Practical demonstration of a large-scale active vibrationisolation system

Tiejun Yang a,*, Yao Sun a, Liubin Zhou b, Michael J. Brennan c, Zhigang Liu a

a College of Power and Energy Engineering, Harbin Engineering University, Harbin, People’s Republic of Chinab Wuhan Second Ship Design and Research Institute, Wuhan, People’s Republic of Chinac Departamento de Engenharia Mecanica UNESP, Ilha Solteira, Brazil

1. Introduction

Active vibration control has been studied for many years, and is now a fairly mature research topic with several text booksbeing written on the topic [1–3]. There have been some successful demonstrations of active control, for example the activecontrol systems described in Refs. [4–8], but there are only a few current commercial systems, for example the active mountfrom Paulstra-vibrachoc [9]. Of particular interest to the authors of this paper is the active control of machinery vibrationsin marine applications [10–16].

Active vibration control concepts are often investigated on small experimental test-rigs in the laboratory, but in marineapplications, there is a large gap between this type of experiment and full-scale implementation onboard a ship. Since vibro-acoustic behavior of a ship structure is often of major interest, a large flexible structure is needed to emulate the hull, rather thanrigid foundations found in many laboratory-based experimental test-rigs. There is thus a need to carry out large-scalelaboratory experiments in controlled conditions to investigate some of the practical issues. This was the motivation for the workdescribed in this article. Established control techniques were applied to a bespoke large-scale test-rig of a typical marine set-upinvolving a floating raft structure attached to a hull-like structure by resilient mounts. Four hydraulic actuators were used in adecentralized feedforward control system and some experiments were performed. The flexible hull-like structure is supported

Case Studies in Mechanical Systems and Signal Processing 1 (2015) 32–37

A R T I C L E I N F O

Available online 14 April 2015

Keywords:

Floating raft

Active vibration isolation

Flexible hull-like structure

A B S T R A C T

Small experimental test-rigs are often used to investigate active vibration control concepts

in the laboratory because of ease of construction and implementation. However, in

marine applications, there is a large gap between this type of experiment and full-scale

implementation onboard a ship. In this article a large-scale laboratory based active

vibration control system is demonstrated. It involves a floating raft system attached to a

hull-like structure by way of four hydraulic actuators, which are placed in parallel with

eighteen passive resilient isolators. The flexible hull-like structure is supported on twenty

six pneumatic springs to simulate a floating ship. A decentralized feedforward control

strategy was implemented resulting in the reduction of vibration levels on the flexible

hull-like receiving structure of up to 36 dB at some tonal excitation frequencies. The

passive isolation results in broadband control and is most effective at higher frequencies.

� 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND

license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

* Corresponding author at: College of Power and Energy Engineering, Harbin Engineering University, No. 145, Nantong Street, Nangang District, Harbin,

Heilongjiang 150001, People’s Republic of China. Tel.: +86 451 82589199x306/18645066270; fax: +86 45182589199x311.

E-mail address: yangtiejun@hrbeu.edu.cn (T. Yang).

Contents lists available at ScienceDirect

Case Studies in Mechanical Systems and SignalProcessing

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http://dx.doi.org/10.1016/j.csmssp.2015.03.001

2351-9886/� 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/

4.0/).

on pneumatic springs and the whole rig behaves dynamically as a section of a floating ship. Details of the experimentsconducted and the results of these experiments are given in this article.

2. Description of the test-rig and the control system

A photograph of the experimental test-rig is shown in Fig. 1(a). It consists of a 6.2 m� 5.2 m� 0.4 m floating raft,constructed by 16 I-shaped steel beams, which are bolted together. It is supported on a flexible hull structure by 18 BE-400type resilient isolators mounted on 18 steel pillars. The 7 m� 6 m� 0.7 m flexible hull structure is made from steel plateswhich are welded together and supported by 26 pneumatic isolators. The details of the test-rig are given in Table 1.

From modal testing, it was found that there were 25 modes below 150 Hz [17]. A Bruel and Kjaer (B&K) type 3628 shaker,positioned at the center of the floating raft as shown in Fig. 1(b), was used as a primary vibration source to excite the raft andsimulate a primary uncontrolled vibration source. Note that this is an intermediate stage of experimentation. In the next[(Fig._1)TD$FIG]

Fig. 1. Details of the active vibration isolation floating raft system connected to the hull structure. (a) Photograph of the experimental test-rig. (b) Schematic

of the test-rig showing the primary B&K shaker and one of the hydraulic secondary actuators.

Table 1

Details of the experimental test-rig.

