Title of Invention

"WORKPIECE INSPECTION METHOD"

Abstract A method of inspecting an artefact (16) using a coordinate measuring apparatus in which an artefact-sensing probe (10) is moved into a position-sensing relationship with each artefact and a position reading taken, the method comprising the following steps in any suitable order: measuring an artefact with an artefact-sensing probe in contact mode (28); scanning said artefact with an artefact-sen sing probe in non-contact mode(32); generating an error map or function corresponding to the difference between the measurement taken with the artefact-sensing probe in contact mode and the artefact-sensing probe in non-contact mode(34); and using the error map or function to correct subsequent measurements (38) with the artefact-sen sing probe in non-contact mode.
Full Text The present invention relates to workmejcejnsBectior method.
This invention relates to a method of inspecting the
dimensions of workpieces using coordinate measuring
apparatus. Coordinate measuring apparatus includes, for
example, coordinate measuring machines (CMM), machine
tools, manual coordinate measuring arms and inspection
robots. In particular, the invention relates to a
method of inspecting the dimensions of a workpiece
using a non-contact probe.
It is common practice after workpieces have been
produced, to inspect them on a coordinate measuring
machine (CMM) having a quill onto which_ a probe is
mounted which can be driven in three orthogonal
directions X,Y,Z within a working volume of the
machine.
Workpiece measuring probes may be divided into contact
probes and non-contact probes. Contact probes comprise
a housing with a workpiece-contacting stylus
deflectable with respect to the housing. There are two
main types of contact probe. In a touch trigger probe,
the stylus is deflected from a rest position to cause a
signal which indicates that the stylus has touched the
surface of the workpiece. Alternatively a contact
probe may comprise a scanning probe in which the
deflection of the stylus is continuously measured as
the stylus is moved along the surface of the workpiece.
Non-contact probes are positioned close to the surface
of the workpiece without touching. The probe detects
the proximity of the surface using, for example,
capacitance, inductance or optical means.
Both contact and non—contact probes suffer from the disadvantage
that scanning at a high speed causes dynamic errors in the
system due to inertia.
Inaccuracies caused by the dynamic deflection of the probe may
be reduced by causing the probe to travel very slowly.
Non-contact probes such as capacitance probes are known, as
described in US patent no. 5,315,259. Methods of scanning a
workpiece are also known, as disclosed in German Patent nos.
19730471 and 10050795.
Saeid Motavalli et al. (Engineering Design and Automation
(1998), 4 (2), 85-100 - XP009027086 describes a method of
creating a dimensionally accurate 3D CAD model of a part. A
machine vision system is 3D CAD image. This image is then used
to guide a CMM to measure a few feature points on the part. The
more accurate CMM data is used to revise the model created by
the non-contact method, to create the final dimensionally
accurate 3D CAD model.
Our previous US Patent No. 4,991,304 discloses a method of
inspecting a series of workpieces using a coordinate
measuring machine (CMM) in which a contact probe is first
calibrated or datumed for each intended direction of probing
movement by touching it at a slow speed against a reference
object such as a datum ball to provide a set of correction
offsets which are stored in the computer and used for subsequent
measurement.
The first workpiece to be measured is put on the CMM table and a
set of points on the surface of the workpiece are measured at a
slow speed to allow accurate readings to be taken. Measurement
of the first workpiece is then repeated at a fast speed. The
difference between the slow speed readings and the fast speed
readings is calculated and stored. The stored error value for
each measured point takes into account the dynamic deflections
of the machine structure at the fast speed.
The next workpiece to be measured is set up on the OMM 30 table
and readings are taken at the fast speed. At this speed the
readings are inaccurate but repeatable. Each fast reading is
adjusted by adding the corresponding stored error value and thus
compensating for errors induced by fast reading.
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This method has the advantage that a whole series of
nominally identical workpieces can be measured at fast
speed by making a dynamic error map from only one
workpiece.
However, a disadvantage of this method is that fast
contact scanning of a workpiece causes significant wear
of the stylus tip of the probe.
Non-contact probes have the advantage that as there is
no contact between the probe and workpieoe, there is no
wear of the probe.
Another advantage of non-contact probes is that there
are no errors due to measurement force. In contact
probes this is the force exerted by the probe on the
workpiece and causes measurement errors due to bending
of the stylus/ coordinate positioning apparatus and
deformable parts of the workpiece.
A further advantage is that non-contact probes have a
higher surface sensing bandwidth that) contact probes
and thus provide more responsive measurement when
scanning or measuring a workpiece at higher speed.
However use of a non-contact probe also has several
disadvantages. The probe may have radial errors due to
the manufacturing process which results in variations
of the measurement data for measurements taken at
different angles around the probe. This could be
corrected for by an elaborate calibration.
In addition non-contact probes, such as inductance and
capacitance probes, are influenced by the geometry of
-Hthe
part being measured and measurement data may vary,
for example, between a straight and curved surface at
the same distance from the probe. The surface finish
of the part may also affect the measurement data from a
non-contact probe, particularly for optical probes.
The present invention provides a method of inspecting
an artefact using a coordinate measuring apparatus in
which an artefact-sensing probe is moved into a
position-sensing relationship with each artefact and a
position reading taken, the method comprising the
following steps in any suitable order:
#
measuring said artefact with an artefact-sensing
probe in contact mode;
scanning said artefact with an artefact-sensing
probe in non-contact mode;
generating an erjror map or function corresponding
to the difference between the measurements taken with
the artefact measuring probe in contact mode and the
artefact measuring probe in non-contact mode; and
using the error map or function to correct
subsequent measurements with the artefact-sensing probe
in non-contact mode.
The step of measuring said artefact with an artefactsensing
probe in contact mode may comprise scanning
said artefact.
The artefact may be scanned with the artefact-sensing
probe in non-contact mode with the artefact-sensing
probe at a nominal offset from the artefact.
The error map or function may be used to calibrate the
artefact-sensing probe in non-contact mode to thereby
