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BACKGROUND OF THE INVENTION
The invention relates to a flat tactile sensor having a network of optical
fibers between a flat support and an elastomer layer resting thereon,
which are respectively arranged to be parallel, and to cross each other at
an angle, and having a readout unit for capturing the changes in intensity
of the light fed into the optical fibers.
DESCRIPTION OF THE PRIOR ART
To extend their area of application, modern robotic systems require tactile
sensors, which measure with fine resolution in a way analogous to human
touch the force distribution affecting a robot gripper. In doing so, such
sensors provide the input signals for the control circuits of the robotic
systems.
Taking the tactile capabilities of the human fingers as a means of
orientation concerning the demands on tactile sensors, an approximation to
the following criteria is to be aimed at:
minimum detectable force 3.6.multidot.10.sup.-4 N (corresponding to a
weight of 36 mg);
maximum precision at 1-8.multidot.10.sup.-2 N with resolvable force
differences of 15-20%;
fine resolution (two-point resolution) 1-2 mm, so that one finger typically
corresponds to a 15.times.20 sensor array.
In a known sensor of the type mentioned at the beginning (GB-A-2,141,821)
the network consists of criss-crossed multimode glass fibers, in which, in
each case, light is fed in via light-emitting diodes at one end and a
photodiode is connected to the other end, in each case. The fibers are
bent via pressure on the sensor. This leads to changes in the intensity of
the light fed into the fibers by the light-emitting diodes. The effective
force is determined by measuring the transported light intensity. By
virtue of the criss-cross arrangement of the fibers, the action of the
force can be located in space. With such a sensor, flexures to which the
fibers are subjected outside the sensor also lead to changes in intensity.
It is not possible, therefore, to distinguish changes in intensity inside
the region of the sensor from such as occur owing to fiber flexure outside
the region of the sensor.
Further, a sensor is known (Journal IEEE Spectrum, August 1985, p. 49), in
which there is a rigid body provided with a matrix having openings in
which the ends of the fibers are fixed in each case. In the elastic
covering opposite the openings is arranged in each case a recess, the base
of which is provided with an optically reflective coating. When pressure
is exerted on the membrane, the optical reflection is changed, and a
signal is thereby emitted. In this connection, the change in the
reflectivity depends on the nature of the deformation of the reflective
surface, which, in turn, depends on the location at which the force acts
on the elastic covering.
SUMMARY OF THE INVENTION
It is the object of the invention to design a flat tactile sensor of the
type mentioned at the beginning in such a way that for very fine lateral
resolution it is simultaneously possible to achieve a high resolution of
force and a measure as precise as possible for the absolute value of the
effective force, and that flexures in the fiber sections lying outside the
sensor do not affect the result of measurements.
This object is achieved according to the invention in that the network
consists of arms of fiber optic two-arm interferometers, both arms of the
two-arm interferometer are arranged as measuring arms on the support and
the readout unit has a circuit for counting interference fringes.
Expedient designs are the subject of the sub-claims.
The construction of fiber optic two-beam interferometers is known, and
investigations are to hand, moreover, on the sensitivity of such
interferometers to strain and temperature--DFVLR-FB 85-86, 1985;
Proceedings "Fiber Optics 86", SPIE, Volume 630, L. R. Baker, ed.
Bellingsham, Washington (1986), pages 220-224; Proceedings "OFS" 86,
Tokyo, the Institute of Electronics and Communication Engineers of Japan,
Tokyo (1986), pages 291-294.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is illustrated in terms of examples in the drawing, and
described in detail below with reference to the drawing, in which:
FIG. 1 shows a schematic representation of a fiber optic Mach-Zehnder
interferometer.
FIG. 2 shows a schematic representation of a fiber optic Michelson
interferometer.
FIG. 3 shows a schematic representation of a two-polarization Michelson
interferometer with readout device.
FIG. 4 shows a schematic representation of a tactile sensor with a network
of four fiber optic Michelson interferometers.
FIG. 5 shows a three-quarter view of a tactile sensor with a network of six
fiber optic Michelson interferometers.
FIG. 6 shows a section along the line VI--VI in FIG. 5, with an additional
arrangement of fiber optic interferometers for measuring temperature with
fine resolution.
