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<p>The purpose of this thesis is to implement and test a semi-active damping system based on a concept from an earlier thesis. The project includes implementation of mechanical, hydraulic and electronic hardware, aswell as controller software. The idea is to measure the movements of the vehicle chassis and based on these measurements set the damping torque using hydraulics. To be able to develop, test and evaluate the system, realistic input data must be available. To acquire such data, driving trials have been conducted on a variety of tracks.</p><p>The first part of the system is the sensors that measure chassis movements. Both accelerometers and a gyro has been used. To remove drift and high frequency vibrations, the signals are filtered. The suggested controller from the earlier thesis requests damping torque based on the dampers vertical velocity. When accelerometer signals are integrated, measurement and rounding errors causes drift in the velocity. To compensate for this, a floating average is calculated and used.</p><p>The main hydraulic component is a pressure reduction valve that controls the pressure inside the damper. Higher pressure will give higher damping torque. The reaction speed of the system is mostly depending on the hydraulic components. It is important to know the time delay from a change in the valve control signal, to when the actual pressure in the damper has been reached. Tests have shown that a large step, going from 10 Bar to 60 Bar takes approximately 46ms, and that a small step from 1 Bar to 20 Bar takes 63ms. The valve is faster when higher pressure levels are requested. In addition to the hydraulic response time the delay through the signal filters, measured to about 14ms, must be added.</p><p>The sensors are affected by vibrations. If these can be reduced, the digital filters can be made less sharp with a lower filter delay as result. It is also important to have a good control computer so that large rounding errors in the filter calculations can be avoided. This would greatly decrease drift in the integrated velocity.</p>

This thesis explores semi-active structural control methods for mitigating damage during seismic events. Semi-active devices offer the adaptability of active devices in conjunction with low power requirements and thus the reliability of passive devices. A number of structural applications utilising semi-active resetable devices in structural control are described and analysed. A distinguishing feature of this research is the novel design of a large-scale resetable device developed, manufactured and extensively tested. This design dramatically extends the capabilities of resetable devices by readily manipulating the device response to the structural demands and specific structural control requirements. In particular, the unique ability to use these devices to reshape or sculpt structural hysteretic behaviour offers significant new opportunities in semi-active structural control. The results indicate improvements in structural performance during seismic events is gained by approaches to structural control and enhanced damping methods that challenge conventional methods. Using an array of performance metrics the overall structural performance is examined without the typically narrow focus found in other studies. Suites of earthquake ground motion records are utilised to avoid bias to any particular type of motion and statistical analysis of the performance over these suites indicates the overall efficacy of the resetable devices in each case considered. A model that accurately captures all the device dynamics is developed, which can be used for a variety of device types and designs. In addition, the testing capabilities of structural control methods is enhanced by the development of a high speed, real-time hybrid test procedure providing a link between pure simulation and full-scale testing to increase confidence before investing in large experiments. Finally, the resetable devices are extended to improve the response force to size ratio, which additionally increases the force-displacement manipulation ability. Large-scale shake table experiments validate the findings of the analytical results. Very close correlation between analytical and experimental results including overall trends and numerical values verifies the analytical methods used and increases confidence in continuing research in this area. Furthermore, these large-scale experiments confirm the efficacy and accuracy of the the device model developed, leading to highly accurate quantitative prediction of the overall structural system response. Overall, this research presents a methodology for designing, testing and applying resetable devices in structural control. The devices developed in this research and the extensive modelling and testing dramatically extend the understanding and scope of these devices. Guidelines developed for these large-scale resetable device designs including a validated dynamic model brings the application of resetable devices closer to real structural control applications.

This thesis investigates the possibility of controlling the response of a general multi-degree of freedom structure to a relatively distant blast load using passive and semi-active devices. A relatively distant blast is one that applies significant momentum to the structure, but does not destroy the face of the structure. Three multi-storey structures, and one single-storey structure, are modelled using non-linear finite elements with structural columns discretised into multiple elements to accurately capture the effects of higher order modes that are typically excited in such blast load responses. The single-storey model structure is subjected to blast loads of varying duration, magnitude and shape, and the critical aspects of the response are investigated over a range of structural periods in the form of blast load response spectra. The optimal device arrangements are found to be those that reduce the first peak of the structural displacement and thus also reduce the subsequent free vibration of the structure. For a given blast load, various passive and semi-active devices, as well as device architectures, are investigated. The optimal device architecture was found to be one that spanned approximately two-thirds the height of the structure. Depending on what damage parameters are considered critical for a given structure, different devices and arrangements are appropriate. The main factors in choosing a semi-active device and its control architecture, or arrangement, are the tradeoffs between permanent deflection, free vibration, base shear and device capacity limitations. Overall, the results present a first analysis on the effectiveness of semi-active devices and the unique force-displacement properties they offer for mitigating non-catastrophic blast loads.

