Vibration Analysis and Control New Trends and Developments Part 5 pot

Acceleration feedback control of human-induced floor vibrations, Engineering Structures, Vol.32, No.1, pp.. 1997 Active control for reducing floor vibrations, Journal of Structural Engi

Fig 15 Root locus of the total transfer function G T for CAFC on the footbridge (×) pole; (ο) zero; (F) footbridge; (A) actuator

−λ

A

F

F

Active Control of Human-Induced Vibrations Using a Proof-Mass Actuator 91

Uncontrolled (m/s2)

Controlled (m/s2)

Reduction (%)

Mass displacement (m)

Table 3 Simulation performance assessment for the footbridge using the peak acceleration

for walking and running excitation

Walking and running tests are carried out to assess the efficacy of the AVC system The

walking tests consist of walking at 1.75 Hz such that the first vibration mode of the structure

(3.5 Hz) could be excited by the second harmonic of walking A frequency of 3.5 Hz is used

for the running tests so that the structure is excited by the first harmonic of running The

walking/running tests consisted of walking/running from one end of Span 2 to the other

and back again The pacing frequency is controlled using a metronome set to 105 beats per

minute (bpm) for 1.75 Hz and to 210 bpm for 3.5 Hz Each test is repeated three times

Table 4 Experimental performance assessment for walking and running excitation (1)

Maximum Transient Vibration value defined as the maximum value of 1s running RMS

acceleration

The results are compared by means of the maximum peak acceleration and the MTVV

computed from the 1 s running RMS acceleration Table 4 shows the result obtained for the

uncontrolled and controlled case It is observed that the AMD designed (with a moving

mass of 30 kg) performs well for both excitations, achieving reductions of approximately 70

% Fig 16 shows the response time histories (including the 1 s RMS) uncontrolled and

controlled for a walking test Fig 17 shows the same plots for a running test

Fig 16 Walking test on the footbridge a) Uncontrolled MTVV 0.207 m s= 2 b) Controlled

2

MTVV 0.067 m s=

a)

b)

Active Control of Human-Induced Vibrations Using a Proof-Mass Actuator 93

Fig 17 Running test on the footbridge a) Uncontrolled MTVV 2.198 m s= 2 b) Controlled

2

MTVV 0.773 m s=

a)

b)

of a first-order compensator (phase-lag network) conveniently designed in order to achieve significant relative stability and damping Note that the compensator could be equivalent to

an integrator circuit leading to velocity feedback, depending on the interaction between actuator and structure dynamics Moreover, the control scheme is completed by a phase-lead network to avoid stroke saturation due to low-frequency components of excitations and

a nonlinear element to account for actuator overloading An AVC system based on this control scheme and using a commercial inertial actuator has been tested on two in-service structures, an office floor and a footbridge

The floor structure has a vibration mode at 6.4 Hz which is the most likely to be excited This mode has a damping ratio of 3% and a modal mass of approximately 20 tonnes Reductions

of approximately 60 % have been observed in MTVV and cumulative VDV for controlled walking tests For in-service whole-day monitoring, the amount of time that an R-factor of 4

is exceeded, which is a commonly used vibration limit for high quality office floor, is reduced by over 97 % The footbridge has a vibration mode at 3.5 Hz which is the most likely to be excited This mode has a damping ratio of 0.7 % and a modal mass of approximately 18 tonnes Reductions close to 70 % in term of the MTVV has been achieved for walking and running tests

It has been shown that AVC could be a realistic and reasonable solution for flexible lightweight civil engineering structures such as light-weight floor structure or lively footbridges In these cases, in which low control forces are required (as compared with other civil engineering applications such as high-rise buildings or long-span bridges), electrical actuators can be employed These actuators present advantages with respect to hydraulic ones such as lower cost, maintenance and level of noise However, AVC systems for human-induced vibrations needs much further research and development to jump into building and construction technologies considered by designers With respect to passive systems, such as TMDs, cost is still the mayor disadvantage However, it is expected that this technology will become less expensive and more reasonable in the near future Research projects involving the development of new affordable and compact actuators for human-induced vibration control are currently on the go (Research Grant EP/H009825/1, 2010)