produce a radial correction for a nominal distance and
direction of the artefact-sensing probe relative to the
artefact surface.
The method may also comprise the steps of: measuring
subsequent artefacts with the artefact-sensing probe in
non-artefact mode and correcting the artefacts using
the error map or function.
The subsequent artefact may comprise a workpiece. The
initial artefact may comprise a workpiece substantially
the same as the subsequent artefact. The initial
artefact may be different from the subsequent artefact.
The same artefact-measuring probe may have both contact
and non-contact modes, or these may be provided by two
different probes.
The error map enables the non-contact scan to be
corrected for measurement errors, and thus the probe
does not need an elaborate calibration.
If the first artefact is substantially identical to the
subsequent artefacts, then the error map or function
also corrects for measurement errors of the non-contact
probe caused by the geometric influence of the
artefact.
In a subsequent embodiment of the invention, the
artefact is measured the first time at a slow speed and
the artefact is measured the second time at the speed
of measurement of subsequent artefacts. Preferably the
speed of measurement of subsequent artefacts is a fast
speed.
produce a radial correction for a nominal distance and
direction of the artefact-sensing probe relative to the
artefact surface.
The method may also comprise the steps of: measuring
subsequent artefacts with the artefact-sensing probe in
non-contact mode and correcting the artefacts using
the error map or function.
The subsequent artefact may comprise a workpiece. The
initial artefact may comprise a workpiece substantially
the same as the subsequent artefact. The initial
artefact may be different from the subsequent artefact.
The same artefact-measuring probe may have both contact
and non-contact modes, or these may be provided by two
different probes.
The error map enables the non-conteict scan to be
corrected for measurement errors, and thus the probe
does not need an elaborate calibration.
If the first artefact is substantially identical to the
subsequent artefacts, then the error map or function
also corrects for measurement errors of the non-contact
probe caused by the geometric influence of the
artefact.
In a subsequent embodiment of the invention, the
artefact is measured the first time at a slow speed and
the artefact is measured the second time at the speed
of measurement of subsequent artefacts. Preferably the
speed of measurement of subsequent artefacts is a fast
speed.
-c7
the artefact is non-prismatic/geometric.
A second aspect of the present invention provides
apparatus for inspecting an artefact using a coordinate
measuring apparatus and at least one artefact sensing
probe, the apparatus comprising a controller adapted to
perform the following steps in any suitable order:
(a) measuring said artefact a first time with an
artefact-sensing probe in contact mode;
(b) measuring said artefact a second time with an
artefact-sensing probe in non-contact mode;
(c) generating an error map or function
corresponding to the difference between the
measurements taken with the artefact measuring probe in
contact mode and the artefact measuring probe in noncontact
mode;
(d) measuring subsequent artefacts with the
artefact measuring probe in non-contact mode; and
(e) correcting the measurements of subsequent
artefacts using the error map.
Preferred embodiments of the invention will now be
described by way of example, with reference to the
accompanying drawings wherein:
Fig 1 is a schematic diagram of a contact probe
mounted on a coordinate measuring machine;
Fig 2 is a schematic diagram of a non-contact
probe mounted on a coordinate measuring machine;
Fig 3 is a schematic diagram showing a contact
probe scanning a bore of a workpiece;
Fig 4 illustrates the paths of the contact probe
and non-contact probe when scanning the bore of Fig 3;
Fig 5 is a flow chart illustrating the scanning
method;
8
Fig 6 is a flow chart illustrating a scanning
method according to the second embodiment of the
invention; and
Fig 7 illustrates a non-contact probe mounted on
an articulating head.
The coordinate measuring machine shown in Fig 1
comprises a machine table 12 on which a workpiece 16
may be placed. Preferably this is done by automatic
means {not shown) which places each of a succession of
substantially nominally identical workpieces from a
production run in at least nominally the same position
and orientation on the table. An analogue probe 14
having a deflectable stylus 18 and workpiece-contacting
tip 20 is mounted on a quill 10 of the machine although
other types of contact probes (including touch trigger
probes) may also be used. The quill 10 and probe 14
may move in X,Y and Z directions under the action of
X,Y and Z drives controlled by a computer. X,Y and Z
scales (which include counters for the outputs of the
scales) show the instantaneous coordinates of the
position of the quill on which the probe is mounted in
three dimensions. Signals from the probe indicating
the deflection of the probe stylus are combined with
the readings from the X,Y and Z scales of the CMM to
calculate the position of the stylus tip and thus the
surface of the workpiece. Alternatively, with a touch
trigger probe a signal indicating that the probe has
contacted the surface of the workpiece freezes the
scales and the computer takes a reading of the
coordinate of the workpiece surface.
As thus far described, the machine is conventional.
The computer contains a programme which causes the
probe to scan the surface of the workplace or for a
touch trigger probe to contact the surface of the
workplace at a plurality of different points sufficient
to take all the required dimensions of the workplace
for the inspection operation required.
The analogue and touch trigger probes described both
comprise contact probes in which the stylus 18 of the
probe 14 is deflected on contact with the workpiece.
Fig 2 shows a non-contact probe 22 mounted on the quill
10 of a coordinate measuring machine, the non-contact
probe 22 may comprise, for example, an optical probe,
capacitance probe or inductance probe. As the quill 10
moves the probe 22 in a path around the workpiece 16,
the probe detects the distance between itself and the
surface of the workpiece. Signals from the probe are
combined with the readings from the X,Y and Z scales of
the CMM to calculate the position of the surface of the
workpiece.
Referring to Fig 5, the following procedure is used in
the present inspection method. An artefact, such as a
calibration artefact or a workpiece, is set up on a
coordinate positioning machine 26, for example a CMM,
and scanned or measured with a contact probe 28, for
example an analogue probe. This contact probe is
calibrated for static errors by conventional means, for
example as described in US Patent No. 4,991,304 in
which a set of correction offsets is calculated by
touching the probe at a slow speed against a reference
object, such as a datum ball, in a plurality of
directions. These correction offsets are then used to
correct all subsequent measurements.
10
The contact probe is exchanged for a non-contact probe,
