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Railway condition monitoring by sensors

  23.06.2016    

The rapid railway system requires the absolute improvement of reliability and safety of passengers and trains, and the increased cost of system maintenance for this is emerging as another problem. Therefore, the effort of new technique development to meet the reliability and safety according to rapid railway system and maintenance cost, and the intelligent railway system of trains must be performed in advance for this. The basis of intelligent railway system lies on the development of continuous real-time detection technique, and the establishment of intelligent monitoring system which IT technique is included in is necessary. Generally, the monitoring system is also necessary to find, prevent, and cope with the risk in advance. It is available to analyze the phenomenon through the analysis of information using sensor that converts the physical transition to electric signal for continuous monitoring. However, the requirement for wire sensor monitoring system is increasing which is generally used in monitoring system of railway until now, and recently the system that uses units, which operate on driving device and replacement railway vehicle, and wireless sensor which is used despite the limitation, of installment and place. Especially, the application of wireless sensor and IT technique to railroad monitoring system makes it possible to maintain on the basis of real-time monitoring during driving, differentiating itself from previous periodic dismantling and inspection, and this fact can increase reliability and safety when driving railroad vehicle. In this research, we analyzed the requirements of intelligent monitoring system which IT technique is included in through examination of monitoring system applied to current railroad system. This study was researched the existing sensors and classification of them, for railway condition monitoring.