7 Acknowledgment

The author would like to acknowledge the financial support of Universidad de Castilla-La Mancha (PL20112170) and Junta de Comunidades de Castilla-La Mancha (PPII11-0189-9979 The author would like to thank his colleagues Dr Paul Reynolds and Dr Donald Nyawako from the University of Sheffield, and Mr Carlos Casado and Mr Jesús de Sebastián from CARTIF Centro Tecnológico for their collaboration in works presented in this chapter

Active Control of Human-Induced Vibrations Using a Proof-Mass Actuator 95

8 References

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active and passive devices, The Shock and Vibration Digest, Vol.35, No.5, pp 347–65,

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Vehicle suspension is used to attenuate unwanted vibrations from various road conditions

So far, three types of suspension system have been proposed and successfully implemented; passive, active and semiactive Though the passive suspension system featuring oil damper provides design simplicity and cost-effectiveness, performance limitations are inevitable due to the lack of damping force controllability On the other hand, the active suspension system can provides high control performance in wide frequency range However, this type may require high power sources, many sensors and complex actuators such as servovalves Consequently, one way to resolve these requirements of the active suspension system is to adopt the semiactive suspension system The semiactive suspension system offers a desirable performance generally enhanced in the active mode without requiring large power sources and expensive hardware

One of very attractive and effective semiactive vehicle suspension systems is to utilize magnetorheological (MR) fluid MR fluids are currently being studied and implemented as actuating fluids for valve systems, shock absorbers, engine mounts, haptic systems, structure damper, and other control systems The rheological properties of MR fluids are reversibly and instantaneously changed by applying a magnetic field to the fluid domain Recently, a very attractive and effective semi-active suspension system featuring MR fluids has been researched widely Carlson et al., 1996 proposed a commercially available MR damper which is applicable to on-and-off-highway vehicle suspension system They experimentally demonstrated that sufficient levels of damping force and also superior control capability of the damping force by applying control magnetic field Spencer Jr et al.,

1997 proposed dynamic model for the prediction of damping force of a MR damper They compared the measured damping forces with the predicted ones in time domain Kamath et al., 1998 proposed a semi-active MR lag mode damper They proposed dynamic model and verified its validity by comparing the predicted damping force with the measured one Yu et al., 2006 evaluated the effective performance of the MR suspension system by road testing Guo & Hu, 2005 proposed nonlinear stiffness model of a MR damper They proposed nonlinear stiffness model and verified it using simulation and experiment Du et al., 2005

proposed H-infinity control algorithm for vehicle MR damper and verified its effectiveness using simulation Shen et al., 2007 proposed load-levelling suspension with a magnetorheological damper Pranoto et al., 2005 proposed 2DOF-type rotary MR damper and verified its efficiencies Ok et al., 2007 proposed cable-stayed bridges using MR dampers and verified its effectiveness using semi-active fuzzy control algorithm Choi et al.,

2001 manufactured an MR damper for a passenger vehicle and presented a hysteresis model for predicting the field-dependent damping force Hong et al., 2008 derived a nondimensional Bingham model for MR damper and verified its effectiveness through experimental investigation Yu et al., 2009 developed human simulated intelligent control algorithm and successfully applied it to vibration control of vehicle suspension featuring

MR dampers Seong et al., 2009 proposed hysteretic compensator of MR damper They developed nonlinear Preisach hysteresis model and hysteretic compensator and demonstrated its damping force control performance

