FGE Power, Command Control & Communications, and Pointing Systems

The power system, developed by Meer Instruments of San Diego, consists of four elements: the solar panels, the charge control unit, the voltage regulators, and the battery stacks. There are ten solar panels, five on either side of the gondola. Each panel contains 16 horizontal and 6 vertical rows of 10 times 10 cm mono-crystalline silicon solar cells, joined in series with protective shadowing diodes across every twelve cells. The total active area is 9.6 m², and with an efficiency of around 12 %, 1.3 kW can be generated at float altitude.

Silicon photovoltaic cells must be operated in a specific manner to achieve peak power production. Setting the operating point is a task performed by the charge controller, which distributes the load across the panels as the power demand changes. The charge controller also ensures that the system's battery stacks are maintained at full charge, sunlight permitting, and it provides on/off switching capability so that the payload can be powered down by a command from the ground, e.g., at the end of the mission.

A set of linear voltage regulators distributes the output from the charge controller to the payload at 31 V over 10 circuits. Inside each pressure vessel, commercial switching regulators provide the voltages required to operate the electronics and actuators.

Three battery stacks are flown, each capable of storing about 1 kW-h of energy. These provide power during periods when the payload cannot be pointed at the Sun, as during the launch-to-float phase, as well as dealing with the short-duration peak currents demanded by the pointing system's torque motors.

Command, Control, and Communications (C³)

An overview of the C³ system is shown in this diagram. The principal component is the Autonomous Control Executive (ACE) computer. It is a 6U VME board manufactured by Motorola with a 68030 microprocessor running a real-time operating system, vxWorks. During development, this machine could be booted via its onboard Ethernet port from a Sun Microsystem Sparcstation II. For flight, the ACE was booted from a Flash ROM disk connected to the onboard SCSI interface. This approach eliminated the need to reprogram a PROM housed inside a pressure vessel anytime an error was detected, and avoided the use of a delicate disk drive.

The primary task of the ACE is to carry out the science mission objectives during the long periods when there may be little or no contact with the ground. Two of the ACE's additional functions are handling the Exabyte tape systems and the CCD camera via an Imaging Technology 150/40 VME framegrabber. There is an Exabyte EXB10i Cartridge Handling Systems associated with an Exabyte 8500C Cartridge Tape Drive. It holds 10 8-mm tapes. The full capacity of the system is about 90 GB.

The adopted CCD camera, a Kodak Megaplus 1.6M, resides in the OPV at the feed from the MT. It has an active area of 1534 columns and 1024 rows, with each pixel being 9 microns by 9 microns and a well depth of approximately 90,000 electrons. When triggered by the IT framegrabber, an exposure in the range of 10 ms to 500 ms is made by opening a blade shutter mounted directly in front of the CCD. Each image is digitized at 10-bit resolution in a separate Camera Control Unit (CCU) located near the camera head. The data is clocked out of the CCU at 10 MHz and delivered to the framegrabber as a 10-bit-wide RS422 differential stream along with separate frame and line synchronization signals. Once captured by the framegrabber, the image is available to the ACE, which can then write it to 8 mm tape or transmit it over the high-speed RF channel via a Mizar I/O board located on the same VME backplane.

The Instrument Control Computer (ICC) provides various services for the ACE, simplifying the interfaces the ACE has to deal with in order to move filter wheels, actuators, temperature controllers and the like. The ICC also handles all communications (except the high speed downlink), collecting and transmitting housekeeping data, handling I/O with the SIP and with all the instrument controllers. In all it deals with nine separate inputs streams and eight output streams, as well as attempting to provide a uniform interface for the ACE to sixteen different devices. The ICC is a Forth-oriented RISC processor (FRISC) developed at JHU/APL. Forth has advantages in instrument development environments such as ours, as it requires only limited host support (to hold a backup of the source), produces compact code, and is readily extensible.

The ICC interfaces to the four instrument controllers, Max1, Max2, Max3, and Max4, via a VMI I/O board. Max1 is a Motorola 68HC11E2 microprocessor which handles the secondary and CCD focus stages as well as the offset pointing motors. Max2 is also a Motorola 68HC11E2 and supplies housekeeping data to the ICC on conditions in the OPV and IMC bench, as well as controlling a wide variety of devices: the positive and negative Fabry-Perot high voltage power supplies, the two liquid-crystal phase-retarder oscillator supplies, the MT beam filter wheels, and the Image Motion Compensation (IMC) system. The IMC is a complex device in itself, and the interface is limited to a few control lines, e.g. "Request Lock."

Max4 is a Microchip PIC16C54 that provides a uniform interface to four temperature controllers: the MT filter wheel, the Fabry-Perot oven, and the liquid-crystal modulator housing. The temperature controllers provide a resolution of 0.01 C and are also based upon a PIC16C54.

The final instrument controller is Max3. This is easily the most complex of the four, having been built up as a special-purpose board designed around a Dallas 87C520 (an upgrade to the 87C51 microprocessor). Max3 collects a large fraction of the housekeeping data for the gondola, including temperatures, pressures, currents, and voltages. It also supplies the control voltages for the servo amplifiers that drive the three torque motors (elevation, reaction wheel, and momentum dump), and the discretes that switch such items as the stow latch. Finally, conversions to and from RS232 for the RF systems are handled on this board.

The most critical function of Max3 is the pointing and control system. In this, Max3 combines input data to determine an "error," and from it and the current state of the payload, produces an output for either the elevation or reaction wheel drives to compensate. Four distinct modes of pointing have been developed:

Track state 0: No tracking. This is the state throughout most of the period from power-up to reaching float altitude. No attempt is made to orient the gondola, so no motors are driven and power consumption is minimized.

Track state 1: Coarse tracking. Four photodiode sensors mounted at 90degree intervals around the gondola provide the position of the Sun in azimuth, while an encoder on the elevation shaft provides that angle. This information is fed to two separate PID (Proportional+Integral+Derivative) control loops, one for azimuth and one for elevation. Pointing accuracy of this loop is only a few degrees, but sufficient to allow hand-off to the next level.

Track State 2: Intermediate Tracking. In this state, a pair of detectors mounted at the front of the pointing telescope bench provide the necessary signals for tracking. Each sensor consists of a cylindrical lens mounted in front of a position-dependent photodiode, one sensitive to the Sun's relative position in azimuth, the other to elevation. With a 10 Hz cadence, these signals are fed to the azimuth and elevation PID loops. Pointing is only to about 0.5 degrees, but it is sufficient to reach the next tracking state.

Track State 3: Fine Tracking. In this critical mode, the error signals are provided by an image of the Sun produced by the PT being formed on a Lateral Effect Diode (LED). The current from each of 4 LED outputs is dependent on both the intensity and the relative distance of the image to the port, allowing the position of the Sun to be estimated accurately (including electronic noise, 5 arcsecs in a 2 degree field of view).