U.S. patent number RE37,030 [Application Number 08/900,447] was granted by the patent office on 2001-01-30 for touch probe and signal processing circuit therefor.
This patent grant is currently assigned to Renishaw PLC. Invention is credited to Peter K. Hellier, Peter G. Lloyd, David R. McMurtry.
United States Patent |
RE37,030 |
Lloyd , et al. |
January 30, 2001 |
Touch probe and signal processing circuit therefor
Abstract
A touch trigger probe incorporates piezoelectric sensors 50,
whose outputs are processed by an interface circuit. The interface
circuit discriminates between signals generated from the
piezoelectric sensors 50 as a result of machine vibration and those
generated as a result of a genuine measurement event, by the use of
a timing circuit 90. The timing circuit 90 compares the time
intervals (t.sub.1 -t.sub.2);(t.sub.2 -t.sub.3) between attainment
of first 1.sub.1 and second 1.sub.2, and second 1.sub.2 and third
1.sub.3 output signal levels from the sensor 50, and upon the basis
of this comparison validates (or rejects) measurements made with
the probe. Additionally, the interface determines whether
measurements made with the probe are taken upon the basis of
outputs generated by the sensors 50 due to a shock wave in the
stylus 24 of the probe, or as a result of strain in the stylus 24;
as an alternative, measurements may be made only on the basis of
strain.
Inventors: |
Lloyd; Peter G. (Bristol,
GB), Hellier; Peter K. (North Nibley, GB),
McMurtry; David R. (Wotton-Under-Edge, GB) |
Assignee: |
Renishaw PLC (Gloucestershire,
GB)
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Family
ID: |
26302226 |
Appl.
No.: |
08/900,447 |
Filed: |
July 25, 1997 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
167027 |
Dec 16, 1993 |
05435072 |
Jul 25, 1995 |
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Foreign Application Priority Data
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Dec 24, 1992 [GB] |
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9226934 |
Jan 29, 1993 [GB] |
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9301822 |
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Current U.S.
Class: |
33/559;
33/561 |
Current CPC
Class: |
G01B
7/002 (20130101); G01B 7/012 (20130101); G01B
5/012 (20130101) |
Current International
Class: |
G01B
7/012 (20060101); G01B 7/008 (20060101); G01B
7/00 (20060101); G01B 007/28 () |
Field of
Search: |
;33/559,555,556,558,561,DIG.3,DIG.13 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0242747 |
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Oct 1987 |
|
EP |
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0301390 |
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Feb 1989 |
|
EP |
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0420416 |
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Apr 1991 |
|
EP |
|
0 420 305 A2 |
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Apr 1991 |
|
EP |
|
501680 A1 |
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Sep 1992 |
|
EP |
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0556574 |
|
Jan 1993 |
|
EP |
|
2094478 |
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Sep 1982 |
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GB |
|
WO94/21983 |
|
Sep 1984 |
|
GB |
|
61-47502 |
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Mar 1986 |
|
JP |
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WO88/01726 |
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Mar 1988 |
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WO |
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92/09862 |
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Jun 1992 |
|
WO |
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93/09398 |
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May 1993 |
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WO |
|
Other References
Offenkundige Vorbenutzung durch Verkauf von Koordinatenmessgeraten
vom Typ PMC durch Carl Zeiss in 73446 Oberkochen, (Date unknown).
.
Kopie eines 1990 erstellten Prospektes fur unsere
Koordinatenmessgeraten von Typ PMC. .
Kopie des gesamten Kapitels 3 unseres Servicehandbuches "Service
Handbuch KMG Komponenten 1" aus dem Jahre 1989. .
Kopie der Seite 2-3 aus Kapitel 3.3 aus der oben genannten
Entgegenhaltung E2b, auf der wir handschriftlich Bezeichnungen
eingefugt haben, (Date unknown). .
Kopie der Seite 2-1 im Kapitel 3.2 aus der oben genannten
Entgegenhaltung E2b, auf der wir handschriftlich Bezeichnungen
eingef ugthaben, (Date unknown). .
Verkleinerte Kopie einer Fertigungszeichnung mit der
Zeichnungsnummer 600660-9009 (3) aus dem Jahre 1983, mit der der
Tastkopfverstarker komplett dargestellt wird. .
Verkleinerte Kopie einer Fertigungszeichnung mit der
Zeichnungsnummer 600660-9008 (2) aus dem Jahre 1983, mit der eine
der beiden Leiterplatten des Tastkopfverstarkers dargestellt wird.
.
Verkleinerte Kopie einer Fertigungszeichnung mit der
Zeichnungsnummer 600660-7008.091 (1) aus dem Jehre 1983, mit der
der beiden Schaltplan unseres Tastkopfverstarkers dargestellt wird.
.
Kopie eines Leiferscheins zur Lieferung eines
Koordinatenmessterates vom Typ PMC an die Firma Werner &
Pfleiderer GmbH in 7000 Stuttgart-Feuerbach vom 04.03./09.03.1988.
.
Kopie einer Rechnung fur das an die Firma Werner & Pfleiderer
GmbH in 7000 Stuttgart-Feuerbach ausgelieferte Koordinatenmessgerat
vom Typ PMC vom 11.03.1988..
|
Primary Examiner: Fulton; Christopher W.
Attorney, Agent or Firm: Oliff & Berridge, PLC.
Claims
We claim:
1. A touch probe having a fixed structure and a stylus supporting
member biased into a repeatable rest position relative to the fixed
structure, from which the supporting member is displaceable when a
deflecting force is applied thereto, and to which the supporting
member may return when said deflecting force is removed, a
direction of biasing action defining an axis, the probe further
comprising a plurality of sensors sensitive to tension and
compression, for sensing force applied to said supporting member
prior to displacement thereof from said rest position, said sensors
being clamped between first and second parts of said stylus
supporting member and each of said sensors having an axis of
maximum sensitivity to tension and compression, said sensors being
grouped into pairs, the axis of maximum sensitivity of each of said
sensors being inclined (a) with respect to the probe axis and (b)
with respect to the axis of maximum sensitivity of the other sensor
in each of said pairs, one of said first and second parts of said
stylus supporting member comprising a plurality of V-shaped rides
extending radially with resect to said probe axis, and each of said
pairs including a sensor located on opening faces of said
ridges.
