Flat tactile sensor

A description is given of a tactile sensor for measuring force with fine resolution which is based on a network of glass fiber optic interferometers. The counting of interference fringes as readout method makes available an inherently digital (incremental) output signal which allows the measured signal to be passed on optically free of interference to evaluation electronics and to data processing. In conjunction with a suitable elastic skin to accept the glass fiber network, the small glass fiber diameter (125 .mu.) and the user of the strain measurement technique employing glass fiber interferometers should make possible a tactile sensor comparable with the human sense of touch. The fields of application for such tactile sensors are robotics, prostheses and advanced controls at the man-machine interface.

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.

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: 
EQU 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 
EQU .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 
EQU .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 
EQU .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.