GB2090662A - Navigational equipment for a vehicle - Google Patents

Abstract

The navigational equipment is based on a magnetic compass which comprises an inductive loop (10) rotated continuously in the earth's magnetic field to generate a sinusoidal voltage waveform and an angular position transducer (30 to 40) and a phase comparator for determining the phase of the sinusoidal waveform relative to the output of the angular position transducer to indicate the direction of travel of a vehicle relative to magnetic north. <IMAGE>

Description

SPECIFICATION Navigational equipment for a vehicle The present invention relates to vehicle navigation and seeks in particular to provide equipment by means of which the position of a vehicle may be constantly plotted. The plotted position may either be used to indicate to the vehicle operator his location or may be relayed by a telecommunication link to a central location for example to enable a fleet operator to determine automatically the location of all the vehicles in the fleet.

It has previously been proposed to sense by means of a compass the direction of movement and to measure from the wheel axis the distance of movement and in this manner compute on board a vehicle the distance and direction travelled from a given reference point. A main difficulty in embodying such a system is that any inaccuracies are cumulative that is to say they are integrated over the period of time during which the system is in operation and this places a serious limitation on the viabiiity of such a system.

With a view to mitigating the above disadvantage, the present invention provides a magnetic compass which comprises an inductive loop, means for rotating the inductive loop in the earth's magnetic field thereby generating a sinusoidal voltage waveform, an angular position transducer for producing a second waveform indicative of the angular position of the inductive loop and phase comparator for determining the phase of the sinusiodal waveform relative to the output signal of the angular position transducer, the said phase being indicative of the direction of travel of the vehicle relative to magnetic north.

The usual compass comprising a pivoted magnet has a torque applied to it by the earth's magnetic field, the torque approaching zero when the magnet points towards magnetic north. To obtain the most accurate reading one requires a totally friction less magnet but this would be impracticable in a vehicle where extensive damping is required to stabilise the motion of the magnet. The conventional compass therefore inherently suffers from severe limitations when attempting to obtain a reading accurate to a few degrees. By contrast, the present invention enables the phase to be determined by the point where the voltage crosses through zero, that is to say at the point of maximum slope so that the reading is inherently more accurate and furthermore readings can be averaged over several cycles enabling an accuracy of one degree to be achieved with relative ease.

Conveniently, the output of the compass is fed together with data derived from the odometer or other distance measuring equipment to a micrd computer serving to determine the position and bearing of the vehicle relative to a starting position.

The micro computer may either be used to control the position of a marker on a map or the output data can be transmitted by means of a radio to a central location.

If desired, an attitude detector may be provided in the vehicle to determine the slope of the vehicle relative to the horizontal, the said information being provided to the micro computer to enable errors resulting from magnetic declination to be compensated. In the latter case, vehicle acceleration may result in incorrect readings of the vehicle inclination but such acceleration may be detected by second order differentiation of the distance readings and the inclination of the vehicle can be modified by the determined value of vehicle acceleration prior to being used in computing the error caused by the magnetic declination.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which, Figure 1 is a side view of a compass, Figure 2 is a plan view from above of the compass in Figure 1.

Figure 3 is a diagramatic front view with an electrical circuit shown in block diagram form, and Figure 4 is a graph illustrating the signals which occur in the compass.

As shown in Figure 1,the compass comprises an inductive loop in the form of a search coil 10 mounted on a verticai shaft which is rotated by an electric motor 14 through a sleeve coupling 16, the motor 14 being mounted on the lower limb of a U-shaped bracket 18. As will be seen, the shaft 12 passes through a suitable bearing 20 in the upper limb of the bracket 18.

Rotation of the coil 10 by the motor 14 will induce an EMF in the coil by its interaction with the earth's magnetic field. The voltage thus produced is fed to an electronic measuring and display circuit shown in Figure 3 by means of sliprings 22 and 24 which cooperate with brushes 26 and 28 (see Figure 2).

Between the two limbs of the U-shaped bracket 18, the shaft 12 carries a transparent disc 30 which is angularly graduated around its perimeter and carries a pointer mark 32 at a reduced radius. The disc 30 is so mounted on the shaft 12 that the pointer mark 32 is aligned with the horizontal axis of the search coil 10.

An optical system comprising a light emitter 34 and a light detector 36 is mounted on a part 38 carried by the bracket 18, the optical system being arranged to intersect the graduation on the disc as it rotates to produce a train of calibration pulses at the detector 36. A second optical system comprising a light emitter 40 and a light detector 42 is also arranged on the part 38 to intersect the pointer mark 32 on the disc 30 as the latter rotates to produce a single pointer pulse at the detector 42 for each revolution ofthe search coil 10.

