Vehicle suspensions have been a major focus of research and design since the introduction of the automobile. The two major characteristics that define the performance of a specific suspension design are the vehicle ride and handling. A simplified view of chassis development is that a desirable vehicle ride is provided by a "soft" suspension, and desirable vehicle handling is provided by "stiff" suspensions, it is impossible to simultaneously maximize both vehicle ride and handling in a passive suspension design. However, with the introduction of active components, such as actuators and semiactive dampers, coupled with dynamic control strategies, a more desirable compromise between the benefits of soft and stiff suspensions can be achieved. The purpose of this research is not to create a better suspension, but to investigate how active control can improve vehicle ride. As with any research area, a method for evaluating new ideas is required. Many computational methods exist for determining the ride performance of various suspension designs, however computational results alone can be hard to interpret and often deceiving. The following thesis outlines the design and validation of an experimental test rig for evaluating fully active or semi-active suspension algorithms. The test rig utilizes a fully active electromagnetic actuator which can simulate the performance characteristics of fully active as well as semi-active components which a suspension design may utilize. The demonstration rig couples visual inspection with computational analysis and provides the tools necessary for the designer to accurately interpret the ride performance of a new design and simultaneously compare it with a passive design. The goal is to provide a visual development platform in which new algorithms can be quickly and easily implemented and compared against existing algorithms to determine the performance characteristics of each on a physical system. The test rig demonstrated its ability to test both fully-active and semi-active skyhook control algorithms, and accurately simulated both fully-active and semi-active suspension components. The test rig provides a simple and cost effective way to evaluate new algorithms both computationally and visually on a physical system. / Master of Science

This experimental study evaluates the dynamic response of five semi-active control policies as tested on a single suspension quarter-car system. Incorporating a magneto-rheological damper, the full-scale 2DOF quarter-car system was used to evaluate skyhook, groundhook, and hybrid control. Two alternative skyhook policies were also considered, namely displacement skyhook and relative displacement skyhook. As well as exploring the relative benefits of each of these controllers, the performance of each semi-active controller was compared to the performance of conventional passive damping. Each control policy is evaluated for its control performance under three different base excitations: chirp, step, and pure tone. Corresponding to the chirp input, transmissibilities and auto spectrums are considered for each control policy. Specifically, transmissibilities between the sprung mass displacement and the unsprung mass displacement are generated relative to the input displacement. Further, the ratio between the relative displacement across the damper and the input displacement is evaluated for each control technique. The chirp input also reveals the results of the auto spectrums of the sprung and unsprung mass accelerations. Both the step input and the pure tone input were used to generate time domain values of RMS and peak-to-peak displacements and accelerations. This study shows that semi-active control offers benefits beyond those of conventional passive damping. Further, traditional skyhook control is shown to outperform the less conventional alternative skyhook policies. / Master of Science

This analytical study evaluates the response characteristics of a two-degree-of freedom quarter-car model, using passive and semi-active dampers, along with a seven-degree-of-freedom full vehicle model. The behaviors of the semi-actively suspended vehicles have been evaluated using skyhook, groundhook, and hybrid control policies, and compared to the behaviors of the passively-suspended vehicles. The relationship between vibration isolation, suspension deflection, and road-holding is studied for the quarter-car model. Three main performance indices are used as a measure of vibration isolation (which can be seen as a comfort index), suspension travel requirements, and road-holding quality. After performing numerical simulations on a seven-degree-of-freedom full vehicle model in order to confirm the general trends found for the quarter-car model, these three indices are minimized using optimization techniques. The results of this study indicate that the hybrid control policy yields better comfort than a passive suspension, without reducing the road-holding quality or increasing the suspension displacement for typical passenger cars. The results also indicate that for typical passenger cars, the hybrid control policy results in a better compromise between comfort, road-holding and suspension travel requirements than the skyhook and groundhook control policies. Finally, the numerical simulations performed on a seven-degree-of-freedom full vehicle model indicate that the motion of the quarter-car model is not only a good approximation of the heave motion of a full-vehicle model, but also of the pitch and roll motions since both are very similar to the heave motion. / Master of Science

A digital controller was designed for a semiactive primary suspension for a class 8 highway truck. The controller used a skyhook policy (where the semiactive damper simulates a damper between the sprung mass and an inertial reference) to control magneto-rheological dampers placed on the truck 's primary suspension in response to measurements made by accelerometers placed on the axle and the truck frame. The completed system was then tested for both random noise (on highway driving) and impulse (speed bump) response. The test results showed that for the damping tuning and controller arrangements used in this study, semiactive dampers do not offer any significant benefits in reducing overall vibration levels at the truck frame or axles. The semiactive dampers, however, provided better control of the dynamic transients, such as roll and pitch induced by hitting speed bumps, as compared to passive dampers. Further assessment of the magneto-rheological damper's tuning and the skyhook control policy is needed to establish any definitive conclusions on the potential benefits of semiactive magneto-rheological suspensions for heavy trucks. / Master of Science

