Thermal dilation fiber optical flow sensor

A thermal dilation type optical flow sensor includes an optical heating sce for heating a flowing fluid and a sensor disposed in a downstream direction from said heating source wherein the sensor includes a probe for varying a reflected light according to the temperature of the probe, a Michelson type interferometer for measuring the temperature of the probe according to the phase of the reflected light, and a calculating device for determining the flow rate based on the temperature of the probe, the distance between the heating source and the probe, and the amount of heat produced by the heating source. In an alternative embodiment, a thermal dilation type optical flow sensor includes a reflecting probe disposed in a flowing fluid for reflecting an incident light with a phase varied according to the probe temperature, an optical heater for heating the probe with a predetermined heat, and a calculating device for calculating a flow rate based on the predetermined heat and the phase of the reflected light.

BACKGROUND OF THE INVENTION 
The present invention relates to an optical flow sensor for measuring the 
flow of a fluid. More particularly, the optical flow sensor is a thermal 
dilation type sensor where a probe is heated and the temperature to which 
the probe is heated varies according to the flow of fluid past the probe. 
The invention more particularly relates to a Michelson type optical fiber 
interferometer and its application in measuring the temperature of the 
probe. 
In order to measure the flow of a fluid accurately, it is necessary to 
employ a flow sensor that measures mass flow. There are two approaches to 
achieve this measurement: a Cordilis based flow sensor and thermal 
dilation flow sensor. In the former type, electrical power is required to 
vibrate the Cordilis sensing element whereas conventional thermal dilation 
sensors require electric power at the sensor head to heat the probe. These 
sensors have a substantial amount of metal (e.g. thermocouples, electric 
wires, etc.) and thus are susceptible to electromagnetic pickup and 
radiation, and have a sparking potential. Such a sensor would be 
undesirable when measuring, for example, the flow of a fuel-air mixture 
into a combustion type engine, or when located in close proximity to 
circuitry whose operation could be harmed by spurious electromagnetic 
radiation. This is especially important in areas where space is at a 
premium, such as aircraft or an oceangoing vessel. 
There are examples of non-standard flow sensors which are relevant to the 
problems discussed above. For example: 
U.S. Pat. No. 4,918,492 to Ferdinand et al. describes an interferometer for 
the measurement of temperatures in, for example, turbo-machines. One arm 
of the interferometer terminates in a sensor sensitive to the physical 
phenomenon to be evaluated and comprised of a hollow and open cell for 
receiving a part of a fluid to be measured and a mirror for returning the 
measurement optical wave. Ferdinand et al. do not disclose a heating means 
which is requisite for constructing a dilation type flow sensor. 
U.S. Pat. No. 3,683,692 to Lafitte describes an apparatus to compute and 
measure the flow of a gaseous fluid by measuring a quantity of heat 
necessary to raise the temperature of a fluid of a given quantity, 
comprising a sensing means disposed in the gaseous flow and a reference 
means disposed in a dead-end cavity in such a manner as to be insensitive 
to the flow of fluid in this cavity. The sensing means and the reference 
means comprise a heating resistor to continually heat the fluid in order 
to raise its temperature and a detecting element sensitive to the 
temperature, the sensing means also including a heat compensating resistor 
whose electrical current supply is regulated by a lack of balance between 
the two detection elements for maintaining the elevation of temperature of 
the fluid flowing past the sensing means, and a means to continually 
measure the amount of current passing through the heat compensating 
resistance. The Lafitte apparatus uses electrical heating means which may 
be dangerous around fuel-air mixtures. 
U.S. Pat. No. 4,755,668 to Davis describes a fiber optic interferometric 
thermometer with serially positioned fiber optic sensors comprising a 
single optical fiber and a means for enabling a temperature to vary the 
phase of light in several well-specified regions of the optical fiber. The 
sensing system consists of a Fabry-Perot type interferometer connected at 
one arm to the end of the optical fiber sensor. The optical fiber sensor 
is separated from the remainder of the optical fiber by a half-silvered 
mirror. The other end of the sensor region is fully mirrored. Thus, light 
is divided by the half-silvered mirror, so that one part of the light 
incident on the sensor is reflected back toward the coupler by the 
half-silvered mirror and constitutes the reference beam. The other part is 
transmitted into the optical fiber sensor portion and constitutes the 
sensor beam. The sensor beam experiences an added phase shift compared to 
the reference beam due to an added path length and the effect of the 
parameter being measured. The sensor component of the beam is then 
reflected by the full mirror at the end and passes once more through the 
sensor region experiencing an additional phase shift and is transmitted 
back through the half-silvered mirror and is interferometrically combined 
at the half-silvered mirror with the reference beam initially reflected by 
the half-silvered mirror. The fiber optic sensor region varies the phase 
of the light reflected from the mirrored end of the sensor according to 
the temperature of the sensor. 
