Abstract:
A motion simulator is constructed from a base driving an intermediate member via a 6 DOF hexapod, and a platform driven by a 2DOF simulator is provided on the intermediate member to supplement pitch and roll.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is the U.S. national phase of International Application No. PCT/EP2013/067521 filed Aug. 23, 2013, which claims priority of British Application No. 1300552.5 filed Jan. 14, 2013, the entirety of which is incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention is concerned with a motion simulator. More specifically the present invention is concerned with a motion simulator for applications requiring a large excursion in at least one rotational degree of freedom. 
       BACKGROUND OF THE INVENTION 
       [0003]    Motion simulators are well known in the art. The Stewart platform (or hexapod) is a well known form of simulator which moves a platform relative to a base. Hexapods have six linear actuators arranged to move the platform in six degrees of freedom (three linear, three rotational) relative to the base depending on which actuators are used in combination. The translational degrees of freedom are commonly known as surge (horizontal movement in the direction of travel), sway (horizontal movement perpendicular to the direction of travel) and heave (vertical motion). The rotational degrees of freedom are known as roll (rotation about an axis parallel to the direction of travel), pitch (rotation about a horizontal axis perpendicular to the direction of travel) and yaw (rotation about a vertical axis). 
         [0004]    Hexapods have finite workspaces defined by the maximum and minimum excursion of the platform, which in turn is defined by the limit of travel of the actuators. For larger workspaces requiring further platform movement in any given degree of freedom, it is known to provide longer hexapod actuators. Although this may achieve the desired result, it substantially increases the cost of the simulator (longer linear actuators are significantly more expensive than short ones), and can sometimes decrease its inherent stiffness. In some cases, hexapods are simply unsuitable for the required degree of excursion. 
         [0005]    Stiffness is an important property of the simulator, because it minimises undesirable vibration and oscillation of the platform, which would otherwise provide false accelerations, and forces on the subject. In known Stewart platforms there is therefore a trade off between maximum platform excursion and stiffness. 
         [0006]    There are various simulations which require a high excursion, or degree of travel, in a specific rotational degree or degrees of freedom. This can be used to simulate gravity or radial accelerations. For example, fuel tank testing, battery testing, fuel metering system testing, inertia measurements of equipment, testing instruments, fixation methods testing, equipment containing/depending on liquids or magnets and any equipment that requires an artificial horizon all require potentially large platform movements in the global roll and pitch degrees of freedom. Providing a hexapod with long stroke actuators would provide the required functionality to a certain extent, but not in all cases. Large hexapods would also provide functionality which is not required—namely additional travel in the remaining four degrees of freedom. 
         [0007]    As such, there is competing requirement to provide a stiff, compact and inexpensive simulator on the one hand, and to provide additional movement in the roll and pitch degrees of freedom on the other hand. 
       SUMMARY OF THE INVENTION 
       [0008]    It is an aim of the present invention to overcome, or at least mitigate this problem. 
         [0009]    According to a first aspect of the invention there is provided a motion simulator comprising: 
         [0010]    a base and an intermediate member connected to the base by a hexapod, the hexapod being configured to move the intermediate member in six global degrees of freedom relative to the base, the six global degrees of freedom including roll, pitch and yaw; 
         [0011]    a platform connected to the intermediate member for movement in at least one local rotational degree of freedom relative thereto; a supplementary actuation assembly arranged to move the platform relative to the intermediate member in the at least one local rotational degree of freedom, so as to supplement global movement of the platform in at least one of the global roll and pitch degrees of freedom. 
         [0012]    Advantageously, the provision of a movable platform on an intermediate member allows a greater range of movement of the platform. It will be noted that although the hexapod is a parallel manipulator (thus providing the required stiffness), the intermediate member and platform are coupled in series (providing a high range of movement). In the embodiment discussed below, a 6 degree of freedom hexapod is supplemented by a 2 degree of freedom system constrained by a universal joint. 
