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September/October 1998· Vol. 62· No. 2

Staying in the Loop: The Search for Improved Reliability of Traffic Sensing Systems Through Smart Test Instruments

The average commuter spends more than 40 minutes on the road going to work and coming home again. Much of this time is spent waiting at traffic signals. Many drivers have had the experience of waiting, waiting, waiting for a signal to change to green.

Much of this waiting could be avoided if the traffic-signal control system could detect that a driver was present or if the system could accurately determine how many vehicles were waiting for the signal. Many times, the traffic-signal controller is not aware of the vehicles waiting at the traffic light because the sensor, a simple loop of copper wire embedded in the pavement, has malfunctioned.

If the sensor, known as an "inductive loop" (IL) detector, does not work correctly, then the traffic-signal controller cannot optimally control traffic. Therefore, the performance of the loop detector system, which includes the wire in the roadway, the sensor electronics in the cabinet, and the lead-in wire connecting the two (see figure 1), is critical to minimizing delay, stops, and air pollution. The purpose of this article is to explain why loops fail, why an inductive loop test instrument is needed to test them, and how the Federal Highway Administration (FHWA) developed the inductive loop tester (ILT).

Why IL Detectors Fail
In the early 1990s, FHWA's Office of Research and Development (R&D) began to explore ways to reduce the high malfunction rate of traffic sensors. One method is to develop new sensor technologies such as laser, video, and radar sensors. That approach is beginning to bear fruit. However, another cost-effective approach is to reduce the problems with the large base of installed inductive loop sensors. The first step to solve the problems with IL sensors is to analyze how the sensor works.

The inductive loop sensor is a relatively simple sensor. When a vehicle reaches the loop, the metal of the vehicle disturbs the magnetic field over the loop, which causes a change in the loop's inductance. Inductance is an electrical property that is proportional to the magnetic field. (See figure 1.) This is how the loop sensor detects a vehicle.

The size of the loop, the shape of the loop, the number of "turns" in the loop coil, and the length of the lead-in wire all combine to form a specific circuit. The current passing through the loop generates an electromagnetic field. When a vehicle passes through the field, it acts as a conductor, changing the inductance of the loop. The sensor detects this change and notifies the traffic-signal controller of its finding.

There are a wide variety of possible causes for IL failure — such as damage to insulation during shipment — but the two most common reasons for failure are poor pavements and poor installation techniques.

Older pavements can be unstable, and in northern states, pavement can be weaken by the freeze-thaw cycle. This can result in significant pavement movement that can stretch or break the wires and connections, causing the loop to malfunction.

Most loops malfunction as a result of mishandling in the installation process; often unskilled technicians set up the system poorly. For example, if the loop were not sealed properly during the installation process, moisture could reach the wire and change its electrical characteristics.

Older loop sensors have additional problems. They were made of large-scale integrated circuits that operated at slower speeds, resulting in slow response times and poor speed-measuring capabilities. Improved technology has significantly reduced these problems. Unless the traffic-control box is struck by lightning or is poorly grounded or a utility company backhoe digs through the loop, the modern systems perform much better. Every loop sensor has an oscillator on the front end. The oscillator has a frequency determined by the loop's inductance. By measuring that inductance and how it changes, the arrival and passage of a vehicle can be detected. The more the inductance changes, the easier it is to detect the arrival or passage of a vehicle.

Unfortunately, some kinds of vehicles cause much smaller shifts in inductance, and thus they are more difficult to detect. These include high-bed trucks, which carry their conductive surfaces much higher — farther from the loop. Similarly, bicycles have a minimal effect on the field of the loop because they have much smaller conductive surfaces, and the surfaces they do have are structured perpendicular to the loop. (Note: Loops sense conductive material while magnetometers sense ferrous materials.) Thus, even when shipped and installed correctly, loop systems may need to have their operating parameters fine-tuned to correctly detect the vehicles operating over them.

To troubleshoot, test for acceptance, and fine-tune these systems, traffic technicians need high-quality, easy-to-use, reliable diagnostic equipment that allows the technician to verify whether the loop is functioning as it should. Therefore, a set of loop detector analysis software was written. The software analyzed the performance of the loop detector in a variety of configurations.