2. An interface circuit for connecting a touch probe, having at
least one analogue sensor for generating an analogue signal, to a
control of a coordinate positioning machine, the interface
comprising:
means defining first, second, and third analogue signal levels;
means for comparing the time interval between attainment of said
first and second, and said second and third signal levels, and
determining upon the basis of said comparison whether said analogue
signal conforms to a predetermined trigger signature; and
means for generating a trigger output responsive to said analogue
signal conforming to said predetermined trigger signature.
3. An interface circuit according to claim 2 further comprising
means for generating a latching signal, instructing the machine
control to store the position of the movable arm, when said
analogue signal attains said first level.
4. A method of processing an analogue output signal from a touch
probe, the method comprising the steps of:
defining first, second and third analogue signal levels;
determining the magnitude of a first time interval between
attainment of said first and second analogue signal levels, and
determining the magnitude of a second time interval between
attainment of said second and third analogue signal levels;
comparing the magnitude of said first and second time
intervals;
on the basis of the result of said comparison, generating a trigger
output signal.
5. An interface circuit for connecting a touch probe to a control
of a coordinate positioning machine, the interface comprising:
latching means for generating a latching signal, for instructing
the machine control to record a measurement, in response to an
amplitude of at least one analogue signal generated by said
interface circuit attaining a predetermined amplitude threshold,
said latching means being responsive to said at least one analogue
signal having a frequency above or below a predetermined frequency
threshold; and
means for determining whether said latching signal was generated in
response to said at least one analogue signal having a frequency
above said predetermined frequency threshold, and for generating a
flag signal accordingly.
6. An interface circuit according to claim 5 wherein said
predetermined threshold of frequency is 500 Hz.
7. An interface according to claim 5, further comprising means for
determining whether said at least one analogue signal conforms to a
predetermined trigger signature, and for generating a confirmation
signal accordingly.
8. An interface according to claim 7, comprising means for
determining whether, at a given instant of time after the emission
of said latching signal, the amplitude of said at least one
analogue signal exceeds said predetermined amplitude threshold.
9. An interface for connecting a touch probe to a control of a
coordinate positioning machine, the probe having at least one
sensor for generating at least one analogue signal, the interface
comprising:
latching means for generating a .Iadd.first .Iaddend.latching
signal, for instructing the machine control to record a
measurement, in response to an amplitude of said at least one
analogue signal attaining a predetermined amplitude threshold, said
latching means being responsive to said at least one analogue
signal having a frequency above or below a predetermined frequency
threshold;
.[.discriminating.]. .Iadd.determining .Iaddend.means for
determining whether said latching signal was generated in response
to said at least one analogue signal having a frequency above said
predetermined frequency threshold, and for generating a flag signal
accordingly; and
suppressing means for suppressing .Iadd.the .Iaddend.emission of
.[.said.]. .Iadd.a second .Iaddend.latching signal .[.in the event
that said discriminating means determines that said at least one.].
.Iadd.which is generated in response to said at least one
.Iaddend.analogue signal .[.attaining said.]. .Iadd.having a
frequency above the .Iaddend.predetermined amplitude threshold
.[.has a frequency in excess of said predetermined frequency
threshold.]. , said suppressing means being selectively
operable.
10. An interface circuit according to claim 9 wherein said
predetermined frequency threshold is 500 Hz.
11. An interface circuit for connecting a probe to a control of a
machine, the probe having at least one analogue sensor, the
interface comprising:
at least one input channel, responsive to signals above and below a
predetermined frequency threshold, which receives an input signal
from said at least one analogue sensor, said at least one input
channel comprising a comparison circuit which generates an output
signal when an amplitude of the input signal exceeds a
predetermined amplitude threshold; and
a frequency discriminator which determines whether the output
signal from the comparison circuit occurred in response to an input
signal having a frequency greater than said predetermined frequency
threshold, and which generates a flag signal accordingly.
12. An interface according to claim 11, further comprising a
trigger signal discriminator which determines whether said at least
one analogue signal corresponds to a predetermined trigger
signature, and which generates a confirmation signal
accordingly.
13. An interface according to claim 12, wherein said trigger signal
discriminator includes a comparator circuit which generates a step
output signal in the event that an amplitude of said at least one
analogue signal exceeds a predetermined threshold at a given
instant of time after the emission of said latching signal.
14. An interface circuit for connecting a probe to a control of a
machine, the probe having at least one analogue sensor, the
interface comprising:
at least one input channel, responsive to signals above and below a
predetermined frequency threshold, which receives .[.an.]. .Iadd.a
first .Iaddend.input signal from said at least one analogue sensor,
said at least one input channel comprising a comparison circuit
which generates .[.an.]. .Iadd.a first .Iaddend.output signal when
an amplitude of the input signal exceeds a predetermined amplitude
threshold; .Iadd.and .Iaddend.
.[.a frequency suppressor which suppresses.]. .Iadd.means for
suppressing .Iaddend.the emission of .[.said.]. .Iadd.a second
.Iaddend.output signal .[.in the event that said.]. .Iadd.which is
generated in response to a second .Iaddend.input signal .[.has.].
.Iadd.having .Iaddend.a frequency greater than said predetermined
frequency threshold.[.; and
a selector enabling selective operation of said frequency
suppressor.]. .Iadd., the suppressing means being selectively
operable.Iaddend..
