WO2025010370A1

WO2025010370A1 - Split stewart platform servicer capture mechanism and associated methods

Info

Publication number: WO2025010370A1
Application number: PCT/US2024/036811
Authority: WO
Inventors: Russell Duane HOWARD; Kieran Wilson
Original assignee: Atomos Nuclear and Space Corporation
Priority date: 2023-07-03
Filing date: 2024-07-03
Publication date: 2025-01-09

Abstract

A capture system attachable to a servicer spacecraft for capturing and controlling a space object includes three triangular arm assemblies. Each triangular arm assembly is independently controllable and includes a cross brace and two linear actuator struts, attached together at one end to form an apex, and connected with the cross brace via joints at the opposing end. The triangular arm assembly further includes an end effector, connected with the two linear actuator struts at the apex. The capture assembly is maneuvered to the space object to enable the end effector to grapple onto a feature of the space object without a priori knowledge of a specific shape of the feature of the space object. An associated method is also disclosed.

Description

Split Stewart Platform Servicer Capture Mechanism and Associated

Methods

REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims the benefit of US Provisional Pat. App. No. 63/524,808, filed 2023-07-03 and titled "Split Stewart Platform Servicer Capture Mechanism and Associated Methods," which application is incorporated hereby in its entirety by reference.

FI ELD OF TH E I NVENTION

[0002] The present invention relates to grappling mechanisms. In particular, but not by way of limitation, the present invention relates to grappling mechanisms for use in space rendezvous and capture operations.

DESCRIPTION OF RELATED ART

[0003] Grappling mechanisms are frequently used in space rendezvous and capture operations, such as the rendezvous of a servicer satellite with a client satellite. Such grappling operations are difficult as adjustment and control are required in multiple dimensions. While a variety of grappling and capture systems exist, there is still a need for an improved servicer capture mechanism to accommodate a variety of capture features on target objects while enabling flexible yet precise control over preferably six degrees of freedom.

SU M MARY OF TH E I NVENTION

[0004] The following presents a simplified summary relating to one or more aspects and/or embodiments disclosed herein. As such, the following summary should not be considered an extensive overview relating to all contemplated aspects and/or embodiments, nor should the following summary be regarded to identify key or critical elements relating to all contemplated aspects and/or embodiments or to delineate the scope associated with any particular aspect and/or embodiment. Accordingly, the following summary has the sole purpose to present certain concepts relating to one or more aspects and/or embodiments relating to the mechanisms disclosed herein in a simplified form to precede the detailed description presented below. [0005] In an embodiment, a capture system attachable to a servicer spacecraft for capturing and controlling a space object by the servicer spacecraft is disclosed. The capture system includes three triangular arm assemblies. Each one of the three triangular arm assemblies includes a cross brace and two linear actuator struts. Each one of the two linear actuator struts includes a first end and a second end, with the first ends of the two linear actuator struts being attached together to form an apex. Each one of the second ends of the two linear actuator struts are connected with the cross brace via a joint. Each one of the three triangular arm assemblies further includes an end effector, which end effector is connected with the two linear actuator struts at the apex. Further, each one of the three triangular arm assemblies is independently controllable for maneuvering the end effector on each one of the three triangular arm assemblies toward the space object to enable the end effector to grapple onto a feature of the space object without a priori knowledge of a specific shape of the feature of the space object.

[0006] In certain embodiments, the end effector includes a wrist joint. In embodiments, the wrist joint is spring-loaded to a nominal position.

[0007] In a further embodiment, the end effector includes a grip actuator configured for capturing the feature and rigidizing a grasp on the feature, once the feature has been captured by the grip actuator.

[0008] In certain embodiments, at least one of the two actuator struts includes at least one of a telescoping ball screw linear actuator, an extendable truss, a rotary joint. In embodiments, each one of the three triangular arm assemblies is configured to maneuver the end effector toward the feature with six degrees of freedom.

[0009] In embodiments, the end effector associated with each one of the three triangular arm assemblies is configured to capture a different portion of the feature of the space object from the end effector of associated with the other ones of the three triangular arm assemblies.

[0010] In certain embodiments, the three triangular arm assemblies are further configured to maneuver the space object to a predetermined attitude and distance with respect to the servicer spacecraft.

[0011] In another embodiment, a method for capturing and controlling a space object using a servicer spacecraft is disclosed. The method includes providing a capture system including three triangular arm assemblies. Each one of the three triangular arm assemblies includes a cross brace and two linear actuator struts. Each one of the two linear actuator struts includes a first end and a second end, wherein the first ends of the two linear actuator struts being attached together to form an apex and each one of the second ends of the two linear actuator struts being connected with the cross brace via a joint. Each one of the three triangular arm assemblies furth includes an end effector connected with the two linear actuator struts at the apex. The method further includes attaching the capture system to the servicer spacecraft, independently maneuvering the end effector on each one of the three triangular arm assemblies toward the space object, and grappling a feature of the space object with the end effector on each one of the three triangular arm assemblies. The grappling is performed without a priori knowledge of a specific shape of the feature of the space object.

[0012] In certain embodiments, the method further includes maneuvering the space object to a predetermined attitude and distance with respect to the servicer spacecraft.

