Active Control_floor Vibration

8/16/2019 Active Control_floor Vibration

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TA15

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Active Control

of

Floor Vibration: Implementation Case Studies

Linda M. Hanagan, Ph.D., P.E.

Departmen t of Civil and Architectural Engineering

University

of

Miami

Coral G ables, FL 33

124

lhanagan@eng rrniami.edu

Thomas M. Murray, PbD ., P.E.

Mo ntague-Betts Professor

of

Structural Steel D esign

Charles E. Via Department of Civil Engineering

Virginia Polytechnic Institute and State University

Blacksburg, VA 24061

[email protected]

Abstract

Progress has been made toward controlling excessive

illustrating the implementation of this control scheme are

presented in this paper.

floor vibration by means of active structural control.

In

this control scheme an electro-magnetic shaker is used to

impart control forces on a floor system, thus, reducing

the floor vibration levels. This paper presents an

overview of the control system setup and describes two

case studies where active control was implemented to

improve floor vibration characteristics.

1.

Introduction

2. Overview of the Active Control System

Control forces are imparted on the floor system by

means of an electro-magnetic shaker. An illustration of

the shaker is presented in Figure

1,

along with the

theoretical model of the shaker. The second order model

of the shaker possesses discrete masses, ma and md,

which represent the reaction (active, 30.4 kg) mass and

the parasitic mass (support frame, etc., 74 kg),

respectively. The spring stiffness, supplied by the

suspension system, is represented by k,. The internal

damping, due to internal motor properties and friction, is

represented by c, and is assumed to be viscous.

The active control of structures is a diverse field of

study, with new applications being developed

continually. One structural system, which is often not

considered a dynamic system, is the floor of a building.

In many cases the dynamics of a floor system are

neglected in the design phase of a building structure.

Occasionally, this omission results in a floor which has

dynamic characteristics found to be unacceptable for the

intended use of the building. Floor motion of very small

amplitudes, often caused by pedestrian movement, is

sometimes found objectionable by occupants of the

building space. Improving an unacceptable floor

system’s dynamic characteristics after construction can

be disruptive, difficult and costly.

In search

of

alternative repair measures, analytical and

experimental research implementing active control

techniques was conducted

to

improve the vibration

characteristics of problem floors. Specifically, a control

scheme was developed utilizing the measured movement

of

the

floor

to

compute

the

input signal

to

an

electromagnetic actuator which, by the movement of the

actuator reaction mass, supplies a force that reduces the

transient and resonant vibration levels. Two case studies

The control law utilized is collocated rate feedback with

a simple command limiter. While adding damping to the

floor system was the key objective, this control law was

selected because it is also robust to system changes and

uncertainties. A presentation of the mathematical model

illustrating the implementation of this control law with

an electro-magnetic shaker is out of the scope of this

paper; however, the formulation is reviewed in

References

[

11and

[2].

The control law is digitally implemented using a 386

computer. With one actuator, the control circuit is

single-inputhingle-output.The input signal, which is a

voltage proportional to the velocity of the floor motion at

the collocated actuator/sensor location, is generated by a

piezoelectric transducer. This input signal is read by an

analog to digital converter housed in the 386 computer.

The output signal is computed by a control program.

The control program implements the control law

discussed previously. Also included in the control

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algorithm is a nonlinear circuit which limits the output

voltage to the limits of the amplifier and shaker. The

digital to analog converter sends the computed output

voltage to the amplifier which drives the shaker.

The only variable quantity involved in controlling

different floors with different dynamic properties is the

control gain applied in the rate feedback control law.

Because of controYstructure interaction, the selection of

this control gain is a tradeoff between the degree of

control for the floor and the actuator stability. This

concept is illustrated in Reference [3]. The control gain

value can be optimized experimentally by trial

and

error.

An analytical model of the structure to be controlled is

not necessary for the implementation of this control

scheme.

Reaction

Mass

a)

Reaction

Mass

Actuator: Electro-Magnetic Shaker

m a

StructureLZz L

b)

Theoretical Shaker Model

Figure

1.

Illustration of Electro-seis Model

400

shaker

and theoretical shaker model

3. Office Floor Case Study

An office floor in a light manufacturing facility, located

in St. Louis, Missouri, was reported to have annoying

levels of occupant induced floor vibrations. A plan of

this floor is shown in Figure

2.