2. Sensor review
There are a multitude of sensors types used in railway condition monitoring for analyzing different aspects of structures, infrastructure, and machinery. Transducers convert energy from one to another. In the case of sensors, the device typically converts a measured mechanical signal into an electrical signal. Most railway sensors fall under the umbrella type micro electromechanical systems (MEMS). MEMS are small, integrated devices, or systems that combine electrical and mechanical components. They are cheap and efficient. Sensor design requires a tradeoff between functionality and power consumption, with functionality often coming at the cost of power. Condition monitoring systems for railways are often deployed in remote or inaccessible locations, where there is no wired power supply available. Hence, the sensors must receive power from either batteries or local energy generation. MEMS sensors have the advantage that they can be produced to consume ultralow power.
Sensor devices may also return additional ambient measurements such as temperature and humidity, and sensor data can be combined with the measurements from other systems. Rabatel et al. noted that the behavior of assets is often affected by external factors. These external factors are encapsulated by contextual data, which describe the ambient running conditions. It is important to take them into account and build models for condition monitoring that consider these ambient conditions. Elia et al. included train speed, position along the line (longitude and latitude from GPS system), and wind speed and direction when analyzing bogie vibrations, whereas Rabatel et al. included structural criteria (route) and environmental criteria (weather, travel characteristics, travel duration) when analyzing bogie temperatures. This paper provides an overview of the sensor types used in railway condition monitoring.
These sensors are divided to below items.
Accelerometers: Railway WSNs generally use accelerometers to measure vibration on infrastructure such as metal track or bridge girders or lateral acceleration of mechanics such as wheels. They are robust, reliable, easy to calibrated and cheap. They are often used in conjunction with strain gauges.
Displacement Transducer: A displacement transducer converts a linear or angular displacement into a signal suitable for recording. They can monitor movement across crack and joints in precast concrete planes in a railway tunnel to ensure the tunnel structure is stable.
Fiber-Optic Sensor, e.g., FBG (Fiber Bragg Grating) Sensors; Interferometric Fibre Sensors:
Fiber Bragg grating can be used as sensing elements in optical fiber sensors to measure temperature, strain or acceleration. A change of temperature or strain causes a shift in Bragg wavelength (The wavelength of  light reflected by the optical fibre). Interferometric fibre sensors use phase detection of the sinusoidal output power signal. The wavelength of electromagnetically noisy environment such as electrified. FBG sensors also have long life (> 20 years).
Gyroscopes (Angular Rate): Gyroscopes precisely measure a vehicle angular velocity around axes i.e., longitudinal, lateral and vertical accelerations as well as pitch, roll and yaw making them ideal for analyzing carriages, chassis and bogies.
Inclinometer (Tilt Meters): Inclinometers measure the slope angle bye generating an artificial horizon and measuring angular tilt with respect to this horizon. Inclinometer can detect distortion of railway structure by detecting changes in incline.
Light (Ultra-Violet): Phototube is a photocathode and a series of electrodes in an evacuated glass enclosure, able to measure the duration of ultraviolet emissions (in the wavelength range 175-195 nm) due to electrical arcing. They are used to duration pantographs.
Magnetoelectric: Magnetoelectric sensors induce electric polarization in a material when a magnetic field is applied. They are passive so do not require an energy supply (battery) as they are powered by induction.
Piezoelectric: Piezoelectric sensors generate a signal when the Piezoelectric material (such as tourmaline or quartz) is compressed. They are used to measure strain, pressure, vibration and shocks. The sensors are rugged and stable over repeated use so can be used on train wheels to monitor any displacement on the wheel surface to identify wheel defects. They are easier to fabricate and cheaper to run than optical sensors.
Piezoresistive pressure sensor: Piezoresistive pressure  sensors have a micro-machined silicon diaphragm with Piezoresistive strain gauges embedded in it, fused to a silicon or glass backplate. They produce a larger signal than other sensor types providing better sensitivity. They are used to measure changes in resistance and can monitor the nuts and bolts on rail fishplatesto ensure They do not work loose.
Acoustic Emission (AE): Acoustic Emission sensors are high-frequency, passive piezoelectric transducers. AEs are transient elastic waves generated when there is a sudden release of strain energy within a material, for example, the energy released by active fatigue cracks when an external force is applied. AE sensors detect these sound waves while filtering out ambient noise. Thus, AE sensors are able to detect internal damage within structures where cracks are growing. One drawback is that AE data is large and requires accurate and efficient algorithms to process and interpret it.
Surface Acoustic Wave (SAW): SAW sensors convert electrical input signal into mechanical waves using a piezoelectric material. They then convert the wave back into an electrical signal for analysis. Mechanical waves are affected by physical phenomena such as temperature so SAW sensors are used to monitor temperature changes in the axle box bearing of railroad vehicles. The sensors are very sensitive and stable but waves are distorted by liquids so SAW sensors need to be dry.
Pressure transducers: A pressure transducer produces a signal to reflect the pressure imposed on sensor. Pressure transducers can determine vertical displacement from a hydraulic reference line. Any displacement will be reflected by changes in pressure. Piezometers have been used in the railways to measure the positive water pressure in the ground water of rail beds. They are hydraulic pressure transducers where external water exerts pressure on internal fluid which in turn exerts pressure on an internal pressure transducer. They are robust and  reliable but need to be calibrated to account for atmospheric pressure.
Time Domain Reflectometer (TDR): TDR converts the travel time of a high frequency, electromagnetic pulse into volumetric water content. They are simple to use and easy to calibrate.
Settlement probes measure the long-term settlement of rail tracks on the rail bed. They are hydraulic pressure transducers with an embedded probe able to measure large settlement. As the probe changes elevation then the pressure and need to be calibrated carefully.
Tensiometer: Tensiometer measure negative water (water tension) in the soil to quantify the soil status. The water exerts a negative pressure on internal fluid and thus on an internal pressure transducer in the sensor. They require regular maintenance.
Vibrating wire piezometers measures changes in ground water pressure by measuring the strain exerted by the water acting on a diaphragm and monitoring the vibration in a wire touching the diaphragm. They are durable, stable and reliable but the vibrating wires limit the frequency of reading to several seconds per reading (e.g., 0.1Hz) so they cannot analyze changes in pore pressure during the passage of individual trains. They are suited to long-term monitoring.
Strain gauge: Strain gauges measure local stresses and generate a signal to reflect the strain applied to the sensor. They are cheap, simple and accurate. A resistance strain gauge contains a steel wire held in tension in the wire. Strain gauges often measure the vertical and lateral force applied to the rail between two sleepers (or ties) as trains pass. When a train is present the strain increases on the rail, which is detected by the sensors.
Temperature sensor: In railway application, temperature sensors are used to monitor the temperature of: the atmosphere (air temperature), the rail bed or train chassis and mechanics.
Ultrasonic strain gauge: Ultrasonic strain gauges record elastic waves in a structure due to vibrations caused, for example, by a passing train. They are easier to install and are more sensitive than standard strain gauges and can be relocated unlike piezoelectric strain gauges.

