Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
1999-03-18
2001-09-04
Lefkowitz, Edward (Department: 2736)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S606000, C340S611000, C356S028500
Reexamination Certificate
active
06285288
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to improvements in or relating to remote air detection, in particular to a remote air detector and a method of remote air detection.
The measurement of air velocity and flow direction is important when controlling the motion of an aircraft in flight. Methods currently used employ a pitot tube extending from the aircraft, which enables velocity measurement in terms of the differential pressure between air in the tube and air outside the tube. This method has the disadvantage of only being able to measure velocity close to an airframe of the aircraft where the flow direction of air may be substantially effected by the airframe and is insensitive to low velocities as the differential pressure between air inside and outside the tube is proportional to the velocity of air squared. These disadvantages are particularly acute in helicopters where the forward velocity of air can be very slow or even negative and the flow direction of air, in close proximity to the airframe, can be greatly effected by the downdraft from the engine rotor.
Additionally high performance aircraft in flight, for example supersonic aircraft, operate in unstable aerodynamic configurations and require measurement of airflow direction relative to the aircraft to enable motion to be stabilised.
From
FIG. 1
, it is known to provide a Doppler anemometer
10
to measure the velocity of flowing air using a laser diode
11
to produce a laser output
12
which is collimated by a collimator lens
13
into a parallel beam
14
. The laser diode is orientated to produce a laser output
12
polarised in the direction of propagation of the parallel beam
14
such that the parallel beam
14
is transmitted by a polarising beam-splitter
15
to form a beam
16
.
The beam
16
then passes through a quarter-wave plate
17
which converts the linearly polarised light of the beam
16
into a circularly polarised light beam
18
. The light beam
18
is expanded by a telescope
19
comprising lenses
20
and
21
and then passes through a window
22
in the aircraft. The beam
18
is brought to focus on a focal point
23
at a distance from the window
22
by adjustment of lenses
20
,
21
.
Air flowing through the focal point
23
contains particles which may be dust volcanic ash or aerosols in the form of microscopic water droplets. Measurements performed in various parts of the world show that such particles are present everywhere in the atmosphere at a concentration sufficiently to cause a measurable amount of back-scatter from the light beam
18
. Hence, a small proportion of light
24
scattered by atmospheric particles close to the focal point
23
is scattered in a direction which returns through window
22
, the lenses
20
,
21
of telescope
19
and quarter-wave plate
17
to form a colimated signal beam
25
. When the light
24
passes through the quarter-wave plate
17
it is converted from circularly polarised light to linearly polarsed light but having a direction of polarisation perpendicular to the propagation direction of the beam
16
.
The signal beam
25
is substantially reflected by polarising beam-splitter
15
and is focussed by lens
26
onto the surface of a photodetector
27
.
A small proportion of beam
16
is reflected from a plane face
28
of the quarter-wave plate
17
positioned closest to lens
20
and passes back through plate
17
to form a reference beam
29
. The portion of beam
16
which is reflected from the plane face
28
of the plate
17
is converted from linearly polarised light to circularly polarised light and then back to linearly polarised light but with a change in the direction of polarisation such that it is perpendicular to the direction of propagation of the reference beam
29
. The reference beam
29
is reflected by beam-splitter
15
and is focussed by lens
26
onto the photodetector
27
.
The orientation of the quarter-wave plate
17
is adjusted such that the reference beam
29
is accurately parallel and collinear with the signal beam
25
. The reference beam
29
and the signal beam
25
form an interference pattern on the surface of the photodetector
27
and, when the plate
17
is properly adjusted, the spacing of interference fringes formed by the beams
25
,
29
is substantially larger than the diameter of either beam
25
,
29
so that the photodetector
27
receives a light intensity modulated at the difference frequency between the beams
25
,
29
which is the Doppler frequency corresponding to the motion of particles at the focal point
23
.
The photodetector
27
produces an output current
30
which is passed to a signal analyser
31
which can consist of a fast Fourier transform analyser or a pulse-pair processor that identifies the Doppler frequency which provides an indication of the corresponding air velocity at focal point
23
. The air velocity may be displayed on a display
32
.
However, the Doppler anemometer described with reference to
FIG. 1
is unable to discriminate between positive and negative directions of air flow, which is a particular problem in helicopters able to fly backwards as well as forwards.
SUMMARY OF THE INVENTION
It is an object of the present invention to obviate or mitigate the problems associated with the prior art.
According to a first aspect of the present invention a remote air detector comprises a remote air detector, comprising a transmitter operably arranged to produce and focus a light beam on a remote focal point, a receiver operably arranged to receive resultant light from the light beam that is scattered by air at the focal point, and a detector operably connected to the receiver and arranged to determine from the resultant light the flow direction of air at the focal point. In this manner the flow direction of air can be measured at a point remote from a source of disturbance of the air. The term light refers to visual, infrared and ultra-violet.
The detector may be operably arranged to determine from the resultant light the velocity magnitude of air at the focal point.
In a first embodiment, the transmitter may comprise a quarter-wave plate operably arranged to divide reference light from the light beam. The detector may comprise a beam-splitter operably arranged to combine the resultant light and reference light. A beam-splitter may be operably arranged to divide the resultant light substantially equally between two arms, each arm operably arranged to produce a signal beam substantially orthogonal and in phase to the signal beam in the other arm. Each arm of the detector may further comprise a photodetector operably arranged to detect frequency shifts between its respective signal beam and reference beam and to produce a photocurrent indicative of frequency shifts. The photocurrent from each photodetector may be operably connected to a phasemeter which is operably arranged to indicate the flow direction of air at the focal point dependant on which arm of the detector leads the other arm in phase. The photocurrent from each photodetector may be operably connected to a combining circuit which forms a combined signal that is passed to a frequency analyser operably arranged to determine the velocity magnitude of air at the focal point.
Alternatively, the transmitter may be operably connected to a variable current supply arranged to generate a modulation signal to modulate the light beam. The variable current supply may be arranged to generate a linearly rising and linearly falling modulation signal.
In a second embodiment, the transmitter may comprise a beam-splitter operably arranged to divide reference light from the light beam and to combine the resultant light and the reference light. A photodetector may be operably arranged to detect frequency shifts between resultant light and reference light and to produce a photocurrent indicative of the frequency shifts. The photodetector may be operably connected to an inversion circuit which is operably connected to the variable current supply and arranged to receive the modulation signal, the inversion circuit may be a
Langdon Roger M
Watts Philip M
Kirschstein et al.
Lefkowitz Edward
Marconi Electronic Systems Limited
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