Frequency Sweep Test Track (FSTT)

Introduction

A lot of effort is put into car interior sound quality because of its importance for customers’ perception of vehicle quality. A classic example is the sound when the door is closed, which often is the first physical interaction between the customer and the vehicle. To further improve vehicle quality, transient or come-and-go sounds such as squeak and rattle (S&R) need to be eliminated. In order to achieve high product sound quality and position the car on the market, it is important to test and evaluate the interior noise in a controlled and systematic way.

S&R is a group of intermittent interior noise that have been reported the third most important factor for customers after owning a car for 3 months and are perceived as a direct indicator of the vehicle’s quality and reliability. Electrical cars are even more sensitive to this kind of weak noise sources due to the lack of masking from the combustion engine. S&R are transient acoustic events that occurs when the car shakes, twists or in other ways are excited in such a way that adjacent parts of the car are in contact with relative motion to each other. There are many parts that can generate these types of problems, e.g. window and door sealing systems, window wipers and dashboard parts. Squeaks are caused by frictional stick-slip contact between two parts, while rattle is a repetitive impact contact of adjacent parts. Both phenomena are caused by vibrations at lower frequencies induced by suspension inputs and the powertrain. The audible squeak and rattle is usually generated in a wide frequency range between 200 Hz and 10 kHz.

Several different methods are used by the car manufacturers to eliminate S&R problems. Computer-aided engineering (CAE) is an attractive tool to investigate potential S&R problems but due to the variety of mechanisms generating S&R CAE-modelling is a challenge. Physical testing is used both for subsystems and complete vehicles both in laboratory environment and at proving grounds. At the proving grounds several different test tracks are used to excite and provoke the car in such a way that potential S&R problems can be identified and corrected. Commonly used test tracks are cobblestone, Vienna block, rough brick, washboards, ropes, and body twist. For these tracks, the obstacles are either equally or randomly spaced. A track with equally spaced obstacles (e.g. washboard) will generate one fundamental frequency given by the obstacle spacing and the speed of the vehicle. In order to identify different frequency dependent S&R noise sources the vehicle must travel at different speeds that will vary the amplitude of the impact force. Tracks with randomly spaced obstacles (e.g. the cobblestone track) will give a more random frequency excitation and will simultaneously excite several S&R noise sources making them harder to identify, quantify and pinpoint a specific location. In order to systematically test for vehicle S&R noise at proving grounds – as when using CAE and laboratory testing – there is a need for a new type of test track that in a controlled and repetitive way excite vehicles at different frequencies.

Frequency Sweep Test Track Design

The Frequency Sweep Test Track (FSTT) is designed to excite a vehicle, driven at constant speed, with a constant force with increasing (or decreasing) frequency. When designing a FSTT a number of properties need to be considered such as:

• Sweep waveform and amplitude
• Frequency range
• Sweep rate and frequency resolution

Sweep waveform and amplitude

Frequency sweep excitation can be designed using different wave forms, e.g. as a sine wave and a pulse train as illustrated in figure 2. A sine-shaped track will not generate strong harmonics as compared to a pulse train but is more expensive to manufacture and maintain. A track consisting of uneven spaced ribs are inexpensive to build and easy to maintain. Independent of the selected track design the amplitude of the FSTT must be selected depending on the quality demands of the actual car model.

Frequency range

S&R is a result of road surface irregularities transferred through the tyre and the wheel suspension. In general S&R is caused by frequencies below 50 Hz as the tyre and suspension act as a low-pass filter. A test track design criterion for detecting S&R is therefore to excite the car in a frequency range up to at least 50 Hz. At frequencies below 5-7 Hz the subsystems of the car will move in phase and there will not be any relative motion causing S&R. An alternative to cover the whole frequency range with a single run of the test track is to run multiple times with different speeds over a shorter test track. However, different speeds will change the force acting on the vehicle.

