Swati Mohan

Assembly of a Large Modular Optical Telescope (ALMOST)

Future space telescope programs need to assess in-space robotic assembly of large apertures at GEO and ESL2 to support ever increasing aperture sizes. Since such large apertures will not fit within a fairing, they must rely on robotic assembly/deployment. Proper assessment requires hardware-in-the-loop testing in a representative environment. Developing, testing, and flight qualifying the myriad of technologies needed to perform such a test is complex and expensive using conventional means. Therefore, the objective of the ALMOST program is to develop a methodology for hardware-in-the-loop assessment of in-space robotic assembly of a telescope under micro-gravity conditions in a more cost-effective and risk-tolerant manner. The approach uses SPHERES, currently operating inside ISS, to demonstrate inspace robotic assembly of a telescope that will phase its primary mirror to optical tolerances to compensate for assembly misalignment. Such a demonstration, exploiting the low cost and risk of SPHERES, will dramatically improve the maturity of the guidance, navigation and control algorithms, as well as the mechanisms and concept of operations, needed to properly assess such a capability.

SPHERES Reconfigurable Framework and Control System Design for Autonomous Assembly

Reconfigurable control system design is a key component for enabling autonomous on -orbit asse mbly. Current resea rch on reconfigurable control system s focuses on adapting to failures. However, for assembly scenarios, the reconfiguration is necessitated by changing mass and stiffness properties. This paper provides a brief description of existing reconfigurable control system technology and develops a framework to incorporate reconfiguration into an existing baseline system to account for mass property variations . The reconfigurable control system framework has been developed and implemented using the SPHERES (Synchronized Position Hold Engage Reorient Experimental Satellites) testbed as the baseline system. The framework highlights the elements that need to be updated, introduces a variable p that captures the configuration, and details the updat es necessary in the key algorithms to calculate the model on line using p. Results are presented from the implementation on the SPHERES , focusing on the reconfigurable estimator . Plans are presented for an integrated assembly test that demonstrates the ma intenance of stability, fuel efficiency, and accuracy throughout configuration changes that occur during assembly.

SPHERES Reconfigurable Control Allocation for Autonomous Assembly

Current research on control allocation emphasizes reconfiguration for adapting to thruster failures. However, in the application of autonomous assembly, the reconfiguration is necessitated by changing physical properties. For the scenario of an assembler tug constructing a large space structure, every docking and undocking maneuver used for the tug to move an individual payload causes a large shift in the dynamics of the tug. Not only do the mass properties change, but so does the thruster configuration. Changes in the center of mass, mass, and inertia of the tug-payload system, causes changes in the equivalent force exerted by each thruster. This paper explores reconfigurable control allocation to adapt to changes in the mass properties. Specifically considered are changes to the center of mass and thruster configuration (number, location, and active thrusters). Results are presented from the implementation of a reconfigurable control allocation algorithm on the SPHERES (Synchronized Position Hold Engage Reorient Experimental Satellites) testbed aboard the International Space Station. Results demonstrate controllability for configurations with large center of mass shifts, varying number of thrusters, as well as maintaining performance from the baseline non-reconfigurable control allocation algorithm on SPHERES. Introduction: Reconfigurable control systems are used in a variety of applications, such as docking, servicing, high efficiency flight, and assembly. Autonomous assembly is a critical technology for future missions such as large space telescopes, on-orbit space stations, and a lunar base. One particular scenario for on-orbit autonomous assembly is to use an assembler tug to maneuver the different payload items. In this scenario, the dynamics of the assembler tug will vary greatly at each docking and undocking maneuver, based on the properties of the payload. Depending on the relative sizes of the tug to the payload, this could be a very significant change. In order to maintain adequate control performance, it is important to account for this property change. The implementation of a reconfigurable control system would be able to account for the frequent mass property changes, while also introducing flexibility into the system. The sequence of assembly would not need to be pre-determined, nor would all configurations have to be precomputed. One aspect of a reconfigurable control system is the control allocation. Previous work on control allocation as primarily focused on being reconfigurable for thruster fault detection and recovery. Work by Dhayagude and Gao [1], Davidson et. al. [2], and Hodel and Callahanz [3] have showed different methods of reconfiguring thruster allocation in the presence of faults. Work by Choi et. al. describes an optimal control allocation for a spacecraft to maximize noise rejection and minimize effect of disturbances [4]. However, for most of these algorithms, the allocation method is fixed in terms of spacecraft properties. This paper’s contribution is to develop a reconfigurable control allocation algorithm that modifies the actuator configuration based on the changing mass properties. The development of the reconfigurable control allocation algorithm was to accommodate the following scenarios. • Docking to an active payload: In some autonomous assembly scenarios, the tug is assembling payload that have maneuvering capability. In these scenarios, it would be beneficial to make use of the actuation capability of the payload. This would save fuel on the tug. It could also possibly allow for greater mobility for the tug-payload system, since the actuators would be placed around the center of mass of the system. • Large center of mass offset: In the scenario when the assembler tug is docking to a passive payload, the center of mass of the system will shift. In cases where the payload is larger than the tug, the center of mass will shift to be outside of the thruster envelope. • Thruster selection: Over the course of an assembly sequence, the specific thrusters that are valid to be used may vary. Three particular cases are considered. First, in a particular docking case some thrusters may need to be disabled to prevent plume impingement. Second, when docking to an active payload and combining actuators, one might to select the thrusters that provide the maximum torque in that configuration. Third, thrusters may fail over the course of a mission, requiring the control allocation to compensate. This paper upgrades the baseline control allocation algorithm on SPHERES (Synchronized Position Hold Engage Reorient Experimental Satellites) testbed to a reconfigurable control allocation algorithm. The following sections gives a SPHERES overview (including description of baseline control allocation scheme), describe the methods for the reconfigurable control allocation algorithms, and show experimental results for the final version of the control allocation algorithm. SPHERES Overview: SPHERES is a formation flight testbed aboard the International Space Station. It consists of three free-flying satellites, about 20cm in diameter. The satellites operate inside a 1.5m volume aboard the ISS, using a pseudo-GPS navigation system. Each SPHERES satellite has twelve thrusters, fueled by a single CO2 tank, for a full six degrees of freedom. [5] (a) SPHERES satellite (b) Satellites aboard the ISS Figure 1: SPHERES hardware pictures The baseline control allocation algorithm on SPHERES uses the symmetry of the thruster placement to specify thruster pairs. These forces/torque pairs are pre-determined using the knowledge of the thruster placement, shown in Table 1. Each thruster in a pair produces the exact opposite force and torque as its’ pair thruster. Thus, the matrix can be simplified to use half the data and extrapolate all twelve thruster commands based on pair forces. Table 1: SPHERES Mixing Matrix and Thruster Pairs (a) Force/Torque matrix for thrusters (b) Thruster Pairs Pair # Thr # Thr # 1 1 7

