100  includes a collection of rotary pistons  812 , and the second actuation section  1102  includes a collection of rotary pistons  822 . By implementing individual rotary pistons  812 ,  822  at various longitudinal positions along the rotary piston assembly  1110 , a range of rotary travel similar to the actuator  800  may be achieved. In some embodiments, the actuator  1100  can rotate the rotor shaft  1110  about 60 degrees total. 
     In some embodiments, the use of the collection of rotary pistons  812  may provide mechanical advantages in some applications. For example, the use of multiple rotary pistons  812  may reduce stress or deflection of the rotary piston assembly, may reduce wear of the seal assemblies, or may provide more degrees of freedom. In another example, providing partitions, e.g., webbing, between chambers can add strength to the pressure chamber assembly  1120  and can reduce bowing out of the pressure chamber assembly  1120  under high pressure. In some embodiments, placement of an end tab on the rotor shaft assembly  1110  can reduce cantilever effects experienced by the actuator  800  while under load, e.g., less stress or bending. 
       FIGS. 12-14  are perspective and cross-sectional views of another example rotary piston-type actuator  1200 . The actuator  1200  includes a rotary piston assembly  1210 , a first actuation section  1201 , and a second actuation section  1202 . 
     The rotary piston assembly  1210  of example actuator  1200  includes a rotor shaft  1212 , a collection of rotor arms  1214 , and a collection of dual rotary pistons  1216 . Each of the dual rotary pistons  1216  includes a connector section  1218  a piston end  1220   a  and a piston end  1220   b . The piston ends  1220   a - 1220   b  are arcuate in shape, and are oriented opposite to each other in a generally semicircular arrangement, and are joined at the connector section  1218 . A bore  1222  is formed in the connector section  1218  and is oriented substantially parallel to the axis of the semicircle formed by the piston ends  1220   a - 1220   b . The bore  1222  is sized to accommodate a connector pin (not shown) that is passed through the bore  1222  and a collection of bores  1224  formed in the rotor arms  1213  to secure each of the dual rotary pistons  1216  to the rotor shaft  1212 . 
     The first actuation section  1201  of example actuator  1200  includes a first pressure chamber assembly  1250   a , and the second actuation section  1202  includes a second pressure chamber assembly  1250   b . The first pressure chamber assembly  1250   a  includes a collection of pressure chambers  1252   a  formed as arcuate cavities in the first pressure chamber assembly  1250   a . The second pressure chamber assembly  1250   b  includes a collection of pressure chambers  1252   b  formed as arcuate cavities in the first pressure chamber assembly  1250   b . When the pressure chamber assemblies  1250   a - 1250   b  are assembled into the actuator  1200 , each of the pressure chambers  1252   a  lies generally in a plane with a corresponding one of the pressure chambers  1252   b , such that a pressure chamber  1252   a  and a pressure chamber  1252   b  occupy two semicircular regions about a central axis. A semicircular bore  1253   a  and a semicircular bore  1253   b  substantially align to accommodate the rotor shaft  1212 . 
     Each of the pressure chambers  1252   a - 1252   b  of example actuator  1200  includes an open end  1254  and a seal assembly  1256 . The open ends  1254  are formed to accommodate the insertion of the piston ends  1220   a - 1220   b . The seal assemblies  1256  contact the inner walls of the pressure chambers  1252   a - 1252   b  and the outer surfaces of the piston ends  1220   a - 1220   b  to form a fluidic seal. 
     The rotary piston assembly  1210  of example actuator  1200  can be assembled by aligning the bores  1222  of the dual rotary pistons  1216  with the bores  1224  of the rotor arms  1214 . The connector pin (not shown) is passed through the bores  1222  and  1224  and secured longitudinally by retaining fasteners. 
     The example actuator  1200  can be assembled by positioning the rotor shaft  1212  substantially adjacent to the semicircular bore  1253   a  and rotating it to insert the piston ends  1220   a  substantially fully into the pressure chambers  1252   a . The second pressure chamber  1252   b  is positioned adjacent to the first pressure chamber  1252   a  such that the semicircular bore  1253   b  is positioned substantially adjacent to the rotor shaft  1212 . The rotary piston assembly  1210  is then rotated to partly insert the piston ends  1220   b  into the pressure chambers  1252   b . An end cap  1260  is fastened to the longitudinal ends  1262   a  of the pressure chambers  1252   a - 1252   b . A second end cap (not shown) is fastened to the longitudinal ends  1262   b  of the pressure chambers  1252   a - 1252   b . The end caps substantially maintain the positions of the rotary piston assembly  1210  and the pressure chambers  1252   a - 1252   b  relative to each other. In some embodiments, the actuator  1200  can provide about 90 degrees of total rotational stroke. 
     In operation, pressurized fluid is applied to the pressure chambers  1252   a  of example actuator  1200  to rotate the rotary piston assembly  1210  in a first direction, e.g., clockwise. Pressurized fluid is applied to the pressure chambers  1252   b  to rotate the rotary piston assembly  1210  in a second direction, e.g., counter-clockwise. 
       FIGS. 15 and 16  are perspective and cross-sectional views of another example rotary piston-type actuator  1500  that includes another example rotary piston assembly  1501 . In some embodiments, the assembly  1501  can be an alternative embodiment of the rotary piston assembly  200  of  FIG. 2 . 