[0013] In another embodiment, a capture system attachable to a servicer spacecraft for capturing and controlling a space object by the servicer spacecraft is disclosed. The capture system includes t least two triangular arm assemblies. Each one of the two triangular arm assemblies includes a cross brace and two linear actuator struts. Each one of the two linear actuator struts including a first end and a second end, the first ends of the two linear actuator struts being attached together to form an apex. Each one of the second ends of the two linear actuator struts is connected with the cross brace via a joint. Each of the at least two triangular arm assemblies further includes an end effector connected with the two linear actuator struts at the apex. Each one of the two triangular arm assemblies is independently controllable for maneuvering the end effector on each one of the two triangular arm assemblies toward the space object to enable the end effector to grapple onto a feature of the space object without a priori knowledge of a specific shape of the feature of the space object. The capture system further comprises a third arm assembly.

[0014] In certain embodiments, the third arm assembly includes at least one of a third triangular arm assembly and a robotic arm assembly, and each of the two triangular arm assemblies and the third arm assembly is configured to maneuver the end effector toward the feature with six degrees of freedom.

[0015] In embodiments, the end effector includes a wrist joint. In certain embodiments, the wrist joint is spring-loaded to a nominal position. [0016] In embodiments, the end effector includes a grip actuator configured for capturing the feature and rigidizing a grasp on the feature, once the feature has been captured by the grip actuator.

[0017] These and other features, and characteristics of the present technology, as well as the methods of operation and functions of the related elements of structure and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention. As used in the specification and in the claims, the singular form of 'a', 'an', and 'the' include plural referents unless the context clearly dictates otherwise.

BRI EF DESCRI PTION OF DRAWINGS

[0018] FIG. 1 illustrates an example of a capture mechanism including one or two serial manipulators.

[0019] FIG. 2 shows a cross-sectional view of a standard Atlas V Type D/Delta IV Type 1666-4/5 Marman ring.

[0020] FIG. 3 shows a cross-sectional view of a standard Atlas V Type B & Bl/Delta IV Type 1194-4/5 Marman ring.

[0021] FIG. 4 shows a cross-sectional view of a standard Atlas V Type A/Delta IV Type 937-4/5 Marman ring.

[0022] FIG. 5 shows a cross-sectional view of a portion of a 15-inch Evolved Secondary Payload Adapter (ESPA) model PAS 381B.

[0023] FIG. 6 shows a cross-sectional view of a portion of a 24-inch ESPA model PAS 610S.

[0024] FIG. 7 illustrates an example of capture system including a Stewart platform suitable for use in servicer capture applications, in accordance with embodiments.

[0025] FIG. 8 shows examples of different standard payload separation interfaces overlayed with a one-meter square standard servicer bus. [0026] FIG. 9 illustrates exemplary translation and attitude envelope requirements for servicer operations, in accordance with embodiments.

[0027] FIG. 10 shows a graphic representation of a Stewart platform-based capture mechanism in relation to different interface diameters, in accordance with embodiments.

[0028] FIG. 11 shows a first step of a notional mechanism for grappling an exemplary interface ring, in accordance with an embodiment.

[0029] FIG. 12 shows a second step of a notional mechanism for grappling an exemplary interface ring, in accordance with an embodiment.

[0030] FIG. 13 shows a third step of a notional mechanism for grappling an exemplary interface ring, in accordance with an embodiment.

[0031] FIG. 14 shows a front view of an exemplary Split Stewart Platform (SSP) arm assembly, in accordance with an embodiment.

[0032] FIG. 15 shows a side view of the exemplary ISP arm assembly of FIG. 14, in accordance with an embodiment.

[0033] FIG. 16 shows a graphic representation of a process for grasping an exemplary Atlas V- type/Delta IV type Marman ring using a gripper, in accordance with an embodiment.

[0034] FIG. 17 shows a graphic representation of a process for operating an ISP system, in accordance with embodiments.

[0035] FIG. 18 illustrates an example of an alternative capture system including a modified Stewart platform suitable for use in servicer capture applications, in accordance with embodiments.

[0036] For simplicity and clarity of illustration, the drawing figures illustrate the general manner of construction, and descriptions and details of well-known features and techniques may be omitted to avoid unnecessarily obscuring the embodiments detailed herein. Additionally, elements in the drawing figures are not necessarily drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of the described embodiments. The same reference numerals in different figures denote the same elements.

[0037] The word "exemplary" is used herein to mean "serving as an example, instance, or illustration." Any embodiment described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments. In the following detailed description, references are made to the accompanying drawings that form a part hereof, and in which are shown by way of illustrations or specific examples. These aspects may be combined, other aspects may be utilized, and structural changes may be made without departing from the present disclosure. Example aspects may be practiced as methods, systems, or apparatuses. The following detailed description is therefore not to be taken in a limiting sense, and the scope of the present disclosure is defined by the appended claims and their equivalents.

DETAI LED DESCRIPTION OF TH E I NVENTION

[0038] As described above, a variety of considerations are required in the capture and grappling operations of objects, especially in space environments. For instance, a servicer spacecraft may be required to rendezvous and capture a client satellite, which may or may not be actively controllable or in a stable altitude and orbit. Similarly, as there is currently no standardized set of capture features (e.g., a launch support ring apparatus such as Marman rings and ESPAs) used in spaceborne operations, the servicer may need to be able to identify and accommodate a variety of capture features.

