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In the railway industry, an instrumented wheelset refers to a wheelset equipped with sensors to measure the dynamic contact forces at the wheel-rail interface. Typically, they can measure forces in all three directions and the more advanced ones can also estimate friction and position of the wheel-rail contact patch on the wheel. They are also known as ‘load measuring wheelsets’ in US.

Typical applications

Historically, the technology is primarily used to verify that new (or substantially modified) rail vehicles are safe and not prone to derailment. They are also used to check that vehicles do not cause unacceptably high levels of rail wear or infrastructure damage. Testing with IWT is required in Europe (Standard EN14363 ), the US (standard 49 CFR-213 ), and is also common in China, Japan, and India.

Other applications for the technology are also under development. One is track condition monitoring where the technology can help in localizing both singular and periodical irregularities, such as poor joints, worn switches or crossings, and corrugations . Estimation of friction is another potential which can help in reducing occurrences of wheel flats due to rapid braking or acceleration. For example, an IWT4 wheelset is shown in this clip, where it is used to measure friction in an experimental test rig. In case of low friction detection, a laser beam is activated to clean the rail head from possible contamination, such as leaves.

There are also other application areas, for example:

• In R&D projects, for example very high speed (400kph), or novel bogie designs (Regina mechatronic)
• For troubleshooting complex problems, e.g. X 2000 wheel failures
• For checking the track, e.g. high-speed lines in Germany, USA, and Finland

Development history

Early years of instrumented wheelset development at SJ

Development of the first instrumented wheelset technology was led by SJ AB in Sweden during the 1950s. A picture taken at SJ shows two of the pioneers in the development of the instrumented wheelset. The technology has been continuously developing and currently in its fourth generation, so called IWT4. The IWT4 technology has changed owners several times since its inception in 2004 due to a number of company acquisitions and takeovers, and today currently owned by TÜV SÜD Sverige.

There are also other countries that have developed their own instrumented wheelsets, each with its own unique implementation technique and characteristics. For example, DB Systemtechnik led the development of a German Instrumented wheelset technology, known as Messradsätze in German.

Operating principles

An instrumented wheelset estimates dynamic forces at the wheel-rail contact through measuring strain values on the wheel disc. When the wheel is exposed to forces, the wheels steel structure is deformed slightly, this deformation (known as strain) is measured by sensors attached to the wheel. If the relations between the inputs (forces) and outputs (strains) are known then the dynamic forces can be calculated from the measured strains. The relationship between force and strain is determined via lab measurements and/or FEM calculations where the wheel is subjected to known forces and the corresponding strain values are measured. Once the strain-force relationship is known the measured strain values can be mapped back to the corresponding dynamic forces at the wheel-rail contact patch.

Limits of the traditional technology

The traditional technology was associated with some limitations and drawbacks. In the following, some of these are discussed.

With the earlier technology, separation of the contact forces into their vertical and lateral components was done by the geometry of the wheel disc itself. The behaviour of the wheel was investigated to find positions that were sensitive to forces applied on one direction but not sensitive to forces applied from another direction. In this manner on set of strain gauges could be applied in an area that is sensitive to vertical forces while non-sensitive to lateral forces, and another set of strain gauges applied in an area that is sensitive to lateral forces while non-sensitive to vertical forces. The behaviour of the wheel is dependent on the geometry of the wheel disc itself. In most cases special wheel discs would need to be manufactured or significant machining of existing wheelsets would need to be undertaken to provide a wheel geometry that enables separation of the vertical and lateral forces.

The manufacturing of special wheel discs proved to be costly and introduced long lead times. While machining of existing wheel discs damaged the structural integrity, and reduced the estimated fatigue life of the wheelset. Furthermore, regular checks of the wheelset for possible cracks would be necessary.

With the earlier generation, a more demanding instrumentation of the wheelset was necessary. Both sides of each wheel disc needed to be instrumented with strain gauges. This was not only more resource demanding but it also meant that the wheel needed to be machined to enable installation of the necessary cabling. Typically, holes were drilled in the wheel disc itself, and also in the axle. This machining reduced the fatigue life of the wheelset and often meant that periodic inspection such as ultrasonic inspection needed to be undertaken.

Transferring power and signals to and from rotating wheels has always been a challenge. Some earlier solutions used slip ring devices which are expensive, unreliable and require regular maintenance. Other systems used axle mounted batteries to avoid the slip ring connection for transfer of power. However, this solution was not attractive either due to limited supply of energy and the regular trouble of mounting and dismounting batteries.

Current state of the art

A wheelset equipped with IWT4 technology

With current state of the art technology, such as IWT4, the shape of the wheel disc does not play an important role any longer and basically any wheel can be converted to an instrumented wheelset without any destructive modification. This is due to the technology no longer relying on the physical geometry of the wheel to separate the forces into their vertical and lateral components, instead the wheel is placed in a test stand and a system identification process is undertaken. In this process, load cases are applied and advanced mathematical algorithms are used to learn the behaviour of the wheel. Hence, the original wheelset of the vehicle in test can be used for preparing the instrumented wheelset leading to lower cost and shorter lead time.

Only one side of each wheel disc needs to be instrumented, this removes the need to bore holes in the wheel and/or axle. Furthermore, the laboratory is able to instrument the side of the disc that offers more space depending on the bogie structure design.

The installation of the current measurement system does not involve any destructive modification of the wheelset. Therefore, no additional periodic ultrasonic inspection is required and the wheelset can be used in normal operation after the test period without any specific safety concerns.

The latest technology includes a telemetry system which allows transferring power and signals to and from the electronics mounted on the wheel. This system is much more robust compared to the slip ring solution used earlier. The picture to the right shows an instrumented wheelset with the power head of the telemetry system fixed on a bracket connected to the journal box. The power head is shaped like a ‘U’ and a disc runs through it. The disc serves two functions first it receives power inductively and transfers it to the electronic components on the wheel disc. Its second function is to transmit the measured signals from the wheel disc to the inductive head mounted on the bracket connected to the journal box.

A common concern with electronics in railway environment is the Electromagnetic Interference (EMI). Different electronic systems should be able to function correctly and safely without disturbing each other. Analogue signals are particularly sensitive to EMI. To minimise this issue, the analogue measurements of the strain gauges are digitalized as close as possible to the sensors. This is done by the analogue to digital converter mounted to the wheel disc, which is shown in the picture, it is the grey box above the axle with black and orange cables connected to it.