Application of Piezoelectric Traffic Sensors in ITS

With the development of global intelligent transportation technology (ITS), like many technologies, the piezoelectric sensors developed and produced by MSI Sensors in the United States have made great progress in the past few years. It provides users with not only good performance, high reliability, simple installation methods, but also gradually reduced prices. Its unique characteristics make it an ideal choice in the ever-expanding applications.

Features of piezoelectric sensors

Capacitive sensors: cannot detect vehicles stationary on the sensor. It can only detect dynamic signals, has high internal resistance, and has a large signal attenuation at low frequencies. At low speeds, a higher circuit input impedance should be considered. The speed range depends on the circuit design, generally 5 km/h to 200 km/h, and the more successful systems reach 10 meters/minute (0.6 km/h). Passive

sensors: can be transmitted over long distances before the preamplifier without power supply.
Long life: more than 40 million ESAL (equivalent single axle load) and up to 100 million times (ESAL) with good installation quality.
Large signal: 200 kg wheel load, when driving at 55 miles, the output is a minimum 250mV signal.
Good dynamic characteristics: can measure bicycles , motorcycles , cars and heavy trucks. High signal-to-noise ratio: the flat structure of the sensor, that is, the width-to-thickness ratio is 6:1, which minimizes the noise in the non-stress direction. Including the road noise and the noise of vehicles in the adjacent lanes. Minimum road damage: the installation cutout is only 19mmx19mm. And it can be consistent with the road profile. Easy to carry: coiled in a 600mmx600mm paper box, the curling diameter is not less than 300mm will not be damaged. One installation to obtain multiple signals: such as axle number, weight, vehicle speed, wheelbase, cooperate with the inductor coil, so as to realize weighing in motion (WIM), vehicle classification statistics, speed monitoring, red light running photography. Application scope of piezoelectric sensors Piezoelectric sensors are mainly used for weighing in motion (WIM), vehicle classification statistics, axle number counting, wheelbase measurement, speed monitoring, red light running photography, parking area monitoring, toll station scales, traffic information collection and statistics (road monitoring) and airport taxiways. 1. Weigh-in-Motion (WIM) is widely used in the United States, Brazil, Germany, Japan and South Korea. Its main purpose is the pre-selection of overweight and overload monitoring of highway vehicles and the bridge overload warning system. It can determine whether the vehicles traveling at high speed, especially those passing through the bridge, are overloaded. The license plate number is recorded by the video system, and then the law enforcement agency uses a high-precision low-speed weighing system to determine the overload and impose fines based on the overload. The performance meets the ASTME1318-94 dynamic weighing standard. The output consistency in the length direction of the sensor is less than ±7%. When it is permanently installed under the road surface, the total weight accuracy is within ±10%, which is suitable for the ASTM-E1318-94 standard Class I dynamic weighing system; when it is temporarily installed on the road surface, the total weight accuracy is within ±15%, which is suitable for the ASTMEl318-94 standard Class II dynamic weighing system. The sensor accuracy is related to the vibration and bounce of the vehicle, the area of ​​the tire pressing on the sensor, and the temperature, and temperature compensation is required. In particular, the quality of the road has a great influence on the accuracy of the system. It is better to use it on cement pavement, and its service life is longer than that of asphalt pavement. The quality of the road used for dynamic weighing should comply with the relevant provisions of ASTM. The accuracy that can usually be guaranteed is ±10%, and the accuracy of some successful systems can reach 1-2%. The speed range can be from 5 km/h to 200 km/h, and the more successful system reaches 10 m/min (0.6 km/h) at the low speed end. The lower limit of the weight is a bicycle, and the upper limit has undergone 500,000 60KN single tire tests, which is equivalent to 70 tons (9 types of vehicles in the US standard). The cement test section was crushed in the actual measurement. 2. Vehicle classification statistics The main purpose of piezoelectric sensors is vehicle type classification, and vehicle speed data can be converted into reliable classification data. Different countries use different classification tables to classify vehicles. In the United States, FHWA defines vehicles as 13 types from motorcycles to multi-purpose trailers (see Standard Specifications and User Requirements and Test Methods for Highway Dynamic Weighing (WIM) Systems ASTMl318-94). The type of vehicle is determined by the number of axles and wheelbase. Wheelbase: Since the vehicle speed is basically uniform within a distance of 3 meters or less, the wheelbase is obtained by multiplying the signal time difference established when the axle passes the sensor by the vehicle speed. Number of axles: Since the sensor detects the force pressing over the tire, it is easy to measure the number of axles even when the vehicles are very close. However, when the traffic is dense, the speed is low, and the vehicle models are similar, it is impossible to distinguish whether the axles counted are the same vehicle or two vehicles. The inductor coil cannot count the number of axles, so the solution of inductor coil + piezoelectric sensor can measure both the number of axles and the number of vehicles. The configuration scheme can be either sensor + coil + sensor, or coil + sensor + coil. In order to obtain the vehicle speed signal and perform other calculations, both schemes are acceptable, but the former configuration is better. Wheelbase: In some countries, such as South Korea, the classification of vehicles requires the detection of wheelbase. There are many types of vehicles in China, and there is a problem of different wheelbases with the same wheelbase, such as Jiefang vehicles and Yellow River vehicles, whose load capacities vary greatly. If the detector can distinguish wheelbases, it will increase the coverage and accuracy of the system. This problem can be solved by burying the sensor at a certain angle. Number of tires: The standard for vehicle classification in other countries, such as Brazil, uses dual tires as the standard for classification. In order to detect dual tires, a sensor is usually installed at a certain angle to the direction of traffic (usually 30 to 45 degrees). When the dual tires pass the obliquely buried sensor, a double-peak pulse will be generated, and the dual tire signal can be identified through circuit processing. The sensor installed perpendicular to the traffic is still used to detect the vehicle speed and the number of axles normally, and compare them with the obliquely buried sensor count. According to the "Regulations on the Management of Highways for Overloaded Transport Vehicles" issued by the Ministry of Transport, the dynamic weighing system should have the ability to identify single and dual tires. This problem can be solved by obliquely buried piezoelectric axle sensors. Due to the rapid growth of vehicle volume, ETC (electronic toll collection system) has become the focus of attention of industry insiders. China has always adopted classification by tonnage and the number of passenger car seats. Now the types of vehicles traveling in China are complex, and it is very difficult to introduce automatic classification in the ETC system according to this classification method. Classification by wheelbase and number of axles, and then considering the load, should be a more reasonable method. Establishing reasonable classification standards is the key to solving ETC problems. The basis for formulating standards is detection methods. The vehicle model recognition system should be designed based on the vehicle's physical characteristics such as the number of axles, wheelbase, number of wheels, length, width, height, etc. by combining video technology, piezoelectric axis sensors and network technology. This requires the organic combination of management departments, system integrators and device suppliers to achieve .





































