CHAPTER 7. MAGNETIC SENSORS

INTRODUCTION

Magnetic sensors can be intrusive and non-intrusive devices that detect the presence or passage of vehicles by measuring the change in the earth’s magnetic field when vehicles enter the detection zone of the sensor. They are rarely used to detect bicycles. They are typically embedded in or mounted on the road surface and are designed to monitor traffic flow, detect vehicles at stop bars, and count vehicles passing through an intersection. Two types of magnetic sensors are used for traffic flow measurement: (a) two- and three-axis fluxgate magnetometers, and (b) induction or search coil magnetometers.

Two- and three-axis fluxgate magnetometers detect variations in the earth’s magnetic field caused by ferrous metal objects, such as vehicles. The two-axis magnetometer has two primary windings and two secondary windings, allowing it to measure both vertical and horizontal magnetic field components. Figure 34 shows examples of magnetometers, a control cabinet, hardware, and two-axis fluxgate magnetometers installed in the middle of a right-turn lane of an intersection.

These magneto-resistive sensors are typically installed in the middle of each lane by drilling holes and covering the sensors with epoxy. Most sensors transmit the data wirelessly to a receiver or gateway, which relays the information to a traffic signal controller or a data collection

server. The data transmission range can be extended by using a repeater between the sensor and the nearest access point. Most wireless sensors are battery-powered, often designed to last 5–10 years depending on usage and traffic conditions. Some advanced models use energy-harvesting technologies (e.g., from vibrations or solar power) to extend battery life.

The second type includes magnetic detectors known as induction or search coil magnetometers. These sensors identify vehicles by detecting changes in magnetic flux lines, which occur when the earth’s magnetic field is disrupted by the movement of metal objects such as vehicles. They typically consist of a coil wound around a magnetic rod, enabling them to sense changes in the magnetic field as vehicles pass over or near the sensor (Klein et al., 2006).

Compared to inductive loops, magnetometers typically require fewer linear feet of sawcut and have a longer life expectancy when installed in crumbly pavements. Magnetometers can also be installed in a conduit drilled underneath the pavement (Figure 35) from the shoulder or median, which avoids cutting the pavement surface (Klein et al., 2006). Some magnetometers cannot detect slow-moving or stopped vehicles.

STRENGTHS AND WEAKNESSES

Table 8 summarizes the main strengths and weaknesses of magnetic sensors, which are primarily used to detect motorized traffic.

Table 8. Strengths and Weaknesses of Magnetic Sensors.

NCHRP Project 03-144 validated two-axis fluxgate magnetometers (Figure 34) installed at an intersection in Michigan. The average accuracy errors were low (WMAPE = 2.0%), with no significant undercounting or overcounting observed. Despite the high accuracy of the sensors validated in this project and reported in the literature (Cheung et al., 2005; Haoui et al., 2008; Taghvaeeyan and Rajamani, 2014; Bao et al., 2016), fluxgate magnetometers have some limitations. For example, each sensor covers a relatively small area, requiring multiple sensors to cover an entire intersection. The initial installation can be complex and may require road closures, leading to temporary traffic disruptions (Gheorghiu et al., 2021). Some sensors are sensitive to changes in temperature (Gheorghiu et al., 2021). Magnetometers can be affected by magnetic interference from nearby sources, such as large metallic objects or underground infrastructure. Sensors may require periodic calibration and tuning to maintain accuracy, especially in areas with changing magnetic fields or high levels of electromagnetic noise. While excellent at detecting vehicle presence and movement, magnetometers typically provide less detailed data than other technologies, such as video-based systems.

Like all sensors, magnetometers may undercount or overcount vehicles. Common causes of undercounting are:

• Low magnetic signature vehicles: Vehicles with low ferrous content (e.g., bicycles) may not generate a strong signal for detection.
• Weak signal sensitivity: Reduced sensor sensitivity can result in missed detections, especially for smaller vehicles.
• Interference from adjacent magnetic fields: Nearby magnetic sources, such as power lines or large industrial equipment, can interfere with the magnetic signature of vehicles.
• Environmental noise: Strong environmental magnetic fluctuations, such as from utility cables or heavy machinery, may disrupt the sensor’s ability to accurately detect vehicles.

