GB2024460A - Improvements in or relating to steering control systems for power- driven ships - Google Patents

Abstract

An automatic leeway correcting auxiliary control system for an automatic pilot in a power-driven ship includes a wind vane (1A) responsive to the apparent wind direction whose response is averaged by integrating over predetermined intervals of time. The difference between the heading of the ship measured by compass (6) and the course set in the automatic pilot (7) is combined with the integrator (2) output to produce a ship's heading control output for automatically substantially preventing departures from a desired ship's track due to leeway. <IMAGE>

Description

SPECIFICATION Improvements in or relating to steering control systems for power driven ships This invention relates to steering control systems for power driven ships and has for its object to provide improved steering control systems which will automatically substantially prevent departures, due to leeway, from a desired ship's track relative to the water (relative track). As will be seen later the invention, in its preferred embodiments, provides control equipment which can be fitted as supplementary to a normal auto-pilot already provided in the ship and which, when so fitted, will automatically modify the-control excerised by the auto-pilot so as substantially to prevent departures, due to leeway, from the desired track.

Leeway can make it quite difficult accurately to navigate or pilot a ship in channels restricted by shallows or the like, especially a ship, such as a container ship or a vessel in ballast, which presents a large windage area, or a ship, such as a large tanker, which has to be manoeuvred at very low speed in restricted waters, and there have been numerous instances of groundings and near groundings due to the failure of a navigator or pilot to correct in time for the effects of leeway. The present invention seeks to make accurate navigation and pilotage in the presence of leeway easier than it would otherwise be.

Although equipment in accordance with the invention is of general advantage, its advantages are most manifest in narrow waters, as in an estuary or when making harbour, for it is then that a vessel has to proceed slowly and is most liable to undesired relatively large alterations in her track by wind.

According to this invention an automatic steering control system for a power-driven ship includes an automatic pilot, an automatic auxiliary control rneans therefor including first means responsive to the apparent wind (i.e. the vector resultant of the actual wind vector and the vector produced by the ship's own heading and speed); second means for integrating or averaging the response of said first means over predetermined intervals of time; third means responsive to the difference between the actual ship's heading and a predetermined desired course which has been set in the auto-piiot; and means for combining the outputs of said first, second and third means to provide an output which is employed for controlling the heading of the ship automatically substantially to prevent leeway.

The auto-pilot may be one already provided in the ship.

In one embodiment of the invention there is provided an apparent wind transducer, which may be a wind venue, so mounted in the ship as to be subjected, with as little interference as possible, to the apparent wind and which produces an electrical output signal substantially proportional to the cosine of the angle between the ship's heading and the apparent wind direction; an averaging or integrating device fed with the output from said transducer and providing an output electrical signal substantially proportional to the average of the transducer output over a predetermined immediately preceding interval of time (typically this interval may be between 20 and 60 seconds, and is chosen to suit the ship, being in general iarger for bigger ships); a summation amplifier having three inputs, and means for controllably attenuating or amplifying the same, one input being the difference between the ship's actual heading (obtainable from the normally provided ship's gyro compass) and the course set in the auto-pilot, the second being the ship's rate of turn (obtainable by differentiating the ship's heading with respect to time or, if the ship is fitted with one, from a yaw rate gyro), and the third being the aforesaid difference multiplied by the output from the integrator or averaging device by means of a multiplier; and an operational amplifier fed with output from said summation amplifier and controtling the normally provided servo-motor controlling the ship's rudder.

The invention is illustrated in and further explained in connection with the accompanying drawings in which: Figure 1 is a block diagram of one embodiment; and Figures 2, 3 and 4 are explanatory graphical figures.

In Figure 1 the various items of equipment are represented schematically and not shown in detail since each is known per se.

Referring to Figure 1, the apparent wind transducer 1 comprises a wind vane 1A feeding its output ct into a cosine potentiometer 1 B providing an output cos a. This is fed to an integrator 1 integrating cos a over a time (chosen at some suitable value between 20 and 60 seconds) and providing an output p cos a dt = cos a. This output constitutes one input to a multiplier 4 which has two inputs.The second input 00 is obtained from a summation amplifier 5 which receives two inputs, namely an input qlr from the normally provided ship's gyro compass 6 and an input 00 which is the course manually set by the navigator in the auto-pilot 7. g0 is thus the course error, i.e. the difference (it can be in either direction) between the course set and the actual course. The gyro compass output 0 is also fed to a differentiator 8, which therefore responds to the ship's rate of turn.

3 is a summation amplifier and gain or attenuation control unit which controls the ship's steering servo-motor 9. It comprises three adjustable gain or attenuation control devices a1, a3 and a4 which provide three respective inputs to a summation amplifier 3A. The input to a1 is the course error the input to a3 is the output from the differentiator 8, and the input to a4 is the output from the multiplier 4. The output S of the amplifier 3A is fed to the rudder-controlling servo-motor 9 of the ship.

Most auto-pilots employ a so-called P.l.D. (position, integral, differential) type of control, the desired rudder angle Areq being given by the equation

where çk = heading angle #0 = desired heading z= integration time a1, a2, a3 are gain settings which are chosen to suit the ship.

Under normal conditions this system of control will ensure that the mean heading of the ship will be the desired heading, and that alterations of the iatter will result in a smooth and reasonably rapid alteration of heading by the ship, with minimal overshoot and subsequent oscillation. The gain a1 is sometimes altered by the ship's officers in order to give a better performance in a rough sea.

