Micromaps host Satellite Design Proposal



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2.3.4 Hardware


The ADCS consist of two types of hardware: attitude sensors and attitude actuators. Attitude sensors take measurements that determine the orientation of the satellite. Attitude actuators apply a torque to the satellite to control its orientation.

2.3.4.1 Determination



The attitude determination hardware used by the VASCAT includes two Earth sensors, two sun sensors, three rate gyros, and a magnetometer. Earth sensors are chosen as attitude sensors for the VASCAT because it is an Earth-referenced satellite, and the sensors meet accuracy requirements. The Earth sensors are Ithaco Conical Earth Sensors (CES). Figure 6 is a diagram of the sensor head of the CES with dimensions.

Figure 6: Ithaco CES sensor head diagram11

The CESs use a rotating scanner that sweeps out a 45 degree half-angle cone.11 A measurement is taken to determine where in that cone the scanner crosses Earth’s horizon. This knowledge, combined with the known curvature of the Earth, is used to determine the pitch and roll of the satellite. The VASCAT uses two CESs mounted along the roll axis of the satellite. They are angled down 45 degrees towards the Earth. The structural and power requirements as well as the performance characteristics are presented in Table 11.
Table 11: Properties of the Ithaco CES11


Description

Value

Mass

3 kg

Sensor Head: 1.1 kg

Electronics: 1.9 kg


Peak Power

8 W

Voltage

22 to 36 V or 31 to 52 V

Dimensions

Sensor Head: 9.9  11.8 cm

Electronics: 16.8  17.0  8.3 cm



Accuracy

0.1 (3), 1’ (avg.)

Scan Cone Half-Angle

45

The VASCAT uses two sun sensors manufactured by Valley Forge Composite Technologies. The sun sensors measure the direction of the sun vector in the body-fixed reference frame. This measurement is compared with the inertial sun vector direction to perform single axis attitude determination. Table 12 summarizes the requirements imposed by the sun sensors and their performance characteristics.

Table 12: Properties of the Valley Forge Composite Technologies Sun Sensor18


Description

Value

Mass

350 grams

Peak Power

2.5 W

Accuracy

1 arc second

Measurement Frequency

10 Hertz

Field of View

100  50

Rate gyros measure the angular rate of the satellite in the body-fixed reference frame. BEI Systron Donner manufactures the rate gyros used on the VASCAT. They measure the angular rate about one axis. Figure 7 is an isometric view of the rate gyro with dimensions and shows about which axis the angular rate is measured.



Figure 7: An isometric diagram of the BEI Systron Donner QRS-11 rate gyro17

Three rate gyros mounted orthogonally determine the satellite’s angular velocity vector. The physical and electrical requirements of the rate gyros and performance characteristics are shown in Table 13.

Table 13: Properties of the BEI Systron Donner QRS-11 rate gyro17



Description

Value

Mass

60 grams

Peak Power

2.1 W

Voltage

5 V

Dimensions

38.1  16.38 mm

Accuracy

3/second

Bandwidth

60 Hertz

An Ithaco IM-103 three-axis magnetometer is one of the sensors in the VASCAT attitude determination system. A three view drawing of the magnetometer is shown in Figure 8.


Figure 8: A three view drawing of the Ithaco IM-103 magnetometer11

A magnetometer measures the direction and magnitude of the local geomagnetic field. The measurement is compared to the same vector in the inertial frame for single-axis attitude determination. The measurement is used to determine the magnetic dipoles required to create a desired control torque using the magnetic torque bars. Table 14 summarizes the characteristics of the Ithaco IM-103 magnetometer.

Table 14: Properties of the Ithaco IM-103 magnetometer11



Description

Value

Mass

227 grams

Peak Power

1 mW

Voltage

15 V

Dimensions

5.5  4.2  3.6 cm

Accuracy

0.5

Frequency Response

3 dB @ >100 Hz

Field Measurement Range

600 mG

2.3.4.2 Control

The attitude control system of the VASCAT is a single-wheel momentum bias system with magnetic torque bars for momentum dumping. The momentum wheel is mounted so that its spin axis is along the pitch axis of the satellite. The purpose of the momentum wheel is to control the attitude about the pitch axis and stabilize the attitude about the roll and yaw axes.

