This report is an archived publication and may contain dated technical, contact, and link information

Federal Highway Administration > Publications > Research Publications > 06139 > Appendj.Cfm Traffic Detector Handbook:Third Edition—Volume II

Publication Number: FHWA-HRT-06-139 Date: October 2006

Traffic Detector Handbook:Third Edition—Volume II

APPENDIX J. NEMA DETECTOR STANDARDS EXCERPTS

INTRODUCTION

This NEMA standard excerpt is reprinted with permission of the National Electrical Manufacturers Association, 1300 North 17th Street,Suite 1847, Rosslyn, VA 22209. For further information, and to order a complete copy of the standard, visit www.nema.org

The following excerpts are provided from NEMA Standards Publication TS 2- 2003 v02.06, Traffic Controller Assemblies with NTCIP [National Transportation Communications for Intelligent Transportation Systems (ITS) Protocol] Requirements. TS 2-2003 v02.06 is a revised and reballoted version of TS 2-1998. Version v02.06 includes the revisions from TS 2 amendment 1 v01 and also includes minor revisions to sections 3, 5, and 6. TS 2-2003 v02.06 was balloted in January 2003 and approved by NEMA in May 2003.

NEMA TS 2-2003 is published by the National Electrical Manufacturers Association, 1300 North 17th Street, Suite 1847, Rosslyn, VA 22209. For further information and to order a complete copy of the standard, visit www.nema.org. NEMA TS 2 was developed and is maintained by the member companies in the NEMA Transportation Management Systems and Associated Control Devices Product Group.

This appendix is reprinted by permission from the National Electrical Manufacturers Association. Excerpt is © 2003 by NEMA. All rights reserved. For clarity, text in [square brackets] has been revised from the original.

  NOTES:

1. NEMA standards have headers and text set in Arial, normal, 10-point type. Heading, figure and table numbers and font type in this appendix correspond to those in the actual NEMA standard rather than those found in rest of the Traffic Detector Handbook.
2. The term detector as used by NEMA is replaced by the terms sensor or electronics unit, depending on context, in the other chapters and sections of this Handbook.
3. In case of conflict with Detector Terms and Definitions beginning on page J-2, those listed in Chapter 1 and Appendix P are recommended for the applications described in this Handbook.

NEMA TS 1 AND TS 2 TRAFFIC CONTROL SYSTEMS

The NEMA TS 2 standard expands on the older NEMA TS 1 Traffic Control Systems standard. The TS1 standard was based on the philosophy that controllers would provide a basic set of features and standard connectors. Manufacturers would compete based on the hardware and software they provided inside the controllers. The NEMA TS1 standard was successful for isolated actuated intersection control, but it lacked sufficient detail for implementing more advanced features, such as coordinated-actuated operation and preemption. Type 1 systems include the controller unit, conflict monitor, and the included features of each. Individual vendors supplemented the TS 1 standard by providing the complement of features necessary for deploying coordinated-actuated traffic signal systems. This introduced incompatibility and procurement issues, particularly when government agencies needed to upgrade existing signal systems at a later date and had to solicit competitive bids. Nevertheless, the competitive market forces continued to rapidly advance the state of the practice and created a following that led many States to adopt the NEMA standard. In the late 1980s and early 1990s, the NEMA TS1 specification was updated with NEMA TS2 to provide coordinated-actuated operation, preemption, and an optional serial bus to simplify wiring.⁽¹⁾

The detectors, referred to as electronics units in other chapters of this Handbook, are described in the TS 2 standard by the following attributes:

• Rack mounted, 16 detector channels per rack, up to four racks.
• Port 1 connector: pluggable, interchangeable Bus Interface unit to convert Port 1 high speed serial data to format required by individual detectors.
• Communications with controller unit, as described above.
• Per channel diagnostic data: open loop; shorted loop; excessive inductance change; watchdog failure.
• Detector reset capability.
• Operation from either 12 volt or 24 volt DC power supply.

DETECTOR TERMS AND DEFINITIONS

These definitions reflect the consensus of the traffic control equipment industry as represented by NEMA and are intended to be in harmony with terminology in current usage, such as is published in the Manual on Uniform Traffic Control Devices and various technical reports of the Institute of Transportation Engineers.

1.2.4 Detection

1.2.4.1 Advisory Detection

The detection of vehicles on one or more intersection approaches solely for the purpose of modifying the phase sequence and/or length for other approaches to the intersection.

1.2.4.2 Passage Detection

The ability of a vehicle detector to detect the passage of a vehicle moving through the zone of detection and to ignore the presence of a vehicle stopped within the zone of detection.

1.2.4.3 Presence Detection

The ability of a vehicle detector to sense that a vehicle, whether moving or stopped, has appeared in its zone of detection.

1.2.5 Detector

A device for indicating the presence or passage of vehicles or pedestrians.

1.2.5.1 Bidirectional Detector

A detector that is capable of being actuated by vehicles proceeding in either of two directions and of indicating in which of the directions the vehicles were moving.

1.2.5.2 Calling Detector

A detector that is installed in a selected location to detect vehicles which may not otherwise be detected and whose output may be modified by the controller unit.

1.2.5.3 Classification Detector

A detector that has the capability of differentiating among types of vehicles.

1.2.5.4 Directional Detector

A detector that is capable of being actuated only by vehicles proceeding in one specified direction.

1.2.5.5 Extension Detector

A detector that is arranged to register actuations at the controller unit only during the green interval for that approach so as to extend the green time of the actuating vehicles.

1.2.5.6 Infrared Detector

A detector that senses radiation in the infrared spectrum.

1.2.5.7 Light-Sensitive Detector

A detector that utilizes a light-sensitive device for sensing the passage of an object interrupting a beam of light directed at the sensor.

1.2.5.8 Loop Detector

A detector that senses a change in inductance of its inductive loop sensor by the passage or presence of a vehicle near the sensor.

1.2.5.9 Magnetic Detector

A detector that senses changes in the earth's magnetic field caused by the movement of a vehicle near its sensor.

