GB2163258A

GB2163258A - Dead reckoning log for ships

Info

Publication number: GB2163258A
Application number: GB08420931A
Authority: GB
Inventors: Michael Bryan
Original assignee: Individual
Current assignee: Individual
Priority date: 1984-08-17
Filing date: 1984-08-17
Publication date: 1986-02-19
Also published as: GB8420931D0; GB2163258B

Abstract

Navigational apparatus for keeping track of the position of a vessel at sea which is particularly useful for small boats. A compass means 1 provides data in the form of an electrical signal, indicating the vessel's bearing. Speed measurement means 3, such as a trailed impellor, provides data in the form of an electrical signal, indicating the vessel's speed through the water. Tide data entry means allows data to be entered, in the form of an electrical signal, indicating the speed 8 and direction 9 (cos and sin outputs) of the tide. Computing means accepts the data and calculates the distance travelled from the starting point. indicating means 17 displays the position of the vessel and, preferably, the total distance travelled. The computing means may be analogue, digital or hybrid, and as shown includes multipliers 5, 6 acting on the sin and cos of the bearing signal, and the speed signal; and adders 10 and 11 for tide correction. <IMAGE>

Description

SPECIFICATION Dead reckoning log for ships This invention relates to apparatus which provides the position of a vessel at sea.

Navigation at sea may be accomplished by a variety of methods. Some methods require considerable skill and experience on the part of the navigator to provide accurate results.

Others are simple to operate but require very complex and expensive equipment. Such equipment is usually beyond the reach of a small boat owner. For example, a two berth 6 metre yacht will carry a simple magnetic compass, a 'log' for measuring speed and distance travelled through the water, a chronometer, charts of the area being sailed and a set of tide tables. This basic equipment may be supplemented with a depth sounder, radio direction finder and a sextant. To calculate the position of a ship using this equipment requires a trained navigator and absence from distraction. In practice the navigator will carry out many other tasks on a small ship which accounts, in part, for the considerable number of small ships who loose their way at sea.

Sometimes poor navigation leads to loss of life.

Radio methods of direction finding exist such as Oecca Navigator, Loran C and satellite navigation. These methods are extremely accurate but the equipment involved is expensive and requires a substantial supply of electricity to operate it.

At the present time there is no simple and inexpensive equipment available which would enabie a comparatively unskilled small boat owner either to know his position at sea or which keeps track of his vessel at all times.

According to the present invention there is provided apparatus for keeping track of the position of a vessel at sea comprising compass means providing data, as an electrical signal, indicating the vessel's bearing; speed measurement means providing data, as an electrical signal, indicating the vessel's speed through the water; tide data entry means allowing data to be entered, as an electrical signal, indicating the speed and direction of the tide; computing means adapted to accept the electrical signals and to provide data indicating the position of the vessel and indicating means for displaying the data provided by the computing means.

Preferably the position of the vessel is indicated as the distance travelled along the NORTH/SOUTH and EAST/WEST axes from the vessel's starting position. Alternatively the position of the vessel can be indicated by its coordinates of latitude and longditude.

Any type of compass, magnetic or gyroscopic, which is capable of providing electrical signals indicating the vessel's bearing may be used as the compass means. The bearing signal may consist of the vessel's bearing in analogue or digital form. Preferably the bearing signal is in the form of two components proportional to the sine and cosine of the bearing respectively. Due to their lower cost and lower power consumption magnetic compasses are preferred.

The magnetic compass may be a direct indicating type, where a pivotted magnet aligns itself with the earth's magnetic field, or the fluxgate type where the position of the earth's magnetic field is sensed directly. Fluxgate compasses provide bearing information in the form of analogue electrical signals proportional to the sine and cosine of the bearing angle. While these signals may conveniently be used directly for computing the vessel's position the sensitivity and output stability of commercial models is poor. In the event of electricity supply failure it is not possible to observe the vessel's bearing from a pure fluxgate compass.

In the most preferred embodiment the compass means consists of a conventional pivotted magnet compass with sensing means to sense the bearing direction shown by the magnet and convert it into an electrical signal in analogue or digital form. The sensing means may be an optical position sensor, a needle following servo mechanism, a Hall effect, digitally encoded or fluxgate sensor.

In the preferred embodiment the fluxgate sensor is fixed to the upper, viewing, surface of a conventional grid steering compass.

Movement of the surface with respect to the needle plane allows corrections for magnetic variation to be set. Similarly corrections may be entered for 'leeway' ie the deviation caused by the wind.