However, none of these sensors are fully satisfactory as a flow sensor, or 
for use in close proximity to electronic circuitry, or volatile chemicals. 
The present invention is aimed at eliminating these problems while at the 
same time affording greater flow measurement sensitivity. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to overcome the 
aforementioned drawbacks in the known art. In particular: 
An object of the invention is to permit the use of flow sensors in close 
proximity to electronics, without the sending or receiving of spurious 
electromagnetic signals. 
Another object is to permit operation of flow sensors safely in the 
vicinity of volatile fluids, without danger of sparking. 
Another object is to permit flow sensors to operate with little or no 
metallic components. 
Another object is to permit such flow sensors to be especially compact. 
In accordance with these and other objects made apparent hereinafter, the 
invention provides an all optical flow sensor comprising an optical 
heating means for heating a flowing fluid and a sensor means disposed in 
the fluid in a downstream direction from the heating means, the sensor 
means determining a flow rate of the flowing fluid and comprising a probe 
means for measuring a temperature of the fluid. 
According to another aspect of the invention, the optical flow sensor 
comprises a probe reflecting means disposed in a flowing fluid for 
reflecting incident coherent light within a light pipe wherein a phase of 
the reflected light within the light pipe with respect to the phase of the 
incident light varies according to a temperature of the probe means. The 
optical flow sensor further comprises an optical heating means for heating 
the probe means and a phase measurement means for measuring the phase of 
the reflected light with respect to the phase of the incident light, and a 
calculating means for determining the flow rate based on the relative 
phase of the reflected light. 
Such a flow sensor is constructed substantially wholly of optical, rather 
than metallic, components. The sensor is thus isolated electromagnetically 
from its surroundings, has no components that can spark, and can be built 
very compactly because of the inherently small nature of optical fibers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates a first embodiment of the invention. In this embodiment, 
optical heating means 14 and probe means 18 are disposed in flowing fluid 
2. Optical heating means 14 is supplied with heating light through light 
pipe 12 from optical heating source 10. In operation, light generated in 
light source 10 passes through optical pipe 12 and a predetermined amount 
of heat is absorbed in heating means 14 to thereby heat a proximal portion 
of flowing fluid 2. 
It will be appreciated that the temperature of heating means 14 will rise 
based on the predetermined amount of heat, and will be limited by the 
amount of heat transferred to flowing fluid 2, which is based on the mass 
flow rate and temperature of flowing fluid 2. It will also be appreciated 
that the portion of flowing fluid 2 proximal to heating means 14 will rise 
according to the amount of heat transferred thereto and the specific heat 
capacity of flowing fluid 2. 
Probe means 18 is disposed at a predetermined distance in a downstream 
direction from heating means 14. Operatively connected to said probe means 
is a sensor means comprised of elements 4, 6, 8, 16, 20, 22, 24 and 26, 
which together comprise a Michelson type interferometer. Coherent light 
from light source 16 drives the interferometer. A first light pipe is 
comprised of first end 22, second end 20 and a portion of coupler 8. A 
second light pipe is comprised of first end 24, second end 26 and another 
portion of coupler 8. 
Coherent light source 16 transmits light through second end 20 into coupler 
8 where the light is divided and a first portion of the light travels 
through first end 22 of the first light pipe toward probe means 18 and a 
second portion of the light travels through first end 24 of the second 
light pipe toward reflecting means 4. Light traveling through first end 24 
toward reflecting means 4 is wholly reflected back through first end 24 
toward coupler 8 where a portion of this reflected light travels through 
second end 26 of the second light pipe toward detector means 6. 
Meanwhile light travelling through first end 22 of the first light pipe 
toward probe means 18 is wholly reflected back through first end 22 of the 
first light pipe toward coupler 8 where a portion of this reflected light 
travels through second end 26 of the second light pipe toward detector 
means 6. In operation, the portion of light that is reflected from each of 
probe means 18 and reflecting means 4 and travels through second end 26 of 
the second light pipe toward detector means 6, constructively or 
destructively interfering with each other at juncture 9 of coupler 20, 22 
and 24, 26, according to the relative phase of the two reflected lights. 
Accordingly, the amplitude detected by detecting means 6 depends on their 
relative phase. 
Probe means 18 reflects a reflected light having a phase relative to an 
incident light that varies according to the temperature of probe means 18. 
