Adaptative high-gain extended Kalman filter and applications

tel-00559107, version 1 - 24 Jan 2011
4.3 real-time Implementation
to be hard real-time whenever the completion of an operation after its deadline is considered
useless. When limited delays in the response time can be accepted, or in other words when
we can afford to wait for the end of computations, the system is said to be soft real-time.
The most common approach consists of defining a real-time task and using a clock that
sends signals to the system at a frequency set by the user. Each time a signal is sent by the
clock, a real-time task is launched whatever the conclusion of the previous task. In other
words, a hard real-time system does not make a task conditional with respect to the real-time
constraints. The system forces the designer of the task to respect the real-time constraints.
We chose a Linux based real-time engine provided with a full development suite: RTAI-Lab
[34].
RTAI-Lab is composed of several software components including:
− RTAI [5]: RTAi is a user friendly RealTime Operating System.
The linux O.S. suffers from a lack of real time support. To obtain real time behavior, it is
necessary to change the kernel source code. RTAI is an add-on to the Linux Kernel core
that provides it with the features of an industrial real-time operating system within a
full non real-time operating system (access to TCP/IP, graphical display and windowing
systems, etc...).
Basically RTAI is a non intrusive interrupt dispatcher, it traps the peripheral interrupts
and when necessary re-routes them to Linux. It uses a concept called Hardware
Abstraction Layer to get information into and out of the kernel with only a few dependencies.
RTAI considers Linux as a background task running when no real time activity
occurs.
− Comedi [4]: Comedi is a collection of drivers for a variety of common data acquisition
plug-in boards. The drivers are implemented as a core Linux kernel module.
− Scilab/Scicos [6–8, 41]: Scilab is a free scientific software package for numerical computation
similar to Matlab. The software was initially developed at INRIA and is now
under the guidance of the Scilab consortium (see the history section of [8]).
Scicos is a a graphical dynamical system modeler and simulator (or Computer Aided
Control System Design Software) developed by the group METALAU at INRIA. It
provides a block oriented development environment that can be found either embedded
into Scilab or in the distribution ScicosLab 13 .
− RTAI-Lib [5, 34]: RTAI-Lib is a Scicos palette, i.e. a collection of blocks to use with
Scicos. This palette is specific to the real-time issues RTAI is dealing with. These
blocks can be used to generate a real-time task (which is not the case of the regular
Scicos blocks).
− Xrtailab [34]: Xrtailab is a oscilloscope-like software that takes care of communications
between the non real-time part of the platform (i.e. the Linux O.S, the graphic displays)
and the real-time executable when it is active. With Xrtailab, it is possible to plot and
record signals and change online the parameter values of the simulation blocks (e.g. PI
and PID coefficients, activate braking).
13 Notice that recent versions of Scilab (2010) doesn’t seem to include Scicos anymore but some similar
utility called xcos.
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