Note: This article is based on chapter 6 of my white paper titled 7 Key CBTC Functions Transit Operators Must Understand. Downloaded it here.
Experienced CBTC Transit Operators keep a laser focus on their diagnostic design because the time it takes for the Operator to identify a problem, localize the problem and fix it is determined by the diagnostics capabilities of the CBTC solution.
Sophisticated diagnostics keep this time to a bare minimum and pinpoint the exact cause of the problem. Whereas rudimentary diagnostics provide basic information and most of the investigation is left to the Operator’s maintenance personnel; wasting critical time while commuters are stuck on the track.
Diagnostics capabilities are key to a running railroad and the capability is defined by the diagnostic architecture. A proper architecture has three levels and each level increases the resolution of the problem:
- Level 1 – Service Affecting Diagnostics - CO alarms indicating problems that affect passenger service.
- Level 2 – Corrective Maintenance Diagnostics - alarms indicating LLRU’s that need to be replaced.
- Level 3 – Predictive Maintenance Diagnostics - predicting an LLRU failure before it fails.
For example, Level 1 diagnostics will report that a train lost communication; level 2 will indicate why the train lost communication (such as a communication board failure); and level 3 should predict the communication board failure before it happens.
At a time when the industry is moving towards more sophisticated software-based signalling systems, diagnostics capabilities must keep pace. The Operators must dictate a diagnostic framework that the Supplier must follow or accept the solution the Supplier provides.
Level 1 Diagnostics – Service Affecting Diagnostics
Level 1 diagnostics are geared towards first responders (CO). It provides the first indication of a problem affecting passenger service; such as a train EB, platform doors failed to open, a switch lost correspondence or a train lost communication. Level 1 diagnostics provide the CO with information to help the CO decide how to keep the trains moving based on the nature of the problem; their purpose is not to fix the problem.
Figure 1 defines a level 1 architecture based on the generic CBTC architecture (illustrated in Figure 2).
Figure 1- Level 1 diagnostic architecture – red path
The alarms generated at this level are called level 1 alarms and they travel on the red path. Each subsystem sends its level 1 alarms to the ATS (repository of all level 1 alarms) for any Service-affecting fault.
- If a train applies the emergency brakes, the VC will send a level 1 alarm to the ATS.
- If the platform doors fail to close, the wayside will send a level 1 alarm to the ATS.
The key to this architecture is that all subsystems must be connected to the red path and the alarms should fall within the category of a level 1 alarm. The Operator must ensure all level 1 alarms are captured because missing alarms prevent the CO from recovering from a Service-affecting fault whereas trivial and non-alarms create clutter and distract the CO from priority alarms that need immediate attention.
The diagnostic architecture is as important as the alarms it generates. The architecture provides a path for the alarms to reach the ATS and the generated alarms allow the CO to make an informed decision. If either one is missing, the CO will either miss a fault or make the wrong decision, delaying recovery from a Service affecting fault.
Level 2 Diagnostics – Corrective Maintenance Diagnostics
Level 2 diagnostics are geared towards maintenance personnel whose primary purpose is to monitor, analyze and pinpoint faults in the system such as a microprocessor board has halted on the wayside or the speed sensor has failed on train 5. Unlike the CO, maintenance personnel are not concerned with keeping the trains moving; they are focused on keeping the system fault free and level 2 diagnostics serve this purpose.
Level 2 diagnostics are conducted at the LLRU (Lowest Line Replaceable Unit) level, resulting in greater demand for data than level 1. Telemetry from every LLRU on every subsystem is required to notify the maintenance personnel of the exact LLRU that needs to be replaced.
The data required to support level 2 diagnostics requires a more sophisticated architecture as shown in Figure 2.
Figure 2 - Level 2 diagnostics architecture – blue path
The blue path feeds level 2 (LLRU health status) alarms to the DMS and the red path feeds level 1 alarms to the ATS.
The individual subsystems (Wayside, VC, IOP) collect the health status of each LLRU in their area and transmit the data to the Diagnostic Maintenance Server (DMS) via the blue path. The DMS stores the health status and processes it for each LLRU in the system. For maintenance personnel, the DMS is a window into the health of the system that pinpoints the exact LLRU that requires attention. Maintenance personnel are aware of the problem before they arrive at the location where the problem occurred, allowing them to arrive on site prepared with replacement parts.
The next diagnostic level is called predictive maintenance (level 3) and that is the topic of my next blog post.