Exo-SPHERES is designed to be highly serviceable and modular. The main structure consists of two box stacks and isogrid panels. One box stack contains the C&DH box, the COMMs box, and the motion controller box. While the battery box and battery balancing box are located in the second stack. Each box stack can be accessed by removing a lid; this allows access to the various electronics without needing to disassemble any panels. Boxes are oversized to allow for future expansion and updates.
Connectors are located on the top of the boxes – which are spaced 2” below the top panel. This allows the boxes to be easily disconnected from the wiring harness. Users can reach through a hole located in the top panel to reach the connectors. Both box stacks can be unplugged and removed from the vehicle by removing these connectors and removing a couple bolts. Individual boxes can be accessed by either removing the entire stack or by simply removing the boxes on top of the stack. Removing boxes from the stack does not require the box stack to be removed from the main vehicle.
The electronics system consists of a main computer in a ruggedized PC/104 form factor, and an I/O expander micro-controller that communicates to the main computer via a serial data interface. Storage for the operating system and data lives on a solid state drive contained within the vehicle. Interfaces to the thrusters are parsed through the I/O expander microcontroller, located in the motion control box. System telemetry information is collected and returned for health monitoring of the prototype spacecraft. The MEMS based IMU is connected to the main computer via RS-422 serial bus.
The battery box for the system is run by a pair of 24 Volt, 5.2 Ah, NiMH batteries, which are combined via diode or-ing. This enforces even discharge in addition to preventing the batteries from charging each other, which can, in severe cases, cause battery-cell death or venting of electrolytes within the vehicle. Additionally, security is provided by a pair of 24V SPST relays wired to an external kill switch. The final combined power and ground lines are passed out via a pair of 37 pin D-Sub connectors, along with dedicated charging, and temperature monitoring data lines.
Thruster quads are mounted to the diagonal panels. The rest of the RCS system is located in the middle of the vehicle and is designed to be easily removed. Quick disconnects are used to attach the thruster quads to the RCS unit. After unbolting the RCS system, it can then slide out of the vehicle.
Exo-SPHERES Experiment is under development at the University of Maryland Space Systems Laboratory, part of the Aerospace Engineering Department and the A. James Clark School of Engineering.