ADCS

Introduction

The ADCS subsystem serves two primary purposes: a) to orient the spacecraft’s solar panels towards the sun and b) to illuminate the payload samples with sunlight.

Simple three-axis control will be obtained using electromagnetic torque rods (developed by York University) for actuation. Dual-redundant three-axis chip-style magnetometers and gyros will supply magnetic field and attitude rate information.

Iris-SAT is assisted with two fine sun sensors and solar arrays. Two sun-sensors from York University will provide fine-pointing sun direction telemetry, while solar panel currents will provide a rough estimate of the sun direction in off-nominal attitudes where the sun is not pointed to the front face of the spacecraft.

The ADCS subsystem hardware interface will consist of the main microcontroller with an SPI interface to the CDH computer. This microcontroller is used for controlling and powering the sun sensors, sensors (magnetometers and gyros connected to microcontrollers via SPI interface) and magnetorquer. The following figures illustrate the functional block and system block diagrams for the ADCS subsystem.

ADCS attitude control strategy will be conducted mostly in a Matlab / Simulink simulation environment.

ADCS Hardware Block Diagram

Functional Block Diagram

PD Attitude Control Strategy

This section presents the PD controller designed for the Manitoba Sat-1 which uses magnetic actuators.

Pointing Budget

The sun sensors' accuracy should be 10 times better than the pointing requirement. The maximum satellite attitude tolerance is +-24 degrees and therefore the sun sensors pointing error should be less than +-2.4 degrees. In Iris the sun sensors provide an accuracy of 0.5 degrees which fulfill the pointing budget.

ADCS Hardware Design

The maximum height of the board including all components is 9.4mm +/- 0.1mm .

Parts List and Bill of Material

In the following the parts list is shown for both LOTAD(UMS-0054) and MOLTRESS(UMS-0380). Each part contains the component along with the part number, the manufacturer and bill of material.

Sun Sensors

York University is developing a novel low-cost sun sensor applicable for Nano-satellites with 0.1-degree accuracy and reliability. The current design consists of two orthogonal arrays of photodiodes with a pattern of slits (shown below) to allow greater precision in sensing and calibration. Micro-fabricated sun sensor will be first demonstrated on the Iris mission to showcase not only the novel concept of low-cost micro-sun sensor but also the micro-fabrication approach in developing satellite attitude sensors. The sun sensors run on an SPI connection and they are digital. Depending on the output you require, 8,4,2,1.5 and 1-bit output modes are available. The sun-sensors would be tied to the common ground bus. The specifications of the sun-sensors as suggested by York is mentioned on the right of the document. The survival and operating temperature ranges of the low-cost sun-sensors are [-50,+150 C] and [-40,+105 C].

Redundant Sensors

There are redundant sensors used to prevent failure from radiation and external torques. Double redundant sensors for the gyros and magnetometers are used. Similarly, two fine sun-sensors are utilized.

Torque Rods

Torque rods, provided by the York University team, are based on the rod concept described in the design page. The proposed design will be demonstrated on the SIGMA mission. The magnetorquers were controlled by PWM signals. For the PWM signals, we included h-bridge circuits. The torque rods will be tied to the common ground bus. The torque rods shall be mounted orthogonally. The maximum operational temperature ranges for the torque rods are 100 C. During the last mission done by York University they had mentioned about the same specification they would use for designing the rods for Iris mission which has a magnetic dipole of 0.1 Am Sq. During torque rod sizing we estimated the power consumption to be around 0.6 W. For each torque rod, a one to one mapping for Dipole vs. Power and Dipole vs. Current are shown in the following images.

Magnetometers

The MEMSIC, high performance and a low cost 3-axis magnetic sensors which has a field range of +/- 8 Gauss with 16 bits operation. This was chosen because it requires very less external components and has a dynamic range with a better accuracy. The RMS noise is 0.4 mG giving a heading accuracy of +/- 1°. Dimensions are 3.0×3.0×1.0 mm which is easy to fit in 3U form factor of the satellite. The main purpose for using this is that it clears the residual magnetization coming from the strong fields and it also has a built temperature sensor for monitoring the temperature throughout the mission. The power requirements are pretty low which suits for the CubeSat mission. It has a 1 µA of power down current and 2.5 V single low power supply. It has a voltage of 1.8 V with I2C interface.

Gyroscopes

The gyros used for the Iris mission are COTS having the model number A3G4250D from STMicroelectronics. They also include an adaptor board with them. These gyros are 3-axis, chip style and surface mounted on the CubeSat with the voltage range of 2.4 V to 3.6 V , eventually having a low power consumption. They work on the SPI/I2C interface and work on 16 bit rate data output.

Detailed Schematics

The LOTAD and MOLTRESS are designed as the ADCS hardware. LOTAD is referred to ADCS PCB(UMS-0054) and MOLTRESS(UMS-0380) points to sun sensor boards.

Phase C Test Plan(EM)

ADCS Budget

Assembly Plan

The following document is the assembly plan including the board assembly, harnessing and mounting the board to the shell.

ADCS Work Packages for Phase C/D

The work packages and the schedule of remaining qualification activities to be performed in Phase C and D are detailed below.

Risk Registry

Single point Failure Analysis

The single point failure(SPF) for ADCS is the failure of torque rods. In order to eliminate the single point failure, the torque rods shall be properly calibrated and tested through out Phase C and D according to the test plans. Regarding the high reliability of magnetorquers, we do not have any redundant actuators.