A remote system for experimentation of an in-scale tug

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1 A remote system for experimentation of an in-scale tug F. J. VELASCOand E. REVESTIDO University of Cantabria Dept. Electronic Technology Systems Engineering and Automatic Control Santander, Spain E. LOPEZ University of the Bask Country Dept. Sciences and Navigation Techniques Engines and Naval Architecture Bizkaia, Spain E. MOYANO and A. BASCONES University of Cantabria Dept. Applied Mathematics and Computing Sciences Santander, Spain Abstract: This paper describes an experimentation system to control and acquire data from an in-scale tug. It has all of the elements required to emulate the installation of a real vessel. The in-scale tug model was built taking into account the hydrodynamic characteristics of the full-scale vessel. This model is autonomous and is controlled remotely from a PC using Wi-Fi communications. It is possible to perform manoeuvering tests such that model and full scale ship exhibit similar behavior. With the in-scale physical model, tests are carried out in open waters that can not be made in a model basin due to its operative dimensional limitation. A simulation of a maneuvering model has been developed based on hull geometry to show the similitude with the acquired data. Key Words: autonomous marine vehicles, sea trials, maneuvering models, web server, simulation 1 Introduction The use of in scale models of ships to predict the behavior of full-scale ships is of great value in the building of a new ship. The first step in the building of a new vessel is to design an in-scale model of the vessel and to carry out trials, in a model basin, in order to obtain the mathematical model of the vessel. With this model, by means of simulation, several sea trials are performed to determine the vessel s manoeuvring characteristics. With these tests, the characteristics of the dynamic behavior of the vessel can be measured, an indication of its stability and steerability can be obtained, its robustness and the limitations of the control system can be assessed and the behavior of the vessel in emergency situations can be evaluated. Once the vessel is built, the sea trials are repeated in the open sea in order to verify whether the same results are obtained as in the simulation. In the literature, some works are found related to the use of small prototypes [5] with trials undertaken in lakes for both of them. More recent works use a small boat for the trials or a small ship for performing tests in a towing tank. In this paper, a system [4] has been developed for remote experimentation with research purposes to study ship manoeuvres. As part of the system, a physical in-scale model (Figure 1), which is a replica of a full-size vessel, was designed to be autonomous and controlled remotely from a Laptop in order to perform sea vessel trials [2]. The model tests are performed such that the model and full-scale vessels exhibit similar behavior, so that the results for the model can be transferred to the full scale vessel by a proportionality factor. Several maneuvering trials can be developed to study different manoeuvring characteristics such as: course keeping, course changing, track keeping (important in restricted waters), speed changing (especially stopping) and also at-scale experimentation of coordination between sea vessels. Autonomous guidance and control technologies are required to perform these purposes and the system is useful for research and education. All the elements that make up the system are of industrial type. The main reason for this choice is the robustness and reliability provided by these elements. With this test environment, the installations of a vessel are efficiently emulated. The tests with the autonomous in-scale physical model were carried out in the surroundings of the Bay of Santander. 2 Remote Control System This remote experimentation environment has an inscale model of a tug, which scale is 1:27 and its mass is 97Kg. The elements that are on-board the tug are: two engines (two at stern for propulsion and one at ISSN: ISBN:

