Embedding Linux to Control Accelerators and Experiments
Linux is being used at the European Synchrotron Radiation Facility to build distributed embedded controllers. The embedded systems are either PC/104-based systems, which boot from a flashdisk, or VME crates which boot from the network. The devices being controlled vary from serial lines to stepper motors to CCD cameras. The control software is written using an object-oriented toolkit that we developed, called TACO (Telescope and Accelerator Control Objects). Using TACO all control points are implemented as device objects. This article will describe how we have implemented embedded controllers using Linux and present some examples.
What is an embedded controller? As the name indicates, an embedded controller is a mixture of control hardware and software which is embedded, i.e., integrated, into the equipment it is supposed to control. Examples of where embedded controllers can be found are abundant in daily life: printers, portable telephones, the brakes of your car, etc. The requirements for embedded controllers are usually that they be physically small, have a small memory footprint, low power consumption and low cost. Most of these requirements are dictated by the fact that controllers will be built into hardware systems and in large production volumes.
Figure 1. Aerial view of the ESRF nestled between the Drac and Isere rivers at the foot of the French Alps (Grenoble).
At the European Synchrotron Radiation Facility (ESRF, see Figure 1), our main job is to connect a wide variety of control hardware with an equally large number of different interfaces. We are faced with the task of interfacing hundreds of power supplies to control the accelerator magnets which guide the electrons, thousands of stepper motors which move and position the experiments on the beamlines and a myriad of other devices. Most of these devices contain embedded controllers which export their functionality via a dedicated interface such as a serial line, parallel interface or computer bus to the outside world. Our task consists of determining how to interface these different hardware devices in a coherent and efficient manner so that higher-level software (often not written by us) can access it easily.
The ESRF, located in Grenoble, France, is a multinational research institute supported by 15 participating countries. The ESRF is operated as a non-profit enterprise under French law. Management is supervised by a Council whose delegates are designated by the member parties.
This large experimental facility performs basic and applied research in physics, chemistry, material and life sciences. The research is facilitated through the use of a powerful source of radiation in the X-ray range. This synchrotron radiation, used at experimental stations called beamlines, has remarkable capabilities. Pushing the technological limits, the ESRF performs novel experiments which have not been feasible before.
The construction of the ESRF started in 1988. The inauguration and opening of the first 15 beamlines to scientific users took place in September 1994. Currently, 40 beamlines are operating 24 hours a day, 7 days a week. More information about the ESRF can be found on their web site at http://www.esrf.fr/.
Our answer to the interface problem has been to use Ethernet and the TCP/IP protocol as the ubiquitous interface for all devices. We have developed an object-oriented toolkit called TACO for wrapping all devices and then exporting them to the external world (our users) as objects on the network, accessible via an application programmer's interface (API). In order to wrap the different pieces of hardware, we often build embedded controllers close to the hardware which run the TACO wrapper software and make the hardware available via the network.
We will describe two types of embedded controllers which we build using Linux—PC/104-based controllers and VME-based controllers.
Why use Linux for embedded controllers? Although the choice to use Linux is obvious to many readers, the reasons are not always the same. In our case, we needed an operating system with the following features: highly configurable, excellent TCP/IP stack implementation, easy to program when it comes to writing device drivers, modern programming standard (e.g., POSIX, CORBA, Java, HTTP) support, stable, well-supported and not too expensive. Linux fulfills all these requirements and more. The fact that Linux is free and comes with the source makes it even more attractive.
Before Linux, we used commercial operating systems for embedded controllers and after two years of working with Linux, we noticed the difference. Gone are the days of flaky TCP/IP implementations, silence as an answer to our bug reports (now that we have the source, we can even fix the bugs ourselves), expensive contracts, lack of modern products and no one to talk to about our work. The choice of Linux for our embedded systems has meant that we now have the same OS from the low level to the desktop to our Beowulf cluster.
Choosing Linux does not mean that all questions have been answered, however. Questions remain, such as “When will industry embrace Linux?” and “How long will Linux last?” The answer to the first one is soon—we hope. The answer to the second is not forever, of course. As with any product/phenomenon, Linux will not last forever. At the ESRF, we renew our control systems roughly every ten years. It is important that Linux continues to exist for this time. In the worst-case scenario, we still have the source.
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