The increased prevalence of semi-active control systems is largely due to the emergence of cost effective commercially available controllable damper technology such as Magneto-Rheological (MR) devices. Unfortunately, MR dampers exhibit highly nonlinear behavior, thus presenting an often over-looked complexity to the control system designer. With regards to controlling dampers, the well-known Skyhook Damping control algorithm has enjoyed great success for both fully active and semi-active control problems. The Skyhook design strategy is to create a control force that emulates what a passive linear damper would create when connected to an inertial reference frame. Skyhook control is device independent since it generates a desired control force command output that must be produced by the control system. For simplicity, MR dampers are often assumed to have a linear relationship between the current input and the force output at a given relative velocity. Often this assumption is made implicitly and without knowledge of the underlying nonlinearity. This thesis shows that the overall performance of a semi-active Skyhook control system can be improved by explicitly inverting the nonlinear relationship between input current and output force. The proposed modification will work with any semi-active control algorithm, such as Skyhook, to insure that the controller performance is at least as good as the performance without the proposed modification. This technique is demonstrated through simulation on a quarter-vehicle system. Hysteretic damping effects are incorporated into the modification by application of simple open loop force compensation. Laboratory testing of the hysteretic inversion process was performed with the goal of emulating an ideal linear damper without hysteresis. These results are compared with the implicit assumption thus providing a basis for validating the benefits of the improved methodology. / Master of Science

This study experimentally evaluates the dynamic response of a single vehicle suspension incorporating a magneto-rheological (MR) damper. A full-scale two-degree-of-freedom (2DOF) quarter-car test apparatus has been constructed at the Advanced Vehicle Dynamics Lab at Virginia Tech to evaluate the response of a vehicle suspension under the different control schemes of skyhook, groundhook, and hybrid semiactive control. The quarter-car apparatus was constructed using materials from 80/20 Incorporated and a hydraulic actuation system from MTS. A dSPACE AutoBox was used both for controlling the MR dampers and acquiring data. The first task was to understand the baseline dynamic response of the quarter-car system with only a passive damper. Next, the passive damper was replaced with a controllable MR damper. The control schemes of skyhook, groundhook, and hybrid semiactive control were applied to the MR damper. The physical response of the quarter-car with the different control schemes was then compared to the analytical prediction for the response, with favorable results. The response of the quarter-car with the semiactive damper was also compared to the response of the quarter-car with a passive damper, and the resulting limitations of passive damping are discussed. Finally, the practical implications of this study are shown in a discussion of the physical implementation of the MR dampers in the Virginia Tech FutureCar, a full-size Chevrolet Lumina. Although the actual skyhook, groundhook, and hybrid semiactive control schemes were not implemented on the vehicle, the results were promising and generated several recommendations for future research. / Master of Science

Truck drivers are exposed to vibrations all day as a part of their work. In addition to repetitive motion injuries the constant vibrations add to the fatigue of the driver which in turn can have safety implications. The goal of this research is to lower the vibrations an occupant of a class 8 semitruck cab sleeper is exposed to by improving the ride quality. Unlike prior research in the area of ride comfort that target the chassis or seat suspension, this work focuses on the cab suspension. The current standard in cab suspensions is comprised of some type of spring and passive damper mechanism. Ride improvements can most easily be accomplished by replacing the stock passive dampers with some type of controllable damper; in this case Magneto-Rheological (MR) dampers. MR dampers can change damping characteristics in real time, while behaving like a passive damper in their OFF state. This means that in case of a failure to the power supply, the dampers still retain their functionality and can provide some level of damping. Additionally, MR dampers can be packaged such that they do not require any redesign of mounting bracketry on the cab or the frame, their use as a retrofitable device. The damper controller is based on the skyhook control policy pioneered by Karnopp et al. in the 1970s. A variation on skyhook control is chosen called no-jerk skyhook control. A controller called Hierarchical SemiActive Control (HSAC) is designed and implemented to allow the no-jerk skyhook controller to adapt to the road conditions. It also incorporates an endstop controller to better handle the limited rattle space of the cab suspension. The development and initial testing of the controller prototype is done in simulation using a model of the cab and its suspension. The model is derived from first principles using bond graph modeling. The controller is implemented in Simulink to ease the transition to hardware testing. The realtime prototype controller is tested on a class 8 semitruck in a lab environment using dSPACE and road input at the rear axles. The laboratory results are veried on the road in a series of road tests on a test truck. The road tests showed a need for HSAC controller. The HSAC is implemented on the test truck in a final prototype system. The test results with this system show signfiicant improvements over the stock passive suspension, especially when dealing with transient excitations. The overall research results presented show that significant ride improvements can be achieved from a semiactive cab suspension. / Ph. D.

