Method of controlling a short-etalon fabry-perot interferometer used in an NDIR mearsurement apparatus

A method of controlling a short-etalon Fabry-Perot interferometer used in an NDIR measurement apparatus includes generating a measurement signal using a radiant source. The measurement signal is provided to a sample point containing a gas mixture to be measured. The measurement signal is bandpass-filtered with an electrically tuneable Fabry-Perot interferometer using at least two wavelengths of the interferometer passband. The measurement signal is passed via an optical filter component prior to detection, and the filtered measurement signal is detected by a detector. During the measurement cycle, the passband frequency of the interferometer is controlled to coincide at least partially with the cutoff wavelength range of the optical filter component.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention is related to a method according to the preamble of 
claim 1 for controlling a short-etalon Fabry-Perot interferometer used in 
an NDIR measurement apparatus. 
2. Description of the Background Art 
Nondispersive infrared measurement equipment are conventionally used for 
gas concentration measurements. The method can be made extremely selective 
with respect to the gas to be measured by limiting the wavelength range 
used for the measurement to coincide with the characteristic absorption 
band of the gas under measurement. In the NDIR method, the wavelength 
range is generally selected by means of a bandpass filter. Disclosed in 
the EP patent application 94300082.8 is a tuneable interferometer suited 
for replacing an optical bandpass filter of an NDIR measurement apparatus. 
The passband wavelength of the interferometer is voltage-controlled thus 
making the interferometer capable of sweeping measurements in which the 
measurement can be made at two or a greater number of wavelengths. Here, 
it is advantageous to measure the gas to be analyzed exactly at its 
absorption band while the reference measurement is made at an adjacent 
wavelength. Thus, the reference measurement facilitates compensation of 
aging processes and temperature dependence in the measurement equipment. 
Further, it is possible to determine the concentrations of a plurality of 
different gases by making the measurements at wavelengths corresponding to 
the absorption bands of said gases. 
When using such a tuneable interferometer, it is crucial to the stability 
of the measurement how well the voltage dependence of the center 
wavelength of the interferometer passband stays constant. In cited EP 
patent application 94300082.8 is further disclosed a tuneable short-etalon 
interferometer intended for gas concentration measurements. The passband 
wavelength of the interferometer is adjusted by altering the distance 
between the interferometer mirrors with the help of an electrostatical 
force. Such an interferometer can be manufactured by surface 
micromechanical techniques so as to comprise a plurality of superimposed, 
IR radiation transmitting thin-film layers, whose thickness is selected to 
make the multilayer structures perform as the mirrors of the 
interferometer. 
Conventionally, the IR radiation used in the NDIR measurement equipment is 
modulated. The purpose of this arrangement is to obtain an AC signal out 
from the detector which is advantageous in terms of noise and drift 
compensation in the electronic circuitry. The IR radiation can be 
modulated by chopping the input power to the IR radiation source. For 
this, however, a sufficiently short thermal time constant is assumed from 
the IR radiation source to facilitate a sufficiently high modulation rate. 
A suitable IR radiation source is formed by, e.g., a microlamp permitting 
a modulation rate as high as about 10 Hz. However, the modulation of the 
glow filament temperature causes an extra stress which shortens the 
service life of the lamp filament. To achieve a higher radiation output 
power, a heatable element of larger radiating area must be used, whereby 
the heating rate is retarded. Thence, the radiation has to be modulated 
with the help of a separate mechanical chopper placed on the optical path 
of the radiation. Unfortunately, the service life of such a mechanical 
chopper is limited. 
In the long run, the internal stress of the interferometer mirror may drift 
causing a change in the curvature of the mirror. This in turn shifts the 
mutual distance of the mirrors at a given level of the control voltage 
thus also shifting the passband wavelength of the interferometer. 
Resultingly, instability occurs in the function of the NDIR measurement 
apparatus. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to overcome the drawbacks of the 
above-described techniques and to achieve an entirely novel type of method 
for controlling a short-etalon Fabry-Perot interferometer used in an NDIR 
measurement apparatus. 
