Abstract. 1 Introduction

Transcription

1 A proposal of an optimal control system for a propulsion unit with a controllable pitch propeller K. Chachulski, W. Kozera, J. Szczesniak Department of Automation, Maritime University of Szczecin, Abstract The paper present a short description of an optimal control system for a propulsion unit with a controllable pitch propeller and proposes how to integrate the system with AUTOCHIEF 7 made by NORCONTROL Also the possibilities of collaboration of a system with shaft generator are discussed 1 Introduction The use of a controllable pitch propeller introduces an additional controlled parameter Possible changes in the propeller pitch extend the engine working field which is particularly visible at partial loads The controllable pitch propeller enables the use of full engine power output in varying conditions of navigation This enhances the propulsion efficiency in comparison with a fixed pitch propeller system and is cost effective Only a properly operated system can turn out to be profitable Incorrect operation of the controllable pitch propeller drive mainly results in increased fuel consumption 2 Optimal control system characteristics The authors have developed, [1,2,3] a microcomputer system of selecting optimal settings of the CP propeller propulsion unit which enables a number of tasks to be executed. The major ones consist in the determination of: optimal rotating speed and propeller pitch settings (relative to maximum efficiency of the propeller and engine) in any navigating conditions and for various ship speeds,

2 490 Marine Technology and Transportation the engine working point in the field of characteristics and defining the current engine load, i.e. the determination if the engine runs in the so called working field without time limits, in the limited-time field or in the prohibited field; * hourly, unit and total fuel consumption for the entire range of ship speed the current propulsion characteristic, maximum and minimum ship speeds at which the engine is overloaded, with navigating conditions being considered, above quantities for anticipated navigating conditions, eg optimal settings (n,p) and fuel consumption at water resistance increase by 20% The foregoing tasks are executed on the basis of continuous measurements of quantities which characterize the current propulsion unit load, such as the net torque A/, engine rotating speed /?, ship speed v, or propeller pitch P The control system may have the structure as shown in Fig 1 The actuator changes the propeller pitch and engine revolutions each time the system determines the optimal settings Mo, litor shi! P L ^ Pro [>lll<»ion unit ]^T -*""* ^ 1 c-ompit El 0^ i 1 n r V P 'v, i Keyb oard 1 Co, ivei-ter VC Figure 1 Block diagram of the control system The system of optimal settings selection was tested on the m/s WINETA during her normal operation. The measurement results showed that the fuel consumption while the settings were determined by the computer was 8% to 12% lower than the consumption with the propulsion unit settings established by the ship's program There were no discrepancies whatsoever between the calculated and real values after the optimal values of the propeller pitch and engine revolutions had been set 3 A new proposal of optimization system application The authors' proposal is based on the existing control system AUTO CHIEF 1 (AC-7), made by NORCONTROL, which has an electronic governor DOS 8800 and a PH-R CP propeller by ABB ZAMECH Ltd The optimization system program will be written into the storage of AC-7 control system In addition to the existing working modes of the AC 7, INDIVIDUAL and COMBINATOR, located in the MODE SELECT controls, an OPTIMUM mode should be introduced as shown in Fig 2

3 Marine Technology and Transportation 491 DDDD Figure 2: A fragment of the control panel AC-7 This new mode would be available from the control room and the bridge The navigating officer would set, e.g. by shifting a telegraph lever, a desired value of the ship speed and the optimizing system would select the propeller pitch and engine rotating speed values so as to maximize the efficiency of the propulsion unit The determined settings of the propeller pitch and engine rotating speed would be transferred to, respectively, the governor and propeller pitch setting device The system would determine optimal settings on the basis of current measurements of the torque A/, rotating speed /?, propeller pitch P and ship speed v entered directly into the AC-7 system The measurements include components from attendant phenomena which accompany the operation of the mam engine - CP propeller system, such as changing wind force or high seas Proper control of the propulsion system requires that the influence of these side effects is eliminated Digital filters are a convenient and simple tool to get rid of the noise from discrete signals obtained from an a/d converter and quantized at equal mervals Examples of the torque signal measured (a) and filtered (b) obtained from a spectral analysis of signals measured onboard m/s WINETA are shown in Fig 3 The examples illustrate two different working states of the system tc-s) Figure 3: A torque signal function