Common causes of overcounting are:

• Detection of non-vehicular metal objects: Magnetometers may mistakenly detect moving metal objects and debris as vehicles, especially when the sensor sensitivity is set too high.
• External disturbances: Construction activities or other mechanical disturbances can trigger false detections.
• Cross-lane interference: Magnetic fields from vehicles in adjacent lanes may cause false detections.
• Long vehicles: Long vehicles (e.g., pick-up trucks with trailers), especially in high traffic conditions, may result in overcounting.

RECOMMENDED PRACTICES

Recommended practices and ideal characteristics of two-axis fluxgate magnetometers and data for traffic monitoring use are described below.

Installation and Calibration

• Strategic positioning: Install magnetometers at strategic positions where they can detect vehicles effectively. Common placements include near the stop bar at signalized intersections or other key points where vehicles tend to stop, decelerate, or queue. Positioning the sensor at these locations ensures that vehicle presence is detected reliably, especially during critical moments like signal actuation.
• Proper depth: Install sensors at the proper depth to avoid interference from large metallic objects such as underground utility lines, rebar in concrete, or metallic debris, which can skew the readings. Placing the sensors too shallow or too deep can result in inaccurate vehicle detection because the magnetic field changes may not be sensed properly.
• Correct alignment: Align the magnetometers correctly according to the traffic flow direction to capture magnetic field changes accurately. Misalignment can lead to detection errors, particularly in capturing the presence or speed of a vehicle as it passes over the sensor.
• Environmental protection: Ensure the sensors are protected from water ingress, moisture, extreme temperatures, and other environmental factors that could affect performance. Proper sealing and protective casings are essential to safeguard the sensor from these elements, ensuring long-term performance and reliability. Sensors installed in areas prone to flooding or heavy rain should be particularly well protected.
• Initial calibration: Calibrate the magnetometers after installation to account for the local magnetic environment and ensure accurate readings. Every location has unique magnetic characteristics due to nearby structures, power lines, and natural variations in the earth’s

• magnetic field. Calibration adjusts the sensor to these factors, ensuring that it can detect vehicles without being affected by background noise from the environment.
• Initial data validation: Validate count and other types of data recorded by the sensors by comparing them against benchmark data (e.g., manual counts or manually reduced data from videos). Make adjustments upon validation or add additional units, if needed.

Maintenance

• Regular recalibration: Regularly recalibrate the sensors to adjust for any shifts in the magnetic environment, sensor drift over time, and potential environmental changes (e.g., snow accumulation and leaf debris). The local magnetic environment can change over time due to construction, installation of nearby equipment, or shifts in traffic patterns. Additionally, magnetometers can experience sensor drift as they age.
• Periodic inspections: Conduct regular inspections and maintenance to ensure the sensors are free from damage and the surrounding area is clear of debris. These inspections include checking for physical damage to the sensor, wear and tear on the protective casings, or any obstructions, such as debris, that might interfere with sensor operation. Keeping the sensors in good working condition ensures consistent vehicle detection. Promptly address any damaged or malfunctioning sensors to maintain the integrity of the traffic monitoring system. Faulty sensors can lead to missed vehicle detections or incorrect traffic counts, compromising the overall accuracy of the traffic counts.
• Electromagnetic interference avoidance: Mitigate the effects of electromagnetic interference (EMI) from nearby power lines, underground cables, or large metallic structures. EMI can distort the sensor’s readings, leading to false detections or missed vehicles. To mitigate this, sensors should be installed at a safe distance from potential sources of interference. Shielding or filtering mechanisms can also be used to reduce the impact of EMI on sensor performance.