Integrated control system for ship motion in inland navigation J. Kulczyk, T. Bielihski Institute ofmachines Desing and Operation, Technical University of Wroclaw, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland EMail: jan@apollo. ikem.pwr. wroc.pl 1. Introduction The automatic steering of the ship already has a long history. The automation encompasses many of the ship's systems. The aspect of automation which this paper deals with is the automatic control of the ship's motion. This control can take place on several levels differing in their time horizons [1]. Five levels are distinguished and the last (5*) level corresponds to the direct control of the ship's motion in real time. The previous levels are connected with the determination of the optimum voyage route, taking into account the weather conditions, the navigational constraints and the performance of the ship in emergency situations. For a given model of the ship's dynamics, the minimum deviation of the ship from the preset trajectory or course is the fundamental criterion of the quality of steering. Apart from emergency situations, the basic factors responsible for deviations from the course are the operating conditions (wind, waves, currents). The autopilot is a key component of the automatic steering system. It enables the realization of the ship's motion for the preset course. The deviation of the ship from the preset course constitutes the input signal. The reaction is an automatic course correction effected through an appropriate operation of the steering system. The autopilot sends a signal to the steering engine which turns the rudder thereby changing the course. A more general course stabilization problem is the maintenance of the ship on a preset trajectory. This trajectory can be: a water-lane route in a strait, a limited depth port-approach water-lane route, an inland water-way. The ship can be steered along the preset trajectory - the so-called trajectory stabilization - manually by the steersman (in the open system) or by the closed system of automatic steering. Currently two structures of the trajectory stabilization system are used:
374 Marine Technology Transactions on the Built Environment vol 42, 1999 WIT Press, www.witpress.com, ISSN 1743-3509 successive changes (Ay/zk) of the preset heading, computed by the master computer and the conventional autopilot operating in a system of the stabilization of preset course vj/^; in addition, the computer can identify the changing ship dynamics parameters and the disturbances (waving, wind, a current) and change appropriately the autopilot adaptation loop settings (loop gain, integration time, differentiation time-damping, zone of insensitivity) to stabilize the desirable dynamic properties of the whole steering system (regulation time,over-regulation) (fig. 1); POL, Y J p(x,y,t) Figure 1: System of trajectory stabilization through successive changes of preset course. direct digital control in real time, realized by a computer hooked up with a conventional autopilot operating in the follow-up system; the computer functions as a digital controller generating set rudder angle value 8% in time or a three-state signal controlling the steering engine and performs the adaptation and filtering functions (fig. 2). P(X,Y,t) Figure 2: System of trajectory stabilization through real-time direct digital control. When the ship's dynamics is described as a trajectory stabilization object, the following assumptions are made:
Marine Technology 375 Transactions on the Built Environment vol 42, 1999 WIT Press, www.witpress.com, ISSN 1743-3509 the effect of sway on the ship's motion parameters is neglected, a full nonlinear model of the ship's dynamics is used, the ship's motion is considered in the system of coordinates X Y. In [2], also the control of the ship's motion in collision situations is distinguished. This is a very difficult task because of the complexity of the equations of the ship's dynamics (nonlinearity, nonstationarity) and several random phenomena. In principle, the control of the ship's motion on an inland water-way is an example of such control. Besides the fact that the ship moves, as a rule, along a curvilinear path, it encounters other vessels or other hydrotechnical objects (bridges, locks). The steering itself is a tedious activity leading quickly to the steersman's fatigue and weariness. A system automatically steering the ship on an inland water-way will relieve the steersman of tedious routine work and it will aid him in exceptionally difficult navigational conditions (navigation at night, in fog). This will improve the safety of traffic on water-ways and reduce considerably the risk of a collision. 2. Components of navigation system The integration of the navigation system consists in the preparation of detailed navigational data and the planning and realization of the motion of an autonomous vehicle (ship) in an navigational environment on the basis of this data. The full autonomy requirement means that the navigation system on the ship must suffice without any additional external devices. To fulfil this task, the integrated navigation system (ENS) needs several data covering: the navigational environment (the image of the current surroundings, objects in the water-way area); the position, course and speed of the ship and other objects; a mathematical model of the ship's dynamics as a priori known data; a representation of the navigational landscape (an electronic map, a street map, etc.) in the form of a knowledge base as a priori known data; a knowledge base containing a description of the traffic rules. The qualifier "integrated" means that the information received from the different sensors and the a priori navigation system data require coordinated handling to solve the ship steering problem. These diverse pieces of information must be processed in real time and ultimately this must result in the generation of a motion trajectory realized through the appropriate operation of the steering engine and the ship's propulsion system. In die case of inland navigation, the following data supplying devices are available for an autonomous ship (fig. 3): an on-board radar and a laser scanner (as sensors plus a tv screen); a course and turn gyroscope; a GPS (Global Positioning System) receiver for position and speed determination, * an ultrasound Doppler for water or land speed determination; a mathematical model of the ship's dynamics; # a bank of mathematical models of the dynamics of foreign ships (for assessment purposes);
376 Marine Technology Transactions on the Built Environment vol 42, 1999 WIT Press, www.witpress.com, ISSN 1743-3509 an electronic map of the water-way as a model of the river and bank environment, including all the data and distinctive features important for navigation; a knowledge base containing the traffic rules; a sonar for the determination of the current water-way depth. STEERSMAN Radar Integrated Navigation System path situation determination, positioning route planning steering electronic i! equations of j navigation map Ij dynamics j \ rules Figure 3: Components of Integrated Navigation System. The different kinds of data in INS will be given different priority, depending on their importance for navigation (precision, reliability). The priority of the particular data changes in time. INS's open and modular structures can be developed further and expanded and new information sources can be added. The navigation system performs the following functions: determines the ship's position, course angle and speed in electronic map coordinates; tracks the position and speed of all the objects within the water-way (other vessels, radar buoys, etc.) to determine the current situation on the river; plots the ship's path in the local navigational environment (possible interaction with the steersman), steers the ship along the preset path by manipulating the steering gear and the propellers, communicates, as regards control and maintenance, with the steersman. The INS principle of operation consists in the comparing of the actual navigational environment recorded by the on-board radar with the computer model. The computer model consists of an electronic map and a dynamic model of the ship.
