U.S. patent application number 10/664852 was filed with the patent office on 2005-01-06 for coordinate measuring instrument with feeler element and optical system for measuring the position of the feeler element.
Invention is credited to Christoph, Ralf, Schwenke, Heinrich, Trapet, Eugen.
Application Number | 20050000102 10/664852 |
Document ID | / |
Family ID | 27217800 |
Filed Date | 2005-01-06 |
United States Patent
Application |
20050000102 |
Kind Code |
A1 |
Christoph, Ralf ; et
al. |
January 6, 2005 |
Coordinate measuring instrument with feeler element and optical
system for measuring the position of the feeler element
Abstract
A method for measuring structures of an object using a feeler
element assigned to a coordinate measuring instrument and extending
from an elastic-to-bending feeler extension is disclosed wherein
the feeler element is brought into contact with an object having
structures to be measured and the position of the feeler is then
determined by comparing the position of the feeler as determined by
the coordinate measuring instrument with the position determined by
the optical sensor.
Inventors: |
Christoph, Ralf;
(Schoffengrund, DE) ; Trapet, Eugen; (Bortfeld,
DE) ; Schwenke, Heinrich; (Braunschweig, DE) |
Correspondence
Address: |
DENNISON, SCHULTZ, DOUGHERTY & MACDONALD
1727 KING STREET
SUITE 105
ALEXANDRIA
VA
22314
US
|
Family ID: |
27217800 |
Appl. No.: |
10/664852 |
Filed: |
September 22, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10664852 |
Sep 22, 2003 |
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09445430 |
Mar 8, 2000 |
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6651351 |
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09445430 |
Mar 8, 2000 |
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PCT/EP98/03526 |
Jun 10, 1998 |
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Current U.S.
Class: |
33/503 |
Current CPC
Class: |
G01B 11/007
20130101 |
Class at
Publication: |
033/503 |
International
Class: |
G01B 005/004 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 12, 1997 |
DE |
297 10 242.7 |
Oct 6, 1997 |
DE |
197 43 969.1 |
Feb 13, 1998 |
DE |
198 05 892.6 |
Claims
1. A method for measurement of structures of an object by means of
a feeler element assigned to a coordinate measuring instrument and
brought into contact with the object and whose position is then
indirectly or directly determined, wherein the position of the
feeler element is determined directly or a position of at least one
target assigned directly to the feeler element is determined with
an optical system for measurement of the structure of the
object.
2-14. (canceled).
15. Array for measurement of structures of an object (12) by means
of a feeler assigned to a coordinate measuring instrument and
comprising a feeler element (14) and preferably a feeler extension
(16,38), wherein the coordinate measuring instrument (22) comprises
a sensor for optical determination of the feeler element (14)
and/or of at least one target directly assigned thereto, and an
evaluation unit with which the structure is calculated from the
position of the optical system relative to the coordinate system of
the coordinate measuring instrument and from the position of the
feeler element and/or of the target measured directly using the
optical system, and wherein the sensor forms the feeler element
with at least one jointly adjustable unit and wherein the feeler
element (14) and/or target (46, 48, 50) is designed as a
reflector.
16. (cancelled).
17. Array according to claim 1 wherein the feeler element (14)
and/or target (46, 48, 50) is designed self-emitting.
18. Array according to claim 1 wherein the feeler element (14)
and/or target (46, 48, 50) is a body such as a ball or cylinder
spatially emitting or reflecting a beam.
19. Array according to claim 1 wherein the feeler extension (38) is
designed at least in some sections elastic to bending and/or as a
light guide comprising a light guide.
20-25. (canceled).
26. Array according to claim 1 wherein the feeler (18) extends from
a holder (20) that is adjustable be at least three degrees of
freedom, preferably five, and preferably interchangeable.
27-28. (cancelled).
29. Array according to claim 1 wherein the feeler element (14)
and/or the target (46, 48, 50) has or is a self-lighting electronic
element such as an LED.
30-34. (cancelled).
Description
[0001] The invention relates to a method for measurement of
structures of an object by means of a feeler element assigned to a
coordinate measuring instrument, where the feeler element is
brought into contact with an object and its position is then
determined. The invention further relates to an array for
measurement of structures of an object by means of a feeler
assigned to a coordinate measuring instrument and comprising a
feeler element and preferably a feeler extension.