     The assembly  1501  of example actuator  1500  includes a rotor shaft  1510  connected to a collection of rotary pistons  1520   a  and a collection of rotary pistons  1520   b  by a collection of rotor arms  1530  and one or more connector pins (not shown). The rotary pistons  1520   a  and  1520   b  are arranged along the rotor shaft  1510  in a generally alternating pattern, e.g., one rotary piston  1520   a , one rotary piston  1520   b , one rotary piston  1520   a , one rotary piston  1520   b . In some embodiments, the rotary pistons  1520   a  and  1520   b  may be arranged along the rotor shaft  1510  in a generally intermeshed pattern, e.g., one rotary piston  1520   a  and one rotary piston  1520   b  rotationally parallel to each other, with connector portions formed to be arranged side-by-side or with the connector portion of rotary piston  1520   a  formed to one or more male protrusions and/or one or more female recesses to accommodate one or more corresponding male protrusions and/or one or more corresponding female recesses formed in the connector portion of the rotary piston  1520   b.    
     Referring to  FIG. 16 , a pressure chamber assembly  1550  of example actuator  1500  includes a collection of arcuate pressure chambers  1555   a  and a collection of arcuate pressure chambers  1555   b . The pressure chambers  1555   a  and  1555   b  are arranged in a generally alternating pattern corresponding to the alternating pattern of the rotary pistons  1520   a - 1520   b . The rotary pistons  1520   a - 1520   b  extend partly into the pressure chambers  1555   a - 1555   b . A seal assembly  1560  is positioned about an open end  1565  of each of the pressure chambers  1555   a - 1555   b  to form fluidic seals between the inner walls of the pressure chambers  1555   a - 1555   b  and the rotary pistons  1520   a - 1520   b.    
     In use, pressurized fluid can be alternatingly provided to the pressure chambers  1555   a  and  1555   b  of example actuator  1500  to urge the rotary piston assembly  1501  to rotate partly clockwise and counterclockwise. In some embodiments, the actuator  1500  can rotate the rotor shaft  1510  about 92 degrees total. 
       FIGS. 17 and 18  are perspective and cross-sectional views of another example rotary piston-type actuator  1700  that includes another example rotary piston assembly  1701 . In some embodiments, the assembly  1701  can be an alternative embodiment of the rotary piston assembly  200  of  FIG. 2  or the assembly  1200  of  FIG. 12 . 
     The assembly  1701  of example actuator  1700  includes a rotor shaft  1710  connected to a collection of rotary pistons  1720   a  by a collection of rotor arms  1730   a  and one or more connector pins  1732 . The rotor shaft  1710  is also connected to a collection of rotary pistons  1720   b  by a collection of rotor arms  1730   b  and one or more connector pins  1732 . The rotary pistons  1720   a  and  1720   b  are arranged along the rotor shaft  1710  in a generally opposing, symmetrical pattern, e.g., one rotary piston  1720   a  is paired with one rotary piston  1720   b  at various positions along the length of the assembly  1701 . 
     Referring to  FIG. 18 , a pressure chamber assembly  1750  of example actuator  1700  includes a collection of arcuate pressure chambers  1755   a  and a collection of arcuate pressure chambers  1755   b . The pressure chambers  1755   a  and  1755   b  are arranged in a generally opposing, symmetrical pattern corresponding to the symmetrical arrangement of the rotary pistons  1720   a - 1720   b . The rotary pistons  1720   a - 1720   b  extend partly into the pressure chambers  1755   a - 1755   b . A seal assembly  1760  is positioned about an open end  1765  of each of the pressure chambers  1755   a - 1755   b  to form fluidic seals between the inner walls of the pressure chambers  1755   a - 1755   b  and the rotary pistons  1720   a - 1720   b.    
     In use, pressurized fluid can be alternatingly provided to the pressure chambers  1755   a  and  1755   b  of example actuator  1700  to urge the rotary piston assembly  1701  to rotate partly clockwise and counterclockwise. In some embodiments, the actuator  1700  can rotate the rotor shaft  1710  about 52 degrees total. 
       FIGS. 19 and 20  are perspective and cross-sectional views of another example rotary piston-type actuator  1900 . Whereas the actuators described previously, e.g., the example actuator  100  of  FIG. 1 , are generally elongated and cylindrical, the actuator  1900  is comparatively flatter and more disk-shaped. 
     Referring to  FIG. 19 , a perspective view of the example rotary piston-type actuator  1900  is shown. The actuator  1900  includes a rotary piston assembly  1910  and a pressure chamber assembly  1920 . The rotary piston assembly  1910  includes a rotor shaft  1912 . A collection of rotor arms  1914  extend radially from the rotor shaft  1912 , the distal end of each rotor arm  1914  including a bore  1916  aligned substantially parallel with the axis of the rotor shaft  1912  and sized to accommodate one of a collection of connector pins  1918 . 
     The rotary piston assembly  1910  of example actuator  1900  includes a pair of rotary pistons  1930  arranged substantially symmetrically opposite each other across the rotor shaft  1912 . In the example of the actuator  1900 , the rotary pistons  1930  are both oriented in the same rotational direction, e.g., the rotary pistons  1930  cooperatively push in the same rotational direction. In some embodiments, a return force may be provided to rotate the rotary piston assembly  1910  in the direction of the rotary pistons  1930 . For example, the rotor shaft  1912  may be coupled to a load that resists the forces provided by the rotary pistons  1930 , such as a load under gravitational pull, a load exposed to wind or water resistance, a return spring, or any other appropriate load that can rotate the rotary piston assembly. In some embodiments, the actuator  1900  can include a pressurizable outer housing over the pressure chamber assembly  1920  to provide a back-drive operation, e.g., similar to the function provided by the outer housing  450  in  FIG. 4 . In some embodiments, the actuator  1900  can be rotationally coupled to an oppositely oriented actuator  1900  that can provide a back-drive operation. 