for example an inductance probe. The workpiece is then
scanned or measured using the non-contact probe 32.
An error map or function is generated 34 from the
difference between the results from the contact scan
and the non-contact scan.
Subsequent artefacts are now placed on the CMM and
scanned or measured using the non-contact probe 36.
Measurement data corresponding to the subsequent
artefacts taken with the non-contact error map may thus
be corrected using this error map or function 38. This
method enables the use of an uncalibrated non-contact
probe to be corrected for measurement errors.
Certain features of a workpiece, such as different
surfaces and corners, may have an effect on the
measurements from a non-contact scan, particularly with
inductance and capacitance probes. An advantage of the
present method is that errors due to these effects
which may occur during the non-contact scan are
corrected by the error map or function as the
measurement data from the contact scan is not effected
by these geometric influences. Thus measurements taken
using the non-contact probe of subsequent workpieces
having the same geometric features will also be
corrected for these geometric influences.
The workpiece may be scanned using a probe which
operates in both contact and non-contact modes. A
single probe may be a combined touch trigger, contact
scanning and non-contact probe. For example a combined
11
touch trigger and non-contact probe may follow a path
around the workpiece taking touch trigger points and
then move around the path a second time taking noncontact
measurements. Alternatively a combined touch
trigger and non-contact probe may be brought into
contact with a surface of the workpiece to obtain a
trigger point and then reversed away from the surface
to enable a non-contact measurement to be taken. This
method allows the non-contact probe to be calibrated.
A second embodiment of the invention will now be
described with reference to Fig 6. In this embodiment,
a workpiece having an unknown surface, from a series of
workpieces to be measured is set up on the CMM 40 and
scanned or measured at a slow speed with the contact
probe 42. At this slow speed, the dynamic errors of the
system are negligible. For example, typically the
speed may be The workpiece is then scanned using the non-contact
probe 44. This scan is carried out at a speed at which
the subsequent workpieces will also be scanned. This
is a fast speed to facilitate high speed inspection.
The fast-speed is preferably greater than 20mm/s, for
example it may typically be lOOmm/s.
As before, an error map or function is generated 46
corresponding to the difference between the results
from the slow speed contact scan and the fast speed
non-contact scan.
Subsequent workpieces in the series of workpieces are
set up on the CMM and scanned by the CMM using the noncontact
probe 48. The data relating to the subsequent
12
workplaces is corrected by the error map 50. The
subsequent parts are measured at substantially the same
speed as before, i.e. the fast speed of the non-contact
probe.
Fig 3 illustrates a bore 24 of a workpiece 16 being
scanned with a contact probe 14. The path of the
workpiece-contacting probe 14 when scanning the bore 24
is shown as A in Fig 4. This profile accurately
depicts the surface of bore 24 as the contact probe is
calibrated to eliminate static errors and the bore is
scanned slowly to reduce dynamic errors.
The data collected from the contact scan may be used to
calculate a path C along which the non-contact probe
travels to scan the bore 24. This path C is offset
from profile A.
The surface of the bore 24 as measured by the noncontact
scan is shown by profile B. This profile B may
less accurately depict the surface of the bore 24 than
profile A as the non-contact probe has not been
calibrated for either static or dynamic errors or
radial errors due to manufacture and geometric features
of the surface. The differences d between profiles A
and B are used to calculate error values by which
subsequent non-contact scans are corrected.
This method thus has the advantage that both dynamic
and static errors of the non-contact measurement method
are compensated for. Dynamic speed errors are
compensated for by the initial slow scan with the
contact probe and static errors are compensated for by
the initial slow scan being carried out with a
13
calibrated probe.
Non-cohtact probes are usually one-dimensional and it
is thus preferable to calculate the path of the noncontact
scan to follow. The measurements taken by the
contact probe may be used to calculate the path for the
non-contact probe to follow. For example this path may
be offset from the measured surface of the artefact a
certain distance X.
It may not be necessary to use data collected from the
contact scan to calculate the path of the non-contact
scan. For example, if the workpiece has nominally
predefined features, the non-contact scan can be easily
ascertained from these features. Furthermore, if a
multi-dimensional non-contact sensor is used, the
workpiece may be easily measured using unknown path
techniques.
The invention is not limited to the coordinate
measuring apparatus providing movement of the probe
relative to the artefact along three orthogonal axes.
For example, the coordinate measuring apparatus may
comprise a rotary table on which the artefact is placed
which allows the artefact to be rotated relative to a
probe.
Alternatively, or additionally, the probe may be
mounted on an articulating head which may have one or
more rotational degrees of freedom. Fig 7 illustrates
a non-contact probe 22, for example an inductance
probe, mounted on an articulating head 52 which is in
turn mounted on a spindle 10 of a coordinate measuring
machine. The articulating head 52 comprises a fixed
calibrated probe.'
Non-contact probes are usually one-dimensional and it
is thus preferable to calculate the path of the noncontact
scan to follow. The measurements taken by the
contact probe may be used to calculate the path for the
non-contact probe to follow. For example this path may
be offset from the measured surface of the artefact by a
certain distance X.
It may not be necessary to use data collected from the
contact scan to calculate the path of the non-contact
scan. For exaimple, if the workpiece has nominally
predefined features, the non-contact scan can be easily
ascertained from these features. Furthermore, if a
multi-dimensional non-contact sensor is used, the
workpiece may be easily measured using unknown path
techniques.
The invention is not limited to the coordinate
measuring apparatus providing movement of the probe
relative to the artefact along three orthogonal axes.
For example, the coordinate measuring apparatus may
comprise a rotary table on which the artefact is placed
which allows the artefact to be rotated relative to a
probe.
Alternatively, or additionally, the probe may be
mounted on an articulating head which may have one or
more rotational degrees of freedom. Fig 7 illustrates
a non-contact probe 22, for example an inductance
probe, mounted on an articulating head 52 which is in
turn mounted on a spindle 10 of a coordinate measuring
machine. The articulating head 52 comprises a fixed


