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Innovative network and antenna design could pave the way for a new generation of robust wireless condition monitoring systems for rail vehicles, says Mario Rossi, Railways Engineering Manager at SKF.
As in many areas of transportation, the drive to improve safety and reliability and to reduce maintenance in the rail sector is leading to increased interest in condition based maintenance approaches. That calls for monitoring technologies: changes in temperature, vibration and other variables during normal operation can provide an early warning of mechanical problems, allowing operators to take action before failures occur. The problem until now has been that installing the array of sensors required to collect this data from key components requires complex networks of additional wiring. Those cables are costly to install, and their presence makes routine maintenance more time consuming and difficult.
Now, working with academics from two leading Italian institutions1, a team of engineers from SKF has shown how a network of sensors can operate using low power wireless communications, greatly simplifying design, installation and maintenance.
Making a wireless network suitable for rail condition monitoring applications is difficult for several reasons. First, the sensors must be able to operate for long periods without being recharged or replaced. The axleboxes on some modern passenger trains may be expected to travel more than a million kilometres between overhauls, for example, and operators have ambitions to double that figure. The need to generate and store their own power means the sensors must be extremely energy efficient, greatly limiting the power available for the transmission of wireless signals.
Set against this need for low power is the large size of rail vehicles. A sensor mounted at the axlebox might have to transmit to a receiver almost 20m away in the centre of the vehicle, for example. Rail carriages are difficult environments for wireless transmission too. Large quantities of conductive material in the bogies, chassis and body of the vehicle can all block or interfere with signals.
To build a wireless network capable of meeting these requirements, the SKF team first had to choose a suitable working frequency for their proposed system. This choice was affected by numerous factors, including the regional regulations governing the use of the electromagnetic spectrum, the likelihood of interference from other equipment on or near the train, and the trade-off between the amount of data that can be carried on each frequency and the size of the hardware needed.
The team initially looked at three potential frequencies: 434MHz, 868MHz and 2.4GHz. They then set about examining the characteristics of the railway system using advanced simulation tools that model the reflection and diffraction of radio waves through and around the structure of a railway carriage.
This simulation explored a number of possible network configurations. This included a system where sensors on each bogie transmit to receivers mounted under the roof of the train, and an alternative approach in which sensors are equipped with both transmitter (Tx) and receiver (Rx) antennae, with each sensor communicating with its neighbors, then sending data along the train to a final receiver in the driver’s cab. The 2.4GHz frequency showed propagation issues, and the potential for interference from on-board Wi-Fi signals, but both 434MHz and 868MHz demonstrated good propagation values. After rejecting 434MHz since it would require large and complex antennae, the team eventually settled on 868MHz as the best compromise for the task. Physical tests using prototype equipment mounted on a real train confirmed the findings of the simulations.
The next essential element of the new approach was a new antenna design, optimised for the unique challenges of the rail environment. To allow sensors to be installed right where they are needed - on the axle-box of a bogie, for example - the overall size of the sensor, control electronics and antenna must be kept very small. And to survive for long periods under a train, any antenna design must be protected from dust and moisture ingress, and able to survive wide temperature swings and high levels of vibration.
To build an antenna that would meet these needs, the team selected a configuration called a Planar Inverted-F Antenna (PIFA). In this design, antenna elements are “built” over a Printed Circuit Board (PCB) which is covered in a conductive material to act as ground plane. A dielectric substrate with metallic layer is added to the other side of the PCB. This configuration is physically robust and transmits in any direction – another important characteristic for components that may have to be squeezed into difficult spaces.
By using a dielectric material with a very high relative permittivity (εr=10.9), the team was able to shrink the antenna to a size small enough to be suitable for a standard railway axlebox. Tests of the new antenna mounted inside an axlebox have shown that it exhibits excellent performance, and is less affected by the proximity of other large metal objects than current commercially available designs. These research results are proving invaluable as SKF develops the next generation of Internet of Things enabled products for the Railway market.
1 The team behind the work included Franco Lambertino and Mario Rossi from SKF’s Railway Segment; Gianluca Dassano, Francesca Vipiana and Mario Orefice from Antenna and EMC Lab (LACE), Dept. of Electronics and Telecommunications, Politecnico di Torino; and Sergio Arianos from the Antenna and EMC Lab (LACE), Istituto Superiore Mario Boella (ISMB), Torino. The group presented its research work at the 11th World Congress on Railway Research in Milan earlier this year.
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