Natural Frequency Measurement

Transcription

1 Natural Frequency Measurement 'Frequently Asked Questions' F 1 What is the motivation for 'natural frequency testing'? There are different applications which make use of this kind of test: A: Checking the microstructure of the castings It is a fast way to continuously keeping track of material characteristics like hardness and modulus of elasticity in a casting process. Cast iron properties are influenced by the added alloying elements, which change the microstructure of the cast. This causes a change of the wave propagation within the test specimen. The method is used to check/distinguish for/between grey iron (GJL), ductile iron (GJS), vermicular iron (GJV) or malleable iron (GJMW or GJMB). B: Assuring part acoustic behavior/influence during operation A brake system consists of multiple components - most of them are casting parts. These parts can be easily made sounding by striking. This ability to sound (or vibrate) is also observable in the assembled condition. If two parts are mounted together having a similar or the same natural frequency, the part vibration will be amplified - they are getting into "resonance". This situation must be avoided. To do so, each part of the brake system has to be designed to its own natural frequency signature. The result is, even if one part is getting into oscillation, the other components do not care about and stay quiet. Brake squeaking can be reduced to a minimum. The automotive industry has defined natural frequencies as part of the specifications and drawings. C: Component sorting Steering columns propagate vibrations from the front axle to the steering wheel. The wheel vibrates when the excited frequency and the natural frequency are nearly the same. When driving the main vibrations come from the tires, elsewise from the engine. By natural frequency measurement and simulation of the steering wheel mass the columns should be grouped. This grouping allows deciding whether the steering columns can be mounted in a car with benzine engine or diesel engine.

2 F 2 F 3 What is the principle of the 'natural frequency test'? The test is based on the acoustic resonance analysis. This method uses the known physical effect that a body having been suitably excited (e. g. by tapping), vibrates in certain characteristic forms and frequencies. These vibrations, also called 'natural frequencies', are so to speak the 'language of the work piece', which can be measured by a microphone (airborne sound) or by a laser vibrometer (structure-borne sound) and digitally analyzed. The decisive factor for the number and spread of natural resonances of a work piece is its geometry: a bar has few natural resonances; a complex work piece has multiple natural resonances. How is the 'natural frequency test' practically performed? First, the part has to be placed on soft tips to avoid damping effects. To stimulate the parts oscillation, a tiny electrically driven metal hammer is used to create an impact on the parts surface. This mechanical impulse makes the part vibrating in its characteristic forms and frequencies. A microphone mounted close to the test specimen surface is collecting these frequencies and converting them into electrical signals. Finally a data acquisition system is converting the signals into numerical values, which are processed by the computer. A special algorithm called 'Fourier Analysis' (FFT) is used to present the typical fingerprint of the tested part. Each vertical peak represents one natural frequency (Fig. 1). The higher the level, the stronger the oscillation and - in the opposite direction - the more sensitivity the part has at this specific frequency. Fig. 1: The acoustical finger print of a test part

3 F 4 What are the different excitation methods? To stimulate the oscillation of the part, different methods are possible: Fig. 2: Excitation methods The excitation has to stimulate the test specimen to vibrate in its resonances. These can then be measured by a microphone, a laser vibrometer or an accelerometer (see F 5). The impulse modal hammer consists of a quartz force sensor mounted on the striking end of the hammer head to measure the excitation force. As the excitation force has only an influence on the resonance frequency amplitude, but not on the value, it is not necessary to use a modal hammer for resonance frequency evaluation. An electrodynamic hammer is designed to have a short contact time, can be adjusted to the necessary excitation force and allows constant amplitudes. This device is sufficient for natural frequency testing and used in automation applications. Manual excitation is used in initial tests. As the excitation force is not constant the level of the resonances varies. Piezo-electric actors vibrate at a specific frequency depending on the supplied voltage. When you contact such a device with the test specimen it will vibrate with the same frequency as the actor. If the actor frequency meets a resonance frequency of the part you can measure this with a second sensor (accelerometer). The advantage is the high frequency range, the disadvantage is that it is necessary to sweep the induced frequencies. This is time consuming and the contact quality and position have a great influence on the result. F 5 Which sensor is the best for measuring the resonance frequencies? Resonance frequencies can be measured contactless by a microphone or laser vibrometer or contacted by an accelerometer. Whereas a laser vibrometer and an accelerometer measure the resonance frequency and amplitude at a single

