As mentioned above, the preceding
work was done in the early nineties at the GSFC Robotics Lab. Our
initial tests with the COS mockup and the Ground Testbed (GT) robot arm
showed us that a sensor such as the Capaciflector would be useful for
the final alignment of the instrument into HST.
Using a modified version of the sensor developed several years ago, I
designed and built a module consisting of eight sensors that are
battery powered and that communicate with a central computer by radio
link. This allowed the robot to grasp the mockup and have sensors
available for final alignment without needing to bother with electrical
Circuit board with the microprocessor and eight sensors. The
is battery powered and works via an RF link. Each of the eight
are connected by the white coaxial cables on the side.
Completed COS science instrument mockup with the sensor circuit.
Range is about one foot for the
sensors and drift was extremely low. The sensors each produce a
frequency in the 50kHz range to
indicate proximity, and the drift is less than 10 Hz over several hours.
The back of the mockup showing one of the eight sensors. Others
are on the right side of the image.
Three sensors are located in the back (facing HST), and the other five
are for aligning to the guiderails
on the top and bottom of the instrument.
View of the robot (same as the one here
performing the extraction and insertion of the
COS mockup in our High Fidelity Mechanical Simulator. The
instrument has to be
positioned with no more than 0.16" of error.
Another view of the task. In the right background the actual
flight instrument (COS) is visible in its storage
container. The bottom guiderail for the instrument can be seen
near the floor level of the robot platform
(yellow/white bar extending into HST). There is also another
similar one in the top right of the instrument.
Both are used to guide the instrument to its final location.
The operator at his workstation. On his left is the translational
handcontroller. This moves
the robot's position (x, y, z). On his right is the rotational
handcontroller, which moves
the robot's orientation (roll, pitch and yaw). This is the same
setup that is used by
Shuttle astronauts to fly the shuttle and run the Shuttle robot arm.
Top view of operator station.
Sensor readings are fed to a central computer which displays for the
which handcontroller commands to use to berth the science instrument
its proper location. Each set of arrows corresponds to one of the
The graphics for the arrows are by Jesse Clark, and
the algorithms and the main program were written by me.
By following the commands on the display, the operator is able to
position the instrument to
no more than 0.04" (1mm) of error. This guidance can not be
obtained from cameras or any
other means showing the importance of proximity sensors for this
The initial tests with the
Capaciflector were performed in July 2004, and then repeated in
September 2004. Consistent performance with both trials show that
the drift and stability of the sensor is very good, and would be a
useful addition to the mission.