Team Vision for System-Level Design PhaseWhat did the team plan to do during this phase?
- The team set out with the goal of completing all systems-level design documents. This involves creating a transform diagram, function tree, risk management, and concept selection Pugh chart. By the end of the phase, the goal was to have all concepts selected for moving forward into design. The team also added a goal of creating the Design of Experiment (DOE) for our space mission while in orbit. This further helped us narrow down on concept selection and systems design.
- Design of Experiment
- Concept Selection:
- Subsystem Functions
- Selection Criteria
- Morphological Chart
- Pugh Chart Screening Matrix
- Feasibility Analysis for Orbit and Sensors
What did the team actually accomplish during this phase?
- The team accomplished all goals that were set out for, as well as additional experimental analysis and feasibility studies on conceptual systems. Prototyping was done in the lab to begin work on the power system, the attitude control system, and experimental detection of the diffractive elements force. Analysis and simulation were done for the mission orbit, plan, and power system. Overall, the systems-level design phase was a success for the team as goals were met and exceeded.
- Feasibility Analysis on:
- Sun Tracking Sensors
- Velocity Sensing
- Power System Testing
- Orbital Parameters
- Experimental Torque
- System Architecture for Avionics
- System Architecture for Power Subsystem
Functional DecompositionA functional decomposition chart was created to help us better understand the customer requirements our CubeSat needs to fulfill. Our sub-functions branch off of two major functions - performing the diffractive experiment and performing mission operations. Our two main customer requirements are satisfied with these top-level functions. Dr. Barbosu and the SPEX customer would like a CubeSat from RIT to orbit the Earth (perform satellite operations) and Dr. Swartzlander would like our platform to support a diffractive solar sailing experiment.
- Collect sensor data for attitude determination
- Control actuators to correct attitude
- Collect solar energy to provide power for electronics
- Communicate with a ground station
- Deploy the solar sail successfully
- Keep the solar sail oriented normal to sun rays
- Measure rotational torque due to diffractive sail elements
The second photo is a transform diagram showing the inputs and outputs of our system and how they relate to our sub-functions.
BenchmarkingExisting solar sailing missions were explored, as well as a few CubeSats developed by nearby universities. Our product is most like the LightSail 2 launched by the planetary society. No one has done the diffractive element experiment before, so there is not a direct comparison of that experiment, only the solar sailing deployment and control as a whole, or CubeSat as a whole.
Rough figures of sensor accuracy and price points were captured in the following table to benchmark and recognize the feasibility of these devices in accomplishing the CubeSat functions.
System FunctionsUsing the Transform Diagram, system functions for which concepts can be brainstormed were defined. Each one was related to a specific customer requirement and its attached engineer requirements.
Design of ExperimentOur current design of the experiment is based on optimizing the amount of perpendicular exposure to the sun's light the sail receives during its orbit. The vector parallel to the sun's rays and running through the center of our sail/CubeSat longitudinal axis is called the heliotropic axis. Heliotropism is a property of certain flower species to move throughout the day to stay pointing at the sun. We have taken the concept of heliotropism to our CubeSat mission design, not only to maximize the effect of the solar radiation on the diffractive experiment but also to simplify the role of the Attitude Determination and Control System (ADACS) which will only have to make corrections to keep the sail pointing at the sun. This rules out the necessity for more complicated in-orbit maneuvers and orientations. The heliotropic axis will be where the diffractive torque is felt and captured with on-board Inertial Measurement Unit. An array of magnetometers is the most feasible method to capture this small experimental torque. Over multiple orbits, the magnetometers will be measuring oscillations in CubeSat's orientation in the magnetic field of the Earth. A small torque will become more substantial over time and contribute to an increase in the frequency of magnetic field oscillations caused by an increase in rotational velocity about the heliotropic axis.
Feasibility: Prototyping, Analysis, Simulation
Brushless DC Motor
Lab bench testing of a quad-copter brushless DC-motor was performed to determine if it was feasible to use for testing of spinning masses for the reaction wheel. Tests determined the motor was too powerful for the needs of ADACS prototyping and too difficult to control to the level of detail needed. We quickly programmed an FPGA to output an adjustable PWM signal to the motor controller. Resolution of PWM is down to 1% of a duty cycle, however, further resolution is easily implementable for finer motor control. Once we begin to set up our microcontroller environment, PWM control for testing will migrate to the target of our primary architecture under development.
Our FPGA implementation of the PWM source allows for the Duty Cycle to be input digitally using the development board's switches. As you can see from the simulation results below, the frequency required by the motor controller is maintained while the percentage of the duty cycle where the signal is high changes with respect to the switch's value.
Sun SensorCoarse sun tracking can be performed by measuring the angle of the sun to the spacecraft in the X and Y-axis. We can accomplish this with simple light-dependent resistors (LDRs), modeling a window in CAD we have 3D printed a functional prototype. Tests demonstrated that measurements of incident light angle can be made in both axes by the method of analog differential measurement of two sensors. This is a cheap way to determine the rough direction of the sun relative to spacecraft.
