Track Mounted Equipment

There are two basic types of Wheel Condition Monitor (WCM) systems. The original systems are based on strain gauges and measure force directly. The Teknis WCM system is based on accelerometers and measures rail displacement.

The strain gage based systems do not always see 100% of the wheels surface and are highly sensitive to axle load. This is a real disadvantage where axle loads vary. It is not uncommon for the force type WILD systems to miss a defect on a wagon that is empty and allow it to be loaded and return to port only to cause an alarm at that time.

The WCM is made up of two components. There is an array of accelerometers mounted to the rail and there is a signal processor in the wayside enclosure.

All sensors are mounted on the track in a six crib clamp-on array where five cribs have one accelerometer on each rail. When the weighbridge option is installed then load cells are mounted under each rail in all six cribs.

Each crib position represents one data acquisition window. It spans from the centre of one sleeper to the centre of the next. There are five of these in a row on each rail for a total of ten sensors. In this way the whole surface of each wheel is measured.

The track mounted array is accurately measured and the dimensions are stored in the signal processor.

As each wheel enters the array its position, direction and speed are recorded by a pair of wheel sensors at each end of the array. The rate of deceleration and acceleration is calculated using the previous axle’s information. (normally negligible) The information is then passed to each sensor module.


Each sensor module acquires data for each wheel, stores it for later processing and waits until the next wheel arrives over its sense zone.

For wheel condition analysis, the accelerometers sense rail motion. Rail motion due to normal train passage is removed leaving impact data. This impact data is recorded axle by axle, combined with the vehicle AVI tag and sent to a PC in the Train Control area where it is analyzed and recorded. The PC in the Train Control Area then prints alarm reports and stores the results in databases situated on various servers and PCs.

The system automatically compensates for changes in track modulus including long term track or subgrade changes. The installation shown in the figure to the right is on an electrified line in northern Norway that undergoes weekly freeze-thaw cycles six months of the year.

Wayside Equipment

Each array is connected to a purpose built signal processor that interfaces to the AVI reader, 10 accelerometers, temperature sensors, 4 wheel sensors, and up to 24 channels of strain gauge bridges.

The architecture is multi-processor distributed. Sampling rates and frequency domains are dynamically selected based on train speed.

The figure below shows a signal processor with the front cover closed.



Data derived from the sensors for each wheelset includes:

• Axle speed
• Inter-axle distance to within 0.2"
• Rail and ambient temperature
• Full AAR spec tag data positioned assigned to nearest axle as read
• Peak lateral forces for both rails at three cribs (bipolar)
• Peak vertical force for both rails at each of the six cribs
• Continual profile of rail motion and acceleration over the full circumference of each wheel with impact characterization and peak locations in each crib. This enablies detection and reporting of multiple defects, long period defects, and runout or ovality. This is independent of sprung mass.
• Self compensating acceleration array ensure linear arrays response over changing conditions.
• In-motion weighing with facilities for auto-calibration to reference vehicles, compensation for frost effects, and exclusion of impact effects. Capability to process mixed single axle and standard three-piece bogie wheelsets.