Dynamic Control Model Calculation: A Model Generation Architecture for Autonomous On-Orbit Assembly

Autonomous on-orbit assembly is a key technology that can enable many space applications in a more cost-effective and lower risk manner than human-assisted assembly. During an assembly sequence, an assembler robot can undergo multiple configurations as it attaches to and releases individual modules. This paper addresses how to account for the mass and stiffness property variations that occur with changes in configuration. Proper model generation for each configuration is critical to maintain control system stability and efficiency. A design, called dynamic control model calculation, is presented in this paper to address a gap where models are aggregated online based on module mass property information. This design uses module information obtained at the time of attachment to generate the model for the current configuration onboard. The design, both framework and algorithm parameterization, has been successfully implemented and validated on hardware. Hardware results show a tracking error performance impro...

On-Orbit Assembly of Flexible Space Structures with SWARM

On-orbit assembly is an enabling technology for many space applications. However, current methods of human assisted assembly are high in cost and risk to the crew, motivating a desire to automate the on-orbit assembly process using robotic technology. Construction of large space structures will likely involve the manipulation of flexible elements such as trusses or solar panels, and automation for assembly of flexible structures has significant challenges, particularly in control systems. This paper presents results of ground-based experiments on the assembly of a flexible space structures using the hardware developed under the Self-Assembling Wireless Autonomous Reconfigurable Modules (SWARM) program. Results are shown for a series of incremental tests that demonstrate control of a flexible structure, docking, and reconfiguration after docking. These results demonstrate the feasibility of the assembly of flexible structures using this methodology.

Docking and reconfiguration of modular spacecraft: preliminary SWARM testing at MSFC

On-orbit servicing and assembly is a critical enabling technology for the advancement of large scale structures in space. The goal of the SWARM project (Synchronized Wireless Autonomous Reconfigurable Modules) is to develop and mature algorithms for autonomous docking and reconfiguration, to be used as the building blocks for autonomous servicing and assembly. Algorithms for approach, docking, and reconfiguration have been implemented and tested through a demonstration of the assembly of two telescope sub-apertures at Marshall Space Flight Center (MSFC) in July 2006. The algorithms developed for reconfiguration set the mass properties based on the configuration. Updatable parameters include the location of sensors and receivers with respect to the geometric center, thruster locations, and control gains specific to each configuration. To test these algorithms in a 2D environment, a ground testbed was developed to provide multiple docking ports and modular payload attachments. Hardware components include nodes, Universal Docking Ports, posts, sub-aperture mirrors, and a SPHERES satellite as the assembler tug. Testing at MSFC successfully demonstrated relative docking and reconfiguration. Valuable information was gained about the performance of the docking under friction, sensitivity to estimator initialization, thrust authority needed for different phases of the test, and control when CM changes during the test.

Inner solar system sample return missions using solar electric propulsion

This study evaluated the effects of solar electric propulsion (SEP) and PowerSail solar array technology on four sample return missions. These missions, to the Moon, Mars, Mercury, and Venus, were compared against previous Jet Propulsion Laboratory (JPL) studies. Compared to these baselines, PowerSail/SEP missions in general had longer trip times, though the Venus mission was of similar duration. Costs for three missions were comparable to JPLs estimates, but in the case of Mars, the PowerSail/SEP design was significantly cheaper. Moreover, for all missions, SEP allows a negative C3 launch, dramatically improving launch vehicle performance and enabling cheaper, lower risk designs. Overall, results indicate that the PowerSail/SEP combination outperforms conventional technology on large-scale missions. As a result of this study, a cohesive four-mission inner solar system sample return program is proposed.