Laser-guided track to safety01 February 2007

The increased loading of the rail network, along with the greater speeds of modern trains, has led to higher stressing of the rails. The track itself is therefore the most important safety aspect and so needs to be inspected regularly to prevent accidents. To that end, several leading rail inspection companies around the world are now using the latest non-contact, laser-based profile measurement sensors from Micro-Epsilon. Infotrans, for example, a leading supplier of inspection systems and services to Russia's sole railway network company Russichen Eisen Bahn (REB), is using Micro-Epsilon sensors on a newly developed wagon inspection system to measure and monitor for wear on the track. This ensures improved safety and reliability, but is also helping the customer to reduce maintenance and downtime costs. Nine Micro-Epsilon ScanCONTROL LLT 2800-100 (205) sensors are being used in the application.

The wear of the rail head itself is clearly an important safety parameter in assessing overall reliability of the track and condition of the rails. If the wear is too high, this can potentially lead to train derailment. Conventional inspection methods are based on manual inspection or, more recently, light-section sensors. However, the manual method is fraught with difficulties: it is slow, inaccurate and hence inefficient for track inspection teams. Before purchasing Micro-Epsilon sensors 12 months ago, wear on the rail head was monitored manually by teams of engineers.

Scanning the track with laser sensors (or light-section sensors) has been available for some time now, but extensive tests have shown that the sensors are too slow and also have problems coping with the different reflective properties of the tracks, which are often made from shiny, metallic materials that have become corroded in places. Also, the resolution of the laser sensor needs to be very high, so that the required number of measurements can be taken as the inspection train moves along the track.

The LLT 2800-100 (205) sensors are unaffected by shiny, metallic rails. They have an integrated, highly sensitive CMOS array, which enables measurements of almost any shiny, reflective or transparent surface, independent of the reflection from the target. This means excellent accuracy, resolution and reliability are achieved, even at high measurement speeds.

A line optical system projects a laser line on to the surface of the rail. The back-scattered light from the laser line is registered on the CMOS matrix by a high quality optical system. Along with distance information (z-axis), the controller also calculates the true position along the laser line (x-axis) from the camera image and outputs both values in the sensor's 2D coordinate system. A moving target or a traversing sensor generates a 3D representation of the object being measured.

"The sensor that Infotrans is using is designed for train speeds of between 120 and 130 kilometres per hour," explains Chris Jones, managing director at Micro-Epsilon (UK). "Infotrans selected Micro-Epsilon sensors because we were willing and able to customise the product to suit the application. The standard sensor has a 15mW laser, but we increased this to 50mW for Infotrans. We also changed the focal distance of the sensor and the profile frequency [speed] from the standard 1kHz, which measures 1,000 profiles each second, to 4kHz, which means Infotrans can measure up to 4,000 profiles per second. Since we sold the sensors to Infotrans," he adds, "feedback from the customer has been very good. There have been no problems with the sensors and the customer is currently conducting more field tests. We are hopeful that series production of the inspection wagons will begin shortly, with around fifty to sixty of our sensors being required."

Micro-Epsilon sensors are not only being used to measure wear on the rail head. A major rail company in Germany is also using the sensors on a test rig to measure the profile of steel tyres on its trains. The tyres are subjected to continuous extreme loads, while wear on the wheels due to high mileage also affects the safety and ride characteristics of the train.

A novel, custom-built test rig has been developed, located in the track bed. This measures the profile sets of a complete train. Point and linear, laser-based optical sensors are being used to measure the profile. By using the latest tyre profile measurement sensors from Micro-Epsilon, which acquire and store the profile data, the costs in terms of time and labour have been significantly reduced. Using application-specific software, the profile data - which forms the basis for the maintenance team to determine the date for re-profiling work - can be inspected to find out if limits have been exceeded.

The test rig consists of two measuring beams, which provide guidance for the train in the track. The complete measurement system is housed in three troughs embedded in the track bed transversely to the direction of travel. In each of the outer troughs, there are two Micro-Epsilon 'optoNCDT 1810 (50)' point laser sensors that acquire the wheel diameter and the position of the steel tyre. The inner trough accommodates two Micro-Epsilon scanCONTROL 2800 profile laser sensors that acquire the wheel profile. Also, two sensors are used to acquire the direction of travel and speed of the rail vehicle. To facilitate measurement, the rails have recesses.

The profile sensors project a laser line transversely on to the wheel surface, which is located transversely to the direction of the track. A high-quality optical system concentrates the diffuse reflected light from this laser line on to a CMOS matrix. In the sensor controller, the transverse profile of the steel tyre is then output from the camera image.

The point sensors in the outer troughs are directed on to the measuring circuit and measure the distance to the surface of the wheel. If the distances of both lasers to the wheel are equally large, three coordinates of the measuring circuit diameter are known by including the measuring beam surface. From this, the exact diameter is calculated, using the required corrections.

Micro-Epsilon, author of this article, is a major global manufacturer of sensors, headquartered in Germany. Web: www.micro-epsilon.co.uk

Sensor stats
The Micro-Epsilon sensors are highly versatile and offer a number of important benefits that include:
- Weather resistant and operate to an accuracy of 0.1mm
- An operating temperature from -25°C up to 50°C
- Four optoNCDT 1810-50 devices for positioning and two scanCONTROL 2800-100 devices for profile detection
- A database for storing the profile data of different data records for determining the re-profiling date
- A significant reduction in workshop reserves and downtime
- The easier disposition of rail vehicles

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