[0039] FIG. 1 illustrates a simplified example of a capture mechanism including one or two serial manipulators. As shown in FIG. 1, a capture mechanism 100 includes a vehicle 110 equipped with a serial manipulator 120. Serial manipulator 120 may be, for example, a 6-DoF manipulator, such as a robotic arm, capable of capturing and manipulating objects in a three- dimensional space. Optionally, a second serial manipulator 122 may be provided with capture mechanism 100 to provide additional range of motion, tensile strength, stroke, and/or force. Each of serial manipulators 120 and 122 may include a gripper apparatus 128 to capture the target object. However, capture mechanism 100 may not provide sufficient stability to the captured object so as to stably maneuver the captured object toward the servicer in the unpredictable conditions of space.

[0040] FIGS. 2 - 6 illustrate examples of capture features commonly used in aerospace applications.

[0041] FIG. 2 shows a cross-sectional view of a standard Atlas V Type D/Delta IV Type 1666-4/5 Marman ring.

[0042] FIG. 3 shows a cross-sectional view of a standard Atlas V Type B ^A Bl/Delta IV Type

1194-4/5 Marman ring. [0043] FIG. 4 shows a cross-sectional view of a standard Atlas V Type A/Delta IV Type 937-4/5 Marman ring.

[0044] FIG. 5 shows a cross-sectional view of a portion of a 15-inch Evolved Secondary Payload Adapter (ESPA) model PAS 381B.

[0045] FIG. 6 shows a cross-sectional view of a portion of a 24-inch ESPA model PAS 610S.

[0046] That is, the grappling mechanism of the capture and grappling system should ideally be able to accommodate such a range of cross-sectional dimensions commonly used in space applications.

[0047] For example, a robotic gripper from NASA (US Pat. No. 10,414,053 B2 to Ashmore; technology overview at https://technology.nasa.gov/patent/GSC-TOPS-190 accessed 2023-06- 21) provides a gripper for securely capturing the launch support ring apparatus in space, suitable for use as gripper apparatus 128 and specifically designed to accommodate a range of capture features, such as those shown in FIGS. 2 - 6. However, the NASA gripper itself is heavy and cumbersome, and a robotic arm (such as shown in FIG. 1) would also need to be highly robust and sturdy to be able to capture and maneuver an object in space. That is, once the capture feature has been identified, an appropriate capture mechanism should be deployed. For instance, the servicer may be maneuvered to bring an end effector, such as a gripper mechanism, in proximity to the capture feature then activate the end effector to successfully capture the capture feature of the target object. Once captured, the target object must be maneuvered by the servicer to perform the servicing operations, such as transport, refueling, and repair.

[0048] Thus, a comprehensive solution to be able to rendezvous, capture, and maneuver a space object would be desirable. Such capture operations solution would require, for instance, one or more of the following features:

[0049] 1) A guidance-navigation-and-control (GNC) system and a variety of sensors

(e.g., long-, medium-, and short-range);

[0050] 2) Autonomous or manual image capture and recognition of the relevant features, and identification of suitable grasping spots on the capture feature, avoiding sensitive instrumentation and connections;

[0051] 3) An end effector capable of surrounding the capture feature prior to gripping, such as gripper with compact stowage geometry, large working volume, quick response, low overall mechanism inertia to minimize disturbance to the servicer, robust and simple local sensor suite, and low friction;

[0052] 4) Sensors to confirm successful capture;

[0053] 5) Dampening mechanism to dampen the motion of the target object before hitting joint limits;

[0054] 6) Low inertia to minimize energy dissipation and disturbance to both the servicer and the object to be captured, e.g., with the end effector effective inertia being similar to point mass near grasp point;

[0055] 7) Sufficient working volume and degrees of freedom (DoF) to position the captured object on the thrust vector of the servicer;

[0056] 8) Ability to provide force and accuracy required to mate the captured object with the servicer (e.g., to accurately position the appropriate interface of the captured object with an interface such as a refueling port on the servicer);

[0057] 9) Image feature recognition to correctly identify orientation of the captured object and to calibrate the internal sensors of the capture mechanism;

[0058] 10) Minimum stiffness of connection (i.e., to exceed a minimum stiffness threshold when grappled - If the connection is not sufficiently stiff, there will be too much relative motion between the servicer and the target object as thrust is applied or solar arrays rotate, etc., potentially causing problems for attitude control of the mated stack formed by the combined servicer and target object);

[0059] 11) Low or no power usage when the grappling mechanism is rigidized and static; and

[0060] 12) Overall simplicity, reliability, lightweight, and low-cost.

[0061] From an unrelated application area of variable motion machines, a Stewart platform is a mechanism commonly used in providing three-dimensional, controlled motion using six linear actuators in applications such as flight simulators. While normally used in providing motion to a flat platform surface, such as the motion base of a machine, it is recognized herein that the linear actuator arms of an "incomplete" Stewart platform (i.e., a Stewart platform without the flat platform surface) may be manipulated to function as a capture mechanism in space applications and elsewhere to provide a stable, flexible, and powerful mechanism for capturing and manipulating space objects. [0062] The "Split Stewart Platform" or SSP described herein is an innovative, lightweight mechanism designed to capture a space object, such as an unprepared client satellite with, minimal disturbance, then precisely control the pose of the space object over a wide range of attitudes and separations. In embodiments, the Split Stewart Platform includes a mechanical arrangement of structure and actuators mounted on the forward, client-facing side of a servicer spacecraft.