We Claim:
1. A method of inspecting an artefact (16) using a coordinate measuring
apparatus in which an artefact-sen sing probe (10) is moved into a position-
sensing relationship with each artefact and a position reading taken,
characterised in that the method comprising the following steps in any suitable
order:
measuring an artefact (16) with an artefact-sensing probe (10) in contact mode (28); scanning said artefact with an artefact-sensing probe (10) in non-contact mode(32);
generating an error map or function corresponding to the difference between the measurement taken with the artefact-sen sing probe in contact mode and the artefact-sen sing probe (10) in non-contact mode(34); and
using the error map or function to correct subsequent measurements (38) with the artefact-sensing probe (10) in non-contact mode.
2. The method as claimed in claim 1, wherein the step of measuring said
artefact with an artefact-sensing probe in contact mode comprises scanning
said artefact.
3. The method as claimed in any preceding claim wherein the artefact is
scanned with the artefact-sensing probe in non-contact mode with the artefact-
sensing probe at a nominal offset from the artefact.
4. The method as claimed in any preceding claim wherein the error map or
function is used to calibrate the artefact sensing probe in non-contact mode to
thereby produce a radial correction for a nominal distance and direction of the
artefact sensing probe relative to the artefact surface.
5. The method as claimed in any of claims 1-3, comprising the additional
steps of:

