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Description  |
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BACKGROUND
FIELD OF THE INVENTION
The present invention relates to stabilized fiber optical measuring
apparatus having structure, when measuring with analog fiber optical
transducers, for compensating the instabilities existing in light-emitting
diodes, photo-diodes, photo-current amplifiers, light-emitting diode
electronics, optical fibers, fiber contacts, fiber holders, transducer
mechanical devices, etc.
SUMMARY OF THE INVENTION
The measuring apparatus according to the invention is intended to be used
for fiber optical measurement of different physical magnitudes such as
position, acceleration, force, pressure, elongation, temperature, etc. An
essential feature of the measuring apparatus is that the light from the
transducer is divided into at least two components, one measuring
component which contains information about the measuring signal and the
status of the optoelectronics, and at least one stabilization component
which contains information about the status of the electronics. The
measuring component and the stabilization component are separated by
electronic filters coupled to the optical detector of the measuring
apparatus, and after demodulation of the stabilization component a
compensating signal is obtained which is utilized to suppress the
dependence of the measuring signal on the above-mentioned instabilities of
the measuring apparatus. The measuring signal can thus be obtained by
dividing the measuring component and the compensating signal, or as a
measuring component directly if the compensating signal controls a
regulator for the power to the light source or the amplification of the
detector amplifier of the measuring apparatus.
BRIEF DESCRIPTION OF THE DRAWINGS
The measuring device will be described with reference to the accompanying
drawings, wherein:
FIG. 1 shows an embodiment of the invention for regulating the light source
with a non-periodic signal;
FIG. 2 shows an embodiment of the invention for regulating the light source
with a periodic signal;
FIG. 2a shows a modified embodiment using electrical modulation;
FIG. 3 shows a modified embodiment using square wave regulation of the
light source;
FIG. 4 shows an embodiment of a transducer arrangement with a bidirectional
fiber and divider;
FIG. 4a shows a modified embodiment of a light-emitting diode and a
photo-diode arrangement;
FIG. 5 shows a transducer with an optical fiber and a half mirror;
FIG. 6 shows a transducer with an optical fiber and two gratings which are
movable relative to each other;
FIG. 7 shows an additional transducer with an optical fiber and two
gratings which are movable relative to each other; and
FIG. 8 shows a transducer with optical thin film modulators.
DETAILED DESCRIPTION
In FIG. 1, light-emitting diode (LED) 1 is arranged to emit a constant
light into optical fiber 2 for conducting light to a transducer S. Between
the output end of fiber 2 and the input end of fiber 7 for conducting
light from the transducer S, there is an air gap, in which screen 3 of
transducer S is able to move. Transducer S comprises mechanical system 4
by means of which measuring quantity 5 is transferred into a movement of
screen 3, which is located in the air gap. System 4 operates within a
specified frequency range and the movement of screen 3 in the air gap
gives rise to an intensity modulation of the light into fiber 7. To
achieve a linear relationship between the amplitude of movement of screen
3 and the light variation in fiber 7, linearizing gap 6 is arranged in the
air gap. Gap 6 may be constructed so that a rectangular beam of rays is
obtained, for example, the cross-section of which is determined by how
large a portion of the gap is covered by screen 3. The light from fiber 7
is detected by photo-diode 8 and is amplified to a suitable level by
amplifier 9. Part of the light in fiber 2 from LED 1 is coupled by oblique
joint 39 to photo-diode 40, the photo-current of which is amplified by
amplifier 41. A correlation of the outputs of amplifiers 9, 41 is
performed in correlation circuit 42, whereby the noise on the output of
amplifier 41 is correlated with the noise on the output of amplifier 9 in
such a way that the output signal from correlation circuit 42 consists of
the detector signal after the noise from LED 1 has been removed by
correlation. The simplest method of performing this correlation comprises
subtracting the output signal of amplifier 41 from the output signal of
amplifier 9 and controlling the amplification of amplifier 41 until a
minimum of noise is obtained in the difference signal, which then also
constitutes the output signal from correlation circuit 42. The output
signal from correlation circuit 42 is supplied both to lowpass filter 12
and to bandpass filter 14, the pass frequency range of which corresponds
to the frequency range of system 4. The output signal U(t) from filter 14
constitutes the measuring signal of the measuring apparatus and is
compared with reference signal V.sub.ref in summation device 11, and the
output signal therefrom controls regulator 13, which regulates drive
electronics system 10 of LED 1 in such a way that the output signal from
filter 12 becomes equal to V.sub.ref. In that way compensation is obtained
for variations of the parameters in electronics system 10, LED 1, optical
fiber 2, optical fiber 7, photodiode 8 and amplifier 9. The dashed line
from the output of regulator 13 to amplifier 9 indicates how regulator 13
can also control the amplification of amplifier 9, electronics system 10
then being controlled with a constant signal.