Sweep rate

Commonly a frequency sweep is either linear or logarithmic. An advantage with a linear sweep is that it is easy to determine the frequency from the position on the track. A drawback is that the low frequencies will be excited with less number of periods than the higher. For a logarithmic sweep rate the number of periods per frequency is constant but it is harder to determine the frequency from the position on the track.

The sweep rate and frequency resolution must be selected to effectively detect S&R noise sources (low sweep rate and long sweep time) while the track length remains reasonable. A lower vehicle speed will decrease sweep rate for a given track length and frequency range. Another advantage with a low speed is that the tyre/road noise and wind noise will decrease. The speed will however set an upper limit of excitation frequency given that the texture wavelength (distance between track irregularities) must be set in relations to the tyre dimensions (see figure 3).

Vehicle Dynamic Modelling

During the development of the FSTT vehicle dynamic modelling has been performed using a quarter-car model and a tyre model, figure 4 and 5. To illustrate the effect of sweep waveform, a 100 m long sinusoidal sweep track (20 mm peak-to-peak value) and a pulse train track consisting of 40 x 20 mm (width x height) square ribs were compared. Simulated body acceleration using the quarter-car and tyre model for the two tracks is shown in figure 6. The frequency range was set to 5-50 Hz and the simulated speed was 50 km/h. The harmonics generated from the square-shaped ribs are effectively reduced by the quarter car dynamics for high frequencies as suspected due to the suspension.

Case Study and Measurements

A 100 m long single path Frequency Sweep Test Track was built, figure 8. In Table 1 the test track specifications are summarized. The track was constructed of plywood plates on which ribs in plastic were secured. The ribs were 20 mm high and 40 mm wide and had a rectangular cross section. The plywood plates were attached on wooden studs that were moulded in the ice on the proving ground. The FSTT was built in sections, each 5 m long, and a schematic drawing of the FSTT is shown in figure 9. The track was built with increasing space between ribs, i.e. sweeping from high to low frequencies.

To verify the sweep excitation of the FSTT the axle acceleration was measured for an executive car (tyre 205/55 R16) using a triaxial accelerometer placed on the left rear wheel hub. Further, the interior noise was measured using a microphone placed close to the centre console. The spectrogram of the recorded axle acceleration as the car run over the FSTT at 50 km/h is presented in figure 10. The car speedometer was used to control the speed and the actual speed was some 47 km/h. The track gave rise to the designed excitation frequencies. The excitation of fundamental frequencies below 15 Hz is weak due to the system response.

While driving at the FSTT an apparent dashboard rattle was detected at the centre-console. Figure 11 shows the spectrogram of the recorded interior noise. The rattle radiated prominent noise around 1.4 kHz that emerged for 1.5 second in the middle of the track. Reviewing the spectrogram, the rattle is also present in the beginning of the track however it is masked by other interior noises.

Discussion

To detect S&R the vehicle shall be excited in the frequency range 5-50 Hz. This was achieved for the built FSTT by driving over the track at both 30 and 50 km/h. The excitation frequency was verified by measuring the axle acceleration. The amplitude of the obstacles has to be determined depending on the quality demands on the specific car model. In this case, the track was built as a pulse train using 40 mm wide and 20 mm high ribs, sufficient to generate dashboard rattle in an executive production car. A logarithmic sweep was selected to excite each frequency same number of periods. The design of the FSTT makes it easy to make repetitive test of S&R for a chosen frequency range. The actual frequency range can easily be selected by changing the speed of the car. However, the force acting on the tyre will change with the present speed. The built FSTT was a single 600 mm wide track, exciting only the wheels on one side of the car. To avoid unnecessary noise, that would make it more difficult to detect S&R, it is important that the other two wheels run on a smooth surface. By making the track wider all wheels can be excited, it is also possible to design the track so that left and right wheels are excited with a phase shift or with different frequencies.

Information material

In the project a video and a brochure has been produced about the Frequency Sweep Test Track. The video can be found at youtube (http://www.youtube.com/watch?v=Bsqs9MMjycE) and below. The brochure can be found below together with building instructions for a 100 m and a 200 m long FSTT.

Project Leader: Roger Johnsson