1.2.5.10 Magnetometer Detector

A detector that measures the difference in the level of the earth's magnetic forces caused by the passage or presence of a vehicle near its sensor.

1.2.5.11 Nondirectional Detector

A detector that is capable of being actuated by vehicles proceeding in any direction.

1.2.5.12 Pedestrian Detector

A detector that is responsive to operation by or the presence of a pedestrian.

1.2.5.13 Pneumatic Detector

A pressure-sensitive detector that uses a pneumatic tube as a sensor.

1.2.5.14 Pressure-Sensitive Detector

A detector that is capable of sensing the pressure of a vehicle passing over the surface of its sensor.

1.2.5.15 Radar Detector

A detector that is capable of sensing the passage of a vehicle through its field of emitted microwave energy.

1.2.5.16 System Detector

Any type of vehicle detector used to obtain representative traffic flow information.

1.2.5.17 Side-Fire Detector

A vehicle detector with its sensor located to one side of the roadway.

1.2.5.18 Sound-Sensitive Vehicle Detector

A detector that responds to sound waves generated by the passage of a vehicle near the surface of the sensor.

1.2.5.19 Ultrasonic Detector

A detector that is capable of sensing the passage or presence of a vehicle through its field of emitted ultrasonic energy.

1.2.6 Detector Mode

A term used to describe the operation of a detector channel output when a presence detection occurs.

1. Pulse Mode: Detector produces a short output pulse when detection occurs.
2. Controlled Output: The ability of a detector to produce a pulse that has a predetermined duration regardless of the length of time a vehicle is in the zone of detection.
3. Continuous-Presence Mode: Detector output continues if any vehicle (first or last remaining) remains in the zone of detection.
4. Limited-Presence Mode: Detector output continues for a limited period of time if vehicles remain in zone of detection.

1.2.7 Inductive Loop Detector System

See 1.6.56.5.1.5.

1.2.8 Inductive Loop Detector Unit

See 6.5.1.6.

1.2.9 Lead-in Cable

See 1.6.56.5.1.4

1.2.10 Output

1.2.10.1 Extension Output

The ability of a detector to continue its output for a predetermined length of time after the vehicle has left its zone of detection.

1.2.10.2 Delayed Output

The ability of a detector to delay its output for a predetermined length of time after a vehicle has entered its zone of detection.

1.2.11 Probe

The sensor form that is commonly used with a magnetometer type detector.

1.2.12 Sensor

The sensing element of a detector.

1.2.13 Vehicle Detector System

See 6.5.1.10.

1.2.14 Zone of Detection (Sensing Zone)

That area of the roadway within which a vehicle will be detected by a vehicle detector system.

ENVIRONMENTAL TESTING OF DETECTORS

[NEMA TS 2-2003] Section 2 relates to environmental standards and operating conditions for intersection traffic control equipment. This section establishes the limits of the environmental and operation conditions in which the Controller Assembly will perform. This section defines the minimum test procedures which may be used to demonstrate conformance of a device type with the provisions of the standard. These test procedures do not verify equipment performance under every possible combination of environmental requirements covered by this standard.

2.8 LOOP DETECTOR UNIT TESTS

The Loop Detector Unit shall perform its specified functions under the conditions set forth in this Section. This Clause defines the test procedures required to demonstrate the conformance of a Loop Detector Unit with the provisions of the standards.

2.8.1 Environmental Requirements

Loop detector units shall operate in accordance with requirements listed herein under the following environmental conditions.

2.8.1.1 Voltage, DC Supply

1. Voltage Range—The voltage range shall be 10.8 VDC minimum to 26.5 VDC maximum.
2. Ripple—The maximum supply ripple shall be 500 millivolts peak to peak.

2.8.1.2 Temperature and Humidity

Temperature and humidity shall be in accordance with 2.1.5.

2.8.1.3 Transients, DC Powered Units

Loop detector units shall operate normally when the test impulse described in 2.1.7 is applied as follows:

1. Between Logic Ground and the DC Supply power input. The test setup shown in Figure 2-4 shall be used for this test.
2. Across the output terminals of each channel while in both the detect and nondetect condition.
3. Between Logic Ground and the control inputs.

Detector loop inputs are specifically excluded from this test.

In the NEMA standard, tables and figures are delineated by a double ruled bar at the top and bottom of the table or figure. When the figure or table has notes at the bottom, the double rule is below the notes. This delineation is kept in this appendix for consistency with the standard.

Figure 2-4 TEST CONFIGURATIONS

TEST CONDITIONS:

• The transient generator is described in 2.1.7.
• The input voltage shall be 10.8 to 26.5 volts DC measured at the input terminal to the loop detector unit under test.
• The DC power source must be capable of supplying at least 50 milliamperes per channel.
• When testing for the reverse polarity transient, the diode shown shall be reversed.

2.8.1.4 Transients, Loop Detector Input Terminals

The loop detector unit shall be capable of withstanding the eight nondestructive transient tests described in Figure 2-5. Each test shall be repeated 10 times with the loop detector unit operating from its normal power source. The time between repetitions shall not exceed 10 seconds.

The energy source shall be a capacitor, oil filled, 10 microfarads ± 5 percent, internal surge impedance less than 1 ohm connected in accordance with Figure 2-5. The voltage on the capacitor shall be adjusted as described herein. The test push button shall be activated for at least one second for each of the 80 test repetitions.

After the foregoing tests, the loop detector unit shall operate normally.

1. Test Numbers 1 and 5. The Loop detector unit shall withstand the discharge of a 10 microfarad capacitor charged to ± 1000 volts applied directly across the loop detector unit sensor loop input pins with no sensor loop load present.
2. Test Numbers 2, 3, 4, 6, 7, and 8. The loop detector unit shall withstand the discharge of a 10 microfarad capacitor charged to ± 2000 volts applied directly across either the loop detector unit sensor loop input pins or from either side of the sensor loop input pins to equipment ground. The loop detector unit sensor loop input pins shall have a dummy resistive load attached equal to 5.0 ohms ± 10 percent.