15. An interface circuit for connecting a probe to a control of a
machine, the probe having at least one analogue sensor, the
interface comprising:
at least one input channel which receives an input signal from said
at least one analogue sensor, said at least one input channel
comprising first, second, and third comparators which generate
first, second and third output signals when the input signal
exceeds first, second and third predetermined thresholds,
respectively; and
a timing circuit which receives said first, second, and third
output signals, and determines whether the time intervals between
the occurrence of the first and second input signals and the
occurrence of the second and third input conforms to a trigger
signature of said probe, the timing circuit further generating a
trigger output signal on the basis of the determination. .Iadd.
16. An interface circuit for processing signals generated by at
least one analogue sensor in a touch probe, and providing output
signals which are useable by a control of a coordinate positioning
machine, the interface comprising:
at least one input channel for receiving signals from the analogue
sensor,
three threshold detectors, each of which generates an output signal
when the analogue signal from the sensor reaches a given threshold
amplitude,
a discriminating circuit which is connected to each of the
threshold detectors and which receives their output signals, the
discriminating circuit determining whether time intervals between
signal emissions from the detectors are representative of a genuine
measurement event..Iaddend..Iadd.
17. An interface according to claim 16, having two input channels
for receiving the analogue signals, each input channel having a
filter, and the two filters transmitting analogue signals of
different frequencies, wherein one of the channels has at least two
threshold detectors..Iaddend..Iadd.
18. An interface according to claim 17, further including a third
input channel which receives a resistance signal corresponding to
the resistance of an electrical contact within the probe, the third
input channel having a further threshold detector which generates
an output signal when the resistance of the electrical contact
reaches a given threshold..Iaddend..Iadd.
19. An interface according to claim 17, further including means for
producing an output that is indicative of which channel a first
occurring output signal was generated..Iaddend..Iadd.
20. An interface for connecting analogue sensors in a touch probe
to a control of a coordinate positioning machine, the interface
having:
a pair of input channels, one for relatively high frequency sensor
signals and one for relatively lower frequency sensor signals;
at least two comparators in one of the input channels, each of
which is set to respond to a different amplitude of sensor signal,
and a further comparator in the other input channel; and
discriminating circuitry connected to the at least two comparators
and the further comparator which determines, from time intervals
separating occurrences of output signals from the comparators,
whether the output signals represent a genuine measurement
event..Iaddend..Iadd.
21. An interface according to claim 20, having three such
comparators in said one of the input channels..Iaddend..Iadd.
22. An interface according to claim 20, further including means for
producing an output that is indicative of which of the two input
channels a first occurring comparator output signal was
generated..Iaddend..Iadd.
23. An interface according to claim 22, further including a third
input channel which receives a resistance signal corresponding to
the electrical resistance of an electrical contact in the probe,
the third input channel having a further comparator which is set to
respond to the resistance of the electrical contact reaching a
threshold level..Iaddend..Iadd.
24. An interface for connecting analogue sensors in a touch probe
to a control of a coordinate positioning machine, the interface
having:
two input channels which are connected to receive sensor signals
from the sensors, each channel having a filter wherein the two
filters pass sensor signals of a different frequency, and each of
the two channels having at least one threshold detector connected
to the corresponding filter, which generates a threshold output
when the sensor signal exceeds a threshold amplitude; and a circuit
connected to the outputs of each of the threshold detectors, which
produces an output that is indicative of the channel in which a
first occurring threshold output was generated..Iaddend..Iadd.
25. An interface according to claim 24, further including a
discriminating circuit which is connected to each of the threshold
detectors and which receives their output signals, the
discriminating circuit determining whether time intervals between
signal emissions from the detectors are representative of a genuine
measurement event..Iaddend..Iadd.
26. An interface according to claim 25, having at least two
threshold detectors in one of the input
channels..Iaddend..Iadd.
27. An interface according to claim 26, further including a third
input channel which receives a resistance signal corresponding to
the resistance of an electrical contact within the probe, the third
input channel having a further threshold detector which generates
an output signal when the resistance of the electrical contact
reaches a given threshold..Iaddend..Iadd.
28. An interface for connecting analogue sensors in a touch probe
to a control of a coordinate positioning machine, the interface
having:
at least three comparators connected to the analogue sensors, which
receive signals from the sensors, and which are each set to respond
to a different amplitude of signal; and
discriminating means to which the comparators are connected, for
receiving output signals from the comparators, and determining on
the basis of time intervals between outputs of each of the
comparators whether the outputs represent a genuine measurement
event..Iaddend..Iadd.
29. An interface according to claim 28, having two input channels,
a first of which has a filter transmitting relatively high
frequency sensor signals, and a second of which has a filter
transmitting relatively low frequency sensor
signals..Iaddend..Iadd.
30. An interface according to claim 29, further including a circuit
connected to the comparators, which produces an output that is
indicative of the channel in which a first occurring comparator
output was generated..Iaddend..Iadd.
31. Apparatus for use on a coordinate positioning machine
including:
a touch probe having a housing and a stylus connected to a
supporting member within the housing, the probe including at least
one analogue sensor sensitive to tension and compression which
generates an analogue signal; and
an interface circuit which connects the probe to a control of a
coordinate positioning machine, the interface circuit having:
a high frequency input channel connected to the analogue sensor,
and having a filter which transmits signals from the analogue
sensor generated as a result of shock in the stylus when the stylus
contacts a workpiece, the high frequency input channel also having
a first comparator for generating a first output signal when the
amplitude of the analogue signal at the first comparator exceeds a
predetermined threshold;
a low frequency input channel connected to the analogue sensor, and
having a filter which transmits signals from the analogue sensor
generated as a result of strain in the stylus when the stylus
contacts a workpiece, the low frequency input channel also having a
second comparator for generating a second output signal when the
amplitude of the analogue signal at the second comparator exceeds a
predetermined threshold; and
means for determining whether, when the second comparator generates
an output signal, an output signal has been generated from the
first comparator..Iaddend.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a touch probe, used on a
coordinate positioning machine such as a machine tool or coordinate
measuring machine to enable the inspection of, for example, the
dimensions of machined components, and to a signal processing
circuit which acts as an interface between the probe and the
control of the machine on which the probe is used. Coordinate
positioning machines typically comprise an arm to which the probe
is mounted, and which is movable relative to a table on which a
component or workpiece to be inspected is supported. The machine
includes one or more transducers which measure displacement of the
arm from a reference position which is usually fixed relative to
the table.
