Note: Descriptions are shown in the official language in which they were submitted.
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FIELD OF THE INVENTION:
This invention relates to optical sensors for
use in thermal spray processes and in particular, in
plasma spray processes.
BACKGROUND TO THE lNV~;N'l'lON:
Plasma spraying is a powerful technique used
widely to produce protective coatings on a large variety
of substrates. For example, thermal barrier coatings
are plasma sprayed in producing aircraft engines, and
ceramic and metal coatings are plasma sprayed for
various purposes. Coating properties depend upon many
spraying parameters, some of them being related to the
spray gun operation. Conse~uently spraying process
control has been implemented by monitoring and
regulating such gun input variables as arc current and
power, arc gas flow rates, powder feed rate, and powder
carrier gas pressure, to keep them at predetermined
optimum values. This control approach has been found to
be complex because a large number of interrelated input
variables must be monitored, and has been found to be
incomplete because some variables, such as electrode
wear state, cannot be monitored at all.
SUMMARY OF THE PRESENT lNV~N'l'lON:
The present invention has been found to be a
more powerful structure and method for controlling the
plasma spray process. In the present invention, the
direct process parameters are monitored rather than (or
in addition to) the indirect gun input variables. It
has been found that the most important parameters that
control directly the coating microstructure and
properties are the temperature and the velocity of the
particles i ~~i~tely before their impact on the
substrate. In the present invention, the temperature
and velocity are measured on-line, and provide an
efficient feedback signal generator performing feedback
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for the gun input parameters to maintain optimum
spraying conditions, and can be used as an efficient
diagnostic tool to detect rapidly any problem during the
coating operation. Since the temperature and velocity
S have a direct influence on the coating quality, the
present invention facilitates obtaining a closer control
of the spraying process, leading to a better
reproducibility of the coating properties than in the
prior art.
Different techniques exist to perform
temperature measurements of in-flight particles.
J. Mishin et al, as reported in the J. Phys.E: Sci~
Instrum., 20 (1987) 620-5, used a pair of monochromators
and fast photomultipliers to determine the surface
temperature of individual particles. In another
approach, as described in U.S. Patent 4,6~6,331 to
Lillquist et al, a mid-infrared (>3~m) sensor is used to
monitor the light intensity emitted by the particle jet,
the collected signal being related to the particle
temperature. In this case, however, information about
the particle temperature distribution is not available
since signals emitted by individual particles are not
time resolved. Thus, radiation from the luminous plasma
may be detected biasing the particle temperature
measurements. Also, the apparent average temperature is
biased toward the highest temperature particles because
of the nonlinearity of the radiance-vs-temperature
emission curves.
There have been two types of techniques
previously available to perform an in-flight particle
velocity measurement. In the first type of techniques,
the velocity information is obtained from light
impinging upon and reflected by the particles, detected
by an appropriate sensor. Laser based techniques, such
as laser Doppler anemometry and laser dual focus
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velocimetry, are included in this first type of
techniques. They use intense laser light beams to form
interference fringes, or use two focused light beams in
the measurement region. When the particle trajectory
intercepts the measurement region, the reflected light
intensity is modulated as the particle travels through
the intensely illuminated zones and the velocity is
computed from the modulation period. Periodic light
distributions may also be obtained using a high
intensity incandescent source and a Ronchi grating.
This technique is inappropriate, being bulky and
requiring high intensity light sources.
The second type of techniques used to perform
the velocity measurement takes advantage of the thermal
radiation emitted by the particles heated to a high
temperature by the plasma. The radiation emitted by
individual particles is detected when the particles pass
through the detector field of view of known dimensions.
The transit time is evaluated and the velocity is
computed knowing the travel length. Since the
dimensions of the field of view change with the distance
from the optical detection assembly, it is necessary to
analyze only particles near the assembly focal plane.
To do that, a laser beam or a second detection assembly
focused in the appropriate region from a different angle
must be used in conjunction with a coincidence detection
analysis system. Such a system is complex and difficult
to keep well aligned under practical operating
conditions. In this same type of techniques, velocity
measurements can also be performed using high speed
cameras. In this case, light emitted by the particles
is used to image them on a high speed film and, from
these images, the particle velocity is determined. Such
a system can be used for a laboratory investigation, but
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it is not suitable for real time operation in the harsh
plasma-spray environment.
