Solar Orbiter's Energetic Particle Detector (EPD) ICU
EPD's Instrument Control Unit (ICU) is composed by the Common Data Processing
Unit (CDPU) and the Low Voltage Power Supply (LVPS). It provides a single point
digital interface to the spacecraft and to all EPD sensors (STEP, HET-EPT and
SIS). It also shares information with the STIX, MAG and SWA investigation to allow
synchronized high data rate burst-mode operations following on-board identification of
predefined triggering events in the EPD data. The ICU is designed to manage sensors
control and monitoring, the sensors timing clock, and sensor data collection,
compression, and packetization for telemetry. ICU is also responsible of the S/C
telecommand reception and delivery, if necessary, to the sensors.
The ICU box is mounted inside the spacecraft body. The box dimensions are 186x152x100 mm. The CDPU contains a LEON 2 microprocessor, RAM, EEPROM and PROM memories, two hot redundant SpaceWire interfaces, and six identical serial sensor interfaces. The CDPU PROM contains boot code that can load flight code from either the EEPROM or from the spacecraft interface via telecommand. The EEPROM contains TBD copies of the flight code. The RAM (up to 1 Gbit) is used for code, variables, and buffers. With 1 Gbit memory, at 3100 kbps download rate, up to four days of data can be stored in this memory. Most of the CDPU RAM is used for a burst memory to save high time resolution interval snapshots from STEP, HET-EPT and SIS, which are then played out slowly via telemetry. In case of power on/off cycling, CDPU will reset and resume operations from a defined mode.
The LVPS board is responsible of filtering, monitoring and switching the spacecraft primary power, ranging from 26 to 29 V (in accordance with EID-A [AD 1] limits), to the sensors. It also provides the power supply to both CDPUs, nominal and redundant. In the latter case, +3.3 V and +1.5 V are provided. The LVPS will be designed to operate with nominal performance within a Power Bus Voltage of 28 V with variations from 26 to 29 volts. The ICU includes following protection mechanisms for the sensors:
- Under-voltage protection: The LCLs responsible of switching on/off the sensors are only enabled if the input power bus voltage is above certain level and are automatically switched off if this power bus voltage drops below it.
- Short-circuit protection: If a short-circuit is produced in the primary power of any sensor, the corresponding LCL limits its current during some milliseconds. After that, this LCL is switched off.