[0063] FIG. 7 illustrates an example of capture system incorporating a Split Stewart Platform suitable for use in servicer capture applications, in accordance with embodiments. For example, a capture system 700 includes a vehicle 710 equipped with three triangular trusses or arm assemblies 720A, 720B, and 720C. Each arm assembly includes two linear actuator struts 722. Each linear actuator strut 722 includes a first end connected together at an apex 724. Furthermore, each linear actuator strut 722 includes a second end connected with a cross brace 726 with a joint 728. Each arm assembly also includes an end effector 730, such as an actuated gripper mechanism. Each arm assembly may be independently controlled, with the base of each triangle fixed in length, mounted around the perimeter of the forward surface of the servicer spacecraft, in certain embodiments.

[0064] Apex 724 may also be pivotable so as to act as a "wrist" joint to allow end effector 730 to rotate thereby. In embodiments, the wrist joint may include actuation mechanisms therein to enable movement along three axes.

[0065] The plane angle of each triangle of the arm assembly, relative to a forward deck of the servicer spacecraft, may controlled, for example, by a rotary actuator acting about the base axis of the triangle arm assembly (i.e., the base axis being coaxial with the cross brace). The two forward-oriented legs of each triangle, i.e., the linear actuator struts, may optionally be variable in length, implemented in various ways such as, for example, two fixed-length struts connected by an actuated rotary elbow joint, a linear actuator, and/or a linear actuator operating a pantograph linkage, which multiplies the reach of the leg, in certain embodiments. Such optional length adjusting and/or linkage mechanism is represented by gray boxes 740 in triangle arm assembly 220A and may be optionally implemented on the other two triangle arm assemblies. Additional trusses and/or joints may be integrated into the triangle arm assemblies, and such modifications are considered to be a part of the present disclosure. The actuators on each triangle can place the gripper at any point within a large working volume. The various actuators in capture system 700 may be controlled by a controller 750, in embodiments. [0066] In certain embodiments, the three wrist axes are not necessarily actuated and, instead, may be spring-centered to nominal positions, such as to constrain wrist attitude prior to the grappling operation. For example, the wrist joint at apex 724 may be spring loaded such that the wrist joint preferentially defaults to a nominal position and, when moved out of the nominal position, the wrist joint tends to return to the nominal position to ensure the end effector is in a suitable aspect for grappling.

[0067] The end-effector may be configured such that it may grip a variety of shapes of features. In embodiments, no a priori knowledge of the actual shape of the feature to be grappled is required for successful grappling operation, as long as the feature to be grappled is one of a variety of common fixtures used on spacecraft, such as launch rings. The end-effector may also include a single grip actuator to capture and rigidize a grasp on a portion of the client satellite's launch interface ring, such as a Marman or ESPA ring.

[0068] With capture system 700, the target object (or an aspect thereof, such as the launch ring attached thereto) essentially becomes the upper platform structure of the Stewart Platform after capture. In embodiments, a first arm assembly may be used for the initial capture of the target object, then the first arm assembly maneuvers the target object to within feasible capture attitudes for the second and third arm assemblies. Once the capture process has been completed, the target object may be manipulated to a predetermined attitude and distance with respect to the servicer spacecraft, onto which the SSS arrangement has been affixed, with full six degrees-of-freedom (6DOF) stiff control using the Split Stewart Platform arrangement.

[0069] Some examples of implementations of the Split Stewart Platform that may be considered include, and are not limited to:

[0070] 1. Telescoping ball screw linear actuators;

[0071] 2. Extendable truss linear actuators (e.g. scissor lift geometry);

[0072] 3. Rotary joint elbow/2 link arms; elbows coplanar with base pivots and wrist; and

[0073] 4. Rotary joint elbow/2 link arms; elbows (more or less) radially outward from Servicer.

[0074] Compared to a single or double robotic arm implementation, such as shown in FIG. 1, the Split Stewart Platform approach provides a number of distinctions. For example, although the Split Stewart Platform requires three end effectors, compared to a single one in the robotic arm implementation, each arm assembly of the Split Stewart Platform is not required to be as stiff as a single robotic arm (i.e., reduced moment stiffness). That is, the grappling of the capture system at three distinct points on the client space object enables a much more stable grappling of the client space object, even while using lighter, less rigid components than a robotic arm.

[0075] The completed parallel truss structure provided by the Split Stewart Platform after completed capture may be much stiffer than a serial manipulator or robotic arm, thus enabling reduced weight and more flexibility in the selection of materials used in the Split Stewart Platform implementation, even with the increased number of actuators and end effectors. Further, whereas inverse kinematics calculations required in manipulating the captured object using serial manipulators (i.e., robotic arms) can be complicated, the inverse kinematics calculation for a parallel manipulator such as the Split Stewart Platform is well known and less complicated.