The Transient Test Configurations data are not listed as a table in the NEMA specifications. Therefore, for consistency they are not listed as a table in this appendix.

 

Transient Test Configurations

Test Number	Test Selector Position	Power Supply Setting +5%	Polarity Selector	Load Across Detector Input	Tested Inputs
1	1	1000	Positive	None	D to E
2	2	2000	Positive	5 ohms	D to E
3	3	2000	Positive	5 ohms	D to L
4	4	2000	Positive	5 ohms	E to L
5	1	1000	Negative	None	D to E
6	2	2000	Negative	5 ohms	D to E
7	3	2000	Negative	5 ohms	D to L
8	4	2000	Negative	5 ohms	E to L

*The pin designations shown are for Channel 1. Similar tests shall be performed on all channels (i.e., pin pairs J and K, P and R, and U and V, respectively and as applicable) of the loop detector unit.

2.8.1.5 Vibration

The loop detector unit shall maintain both its physical integrity and operating characteristics after being subjected to the vibration test in 2.2.8. This test shall be run at nominal voltage and room environmental conditions.

2.8.1.6 Shock

The loop detector unit shall suffer neither permanent mechanical deformation nor any damage which renders it permanently inoperable after being subjected to the shock test described in 2.2.9.

This test shall be run at room environmental conditions without power applied to the unit.

MINIMUM REQUIREMENTS FOR DETECTORS

[NEMA TS 2-2003 Section 6] defines the minimum requirements for auxiliary devices within the cabinet, consisting of solid state load switches, solid state flashers, flash transfer relays, and inductive loop detector units.

[In the] TS 2-1998 update of TS 2-1992, the SECTION 6 [was revised to include these changes in detector specifications:]

Detector Configurations has been modified to add four new types (AC, BC,CC, & DC) with communications port TX & RX capability.

Detection Outputs & Status Outputs condition has been added for the Disable and Reset states.

Detector Connector Terminations has been modified to add Detector Address Bit #3.

6.5 INDUCTIVE LOOP DETECTOR UNITS

Clause 0 responds to the need for a series of Inductive Loop Detector Units which provide inputs for traffic-actuated or traffic-responsive control, surveillance, or data collection systems. The Inductive Loop Detector Unit responds to the presence of vehicles on the roadway by relying upon the effect of the conductive mass of the vehicle on the alternating magnetic field of a loop. When a vehicle passes over the loop of wire embedded in the surface of the roadway, it reacts with the alternating magnetic field which is associated with that loop. On standard loops this reaction is a reduction in loop inductance.

These standards cover the performance and design requirements of interchangeable Inductive Loop Detector Units. A Detector Unit used with a sensor loop embedded in the surface of a roadway detects vehicles moving or standing in the detection zone of the sensor loop. The output of the Detector Unit may be used directly to provide an input to a vehicle-actuated traffic CU or provide inputs to traffic-responsive control and surveillance systems. Detector Units generate outputs indicative of vehicles passing through the sensor loop zone of detection. This output may be used for counting (volume) or for detecting presence time representative of the time that vehicles are in the sensor loop zone of detection (occupancy), or both. When two loops are placed in a lane, one downstream from the other, the time between the sequential responses of detectors attached to these loops may be used to measure speed.

6.5.1 Loop Detector Unit Definitions

6.5.1.1 Channel

Electronic circuitry which functions as a Loop Detector System.

6.5.1.2 Crosstalk

The adverse interaction of any channel of a Detector Unit with any other channel.

6.5.1.3 Detector Mode

A term used to describe the duration and conditions of the occurrence of a detector output.

6.5.1.4 Lead-In Cable

The electrical cable which serves to connect the sensor loop(s) to the input of the Detector Unit.

6.5.1.5 Loop Detector System

A vehicle detector system that senses a decrease in inductance of its sensor loop(s) during the passage or presence of a vehicle in the zone of detection of the sensor loop(s).

6.5.1.6 Loop Detector Unit

An electronic device which is capable of energizing the sensor loop(s), of monitoring the sensor loop(s) inductance, and of responding to a predetermined decrease in inductance with output(s) which indicates the passage or presence of vehicles in the zone(s) of detection.

6.5.1.7 Reset Channel

A command for the Detector Unit to calculate a new reference frequency (the frequency that the loop oscillates at when no vehicle is influencing the loop) for the channel being reset and to appropriately adjust other channel related parameters.

6.5.1.8 Reset Unit

A command for the Detector Unit to set all parameters to the states they would be set at if power had been applied at the moment the RESET command is received or released.

6.5.1.9 Sensor Loop

An electrical conductor arranged to encompass a portion of roadway to provide a zone of detection and designed such that the passage or presence of a vehicle in the zone causes a decrease in the inductance of the loop that can be sensed for detection purposes.

6.5.1.9 Vehicle Detector System

A system for indicating the presence or passage of vehicles.

6.5.1.9 Zone of Detection

That area of the roadway within which a vehicle is detected by a vehicle detector system.

6.5.2 Functional Standards

6.5.2.1 Operation

The Detector Unit defined and described in this standard shall respond to changes in the inductance of the sensor loop/lead-in combination(s) connected to its loop input terminals. It shall develop a detection output when there is a sufficiently large decrease in the magnitude of the connected inductance.

The sensor loop(s) connected to the Detector Unit input terminals shall be located at the intended zone(s) of detection. The sensor loop(s) shall be connected to the Detector Unit by means of lead-in cable.

The sensor loop(s) shall be so configured that the presence of a vehicle in each zone of detection causes a sufficient decrease in inductance to cause an output response from the Detector Unit.

6.5.2.2 Configurations and Dimensions

6.5.2.2.1 Configurations

This standard covers Detector Unit configurations shown in Table 6-1.

Table 6-1 DETECTOR UNIT TYPES

Rack Mount Detector Unit Type	NEMA Detector Unit Designations	Delay/Extension Timing	Communication Port RX & TX
2 Channel	Type A	None	No
4 Channel	Type B	None	No
2 Channel	Type C	Included	No
4 Channel	Type D	Included	No
2 Channel	Type AC	None	Yes
4 Channel	Type BC	None	Yes
2 Channel	Type CC	Included	Yes
4 Channel	Type DC	Included	Yes

Each channel shall be provided with independent loop input terminals and shall deliver detection information on independent output terminals.

