As the need for reduced noise and vibration increases, the suspension systems of machines and structures are becoming even more complex and more important than ever. Many researches on the suspensions, especially mounts, are actively conducted to improve the vibration isolation performance. Mounts support the static weight of a mechanical structure and isolate the vibration from the structure in order to minimize the force transmitted to the support. But in order to execute such two roles all together, stiffness of the mount must satisfy conflicting design conditions: Theoretically, to support the static weight of an structure, the dynamics stiffness of the structure must be hard. On the other hand, to minimize a force transmitted from the structure to the support, the dynamic stiffness must be soft. So, it is clear that passive mounts, which are still in wide usage, often fail in meeting such stringent requirement. Although passive mounts, especially hydraulic mounts, have been developed for many years to meet such conflicting requirements, they cannot fundamentally change their stiffness freely according to situations and have complex mechanisms that cause difficulties in design and manufacture. To overcome such fundamental restrictions of passive mounts, fully active mounts, which measure the force or vibration transmitted from a structure to a support and suppress it utilizing forces from actuators, are introduced. They are more effective than passive ones, because they can fundamentally change their stiffness freely according to situations. However the active mount is often costly because of sensors and actuators. It is susceptible to the so-called spillover problem, which causes damage to stability of the suspension system. Thus, as an alternative to fully active mounts, vibration control with semi-active actions has been of popular interest in recent years, because semi-active mounts can change their stiffness properly according to situations. Among others, the controllable fluid-based device, especially magneto-rheological fluid (MRF) based mount, is known to be a good candidate for such applications. MR fluid-based mount does not have mechanically moving parts and can be easily controlled by simple electric circuits. So, it is considered as a simple, quiet, rapid-response semi-active actuator. It is more effective than passive ones and never causes damage to stability of the system because it always dissipates vibration energy from the system.
The main goal of this thesis is to provide good vibration isolation performance using MR fluid based devices. For this, first, development of effective MR devices and deep understanding of the device performance are needed. Among the factors affecting the dynamics of the devices, the effect of power supply voltage on the performance limits in a laboratory Magneto-rheological fluid based device was identified by experiments. It suggests that the frequency range of motion for control is limited by the voltage attenuation due to the coil inductance and the maximum power supply voltage set for practical use of MR devices. In this work, the magnetic and electrical characteristics of MRF device are investigated and a design procedure is formulated to achieve the desired performance for a given power supply.
A practical and effective semi-active on-off damping control law using semi-active actuators is developed for vibration attenuation of a natural, multiple degree-of-freedom suspension system, when its operational response mode is specified. It does not need the accurate system parameters and semi-active actuator dynamics. It reduces the total vibratory energy of the system including the work done by external disturbances and the maximum energy dissipation direction of the semi-active actuator is tuned to the operational response mode of the structure. The effectiveness of the control law using a single semi-active linear mount is demonstrated via simulation by considering two applications, i.e. a three d-o-f excavator cabin suspension model and a two d-o-f quarter car suspension model.
A semi-active mounting system of a variable displacement engine, which consists of two MR devices, is constructed. The developed semi-active damping control reduces the motions of the system. In order to reduce the transmitted force form the system to the support, the control algorithm must be altered. By considering the mechanisms, which cause the excessive vibration of the engine block, modified semi-active damping control to reduce the transmitted force is established. Experimental results show that the semi-active mounting system effectively reduces the vibration levels at steering wheel during resonance.