[0076] FIGS. 8 and 9 show examples of different standard payload separation interfaces overlayed with a one-meter square standard servicer bus, shown here to illustrate the reach requirements of a capture mechanism for space applications as well as exemplary translation and attitude envelope requirements for servicer operations. As shown in FIG. 8, the payload separation interfaces (e.g., launch rings) are provided in several standardized diameters, namely 15 inch, 17 inch, 24 inch, 37.215 inch, 47.835 inch, 65.6 inch, 104 inch, and 110 inch, as represented by the rings shown in FIG. 8. A servicer bus, as commonly used in space applications, usually has the dimensions of a one-meter square.

[0077] It would be desirable for a Split Stewart Platform arrangement to be able to accommodate all of these standard ring diameters as mounted on the standard servicer bus dimensions. For example, if the largest ring (i.e., <|> 110 inch) is the driving case, then the smaller rings may provide greater translation and tilt ranges, as illustrated in FIG. 9. A maximum offset in the z-direction may be, for example, 0.5 meter at center, with a minimum z-offset of 0.1 meter at center. A translation range of 0.5 meter in the x-y plane may also be desirable. Further, a maximum 25-degrees tilt/rol I range about any single axis at maximum z-offset may also be desirable.

[0078] FIG. 10 shows a graphic representation comparing the capture mechanism mounting requirements of a serial manipulator compared with a Stewart platform-based capture mechanism in relation to different interface diameters, in accordance with embodiments. In the case of serial manipulator arms, the base of the serial manipulator arms are usually attached to a side of a servicer bus, such as near the front face (e.g., 0.1 meter setback from a front face). A second arm, if present, may be attached on an opposing face in a mirrored fashion. In this case, the maximum extension required for a single arm, to be able to reach the full range of a 110- inch standard ring is 1.8 meters, and the minimum extension required for a single arm is 0.2 meter, driven by the requirements of smaller separation rings at minimum offset and worst case roll.

[0079] In contrast, the Split Stewart Platform arm assemblies may be mounted at edges of a front face of the servicer. The first actuation axis may be along the long sides of the forward servicer face shown in FIG. 7, with pivots at the platform ends of the arm assemblies. In this case, the distance from the servicer centerline is approximately 0.5 meter, with an approximately 0.85 meter distance between the pivots. Then, the maximum extension required for the arm assemblies may be 1.5 meters, which is less than the 1.8 meters required with a serial manipulator, while the minimum extension may be 0.15 meter.

[0080] Further, due to the wide variety of capture features that are commonly used in the aerospace industry, the grapple end-effector must be designed to flexibly accommodate a range of shapes and sizes of capture features, such as illustrated in FIGS. 2 - 6. As discussed above, payload separation rings are good candidates for use as grapple points or capture features, and are somewhat standardized, such as the Atlas and ESPA configurations illustrated.

[0081] These features are also structurally strong and generally not covered with multilayer insulation (MU), which can impede gripping operations. Further, as these features have already performed their intended function of supporting the payloads during launch, any gripping operation would not affect their future usability. Additionally, such payload separation rings are generally disposed on an aft face of the target object, with no delicate appendages located nearby. Many of the payload separation rings include features, such as a lip, an indentation, or other features that make them particularly suited for a "capture then grapple" type strategy.

Still, the grappling mechanism would need to avoid features, such as electrical connectors, push- off spring seats, kick nozzles, and others, such that accurate placement of the grappling mechanisms would be highly desirable.

[0082] FIGS. 11 - 13 illustrate exemplary steps of a notional mechanism for grappling an exemplary interface ring, in accordance with an embodiment. In the illustrated embodiment, as shown in FIG. 11, the grappling mechanism may first be maneuvered to hold a fixed finger just inside the ring at a clear spot between ring features, with a fixed flat surface orthogonal to the finger placed just below the ring. Proximity sensors may be used to confirm that the grappling mechanism is ready to initial grappling operations. In the configuration as shown in FIG. 11, the grappling mechanism is not yet fully engaged, although confirmed to be in position for next steps in the grappling process. Then, as further indicated in FIG. 11 by a curved arrow, the capture mechanism may be transitioned from a position control operation to a low friction/stiffness mode so as to be able to accommodate capture forces and moments, without resisting such capture forces.

[0083] As shown in FIG. 12, the grapple mechanism may then be actuated by rotating two opposing fingers inward from the outside of the ring, thus clamping the ring wall against the first finger and capturing the ring lip. When clamping pressure exceeds a set threshold (e.g., as sensed by a pressure sensor or mechanically via a differential), a downward motion of the two inward-moving figures may be initiated while maintaining inward pressure, as shown in FIG. 13, to clamp the ring lip against the fixed flat surface of the grapple mechanism/end-effector. When clamp pressure exceeds a set threshold, the grappling fingers may be locked using, for example, a power-off braking mechanism or ratchet.

[0084] Further, the grapple mechanism may be further configured to dampen any relative motion between the grapple mechanism and the capture feature, provide additional mechanisms to actively control the positioning of the capture feature and the target object attached thereto, and to rigidize the capture mechanism, e.g., using power-off brakes, when a desired position or pose of the target object is achieved.

