PAGE CONTENTS
Objectives
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.
Challenges
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.
System Architecture
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)
- Software:
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.
Plan
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.
Current Status
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.