[0002] For measurement of structures of an object, coordinate
measuring instruments with electromechanically operating feelers
are used with which the structure position is determined
indirectly, i.e. the position of the sensing element (ball) is
transmitted via a feeler pin. The attendant deformations of the
feeler pin in conjunction with the active friction forces lead to a
falsification of the measurement results. Because of the strong
force transmission, measurement forces also result that are
typically in excess of 10 N. The geometric design of such feeler
systems limits these to ball diameters greater than 0.3 mm. The
three-dimensional measurement of small structures in the range of a
few tenths of a millimeter and the sensing of easily deformed test
specimens is therefore problematic, if not impossible. As a result
of the not completely known error influences due to deformation by
the feeler pin and feeler element, and the unknown sensing forces
due to stick-slip effects for example, measurement uncertainties
occur that are typically in excess of 1 .mu.m.
[0003] A corresponding mechanical-feeler coordinate measuring
instrument is shown for example in DE 43 27 250 A1. Here a visual
check of the mechanically sensing process can be made with the aid
of a monitor by observation of the feeler head using a video
camera. This feeler head can if necessary be designed in the form
of a so-called oscillating crystal feeler that is cushioned upon
contact with the workpiece surface. The video camera therefore
permits tracing and control on the monitor of the position of the
feeler ball relative to the workpiece or to the hole therein which
is being measured. The measurement proper is conducted
electromechanically, so that the above drawbacks remain valid.
[0004] An optical observation of a feeler head in a coordinate
measuring instrument is also shown in DE 35 02 388 A1.
[0005] To determine the precise position of the machine axes of a
coordinate measuring instrument, at least six sensors are attached
on a sleeve and/or to a measuring head in accordance with DE 43 12
579 A1, for enabling the distance from a reference surface to be
determined. The sensing of the object geometries is not dealt with
in detail here, instead a proximity-type process as a substitute
for the classic incremental path measurement systems is
described.
[0006] U.S. Pat. No. 4,972,597 describes a coordinate measuring
instrument with one feeler, of which the feeler extension is
pretensioned in its position by a spring. A feeler extension
section passing inside the housing has two light-emitting elements
at a distance from one another for determining by means of a sensor
element the position of the feeler extension, and hence indirectly
that of a feeler element arranged on the outer end of the feeler
extension.
[0007] The optical system here also replaces the classic path
measurement systems of electromechanical feeler systems. The
sensing process proper is again achieved by force transmission from
the feeler element to the feeler pin via spring elements to the
sensor. The aforementioned problems with bending and sensing force
remain here too. This method is indirect.
[0008] To measure large objects such as aircraft components, feeler
pins with light sources or reflecting targets are known, the
positions of which are optically measured (DE 36 29 689A1, DE 26 05
772 A1, DE 40 02 043 C2). The feelers themselves are moved manually
or using robotics along the surface of the body to be measured.
[0009] With this method, the position of the feeler element is
stereoscopically determined in its position by triangulation or
similar. The resolution of the overall measurement system is hence
directly limited by the sensor resolution. The use of such systems
is therefore possible only in the case of relatively low
requirements as regards the relationship of measurement area and
accuracy. In practice their use is limited to the measurement of
large parts.
[0010] Aiming at the position of the feeler element using a
microscope is also known. In this case, the transmitted-light
method is used, so that only structures such as all-through holes
or similar can be measured in respect of their diameters. In view
of the visual evaluation in the microscope and the separate
arrangement of feeler element and optical observation system,
neither measurement of more complex structures (distances in
complex geometries, angles etc.) nor automatic measurement is
possible. Systems of this type are as a result highly prone to
faults and are therefore not offered on the market.
[0011] The problem underlying the present invention is to develop a
method and a device of the type mentioned at the outset such that
any structures can be determined with a high degree of measurement
accuracy, with the aim of precisely determining the position of the
feeler element to be brought into contact with the object. In
particular, it should be possible to measure out bores, holes,
undercuts or similar, and to determine structures in the range
between 50 and 100 .mu.m with a measuring accuracy of at least
.+-.0.5 .mu.m.
[0012] The problem is solved in accordance with the invention
substantially in that the position of the feeler element is
determined directly or a position of at least one target assigned
directly to the feeler element is determined with an optical system
for measurement of the structure. Here the feeler element and/or
the at least one target is moved from the area of the optical
sensor into the position to be measured. In other words, the feeler
is moved towards the object from its side facing towards the
sensor. The feeler and sensor are here adjustable as a unit inside
a coordinate measuring instrument and their joint position can be
measured with high precision. This is followed by a linked movement
that ensures low uncertainty in the results. Here the position in
particular of the feeler element and/or of the at least one target
is determined using the sensor by means of light beams reflecting
from and/or penetrating the object and/or emanating from the feeler
element or target.
