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