The possible benefits of employing inerters in automotive suspensions are explored for passenger comfort and handling. Different suspension strut designs in terms of the relative arrangement of springs, dampers and inerters have been considered and their performance compared with that of a conventional system. An alternate method of electrically realizing complex mechanical circuits by using a linear motor (or a rotary motor with an appropriate mechanism) and a shunt circuit is then proposed and evaluated for performance. However, the performance improvement is shown from simulations to be significant only for very stiff suspensions, unlike those in passenger vehicles. Hence, the concept is not taken up for prototyping. Variable damping can be implemented in suspension systems in various ways, for example, using magneto-rheological (MR) fluids, proportional valves, or variable shunt resistance with a linear electromagnetic motor. Hence for a generic variable damping system, a control algorithm is developed which can provide more comfort and better handling simultaneously compared to a passive system. After establishing through simulations that the proposed adaptive control algorithm can demonstrate a performance better than some controllers in prior-art, it is implemented on an actual vehicle (Cadillac STS) which is equipped with MR dampers and several sensors. In order to maintain the controller economical so that it is practically viable, an estimator is developed for variables which require expensive sensors to measure. The characteristic of the MR damper installed in the vehicle is obtained through tests as a 3-dimensional map relating suspension speed, input current and damping force and then used as a look-up table in the controller. Experiments to compare the performance of different controllers are carried out on smooth and rough roads and over speed bumps.

Abstract The tuned mass damper (TMD) is a well-known and approved concept for resonance vibration control. However, as a fully passive device, the traditional TMD has a limited operating band and rather poor robustness against parameter variations. To overcome these weaknesses, a semi-active control can be applied to TMD. As a result, a more effective and flexible device can be attained. In theory, the application of the semi-active scheme is straightforward and the gain in performance is considerable. In practice, however, the non-idealities associated with actuators and control systems degrade the performance. In this thesis, the dynamic behaviour of a semi-active TMD with groundhook control was studied both numerically and experimentally. The semi-active scheme studied is based on groundhook control and a dry-friction damper is used as an actuator in rapid damping modulation. The performance of the semi-active TMD was evaluated in terms of two performance indices which are calculated from the normalised displacement response in the frequency domain. Also, parametric studies were conducted to find out how the different parameters influence the system performance. It is shown that the non-idealities in the semi-active damper have a significant influence on the performance of a groundhook controlled semi-active TMD. On the basis of simulations, a new parameterised semi-active control method was developed. The method is treated as a generalised groundhook control, and it involves a parameter through which the dynamic behaviour of a semi-active TMD can be affected both online and offline. The new method does not require an actuator model. The method developed opens the way for effective use of a non-ideal semi-active actuator, thus ensuring the good performance of the semi-active TMD. Also, the semi-active TMD’s sensitivity for certain parameter variation decreases considerably.

A well-known challenge in vehicle dynamics is to design a vehicle that will not only keep the occupants comfortable, but will also ensure safe and stable operation during various manoeuvres over multiple driving surfaces. A soft and compliant suspension is generally required for good ride comfort, while a stiff suspension with a low centre of mass is required for improved handling. These contradicting factors in the design process is commonly referred to as the ride comfort versus handling compromise. A newly developed semi-active hydropneumatic suspension system is proposed to reduce or negate this compromise by being able to change its characteristics according to the dynamic state of the vehicle. The unit is equipped with two proportional solenoid valves that can provide continuously variable damping. In addition, the valves are able to completely close off flow to compressible gas volumes to provide four discrete stiffness characteristics. This suspension system is based on a previously developed suspension that had only two state (open or closed) valves, which provided discrete damping characteristics. A thorough investigation of the older system proved that the system was capable of addressing the ride comfort versus handling compromise. The purpose of this study was to investigate whether the updated design could deliver improved performance and to recommend focus areas for future research initiatives. The suspension system’s characteristics were determined experimentally by actuating the unit on a test bench. Results indicated that the unit produced the desired stiffness, low damping and response time characteristics. A mathematical model of the suspension unit was developed and validated against experimental data. The model was used in single degree of freedom simulations to investigate both passive and semi-active controlled performance. Results indicated that the suspension could be semi-actively controlled for improve ride comfort. However, the magnitude of improvements with semi-active control, which includes a suitable response time, proved to be rather insignificant compared to the optimum passive suspension. / Dissertation (MEng)--University of Pretora, 2019. / Mechanical and Aeronautical Engineering / MEng (Mechanical) / Unrestricted