[0013] In accordance with the invention, the position of the feeler
element resulting from contact with the object is determined
optically, in order to measure the shape of a structure directly
from the position of the feeler element itself or of a target. Here
the deflection of the feeler element can be measured by
displacement of the image on a sensor field of an electronic
image-processing system using an electronic camera. It is also
possible to determine the deflection of the feeler element by
evaluation of a contrast function of the image. A further
possibility for ascertaining the deflection is to determine it from
a size change of the target image, from which results the
geometrical-optical correlation between object distance and
enlargement. Also, the deflection of the feeler element can be
determined by the apparent target size change resulting from the
loss of contrast due to defocusing. As a general principle, the
deflection vertical to the optical axis of the electronic camera is
determined here. Alternatively, the position of the feeler element
or of the at least one target assigned thereto can be determined by
means of a photogrammetric system. If several targets are present,
their position can be optically measured and then the position of
the feeler element computed, as there is a clear and firm
correlation between this and the targets.
[0014] In accordance with the invention, and in a divergence from
the previous prior art, indirect measurement of the position of the
feeler element or of the target assigned thereto takes place in
order to determine the structure of an object. Here the feeler
element and the target have a clear spatial correlation to the
extent that a relative movement to one another does not take place,
i.e. short spacings are maintained. It is immaterial here whether
the feeler extension from which the target or feeler element
extends is deformed during the measuring process, since the feeler
element or the target is not indirectly measured, as in the prior
art, but directly. With the method in accordance with the
invention, holes, bores, depressions, undercuts or other structures
with an extent in the range of at least 50-100 .mu.m can be
measured with an accuracy of at least .+-.0.5 .mu.m. This enables
three-dimensional measurements of very small structures to be
performed, a requirement which has long been felt for example in
medical technology for minimally invasive surgery, in microsensor
systems, or in automotive engineering to the extent that injection
nozzles, for example, are concerned, but which has not yet been
satisfactorily solved. Thanks to the direct measurement of the
feeler element position or of the target clearly assigned and not
movable relative thereto, a direct mechanical/optical measuring
method using a coordinate measuring instrument is provided that
operates with high precision and does not lead to falsifications of
measuring results even if the feeler extension becomes deformed
during the measuring process.
[0015] An array for measurement of structures of an object is
characterized in that the coordinate measuring instrument comprises
a sensor for optical position measurement of the feeler element
and/or of at least one target directly assigned thereto, and an
evaluation-unit using which the structure is calculated on the one
hand from the position of the optical system relative to the
coordinate system of the coordinate measuring instrument and on the
other hand from the position of the feeler element and/or of the at
least one target measured using the optical system, and in that the
sensor forms a jointly adjustable unit with at least the feeler
element. The feeler element and/or the at least one target can here
be designed self-radiating and/or as a reflector.
[0016] The feeler element and/or the target should preferably be
designed as a body such as a ball or cylinder spatially emitting or
reflecting a beam.
[0017] In accordance with a further embodiment of the invention,
the feeler element is connected to a feeler extension such as a
shaft that is designed elastic to bending. The connection can be
made by gluing, welding or any other suitable type of fastening.
The feeler element and/or the target can also be a section of the
feeler extension itself. In particular, the feeler extension or the
shaft is designed as or incorporates a light guide via which the
necessary light is supplied to the feeler element or to the
target.
[0018] The shaft itself can be designed as a feeler at its end or
can incorporate a feeler. In particular, the feeler element and/or
the target should be interchangeably connected to the feeler
extension such as a shaft.
[0019] In order to determine almost any structure, it is
furthermore provided that the feeler extends from a holder
adjustable in five degrees of freedom. The holder itself can in
turn form a unit with the sensor or be connected to the sensor.
[0020] It is also possible for the feeler element and/or the target
to be designed as or to incorporate a self-lighting electronic
element such as an LED.
[0021] In accordance with the invention, a feeler system for
coordinate measuring instruments is proposed that combines the
advantages of optical and mechanical feeler systems, and which can
be used in particular for the mechanical measurement of very small
structures where conventional feeler systems can no longer be
employed. However, simple attachment and changing of optical
measuring instruments for mechanical measuring tasks is also
possible as a result.
[0022] For example, it is provided that a feeler element or sensing
element or a target assigned thereto can be determined in its
position by a sensor such as an electronic camera once the former
has been brought into mechanical contact with a workpiece. Since
the position of either the feeler element itself or the target
connected directly to the feeler element is measured, deformations
of a shaft receiving the feeler have no effect on the measuring
signal. In the measurement, it is not necessary for the elastic
behavior of the shaft to be taken into consideration, and plastic
deformations, hystereses and drift effects of the mechanical
connection between the feeler element and the sensor cannot impair
measurement accuracy. Deflections in the direction vertical to the
sensor axis such as the camera axis can be determined directly by
displacement of the image in a sensor field in particular of an
electronic camera. The evaluation of the image can be performed
with an image-processing system already installed in a coordinate
measuring instrument. This provides a two-dimensionally operating
feeler system which can be connected very easily to an optical
evaluation unit.