In this first embodiment, probe means 18 takes on the temperature of 
flowing fluid 2 which is proximal to the probe means. In operation, 
heating means 14 is heated by a predetermined quantity of heat which in 
turn heats flowing fluid 2 according to the temperature of heating means 
14 relative to the temperature of flowing fluid 2. The heated flowing 
fluid flows past probe means 18 decreases in temperature by a 
predetermined amount by diffusion of heat as the fluid flows from heating 
means 14 to probe means 18 which takes the temperature of the heated 
flowing fluid proximal to probe means 18. Dilation principles determine 
the temperature to which the flowing fluid is heated by heating means 14 
according to the mass flow rate of flowing fluid 2. That is, the slower 
that fluid 2 flows, the longer any portion of it will be adjacent heater 
14, and the hotter the fluid will become. The exact relationship between 
temperature and flow rate can be calibrated in situ. Therefore, by 
measuring the temperature induced deformation, or change in refractive 
index, of probe means 18 which has taken on the temperature of the 
proximal flowing fluid, the mass flow rate can be determined. Probe means 
18 expands or contracts, thermally, in response to the temperature of the 
flowing fluid so that the phase of light reflected from probe means 18 
relative to the light incident on probe means 18 varies according to the 
temperature of the probe means 18. It will be appreciated that other means 
for varying reflected phase according to temperature may substitute for 
the thermal expansion means. For example, properly selected materials that 
vary refractive index with temperature may be incorporated in probe means 
18. The temperature dependent phase of light reflected from probe means 18 
causes constructive and destructive interference with reflected light from 
reflecting means 4 as viewed at detecting means 6. This interference 
causes the signal detected by detecting means 6 to vary in amplitude 
according to the temperature of probe means 18. Calculating means 7 
transforms the amplitude of the detected signal from detecting means 6 
into a signal representative of the mass flow rate of flowing fluid 2 
according to conventional dilation sensor principals. 
Coupler 8 can be a conventional fused-fiber coupler, with fiber arm 20, 22 
fused to arm 24, 26 at 9. Heater 12, 14 could be a simple heated metal 
wire, although this might lead to possible short circuits, and/or 
sparking. To avoid this, heater 12, 14 is preferably made entirely of 
optical material, with an absorptive termination to effectively dissipate 
optical energy from source 10 in fluid 2. 
FIG. 2 illustrates a second embodiment of the invention. In this 
embodiment, flowing fluid 2 flows past probe means 50. Probe means 50 is 
operatively connected to first end 48 of a first light pipe. In operation, 
two lights, a coherent light and a heating light, are transmitted from 
coupler 8 through first end 48 toward probe means 50. Probe means 50 
absorbs at least a portion of the heating light and wholly reflects 
substantially all of the coherent light. In this embodiment, heating 
source 30 serves an analogous function to heating source 10 of the first 
embodiment and light source 40 serves an analogous function in this 
embodiment to light source 16 of the first embodiment. Heating source 30 
transmits an optical heating light through light pipe end 32 into 
fused-fiber coupler 38 where at least a portion of the heating light is 
coupled into light pipe end 44 traveling towards coupler 8. Light source 
40 transmits coherent light through light pipe end 42 into coupler 38 
where at least a portion of the coherent light travels through light pipe 
end 44 toward coupler 8. Any portion of either heating light from source 
30 or coherent light from source 40 that does not travel through light 
pipe end 44 will travel through light pipe end 34 into absorbing means 36 
where it is dissipated as unused heat. 
The combination of heating light and coherent light travels through light 
pipe end 44, through light pipe end 46 and into coupler 8. A portion of 
the combined light travels through coupler 8 through light pipe end 48 
toward probe means 50. Heating light from source 30 is at least partially 
absorbed by probe means 50 so as to heat probe means 50 with a 
predetermined quantity of heat. Coherent light from source 40 traveling 
through light pipe end 48 toward probe means 50 is wholly reflected. Light 
reflected from probe means 50 travels back through light pipe end 48 into 
coupler 8 where at least a portion travels through light pipe end 26 
toward detecting means 6. 
Meanwhile the combined light from both source 30 and source 40 which 
travels through light pipe end 44 through light pipe end 46 through 
coupler 8 and through light pipe end 24 towards reflecting means 4 is 
wholly reflected at reflecting means 4. This reflected combined light 
travels back through light pipe end 24 through coupler 8 where at least a 
portion of this reflected light travels through light pipe end 26 toward 
detecting means 6. Detecting means 6 is responsive to coherent light from 
light source 40 and non-responsive to heating light from heating source 
30. It will be appreciated that heating light from heating source 30 need 
not be coherent and may be of considerably different wavelength than 
coherent light from light source 40. Well known filters are available 
which permit the separation of optical signals which are very close to one 
another in frequency. It will also be appreciated that detecting means 6 
need not be non-responsive to heating light from light source 30 if 
substantially all of incident heating light on probe means 50 and on 
reflecting means 4 is absorbed in probe means 50 and reflecting means 4, 
respectively. Again, this can be done by the judicious choice of well 
known and commonly available optical filters. 