4 position the microphone is the only device that can measure integral, i.e. all frequencies regardless of the position. To measure all vibration modes with significant amplitude two or more microphones can be used. F 6 F 7 F 8 How long does it take to get the natural frequencies of a part? This depends on the number of impacts and the part itself. When testing a brake disc, a caliper or a bracket, typically two or three excitations are used to induce the natural frequencies. After the excitation, it takes less than 1 second to read and analyze the sound. So in total, within two seconds or even less all natural frequencies are available for evaluation. What about the environmental conditions to perform this test? In most cases nothing. This measurement is not influenced by environmental noise and vibrations. The sensors are directional microphones which are located close to the part surface. If necessary, simple damping elements are placed in the area surrounding the specimen to ensure a good signal-to-noise ratio. Which influences have an impact on the 'natural frequencies' of the part? There are two different types of influences: A: Static influences: Static influences are based on the production process and the part material itself. They are invariant over the time and primary responsible for the position of the natural frequencies. These are mainly geometrical dimensions, coefficient of elasticity, density, hardness, cast structure and some others. B: Temporary influences: Temporary influences are work piece temperature and age of the part. The following example explains the temperature influence (Fig. 3): If a part temperature is 50 C and the reference values are typically measured at 20 C we can expect a frequency shift of about 0.75 %. This is a respectable value compared to a tolerance range of ±3 %. In fact, by measuring the part temperature, this effect can be compensated by the software. The age of the part is responsible for a shift of natural frequencies within a time period of e. g. 30 days (Fig. 4). During this time the natural frequencies of the part are shifted to higher frequencies following an exponential curve. This effect is caused by changes in the microstructure as well as by reduction of material tensions (especially with grey iron).

5 The reference values in a drawing are referred to 'aged' parts. A very theoretical solution would be to wait 30 days before performing the measurement. The more efficient way is to input the age of the part into the measurement software which uses this information to calculate the values as they would be at an age of 30 days. All types of compensation methods mentioned above are part of RTE's test software SonicTC.EigenFrequency. Fig. 3: Influence of the part temperature Fig. 4: Influence of the part age F 9 Measure cast blank or machined part? Both - with different focus. In the casting area the main issue is to check the averaged values over one batch. For brake discs the tolerances of raw parts are often defined as acceptable within ±5 % and for machined parts within ±3 % of the reference values. The measured statistical mean values should be as close as possible to the reference values to avoid increased reject later on. Machining changes the natural frequencies (see above). Batch inspection by samples may be sufficient, but depends on customer requirements. F 10 Is this technology applicable in laboratory and shop floor? Yes - as long as the measurement conditions follow the rules mentioned above. It can be used for low volume/laboratory (Fig. 5), medium volumes in labs/shop floors (Fig. 6) and manufacture-integrated fully automatic units (Fig. 7, Fig. 8). Fig. 5: SonicTC.Cube: manually loaded test device (low volume, exchangeable fixture, manual type adaption, special fixtures for brake discs)

6 Fig. 6: Semi-automatic test benches (medium volume, type-specific work piece carriers) Fig. 7: Semi-automatic test bench (high volume, automatic changeover, optimized for brake discs) Fig. 8: Inline 100 % test bench (high volume, automatic changeover, optimized for brake discs) F 11 Are there any specifications concerning 'natural frequency test'? Yes - there are some. Common specifications concerning brake parts are the EKB Technical Standards from the European Brake Noise Experts Group (EKB 2001/2002 for brake discs, EKB 4001 for brake calipers). It contains some hints regarding theory and practical application of that test. In addition the brake manufacturers and OEMs often have proprietary specifications. Any further questions? Feel free to contact RTE the experts in acoustic testing for production and development.