[0023] For sensing the deflection in the direction of the optical
system axis such as a camera axis, there are several possibilities
in accordance with the invention, for example:
[0024] 1. The deflection of the feeler element in the direction of
the sensor axis (camera axis) is measured by a focus system as
already known in optical coordinate measurement technology for
focusing on workpiece surfaces. Here the contrast function of the
image is evaluated in the electronic camera.
[0025] 2. The deflection of the feeler element in the direction of
the sensor axis or camera axis is measured by the imaging size of a
target being evaluated, e.g. in the case of a circular or annular
target the change in the diameter. This effect is the result of the
geometrical-optical imaging and can be selectively optimized by the
design of the optical unit. In coordinate measurement technology,
so-called telecentric lenses are frequently used and are intended
to achieve a largely constant enlargement even in the event of
deviation from the focal plane. This is achieved by moving the
optical entry pupil into "infinity". For the evaluation as
described above, an optimization the other way round would be
useful: even a minor deviation from the focal plane should result
in a clear change of the imaging scale. This is achieved by for
example moving the optical entry pupil to the level of the focal
point on the object side. If possible a high depth of field should
be achieved to permit high-contrast imaging of the target over a
relatively wide distance range. An ideal optical unit as regards
its imaging properties for the application described above would be
for example a pin camera. By the use of an annular target, size
changes resulting from lack of focus can be minimized: it is not
the mean ring diameter that changes due to lack of focus in the
first approach, but only the ring width.
[0026] 3. In a third option too, the size change of the target is
evaluated, however this change results from the combination of
geometrical-optical size change and the apparent enlargement by
out-of-focus edges. In comparison with the evaluation of the
lack-of-focus function, this method takes advantage of the fact
that the actual size of the target is invariable.
[0027] In accordance with the invention, direct measurement of a
feeler element position is used for determining the structures of
objects. Generally speaking, many different physical principles are
usable for the direct measurement. Since the measurement of the
feeler element deflection in a very large measurement range in the
spatial sense must be very precise, for example to permit
continuous scanning operations, and to allow for a large excess
stroke during object sensing (e.g. for safety reasons, but also to
reduce the effort needed for precise positioning), a
photogrammetric method can also be used. Two camera systems with
axes oblique to one another can be used. In general the evaluation
techniques known from industrial photogrammetry can be used.
[0028] With two cameras "looking" for example obliquely toward the
longitudinal direction of the feeler element or to the ends facing
said feeler element of a feeler extension such as a shaft, all
measuring tasks can be performed in which the feeler element does
not "disappear" behind undercuts. The use of a redundant number of
cameras (e.g. three) permits measurement of objects with steep
contours too. For measurement in small bores, a camera can be used
that is arranged such that it is "looking" onto the feeler element
in the longitudinal direction of the feeler element or feeler
extension. As a general principle, a single camera aligned with the
longitudinal direction of the feeler extension such as shaft
holding the feeler element is sufficient in the case of
two-dimensional measurements (e.g. for measurements in bores).
[0029] For the use of the feeler in accordance with the invention,
an actively light-emitting feeler element or other active target is
not essential. Particularly high accuracies can be achieved with
light-emitting feeler balls or other light-emitting targets on the
feeler extension. The light from one light source is here supplied
to the feeler element such as ball or to other targets of the
feeler extension for example via a light guide fiber which can
itself be the feeler shaft or feeler extension. The light too can
be generated inside the shaft or in the targets if these contain
LEDs, for example. The reason for these designs is that electronic
image systems such as photogrammetric systems, in particular those
for microscopically small structures, require a high light
intensity. If this light is directly supplied to the feeler element
in targeted form, the necessary light intensity can be reduced
considerably, and hence also the thermal load on the object during
the measurement. If a ball is used for the feeler element, the
result is an ideally high-contrast and ideally circular image of
the feeler ball from every direction viewed. This applies in
particular in the use of a volume-dispersing ball. Errors from
imaging of structures of the object itself are avoided, since the
object itself is only brightly illuminated in the immediate
vicinity of the feeler ball. Here however the feeler ball image
resulting from reflections on the object in practice always appears
less bright than the feeler ball itself. As a result, errors can be
corrected without difficulty. Externally illuminated targets do not
necessary have these advantages. It is also possible to design the
targets fluorescent, so that incoming and outgoing light is
separated in terms of frequency, and hence the targets too can be
more clearly isolated from their surroundings in the image. The
same considerations apply for the feeler element itself.
