Second of two parts
By John McHale
GREENBELT, Md. - NASA Astronauts improved the Hubble Space Telescope`s pointing control system during their recent maintenance mission by installing two 6-pole, unipolar direct current (DC) stepper motors to control the position of the articulating mirror on an enhanced Fine Guidance Sensor.
"The stepper motor is more sophisticated than a normal DC motor and allows fine positioning without the need for a servo control system," says David Scheve, observatory development manager at the NASA Goddard Space Flight Center in Greenbelt, Md.
The stepper motors on Hubble`s guidance sensor "are controlled by custom- designed Actuator Mechanism Electronics built by Jackson and Tull Chartered Engineers for NASA," he continues. "Custom electronics were required because the stepper motors were being integrated into an existing design."
The electronics are on two small printed circuit boards in a 4.5-by-2.5-by-3.5-inch enclosure inside the guidance sensor, Scheve explains. The digital electronics board receives the step count from optical control electronics on Hubble and provides the proper timing signals and number of pulses to the sensor electronics assembly also located in the sensor.
Then the assembly decodes the timing signals and pulse count and provides the proper step sequence to "move the stepper motor to the Actuator analog board," Scheve says. "The analog board provides the 28 volts DC drive signals to the stepper motor through a 4N48 optocoupler and 2N6351 high-power darlington transistor."
The stepper motor rotates 30 degrees for each motor step, requiring 12 steps per complete revolution, he says. "Fine control over the mirror position is achieved by attaching a high-precision cam mechanism to the output shaft of each stepper motor," he says. "The profile of the cam is such that it requires approximately four motor steps to detect a noticeable change in performance."
NASA flight technicians on Earth control the stepper motor by a series of commands they issue through Hubble`s normal radio frequency communication link.
Radiation hardness
"These electronics have proven themselves to be highly immune to space based sources of noise due to the components used in their design," Scheve explains.
"The Hubble data management subsystem also uses an error-detection mechanism to ensure that commands are properly received before they are acted upon to prevent erroneous commands from being executed," he says.
"The stepper motor control system on Hubble operates open loop, not requiring feedback for proper positioning. The disadvantage of this system is not knowing the precise location of the stepper motors at any given time," Scheve cautions. "This approach was necessary, however, because there were no telemetry points available in the existing system to provide the position information to the ground."
NASA experts have been using the new fine guidance sensor installed in February for several weeks, and the performance "has been fantastic," Scheve says.
The new guidance sensor has the potential of carrying out astrometric and photometric observations such as measuring the separations and magnitude differences of binary stars, and measuring stellar angular diameters.
The fine guidance sensor consists of a collection of mirrors, lenses, servos, prisms, beam-splitters, and photomultiplier tubes.
Principal hardware
Hubble`s pointing control system has two principal hardware components. Rate gyros are the guidance sensors for large maneuvers and high-frequency pointing control, and the three fine guidance sensors perform at lower frequencies.
Optics within the guidance sensors, using precision motor-encoder combinations, select a 5-by-5-inch region of sky into an x, y interferometer system. Once the mechanism locks onto a star, the motor-encoders track the interference fringe of the guide star. The pointing control system software uses encoder positions to update the current telescope attitude and pointing.
The mechanism can provide star positions that are about 10 times more precise than those observed from a ground-based telescope, Scheve says. This level of stability and precision is comparable to holding a laser beam steadily enough in New York to focus on a dime in Washington.