[0085] For example, in the first phase of capture, the three actuators of one of the arm assemblies of the Split Stewart Platform may be operated in closed-loop position control mode to achieve end-effector placement adjacent to (and, optionally, not in contact with) the target feature on the client satellite. Proximity sensors on the end-effector may confirm a ready-to- capture state, and the grasp geometry may ensure positive confinement of the capture feature prior to actual contact, thus avoiding the possibility of contact-induced departure of the target object (e.g., knocking a client satellite away from the capture mechanism by unintentional contact). In an embodiment, before actuating the grasp operation, the actuators of the arm assembly may be placed in a low-friction (i.e., undriven) mode, thus minimizing any reaction forces on the target object due to grasp contact. After the grasp is complete, the actuators of the arm assembly may be placed in a damping mode, to dissipate any end-effector motion relative to the servicer. Once this motion is damped out, the actuators may again be operated in closed-loop position control mode. [0086] In embodiments, to complete capture, the remaining two arm assemblies, in sequence or in parallel, may grasp and rigidize connections to the target object while the first triangle stabilizes the target object pose. When all three grapples are complete, the capture feature (e.g., launch ring) of the target object may provide a rigid connection between the apexes of the three triangular trusses of the arm assemblies, thus forming the upper platform of a complete Stewart Platform joining the target object with the servicer. The base actuators of the three arm assemblies of the Split Stewart Platform may then be disengaged, and the 6DOF relative pose of the target object with respect to the servicer may be fully controlled over a large workspace by the six actuators in the three arm assemblies.

[0087] Disengagement of the target object from the servicer may be performed as a reverse of the capture process, with two arm assemblies disengaging their grapples in sequence or in parallel, and moving away from the target object. The final arm assembly may then be extended to a maximum separation distance, for example, the actuators are placed in a low- friction mode, the final grapple is gradually released, and the third arm assembly may then be moved away from the target object. In the event of any failure (such as an inoperable grapple or arm assembly actuator) preventing the nominal release sequence, an independent backup method may be provided to separate any grapple mechanism from the arm assembly. In this way, while the mechanical grappling element may be left clamped to the target object, all connections between the target object and the servicer may be cleanly separated.

[0088] FIGS. 14 and 15 show a front view and a side view, respectively, of an exemplary Split Stewart Platform arm assembly, in accordance with an embodiment. As shown in FIGS. 14 and 15, an arm assembly with radially-extending elbows enable the flexibility and adaptability not currently available with existing capture mechanisms.

[0089] The exemplary arm assembly illustrated in FIGS. 14 and 15 include two link struts with rotary joint elbows, with the elbows extended outward and inward from the servicer, onto which the arm assembly is attached. In this way, the possibility of elbow collisions may be avoided. The elbow actuators may be located near the base of the struts, such as including a cable drive as an example, to reduce arm endpoint inertia. Roll bearings in the second links may be used to implement a planar joint at the wrist portion of the arm assembly. In this way, the inverse kinematics calculations in controlling the grappled object when the platform is complete may be straightforward. Even prior to grappling by all three arm assemblies, the kinematics are still tractable since there are only three degrees of freedom involved in the calculations for each arm assembly.

[0090] The two elbow actuators and the shoulder axis actuators define the wrist position, with optionally an additional 3D0F provided for the wrist attitude. For instance, all 3D0F of the wrist portion may be free to pivot, once the "ready to grapple" indication has been sensed, such as described above in FIGS. 11 - 13. To maintain a predictable wrist attitude for grapple, restriction of the wrist attitude may be required prior to actual contact of the grappling mechanism with the target object. For example, a mild spring centering about a nominal attitude relative to the links may be provided to control for roll & yaw at the wrist. Optionally, an active control mechanism, such as one or more actuators per strut pair, may be provided at the wrist to actively control the wrist attitude and pose.

[0091] A variety of alternative configurations of the Split Stewart Platform may be contemplated including, and not limited to: ball-screw linear actuators; scissor-lift type extendable truss; two-link struts with rotary-joint elbows, such as with elbows in plane of shoulders and wrist or plane rotated by actuator on axis connecting shoulder pivots of each pair of struts, with elbow collision avoidance restrictions.

[0092] FIG. 16 shows a graphic representation of a process for grasping an exemplary Atlas V- type/Delta IV type Marman ring using a gripper, in accordance with an embodiment. FIG. 16 essentially illustrates an alternative implementation of the process described above in association with FIGS. 11 - 13. FIG. 17 shows a graphic representation of a process for operating an ISP system, in accordance with embodiments, from a macro view.

[0093] As described above, embodiments of the Split Stewart Platform system may involve the manipulation of the following elements:

[0094] 1. A launch interface (e.g., Marman ring) on the target object, nominally in a 24-inch to

104-inch diameter range;

[0095] 2. A rigid base platform (i.e., the Split Stewart Platform) supporting six mounting points at a forward end of servicer spacecraft or satellite;

[0096] 3. Three arm assemblies (e.g., triangular sets of struts) with optionally disengageable rotation control of each base axis and control via either linear actuators or rotational elbow joints of the forward-oriented legs of the triangles.