[0030] To measure in small bores or on very steep structures too,
when the feeler element cannot be measured itself or not measured
by several cameras due to shading, the position, the orientation
and the curvatures of the light guide fiber in the visible
part-areas can be measured in accordance with the invention by
sensors or by photogrammetry. From this the position of the feeler
element can be calculated, e.g. by applying the fiber curvature in
the form of a parabola with linear or square term. The measurement
with different excess strokes (more or less positioned into the
object) and then taking the mean of the feeler element position
increases measuring precision. Both optical and photogrammetric
measurement of the fiber is facilitated by a steady light emission
of the fiber, which can be improved by the use of volume-dispersing
fiber material, the application of a diffusely emitting layer on
the fiber surface or another suitable selection of fiber
composition and fiber geometry (e.g. production using material with
relatively low refractive index).
[0031] It is also possible in accordance with the invention to
attach further illuminated balls or other targets on the light
guide fiber, to measure the position of these targets by
photogrammetry in particular, and to calculate the position of the
feeler element accordingly. Balls are here relatively speaking
ideal and clear targets that are not otherwise present on the
fiber. A good light incidence into the balls is achieved by
disrupting the light guidance properties of the shaft, for example
by mounting the volume-dispersing balls with through-holes onto the
shaft, i.e. the feeler extension, and gluing them there. The
volume-dispersing balls can also be affixed to the side of the
shaft, which also permits light incidence, provided that the shaft
carries light up to its surface, i.e. does not have a sheath at the
fastening point. A particularly high accuracy is achieved when the
feeler element position is experimentally measured (calibrated) as
a function of the fiber position and fiber curvature (zones of
fibers at some distance from feeler element). Here too the
measurement of targets attached along the fiber is possible instead
of measurement of the fiber itself.
[0032] Calibration can for example for achieving by sensing a ball
from different directions and with different forces (more or less
"positioned into" the object), or by a known relative positioning
of the feeler system relative to the clamped feeler ball.
[0033] The separation of the feeler element--such as feeler
ball--and targets additionally reduces the possibility of
disruption of the feeler element position measurement by
reflections of the targets on the object surface.
[0034] In accordance with the invention, several feelers can be in
use consecutively; for example, various feeler elements or feeler
pins can be rotated into view with a simple changing unit (e.g.
turret with several feelers). It is also possible in accordance
with the invention for several feeler elements to be in operation
at the same time. The active feeler element or feeler pin can for
example be identified by switching off the lights of the non-active
feeler pins or by other coding means such as target size, light
color, target position in feeler coordinate system, modulation of
the light and/or using attached models. Feeler pin measurements as
standard in classic coordinate measurement technology are no longer
essential in the feelers in accordance with the invention, since
the feeler ball position and the feeler ball diameter can be
measured with often sufficient precision by photogrammetric
means.
[0035] Measurement with small feeler elements often entails a large
number of destroyed feeler pins (feeler element, feeler extension).
With the system in accordance with the invention, the feeler pins
are inexpensive and easy to replace. Expensive sensors and the
movement axes are generally not damaged or altered by collisions,
since the distance from the feeler element can be quite large. For
example, the shaft length can be greater than the movement range of
the system, so a collision is not possible. A large feeler or ball
deflection relative to the shaft length is possible without
difficulty. The result is a high inherent safety of the system and
good scannability. Also, high sensing speeds are possible without
damaging the object surface.
[0036] Photogrammetric systems or other known optically operating
sensor systems permit a mathematical alignment of the object before
the actual start of measurement thanks to the image information
from the lens. This permits accurate sensing of the object in the
actual tactile measurement.
[0037] There are in this system two types of elastic influences
that can lead to measurement deviations.
[0038] 1. The resilience of the object itself (in large ranges);
influences from this can be extrapolated to zero by measurement
with at least two sensing forces.
[0039] 2. The local resilience from Hertzian stress between ball
and object surface; these effects can if required (i.e. for
high-precision measurement or for resilient objects) be eliminated
by a measurement with at least two different sensing forces and
extrapolation to the fictitious sensing force "zero".
[0040] The extrapolation to the force "zero" in the second case is
possible since the deformation according to Hertz is equal to a
constant multiplied with the (sensing force).sup.2/3.
D=K.times.F.sup.2/3
[0041] where:
[0042] D: deformation at the point of contact between object and
feeler ball
[0043] F: force (or a quantity proportional to the sensing
force)
[0044] K: constant
1 D.sub.1 = K .times. F.sub.1.sup.2/3 D.sub.2 = K .times.
F.sub.2.sup.2/3 D.sub.1 - D.sub.2 = K .times. (F.sub.1.sup.2/3 -
F.sub.2.sup.2/3)
[0045] From the above is derived the value of K when the difference
(D.sub.1-D.sub.2) is known from the measurement and when F1 and F2
are known. It is now possible to calculate the deformations
D.sub.1-D.sub.2 in relation to sensing with "zero" force. The
force-proportional values are for example the movement distances
calculated starting from the first object contact.