3. Sensor for Fixed Monitoring
Many authors have investigated condition  monitoring of infrastructure, including rail bridges, tunnels, rail tracks and other track infrastructure. Krüger et al. noted that structural  health monitoring is becoming ever more important as both train speeds and the axle loads exerted by trains increase. Until recently, inspection has been performed visually, but this only examined the structure superficially and intermittently. Additionally, the visual analysis needs to be interpreted by an expert, which can be subjective. Sensors are objective and can provide data from all of the structure (including internally) to allow the whole structure’s health to be assessed and to analyze its durability and remaining life time.
Bridges: Bridges suffer structural defects exacerbated by the constant strains and vibrations of passing trains. Human inspection of bridges is difficult and much of the structure may be inaccessible. Sensors enable constant monitoring of the whole structure,  including the internal structure of  the bridge. The monitoring systems often comprise two units, as detailed in Figure 1*. The first unit is low power. It detects the approach of a train and wakes the second unit. This second unit generates the measurements, whereas the bridge is under load (a train is crossing) to assess the bridge’s health.
Tunnels: Tunnels can be damp leading to rapid corrosion damage and other defects; thus, monitoring is essential. Monitoring of tunnels has many similarities to bridge monitoring. However, rail tunnels are often difficult to access and inspect. An added complication of tunnel monitoring compared with bridge monitoring is data transmission in a confined environment. Data generated by sensors inside the tunnel have to be transmitted using a suitable relay mechanism (often multiple hop using relay nodes) from the various sensor nodes to the outside.
Rail Tracks: Track monitoring systems also play a vital role in maintaining the safety of the railways. Monitoring bridges and tunnels (discussed in the previous paragraphs) uses sensors to identify and analyze defects (cracking and displacement) in large structures. In contrast, track monitoring involves identifying and analyzing defects in long narrow metal rails. Tracks can crack and displace like bridges but also twist and tilt (incline). Hence, track monitoring can vary from detecting settlement and twist such as that caused by nearby tunneling or excavation; to measuring the forces exerted by train wheels on the tracks; to monitoring the development of cracks and structural flaws as trains pass and over the longer term. A dedicated Track Recording Vehicle (TRV) passes over track sufficiently often that deterioration of the track geometry and new irregularities can be detected and suitable maintenance scheduled. The frequency of TRV operation is often insufficient to observe how track geometry irregularity actually arises and develops into a fault that needs to be repaired. A comparatively inexpensive IMU mounted on the bogie of an in-service vehicle provides a large amount of information on the trajectory and orientation of the bogie over the same track, often many times each day. As well as the higher temporal resolution of information, such an IMU  is easily installed compared to many other forms of measurement equipment, and is cheap to operate.

4. Sensor for Rolling Stock Monitoring
Sensors are able to monitor the condition of a range of mechanics, systems, and environments using on-board sensors to measure parameters such as temperature, shocks, tilts, and humidity. Thus, sensors demonstrate a promising platform for real-time movable condition monitoring of transport systems, allowing the early detection and diagnosis of problems. Measurements can all be taken during normal train service that allows in-service condition monitoring of the train chassis and mechanics and the rail track that the train is running on. The sensor nodes used for movable monitoring are often mounted on the train bogies or carriages.
Train Engines, Passenger Carriages, and Freight Wagons: Wagon or container failure could lead to damage of the rail infrastructure or the environment and could even result in loss of life; thus, it is important that the wagons, containers, and their respective contents are also monitored. Additionally, freight trains use unpowered and unwired railroad wagons, thus an on-board WSN can analyze the condition of the freight wagons without power from the train.

5. Sensor Node
Sensor nodes are deployed on function units of large-scale railway maintenance equipment to collect the basic working status information. Sensor nodes are the basic units of the fault detection application with the functions of collecting information, processing data, sending data. Low-cost, energy efficiency is the basic requirement, to fit this requirement the sensor nodes used processor unit. Sensor nodes had three main parts: processor unit, communication unit, and sensor unit. The basic design is shown in Fig. 6.

6. CONCLUSION

This paper has reviewed the range of sensors used for condition monitoring in the railway industry. The emphasis is on the practical engineering solutions, principally which sensors devices are used and what they are used for; and, identification of sensor node configurations and network topologies. Until recently, railway inspection has been visually performed, but this only examines objects superficially and intermittently, and the analysis needs to be interpreted by an expert, who can be subjective. Sensors are objective and can provide data from the entire object (including internally) to allow the whole object’s health to be fully assessed and to analyze its durability and remaining life time. A broad range of sensors are used in railway monitoring to provide an extensive range of data and allow monitoring of different structures, vehicles and machinery. The paper divides railway condition monitoring into fixed monitoring for infrastructures such as bridges, tunnels, tracks and associated equipment, and movable monitoring for vehicles and their mechanics. Fixed monitoring uses sensors to monitor vibrations, stresses and sound waves passed through structures caused by passing trains and also changes in stresses, pressures and sound waves passed through structures over the longer term.

* A full version of this research is available upon request.


First Author: Reza Sharifi  
Islamic Azad University - Science and Research Branch
E-mail: r.sharifi@srbiau.ac.ir
Second Author: Dr.Ehsan Arfa
Islamic Azad University - South Teheran Branch
E-mail: e_arfa@azad.ac.ir




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