A goal of the invention is achieved by adjusting the passband wavelength of 
said short-etalon Fabry-Perot interferometer to coincide at least 
partially with the cutoff wavelength range of the optical filter in said 
NDIR measurement apparatus. According to a preferred embodiment of the 
invention, the entire passband of the interferometer is controlled in a 
cyclically repetitive manner sufficiently far into the cutoff wavelength 
range of the optical filter in order to use the interferometer as an 
amplitude modulator of the IR radiation. According to another preferred 
embodiment of the invention, the voltage dependence curve of the 
interferometer length is calibrated by controlling the interferometer 
passband to coincide with the optical filter cutoff edge wavelength, 
whereby such a stable passband wavelength gives a fixed reference point 
for the voltage dependence curve of the interferometer. 
The invention offers significant benefits. 
The method according to the invention for controlling the interferometer 
replaces the use of a mechanical chopper or electrical modulation of the 
IR radiation source. 
Hence, an embodiment according to the invention provides both a lower cost 
and longer service life. According to the invention, the IR radiation 
source can be driven by a DC source, which is more cost-efficient and 
imposes no additional stress on the IR radiation source due to its 
temperature modulation. 
Furthermore, the automatic calibration method according to the invention 
gives the NDIR measurement equipment a good long-term stability and 
removes the need for a separate calibration step.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, an NDIR measurement apparatus utilizing a short-etalon 
interferometer is outlined. The measurement apparatus comprises the 
following components: 
a radiation source 1, 
a measurement channel 2, 
an optical longpass filter 3, 
a voltage-tuneable short-etalon interferometer 4, and 
a detector 5. 
The radiation source 1 is formed by a wideband thermal IR radiation source 
such as an incandescent lamp, for example. The radiation emitted by the 
source is passed into the measurement channel 2 containing the gas under 
measurement. The amount of radiation passed through the measurement 
channel is detected by means of the detector 5. Prior to detection, a 
wavelength range useful for the measurement is selected from the wideband 
spectrum of radiation by means of the optical longpass filter 3 and the 
interferometer 4. The interferometer 4 is utilized so that the measurement 
is performed by virtue of the voltage control at two passband wavelengths 
corresponding to: the absorption band wavelength and the reference 
wavelength. The absorption band wavelength is selected to coincide with 
the characteristic absorption spectrum of the gas under measurement so 
that concentration-dependent absorption caused by the gas under 
measurement causes a decrease in the amplitude of the signal obtained from 
the detector output. The reference wavelength is selected adjacent to the 
absorption band wavelength. The purpose of the measurement at this 
wavelength is to provide a signal independent from the concentration of 
the gas under measurement that represents the basic intensity of radiation 
passing the measurement channel without absorption and that can be used 
for the error compensation of changes in the intensity of radiation 
transmitted from the source. 
Referring to FIG. 2, shown therein is a schematic diagram of an 
electrostatically tuneable short-etalon interferometer having the lower 
mirror 6 forming the stationary part of the interferometer 7 and the upper 
mirror 8 acting as the part which is movable by means of a control voltage 
U. The interferometer 7 may be fabricated by surface micromechanical 
techniques, whereby the upper mirror 8 is formed by a flexible multilayer 
thin-film structure. The distance L between the mirrors of the IR-band 
short-etalon interferometer is typically in the range 0.5-5 .mu.m. 
The distance L between the mirrors 6 and 8 is controlled by means of an 
external voltage U. The force of electrostatic attraction between the 
mirrors is obtained from the formula 
EQU F.sub.s =.epsilon.A/2(U/L).sup.2 (1) 
where .epsilon. is the dielectric constant of a vacuum and A is the surface 
area of the mirror. The force opposing the movement of the upper mirror 8 
can be described with sufficient accuracy by a single spring constant k. 
Denoting the distance between the mirrors at rest by L.sub.0, the spring 
force F.sub.j may be written 
EQU F.sub.j =k (L.sub.0 -L) (2) 
The change in the distance between the mirrors 6 and 8 caused by a given 
control voltage can be written assuming that in a static situation the 
electrostatic force and the spring force are equal in magnitude (F.sub.s 
=F.sub.j) but acting in opposite directions 
EQU .epsilon.A/2(U/L).sup.2 =k (L.sub.0 =L) (3) 
In FIG. 3 are shown the relationship between the absorption band a and the 
reference wavelength band b when the distance between the mirrors of the 
short-etalon interferometer is controlled to 2.1 .mu.m and 2.0 .mu.m, 
respectively. The distance between the mirrors 6 and 8 is selected for 
measurements of carbon dioxide concentration. The absorption spectrum of 
carbon dioxide is centered at 4.26 .mu.m wavelength. 