2 3 The Model The linear model subject of study, takes the following form [6]: M ν + N(u )ν = τ δ (1) Figure 1: in-scale tug model. where ν = [v, r] T is the state vector with velocities according to figure 2, τ δ is the hydrodynamic forces generated by the steerable nozzle. The vector of external forces is generated by the propulsion system and the environment (wind and waves). For the manoeuvering model in equation 1, the experimental tests are performed in calm water, hence the external forces are assumed to be only due to propulsion system. bow for dynamical positioning), two servomotors, two speed variators, two steerable kort nozzles for propulsion and steering. The vehicle also features control circuitry for pulse-width modulation (PWM). Instruments include a global positioning system (GPS), an inertial measurement unit (IMU). The onboard system also incorporates an industrial PC with Windows XP and an access point for wireless communications along with a data acquisition (DAQ) device. On land, the receiving system features another industrial PC with Windows XP and one router. This system makes it possible to perform the sea trials most widely used to determine the main characteristics of the steering and manoeuvring of a sea vessel, such as: turning circle, zig-zag, pull-out and spiral manoeuvres. A software support has been implemented in Lab- VIEW which can capture and store data at a distance of all the instruments of the platform. In this software support, all of the manoeuvres are programmed so that they can be performed automatically. To do this, the Industrial PC is equipped with an application, which is published on the web server which also resides in the Industrial PC. The communication between the computer located on land and the one positioned on the sea vessel (Industrial PC) is made through a wireless network using Wi-Fi (Wireless Fidelity), which is capable of communicating the laptop with the industrial PC. Once it has been verified that the establishment of communications through the wireless network is correct, it is possible to access from the laptop the data published on the web server of the Industrial PC by keying the corresponding URL in a web browser, such as Internet Explorer, in the laptop. LabVIEW permits the creation of an HTML document, in which a VI running in another computer can be embedded. Figure 2: Variables that describe the movement of the physical model on the horizontal plane. Equation (1) is formulated about the origin of a coordinate system fixed the vessel, which is called, the body fixed point O b (Figure 1). The adopted coordinate system is fixed to the point determined by the intersection of the port-starboard plane of symmetry, the waterline plane and a transverse vertical plane at Lpp (see Appendix). The matrixes, that make up the equation (1) are defined below. The total mass matrix M is composed of a rigid body and added mass terms: [ M = m Y v mx G N v ] mx G Yṙ I z Nṙ where the hydrodynamic derivatives are denoted according the SNAME [1] notation. In the matrix N(u ) is included the linear damping, coriolis and centripetal terms. [ ] Yv mu N(u ) = Y r N v mx G u N r The estimation of the hydrodynamic derivatives in the expression (1) was made with Clarke formulae [1] where the derivatives with the subindex h are calculated from a knowledge of the overall hull geometry ISSN: ISBN:

3 (see appendix). (Y v) h = π( T ) 2 (1 +.16C BB 5.1( B ) 2 ) T (Y ṙ ) h = π( T ) 2 (.67B/.33(B/T ) 2 ) (N v) h = π( T ) 2 (1.1B/.41B/T ) (N ṙ) h = π( T ) 2 ( C BB.33 B )) T (Y v) h = π( T ) 2 (1 +.4C B B/T ) (Y r ) h = π( T ) 2 ( B.8B ) T (N v) h = π( T ) 2 ( (T/ )) (2) (N r) h = π( T ) 2 ( B.56( B )) T 3.1 Hydrodynamic Derivatives of Ship-Fin The in-scale tug has hull-deadwood and two fins as shown in figure 3. Jacobs [12] presented that the hydrodynamic derivatives of the bare hull-deadwood can be computed with reasonable accuracy by simply adding the contributions of the appropriate fixed fin to the bare hull derivatives. For this kind of the dead- Based on an approach developed by Lewis [11], in which the lift and drag coefficients of the fin area are used. It is obtained for a β f = : (Y v ) f = ρ/2a f U(( C L β ) f + (C D ) f ) (3) Because of the hight aspect ratio of the deadwood and the fins, Jacobs uses the following relation to estimate values of the lift-curve slope, C L / β, needed for equation (3): C L β = 2π (4) 1 + 2/a where a is the effective aspect ratio. The rest of the hydrodynamic derivatives can be calculated simply by: (N v) f = x f (Y v) f (Y r ) f = x f (Y v) f (N r) f = x 2 f (Y v) f (5) It must be noticed that x f is a negative value due to the position of the body fixed point b and the stern location of the deadwood and the fins. Similarly, the contributions of a fixed fin to the acceleration derivatives of a ship s hull are functions of the acceleration-dependent force derivative of the fin, (Y v ) f. This derivative corresponds to the added mass of the fins and can be approximated as follows: (Y v) f = 2πbρA f (4α G 2 + 1) 1/2 1 ρ/2l 3 U (6) where A f is the area