Last summer, a train derailment in Spain – one caused by speeding – resulted in 80 deaths and dozens of injuries. This and other accidents have highlighted the importance of Positive Train Control (PTC), which can help prevent incidents like these. PTC systems require sophisticated Machine-to-Machine (M2M) communication, and are an example of an instance where M2M can save lives.
PTC systems encompass a large number of integrated components, including database servers, wired and wireless communication networks, rail signaling systems interfaces, and locomotive control systems in order to perform train safety functions. It is beyond the scope of this article to fully address the complexities of PTC, but, at the highest level, PTC systems are computer-based systems used to minimize human errors that occur when drivers become distracted.
PTC is an umbrella term used for technologies built to stop or slow a train without human intervention in the event that a train begins operating outside of safe operating limits. PTC systems are primarily focused on freight rail but also include provisions for passenger trains operating on freight tracks, and were first mandated by the Rail Safety Improvement Act (RSIA) of 2008 after a commuter train operator who was texting ran a red signal and collided head-on with a freight train. RSIA 2008 stipulates that passenger and Class I railroads must deploy PTC by the end of 2015, and that the systems must include provisions for a number of different situations, including collision avoidance, speed limit enforcement to prevent derailment, and worker safety measures during maintenance and repair.
In the past, train operators read traffic signals the same way that car drivers do: by looking out the window. Later on, some locomotives were fitted with computer-based dashboards that displayed signal status information within the cab. Today with PTC, computers on locomotives are able to communicate with wayside computers using M2M, and, most importantly, they're actually capable of control. Although today's instances of M2M within PTC system do not (yet) fully automate train operation, they do offer a safety net for instances in which the operator is somehow incapacitated.
What follows highlights recent efforts in PTC, specifically in the area of base (wayside) and mobile (locomotive cab) communications manager development. Using a Time Division Multiple Access (TDMA) communications technique deployed by AMTRAK railroads in the Northeast Corridor, PTC communication managers assign and manage these TDMA timeslots to control radio communications between the wayside network and locomotives. The communications manager requirements were met with a System-on-Module (SoM).
PTC communications managers: What's needed
An Advanced Civil Speed Enforcement Systems (ACSES) is an implementation of PTC that determines a safe braking curve based on permanent and temporary speed restrictions before automatically slowing the train if the operator fails to maintain a safe operating speed. The critical requirement of ACSES communications managers is that they consistently and reliably transfer ACSES-formatted data packets over radio link, maintaining a consistent communication heartbeat between the train and wayside network. If communications fail and/or the train operator is not operating the train within a safe operating speed, the ACSES PTC system takes control of the locomotive and applies the brakes.
In an ACSES implementation of PTC, a number of high-level elements are involved to make remote manageability possible, including the locomotive's onboard computer system, the wayside signaling system, the central office (operations center) that stores temporary speed restrictions and other information, and the communications network linking the various components (Figure 1). As such, ACSES-compliant base and mobile communications managers are required to perform and manage several primary functions, including:
- Accessing and packetizing data for transmission to/from the wayside PTC network
- Controlling the TDMA train/wayside radio communications timeline
- Sending packets to radios at the appropriate time for each train-to-wayside and wayside-to-train communication slot
- Processing received packets from radios
- Verifying data integrity of received packets and discarding erroneous transmissions
- Passing received transmissions to the train's onboard computer (communications to the locomotive) or the wayside communications network (communications from the locomotive)
These functions require a highly reliable, general-purpose microprocessor solution with enough horsepower to organize and control TDMA communications while maintaining simultaneous communications with both the wired wayside network and locomotive's onboard PTC computer.
Selecting a solution for ACSES base and mobile communication managers
To meet the aforementioned ACSES requirements, Critical Link employed the MityDSP-L138F System-on-Module (SoM) based on the Texas Instruments (TI) OMAP-L138 dual core ARM-DSP processor because of its good general-purpose compute capabilities; support for Ethernet and serial interfaces; lack of dependence on active cooling; and proven track record applications with similar communications requirements, which significantly reduced development time and risk.
To handle the ACSES system's TDMA connectivity requirements, the MityDSP-L138F is equipped a Xilinx Spartan-6 FPGA that implements High-level Data Link Control (HDLC) serial communications, a data link layer protocol used to ensure synchronization of device-to-device connections (Figure 2). In conjunction with the ARM processor, this afforded the ACSES communications managers a robust mix of processing that included:
- TDMA timeline control of the radio link between the trains and the wayside PTC systems
- Ethernet and serial communications with PTC devices both onboard the train and in the wayside network
- Link layer, network layer, and transport layer communication functions on both wired and wireless PTC networks
- Real-time diagnostics of communications manager hardware
- Data logging
Another important element of the ACSES communications managers was a design that incorporated parts that will be available for many years to come. PTC systems are expensive and have long life cycles, requiring technology that can be manufactured for many years. As such, it was ensured that key components of the SoM – the TI OMAP-L138F, for instance – would be available from manufacturers long into the future.
Critical Link also developed an ACSES simulator for the AMTRAK deployment that simulates data moving among multiple trains within a PTC system. The simulator is used to analyze the communications data flow throughout the system, highlighting system bottlenecks and overall throughput. This simulator also supports interoperability testing of components from multiple suppliers without having to test in the field.
M2M communications make an impact
PTC is an instance of M2M that has the potential to directly impact the lives of the millions who rely on train transportation. It's largely invisible to most of us, but it can and does bring with it the prospect of saving lives. Thousands of PTC base and mobile communications units, each containing a Critical Link SoM, are in the process of being deployed in the Northeast Rail Corridor.
Critical Link, LLC.