The present invention is a method and an
apparatus for monitoring simultaneously the temperature
S and the velocity of sprayed particles without the
limitations and problems described above, for previously
developed techno~ogy. Temperature measurements are
performed using a two-colour pyrometry technique, while
the velocity measurements are done using a two-slit or a
multiple-slit system that collects radiation emitted by
the hot particles. The detection assembly permits the
simultaneous determination of the temperature and the
velocity of each individually-detected particle.
The system is comprised of a sensor head
attached to the spray gun, an optical fibre transmitting
the collected radiation to detection apparatus, and a
protective detection cabinet having the detection
apparatus that incorporates two detectors. A two-slit
or multiple-slit mask is located in the sensor head at
the end of the optical fibre.
The result is a rugged optical sensor that
monitors the temperature and velocity distributions of
plasma-sprayed particles simultaneously, i ?~;ately
before their impact, in which the optical fibres permit
the location of the fragile optical and electronic
components away from the aggressive environment around
the plasma gun. The sensor head is located in the harsh
environment close to the plasma, and indeed is
preferably attached to the plasma gun for collecting
radiation emitted by the hot particles.
For the temperature measurements, the particle
emitted radiation collected by the sensor head is
transmitted to two photodetectors, filtered by
interference filters at two adjacent wavelengths. The
particle temperature may be computed from the ratio of
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the detector outputs. To measure the velocity, the two-
slit system collects radiation emitted by the in-flight
particles travelling in the sensor field of view, which
generates a double peak light pulse transmitted through
S the optical fibre. The time delay between these two
peaks may ~e evaluated automatically and the particle
velocity computed knowing the distance between the two
slit images. The velocity measurement can be performed
also with a system of three or more slits.
The sensor head can also include a linear
fibre bundle that provides a continuous monitoring of
the position of the sprayed-particle cone. The light
collected by the fibre bundle may be detected by a
linear CCD camera. This permits the automatic centering
of the sensor head field of view relative to the
sprayed-particle cone and the detection of any changes
in the particle injection conditions.
The above-computation is preferably performed
by a processor, e.g. a personal computer, which can be
programmed to continuously perform statistical
computations to obtain the mean and standard deviation
of the temperature and velocity distributions. These
values and the particle cone position are directly
related to the deposition process and are provided to
the control apparatus as feedback signals whereby the
main spraying variables or arc current, powder fee~;ng
gas pressure, etc. as noted above may be controlled.
In accordance with an embodiment of the
invention, a method of detecting a characteristic of
plasma sprayed particles in a plasma jet during flight
between a plasma jet gun and a substrate, is comprised
of the steps of focusing radiation emitted from a
particle on a first photodetector through a slit mask
formed of at least two parallel slits, and transmitting
signals from the photodetector to a processor for
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determining the velocity of the particle from the
difference in time slit images are detected resulting
from said radiation.
In accordance with another embodiment, the
S invention further includes the step of splitting the
radiation, filtering the radiation into separate
adjacent wavelengths, passing one wavelength to the
first photodetector, passing the other wavelength to a
second photodetector, and transmitting signals from the
second photodetector to the processor for integrating
the signals from both photodetectors and determining the
temperature of the particle from the ratio of the
integrated signals.
In both of the above embodiments, it is
preferred that the focusing step includes carrying the
radiation via an optical fibre from a position adjacent
the plasma jet to a protected location remote from the
plasma jet, and locating the photodetectors in the
protected location.
In accordance with another embodiment an
optical sensor for plasma sprayed particles in a plasma
jet is comprised of a sensor head mounted rigidly
adjacent the plasma jet; the head comprising an optical
fibre for carrying radiation emitted by a particle in
the jet to a protected location remote from the jet,
optical apparatus for focusing the radiation on a first
end of the fibre, and a slit mask formed of a pair of
parallel slits disposed over the end of the fibre
through which the radiation may pass; and at the
protection location, a pair of photodetectors, apparatus
for splitting the radiation from a second end of the
fibre, apparatus for filtering the split radiation into
two separate adjacent wavelengths, and apparatus for
passing the separate wavelengths into respective ones of
the photodetectors.
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BRIEF INTRODUCTION TO THE DRAWINGS:
A better understanding of the invention will
be obtained by reference to the detailed description
below, in conjunction with the following drawings, in
s which:
Figure 1 is a sectional view of a sensor head
in accordance with the present invention and of a plasma
torch to which the sensor head is rigidly attached,
Figure lA illustrates an optical mask used in
the invention,
Figure 2 is a block diagram of another portion
of the invention, which is attached to the sensor head
of Figure 1,
Figure 3 illustrates the field of view of the
pair of slits shown in Figure lA, and
Figure 4 is a graph of the amplitudes of the
output signals of the two detectors shown in Figure 2 as
a function of time.
DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE
INVENTION:
Turning to Figure 1, a substrate 1 is given a
coating 2 by means of a plasma spray 3 of hot particles,
emitted by a plasma spray gun 4. A sensor head 8 is
comprised of a lens 9 that images, after reflection on a
flat mirror 10, the first end of an optical fibre 11
into the particle jet of hot particles 3 preferably to a
single particle. Ray lines 12 illustrate the reciprocal
imaging of a particle onto the end of the optical fibre
11 .
The first end of the fibre is covered by an
optical mask 14 as shown in Figure lA. The optical mask
contains two transparent parallel slits 15. Preferably
the slits are about 25~m wide, 50~m long and 50~m center
to center. With an optical magnification of 3, the
slits formed by the lens 9 are about 75~m wide, 150~m
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long, and 150~m center to center. The slit size and
arrangement are important for reliable temperature and
velocity measurements even when high particle flow rates
are used.
S Turning now to Figure 2, the radiation
collected by the sensor head, i.e. at the output (second
end) of the optical fibre 11, is sent to a system which
is located away and protected from the harsh environment
of the plasma spray gun. It should be well shielded
electronically and kept in a quiet environment far from
the operating spray gun and torch. Radiation from the
optical fibre is imaged via a dichroic mirror 17, via a
convex lens 18, on two photodetectors, Dl and D2,
through respective interference filters 20 and 21.
Output signals from detectors D1 and D2 are digitized in
analog-to-digital converters (not shown) and may be
analyzed by a computer 24 which computes the temperature
and velocity of in-flight particles from the signals, as
described below.
Figure 3 illustrates the field of view of the
two slits 15 of the sensor head. The depth of field is
shown by the width between the horizontal arrow heads.
A particle 27 of the mass of hot particles travelling
through the focal plane will generate a double peak
light (radiation) pulse as it moves from the field of
view of the first slit to the field of the view of the
second slit.
Examples of the light (radiation) pulses are
illustrated in Figure 4, which show signals output from
both detectors D1 and D2 drawn as a function of time
during the passage of a few particles in the sensor
field of view. From the time delay between the two
components of each pulse, the particle velocity can be
computed, since the distance between the two slit images
in the focal plane is known. The particle temperature
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may be determined from the ratio of the integrated light
pulses at both wavelengths, i.e. from detectors ~1 and
D2. The slit dimensions should be established such that
the radiation is collected from the smallest possible
S volume in the particle jet, to reduce to a minimum the
background signal intensity, but the slit images must be
larger than the particle diameter in order to collect
intense light signals from each single particle.
The computer 24 analyzes the detector outputs
by performing continuously statistical computation to
obtain the mean and standard deviation of the
temperature and velocity distributions. These values
are used to provide continuous feedback to the plasma
torch main spraying variables, i.e. arc current, powder
feeding gas pressure, etc.
It may be seen that since the properties of
the particles themselves ; ~~iately before their impact
on the substrate are directly measured, the effect of
plasma gun wear, etc., may be automatically compensated.
Moreover, the direct particle localization may permit
the measurement of temperature and velocity at many
points within the particle jet, permitting obt~in;ng a
precise characterization of the spraying process. It
may be seen that this has significant advantages over
the indirect measurement techniques of monitoring arc
current and power, arc gas flow rates, powder feed
rates, and powder carrier gas pressure.
An advantage of the present invention over
active projection particle velocity measurement
techniques lies in the fact that the present invention
does not require the use of fragile laser devices or
intense light sources. Accordingly a more compact and
rugged sensor is obtained that does not require any
special eye protection for the operator.
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The present invention may also be used in
conjunction with the two-colour pyrometer described
earlier, without the use of any additional detectors or
electronics.
As compared to passive techniques, the use of
the two-slit mask permits the particles to be measured
directly without the use of a laser beam or a second
detection assembly focused in the same region within the
particle jet. The distance between the axes of the two
focused beams is nearly constant through the depth of
field, as shown in Figure 3, while the width of a single
beam, proportional to the time of flight in a single-
slit configuration, changes very quickly. This requires
a second coincidence detection to localize the particle,
not required in the present invention. These advantages
are particularly important in an industrial environment.
A person understAn~ing this invention may now
conceive of alternative structures and embodiments or
variations of the above. All of those which fall within
the scope ~f the claims appended hereto are considered
to be part of the present invention.