6.5.2.2.2 Dimensions

1. Two-channel card rack units shall be 28.96 mm (1.14 in.) max. W x 114.3 mm (4.5 in.) H x 177.8 mm (7.00 in.) D, excluding the handle as shown in Figure 6-4.
   
   
   Figure 6-4 TWO CHANNEL CARD RACK UNIT
2. Four-channel card rack units shall be 59.44 mm (2.34 in.) max. W x 114.3 mm (4.5 in.) H x 177.8 mm (7.00 in.) D, excluding the handle as shown in Figure 6-5.
   
   
   Figure 6-5 FOUR CHANNEL CARD RACK UNIT

6.5.2.3 Accessibility

The Detector Unit shall be easily disassembled to gain access for maintenance. When thus disassembled, the Detector Unit shall be operational for trouble shooting.

6.5.2.4 Material and Construction of Rigid Printed Circuit Assemblies

6.5.2.4.1 Materials

All printed circuit boards shall be per 3.2.3.1.

6.5.2.4.2 Component Identification

All components shall be identified per 3.2.3.3.

6.5.2.5 Power Inputs

Detector Unit DC Supply Voltage

1. Voltage Range–The voltage range shall be 10.8 VDC minimum to 26.5 VDC maximum.
2. Ripple–The maximum supply ripple shall be 500 millivolts peak to peak.

This input supplies power. The current consumption shall not exceed 50 milliamperes per channel. The return for this input is Logic Ground as described in 6.5.2.6. This input shall not be connected within the unit to any loop input. The input shall draw a surge current not to exceed 5 amperes at the time of power application to the input. The Detector Unit shall not be damaged by insertion to or removal from a powered Detector Rack.

6.5.2.5.2 Low Supply Voltage Automatic Reset

A Reset Unit condition shall be activated anytime the input DC supply voltage falls below that which is required for operation, as defined in this standard.

A Reset Unit condition shall not be activated when the DC Supply Input falls below 10 volts for less than 0.25 milliseconds.

6.5.2.6 Logic Ground

This input is the return for the Detector Unit DC Supply input. This point shall not be connected within the unit to AC Neutral, Earth Ground, or to any loop input terminal.

6.5.2.7 Earth Ground

The loop Detector Unit shall have a terminal for connection to the chassis of the unit. This input shall not be connected to Logic Ground, AC Neutral, or to any other point within the unit, except that it shall be permissible to use this input as a return for transient protection devices.

If the unit has a metallic case or front panel, the case and/or front panel shall be connected to Earth Ground.

6.5.2.8 DC Control Inputs

Control inputs shall have the following characteristics as measured from Logic Ground.

6.5.2.8.1 Low or Active State

A voltage between 0 and 8 volts shall be considered the Low or active state.

6.5.2.8.2 High or Inactive State

A voltage greater than 16 volts shall be considered the High or inactive state.

6.5.2.8.3 Transition Voltage Zone Of Input Circuitry

Transition zone of input circuitry from Low state to High state and vice versa shall occur between 8 and 16 volts.

6.5.2.8.4 External Transition Time

External transition from Low state to High state and vice versa shall be accomplished within 0.1 milliseconds.

6.5.2.8.5 Maximum Current

Over the voltage range 0 to 26 volts DC, the maximum current In or Out of any input control terminal shall be less than 10 milliamperes. The input circuitry shall be returned to the Detector Unit DC supply in such a manner that the removal of all connections to the input shall allow the voltage at the input terminal to rise to the High or inactive state. Rising to Detector Unit DC supply voltage is permitted.

6.5.2.8.6 Signal Recognition

Any input signal (including External Reset, Pin C) shall respond as defined in 3.3.5.1.3.

All channels shall remain in a RESET UNIT or RESET CHANNEL condition, see 6.5.1.7 and 6.5.1.8, while Pin C is held in an Active State (Low) voltage.

6.5.2.8.7 Activation of Delay/Extension Feature

The application of a Low state voltage to a Delay/Extension input shall function to inhibit the delay timing function and/or enable the extend timing function.

This input is provided for downward compatibility. (Authorized Engineering Information.)

6.5.2.8.8 Activation of Detector Unit Address Feature

The application of a Low state voltage to a Detector Unit Address input shall activate the input, e.g. it is a logic 1. Four address input pins shall provide for a maximum of 15 hard wired addresses. Address FF(hex) shall be a broadcast address.

6.5.2.9 Data Receive (RX) Input

The Data Receive input, RX, must have an input impedance 45 kohm to 105 kohm and an input capacitance less than 250 picofarads. This input impedance shall terminate to DC common. The input open circuit voltage shall be less than 0.9 VDC. The input shall be functional when voltages over the range of ±25 VDC are applied to it. The Transition Region is any voltage between a Mark and a Space.

6.5.2.9.1 Mark State (Binary 1)

During Mark, the RX input shall recognize as a Mark voltages less than 0.9 VDC.

6.5.2.9.2 Space State (Binary 0)

During Space, the RX input shall recognize as a Space voltages greater than 3.0 VDC.

6.5.2.9.3 Other States

The RX input shall not be damaged when connected to any voltage up to 26 VDC and:

1. Shall rise to >8.5 VDC when connected to +12 VDC through 30 kohm.
2. Shall rise to >16 VDC when connected to +22 VDC through 11 kohm.

6.5.2.9.4 Transient Withstand

The RX input shall not be damaged by application of the transients described in 2.1.7.1 through 2.1.7.5.

6.5.2.10 Loop Inputs

Two loop input terminals shall be provided for each sensor channel. These inputs shall be isolated (resistance > 10⁶ and breakdown voltage >1000 V rms) from Logic Ground, AC Neutral, and the control input and output circuits.

