Adding accuracy and availability to automatic train wheelset measurement
Automated on-track measurement of wheelsets for the rail industry
CALIPRI X is a real breakthrough for overrun measurement systems. Why is its design so different to traditional solutions? Let’s take a deep dive.
We demand a great deal from train wheels, so regular condition checks are vital. Compared to manual inspection, on-track automated measurement is much faster and cuts labour costs significantly.
But no matter how convenient, automation must still be accurate and consistent. Errors lead to superfluous, wasteful maintenance interventions or, much worse, unsafe operations. Overrun systems must also survive the harsh railway environment with minimal attention.
Because it uses laser-light-section technology, CALIPRI X might sound similar to other vendor solutions. In fact, it’s very different. There is no device like it on the market. To understand why, you have to look inside.
Practical challenges, engineering answers
When we started to develop CALIPRI X in 2017, our first priority was to speak to rail operators. They told us about the multiple issues they faced with on-track wheel measurement. It wasn’t a happy story.
Despite supposedly complying with the EN 15313 wheel measurement standard, many of their systems failed to deliver consistent measurement accuracy year-round or were only accurate for a narrow wear range – or both. There were also longstanding, expensive problems with availability, reliability and excessive maintenance.
Let’s consider measurement first. With many wheelsets and in average conditions, complying with EN 15313 is actually straightforward. But it’s much harder to automatically measure accurately and consistently in extreme conditions, whether that’s in hot summers, very cold winters or with heavily worn wheelsets.
At NEXTSENSE, part of Hexagon, metrology – the science of measurement – is our speciality. We’re proud to say we are domain experts. Our C4X range has led handheld rail wheel measurement for the last 17 years, largely because we engineer our designs to control the many influences on accuracy and repeatability. Many on-track system vendors’ core expertise lies elsewhere and their designs reflect this.
Despite supposedly complying with the EN 15313 wheel measurement standard, many of their systems failed to deliver consistent measurement accuracy year-round or were only accurate for a narrow wear range – or both. There were also longstanding, expensive problems with availability, reliability and excessive maintenance.
Let’s consider measurement first. With many wheelsets and in average conditions, complying with EN 15313 is actually straightforward. But it’s much harder to automatically measure accurately and consistently in extreme conditions, whether that’s in hot summers, very cold winters or with heavily worn wheelsets.
At NEXTSENSE, part of Hexagon, metrology – the science of measurement – is our speciality. We’re proud to say we are domain experts. Our C4X range has led handheld rail wheel measurement for the last 17 years, largely because we engineer our designs to control the many influences on accuracy and repeatability. Many on-track system vendors’ core expertise lies elsewhere and their designs reflect this.
Coping with temperature
Temperature-related drift is a serious issue for overrun systems. Electronic characteristics change while physical expansion and contraction affect laser and sensor positions. Multiple design features help overcome this classic measurement engineering challenge.
CALIPRI X has four sensor boxes grouped in two pairs, one pair per rail; the boxes in each pair face each other on opposite sides of a rail. There are three lasers and camera sensors in one box and two in the other.
Simple, straight steel bars link and support each pair of sensor boxes, so the boxes only move in a single dimension when the bars expand and contract. That means we can accurately predict their position, based on previous measurements at different temperatures. During operations, we monitor ambient temperature and our software algorithms compensate for any predicted box movement.
But inside each box, temperature-driven frame and sensor movement is three-dimensional which is impossible to map accurately enough in advance. Instead, we maintain the box internals, electronics and cables at a steady 20°C using fluid-based cooling or heating. Certain critical components are made of Invar, a nickel-iron alloy with a uniquely low coefficient of thermal expansion.
This practically eliminates measurement error due to physical expansion and contraction and, because the internal electronics stay at 20°C too, they don’t drift either. This also significantly improves electrical component lifetime; CALIPRI X’s trackside UPS (power supply) is similarly cooled for a much longer lifespan and lower lifecycle costs.
CALIPRI X has four sensor boxes grouped in two pairs, one pair per rail; the boxes in each pair face each other on opposite sides of a rail. There are three lasers and camera sensors in one box and two in the other.
