OBLIQUE TOWING TEST AND MANEUVER SIMULATION AT LOW SPEED AND LARGE DRIFT ANGLE FOR DEEP SEA OPEN-FRAMED REMOTELY OPERATED VEHICLE*

FAN Shi-bo, LIAN Lian

State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University, Shanghai 200240, China, E-mail: fredtalent@sjtu.edu.cn

REN Ping

Chinese Underwater Technology Institute, Shanghai Jiao Tong University, Shanghai 200231, China

HUANG Guo-liang

State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University, Shanghai 200240, China

OBLIQUE TOWING TEST AND MANEUVER SIMULATION AT LOW SPEED AND LARGE DRIFT ANGLE FOR DEEP SEA OPEN-FRAMED REMOTELY OPERATED VEHICLE*

FAN Shi-bo, LIAN Lian

State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University, Shanghai 200240, China, E-mail: fredtalent@sjtu.edu.cn

REN Ping

Chinese Underwater Technology Institute, Shanghai Jiao Tong University, Shanghai 200231, China

HUANG Guo-liang

State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University, Shanghai 200240, China

This article presents the newly designed oblique towing test in the horizontal plane for the scaled model of 4 500 m deep sea open-framed Remotely Operated Vehicle (ROV), which is being researched and developed by Shanghai Jiao Tong University. Accurate hydrodynamics coefficients measurement is significant for the maneuverability and control system design. The scaled model of ROV was constructed by 1:1.6. Hydrodynamics tests of large drift angle were conducted through Large Amplitude Horizontal Planar Motion Mechanism (LAHPMM) under low speed. Multiple regression method is adopted to process the test data and obtain the related hydrodynamic coefficients. Simulations were designed for the horizontal plane motion of large drift angle to verify the coefficients calculated. And the results show that the data can satisfy with the design requirements of the ROV developed.

Remotely Operated Vehicle (ROV), scaled model, large drift angle, multiple regression, Large Amplitude Horizontal Planar Motion Mechanism (LAHPMM)

Introduction

Remotely Operated Vehicle (ROV) has been applied in many fields, such as ocean resource exploration and exploitation, military defense and protection, seafloor geography mapping[1-3]. It has become an important assistant tool to accomplish different underwater missions. However, the lack of experimentally verified mathematical models, which is the critical request for reliable control system design and maneuverability, has been obstructing the research and development of model-based motion controlling technology. Accordingly, the determination and analysis of hydrodynamics forces and moments on model based control systems through experiment and theoretical calculation, are of great significance for the structure designing and parameter selections for underwater vehicles[4-5].

Generally, deep sea underwater vehicles operate some missions in the underwater space without considering the effects of free surface and wave resistance[6,7]. When moving under seafloor, the vehicles could only be affected by viscous hydrodynamics and inertial forces. For the calculation of the inertial forces, the empirical formula or potential flow theory can be used in researches besides the Planar Motion Mechanism (PMM) and vibration test. Currently, the following methods are usually adopted to compute the viscous forces: the constrained or free model test, CFD, system identification and empirical estimation[8-11]. System identification approach considers the performance of experimental runs with the propelled prototype, and can make full use of the data of onboard sensors. The method is cost-effective and with high repeatability[12,13]. CFD is mainly aimed at the vehicles with streamline configuration. However, itgreatly limits the application into the vehicles with irregular open-framed structure. The model test through using PMM is still the most effective method although it needs high cost and long time.

Fig.1 Layout and structure of Open-framed ROV

This article makes an effort to the nonlinear hydrodynamics coefficients test for the ROV developed by using towing mechanisms. The ROV developed is a deep sea open-framed ROV, and its maximum operation depth is 4 500 m. Figure 1 shows the configurations and layout of the real ROV body. There are some unique features about its structure and layout of the ROV, which can be summarized as follows:

(1) The vehicle manufactured has the features of open-framed body structure and rectangle shape.