In harbours and other restricted waterways when the ship is proceeding slowly and the wind speed may be several times the ship speed, this system of control does not work well and it is normai to use manual steering. Nevertheless vessels often leave the centre line of their desired channel and sometimes run aground because of the leeway caused by the wind.

With the present invention it is desirable to retain the auto-pilot in control of the steering but with a different control equation. The equation is:

where a = angle of apparent wind to ship's head; a1 is the course error gain (this will usually be rather higher than if P.l.D. control was used); and a4 is the wind correction gain (normally negative).

The gains ar, a3 and a4 should in practice be adjustable individually in order to cater for different loading conditions of the ship and depths of water, and recommended values would be calculated from a knowledge of the ship's above- and below-water form and provided for the use of the ship's officers in the form of tables. However, it is possible to provide for automatic control of the gains in some cases in dependence upon the condition of loading and the depth of water. As will be appreciated from equation (2) above, good results may be expected for all wind speed/ship speed ratios and all wind directions, even if either or both change rapidly. However, there will inevitably be some brief transients when the track will depart from that desired when there has been a sudden change in wind stcength or direction.

Equation (2) involves deliberately offsetting the ship's head, usually to windward, in order to maintain a track of the desired direction after allowance for leeway. It is important that there should be no "integral" term in the control equation which would seek to equate ship's heading and desired heading.

The curves of Figure 2 simulate the manoeuvring of a 30,000 D.W.T. tanker, loaded, proceeding on a course of 0000 in a fluctuating wind. The initial speed (UO) is assumed to be 5 knots and the control equation of the auto-pilot is assumed to be:

The uppermost curve in Figure 2 is a curve of the ratio windspeed/UO; the next is a curve of wind direction (True); the next curve (full line) is a curve of actual ship headings; the chain line curve beneath it shows the instantaneous track; the broken line curve crossing the chain line curve shows the track from T = 0; while the bottom curve shows the track offset, to port and to starboard, in cables against nautical miles travelled. It will be seen that, although the ship's head is at one time as much as 120 to windward of the desired heading the instantaneous track "made good" (the term "made good" is here used on the assumption that there is no tidal or other stream or, if there is, that it has already been allowed for in setting the course) is never more than 40 from that desired and the overall track "made good" is correct to within 0.50C.

The reason why these good results are achievable can be seen from a comparison with Figure 3, which is a set of curves for a 30,000 D.W.T. tanker, loaded, in equilibrium with a steady relative wind of 30 knots when the ship revolutions are set for 5 knots. If a ship is steered with only "position" and "differential" terms in its auto-pilot control equation, the curve of leeway angle against apparent wind direction depends significantly upon course error gain, a1. An example is shown in Figure 3. From this can be deduced the gain required at any apparent wind direction to make the leeway angle equal to zero. In Figure 3 the curves show the effects of varying course error gain in the auto-pilot control equation with an integral gain a2 = 0 and a differential gain (yaw rate) a3 = 3.5. Figure 4 is deduced from the curves of Figure 3 and shows the course error gain required to give zero leeway. As will be seen the broken line curve is an approximation to the true one (represented by a dotted curve with crosses) but departs from it for wind angles more than 90 (i.e. wind from abaft the beam). However, as Figure 3 shows, in these conditions the leeway is extremely small if the auto-pilot course error gain is 4 or higher.

The invention may be used with advantage in all types of power vessel with automatic pilot control though, obviously, the more the vessel is liable to leeway, whether by reason of its freeboard windage area or by the necessity to proceed at low speeds in restricted waters, or both, the more advantageous is the invention.

Claims (6)

1. An automatic steering control system for a power-driven ship including an automatic pilot and automatic auxiiiary control means therefor including first means responsive to the apparent wind; second means for integrating or averaging over predetermined intervals of time the response of said first means; third means responsive to the difference between the heading of the ship and the course set in the automatic pilot; and means for combining outputs from said first, second and third means to produce a ship's heading control output for automatically substantially preventing departures from a desired ship's track due to leeway.

2. An automatic steering control system for a power-driven ship including an automatic pilot; an apparent wind transducer producing an output cos a, where a represents the direction of the apparent wind; an integrator or averager to which the output cos a is fed as input and which produces therefrom an integrated or averaged output cos a; a compass device producing an output # where qlr represents the direction of the ship's head: means for producing a course error output #+0 where +0 represents the course set in the automatic pilot; a multiplier having two inputs to one of which is fed said integrated output and to the other of which is fed said course error output; a differentiator fed with said output glr; a summation circuit having three inputs to one of which is fed an input proportional to the output from the multiplier, to another of which is fed an input proportional to the output from the differentiator and to the third of which is fed an input proportional to the course error; and means for utilising the output from said summation circuit to effect automatic auxiliary control of the ship's steering gear so as substantially to prevent departures from a desired ship's track due to leeway.

3. A system as claimed in claim 2 wherein the inputs to the three-input summation circuit are fed thereto through respective individually adjustable gain or attenuation circuits.

4. A system as claimed in any of the preceding claims wherein the integrating or averaging is effected over time intervals of a value chosen between 20 and 60 seconds.

5. A system as claimed in any of the preceding claims wherein the automatic pilot is arranged to give an automatic control of ship's steering which is represented by req and is in accordance with the equation.

where # represents the direction of the ship's head; +0 represents the course set in the automatic pilot; a represents the direction of the apparent wind; and a, a3 and a4 are constants.

6. Automatic steering control systems for power-driven ships substantially as herein described with reference to Figure 1 of the accompanying drawings.