     In some embodiments, the rotary pistons  1930  can be oriented in opposite rotational directions, e.g., the rotary pistons  1930  can oppose each other push in the opposite rotational directions to provide bidirectional motion control. In some embodiments, the actuator  100  can rotate the rotor shaft about 60 degrees total. 
     Each of the rotary pistons  1930  of example actuator  1900  includes a piston end  1932  and one or more connector arms  1934 . The piston end  1932  is formed to have a generally semi-circular body having a substantially smooth surface. Each of the connector arms  1934  includes a bore  1936  (see  FIGS. 21B and 21C ) substantially aligned with the axis of the semi-circular body of the piston end  1932  and sized to accommodate one of the connector pins  1918 . 
     Each of the rotary pistons  1930  of example actuator  1900  is assembled to the rotor shaft  1912  by aligning the connector arms  1934  with the rotor arms  1914  such that the bores  1916  of the rotor arms  1914  align with the bores  1936 . The connector pins  1918  are inserted through the aligned bores to create hinged connections between the pistons  1930  and the rotor shaft  1912 . Each connector pin  1916  is slightly longer than the aligned bores. About the circumferential periphery of each end of each connector pin  1916  that extends beyond the aligned bores is a circumferential recess (not shown) that can accommodate a retaining fastener (not shown), e.g., a snap ring or spiral ring. 
     Referring now to  FIG. 20  a cross-sectional view of the example rotary piston-type actuator  1900  is shown. The illustrated example shows the rotary pistons  1930  partly inserted into a corresponding pressure chamber  1960  formed as an arcuate cavity in the pressure chamber assembly  1920 . 
     Each pressure chamber  1960  of example actuator  1900  includes a seal assembly  1962  about the interior surface of the pressure chamber  1960  at an open end  1964 . In some embodiments, the seal assembly  1962  can be a circular or semi-circular sealing geometry retained on all sides in a standard seal groove. 
     When the rotary pistons  1930  of example actuator  1900  are inserted through the open ends  1964 , each of the seal assemblies  1962  contacts the interior surface of the pressure chamber  1960  and the substantially smooth surface of the piston end  1932  to form a substantially pressure-sealed region within the pressure chamber  1960 . Each of the pressure chambers  1960  each include a fluid port (not shown) formed through the pressure chamber assembly  1920 , through with pressurized fluid may flow. 
     Upon introduction of pressurized fluid, e.g., hydraulic oil, water, air, gas, into the pressure chambers  1960  of example actuator  1900 , the pressure differential between the interior of the pressure chambers  1960  and the ambient conditions outside the pressure chambers  1960  causes the piston ends  1932  to be urged outward from the pressure chambers  1960 . As the piston ends  1932  are urged outward, the pistons  1930  urge the rotary piston assembly  1910  to rotate. 
     In the illustrated example actuator  1900 , each of the rotary pistons  1930  includes a cavity  1966 .  FIGS. 21A-21C  provide additional cross-sectional and perspective views of one of the rotary pistons  1930 . Referring to  FIG. 21A , a cross-section the rotary piston  1930 , taken across a section of the piston end  1932  is shown. The cavity  1966  is formed within the piston end  1932 . Referring to  FIG. 21B , the connector arm  1934  and the bore  1936  is shown in perspective.  FIG. 21C  features a perspective view of the cavity  1966 . 
     In some embodiments, the cavity  1966  may be omitted. For example, the piston end  1932  may be solid in cross-section. In some embodiments, the cavity  1966  may be formed to reduce the mass of the rotary piston  1930  and the mass of the actuator  1900 . For example, the actuator  1900  may be implemented in an aircraft application, where weight may play a role in actuator selection. In some embodiments, the cavity  1966  may reduce wear on seal assemblies, such as the seal assembly  320  of  FIG. 3 . For example, by reducing the mass of the rotary piston  1930 , the amount of force the piston end  1932  exerts upon the corresponding seal assembly may be reduced when the mass of the rotary piston is accelerated, e.g., by gravity or G-forces. 
     In some embodiments, the cavity  1966  may be substantially hollow in cross-section, and include one or more structural members, e.g., webs, within the hollow space. For example, structural cross-members may extend across the cavity of a hollow piston to reduce the amount by which the piston may distort, e.g., bowing out, when exposed to a high pressure differential across the seal assembly. 
       FIGS. 22 and 23  illustrate a comparison of two example rotor shaft embodiments.  FIG. 22  is a perspective view of an example rotary piston-type actuator  2200 . In some embodiments, the example actuator  2200  can be the example actuator  1900 . 
     The example actuator  2200  includes a pressure chamber assembly  2210  and a rotary piston assembly  2220 . The rotary piston assembly  2220  includes at least one rotary piston  2222  and one or more rotor arms  2224 . The rotor arms  2224  extend radially from a rotor shaft  2230 . 
     The rotor shaft  2230  of example actuator includes an output section  2232  and an output section  2234  that extend longitudinally from the pressure chamber assembly  2210 . The output sections  2232 - 2234  include a collection of splines  2236  extending radially from the circumferential periphery of the output sections  2232 - 2234 . In some implementations, the output section  2232  and/or  2234  may be inserted into a correspondingly formed splined assembly to rotationally couple the rotor shaft  2230  to other mechanisms. For example, by rotationally coupling the output section  2232  and/or  2234  to an external assembly, the rotation of the rotary piston assembly  2220  may be transferred to urge the rotation of the external assembly. 
