GB2221591A

GB2221591A - Optical vehicle speedometer

Info

Publication number: GB2221591A
Application number: GB8818729A
Authority: GB
Inventors: Duncan Peter Rowe
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1988-08-06
Filing date: 1988-08-06
Publication date: 1990-02-07
Also published as: GB8818729D0

Abstract

A vehicle speedometer comprises a laser 4 directing a beam onto the ground and detecting the Doppler frequency shift in the return, with reference, either Fig 2 to a second beam also directed onto the ground but in a direction to have an opposite sense Doppler shift or Fig 3 to part of the beam not directed and not Doppler shifted. <IMAGE>

Description

Velocity Sensing Apparatus This invention relates to velocity sensing apparatus.

It arose in consideration of the design of a speedometer for a motor vehicle. It could however be applicable in other situations for example for measuring the forward and/or sideways movement of other vehicles such as hovercraft or aircraft. It could also be used for measuring the velocity of certain mechanical components such as conveyers.

Previously speedometers have consisted of mechanical clocking devices which have been used to calculate the speed of a vehicle. This has been performed by utilising the number of revolutions of a wheel or gearing mechanism, linked directly or indirectly to the engine or the wheel itself.

An alternative method of providing an indication of the speed of a vehicle has been to attach a dynamo-like arrangement onto a moving portion of the vehicle, or a gearing mechanism, and to construct this so that a small EMF is generated. This EMF has then been related to the speed of the vehicle so as to give an indication of its speed. Both these previous techniques have given rise to non-linearities. These readings were therefore. only accurate over a small speed range; have often required implementation of complex secondary equipment and have in fact not given a true indication of the speed of the vehicle, due to the fact that slippage of tyres on the road often occurs.

This invention provides velocity sensing apparatus comprising means for generating electromagnetic radiation and directing it onto a surface and detecting means for detecting a Doppler shift in the radiation after reflection from the surface.

It is believed that by employing the invention it is possible to remove the problem of non-linearaties previously mentioned with reference to the prior art.

Also of course the problem of inaccuracies due to slippage of the tyres can be entirely removed.

The Doppler shift can be measured by splitting the beam into first and second parts and then reflecting just one of the parts from the road surface or reflecting the different parts from different points on the road surface.

If the two parts are then mixed together the difference frequency can be detected to give an indication of the Doppler shift. This technique relies on the source of radiation being coherent and for that reason it is preferable to use a laser source.

It may be advantageous in some situations to include, on a single vehicle, two devices, each constructed in accordance with the invention and arranged to sense components of velocity in different directions. Thus an indication of sideways drift can be obtained in addition to an indication of forward or backward velocity. Two ways in which the invention may be performed will now be described with reference to the accompanying drawings in which:- Fig. 1 illustrates a motor vehicle carrying a velocity sensing apparatus constructed in accordance with the invention; Fig. 2 is a schematic illustration showing the velocity sensor of fig. 1; and Fig. 3 illustrates another velocity sensor which can be-used as an alternative to that shown in fig. 2.

Referring to figure 1 there is shown a motor vehicle 1 travelling along a road surface 2 and carrying a speedometer 3. The speedometer is shown in greater detail in figure 2. It has a laser 4, the output of which is passed along an optical fibre 5 and is emitted from a termination 6. The emitted light is collimated by a lens1 indicated schematically at 7, and the collimated beam is split, by a half-silvered mirror 9, into first and second parts.

The first beam part is focussed by a lens, indicated schematically at 10, onto the road surface 2 at a point 11 which is behind the beam splitter 9. On reflection from the point 11 the light undergoes a Doppler shift in the form of a drop in frequency. The reflected, Doppler shifted, light is focussed by the lens 10 and passes through the beam splitter 8 onto a termination 12 of a second optical fibre 13. The fibre 13 leads to an optical detector 14. In an alternative arrangement the detector 14 may be positioned so that no waveguide 13 is required.

The second beam part from the beam splitter 8 is focussed by a lens, shown schematically at 15, onto the road surface at a point 16 which is forward of the beam splitter 8. Light reflected from this point 16 is directed by the lens 15 and the beam splitter 8 onto the termination 12 of the fibre 13.

The detector 14 thus receives a mixture of signals which have been subjected to Doppler shifts in opposite directions. The output of the detector 14 thus contains a modulation equal to the difference between two Doppler shifted inputs. A processor 17 detects the difference frequency and calculates from it the velocity of the vehicle. This is displayed on a display 18 positioned so as to be visible to the driver of the vehicle.

The alternative arrangement shown in figure 3 is similar to that shown in figure 2 and identical parts are denoted by identical reference numerals. The only difference is that the lens 15 is replaced by a mirror 19 so that the light reflected from the point 11 on the road surface is mixed, in the detector, with light reflected from the mirror 19 without Doppler shift. The effect of this is to reduce by half the modulation frequency at the output of the detector 14 and this needs to be taken into account by the processor 17 when calculating the velocity.

An advantage of the figure 3 arrangement is that, because losses of energy on reflection from point 16 of figure 2 are not incurred, there is a greater amount of light energy received by the detector 14 and consequently better mixing is achieved. The resulting signal to noise ratio is considerably improved. The coherence length of the source however will be as a result longer as the difference in path lengths is greater.

It will be appreciated that the illustrated embodiments have been described just by way of example and that many other variations are possible within the scope of the invention. For example a vehicle may be provided with two sensors to measure relative velocity components in different directions such as forward velocity and sideways drift. Also the invention may be used for example to measure the velocity of a conveyer or other moving parts of machinery.

Claims

1. Velocity sensing apparatus comprising means for generating electromagnetic radiation and directing it onto a surface and detecting means for detecting a Doppler shift in the radiation after reflection from the surface.

2. Apparatus according to claim 1 in which the means for generating" includes a laser; and including a beam splitter for splitting the radiation into a first part which is directed towards the surface so as to be subjected to the said Doppler shift and a second part which is not directed so as to be subjected to that same Doppler shift; the detecting means being arranged to receive radiation form the first and second parts and being constructed so as to detect the difference in frequency, due to Doppler shift, between the radiation in the two parts.

3. Apparatus according to claim 1 or 2 in which the second part is directed so as to be reflected from the surface and so as to be subjected to a second Doppler shift-, different from the first mentioned Doppler shift.

4. Apparatus according to claim 3 in which the first and second parts are directed forwardly and rearwardly so as to be subjected to an increase and decrease in frequency respectively due to Doppler shift.

5. Apparatus according to claim 1 or 2 in which the second part passes to the detector without reflection from the said surface.

6. A road vehicle comprising a speedometer constructed according to any preceding claim.

7. Apparatus according to claim 6 comprising two or more speedometers, each constructed in accordance with any one of claims 1 to 5 and arranged to give an indication of velocity components in different directions.

8. Apparatus substantially as described with reference to figure 2 or figure 3 of the accompanying drawings.