Written by Tony Rucker, City of Campbell, CA.
What’s a PLC?
Generally speaking, a Programmable Logic Controller, or PLC, is a solid state control system which has a user-programmable memory for storage of instructions to implement specific tasks, such as: I/O control logic, timing, counting, arithmetic, and data manipulation. PLC’s are generally programmed in what is known as Ladder Logic. This method of programming was established because it could be closely related to hardwired relay logic that PLC’s were developed to replace. PLC’s are used in many industries for process control such as parts manufacturing, lumber mills, food processing and auto manufacturing.
The City of Campbell uses PLC’s to supplement the operational needs of several signalized intersections and were chosen because of their high performance-to-cost ratio over relays and 24 volt external logic cards. The PLC receives inputs from the signal controller’s NEMA outputs, makes logic decisions based on its operator-written stored program, and then outputs commands to the signal controller’s NEMA inputs. The PLC used in Campbell for traffic signal operation, the “IDEC Micro-1® “, is a fixed, 8 input/6 output, “brick type” PLC that can be expanded to a total of 16 inputs and 12 outputs. Although several I/O types are available, the model used in Campbell has “source” inputs and “sink” outputs, so that like a NEMA signal controller, it recognizes a ground as a “true” input and outputs a ground as a “true” output, in reference to the cabinet’s +24 volt dc power supply. It has EEPROM memory capable of storing 600 steps of user program and numerous internal logic components such as “AND” gates, “OR” gates, latches, 80 timers, and 48 counters. It can be programmed with either a hand-held “Boolean type” loader with LCD display or ladder logic software that runs on an IBM, or equivalent computer. Other PLC models and brands are available that can be modularly expanded as needed to provide up to 512 I/O points, floating point math, high speed counting, line voltage I/O and analog I/O. These more costly, higher-end units can be configured with RS-232/422/485 serial interfaces for peer-to-peer networking and telephone modem interfacing, providing remote control and monitoring. Of course, the concept of using external logic cards has been available for some time from controller manufacturers, e.g. “Econologic”, “MultiLogic”, etc.. However, this PLC only occupies 1/4 cubic foot of space and for only $300 it can replace relays and timers costing 10 times as much and taking up 50 times more space.
A Sample PLC Application in Campbell
Refer to Figure 1 below. The intersection of Hamilton Avenue and Eden Avenue utilizes 6 phases. Hamilton, the arterial, utilizes phases 2 for EB Thru, 1 for WB LT, 5 for EB LT, 6 for WB. Eden, the SB side street, is phase 3 and a commercial driveway is phase 4 for NB. Pedestrian movements are with phases 2 for EB, 6 for WB and 3 for SB. Figure 1 shows the signal phase sequence and intersection configuration. The goal was to provide alternate pedestrian timing for the SB movement phase 3 which could be enabled by a crossing guard. The guard would flip a toggle switch inside the locked police door to enable or disable the alternate ped timing. The alternate ped timing was necessary for the guard to be able to accompany a large group of students across the wide arterial and still be able to return to the appointed post before the arrival of the next large group. The operation was designed with the following logic: Utilize phase 3 ped timing during “normal” ped timing operation, allowing the 3 ped to operate concurrently as it normally does with phase 3 vehicle. However, when “alternate” ped timing is selected by the crossing guard, phase 8 ped timing will be selected to time AND display the “WALK” and “DON’T WALK” display for the SB ped movement, concurrently with phase 3 for the vehicles. The logic will also insure that this “alternate” phase 8 ped will only serve with phase 3 SB and NOT with phase 4 NB, as that combination (8 ped with 4 vehicle) would be allowed in normal NEMA controller operation and configuration. At this intersection, however, NB and SB are each protected movements with left turn green arrows, hence, the SB ped can operate concurrently ONLY with the SB vehicle, phase 3.
The following is a description of the ladder logic program in the PLC (refer to figure 2):
Line 0 allows that the enabling of the alternate ped occurs only when the toggle switch (Input #1) is closed AND phase 3 is NOT in green (Input #7). Thus, the alternate ped is enabled when internal relay #401 is SET.
Line 3 allows that the disabling of the alternate ped occurs only when the toggle switch (Input #1) is NOT closed and phase 3 is NOT green (Input #7). Thus, the alternate ped is disabled when internal relay #401 is RESET.