6.5.2.11 Loop/Lead in Electrical Properties

Each channel of the Detector Unit shall function in accordance with the specific requirements of this standard and in addition shall operate without significant degradation with any sensor loop/lead-in combination which exhibits the following electrical properties as measured at the Detector Unit terminals of the lead-in:

1. Inductance at 50 KHz – 50 to 1000 microhenrys.
2. Q at 50 KHz–greater than 5.
3. Resistance to earth ground–greater than 1 megohm.
4. Field installation practices or Detector Unit design may require grounding the shield of the loop lead-in cable. Such grounding should be in accordance with the Detector Unit manufacturer's recommendation.

6.5.2.12 Test Loop Configurations

Sensor loop and lead-in combinations used to verify the performance requirements of this standard shall consist of the following combinations of 1.828 m x 1.828 m (6 ft. x 6 ft) three-turn loops and shielded lead-in cable as illustrated in Figure 6-6.

1. Single-loop 1.828 m x 1.828 m (6 ft by 6 ft), three turns, with 30.48 m (100 ft) of lead-in (80–105 microhenrys).
2. Single-loop 1.828 m x 1.828 m (6 ft by 6 ft), three turns, with 304.8 m (1,000 ft) of lead-in (260–320 microhenrys).
3. Four loops 1.828 m x 1.828 m (6 ft by 6 ft), three turns, in a row in the direction of travel and separated by 2.743 m (9 ft), series/parallel connected with 76.0 m (250 ft) of lead-in (100–140 microhenrys).

6.5.2.13 Test Vehicle Definition

Detector Units shall detect all vehicles which ordinarily traverse the public streets and highways and which are comprised of sufficient conductive material suitably located to permit recognition and response by the Detector System.

Vehicles are classified by this standard in accordance with the reduction in inductance resulting when they are centered in the single 1.828 m x 1.828 m (6 ft x 6 ft), three-turn-test loop with 30.48 m (100 ft) of lead-in.

These minimum reductions are as follows:

1. Class 1: 0.13 percent (ΔL/L) or 0.12 microhenrys (ΔL) inductance change with a single 1.83 m x 1.83 m (6 ft x 6 ft), three turn loop, with 30.48 m (100 ft) lead-in (small motorcycle).
2. Class 2: 0.32 percent (ΔL/L) or 0.3 microhenrys (ΔL) inductance change with a single 1.83 m x 1.83 m (6 ft x 6 ft), three turn loop, with 30.48 m (100 ft) lead-in (large motorcycle).
3. Class 3: 3.2 percent (ΔL/L) or 3 microhenrys (ΔL) inductance change with a single 1.83 m x 1.83 m (6 ft x 6 ft), three turn loop, with 30.48 m (100 ft) lead-in (automobile).

• The maximum reduction caused by a class three vehicle shall be 5.4 percent (ΔL/L) or 5 microhenrys (ΔL) inductance change with a single 1.83 m x 1.83 m (6 ft x 6 ft), three-turn loop with 30.48 m (100 ft) lead-in.

Construction–Loop dimension tolerances shall be +50.8 mm (2 in.). Connections shall be soldered and waterproofed. Loops shall be installed in a non-reinforced pavement and located at least 0.914 m (3 feet) from any conductive material. Lead-in cable shall be spooled. Loop leads shall exit at one corner of the loop structures. All loop corners shall be chamfered 0.305 m (12 in.).

Loop Wire–Each loop shall be three turns of AWG #14 cross-linked polyethylene insulated, stranded copper wire, such as IMSA (International Municipal Signal Association) Specification 51-3, 1984, or equivalent. Loop inductance shall be between 60–80 microhenrys.

Lead-in Wire–The lead-in wire shall be AWG #14 twisted pair, aluminum polyester shield, polyethylene insulation, polyethylene jacket, inductance between 20 uH and 24 uH per 30.48 m (100 ft), such as IMSA Specification 50-2, 1984, or equivalent. For standardized test purposes, the shield shall be insulated from ground.

Field installation practices or Detector Unit design may require grounding the shield of the loop lead-in cable. Such grounding should be in accordance with the Detector Unit manufacturer’s recommendation. (Authorized Engineering Information).

Sawslot–The conductors shall be placed at the bottom of a 38.1 mm + 6.35 mm (1-1/2" +1/4 in.) deep by 6.35 mm (1/4 in.) wide sawslot. Pavement sawslot shall be filled with a suitable polyurethane or equivalent sealant.

6.5.2.14 Sensitivity

The Detector Unit shall be capable of detecting any of the vehicles defined in 6.5.2.14 on any of the test loops defined in 6.5.2.13.

6.5.2.15 Sensitivity Control

When detecting test vehicles as described in 6.5.2.13 and operating on any of the test loop configurations described in 6.5.2.12, each channel of the Detector Unit shall include means to adjust the sensitivity such that it shall not produce an output when the nearest point of any test vehicle of 6.5.2.13 is 0.914 m (36 in.) or more outside the loop(s) perimeter. A minimum of three sensitivity selections shall be provided for each detection channel.

6.5.2.16 Approach Speed

The Detector Unit shall detect any vehicle described in 6.5.2.13 over any of the single loops described in 6.5.2.12 traveling within the speed range of 8.045 km to 128.72 km (5 to 80 miles) per hour.

The Detector Unit shall detect any vehicle described in 6.5.2.13 over all of the loops of the four loop configurations described in 6.5.2.12 traveling within the speed range of 8.045 km to 64.36 km (5 to 40 miles) per hour.

All channels of a multichannel Detector Unit shall be operating at the same sensitivity and connected to equivalent inductances for the purpose of these tests.

6.5.2.17 Modes of Operation

Each Detector Unit channel shall be capable of functioning in the following two front panel selectable modes:

6.5.2.17.1 Presence

When a Class 2 vehicle defined in 6.5.2.13, or larger vehicle occupies the center of any test loops described in 6.5.2.12, the Detector Unit shall be capable of maintaining a detection output for a minimum of 3 minutes.

