Control Systems

Control Systems

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Terminology

In discussion of control equipment, two phrases used extensively are defined as follows:

Control assembly- The complete electrical mechanism mounted in a cabinet for controlling the operation of a traffic control signal. Unless excluded specifically, the cabinet is also generally considered to be included in the controller assembly.

Controller unit- That portion of a controller assembly that is devoted to the selection and timing of traffic signal displays.

Hardware Evolution

Clearly, the evolution of traffic signal controller hardware parallels that in related industries. Undoubtedly, the phenomenal pace in electronics development during the past 20 years in all fields has been spurred by innovation an miniaturization required in the space industry. Signal controller unit hardware has evolved from the motor driven dials and camshaft switching units to the adaptation of general use microprocessors for a wide variety of intersection-control and special-control concepts.

Types of Operation

Pre-timed operation- The duration of all intervals is predetermined, with interval timing handled by the local controller unit at an isolated intersection. Under supervised control, the duration of certain intervals is determined by a master controller or other remote device.

Electro-mechanical pre-timed controller (Econolite)

Full-actuated operation- All signal phases are actuated, and the change and clearance intervals are of predetermined duration. The length of each of the respective variable intervals, such as greens and green arrows, is determined by the local controller unit based on detected traffic demand on the associated approach, and is limited by the pre-selected maximum timing set on the controller. Phase associated WALK intervals are of a guaranteed duration, but are timed and displayed only upon respective pedestrian push button actuation.

Fully actuated model 170 controller in 332 cabinet

Semi-actuated operation- Controller units operating in the semi actuated mode provide a guaranteed minimum green time for the non-actuated, major street phase. The green interval on the major street is relinquished only when a conflicting call is received on the actuated phase(s). After timing the appropriate major street change and clearance intervals, green is displayed to the calling phase for the minimum green interval. The phase being serviced will retain right-of-way as long as vehicles are detected and the vehicle (passage) interval portion is reset. The actuated phase green interval will terminate when the vehicle passage time is allowed to time out, when the maximum green time has been reached, or when force-off is applied. The change and clearance intervals are timed and green is displayed to the next calling phase for the minimum green interval (in the case of multiple actuated phase operation), or green for the non-actuated major street phase (2 phase operation).

Semi actuated controller assembly (Pre-NEMA)

In most of these installations, pedestrians are detected through the use of special pedestrian push buttons which are manually operated by the pedestrians. This provides the advantage of only servicing the pedestrian signal "WALK" and "DON'T WALK" when it is required, thereby eliminating wasted time when no pedestrians are present. Many urban areas with high pedestrian traffic will set the traffic controller to service the pedestrians every cycle.

NEMA Controllers

Some electrical equipment manufacturers have voluntarily agreed to conform to the traffic signal controller units they manufacturer to a developed standard. These standards reflect inputs from traffic engineers, installers, and professionals in the field of traffic control, and describe physical and functional requirements for full actuated traffic signal controller units. Most of the functions described in the standards include 2 phase through 8 phase controllers. Input/ output formats, environmental standards, and test procedures are also covered. These standards were developed to provide compatibility and interchangeability in controller units.

Typical NEMA-style cabinet

Model 170 Controllers

This system was originally jointly developed by the states of California and New York. The conceptual approach of the Model 170 system is based on hardware standardization, and structures the controller assembly as a package consisting of standardized modules, complementary hardware and wiring harnesses, all housed within a standard cabinet enclosure. The controller unit uses a microcomputer which, with the addition of appropriate software, can be utilized in a variety of control applications.

170 controller installed in smaller 337 Cabinet

Thus, the concepts of the NEMA and model 170 controllers are quite different. NEMA controller units are designed and manufactured in various electronic arrays, but all must conform functionally to the NEMA standards. Certain physical requirements for the input/output functions must also be met to maintain compatibility between certain components. Model 170 units, on the other hand, are microprocessors conforming to a 170 hardware specification, but the software to make it a traffic signal controller, must be procured from a different source and installed in the traffic PROM module of the unit. This is the actual "brain" of the unit.

Conflict Monitor Unit

All solid-state controller assemblies would not be complete without the inclusion of a separate signal monitor unit. Be detecting the presence or absence of voltages at field terminals, the conflict monitor detects conflicting signal indications and basically "keeps the electronics honest"! Upon detection of conflicting indications or an unsatisfactory operating voltage, the monitor will cause immediate transfer of the signal operation to a preprogrammed flashing condition. This is also a means of lessening the liability and insuring that faulty electronics in the controller will not cause tragedy on the roads!

Loop and Video Detectors

A loop is comprised of several turns of wire implanted into the pavement and then led back to the traffic controller cabinet via underground conduits. There are different shapes for different applications with the most common being circles and quadrangles. They operate on the principal of electromagnetic induction. The loop detector, in the traffic controller cabinet, sets up a resonant frequency within the loop. When the steel mass of a vehicle passes over the loop, by the principals of induction, the frequency of the signal in the loop is altered. The loop detector recognizes this deviation and inputs a digital signal into the traffic controller mainframe. Loops have the advantages of being inexpensive to install and maintain, which is why they are the most common form of detection. Video detection is a new and expanding field which holds the promise of increased performance and reduced maintenance but unfortunately, it is currently very expensive. Utilizing modern advancements in machine vision technology, this system detects vehicles in busy multilane environments using modern video technology. Other drawbacks to this system, when located in windy environments, are that the cameras can shake or be knocked out of alignment, causing false readings. Snow and icy localities have also caused problems with camera detection.