The construction of this

floor consists of a 2 12 in. lightweight concrete slab on

metal deck supported by

joist framing

members as

indicated on the plan. The 28

ft

4 in. span was found

to be the problem area.

In this span, two long rows of

desks are separated by an aisle near the center of the

span. This open of ice area is used primarily for order

processing with personal computers on nearly every

desk. Walking in the aisle causes computer monitors to

rock, thus intensifying the degree of annoyance. One

particularly disturbing characteristic of this floor is that

annoying levels of vibration are felt even when the

occupant movement is several bays away.

An attempt was made to actively control the floor

movemeint at a location where the problem was

particularly acute. The control actuator and sensor were

placed ai: the controller location noted in Figure 2. The

floor response due to a person walking in the aisle

between the desks was measured for the uncontrolled

and controlled system. To provide a valid comparison,

care was taken to keep the walking excitation

as

consistent

as

possible for the two measurements. A

comparison of the results for the uncontrolled and

controlled system is shown in Figure 3. For each

vibration measurement the rms. acceleration was

calculated. The uncontrolled floor system had a

rms.

acceleration level of 0.57%g while the controlled system

had a level of 0.17%g. This represents more than a

300

reduction in the vibration level.

4 Chemistry Laboratory Floor Case Study

Excessivle floor vibration due to occupant movement was

reported to exist in a Vermont university chemistry

laboratory where sensitive microscopes were

in

use.

A

partial pllan of the floor system is shown in Figure 4.

The 7

ft

span is a corridor with laboratory rooms on

either sidle. The floor construction consists of a 31/2 in.

concrete slab on metal deck supported by joist members

as shown in the plan. The problem area, in the

laboratory with the 28 e.- 7 in. span, contains three

island type workbenches where the function has been

severely impaired due to disturbing levels of floor

vibration.

The active control scheme was implemented to reduce

the floor motion. Several tests were performed to assess

the impact of the control. Results from the walking

excitation tests are shown in Figure

5 .

For these test, the

control actuator and sensor were placed between

two

of

the workbenches. This location is noted in Figure 4.

From the data shown in the graphs, the uncontrolled rms.

acceleration was computed to be 0.37%g for the

uncontrolled response and O.O9%g for the controlled

response. This represents over a 400% reduction in the

vibration level.

A Assessment

of

the Control Scheme

The most valuable assessment of the control scheme lies

the human perception of the floor behavior with and

without control. To those present on each of the floor

systems (during testing, the improvements due to the

active control were dramatic. These improvement, with

respect to human perception, can also be illustrated

by

plotting the data from the experimental studies on the

International Standards Organization

ISO)

cale [4] or

assessing floor vibration levels as shown in Figure 6.

The scale: is represented by a baseline acceleration vs.

frequency curve with multipliers for different occupancy

and vibration types. A logarithmic plot of the baseline

1912



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U

20 -

0

Stair

28' - 4

0.1

0

Figure 2. Off ice Floor Plan

--

Velocity

i n / S e C )

Uncontrolled Time History

0.2

0.1

0

-0.1

-0.2

0

1

2

3

4

5 6

7

8

Time (sec)

Velocity

(in/seC)

Controlled Time History

.2 1

6

I

-0.2

6 7 8

-OS1

t....

0

2

3 4

Time (sec)

Figure 3 Uncontrolled and Controlled Response of an Office Floor

Due to Walking Excitation

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4/5

28

7

21'

-

7

Velocity

(idsec)

Direction of

Walking

Controller

Location

16K7

@24 0 C.

4

~

1 6 K 2 2 2 4 O . C .

Figure 4. Chemistry Laboratory Floor Plan

Uncontrolled Time History

0

1 2 3

4

5 6 7 8

Time (sec)

Velocity

(idsec)

0.1

0.05

0

-0.05

-0.1

Controlled Time History

0

1

2

3

4

5

6

7 8

Time sec)

Figure 5. Uncontrolled and Controlled Response of a Chemistry Laboratory Floor

Due to Walking Excitation

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curve (satisfactory magnitude curve for critical working

areas) and a satisfactory magnitude curve for office

environments subjected to intermittent and continuous

vibration is shown in the figure.

