True ground speed sensor

A speed sensor for a moving vehicle. A transmissive grating is used in conjunction with an electromagnetic wave source and beam combining waveguide to mix two beams of frequency f and f+.DELTA.f, where .DELTA.f is proportional to the speed of the grating relative to the ground. The source energy is reflected off the road to the grating. The zeroth and first order transmission from the grating are mixed to give the beat frequency .DELTA.f proportional to the ground speed. The sensor has no moving parts, and is independent of any gear ratios.

BACKGROUND OF THE INVENTION 
The present invention relate to the field of ground speed sensor apparatus, 
and more particularly to such apparatus for measuring the true ground 
speed of moving vehicles. 
Conventional speedometers pick off the vehicle speed assuming a constant 
gear ratio from the tire to axle to drive train. If the tire(s) pressure 
becomes low, or if tires are installed which are larger or smaller in 
diameter than the original equipment tire size, then the gear ratio, and 
hence the calibration of the speedometer, changes. 
It would therefore represent an advance in the art to provide a ground 
speed measuring apparatus which is independent of height of the car from 
the road and any gear ratios. 
It would further represent an advance in the art to provide a ground speed 
sensor for moving vehicles which is compact, has no moving parts, and is 
rugged. 
SUMMARY OF THE INVENTION 
A ground speed sensor for moving vehicles is described, and comprises a 
source of electromagnetic energy arranged to generate a beam of energy 
downwardly toward the ground adjacent or underneath the vehicle. 
A diffraction grating is arranged adjacent the vehicle undercarriage so 
that electromagnetic energy reflected from the ground is incident on the 
grating, which diffracts the beam into a plurality of beams. 
The sensor further includes means for combining the energy diffracted by 
the grating to develop an energy beat frequency which is dependent on the 
speed of the grating relative to the ground. Means responsive to the 
energy beat frequency provides a sensor signal indicative of the speed of 
the vehicle relative to the ground. 
In a preferred embodiment, the energy combining means includes means for 
combining the zero order and first order diffracted energy beams from the 
grating. The beat frequency is the difference between the frequency of the 
zero order beam and the frequency of the first order diffracted beam. 
The source of electromagnetic energy may comprise a source of infrared 
electromagnetic energy such as a semiconductor infrared laser. 
Alternatively, the source can be an optical light source or an RF energy 
source. 
The sensor grating is characterized by a grating having a number of slits 
of width b, wherein the beat frequency is substantially equal to v/b, 
where v represents the speed of the vehicle relative to the ground.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
It has been shown that optical frequency shifts can be produced in various 
diffraction orders of a linear grating moving in a direction perpendicular 
to the rulings. See, e.g. "Interferometry," W.H. Steel, Second Edition, 
Cambridge University Press, 1983, pages 57-58; "Optical Frequency Shifting 
by Means of a Rotating Diffraction Grating," W. H. Stevenson, APPLIED 
OPTICS, Vol. 9, No. 3, March 1970, pages 649-652. 
Consider the case of Fraunhoffer type diffraction by a single slit of width 
b and length L. (It is simpler to start with describing the field for one 
slit. To generalize for a grating (multiple slits equally spaced), the 
field is summed over N, the total number of slits illuminated by the 
source.) FIG. 1 illustrates the geometry. A plane parallel wave of 
frequency w.sub.0 is incident on a slit 5 formed in a grating 6, where the 
slit edges diffract the incident beam through the angle .theta.. A lens 7 
converges the diffracted beams at a lens focal plane 8. The distribution 
of the field at point P is given by the Fresnel-Kirchoff formula, 
EQU U.sub.p =C .intg..intg.e.sup.ik o.sup.r dxdye.sup.iw o.sup.t (1) 
where C is a constant, dxdy=Ldy, w.sub.o =2.pi.f, .theta.=n(.lambda./b), 
k.sub.o =2.pi./.lambda..sub.o, r is the distance from the slit edge to the 
focal plane illustrated in FIG. 1, and n=0, .+-.1, .+-.2. For the slit 
moving with velocity v, r can be expressed as 
EQU r=r.sub.o +(y+vt) sin.theta., (2) 
where r.sub.o is the value of r for y=0. The integral for U.sub.p reduces 
to 
##EQU1## 
where.beta.=(1/2)kbsin.theta.. The frequency shift .DELTA.f due to the 
moving slit is 
EQU .DELTA.f =(kv/2.pi.)sin.theta.. (5) 
For .theta.=n(.lambda./b)&lt;&lt;1, sin.theta..apprxeq..theta.and 
.DELTA.f=n(v/b). 
Since .DELTA.f=0 for n=0 (zero-th order, the non-diffracted beam), the 
zero-th order and first order (n=.+-.1) diffracted beams are combined in 
accordance with this invention. The beams interfere with one another to 
produce a beat frequency .DELTA.f. The beat frequency provides a value for 
the speed v. 
Diffracted beams other than the n=0 and n=.+-.1 can be combined to produce 
a beat frequency other than .DELTA.f. For example, the n=+2 order beam can 
be combined with any of the n=.+-.1, -2, .+-.3, . . . order beam to 
produce a beat frequency that is an integer multiple of .DELTA.f. 
A simplified block diagram of an apparatus embodying the invention for use 
in sensing true ground speed of a moving vehicle 15 is illustrated in FIG. 