[0046] Alternatively, these can also be measured with force
sensors. A force sensor for example can be the fiber itself if its
curvature is photogrammetrically measured or on the basis of
changes in the light reflected/diffused back internally to the
light source or in the emitted light. It is best to perform the
measurement with several sensing forces for all high-precision
measuring tasks, since the effective radii in the contact point
between the object and the feeler element can vary greatly due to
local waviness and roughness features.
[0047] If the Hertzian and the linear resilience are of the same
order of magnitude, sensing with at least three forces is
necessary, and both the linear and the Hertzian resilience constant
must be determined in order to extrapolate to the fictitious "zero"
force.
[0048] If the divergences from the ideal spherical form in small
balls used as feelers are not negligible, with diameters of less
than 0.1 m, a direction-dependent correction of the sensing point
coordinates may be necessary. To measure the correction values, two
methods are possible:
[0049] 1. measurement of the deviations of the feeler element from
the spherical form, performed independently of the feeler system
with special measuring instruments;
[0050] 2. measurement of the deviations of the feeler element from
the spherical form, performed by measurement of a reference ball
with the feeler system itself.
[0051] As a general principle, it is also possible to select a
different geometry form for the feeler elements than that of a
ball, e.g. a cylinder, which can represent the fiber itself or the
rounded end of the fiber itself as the feeler extension.
[0052] Since the feeler element (e.g. a ball) is more or less
completely imaged depending on the direction of observation, and
since dirt too has a very disruptive effect, it is best to
determine the position of the feeler element with so-called robust
compensation algorithms.
[0053] These include for example the minimization of the sum of
deviation amounts (so-called L1 standard).
[0054] Correction methods set forth above are however only
necessary in extreme cases, without the teachings in accordance
with the invention being generally affected as a result.
[0055] Generally speaking, the illumination of the feeler element,
the targets or the shaft can be not only from the inside through
the shaft, but also from the outside using suitable illumination
devices.
[0056] One variant that is possible here is for the feeler element
or targets to be retro-reflectors (triple reflectors, cat's eyes,
reflecting balls) that are externally illuminated from the camera
viewing angle.
[0057] The feeler in accordance with the invention is generally not
itself restricted to certain sizes of the measurement objects and
feeler element itself. It can be used for measurement of
single-dimensional, two-dimensional or three-dimensional
structures. In particular the feeler extension can be designed as a
light guide and have a diameter of 20 .mu.m. The diameter of the
feeler element such as feeler ball should then be preferably 50
.mu.m.
[0058] In particular, it is provided that the diameter of the
feeler element is about 1 to 3 times greater than that of the
feeler extension.
[0059] To increase the fracture strength of the feeler extension
when light guides are used, the latter can have a surface coating
such as Teflon or another fracture-inhibiting substance. Sheathing
can be applied by sputtering, for example.
[0060] The spatial position of the feeler element can be determined
using a two-dimensional measuring system when the feeler element
has at least three targets assigned to it, the images of which are
evaluated for determining the spatial position of the feeler
element.
[0061] The invention also permits a scanning method for determining
workpiece geometries. In particular, the images to be evaluated can
be generated by a position-sensitive surface sensor.
[0062] Compared with purely mechanically measuring feeler systems,
the teachings in accordance with the invention have the following
advantages, among others:
[0063] Elastic and plastic influences and creepage effects of the
mechanical holder and the sensing shaft do not affect the measuring
result.
[0064] Very low sensing forces (<1 N) can be attained.
[0065] No precision mechanics are necessary.
[0066] Very small feeler elements and shaft diameters can be
used.
[0067] The positioning of the feeler system can be optimally
monitored by the operator using the optical system.
[0068] The systems can be directly attached to the existing optical
system of a coordinate measuring instrument and the image signal
evaluated using an existing image processor.
[0069] Low equipment expenditure thanks to adaptation to existing
optical coordinate measuring instruments.
[0070] Compared with purely optically measuring feeler systems, the
advantages are as follows:
[0071] The actual mechanical quantities are measured. Surface
properties such as color and reflection characteristics do not
affect the measurement result;
[0072] Measurements can be made on three-dimensional structures not
accessible for purely optical systems. For example, the diameter
and the form divergence of a bore can be measured at different
height sections.
[0073] Further details, advantages and features of the invention
are shown not only in the claims and in the features they
contain--singly and/or in combination--but also in the following
description of preferred embodiments shown in the drawing.