In operation, a predetermined quantity of heat is absorbed in probe means 
50. Because probe means 50 is disposed in said flowing fluid and is heated 
by a predetermined quantity of heat, probe means 50 will experience a 
temperature rise limited by the amount of heat transferred to flowing 
fluid 2, which is based on the mass flow rate and temperature of flowing 
fluid 2. Thus, in this embodiment, the temperature of probe means 50 is 
responsive to the mass flow rate and temperature of flowing fluid 2 in a 
way analogous to the way probe means 18 of the first embodiment is 
responsive to the mass flow rate and temperature of flowing fluid 2, that 
is to say the phase of coherent light reflected from probe means 50 
relative to coherent light incident on probe means 50 varies according to 
temperature. It will be appreciated that elements 4, 6, 8, 24, 26, 46 and 
48 of FIG. 2 comprise a Michelson type interferometer for measuring the 
temperature of probe means 50. It will be appreciated that the calculation 
of the mass flow rate of flowing fluid 2 by calculating means 7 in this 
second embodiment is substantially the same as the method for calculating 
the flow rate in the first embodiment, and will not be further described 
here. 
FIG. 3 illustrates a third embodiment of the invention. In this embodiment, 
flowing fluid 2 flows past probe means 52. Light source 16, detecting 
means 6, coupler 8, reflecting means 4, and light pipes 20, 22, 24 and 26 
comprise a Michelson type interferometer substantially identical to the 
Michelson type interferometer of the first embodiment. Probe means 52 is 
comprised of a material that partially absorbs and partially reflects 
light from light source 16. In this embodiment, the portion of the light 
from light source 16 that travels through light pipe end 20 and light pipe 
end 22 and is absorbed by probe means 52 to produce a predetermined 
quantity of heat which constitutes the heat from the heating means. It 
will be appreciated that in all other aspects this embodiment functions 
substantially identical to the first embodiment and will not be further 
described here. 
FIG. 4 illustrates a schematic view of reflecting means 4. In this view, 
reflecting material 60 is disposed on the end of optical fiber 62 so as to 
reflect substantially all of light incident on reflecting means 4. 
FIG. 5 illustrates a schematic view of a first variant of probe means 50. 
In this view, optical fiber 62 is clad with absorbing cladding material 
64. At least a portion of the heating light incident on probe means 50 is 
absorbed in the absorbing cladding material 64 to thereby heat probe means 
50 with a predetermined heat. Coherent light incident on probe means 50 is 
reflected from reflecting material 60 disposed on the end of optical fiber 
62. In this variant, a portion of heating light incident on probe means 50 
may be reflected back through optical fiber 62 while another portion is 
absorbed in absorbing cladding 64. 
FIG. 6 illustrates a second variant of probe means 50. Light incident on 
probe means 50 travels through optical fiber 62 and is reflected back 
through optical fiber 62 by reflecting material 60 disposed on the end of 
optical fiber 62. Optical fiber 62 is clad in a non-absorbing cladding 66. 
The optical fiber clad in non-absorbing cladding is encased in absorbing 
fiber jacket 68. A portion of heating light incident on probe means 50 may 
be reflected back through optical fiber 62 and another portion is absorbed 
in absorbing fiber jacket 68 to generate a predetermined heat. 
FIG. 7 illustrates a schematic view of probe means 52. Light incident on 
probe means 52 travels through optical fiber 62, then through probe 
material 70 to be reflected by reflecting material 60 disposed on one side 
of material 70 so as to travel back through probe material 70 and back 
through optical fiber 62. Probe material 70 partially absorbs the incident 
light, converting the absorbed portion of light into a predetermined 
quantity of heat. 
It will be appreciated that additional embodiments are apparent from these 
teachings and consideration of various combinations of the described probe 
means and the descriptions of the operation of the first, second and third 
embodiments. The optical flow sensor described herein has advantages over 
prior art structures that heat a probe with electric current, in 
particular the sensitive measurement of flow rate without the hazards of 
sparking potential or interference from spurious electromagnetically 
radiation. These and other advantages will be appreciated from the 
disclosure herein. 
The invention has been described with reference to its preferred 
embodiments which are intended to be illustrative and not limiting. 
Various changes may be made without departing from the spirit and scope of 
the invention as defined in the following claims.