FIG. 7 shows a schematic representation of a tactile sensor with a network
of two fiber optic Michelson interferometers.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The fiber optic Mach-Zehnder interferometer represented in FIG. 1 has two
parallel optical fibers 2, 4, each of which is routed through two 3 dB
monomode couplers 6, 8, arranged at a distance from one another. The two
fiber lengths of fiber arms 10, 12 between the couplers 6 and 8 form the
measuring and reference arm of the interferometer. Light having an input
power P.sub.0 is fed into the fiber 2 from a light source 14, preferably a
laser light source. This light is coupled in or out of the fiber 4 by
means of the couplers 6 and 8. The output power P.sub.- and P.sub.+ at
the opposite end of the fibers 2 and 4 is given to the photodetectors 16,
18. Strains in one of the arms (=measuring arm) 10 or 12 relative to the
other arm (=reference arm) lead to a change in the difference of the
optical path length between 10 and 12, and thus to a change in intensity
that can be measured with the photodiodes 16, 18.
Details will be given further below.
In the fiber optic Michelson interferometer, represented schematically in
FIG. 2, two optical fibers 20, 22 are likewise provided, which are routed
through a fiber optic 3 dB monomode coupler 24. Measuring and reference
arm 26, 28 are silvered at their ends 30, 32 so that the light is
reflected. Here, too, light, preferably laser light, having a power
P.sub.0 is fed into the fiber 20 from a light source 14. This light is
reflected via the mirrors 30, 32, and the output power P.sub.- or P.sub.+
is given to a photodetector 34 at the other end of the glass fibers 20,
or absorbed by an optical isolator 36 in front of the light source 14 (for
example a laser diode), in order to avoid instabilities. Instead of
silvered end faces 30, 32 of the fibers 26, 28, it is also possible to
provide mirrors.
The Michelson type two-polarization interferometer according to FIG. 3
shows the complete wiring of such an interferometer in the form of a block
diagram. Laser light is fed into the fiber 38, which has the form of a
polarization preserving monomode fiber, via a laser 48, which is designed
either as a laser diode or as an He-Ne gas laser, via a first microscope
objective 50 or a suitable lens, an optical isolator 52 and a second
microscope objective 54. This supply unit is connected to the input arm of
the interferometer via a splice or a monomode fiber plug-and-socket
connector 56. The input arm and the fibers 40, which are routed through
the fiber optic 3 dB monomode coupler 42, are silvered at the ends of the
sections 44, 46, which form the measuring and reference arms,
respectively. A fiber optic polarization control 58 is arranged between
the plug-and-socket connector 56 and the coupler 42. A corresponding
polarization control 60 is arranged in the fiber 40, which is connected to
a Gradient-index lens 62, behind which is arranged a polarization beam
splitter 64. To the outputs of this splitter 64 there is connected, in
each case, a multimode fiber 66 for the vertical, and a multimode fiber 68
for the horizontal components of the output intensity. Instead of the
combination 62, 64 it is also possible to use a fiber optic polarization
beam splitter between 40 and 66, 68. The multimode fibers 66 or 68 can be
plastic fibers. In each case, they are connected to a
photodiode-preamplifier combination 70, 72, the outputs of which are
switched to readout electronics 74, to which is joined an up-down counter
76.
The sensor elements have the function of fiber optic strain sensors. In
terms of the corresponding relative phase shift of the light waves in the
two arms, they measure the change in optical path caused by the
flexure-induced strain of one of the two interferometer arms (measuring
arm) relative to the unaffected (reference) arm. By means of superposition
of the two light waves in a fiber optic coupler, the phase shift is
converted into a change in intensity (the interference signal), that can
be measured with a photodiode. In the simplest case, the output
intensities from the two out put arms of the Mach-Zehnder interferometer
or Michelson interferometer have the following form:
I.sub..+-. =1/2(1.+-.cos .DELTA..phi.), (1)
the two intensities being distinguished by the algebraic sign. .DELTA..phi.
is the phase difference between the two arms
##EQU1##
where .lambda..sub.0 is the vacuum wavelength, n the refractive index, and
L the geometrical length. The interference signal can be read out in the
form of alternations of brightness and darkness. For example, counting the
brightness maxima passing at the detector yields an absolute measure for
the strain, if the initial value (for example counter reading=0 for
unloaded fiber) is preset.
The strain .epsilon.=.DELTA. L/L parallel to the fiber longitudinal axis
that is required for an alternation between two neighboring interference
maxima of the intensity is taken as the sensitivity to strain. It
corresponds to a phase shift between the light waves in the two
interferometer arms of .DELTA..phi.=2.pi.. In general, we have
##EQU2##
the material parameters of refractive index n, elasto-optic constants
p.sub.11, p.sub.12 and Poisson number .nu. being a function of wavelength
and available from the literature.