       FIG. 23  is a perspective view of another example rotary piston-type actuator  2300 . The actuator  2300  includes the pressure chamber assembly  2210  and a rotary piston assembly  2320 . The rotary piston assembly  2320  includes at least one of the rotary pistons  2222  and one or more of the rotor arms  2224 . The rotor arms  2224  extend radially from a rotor shaft  2330 . 
     The rotor shaft  2330  of example actuator  2300  includes a bore  2332  formed longitudinally along the axis of the rotor shaft  2330 . The rotor shaft  2330  includes a collection of splines  2336  extending radially inward from the circumferential periphery of the bore  2332 . In some embodiments, a correspondingly formed splined assembly may be inserted into the bore  2332  to rotationally couple the rotor shaft  2330  to other mechanisms. 
       FIG. 24  is a perspective view of another example rotary piston  2400 . In some embodiments, the rotary piston  2400  can be the rotary piston  250 ,  260 ,  414 ,  712 ,  812 ,  822 ,  1530   a ,  1530   b ,  1730   a ,  1730   b ,  1930  or  2222 . 
     The example rotary piston  2400  includes a piston end  2410  and a connector section  2420 . The connector section  2420  includes a bore  2430  formed to accommodate a connector pin, e.g., the connector pin  214 . 
     The piston end  2410  of example actuator  2400  includes an end taper  2440 . The end taper  2440  is formed about the periphery of a terminal end  2450  of the piston end  2410 . The end taper  2440  is formed at a radially inward angle starting at the outer periphery of the piston end  2410  and ending at the terminal end  2450 . In some implementations, the end taper  2440  can be formed to ease the process of inserting the rotary piston  2400  into a pressure chamber, e.g., the pressure chamber  310 . 
     The piston end  2410  of example actuator  2400  is substantially smooth. In some embodiments, the smooth surface of the piston end  2410  can provide a surface that can be contacted by a seal assembly. For example, the seal assembly  320  can contact the smooth surface of the piston end  2410  to form part of a fluidic seal, reducing the need to form a smooth, fluidically sealable surface on the interior walls of the pressure chamber  310 . 
     In the illustrated example, the rotary piston  2400  is shown as having a generally solid circular cross-section, whereas the rotary pistons piston  250 ,  260 ,  414 ,  712 ,  812 ,  822 ,  1530   a ,  1530   b ,  1730   a ,  1730   b ,  1930  or  2222  have been illustrated as having various generally rectangular, elliptical, and other shapes, both solid and hollow, in cross section. In some embodiments, the cross sectional dimensions of the rotary piston  2400 , as generally indicated by the arrows  2491  and  2492 , can be adapted to any appropriate shape, e.g., square, rectangular, ovoid, elliptical, circular, and other shapes, both solid and hollow, in cross section. In some embodiments, the arc of the rotary piston  2400 , as generally indicated by the angle  2493 , can be adapted to any appropriate length. In some embodiments, the radius of the rotary piston  2400 , as generally indicated by the line  2494 , can be adapted to any appropriate radius. In some embodiments, the piston end  2410  can be substantially solid, substantially hollow, or can include any appropriate hollow formation. In some embodiments, any of the previously mentioned forms of the piston end  2410  can also be used as the piston ends  1220   a  and/or  1220   b  of the dual rotary pistons  1216  of  FIG. 12 . 
       FIG. 25  is a flow diagram of an example process  2500  for performing rotary actuation. In some implementations, the process  2500  can be performed by the rotary piston-type actuators  100 ,  400 ,  700 ,  800 ,  1200 ,  1500 ,  1700 ,  1900 ,  2200 ,  2300 , and/or  2600  which will be discussed in the descriptions of  FIGS. 26-28 . 
     At  2510 , a rotary actuator is provided. The rotary actuator of example actuator  2500  includes a first housing defining a first arcuate chamber including a first cavity, a first fluid port in fluid communication with the first cavity, an open end, and a first seal disposed about an interior surface of the open end, a rotor assembly rotatably journaled in the first housing and including a rotary output shaft and a first rotor arm extending radially outward from the rotary output shaft, an arcuate-shaped first piston disposed in the first housing for reciprocal movement in the first arcuate chamber through the open end. The first seal, the first cavity, and the first piston define a first pressure chamber, and a first connector, coupling a first end of the first piston to the first rotor arm. For example, the actuator  100  includes the components of the pressure chamber assembly  300  and the rotary piston assembly  200  included in the actuation section  120 . 
     At  2520 , a pressurized fluid is applied to the first pressure chamber. For example, pressurized fluid can be flowed through the fluid port  320  into the pressure chamber  310 . 
     At  2530 , the first piston is urged partially outward from the first pressure chamber to urge rotation of the rotary output shaft in a first direction. For example, a volume of pressurized fluid flowed into the pressure chamber  310  will displace a similar volume of the rotary piston  260 , causing the rotary piston  260  to be partly urged out of the pressure cavity  310 , which in turn will cause the rotor shaft  210  to rotate clockwise. 
     At  2540 , the rotary output shaft is rotated in a second direction opposite that of the first direction. For example, the rotor shaft  210  can be rotated counter-clockwise by an external force, such as another mechanism, a torque-providing load, a return spring, or any other appropriate source of rotational torque. 
     At  2550 , the first piston is urged partially into the first pressure chamber to urge pressurized fluid out the first fluid port. For example, the rotary piston  260  can be pushed into the pressure chamber  310 , and the volume of the piston end  252  extending into the pressure chamber  310  will displace a similar volume of fluid, causing it to flow out the fluid port  312 . 
