Linux for Embedded Systems
Fujitec is a global manufacturer and provider of a complete range of elevators, escalators and other vertical transportation equipment. Nearly all of our products use embedded computers in one form or other. To see the implications, a few features of the elevator business to note are:
The lifetime of the product is very long, on the order of 20 years or more.
Safety and availability of elevator service is a deciding factor for the users, especially for medium to high-rise buildings.
Continuous service and maintenance during the full life cycle is vital; usually, it is even required by law.
To meet the requirements of long term, disruption-free service and maintainability for embedded computers, we have recently started to investigate the possibility of using the freely available Linux OS and the GNU utilities.
We have in the past shipped several products using Linux. We will describe here only one of them—a monitoring display for group controllers (see Figure 1).
Another application, a user-interface system for editing scrolling messages in elevator cars, is shown in Figure 2, but we do not have space to introduce it in detail. (Notice the Japanese characters—Linux works fine in Japan, too.)
To put things into perspective, we need a few facts about elevators. If a building has more than 2 or 3 elevators, they are usually working in groups sharing the passenger traffic. A group of elevators is connected to a “Supervisory Group Controller”, which shares the service of each elevator among users while attempting to give each user the illusion that he/she has a personal elevator.
The modern elevator group controller is rather sophisticated; it has to run difficult, optimal-scheduling algorithms in real time. The latest Fujitec models use neural networks for on-line learning and optimization. Without proper tools, the support personnel who perform installation and testing tasks wouldn't understand what is happening with the elevator group.
For maintenance checks, upgrades and occasional troubleshooting, the GSP (Group Supervisory Panel) has a graphical display showing the status of the elevators. This monitoring display was the target of our first Linux application.
Figure 1 shows the graphic screen of the monitor display. Information for each elevator, such as the present floor, traveling direction, door status, registered calls, etc. is indicated by graphic symbols. This screen design has evolved over several years, and it is now familiar to our technical and maintenance personnel.
We built most of the original monitoring display system with custom-designed components:
A custom graphic board with custom graphics software
Control software common with our sequence-controller, CPU board
Serial interface with our proprietary protocol to communicate with the group controller
A multiscan VGA display (the only commercial part)
These components were chosen to give us the following advantages:
Stand-alone operation in the elevator, machine-room environment
Long-term supply and maintenance
Even though we saw several problems with our custom-designed approach and wanted to move to an established, popular platform, until recently it was not feasible.
For the hardware, our first choice would have been the IBM- compatible PC, perhaps in the form of PC/104 cards. However, the dominant commercial OS (first DOS, then Windows), was unsuitable for many reasons. It is neither real-time capable nor reliable, and there is no chance that a particular release could have long-term support from its vendors.
Although there were many alternatives, such as the excellent Lynx OS, none of them satisfied all of our requirements, especially the long-term stability and availability criteria.
With the appearance of Linux, all this has changed.
Over the past few years, several engineers at Fujitec became familiar with Linux. Although we started it as a hobby, gradually the wider applicability of Linux became clear to us.
Our motivation for embracing Linux for product development has several roots:
The availability of the complete source tree of the system, without any restrictions on use, distribution or revisions, is vitally important to us. It can ensure that we will have some way of maintaining our systems at a site 5, 10, 15, ... years from now. If we wanted to guarantee the same with a commercial system, it could easily become a nightmare. We could imagine the vendor in 2010 (if still in business) just staring at us with mouth agape, when we ask them about bug fixes for their 1997 system. However, when we have the source, maintenance becomes “difficult” instead of “impossible”, just as with in-house, custom-made systems.
An added bonus is the freedom from administration of a royalty-based, license system for some proprietary OS; not to mention the savings on the license fee.
We have been using Unix for system development for many years. After some rather frustrating experiences with other systems on the PC platform, our developers were quite eager to get back to a Unix-like environment.
Even among Unix systems, Linux feels unique: it is light, fast, runs on almost any platform, contains the latest version of all free software and is now so visible that even non-computer people are starting to talk about it. It is a good feeling to be back in the mainstream again.
However, not everything is rosy. Since Linux is essentially Unix, a few things had to be fixed before we could deploy it in an embedded controller.
Ideally, we would like to use a ROM-based system, such as the one described in the article “Booting Linux from EPROM” by D. Bennett, Linux Journal, January, 1997. In our case, however, we needed X11, Tcl/Tk, libraries, fonts, etc., so we were forced to use a hard disk-based system. This means that we have to think about disk-buffer flushing, cleanup and recovery after messy shutdowns, etc.
Linux is not a “hard” real-time system as it stands, although people are already working on real-time patches. (See “Introducing Real-Time Linux”, M. Barabanov and V. Yodaiken, Linux Journal, February 1997.) For our current application, this was not directly a problem, but we would need guaranteed response time if we wanted Linux to control an elevator car.
Installation must be made foolproof, especially for re-installation or upgrades. We have found that our customized procedure works, but it still needs to be improved.