4 492 Marine Technology and Transportation All information on changes of weather conditions, ship draft, hull condition etc are contained in a set of values M,/;,P,v. The control system AUTOCHIEF 7 should be equipped with a monitor The operator of the system (engineer or officer of the watch) could display on its screen the system tasks mentioned in section Optimal control of the propulsion unit co-operating with a shaft generator The overall efficiency of the power plant can be increased by using a shaft generator for electricity production driven by a PTO gear and a planetary gear The application of a shaft generator is particularly beneficial on ships driven by a CP propeller whose pitch and revolutions are set by an optimizing system of the propulsion unit Fig 4 presents the results of a cost analysis of the fuel used for electricity production onboard m/s WINETA Individual curves refer to a generating set, a shaft generator running at constant ME revolutions (//^const) and a generator shaft running at varying ME revolutions (/?=~var) (USD/h) / = 13 w,n = const v = 15 w, n = const Figure 4 Comparison of fuel costs of electricity production onboard m/s WINETA The devised optimal control system performs the following functions 1. determines optimal settings (P,n) for a set ship speed and desired electrical power, 2 plots a current propulsion characteristic, 3. determines the propeller pitch value P, for a set ship speed, at constant rotating speed (/?=const) and signals that the shaft generator can be switched on, 4. checks if the shaft generator can be switched on at current or prognosed external conditions, 5 analyzes costs of electricity production by a shaft generator and by a generating set. The above functions lead to the conclusion that the system can be applied in ships where the shaft generator drive requires constant engine revolutions (/?=const) and in ships where a shaft generator is driven via BWill/RFC or CONSPEED type gears, i.e. at varying revolutions (/r=var)

5 Marine Technology and Transportation 493 On ships where the engine rotating speed can be changed the optimizing system determines, for a set ship speed, the settings of propeller pitch and the revolutions which guarantee the minimum fuel consumption It also informs that the shaft generator can be switched on when electric power demand (with allowance for losses in the generator and gear) is smaller than or equal to the engine power exceeding the power required by the propeller If the electric power demand exceeds the excess of engine power obtained at optimal settings, the system seeks such settings (f\n) lying as close to the optimal ones as possible which will enable the shaft generator to be switched on and nonoverload running of the ME while the desired ship speed is maintained. The location of the optimal working points of the propulsion unit for both mentioned cases are presented in Fig 5 Point A in the chart defines the position of the optimal point for speed v? without a shaft generator while point H determines settings at which the shaft generator can be switched on Point B is a working point lying possibly the closest to the optimal point thus quaranteeing a constant speed v > with the shaft generator switched on Figure 5 Positions of the optimal working points On ships where a constant ME rotating speed is needed to drive a shaft generator, both the set ME rotating speed at which the shaft generator runs and the set ship speed should be entered. The optimizing system finds the current electric power demand and determines a propeller pitch value ensuring that the set ship speed is maintained (point C, Fig 5), it also analyzes the possibilities of switching on the shaft generator. If possible, the system displays the engine load and the corresponding fuel consumption When the power reserve is too small, the system sends two pieces of information 1 by how much the electric power demand should be reduced to maintain the set ship speed, 2. by how much the ship speed should be reduced to ensure the electric power demand

6 494 Marine Technology and Transportation The optimizing system makes it possible to analyze the costs of fuel used for the generation of electricity without the need to switch on the shaft generator The choice of the option analyzing the switching of the generator would be carried out by a proper control button placed in the group OTHER FUNCTIONS in the control panel of the AC-7 The shaft generator would be switched on after pressing a button in the MODE SELECT group References 1. Paper in a journal: 1 Chachulski, K, Kozera, W., & Szczesmak, J Mikrokomputerowy system doboru optymalnych nastaw ukiadu nap^dowego ze srub% nastawn% Transport i Gospodarka Morska, 1987,11 2. Paper in conference proceedings: 2 Chachulski, K., Kozera, W., & Szczesmak, J Optimal control system for propulsion unit with controllable pitch propeller, The 3rd II'AC Workshop on Control Applications in Marine Systems (CAMS''95), Trondheim, Norway, Chachulski, K., Kozera, W., & Szczesmak, J Filtracja sygnalow pomiarowych stanu obci^zema ukiadu napgdowego ze srub^ nastawn% pp 21-28, XI International Symposium of Ship Power Plants, Gdansk, Poland, 1989