Experimental Testing of an Autonomous Underwater Vehicle with Tunnel Thrusters

Transcription

1 First International Symposim on Marine Proplsion smp 9, Trondheim, Norway, Jne 29 Experimental Testing of an Atonomos Underwater Vehicle with Tnnel Thrsters Alistair Palmer 1, Grant E. Hearn 1, Peter Stevenson 2 1 School of Engineering Sciences, University of Sothampton, Sothampton, UK 2 Underwater Systems Laboratory, National Oceanography Centre, Sothampton, UK ABSTRACT Preparing a manscript for delivery in Acrobat pdf format. Manscripts mst be prepared in English and mst consist of: title, fll names of all athors, affiliation(s), abstract, keywords, text, reference, tables and figres. The manscript generally shold not exceed eight pages. The complete manscript inclding figres and tables shold be spplied by to the Secretariat of smp 9 secretariat@marineproplsors.com. A name of an athor to which all correspondence are to be forwarded mst be shown clearly therein along with complete address, and e- mail address. The abstract shold not exceed 2 words and be informative of the prpose, methods, reslts, and conclsions. Keywords Atonomos Underwater Vehicle, Simlation, Tnnel Thrster, Experimentation. 1 INTRODUCTION Atonomos Underwater Vehicles (AUVs) are a class of nderwater vehicles which operate independently of any hman control. These vehicles are controlled by onboard systems which se the information recorded by sensors to determine demands for the vehicle actators. The complexity of these control systems is a fnction of the sensors and actators employed and the desired vehicle performance. Frthermore, these vehicles are constrained by the limited energy spply carried onboard. There are many different types of AUV crrently in se and generally these vehicles are designed for a specific prpose. Example missions ndertaken by AUVs inclde oceanographic srveying, mine-sweeping and pipeline inspection. As the performance of these vehicles improves so the desire to se these vehicles in a greater nmber of scenarios increases. Therefore, the next stage in the development of AUVs is the creation of a mltiprpose vehicle capable of combining long range srvey missions with low speed interaction and investigation style tasks. A key performance indicator for a srvey vehicle is the range it can achieve. Therefore the design of srvey vehicles focses on combining a hydrodynamically shaped hll form and a high efficiency proplsion system with the ability to carry sfficient energy and the necessary mission dependent payload. This has reslted in a common srvey style vehicle design consisting of a torpedo-shaped hll form with a stern monted propeller and control srfaces for control at speed. These vehicles tend to be ballasted to be positively boyant to ensre that the vehicle rises to the relative safety of the srface shold the proplsion systems fail. In order to overcome the positive boyancy at srvey speeds the vehicles operate with a small pitch angle, controlled by the hydroplanes, to generate a downwards force hydrodynamically. As a srvey vehicle slows down a speed limit is reached beyond which the control srfaces can no longer provide sfficient forces to, firstly, maintain a pitch angle to control the positive boyancy, and secondly, manoevre the vehicle in the desired manner. Ths the creation of a mlti-prpose vehicle reqires additional control devices to provide low speed control. The majority of nderwater vehicles se propeller based thrsters to provide low speed control de to their responsiveness, reliability and ability to generate forces throghot the speed range. To maintain the srvey efficiency of the vehicle these thrsters can be placed within throgh-body tnnels. An example thrster configration for a mlti-prpose vehicle is shown in Figre 1. Figre 1 Example mlti-prpose AUV based on srveystyle configration with for additional tnnel thrsters 2 TUNNEL THRUSTER PERFORMANCE The vehicle shown in Figre 1 will need to se the tnnel thrsters for two key tasks. These are the control of the positive boyancy at speeds below the limit of control srface control and for low and zero speed manoevring control. This provides two different operational envelopes for the tnnel thrsters, namely, low and zero speed operation at a wide range of vehicle orientations

