The objective of this study is to identify and select one technology device which is suitable as best as possible to be used as controller for a digital reaction wheel in space applications.
Within this study the considered technologies are: Field Programmable Gate Arrays (FPGA), microcontrollers and Application Specific Integrated Circuits (ASIC) and Microcontrollers (MC).
During the first phase of this study, the theoretical requirements of a digital reaction wheel will be analysed and several devices - which are currently available on the market - will be investigated. Based on these results, one suitable device will be selected for the application.
In the second phase of the study, this specifically identified component is tested in a laboratory setup to be compared with the defined key requirements
In addition the ESA InDiCAtor study constitutes the entry into the design and qualification of a digital controlled electronics as additional option for the “family of systems” product line at Collins.
The digital controller (and digital interface) introduces the first major performance enhancements to the new design family. Because it is intended to re-use the hardware in the follow-up design improvements at Collins, a thorough and comprehensive requirements analysis and design trade-off is targeted.
Additionally, possible benefits and improvements that are enabled by new space technologies for electronics are evaluated to further reduce the cost of the electronics subassembly at Collins.
The following challenges are identified to be associated with the development and performance of the ESA InDiCAtor study.
- Despite theoretical compliance of a product (based on datasheet, algorithm check, simulation …), the performance or functionality when employed in actual hardware may not fulfil the defined requirements.
- The selected suitable component (identified during this project) becomes obsolete and is not available for later design phases.
- Long delivery time of evaluation items.
- Required resolution of A/D conversion for motor current measurement cannot be achieved with A/D converter of selected component.
- Requirements for torque controller software cannot be met with available components on the market.
- Time resolution for accurate speed evaluation cannot be realized with available clock rate of digital controller.
- Functional blocks executing safety or failsafe routines might demand very high resources in a digital control technology.
A digital reaction wheel design exhibits two main advantages.
Firstly the communication via a digital bus interface reduces overall hardware cost of the satellite, because data converting from analogue to digital and vice versa will not be required anymore, as well as allows an ideal commanding structure without errors as in an analogous system. This allows the attitude and orbit control system of any spacecraft to send an ideal torque command to the RWA.
Furthermore a digital interface increases the flexibility of data exchange. It could be possible to provide Collins customers different types of interface architectures like CAN or Mil-BUS 1553.
Secondly the integration of the digital controller inside the RWA allows a control loop structure that controls the reaction torque of the RWA rather than the motor torque. This shifts functionality from the AOCS inside the RWA and allows the torque control to be optimized from the manufacturer specifically for the RWA. Due to the enhanced torque controller design it is possible to increase the resolution of the torque command. Based on this higher resolution the pointing accuracy of the space ship can be increased. Further additional functionalities as extended status and health messages as well as self-test information are possible to implement, enhancing the RWAs functionality.
The replacement with a digital controller offers many benefits. The present-day functionality can be improved by additional functions, which only can be implemented in a digital device. Mandatory functions like the current controller is presently realized through multiple analogue components and can be realized in various digital devices. But it is part of this project to find the most suitable digital device regarding RCD wheel electronics to guarantee the satisfaction of the RCD customers. Therefore it is mandatory to preserve, respectively to increase the safety and reliability of RCD space wheels.
This project will investigate the improvement of the cost-to-benefit ratio of future space wheels, combining the heritage design of RCD reaction wheels and new interfaces / controller to provide the best solution for our customers.
High longevity space programs remain RCD’s primary focus. However, the newly coined and not yet well-defined “New Space” segment may benefit from the results of the InDiCAtor project as well.
The ESA InDiCAtor activities are grouped in a theoretical literature phase followed by a practical verification.
During the theoretical literature phase, requirements for core-functionalities and high-level functionalities are investigated and defined.
During the verification, first implementation steps and corresponding tests of core and high-level functionalities are performed, excluding the qualification of those.
- Literature research phase
- Assessment & definition of requirements for digital wheel controller based on Collins internal specifications, customer requirements and ECSS norms for future space reaction wheel missions.
- After the determination of core functionality, depending on the resources of the selected controller some hardware (e.g. memory, I/O’s, ADC) will be unused. Remaining resources can be used for high-level functions & these opportunities depending on different digital technologies are evaluated.
- Based on requirements definition a questionnaire will be generated to evaluate every digital controller separately
- Evaluation of available & representative parts on market for radiation hard microprocessors, FPGAs, and ASICs.
- Analysis of relevant function & building blocks of representative parts & evaluation of their specified performance & limits.