When .lambda..sub.0 =786 nm (semiconductor laser diode) the following holds
for a Mach-Zehnder interferometer:
##EQU3##
In the case of the Michelson interferometer as sensor element, the
sensitivity is doubled, because the light waves traverse the extended
fiber section twice. The sensitivity can be doubled a further time, if in
addition to the interference maxima the minima are counted or if the zero
crossings of the signal according to equation (1) are counted, the
constant fundamental intensity having been previously subtracted
(electronically).
The longitudinal strain of a fiber fixed at the ends and subjected to
stress by a force acting transverse to the longitudinal axis of the fiber
is given, for small deflections .DELTA.H, by the quadratic relationship:
##EQU4##
The order of magnitude of the maximum permissible strain is determined by
the breaking limit. A value of
.epsilon..sub.max =0.4% (6)
can be taken as a reliable value, applying also to long-term stresses. For
a typical value of L=5 cm, the maximum deflection transverse to the fiber
axis is obtained as
.DELTA.H.sub.max =2.2 mm. (7)
With these values, there is a digital resolution of at least 8 bit, the
corresponding number of increments being distributed quadratically over
the measuring area, as befits the case described here (5).
The basic principle of the interferometric sensor element as described
above still does not enable the algebraic sign of the change in the
measurand in association with incremental readout to be recognized,
because the counting process itself sums up only absolute values (number
of intensity maxima).
The problem of the recognition of the algebraic sign can be solved in
various ways, which are known and described in the literature. The methods
are based on producing two interference signals which are phase-shifted
by, for example, .pi./2, but are otherwise identical and are recorded
simultaneously with two detectors. With reverse of the algebraic sign of
the measurand, the algebraic sign of the phase shift also reverses, and
this can be recorded by means of a simple logic circuit. Dependent on the
algebraic sign, the circuit carries the counting pulses into either the up
or down input of an up-down counter, so that the counter reading gives the
fiber strain in relation to the initial condition.
The production of a second, phase-shifted interference signal can be
effected electronically (by differentiating the signal downstream of the
detector) or optically. An optical method is to be preferred because of
the naturally enhanced noise in the differentiated signal. One possibility
is based on splitting the output light wave of the interferometer into two
orthogonally polarized components by means of a polarization beam
splitter, which is realized in FIG. 3 by the polarization beam splitter 64
or alternatively in a fiber optic fashion. The desired phase shift can be
set between these two fractions of the interference signal which are to be
read out separately, for example, by selecting a suitable polarization of
the input wave into the interferometer by means of the polarization
regulator 58.
The two phase-shifted, orthogonally polarized (for example horizontally H
and vertically V) output light waves are described by the following
equations for the intensities:
##EQU5##
a.sub.H,V and b.sub.H,V are complex functions of the input polarization,
birefringence of the fibers and of the angle between the fast or slow
fiber axes and the axes H, V of the polarization beam splitter.
In essence, the measurand M affecting the measuring arm of the
interferometer influences only the phase term .DELTA..PHI., insofar as the
birefringence of the fibers does not change too strongly. The phase
difference between the output intensities I.sub.H, I.sub.V, which is
independent of the measurand, is then given by the angular difference
.DELTA..phi.=arc (a.sub.H a.sub.H *)-arc (a.sub.V a.sub.V *). (9)
For the purpose of recognizing the algebraic sign (distinguishing between
strain or unloading of the measuring fiber (+ or -) or distinguishing
between strain of the measuring or reference fiber) by means of the logic
unit of the readout electronics, we set .DELTA..phi.=.+-..pi./2.
.DELTA..phi. changes its algebraic sign with change in the algebraic sign
of the variation of the measurand.
The setting of the .DELTA..phi. is done by setting a suitable input
polarization by means of the fiber optic polarization control 58--FIG.
3--in the input fiber. the intensity ratio I.sub.+H /I.sub.+V between the
output fibers 66, 68 can be set using the polarization regulator 60.