2. Description of Related Art
A known probe includes a fixed structure such as a housing by which
the probe is mounted on the arm of the machine, and a stylus
supporting member supported relative to the housing in a repeatable
rest position, from which the supporting member may be displaced
when a deflecting force is applied thereto, and to which it may
return when the deflecting force has been removed. Measurements are
taken by operating the machine to move the arm until a stylus
connected to the supporting member comes into contact with the
surface of the part to be inspected, and, upon detecting such
contact, taking readings from the transducers of the machine to
determine the position of the movable arm relative to the reference
position. The probe includes one or more analogue sensors which
emit signals indicating contact between the stylus and the surface
whose position is to be measured. These sensors may sense
displacement of the stylus supporting member relative to the
housing, or, in high accuracy probes, the sensors may sense
deformation of the stylus and/or the stylus supporting member which
takes place before such a displacement occurs. Probes employing
displacement or deformation sensors are shown in U.S. Pat. No.
4,153,998 or U.S. Pat. No. 4,177,568 respectively.
Signal processing circuitry, which acts as an interface between the
probe and the machine control, emits a step-change, or "trigger"
signal when the analogue signal from the sensor inside the probe
has attained a predetermined threshold set in the interface. The
trigger signal instructs the machine control to determine the
position of the movable arm and arrest motion of the arm in order
to prevent damage to the machine. A small scale movement of the arm
relative to the part to be inspected after contact between the
stylus and the surface has occurred, known as "overtravel", is
accommodated by the ability of the stylus supporting member to
deflect relative to the housing of the probe.
SUMMARY OF INVENTION
A first aspect of the present invention relates to the optimum
location of deformation sensors in such a probe. According to a
first aspect of the present invention, a touch probe includes a
fixed structure, by which the probe may be supported on the movable
arm of a coordinate positioning machine, and a stylus supporting
member biased into a repeatable rest position relative to the first
structure, from which the supporting member is displaceable when a
deflecting force is applied thereto, and to which it may return
when said deflecting force is removed, the direction of biasing
action defining a probe axis, the fixed structure and stylus
supporting member forming at least part of a load path between a
stylus securable to said stylus supporting member and said movable
arm, wherein the probe comprises a plurality of sensors, sensitive
to tension and compression for sensing force applied to said
supporting member prior to displacement thereof from said rest
position, each said sensor having an axis of maximum sensitivity to
tension and compression, wherein said sensors are provided in said
load path and are grouped into pairs, the axis of maximum
sensitivity of each sensor in a pair of sensors being inclined (a)
with respect to the probe axis and (b) with respect to the axis of
maximum sensitivity of the other sensor in the pair.
In one embodiment, the sensors are provided between two parts of
said stylus supporting member. In a further embodiment, the sensors
are provided on the fixed structure, and are pre-stressed by the
weight of, and/or biasing action on the stylus supporting
member.
The deformation sensors may, for example, be provided by strain
gauges (and associated load cells), or by piezoelectric sensors.
The output from such sensors increases with increased deformation
of the stylus and/or the stylus supporting member, up to a maximum
value at which displacement of the stylus supporting member
relative to the housing occurs. A further independent aspect of the
present invention relates to an advantageous configuration of
piezoelectric sensor which provides good signal to noise
characteristics. According to a further aspect of the present
invention, a piezoelectric sensor for sensing tension and
compression between two conducting surfaces comprises first and
second piezoelectric elements provided between said surfaces, each
of which generates a polarisation of electric charge in a direction
extending between said surfaces upon tension or compression thereof
in said direction, wherein said elements are stacked one upon
another and have oppositely directed polarities, the sensor further
comprising means for insulating each of said surfaces from said
elements, means for equalising the electric potential of said
surfaces, a first electrode connected to one of said elements at a
point adjacent one of said surfaces, and a second electrode
connected to said one element at a point adjacent its abutment with
the other of said elements.
This arrangement enables a larger manifestation of the electric
charge polarisation, created by e.g. compression of a stack of
elements, in the form of a voltage, by minimising the effect of
stray capacitance between the electrodes and the surfaces.
A problem which occurs particularly with probes which have analogue
sensors indicating deformation of the stylus and/or stylus
supporting member, is that vibration of the machine during movement
of the movable arm causes the sensors to emit signals whose value
exceed the threshold set in the interface, causing the interface to
emit a "false" trigger signal (i.e. a trigger when no contact
between the stylus and a surface has occurred). To overcome this
problem, interfaces have been provided which generate an initial
"latching" signal when the signal level from the probe reaches a
predetermined threshold, and a subsequent confirmation signal some
time later if the signal level from the sensor is still above the
threshold. The latching signal causes the machine control to
register the position of the movable arm; the confirmation signal
validates the "latched" position reading and is also used to arrest
movement of the movable arm. An interface of this type is described
in U.S. Pat. No. 4,177,568. Machine vibrations causing isolated
increases in the analogue signal level above the predetermined
threshold thus fail to cause false trigger because the signal
output from the analogue sensor does not correspond to the trigger
signature required by this interface. However, a machine vibration
resulting in the generation of a latching signal may occur
sufficiently close to a genuine contact between the stylus and a
surface for the confirmation signal emitted by the interface in
respect of the genuine measurement event to confirm the validity of
the position measurements latched in respect of the analogue signal
resulting from the machine vibration; the resulting measurement
will thus be the position of the movable arm at which the machine
vibration occurred, rather than the position at which the stylus
contacted the surface (an event which occurred a very short space
of time afterwards).
