Traffic Detector Handbook

As with traffic signal controllers, loop detector electronics units were developed and marketed by numerous manufacturers, each using a different type of harness connector and detection technique. To overcome subsequent interchangeability problems, JUTAI developed a set of standards known as "Section 7. Inductive-Loop Detectors." These were released early in 1981. This section of the JUTAI Standards defined functional standards, physical standards, environmental requirements, and interface requirements for several inductive-loop electronics unit configurations.

Section 7 described only the basic functions associated with inductive-loop detector electronics units. Users identified the need for additional functions for specific locations, particularly delay and extension timing. To cover this gap, JUTAI developed and in 1983 released "Section 11. Inductive-Loop Detectors with Delay and Extension Timing." This section was basically identical to Section 7 with the addition of requirements for the timing of delayed call and extended call features. A further revision resulted in a new Section 15, which was released February 5, 1987.. This new standard combines, updates, and supersedes Sections 7 and 11.

JUTAI Electronics Unit Configurations

The JUTAI Standards define two basic types of electronics unit configurations: shelf mounted and card-rack mounted. Shelf mounted units are commonly used in JUTAI controllers and are available in both single-channel and multichannel (two- or four-channel) configurations. Figure 2-31 shows an example of a shelf-mounted unit, which is powered by the 120-volt AC supply in the cabinet. Outputs are generated by electromechanical relays or by electrically isolated solid-state circuits. Physical dimensions and connector requirements are included in the JUTAI Standards in traffic road

Card-rack mounted electronics units, illustrated in Figures 2-32 and 2-33, fit into a multiple card rack and operate with external 24-volt DC power generated in the rack assembly or elsewhere in the controller cabinet. These devices are an effective way to reduce cabinet space requirements where large numbers of inductive-loop detector electronics units are needed.

Output Types

Electronics units have one of two output types: relay or solid-state optically isolated. Relay outputs use electromechanical relays to generate a circuit closure and generate a detection call to the controller. Solid-state outputs have no moving parts and are, therefore, generally more reliable and more accurate in tracking the presence of vehicles. This factor can be important in some traffic signal timing operations.

The relay outputs are designed to fail "on" (contacts closed) when power to the electronics unit is interrupted. The solid-state output fails "off" (nonconducting) in the same circumstances. Therefore, a relay output may be more desirable for use with intersection actuation because a constant-call would be safer than a no-call situation. A solid-state output is more desirable where accurate presence detection is desired.