NTS Ultrasonics
NTS Ultrasonics Pty Ltd
NTS ultrasonics offers two basic wheel probe configurations based on the PL180 wheel probe: the "standard PL180" and the "phased array PL180". Each design can be customized. The internal transducer mountings can be altered to support different numbers of transducers and different beam angles. These wheel probes can be used with ultrasonic NDT systems for applications such as corrosion mapping and ultrasonic flaw detection. Note that wheel probes are also called "roller search units" or "RSUs".
The standard PL180 is a 180 mm diameter wheel probe is intended for heavy duty use with multiple transducers. It is suitable for rail flaw detection and steel mill applications. The standard design can be easily modified to accept phased array transducers.
180 mm diameter (approx.)  when filled.
Internal water paths in excess of 100 mm possible.
Can accept multiple transducers, e.g. 0 degree. 40 degree, and 70 degree as used in rail flaw detection.
Wide enough to accept paintbrush style transducers or phased array transducers.
Transducers are electrically isolated from metal parts of the wheel.
Uses a tough polyurethane tyre with low sidewalls to prevent the tyre from being pulled out of the rim.
Easily customized.
PL180 Accessories:
Caliper for holding the wheel probe.
Seal making system.
Filling pump.
PL180 Downloads (pdf):
PL180 information
PL180 outline drawing
Material   Attenuation dB/mm Hardness
@ 1 MHz @ 2.25 MHz @ 5 MHz Shore A
Rubber 1.6 2.0 2.4 40-50
Polyurethane 1.2 1.43 3.4 90-95

Properties of wheel probe tyre materials
ultrasonic roller search unit for phased array transducer
Phased array PL180 with caliper. Note the larger diameter axle.
Ultrasonic Wheel Probes
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NTS Ultrasonics Pty Ltd, Perth, Western Australia,  Tel: +61 8 9367 1578, Email: ntsu@ntsu.net.au
PL180 ultrasonic wheel probes
 
The standard PL180 ultrasonic wheel probe is suitable for use in ultrasonic rail flaw detection and similar applications. The standard probe configuration produces 0, 40, and 70 degree refracted shear wave beams in steel. The preferred configuration is for the 40 and 70 degree forward facing beams to be produced in one wheel probe, and the 40 and 70 degree backward facing beams produced in a second (assymteric configuration). This configuration minimizes the possiblity of strong reflections being produced internally in the wheel which may interfere with echoes produced in the rail, as occurs with the symmetric configuration. However, the assymetric configuration requires two wheel probes per rail, whereas the symmtric configuration needs only one, and is favoured by some for walking stick applications.
Assymetric transducer configuration (preferred)
transducer arrangement in ultrasonic wheel probe
Symmetric transducer configuration
transducer arrangement in roller search unit
NTS Ultrasonics Pty Ltd
Standard PL180 wheel probe with caliper
heavy duty ultrasonic roller search unit
~ Ultrasonic Wheel Probes
Wheel probes can contain transducers set at angles to produce specific refracted beam angles in a test item. For this, it is necessary to ensure that the fluid used to fill the wheel has minimal variation of ultrasonic velocity with temperature. For example, for a wheel probe intended for use in ultrasonic rail flaw detection it is necessary to ensure that the refracted 40 degree and 70 degree beams vary as little as possible over the working temperature range. Water has a positive temperature coefficient of ultrasonic velocity. Using water to fill a wheel probe, for a temperature range of 10 degrees C to 40 degrees C, a nominal refracted angle in steel of 70 degrees will actually vary from 75 to 66 degrees. However, most liquids have a negative temperature coefficient of ultrasonic velocity, and certain fluids, when mixed with water, can create a fluid mixture with minimal variation of ultrasonic velocity with temperature. NTS Ultrasonics recommends a particular fluid mixture that users can mix themselves using readily available components.
graph: temperature variation of the ultrasonic velocity of wheel probe fluid
The graph above shows the variations of ultrasonic velocity with temperature for water and for the wheel probe fluid mixture recommended by NTS Ultrasonics. The graph below shows the resulting errors in a nominal 70 degree angle refracted shear wave from temperature variations of the NTS Ultrasonics fluid mixture, and water.
graph: refracted angle errors with temperature variation in ultrasonic wheel probes

Customizing

Transducer configurations for railway inspections

Wheel probe filling fluid

The standard PL180 wheel probe uses a machined plastic transducer holder. Users can be supplied with mounting details for the holder and machine new holders for their own transducer configurations. The transducer holder system also allows users to fit side looker transducers.
The standard PL180 wheel probe uses a machined plastic transducer holder. Users can be supplied with mounting details for the holder and machine new holders themselves for their own transducer configurations. The transducer holder system also allows users to fit side looker transducers. Complex shaped plastic transducer holders can be fabricated using 3D printing.
Simple calibration rig for PL180 wheel probe

Calibration of PL180 Ultrasonic Wheel Probes

At NTS Ultrasonics PL180 wheel probes are calibrated in a similar way to conventional manual transducers using an IIW calibration test block in a special test rig. The index points for the various transducers in the wheel and refracted angles in steel are recorded. Any adjustments required are made during the calibration process. Old fashioned "zeroing" is not required, as this favours the less critical 0 degree probe over the more critical 40 degree shear wave probe.
roller search unit calibration
Download "An introduction to ultrasonic wheel probes" (pdf)

Ultrasonic Wheel Probes