Scott Gruenberg

Dynamic testing of shock absorbers under non-sinusoidal conditions

Abstract This paper deals with the dynamic characterization of an automotive shock absorber, the continuation of an earlier work [1]. The objective of this ongoing research is to develop a testing and analysis methodology for obtaining dynamic properties of automotive shock absorbers for use in CAE-NVH low-to-mid-frequency chassis models. Stepped sine sweep excitation is currently used in industry to obtain shock absorber parameters along with their frequency and amplitude dependence. Sine-on-sine testing, which involves excitation using two different sine waves, has been done in this study to understand the effects of the presence of multiple sine waves on the estimated dynamic properties. In an effort to obtain all frequency dependent parameters simultaneously, different types of broadband random excitation have also been studied. Results are compared with stepped sine sweep tests. Additionally, actual road data measured on different road profiles have been used as input excitation to obtain the shock absorber parameters for broad frequency bands under realistic amplitude and frequency conditions. These results are compared with both simulated random excitation and stepped sine sweep test results.

The effects of different input excitation on the dynamic characterization of an automotive shock absorber

This paper deals with the dynamic characterization of an automotive shock absorber, a continuation of an earlier work [1]. The objective of this on-going research is to develop a testing and analysis methodology for obtaining dynamic properties of automotive shock absorbers for use in CAE-NVH low-to-mid frequency chassis models. First, the effects of temperature and nominal length on the stiffness and damping of the shock absorber are studied and their importance in the development of a standard test method discussed. The effects of different types of input excitation on the dynamic properties of the shock absorber are then examined. Stepped sine sweep excitation is currently used in industry to obtain shock absorber parameters along with their frequency and amplitude dependence. Sine-on-sine testing, which involves excitation using two different sine waves has been done in this study to understand the effects of the presence of multiple sine waves on the estimated dynamic properties. In an effort to obtain all frequency dependent parameters simultaneously, different types of broadband random excitations have been studied. Results are compared with stepped sine sweep tests. Additionally, actual road data measured on different road profiles has been used as input excitation to obtain the shock absorber parameters for broad frequency bands under realistic amplitude and frequency conditions. These results are compared with both simulated random excitation and stepped sine sweep test results. INTRODUCTION The shock absorber is one of the most important elements in a vehicle suspension system. It is also one the most non-linear and complex elements to model. The current method of characterizing the dynamic properties of shock absorbers for CAE models involves testing at discrete frequencies, displacements, and preloads using an MTS test machine. The dynamic stiffness (K) and damping (C) are extracted by fitting a linear model of the form F(ω)=K*x(ω)+C*v(ω) to the measured input displacement (x), velocity (v), and output force (F). The excitation technique is a pure sine excitation at the desired frequency and amplitude. These harmonic excitations are then swept through all desired frequency and amplitudes. Parametric and non-parametric models also exist for the shock absorber. A non-parametric model based on a restoring force surface mapping has been developed [2,3,4]. The model considers the force to be a function of displacement and velocity. Although, this model is more applicable to a single frequency excitation, it serves as a useful tool for identifying the non-linearity’s in the system. A comprehensive physical model was developed by Lang [5], later condensed and validated by Morman [6]. Lang’s model has more than 80 parameters, is computationally complex and is not suitable for comprehensive vehicle simulation studies. Morman’s model has been shown to be useful for studying the effects of design changes for a particular shock. Reybrouck [7] has developed a physical model, which has 14 parameters, valid for frequencies up to 20 Hz, but has limited appeal for the analysis of shock absorbers for NVH applications.

Measurement of Dynamic Properties of Automotive Shock Absorbers for NVH

This paper describes a project on the dynamic characterization of automotive shock absorbers. The objective was to develop a new testing and analysis methodology for obtaining equivalent linear stiffness and damping of the shock absorbers for use in CAE-NVH lowtomid frequency chassis models. Previous studies using an elastomer test machine proved unsuitable for testing shocks in the mid-to-high frequency range where the typical road input displacements fall within the noise floor of the elastomer machine. Hence, in this project, an electrodynamic shaker was used for exciting the shock absorbers under displacements less than 0.05 mm up to 500 Hz. Furthermore, instead of the swept sine technique, actual road data were used to excite the shocks. Equivalent linear spring-damper models were developed based on leastsquares curve-fitting of the test data. The type of road profile did not influence the stiffness and damping values significantly for the range of amplitudes and frequencies considered. Finally, sensitivity of the vehicle level responses to the shock absorber rate change was studied, to finalize whether or not an upgrade to the existing shock absorber test procedure is necessary.