The arrangement according to FIG. 1 does not compensate for the zero point
drift of the photo-detector. If such compensation is required, alternating
light sources are used for both the measuring and the regulating
components according to FIG. 2. Parts 1-10 are identical with those of
FIG. 1. Drive device 10 for LED 1 is fed by an alternating voltage from
oscillator 18 operating at frequency f.sub.0 through controllable
amplifier 17. The output of amplifier 9 is connected to band-pass filter
15 having pass frequency f.sub.0. The filtered signal is rectified in
rectifier 16 and is supplied via low-pass filter 12 to summation device
11, where the signal is compared with reference voltage V.sub.ref. The
output signal from summation device 11 is supplied as an error signal to
regulator 13 which controls amplifier 17, the output signal of which
enters driver 10. LED 1 is controlled so that the output signal from
low-pass filter 12 is kept equal to V.sub.ref. Amplifier 17 controls the
amplitude of the alternating signal which is generated by oscillator 18.
In its simplest form controllable amplifier 17 may consist of a multiplier
circuit or a controllable voltage divider with a field effect transistor,
the voltage of which is controlled by regulator 13. The frequency f.sub.0
of oscillator 18 is at least twice greater than the highest occurring
frequency of the measuring signal to fulfill the sampling thereom. The
measuring signal is obtained by demodulation in filter 14 of the output
signal from amplifier 9. The frequency f.sub.0 can be called the carrier
frequency of the measuring apparatus and the carrier wave can be said to
be amplitude modulated by the movement of screen 3 in the transducer.
FIG. 2a shows that the measuring signal can consist of an electrical
signal. The light from LED 1 passes through optical fiber 2 to photo-diode
35 placed in the transducer, the photo-current of the photo-diode being
amplified in amplifier 36 and thereafter supplied to controllable
amplifier 37 as a control signal. The measuring voltage U.sub.m is
amplified in amplifier 37 and drives LED 38 which sends light into optical
fiber 7, to photo-diode 8. The electronics system for producing the
measuring signal and regulating of LED 1 is the same as that shown in FIG.
2. The modulation of the signal through optical fiber 1 is performed
electrically in this case, instead of mechanically as previously shown.
FIG. 3 shows a measuring device with a somewhat simpler electronics system
than that shown in FIG. 2. This can be achieved if the carrier wave
consists of square wave 28, which is obtained from the output of analog
switch 20. Square wave 29 which is amplitude modulated by the sensor is
then obtained from photo-current amplifier 9. This square wave is fed
through high-pass filter 21 to multiplier 22 in which the output signal
from filter 21 is multiplied by output signal 30 from square wave
oscillator 23. The measuring signal is taken out by band-pass filter 24,
whereas the status of the optoelectronics is taken out after low-pass
filter 25. The upper limit frequency of this filter is chosen lower than
the lower limit frequency of the measuring signal. The output signal from
low-pass filter 25 and reference signal V.sub.ref are compared in
summation device 27, from which a difference signal is taken out and
supplied to integrator 26 which controls the amplitude of the square wave
to driver 10 so that the output signal from filter 25 becomes equal to
V.sub.ref. In this way compensation is ensured for parameter variations of
the optocircuitry. Signal 30 from oscillator 23 is used to control switch
20 and the frequency f.sub.0 must be chosen at least twice as great as the
highest measuring frequency occurring.
The previously described devices are also suited for fiber optical
measuring devices having only one fiber between the transducer and the
light-emitting and photo-diodes, such as is shown in FIG. 4. The light
from LED 1 is switched via fiber 2 into fiber 31, which conducts the light
to the transducer. This consists of linearizing column 6 in the air gap in
which screen 3 moves, and mirror 32 which reflects part of the emitted
light back into fiber 31. Part of the reflected light is also reflected
into fiber 7 and thereafter to photo-diode 8. The photo-current from
photo-diode 8 is amplified in amplifier 9 and is divided into measuring
and stabilization components by filters 14 and 12, respectively. The
output signals from the two filters are supplied to divider 19, the output
signal of which consists of the quotient between the aforesaid two
components and constitutes measuring signal U(t). This quotient formation
compensates the measuring signal for instabilities in the optoelectronics
and fiber optics. Instead of the illustrated optical Y-connection between
fibers 2, 7 and 31, the LED and photo-diode arrangement according to FIG.