To overcome this problem, a third independent aspect of the present
invention provides an interface for connecting a measuring probe to
a control for a machine on which said probe is used, the interface
emitting a latching signal when the analogue signal level from the
probe attains a first threshold value, and a confirmation signal
responsive to a trigger signature from the analogue sensor
characterised by at least three successively increasing signal
levels, the time interval between attainment of the first and
second signal levels determining the requisite time interval
between the second and third signal levels. Preferably, the first
threshold value of analogue signal corresponds to the signal level
required to generate the latching signal.
The interface of the present invention is thus more discriminating,
while simultaneously accommodating all conceivable signal profiles
occurring as a result of a genuine measurement event. This has
particular relevance in relation to measurement operations in which
different individual measurements are made at different speeds of
the movable arm relative to the part to be inspected. If, for
example, the part is inspected at a relatively slow speed, then the
time interval between the attainment of the first and second
thresholds by the analogue signal will be relatively large.
However, since this time interval is used to determine the
requisite time interval after which the analogue signal level
should be at, or above, the third threshold, the signature of this
event will correspond to a trigger signature recognisable by the
interface. Conversely, the same will be true for a probing
operation which occurs at a relatively fast speed.
Deformation sensors such as strain gauges or piezoelectric elements
are typically sensitive to tension and compression over a large
range of frequencies. Thus, a probe employing such sensors to
detect deformation of the stylus and/or stylus supporting member,
will be susceptible to generating analogue signals in excess of a
threshold set in the interface responsive to both high frequency
deformation, i.e. a shockwave generated upon contact between the
stylus and a surface, and low frequency deformation, i.e. strain
resulting from contact between the stylus and a surface. Trigger
signals generated by the interface in response to these two
different types of probe output have different response times, i.e.
different time intervals between initial contact of the stylus with
the surface, and emission of a latching signal by the interface.
This affects the accuracy of measurements made with the probe,
since the distance travelled by the movable arm of the machine
during the time interval between initial stylus-surface contact and
emission of a latching signal by the interface (known as the
"pre-travel"), is dependent upon the time interval between these
two events. Pre-travel for a given probe/stylus combination is
normally calibrated prior to measurement. However, a variation in
pre-travel, which occurs if latching signals are generated
sometimes as a result of shockwaves and sometimes as a result of
strain will result in a measurement error. A fourth independent
aspect of the present invention provides an interface which
discriminates between signal outputs from the probe resulting from
(a) shockwaves generated in the stylus and stylus supporting member
when the stylus contacts the workpiece, and (b) strain in the
stylus and/or stylus supporting member when a stylus contacts the
surface.
Accordingly, a fourth aspect of the present invention provides an
interface for connecting a touch probe, having at least one sensor
for generating analogue signals, to a control of a coordinate
positioning machine, the interface comprising:
means for generating a latching signal, for instructing the machine
control to record a measurement, responsive to said analogue
signals attaining a predetermined threshold; and
means for determining whether said latching signal was generated in
response to analogue signals of above a predetermined frequency,
and for generating a flag signal accordingly.
The user of the machine is therefore able to determine from the
flag signal whether the measurement was taken as a result of a
shockwave or as a result of strain, and, consequently, may either
assign the appropriate pre-travel to the measurement (where
pretravel calibration values have been obtained in respect of both
shock and strain), or where e.g. the shockwave was not present,
perform a further measurement operation to attempt to obtain a
measurement value on the basis of a shockwave.
In an alternative form of interface, latching signals are not
generated when analogue signals in excess of the predetermined
threshold have a frequency in excess of the predetermined
frequency, i.e. the shockwave is ignored.
BRIEF DESCRIPTION OF DRAWINGS
An embodiment of the invention will now be described, by way of
example, and with reference to the accompanying drawings in
which:
FIG. 1 shows a section through a probe according to the present
invention;
FIG. 2 shows a section on II--II in FIG. 1;
FIG. 3 shows a section on III--III in FIG. 2;
FIGS. 4 shows a detail of FIG. 3;
FIG. 5 is a circuit diagram for the detail of FIG. 4;
FIG. 6 is a circuit diagram showing an embodiment of interface
according to the present invention; and
FIGS. .[.7a-n.]. .Iadd.7a-h .Iaddend.are signal diagrams
illustrating the operation of an interface according to the present
invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
Referring now to FIG. 1, a touch probe which employs a number of
analogue sensors will now be described. The probe includes a fixed
structure provided by a cylindrical housing 10, which defines an
axis A, and a stylus supporting member 12 supported in a kinematic
rest position with respect to the housing 10 provided by the
engagement of rollers 14 on the supporting member 12 with pairs of
balls 16 on the housing 10. A support of this type enables
displacement of the supporting member 12 from the rest position
when deflecting force is applied thereto, and return of the
supporting member 12 to the rest position when the deflecting force
is removed. It is not essential to use a kinematic support for the
stylus supporting member 12, other types of support mechanism which
provide repeatable location of the supporting member 12 relative to
the housing 10 may be used such as those described in GB
2094478.
The stylus supporting member 12 consists of an upper body 20 and a
lower body 22 which supports a stylus 24, having a spherical
sensing tip 26 at its remote end. The stylus 24 is mounted on a
circular retaining plate 28. The retaining plate 28 is magnetically
urged into engagement with the lower body 22 by means of the
magnetic interaction between a permanent magnet 30 supported on the
upper body 20 with a steel striker plate 32 provided on the
retaining plate 28. The magnetic attraction force between the lower
body 22 and the retaining pate 28 may be adjusted by altering the
position of the plate 32 within the screw-threaded bore 34 provided
in the retaining plate 28.
Location of the retaining plate 28 relative to the lower body 22 is
provided by a further kinematic support; three pairs of radially
extending rollers 36 being provided on the lower body which are
respectively engaged by three balls 38 provided on the retaining
plate 28. The rollers 36 are included in a series electrical
circuit in order to detect this engagement of the retaining plate
28, or, alternatively (depending upon the length of the stylus and
the magnetic attraction), deflection of the retaining plate 28
relative to the lower body 22 to accommodate overtravel during a
measurement. The magnetic mounting of the stylus 24 on the lower
body 22 enables automatic exchange of styli, for example in the
manner described in our earlier filed International patent
application PCT/GB92/02070. The automatic exchange of styli enables
the mounting of different configurations of styli on the probe, in
order to inspect differently oriented surface of a part.
