GB2035570A - Electromagnetic compass - Google Patents

Abstract

A navigational instrument having a coil, 1, for rotation in the earth's magnetic field and having means, as peak occurrence detector 5, to indicate when the plane of the coil passes through magnetic north, means 10, 11 to indicate when the plane of the coil is aligned with a reference direction and means 8 for generating a signal representative of the angle between this reference direction and the magnetic north. <IMAGE>

Description

SPECIFICATION Navigational instrument This invention relates to navigation instruments.

Atthe present time two instruments, namely the magnetic compass and the gyroscope form the basis of most navigational installations and have done for many years. The conventional magnetic compass suffers from a number of disadvantages in particular it has poor readability, it has a slow response speed due to the fact that its movement must be damped in order to prevent oscillation, its accuracy can be affected by extraneous metal objects and it is unsuitable for use and adaption in a completely electronic navigational system. Furthermore the gyroscope suffers from the disadvantages of being slow and energy consumptive during starting up, bulky and extremely expensive to manufacture.

An object of the present invention is to provide a navigational instrument which obviates or mitigates the aforementioned disadvantages.

According to the present invention there is provided a navigational instrument comprising a coil adapted to be rotated in the earth's magnetic field, first means associated with said coil for producing a first signal when the plane of the coil coincides with the direction of the lines of action of said magnetic field, second means associated with said coil for producing a second signal when the plane of the coil coincides with a reference direction, and output means for comparing the recurrence of said first and second signals and producing therefrom an output signal representative of the angle between said direction of the lines of action said magnetic field and said reference direction.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is an electrical block-diagram of a navigational instrument according to the invention; Fig. 2 is an electrical circuit diagram of the instrument shown in Fig. 1: Fig. 3 is a circuit diagram showing the peak detector in Fig. 2 and showing schematic oscillographs of the input and output signals of the detector; Fig. 4 is a circuit diagram of an alternative form of peak detector; Fig. 5 is an isometric view of the working components of an instrument according to the invention; Figs. 6a and 6b are views showing the display from the instrument according to Fig. 5 and with the system at two different headings; and Fig. 7 is an isometric view of part of an instrument according to the invention.

Referring now to the drawings, the components of a navigational instrument in the form of a compass are shown in Fig. 1 and in Fig. 5 and comprise a magnetic field transducer in the form of a circular coil 1 formed of 3,500 turns of 26 SWG wire and having a mean diameter of 55 mm. The coil 1 is mounted on a shaft 2 adapted to be rotated by a constant speed motor 3 at a speed of around 300 r.p.m. and in a clockwise direction when viewed from above. As is well known, the earth has a magnetic field the direction of which will hereinafter be referred to as the north-south axis of the field.

This north-south axis is commonly used at the present time in standard magnetic compasses for navigation. Thus, using the well known dynamo effect the rotation of the coil 1 within the earth's magnetic field provides a sinusoidal voltage output from the coil, the amplitude of which is a maximum when a vertical plane through the coil 1 is aligned along the north-south axis of the earth's magnetic field. As shown in Fig. 1, the signal from the coil 1 is fed to an amplifier 4 and thereafter to a peak value detector 5.

The input to the peakvalue detector 5 is a sinusoidal wave 6 and the output is a lineal signal with a pulse 7 corresponding in time to the peak value of the signal 6. The pulse 7 is then fed to the "set" terminal of a reset-set flipflop 8.

The electronic components of the compass are mounted on a disc 9 secured to the shaft 2 and rotating therewith and a stationary light source 10 is mounted on the housing of the compass (not shown) which in turn is firmly secured to the ship or vehicle in which the compass is mounted and the heading of which is required relative to the earth's magnetic field. As can be seen in Fig. 5 the light source 10 is located adjacent the perimeter of the disc 9 and a photo-sensitive detector 11 is provided on the rotated ing disc.Thus, if the light source 10 is provided on a housing in a position corresponding to the normal direction of travel of the ship or vehicle and the photo-sensitive detector is provided on the disc at a position in the vertical plane through the coil 1, as the disc rotates the photo-sensitive detector 11 will produce a signal 12 having a peak 13 at the point in time when the vertical plane through the coil coincides with the light source 10. The signal 12 is then passed through a monostable multivibrator so as to produce a lineal signal with a pulse 14 which occurs at the point in time when the vertical plane of the coil coincides with the light source 10 which pulse is then fed to the reset terminal of the flipflop 8.The output signal from the flipflop 8 is a stepped signal 15 in which the time elapse T between the step up and the step down corresponds to the time elapse between the pulse 7 and the pulse 14 being fed to the respective inputs of the flipflop 8, i.e. T corresponds to the time elapse between the vertical plane of the coil 1 passing the magnetic north of the earth's magnetic field and the vertical plane of the coil 1 as represented by the photo-sensitive detector 11 passing the light source 10. The signal 15 from the flipflop 8 in the embodiment shown in Figs. 1 and 5 is passed to a display light source in the form of a light emitting diode 16 mounted at one end of a display arm 17 which is in turn secured to the shaft 2 and rotates therewith.The display arm 17 extends in the vertical plane through the coil 1 and the signal 15 is arranged to illuminate the light source 16 during the time period T, i.e. during the time period between a vertical plane of the coil passing through the magne tic north direction and until the vertical plane passes the light source 10, the length of which time period depends on the heading of the ship or vehicle rela tive to magnetic north.

