TableSat

The University of Michigan.s TableSat platform [6] is a one degree-of-freedom .Tabletop. satellite that emulates the dynamics, sensing, and actuation capabilities required for spacecraft attitude control. Illustrated in Figure 3, TableSat is driven by two .computer fan. thrusters commanding clockwise and counter-clockwise torques, respectively, and experiences extremely low friction on its central pivot point. TableSat contains a high-precision rate gyro to measure angular velocity along with a three-axis magnetometer and set of four core sun sensors (CSS) to measure pointing direction. An onboard Diamond Systems Prometheus PC/104 computer running the QNX real-time operating system communicates to a ground station via a wireless 802.11b interface. The computer interfaces to sensors through 16-bit analog-to-digital converters and to actuators through amplified 16-bit digital-to-analog channels.

TableSat exhibits nonlinear dynamics, including off-axis wobble given certain actuation magnitudes and switching profiles. TableSat provides a variety of practical challenges in hardware and software for control engineers with limited experience using real physical systems. In its current lab environment, magnetometer and CSS sensor calibrations are nonlinear and dependent on location and time of day (e.g., proximity to the ferrous building frame, lighting conditions). Control system designs have ranged from rate control based on gyro readings to pointing with single or multiple sensor measurements.

TableSat onboard software is composed of the following four threads, each of which executes as a periodic real-time task of constant frequency:

• State Estimator thread that reads sensors and estimates the current state of TableSat;
• Controller thread that applies the specified control law to compute output torque;
• Actuator thread that outputs computer fan voltages to achieve the commanded control torque;
• Communication thread that supports data/command transmission from/to an external client program.

"Safe Core" Control System - Due to the atmosphere-free spacecraft environment and the low friction experienced by TableSat, uncontrolled actuation can lead to undesirable high-speed rotations that impart dangerous forces to system components. For space operations, damping rotational motion to zero, or near-zero, is viewed as a capability all spacecraft must possess at all times, even after being .safed. due to other exception(s). We have implemented a core (safe) controller for TableSat that relies only on its rate gyro to stabilize TableSat motion should other sensors or controllers be compromised. Rate control is a straightforward algorithm. A simple proportional controller approximately achieves commanded rates; augmentation with an integral term drives steady-state rate error to zero and has been demonstrated effective given TableSat.s relatively high thread execution rates and small external disturbance magnitudes. Although subject to bias thus typically small but non-zero steady-state drift, rate control is not difficult to prove correct so long as the rate gyro and computer fans function properly, a simplifying assumption we make to constrain the scope of our core controller for this work. Because it uses only rate gyro data, the proportional-integral rate control law deployed as the TableSat safe core control law is impervious to changes in lighting conditions and magnetic field disturbances.

Augmented (Risky) Control System - Rate control is a safe backup but cannot accurately point TableSat in any particular direction. Under normal operating conditions, TableSat incorporates inertial attitude measurements in its feedback control law. Three inertial attitude sensing strategies are possible, all relying on the gyro for angular rate estimates: 1) Magnetometer-based (North-referenced) pointing, 2) Sun sensor (CSS) pointing, and 3) Pointing over a fused estimate of magnetic field and light sensor measurements. The initial implemented controllers [6] relied strictly on the magnetometer for pointing information, but CSS-based control is more capable in TableSat.s University of Michigan locale due to ferrous building structure disturbance of magnetic field.

Failure of attitude sensors is the primary reason the risky controller will malfunction in our experiments. To conduct controlled, repeatable tests, we induce sensor failures. The CSS fails in two ways: by shutting down the single light source or by adding a second "competing" light source. Although we have identified an ideal lab location at which our magnetometer is calibrated, we can similarly induce magnetometer failures by carrying TableSat through the lab during testing. A magnetometer-based control law destabilizes when magnetic North and a building beam exert approximately equal magnetic influences. Pointing direction is marginally stable but incorrect when TableSat further approaches a ferrous building column due to the dominance of the structure.s local magnetic field but with a less-localized directional signal.

Regardless of the pointing angle data source, TableSat applies baseline proportional-derivative and proportional-integral-derivative controllers over pointing direction and angular rate to achieve and maintain pointing commands. For this project, it is sufficient to command TableSat to achieve a single reference attitude or constant angular rate (likely zero for the safe controller) since we are studying resource (sensor) failure rather than logic failure. However, as an analogue to spacecraft science missions, TableSat will ultimately be extended to execute uploaded pointing command sequences of specified durations, enabling more complex analysis of the long-term interaction of safe core and risky control laws.