Device for measuring small distances

A device for measuring small distances, provided with a sensing tip mounted to be movable in the direction of the length to be measured, a transducer for converting the movements of the sensing tip into corresponding electric signals, and indicating means to indicate such signals. The transducer comprises an optical interferometer provided with a measuring waveguide. One end of the waveguide is connected to a laser and the other end to optical means for directing the light onto a measuring mirror disposed at a distance. The light is reflected by the mirror back to the optical means. The measuring mirror is connected to the sensing tip. The interferometer further includes a reference waveguide linked to the measuring waveguide. The reference waveguide has a mirror disposed at one end and a photoelectric transduer at the other end. The transducer is connected to the indicating means for indicating the output signals of the transducer. The device permits a cost-effective and uncomplicated construction as an integrated unit. The device affords precision measurements at accuracies in the range of one half of the wavelength of the lasr beam, or even far below such a value. The device also exhibits a high linearity for substantially greater distances. The device is also suitable for measuring extremely rapid changes in length while using a small bearing pressure on the sensing tip.

BACKGROUND OF THE INVENTION 
The present invention relates to a device for measuring small lengths or 
distances. Such devices may include a sensing tip which is movable along 
the length to be measured, a transducer for converting the movements of 
the sensing tip into a corresponding electric signals, and an indicator 
for indicating the electrical signals. 
Devices of this kind, for measuring small lengths or sections, have been 
known for some time. For example, a device of this type, in the form of a 
mechanically inductive transducer, is shown in FIG. 9 of German patent 
specification No. 11 00 978. The sensing tip of this prior device moves an 
iron core in a system of coils. The coils are enclosed in a measuring 
bridge which supplies an electric output signal. The output signal is 
dependent on the movements of the sensing tip. The relatively heavy weight 
of the iron core being moved, however, substantially reduces the speed at 
which the device may measure distances. In addition, the weight of the 
iron core subjects the sensing tip to a great deal of wear, and this wear 
becomes even more acute the finer, or more delicate, the sensing tip is. 
Such delicate sensing tips, however are required to properly sense the 
fine structure of a surface. Another disadvantage of the foregoing device 
is its limited linearity, especially when it is executing large measuring 
strokes. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a device of the type 
discussed above which is for measuring small distances or lengths. The 
movable parts of such a device should be relatively light weight. The 
sensing tip of the device may have a small rounding off radius. Thus, the 
device exhibits a high sensing speed in sensing the fine structure of a 
surface. This enables the device to exhibit a high degree of linearity and 
high resolution when the device is executing large measuring strokes. 
The invention is based on the principle that the position and/or the 
movements, respectively, of the mechanical sense organ for sensing a given 
length, or distance, can be achieved practically weightlessly solely with 
the assistance of a reflecting surface disposed at the sense organ. In 
connection with optical phase measurement which is performed by means of 
an interferometer of the type described, in part, in the publication 
"Laser und Optoelektronik", Vol. 1, 1984, page 19, FIG. 3. This known 
interferometer was, however, modified so that the light from the measuring 
waveguide is deflected by optical means, passed onto the reflector 
connected to the sensing tip (or sensor), and returned the same way by the 
reflector to the measuring waveguide. Except for the means for holding and 
guiding the sensing tip, no other weight is placed on the sensing tip. 
Thus, the weight on the sensing tip is minimized, and only a small amount 
of bearing pressure is necessary to enable the sensing tip to accurately 
follow the particular configuration of a surface. Such accurate following 
is required, for example, to sense the fine contours of the surface of a 
workpiece and to sense and determine any changes in length that correspond 
to the surface contour. Any such length variations are subsequently 
converted by the interferometer into electric signals. The sensing speed 
may be selected to be very high. The interferometer provides high 
resolution over a large measuring range. 
The optical means for shutting out or deflecting the light on the measuring 
waveguide consists preferably of a diffraction grating generally of the 
type described in the periodical "LASER MAGAZINE", No. 4, 1985, page 75. 
The disclosed device is used in connection with a scanner for sensing 
phonographic records with optical recording grooves. However, other 
optical means for focusing the light from the measuring waveguide onto the 
reflector are also suitable. Preferably, the light is in the form of 
parallel rays or parallel beams (bundle of rays). However, in some cases, 
a partial or complete focusing onto the measuring reflector (or mirror) 
may be appropriate and called for. 
