Jeong-Hoi Koo

Investigation of the response time of magnetorheological fluid dampers

The primary purpose of this paper is to investigate the response time of magnetorheological (MR) dampers and the effect of operating parameters. Rapid response time is desired for all real-time control applications. In this experimental study, a commercially available MR damper was tested and the response time was found for various operating conditions. The parameters considered include operating current, piston velocity, and system compliance. The authors define the response time as the time required to transition from the initial state to 63.2% of the final state, or one time constant. Using a triangle wave to maintain constant velocity across the damper, various operating currents ranging from 0.5 Amps to 2 Amps were applied and the resulting force was recorded. The results show that, for a given velocity, the response time remains constant as the operating current varies, indicating that the response time is not a function of the applied current. To evaluate the effect of piston velocity on response time, velocities ranging from 0.1 in/s to 3 in/s were tested. The results show that the response time decreases exponentially as the velocity increases, converging on some final value. Further analysis revealed that this result is an artifact of the compliance in the system. To confirm this, a series of tests were conducted in which the compliance of the system was artificially altered. The results of the compliance study indicate that compliance has a significant effect on the response time of the damper.

Improving Robustness of Tuned Vibration Absorbers Using Shape Memory Alloys

A conventional passive tuned vibration absorber (TVA) is effective when it is precisely tuned to the frequency of a vibration mode; otherwise, it may amplify the vibrations of the primary system. In many applications, the frequency often changes over time. For example, adding or subtracting external mass on the existing primary system results in changes in the system’s natural frequency. The frequency changes of the primary system can significantly degrade the performance of TVA. To cope with this problem, many alternative TVAs (such as semiactive, adaptive, and active TVAs) have been studied. As another alternative, this paper investigates the use of Shape Memory Alloys (SMAs) in passive TVAs in order to improve the robustness of the TVAs subject to mass change in the primary system. The proposed SMA-TVA employs SMA wires, which exhibit variable stiffness, as the spring element of the TVA. This allows us to tune effective stiffness of the TVA to adapt to the changes in the primary systems natural frequency. The simulation model, presented in this paper, contains the dynamics of the TVA along with the SMA wire model that includes phase transformation, heat transfer, and the constitutive relations. Additionally, a PID controller is included for regulating the applied voltage to the SMA wires in order to maintain the desired stiffness. The robustness analysis is then performed on both the SMA-TVA and the equivalent passive TVA. For our robustness analysis, the mass of the primary system is varied by ± 30% of its nominal mass. The simulation results show that the SMA-TVA is more robust than the equivalent passive TVA in reducing peak vibrations in the primary system subject to change of its mass.

Semi-active controller dynamics in a magneto-rheological tuned vibration absorber

The primary objective of this study is to experimentally implement semi-active control to a Magneto-Rheological Tuned Vibration Absorber (MRTVA) and evaluate the dynamic performance of the MRTVA. The MRTVA is a semi-active TVA that employs an MR damper as its damping element. A test apparatus was built to represent a two-degree-of-freedom system-a primary structure coupled with an MRTVA. Using this test setup, a series of tests were performed to assess the dynamics of the MRTVA and to compare them with those of a passive TVA. The TVA used displacement-based, on-off groundhook (on-off DBG) control to regulate the MR damper. Unlike a passive TVA, the MRTVA was able to effectively control the resonant vibrations without sacrificing the isolation valley at high damping. To interpret the dynamics of the passive and semi-active system, the damper lock-up dynamics were investigated. The lock-up analysis further explains the actual implementation of the on-off DBG control policy in the system. The results of the lock-up analysis indicated that the dynamics of the control logic prevented lock-up in the MRTVA. This paper demonstrates that the MRTVA with the on-off DBG semi-active control can offer the benefits of high damping at the resonant peaks while still maintaining good isolation at the natural frequency of the structure. In other words, the semi-active TVA that employed an MR damper was more effective than an equivalent passive system in reducing vibrations of the primary structure.