Simple, straight steel bars link and support each pair of sensor boxes, so the boxes only move in a single dimension when the bars expand and contract. That means we can accurately predict their position, based on previous measurements at different temperatures. During operations, we monitor ambient temperature and our software algorithms compensate for any predicted box movement.
But inside each box, temperature-driven frame and sensor movement is three-dimensional which is impossible to map accurately enough in advance. Instead, we maintain the box internals, electronics and cables at a steady 20°C using fluid-based cooling or heating. Certain critical components are made of Invar, a nickel-iron alloy with a uniquely low coefficient of thermal expansion.
This practically eliminates measurement error due to physical expansion and contraction and, because the internal electronics stay at 20°C too, they don’t drift either. This also significantly improves electrical component lifetime; CALIPRI X’s trackside UPS (power supply) is similarly cooled for a much longer lifespan and lower lifecycle costs.
Applying the CALIPRI principle
For reliable measurement data, sensors must be perpendicular to rail wheels when capturing their cross sections. If a wheel is not exactly in the expected position, the lasers in most laser-light-section-based systems will hit slightly different positions on the wheel and that effectively tilts one sensor relative to the others. This harms accuracy.
Because CALIPRI X employs the patented CALIPRI Principle in its algorithms, it automatically corrects for any sensor tilt during measurement. So, unlike many other laser systems, CALIPRI X’s wheel profile measurement does not rely on the wheelset's position above the rail and so copes easily with very worn wheels or wheels of different sizes.
Then there’s rail movement. With up to 30t per wheel passing overhead, rail deflections can be significant – 2cm or even more – and there’s plenty of vibration too. This can be transmitted to and affect the stability of sensor boxes attached to rails or sleepers (their position relative to each other).
To combat this, 2m-long screw piles secure CALIPRI X’s sensor box supports to the ground. The sensors still sit close to passing wheels but are completely decoupled from the rails, sleepers and ballast so no loads are transmitted to them.
Because CALIPRI X employs the patented CALIPRI Principle in its algorithms, it automatically corrects for any sensor tilt during measurement. So, unlike many other laser systems, CALIPRI X’s wheel profile measurement does not rely on the wheelset's position above the rail and so copes easily with very worn wheels or wheels of different sizes.
Then there’s rail movement. With up to 30t per wheel passing overhead, rail deflections can be significant – 2cm or even more – and there’s plenty of vibration too. This can be transmitted to and affect the stability of sensor boxes attached to rails or sleepers (their position relative to each other).
To combat this, 2m-long screw piles secure CALIPRI X’s sensor box supports to the ground. The sensors still sit close to passing wheels but are completely decoupled from the rails, sleepers and ballast so no loads are transmitted to them.
Always on, always available
To maximise availability, we were determined to avoid long, sliding doors, a familiar feature of other systems. They require regular lubrication and are a failure risk – a single stone could jam them.
Opening these doors also delays measurement and requires separate triggering sensors further along the track. One of our customers was forced to leave 15m between its old on-track measurement system and parked vehicles. The gap gave that system time to wake up, but reduced storage area and disrupted depot operations.
CALIPRI X’s small, tough sensor windows and their safe location make the risk of stone damage very low, so they need no other protection. So there’s no doors to grease. In fact, there are no moving parts at all. CALIPRI X is always on, ready to measure instantly – with a lower risk of mechanical failure.
If gravel does crack a window, an internal humidity sensor triggers a remote alarm and maintenance can change a window from the outside in minutes by removing four screws. But after years of testing and operations, we have yet to see this happen.
Operators also told us that their devices were often choked by dirt, leaves and snow so we deliberately made CALIPRI X’s sensor boxes low and smooth. There are no edges or holes to trap wayside trash. Instead, debris just falls off the sloping sides and there’s plenty of clear space for dispersal – there’s no need for a dirt collector. In winter, its heated boxes not only protect measurement accuracy, they also melt snow away so it has no chance to block the sensor windows.
Opening these doors also delays measurement and requires separate triggering sensors further along the track. One of our customers was forced to leave 15m between its old on-track measurement system and parked vehicles. The gap gave that system time to wake up, but reduced storage area and disrupted depot operations.
CALIPRI X’s small, tough sensor windows and their safe location make the risk of stone damage very low, so they need no other protection. So there’s no doors to grease. In fact, there are no moving parts at all. CALIPRI X is always on, ready to measure instantly – with a lower risk of mechanical failure.
