2.2.4 Power

The Power Supply and Mechanism Controller (PSMC) provides power and control to the Detector Electronics Assembly (DEA), Digitial Processor Assembly (DPA) and the Detector Assembly (DA). The major components of this unit are the DPA power supply, the DEA power supply, Vent Valve and Mechanism Control (VVMC), Detector Housing Temperature Control (DHTC), Input/Output EMI cavity (I/O EMI), and Serial Digital Telemetry (SDT).

The power system is composed of two fully redundant sides, known as side A and side B. All of the previously mentioned components are fully block redundant (i.e. A can completely replace B and vice versa), and single fault tolerant (i.e. no single point failure will cause the loss of the experiment), with one exception. The DPA power supply is not fully redundant, but each independently commandable DPA power supply (A & B) is hardwired to power a subset of three FEPs and one BEP (half of the DPA), which allows a graceful degradation of ACIS function in the event of a DPA power supply failure.

The DPA and DEA power supplies provide regulated electrical power to the DPA and DEA electronics. Telemetry monitoring of the power supply voltages and input current monitoring are provided to the RCTU. Primary power is supplied by the AXAF-I spacecraft power bus. Each DC-DC converter is synchronized to a DPA clock to assure that no operational transients occur during the time science data is being taken.

Each power supply output contains overvoltage protection to protect the load from over-voltage conditions, including turn-on transients and transients induced by load switching. Power Supply outputs also contain a current limit function such that source and load are both protected during faulty conditions.

All PSMC power supplies are designed with internal preloads to ensure stable operation with no DEA or DPA load present. Load switching of the DEA and DPA are supported on a board-by-board basis, allowing any number of DEA analog or DPA processor boards to be brought on or off-line during normal mode operations.

The VVMC controls three mechanisms: (1) the door mechanism, which is activated to open and close during ground operations, and opens once on-orbit; (2) the high-conductance vent valve mechanism, which is activated to open and close during ground operations, and opens once on-orbit; and (3) the low-conductance vent valve, which is used to equalize pressure between the DA internal volume and ambient.

Redundant actuator limit switches are employed in both the door and high-conductance valve mechanisms to report mechanism position and terminate when mechanism travel is completed. Actuator temperature is also monitored to terminate operation and preclude possible damage of the paraffin (hot wax) actuator, should mechanical fault conditions exist. Temperature measurements are also continuously reported as analog telemetry during operation of the Detector Housing (DH) door and high-conductance valve mechanisms. Limit switch positions are reported as digital telemetry, with analog voltage representing the actuator temperature also reported. The VVMC also provides internal housing pressure information to ensure proper system venting during ground and on-orbit operations.

The DHTC is a Pulse Width Modulated (PWM) heater control circuit that uses redundant heaters and thermistors installed on the DA to control the Detector Housing temperature to -60^oC$\pm 1 ^o$C over one orbit during normal mode operations, and +25^oC$\pm 1 ^o$C for on-orbit contamination bake out mode operation. The Thermal Controller circuitry monitors the Detector Housing temperature and reports an analog voltage representing the housing temperature.

The serial digital telemetry provides status of various PTS subsystem states, and in concert with various passive and active analog measurements allows visibility into PTS operational condition. All PSMC control circuitry is commandable by RCTU High Level-Pulse Commands, requiring `Enable and On' commands or `Disable and Off' commands to either activate or deactivate any given PSMC function, providing single fault tolerance to inadvertant operation.

The PSMC includes under voltage protection such that PSMC outputs latch off should the spaceraft input voltage drop below specification. The PSMC output then returns to active state when the `ON' command is reiniatiated.

  

ACIS Configuration 2|c|Total Power 2|c|Consumption

Normal (6 CCDs on) 155 W

Calibration (2 CCDs on) 100

Standby (with thermal control) 64

Memory Save (no thermal control) 20

Bake out (initial 8 hours) 168

Bake out (stable operations) 128

ACIS normal operations power consumption (with six CCDs active, and the focal plane regulating at -120 C) is 155 W, as measured in several ACIS functional test procedures. ACIS power consumption under various configurations is listed in Table 2.3, derived from document 36-01510.218, ``ACIS CEI Specification: Electrical Power Requirements''. Note: If all ACIS systems are switched on together the total power consumption is 276 W; this represents a maximum possible power consumption but is neither planned nor useful.

The only known constraint on power switching is a need to prevent ACIS from operating the DEA/DPA electronics and carrying out an instrument Bake-out at the same time. (If such a condition occurred, it could draw enough power to blow the ACIS fuses on the spacecraft.)

The ability to command the focal plane array into bakeout has been removed from the current version of the DPA flight software (Rev. 1.5) stored in the flight EEPROM. A patch to the software must be uplinked before the software can command ACIS to enter a bakeout mode (but there is no hardware lock-out to prevent simultaneous DEA/DPA-Bakeout operation). The DA bakeout mode is not controlled by the DPA software (and hence is not `safed' by Rev. 1.5). However, as is true for all PSMC commands, an ``enable'' and an ``on'' command must both be sent erroneously for a problem to develop.