Laser triangulation sensors are being used to measure the lateral position of rail bogie wheels relative to the rail head in the ‘Acti Wheel’ rail traction technology project being undertaken by Derby-based Stored Energy Technology Ltd (SET). These sensors have been supplied by Micro-Epsilon UK & Ireland Ltd, Birkenhead (www.micro-epsilon.co.uk). Acti Wheel is a traction system that uses arti-ficial intelligence to ‘guide’ trains along the tracks, enabling quicker, smoother and more-economical rail travel.

SET director Martin Whitley said: “This revolutionary wheel motor can be controlled to produce more driving force on one side or the other in order to ‘steer’ the wheel set down the centre-line of the track. For the last 200 years, conventional rail vehicles have only had a solid axle and ‘wheel coning’ providing this capability, which drives some significant compromises and issues. What we want to do is to see this technology implemented as ‘the technology of choice’ for rail passenger vehicles of the future.”

The wheels of rail vehicles are never made flat; they are in the shape of a cone with a slope of about 1:20. This is known as ‘coning’ and is mainly done to keep the vehicle in the centre of the track. In the Acti Wheel solution, the motor is integrated in the wheel, which means that there is no transmission between the two and no moving parts beyond a bearing, which every wheel and axle has. In addition, there is no friction-based braking system, significantly reducing the amount of maintenance required.

Avoiding rolling-contact fatigue
Neil Cooney, technical director at SET, said: “One of the biggest issues facing the rail infrastructure is rolling-contact fatigue (RCF), which occurs due to the energy in the contact between the wheel and the rail. Conditions under the contact patch are always severe, and the yield stress of the rail wheel is always exceeded — on at least a micro-scale — due to the surface roughness of the wheel and the rail.

“Actively ‘yawing’ the wheels, along with active torque control, manages the contact patch at the optimum point of the rail, practically eliminating RCF. This is because ideal ‘close to radial’ steering reduces the energy in the contact patch to a point where RCF does not occur, so wear is very small.”

Mr Cooney expects SET’s Acti Wheel wheels to last between four- and 10-times longer than a conventional set-up. Moreover, the system is significantly lighter, which means that it is easier to accelerate and decelerate the train — and there is additional carrying capacity for passengers.

For almost the last year, Acti Wheel has been put through its paces in a technical demonstrator project. An ex-London Underground train with a single carriage was fitted with an Acti Wheel system on each of its eight wheels.

Mr Cooney says the project proved that wear and RCF were almost negligible, and SET is further developing the system for rail network application. Each Acti Wheel system incorporates an optoNCDT 1420 laser triangulation sensor from Micro-Epsilon.

He said: “A critical part of the Acti Wheel system is to understand the lateral position of the wheel relative to the rail. We can then control the wheel and avoid the flange making contact with the rail head. We’re really using the sensors to confirm that our complex controllers are actually working correctly.”

Demonstrator project
The optoNCDT 1420 laser triangulation sensor with integral controller is designed for high-precision and high-speed dynamic displacement, distance and position measurement applications.

The measuring rate is adjustable up to 4kHz, and a range of different output signals enables easy integration of the sensor into plant or machine control systems. As well as analogue voltage and current outputs, a digital RS422 interface provides distance information from the sensor.

All optoNCDT 1420 sensors operate using a Web interface for fast set-up and configuration; other features include video signal display, signal peak selection and freely adjustable signal averaging for optimisation of the measurement task.

SET engineers built a special frame that is located beneath the train’s wheel axle and positions the optoNCDT 1420 sensors 400mm from the rail head — just in front of the flange radius, pointing directly at the rail head. Measurement data from the sensors is sent to the Acti Wheel control system.

Mr Cooney said: “We initially approached Micro-Epsilon for a suitable sensor and were suitably impressed by the demonstration of an optoNCDT 1420 sensor. It met all our technical requirements in terms of its flexibility, resolution and robustness. We are measuring to an accuracy of 0.1mm on a lateral movement that can be up to a maximum of 20mm.”

“The sensors have performed very well in the demonstrator project and on other projects where we’ve tested the Acti Wheel system. They are reliable even in the harsh environment underneath a train, where dust, dirt and moisture are present. In the demonstrator project, we didn’t even have to clean the sensors after a couple of thousand miles of testing.”