         [0013]    Preferably, the platform and intermediate member are connected by a joint is fixed to the intermediate member at a first side, and fixed to the platform at a second side, which joint has degrees of freedom in the local pitch and roll axes of the intermediate member. 
         [0014]    Preferably the supplementary actuation assembly comprises a first supplementary linear actuator mounted to the intermediate member at a first end and to the platform at a second end. More preferably the platform is connected to the intermediate member via a joint, and in which the first supplementary linear actuator is connected to the platform at a position spaced from the joint so as to produce a moment on the platform. This results in a rotation of the platform using a linear actuator. 
         [0015]    Preferably the joint is a universal joint, such as a cardan joint or a spherical joint. This allows rotation of the platform in two notional horizontal degrees of freedom of the intermediate member only. The term “universal joint” is used here to denote a joint having at least two rotational degrees of freedom. Preferably the platform is constrained relative to the intermediate member in all local degrees of freedom except roll and pitch. 
         [0016]    Alternatively the joint may be a joint constrained in all but one rotational degree of freedom—i.e. a hinge joint. 
         [0017]    Preferably the supplementary actuation assembly is a parallel manipulator having at least two functionally parallel actuators. In this context, “functionally parallel” means operating in parallel—i.e. both being joined to the intermediate member and platform. This further enhances the stiffness of the overall manipulator. Preferably the hexapod and the supplementary actuation assembly overlap in three dimensional space. This provides a stiff, compact arrangement. Preferably the actuators are not parallel in a geometric sense—i.e. they are at an oblique angle relative to each other. 
         [0018]    The supplementary actuation assembly may comprise a second supplementary linear actuator mounted to the intermediate member at a first end and to the platform at a second end. Preferably the second ends of the first and second actuators are spaced apart on the platform. Combinations of movement of the first and second actuators can thereby move the platform in the two degrees of freedom. 
         [0019]    Preferably the hexapod is attached to the intermediate member at least three fixing points defining a first plane, and in which the first end or ends of the supplementary linear actuator or actuators are positioned on a first side of the first plane, opposite to the platform. 
         [0020]    Preferably the hexapod is attached to the intermediate member at least three fixing points defining a first plane, and in which the supplementary actuation assembly crosses the first plane. More preferably the at least three fixing points define a first surface bounded by lines joining the at least three fixing points, and in which the supplementary actuation assembly crosses the first surface. This provides a stiff, compact simulator. 
         [0021]    Preferably the intermediate member comprises a central region and a plurality of legs, in which the hexapod is attached to the legs. This allows for a lightweight intermediate member with low inertia, and also allows the supplementary actuation assembly to pass between the legs to make a more compact simulator. The legs may extend in the local horizontal plane of the intermediate member. 
         [0022]    Preferably the intermediate member comprises a leg extending into a volume defined by the hexapod, in which the supplementary actuation assembly is attached to the leg. By “volume defined by the hexapod” we mean a notional three dimensional space bounded by the hexapod actuators. Such a volume is bounded by surfaces extending the shortest possible distance between adjacent actuators, and by a top surface joining the three areas where pairs of actuators are attached to the intermediate member. 
         [0023]    Preferably the supplementary actuation assembly is attached to the leg at a foot, the foot defined at an end distal to the platform. 
         [0024]    Preferably the supplementary actuation assembly is configured to actuate the platform relative to the intermediate member about two notional horizontal axes in the local coordinate system of the intermediate member. 
         [0025]    Preferably the hexapod comprises a plurality of linear actuators, in which the supplementary actuation assembly comprises at least one linear actuator having an excursion less than any of the linear actuators of the hexapod. In other words, instead of the prior art approach of providing six longer actuators in the hexapod, six “normal” length actuators are supplemented by two further actuators. Provision of 8 normal-length actuators instead of 6 longer actuators is both less expensive and stiffer. 