measuring subsequent artefacts with the artefact measuring probe in non-contact mode; and
correcting the measurements of subsequent artefacts using the error map.
6. The method as claimed in any preceding claim wherein the artefact-
sensing probe in contact mode and the artefact-sen sing probe in non-contact
mode comprise a single artefact-measuring probe with both contact and non-
contact nodes.
7. The method as claimed in any of claims 1-5, wherein the artefact-sensing
probe in contact mode and the artefact-sensing probe in non-contact mode
comprise separate probes.
8. The method as claimed in any preceding claim, wherein the artefact is
measured with the artefact-sensing mode in contact mode at a slow speed and
with the artefact-sensing mode in non-contact mode at the desired speed of
measurement of subsequent artefacts.
9. The method as claimed in claim 8, wherein the speed of measurement of
subsequent artefacts is a fast speed.
10. The method as claimed in any preceding claim, wherein the artefact is
measured with the artefact-sen sing probe in contact mode using a high
accuracy reference co-ordinate measuring apparatus and the artefact is
measured with the artefact-sensing probe in non-contact mode using a
repeatable co-ordinate measuring apparatus.
11. The method as claimed in any preceding claim, wherein the
measurements of the artefact gained from measurement with the artefact-
sensing probe in contact mode are used to calculate a path for the artefact-
sensing probe in non-contact mode to follow.

