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System Status: March 2001

Hysteresis Rods

Analysis on the hysteresis rods was done to determine the minimum volume needed to counteract Spartnik's spin due to the Solar Pressure Paddles. In order to dissipate spin energy beyond the mission requirement of 0.4 rev/min, it was necessary to translate the analysis data from Walker Scientific Inc, into physical dimensions. Heat dissipation from the hysteresis effect and eddy currents, due to the ferromagnetic rods moving through an external field was investigated. Analysis¹ showed that the alignment of Spartnik's spin axis with the local geomagnetic field would render these two forms of heat dissipation invalid. (See Analysis of Hysteresis for Attitude Control of Microsatellites for more information.)

Since the hysteresis rods are not going to help despin Spartnik, other means of heat dissipation or despin mechanisms need to be investigated. Currently, Spartnik's tumble and random nutation are being looked at as a viable means of heat dissipation. Also, if the hysteresis rods don't effectively despin Spartnik, the satellite will spin fast enough to achieve gyroscopic stiff, thereby loosing it's alignment with the geomagnetic field, and allowing the rods to work temporarily. However, this would make attitude in support of the payload difficult to predict.

A modification of the Launch Vehicle Adaptor, headed by Philip Canlas, the lead of the Integration and Manufacturing subsystem, is currently being considered. Rifling on the inner housing of the adaptor, coupled with bearings on the shaft of the adaptor, would provide an initial spin rate. This would require that the antennas be used for communication purposes only, not as solar pressure paddles.

Due to the wide functional range of motor fluids, testing to obtain a qualitative viscosity profile was done on the following fluids: SAE 30, 10W 40, transmission fluid and antifreeze. Ethyl alcohol was also tested. Research showed that the nutation damping constant could be maximized by minimizing the viscosity of the fluid slug. Hence, Chris Lund and Gerardo Penaloza will soon be manufacturing the ring with ethyl alcohol. Braizing was attempted on an aluminum prototype and was not successful. Therefore, the ring will be welded shut, pierced and filled up to 135° with a syringe, and resealed.

IR sensors from Infratec have been located for purchase. Infratec recommended that, in the interest of cost, we supply the 15-micron filters for integration. Lockheed has donated filters, but they need to be cut. Barr Associates has agreed to cut our filters to size.

Greg Rodgers, the past Systems Engineer, along with this year’s Team Lead and Gerardo, are researching different fluids for their potential damping properties, within the limitations of the space environment. The amount of fluid in the nutation damper will be finalized when the research is complete.

Robert Benzio and Peter Mazanec are continuing the research on infrared Earth sensors. This research includes commercial availability of IR sensors in the 15 micron range, and their durability in the space environment.

Subsystem Overview

Beyond general stabilization, Spartnik’s attitude is designed to support one of its two payloads: the digital camera receiving light through the periscope attached to the top plate. The Mission Objective is to obtain photographs of San Jose, Ca.

Attitude determination, stabilization and control are acquired through purely passive means. All determination and control hardware is self-contained and requires no input.

Solar Pressure Paddles

Spartnik’s communication antennas will double as Solar Pressure Paddles (SPP), in order to induce a spin about Spartnik’s Z-axis. These antennas are coated with a highly reflective tape, Aluminum Flexible Optical Solar Reflector (FOSR), on one side and black anodized on the other. This surface difference will create a torque upon the satellite and subsequent spin. Spartnik will spin at a rate of 0.4 revolutions per minute.

*Note: Many satellites use spin for stability, as a rapidly spinning top remains upright and stable. For our purposes, Spartnik’s spin is a means to evenly distribute and dissipate heat.

Hysteresis Rods

To prevent the spacecraft from continuously spinning up, hyperm steel rods will be mounted perpendicular to the spacecraft’s Z-axis. As these "hysteresis" rods rotate about the spin axis, and through the Earth’s magnetic field, the magnetic dipoles, inherent in the material, will continuously realign themselves to run in the same direction as Earth’s magnetic field. This realignment generates eddy currents and a torque, countering the spin of the satellite. At each half turn of the rods, the current flow will reverse and the poles will switch. This flipping of the poles and reversal of the current in the rods will create heat within the rods. This heat is the conversion of the spin energy of the satellite. Thus, as heat is radiated into space, the spin energy is also dissipated.

Permanent Magnets

Eight permanent bar magnets have been distributed throughout Spartnik’s shell, their dipoles parallel to the satellite’s spin axis. The purpose of the magnets is to orient Spartnik such that its spin axis will align with Earth’s magnetic field. With this in mind, and considering the dip of Earth’s magnetic field, Spartnik will experience two tumbles per orbit: one at the Magnetic North Pole, and one at the Magnetic South Pole.

This controlled tumble will ensure that the camera lens will be pointing toward Earth in the Northern Hemisphere only. Simulations show that San Jose, Ca. will be in the camera’s field of view during a North American pass. In the Southern Hemisphere, magnets will align the satellite so that the camera is pointing toward deep space.

Nutation Damper

To control nutation about the z-axis of the spacecraft, a nutation damper will be mounted perpendicularly to the Z-axis of the satellite. The nutation damper is a hollow ring that will be filled with a predetermined amount of fluid, with room left for “sloshing”. As the hoop rotates, the fluid will initially lag behind, acting as a spin damper until it reaches equilibrium with the satellite. If the spacecraft starts to nutate in small angles, waves will be formed on the surface of the fluid. The waves will propagate around the hoop and dampen the small nutations. If the satellite experiences large nutations, the viscous fluid will act as a slug to dampen the unwanted nutations.