6.5.2.17.2 Pulse

A detection output between 100 and 150 milliseconds shall be initiated when a vehicle enters the sensor loop zone of detection.

If this vehicle remains in the zone of detection, the Detector Unit shall become responsive within a maximum of 3 seconds to additional test vehicles entering the zone of detection. The Detector Unit shall produce one and only one output pulse for a test vehicle traveling at 16.09 km (10 miles) per hour across the zone of detection of the single sensor loops defined in 6.5.2.12.

6.5.2.18 Recovery from Sustained Occupancy

When operating in the presence mode, and following an occupancy of any duration, the Detector Unit shall recover to normal operation with at least 90 percent of its selected sensitivity within five seconds after the zone of detection is vacated.

6.5.2.19 Response Time

When operating in the presence mode, the Detector Unit shall be capable of being set to produce an output in response to a step decrease in inductance equivalent to the minimum decrease from a class one vehicle as defined in 6.5.2.13 within not more than 100 milliseconds when tested on either of the single loop test configurations described in 6.5.2.12. In response to step return to the original inductance, the Detector Unit shall terminate its output within no more than 100 milliseconds.

6.5.2.19.1 Variation in Response Time

The difference between the minimum measured response time and the maximum measured response time for input changes in either direction, over any number of tests, to an input change equivalent to a Class 1 vehicle shall not exceed 10 milliseconds per channel multiplied by the number of active channels. The Detector Unit must be set to the proper sensitivity to detect a Class 1 vehicle.

The difference between the minimum measured response time and the maximum measured response time for input changes in either direction, over any number of tests, to an input change equivalent to a Class 3 vehicle shall not exceed 5 milliseconds per channel multiplied by the number of active channels. The Detector Unit must be set to the proper sensitivity to detect a Class 3 vehicle.

All channels of a multichannel Detector Unit which are ON shall be operating at the same sensitivity and connected to equivalent inductances for the purpose of these tests.

For certain specific surveillance applications involving vehicle speeds in excess of 72.405 km (45 miles) per hour, a more precise response time will be required. (Authorized Engineering Information.)

6.5.2.20 Tuning

Each Detector Unit channel shall include means for accommodating the range of sensor loop/lead-in inductance.

The unit shall tune automatically upon the application of power. It shall operate with at least its minimum sensitivity within 2 seconds after application of power, and at 90 percent of its selected sensitivity within 5 seconds after application of power.

6.5.2.21 Self-Tracking

The Detector Unit shall automatically accommodate those after-tuning changes in the loop/lead-in electrical characteristics as might reasonably be expected to occur in undamaged loops, properly installed in sound pavement and exhibiting the electrical properties outlined in 6.5.2.11, without producing a false output or change in sensitivity.

6.5.2.22 Recovery From Reset

After any reset, Reset Unit or Reset Channel, the Detector Unit shall operate with at least its minimum sensitivity within 2 seconds after the removal of the reset condition, and at 90 percent of its selected sensitivity within 5 seconds after the removal of the reset condition.

6.5.2.23 Crosstalk Avoidance

Each Detector Unit channel shall include means to prevent that channel from adversely interacting with any other channel. The means to prevent such interaction shall be either inherent, automatic, or manual switch.

6.5.2.24 Delay/Extension

Each channel of a unit with delay/extension shall have at least three modes of operation–delay, extension, and normal (neither delay nor extension). Channels 1 and 2 of four-channel Detector Units shall be the channels to include the delay/extension feature. Delay and extension timing shall be setable on a per channel basis with the timing programmed independently.

6.5.2.24.1 Delay

When selected, the output is delayed for the time set. If the vehicle departs before the time set, an output does not occur and the timer is reset. If a vehicle is present and the delay timer is active, when the delay inhibit is applied, the output shall become active. See Figure 6-7. This delay timing is controlled by the Delay/Extension input defined in 6.5.2.8.7.

The delay time shall be adjustable over the range from 0 to 30 seconds. The setability shall be within one second in the 0 to 15 second range and within two seconds in the 16 to 30 second range. The accuracy shall be + 1/2 second or + 5 percent of the setting, whichever is greater. When the Delay/Extension input is active, the delay shall be zero (0 to 0.1 second).

6.5.2.24.2 Extension

When selected, the output is extended after the vehicle departs the zone of detection for the time set. If a new vehicle arrives before the extension timer times out, the timer is reset, the output is maintained, and the timer resumes timing when the vehicle departs. See Figure 6-8. This extension timing is controlled by the Delay/Extension input defined in 6.5.2.8.7.

The extension time shall be adjustable in the range from 0 to 7-1/2 seconds. The setability shall be within 1/2 second. The accuracy shall be + 1/2 second. When the Delay/Extension input is inactive, the extension shall be zero (0 to 0.1 second).

6.5.2.25 Controls and Indicators

All controls and indicators necessary for the operation of the Detector Unit shall be located on the front panel of the unit except as noted below. Multiple functions combined in a single control shall be permitted. The controls and indicators shall include, but are not limited to:

1. Output Indicator—Means to visually indicate the output state of each channel. Each channel shall have a separate indicator.
2. Sensitivity Control—Means to permit selection of the sensitivity of each channel as described in 6.5.2.15.
3. Reset—(Reset Unit or Reset Channel or both) A control which unconditionally causes the Detector Unit or detection channel to retune to a non-vehicle present condition.
4. Mode Selector—Shall provide for selection of pulse or presence mode operation of each channel. Card mounting of this control shall be permitted where actuation does not require disassembly.
5. Crosstalk Control—As required, shall provide means to prevent interaction of channels as described in 6.5.2.23.
6. Delay/Extension Selection Control—The type C, D, CC, and DC Detector Units shall have either a control or combination of controls to allow for the selection of one of at least three operating modes—delay, extension, and normal (neither delay nor extension). It shall be permissible to have the normal mode selected by either setting a three position selector switch to the normal position or setting the delay/extension timing control to zero. Card mounting of the control(s) shall be permitted where actuation does not require disassembly.
7. Delay/Extension Timing Control—The type C, D, CC, and DC Detector Units shall provide a means to permit setting of the time duration of the delay/extension period for each channel as described in 6.5.2.24. Card mounting of this control shall be permitted where actuation does not require disassembly.
8. Enable/disable—Means to turn ON (enable) or OFF (disable) a channel or number of channels on a Detector Unit.