         [0026]    Preferably at least one of the hexapod and supplementary actuation assembly comprises at least one linear actuator, the at least one linear actuator comprising an electric motor driving a ball screw to advance a piston. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING VIEWS 
         [0027]    An example motion simulator according to the present invention will now be described by way of example with reference to the accompanying figures in which: 
           [0028]      FIG. 1  is a perspective view of a motion simulator in accordance with the present invention; 
           [0029]      FIG. 2  is a plan view of the motion simulator of  FIG. 1 ; 
           [0030]      FIG. 3  is a front view of the motion simulator of  FIG. 1 ; 
           [0031]      FIG. 4  is a side view of the motion simulator of  FIG. 1 ; 
           [0032]      FIG. 5  is a perspective view of a part of the motion simulator of  FIG. 1 ; 
           [0033]      FIG. 6  is a first perspective view of a sub-assembly of the motion simulator of  FIG. 1 ; 
           [0034]      FIG. 7  is a further perspective view of a sub-assembly of a motion simulator of  FIG. 1 ; and 
           [0035]      FIG. 8  is a perspective view of the motion simulator of  FIG. 1  in an actuated state. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0036]    Turning to  FIG. 1 , a motion simulator  100  generally comprises a base  102 , an intermediate member  104 , and a platform  106 . The intermediate member  104  and the base  102  are joined and driven by a hexapod  108  and the platform  106  and the intermediate member  104  are joined by a joint assembly  110  and driven by supplementary actuation assembly  112 . 
         [0037]    The base  102  is generally triangular in shape having a first, second and third vertex  114 ,  116 ,  118  respectively, as shown in  FIG. 2 . The base  102  is attached to a solid immoveable surface such as the floor of a workshop by a known method. The base is positioned to globally immovable global axes XG, YG and ZG. Rotation about XG is roll, and rotation about YG is pitch. Rotation about ZG is yaw. 
         [0038]    The intermediate member  104  is shown in more detail in  FIGS. 6 and 7 . The intermediate member  104  comprises three arms  120 ,  122 ,  124  respectively, extending radially from a central region  126 . The intermediate member  104  has a notional local co-ordinate system having axes XL, YL and ZL, which is slightly vertically offset from the top of the central region  126 . The local coordinate system moves with the intermediate member  104 . In the neutral position shown in  FIG. 1 , ZG and ZL are aligned, XG and XL are parallel, and YG and YL are parallel. 
         [0039]    Each of the arms  120 ,  122 ,  124  are equally spaced about the local vertical axis ZL. Extending from the central region  126 , parallel to and along the local vertical axis ZL, there is provided a leg  128 . The leg is tubular and cylindrical and terminates in a foot  130  at an end opposite to the arms  120 ,  122 ,  124  and central region  126 . The foot  130  is in the form of a radially extending flange. 
         [0040]    Extending in the 90 degree corner defined between the leg  128  and each individual arm  120 ,  122 ,  124 , there is provided a web  132 ,  134 ,  136  respectively which acts to stiffen the intermediate member  104 . 
         [0041]    The platform  106  comprises a plate member  138  which has a generally flat support surface  140 . The platform  106  defines a support  142  extending from the plate member  138  opposite to the support surface  140 . The support  142  is a generally solid, cylindrical member. The support  142  terminates in a platform joint flange  144 . 
         [0042]    A plurality of webs  146  extend between the platform joint flange  144 , support  142 , and the underside of the member  138  opposite the support surface  140 . 
         [0043]    The hexapod  108  comprises six linear actuators  150  to  160  respectively. Each of the linear actuators is substantially identical and, as such, only the actuator  150  will be described here, with reference to  FIG. 5 . The linear actuator  150  comprises a first universal joint  162  and a second universal joint  164 . Universal joints  162 ,  164  are at opposite ends of the actuator  150 . Intermediate the universal joints  162 ,  164 , there is provided a cylinder  166  which houses a piston  168  (shown more clearly with respect to the third linear actuator  154  in  FIG. 8 ). The piston  168  is mounted inside the cylinder  166  with a ball screw which is actuable via an electric motor  170  connected to the linear actuator  150  proximate the first universal joint  162 . A belt drive  172  connects the motor  170  to the ball screw such that the piston  168  can be driven in and out of the cylinder  166  by the motor  170 . 
