Linux on Track
The Fraunhofer Gesellschaft is a non-profit research organization specializing in applied research and acting as a proponent of technology transfer between basic research and industry. With nearly 50 institutes in Germany, almost all aspects of science are covered. Part of the IITB (Institut für Informations- und Datenverarbeitung) specializes in applications of computer-based monitoring, control and diagnosis of industrial processes and equipment by means of signal and image analysis. As such, our group was involved in two projects requiring data acquisition and analysis in one of the German high speed trains, the InterCity Express (ICE). This article describes how the data acquisition was implemented using Linux.
Project UNRA (unrunde Räder) tried to discover the reasons wheels of high speed trains become out-of-round sooner than expected. As railway experts know, train and coach wheels become out-of-round, i.e., the difference between the minimum and the maximum radius of a wheel becomes non-negligible. For ICE wheels, DB-AG uses 0.6 mm as the threshold above which wheels must be changed. Despite being only 0.1% of the wheel radius, such differences induce low-frequency vibrations into the coach structure. They not only put additional stress on structural materials but also cause an unpleasant ride for passengers.
As part of this project, a measuring axle developed and patented by Forschungs- und Technologiezentrum der DB-AG (Minden, Germany) was installed in a regular running ICE to measure triaxial forces in the stand-up point of the wheel. In addition, four accelerometers were installed, three on the axes at the bearing and one in the coach measuring vertical acceleration. In addition, a resolver was attached to the axle. It delivers 1440 TTL-edges for one turn of the wheel. The edges are used to clock 90 A/D conversion for one turn of the wheel, resulting in a sampling rate dependent on the wheel's rotation speed but synchronous with its rotation.
With additional channels not mentioned above, a total of 17 channels had to be measured. Since the radius of an ICE-wheel is 460 mm, 1 km amounts to:
samples, resulting in:
of data every day.
The second project, called ICE-D, did not have such a great demand on bandwidth, but it required some data analysis to be computed on-line. Since the system is quite similar to the one used in the UNRA project, this project will not be described in such detail. Suffice it to say that another Linux computer will run for two years in a coach car with a new type of bogie (the assembly of four wheels on which the coach rests) to acquire and analyse 50 data channels.
The computer hardware used in both projects is not identical but not very exciting in either case. In UNRA we use a garden variety Pentium 90 system with Adaptec 2940 SCSI controller, a sufficiently large Quantum disk, which seems to have settled its disagreements with the SCSI controller, and an HP DAT streamer. For the ICE-D project very similar hardware is used. Comparatively expensive were the 19-inch cases which were necessary to mount the computer in a rack in the train.
Of more interest might be the measurement hardware. In UNRA there is an Analog Devices RTI-860, which is a 16-channel, 12-bit A/D (analog/digital) conversion board. What makes it particularly suited for use with Linux is its on board memory of 256K samples, relieving us from hard real-time constraints.
Another board, called ADCO, was developed along the specifications of IITB by CMS. It is basically a device to measure times with a 40 MHz clock driving a 16-bit counter. On external events, the counter is read into a FIFO of one kilometer word length and then reset to start again from zero. Consequently the FIFO collects counter values representing a sequence:
of times between events. The events are generated by the resolver mounted on the axle and happen once for every four degrees of wheel rotation. Knowing how much time the wheel needs to rotate four degrees, we can calculate the rotational speed of the wheel with high precision. The one kilometer sample FIFO on the ADCO is small compared to the buffer on the RTI-860 and actually proved to be too small for the first driver developed (see below).
Project ICE-D required another kind of measurement hardware because the task was not to acquire data at high speed but to measure up to 64 channels. We decided on an RTI-834 from Analog Devices because it can measure 32 channels and is probably the only board around with a useful programmer's manual. The manual is not free (approximately 250 DM) but, believe it or not, it contains almost all details about programming the hardware, including examples that made it comparatively easy to write a device driver.
In addition to these devices, a GPS (Global Positioning System) device was mounted on the coach. Position information was recorded with the data to be able to correlate it later with actual locations on the track.