The fluxgate sensor consists of a set of coils wound on a toroidal ferrite former with a suitable energising and detection system which provides signals whose voltage is proportional to the sine and cosine of the needle's position. A fluxgate compass of this type but using the earth's magnetic field is described in Wireless World, October 1 982.

By using appropriate circuit arrangements it is possible to compensate for any errors caused by variations in the ambient temperature. A system of this nature can provide bearing information having an accuracy of 1 degree.

A pivotted magnet compass with a fitted fluxgate position sensor, as described above, is most easily installed in the vessel. Furthermore it remains possible to view the needle and visually assess the vessel's direction at all times. In dire emergencies the compass may be used to steer the vessel.

Various methods are available for measuring the speed of a vessel through the water the simplest is the so-called 'paddle wheel' type log. This device consists of a small propellor which is fitted in the hull or trailed behind the vessel. The rotational speed of the propellor is proportional to its speed through the water. The rotation may easily be sensed by optical, magnetic or other means. A common commercially available speed measurement device consists of a small paravane carrying a propellor towed by the vessel. The propellor has a small magnet attached to its rotating shaft which operates a reed switch inside the paravane. The switching signal is sensed in the towing vessel through a cable which also acts as the towing cable.The frequency of the switching pulses is proportional to the vessel's speed and their integral, normally displayed on a counter, represents the distance travelled by the vessel.

When such speed measuring means are fitted into the hull of a vessel corrections are necessary to allow for errors caused by its position. Ultrasonic sensors may also be fitted to the hull of a vessel to sense its speed through the water by means of the Doppler effect. Such devices are only satisfactory on metal and resin bonded fibreglass vessels as wood hulls are too absorbent of ultrasonic radiation.

When calculating the actual position of the vessel the course of the vessel and the drift caused by the tide must be taken into account. Accordingly it is the custom for navigators to use tide tables published by the Admiralty to find out the speed and direction of the tide in their locality.

In the preferred form of the present invention the tide speed, obtained from tables, is entered as by means of a manually rotated knob. For example an a.c or d.c. voltage proportional to the speed can be obtained using a potentiometer supplied with a stable a.c. or d.c. voltage source. The direction of the tide is preferably entered on a second manually rotated knob having 360 degrees rotation. This second knob preferably provides two a.c. or d.c. voltage outputs proportional to the sine and cosine of its shaft position respectively. A.c. or d.c. voltages of this nature can be obtained using a suitably energised sine/ cosine potentiometer or, in the case of a.c. signals, a synchro. Alternatively the speed and/or direction of the tide may be entered using a knob attached to a shaft carrying encoding means for indicating the shaft's angular position in digital form.Push button entry of data may be used but this is less convenient on small boats.

The computing means can be analogue, digital or a hybrid system. The choice of computing means chosen will depend upon the type of electrical data signals available from the compass means, speed measurement means and tide data entry means. When these signals are in analogue form it is preferable to compute with them in this form rather than include analogue to digital converters.

Both analogue and digital computing systems are well known.

In essence the sine and cosine of the vessel's bearing angles are each multiplied by the speed to obtain the NORTH/SOUTH and EAST/WEST velocities. To these velocities are added the tide velocities and the sums of the velocities are integrated with respect to time to provide the distance travelled in the NORTH/SOUTH and EAST/WEST directions.

If the electrical signals are available in digitally encoded form it is possible to supply them to a microprocessor interface. With appropriate programming the values may be polled and computed to provide the necessary distance information in digital form.

In the preferred form of the invention the bearing signals consist of the sine and the cosine of.the vessel's bearing in voltage analogue form. The speed signal consists of a series of pulses whose repetition rate is the analogue of the speed. The tide direction and speed signals consist of the sine and cosine of the tide's direction mulitplied by its speed manually entered after consulting tables.

The speed signal is converted to a binary coded decimal signal in a conventional converter circuit. Such circuits are well known and available in integrated circuit form. This signal can be multiplied by the sine and the cosine bearing signals in a muliplier circuit; such circuits are well known. It is possible to multiply a digitally encoded signal by an analogue signal using a comercially available integrated circuit.

The output from the computing means is displayed on indicating means which allow the navigator to see the distance travelled by the vessel. The indicating means may be needle pointer meters but due to the poor accuracy with which they may be read they are not satisfactory. So called 'digital' displays which show information in the form of numeric characters are preferred. These devices can be self illuminating using light emitting diodes or incandescent filaments. The preferred form of numeric indicator is the liquid crystal display (LCD) which requires very little electrical energy to operate it.