(2) The sway damping is so large that the relating coefficients with p can’t be tested by current devices.

(3) The vehicle will mainly operate in the horizontal planar, while the motion and maneuverability in the vertical planar has only small attack angles.

According to the above characteristics and the requirements of operation, such as maneuvering under the whole range of drift angles, we manufacture the scaled model by 1:1.6. The test procedures at low speed and large drift angles by using Large Amplitude Horizontal Planar Motion Mechanism (LAHPMM) are proposed. And the test data are processed through multiple regression method to figure out the related nonlinear hydrodynamics coefficients. Matlab is employed to simulate the horizontal plane motion of large drift angle at low speed.

1. Test model of open-framed ROV

As is illustrated in Fig.2, the ROV model was constructed by 1:1.6 according to the layout of hydraulic power systems, valves, manipulators, thrusters. The buoyancy material of model is made of woods, and the bottom chassis is made of steel. We only manufactured ROV itself without considering umbilical cable and sampling chassis. The thrust direction between the x-axis is 35oin the horizontal plane, and the vertical thrust between the z-axis is 10o. And the main parameters of ROV model are summarized in Table 1, including body size of the model, mass in air, and displacement of volume.

Fig.2 Test model of open-framed ROV

Table 1 Parameters of ROV model

Fig.3 Hydrodynamics components of oblique motions

2. Oblique towing test at low speed and large drift angle

2.1 Test prnciples

The drift angle (β=atan(-v,u)) will be changed to large extent due to the very slow rotation motions of underwater vehicle, and the forces affected byposition will also be altered. To obtain the hydrodynamics model of oblique motions for the vehicle, the fluid dynamics are divided into the following components: ideal fluid force (munk), linear lift (Lv), induced drag (Dst), transverse flow resistance (-Yc), longitudinal resistance (Xf) and asymmetrical lift along longitudinal direction (Xlas). Figure 3 illustrates the forces acted on the vehicle in detail.

According to the above forces, the fluid dynamics model of underwater vehicles at low speed and

where V is the resultant velocity, and Cx, Cy,Cnare fluid dynamics coefficients, which are usually the functions of β in the towing test results.

Equations (4)-(6) can be transformed as the following dimensionless formulas:

Submitting Eqs.(1)-(3) into Eqs.(7)-(9), we may obtain the following equations:

For the convenient of the multiple regression calculation, Eq.(17) can be rewritten as

It is assumed that Y is the predictive objective,which is determined by multiple factors,x1,x2,…, xm. It is also hypothesized that the effective factors has the linear relations with Y, thus, we can establish the following multiple regression model

Submitting the ith observed value of predictive objective and effective factors, yi,x1i,x2i,…,xmi, into Eq.(19), respectively. Then

where eiis the regression error, and n is the observed number.

Equation (20) can be expressed as a matrix form

where

The coefficients could mainly be figured out by observed value for the multiple linear regression. According to the least square method and extremum principle matrix, we have

According to the above algorithm, the nonlinear hydrodynamics coefficients could be obtained by using the computer.

Under low speed and large drift angles, the longitudinal resistance affects the underwater vehicle very slightly when moving at oblique direction. So we make the following assumption

Additionally, the purely resistance test along the x-axis were conducted to obtain the coefficient, Xu＇u. Here, we mainly discuss Y(.)and N(.).

2.2 Test procedures

The towing test was carried out in the ship model towing tank of Science Research Center of Marine Industry at Shanghai, which is the member of ITTC, and the tank dimensions is as follows: length 198 m× width 10 m×depth 4 m. LAHPMM is adopted for the towing tests, and the device is shown as Fig.4.

Fig.4 Large Amplitude Horizontal Planar Motion Mechanism (LAHPMM)

Table 2 Test conditions of oblique towing test

Fig.5 Oblique towing test by 30o

The model was installed in two ways when testing, including upright and side. And four resistance sensors are installed on the model along the xaxis and y-axis, the measuring range of x-axis and y-axis is 1 000 N and 700 N, respectively. The water temperature t=16oC , and Table 2 illustrates the testing parameters.

