I’m an electronic tech for a large transit agency. In 2004, we opened a new light rail vehicle maintenance facility equipped with a network of 16 track switches and colored light signals.
The switches are controlled by a Train to Wayside Communicator (TWC) consisting of a data transmitter unit on each rail vehicle and 16 inductive loop antennas that are embedded in various locations of the track. All of the antennas tie into a receiver unit, which converts the data transmitted from the vehicle to a switch/signal request. The request is then forwarded to a checked redundant PLC that performs all the safety-critical functions required to execute the operation of the track switch and then display a signal to proceed.
The switch/signal installation is also provided with a local control panel (LCP) in the yard attendant’s office that indicates which sections of track are occupied, the position of the track switches, and the status of the signal lights. In practically all installations, the LCP would have pushbuttons from which the attendant could manually enter a request for any particular track switch/signal. Technicians also need to enter track switch/signal requests for maintenance and troubleshooting.
However, this particular installation was one of the rare exceptions where the LCP had indicating lights only. It did not have any pushbuttons for switch/signal requests. In the absence of pushbuttons on the LCP, we had to carry a handheld version of the TWC transmitter out to a particular inductive loop in order to enter a request. Considering the inconvenience of having to walk out to the yard with the handheld, we decided to add real control capability to the panel.
The TWC receiver has an output buffer card with 16 relays that interfaced it to an input buffer card of the checked redundant PLC. A switch/signal request would pick one of the relays and apply a “Logical High” (in the form of positive 12V DC) to the corresponding input terminal of the PLC buffer card. After the request was stored in the buffer, the PLC would check all the safety critical variables in the track/switch network and execute the request.
In essence, a request could be entered directly to the PLC by applying positive 12V DC to the appropriate terminal of its input buffer. Simply adding a switch that would provide another path around each of the TWC relay contacts would provide the alternate control capability. The TWC-to-PLC interface was on a terminal block with 16 connections so we had a convenient tie-in point for the additional pushbuttons. Accordingly, we mounted 16 pushbutton switches in a blank 19-inch aluminum panel and installed it directly above the existing panel.
Now, our LCP has real control capability!
This entry was submitted by Galen L. Dutch and edited by Rob Spiegel.
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