Implementation of three axis magnetic control mode for PISAT

Shashank Nagesh Bhat, Arjun Haritsa Krishnamurthy Student, PES Institute of Technology, Bangalore

Prof. Divya Rao, Prof. M. Mahendra Nayak

CORI Lab, PES Institute of Technology, Bangalore

Dr. V.K Agrawal Director,

CORI Lab, PES Institute of Technology, Bangalore

PISAT Overview

5/30/2014 PISAT PDR 2

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Low Cost Three Axis Stabilized Imaging Satellite

MAIN FEATURES: • Imaging payload (2048 x 1536

pixels), High resolution images of earth • Three axis stabilized with active magnetic control • S-band Communication system. • AVR 32 microcontroller for OBC. • Structure dimensions: 254 x 226 x 181 mm. • In-house developed subsystems

except payload

Subsystems

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NANOCAM C1U Parabolic

Antenna

AOCS

OBC

EPS

Subsystem Features

Payload NANOCAM C1U (CMOS, 2048 X 1536)

ADCS Magnetic Torquers, Sun sensors, MEMS Inertial sensor and 3 axis Magnetometer, Three axis stabilized.

RF & Ground System

Parabolic Antenna , S-band Tx and Rx

On Board Computer

ATMEL AVR 32 (AT32UC3A0512), 32-bit μC, On board 512 KB EEPROM

Telecommand PCM/ FSK , Data Rate: 100bit/second

Telemetry 128 words /frame , 8 bits Word length Data Rate: 10000bits/second

Electrical Power System

Triple Junction Solar Cell, Li-Ion battery

Structure & Thermal

Dimensions and ejection system design under study, Thermal analysis to be carried out

Mission & Ground Software

Orbit determination using SGP4 model, Ground station software developed & implemented.

ADCS Configuration and Three Axis Mode Design Flow

PISAT ADCS Configuration

• PISAT is designed with an imaging payload.

• PISAT is a three axis stabilized satellite with active magnetic control system.

• PISAT is equipped with three magnetic torquer rods along all three axes as actuators.

• Sensors Selected: MEMS based Inertial Measurement Unit (IMU) with magnetometers, gyroscopes and accelerometers(range is about ±3,50,000 nT, initial sensitivity of 50 nT and initial bias error of ±400 nT) , Four Pi Steradian Sun Sensor (FPSS).

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ADCS Specifications

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ORBITAL REQUIREMENTS Altitude 630km

Inclination 97.866 deg

Orbit type Polar sun synchronous

Payload Pointing Accuracies Yaw 5 deg (3 sigma) Roll 5 deg (3 sigma) Pitch 5 deg (3 sigma)

Drift Rate (max) 8.72*10-2(3 sigma)

ADCS Mode transition under normal operation

Suspended Mode • This mode starts once satellite separates from the launcher • All the Magnetic Torquer’s are disabled • The operations initialized in this mode by OBC are:

• Activate Telemetry function (if any) • Activate Tele-command function • Monitor magnetic field data, rate data, temperature and other

sensor data.

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Detumbling Mode • This mode is provided to reduce the body rates before

entering the three axis attitude control mode.

Safe Mode Safe mode is transited during any unexpected contingency conditions, during which satellite may lose its attitude and

result in onboard power loss

Three Axis Magnetic Control • This mode ensures proper power generation and payload

operation.

ONBOARD DESIGN OF THREE AXIS MAGNETIC CONTROL MODE

• The magnetometer and gyro data is produced independently by IMU

• The orbit model provides the state vectors (position and velocity) in Earth Centered Inertial (ECI) and Earth Centred Fixed (ECF) reference frames.

• The attitude determined in terms of quaternions is based on the construction of attitude profile matrix which requires the set of four normalized reference field vectors and bias eliminated magnetometer measurement vectors, each vector is spaced at an interval of 4 sec.

ONBOARD DESIGN OF THREE AXIS MAGNETIC CONTROL MODE(Continued…)

• The attitude error is corrected based on the dipole moment computed and the corresponding control torque is provided in duty cycle mode.

Implementation of Design on OBC

OBC Software Cycle

IMPLEMENTATION AND TESTING

• Each Module is written as function in the C language using Atmel Studio 6 as IDE which was then implemented on ATMEL AVR 32 (AT32UC3A0512).

• Control Byte is assigned to each module to make sure that modules are executed as per the OBC Software cycle.

• Percentage error calculation done for each module , making sure that they are within agreeable limits.

• Time of execution is measured for every module at clock speed of 12 MHz

• Tested using both single precision and double precision floating point. single precision floating point variables were found to be suitable for the computation giving a good accuracy as well as a suitable execution time.

IMPLEMENTATION AND TESTING(continued…)

• overall accuracy and execution deadline has to be maintained to ensure proper orientation. single precision arithmetic was found to be suitable for the computation giving a good accuracy

• Up to 99.99% accuracy was observed in All the tested modules.

Reduction of execution time for Reference Magnetic Field Computation • Consisted 5 cosine and 4 sine functions which needs to be

executed in a loop for 105 times to get a single value of reference magnetic field.

• Use of inbuilt trigonometric functions from math library uses Taylor’s series for calculating trigonometric values.

• Execution time observed with use of inbuilt trigonometric functions mounted upto 2056.32μsec.

• Look up table designed for each of the trigonometric function with a resolution of 0.001 rad for input angles.

• Execution time reduced to 172.33μsec after using look up table.

Execution time of three axis magnetic control

Three axis magnetic control mode module distribution

Output Table of Magnetic Bias Estimation and Correction

Thank You