Two sensors are usually installed on each lane, which makes it easy to collect data from each lane separately. The speed of the vehicle can be calculated using two sensors. When the tire passes sensor A, the electronic clock is started, and when the tire passes sensor B, the clock stops. The distance between the two sensors is generally 3 meters, or less than 3 meters (can be determined as needed). The distance between the sensors is known, and the distance between the two sensors is divided by the time period of the two sensor signals to obtain the vehicle speed. According to a report by the German PTB, the measurement accuracy can reach 1% when the car is driving at a constant speed of 200 kilometers per hour.

Piezoelectric sensors can distinguish vehicles with very small differences, which makes them useful in fixed locations with speed camera triggers. Usually 2 sensors are installed as a group, and some countries also install 3 (with additional calibration). When the tire passes the sensor, the speed is calculated based on the time from A to B, then from B to C, and finally from A to C. Then these speeds are compared, and they should all be within the specified range, usually not more than 2%. If the vehicle exceeds the specified speed, the vehicle is photographed immediately when the front wheel passes the last sensor, and the speed is calculated. A second photo is taken at a fixed time after the first photo is taken, so that the scope can verify the speed of the vehicle. Even in the case of high traffic volume, information can be obtained for each lane. The sensors can be staggered so that the camera has a stable focus, making the photos clear and readable.

Speed ​​monitoring can not only fine speeding vehicles, but also establish variable speed limit signs and variable information boards according to traffic volume. When the traffic volume is high, a lower speed limit is set; when the traffic volume is low, a higher speed limit is set, and a dynamic management system is established to realize intelligent road management.

4. Toll station weighbridge

One application of piezoelectric shaft sensors is toll station weighbridges. The sensor can record data from vehicles traveling at high speeds. At low speeds, the interface between the shaft sensor and the circuit is critical. The piezoelectric sensor will attenuate low-frequency signals. The low-frequency attenuation is determined by the capacitance of the sensor and the input impedance of the circuit. Another improvement in the circuit part of the piezoelectric shaft sensor allows the sensor to be used at a speed of 10 meters per minute (0.6 kilometers per hour).

Although the piezoelectric shaft sensor can detect a tire that presses on the sensor and then moves away from the sensor, it cannot detect a vehicle that is stationary on the sensor. Multiple sensors can be applied simultaneously within a very small distance. To prevent false counts and improve the verification of counts. Piezoelectric sensors are very suitable for use in automatic classification lanes at toll booths, where vehicle speeds vary greatly.