Vibration testing and dynamic modeling of automotive shock absorbers

This paper is a continuation of previously presented research work involving the dynamic characterization of automotive shock absorbers. The objective was to develop new testing and analysis methodologies for obtaining equivalent linear stiffness and damping of the shock absorbers for use in CAE- NVH low-to-mid frequency chassis models. It is well known that a hydraulic actuated elastomer test machine is not suitable for testing shocks in the mid-to-high frequency range where the typical road input displacements fall within the noise floor of the hydraulic machine. Hence, initially in this project, an electrodynamic shaker was used for exciting the shock absorbers under displacements less than 0.05 mm up to 500 Hz. Furthermore, instead of the swept sine technique, actual road data were used to excite the shocks. Equivalent linear spring-damper models were developed based on least- squares curve-fitting of the test data. The type of road profile did not influence the stiffness and damping values significantly for the range of amplitudes and frequencies considered. The success of the characterization of shock absorbers on the electrodynamic shaker using non-sinusoidal input has led to the development of a similar methodology to be employed on the hydraulic actuated elastomer test machine.

Measurement of dynamic parameters of automotive exhaust hangers

Different methodologies to test and analyze the dynamic stiffness (K) and damping (C) properties of several silicone and EPDM rubber automotive exhaust hangers were investigated in this research. One test method utilized a standard MTS hydraulic test machine with a single sine excitation at discrete frequencies and amplitude levels, while a second method utilized an electrodynamic shaker with broadband excitation. Analysis techniques for extracting the equivalent stiffness and damping were developed in the shaker tests using data from time domain, frequency domain, as well as force transmissibility. A comparison of all of the shaker testing methods for repeatability and accuracy was done with the goal of determining the appropriate method that generates the most consistent results over the range of testing. The shaker testing in the frequency domain using a frequency response function model produced good results and the set-up is relatively inexpensive. The hydraulic excitation method, however, is more suitable for large displacements and is ideal to study the variations of K and C with frequency, displacement, temperature and pre-load.

Dynamic characterization of automotive exhaust isolators

Abstract Different approaches to measure the dynamic stiffness and damping properties of several silicone and EPDM rubber automotive exhaust isolators were investigated in this research. Shaker excitation and electrohydraulic actuator methods were used to extract the dynamic properties, both under swept sine and random excitations. The shaker testing in the frequency domain using a frequency response function from linear theory produced good results, and the experimental set-up for this case is relatively inexpensive. The electrohydraulic excitation method, however, is more suitable for testing under low frequencies and large displacements. It is ideal to study the variations in stiffness and damping with frequency, displacement, temperature and preload. This study has confirmed that the stiffness and loss factor of the samples are highly dependent on preload and input force levels. It has also been shown that the stiffness and loss factor values are relatively insensitive to the manner in which the exhaust isolator is excited, whether it be with random, burst chirp or swept sine excitation. Of particular interest is the fact that stiffness and loss factor results are very similar between the electrohydraulic and shaker excitation, except at low frequencies.

Time history-based excitation in the dynamic characterization of automotive elastomers

Abstract Different input excitation approaches to measure the dynamic stiffness and damping properties of two automotive elastomer mounts were investigated in this research. The traditional methods of dynamic characterization of elastomers are based largely on sinusoidal input excitation where discrete frequency sine wave signals at specified amplitudes are used to excite an elastomer sample in a step-sine sweep fashion. This method is straight-forward in its signal processing and can easily be performed with a wide variety of available test equipment. However, many questions remain unanswered with respect to the behaviour of an elastomer during broadband frequency excitation. This paper examines how various other types of excitation affect the dynamic characterization results. These excitation inputs include continuous sine sweep (chirp), shaped random, and acquired road profile data. Use of the broadband data types is expected to provide a more accurate representation of conditions seen in the field, while helping to eliminate much of the interpolation that is inherent with the classic discrete step-sine technique. This paper elaborates on the use of time histories to facilitate broadband excitation, as well as to provide a description of accurate digital signal post-processing methods recommended for the various types of input.

The use of unique time history input excitation in the dynamic characterization of automotive mounts

The traditional method of dynamic characterization of elastomers used in industry has largely been based on sinusoidal input excitation. Discrete frequency sine wave signals at specified amplitudes are used to excite the elastomer in a step-sine sweep fashion. This paper will examine new methods of characterization using various broadband input excitations. These different inputs include continuous sine sweep (chirp), shaped random, and acquired road profile data. Use of these broadband data types is expected to provide a more accurate representation of conditions seen in the field, while helping to eliminate much of the interpolation that is inherent with the classic discrete step-sine technique. Results of the various input types are compared in this paper with those found using the classic discrete step-sine input. One of the challenges lies in accurate reproduction of specific time histories, which is achieved using an iterative method in a hydraulic high-frequency elastomer test machine. This paper will elaborate on the time-history based method, as well as provide a description of accurate digital signal post-processing methods recommended for the various types of input.