This research will compare the transient response characteristics of a four-degree-of-freedom, roll-plane model, representing a class 8 truck, using passive and semiactive dampers. The semiactive damper control policies that are examined include the previously developed policies of on-off skyhook, continuous skyhook, and on-off groundhook control, along with a newly developed method of fuzzy logic semiactive control. The model input will include body forces and torques, as well as transient displacements at the tires. The model outputs include the vehicle body heave and roll displacements, the vertical displacement of the tire (wheel hop) and the vertical acceleration of the vehicle body. For each output, the maximum peak-to-peak and RMS values of the response are examined. The results of the study show that semiactive dampers have minimal effect on improving the vehicle body and tire transients due to forces or torques applied to the body, as compared to passive dampers. For road inputs, however, semiactive dampers are able to provide a more favorable compromise between the body and axle transient dynamics, when compared to passive dampers. The fuzzy logic semiactive control policy that is proposed in this research is better able to balance the body and axle dynamics than the conventional semiactive damping control policies that are investigated. Further research on the application of fuzzy logic semiactive control concepts is suggested, in order to fully investigate the potential of such control schemes for vehicle suspensions. / Master of Science

This study evaluates and provides solutions to the problem of poor subjective feel of seat suspensions that employ magnetorheological (MR) dampers and skyhook control. An Isringhausen seat suspension that had been modified to replace the stock passive damper with a controllable MR damper was used to evaluate the problems and potential solutions. A seat suspension tester was built using materials from 80/20 Incorporated and a hydraulic actuation system from MTS. An HP Dynamic Signal Analyzer was used as the main piece of data acquisition equipment, along with a Pentium PC and National Instruments Data Acquisition card. All of the hardware is installed in a controlled laboratory facility at Virginia Tech's Advanced Vehicle Dynamics Lab. The first task was to analyze the source of the unexpected peak in the acceleration spectrum of the suspended seat. This analysis was accomplished using a combination of pure tone inputs and a Fourier analysis of a simple model of the system. This analysis indicated that the peak is actually three times the resonant frequency of the seat suspension. The analysis also indicates that the frequency components continue at odd multiples of the resonant frequency, however, the third peak is the most noticeable. The third multiple is in the resonant frequency range (4-8 Hz) of the human body, so it was initially blamed for the poor subjective feel of the seat. However, solutions to remove this harmonic were tested without success. The work progressed to a time domain analysis, which eventually led to determining the source of the poor subjective feel. The seat suspension was excited with a variety of inputs. The seat acceleration and damper control current were examined in the time domain to show that the cause of the poor subjective feel is the control signal discontinuities. The control policy was modified to remove the control signal discontinuities and was found to improve the subjective feel of the seat. Finally, several two-degree-of-freedom control policies were implemented and tested. Although the results from this testing are inconclusive, they generated several recommendations for future research. / Master of Science

Les travaux présentés dans cette thèse concernant la modélisation et la commande des suspensions pour améliorer le comportement et les performances des véhicules, face aux diverses sollicitations (conducteurs,route,...). Après avoir présenté différents modèles simple, destinés à la commande de suspension d'un quart, d'un demi ou d'un véhicule complet, des critères permettant de quantifier les performances des suspensions sont donnés. Un bref état de l'art de la commande des suspensions et des méthodes de commandes utilisées dans ce travail sont aussi présentées. La contribution principale de ce travail concerne la commande d'un modèle d'un quart de véhicule à deux degrés de liberté. Deux méthodologies de commande génériques (placement de pôle, H-infini) et une spécifique aux suspensions (SKyhook) sont étudiées. La robustesse en stabilité et en performances (vis-à-vis de variations paramétriques) de ces trois systèmes de commande, est ensuite étudiée à l'aide de la mu-analyse. Ces trois commandes sont ensuite appliquées sur un modèle non-linéaire plus précis. Ce travail se termine par la synthèse d'une commande H-infini d'un modèle demi-véhicule de type essieu, puis, par l'application d'une loi de commande par retour d'état, permettant simultanémant le découplage ligne par ligne et le rejet des perturbations, sur un modèle complet de véhicule à 7 degrés de liberté.