The Quest for a Better System
In the past, it was difficult to accurately measure the quality and performance of installed loops because of problems with the measuring instruments and with knowing what values to expect from the measurement.

For years, traffic technicians working in the field used standard equipment borrowed from the electronics repair shop. Because of technological limitations and the difficulties of measuring a variety of electrical parameters with a single device, technicians typically used three or more different instruments to measure parameters such as inductance, resistance, and quality factors. For example, the technician would use an ohm meter, a megger, a voltmeter, a frequency counter, and a quality-factor meter to measure these different parameters.

The electrical theory of how loops worked was poorly understood. Thus, many technicians did not really know what values to expect, and they could not compute the correct readings for a loop. As a result, the technicians' dominant diagnostic approach was the old-fashioned trial-and-error method. Such methods were labor-intensive and relied on the technician's training and experience with detector diagnostics and troubleshooting.

In the early 1990s, FHWA R&D set up a small business innovative research (SBIR) study to produce an inductive loop detector test system. The goal was to produce a single instrument that would allow a technician to perform all of the critical measurements needed to determine whether a loop was operating correctly.

This requirement has become more important with the passage of time because of the shortage of qualified electrical and electronics technicians. Most state and city transportation departments can no longer afford to compete with the higher wages offered in the private sector.

FHWA selected DVP (Digital Video Processing), a company with expertise in advanced data signal processing (DSP) and hardware design, for the SBIR study. They were able to use DSP techniques to develop a multiple-use test instrument in which most of the key functions were embedded in software rather than hardware.

Digital Signal Processing (DSP)

ILT I
DVP's first effort was an experimental concept design and was called the ILT I. Their first hurdle in creating ILT I was to measure the loop's inductance and quality factor at different operational frequencies. To get an accurate measurement, a digital frequency synthesizer was developed, and a digital inductance quality-factor meter, which covered a frequency range of 10,100 kilohertz, was designed.

The new instrument was able to take measurements at any test frequency and provide a more accurate depiction of the loop's actual operation. Similarly, the new instrument measured inductance at different frequencies. By measuring loop frequency and impedance, the instrument could calculate and display the loop's inductance.

This new system also measured the quality factor of the loop. The quality factor is proportional to loop frequency and inductance and is inversely proportional to the resistance of the loop circuit. If the loop circuit resistance increases because of breakdowns in the insulation of the loop wires, the loop quality factor decreases. Because the loop resistance and inductance are dependent on the operating frequency, which may vary, the measurement of these parameters and the measurement of the loop's quality factor had been difficult to determine in the past. An accurate measurement is essential to the loop's functionality. The quality factor is a key element in determining whether to accept or reject a loop. Punctures in the insulation of the loop or in lead-in cable wires will significantly reduce the loop's quality factor.

To get an accurate measurement of detector frequency, frequency meters used by traffic departments typically needed the loop detector frequency to be relatively continuous. This can be a problem in dealing with multichannel loop detector systems because the systems operate several loops for short sample periods, making it difficult to get an accurate loop frequency measurement. Loop frequency measurements are primarily useful to troubleshoot loop detector electronics in the field. A robust frequency meter was designed that could measure the frequency of multichannel loop detector systems with short sample times, using DSP.

Recent advances in DSP techniques allowed a sophisticated digital test instrument to be designed using basic off-the-shelf components. By processing the data digitally, the system can be more flexible in its measurements, provide more consistent results, and provide more detailed analysis than analog processing would permit.

ILT II
Because ILT I was never actually commercialized, ILT II was the first true product produced under the ILT research. It was a reduction of concepts and technology developed with the ILT I. FHWA's Office of Technology Applications field tested ILT II in many cities and states under a wide range of field conditions. Field engineers and technicians uncovered a variety of limitations in using the device, and a report was written about their findings.1 This feedback provided the basis for the development of ILT III.