The sizing of the momentum wheel is based on the maximum disturbance torque applied to the satellite, and the accuracy requirement of 2.5 degrees. The reaction torque capability must cope with the maximum total disturbance torque, estimated to be 56 N-m. The amount of stability required by the satellite and the maximum disturbance torque determines the momentum capacity of the momentum wheel.





2-1320

The disturbance torque, TD, the semi-major axis of the orbit, a, and the allowable angular deviation, a, affect the required momentum storage capability. 20 The calculations are performed using the values shown in Table 15. The momentum wheel is required to maintain 1.78 N-m-s of momentum.
Table 15: VASCAT orbital and environmental properties

Description

Symbol

Value

Semi-major axis

a

6778 km

Disturbance torque

TD

56 N-m

Allowable angular deviation

a

2.5

The momentum wheel used in the VASCAT is an Ithaco TW-4A12 momentum wheel. Figure 9 is a cut-away diagram depicting the internal configuration of this momentum wheel.



Figure 9: A cut-away diagram showing the interior of an Ithaco Type A momentum wheel11

This momentum wheel is capable of 12 mN-m of torque, which exceeds the maximum disturbance torque of 56 N-m. The maximum momentum capacity of the wheel is 4 N-m-s.11 The momentum wheel operates with at least 2 N-m-s of momentum stored at all times to satisfy the stability requirement. The mass and power properties and performance characteristics of the momentum wheel are summarized in Table 16.

Table 16: Properties of the Ithaco TW-4A12 momentum wheel11



Description

Value

Mass

3.46 kg

Motor Drive: 2.55 kg

Wheel: 0.91 kg


Peak Power

25 W

Dimensions

Motor Drive: 15  19  32 cm

Wheel: 20.5  6.4 cm



Momentum Capacity

4 N-m-s

Reaction Torque

12 mN-m

Steady State Power

Max. Speed: 9 W

@ 1000 rpm: 5 W



Speed Range

5100 rpm

Momentum dumping is performed using magnetic torque bars (MTBs). Magnetic torque bars induce a magnetic dipole that interacts with the geomagnetic field to create a torque on the satellite. 20 The MTBs are mounted orthogonally. This arrangement allows greater flexibility in inducing the direction of the magnetic dipole.



The MTBs are sized to provide sufficient torque to dump enough of the momentum wheel’s momentum within a reasonable amount of time. The maximum storage capacity of the momentum wheel is 4 N-m-s, and the minimum storage capacity required for attitude stabilization is 2 N-m-s. Therefore, a momentum-dumping maneuver is required whenever 2 N-m-s of momentum is built up in the wheel. The amount of added momentum stored in the wheel during a single orbit is calculated using the following equation.



2-1420

The momentum stored per orbit is 0.22 N-m-s. Approximately every 9 orbits a momentum-dumping maneuver is required. The maneuver for the VASCAT takes approximately 22 minutes. The torque required to accomplish this is 1.52 mN-m. The magnetic dipole necessary to meet the torque requirement is the magnitude of the torque divided by the magnitude of local magnetic field.17 The magnitude of the local magnetic field is assumed to be 4.5  10-5 Tesla in the worst case. Using the worst case results in a required magnetic dipole of 33.67 A-m2.

Ithaco TR30CFRs are the MTBs that are used by the VASCAT (shown above in Figure 10). These MTBs generate a 35 A-m2 magnetic dipole (Ithaco). This value meets the magnetic dipole requirement determined above. Other performance characteristics and physical and power requirements are presented in Table 17.



Figure 10: External configuration diagram of an Ithaco TR30CFR magnetic torque bar11



Table 17: Properties of the Ithaco TR30CFR magnetic torque bar11

Description

Value

Mass

1 kg

Peak Power

5.4 W

Voltage

26.2 W

Dimensions

49.6  2.3 cm

Linear Moment

35 A-m2

Saturation Moment

40 A-m2




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