I S 0

Scale for Limits of Satisfactory

Magnitudes

of

Floor Vibration

with Respect to Human Perception

h4S

Acceleration

10

(%g)

- -

*

_ I _ , - - - - rf - - - I- - , - 1 - 1 - 1

_ _ _

_ _ _

_ _ _ _ _ _

- - _

_ _

_ _ _ _ _ _ _ _ _ _ _ _

+ - 1

4 - I - I ~ I - l i

- - -

- - I - 1 - 1 - 1

~ - - I - ~ - I - I - I - I T

-

I

- 1 I I M

T

_ _

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

- - l - - l - l - I - l - - - - + - -I 1 - 1 - 1 - 1 +

-

, - I -

- - -

_ _

- - - _

_ _ _

0.1

I I I I

I l l 1

- -

-

+

1

- 1 - I - l - l i - - - +

- - I - - 1 - 1 - 1 - 1 1

I I I I

I l l

Frequency

hz)

Uncontrolled office floor due to walking excitation

A Uncontrolled laboratory floor due to walking excitation

H Controlled office floor due to walking excitation

A

Controlled laboratory floor due to walking excitation

Figure

6.

Evaluation of floor vibration levels using

I S 0

human perception scale [4]

As represented in Figure 6, the active control scheme has

reduced the office floor response to walking excitation to

within an acceptable level, with respect to the I S 0 scale,

for an office environment at the measured location.

While the chemistry laboratory floor response also

shows a significant reduction in vibration levels, its

classification as a “critical work area” has much more

stringent requirements for satisfactory magnitudes

of

vibration and is therefore still unacceptable with respect

to the IS0 scale. Additional damping, beyond what is

provided by the active control scheme, would do little to

further reduce the vibration levels in the laboratory floor.

Alternate repair measures affecting the stiffness of the

system would be necessary to bring the vibration levels

to within satisfactory limits represented by the baseline

curve.

6.

Conclusions

The active control scheme studied in this research

presents several advantages over many of the traditional

methods used in repairing problem floors. An actively

controlled mass provides a larger degree of control than

a passive device with an equivalent reactive mass.

The

active system is also less disruptive to the building

function than most other repair measures. The active

device is rather compact and can be installed with

relative speed and ease in the ceiling cavity present in

most commercial buildings.

There are also disadvantages to the active control

scheme. The cost of the components to provide a single

control circuit are currently very high. The hardware

components alone have a total cost of $21,300 for a

single control circuit. This results in an estimated cost of

$24 per square foot, assuming one actuator is necessary

to control a 30 ft x 30 ft bay. One must keep in mind,

however, that any new technology is expensive and often

becomes more reasonable in time. Maintenance and

reliability issues also detract fiom the attractiveness of a

active system. These issues are not necessarily

prohibitive. Maintenance and repair is necessary for

many building systems. As this technology matures,

maintenance and repair could be considered similar to

changing a filter or overhauling a boiler.

The potential of this application far exceeds the

drawbacks. The results of this research, in addition to

future research, will move this technology toward

acceptance as an alternative

lo

traditional methods in

repairing problem floors and provide desperate building

owners a practicable solution to a very difficult problem.

References

[I]

Hanagan,

L.

M. and Murray, T. M., “Active Control

of Floor Vibrations,” Technical Report CENPI-ST

94/13, Charles E. Via Department of Civil Engineering,

Virginia Polytechnic Institute and State University,

Blacksburg, Virginia, 1994.

[2] Hanagan,

L.

M., and Murray, T. M., “Floor

vibration: A new application for active control,”

Presented at the Fourth Pan American Congress of

Applied Mechanics, Universidad del Salvador, Buenos

Aires, Argentina, January, 1995.

[3] Hanagan, L. M. and Munay, T. M., “Experimental

Results from the Active Control of Floor Motion.”

Proceedings of the

First W orld Conference on Structural

Control, August 3-5, 1994,Los Angeles, CA, 1994.

[4] Intemational Standards I S 0 10137, “Basis for the

design of structures Serviceability of buildings against

vibration,” Intemational Standards Organization,41-43,

1992.

Acknowledgments.

The work described in this paper has been supported in

part by the National Science Foundation

NSF)

Grant

No. MSS-9201944 and by a grant from NUCOR

Research and Development, Norfolk, Nebraska.

1915

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Active Control_floor Vibration