2 and a higher speed range with a limited range of pitch angle. The exact vales applied to these ranges are a fnction of the detailed design of the vehicle and control srfaces. In this paper the focs is on the latter operational envelope, that is, the performance of a tnnel thrster on a vehicle moving with a forward speed. The analysis will also be limited to zero pitch as the performance of the tnnel thrster is expected to be relatively consistent over the range of small pitch angles expected at these speeds. The performance of small diameter tnnel thrsters has been investigated in static conditions (McLean 1991; Cody 1992) leading to the development of a model of the dynamic performance of the thrster (Healey et al 1995). These experiments demonstrated the steady state performance of these devices to be similar to other propeller based thrsters, that is, the thrst generated is proportional to the sqare of the rotational speed. The performance of tnnel thrsters in an AUV hll form over the fll range of operational vehicle speeds and yaw angles in the range ±9º has been investigated in (Sanders & Nahon 22). However, for these experiments the thrster was isolated from the vehicle hll form and the forces recorded were those generated by the thrster and not those experienced by the vehicle. These reslts showed a variation in thrst of arond 15% over the range of forward speeds tested. A frther set of reslts giving the performance of a tnnel thrster on a sbmersible, in terms of the forces experienced by the vehicle, are shown in Figre 2 (Beveridge 1972). KF, KN Figre 2 Force and moment data for a tnnel thrster on a sbmersible Figre 2 ses the following coefficients to represent the performance of the thrster, see Eqation 1. A force coefficient, K F, gives the ratio of the force experienced by the vehicle to the corresponding zero speed thrster force. A moment coefficient, K N, gives the ratio of the moment experienced by the vehicle to the corresponding zero speed moment. These coefficients are plotted against the speed ratio of the vehicle forward speed,, to the thrster jet exit speed, j. KF KN K F K N Speed Zero Speed = (1a) N Speed Zero Speed = (1b) j = T (1c) ρa The data shown in Figre 2 shows a variation in force of p to 95%, which is considerably more than that recorded in (Sanders & Nahon 22). This indicates that variation in force is not solely de to the performance of the thrster nit itself. 2.1 Lateral Thrsters on Srface Vessels The performance of a tnnel thrster on a moving nderwater vehicle is analogos to the performance of a lateral thrster on a srface vessel. The performance of lateral thrsters has been investigated in (Nienhis 1992; English 1972; Brix & Bssemaker 1973; Chislett & Björheden 1966). This research incldes measrements of the forces and moments on the vessels, simple flow visalisation experiments and pressre measrements arond the thrster. These reslts, and the conclsions drawn, provide an insight into the mechanisms casing the variations in the performance of a lateral thrster. The thrster itself can be considered as a jet prodcing device. Hence when the vessel is stationary the jet flows away from the vehicle. However when the vessel is moving forwards the thrster jet is deflected as a fnction of the relative strength of the jet to the ambient flow. As the thrster jet flow develops flid is entrained into the jet, casing a sction effect arond the jet. When the jet is deflected backwards this sction region interacts with the vehicle, indcing a force on the vehicle opposite to the desired thrster force. The offset of this sction force from the thrster force cases a frther variation in moment experienced by the vehicle. The complexity of the interaction between the ambient flow (inclding the bondary layer), thrster jet and vehicle means that the performance of each different configration is niqe. Therefore to be able to characterise the performance of a tnnel thrster on an AUV an experimental approach was adopted. 2.2 AUV Simlation Simlations are commonly sed in the development of AUVs to aid in control system design and to gain insight into the performance of the vehicle. In order for the simlations to accrately reflect the performance of the vehicle it is necessary to model the inflence of the actators employed. However no common modelling approach for the performance of a tnnel thrster is readily available. This is thoght to be de to the complexity of the interactions involved and the niqeness of each configration.