Conventionally, the radiation passed in an NDIR measurement apparatus 
through the measurement channel is amplitude-modulated either by 
electrically chopping the input power to the radiation source or 
mechanically using a separate optical chopper. Then, the detector output 
provides an AC signal from which the offset component of the detector dark 
signal is eliminated. The AC signal is also useful in drift compensation 
of the detector signal amplifier circuit. Furthermore, the noise component 
of the signal can be reduced by passing the signal through a narrowband 
filter. When using a pyroelectric detector, the radiation must necessarily 
be modulated, because a pyroelectric detector is sensitive to intensity 
changes of the radiation alone and does not give any DC output signal as a 
response to a constant level of impinging radiation. 
In the embodiment according to the invention, the amplitude-modulation of 
radiation intensity is implemented, e.g., by means of the interferometer 7 
shown in FIG. 2. Modulation is achieved as shown in FIG. 3 by setting the 
control voltage of the interferometer 7 so that the interferometer 
passband is shifted outside the passband of the optical longpass filter, 
into its cutoff wavelength range d. Thus, the use of the "blanked" 
passband c obtained as shown in FIG. 3 by controlling the interferometer 
passband wavelength sufficiently far to the optical longpass filter cutoff 
wavelength range d replaces the conventional method of chopping the 
radiation source. The detector provides an AC signal when the 
interferometer passband wavelength is alternated between the "blanked" 
passband and the active passbands a and b. 
The "blanked" passband shown in FIG. 3 is achieved by controlling the 
distance between the interferometer mirrors to 1.9 .mu.m. The distance 
between the mirrors of a voltage-tuneable interferometer can be brought 
down to approx. 25% of the distance between the mirrors in an 
interferometer at rest. Hence, the different passbands shown in FIG. 3 are 
clearly within the wavelength sweep range of a single interferometer 
structure. 
The "blanked" passband can basically be used in two different ways for the 
control of the interferometer: 
1. The interferometer passband wavelength is cyclically shifted between 
wavelengths of the blanked passband c and the absorption passband a of the 
gas under measurement. Then, the detector provides an AC output signal 
whose amplitude is proportional to the intensity of radiation impinging on 
the detector within the wavelength range of the absorption passband. 
Correspondingly, the reference output signal is obtained by shifting the 
wavelength of the interferometer passband between the blanked passband c 
and the reference passband b. 
2. The interferometer passband wavelength is cyclically shifted in a 
sequence between the wavelengths of the passbands a, b and c and the 
corresponding output signals S.sub.a, S.sub.b and S.sub.c, of the detector 
are recorded synchronized with the wavelength shifts of the interferometer 
passband, respectively. The output signal values are stored in the memory 
of a microprocessor used for controlling the measurement apparatus, after 
which the value S.sub.c, of the detector "blanked" output signal is 
deducted from the signal values S.sub.a and S.sub.b. To improve the 
signal-to-noise ratio, the measurement sequence can be repeated cyclically 
several times for averaging the measurement results. 
For stable operation of the interferometer 7, it is important that the 
wavelength of interferometer passband at a given value of the control 
voltage stays maximally constant. A change in the spring constant of the 
upper mirror 8 causes a change in the distance between the interferometer 
mirrors resulting in a corresponding drift of the passband wavelength. 
Such a drift may be caused by, e.g., a change in the internal stresses of 
the upper mirror 8. The calibration method according to the invention 
utilizes an integral wavelength reference by virtue of which the effect of 
the change in the spring constant of the mirror at the wavelength of 
interferometer passband can be eliminated by computing a suitable factor 
of correction to be used in the drift correction of the interferometer 
control voltage. 
The calibration method is based on utilizing on the path of the measurement 
channel an IR radiation transmitting element with such a suitable shape of 
the transmittance curve that has a cutoff edge, a transmittance minimum or 
a transmittance maximum capable of performing as a wavelength reference. 
Thence, the method can employ an optical longpass filter such as the one 
illustrated in FIG. 3 with the passband cutoff edge wavelength tuned at 
approx. 3.8 .mu.m. 