ILT III
State and city technicians recommended that ILT III include a megger to measure the condition of the insulation of the loop and lead-in cables. A megger generates a 500-volt charge, which breaks down the loop-wiring insulation if any breaks exist. The megger causes a total loop failure if the insulation is defective. By using a megger, a technician can determine whether the insulation is faulty right away, rather than allowing moisture to eventually seep through the break in insulation, causing the loop to malfunction. A low-voltage measurement of wire resistance in a loop will not reveal a small break in the insulation; therefore, the megger, which generates high voltages, was added to ILT III. Users also wanted additional data-storage capacity so that information could be better stored and processed. This was done by incorporating a high-end Hitachi microprocessor (better known for its use in Nintendo games) in the design of the ILT III.

ILT III is configurable via PCMCIA cards. PCMCIA cards are small cards designed to fit into laptop computers, hand-held instruments, and high-end scientific calculators. The card for the ILT III stores programs and field-data and allows the information to be portable between ILT units and computers. This allows the software in the unit to be easily upgraded without having to "perform surgery" on the unit. It also allows the user to easily make changes in the measured characteristics by changing the PCMCIA card. For example, the unit can be used for measuring airflow in pediatric asthmatics by using a different PCMCIA card and a custom sensor.

Future Needs
ILT III, in its current form, still does not meet all the needs of a complete inductive loop diagnostic system. An expert system — a system that could guide the technician in selecting which advanced diagnostics tests to run — is needed. The expert system can use electrical loop design theory to calculate the expected electrical characteristics of the design for the technician. The expert system could then control the instrument to make the field measurements. Field measurements could then be compared to the theoretical measurements, and recommendations would be made on how to improve the system. Such a system would enable a less-skilled technician to handle the complex jobs associated with intelligent transportation systems (ITS) detection technologies, such as troubleshooting inductive loop detector systems. The problem in developing this system is that such expert systems require the collection of a lot of performance data, and no database has yet been developed to store the data.

The development of ILT is currently constrained by its high cost. At $2,000 per unit, the device is a good value for the money, but it is too expensive to be given to every technician. As more units are purchased, it may be possible to produce a custom-injection molded unit that will significantly reduce the manufacturing costs.

ILT III is now undergoing the same kind of field tests as its predecessors. FHWA's Office of Technology Applications will be releasing a final report on these tests in the near future. The active cooperation of the Advanced Research Team and the Enabling Technologies Team of FHWA's Office of R&D, the Office of Technology Applications, and the Innovations Deserving Exploratory Analysis (IDEA) program of the Transportation Research Board enabled the development of this product. ILT III now provides signal technicians with the capabilities they need to diagnose and troubleshoot inductive loop detectors.

Reference 1. Inductive Loop Tester ILT II, Summary Report, Publication No. FHWA-SA-94-077, Federal Highway Administration, Washington, D.C., 1994.

David Gibson is a traffic research engineer on the Enabling Technologies Team of the Office of Safety and Traffic Operations Research and Development. He is a registered professional traffic engineer and has a master's degree in transportation from Virginia Polytechnical Institute and State University. His areas of interests include traffic sensor technology, traffic-signal controls, traffic modeling, computers, and traffic engineering education (specifically how applying advanced technologies can simplify an engineer's daily work). He worked with Milton K. (Pete) Mills to develop the first two editions of the Traffic Detector Handbook and to develop the original type I-70 traffic-signal controller system.

Milton K. (Pete) Mills is an electronics engineer on FHWA's Advanced Research Team, Office of Safety and Traffic Operations Research and Development. From 1963 to 1966, he tested and evaluated aircraft antenna systems at the U.S. Naval Air Test Center, Patuxent River, Md. From 1966 to 1968, he designed and patented a number of spacecraft antenna systems at NASA's Goddard Space Flight Center in Greenbelt, Md. Since 1968, he has managed the development and evaluation of vehicle sensor systems at the FHWA's Turner-Fairbank Highway Research Center in McLean, Va. He received his bachelor's degree in electrical engineering from North Carolina State University and his master's degree in electrical engineering from Catholic University in 1975.

Doug Rekenthaler Jr. is a freelance writer and editor. His experiences as a writer and editor include cub reporter covering Capitol Hill and Pentagon news beats; managing editor responsible for 12 newsletters that covered a wide array of communications technologies; founder of the multimedia industry's first daily fax news service; and corporate communications manager for America Online Inc., the largest commercial online service in the world.

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