The display arm 17 is preferably mounted below a translucent display disc 18 as shown in Figs. 6a and 6b and which is segmented anticlockwise into 3600.

The effect of the light source 16 being illuminated for the time period T, which is illustrated as an arc of a circle in Figs. 6a and 6b with the speed of rotation of the arm is to provide an illuminated arc 19 which terminates on the scale at the heading, measured in degrees, of the ship or vehicle. Fig. 6a shows ahead- ing of 40 and Fig. Sb shows a heading of 210 rela tiveto magnetic north.

Atypical circuit diagram for the electronics of the compass is shown in Fig. 2 from which it can be seen that the amplifier 4 in which the signal from the coil 1 is amplified, consists of a two-stage amplifier formed by two operational amplifiers IC1. The com ponentvalues chosen in the amplifier4 give a total stage gain of 1000. The output from the amplifier 4 is a.c. coupled by a capacitor C3 to the peak value detector 5 which has as its main component an integrated circuit IC2. The peak value detector 5 functions by slightly delaying the signal atthe noninverted input by means of the combination of the resistor R8 and the capacitor C4.As the circuit IC2 is wired as a voltage comparator its output will be as shown at 20 in Fig. 3 being a detailed diagram of the circuitry of the peak value detector 5. An alternative design for the peak value detector 5 is shown in Fig.

4. The square wave output 20 from the peak value detector 5 is inverted by a transistor Q1 and the leading negative going edge is differentiated by capacitor C5 and resistor R13 to provide the short negative going pulse required to set the flipflop 8 formed by an integrated circuit designated by IC3.

The flipflop 8 consists of a 555 timer wired in the monostable mode and having a period of 300 m.s.

Although connected in the monostable configuration the period of the reset signal which is described hereafter leads to operation as a reset-set flipflop.

A photo-sensitive detector 11 is formed by a single integrated circuit designated by IC4 (15 volt light activated switch) whilst the light source 10 is avis- ible green light emitting diode (LED 1). The 15 volt light activated switch consists of a combined diode and monolithic integrated circuit. One external resistor and capacitor (designated R14 and C6) sets the light switch in threshold level, and when the normally incident light is greater than the threshold level the device output switches from low to high.

The device is contained in a TO-18 case with an end window. The output from the photo-sensitive detector 11 is a square wave which after inversion by a transistor Q2 has its leading edge differentiated by a resistorR18 and capacitor C7 to provide a suitable reset pulse for the flipflop 8. With the integrated circuit IC3 set for a monostable period of 300 m.s. it can be both set and reset within the 200 m.s. required for a single rotation of the transducer coil 1. A higher speed of rotation can be chosen (with a consequent increase in the output voltage of the transducer coil 1), however, the monostable period of the 555 timer (IC3) may require alteration since the component values indicated in the circuit diagram will tolerate only a maximum increase of 50 per cent in the speed of rotation.

The output of the flipflop 8 controls the light emitting diode LED2 which forms the display light 16.

This diode may either be used directly as the display device as previously described or alternatively it can be used as shown in Fig. 7 mounted directly on the shaft 2 to transmit the compass information to a stationary photo-sensitive device 21 which in turn can feed the information to other sources externally of the compass. In this case the magnetic compass heading can be computed easily using the following formula: + = -x 360 T wherein:-T = timefora single rotation ofthefield transducer (i.e. the period of the sinusoidal wave form; t = time for which the display light source is illuminated; and (p = the magnetic compass heading (in degrees).

By use of the calculation shown above the compass heading can be calculated directly by a micro processor based system. In addition it can check the validity of calculated data and assume direct control of the display system. Given a basic micro processor based control unit the possibilities of constructing a centralised marine instrumentation system is made easy. By expansion of the program memory capacity such a system could provide (a) calculation and display of digital information from the compass (as described), echo sounder and other peripheral units, (b) make logical decisions on the data obtained above, for example, in the control of an auto pilot system using compass data, (c) control of all timekeeping-functions-displays of current time and its comparison with set alarm times, and (d) given a peripheral for determining the true speed of a vessel, a processor could use in conjunction with the compass and time information to compute ship positions.

It should be emphasised that the system outlined above would differ very little from that proposed for the control and display of compass information only.

Improvements or modifications may be incorporated without departing from the scope of the invention.

Claims (6)

1. A navigational instrument comprising a coil adapted to be rotated in the earth's magnetic field, first means associated with said coil for producing a first signal when the plane of the coil coincides with the direction of the lines of action of said magnetic field, second means associated with said coil for producing a second signal when the plane of the coil coincides with a reference direction, and output means for comparing the recurrence of said first and second signals and producing therefrom an output signal representative of the angle between said direction of the lines of action said magnetic field and said reference direction.

2. An instrument as claimed in Claim 1, wherein said first means includes an amplifier and a peak amplitude detector.

3. An instrument as claimed in Claim 1 or Claim 2, wherein said second means includes a fixed light source and a photo-sensitive detector adapted to rotate with said coil.

4. An instrument as claimed in any preceding claim, wherein said output means includes a reset/set flipflop.

5. An instrument as claimed in any preceding claim, and including display means for displaying said output signal.

6. A navigational instrument substantially as hereinbefore described with reference to the accompanying drawings.