Experimental Approach for Finding the Response Time of MR Dampers for Vehicle Applications

This paper offers a method and an experimental example of determining the response time of Magneto-Rheological (MR) dampers. The response time of MR dampers for automotive suspension applications is valuable information because it is one of the key factors that determine the practical effectiveness of the use of MR dampers in vehicles. However, a detailed description of the response time of MR dampers is seldom given in the literature. Furthermore, the methods of computing the response time are not discussed in detail. Therefore, this study intends to develop a method for experimental determination of the response time of MR dampers for automotive suspensions. A triangle wave that maintains a constant velocity across the damper is proposed as the input to use in experiments. This triangle wave ensures a constant velocity across the damper in order to accurately evaluate the response time of the MR damper. The response time was defined as the time required to make the transition from the initial state to 63.2% of the final state, or one time constant. The time constant is a measure of how long it takes a system to respond to a given input. In other words, the response time is the time necessary for the damper to achieve the desired force upon activation. To demonstrate the method, the response time was found for an MR damper particularly designed and fabricated for vehicle applications. Two cases were considered: activation response time of the damper and deactivation response time of the damper. Both cases were studied during the rebound stroke of the damper. It was found that the response time of the MR damper under activation and deactivation was 15.4 ms and 13.9 ms respectively. The results are comparable to those found in the literature.Copyright

Experimental evaluation of magnetorheological dampers for semi-active tuned vibration absorbers

The main purpose of this study is to experimentally evaluate the dynamic performance of a semi-active Tuned Vibration Absorber (TVA) with a Magneto-Rheological (MR) damper. To this end, a test apparatus was built to represent a two-degree-of-freedom primary structure model coupled with a MR TVA. The primary structure mass, which is modeled with steel plates, was excited by a hydraulic actuator through four air springs. The air springs represent the stiffness of the primary structure and offer the ability to change the stiffness. The semi-active TVA consists of a steel plate, a MR damper, and four coil springs for physical representation of the mass, the damping element, and the stiffness of the TVA, respectively. Mounted on top of the primary structure, the TVA is connected to the primary structure plates by hardened linear bearing shafts. A series of transducers along with a data acquisition system was used to collect sensory information and implement real time control of the MR TVA. Using this test rig setup, a parametric study was performed to analyze the dynamics of the semi-active TVA and to compare the performance of the semi-active TVA with a passive TVA. Displacement based on-off groundhook (on-off DBG) control was used as the control policy for the semi-active TVA. In the parametric study, the effects of on/off-state damping of the MR damper were investigated and compared with a passive TVA to analyze the relative benefits of a semi-active TVA. When damping increased in the passive TVA, the two resonant peaks merge into one peak, and the peak grows. This indicates that the primary structure and TVA are linked together, disabling the TVA, and it eventually magnifies the vibrations. For a semi-active TVA, however, the two resonant peaks decrease as on-state damping increases (keeping low off-state damping), indicating reduction of vibrations. It is shown that semi-active TVAs outperform passive TVAs in reducing the peak transmissibility, implying that semi-active TVAs are more effective in reducing the vibrations of the primary structure.

Robustness Analysis of Semi-Active Tuned Vibration Absorbers With Magneto-Rheological Dampers: An Experimental Study

This paper offers an experimental robustness analysis of a semiactive tuned vibration absorber (TVA) as well as a passive TVA. A conventional passive TVA is only effective when it is tuned properly; otherwise, it may amplify the vibrations of the primary system. In many practical applications, inevitable off-tuning of a TVA often occurs because of system’s operating conditions or parameter changes over time. For example, adding or subtracting external mass on the existing primary system results in changes in the system’s natural frequency. The frequency changes of the primary system are responsible for “off-tuning” of TVAs. When TVAs are off-tuned, their effectiveness is sharply reduced. In our experimental robustness analysis, we focused on the dynamic performance of both the passive and the semiactive TVAs when the mass of the primary system changed (mass off-tuning). To this end, a test apparatus was built to represent a two-degree-of-freedom structure model coupled with a TVA. The semiactive TVA considered in this study employed a Magneto-Rheological (MR) damper as its damping element to enhance overall performance. Using this test apparatus, a series of tests were conducted to identify the optimal tuning parameters of each of the TVAs. The optimal tuning parameters were obtained based on equal peak transmissibility criteria. The mass off-tuning tests were then performed on the optimally tuned semiactive TVA and the optimally tuned passive TVA. In order to off-tune the primary mass, the mass of the primary system varied from −23% to +23% of its nominal mass. The experimental results showed that the semiactive TVA with MR dampers are more robust to changes in the primary mass (off-tuning) than the passive TVA.Copyright