A fine sun sensor with higher precision is more expensive but will provide precision for small attitude correction. Since our mission plan requires the satellite to spin along the axis parallel to the sun's rays, our sun sensor must be located along the axis of rotation in the front of our sail. This level of precision of the spacecraft's spin (and precession) will help guide the attitude actuators to a normal spin axis pointing the sail orthogonally to the sun.
The sun sensor's price will determine the level of precision as flight-tested sun sensing equipment can cost over $10,000. Implementing our own sensors is much cheaper but will require much more work to validate and calibrate under an appropriate testing environment. Ideally, our team would like to take the approach of finding cheap, reliable methods employed in published nano-satellite orbits. The International Journal of Aerospace Engineering has published methods of sensing solar angle which can be utilized for course (+/-5 degree) measurements.
MagnetometerWe prototyped two magnetometers angled at 180 degrees from each other; the Earth's magnetic field was measured using a differential amplifier in the analog domain. This cancels out any slight variation in the magnetic field, assuming the difference between the sensors is negligible (2"), to obtain an accurate relative velocity measurement over a long period of time at very low frequencies.
We plan on using an array of tri-axis magnetometers across our Intertial Measurement Unit (IMU) to sense our rotation and help guide torque movements during detumbling before sail deployment. Relatively accurate tri-axis magnetometers are available for less than $10 / piece. The most concerning hurdle to deal with is the calibration routine which must be developed and controlled by the microcontroller of the flight controller to interpret magnetic orientation. This hurdle must be overcome if we are to use any magnetic solutions, also including magnetorquers. Again, The International Journal of Aerospace Engineering has published methods of in-orbit triaxis magnetometer calibration.
Accelerometer & GyroscopeAccelerometers on the earth's surface, have the ability to measure orientation with respect to the force of gravity. Satellites in orbit do not have this luxury since they are experiencing free fall. Therefore, accelerometers will be utilized as feedback from the torque mechanisms such as reaction wheels. Accelerometers will also require self-calibration routines as the ADAC System comes online. Also like magnetometers, they are relatively cheap for the accuracy desired. Accelerometers are not accurate enough to measure the experimental torque provided by the diffractive elements on the sail, so they will not be employed in measuring experimental data, only as feedback to the ADACS of torque effects.
Power SystemFor testing of the power management system on the CubeSat, a solar cell simulator was designed and constructed with settings allowing for all characteristics of a solar cell to be changed and tested. The simulator is capable of simulating different lighting conditions, efficiency, number of solar cells in parallel and series in the array, and temperature effects.
Orbit PredictionsBased on research into the orbits of other CubeSats and available launch vehicles, we have selected values for a possible orbit. There are several launch vehicles that can provide a Sun- Synchronous Orbit (SSO) at 600 km including several provided by Spaceflight. This orbit will have an inclination of 97.8 degrees. We are assuming an eccentricity of 0.00001 based on the orbit of several other CubeSats at that altitude. These values will give us an orbital period of 96.7 minutes. This orbit will always have a view of the sun and out of areas of high atmospheric drag while ensuring that communications will be possible over Rochester. The frequency of communication passes is yet to be determined. Higher altitudes would be preferred, launch permitting.
Preliminary torque calculations from the diffractive sails in the z-axis are shown below. It is assumed that all sail is diffractive, and the force location is at the end of the sail. These are preliminary calculations before further analysis is done.
Morphological Chart and Concept SelectionNext, we took the sub-functions from our transform diagram and brainstormed ideas for each one. Shown below is our concept matrix detailing these ideas.
Designs and Flowcharts
Risk AssessmentThe risk management list has been updated to reflect new concepts generated and system-specific items. Each team member has been assigned a risk to manage and thought up careful considerations to exercise in order to minimize their risk.
Design Review Materials
- This page serves as the pre-read material for the Systems Level Design Review.
- Systems Level Design Review Presentation
- Notes from review
- Action Items
Plans for next phaseBy the Preliminary Detailed Design Review
- A draft of the bill of materials.
- CAD modeling of modified deployment mechanism
- CAD modeling of modified hinge
- Detailed torque calculations
- Draft of missions operations
- Draft of design of experiment
- Prototype power system working on lab bench.
- ADCS detailed calculations
- Rough CAD model of reaction wheel.
- Component selection for final system.
- Detailed orbital simulations
- test plan
- compute detailed torque model
- complete orbital simulations
- placement and bonding of diffractive elements
- draft of mission operations
- CAD sail and deployment mechanism
- Complete mathematical model for gyroscopic stabilization of the CubeSat using reaction wheels
- Manufacture a reaction wheel
- Test reaction wheel with brushed motor to validate mathematical model
- CAD model of reaction wheel initial concept design
- Figure out sail folding procedure.
- Finish draft of diffractive experiment design.
- Re-design panel hinge and make a prototype
- CAD sail and deployment mechanism
- Select sensors for further prototyping
- Choose microcontroller and setup development environment
- Develop test plans for hardware/software
- Explore testbed ideas for ADACS testing/Imagine demo
- Choose specific microcontroller family.
- Draw up specific software flow.
- Work on test plan for controller based on sensors.
- Complete a lab bench prototype of power subsystem.
- Hand off first processor board for prototyping flight software to software engineers, as well as SPEX communications team.
- Begin design on attitude actuation drivers and evaluation of determination sensors.