6.5.2.26 Outputs

6.5.2.26.1 Solid State Channel Detection Outputs

The output interface of each channel shall be an independent, isolated, solidstate output. Each output device shall have the following characteristics:

1. Output Solid-State Device—The output shall be conducting when a vehicle is detected or when the loop circuit is in a failed state (i.e., open loop or shorted loop). The channel Disabled (OFF) condition shall have the output in the nonconducting state. During a Reset (Reset Unit or Reset Channel) condition the output shall be in the conducting state.
2. DC Supply Voltage Failure Condition—The output shall be conducting when it is terminated to a NEMA defined input, indicating a detection output condition.
3. Output Circuit Isolation—The isolation between each output device terminal pair and all other terminals shall exceed:
   1. Resistance >10⁶ ohms
   2. Breakdown >1000 V rms
4. Output Rating—The output shall conduct a minimum of 20 milliamperes with a maximum 3.5 volt drop across the output terminals in the conducting state. The output shall conduct a maximum of 500 microamperes with any voltage between 0 and 26 VDC applied across the output terminals in the non-conductive state.
5. Transition Time—When switching to or from a steady state current in the range of 2.4 to 20 milliamperes, the transition time from 8 to 16 volts and vice versa shall be 0.1 milliseconds or less. The circuit(s) to which the output is connected is defined in 3.3.5.1.3.
6. Maximum Voltage—When in the non-conducting state, the output shall tolerate a voltage as high as 30 VDC without damage.

6.5.2.26.2 Channel Status Outputs

Each Detector Unit channel shall provide a channel status output. The channel status output shall be a solid state device with the following characteristics:

1. Output Solid State Device—The output shall be conducting when the channel is operating properly. The channel Disabled (OFF) condition shall have the output in the non-conductive state. During a Reset (Reset Unit or Reset Channel) condition the output shall be in the non-conducting state.
2. DC Supply Voltage Failure Condition—Output device is nonconductive,indicating a fault condition.
3. Status Outputs shall be referenced to logic ground.
4. Output Rating—The output shall conduct a minimum of 20 milliamperes with a maximum 3.5 volt drop across the output terminals in the conducting state. The output shall conduct a maximum of 100 microamperes with any voltage between 0 and 26 VDC applied across the output terminals in the non-conductive state.
5. Transition Time—When switching to or from a steady state current in the range of 2.4 to 20 milliamperes, the transition time from 8 to 16 volts and vice versa shall be 0.1 milliseconds or less. The circuit(s) to which the output is connected is defined in 3.3.5.1.3.
6. Maximum Voltage—When in the non-conducting state, the output shall tolerate a voltage as high as 30 VDC without damage.

6.5.2.26.3 Channel Status Reporting

The Channel Status Output shall provide for the communication of eight distinct status states as defined below:

1. Normal operation (Detector Unit and loop OK).
2. Detector Unit failure (watch dog time-out, channel currently in RESET, or channel disabled).
3. Open loop (An open loop may be reported when the terminal inductance is >1000 microhenrys and <2500 microhenrys. The open loop shall be reported when the terminal inductance is >2500 microhenrys).
4. Shorted loop (A shorted loop may be reported when the terminal inductance is <50 microhenrys and >20 microhenrys. A shorted loop shall be reported when the terminal inductance is <20 microhenrys).
5. Excessive inductance change (+25%).
6. Reserved.
7. Reserved.
8. Reserved.

Pulse width modulation shall be utilized to encode the eight possible states as described below:

1. Continuous Low or On state.
2. Continuous High or Off state.
3. 50 milliseconds Off time.
4. 100 milliseconds Off time.
5. 150 milliseconds Off time.
6. 200 milliseconds Off time.
7. 250 milliseconds Off time.
8. 300 milliseconds Off time.

The tolerance for all times listed above shall be + 10 milliseconds; the On time between pulses shall be 50 milliseconds + 10 milliseconds.

The Channel Status Output shall reflect the current fault status of the channel (i.e., the fault status shall be self clearing in the event that a fault condition clears).

The Channel Status Output shall maintain an output state, other than a condition generated during a Reset, for a minimum of 5 seconds. The Channel Detection Output shall maintain the conducting state during a fault condition only while the fault condition exists.

The Channel Status Output shall be in State 2 during RESET. Once the RESET is complete and the Detector Unit resumes normal operation, the channel status output shall return to State 1.

The Channel Status Output shall be in State 2 for channels that are disabled (OFF). The Channel Detection Output shall maintain the non-conductive state (State 2) for channels that are disabled.

The last channel failure status shall be stored in memory for future reference. Enunciation of the last failure status shall be accomplished through the use of the front panel indicators. If power is removed or the Reset is activated, the last channel status information shall be cleared.

The particular method employed to enunciate past failure status is not specified and will be manufacturer specific. (Authorized Engineering Information.)

6.5.2.26.4 Data Transmit Output (TX)

This output shall have 3 states: Mark, Space, and High Impedance. The output shall be in the High Impedance state except when transmitting to the BIU. The transmitter output shall go from the High Impedance state to the Mark state until the first Start Bit is sent. The Transition Region shall be defined as any voltage between a Mark and a Space.

6.5.2.26.4.1 Mark State (Binary 1)

During Mark, the TX output shall be less than 0.5 VDC when sinking 4 milliamperes current.

6.5.2.26.4.2 Space State (Binary 0)

During Space, the TX output shall be greater than 3.5 VDC when sourcing 4 milliamperes current.

6.5.2.26.4.3 High Impedance State

The TX output shall have an impedance greater that 1 Megohm in the High Impedance state. When connected to the following loads, the TX output in the High Impedance state:

1. Shall be >8.5 VDC when connected to +12 VDC through 30 kohm.
2. Shall be >16 VDC when connected to +22 VDC through 11 kohm.