If gravel does crack a window, an internal humidity sensor triggers a remote alarm and maintenance can change a window from the outside in minutes by removing four screws. But after years of testing and operations, we have yet to see this happen.
Operators also told us that their devices were often choked by dirt, leaves and snow so we deliberately made CALIPRI X’s sensor boxes low and smooth. There are no edges or holes to trap wayside trash. Instead, debris just falls off the sloping sides and there’s plenty of clear space for dispersal – there’s no need for a dirt collector. In winter, its heated boxes not only protect measurement accuracy, they also melt snow away so it has no chance to block the sensor windows.
Designed for hands-off operation
The recessed sensor windows on many systems annoy rail operators. They trap dirt and must be regularly manually cleaned. But cleaning them is awkward and time-consuming; one team has to use cotton swabs.
Every 15 minutes, CALIPRI X’s sensor windows clean themselves automatically with bursts of compressed air. Passing trains also frequently drop water or dirt so, during active measurement, air constantly blows across the windows. The windows are the highest point on the boxes and flush with the top cover, again helping them stay dirt-free and also making them easy to clean manually if required.
Many other modern innovations make managing CALIPRI X low cost and low effort. Our remote server handles tasks like controlling cooling and compressed air pressure, minimising on-track electronics. Measurement data goes straight to the cloud via secure 4G and is available for use instantly – but there’s no obligation to link to your own systems.
The same 4G link supports sophisticated failure alerting and status updates plus remote diagnostics and self-calibration checks, accessed via a simple browser interface. This will cut on-track maintenance to a single annual visit. On-track physical calibration with reference blocks takes around 15 minutes but will rarely be required.
Every 15 minutes, CALIPRI X’s sensor windows clean themselves automatically with bursts of compressed air. Passing trains also frequently drop water or dirt so, during active measurement, air constantly blows across the windows. The windows are the highest point on the boxes and flush with the top cover, again helping them stay dirt-free and also making them easy to clean manually if required.
Many other modern innovations make managing CALIPRI X low cost and low effort. Our remote server handles tasks like controlling cooling and compressed air pressure, minimising on-track electronics. Measurement data goes straight to the cloud via secure 4G and is available for use instantly – but there’s no obligation to link to your own systems.
The same 4G link supports sophisticated failure alerting and status updates plus remote diagnostics and self-calibration checks, accessed via a simple browser interface. This will cut on-track maintenance to a single annual visit. On-track physical calibration with reference blocks takes around 15 minutes but will rarely be required.
CALIPRI – industry-standard measurement
One final example. When we store any CALIPRI dimensional data, we standardise it to an equivalent wheelset temperature of 20°C in accordance with ISO-1. So whether you measure automatically with CALIPRI X in summer or in winter, the results will be directly comparable.
This typifies our standards-based, engineering-led approach. Few in the rail industry know about ISO-1 and even fewer comply with it. Not standardising data might only introduce a tiny error, but these small errors add up.
Temperature drift, wheel centrality, track deflection: if you don’t control these factors, then your wheel profile measurements will be both inaccurate and inconsistent. That’s bad news for condition monitoring.
We designed CALIPRI X to be the best overrun system available: fast and accurate in all possible conditions and easy to use with very low lifecycle costs. Austria’s Wiener Linien now measures whole vehicles in seconds instead of hours and reports that CALIPRI X’s “measurement repeatability is in the tenth of a millimetre range”.
How did that happen? By listening to our customers, domain expertise and solid engineering.
This typifies our standards-based, engineering-led approach. Few in the rail industry know about ISO-1 and even fewer comply with it. Not standardising data might only introduce a tiny error, but these small errors add up.
Temperature drift, wheel centrality, track deflection: if you don’t control these factors, then your wheel profile measurements will be both inaccurate and inconsistent. That’s bad news for condition monitoring.
We designed CALIPRI X to be the best overrun system available: fast and accurate in all possible conditions and easy to use with very low lifecycle costs. Austria’s Wiener Linien now measures whole vehicles in seconds instead of hours and reports that CALIPRI X’s “measurement repeatability is in the tenth of a millimetre range”.
How did that happen? By listening to our customers, domain expertise and solid engineering.