As is evident from the previous research work, MR damper is very effective solution for vibration control of vehicle suspension system So in this chapter, we formulate various vibration control strategies for vibration control of MR suspension system and evaluate their control performances In order to achieve this goal, material characteristics of MR fluid are explained Then the MR damper for vehicle suspension system is designed, modelled and manufactured The characteristics of manufactured MR damper are experimentally evaluated For vibration control, the quarter vehicle suspension system featuring MR damper is modelled and constructed Then, various vibration control strategies such as skyhook control, PID control, LQG control, H∞ control, Sliding mode control, moving sliding mode control and fuzzy moving sliding mode control are formulated Finally, control performances of the proposed control algorithms are experimentally evaluated and compared

2 Suspension modelling

2.1 MR fluid

Since Jacob Rabinow discovered MR fluid in the late 1940s, of which yield stress and viscosity varies in the presence of magnetic field, various applications using MR fluid have been developed such as shock absorbers, clutches, engine mounts, haptic devices and structure dampers, etc (Kim et al., 2002) Physical property changes of MR fluid are resulted from the chain-like structures between paramagnetic MR particles in the low permeability solvent At the normal condition, MR fluid shows the isotropic Newtonian behavior because the MR particles move freely as shown in Fig 1 (a) However, when the magnetic field applied to the MR fluid, MR fluid shows the anisotropic Bingham behavior and resist to flow or external shear force because the MR particles make a chain structure as shown in Fig 1 (b) From this property, force or torque of application devices can be easily controlled

by the intensity of the magnetic field

2.2 MR damper

The schematic configuration of the cylindrical type MR damper proposed in this work is shown in Fig 2 The MR damper is composed of the piston, cylinder and gas chamber The floating piston between the cylinder and the gas chamber is also used in order to compensate for the volume induced by the motion of the piston Also the gas chamber

Control Strategies for Vehicle Suspension System Featuring Magnetorheological (MR) Damper 99

(a) no magnetic field applied

(b) magnetic field applied Fig 1 Phenomenological behavior of MR fluid

which is filled with nitrogen gas acts as an accumulator for absorbing sudden pressure

variation of lower chamber of the MR damper induced by the rapid motion of the piston The

MR damper is divided into the upper and lower chambers by piston, and it is fully filled with

the MR fluid By the motion of the piston, the MR fluid flows through the annular duct

between inner and outer piston from one chamber to the other The magnetic poles in the

piston head is placed to control the yield stress of the MR fluid by supplying current to the

coil In order effectively to generate the magnetic field in the magnetic pole, the outer cylinder

and both ends of inner piston are made of ferromagnetic substance, while the center of the

inner piston is a paramagnetic substance In the absence of a magnetic field, the MR damper

produces a damping force caused only by fluid viscous resistance However, if a certain level

of magnetic field is supplied to the MR damper, the MR damper produces an additional

damping force owing to the yield stress of the MR fluid This damping force of the MR

damper can be continuously tuned by controlling the intensity of the magnetic field

In order to simplify the analysis of the MR damper, it is assumed that the MR fluid is

incompressible and that pressure in one chamber is uniformly distributed The pressure drops

due to the geometric shape of the annular duct and the fluid inertia are assumed to be negligible

For laminar flow in the annular duct, the fluid resistance is given by (Liu et al., 2006; White, 1994)

= 8η

⁄

(1)

Particle Base Oil

Magnetic Pole

N

S

(a) MR damper

(b) piston (3-D view) Fig 2 Schematic configuration of the proposed MR damper

where η is the viscosity of the MR fluid and is the length of the annular duct and

are the inner radius of the outer piston and outer radius of the inner piston respectively

By assuming that the gas does not exchange much heat with its surroundings, and hence

considering its relation as adiabatic variation, the compliance of the gas chamber is

obtained by

where and are the initial volume and pressure of the gas chamber respectively, and

is the specific heat ratio On the other hand, the pressure drop due to the increment of the

yield stress of the MR fluid is given by

where is a coefficient that depends on flow velocity profile and has a value range from 2.0

to 3.0, is the length of the magnetic pole, ℎ is the gap of the annular duct, and ( ) is the