[0097] In embodiments, free (i.e., not actuated) yaw may provide pivoting motion between the base axis and forward legs of each arm assembly/triangle. A free (not actuated) roll joint between the two arm assemblies/triangles may be provided at their apex, to ensure a planar connection, in certain embodiments. Optionally, a 3DOF wrist may be provided at each arm assembly/triangle apex, with all axes intersecting at a point. As a further option, a singleactuator end-effector may be provided at each wrist, configured for grasping a wide range of launch ring geometries, providing capture-before-grasp and 6DOF grasp rigidizing functions. Additional components may include, for example and not limited to, proximity sensors on the end-effector for confirming "ready-to-grasp" state before contact, power-off (i.e., passive) brakes on the six-arm assembly/triangle leg actuators and three grasp actuators, and one or more single-use grapple release devices (e.g., paraffin or nitinol) on each gripper for fail-safe disengagement.

[0098] FIG. 18 illustrates an example of an alternative capture system 1800 including a modified Stewart platform suitable for use in servicer capture applications, in accordance with embodiments. In contrast to the capture system shown in FIG. 7, one of the triangular arm assemblies has been replaced with a robotic arm 1820. While the inclusion of the robotic arm may add complexity to the capture system, various advantages of the Split Stewart Platform are still present, such as the three-point contact with the client space object and the use of lighter weight and less stiff components. In certain embodiments, robotic arm 1820 is operatable with least three actively-controlled degrees of freedom.

[0099] It is noted that, if a typical robotic arm is used in place of one of the triangular arm assemblies, the control system may be required to be quite sophisticated in order to balance the forces applied to the grappled, client space object. For example, it would be desirable to avoid imposing forces and moments on, for instance, the captured Marman ring while closing a torque loop on the wrist portion of the end effectors to ensure the applied moments at the Marman ring adds to zero. Additionally, the control system would need to ensure the two triangular arm assemblies and the robotic arm avoid applying radial force around the Marman ring. Essentially, the robotic arm would be required to simulate the third triangular arm structure of a normal SSP configuration.

[0100] As an example, if a typical 6DOF manipulator arm is used in place of one of the triangular arm assemblies, as shown in FIG. 18, the manipulator arm would require a sophisticated control system to avoid imposing unwanted forces and moments on the Marman ring which would fight the two triangular arm structures, otherwise the operations of the resulting SSP would not be statically determinate. [0101] The embodiments described above provide a wide range of heretofore unavailable benefits in spaceborne grappling and capture processes. For example, a wide range of launch ring diameters can be accommodated, since the ring itself forms the upper platform, in contrast with traditional Stewart platforms with fixed upper platform assemblies. This servicer capture system may be designed to be lighter and more efficient compared to serial manipulator-based systems. The existing launch ring structure may be repurposed as part of the Split Stewart Platform, rather than requiring additional or customized capture features. Further, the launch ring has already fulfilled its role in launch and separation; thus further utility as a grapple fixture may be a bonus functionality of the presently described system. The completed Stewart Platform provides full 6DOF control of the relative pose between the target object and the servicer over a large workspace. Additionally, this parallel structure establishes a high-stiffness connection between the target object and the servicer, as moments are reacted over a large baseline (i.e., the diameter of the launch ring) rather than a single localized grapple point (as in a single-manipulator grapple). The completed Stewart Platform may be operated as a statically determinate structure, unlike captures using multiple manipulator arms which may require active control of all 6DOF. That is, the actuators in the Split Stewart Platform do not work against each other and binding or stress from strut thermal differences may be avoided. Moreover, since any of the three arm assemblies/triangular strut sets can perform the initial capture, the design is tolerant to a variety of component failures. The second and third triangle connections are less demanding, as the first connection controls the client satellite pose for subsequent grasps, and the first connection may be established by any of the three arm assemblies of the Split Stewart Platform configuration.

[0102] In other words, the above-described embodiments include a variety of advantages, such as:

[0103] • The SSP is attachable to standard launch support rings on client satellites, thus enabling grasping operations without requiring specialized grapple fixtures to be pre-installed on the target spacecraft.

[0104] • The SSP uses multiple, separated points of contact on the client satellite, such that the broad base of support enables highly precise control of the client spacecraft pose.

[0105] • The wrists/grippers of the SSP triangles are freely pivoting when grappled.

[0106] o No moments are applied to the client spacecraft at a grasp point; only forces. [0107] o Thus the local stresses on the satellite are minimized to reduce the possibility of damage or deformation on the client spacecraft

[0108] o Stresses on the SSP triangle legs are also reduced, thus enabling a lightweight structure for the SSP.

[0109] o In embodiments, the local forces needed to apply a reorientation torque to a client satellite with an SSP grasping a 1.5 m launch ring are a factor of 30 lower than those needed with a single 5-cm grasp point (typical for a serial manipulator grapple).

[0110] • The SSP is a parallel mechanism rather than being a serial manipulator, such as the SSRMS on the International Space Station, the arm on the Northrop Grumman servicer, and many others existing grappling systems.

[0111] o All of the six SSP actuators used to manipulate the client satellite may directly connect the client to the servicer, thus enabling a stiffer, more accurate connection for a given actuator compliance and accuracy compared to a serial chain in which the errors and compliance of each actuator are cumulative.

[0112] o The completed SSP structure is statically determinate so thermal gradients, tolerance buildup, or controller disturbances generally cannot cause binding or conflicting internal stresses in the manipulator system or client structure.