12. The method as claimed in any of claims 1-10, wherein the path for the artefact sensing probe in non-contact mode is ascertained using predefined features of the workpiece.
13. The method of inspecting anartefact foruse in a coordinate measuring
machines


Documents:

2243-DELNP-2005-Abstract-02-05-2008.pdf

2243-DELNP-2005-Abstract-21-04-2008.pdf

2243-delnp-2005-abstract.pdf

2243-DELNP-2005-Claims-02-05-2008.pdf

2243-delnp-2005-claims-12-05-2008.pdf

2243-DELNP-2005-Claims-21-04-2008.pdf

2243-delnp-2005-claims.pdf

2243-DELNP-2005-Correspondence-Others-02-05-2008.pdf

2243-delnp-2005-correspondence-others-12-05-2008.pdf

2243-DELNP-2005-Correspondence-Others-21-04-2008.pdf

2243-delnp-2005-correspondence-others.pdf

2243-delnp-2005-description (complete)-02-05-2008.pdf

2243-delnp-2005-description (complete)-12-05-2008.pdf

2243-DELNP-2005-Description (Complete)-21-04-2008.pdf

2243-delnp-2005-description (complete).pdf

2243-DELNP-2005-Drawings-02-05-2008.pdf

2243-DELNP-2005-Drawings-21-04-2008.pdf

2243-delnp-2005-drawings.pdf

2243-delnp-2005-form-1.pdf

2243-delnp-2005-form-18.pdf

2243-DELNP-2005-Form-2-02-05-2008.pdf

2243-DELNP-2005-Form-2-21-04-2008.pdf

2243-delnp-2005-form-2.pdf

2243-DELNP-2005-Form-3-21-04-2008.pdf

2243-delnp-2005-form-3.pdf

2243-delnp-2005-form-5.pdf

2243-DELNP-2005-GPA-21-04-2008.pdf

2243-delnp-2005-gpa.pdf

2243-delnp-2005-pct-101.pdf

2243-delnp-2005-pct-210.pdf

2243-delnp-2005-pct-304.pdf

2243-delnp-2005-pct-308.pdf

243-DELNP-2005-Correspondence-Others-(07-07-2010).pdf

243-DELNP-2005-Form-3-(07-07-2010).pdf

abstract-02-05-2008.jpg


Patent Number 250381
Indian Patent Application Number 2243/DELNP/2005
PG Journal Number 01/2012
Publication Date 06-Jan-2012
Grant Date 30-Dec-2011
Date of Filing 26-May-2005
Name of Patentee RENISHAW PLC
Applicant Address NEW MILLS, WOTTON-UNDER-EDGE, GLOUCESTERSHIRE GL12 8JR, ENGLAND.
Inventors:
# Inventor's Name Inventor's Address
1 GEOFFREY MCFARLAND THE MANSE, 22 CHURCH ROAD, UPPER CAM, DURSLEY, GLOUCESTERSHIRE GL11 5PG, ENGLAND.
PCT International Classification Number G01B 5/008
PCT International Application Number PCT/GB2003/005326
PCT International Filing date 2003-12-05
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 0228371.7 2002-12-05 U.K.