6.5.2.26.4.4 Output Impedance During Power Off

When power is OFF, the output impedance shall be greater that 300 ohms when measured with an applied voltage not greater than 2 volts in magnitude to circuit common.

6.5.2.26.4.5 TX Output Shorts

The TX output shall not be damaged under these situations:

1. The TX output shall be capable of withstanding a continuous short to DC common.
2. The TX output shall be capable of withstanding a continuous short to +26 VDC through 600 ohms.

6.5.2.26.4.6 Rise/Fall Time

The transition time from the Space to the Mark voltage or from the Mark to the Space voltage shall be less than 2.5 microseconds or 4% of a bit time, whichever is less. This shall apply with 3750 picofarads attached to the TX output. There shall be no reversal of the direction of voltage change while the signal is in the Transition Region. The Transition Region shall not be reentered until the next change of signal condition.

6.5.2.26.4.7 Transient Withstand

The TX output shall not be damaged by application of the transients described in 2.1.7.1 through 2.1.7.5

6.5.2.27 Communication Port Functional Requirements

The protocol for this port is under definition.

Inductive Loop Detector Unit types AC, BC, CC, and DC have this communication port.

6.5.2.27.1 Communication Port Electrical Requirements

This port, RX and TX and DC Common, is an unbalanced, +5 volt nominal, standard NRZ (Mark/Space) format, asynchronous serial communication port. For compatibility with systems currently in use, when TX is shorted to the Ch 2(+) Output and when RX is shorted to the Ch 4(+) Output, TX and RX must not interfere with Ch 2(+) or Ch 4(+) operation. See 6.5.2.9 for RX specification details and 6.5.2.26.4 for TX specification details.

6.5.2.27.2 Baud Rate

The port shall be capable of operation at a baud rate of 9600 bps ±1%. The width of 1 bit shall be the reciprocal of the baud rate.

6.5.2.27.3 Communication Parameters

The standard NRZ (Mark/Space) format shall be used. Further specifications are under definition.

6.5.2.27.4 Slot Addresses

The address of a Detector Unit slot is "hard-wired" at each Detector Rack connector. A logic 1 shall be created by wiring an address pin to DC Common. Open or +24VDC shall be a logic 0. Slot 1 shall be Address 0 and Slot 15 shall be Address 14.

6.5.2.28 Electrical Connections

6.5.2.28.1 Connector Description

Two and four channel card rack units shall mate with a 44 terminal, double row, 3.2 mm (0.156 in.) contact spacing, Cinch Jones card edge connection 50-44A- 30M, or equivalent.

6.5.2.28.2 Connector Terminations

Input / Output connector pin terminations shall be as shown in Table 6-2.

Table 6-2 CONNECTOR TERMINATIONS

Pin	Function	Pin	Function
A	Logic Ground	1	Channel 1 Delay/Extension Input
B	Detector Unit DC Supply	2	Channel 2 Delay/Extension Input
C	External Reset	3	Detector Unit Address Bit #3
D	Channel 1 Loop Input	4	Channel 1 Redundant Loop Input (Optional)
E	Channel 1 Loop Input	5	Channel 1 Redundant Loop Input (Optional)
F	Channel 1 Output (+)	6	Detector Unit Address Bit #0
H	Channel 1 Output (-)	7	Channel 1 Status Output
J	Channel 2 Loop Input	8	Channel 2 Redundant Loop Input (Optional)
K	Channel 2 Loop Input	9	Channel 2 Redundant Loop Input (Optional)
L	Chassis Ground	10	Detector Unit Address Bit #1
M	Reserved (AC Neutral)	11	Reserved (AC Neutral)
N	Reserved (AC Line)	12	Reserved (AC Line)
P	Channel 3 Loop Input	13	Channel 3 Redundant Loop Input (Optional)
R	Channel 3 Loop Input	14	Channel 3 Redundant Loop Input (Optional)
S	Channel 3 Output (+)	15	Detector Unit Address Bit #2
T	Channel 3 Output (-)	16	Channel 3 Status Output
U	Channel 4 Loop Input	17	Channel 4 Redundant Loop Input (Optional)
V	Channel 4 Loop Input	18	Channel 4 Redundant Loop Input (Optional)
W	Channel 2 Output (+)	19	Data Transmit Output (TX)
X	Channel 2 Output (-)	20	Channel 2 Status Output
Y	Channel 4 Output (+)	21	Data Receive Input (RX)
Z	Channel 4 Output (-)	22	Channel 4 Status Output

6.5.2.28.3 Type A Two Channel Without Delay / Extension Timing

The following pins shall be inactive: P, R, S, T, U, V, Y, Z, 1, 2, 13, 14, 16, 17, 18, and 22.

6.5.2.28.4 Type B Four Channel Without Delay / Extension Timing

The following pins shall be inactive: 1 and 2.

6.5.2.28.4 Type C Two Channel With Delay / Extension Timing

The following pins shall be inactive: P, R, S, T, U, V, Y, Z, 13, 14, 16, 17, 18, and 22.

Pin 1 through 22 is on the top (component) side and pin A through Z is on the back (solder side). Polarization keys shall be located at three positions:

Between B/2 and C/3

Between M/11 and N/12

Between E/5 and F/6

Pins 3, 6, 10, and 15 are address pins for Type AC, BC, CC and DC Inductive Loop Detector Units. When one of these Detector Unit types are installed, it will be assigned an address associated with the Detector Unit position in the Detector Rack.

Pins 19 and 21 are the TX output and RX input for communication with the Detector Unit. The communication protocol is under definition. (Authorized Engineering Information.)

REFERENCES

1. Bullock, D., and T. Urbanik. "Traffic Signal Systems: Addressing Diverse Technologies and Complex User Needs," TRB A3A18 Committee on Traffic Signal Systems. http://onlinepubs.trb.org/onlinepubs/millennium/00116.pdf, accessed June 22, 2007.

Previous | Table of Contents | Next

FHWA-HRT-06-139