katysat adcs update 2006 01 31

ADCS Team Update31 January 2006: 

ADCS Team Update 31 January 2006 David Stauffer Dave Matthews Alan Moore Bill Rice

Agenda: 

Agenda ADCS team requirements Potential requirements based on different designs Description of two paths forward Task list for each path KatySat Modular design Sensor and actuator data MEMS sun sensor MEMS yaw rate gyro

Requirements: 

Requirements ADCS Team Requirements The ADCS shall be able to determine the spacecraft attitude to an accuracy of 10 degrees in all 3 axes. The ADCS shall be able to determine the spacecraft attitude within TBD hours after deployment from the P-Pod. The ADCS shall be able to estimate the spacecraft attitude at a frequency of 1 Hz. The ADCS shall be able to output an estimated attitude to any on board subsystem. Requirements flowed down from Spacecraft The ADCS shall be designed for a 6 month expected lifetime. Related Requirements The bus shall be able to provide a timer accurate to within TBD (1?) seconds throughout the duration of the mission. The Comm System shall be able to relay the attitude when requested by a ground station.

Potential Requirements: 

Potential Requirements If using passive control such as magnets then, • The ADCS shall be able to passively control its attitude to 5 degrees in 2 axes. If using electromagnets or magnetic torquers then, The ADCS shall be able to provide 2 axes control to TBD (1-5?) degrees. If using thrusters then, The ADCS shall be able to measure rotational and translation accelerations in 3 axes to TDB m/s^2. The ADCS shall be able to determine the spacecraft attitude to an accuracy of 1 degree in all 3 axes and control the spacecraft attitude to an accuracy of TDB degrees in all 3 axes.

Two paths forward: 

Two paths forward There are two paths being investigated KatySat This path deals with finishing the capabilities that the previous year’s KatySat team had planned – attitude determination using solar cell currents and a magnetometer, and possibly using permanent magnets for a passive two axis attitude control. Modular Design This path assumes we would be either adding sensors and actuators to the KatySat cube or integrating into an extra half cube, possibly combined with the thrusters group, for a more precise attitude determination and possibly active three axis attitude control system.

Task List – KatySat path: 

Task List – KatySat path Review or add requirements to the ADCS system for KatySat. Write the software task to read solar panel current. Write software to calculate sun angle based on solar panel current. Determine where the magnetometer is going and integrate it to a board. Write the task to read the magnetometer data. Write software to determine the magnetic field based on magnetometer data. Complete any integration tasks still remaining (hardware and software). Write the software to calculate the attitude based on orbit, time and sun angle. Develop and run simulation to test attitude determination algorithm. Write a task to output attitude to the ground or other subsystems that want it. Write software to calculate attitude based on orbit, time and magnetic field. Combine solar panel current and magnetic field methods to improve attitude determination.

Task List – Modular path: 

Task List – Modular path Determine what other sensors would be useful to add (such as a rate sensor like a MEMS gyro). Integrate new sensors (hardware and software integration). Write tasks to measure new sensor data. Expand attitude determination software to incorporate new sensors. Determine what actuators or passive control systems would be feasible. Integrate actuators to board. Write tasks to use actuators. Expand software to incorporate control system.

MEMS Sun Sensor: 

MEMS Sun Sensor The DTUsat project, a CubeSat project of the Technical University of Denmark, involved the design of a sun angle sensor on a single chip. The sensors are 7 by 8 mm, and weigh 116 mg, and are able to determine the direction to the sun with an accuracy of under 1°. The fundamental unit of the sensor consists of a pair of triangular photocells exposed to the sun through a narrow slit. As the unit is rotated about the axis of the slit, the relative areas of the two photocells exposed to the sun changes. Relative exposure to the sun is independent of rotation about the perpendicular axis, over a fairly wide range of angles. A pair of these units arranged perpendicularly to each other then gives the direction to the sun in both axes, over a range of angles greater than 90 degrees. Such a pair mounted on each face of the cubesat gives complete coverage of the celestial sphere, and permits the attitude of the cubesat to be determined to within 1 degree or better, with a single degree of freedom (the cubesat can be rotated about the satellite-sun axis without altering the sun’s direction). The illustration below is a photograph of one of the sensor pairs mounted on a circuit board. The boards contemplated for KatySat’s external use appear to offer ample space for mounting such devices.

