The DBA (red box) sits on the exterior of HST near the boom of the Solar Arrays. Since everything is covered in silver mylar,
it may be difficult to distinguish the box precisely. This image shot in December 1999 during the third Servicing Mission.
For the fourth servicing mission in December 2000, the HST project decided to replace the floppy Solar Arrays with rigid (solid) Solar Arrays from the Iridium program. These would offer important improvements in terms of higher power output, smaller size, and better jitter performance. This last point is important as the old floppy Solar Arrays were very delicate, and we constantly had to worry about damage to them during Servicing Missions. This rigid set of arrays will actually be the third set of 'wings', as another set was already installed during the first mission (SM1).
The DBA is visible here and highlighted by the red box. The mast or boom of the Solar Array is visible
above the DBA. This image was shot during SM3A (Dec 1999) just prior to the astronauts starting a servicing task.
The replacement of the Solar Arrays with a new set meant that a new DBA would have to be built. Recall that the DBA combines the power from the Arrays into outputs for the batteries. This new unit became known as the DBA2. The Hubble Project decided to add a set of relays into the new DBA2, so that power from the Arrays could be controlled from outside of HST. However, due to the short development schedule, it was decided not to build a controller, but to leave the relays dormant for a potential future mission.
In the Spring of 1999, a internal failure of the HST charging system
added new urgency to the situation, and the Project decided to add a controller
to the DBA2 after all. This is where I came in. Since the DBA
was never meant to be controlled by the HST computer, there is no path
from the computer to the DBA. One of the design goals was to minimize
the astronaut (EVA) time needed to install the system, but adding cabling
from the computer would take too much time. My idea was to reuse
a unneeded signal so that the computer could send pulses down the line
which would be read by the DBA's controller. With this idea, the
project decided to go ahead the the Diode Box Controller (DBC). The
many commands that the DBC would need to accept are encoded in the width
of the pulse on this line.
The DBA2 with the added DBC in the back. On the top of the DBA2 are the rows of connectors that take in power
from the Solar Arrays, and feed out power to HST.
The DBC's circuitry is a mixture of digital and analog circuitry. The digital circuitry is implemented in a radiation hardened and flight proven Field Programmable Gate Array from the Actel Corporation. This takes the command signal from the HST computer and decodes the pulse width into an appropriate set of relays that need to be opened or closed. For example, a 1 second command directs a certain set, and a 2 second command another set, etc. The analog circuitry is used when the computer is down, and autonomously looks at the battery voltage and temperature to perform self-contained charging of the batteries. This assembly is thus being entrusted with the most valuable resource on a spacecraft : its power. Designing and building this controller was not as much fun as the HOST Controller, but it is a lot more important.
Of course, I did not build the DBA2/DBC alone. A large team of
mechanical, thermal and electrical engineers along with technicians and
assemblers have worked on the four units that were built. However,
I did have the privilege of being the originator of the control method,
and the electrical designer for the DBC.
Shot of the DBA2 wiring. Built by John Rodd and Robert Lawton (Jackson and Tull). They did a wonderful job.
This effort was followed by the build of the Engineering Unit, which was then integrated into a corresponding DBA2. The resulting unit is shown below. The gold colored bar on the front of the unit is the astronaut handle. Into this plugs the 'cross-strap' cable that connects the two DBC's on HST together. At the top right of the box are the four connectors that feed in power from the new Solar Arrays. The next four in the middle of the top of the DBA2 is the bundle that carries the power from the DBA2 to HST. The two remaining connectors (one is capped with a gold cap), are for control of the internal relays.
The Engineering Model DBA2/DBC just before its thermal balance test (Dec. 2000).
Here the assembly is undergoing checkout prior to the test.
The Test System on the left with the DBA2/DBC on the right. In the
background one can see the thermal-vacuum chamber used for the test.
The DBA2/DBC is placed inside the chamber, and the air pumped
out to simulate the environment of space.
In addition to the thermal-vacuum test performed above, the flight units are also tested in the
Electro-Magnetic Compatibility Lab. This verifies that it will not emit noise that will
disturb the other instruments on the telescope. (3/01).