Marine Technology 377 The computer model includes the ship's optimum trajectory. The comparison of the actual position with the model one leads to a course correction. The correction must take into account other vessels present in the current navigational environment. 3. INS design assumptions The design of INS is based on the application of a GPS receiver which uses the differential global positioning technique (DGPS). The latter requires reference stations. DGPS ensures the position measurement accuracy of below 5 m. The ship is also equipped with devices which determine its basic operational parameters (sailing speed, propeller rpm, current velocity, water-way depth, etc.). Another necessary component is an electronic map of the water-way. It should include all the navigational information necessary for safe navigation. These are data on the obstructions to navigation, the hydrotechnical structures, the axes of the ideal downstream and upstream courses, the shape of the shore-line and so on. If a difference between the current position and the preset axis of the ideal course is detected, a course correction must be initiated. The correction can be effected by a propeller manoeuvre (two-propeller propulsion) or by turning the rudder. The rudder angle is determined on the basis of the known dynamic model of the ship. This model includes. the equation of motion of the ship, the external forces and the hydrodynamic moments acting on the ship. Assuming that the ship moves only in the horizontal plane and the coordinate system is bound with the hull, it follows from the principle of conservation of momentum that the equations of motion have the following form [3]: (1) where: m, Iz - the ship's mass and moment of inertia together with the mass of the associated water; these quantities are a function of the loading state and the water-way depth; u, v - velocity vector components in the longitudinal and transverse direction, respectively; r = **V- - angular velocity relative to a fixed coordinate system Angle if/ is a dt course angle. The "dot" stands for a derivative over time.
378 Marine Technology The right side of system of equations (1) defines the forces and the hydrodynamic moments acting on the ship. These are functions of such quantities as: v, w, v, u, r, r, rudder angle 8, propeller thrust and water-way depth. It is assumed that these quantities are known. This means that the right side of equation (1) is determinate and therefore the ship's trajectory can be related to the rudder angle. As a result, the ship's course can be corrected and adapted to the preset ideal trajectory for a given water-way. It is assumed here that the relationships which make it possible to determine all the forces and moments acting on the ship's hull on the basis of the measured motion parameters are known. In practice, these relationships are not always determined with sufficient accuracy. This requires further theoretical and model studies. 4. Example of motor cargo boat adaptation to INS A schematic of a motor cargo boat showing the arrangement of the necessary navigational instruments is presented in fig. 4. All the instrument indications are transmitted to the on-board computer (it. 8 infig.4). The necessary navigational devices are available on the market and they can be hooked up to a computer system. The system checks if the ship's current position conforms to the preset one. If there is a divergence, the equations of motion are used to determine the required rudder angle to correct the ship's course. The rudders are turned automatically. This is possible through the use of an electrically controlled proportional distributor in the steering system. The distributor is the only element which should be incorporated into the rudder hydraulic installation on motor cargo boat BM 500. A diagram of this installation is shown infig.5. INS requires investments in the water-way, i.e. an electronic map of the water-way should be made and a reference station for DGPS should be built. 67 ^JM Figure 4: The motor cargo boat type BM-500. Arrange of navigation instruments: 1-sonar, 2-log, 3-gyrocompass, 4-tahometer, 5-deflection meter of the rudder, 6- GPS, 7-radar, 8-integrated navigation system.
Marine Technology 379 14, 13. Manometer OH tank 12 Manometer (ocean version) 10. 11, stop Cock cock 9. 8. Cut-off Non return 7. High presure 6. Overflow fine filter 3. 6. Stern Overflow distributor 3, Proportional distribution RBS 2. Hydraulic cylinder 1. Gear pump Figure 5: Hydraulic installation of motor cargo boat BM-500. References 1. Lipowski J: Ship as object of numerical control (in Polish). Wydawnictwo Morskie, Gdansk 1980. 2. 12* Domestic Conference on Automation, Wyzsza Szkola Morska, Gdynia 1994. 3. Welnicki W.: Mechanics of ship's motion (in Polish), Gdansk Polytechnic, Gdansk 1989.