Solar Cells

The ten panels of Spartnik’s shell are covered with arrays of solar strings (made up of individual solar cells, connected in series), that convert the Sun’s energy into electrical energy, in order to recharge the system power supply(see power subsystem). The current readings from these solar strings will provide the input with which to determine Spartnik’s attitude.

To verify the passive attitude control system, software was developed, in-house, by SJSU students to simulate the attitude dynamics of the satellite. The C code takes into account the Earth’s magnetic field and the attitude control hardware.

The algorithm is based on the fact that the angle between the solar vector and the solar string is directly proportional to the current going through the string. Knowing this, a current incidence curve can be calibrated and created. Using these current readings, the onboard computer will calculate the orientation of the spacecraft, relative to its local inertial coordinate system. This will give the spacecraft’s attitude at any time in orbit and will provide a means of predicting the attitude at any future time. This will allow us to determine when a picture of Earth’s surface can be taken.

Infrared Sensors

To aid the current sensor calculations in determining when the camera lens is pointing nadir (toward the Earth), two 15 micron infrared sensors will be mounted equidistant of the camera lens, one on either side. When both IR detectors have equal readings, then the surface of Earth is in view of the camera lens.

If you have any questions, please feel free to contact the ADAC team.

System Status: December 1999

The ADAC team has finished all of the dynamic simulations for microsatellite, Spartnik. The results of these simulations have led to a final attitude control system design. The old control system, which includes two permanent magnets and a spin rate of 0.5 rev/min, was upgraded to include eight magnets and a spin rate of 0.4 rev/min. Simulations were run on this new control system design. The results of these simulations show that Spartnik will tumble two times per orbit as desired.

Irma Franco and Ruben Garza are researching the nature of hysteresis rods and their effect on the spin rate of Spartnik.

John Van Arsdall along with this year’s Team Lead is researching the nutation damper. It was determined that the nutation damper will be braized shut instead of using a pressure fitting. The amount of fluid in the nutation damper will be finalized when the research is complete.

The infrared detectors are being tested. The tests will indicate the voltages produced by the detectors. The tests will also determine the useful angle range of the detectors.

As Spartnik does not yet have an orbit selected, because we do not yet have a donated launch [hint hint! :>)] Therefore, the team had to design for a wide range of possible orbits.

The attitude determination and control subsystem is completely passive and uses spin stabilization and a controlled tumble to maintain an acceptable orientation. A spin will be induce by solar radiation pressure on the communications antennas. These antennas are coated with a highly reflective tape, Aluminum Flexible Optical Solar Reflector (FOSR), on one side and black anodized on the other.

To prevent the spacecraft from continuously spinning up, four soft iron rods have been mounted perpendicular to the spacecraft’s z-axis. As these “hysteresis” rods rotate within Earth’s magnetic field, the magnetic dipoles within the rods will realign themselves to run in the same direction as Earth’s magnetic field. This realignment generates a torque, countering the spin of the satellite. At each half turn of the rods, the current flow will reverse and the poles will switch. This flipping of the poles and reversal of the current in the rods will create heat within the iron rods. The heat is the conversion of the spin energy of the satellite. Thus as heat is radiated into space, the spin energy is also dissipated.

This control system is useful in keeping the spacecraft pointed in one direction. It was necessary to find a way to ensure proper pointing for the camera lens, which will be mounted on the +z-face of the satellite. This is done by inducing a controlled tumble of the spacecraft’s z-axis. Eight bar magnets will be placed inside the shell, with the poles parallel to the satellite’s z-axis and the local magnetic field. This is a controlled tumble because we can approximate the angle between the spacecraft’s z-axis and an inertial frame. This controlled tumble will ensure that the camera lens will be pointing toward Earth in the Northern Hemisphere only. The magnets will align the satellite so that the camera is pointing out to space in the Southern Hemisphere.

To control nutation about the z-axis of the spacecraft, a nutation damper hoop will be mounted parallel to the xy-plane of the satellite. The nutation damper will be filled with a predetermined amount of 10-40 weight grade oil. As the hoop rotates, the fluid will lag behind, acting as a spin damper until it reaches equilibrium with the satellite. If the spacecraft starts to nutate in small angles, waves will be formed on the surface of the fluid. The waves will propagate around the hoop and dampen the small nutations. If the satellite experiences large nutations, the viscous fluid will act as a slug to dampen the unwanted nutations.

To verify the passive attitude control system, software was developed in-house by SJSU students to simulate the attitude dynamics and take into account Earth’s magnetic field and the attitude control hardware.

In order to determine the orientations of Spartnik, measurements of current will be taken from the solar strings mounted on the exterior panels of the satellite. The angle between the solar vector and the solar string is directly proportional to the current going through the string. Knowing this, a current incidence curve can be created. Using these current readings, the onboard computer will calculate the orientation of the spacecraft relative to its local inertial coordinate system. This will give the spacecraft’s attitude at any time in orbit and will provide a means of predicting the attitude at any future time. This will allowus to determine when a picture can be taken of Earth’s surface or of the moon.

To aid the current sensor calculations in determining when the camera lens is pointing nadir (toward the Earth), two 15 micron infrared sensors will be mounted equidistant of the camera lens. When both IR detectors are reading, then the surface of Earth is in view of the camera lens.