spacecraft formation flight
The Orbital Robotic Interaction, On-orbit servicing, and Navigation (ORION) laboratory at Florida Institute of Technology enables experimental research in controlling and operating teleoperated and autonomous ground, air, and space robots. The ORION Lab was designed, developed, and integrated by Dr. Markus Wilde in the period 2014 – 2018. The lab is built around a maneuver simulator and a motion tracking system. It also contains a control room, a supporting electronics workshop, and office space for graduate students and visiting researchers.
The ORION maneuver simulator is located in a dedicated testbed space with a usable volume of 9.6 m × 9.4 m × 3 m. ORION integrates an acrylic flat-floor into a planar, cartesian Maneuver Kinematics Simulator with a workspace of 5.5 m × 3.5 m. The primary component of the Maneuver Kinematics Simulator is a horizontal two degrees-of-freedom (DOF) motion table capable of positioning a payload of 80 kg at a maximum speed of 0.25 m/s and a maximum acceleration of 1 m/s2 along both linear axes.
The motion table is designed to carry a wide range of equipment, such as small industrial manipulators or pan-tilt mechanisms. The ORION pan-tilt mechanisms are custom designed to carry a test article with mass 20 kg and dimensions 0.5 m × 0.5 m × 0.5 m. The motion envelope is ±90° in elevation and infinite rotation in azimuth, with maximum rotation rate 60°/s and maximum acceleration 60°/s2 about each axis. The test article is supplied with power and Ethernet connections via a slip-ring around the azimuth axis.
The ORION Maneuver Kinematics Simulator currently employs two pan-tilt mechanisms. The stationary pan-tilt head is typically used to generate the attitude motion of a target spacecraft model. The target model has geometrical and surface features typically found on a satellite, such as e.g. a parabolic antenna, spherical propellant tanks, thruster nozzles, solar arrays, etc. The moving pan-tilt head is designed to carry a spacecraft robotics test vehicle equipped with a number of robotic manipulators, a capture tool, multiple cameras and distance sensors. In past experiments, the pan-tilt head has also been used to position the base of continuum robots to study capture dynamics. Teleoperation and human-in-the-loop experiments commonly use the two vehicle models mounted on the Maneuver Kinematics Simulator, for maximum positioning accuracy and repeatability.
The motion of the resulting 6 DOF system is controlled by a combination of custom simulation software for orbital relative motion of spacecraft and a real-time soft-motion controller running on a National Instrument CompactRIO system. The motion controller then drives stepper motors in microstepping mode. This enables high precision and constant knowledge of the position and orientation of the spacecraft models. Both open-loop and closed-loop control modes are available, with the position and orientation of the Maneuver Kinematics Simulator components being tracked by the external motion tracking system.
The base frame of the Maneuver Kinematics Simulator forms the bounds of the Integrated Flat-Floor Motion Dynamics Testbed. The 5.94 m × 3.60 m flat-floor is covered with acrylic plates and provides a flat and level surface on which air-bearing vehicles can glide on thin layers of pressurized air to eliminate friction. This enables the simulation of orbital maneuver dynamics and contact dynamics. Every point on the flat-floor is accessible by sensors, capture mechanisms, or robotic manipulators mounted on the Maneuver Kinematics Simulator. This enables the combined use of the integrated kinematics and dynamics simulators in experiments. For testing of capture mechanisms and validation of contact dynamics prediction models, the mechanisms and their control electronics are typically mounted on the Maneuver Kinematics Simulator, the target objects will be floating on the integrated flat-floor.
The High-Precision Air-Bearing Table is located next to the Integrated Maneuver Kinematics and Dynamics Simulator. It is based on a 4.57 m × 1.83 m vibration isolated optical bench covered with a 3.66 m × 1.80 m tempered glass plate. The air-bearing table has superior flatness and levelness compared to the integrated flat-floor. The table is used with air-bearing vehicles to test control systems for satellite formation flight and attitude control and to validate multi-body dynamics models. Multiple air-bearing vehicles are available for experiments, e.g. the pair shown in Figure 4. One of the pair is equipped as “chaser” vehicle with relative navigation sensors and a grasping mechanism. The other is the “target” vehicle equipped with a grasping feature representing a segment of a rocket payload adapter ring. All air-bearing vehicles are self-contained units with their own gas tanks, batteries, on-board computers, control electronics, and communicate with the other elements of the ORION Lab via WiFi.
For experimentation involving air and ground robots, the lab currently uses a commercial-off-the-shelf DJI Matrice 100 UAV and the Pantherbot. The AR Drone enables testing and evaluation of teleoperation and autonomous control modes for uninhabited aerial vehicles. The Pantherbot is based on a MobileRobots PowerBot Autonomous Ground Vehicle equipped with an Amtec 6 DOF manipulator. Pantherbot thus enables ground navigation and robotic manipulation experiments.
All objects within the ORION Lab can be tracked by a twelve camera OptiTrack Prime 17W object tracking system. The cameras are installed along the walls of the testbed space at a height of 2.5 m. The OptiTrack system works by measuring the position of individual infrared reflectors within its workspace. In the OptiTrack Motive software, multiple markers can be combined to represent one rigid body. Thus the system is capable of tracking the position and orientation of a virtually unlimited number of rigid bodies with sub-millimeter and sub-degree resolution. The tracking data is recorded and used for the analysis of experiments involving the motion table, air-bearing vehicles, UAVs, and ground robots. The data is also streamed over a local WiFi network in order to close the control loop for floating and flying vehicles without having to equip them with position and orientation sensors. The common limitations of onboard sensors can then be reproduced by introducing noise or controlled errors into the OptiTrack data. For computationally intensive control algorithms overwhelming typical onboard computers of small UAVs or small robotic vehicles, the control loop can also be closed on a laboratory workstation and the resulting commands be streamed to the vehicles. This introduces transmission delays, but these are typically below 30 ms within the local ORION network.
The ORION simulator uses commercial-off-the-shelf components to generate a light source sufficiently bright to exceed the dynamic range of common optical sensors while providing a narrow beam angle. The walls, floor, and ceiling of the testbed are painted a low-reflectivity black and all windows are covered with black-out blinds to fully control the lighting conditions and to reproduce orbital conditions. The selected light source is a Litepanels Hilio D12 LED panel. The panel generates light with a color temperature of 5,600 K (daylight balanced) with 350 W of power. The intensity is equivalent to a 2,000 W incandescent lamp. The intensity can be continuously dimmed from 100% to 0%, and the beam angle can be varied between 10° and 60° using lens inserts. Therefore, the light can be used to simulate solar illumination and also the weaker and diffuse Earth albedo. The LED panel is mounted on a wheeled tripod for quick positioning anywhere within the lab space.
The control room contains the simulator control workstation, control computers, and an operator workstation for teleoperation experiments. This workstation features a multi-monitor setup for immersive display of video streams, virtual reality representations, and system status data. A number of input devices including joysticks, 3dConnexions Space Navigators, and Novint Falcons are available.
The ORION lab is fully networked. All control signals and telemetry within the lab are transmitted using a combination of point-to-point UDP streaming and RTI Data Distribution System channels. This enables experiments in the lab to be controlled from remote sites.