2. A source 20 of electromagnetic energy of wavelength .lambda. is mounted 
adjacent the undercarriage 22 of the vehicle 15, and arranged to transmit 
a beam 24 toward the ground beneath the undercarriage. A diffraction 
grating 26 is arranged adjacent the undercarriage 22 and substantially 
parallel thereto. The sensing apparatus further comprises a zero-th order 
focusing lens 28, a first order focusing lens 30, electromagnetic 
transmission lines 29, 31, and 33, combiner 32 and an opto-electric 
detector and .DELTA.f-to-ground speed converter 34. 
The purpose of the respective focussing lens 28 and 30 is to converge the 
respective beam contributions from many slits at the focal plane of the 
lens. Focussing lens 28 converges all the zero-th order contributions from 
the various slits and focussing lens 30 converges all the first order 
contributions from the various slits. 
A beam of electromagnetic radiation of wavelength .lambda. is reflected off 
the ground from the vehicle 15 moving with speed v. The ground acts as an 
extended source. The reflected radiation (beam 25) illuminates the grating 
26 which transmits diffracted beams of order zero to n. Two beams 36 and 
38, say of order n=0 and n=1, are passed through the respective focusing 
lenses, conducted by the respective transmission lines 29 and 31 to the 
combiner 32 and combined in the combiner 32. The combined energy, wherein 
the respective beams interfere to produce the beat frequency .DELTA.f, is 
conducted to the detector 34 by transmission line 33. The detector and 
electronics 34 converts the beat frequency .DELTA.f to a sensor signal 35 
indicative of the true ground speed of the grating. 
The physical origin of .DELTA.f produced by a moving grating is the Doppler 
effect. The advantage of the grating 26 is its efficiency, i.e., a large 
amount of energy is directed in a particular direction and at a single 
frequency. It is important to note that the beat frequency and, hence, 
ground speed (.DELTA.f=(v/b)) is independent of the wavelength or 
intensity of the source 20 or the height of the vehicle from the ground. 
An estimate can be made for a given grating spacing b (its slit spacing and 
slit width) as to what frequencies to expect for a range of vehicle ground 
speeds. For example, the velocity v might be in the range 0-150 miles per 
hour, or 0-67 m/sec. So, for b=10.mu.m, .DELTA.f=0-6.7 MHz. For the 
millimeter wave region b.apprxeq.1 cm, .DELTA.f=0-670 Hz. Detection at 
these beat frequencies is easily attained. 
In one preferred embodiment, the source emits light in the infrared 
wavelength region so that effective operation can be obtained with some 
dirt obscuration of the source and grating elements. Preferably the light 
source and grating are recessed into respective tubular receptacles to 
lessen the possibility that dirt could obscure the optics of the system. 
In such a sensor system, the light source 20 could comprise an infrared 
semiconductor laser operating at 1300 nm, such as the model C86013E, 
marketed by RCA Electro-optics, 733 Donegal Business Center, Mount Joy, PA 
17552. The grating 26 should include a large number of lines, at least 
several hundred or thousand, since the frequency resolution of the sensor 
increases with the number of lines. For infrared operation, the grating 
spacing is preferably in the range of one to five microns. The grating 26 
could be an acetate film grating with 2,700 grooves/inch (5 micron 
spacing), available from Edmond Scientific, 101 E. Gloucester Pike, 
Barrington, NJ 08007. The focusing lens 28 and 30 could be the lens model 
010 at the IR wavelength with antireflective coatings at the wavelength 
of interest, available from Newport Research Corporation, 18235 Mt. Baldy 
Circle, Fountain Valley, California 92728. The optical fibers 29 and 31 
are single mode optical fibers operable at the IR frequency of interest. 
The optical combiner 32 can be a model SF4-D-1300-B combiner, available 
from CANSTAR, 3900 Victoria Park Avenue, North York, Ontario, Canada, 
M2H3H7. The detector circuit and conversion electronics can comprise an 
Indium Gallium Arsenide photodiode photodetector with a frequency counter 
circuit. A suitable diode detector is the C30617E detector available from 
RAC Electro-optics, 733 Donegal Business Center, Mount Joy, PA 17552. A 
suitable frequency counter can include the frequency-to-voltage converter 
part LM2907, available from National Semiconductor Corporation, 2900 
Semiconductor Drive, Santa Clara, California. The diode detector envelope 
detects the beat frequency .DELTA.f, not the optical frequency, and 
produces a voltage output signal. The frequency-to-voltage converter 
generates an output signal indicative of the beat frequency .DELTA.f. This 
converter output signal may be applied to an indicator apparatus (e.g., a 
speedometer indicator on the vehicle dashboard) to indicate the vehicle 
ground speed to the driver, or to other apparatus utilizing the speed 
data, such as a cruise control, anti-lock brake system, odometer 
apparatus, or the like. 
One concern is the alignment when the vehicle is tilting from a sharp turn 
or from braking action. The misalignment would render the instrument 
useless if the source were collimated with a narrow beam width, since the 
reflected beam 25 might miss the grating 26 altogether. The sensor system 
can be designed to accommodate tilt by emitting a diverging beam from the 
source 20 and increasing the entrance aperture and field of view of the 
equivalent lens 28 and 30 in FIG. 2. 
The purpose of this invention is to provide a measure of true ground speed 
for moving vehicles. It has the advantage of no moving parts, compactness 
and can be made rugged. 
It is understood that the above-described embodiments are merely 
illustrative of the possible specific embodiments which may represent 
principles of the present invention. For example, instead of a laser for 
the light source, a longer wavelength source such as a millimeter wave 
transmitter could be use. Other arrangements may readily be devised in 
accordance with these principles by those skilled in the art without 
departing from the scope and spirit of the invention.