Experimental Dynamic Analysis of Magneto-Rheological Tuned Vibration Absorbers

The primary purpose of this study is to experimentally evaluate the dynamics of a Magneto-Rheological Tuned Vibration Absorber (MR TVA) with several semi-active control schemes. A test rig was built to represent a two-degree of freedom primary structure model coupled with an MR TVA, and four semi-active control policies were considered. The four control policies include: velocity-based, on-off groundhook control (on-off VBG); velocity-based, continuous groundhook control (continuous VBG); displacement-based, on-off groundhook control (on-off DBG); and displacement-based, continuous groundhook control (continuous DBG). Using the test apparatus, a series of tests were conducted to investigate the dynamics of the MR TVA with each control policy. The performances of each of the cases were then analyzed along with the equivalent passive TVA. The performance index was the transmissibility between the input and the output displacement of the structure. The experimental results indicated that the MR TVA with all of the semi-active control policies, outperformed the passive TVA in reducing structural vibrations. Furthermore, the displacement-based groundhook control policies perform better in reducing the resonant vibrations of the primary structure than the velocity-based groundhook control schemes.Copyright

Backstepping Control of an SMA-Actuated Manipulator

In this paper a nonlinear model-based controller is designed to globally asymptotically stabilize a single-degree-of-freedom shape memory alloy (SMA) actuated manipulator. A three part model was constructed based on the dynamics/kinematics of the arm, the thermomechanical behavior of SMA’s, and an assumed heat transfer model consisting of electrical heating and natural convection. The backstepping control is used to calculate the applied voltage to the SMA wire. Initially, the SMA’s wire stress is assumed to be the control input of the system. The stress is then chosen to asymptotically stabilize the desired position. The applied voltage to the SMA wire is the actual control input. This voltage is calculated based on the desired stress and the SMA’s thermomechanical and heat transfer models. It is shown that the calculated voltage can globally asymptotically stabilize the system. Numerical simulations are performed to investigate stabilizing performance as well as other issues such as robustness. The results demonstrate that the backstepping controller designs is highly accurate in stabilization.© 2004 ASME

Shape Memory Alloy Tuned Vibration Absorbers: Robustness Analysis

A conventional passive tuned vibration absorber (TVA) is effective when it is precisely tuned to the frequency of a vibration mode; otherwise, it may amplify the vibrations of the primary system. In many applications, the frequency often changes over time. For example, adding or subtracting external mass on the existing primary system results in changes in the system’s natural frequency. The frequency changes of the primary system can significantly degrade the performance of TVA. To cope with this problem, many alternative TVAs (such as semiactive, adaptive, and active TVAs) have been studied. As another alternative, this paper investigates the use of Shape Memory Alloys (SMAs) in passive TVAs in order to improve the robustness of the TVAs subject to mass change in the primary system. The proposed SMA-TVA employs SMA wires, which exhibit variable stiffness, as the spring element of the TVA. This allows us to tune effective stiffness of the TVA to adapt to the changes in the primary system’s natural frequency. The stimulation model, presented in this paper, contains the dynamics of the TVA along with the SMA wire model that includes phase transformation, heat transfer, and the constitutive relations. The robustness analysis is then performed on both the SMA-TVA and the equivalent passive TVA. For our robustness analysis, the mass of the primary system is varied by 30% of its nominal mass. The simulation results show that the SMA-TVA is more robust than the equivalent passive TVA in reducing peak vibrations in the primary system subject to change of its mass.Copyright

A Robust Semi-Active Tuned Vibration Absorber for Reducing Vibrations in Force-Excited Structures

A passive TVA is only effective when it is tuned properly; otherwise, it can magnify the vibration levels. Often, inevitable off-tuning of a TVA occurs due to changes in the primary structure mass and stiffness for force-excited structural systems such as a floor. The main purpose of this study is to evaluate the robustness of semi-active groundhook TVAs to structure mass and stiffness off-tuning. In the case of floor systems, adding external mass to an existing floor, such as people and furniture, will increase the floor mass, and reduce the mass ratio. Theses changes result in off-tuning of the frequency ratio, which is defined by the ratio of the natural frequency of the TVA to the primary structure natural frequency. In order to study the effect of off-tuning, a force-excited equivalent model of a groundhook TVA is developed and its closed-form solutions are obtained for dynamic analysis of such systems. Moreover, the optimal design parameters of both passive and groundhook equivalent semiactive TVA models are obtained based on minimization of peak transmissibility. The two optimally tuned models are compared as the primary mass and primary structure stiffness changes. The results indicate that the peak transmissibility of the groundhook TVA is lower than that of passive, implying that the groundhook TVA is more effective in reducing vibration levels. The results further indicate that the groundhook TVA is more robust to changes in primary structure mass and stiffness.Copyright