Using 3D Scanning Vibrometry for Automotive Testing

Rolling tires stimulate a considerable amount of passenger car noise (NVH) inside and outside the passenger cabin for a large range of driving conditions. These are three important reasons this rolling tire noise must be dealt with:

• With no internal combustion engine noise, the otherwise exceptional quiet of electric cars can be ruined by tire-generated noise and the driving expoerience can be affected
• New regulations restricting tire noise are predicted in several countries concerned with urban area noise.
• Road noise from rotating tires disturbs neighborhoods alongside the roadway

Figure 1. Laboratory Setup. The three Vibrometer Sensor Heads are mounted to a common frame (right side). The roller test bench is to the left with the tire on top and the motor on the bottom. The mirror on the far left is used to simplify the repositioning of the vibrometer scan pattern.

Therefore, a better understanding of the dynamic mechanism producing the noise is critical in order to restrict the amount of tire noise, and, for that understanding, the precise measurement of the vibration behavior of the rotating tire surface is essential. Traditional measurements which employ simple microphones to quantify the noise amplitudes inside and outside a car give hardly any insight into the precise physical origins of the noise.

Even accelerometers positioned at crucial locations cannot provide the spatial and frequency resolution required for accurate characterization. Classical NVH measurements and techniques are not helpful to solve this problem.

Non-contact 3D Scanning Vibrometry as a Measurement Solution

Full-field or Scanning Vibrometry is the ideal solution for modal testing of tires. For this application, the Xtra model with high optical sensitivity exhibits its full potential, allowing measurements directly on black rubber without any surface preparation required. In this case, the Xtra Polytec Scanning Vibrometers allowed measuring on rotating tires with rotational speed equivalent to a driving speed of around 100 km/h.

Polytec developed the Xtra Vibrometer using an infrared laser source to allow measuring on uncooperative surfaces,  The Xtra Vibrometer has a visible pilot laser, is eye safe (class II), and has extra sensitivity in addition to a 2.5x increase in maximum velocity up to 30 m/s compared to its HeNe sister.

The Xtra technology Scanning Vibrometer can now measure deflection shapes of rotating tires much more easily, giving a more accurate and faster insight into the origins of tire noise.

Figure 2. Complete spectrum averaged over all points. Orthogonal vibrations x,y,z, are represented as separate colors.

Figure 3. Detailed spectrum from Fig. 2 shown centered around 300 Hz.

Test Setup

A laboratory version of a “roller test bench” was built in order to confirm the capabilities of the new Xtra 3D Vibrometer system to simplify and enhance the collection of data on a real rolling tire. The rudimentary test fixture was made up of a small 25 cm diameter tire rolling on top of the drive shaft of an electric motor. The Xtra 3D Scanning Vibrometer had its three heads mounted onto a common frame and was located on the lab table close to the test bench.

When covering different portions of the tire, this common frame made the repositioning of the heads a lot simpler. To access different portions of the tire without movement of the common frame, a large mirror was also utilized. Similar results were achieved when compared to those obtained with a real car tire on a commercial roller test bench by using this simple measurement setup.

The averaged spectrum across all points along the three directions (see the three different colors) can be observed in Fig. 2. The details of the spectrum from 200 Hz to 400 Hz are shown on an expanded scale in Fig. 3. A comb-like feature is easily identifiable in the spectrum. This feature is also seen in real roller test bench assessments employing the Xtra Scanning Vibrometers.

The peaks in this comb feature happen at multiples of the rotation speed of the tire. This can be explained by the periodic contact of the treads (and the air pockets in-between) with the road surface for tires which have a tread pattern. Employing this measurement setup clearly permits the effect of tread design on the structural resonances of the tire to be observed under different loading conditions and at different speeds.

Simplified Capturing of Deflection Shapes

A far bigger area of the rolling surface can be measured with the Xtra Sensor Heads. The heads are repositioned in order to capture the second area of the rolling surface after the first measurement is finished. By utilizing reference points with known position coordinates detected by a built-in distance sensor, the second position is known relative to the first.

Using this method, the results of each surface measurement may be stitched together, resulting in just one animation and one overall deflection shape. Facilitated by the mirror, employing this stitching method permits even the side wall measurement to be included. By extending this method, the tire can be covered quickly, and relatively large surface portions measured with the Xtra Vibrometer Sensor Heads and then stitched together.

Figure 4. Measured deflection shapes at 396 and 468 Hz.

The Xtra option supplies an excellent signal-to-noise ratio (SNR) despite the larger measurement areas and fast rotating black tire surface. Some usual deflection shapes can be seen in Fig. 4, respectively at 396 and 468 Hz. The typical pattern of multiple maxima on the rolling surface can be seen very clearly. Extremely similar results are gathered in commercial tire test stands.

To summarize, the latest infrared sensing technology for 3D Scanning Vibrometers has captured the deflection shapes and spectral patterns of fast-moving rolling surfaces clearly, supplying vital information on the vibration behavior of the tire surface which is at the origin of the emitted noise.

Combined with numerical simulations of tires, these precise results will permit the refinement of FE models and the minimization and control of rolling tire noise via the design of better tires.

This information has been sourced, reviewed and adapted from materials provided by Polytec.

For more information on this source, please visit Polytec.