The interferometer can have any desired embodiment. Instead of limited 
photoconductors, photoconductive laminar layers can also be used. 
The measuring mirror is located on a sensing tip or a part that moves with 
the sensing tip, so that the length is scanned mechanically. However, it 
is still within the framework of the invention to use one surface directly 
as a measuring reflector, where the distances to the surface are to be 
measured as length values, and the surface has adequate reflective 
properties. 
The movable mounting means of the sensing tip may take the form of a 
two-arm lever, with the sensing tip being attached to the end of one arm 
and the measuring mirror to the other arm. It is also possible to use a 
one-arm lever, with the mirror being disposed in the region of the sensing 
tip. This has the advantage that the mass being moved by the deflections 
of the sensing tip is further reduced. The measuring mirror may also be 
disposed at one end of a parallel extending measuring rod, the other end 
of which having the sensing tip affixed thereto. 
With both types of the movable mounting of the sensing tip, and 
particularly with the lever type mounting, it has been found most useful 
for the measuring mirror to take the form of a triple mirror or a glass 
sphere. Such an arrangement helps ensure that the light is always 
reflected in the same direction back to the optical means, even if changes 
in the angular position of the measuring mirror occur. In this way the 
optical means may shunt or deflect light out of the measuring waveguide as 
well as cause the light to return. 
For length measurements in the range of one-half of the waveguide of the 
light emitted by the laser, an indicator in the form of a voltage or 
current meter may be used. The deflections of the meter correspond to the 
intensity of the light intercepted by the photoelectric transducer and the 
voltage or electric current supplied by the transducer. In addition, it 
may be useful to employ a counter as an indicating means when an extensive 
measuring range, having a multiple of the wavelength of the measuring 
light, is to be covered. The counter functions to count the variations in 
light intensity that correspond to the measured changes in length brought 
about by interference. Thus, the counter displays a count value which, 
considering the wavelength of the measuring light, is completely 
proportional to the length, or amount of change in length, encountered by 
the sensing tip. A measurement is performed, for example, by first placing 
the sensing tip upon the starting point of the length to be measured. The 
counter is simultaneously reset to zero. The sensing tip is then allowed 
to proceed to the end value of the measured length, and the counter is 
read off. The procedure is repeated when another distance, or length, is 
to be measured. 
To prevent a resetting of the counter and to be able to measure accurately 
the distances along different directions of the deflections of the sensing 
tip, it is useful to determine the direction of movement of the sensing 
tip. Accordingly, it is useful for the counter to operate in dependence on 
the particular direction that the sensing tip moves, i.e. count either up 
or down. The counter may thus be in the form of an up and down counter. 
The direction of the movement of the sensing tip is determined by the 
measuring line which is effective, with the assistance of a coupling, to 
couple out a portion of the light from the reference waveguide. Such 
coupling has been described in the dissertation submitted by Dipl.-Ing. H. 
Schlaak, Berlin 1984, pages 28-29. The portion of light just described as 
having been "coupled out" is supplied, by means of a phase modulator, to 
another photoelectric transducer. Consequently, the phase position is 
shifted by 90.degree. with respect to the phase position at the end of the 
reference waveguide. A phase modulator of the foregoing type has become 
known from the publication "Laser und Optoelektronik" No. 1, 1984, p. 27. 
FIG. 33. As the sensing tip executes its movements, the two photoelectric 
transducers emit two measuring potentials displaced by 90.degree. with 
respect to each other. Together, the two measuring potentials form a 
rotary field having a direction of rotation which corresponds to the 
direction of movement of the sensing tip. The direction of counting by the 
up and down counter is thus controlled by the rotary direction. This mode 
of determining the direction of movement has been described separately for 
example, in the dissertation entitled "A Laser Interferometer for 
Photoelectric Motion Measurement in the Two Lateral Coordinates", 
submitted by Gerd Ulbers, 1981, University of Hannover, p. 58 ff, 
particularly p. 68, FIG. 29. 