The stylus 24 is thus supported on the movable arm of the machine
by a load path which includes the lower and upper bodies of the
stylus supporting member 12, and the housing 10. Thus all forces
applied to the stylus 24, prior to displacement of the supporting
member 12 from its rest position, will be transmitted through this
load path.
Biasing of the stylus supporting member 12 into the rest position
with respect to the housing 10 is provided by a biasing spring 40.
The biasing force applied by the spring 40 acts parallel to axis A,
and may be adjusted by rotation of a rotatable collar 42 mounted on
the housing 10. The collar 42 has, at its inner side, a
screw-threaded engagement with a plate 44 which provides an
abutment for the spring 40. Rotation of the collar 42 relative to
the housing 10 results in axial movement of the plate 44 and a
consequent change in the biasing force applied by the spring 40 to
the supporting member 12.
Referring now additionally to FIG. 2 and 3, the lower body 22 of
the stylus supporting member 12 is formed as a circular dish having
three upwardly directed, radially extending v-shaped ridges 46. The
upper body 20 has, in its lower surface, three correspondingly
arranged radially extending u-shaped channels 48 which receive the
ridges 46. An analogue sensor in the form of a stack 50 of
piezoelectric elements 52 is mounted between each of the inclined
surfaces on each of the ridges 46 and the corresponding sides of
each of the u-shaped channels 48. The stacks 50 of elements 52 are
compressed by clamping bolts 54, which clamp the upper and lower
bodies 20,22 together. As can be seen in FIG. 2, the stacks 50 of
elements are grouped in pairs at three points about the axis A. The
direction of electric charge polarisation of each piezoelectric
element 52 in a stack 50 extends substantially perpendicular to the
surfaces of the upper and lower bodies 20,22 against which the
elements are compressed and thus is inclined both with respect to
the probe axis A, and with respect to the direction of electric
polarisation of the adjacent stack 50 in a given pair. Such a
configuration provides optimum sensitivity to forces applied to
stylus 24 in a wide range of directions.
Referring now to FIG. 4, an individual stack 50 of piezoelectric
elements 52 consists of first and second elements 52A,B, a positive
electrode 56 spaced between the elements 52, and two negative
electrodes 58A,B spaced between each element and the upper or lower
body 20,22 respectively. The polarity of the elements 52A,B is
arranged so that they will generate oppositely directed electrical
charge polarisations upon tension or compression. The electrodes
58A,B are insulated from the upper and lower bodies 20,22, in this
example, by an anodising layer; bolts 54 which connect the upper
and lower bodies 20,22 ensure that the bodies remain at the same
potential. The output signals are taken from the element 52B, via
electrodes 56 and 58B. As will be described, element 52A serves to
reduce the effect of stray capacitance, between electrode 56 and
the upper body 20 for example.
When the stack 50 is compressed, each of the elements 52A,B will
generate an electric charge polarisation Q. This charge
polarisation Q manifests itself as a potential difference V across
a capacitor, with V being equal to Q/C, where C is the capacitance.
Thus, the lower the effective capacitance of a system of this type,
the higher the output voltage for a given charge polarisation (and
therefore for a given compressing force) i.e. the higher the
sensitivity of the system. Referring now to FIG. 5, the capacitance
provided by piezoelectric element 52B, and electrodes 56 and 58B is
denoted as C.sub.p1 ; the capacitance provided by the element 52A
and electrodes 56 and 58A is denoted C.sub.p2. Stray capacitances
C.sub.1 and C.sub.2 are the result of the capacitances between
electrodes 58A and 58B, and upper and lower bodies 20,22
respectively. Upon compression (e.g.) of a stack 50 a positive
charge will appear at electrode 56 and negative charges will appear
at electrodes 58A,B. In the absence of element 52A, (i.e.
C.sub.p2), parts of the charge at electrodes 56 and 58B would
migrate to stray capacitances C.sub.1,C.sub.2, thus reducing the
output voltage across electrodes 56,58B (and hence the magnitude of
the detectable signal). However, because electrode 58A is at the
same potential as electrode 58B, and the upper and lower bodies are
at the same potential by virtue of bolts 54, no charge is taken up
by capacitors C.sub.1,C.sub.2, and the entire electric charge
polarisation in element 52B is converted to a voltage across
electrodes 56,58B.
Usually, when the sensing tip 26 of the stylus 24 contacts the
surface of a part to be inspected, a shockwave is generated which
travels up the stylus 24 and causes a high frequency voltage
oscillation, or "ringing", in the output of the piezoelectric
elements 52. Occasionally however, as a result of grease or other
contaminants on the surface of the part, this shock wave is not
present and the earliest indication of contact between the sensing
tip 26 and a surface is the steady increase in voltage caused by
strain in the stack 50 of piezoelectric elements 52 as a result of
a microscopic movement of the stylus 24, retaining plate 28 and
lower body 22 of the stylus supporting member relative to the upper
body 20. To detect each of these type of output signals, an
interface (shown in FIG. 6) is provided which has, in respect of
each stack 50, a high frequency channel 60, and a low frequency
channel 62. The high frequency channel 60 includes a high-pass
filter 64, having a lower cut-off frequency of 500 Hz, in series
with a window comparator 66 set at a threshold level 1.sub.1. The
outputs of each of the six high frequency channels 60 are OR'd
together and passed through a debounce circuit 68, which generates
a high output when the output of the OR gate 70 is high, but does
not return to the low voltage state until some time after the last
low output of the OR gate 70. The low frequency channel 62 includes
a low-pass filter 72, having an upper cut-off frequency of 200 Hz,
in series with a rectifier 74. The outputs of each of the low
frequency channels 62 are sent to a summing circuit 76 whose output
is then connected in parallel to three comparators 78,80,82, set at
three threshold levels: 1.sub.1, 1.sub.2, and 1.sub.3. The output
of comparator 78 is connected in series to a further debounce
circuit 84; debounce circuits are not required in respect of
comparators 80 and 82 because the thresholds 1.sub.2 and 1.sub.3
are set at sufficiently high level.