Floating raft Dimensions 6.2 m� 5.2 m� 0.4 m

Number of I-shaped beams 16

BE-400 resilient isolators Number 18

Vertical stiffness 1.61e6 N/m

Hull structure Dimensions 7 m� 6 m� 0.7

Weight 4.8 t

Pneumatic isolators Number 26

Vertical stiffness 2.3e4 N/m

Horizontal stiffness 4.7e4 N/m

T. Yang et al. / Case Studies in Mechanical Systems and Signal Processing 1 (2015) 32–37 33

stage multiple sources and then a diesel engine will be fitted to the test rig. Four hydraulic actuators, which were used assecondary sources and could each generate a force of 8.9 kN, are located symmetrically between the raft and the hull,as shown in Fig. 1(b) and Table 2. The whole system weighs about 8 tons.

The active vibration isolation is depicted in Fig. 2. Fig. 2(a) is a photograph of the system in which the primary shaker,which was suspended on a crane, and the control instrumentation, can be seen. A block diagram of the control system isshown in Fig. 2(b). The vibration of the system was measured using 8 accelerometers. Four accelerometers (Nos. 1–4) callederror sensors were used to measure the vibration of the hull structure at the positions where the four hydraulic secondaryactuators were located, and four accelerometers (Nos. 5–8) were placed on the raft at the positions of the hydraulic actuatorsto determine the effect of the control system on the raft.

A B&K 3560D system with Pulse software was used as a signal source for the primary shaker and for subsequentanalysis of the resulting vibration. Four hydraulic actuators with a four-channel valve driver acted as the secondarysources. A TMS320VC33 based DSP board with 4-channel A to D and 4-channel D to A converters was used as thecontroller. The system also included four-channel high-pass and low-pass filters. The transducers and instrumentationare listed in Table 2.

The signal used to drive the primary shaker through a power amplifier was also used as a reference signal for the adaptivecontroller. Four hydraulic actuators were operated simultaneously by feeding the acceleration from their locations on thehull structure (measured by 4 error sensors) back to the adaptive controller. The frequency responses between the electricalinput to one of the hydraulic actuators and the four error sensors outputs, which are called error paths in the active controlsystem, are shown in Fig. 3. It can be seen that, except at some frequencies close to the natural frequencies of the wholesystem, in which there is strong coupling between the raft and the hull as described in [17], the response of the structureat the point where it is excited (point response) is almost 10 dB greater than at the other positions (transfer responses).This means that a decentralized control system could be implemented, reducing the complexity drastically. Fourindependent adaptive x-LMS based filters [18], which are described by the following equations, were used as controllers,with each actuator operating independently.

yiðnÞ ¼ wTi ðnÞxðnÞ ¼

XL�1

l¼0

wi;lxðn� lÞ ðaÞ

eiðnÞ ¼ diðnÞ � siðnÞ � yiðnÞ ðbÞwiðnþ 1Þ ¼ wiðnÞ þmix

0iðnÞeiðnÞ ðcÞ

x0iðnÞ ¼ x0iðnÞ; x0iðn� 1Þ; . . . ; x0iðn� Lþ 1Þ� �T ðdÞ

x0iðnÞ ¼ cTi ðnÞxðnÞ ¼

XH�1

h¼0

ci;hxðn� hÞ ðeÞ

8>>>>>>>>>>><>>>>>>>>>>>:

(1)

where n is iteration number, si(n) (i = 1–4) is impulse response function of the ith error path, x(n) is the reference inputsignal and yi(n) is the ith controller output signal, * denotes linear convolution, L is the tap number of the adaptive controlfilters, H is the tap number of the estimated error path filters, mi is the convergence coefficient of the adaptive filter, x(n),

Table 2

Details of the active floating raft vibration isolation system.

Transducers Type B&K 2258a-100

Number 10

High-pass filters Type YE3762A

Number 4

Low-pass filters Type YE3770A

Number 4

Signal source and analyzer Type Bruel and Kjaer type 3560D (multi-function analyzer PULSE 3560D)

Controller Type TMS320VC33 DSP system

Number of A to D 4

Number of D to A 4

Charge amplifiers Type Bruel and Kjaer type 2692-014

Number 4

Primary actuator Make Bruel and Kjaer type 3628

Type Electrodynamic

Output force 1000 N

Secondary actuators Number 4

Type Hydraulic

Output force 8.9 kN

Locations (2 m, 2 m), (2 m, 5 m), (4 m, 2 m), (4 m, 5 m) from the origin shown in Fig. 1

T. Yang et al. / Case Studies in Mechanical Systems and Signal Processing 1 (2015) 32–3734

[(Fig._3)TD$FIG]

Fig. 3. Frequency response function (FRF) between the input to one of the secondary actuators and the four error sensors (point response: blue solid line; transfer

response 1: red dashed line; transfer response 2: green dashed line; transfer response 3: green solid line; line 1: mode 11 at 28 Hz; line 2, mode 12 at 32 Hz; line

3, mode 13 at 37 Hz; line 4, mode 20 at 58 Hz; line 5, mode 22 at 67 Hz; line 6 mode 23 at 107 Hz; line 7, mode 25 at 144 Hz; line 8, mode 26 at 162 Hz).