4a can be used, resulting in reduced light losses. LED 1 sends light into
the center of the end surface of fiber 31, whereas annular photo-diode 9,
which is positioned concentrically around the LED, receives the light
reflected from mirror 32 into fiber 31.
FIG. 5 shows a modified embodiment of the optical transducer according to
FIG. 4. In this embodiment fiber 31 is attached to transducer housing 33
and portion 34 of the fiber located inside the transducer housing is
rotatable around the attachment, whereby the end surface of fiber portion
34 moves partly in front of mirror 43. When the fiber end surface moves
upwardly, less light is reflected into the fiber, and when the fiber end
moves downwardly, more light is reflected back into the fiber. However,
the transducer is insensitive to movements perpendicular to the plane of
the paper. Since the fiber end functions as a mechanical oscillating
system of the second order, the transducer constitutes an optical
accelerometer.
To increase the sensitivity, and simultaneously the linearity of the
geometrical-optical transducer, the arrangement of gratings according to
FIGS. 6 and 7 can be used. According to FIG. 6, the light from fiber 31 is
switched via index-matching material 44 to light conducting plate 45 of
glass or quartz. Plate 45 is coated on its bottom side with thin
reflecting grating 46, for example by means of a vaporization technique.
Through this grating part of the light is switched by index-matching
material 47 and another grating 50 to light conducting plate 48. Plate 48
is covered on its end surfaces with reflecting layers 49, by means of
which the light is reflected back through material 47 to plate 45 and into
fiber 31. The amount of light fed back into fiber 31 is determined by the
coupling between plates 45 and 48, which in turn is determined by the
relative position between gratings 46 and 50. Plate 48 is fixed to the
bottom of transducer housing 51 and plate 45 is fixed to movable compound
52, which is pressed downwards by spring 53 to minimize the distance
between gratings 46 and 50. Laterally compound 52 is retained by springs
54. The shown and described transducer functions as an accelerometer,
which is sensitive in the direction of arrow 55, the gravitational
compound 52 and the spring constant being determined by the springs and
possibly by index-matching material 47. Alternatively, the light can be
switched in from fiber 31 to lower plate 48, upper plate 45 then being
provided with short reflecting sides.
FIG. 7 shows that the light from fiber 31 is led into the gratings
perpendicular to the surfaces of plates 45 and 48. The light passes via
index-matching material 44 into plate 45 and further through grating 46,
material 47, grating 50 and plate 48, and is reflected by reflecting
surface 56 on the back side of plate 48 and returns through the grating,
whereby part of the reflected light is conducted back into fiber 31. Such
a transducer functions as a strain gauge. At point 58, plate 48 is
attached by its lower end to load cell 57, whereas plate 45 is connected
by its upper end to load cell 57 by clamp 59. When a force F acts on load
cell 57, the load cell is compressed, whereby gratings 46 and 50 move
relative to each other and give rise to a modulation of the light passed
in from fiber 31. To minimize the distance between the two gratings,
spring 60 is arranged between transducer housing 51 and plate 45.
FIG. 8 shows a measuring device having optically thin film modulators 76
and 77, which modulate the light in thin film light conductor 86 in thin
film substrate 85. Light from semiconductor laser 74, which is driven from
driver 73, is switched into monomode fiber 75, which is connected to light
conductor 86 which may be, for example, a Ti-diffused LiNbO.sub.3 crystal.
This light is modulated by electro-optical modulators 76 and 77 before
being switched over to fiber 78, which conducts the modulated light to
photo-detector 79, the photo-current of which is amplified in amplifier
80. Modulator 77 is controlled by measuring signal U.sub.in, whereas the
other modulator 76 is controlled by photo-diode 71 via current/voltage
convertor 72. Photo-diode 71 receives modulated light via fiber 70 from an
LED or laser 69, which is driven from driver 68 and oscillator 67 with
frequency f.sub.o. The received signal which occurs at the output of
amplifier 80 is divided into a measuring component and a stabilization
component by two band-pass filters 81 and 82, respectively. After
demodulation of the stabilization component f.sub.o in demodulator 83 and
quotient formation between measuring component f.sub. m and the
stabilization signal, which corresponds to the demodulated stabilization
component f.sub.o, a measuring signal is obtained at the output of divider
84, which measuring signal, through the quotient formation, is compensated
for instabilities in the optoelectronics and the fiber optics.
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