     In some embodiments, the example process  2500  can be used to provide substantially constant power over stroke to a connected mechanism. For example, as the actuator  100  rotates, there may be substantially little position-dependent variation in the torque delivered to a connected load. 
     In some embodiments, the first housing further defines a second arcuate chamber comprising a second cavity, a second fluid port in fluid communication with the second cavity, and a second seal disposed about an interior surface of the open end, the rotor assembly also includes a second rotor arm, the rotary actuator also includes an arcuate-shaped second piston disposed in said housing for reciprocal movement in the second arcuate chamber, wherein the second seal, the second cavity, and the second piston define a second pressure chamber, and a second connector coupling a first end of the second piston to the second rotor arm. For example, the actuator  100  includes the components of the pressure chamber assembly  300  and the rotary piston assembly  200  included in the actuation section  110 . 
     In some embodiments, the second piston can be oriented in the same rotational direction as the first piston. For example, the two pistons  260  are oriented to operate cooperatively in the same rotational direction. In some embodiments, the second piston can be oriented in the opposite rotational direction as the first piston. For example, the rotary pistons  250  are oriented to operate in the opposite rotational direction relative to the rotary pistons  260 . 
     In some embodiments, the actuator can include a second housing and disposed about the first housing and having a second fluid port, wherein the first housing, the second housing, the seal, and the first piston define a second pressure chamber. For example, the actuator  400  includes the outer housing  450  that substantially surrounds the pressure chamber assembly  420 . Pressurized fluid in the bore  452  is separated from fluid in the pressure chambers  422  by the seals  426 . 
     In some implementations, rotating the rotary output shaft in a second direction opposite that of the first direction can include applying pressurized fluid to the second pressure chamber, and urging the second piston partially outward from the second pressure chamber to urge rotation of the rotary output shaft in a second direction opposite from the first direction. For example, pressurized fluid can be applied to the pressure chambers  310  of the first actuation section  110  to urge the rotary pistons  260  outward, causing the rotor shaft  210  to rotate counter-clockwise. 
     In some implementations, rotating the rotary output shaft in a second direction opposite that of the first direction can include applying pressurized fluid to the second pressure chamber, and urging the first piston partially into the first pressure chamber to urge rotation of the rotary output shaft in a second direction opposite from the first direction. For example, pressurized fluid can be flowed into the bore  452  at a pressure higher than that of fluid in the pressure chambers  422 , causing the rotary pistons  414  to move into the pressure chambers  422  and cause the rotor shaft  412  to rotate counter-clockwise. 
     In some implementations, rotation of the rotary output shaft can urge rotation of the housing. For example, the rotary output shaft  412  can be held rotationally stationary and the housing  450  can be allowed to rotate, and application of pressurized fluid in the pressure chambers  422  can urge the rotary pistons  414  out of the pressure chambers  422 , causing the housing  450  to rotate about the rotary output shaft  412 . 
       FIGS. 26-28  show various views of the components of another example rotary piston-type actuator  2600 . In general, the actuator  2600  is similar to the example actuator  100  of  FIG. 1 , except for the configuration of the seal assemblies. Whereas the seal assembly  320  in the example actuator  100  remains substantially stationary relative to the pressure chamber  310  and is in sliding contact with the surface of the rotary piston  250 , in the example actuator  2600 , the seal configuration is comparatively reversed as will be described below. 
     Referring to  FIG. 26 , a perspective view of the example rotary piston-type actuator  2600  is shown. The actuator  2600  includes a rotary piston assembly  2700  and a pressure chamber assembly  2602 . The actuator  2600  includes a first actuation section  2610  and a second actuation section  2620 . In the example of actuator  2600 , the first actuation section  2610  is configured to rotate the rotary piston assembly  2700  in a first direction, e.g., counter-clockwise, and the second actuation section  2620  is configured to rotate the rotary piston assembly  2700  in a second direction substantially opposite the first direction, e.g., clockwise. 
     Referring now to  FIG. 27 , a perspective view of the example rotary piston assembly  2700  is shown apart from the pressure chamber assembly  2602 . The rotary piston assembly  2700  includes a rotor shaft  2710 . A plurality of rotor arms  2712  extend radially from the rotor shaft  2710 , the distal end of each rotor arm  2712  including a bore (not shown) substantially aligned with the axis of the rotor shaft  2710  and sized to accommodate one of a collection of connector pins  2714 . 
     As shown in  FIG. 27 , the first actuation section  2710  of example rotary piston assembly  2700  includes a pair of rotary pistons  2750 , and the second actuation section  2720  includes a pair of rotary pistons  2760 . While the example actuator  2600  includes two pairs of the rotary pistons  2750 ,  2760 , other embodiments can include greater and/or lesser numbers of cooperative and opposing rotary pistons. 
     In the example rotary piston assembly shown in  FIG. 27 , each of the rotary pistons  2750 ,  2760  includes a piston end  2752  and one or more connector arms  2754 . The piston end  252  is formed to have a generally semi-circular body having a substantially smooth surface. Each of the connector arms  2754  includes a bore  2756  substantially aligned with the axis of the semi-circular body of the piston end  2752  and sized to accommodate one of the connector pins  2714 . 