The above prior publication also describes a method of improving the 
accuracy of the measurement by subdividing the cycle of the rotary 
potential. See Section 10.4 of the publication. A further modification of 
the invention resides in such signal cycle subdivision being employed and 
in the inventive device for resolution enhancement. 
In a particularly useful embodiment of the present invention the measuring 
waveguide, the reference waveguide, and the branch waveguide are formed, 
in a manner known per se, in or on the surface of a common plate (or 
board). Laser and/or photoelectric transducers are secured, in a 
straightforward manner, to the lateral edges of the plate where the 
waveguides also terminate. The mirror of the reference waveguide consists 
conveniently of a reflecting coating applied to the edge of the plate in 
the area where the reference waveguide termintes. The waveguides may be 
constructed in or on the plate by known means, either by the application 
of glass fibers onto the plate or by structuring the waveguides in the 
surface of the plate as it has been described, for example, in the 
publication "Laser und Optoelektronik", No. 1, 1984, p. 26, FIG. 31. Also, 
the phase adjusting means can be directly applied to the surface of the 
plate, as it has become known from the above cited periodical "Laser und 
Optoelektronik", 1983, p. 112, FIG. 3. Finally, in accordance with yet 
another embodiment of the present invention, the optical means focuses the 
light from the measuring waveguide onto the measuring mirror and 
intercepts the light again as it is reflected by the mirror. The optical 
means may be disposed directly upon the surface of the plate in the form 
of a diffraction grating, pursuant to the above cited reference "LASER 
MAGAZINE", 1985, p. 75. As a whole, all optical or optoelectrical 
component parts, as well as the photoelectric transducers and/or the laser 
of the inventive device, may be integrated in one plate, with the smallest 
possible dimensions. Consequently, the device may be manufactured in a 
straightforward, simple and inexpensive manner. To keep out any effects of 
temperature fluctuations, the entire plate, including its optoelectronic 
components, is seated on a Peltier element. 
In one particularly suitable addition to the invention an optical 
electronic range finder is used. With a preset range value, the range 
finder emits an electrical signal, which sets the counter used for the 
indicator device to zero. This optical electronic range finder does not 
need to have great precision over a wide range of measurement. Rather, it 
need only have great precision at the present range value or length of 
measuring value, in order to be able to set the counter precisely to zero 
there. 
A focus position measuring system is well suited for the purpose of an 
optical electronic range finder. The system has a high level of precision, 
if the reflecting surface measured at a distance is precisely in the 
focusing position. This is used in the invention for setting the device at 
zero. The interferometer measures extremely precisely in the nm-range over 
large distances, but only in a relative manner. The system consequently 
provides the interferometer with a reference value or zero value, so that, 
in addition to the valuable properties stated, there is also the 
possibility of being able to measure absolute lengths. 
Lenses can be used as focusing devices, as can focusing optical screens. 
Thus, it is suitable to construct the entire optical system in a 
photoconductive layer of a plate, whereby laser and photodiodes are 
coupled to or coupled into the corresponding points. Such a type of 
optical electronic system is known from the periodical "LASER MAGAZINE", 
4/85, page 75. 
The focus position measuring system can be a system which is completely 
independent of the interferometer. Such a system uses a separate mirror on 
the sensing tip, or on its part which moves with the sensing tip. This 
embodiment has the advantage that, with a focused beam from the 
interferometer, the signal-to-noise ratio of the interferometer signal is 
large relative to parallel light. 
It is, however, in accordance with another embodiment of the combination of 
the interferometer and focus position measuring system, also fundamentally 
possible to use the same laser for both systems. Such an arrangement is 
possible if, out of the light which has been reflected from the measuring 
reflector, one part is diverted and then reused in the manner which is 
typical for the focus position measuring system. The light may be diverted 
by means of a semi-transparent mirror. 
