Written by Tony Rucker and Jessy Pu, City of Campbell
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
The intersection of Winchester Blvd. and Budd Ave. has a 6-phase signal with high left-turn/right-turn overlap traffic and pedestrian/right-turn conflicts. Figure 1 shows the signal phase sequence and intersection configuration. The goals were to provide the right-turn ( 1+ 4) overlap operation while protecting pedestrians from the right-turn traffic when they are in a crosswalk. The operation was designed with the following logic: turn on the right-turn green arrow to overlap the left-turn traffic ( 1 or 4) when there is no pedestrian conflicts, and stop right-turn traffic (red ball) when pedestrians cross Winchester Blvd. ( 4). In order to accomplish the operational objectives, a PLC was utilized to perform the logic decisions when the overlap is allowed and also detector switching and delay for the right turn lane ( 1+ 4). The following is a brief description of the programmed logic in the PLC:
When 4 turns green and there is no concurrent pedestrian service, the overlap right turn green arrow will be displayed along with a green ball on the same signal head. If there is demand on 1 at the time 4 terminates and 1 is next, the overlap right turn green arrow will be maintained while a yellow ball on the same signal head is displayed for 4. A red ball is then displayed for 4 when the 4 yellow times out. This overlap right turn green arrow will remain on as long as 1 is green. When 1 terminates and goes to yellow, the overlap right turn green arrow then is replaced by an overlap right yellow arrow.
If there had been phase pedestrian service at the start of 4 green, the overlap right turn green arrow and overlap right yellow arrow will not be allowed by the PLC. It will not be allowed during 1 if not allowed during 4. In other words, the overlap right turn arrows are allowed only at the beginning of green for 1 if 4 is skipped or at the beginning of green for 4 and no concurrent 4 pedestrian service.
The cabinet wiring was modified and the Conflict Monitor was programmed to additionally protect the 4 walk from the overlap right turn green arrow. The PLC, besides controlling the signal controller’s overlap output, also provides some detector switching and delay logic functions. Its operation is as follows: When a vehicle arrives on the 4 right turn loop and 4 is not in green, a timer in the PLC starts. Upon time out (8 seconds), the PLC calls 1. If 2 terminates and there is no demand on 4’s left lane, then 1 is served, concurrently with the northbound thru 6, and the overlap right turn green arrow comes on. During this 1 green, vehicles on 1 or the east to south right turn can extend 1. If, however, 4 is green because of demand for service in the left turn lane of 4, then vehicles on the right turn will call and extend 4 if there is no demand on 1. If right turn traffic is heavy and there is demand on 1, then 4 can gap out if there are no left turners and the overlap right turn can remain on as 4 is terminated and the actual demand on 1 is served.
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 generally 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, fuseholders 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 and experience in 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.