     In some implementations, each of the rotary pistons  2750 ,  2760  includes a seal assembly  2780  disposed about the outer periphery of the piston ends  2752 . In some implementations, the seal assembly  2780  can be a circular or semi-circular sealing geometry retained on all sides in a standard seal groove. In some implementations, commercially available reciprocating piston or cylinder type seals can be used. For example, commercially available seal types that may already be in use for linear hydraulic actuators flying on current aircraft may demonstrate sufficient capability for linear load and position holding applications. In some implementations, the sealing complexity of the actuator  2600  may be reduced by using a standard, e.g., commercially available, semi-circular, unidirectional seal designs generally used in linear hydraulic actuators. In some embodiments, the seal assembly  2780  can be a one-piece seal. 
       FIG. 28  is a perspective cross-sectional view of the example rotary piston-type actuator  2600 . The illustrated example shows the rotary pistons  2760  inserted into a corresponding pressure chamber  2810  formed as an arcuate cavity in the pressure chamber assembly  2602 . The rotary pistons  2750  are also inserted into corresponding pressure chambers  2810 , not visible in this view. 
     In the example actuator  2600 , when the rotary pistons  2750 ,  2760  are each inserted through an open end  2830  of each pressure chamber  2810 , each seal assembly  2780  contacts the outer periphery of the piston end  2760  and the substantially smooth interior surface of the pressure chamber  2810  to form a substantially pressure-sealed region within the pressure chamber  2810 . 
     In some embodiments, the seal  2780  can act as a bearing. For example, the seal  2780  may provide support for the piston  2750 ,  2760  as it moves in and out of the pressure chamber  310 . 
       FIGS. 29A-29E  are various views of another example rotary piston-type actuator  2900  with a central actuation assembly  2960 . For a brief description of each drawing see the brief description of each of these drawings included at the beginning of the Description of the Drawings section of this document. 
     In general, the example rotary piston-type actuator  2900  is substantially similar to the example rotary piston-type actuator  1200  of  FIGS. 12-14 , where the example rotary piston-type actuator  2900  also includes a central actuation assembly  2960  and a central mounting assembly  2980 . Although the example rotary piston-type actuator  2900  is illustrated and described as modification of the example rotary piston-type actuator  1200 , in some embodiments the example rotary piston-type actuator  2900  can implement features of any of the example rotary piston-type actuators  100 ,  400 ,  700 ,  800 ,  1200 ,  1500 ,  1700 ,  1900 ,  2200 ,  2300 , and/or  2600  in a design that also implements the central actuation assembly  2960  and/or the central mounting assembly  2980 . 
     The actuator  2900  includes a rotary actuator assembly  2910 , a first actuation section  2901  and a second actuation section  2902 . The rotary piston assembly  2910  includes a rotor shaft  2912 , a collection of rotor arms  2914 , and the collection of dual rotary pistons, e.g., the dual rotary pistons  1216  of  FIGS. 12-14 . 
     The first actuation section  2901  of example actuator  2900  includes a first pressure chamber assembly  2950   a , and the second actuation section  2902  includes a second pressure chamber assembly  2950   b . The first pressure chamber assembly  2950   a  includes a collection of pressure chambers, e.g., the pressure chambers  1252   a  of  FIGS. 12-14 , formed as arcuate cavities in the first pressure chamber assembly  2950   a . The second pressure chamber assembly  2950   b  includes a collection of pressure chambers, e.g., the pressure chambers  1252   b  of  FIGS. 12-14 , formed as arcuate cavities in the second pressure chamber assembly  2950   b . A semicircular bore  2953  in the housing accommodates the rotor shaft  2912 . 
     The central mounting assembly  2980  is formed as a radially projected portion  2981  of a housing of the second pressure chamber assembly  2950   b . The central mounting assembly  2980  provides a mounting point for removably affixing the example rotary piston-type actuator  2900  to an external surface, e.g., an aircraft frame. A collection of holes  2982  formed in the radially projected section  2981  accommodate the insertion of a collection of fasteners  2984 , e.g., bolts, to removably affix the central mounting assembly  2980  to an external mounting feature  2990 , e.g., a mounting point (bracket) on an aircraft frame. 
     The central actuation assembly  2960  includes a radial recess  2961  formed in a portion of an external surface of a housing of the first and the second actuation sections  2901 ,  2902  at a midpoint along a longitudinal axis AA to the example rotary piston-type actuator  2900 . An external mounting bracket  2970  that may be adapted for attachment to an external mounting feature on a member to be actuated, (e.g., aircraft flight control surfaces) is connected to an actuation arm  2962 . The actuation arm  2962  extends through the recess  2961  and is removably attached to a central mount point  2964  formed in an external surface at a midpoint of the longitudinal axis of the rotor shaft  2912 . 
     Referring more specifically to  FIGS. 29D and 29E  now, the example rotary piston-type actuator  2900  is shown in cutaway end and perspective views taken though a midpoint of the central actuation assembly  2960  and the central mounting assembly  2980  at the recess  2961 . The actuation arm  2962  extends into the recess  2961  to contact the central mount point  2964  of the rotor shaft  2912 . The actuation arm  2962  is removably connected to the central mount point  2964  by a fastener  2966 , e.g., bolt, that is passed through a pair of holes  2968  formed in the actuation arm  2962  and a hole  2965  formed through the central mount point  2964 . A collection of holes  2969  are formed in a radially outward end of the actuation arm  2962 . A collection of fasteners  2972 , e.g., bolts, are passed through the holes  2969  and corresponding holes (not shown) formed in an external mounting feature (bracket)  2970 . As mentioned above, the central actuation assembly  2960  connects the example rotary piston actuator  2900  to the external mounting feature  2970  to transfer rotational motion of the rotor assembly  2910  to equipment to be moved (actuated), e.g., aircraft flight control surfaces. 
