PLCs and robust HMI help rein in costs associated with wireless SCADA systems in mass transit applications

Ask anyone who lived in the golden age of passenger rail, and they’ll tell you how you could have set your watch by a train’s on-time performance. Today’s commuter rail systems are still highly reliable, and on-time trains are the most visible evidence of that reliability. Yet, there’s a huge interconnected system of switches, substation and other equipment operating like clockwork to make those railways so reliable.

Monitoring and controlling equipment over a computer network is a daunting challenge within the confines of a single location. But when your network comprises a large geographic area within one of the most densely populated cities in the world, and much of it operates outdoors, establishing a robust and reliable network becomes exponentially more challenging, as is the case with communications networks in large subway system.

Managing heating installations more efficiently
Many subway trains get their power from the “third rail” as opposed to diesel or catenary (overhead) power. The power transfers to the subway train’s motors by a sliding shoe which contacts the third rail. In the wintertime, the third rail that is exposed on the above-ground portion of the railway is prone to freezing when snow and ice accumulate, so a manual set of heaters — heat-trace trusses — is turned on during autumn and run continuously until spring. This is one area in which transit agencies would like to reduce the wasted energy and costs associated with manually managing the heat traces via a remote control and monitoring system.

With large track distances to cover along with thousands of heaters and control points to supervise, a wireless SCADA (Supervisory Control and Data Acquisition) network  is one true option. Here, a system has to be able to send and receive data about the heater switches back to a central, master host, it has to reduce energy and cost, and it has to be implemented without disrupting the bustling transit system.

Systems integrator Transdyn, Inc. of Duluth, Georgia, has experience designing wireless SCADA for rail transit systems.  In one recent installation, two main requirements for the communications system were: wireless media and a single, robust HMI (human machine interface) front end along with its associated software to make communication to the many devices out in the wayside easy and seamless.

Power supply and PLC selection
The power typically is converted from the high voltage distributed throughout a subway system down to a lower voltage at each control node. Locally, power supplies tap into the higher voltage, which both helped eliminate the need to draw power from other structures or buildings while helping maximize surge protection. “We provide surge protection elements in there that would not only take care of anomalies and surges caused by Mother Nature, but also track anomalies. And on the secondary side, electronic protection for those let through voltages and currents that would affect electronics as well,” says Dick March, senior business development engineer at Phoenix Contact. At local nodes, hybrid MOVs (metal oxide varistors), and gas tubes are incorporated as pluggable devices that are hot swappable for ease of integration, maintenance, scalability and diagnostics feedback should the system requirements change.

Since reduced cost is typically a goal, wireless radio telemetry may be preferable instead of a fiber hardwired system. Using the license-free, 900-MHz ISM band, an agency does not have to incur the monthly costs typical with other wireless bands that would require services from an outside vendor. The ISM band radio system is also desirable because it can cover a greater distance than 2.45 GHz, and it is more adaptable to the “busy” environment of a transit system which is generally rich with radio energy, EMI, large buildings and tight spaces. Transdyn often chooses radios that are able to be mounted both high on top of buildings and low at track level. They operate within FCC regulations up to 1 Watt of EIRP (Equivalent Isotropically Radiated Power). In addition, they are designed to communicate with the processor via SNMP (simple network management protocol), which allows for diagnostics and rebooting, and parameters can be easily changed. The radios can also be configured as “store and forward” repeaters, able to send and receive data both up and downstream to hundreds of control points in the system.

Repeaters are particularly important in a large, self-contained system such as a transit project. By making sure that signals go from the master location to many local areas, often sending the signals around buildings and curves in the tracks, conditions at the heater such as currents, their ON/OFF states, and hand switch positions can be communicated back to  the central host.

While the system level is self-contained, the control points themselves benefit from self-contained PLCs. Meeting IEC 61131 specifications, the company employs processors whose chipsets do the code solving and can add IO modules whenever they are needed at the location. Although the media is wireless, the PLCs in many transit systems still connect to and reside on an Ethernet infrastructure. With several hundreds of control points, controllers must be directly programmable and have plenty of onboard memory to accommodate both retentive and non-retentive variables. Since a system may have hundreds of control points, the PLCs must be easily expandable for lots of additional IO. Control panels, meanwhile, are often situated in very tight spaces, so the smaller the controller, the better. The Phoenix Contact ILC-150 PLC used in a recent Transdyn subway project is small enough to sit directly on the DIN rail without a metal rack and operates from the 19 V to 36 VDC range.

“Documentation, documentation, documentation”
Transdyn and Phoenix contact collaborated extensively to make sure their most recent wireless SCADA system is expandable and easily adaptable to changing conditions while simplifying any manual labor that must still be performed. “Documentation, documentation, documentation,” says Dan Boone, senior field applications engineer at Phoenix Contact, who stressed that the system was designed so equipment can be easily replaced, the technician should be able to reload the code and radio information in minutes, and the system should perform as it did before. Since the code is intended for hundreds or thousands of points in the system, the code has to be simple for the technician to troubleshoot, and the documentation easy enough for any technician to understand.

To tie the control points together with the operator, a large SCADA system requires robust HMI software. In any type of automation application, equipment must be monitored and controlled. When working with trackside heaters, voltage level monitoring and other measurements often requires personnel to ride a train to the heater site and manually make the observations. A wireless SCADA system, however, lets operators monitor the health of the heaters remotely, saving both time and energy costs. Using DYNAC HMI software, operators can use its external data warehousing features so historical data pertaining to the heaters’ operation can be checked to make comparisons to an earlier time and to establish trends. It includes a mapping feature tied to satellite images, so the whole system can be seen along with color codes to reflect various conditions at the heaters. Using this SCADA system and software to automate the process, Dave Hernandez, Senior Account Executive at Transdyn notes, “If it’s getting cold, (the operator) can put the system into an auto-state mode and have the heater elements turn on using using the integrated weather sensing probe ... or, DYNAC also gives them the ability to turn things on using a schedule.” This helps prevent, as when the heater elements all turn on at the same time, an overload of the power system or power grid.

SCADA systems are increasingly used in railway environments to deliver reliable and efficient communications to monitor and control not only heaters but any type of wayside equipment with less need for human interaction. In the ongoing collaboration between Transdyn and Phoenix Contact, the client is already saving thousands of dollars each day in energy costs and personnel. Thanks to equipment built to withstand vibration, noise, vandalism, freezing temperatures and high winds, and a very durable package from Transdyn, the system was even able to withstand a major storm that otherwise crippled much of the region’s transit infrastructure.

Most importantly, a successful, large-scale wireless SCADA system for transit applications hinges on extensive knowledge and collaboration between the systems integrator and the component manufacturer. Dick March explains, “If you relied on separate trades to do their portion of it, it would be very difficult to write a spec to do this kind of a project.”

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