In general a four and a half figure display (1 plus four digits) is found most satisfactory for the log of the invention. It is possible to have a series of numeric displays showing the NORTH/SOUTH and EAST/WEST distances travelled simultaneously together with the total distance travelled and the speed. In most cases it is sufficient to have a single display and switch from one source of data to another. It is convenient to use the same display to show the vessel's speed through the water and, if the computing means is capable of deriving the vessel's acceleration from the speed data, its acceleration.

The operation of the apparatus according to the invention will now be described with reference to the accompanying drawings in which: Figure 1 shows the apparatus in block dia gram form, Figures 2 and 3 show the magnetic compass used to measure the heading of the vessel and its correction for magnetic variation and leeway, and Figure 4 shows the layout of the display system.

Compass means consisting of a grid steering compass modified by the addition of a fluxgate type magnetic sensor 2 attached to the moving grid of the compass. The magnetic sensor 2 provides two electrical signals whose amplitudes are proportional to the sine and cosine respectively of the vessel's true heading.

Speed measurement means in the form of a 'paddle wheel' type motion sensor 3 provides an electrical output consisting of a series of pulses whose repetition rate is proportional to the speed of the vessel through the water.

The pulses from the paddle wheel 3 are fed to a frequency digitiser 4 which provides a digital output proportional to the speed of the vessel.

The sine and cosine signals from the compass sensor 2 are each multiplied by the speed signal from the converter 4 in analogue multiplier units 5 and 6. The amplitude of the outputs from the multipliers 5 and 6 are proportional to the velocity of the vessel in the NORTH/SOUTH and EAST/WEST directions respectively due to its movement in the water.

Tide data entry means consists of two potentiometers 8 and 9. The linear law track potentiometer 8 is supplied from a constant voltage source 7 and provides an output voltage directly proportional to its shaft setting.

The shaft is set manually to a value for the local tide velocity found from published tables. The output voltage from the potentiometer 8 is fed to the sine/cosine potentiometer 9. The potentiometer 9 provides two voltage outputs whose amplitudes are proportional to the sine and cosine of the shaft setting respectively. The shaft is set manually to a value for the local tide direction found from published tables. The combined settings of the two potentiometers 8 and 9 provide voltage outputs whose amplitudes are proportional to the velocity of the tide in the NORTH/SOUTH and EAST/WEST directions respectively.

The velocity signals proportional to the vessels motion and those proportional to the tide's motion are added together in analogue adding units 10 and 11. The two adding units 10 and 11 provide voltage outputs whose amplitude is proportional to the true surface velocity of the vessel in the NORTH/ SOUTH and EAST/WEST directions respectively. These voltage outputs are fed to two voltage to frequency converter units 1 2 and 1 3. The units 1 2 and 1 3 provide output signals in the form of electrical pulses whose rate is proportional to the vessel's true surface velocity in the NORTH/SOUTH and EAST/WEST directions respectively.The pulses from the converters 1 2 and 1 3 are fed to two counter units 14 and 1 5. The count recorded by each of these units will be the integral of the velocity signal and thus represent the true distance travelled by the vessel in the NORTH/SOUTH and EAST/WEST direction.

The outputs from the counter units 14 and 1 5 are in digitally encoded form and are supplied to a data selector unit 1 6. The digitally encoded output from the frequency digitiser converter 4 is also supplied to the data selector unit 1 6. By means of a switch the contents of any of the three digital inputs to unit i6can be displayed by a display unit 17. According to the switch setting either the distance travelled on either of the two axes or the speed of the vessel through the water will be displayed. Normally the speed will be displayed and the distance travelled on either axis is selected by a push button.

The counter units 14 and 1 5 are caused to count upwards or downwards according to the sign of the voltage from the adding units 10 and 11. After a zero count is reached the count increases again and an indicator lamp shows the sign of the count. In practice one of a pair of lamps for each counter unit 14 and 1 5 will be illuminated and show whether the vessel has travelled north or south on one axis and east or west on the other from its starting point. 3The preferred magnetic compass, see figures 2 and 3, is based on a grid steering compass such as a Type 33 made by Silva of Sweden. The body of the compass 1 is mounted on a vessel in the conventional manner along its fore and aft centre line 2.

The housing for the pivotted magnet 3 is mounted on gimbals 4. The top of the housing carries a rotatable grid 5 which carries angular graduations which can be aligned with a reference mark 6 on the body of the compass 1. A fluxgate sensor is fixed centrally over the centre of rotation of the pivotted magnet 3 within the area denoted by the circle 7. It is aligned with the zero datum line 8 on the rotatable grid 5 so that it will sense the difference between the vessel's heading and magnetic north. In figure 2 the rotatable grid 5 is shown in its zero position.