     In some embodiments, one of the central actuation assembly  2960  or the central mounting assembly  2980  can be used in combination with features of any of the example rotary piston-type actuators  100 ,  400 ,  700 ,  800 ,  1200 ,  1500 ,  1700 ,  1900 ,  2200 ,  2300 , and/or  2600 . For example, the example rotary piston-type actuator  2900  may be mounted to a stationary surface through the central mounting assembly  2980 , and provide actuation at one or both ends of the rotor shaft assembly  2910 . In another example, the example rotary piston assembly  2900  may be mounted to a stationary surface through non-central mounting points, and provide actuation at the central actuation assembly  2960 . 
       FIGS. 30A-30E  are various views of an example rotary actuator  3000  with a central actuation assembly  3060 . For a brief description of each drawing see the brief description of each of these drawings included at the beginning of the Description of the Drawings section of this document. 
     In general, the example rotary actuator  3000  is substantially similar to the rotary piston-type actuator  2900  of  FIGS. 29A-29E , where the example rotary actuator  3000  also includes a central actuation assembly  3060  and a central mounting assembly  3080 . In some embodiments, the example rotary actuator  3000  can be a modification of the example rotary piston-type actuator  2900  in which rotational action can be performed by a mechanism other than a rotary piston-type actuator. For example, the example rotary actuator  3000  can be include a rotary vane type actuator, a rotary fluid type actuator, an electromechanical actuator, a linear-to-rotary motion actuator, or combinations of these or any other appropriate rotary actuator. Although the example rotary actuator  3000  is illustrated and described as modification of the example rotary piston-type actuator  2900 , in some embodiments the example rotary actuator  3000  can implement features of any of the example rotary piston-type actuators  100 ,  400 ,  700 ,  800 ,  1200 ,  1500 ,  1700 ,  1900 ,  2200 ,  2300 ,  2600  and/or  2900  in a design that also implements the central actuation assembly  3060  and/or the central mounting assembly  3080 . 
     The actuator  3000  includes a rotary actuator section  3010   a  and a rotary actuator section  3010   b . In some embodiments, the rotary actuator sections  3010   a  and  3010   b  can be rotary vane type actuators, a rotary fluid type actuators, electromechanical actuators, a linear-to-rotary motion actuators, or combinations of these or any other appropriate rotary actuators. The rotary actuator section  3010   a  includes a housing  3050   a , and the rotary actuator section  3010   b  includes a housing  3050   b . A rotor shaft  3012   a  runs along the longitudinal axis of the rotary actuator section  3010   a , and a rotor shaft  3012   b  runs along the longitudinal axis of the rotary actuator section  3010   b.    
     The central mounting assembly  3080  is formed as a radially projected portion  3081  of the housings  3050   a  and  3050   b . The central mounting assembly  3080  provides a mounting point for removably affixing the example rotary actuator  3000  to an external surface or an external structural member, e.g., an aircraft frame, an aircraft control surface. A collection of holes  3082  formed in the radially projected section  3081  accommodate the insertion of a collection of fasteners (not shown), e.g., bolts, to removably affix the central mounting assembly  3080  to an external mounting feature, e.g., the external mounting feature  2090  of  FIG. 29 , a mounting point (bracket) on an aircraft frame or control surface. 
     The central actuation assembly  3060  includes a radial recess  3061  formed in a portion of an external surfaces of the housings  3050   a ,  3050   b  at a midpoint along a longitudinal axis AA to the example rotary actuator  3000 . In some implementations, an external mounting bracket, such as the external mounting bracket  2970 , may be adapted for attachment to an external mounting feature of a structural member or a member to be actuated, (e.g., aircraft flight control surfaces) can be connected to an actuation arm  3062 . An actuation arm, such as the actuation arm  2962 , can extend through the recess  3061  and can be removably attached to a central mount point  3064  formed in an external surface at a midpoint of the longitudinal axis of the rotor shafts  3012   a  and  3012   b.    
     Referring more specifically to  FIGS. 30D and 30E  now, the example rotary piston-type actuator  3000  is shown in end and cutaway perspective views taken though a midpoint of the central actuation assembly  3060  and the central mounting assembly  3080  at the recess  3061 . The actuation arm (not shown) can extend into the recess  3061  to contact the central mount point  3064  of the rotor shafts  3012   a ,  3012   b . The actuation arm can be removably connected to the central mount point  3064  by a fastener, e.g., bolt, that can be passed through a pair of holes (e.g. the holes  2968  formed in the actuation arm  2962 ) and a hole  3065  formed through the central mount point  3064 . Similarly to as was discussed in the description of the rotary piston-type actuator  2900  and the central actuation assembly  2960 , the central actuation assembly  3060  connects the example rotary actuator  3000  to an external mounting feature or structural member to impart rotational motion of the actuator sections  3010   a ,  3010   b  to equipment to be moved (actuated), e.g., aircraft flight control surfaces, relative to structural members, e.g., aircraft frames. 
     In some embodiments, one of the central actuation assembly  3060  or the central mounting assembly  3080  can be used in combination with features of any of the example rotary piston-type actuators  100 ,  400 ,  700 ,  800 ,  1200 ,  1500 ,  1700 ,  1900 ,  2200 ,  2300 ,  2600  and/or  2900 . For example, the example rotary actuator  3000  may be mounted to a stationary surface through the central mounting assembly  3080 , and provide actuation at one or both ends of the rotor shafts  3012   a ,  3012   b . In another example, the example rotary actuator  3000  may be mounted to a stationary surface through non-central mounting points, and provide actuation at the central actuation assembly  3060 . In another example, the rotary actuator  3000  may be mounted to a stationary surface through the central mount point  3064 , and provide actuation at the central mounting assembly  3080 . 
