3.1 Wheel roughness measurement/in-depot (offline) wheel inspection

Wheel tread roughness measurement (in-depot inspection) is a direct way of collecting wheel condition information for maintenance, monitoring profiles in conjunction with wear problems. With wheel roughness measurement data, the wheel re-profiling strategy can be optimised using data-driven wear model [24]. The NDT technologies employed by roughness measurement include linear variable differential transformer (LVDT), the mechanical displacement probe, the rotation sensor, electromagnetic acoustic transducer (EMAT) [25], laser-ultrasonics [26], laser-air hybrid ultrasonic technique (LAHUT) [27] and other novel NDT techniques [28–30]. Some even allow the trains run at a low speed during inspection [30]. However, for measurement methods using ultrasound pulse-echo technique, it is sometimes difficult to detect wheel flats because they usually have smooth edges that do not generate echoes [31]. There are now many commercial devices that allow the measurement to be done in depot with high efficiency, such as ØDS measurement instrument, Miniprof, MÜLLER-BBM, etc.
