Hayden Harrington, Application Engineer
When the first trains started ambling down their tracks, excessive speed that could cause derailment was not a factor. Nor was there concern about the amount of traffic travelling the rails. But obviously over the years, the railway system has undergone significant evolutions that moved it from merely an efficient method of transportation to a vital means of economic development. This has significantly increased both the dependence on railroads for mass transportation of goods and people as well as the need for a more efficient system to manage the larger traffic network.
Trains now travel at far higher speeds and at much greater frequency than when the railway infrastructure was built, and there are large elements of safety and reliability that need to be considered to accommodate the modern systems.
Electronics are now an integral part of safe, reliable and functional railway operations. Unfortunately, recent crashes that have resulted in loss of life have shown the importance of high levels of offsite control, remote access and remote monitoring, which requires reliable, dense computing power to ensure safe operation across all channels. All of these critical functions are controlled by computing equipment, both inside the train and by the wayside that must be housed in an enclosure that can withstand the shock and—specifically on board the train—the enduring levels of vibration. (Figure 1)
Some may say that railway electronics are subject to the same rigor as military and seismic environments, which require significant protection from extreme shock and vibration as well as other environmental factors.
While the electronics may have ruggedization built inherently into their design, the added protection and stabilization comes from the enclosures that surround this sensitive equipment. No board or system is designed to fully take the brunt of any mobile or harsh environment, it’s always the housing that provides the first line of defense.
Ensuring Reliable Operation
The shock and vibration that the tracks, the wayside equipment and the car themselves have to endure in a railway environment is relentless. Fortunately, standards designed to incorporate key test requirements for shock and vibration have been developed for both North American as well as European railways.
Unfortunately, these standards do not always account for the fact that much of the available space where these railway electronics need to be placed is a fixed dimension. Just as small form factor systems face challenges produced by Moore’s Law, railway applications do not have the luxury of limitless space in which to place electronics. And although it may not be a compact environment, it is still an area where dense, heat-producing electronics are crammed into a defined area and expected to operate reliably.
The rail specification standards that ensure electronics and enclosures can withstand the ongoing impact of a railway car barreling down the tracks are AREMA in North America and CENELEC for Europe. Both publish required practices for the design, construction and maintenance of railway infrastructure as well as define the degree of shock and vibration that can be safely endured in a railway environment.
Typically, for an enclosure to pass these rigid test specifications, the use of resilient mounts or shock and vibration isolators is needed, but these seemingly innocuous components take up valuable real estate by increasing the footprint of the enclosure itself.
Meeting the standards
The AREMA test, based on lab experience, is the more difficult test to pass because of the vibration testing. Shock requirements are 10 G for a 11 ms pulse in the x, y and z-axis, three times in each direction. While moderately severe, this would not typically cause problems for a robustly designed rack.
But the vibration testing is done in the form of a sine sweep through a frequency range of 10 Hz to 200 Hz, with a peak acceleration of 1.5 G. Each sweep takes 12 minutes and must be completed 20 times for a total of four hours per axis (x, y, z). Using a sine sweep means that the rack will have to sit within the natural frequency band of the rack for a significant amount of time (approximately 30 seconds), which proves to be quite severe.
The shock portion of CENELEC test requires a shock input of 30 m/s2 for 30 ms in the vertical direction as well as in the transverse direction and a shock input of 50 m/s2 for 30 ms in the longitudinal direction. This translates to approximately 3 G, 3 G and 5 G, respectively.
The vibration testing is a little bit more difficult to define, and is intended to simulate random vibration. Conducted over a frequency range of 50 Hz to 150 Hz for 5 hours in each axis, this is an intense vibration test. The test inputs are defined as ratios of maximum and minimum accelerations to the frequency input.
When designing a rack to meet these rail specifications, it’s important to consider the potential points of weakness during the 15 hours of endurance vibration testing across a full spectrum, which will most likely include the natural frequency of your enclosure.
A Unique approach
Housing railway electronics in an enclosure that can withstand the shock and endure the vibration of a rugged railway environment, both inside the train and at the wayside, while meeting the AREMA and CENELEC test specifications of is one challenge. Doing so without the use of the typical resilient mounts or shock and vibration isolators is another.
But being able to remove the isolators is a key advantage in a few ways, not only from a design perspective but in terms of installation as well. Since there are no isolators to size and specify, this extra layer of design is eliminated. It also removes an extra step during assembly, since there is no need to account for an isolator bolting pattern, allowing the enclosure to be mounted in its own footprint. This is especially important if there is a pre-determined footprint that must be met.
Such was the situation recently faced by the design team at Optima Stantron, an Elma company, tasked with developing a rail-based rack system to house control equipment on board trains intended for mass transportation.
Aluminum extrusions, when constructed together properly, can form quite robust structures that have an excellent strength-to-weight ratio, and this is where the design team started. A welded aluminum platform served as the basis for a rack that would ultimately pass the CENELEC and AREMA standard’s testing without the assistance of the typical shock and vibration damping devices. (Figure 2)
First step in the enclosure’s enhancement was to reinforce the bottom four corners of the frame with the addition of welded aluminum gussets, parallel to the floor. After this modification, initial testing showed a more-than-acceptable level of movement and stress occurring in the front-to-rear direction, so gussets that were perpendicular to the floor were added. The issue with this axis was the depth of the rack. It was only around 18” deep which resulted in a significant reaction to the vibration testing.
Riveting all the nuts and bolts of the enclosure, from the cross braces and distribution plate to the front-to-rear stiffeners and vertical mounting rails, was a simple, yet effective way to prevent the hardware from vibrating loose. It proved to be a critical design aspect that significantly enhanced the strength of the rack.
Further testing revealed that the outer portion of the frame had some additional bowing at the bottom and whipping at the top, due to the initial location of the bolt-down points a few inches inboard. The force was dispersed around the entire bottom edge of the frame from the four bolt-down points by incorporating a distribution plate. It was this design element that stabilized the frame enough to eliminate the vibration isolators typically required to meet CENELEC and AREMA test specifications.
Keeping it all on track
Today’s railway systems rely more heavily on electronics to streamline processes, manage traffic and ensure passenger safety, meaning reliable operation is paramount in this rugged environment, whether on the train itself or on the side of the tracks. Methods that maximize existing footprints, while protecting these more complex computing systems, mean the difference between safe operation and catastrophic failures.