If both the interferometer and the focus position measuring system are 
constructed in or on optically conductive plates, it is practical to 
construct these plates as a common piece. From this there results a stable 
and slight height of construction. This is also made possible by another 
further form of the invention. In such an embodiment, by means of the 
first focus screen, a deflecting mirror is positioned with an inclination 
of approximately 45.degree. to the plate. The deflecting mirror deflects 
the essentially vertical beam issuing from the plate into the level of the 
plate. Consequently, the course of the beams outside the plate runs in the 
direction of the plate. This is particularly advantageous if the measuring 
reflector is located on the rear end of a measuring striker unit as the 
forward end of the unit has the sensing tip.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The optoelectronic device of FIG. 1 comprises a plate 1 (or board) made of 
lithium niobate crystal or silicon crystal. Structured in the surface of 
the plate 1, by a method known per se and explained in the foregoing 
portions of this specification, are a measuring waveguide 2, a reference 
waveguide 3, and a branch waveguide 4. The measuring waveguide 2 leads to 
an edge 5 of the plate 1, where a laser 6 is directly applied. The laser 6 
is applied, by means of an adhesive, for example, so that its light beam 
is being passed into the measuring waveguide 2. At its other end, the 
measuring waveguide 2 terminates in a diffraction grating 7 by which the 
light is deflected substantially vertically with respect to the plate 1, 
as it is indicated by the arrow 8 in the side view of FIG. 2. The light 
impinges upon a triple mirror 9 by which it is reflected back to the 
diffraction grating 7, as indicated by the arrow 10. From the diffraction 
grating 7, the light is returned to the measuring waveguide 2. The beams 
of light extend substantially parallel, as it is indicated by the arrows 8 
and 10. 
The triple mirror 9 is attached to one end of an arm 11 of a two-arm lever 
12. The other arm 13 of the lever 12 has attached to its extreme end a 
sensing tip 14 which is illustrated as being in contact with the surface 
15 of a workpiece 16. The elevational profile of the surface 15 of the 
workpiece 16 is to be determined, for example, as a measure of the change 
in length in dependence of the profile direction. 
The reference waveguide 3 shown in FIG. 1 extends up to the edge 5, where a 
photoelectric diode 17 is affixed so as to be capable of intercepting the 
light from the reference waveguide 3 and converting it into a 
corresponding electric current or voltage signal. The other end of the 
reference waveguide 3 extends up to the edge 18 of the plate. The edge is 
smoothly polished and is provided with a reflective coating. Thus, at 
least in the area where the reference waveguide 3 terminates, a reflector 
19 is formed at that end of the reference waveguide 3. 
For a short distance, the reference waveguide 3 approaches and runs 
alongside the measuring waveguide 2 so as to give rise to a coupling 
element 20. The branch waveguide 4 approaches with one end the reference 
waveguide 3 to form a coupling element 21. With its other end, the branch 
waveguide 4 terminates in the area of the photoelectric diode 22 by which 
the light from the branch waveguide 4 is intercepted and subsequently 
converted into a corresponding electric voltage signal. Over a short 
distance, electrodes 23 and 24 are disposed on either side of the branch 
waveguide 4. The electrodes 23 and 24 are connected by lines 25 and 26 to 
a variable direct voltage source. This is schematically indicated by plus 
and minus symbols. By adjusting the direct voltage at the electrodes 25 
and 26, the phase position of the light at the photoelectric diode 22 is 
set with respect to the light at the photoelectric diode 17. Accordingly, 
the phase of the electric current at the outlet of the photoelectric diode 
22 is displaced by 90.degree. with respect to the phase of the electric 
current at the photoelectric diode 17. 
To enhance stability, the plate 1 of FIGS. 2 and 3 is fixedly secured on a 
Peltier element 32 whose function it is to stabilize the temperature of 
the interferometer and to maintain it at a constant value. 
To eliminate noise and drifting, the laser or the laser beam may be 
modulated by a carrier frequency which is subsequently modified by 
conventional techniques to be used in carrier frequency signal processing. 
If, during operation of the device of FIGS. 1 and 2, the sensing tip 14 is 
displaced relative to the surface 15 of the workpiece 16 in the plane of 
the surface 15, the elevation position of the sensing tip 14 is changed 
pursuant to the fine profile of the surface 15. Correspondingly, the 
triple mirror 9 changes its position relative to the diffraction grating 7 
on the plate 1, so that the path of the light along the arrows 8 and 10 
toward the triple mirror 9 and back is also changed. The result is that 
the interference pattern in the reference waveguide 3 is modified in 
proportion to the movement of the sensing tip 14 and shows up at the 
photoelectric diode 17 as a change in the output current. 