A tactile sensor constructed from four two-arm interferometers of the
Michelson type is shown schematically in FIG. 4. The four interferometers
78, 80, 82, 84 are shown in the simplified representation corresponding
to FIG. 2. The arms of the interferometers, which both function as
measuring arms and are therefore also designated as measuring arms in the
following, are embedded in an elastomer layer 86, or arranged under an
elastic membrane in contact with the latter, which layer is indicated here
by its dotted outline, and which is attached, in turn, to a rigid support
92 (cf. FIG. 5). The embedding is done in such a way that the glass fibers
forming the measuring arms are stressed in the axial direction and fixed
in or under the layer. The arrangement of the measuring arms has the form
of a network with equal spacings a between the measuring arms which are
arranged to run parallel, namely x.sub.1 -x.sub.4 in the horizontal
direction and y.sub.1 -y.sub.4 in the vertical direction in the
representation according to FIG. 4. In this connection, the measuring arms
x.sub.1 /x.sub.3, x.sub.2 /x.sub.4, y.sub.1 /y.sub.3 and y.sub.2 /y.sub.4
of the interferometers are arranged at a spacing 2a, so that between the
arms of one interferometer, there is, in each case, arranged an arm of the
other interferometer. This reduces the undesirable mechanical coupling
between associated measuring and reference arms of the interferometers,
which is conveyed by the elastic skin. Altogether, the network of the arms
of the four interferometers has 16 points of intersection m.sub.11
-m.sub.44. At these points of intersection the fibers lie spaced above one
another in the elastic covering, so that there is no direct contact
between the fibers at the points of intersection.
The points of intersection of the interferometer arms define a matrix of
measuring points m.sub.ij, formed by the points of intersection, which
make it possible to measure force with fine resolution given a sufficient
mechanical decoupling between the two arms of the individual sensor
elements. In the ideal case, a force F.sub.ij acts on one of the points
m.sub.ij, leading to a strain in the interferometer arms x.sub.i, y.sub.j.
The point m.sub.ij is uniquely determined by the counters mentioned and
the algebraic signs of the changes in counter reading.
For example, an orthogonal component of force f.sub.z (x,y)=F.sub.23 acts
on the point m.sub.23. In this connection, it is assumed that there is
sufficient mechanical decoupling between the individual measuring points
m.sub.ij, for example through suitable fixing in the "skin", which serves
to receive the glass fibers.
The detectors d.sub.x24 and D.sub.y13 then record a measuring signal. It is
assumed that the strain of the fibers x.sub.1,2, y.sub.1,2 delivers
positive counting pulses and that the strain of the fibers x.sub.3,4,
y.sub.3,4 delivers negative counting pulses (see above). Working from
measuring point m.sub.23 in the next stage, the detector unit D.sub.x24
records positive (N.sub.x2), and D.sub.y13 negative (N.sub.y3) counting
pulses. Since each of the, in each case, two readout units for the x and y
co-ordinates can distinguish the algebraic signs of the two allocated
interferometer arms, it is possible to distinguish all sixteen measuring
points uniquely using four readout units.
Generally, it will be not a point force, but a force distribution f(x,y)
which acts on the tactile sensor. Here, situations are conceivable in
which no measuring signals will be produced despite f(x,y).noteq.0. This
will always be the case if both interferometer arms of a sensor element
are simultaneously strongly strained to the same extent. The task in
designing the sensor consists in guaranteeing for all force distributions
that arise a sufficient decoupling of the in each case two arms of the
sensor elements. As described above, one possibility consists in making
the distance between the two arms greater, namely greater than the maximum
lateral strain occurring in the distribution f(x,y). Another possibility
is to arrange one of the two arms of the sensor elements insulated on the
side of the elastic skin turned away from the force, so that in each case
only one arm is strained, and therefore functions as measuring arm while
the other arm functions as reference arm. However, with the same fine
resolution this requires double the number of interferometers and readout
units (detectors, readout electronics, counters). The first solution is
therefore to be preferred, insofar as this is permitted by the measuring
task. Sensor characteristics, such as measuring range, fine resolution and
frequency response are essentially co-determined through the "skin", in
which the interferometer arms are embedded. Silicone rubber, latex and
neoprene are examples of materials that have been examined in the
literature for an elastic skin in tactile sensors. In this connection, the
stress-deformation characteristic curve of neoprene exhibits the lowest
hysteresis, so that this material seems to be best suited.
Again, generally speaking the elastic properties of the skin entail that,
even for a force acting only at a point, more than one sensor element will
deliver an output signal, depending on the spacing of the sensor arms of
the individual interferometers.
There are several, mutually complementary methods for the quantative
determination of a measured unknown force distribution. On the one hand,
the measured values registered by the counters can be converted into the
force distribution (force matrix) using an analytical model of the sensor
and scaling measurements (sensitivity to deformation, couplings between
interferometer arms, etc.). Because of the possible couplings between the
individual sensor elements, which are transmitted via the elastic skin on
occasion, a more expensive, computer-aided evaluation of the measured data
may be necessary. In this connection, a type of expert system for tactile
sensations could identify the unknown force distribution by comparing the
measured values (counter readings) with a standard distribution stored in
a data bank.