         [0044]    The joint assembly  110  comprises a universal joint  174  in the form of a cardan joint positioned on the local axis ZL and actuable about the local horizontal axes XL and YL. 
         [0045]    Referring to  FIG. 8 , the supplementary actuation assembly  112  comprises a first supplementary linear actuator  176  and a second supplementary linear actuator  178 . The supplementary actuators  176 ,  178  are similar to the linear actuators  150  to  160  with the exception that they are generally shorter and have less travel; that is a lower range of motion from their compact state as shown in  FIG. 5 , to their extended state as shown, for example, in  FIG. 8 . 
         [0046]    The motion simulator  100  is assembled as follows. 
         [0047]    The base  102  is installed on a stationary, horizontal, flat surface such that it is immoveable in use. The intermediate member  104  is then suspended above the base  102  via the hexapod  108 . 
         [0048]    The actuators of the hexapod  108  are arranged as follows. 
         [0049]    Firstly, the platform  106  is oriented such that each of the arms  120 ,  122 ,  124  is interspersed between two of the vertices  114 ,  116 ,  118  of the base  102  when viewed from above (see  FIG. 2 ). The first actuator  150  then extends diagonally from the first vertex  114  to the end of the first arm  120 . The second linear actuator  152  extends from the second vertex  116  to the first end of the first arm  120 . The third linear actuator  154  extends from the second vertex  116  to the end of the second arm  122 , and the fourth linear actuator  156  extends from the third vertex  118  to the end of the second arm  112 . The fifth linear actuator  158  extends from the third vertex  118  to the end of the third arm  124  and finally, the sixth linear actuator  160  extends from the first vertex  114  to the end of the third arm  124 . In this manner a hexapod or Stewart platform is formed. It will be noted that the volume formed by the hexapod defined by the linear actuators  150  to  160  is penetrated by the downwardly depending leg  128  of the intermediate member  104 . 
         [0050]    The platform  106  is then attached to the central region  126  of the intermediate member  104  via the joint assembly  110  for rotation about local axes XL and YL. The supplementary actuation assembly  112  is then installed in which the first supplementary linear actuator  176  extends from the foot  130  of the intermediate member  104  between the first and second arms  120 ,  122  of the intermediate member  104  to a corner of the plate member  138  of platform  106 . Similarly, the second supplementary linear actuator  178  extends from the foot  130  of the intermediate member  104  between the second and third arms  122  and  124  of the intermediate member  104  to an adjacent corner of the plate member  138  of the platform  106 . 
         [0051]    The first and second supplementary actuators  176 ,  178  are at a mid-travel point when the platform  104  is horizontal. Retraction of the first supplementary actuator  176  and lengthening of the second supplementary actuator  178  rotates the platform  104  about local axis XL, and simultaneous lengthening or shortening of both supplementary actuators  176 ,  178  rotates the platform  104  about joint axis YL. 
         [0052]    Roll of the intermediate member  104  about the axis XG via the hexapod, and roll of the platform  106  about the local axis XL relative to the intermediate member, is shown in  FIG. 8 . It will be noted that a large roll of the platform  106  about the global axis XG is achieved. 
         [0053]    Variations fall within the scope of the present invention. 
         [0054]    The free ends of the legs of the intermediate member  104  may be joined by a peripheral structure (which may be circular—i.e. a ring—or any other shape) which bounds the intermediate member. 
         [0055]    In an alternative embodiment, motion of the universal joint  174  about the local horizontal axes XL and YL may be performed by a pair of motors with rotary output shafts directly driving the joint.