The scanning microwave limb sounder (SMLS) will use technological improvements in low-noise mixers to provide precise data on the Earth’s atmospheric composition with high spatial resolution. This project focuses on the design and implementation of a real-time control system needed for airborne engineering tests of the SMLS. The system must coordinate the actuation of optical components using four motors with encoder readback, while collecting synchronized telemetric data from a GPS receiver and 3-axis gyrometric system. A graphical user interface for testing the control system was also designed using Python.
Although the system could have been implemented with an FPGA(field-programmable gate array)-based setup, a processor development kit manufactured by XMOS was chosen. The XMOS architecture allows parallel execution of multiple tasks on separate threads, making it ideal for this application. It is easily programmed using XC (a subset of C). The necessary communication interfaces were implemented in software, including Ethernet, with significant cost and time reduction compared to an FPGA-based approach.
A simple approach to control the chopper, calibration mirror, and gimbal for the airborne SMLS was needed. The XMOS board allows for multiple threads and real-time data acquisition. The XC-2 development kit is an attractive choice for synchronized, real-time, event-driven applications. The XMOS is based on the transputer microprocessor architecture developed for parallel computing, which is being revamped in this new platform.
The XMOS device has multiple cores capable of running parallel applications on separate threads. The threads communicate with each other via user-defined channels capable of transmitting data within the device. XMOS provides a C-based development environment using XC, which eliminates the need for custom tool kits associated with FPGA programming. The XC-2 has four cores and necessary hardware for Ethernet I/O.
This work was done by Robert F. Jarnot of Caltech and William J. Bowden of the University of British Columbia for NASA’s Jet Propulsion Laboratory. The software used in this innovation is available for commercial licensing. Please contact Daniel Broderick of the California Institute of Technology at
This Brief includes a Technical Support Package (TSP).
XMOS XC-2 Development Board for Mechanical Control and Data Collection
(reference NPO-48054) is currently available for download from the TSP library.
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Overview
The document is a Technical Support Package from NASA's Jet Propulsion Laboratory (JPL) detailing the XMOS XC-2 Development Board, which is utilized for mechanical control and data collection in the context of the Atmospheric Scanning Microwave Limb Sounder (A-SMLS) project. The A-SMLS is designed to collect thermal emissions and synchronize data with the spatial location and orientation of the detector, which is crucial for atmospheric studies.
The data acquisition process involves counting index marks and determining the correct system response, which includes controlling the gimbal and setting the mirror position. Telemetric data is collected and transmitted to a host computer at specified data rates. The system enters an idle state until an initialization command is received.
The mechanical control system is implemented using the XC-2 development board, chosen for its capabilities over traditional FPGA solutions. The control system coordinates four motors that actuate various optical components and manage telemetric data from GPS receivers and three-axis gyro packages. This coordination is essential for defining atmospheric scan paths and regulating the sampling of radiometric reference views.
The document also discusses the interfacing requirements of the XMOS board, which connects to various system subcomponents through serial connections, including RS-422 for microcontroller units (MCUs) and RS-232 for gyro packages. Specific processor cores are designated to control different systems, ensuring efficient operation.
Future design modifications are anticipated, particularly regarding the calibration of system subcomponents. The chopper attachment for the motor shaft and the scan mirror for the gimbal are currently unavailable, which limits precise calibration. Once these components are available, various time constants will need to be measured, including the time to reach an angular velocity of 10 Hz and the timing between index marks and the chopper.
The document emphasizes the importance of the Ethernet connection between the XMOS board and the host computer, which is based on example source code from XMOS. Adjustments have been made to ensure smooth data transmission and reception.
Overall, this Technical Support Package provides a comprehensive overview of the A-SMLS control system's design, implementation, and future considerations, highlighting its significance in advancing atmospheric research and technology.