If not only orthogonal forces (with reference the sensor surface), but also
arbitrary force vectors act on a tactile sensor, it would be advantageous
to be able to distinguish tangential (shear) components from orthogonal
components. Given the principle described here, this is possible during
the data processing via the different stress-strain characteristic curves
for orthogonal forces (quadratic dependence of the fiber strain on the
flexure, see (5)) and tangential forces (fiber strain proportional to the
applied mechanical stress).
For an arrangement with six interferometers, as represented in FIG. 5, the
spacings of the measuring arms of the individual interferometers are 3a in
each case. In each case, then, one measuring arm of the two other
interferometers lies between these measuring arms. The figure shows one of
the altogether six interferometers with the wiring as two-polarization
interferomweter. A corresponding wiring is also envisaged for the other
interferometers. Here, individual components of the wiring correspond to
those described above with reference to FIG. 3. Accordingly, the same
reference numerals are also employed for the same parts. When a force
acts, the flexure of the fibers is restricted by the rigid housing 92,
which serves to receive the optical components of the six interferometers.
It is known that, next to the mechanical strain, a thermal strain causes
the greatest measuring effect when there are temperature differences
between interferometer arms of the sensor elements. Consequently, the
sensor must be constructed in such a way that temperature influences are
essentially suppressed or compensated. For a setup according to FIG. 4 or
5, in which the two arms of the interferometers serve as measuring arms,
this can, for example, be achieved with a (metal) layer which is a good
conductor, on the elastic skin. This layer essentially compensates locally
inhomogeneous temperature distributions, so that the two arms of the
sensor elements are exposed to the same temperature and do not deliver a
temperature-induced signal.
In accordance with FIG. 6, an elastic film 88, made of material which is a
good conductor, is arranged on the topside of the elastomer layer 86. A
Mylar film, for example, can be provided for this purpose. The thermal
compensation which has been mentioned then takes place over the surface of
the sensor via this layer 88. The rigid support 92 is provided with
depressions 93 into which the glass fibers are pressed during maximum
flexure of the elastomer layer, in order to prevent birefringence induced
by transverse stress or damage in conjunction with transverse forces that
are too strong.
In addition, the tactile sensor can be provided with a temperature sensor.
Such a temperature sensor can likewise be assembled from a network
consisting of the measuring arms of a plurality of interferometers, which,
in their turn, lie in an elastic layer 90 which, as represented above in
FIG. 6, is arranged above the heat conducting film 88. In this connection,
the number of interferometers in the layers 86 and 90 can be identical.
However, it could also be different. Given the locally different thermal
action, different thermal strains arise in the measuring arms, leading, in
their turn, to phase shifts, which in this case are a measure of the local
temperature concerned.
By contrast with the plane of the network lying below, the arms of the
interferometers in the network in the layer 90 are, in addition, directly
exposed to the temperature distributions at the sensor surface, and
experience deformation through the action of force. The measured
temperature distribution is derived by combining the measured values from
the two sensor planes (essentially subtraction of the appropriate counter
readings from the two planes).
A further embodiment of a sensor is shown in FIG. 7. Here, it is a question
of a point sensor which operates with two interferometers 92, 94,
according to FIG. 1 or 2. The two glass fibers 96, 98, or 100 and 102,
which function as measuring arms and are embedded stressed in the elastic
layer 14 or are arranged beneath an elastic membrane in contact with the
latter are, once again, parallel to one another and the measuring arms of
the two interferometers cross one another as in the embodiments according
to FIGS. 4 and 5. If the sensor according to FIG. 7 is actuated at the
point of intersection of the two diagonals through the points of
intersection of the measuring arms, that is concentrically, then in each
case the two measuring arms of the interferometers are strained to the
same extent. Consequently, the output signal is equal to zero. If
actuation takes place outside the center point, different strains arise in
the two interferometers, from which the point of actuation can be
determined locally, insofar as for repeated measurements the flexure of
the elastic layer is identical and known, in each case. This applies to
the entire sensor area formed here by the areas of the elastic layer 104.
The wiring of the interferometers corresponds to that of the other
embodiments.
The sensors according to the invention have the advantage that flexural
stresses of the input and output arms of the interferometers lying outside
the sensor area do not affect the measured result.
* * * * *
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