3 Pblished AUV simlations tend to assme that the forces experienced by the vehicle are eqal to those generated by the thrster and that the moment can be calclated according to geometric considerations (Ananthakrishnan et al 1998). Sanders & Nahon (22) do attempt to modify the model from (Healey 1995) bt since this model does not accont for ambient flow effects it does not model the complete performance. Hence the reslts obtained from the experiments ndertaken will be sed to develop a modelling approach for the performance of a tnnel thrster on a srvey style AUV. 3 EXPERIMENTAL SETUP A 2.5m, approximately one-third scale, model of the srvey AUV Atosb (Fallows 24) was modified to accommodate two throgh-body tnnels, one forward and one aft, as shown in Figre 3. Each tnnel has a diameter eqal to that of the thrster monted within the tnnel and the tnnel is symmetrically faired into the shape of the vehicle at the inlet and otlet. The particlar thrsters sed are 7mm diameter rim driven thrster nits (Ab- Sharkh et al 23). These thrster nits are well sited to this application as they offer symmetrical performance and minimise the blockage in the tnnel cased by the hb. The thrsters were driven sing an electronic speed controller with the rotational speed of the thrster controlled by varying the voltage of the signal. The test plan for the experiments was designed to cover the range of operational conditions expected. This incldes testing over a range of forward speed and thrster rotational speed. 4 RESULTS The drag of the vehicle was recorded, withot the thrsters operating, to assess the srvey drag impact of adding thrster tnnels to an AUV hll form. The increase in the drag of the vehicle with thrster tnnels compared to withot thrster tnnels, at srvey speeds, was less than 2%. The force generated by the thrster at zero speed was recorded throghot the range of rotational speeds. These reslts are shown in Figre 4. These reslts show a linear trend with the sqare of the rotational speed as expected and closely match the data pblished by the manfactrer. T (N) n^2 (s^-2) Figre 4 Tnnel thrster performance nder static conditions showing thrst, T, against the sqare of the thrster rotational speed, n 2 The reslts for the forward and aft thrsters across the range of speed ratio are shown in Figre 5. The data recorded is presented sing the coefficients defined in Eqation 1. The forces indced by the operation of the thrster are determined by calclating the difference between the forces recorded at a given speed with and withot the thrster operating. Figre 3 CAD drawing of the Atosb model showing forward and aft thrster tnnels (control srfaces and stern proplsor not shown) The tank sed for the testing was the Sothampton Solent University Towing Tank which is 6m long, 3.7m wide and 1.85m deep and has a carriage which can rn p to 4.25m.s -1. The model was monted onto a prpose designed and bilt dynamometer and spporting framework which incorporates for force blocks. Each force block ses a linear variable differential transformer to measre the transverse displacement indced by a force applied between the top and bottom of the block. The force blocks are monted in orthogonal pairs to measre drag and side force. Each force block was calibrated sing a mlti-point calibration sing calibrated weights. The signals are digitised and passed to a PC for atomated data logging. The data was recorded at 6Hz. KF, KN Aft KF Fwd KF Aft KN Fwd KN Figre 5 Tnnel thrster performance for forward (hollow symbols) and aft (solid symbols) thrsters on an AUV moving with a forward speed These reslts show a large drop off in the force experienced by the vehicle with increasing speed ratio for

4 both the forward and aft thrsters. This data shows a similar form to that obtained by (Beveridge 1972) (Figre 2). The differences between the two force crves are an indication of inflence of the differing form of the vehicle arond the two tnnel exits. The hll form aft of the rear tnnel slopes away from the tnnel whereas the hll form aft of the forward tnnel is flat. This variation will give a differing interaction between the deflected jet and vehicle and conseqently differing performance characteristics. Both thrsters were also rn at the same time, in the same and opposite directions, and no interaction effects were experienced at the large thrster separation sed. Figre 5 also shows the moments generated