Controlling Laser Ranging with RTAI- based Real-Time Linux - NASA
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Controlling Laser Ranging with RTAI- based Real-Time Linux - NASA
proprietary real-time, Unix-like LynxOS for controller systems. ○ Several real-
time variants of Linux are available. ○. RTAI was chosen, as it is free (open.
Controlling Laser Ranging with RTAIbased Real-Time Linux
Evan Hoffman, HTSI Randall Ricklefs, UT/CSR
Introduction - 1
Current NASA laser systems use costly proprietary real-time, Unix-like LynxOS for controller systems Several real-time variants of Linux are available RTAI was chosen, as it is free (open source) and there is an active user community
Introduction - 2
Advantage of Linux: −
“Free”, stable, and secure
−
Development and many other tools are available with the distributions
−
Active user community and support
−
Many distributions to choose from
Disadvantage −
No vendor to blame if things go wrong :-)
RTAI (Real Time Application Interface)
Free, and supported Provides for soft and hard real time for interrupts and timers Requires modification of a standard kernel Does not require major rewrite of existing software Linux runs as guest of RTAI – not as bad as it sounds
Other Real Time Options • RTLinux • Commercial and Free version
Xenomi
Emphasis on extensibility and portability RTAI focuses more on lowest possible latency and interrupts
• RTAI seems to be under the most active development of the Free options
Concerns
RTAI is a number of kernel versions behind the state of the art: RTAI 3.8.1 support kernels up to 2.6.32.11 (2.6.38.2 latest) Documentation is less than coherent
Arch Linux • Advantages • Rolling release (latest bug fixes and features during development) • Feature rich package manager • Extremely customizable, can be as lean as you want it to be • Designed for simple configuration structure (BSD style)
• Disadvantages • Rolling release (updates require care and review) • Requires knowledge about Linux/OpenBSD system management (not a beginner distribution)
CentOS
Advantages −
Long term release based on RHEL means security updates are available for years
−
Updating process allows omitting updates for packages such as the kernel
Many tools and packages immediately available
Disadvantages −
Not lean
−
Slow (re)boot compared to LynxOS
−
Older kernel (2.6.22.1)
Other Linux Options • Ubuntu/Ubuntu Studio • Most popular distro, very user friendly • Some versions designated “Long Term Support” • Studio: soft real time, not hard real time • Bloat
• Debian • Reputation for stability • Very long release time
At the GGAO 48”... Major Code Ovehaul using: * the GTK+ toolkit (GUI) * RTAI shared memory and FIFOs to communicate between RTAI and regular Linux
MLRS Conversion of existing code Phase I – LynxOS to Linux
Replaced a number of library calls Fixed some memory violations (not caught under LynxOS)
Used existing Linux-based CAMAC PCI device driver
Wrote CAMAC simulator device driver (for Austin)
Installed patches for Blueheat serial driver into Linux serial driver module (kernel patch) Handled priority numbering differences from LynxOS (still in progress)
MLRS Conversion of existing code Phase II – Linux to RTAI Linux
Added RTAI kernel patches Added “helpers” to each RT thread to use RTAI scheduler in “user” mode May need to convert from POSIX to RTAI semaphores May require device driver modifications for real time
Current Status – GGAO 48”
Conversion to RTAI is part of an overall hardware and software modernization effort Software being developed to run with PCI bus and without the CAMAC interface Currently some analog signal controls require hard real time
Current Status - MLRS •
Controller software suite can now run under RTAI Linux –
•
Serial and CAMAC I/O and interrupts working properly •
Met sensors, radar, handpaddle
•
clock, timers, laser, detector
–
Hardware-simulated ranging (100% returns) works well
–
Have tracked stars and satellites
–
Cals look OK, but no satellite data yet (1 pass attempted)
Still have issues with hard real time
Conclusion
Linux and RTAI appear to be a good match for SLR controller upgrades Updates so far have been only slightly painful for various reasons More at the next workshop!