Piezoelectric axis sensors provide a very effective advantage for toll booth scales. The life of piezoelectric axis sensors is much longer than that of ordinary resistive scales. Due to the solid-state structure of the sensor, piezoelectric axis sensors have no moving parts. The visible deformation in the sensor is in the micrometer (um) range, while resistive scales usually have a deformation of several millimeters in the rubber sleeve, thus introducing a fatigue element. The life of resistive sensors is 1-5 million axis times, while piezoelectric sensors exceed 100 million axis times.

5. Red light running camera

Piezoelectric axis sensors can also be used as triggers for red light cameras. Two sensors are installed in front of the red light line at the intersection, and the minimum distance between the sensor and the red light line is generally 2 meters. The distance between the two sensors is 1 meter or less. They can be installed above the ground sensor coil. All data is collected by the front wheel. The signal collection is completed before the vehicle moves 6'' (150mm). The signal collection is independent of speed and vehicle type. It can be used in high-density traffic flow. The camera controller is connected to the traffic light controller so that the action is completed only when the light is red.

The two sensors are used to determine the speed of the vehicle before the stop line. If the red light is on and the speed is greater than the preset value, the first photo will be automatically taken. The first photo proves that the red light is on and the vehicle has not exceeded the stop line when the red light is on, and can prove the speed and the time the red light has been on. The second photo is taken at a fixed time after this according to the speed of the vehicle, generally 1 to 2 seconds. The second photo proves that the vehicle actually crossed the stop line into the intersection and ran a red light.

In the United States, due to privacy reasons, most photos are taken at the rear of the car, and then the vehicle is issued a ticket in a similar way to a parking ticket. The registered owner will receive a ticket, which includes two photos and the license plate number is enlarged. Although digital cameras have been accepted, most systems still use wet film of 35 mm or larger format to shoot. Preliminary evidence is not favorable to the offender when wet film or digital images are used. This prevents digital tampering of the evidence.

In Howard County, Maryland, USA, after installing a red light camera system using piezoelectric axis sensors, red light violations were reduced by 53% in one year, only 3.2% were brought to court, and 90% of the court trials proved the offender. At the first intersection with a camera, conflicts were reduced from 15/year in 1992 to 8/year in 1998. 20 cameras, 21,000 penalty notices issued.

VI. Trigger

The reason for choosing piezoelectric sensors as triggers for red light cameras is the same as their application in speed cameras. In the photo, it can be seen that the vehicle is still above the sensor. The sensor is wired to the camera to specific the information to a certain lane. Even if two vehicles are close together in two adjacent lanes, the data transmitted to the camera controller will be the data of the lane where the sensor is located. Sensors are not as difficult to distinguish between adjacent vehicles as fixed radar devices, so piezoelectric sensors are suitable for multiple lanes. People who receive tickets are reassured that they have indeed run a red light or violated the speed regulations because they can clearly see the evidence displayed in front of them.

7. Traffic Information Collection and Statistics (Road Monitoring)

The application of piezoelectric axis sensors has expanded into a technology that is more reliable and affordable. This technology originated from the Federal Highway Administration (FHWA) Long-Term Road Performance Project (LTPP) in the United States. Under this project, traffic load, type and weight on some roads are monitored to determine the wear, type and grade of the road. In this way, periodic information collection is usually used, and there is almost no real-time data collection.

At present, since China's highway construction is still in its infancy, some sections of the road are severely overloaded and prematurely damaged before the design service life, resulting in rising maintenance costs. Most management departments focus their main efforts on the formulation of toll (especially non-stop toll) standards and system technical issues. This is necessary at present, but with the increase of traffic volume and the increase of road load, traffic accidents will increase, the traffic jam time will be longer, the road damage repair period will be shortened, the number of times will increase, and the monitoring of road conditions will become more and more important.

If network technology, video technology, ground sensing coils buried under the road surface and piezoelectric shaft sensors are combined to achieve short-term collection of traffic information, traffic volume, number of axles, speed, wheelbase, classification information, load and other information can be collected and analyzed, and the Automated Traffic Information Survey System can provide maintenance solutions for road load to owners, and also provide reliable and comprehensive data for highway planning, design, maintenance and decision-making. The traffic management system of Highway 401 in Toronto, Canada is a typical example. In the past five years, piezoelectric shaft sensors have significantly improved in performance, while prices have continued to decline. In terms of installation price, it is only slightly higher than the induction coil, but it provides more effective information than the induction coil, such as improved speed information, vehicle classification, etc. In addition, the ability to weigh while driving has been added to determine and monitor the weight of the vehicle. Combined with induction coils, it will make the collection of traffic information more accurate and comprehensive. Obviously, piezoelectric shaft sensors, as a technology, should be considered for widespread use in intelligent transportation systems (ITS).