       FIGS. 31A-31E  are various views of an example rotary actuator  3100  with a central actuation assembly  3160 . For a brief description of each drawing see the brief description of each of these drawings included at the beginning of the Description of the Drawings section of this document. 
     In general, the example rotary actuator  3100  is substantially similar to the rotary actuator  3000  of  FIGS. 30A-30E , where the example rotary actuator  3100  also includes a central actuation assembly  3160  and a central mounting assembly  3180 . In some embodiments, the example rotary actuator  3100  can be a modification of the example rotary piston-type actuator  3000  in which rotational action can be performed by a mechanism other than a rotary fluid actuator. The example rotary actuator  3100  is an an electromechanical actuator. Although the example rotary actuator  3100  is illustrated and described as modification of the example rotary actuator  3000 , in some embodiments the example rotary actuator  3100  can implement features of any of the example rotary piston-type actuators  100 ,  400 ,  700 ,  800 ,  1200 ,  1500 ,  1700 ,  1900 ,  2200 ,  2300 ,  2600  and/or  2900  and/or the rotary actuator  3000  in a design that also implements the central actuation assembly  3160  and/or the central mounting assembly  3180 . 
     The actuator  3100  includes a rotary actuator section  3110   a  and a rotary actuator section  3110   b . In some embodiments, the rotary actuator sections  3110   a  and  3110   b  can be electromechanical actuators. The rotary actuator section  3110   a  includes a housing  3150   a , and the rotary actuator section  3110   b  includes a housing  3150   b . A rotor shaft  3112   a  runs along the longitudinal axis of the rotary actuator section  3110   a , and a rotor shaft  3112   b  runs along the longitudinal axis of the rotary actuator section  3110   b.    
     The central mounting assembly  3180  is formed as a radially projected portion  3181  of the housings  3150   a  and  3150   b . The central mounting assembly  3180  provides a mounting point for removably affixing the example rotary actuator  3100  to an external surface or an external structural member, e.g., an aircraft frame, an aircraft control surface. A collection of holes  3182  formed in the radially projected section  3181  accommodate the insertion of a collection of fasteners (not shown), e.g., bolts, to removably affix the central mounting assembly  3180  to an external mounting feature, e.g., the external mounting feature  2090  of  FIG. 29 , a mounting point (bracket) on an aircraft frame or control surface. 
     The central actuation assembly  3160  includes a radial recess  3161  formed in a portion of an external surfaces of the housings  3150   a ,  3150   b  at a midpoint along a longitudinal axis AA to the example rotary actuator  3100 . In some implementations, an external mounting bracket, such as the external mounting bracket  2970 , may be adapted for attachment to an external mounting feature of a structural member or a member to be actuated, (e.g., aircraft flight control surfaces) can be connected to an actuation arm  3162 . An actuation arm, such as the actuation arm  2962 , can extend through the recess  3161  and can be removably attached to a central mount point  3164  formed in an external surface at a midpoint of the longitudinal axis of the rotor shafts  3112   a  and  3112   b.    
     Referring more specifically to  FIGS. 31D and 31E  now, the example rotary piston-type actuator  3100  is shown in end and cutaway perspective views taken though a midpoint of the central actuation assembly  3160  and the central mounting assembly  3080  at the recess  3161 . The actuation arm (not shown) can extend into the recess  3161  to contact the central mount point  3164  of the rotor shafts  3112   a ,  3112   b . The actuation arm can be removably connected to the central mount point  3164  by a fastener, e.g., bolt, that can be passed through a pair of holes (e.g. the holes  2968  formed in the actuation arm  2962 ) and a hole  3165  formed through the central mount point  3164 . Similarly to as was discussed in the description of the rotary piston-type actuator  2900  and the central actuation assembly  2960 , the central actuation assembly  3160  connects the example rotary actuator  3100  to an external mounting feature or structural member to impart rotational motion of the actuator sections  3110   a ,  3110   b  to equipment to be moved (actuated), e.g., aircraft flight control surfaces, relative to structural members, e.g., aircraft frames. 
     In some embodiments, one of the central actuation assembly  3160  or the central mounting assembly  3180  can be used in combination with features of any of the example rotary piston-type actuators  100 ,  400 ,  700 ,  800 ,  1200 ,  1500 ,  1700 ,  1900 ,  2200 ,  2300 ,  2600  and/or  2900  and/or the rotary actuator  3000 . For example, the example rotary actuator  3100  may be mounted to a stationary surface through the central mounting assembly  3180 , and provide actuation at one or both ends of the rotor shafts  3112   a ,  3112   b . In another example, the example rotary actuator  3100  may be mounted to a stationary surface through non-central mounting points, and provide actuation at the central actuation assembly  3160 . In another example, the rotary actuator  3100  may be mounted to a stationary surface through the central mount point  3164 , and provide actuation at the central mounting assembly  3180 . 
     Although a few implementations have been described in detail above, other modifications are possible. For example, the logic flows depicted in the figures do not require the particular order shown, or sequential order, to achieve desirable results. In addition, other steps may be provided, or steps may be eliminated, from the described flows, and other components may be added to, or removed from, the described systems. Accordingly, other implementations are within the scope of the following claims.