FIG. 3 illustrates a modification of the embodiment according to FIG. 1. 
Like parts are provided with like reference numerals. The difference is 
that the triple mirror 9 is disposed at the upper end of a measuring rod 
28 vertically slidably mounted in a guide 27. Secured to the lower end of 
the measuring rod 28 is a sensing tip (sensor) 29 shown to be in contact 
with a workpiece 30 placed on a table 31. 
FIG. 4 is a block diagram for evaluation and indication of the output 
signals of the photoelectric diodes 17 and 22 which are shown as sine and 
cosine signals to clearly illustrate their phase position with respect to 
each other. Following voltage/current transformation and amplification in 
amplifiers 33 and 34, the rotating field interpolation is effected by a 
rotating field interpolator 35. The rotating field interpolator 35 is 
capable of subdividing a complete cycle of 360.degree. of the rotating 
field into n divisions. A complete rotating field cycle corresponds to a 
displacement of the measuring mirror 1/2 of the laser wavelength. The 
signal for the up and down counter 38 is worked up by a serially following 
square wave pulse generator 36 having a counting direction discriminator 
37. Connected to the counter 38 is an indicator 39. Movement of the triple 
mirror 9 in one direction causes the signal at the photoelectric diode 22 
to lead the signal at the photoelectric diode 17 by 90.degree., and the 
counter 38 is counting in one particular direction. Reversal of the 
direction of movement of the triple mirror 9 causes the signal at the 
photoelectric diode 22 to trail the signal at the photoelectric diode 17 
by 90.degree.. The up and down counter 38 also reverse its direction of 
counting. In each instance, the counter 38 counts n divisions (or parts) 
of cycles of the rotating field. The resolution thus achievable for the 
measuring stroke depends therefore solely on the magnitude of the 
subdivisions of the rotating field and on the stability of the system as a 
whole. 
FIG. 5 shows a cross-sectional view of a length measuring sensor for 
absolute measurement. Inside a casing 40, a striker unit 41 is supported 
by a roller guide 42. The striker unit may be displaced in the axial 
direction. The striker 41 projects out of the casing 40, and is provided 
there with the part 43, which is depicted separately. On the end of the 
part 43, there is a spherically shaped sensing tip 44. The striker unit 41 
is protected against dust by a bellows unit 45. 
The striker unit 41 is prestressed in the support direction of the scanning 
point 44 by a helical spring 46. The movement of the striker unit 41 is 
limited by a page 47, which extends into a groove 48. 
The striker unit 41 has, on its rear end, a bar 49. A plate 50 is located 
on the end of the bar 49. A plane mirror 51 and a glass sphere 52 are 
attached to the plate 50. 
An optical electronic component 53 is positioned at a distance from the 
plane mirror 51 and the glass sphere 52. The electronic component 53 is 
attached by means of plug contacts 53'. The plug contacts 53' are 
interconnected to a counter of the interferometer (which is not depicted) 
and a comparator for the focus position measuring system (which is also 
not depicted). 
FIG. 6 shows, in a simplified manner, the optical electronic unit 53 and 
clarifies its construction from two plates 1 and 54, with photoconductive 
layers 55 and 56. The plate 1 is the same is the same as that in the 
example of an embodiment shown in FIG. 1. Thus, in the photoconductive 
layer 55, the same optical means are provided as in FIG. 1, so that the 
same reference numbers are used here. Only the diffraction screen 7 has 
been eliminated, so that the light from the measuring oscillation 
conductor 2 exits in the direction of the plate on the side, as is shown 
in FIGS. 5 to 7. The function of the optical elements on the plate 1 as 
interferometer is the same as that described in connection with FIG. 1. 
When changing the distance of the sphere 52 from the plate 1, electrical 
currents arise on the photodiodes 17 and 22. These currents corresponds to 
the interference and, thus, the relative change of the distance between 
sphere 52 and plate 1. 
Corresponding to the construction in accordance with FIG. 6, the plate 1 is 
adhered by means of another 54. The photoconductive layer 56 is uniform 
over its entire extent, so that a path of beams can form in accordance 
with FIG. 7. A laser 57 is positioned laterally on the plate 54, and 
radiates on a mirror 58 inclined by 45.degree., which directs the light on 
the focusing screen 59 and 60. Thus, the rays 61 ad 62 issuing out of the 
laser 57 are focused on the surface of the mirror 51. 