Fig.6 Oblique towing test by 90o

Fig.7 Oblique towing test by 180o-

ROV model was fixed on the two trestles of the tow truck. The tests were carried out at designed speed, u=1m/s. Figure 5 shows the oblique towing test of the ROV model at β=30o, Figure 6 shows the oblique towing test of the ROV model installed by side at β=90o, Figure 7 shows the oblique towing test of the ROV model at β=-180o.

2.3 Test results

During the oblique test with large drift angles, the towing speed keeps at 1 m/s. And the forces and moments are obtained through resistance sensors, and the tested discrete results of Y and N are illustrated in Figs.8 and 9, respectively. Through examining the original results, we can see that Y arrives at the maximum value whenβ=±90o, while N arrives at the maximum value when β=±45oand β=±120o. Meanwhile, the rotation hydrodynamics induced by instantaneous angular rate alters not very obviously.

Fig.8 Test result of Y

Fig.9 Test result of N

Fig.10 Fitting curve of lateral force

The test results of Y and N are addressed and analyzed by the multiple regression method employed, Figs.10 and 11 give the related fitting curve of Y and N, respectively. Through seeing the curves, the relationship between Y , N andβ has high nonlinear features, and in the range of β=0o±180o, the force and moment are similar with sine distribution. And it is verified that the dynamics model and multiple regression method employed can satisfy the correction of the tested data.

The nonlinear hydrodynamics coefficients for theROV model at low speed and large drift angles are figured out, shown as Table 3. And the coefficients obtained will be used to design the motion controller in the applications of ROV operations.

Fig.11 Fitting curve of yawing moment

Table 3 Nonlinear hydrodynamics coefficients of ROV

Fig.12 Simulation results of linear and angular speed

3. Maneuvering simulation

Matlab and GNC tool box were employed to simulate the dynamics model according to the test results at low speed and large drift angles[17,18]. The simulation designed mainly includes linear and angular speed control, drift angle response and rotation trajectory. The original thrust force 1 500 N along the axis is set as the input of the simulating model, the time step is 0.1 s and the initial speed is 1 m/s.

Fig.13 Simulation results of drift angle

Fig.14 Simulation results of rotation trajectory on horizontal plane

Figure 12 gives the speed control results in the horizontal plane, Fig.13 shows the response trend of the drift angles, and Fig.14 illustrates the trajectory that ROV moves along. The results demonstrate the ROV has good stability and maneuverability, and the model can be used to design motion controller:

(1) Linear and angular speed control simulation.

(2) Drift response simulation.

(3) Trajectory simulation on horizontal plane.

4. Conclusion

The nonlinear hydrodynamics coefficients for the ROV model at low speed and large drift angles have been determined through the model test. The scaled model of 4500 m deep sea open-framed remotely operated vehicle is manufactured, which is researched and developed by Shanghai Jiao Tong University. The oblique towing tests are conducted through LAHPMM at designated low speed and the whole range drift angles. Multiple regression method is adopted to address the testing data and obtain the related hydrodynamic coefficients. Simulations are designed for themaneuvering of large drift angle to verify the coefficients calculated. And the simulating results show that the data can meet the designing requirements of the ROV developed. We only manufactured the ROV model itself without considering the cable[19,20]. Accordingly, for the future application and operation, we will focus on the effects of the cable and select the appropriate algorithm to guarantee the good maneuverability and control capability.

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January 4, 2012, Revised February 21, 2012)

* Project supported by the National High Technology Research and Development Progm of China (863 Program, Grant No. 2008AA092301).

Biography: FAN Shi-bo (1981-), Male, Ph. D. Candidate

LIAN Lian, E-mail: llian@sjtu.edu.cn