[(Fig._2)TD$FIG]

Fig. 2. Details of the active control system. (a) Photograph of the active vibration control experimental test-rig showing the instrumentation. (b) Block

diagram of the active control system.

T. Yang et al. / Case Studies in Mechanical Systems and Signal Processing 1 (2015) 32–37 35

wi(n), ci(n) are reference input signal vector, control filter vector and error path filter vector respectively, which can beexpressed as

xðnÞ ¼ ½xðnÞ; xðn� 1Þ; . . . ; xðn� Lþ 1Þ�T (2)

wiðnÞ ¼ ½wi;0;wi;1; . . . ;wi;l; . . . ;wi;L�1�T (3)

ciðnÞ ¼ ½ci;0; ci;1; . . . ; ci;h; . . . ; ci;H�1�T (4)

x0(n) is the reference signal input after being filtered by error path filter.

[(Fig._4)TD$FIG]

Fig. 4. Reduction of the outputs from the error sensors at discrete frequencies due to control.

[(Fig._5)TD$FIG]

0 50 100 150 200 250 30070

75

80

85

90

95

100

105

110

Frequency (Hz)

Acc

eler

atio

n (d

B re

1e-

6 m

/s2 )

24Hz

54Hz

0 50 100 150 200 250 30070

75

80

85

90

95

100

105

110

Frequency (Hz)

Acc

eler

atio

n (d

B re

1e-

6 m

/s2 )

71Hz

54Hz

0 50 100 150 200 250 30070

75

80

85

90

95

100

105

110

Frequence(Hz)

Acc

eler

atio

n (d

B re

1e-

6 m

/s2 )

71Hz

100Hz

0 50 100 150 200 250 30070

75

80

85

90

95

100

105

110

Frequency (Hz)

Acc

eler

atio

n (d

B re

1e-

6 m

/s2 )

130Hz

155Hz

(a) (b)

(c) (d)

Fig. 5. Square root of the mean squared accelerations measured by the four error sensors. (a) Excitation frequencies 24 Hz & 54 Hz. (b) Excitation frequencies

54 Hz & 71 Hz. (c) Excitation frequencies 71 Hz & 100 Hz. (d) Excitation frequencies 130 Hz & 155 Hz (raft without control, green dashed line; hull without

control, red thin solid line; hull with control, blue thick solid line).

T. Yang et al. / Case Studies in Mechanical Systems and Signal Processing 1 (2015) 32–3736

3. Experimental tests

To demonstrate the active control system for at least two rotating machines on the floating raft (but at the same location),several two-frequency combinations of 24 Hz & 54 Hz, 54 Hz & 71 Hz, 71 Hz & 100 Hz, and 130 Hz & 155 Hz were used as theinput signals to the primary shaker.

The details of the vibration reductions at each frequency component are shown in Fig. 4. It can be seen that reductions inthe vibration of the hull structure (as measured by the error sensors) of up to 36 dB were achieved at each frequency exceptat 54 Hz, where the vibration at error sensor 3 was increased slightly when the 24 Hz & 54 Hz combination of excitationfrequencies were used. The square roots of the mean squared acceleration at the four error sensor positions with and withoutcontrol are plotted in Fig. 5 for the excitation frequencies given above. Also plotted is the vibration of the machinery raftwithout control (as measured by sensors 5–8). It can be seen that although the floating raft reduces the vibrationtransmission from the raft to the hull structure greatly, especially for frequencies greater than about 100 Hz, the responsesat the excitation frequencies are still distinct. It is only with active control that all the responses at the excitation frequencieswere suppressed.

4. Conclusions

A practical demonstration of a large-scale active vibration system has been described in this article. Decentralizedfeedforward control was applied using four hydraulic actuators which were attached between a floating raft and a flexiblehull-like structure. There were also eighteen rubber isolators in parallel with the actuators. Up to 36 dB reduction in thevibration of the hull-like structure was achieved at some tonal excitation frequencies, while the passive isolators wereeffective at controlling broadband vibration transmission, especially at high frequencies. It should be noted that althoughvibration suppression has been demonstrated at the control positions, further work is needed to estimate the positioningof additional sensors to determine the effect of active control on the global vibration of the hull structure.

Acknowledgement

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China(No. 51375103).

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