[0074] In the drawing,
[0075] FIG. 1 shows a embodiment of an array for measurement of
structures of an object,
[0076] FIG. 2 shows a second embodiment of an array for measurement
of structures of an object,
[0077] FIG. 3 shows a third embodiment of an array for measurement
of structures of an object,
[0078] FIG. 4 shows a fourth embodiment of an array for measurement
of structures of an object,
[0079] FIG. 5 shows a fifth embodiment of an array for measurement
of structures of an object,
[0080] FIG. 6 shows a sixth embodiment of an array for measurement
of structures of an object,
[0081] FIG. 7 shows a section through a first embodiment of a
feeler,
[0082] FIG. 8 shows a section through a second embodiment of a
feeler,
[0083] FIG. 9 shows a section through a third embodiment of a
feeler,
[0084] FIG. 10 shows a seventh embodiment of an array for
measurement of structures of an object,
[0085] FIG. 11 shows an eighth embodiment of an array for
measurement of structures of an object, and
[0086] FIG. 12 shows a block diagram.
[0087] In the figures, in which identical elements are generally
provided with the same reference numbers, various embodiments of
arrays for the measurement of structures of an object are shown in
principle by means of a feeler assigned to a coordinate measuring
instrument. As a embodiment, the structure of a bore 10 in an
object 12 is to be determined. The edge of the bore 10 is sensed by
a feeler element 14 which in turn extends from a feeler extension
16 and forms with the latter a feeler 18.
[0088] The feeler 18 extends from a holder 20 that is adjustable by
at least three degrees of freedom, preferably five. An optical
system of a coordinate measuring instrument 22 is preferably
mounted on the holder 20 itself. A different type of connection is
also possible. It is however crucial that the optical system or a
sensor of the coordinate measuring instrument 22 is adjustable as a
unit with the feeler element 14 in the X, Y and Z directions.
Regardless of this, an adjustment of the feeler element 14 relative
to the optical axis 24 and to the focal plane takes place. There
are here various possibilities for positioning the feeler element
14, i.e. in the embodiment a feeler ball in the intersection of the
optical axis 24 with the focal plane. It is therefore possible in
accordance with the embodiment in FIG. 1 for the feeler extension
16 to be inserted laterally from the holder 20 into the optical
axis 24.
[0089] In the embodiment in FIG. 2, fastening arms 26, 28 extend
from the holder 20, end outside the focal plane and are used as a
receptacle for a feeler extension 16 inserted laterally into the
optical axis 24, said feeler extension being connectable by a
coupling piece 30 to the feeler element 32 which, via a rod-like
section 34 passing along the optical axis 24, merges into the
feeler element 14 proper in the form of a ball, using which the
structure of the edge of the hole 10 is determined.
[0090] In the array according to FIG. 3, an L-shaped curved feeler
extension 38 is held by a receptacle 36 extending from the holder
20, with a straight-lined end section 40 of the feeler extension 38
running parallel to the optical axis 24 and merging at the end into
the feeler element such as feeler ball 14.
[0091] Once the feeler element 14 has been adjusted, it can be
observed through the existing optical system of the coordinate
measuring instrument 22 or an appropriate sensor. When the edge of
the bore 12 is sensed, the feeler element 14 changes its position
in the camera or sensor field. This deflection is evaluated by an
electronic image-processing system. This achieves a mode of
operation with a similar effect to a conventionally measuring
feeler system. The coordinate measuring instrument 22 can here be
controlled in the same way as a conventional mechanically measuring
feeler system.
[0092] There are various possibilities for optical measuring of the
feeler element 14, and these are shown in principle in FIGS. 4 to 6
and 10 and 11.
[0093] In FIG. 4, for example, a transmitted-light method is
proposed, where the shadow of the feeler element 14 on the sensor
or camera field is viewed or measured. Essential for the
transmitted-light method as shown in FIG. 4. is however that the
workpiece 12 is passed through completely.
[0094] In the embodiment in FIG. 5, the feeler element 14 is
subjected to light by reflecting in the light along the optical
axis 24. To that end, there is a mirror 42 above the coordinate
measuring instrument 22 via which light is reflected in through the
coordinate measuring instrument 22 and the holder 20 along the
optical axis 24.
[0095] A light guide fiber is preferably used for the feeler
extension 30. This has the advantage that the light is passed
through it to the feeler element 14, as shown in FIG. 6. The light
source itself is numbered 44 in the Figure.
[0096] The feeler element 14 has in the embodiments a
volume-emitting ball form. The feeler element 14 here can be firmly
connected to the feeler extension 30 for example by gluing or
welding. However an interchangeable connection using a coupling is
also possible.
[0097] While in the embodiment in FIG. 7 the feeler element 14 is
glued to the end 40 of the feeler extension 30, in the embodiment
in FIG. 8 the feeler element 30, i.e. its end section 40, is itself
designed as the feeler element. To that end, the end section 40 is
appropriately shaped at its end. It is however also possible to
provide the end face of the feeler extension 30 with a reflecting
cover in order to fulfill the function of the target.