From the mirror 51, the light travels back again to the focusing screen 60 
and 59, whereby the latter functions as a beam separator, and focuses half 
of the light on two different focus points 63 and 64. The focus point 63 
lies exactly between the diodes 65 and 66, on the one hand, and the focus 
point 64 lies exactly between the diodes 67 and 68, on the other, if the 
mirror 51 is precisely within the focus of the beams 61 and 62. 
Comparators are connected to the diodes 65, 66, 67 and 68. The 
comparators, in focus position represented here, emit a signal for the 
interference values at the counter stated previously, in order to set the 
counter to zero. 
If the mirror 51 moves because of a movement of the sensing tip 44, the 
counter is then always set at zero. As a result, the absolute value is 
produced if the mirror 51 runs through the precise focus position. 
FIG. 9 shows, in a top plan view, and FIG. 10 shows, in a side view, a 
plate 69 with a photoconductive layer 70. On the layer 70, on the right 
side in FIG. 9, an interferometer is constructed in the manner depicted in 
FIG. 1. The same reference numbers are therefore used here for the same 
parts. The position of the measuring striker 28 is that given in FIG. 3, 
so that the same reference numbers are used here again. Instead of the 
triple mirror 9 on the upper end of the measuring striker 28, a glass 
sphere 71 is provided. The light reflected from this moves into the 
interferometer in precisely the same manner as has been described in 
connection with FIG. 1. In addition, a deflecting mirror 72 is provided 
here, so that the measuring striker 28 can extend approximately in the 
direction of the plate 69. Thus, an overall flat structure is present, 
through which a very low overall construction height or width can be 
attained. 
The light reflected from the glass sphere 71 is partly diverted by a 
partially transparent mirror, and, through a deflection mirror 74, reaches 
the focus screen 75 and 76. Each of the focus screen 75 and 76 focuses 
half of the light on one of the focus points 77 and 78. The focus points 
77 and 78 are positioned precisely between pairs of photodiodes 79, 80 and 
81, 82, if the reflecting point of the glass sphere 71 is precisely in 
focus. Comparators are connected to the photodiodes 79-82 in precisely the 
same manner as in the embodiment shown in FIG. 7. If the photodiodes of 
the equivalent pairs are illuminated, that is, the focus points 77, 78 lie 
precisely between the photodiodes, the comparators emit a signal. The 
signal is emitted in order to set the counter connected to the photodiodes 
17 and 22 to zero. 
FIG. 11 shows an example of an embodiment with a cylindrical casing 83. As 
in the example of an embodiment shown in FIG. 5, an optical electronic 
unit 84 (similar to the optical electronic unit 53 in FIG. 5) is 
positioned in the casing 83. The fundamental construction is the same as 
in FIGS. 6 and 7. One difference is that, in addition, a deflecting mirror 
85 is provided for the interferometer. The mirror 85 deflects the light 
from the measuring oscillation conductor 2 (FIG. 6) by 90.degree., 
specifically in the direction of motion of a sphere 86. The sphere 86 is 
positioned on the rear end of a double lever 88. A sensing tip 90 is 
located on the external arm 89 of the double lever 88. The double lever 88 
rotates around a peg 91, and is prestressed in the scanning direction by 
means of a helical spring 92. 
A further difference regarding the optical electronic unit 84, relative to 
the optical electronic unit 53 is accordance with FIG. 5, consists of the 
fact that beams 93 and 94 emerge laterally out of the optical electronic 
unit 84. The beams are then directed to a mirror 95, which corresponds to 
the mirror 51 in FIGS. 6 or 7. Inside the optical electronic unit 84, 
there is located a deflection mirror (which is not depicted) so that the 
path of the beams is therefore the same as in FIG. 7. 
The arm 89 is protected by a tube 96 out of which the sensing tip 9 
projects through an opening in the tube 99. On the other end of the casing 
83, there are again located plug contacts 96, which correspond to the plug 
contacts 53' in FIG. 5.