Referring to FIG. 4, the detector output signal amplitude is plotted 
therein for different values of the interferometer control voltage when 
the optical path is provided with a longpass filter 3 illustrated in FIG. 
3. As is evident from the diagram, when the control voltage is increased, 
the amplitude of the detector output signal obtained from the 
interferometer channel starts to drop by the cutoff effect of the optical 
longpass filter 3. The interferometer passband control curves L1 and L2 
shown in FIG. 4 differ from each other due to a change in the spring 
constant of the upper mirror 8. In more detail, owing to a change in the 
spring constant of the upper mirror 8, the passband control curve L1 has 
the passband control voltage of the interferometer 4 corresponding to the 
cutoff edge wavelength of the optical longpass filter 3 shifted by approx. 
0.4 V in comparison with curve L2. Obviously, the distance L between the 
interferometer mirrors 6 and 8 is equal on both curves L1 and L2 for equal 
transmittance percentage values of, e.g., 50%, corresponding to a control 
voltage of 8.4 V on curve L1 and 8.8 V on curve L2. 
The automatic calibration method based on utilizing an optical longpass 
filter 3 as a wavelength reference can be applied in, e.g., the following 
way: 
the reference wavelength is defined to be, on the cutoff edge of the 
optical longpass filter transmittance curve, that wavelength at which the 
detector output signal is reduced to 50% from its maximum value, 
the factory calibration is made so that the interferometer control voltage 
is swept over a suitable voltage span with the help of a microprocessor, 
the measurement values are stored in the memory of the microcomputer and 
the control voltage value U.sub.a corresponding to said 50% reduction in 
detector output signal amplitude is computed by, e.g., interpolation, 
the thus obtained value U.sub.a is stored in the memory of the 
microcomputer, 
the automatic calibration cycle of the measurement apparatus is performed 
analogously to factory calibration in order to identify a possible change 
in the transmittance properties of the apparatus and the new value U.sub.b 
corresponding to said 50% reduction in detector output signal amplitude is 
computed, 
the gas concentration measurements are subsequently performed using a 
control voltage value corrected by the factor U.sub.b /U.sub.a as the 
interferometer control voltage. Hence, for example, if the absorption band 
of carbon dioxide during factory calibration was coincident with a value 
UC.sub.CO2 of the interferometer control voltage, a corrected value 
(U.sub.b /U.sub.a) * U.sub.CO2 of the control voltage will be used during 
measurements. 
The longpass filter 3 is selected so that no absorption spectrum components 
of other gases possibly disturbing the measurement can occur at the 
wavelength of the cutoff edge of the filter spectral transmittance curve. 
With the help of the microprocessor incorporated in the measurement 
apparatus, the wavelength reference obtained from the cutoff edge 
wavelength of the optical longpass filter may also be resolved using more 
advanced curve-fitting computational algorithms. Hence, the 
above-described technique using the 50% reduced signal value must be 
understood as an exemplifying method only. 
On the basis of Equation 3 above, it can be shown that the same correction 
factor U.sub.b /U.sub.a may be universally used for all values of the 
interferometer control voltage. A precondition to this is, however, that 
the model of a single spring constant can describe the motion of the upper 
mirror 8 with a sufficient accuracy. 
The longpass filter 3 typically is an interference filter manufactured as a 
multilayer thin-film structure. A disadvantage in the use of an 
interference filter for an automatic calibration process is related to the 
temperature dependence of the filter spectral transmission curve. In this 
respect, a better alternative is to use, e.g., a suitable glass grade 
having a transmittance minimum within the wavelength sweep range of the 
interferometer. For example, the spectral transmittance curve of a thin 
Vycor glass plate is suitable for carbon dioxide gas concentration 
measurement, since this glass grade has a distinct transmittance minimum 
at approx. 4 .mu.m. Analogously to the edge wavelength of a longpass 
filter, such a transmittance minimum can be utilized as a wavelength 
reference in the calibration of an interferometer. Here, the measurement 
results of the spectral sweep obtained by means of the interferometer must 
be corrected according to the spectral transmittance curve of the glass. 
Alternatively, a suitable type of radiation-transmitting polymer may be 
used as the wavelength reference.