- Definition of digital controller systems depend on selected controller. That includes controller & required periphery components
- Based on these analysis results: evaluation of suitability of representative technology (microprocessor, FPGA, ASIC) for digital reaction wheel control & digital wheel interface application on system level
- Selection of suitable, most promising technologies & components for verification & proof of concept.
- Verification phase
- In a proof-of-concept hardware test-setup environment, verification of performance of one suitable component for digital wheel control (lab setup with OEM development boards).
- Verification of applicability based on literature study dealing with practical investigations in the field.
- Implementation & test of core functionality to investigate suitable component.
- Implementation & test of first approaches of high level functions.
The ESA InDiCAtor study is a device evaluation in relation to a digital controller for Collins space wheel electronics, which includes no hardware development but minor software development.
The main architecture for a digital reaction wheel comprises the mechanical hardware, the electrical hardware and the software:
- Mechanical Hardware
Function and features: Via the bearing unit the flywheel mass is connected to the motor. The motor delivers torque around the flywheel’s axis which results in the corresponding torque of RWA, which causes the rotation of flywheel mass.
Design Concept: BLDC motor and bearing unit with a directly connected fly wheel mass. All components share a common housing.
Critical Technologies: n/a
- Electronic Hardware:
Function and features: The electronic hardware on the PCB of the RWA incorporates all power electronics for supply voltage generation (DC/DC) as well as the power electronics required for motor control. The digital control chip is the heart and controls the DC/DC and Power Electronics. Further, a digital interface is integrated for communication with the space craft’s AOCS. The signal conditioning function block is part of the RWA electronics.
Design Concept: Multilayer PCB. Analog implemented power electronics function blocks controlled by digital function blocks
Critical Technologies: Rad-hard memory (data retention), power electronic devices (power dissipation), signal conditioning modules and controller architecture (SEE mitigation)
Function and features: The software includes the control of the reaction wheel and the interface to the spacecraft’s AOCS.
Design Concept: The software is divided into function blocks which exchange the corresponding information among each other to control the reaction wheel and guarantee the communication with the AOCS.
Critical Technologies: Digital wheel interface, motor current controller, PWM generation, commutation logic, motor current measurement, Hall sensor event detection, tacho event evaluation, safety functions, DC/DC control algorithm, reaction torque controller.
The ESA InDiCAtor study at Collins Aerospace is foreseen to be performed within a project period of 12 months, including the following planned review milestones, to be attended by the Agency’s representative:
- Mid-Term Review:
The MTR will show the results of the literal research in form of a formless technical presentation. It summarizes the content and the results of the requirements definition, the test structure, the technology investigation, the market analysis as well as the current state of technology evaluation and selection (excluding proprietary information).
- Final Review:
The FR will describe the major technical, operational and commercial accomplishments of this contract (excluding proprietary information), especially:
- objective of activity
- key issues
- results of work incl. description of new product / service developed
- main benefits
- perspective (further technical & commercial evolution).
Complementarily regular progress reports or meetings are held to present a summary of the current status of the activity and to report on any problems and schedule slippages.
The goal of selecting and verifying a digital controller unit (DCU) for a digital controlled reaction wheel assembly (RWA) was carried out in the scope of this study and is finished. As a result of the Market Analysis, Requirements Definition and Device Selection the GR716 space grade Microcontroller accompanied by a dedicated Timer ASIC was chosen as the technical, economic and strategic best solution for RCD RWAs. The GR716 is technical a very good fit for the applications and is free of ITAR and EAR export restriction. The Timer ASIC is important for the design because the GR716 is not capable of measuring the timings of the hall sensor signal edges required for rotational speed and reaction torque calculation. The additional development and part cost for a dedicated Timer ASIC have been considered during the selection process.
As verified by simulation and test activities, the chosen design approach is capable of achieving a 0.01rpm rotational speed measurement precision with a 10Hz update rate. It was further shown that a target conflict exists between rotational speed measurement precision, measurement update rate and flywheel moment of inertia. The flywheel inertia plays a role in the target conflict, because torque disturbances in the RWA cause a smaller rotational speed deviation for a higher moment of inertia, which influences the achievable rotational speed measurement precision.
The concept of a dedicated Timer ASIC was verified by a proof of concept prototype. The prototype was based on a FPGA Board with a custom implemented timer module. The captured data were compared against a high precision lab setup. Although future improvements are required, the general concept and design approach could be verified.
The presented work highlights the critical aspects of digital RWA control and verifies them against the respective requirements. This ultimately verifies and adds confidence to the chosen design approach. Further it allows the technical configuration of the target design for the next stages of product development.