It will be appreciated that the disc 30 and the associated light emitters and light detectors constitute an angular position encoder and any alternative angular position encoder can be used. In particular, it is preferred in some applications to simplify the encoder by providing a single marking and to omit the graduated scale, the angular position being determined instead electronically by dividing the time interval between the instants that the pointer mark 32 passes through the light detector 42.

Figure 4 shows the nature of, and the relationship between, the three electronic outputs generated by the assembly illustrated in Figures 1 and 2. As will be seen, the EMF generated by the search coil 10 is a sinusiodal curve 44, the value of which is zero when the horizontal axis of the search coil 10 is aligned with the earth's magnetic field and at a maximum when at right angles to the said field. The output of the light detector 42 is a single pulse 46 for each revolution whose displacement in time from the minimum value of the sinusiodal curve 44 serves to indicate the angular dispacement position of the optical system associated with the pointer mark 32 and the earth's magnetic field. The calibration pulses 48 are those produced by the detector 36.These can be counted between the negative transitional zero value of the sinusiodal curve 44 and the pulse 46 to calibrate the bearing.

Figure 3 illustrates the way in which the magnetic bearing given by the compass is measured. The rotating search coil 10 produces and EMF which is conveyed to an amplifier 50 via sliprings 22 and 24 and the brushes 26 and 28. After amplification, the signal is passed to a zero crossing detector 52 where the negative transitional zero value of the pulse is used to open a signal gate 54. Calibration pulses from the light detector 36 are similarly amplified by an amplifier 56 and passed through the signal gate 54to a pulse counter 58. The pointer pulse generated at the light detector42 is amplified in an amplifier 50 and is also passed through the signal gate 54 to pre vent further calibration pulses reaching the pulse counter 58. The contents of the counter 58 are then displayed on a digital display unit 62 to disclose the bearing given by the compass.

In addition to providing a display of the bearing of the vehicle to the driver, the output signal ofthe compass is fed to a micro computer (not shown) which is also connected to receive signals from an odometer so that it may computer the distance and direction travelled from a given reference point. This information can be used to plot the position of the driver on a map provided in the vehicle. The computer in moving the marker relative to the map additionally requires an indication of the scale of the map and this may either be entered by means of a key board or by manually setting a divider train associated with the odometer.

Additionally, the position of the vehicle on a map can be relayed by the computer incode over a radio link to transmit the location to a central monitoring station.

Despite the improved activity of the compass of the present invention, it may still be possible for a cumulative error to arise if the system is operated for a substantial time without the calibration. It is thus possible additionally to provide radio transmitters along a given route having a very short range and arranged to reset the stored data in the computer relating to the actual position of the vehicle when such a radio transmission is received. It is also poss ible for a central monitoring station to reset the computer remotely whenever the actual position of the vehicle is known and differs from that stored in the micro computer.

One possible source of errors is that a vehicle on an incline might derive an inaccurate reading from the compass on account of magnetic declination. It is conceivable that the micro computer may effect a correction of an indication of the inclination of the vehicle is also supplied to the micro computer. Such an indication can be derived from an inclinometer operating for example on the principle of a pendulum or of the level of liquid in a U-tube. Any inaccuracies in the reading of the inclinometer as a result of acceleration or deceleration of the vehicle can be predicted by second order differentiation by the computer of the distance travelled.

Claims (8)

1. A magnetic compass comprising an inductive loop, means for rotating the inductive loop in the earth's magnetic field thereby generating a sinusoidal voltage waveform, an angular position transducer for producing a second waveform indicative of the angular position of the inductive loop and a phase comparator for determining the phase of the sinusiodal waveform relative to the output signal of the angular position transducer, the said phase being indicative of the direction of travel of the vehicle relative to magnetic north.

2. A navigation system comprising a magnetic compass as claimed in claim 1, means for measuring the distance travelled by the vehicle and a micro computer connected to receive data from the com pass and the distance measuring means operative to determine the position and bearing of the vehicle relative to a starting position.

3. A navigation system as claimed in claim 2 additionally comprising a map or position indicator underthe control of the said micro computer.

4. A navigation system as claimed in claim 2 or 3 further comprising a radio transmittor/receiver arranged to communicate the vehicle location to a remote monitoring station.

5. A navigation system as claimed in any of claims 2 to 5 wherein the micro computer comprises an additional reset facility whereby the actual position of the vehicle may be entered into the computor by the radio transmittor/receiver.

6. A navigation system as claimed in any of claims 2 to 6 additionally comprising means for determining the inclination of the vehicle connected to the computer, the computer being operative to apply a correction to the bearing of magnetic north to compensate for errors caused by magnetic declination.

7. A navigation system as claimed in claim 7, wherein the computer is operative additionally to determine the vehicle acceleration or deceleration by second order differentiation of the distance travelled and to apply a correction to the inclination reading prior to computing the error resulting from magnetic declination.

8. A navigation system comprising a compass as hereinbefore described with reference to and as illustrated in the accompanying drawings.