[0098] Instead of observation of the feeler element 14 itself, a
preferably spherical target 46 can be assigned to it in a fixed
location, and is a section of the feeler extension 30 or is mounted
thereon, as made clear in FIG. 11. The feeler extension 30
therefore has at its end the spherical feeler element 14.
Furthermore, spherical targets 46, 48, 50 are provided at intervals
from one another on the feeler extension 30. As a result, it is
possible to observe either the position of the feeler element 14
directly or the targets 46 or 46, 48 or 46, 48, 50 clearly assigned
to it.
[0099] The feeler element 14 or the target 46, 48, 50 can be of
various materials such as ceramics, ruby or glass. In addition, the
optical quality of the appropriate elements can be improved by
coatings of diffusing or reflecting layers.
[0100] The diameter of the feeler extension 30 is preferably less
than 100 .mu.m, and preferably 20 .mu.m. The feeler element 14 or
the target 46, 48, 50 has a greater diameter, preferably one
between 1.5 and 3 times larger than that of the feeler extension 30
such as the light guide.
[0101] In the area where the sheath of the feeler extension 30 does
not have to be traversed by light, a surface coating of Teflon or
another fracture-inhibiting substance can be provided.
[0102] The image of the feeler element 14 or of a target 46, 48, 50
assigned thereto can be displayed on, for example, a CCD field of
an optical coordinate measuring machine.
[0103] The displacement of the light dot in the CCD field can be
measured with subpixel precision. With the method in accordance
with the invention, reproducible measurements with a precision in
the .mu.m range are possible.
[0104] In the embodiment in FIGS. 10 and 11, a photogrammetric
method is used: two optical imaging systems such as cameras 52, 54
aligned with the feeler element 14 extend from a common holder 20.
The cameras 52, 54 optically aligned with the feeler element 14
permit a spatial determination of the feeler element 14 using
conventional evaluation techniques known from industrial
photogrammetry. The use of a redundant number of cameras (for
example three) also permits measurement of an object when one of
the three cameras is shaded. For small bores, the use of one camera
is sufficient, and in this way is optically aligned on the feeler
element 14. Independently of this, either an actively
light-emitting, light-reflecting or light-shading feeler element 14
or a target 46, 48, 50 is used to determine the structure in the
object, with light being supplied from the light source 44 to the
feeler element 14 or to the targets 46, 48, 50 via the feeler
extension 30 designed as light guide fiber. Alternatively, it is
possible to generate the light itself in the feeler extension 30,
or in the targets 46, 48, 50 or feeler element 14 by these being or
containing electrically illuminated modules such as LEDs, for
example. With the teachings in accordance with the invention, an
ideally high-contrast image and an ideally circular image of the
feeler element 14 or of the targets 46, 48, 50 are obtained,
provided the latter are of spherical form. Additionally or
alternatively, it is possible to design the feeler element 14 or
the targets 46, 48, 50 fluorescent, so that incoming and outgoing
light are separated in frequency such that the image generated by
the feeler element 14 or the targets 46, 48, 50 can be separated
from its surroundings.
[0105] With the design in accordance with the invention of a
coordinate measuring instrument using which a feeler element such
as a feeler ball or a target clearly assigned thereto spatially is
directly measured optically, in order to determine the structure of
the body from this direct optical measurement of the feeler element
or target, structures in the order of magnitude of 100 .mu.m and
less, in particular in the range up to 50 .mu.m, can be determined
with a measurement uncertainty of .+-.0.5 .mu.m. With the
coordinate measuring instrument, standard measurement volumes of,
for example, 0.5.times.0.5.times.0.5 m.sup.3 can be measured.
[0106] FIG. 12 shows a block diagram in order to determine, in line
with the teachings in accordance with the invention, the structure
of an object in a coordinate measuring instrument 56 by direct
optical measurement of the position of a feeler element 58, where
the object is to be sensed by the feeler element 58 using CNC
control.
[0107] The coordinate measuring instrument 56 is of standard design
for example the feeler element 58 extends from a holder attached to
a sleeve 62 adjustable in the X direction along a cross-piece 60
which in turn is adjustable in the Z direction. The object itself
is fastened to a measurement table 64 movable in the Y direction.
When the feeler element 58 senses the object, the coordinates X',
Y' and Z' of the feeler element 58 are calculated from the video
signals corresponding to the position of the feeler element 58 by
an image-processor 66, and then fed to a measurement computer 68
and there linked to the coordinate values X, Y, Z of the coordinate
measuring instrument 56, which are determined using a counter 70.
From the values computed in this way, on the one hand the object
geometry is determined and on the other hand the CNC operation of
the coordinate measuring instrument 56 is controlled using a CNC
control 72.
* * * * *