Line 6 allows that the walk output (Output #200) to the loadswitch will be driven by phase 3 walk (Input #2) when the alternate ped is disabled (relay #401 has NOT been SET) OR by phase 8 walk (Input #4) when the alternate ped is enabled (relay #401 has been SET).
Similarly, line 12 allows that the don’t walk output (Output #201) to the loadswitch will be driven by phase 3 don’t walk (Input #3) when the alternate ped is disabled (relay #401 has NOT been SET) OR by phase 8 don’t walk (Input #5) when the alternate ped is enabled (relay #401 has been SET).
Line 18 allows that phase 8 will be omitted (Output #202) when NOT in phase 3 green (Input #7) OR when the alternate ped is disabled (relay #401 has NOT been SET).
Line 21 allows that a hold will be placed on phase 3 (Output #203) when phase 8 is displaying walk (Input #4) OR ped clearance (Input #6). To clarify, ped clearance is TRUE whenever a NEMA controller is timing the flashing don’t walk interval.
The cabinet wiring was modified and the Conflict Monitor was programmed to additionally protect the SB “WALK” from NB phase 4. Also, a jumper plug that mates with the PLC’s connector was added so that if the PLC must be removed, the technician connects the cabinet harness to the jumper plug instead of to the PLC. This passes the NEMA controller’s phase 3 “WALK” and “DON’T WALK” outputs directly to the loadswitch so that the phase 3 ped operates “normally” (concurrently) with phase 3 vehicle until the PLC is returned to service.
Another Distinct Advantage
Another distinct advantage of PLC’s is that they can be easily modified or reprogrammed to meet changing intersection operational needs without having to purchase and install more connectors, sockets, cards, or relays. Usually the required modifications are limited to running a couple of wires between I/O points of the NEMA controller and the PLC. After the new logic operation has been checked thoroughly in the shop for the correct operation, the technician can download the revised program in the field with a loader or a laptop PC and then field check the operation to insure its conformance. However, if the PLC-to-Cabinet wiring is installed as described below, in a matter of seconds you can replace the existing PLC with a spare PLC already pre-programmed with the revision.
PLC Wiring in Controller Cabinet
PLC’s are usually hardwired in their more familiar process control environment such as plants and factories. This is generally not acceptable in the realm of traffic signals, as technicians prefer the modular, connectorized concept of the controller, conflict monitor and other cabinet equipment applied to all active components. This facilitates maintenance and decreases down time. To achieve this, the City of Campbell selected a 24 pin “AMP” brand circular plastic connector (CPC) cable system. The chassis-mounted plug with male contacts is mounted on a small piece of sheet aluminum that is mounted with stand-offs to the PLC’s mounting holes. Standard nylon-jacketed, 19 strand #22 AWG wire is used to connect the pins from the rear of the CPC to the terminal points on the PLC. Only the AC power, neutral and ground should be carried on shielded #18 AWG cable. A mating CPC receptacle with female contacts is wired with conductors of sufficient length to be connected to the appropriate controller I/O terminals in the cabinet. A 35 mm DIN rail is secured to the cabinet’s inside wall where appropriate and then the PLC is snapped in place. Terminal blocks, fuse holders and relay bases are also available that are specifically designed to snap onto the DIN rail, further facilitating the process. Installation is completed by screwing on the female harness to the PLC. A standard wire list of PLC I/O function assignments was adopted early and adhered to, so that PLC’s are interchangeable throughout the City. A PLC can be removed and replaced with another unit in less than 30 seconds. The only unique feature of the PLC installation at a site is the PLC’s internal logic program.
This article describes the use of reasonably priced, readily available alternative control tools to enhance the signal operation beyond what a typical NEMA signal controller offers. A PLC application has been successful in Campbell but it should be noted that, to successfully program and install a PLC requires above-average knowledge of NEMA controller operational specifications in addition to PLC programming. An experienced signal technician can obtain the knowledge required to implement PLC operation by taking courses in PLC operation and Ladder Logic Programming, beginning with basics and fundamentals and then graduating to the more advanced classes. Most major manufacturers offer factory courses and many are free. Contact the manufacturer’s representative for the PLC of your choice for details.
It would be ideal if a signal controller had a user programmable area for a traffic engineer or signal technician to program the desired logical operation without having to rely on external devices. But until that time arrives, you might consider using a PLC to enhance or optimize your signal operation. There are four PLC’s currently in use in Campbell. They have provided continuous, trouble-free operation since installation in 1994. And although there have been numerous power outages in that time frame, not once has any of the PLC’s caused a problem.