FIG. 7a illustrates the output from the positive electrode 56 of a
piezoelectric stack 50. From the signal diagram it can be seen that
the threshold level 1.sub.1 is first attained at a time t.sub.1,
and as a result of the high frequency voltage oscillation generated
by the shock wave upon initial contact of the sensing tip 26 with
the surface. The instant at which the output of the debounce
circuit 68, in the high frequency channel 60 of the interface, goes
high thus corresponds to the time t.sub.1 at which the level
1.sub.1 is attained at window comparator 66, and this is
illustrated in FIG. 7b. As the shock wave is attenuated, and the
high frequency oscillations are correspondingly damped, the probe
continues to move relative to the surface of the part. This causes
a steady increase in the strain in the piezoelectric stack 50, and
corresponding steady increase in the voltage output from the
electrode 56. At a time t'.sub.1, the threshold level 1.sub.1 is
once again attained as a result of this steady increase in voltage,
and the output of the debounce circuit 84, provided in respect of
the low frequency channels 62, goes high; this is illustrated in
FIG. 7c. The outputs of both debounce circuits 68 and 84 are
combined at a LATCH output terminal 86, which is sent to the
machine control, and instructs the machine control upon receipt of
an output signal at this terminal to register the position of the
movable arm of the machine. As is mentioned above, the shock wave
generated by initial contact between the sensing tip 26 and the
surface of the part is not always present. The output of debounce
circuit 68 cannot therefore be reliably used to provide the
latching signal at output terminal 86. It is for this reason that
the outputs of the high and low channels 60,62 are combined at
output 86.
In order to provide confirmation that the latching signal generated
at output 86 represents a genuine contact between the sensing tip
26 and the workpiece, rather than the generation of a voltage which
exceeds the threshold level 1.sub.1 as a result of vibration of the
machine, comparators 80 and 82, and a timing circuit 90 are
provided to enable the interface to respond only to signal profiles
which closely mimic a genuine trigger signature from the stacks
50.
The timing circuit 90 includes a supply rail 92 to which a 2mA
constant current source i.sub.1 is connected, which is in turn
connected in series with a capacitor 94. The capacitor 94 is
connected in parallel with a transistor TR1. Constant current
source i.sub.1 is also connected in series to a 1mA constant
current source i.sub.2, which in turn is connected in parallel with
a second 2mA current source i.sub.3. The constant current source
i.sub.3 is connected in series with a transistor TR2. The base of
transistor TR1 is connected in parallel with the output signal 86,
and the base of transistor TR2 is connected to the output of
comparator 80.
In normal operation, when no signals are generated from the
piezoelectric stack 50 the transistor TR1 is switched on, and the
transistor TR2 is switched off. However, upon generation of a
latching signal at output 86, the transistor TR1 is switched off.
This causes the capacitor 94 to be charged with a current (I.sub.c)
of 1mA. The voltage V.sub.cap across the capacitor 94 is
illustrated in FIG. 7f, and it can be seen from this Figure that
the voltage rises linearly. When continued movement of the movable
arm of the machine relative to the part to be inspected increases
the strain in the piezoelectric stack 50 to such an extent that the
output voltage from the stack 50 attains the threshold level
1.sub.2, the comparator 80 outputs a step signal CONF1 (illustrated
in FIG. 7d) which switches the transistor source i.sub.3 ; the
current I.sub.c now being equal to -1mA. The TR2 on. This causes
the capacitor 94 to discharge, in voltage V.sub.cap now decreases
linearly. Further movement of order to satisfy the requirements of
the constant current the probe relative to the workpiece, causing
further strain in the piezoelectric stack 50, will result in a
further increase in the voltage output from the stack to a level
above the threshold level 1.sub.3. This will cause the comparator
82 to emit a signal CONF2 (illustrated in FIG. 7e), which latches,
by means of a D-type flip-flop 96, the output of a comparator 98.
Comparator 98 compares the voltage V.sub.cap to a predetermined
threshold voltage V.sub.t ; the output of the comparator being high
unless the voltage V.sub.cap lies below the voltage threshold
V.sub.t. An error signal is thus generated at output terminal 100
of the flip-flop 96 if, at the time signal CONF2 is generated, the
voltage V.sub.cap exceeds the threshold voltage V.sub.t when the
voltage from the piezoelectric stack 50 passes through the
threshold level 13 (in FIG. 7 no such error signal is
generated).
This situation arises when the time interval between the attainment
of the threshold voltage levels 1.sub.2 and 1.sub.3 does not
correspond to the time interval between the attainment of the
levels 1.sub.1 and 1.sub.2 ; in other words, where the initial
latching signal at output 86, caused by high frequency ringing or a
small strain in the piezoelectric stack 28, occurs a relatively
long period of time before the strain in the piezoelectric stack 50
increases to the level 1.sub.2, but where the rate of increase of
strain is such that the level 1.sub.3 is attained a relatively
short time after the level 1.sub.2. The are two possible reasons
for this: firstly, it is possible that the movable arm of the
machine was moving at a relatively slow speed when initial contact
was made with the surface, thus explaining the relatively large
time interval between attainment of the threshold levels 1.sub.1
and 1.sub.2. A subsequent acceleration of the machine would result
in an increased rate of compression of the piezoelectric stack 50,
causing a corresponding increase in the rate of increase of the
output voltage, thereby decreasing the time interval between the
voltage passing through threshold levels 1.sub.2 and 1.sub.3.