When the grid 5, see figure 3, is rotated it will also rotate the axis of the fluxgate sensor attached to it. By this means it is possible to offset the datum line for the fluxgate sensor to allow for local magnetic variation. This information is normally present on navigation charts; as illustrated the grid has been offset 10 degrees West.

It is also possible to allow for the vessel's leeway by moving the grid 5 so that the angle of leeway is added algebraically to the correction for magnetic variation.

The display means, see Figure 4, shows the vessels position with respect to its starting position on the four and a half digit display 1.

For this reason it is necessary to show not only the distance travelled along the two axes but also the direction i.e. northwards or southwards. These directions are shown by luminous indicator lamps 3,4,5 and 6 preferably LED types, placed on the four corners of the numeric display as shown. The direction indicator lamps are selected by the circuits associated with the counter units 14 and 1 5 described above.

Claims

1. Apparatus for keeping track of the position of a vessel at sea comprising compass means providing data, as an electrical signal, indicating the vessel's bearing,; speed measurement means providing data, as an electrical signal, indicating the vessel's speed through the water; tide data entry means allowing data to be entered, as an electrical signal, indicating the speed and direction of the tide; computing means adapted to accept the data as electrical signals and to provide data indicating the position of the vessel and indicating means for displaying the data provided by the computing means.

2. Apparatus as claimed in claim 1 in which the compass means is a magnetic compass.

3. Apparatus as claimed in claim 2 in which the magnetic compass is of the suspended magnet type and carries sensing means to provide electrical signals indicating the magnet position.

4. Apparatus as claimed in claim 3 in which the sensing means is a fluxgate sensor.

5. Apparatus as claimed in claim 3 in which the sensing means is located on a transparent cover over the compass needle and the cover may be rotated to allow modification of the bearing data.

6. Apparatus as claimed in any of the preceding claims in which the tide data entry means comprises two rotatable knobs, one having 360 degrees rotation, for entering the tide direction and speed.

7. Apparatus as claimed in any of the preceding claims in which the tide data entry means comprises a linear law track potentiometer for entering the tide speed and a sinecosine potentiometer for entering the tide direction.

8. Apparatus as claimed in any of the claims 1 to 6 in which the tide data entry means includes a synchro for entering the tide direction.

9. Apparatus as claimed in any of the claims 1 to 6 in which the tide data entry means includes at least one rotary shaft carry ing encoding means for indicating the shaft's position in digital form.

10. Apparatus as claimed in any of the preceding claims in which the speed measure ment means comprises a propellor travelling in the water at the same speed as the vessel and providing electrical signals related to the rate of rotation of the propellor shaft.

11. Apparatus as claimed in any of the claims 1 to 9 in which the speed measurement means comprises an ultrasonic sensor which senses the vessel's speed through the water by means of the Doppler effect.

12. Apparatus as claimed in any of the preceding claims in which the computing means includes a frequency converter to provide an output, in analogue or digital form, proportional to the speed of the vessel through the water.

1 3. Apparatus as claimed in claim 1 2 in which the frequency converter provides an output in digitally encoded form proportional to the speed of the vessel through the water.

14. Apparatus as claimed in any of the preceding claims in which the indicating means displays data in numeric form together with an indicator showing the direction axis from the start point.

1 5. Apparatus as claimed in any of the preceding claims in which the indicating means is adapted to show simultaneously or selectively the distance travelled from the vessel's start point in the NORTH/SOUTH axis, the EAST/WEST axis.

1 6. Apparatus as claimed in any of the claims 1 to 14 in which the indicating means is adapted to show simultaneously or selectively the latitude and the longditude of the vessel's position.

1 7. Apparatus as claimed in any of the preceding claims in which the indicating means is adapted to show the vessel's speed through the water.

1 8. Apparatus as claimed in any of the preceding claims in which the indicating means is adapted to show the total distance travelled by the vessel from its start point.

1 9. Apparatus as claimed in any of the preceding claims in which the computing means includes means to derive the vessel's acceleration from the speed data and the indicating means is adapted to show the vessel,s acceleration through the water.

20. Apparatus as claimed in any of the preceding claims in which the computing means is an analogue, digital or hybrid computer.

21. Apparatus as claimed in claim 20 in which the computing means is a microprocessor.

22. Apparatus as claimed in claim 1 and as herein described with reference to the accom panying drawings.