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
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 5/30/2014 PISAT PDR 3
Subsystems 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 On Board Computer Telecommand Parabolic Antenna, S-band Tx and Rx ATMEL AVR 32 (AT32UC3A0512), 32-bit μc, On board 512 KB EEPROM PCM/ FSK, Data Rate: 100bit/second NANOCAM C1U Parabolic Antenna AOCS Telemetry Electrical Power System Structure & Thermal 128 words /frame, 8 bits Word length Data Rate: 10000bits/second Triple Junction Solar Cell, Li-Ion battery Dimensions and ejection system design under study, Thermal analysis to be carried out OBC EP S Mission & Ground Orbit determination using SGP4 model, Software Ground station software developed & 5/30/2014 implemented. PISAT PDR 4
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). 6 5/30/2014
ADCS Specifications ORBITAL REQUIREMENTS Altitude Inclination Orbit type 630km 97.866 deg 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) 5/30/2014 PISAT PDR 7
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. Detumbling Mode This mode is provided to reduce the body rates before entering the three axis attitude control mode. 5/30/2014 PISAT PDR Safe Mode Safe mode is transited during any unexpected contingency conditions, during which satellite may lose its attitude and result in onboard power loss 9
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
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