[0113] As used herein, the recitation of "at least one of A, B and C" is intended to mean "either A, B, C or any combination of A, B and C." The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present disclosure. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the disclosure. Thus, the present disclosure is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

[0114] The terms and expressions employed herein are used as terms and expressions of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof. Each of the various elements disclosed herein may be achieved in a variety of manners. This disclosure should be understood to encompass each such variation, be it a variation of an embodiment of any apparatus embodiment, a method or process embodiment, or even merely a variation of any element of these. Particularly, it should be understood that the words for each element may be expressed by equivalent apparatus terms or method terms— even if only the function or result is the same. Such equivalent, broader, or even more generic terms should be considered to be encompassed in the description of each element or action. Such terms can be substituted where desired to make explicit the implicitly broad coverage to which this invention is entitled.

[0115] As but one example, it should be understood that all action may be expressed as a means for taking that action or as an element which causes that action. Similarly, each physical element disclosed should be understood to encompass a disclosure of the action which that physical element facilitates. Regarding this last aspect, by way of example only, the disclosure of a "protrusion" should be understood to encompass disclosure of the act of "protruding"— whether explicitly discussed or not— and, conversely, were there only disclosure of the act of "protruding", such a disclosure should be understood to encompass disclosure of a "protrusion". Such changes and alternative terms are to be understood to be explicitly included in the description.

Claims

1. A capture system attachable to a servicer spacecraft for capturing and controlling a space object by the servicer spacecraft, the capture system comprising: three triangular arm assemblies, each one of the three triangular arm assemblies including a cross brace, two linear actuator struts, each one of the two linear actuator struts including a first end and a second end, the first ends of the two linear actuator struts being attached together to form an apex, and each one of the second ends of the two linear actuator struts being connected with the cross brace via a joint, and an end effector connected with the two linear actuator struts at the apex, wherein each one of the three triangular arm assemblies is independently controllable for maneuvering the end effector on each one of the three triangular arm assemblies toward the space object to enable the end effector to grapple onto a feature of the space object without a priori knowledge of a specific shape of the feature of the space object.

2. The capture system of claim 1, wherein the end effector includes a wrist joint.

3. The capture system of claim 2, wherein the wrist joint is spring-loaded to a nominal position.

4. The capture system of claim 1, wherein the end effector includes a grip actuator configured for capturing the feature and rigidizing a grasp on the feature, once the feature has been captured by the grip actuator.

5. The capture system of claim 1, wherein at least one of the two actuator struts includes at least one of a telescoping ball screw linear actuator, an extendable truss, a rotary joint.

6. The capture system of claim 5, wherein each one of the three triangular arm assemblies is configured to maneuver the end effector toward the feature with six degrees of freedom.

7. The capture system of claim 1, wherein the end effector associated with each one of the three triangular arm assemblies is configured to capture a different portion of the feature of the space object from the end effector of associated with the other ones of the three triangular arm assemblies.

8. The capture system of claim 1, wherein the three triangular arm assemblies are further configured to maneuver the space object to a predetermined attitude and distance with respect to the servicer spacecraft.

9. A method for capturing and controlling a space object using a servicer spacecraft, the method comprising: providing a capture system including three triangular arm assemblies, each one of the three triangular arm assemblies including a cross brace, two linear actuator struts, each one of the two linear actuator struts including a first end and a second end, the first ends of the two linear actuator struts being attached together to form an apex, and each one of the second ends of the two linear actuator struts being connected with the cross brace via a joint, and an end effector connected with the two linear actuator struts at the apex, attaching the capture system to the servicer spacecraft, independently maneuvering the end effector on each one of the three triangular arm assemblies toward the space object, and grappling a feature of the space object with the end effector on each one of the three triangular arm assemblies, wherein grappling is performed without a priori knowledge of a specific shape of the feature of the space object.

10. The method of claim 9, further comprising maneuvering the space object to a predetermined attitude and distance with respect to the servicer spacecraft.

11. A capture system attachable to a servicer spacecraft for capturing and controlling a space object by the servicer spacecraft, the capture system comprising: at least two triangular arm assemblies, each one of the two triangular arm assemblies including a cross brace, two linear actuator struts, each one of the two linear actuator struts including a first end and a second end, the first ends of the two linear actuator struts being attached together to form an apex, and each one of the second ends of the two linear actuator struts being connected with the cross brace via a joint, and an end effector connected with the two linear actuator struts at the apex, wherein each one of the two triangular arm assemblies is independently controllable for maneuvering the end effector on each one of the two triangular arm assemblies toward the space object to enable the end effector to grapple onto a feature of the space object without a priori knowledge of a specific shape of the feature of the space object, and wherein the capture system further comprises a third arm assembly having at least three actively-controlled degrees of freedom.

12. The capture system of claim 11, wherein the third arm assembly includes at least one of a third triangular arm assembly and a robotic arm assembly, and wherein each one of the two triangular arm assemblies and the third arm assembly is configured to maneuver the end effector toward the feature with six degrees of freedom.

13. The capture system of claim 11, wherein the end effector includes a wrist joint.

14. The capture system of claim 13, wherein the wrist joint is spring-loaded to a nominal position.

15. The capture system of claim 11, wherein the end effector includes a grip actuator configured for capturing the feature and rigidizing a grasp on the feature, once the feature has been captured by the grip actuator.