Alternatively, if the movable arm was travelling at a constant
speed relative to the surface of the part, a vibration of the
movable arm resulting in the generation of a latching signal at
output 86, which occurred a short period of time before contact
between the sensing tip 26 of the stylus and the surface of the
part, would cause the capacitor 94 to start charging too early. The
time interval between attainment of the voltage levels 1.sub.2 and
1.sub.3 would thus be insufficient for the capacitor to lose enough
charge for the voltage level V.sub.cap to pass below the threshold
level V.sub.t.
The interface of the present invention thus operates by
discriminating genuine trigger signatures from the piezoelectric
stack 50 on the basis of a correspondence in the time intervals
between the attainment of threshold levels 1.sub.1 and 1.sub.2, and
the levels 1.sub.2 and 1.sub.3. A result of this is that the
interface is insensitive to variations in probing speed from one
probing operation to the next; it is, however, a requirement that
each individual probing operation is made under constant speed. For
example, if the measurement is taken at a relatively slow speed,
the strain in the piezoelectric stack 50 will increase at a
relatively low rate as will the output voltage. Because the probe
is moving at a constant speed, the voltage will increase at a
substantially constant level, and therefore the relatively large
time interval between attainment of the voltage thresholds 1.sub.1
and 1.sub.2 will be balanced by correspondingly large time interval
between attainment of the voltage thresholds 1.sub.2 and 1.sub.3.
The converse is true for a probing operation occurring at a
relatively fast speed.
The relative values for the capacitance of capacitor 94, and the
value of the constant current sources are determined by the
characteristics of the piezoelectric stacks 50; and the absolute
value of the capacitor 94 is chosen with regard to practical
opening voltages for the comparator 98.
The peak output voltage from the piezoelectric stack 50 corresponds
to the instant in time at which the stylus supporting member 12 is
displaced from its kinematic rest position with respect to the
housing. At this instant in time, a HALT signal is emitted from a
further circuit (not shown) in the interface, which incorporates
the serial connection of each of the rollers 14 ad balls 16. The
HALT signal is emitted when the resistance in the aforementioned
circuit reaches a predetermined threshold (such a circuit is shown
in, e.g. W092/09862), and is used to instruct the machine control
to arrest movement of the movable arm. The small movement or
"over-travel" of the movable arm after the sensing tip 26 has come
into contact with the surface of the part to be inspected is
accommodated by the ability of the stylus supporting member 12 to
be displaced from its rest position relative to the housing 10.
During the time period between contact of the surface of the part
by the sensing tip 26 of the stylus 24, and the instant of time at
which the LATCH signal from terminal 86 causes the machine to
register the position of the movable arm, the probe (and thus the
movable arm) will move relative to the part. This movement is known
as "pre-travel", and in order to obtain an accurate measurement of
the position of the surface relative to a reference position on the
machine it is necessary to calibrate or "datum" the magnitude of
the pre-travel. Such calibration procedures are well known.
However, in a probe of the present type the LATCH signal at output
terminal 86 may result from analogue signals generated in the
piezoelectric elements as a result of two different physical
phenomena; the LATCH signal at output terminal 86 may be generated
as a result of a voltage from the piezoelectric stack 50 cad by a
shock wave occurring as a rest of the impact between the stylus tip
26 and the surface of the part to be inspected; alternately, it is
possible that the shock wave may not be present, in which case the
LATCH signal at output terminal 86 will be generated as a result of
the steady increase in compression of the piezoelectric stack 50
after initial contact of the tip 26 with the surface of the part,
caused by continued movement of the movable arm (and thus the
probe) relative to the surface. As can be seen from FIGS. 7b and
7c, the LATCH signal will occur at different instances in time
depending upon whether the shock wave was present or not. Because
the pre-travel of the probe is related directly to the time delay
between the instant of contact of the sensing tip 26 with the
surface and the moment at which the machine registers the position
of the movable arm, measurements resulting from a LATCH signal
caused by the shock wave will have a different pre-travel to
measurements resulting from a LATCH signal generated upon the basis
of an increase in the strain in the stack 50. For improved accuracy
therefore, it is necessary to determine whether the LATCH signal is
generated as a result of a shock or as a result of strain. The
probe may then be calibrated so that a pre-travel value is obtained
for LATCH signals generated by shock, and a further pre-travel
value is obtained for LATCH signals generated by strain.
Referring again to FIG. 6, a D-type flip-flop 110 has its clock
input 112 connected to the output terminal 86 and its data input
114 connected to the output of comparator 78 provided in respect of
the low frequency channel 62. The LATCH signal generated at output
terminal 86 thus performs a clock function at the input 112,
switching the data input 114 on to the output line 116 of the
flip-flop 110. If the LATCH signal is generated as a result of
shock in the stylus 24, then, when the LATCH signal is sent to the
clock input 112 the data input 114 of the flip-flop 110 will be
low, since the level of strain in the low frequency channel 62 will
not yet be high enough to cause the output of comparator 78 to go
high. The signal level on output line 116 of the flip-flop 110 will
thus be low, indicating that the trigger was caused by shock in the
stylus 24. Alternatively, if the LATCH signal is generated as a
result of strain, then the output of comparator 78 will be high at
the instant the LATCH signal is generated, causing the LATCH signal
at clock input 112 to switch a high signal on data line 114 to the
output 116; a high output signal on line 116 thus indicates that
the LATCH signal is generated as a result of strain.
In a modification, the interface may be adapted to ignore signals
from the piezo stacks 50 in excess of a certain frequency, e.g. by
providing a switch which switches off channel 60, thereby
triggering only by signals generated by strain.
The present invention has been described with reference to the use
of piezoelectric elements as analogue sensors which detect
deformation of the stylus and stylus supporting member before
movement of the stylus supporting member 12 relative to the housing
10. Alternative analogue sensors may be used for this purpose, one
example being strain gauges and an associated load cell.
Additionally, an electrical circuit has been shown as an example of
an analogue sensor which detects displacement of the stylus
supporting member relative to the housing 10; alternative such
analogue sensors may be employed, such as capacitive sensors or
optical sensors employing photodiodes or position sensitive
detectors.
* * * * *