of the fin, b is the geometric span, and α G is the geometric aspect ratio b 2 /A f. The expressions for the contributions to the other acceleration derivatives of a fixed fin remote from the origin of the ship O b Figure 3: Hull-deadwood configuration of the in-scale tug. wood configuration Jacobs assumes that the effective span of the deadwood (fin) is equal to the height of the deadwood, h f, at the trailing edge of the deadwood. The deadwood is also assumed to be sufficiently low aspect ratio that the Jones formula for the lift curve is applicable. The fin length along the keel is taken as l f, fin area A f is h f l f /2; the mean chord, c is A f /h f = l f /2 and the fin effective aspect ratio is 4h f /l f since the hull provides full groundboard effect for the deadwood. (N v) f = x f (Y v) f (Y ṙ ) f = x f (Y v) f (N ṙ) f = x f 2 (Y v ) f (7) Then, the total derivatives for the ship are the sum of the two fins contribution, the deadwood and the hydrodynamic hull contribution in each degree of freedom. From results in appendix (table 3), it must be noticed that the contribution of the fins are obviously small compared to the total hull and fins contribution. The contribution of the added mass terms (such as (Y v ) f ) is very small that is why some authors reject ISSN: ISBN:

4 those terms [9] and also the small contribution of the rest of the terms(such as (Y v) f ). There are some terms which are always negative despite the hull dimensions like Y v or Y v. According to the SNAME [1] notation; for instance the hydrodynamic force Y v along the y-axis (see Figure 1) due to the velocity v in the x- direction is always a negative value which is opposite to the movement. 3.2 Thrust Characteristics An identical model with scale 1:54 was tested in The Pardo Model Basin (CEHIPAR) in Madrid. The data depicted in table 1 were the result of transferring data of old scale 1:54 into the new scale 1:27. This was made by means of the Froude number were the relatives speeds are: U in scale λ = Ureal (8) being λ the scale of the model. Table 1: Tug tests The power of the Speed Speed Resistance Power Engine Real in-scale (Kg) (Kw) (Kw) (Kn) (m/s) engine has been obtained from the power due to the thrust and considering a thruster efficiency of.25. Then, two engines of 25 watt were mounted at stern to support the maximum power of 495 watt in table 1. The Thrust is obtained from the table (1) considering that P T = T a V A where V a(m/s) is the advance speed. It is common to define the relative speed reduction by introducing the advance speed at the propeller (speed of the water going into the propeller) as: V a = (1 w)u (9) where w is the wake fraction number and U(m/s) is the forward speed of the ship previously defined. The wake factor at the stern can be given by the following formulae [3]: w =.5C B.5 (1) Therefore, the resultant thrust for the nominal speed is T a = 15.8N. 3.3 Kort Nozzle The in-scale tug was mounted with two steerable kort nozzles and two propellers as the full-scale model has. The model for the nozzles is based on that of Perdon [8]. The forces generated by the nozzle and the propeller are expressed as functions of thrust and nozzle deflection δ. X δ = T cos 2δ The thrust T is expressed as, Y δ = T sin 2δ (11) T (v, r, δ) = (1 v.33r ).6 1.4sin δ δt a (12) where, T a is the total trust at the initial speed on a straight course. The first term express a reduction in thrust caused by the local sway velocity. The second term expressed the loss in thrust due to nozzle deflection. 3.4 Filtering The term ψ is computed through the in-scale tug measurements acquisition system including GPS heading data. When the derivatives are not measured a local polynomial fixed-point smoother can be used to obtain them [7]. That is, for each sequential measurement one fits a local polynomial based on a number of measurements close to the measurement of interest and estimate a model: ˆψ j (t k ) = â + â 1 (t k+i t k ) + â 2 (t k+i t k ) 2 + â 3 (t k+i t k ) 3 (13) where i = M,..., 1,, 1,..., M. Then, the smoothed measurement and its derivative are obtained by taking i = : ˆψ j (t k ) = â ˆ ψ j (t k ) = â 1 ˆ ψ j (t k ) = â 2 (14) On the top of the figure 4 is depicted the measured heading and the filtered heading for a turning circle manoeuvre. In this case the measured heading ISSN: ISBN:

