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Frequency Sweep Excitation Test Track
– A test track to identify NVH problems in cold climate –
The behaviour of a vehicle and its parts changes with temperature, for example seals become stiffer as the temperature drops. The effect of temperature will not only alter the dynamic behaviour of the car but also affect the acoustic properties. Examples of NVH problems that are more pronounced in cold climate are squeak and rattle. An important part of cold climate testing is to identify parts that squeak and rattle. Especially disturbing sources inside the car cabin are of great interest to address. In cooperation between IceMakers, Colmis, CASTT and Luleå University of Technology a new test track has been developed. The aim of the new test track named Frequency Sweep Excitation Test Track is to identify NVH problems in cold climate.
The basic idea behind the Frequency Sweep Excitation Test Track is that a vehicle travelling along the test track with a constant speed will be exposed to a constant force with a changing frequency. The excitation frequency increases (or decreases) as the vehicle moves along the track. The force is created from the obstacles mounted on the test track. Since the distance between the obstacles varies along the test
track a frequency sweep is achieved. If the distance between obstacles increases or decreases a frequency that decreases or increases correspondingly results. The amplitude of the force will however be constant independent of the current excitation frequency. Due to the varying frequency a noise source can be attributed to a certain frequency which facilitates the work with correcting them.
Because the frequency varies in a controlled manner, various noise sources inside the car can be detected, located and identified in a systematic way. Currently, the work is less systematically using wooden step tracks which only excite one frequency at a time and cobblestone tracks, which cause a randomly varying force. The Frequency Sweep Excitation Test
Track generates a more systematically process with better opportunity for repeatability and ability to control both the amplitude and frequency of the force as well as assuring that a given frequency range is excited during the test. The Frequency Sweep Excitation Test Track is a complement to wooden step tracks, cobblestone tracks and twist hills.
During the development process a number of different versions of the Frequency Sweep Excitation Test Track have been modelled and evaluated in Matlab. The goal has been to achieve a test track that excites the vehicle with a linearly increasing (or decreasing) frequency. A master thesis project has investigated the influence of the shape of the obstacles on the force between the obstacle and the tyre.
A prototype of the Frequency Sweep Excitation Test Track was built and evaluated at IceMakers testing ground in 2010. The development of the new test track is a cooperation project between IceMakers, CASTT and Division of Sound and Vibration at Luleå University of Technology. The prototype track is 100 m long and consists of some 300 obstacles. If the vehicle travels at a speed of 40 km/h the frequency range is 6‐40 Hz. If the speed increases to 100 km/h the frequency range changes to 15‐100 Hz.
Prototype of the Frequency Sweep Excitation Test Track built and evaluated at IceMakers’ testing ground, in Arjeplog, Sweden.
Preparation of the test track for a new day at the testing ground.
A car being equipped with a triaxial accelerometer for a test run on the Frequency Sweep Excitation Test Track.
The major advantage with the Frequency Sweep Excitation Test Track is that it more quickly can detect, locate and identify the natural frequencies, weaknesses, squeak and rattling compared to the test tracks available today.
The figure below shows a result from a measurement on a vehicle driving 50 km/h along the Frequency Sweep Excitation Test Track. The lower graph shows the acceleration in vertical direction as the vehicle drives along the test track. The upper graph shows a spectrogram of the acceleration signal. At the beginning of the test track the excitation frequency is 50 Hz which drops to 8 Hz at the end of the track.
Measurements being performed at the Frequency Sweep Excitation Test Track.
Contact: If you are interested in information about the Frequency Sweep Excitation Test Track please contact:
Luleå University of Technology (www.ltu.se) Roger Johnsson, E‐mail: [email protected]