The purpose of this research is to establish the effectiveness of tuned-mass-dampers (TMD) using semi-active magneto-rheological (MR) dampers to mitigate annoying floor vibrations. Annoying floor vibration is becoming more common in today's building structures since building materials have become stronger and lighter; the advent of computers has resulted in "paperless" offices; and the use of floors for rhythmic activities, such as aerobics and concerts, is more common. Analytical and experimental studies were conducted to provide an understanding of the effects of incorporating the semi-active-TMD as a remedy to annoying floor vibration. A pendulum tuned mass damper (PTMD) in which the tuning parameters could independently be varied was used. Closed form solutions for the response of the floor using passive dampers were developed. In addition, a numerical integration technique was used to solve the equations of motion where semi-active dampers are utilized. The optimum design parameters of PTMDs using passive and semi-active dampers were found using an optimization routine. Performances of the PTMD in reducing the floor vibration level at the optimum and when subjected to off-tuning of design parameters using passive and semi-active dampers were compared. To validate the results obtained in the analytical investigation, an experimental study was conducted using an 8 ft x 30 ft laboratory floor and a commercial PTMD. Comparative studies of the effectiveness of the PTMD in reducing floor vibrations using semi-active and passive dampers were conducted. / Master of Science

Vehicle suspension systems play an integral role in influencing the overall performance of a vehicle. The suspension system of a vehicle performs multiple tasks, such as maintaining contact between the tires and the road and isolating the frame of the vehicle from road-induced vibration and shocks. A significant amount of research has been directed to improving the performance of the suspension system by varying the damping coefficient so as to alter the frequency response of the system. This study describes the development of such a damper. The goal of this research has been to design, model, fabricate and test a novel semi-active damper. The damper consists of two independent electronically controlled units placed in series with one another. The system was initially simulated using a 2 DOF quarter-car model and the performance characteristics of the damper were outlined. Following that, multiple design iterations of the damper were created and a MATLAB/Simulink model was used to simulate physical and flow characteristics of the damper. After the design and analysis was complete, the damper was fabricated and tested using a shock dyno at CenTiRe. The test results were then compared to the simulation results so as to confirm performance of the damper. Additionally, the results obtained on the dyno were then compared against that of a relative single semi-active and passive damper. / Master of Science

Semi-active suspension systems promise to significantly reduce the necessary trade-off be-tween handling and passenger comfort present in conventional suspension systems by enabling active chassis and wheel control. Öhlins Racing AB have developed a semi-active suspension technology known as CES, Continuously controlled Electronic Suspension, based on solenoid control valves which are integrated into specially designed hydraulic dampers, and are currently developing control and estimation systems which will enable their application in advanced motorcycle suspensions. In these systems an important aspect is being able to accurately control the forces produced. Öhlins’ current system uses an open loop control strategy in which currents sent through the solenoid valves, to achieve the requested damping force under the prevailing circumstances, is calculated using experimentally derived static lookup tables. In this thesis a new closed loop control system, based on the direct measurement of the damper force, is developed and its performance is evaluated in comparison to the old one’s. Sufficient understanding of the system requires extensive modeling and therefore two different models have been developed; a simpler one used for model based control design and a more extensive, high fidelity model used for high accuracy simulations. The developed simulation model is the first of its kind that is able to capture the studied systems behavior with satisfactory accuracy, as demonstrated against real dynamometer measurements. The valves and damper behave in a highly non linear manner and the final controller design uses a combination of exact linearization, non linear state estimation, dynamical inversion and classical control theory. Simulation results indicate that the new controller reduces the root mean square force tracking error to about 63% of that of the existing controller in the evaluation scenarios used. Cascaded within the system is also closed loop current controllers. A developed model based controller is shown to reduce the rise time to less than 30% of that of the existing PID-controllers, reduce the overshoot and provide online estimates of the winding series resistance, providing the basis for future solenoid diagnosis and temperature tracking systems.