5 present a low level of noise and then the filtered heading is slightly smoothed. At the bottom of the figure 4 is shown the derivative of the heading computed with this method, which provides good results with a smooth curve. It can be checked that when the slope of the heading is zero the derivative of the heading is zero. Heading(º) Measured Filtered r(deg/s) Figure 4: Filtering data with a smoother. 4 Simulations The simulations were made using the Marine Systems Simulator (MSS), which is a Matlab/Simulink library and simulator for marine systems. The library also contains guidance, navigation, and control (GNC) blocks for real-time simulation. It has been simulated a 3 zig-zag test in order to see the behaviour of the ship, figure 5. It can be made a comparison between figure 4 and figure 5. In both cases the rate of heading gets the stationary state around 8 deg/s for a nozzle angle of 3. The rate of heading computed from real data measured with the in-scale tug system exhibits non-linear behavior which is in part attributed to slight perturbation effects of current and wind. The trials were made in calm water but there is always a small effect of wind and current. 5 Conclusion It has been presented a remote system for experimentation of an in-scale tug and its possibilities as a research and educational tool. It has been compared real data from trials in open waters with simulated v(m/s) r(deg/s) Heading(deg) Waterjet(deg) Figure 5: Simulation results corresponding to the inscale tug under a 3 zig-zag test. data from theoretical models. It is possible to perform different kinds of sea trials commonly applied to fullscale vessels in open waters and also simulations in simulink in order to compare theory with the real data acquired and see the differences between. Acknowledgements: This work has been partially supported by MEC with the project DPI Appendix Principal Hull data for the in-scale tug model Figure 6: Main particulars and reference frames: geometric (origin O g ) and body fixed (origin O g ); CGcenter of gravity; Ap-aft perpendicular; F P -front perpendicular; -length between perpendiculars; T - draught; DW L-design water line and BL-base line. NOTE: Sea water density ρ = 125Kg/m 3 ; Gravity constant g = 9.81M/s 2. The block coeffi- ISSN: ISBN:

6 Table 2: tug dimensions Quantity Symbol In-scale (full load) Length between 1.71 m perpendiculars Beam over all B.535 m Nominal Speed U.9 m/s Draugtht T.159 m Displacement vol m 3 mass m 97 Kg Inertia Moment Roll I xx Kgm 2 Inertia Moment Yaw I zz Kgm 2 cient is the ratio of the displacement volume,, to the volume of the parallelepiped(rectangular block) with the dimensions L, B and T : C B = LBT (15) Table 3: Hydrodynamic derivatives in nondimensional form and Nomoto parameters for the in-scale model tf-12 (Y v ) d e-4 (Y v) d -.11 (Y v ) f e-4 (Y v) f -.1 Y v Y v Y ṙ -.37 Y r.245 N v.65 N v.27 N ṙ -.13 N r Table 4: Deadwood and fin data Quantity Symbol Deadwood Fin Area A R m 2 Span sp m Mean chord c m Eff. aspect ratio a m Moment Dist. to O b r f.7.8 m References: [1] Clarke D., P. Gedling, and G. Hine. The application of manoeuvring criteria in hull design using linear theory. In The Royal Institution of Naval Arquitects, [2] Revestido E., F. J. Velasco, T. M. Rueda, E. Moyano, E. Lopez, and L. A. Esquibel. Turning circles of an autonomous high speed craft model. In WSEAS Int. Conf. on System Science and Simulation in Engineering, 26. [3] Clark I. Ship Dynamics for Mariners. The Nautical Institute, 24. [4] Velasco F. J., T. M. Rueda, E. Revestido, E. Lpez, E. Moyano, and L. A. Esquibel. Experimentation environment for marine vehicles. Journal of Maritime Research, 3:3 18, 26. [5] Witt N. A. J., K. M. Miller, and M. P. Russell. Mathematical and scale model platforms for ship guidance trials. Technical report, The Institution of Electrical Engineers, [6] Davidson K. S. M. and L. I. Schiff. Turning and course keeping qualities. In Transactions of SNAME, volume 54, [7] Vladislav Klein Eugene A. Morelli. Aircraft System Identification Theory and Practice. AIAA Education Series, 26. [8] Perdon P. Rotating arm manoeuvering test and simulation or waterjet propelled vessels. In Proc. International Symposium and Workshop on Force Acting on a Manoeuvering Vessel, Val de Reuil, France, September [9] Tristan Prez. Ship Motion control Course keeping and roll Stabilisation using rudder and fins. Springer, 25. [1] SNAME. Nomenclature for treating the motion of submerged body through a fluid. Technical and Research bulletin 1-5, The Society of Naval Architects and Marine Engineers, 195. [11] Lewis Edward V. Principles of Naval Arquitecture. The Society of Naval Architects and Marine Engineers, [12] Jacobs W.R. Estimation of stability derivatives and idices of various ship forms and comparations with experimental results. Technical report, Davidson laboratory R-135, ISSN: ISBN: