TPF-I SWG Report - Exoplanet Exploration Program - NASA

T E C H N O L O G Y R OADMAP FOR TPF-I
Two commands were sent in this demonstration, both of which specify stellar targets to observe. The
stop-and-stare observations consist of holding a constant relative position with the thruster quiescence
requirement. The commands include the baseline to hold while observing, and the time allotted for
retargeting. In response to a retarget command, the formation guidance algorithm first plans collisionavoidance
constrained, energy-optimal relative spacecraft trajectories to achieve the desired baseline. This
process is non-trivial, since the unconstrained, energy-optimal trajectories for the second retarget lead to a
collision. Figure 6-6 shows several visualizations from this distributed, real-time demonstration of
formation flying.
Two-Robot FCT
The purpose of this demonstration is to validate the FACS used on the FCT robots before implementation
on the actual robotic hardware, including flight-like ISC and control-cycle synchronization. The
formation software used the Real-Time protocol, wireless links, and time-offset estimation via echo
packets, and control cycle synchronization. Additionally, the sensors models use measured sensor noise
values. For example, each fiber-optic gyroscope on the robots was calibrated, and measured values of
rate-random walk, angle-random walk, and angle white noise were used in the gyroscope model. The
control cycles were initially 0.5 s out of synchronization, and the clock models had a relative drift of 0.1
ms/s.
In addition to autonomous reconfiguration, synchronized rotations were demonstrated in which the
formation rotation rotates as a virtual rigid body. Synchronized rotations are used for observations “onthe-fly”
and can also be used to retarget a formation. In the latter case, the rotation axis is not along the
formation boresight.
Figure 6-8 shows visualization from the two-robot FAST demonstration. The top of the figure shows the
modes that the “Combiner” and “Collector” robots go through. In the figure, they are in Formation
Observation mode and performing the first synchronized rotation. The lower part of the robots with the
white tanks serves as a translation stage. The upper, cylindrical portions are referred to as the attitude
stages and emulate spacecraft. As can be seen in the lower left, the attitude stages are a rotating and
translating in a plane inclined to the experiment floor. This plane is normal to the star direction.
These two FAST simulations demonstrated formation software for autonomous formation flying with
realistic inter-spacecraft communication and asynchronous clocks. In particular, formation algorithms for
actuation-constrained formation control, autonomous collision-free reconfiguration, and synchronized
rotation were demonstrated. This formation software has been integrated with the Formation Control
Testbed robots for a flight-like, hardware demonstration of precision-formation flying.
6.3.3 Formation Control Testbed (FCT)
The Formation Control Testbed (FCT) is the testing-ground for flight software developed for formation
flying for TPF-I. It includes the two robots pictured in Fig. 6-8. Each robot uses cylinders of compressed
air and linear air bearings (the circular metal pads seen in the photo) to float freely above a polished metal
floor. A spherical air bearing supports a stage (shown tilted in the photograph) upon which are housed
the avionics and processors of each robot. The robots have a master–slave relationship and algorithms for
autonomous guidance. They can either be operated independently or together in “cooperative” mode.
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