MEMS Yaw Rate Gyro: 

MEMS Yaw Rate Gyro

MEMS Gyro Highlights: 

MEMS Gyro Highlights Use 3 gyro sensors to sense angular rates in 3-Axis Angular rates can be integrated to update spacecraft attitude during eclipse Two auxiliary analog input channels can interface with non-SPI sensors Small 8x8 mm footprint Low Power ~40mw On chip A/D converter Serial SPI interface Wide temperature range

ADCS Processing Scope: 

ADCS Processing Scope Short-Term: Rotation of S/C body, relative to Sun and Earth. This can be sensed and controlled using local (on-board) time-synchronized events with minimal algorithmic complexity and control energy. Mid-Term: Orientation of S/C body relative to external reference frame(s) (e.g., alignment of S/C axes with Nader and/or Earth poles). This requires some knowledge of “orbital” S/C time (possibly by upload), with greater processing complexity and control energy. Long-Term: Orbital maneuvering / stationkeeping. This requires both local and “orbital” S/C time synchronization, with the greatest processing complexity and control energy.

ADS Processing Tasks: 

ADS Processing Tasks Verify / Update Timer, as needed Read Command Data Decode, scale and “float” Command Data, as needed Read Sensor Data (synchronized with Timer) Decode, scale and “float” Sensor Data, as needed Calculate Position and Rate information, updating Quaternians (or Keplarian Vectors). (This involves calculating sines and cosines, square roots and some simple matrix algebra.) Format Telemetry Data Queue / Transmit Telemetry Data

Additional Control Processing: 

Additional Control Processing Verify / Update Timer, as needed Read Command Data Decode, scale and “float” Command Data, as needed Read Sensor Data (synchronized with Timer) Decode, scale and “float” Sensor Data, as needed Calculate Position and Rate information, updating Quaternians (or Keplarian Vectors). This involves calculating sines and cosines, square roots and some simple matrix algebra. Calculate Control Outputs (Wheel Speed(s), Torquer Coil Current(s), Thruster Firing Time(s), etc.). This is more complex than calculating Position and Rate information, requires the equivalent of a matrix inversion, and probably involves highly non-linear control functions. Format Control Outputs Queue / Transmit Control Outputs (synchronized with Timer). Format Telemetry Data Queue / Transmit Telemetry Data

ADCS Timing: 

ADCS Timing Sensor Data Measurement Synchronization- “like” data should be measured at fixed intervals (“jitter” can be filtered-out (as with over-sampling), but winds up as noise. Control Actuator Output Synchronization- more or less important, depending on type of actuator and on magnitude of actuator output relative to mass or inertia of S/C. ADS Sensor Input Timing Requirements- minimum sampling rate based on Nyquist’s Law. ADCS Control Process Timing Requirements- related to synchronization of actuator outputs, this is the minimum control process cycle rate required for Control Law Stability. Initial “WAG”: Try 32Hz Processor cycle (needs analysis).

ADS/ADCS Process Home: 

ADS/ADCS Process Home MSP430 Base Processor- Will ADS/ADCS overburden the Pumpkin/CubeSat processor? Auxiliary Processor Options- Ground Processing? Another MSP430? A Different On-Board Processor? A General Purpose Processor? A Specialized Processor (e.g., a TMS320VC5401 or 5507 DSP)?

TMS320VC5401 Block Diagram: 

TMS320VC5401 Block Diagram

TMS320VC5507 Block Diagram: 

TMS320VC5507 Block Diagram