by the thrsters to drop off considerably with increasing speed ratio. There is a notable difference between the variations for the two thrsters. In order to gain some insight into these variations it is necessary to nderstand the forces acting on the vehicle. A simplified representation of a tnnel thrster ses two forces. These are the thrst force generated, assmed to act at the thrster axis, and a sction force, which acts at a variable point as a fnction of the speed ratio. The sction force, F S, is defined as the difference between the expected (zero speed) force and the force experienced by the vehicle. The sction moment, N S, is defined as the difference between the expected (zero speed) moment and the moment experienced by the vehicle. F S = ZeroSpeed Speed (2a) N S = Zero Speed Speed (2b) The centre of action of the sction force is then defined as the ratio of the sction moment to the sction force: N S x S = (3) F Figres 6 and 7 show the variation of the centre of action of the sction force, with speed ratio, for the forward and aft thrsters, respectively. These reslts show that the centre of action of the sction force for the forward thrster moves aft with increasing speed ratio. This movement is towards the central pivot of the vehicle and ths redces the impact of the sction moment, giving the limited redction shown on Figre 5. For the aft thrster the centre of action is roghly constant and aft of the thrster, giving a relatively greater inflence of the sction moment. This leads to the point at a speed ratio of approximately.5, where the aft thrster effective moment changes sign as the sction moment dominates the desired thrster moment. The reasoning behind this relatively constant centre of action for the aft thrster is thoght to be cased by the trncation of the hll form after the thrster tnnel exit. S xs (m) Figre 6 Centre of action of the sction force, x S, for the forward tnnel thrster against speed ratio, / j xs (m) Figre 7 Centre of action of the sction force, x S, for the aft tnnel thrster against speed ratio, / j The drag force on the vehicle was also recorded dring these experiments. Figre 8 shows the increase in volmetric drag coefficient, compared to the thrster-off case, against speed ratio. CD Aft Forward Figre 8 Volmetric drag coefficient increase, C D, against speed ratio, / j, for the forward (hollow symbols) and aft (solid symbols) thrster operation These reslts show that the increase in drag decreases as speed ratio increases. At the low speed ratios, where the jet dominates the ambient flow and effectively forms a cylinder in the flow, the increase in drag is the largest. As the speed ratio increases, and the jet is deflected more by the ambient flow, the increase in drag redces. Note shold be made that the drag at low speeds is small and hence a large increase in drag coefficient does not correspond to a large increase in actal force.

5 5 TUNNEL THRUSTER MODELLING To accrately simlate the performance of an AUV eqipped with tnnel thrsters reqires a model of how the operation of these thrsters affects the vehicle. Since no established modelling procedre is readily available, the data obtained from these experiments will be sed to develop a simple and easily applicable model. Yoerger et al (199) states that at low speeds the control of an AUV can be dominated by the dynamics of the thrsters employed. Therefore it is important to inclde the dynamic effects of the thrster in the modelling procedre. Sanders & Nahon (22) concldes that the dynamic performance of the tnnel thrster tested was nchanged by the range of experimental conditions experienced. This conclsion is backed p by a series of dynamic experiments ndertaken sing the experimental setp tested here. Therefore existing models of the dynamic performance can be employed, for example, Healey et al (1995). To model the steady state performance of the thrster an exponential has been fitted to the force reslts of the form: 2 K F = exp c (6) j The force experienced by the vehicle can readily be determined by applying a model of the performance of the thrster at zero speed. The selection of the constant, c, is a fnction of the individal configration tested. For the forward thrster c 7 and for the aft thrster c 3. To model the moment experienced by the vehicle the simplified representation consisting of only two forces is sed. Ths the moment is given by: N = F x + F x (4) T The thrster force, F T, is determined from a zero speed thrster performance model and the thrster moment arm, x T, is determined sing the geometry of the vehicle. The sction force, F S, is determined from Eqation 2 and the sction moment arm, x S, is determined sing a simple model. Chislett & Björheden (1966) conclde that the centre of action of the sction force moves linearly aft with increasing speed ratio. The reslts for the forward thrster, presented in Figre 6, show some agreement with this conclsion, giving a model of the form: x T S S S = xt kd (5) j However the reslts for the aft thrster do not follow this linear trend and hence a constant vale is applied here. These reslts show that it is important to accont for the trncation of the body when selecting the model to be sed for the sction moment arm. 6 CONCLUSIONS To create a mlti-prpose AUV capable of both srveystyle missions and low speed interaction with the environment encontered reqires the addition of frther control devices to common srvey AUV configrations. In order to retain the existing srvey efficiency where possible these additional control devices can take the form of throgh-body tnnel thrsters. This paper reviews the available pblished data for the performance of tnnel thrsters on AUV type bodies and finds a need for additional experimental testing. Therefore experiments were ndertaken sing rim driven thrsters monted in fore and aft tnnels on a torpedoshaped AUV model. The reslts of these experiments are presented to show how the forces and moments experienced by the vehicle, de to the operation of the tnnel thrster, vary as a fnction of the operational conditions. To aid in control system design and AUV performance analysis a modelling procedre for the performance of a tnnel thrster on an AUV type body, as determined from the experiments, is presented. ACKNOWLEDGEMENTS The athors wold like to thank TSL Technology Ltd for the loan of the rim driven thrster nits sed in the experiments, the technical staff in the School of Engineering Sciences at the University of Sothampton for their help in setting p the experiments and the Engineering and Physical Sciences Research Concil (UK) for financial spport. REFERENCES Ab-Sharkh, S.M., Trnock, S.R. & Hghes, A.W. (23). Design and Performance of an Electric Tip- Driven Thrster. Jornal of Engineering for the Maritime Environment, Proceedings of Instittion of Mechanical Engineers, 217(M), pp Ananthakrishnan, P., Vantorox, S., Zhang, K. & Smith, S. (1998). Nonlinear Dynamics and Hydrodynamics Considerations in the Design of an Atonomos Underwater Vehicle. Proceedings of Oceanology International 1998, Brighton, UK, pp Beveridge, J.L. (1972). Design and Performance of Bow Thrsters. Marine Technology, 9(4), pp Brix, J.E. & Bssemaker, O. (1973). Lateral Thrsters with Anti-Sction Tnnels. Proceedings of the 1 st North American Tg Convention, Ship & Boat International, Vancover, Canada. Chislett, M.S. & Björheden, O. (1966). Inflence of Ship Speed on the Effectiveness of a Lateral-Thrst Unit. Report Hy-8, Hydro-og Aerodynamisk Laboratorim, Lyngby, Denmark. Cody, S.E. (1992). Experimental Stdy of the Response of Small Tnnel Thrsters to Sqare and Trianglar Wave Inpts. Masters Thesis, Naval Postgradate School, Monterrey, CA, USA.

6 English, J.W. (1963). The Design and Performance of Lateral Thrst Units for Ships. Transactions of RINA, 15(3), pp Fallows, C.D. (24). Characterisation of the Proplsion Systems of Underwater Vehicles. PhD Thesis, University of Sothampton, Sothampton, UK. Healey, A.J., Rock, S.M., Cody, S.E., Miles, D. & Brown, J.P. (1995). Toward an Improved Understanding of Thrster Dynamics for Underwater Vehicles. IEEE Jornal of Oceanic Engineering, 2(4), pp McLean, M.B. (1991). Dynamic Performance of Small Diameter Tnnel Thrsters. Masters Thesis, Naval Postgradate School, Monterrey, CA, USA. Nienhis, U. (1992). Analysis of Thrster Effectivity for Dynamic Positioning and Low Sped Manoevring. PhD Thesis, Delft University, Delft, Netherlands. Sanders, A. & Nahon, M. (22). The Effect of Forward Vehicle Velocity on Throgh-body AUV Tnnel Thrster Performance. Proceedings of Oceans 22 Conference, MTS/IEEE, Biloxi, MI, USA. Yoerger, D.R., Cooke, J.G. & Slotine, J.-J.E. (199). The Inflence of Thrster Dynamics on Underwater Vehicle Behavior and Their Incorporation into Control System Design. IEEE Jornal of Oceanic Engineering, 15(3), pp