 |
Claims  |
|
|
We claim:
1. A coordinate-measurement machine having one or more probe heads (10)
with associated replaceable probes (15, 61 to 64) and an electronic system
(20) for processing probe signals (a, b, c, k) produced in the course of a
work-probing procedure, wherein the electronic system (20) includes
resettable means that permits parameters to be set to determine the manner
and nature of signal processing, the parameters being set digitally, a
plurality of data files for storage of a plurality of different sets of
parameter values, and means for calling up a selected set of values of
different parameters stored in a data file (*.ST3) to reset said
resettable means in accordance with a selected measurement task.
2. A coordinate-measurement machine according to claim 1, in which the data
files (*.ST3) are associated with a plurality of selected probe pins (61
to 64), and the values of the parameters stored in the corresponding data
file (*.ST3) can be set upon workpiece contact with the corresponding
probe pin (61 to 64).
3. A coordinate-measurement machine according to claim 2, in which an
identification code (61) is associated with the replaceable probes to be
introduced, and in which the selection of the corresponding parameter data
file is effected under the control of the code.
4. A coordinate-measurement machine according to claim 1, in which the data
files (*.ST3) are associated with selected types of machine operation
within the capability of the coordinate-measurement machine, and are
activated upon a change in the type of machine operation, the values of
the stored parameters being set for operation of the
coordinate-measurement machine upon making such a change in machine
operation.
5. A coordinate-measurement machine according to claim 1, in which the
electronic system includes a digitally encoded analysis program for
analysis of the course of a probe signal at different points in the
probe-pin electronic system and for setting parameters in accordance with
the analysis.
6. A coordinate-measurement machine according to claim 1, in which the
electronic system includes a digitally encoded module which sets different
parameter combinations during a trial run in which a workpiece is
contacted several times and which sets the parameters in accordance with
the measurement results thereby obtained.
7. A coordinate-measurement machine according to claim 1, in which the
machine includes a computer and in which the electronic system (20) is
connected via a serial bus (CAN) to control means (21), the control means
(21) being connected to the computer (22) of the coordinate-measurement
machine.
8. A coordinate-measurement machine according to claim 1, in which the
electronic system contains a microprocessor having several analog inputs
(A1 to A8) which are respectively connected with different points (A1 to
A8) within the electronic system for probe-signal processing, and that the
course in time of the probe signals occurring during a work-contacting
procedure at the analog-input points (A1 to A8) are stored in a memory
component (RAM 26) of the microprocessor (23).
9. A coordinate-measurement machine according to claim 8, in which the
electronic system (20) includes means for connecting the electronic system
to a modem over which stored signal courses (A1 to A8) can be remotely
transmitted.
10. A coordinate-measurement machine according to claim 8, including means
for serial-bus (CAN) transmission of signal courses stored in the memory
component (RAM 26) of the microprocessor.
11. A coordinate-measurement machine according to claim 1, in which the
values of the parameters set by the electronic system (20) are stored in a
random-access memory (25).
12. A coordinate-measurement machine according to claim 1, in which the
probe head supplies at least two different contact signals (a, b, c, k),
and the electronic system concurrently processes each of the at least two
signals (a, b, c) in parallel in separate signal branches (N1M1, N1M2,
MESK).
13. A coordinate-measurement machine according to claim 1, in which the
machine includes a computer (22), and in which the electronic system (20)
is connected via a serial bus directly to the computer (22) of the
coordinate-measurement machine.
14. A coordinate-measurement machine according to claim 1, in which the
data files (*.ST3) are associated with a plurality of selected probe
configurations, and the values of the parameters stored in the
corresponding data file (*.ST3) are set upon workpiece contact with the
selected probe configuration.
15. A coordinate-measurement machine according to claim 1, in which the
data files (*.ST3) are associated with a plurality of selected different
types of probe heads, and the values of the parameters stored in the
corresponding data file (*.ST3) can be set upon workpiece-contact
involvement of a selected probe head.
16. A coordinate-measurement machine according to claim 1, in which the
data files (*.ST3) are associated with different measurement tasks, and
are activated upon a change in measurement task, the values of the stored
parameters being set for operation of the coordinate-measuring machine
upon making a change in measurement task.
17. A coordinate-measurement machine according to claim 1, in which the
machine includes a computer and in which the electronic system (20) is
connected via a serial interface to control means (21), the control means
(21) being connected to the computer (22) of the coordinate-measurement
machine.
18. A method for coordinate-measurement on workpieces using a probe head
(i) adapted for interchangeable use of a selected one of a plurality of
associated replaceable probes (61 to 64) and (ii) having an electronic
system (20) for processing signals (a, b, c, k) produced upon probe
contact with a workpiece, (iii) wherein the electronic system permits the
resetting of at least two different parameters which determine the nature
and manner of signal processing, and further wherein
for specific measurement tasks and for each selectable probe, particularly
suitable sets of parameter values are determined and the thus-determined
sets of parameter values are stored in different data files;
the corresponding data files are called up upon a change in measurement
task, and the applicable stored set of parameter values is loaded into the
electronic system to reset parameter values therein; and
signal-processing then proceeds for the changed measurement task using the
newly loaded set of parameter values.
19. A method according to claim 18, in which different measurement tasks
are selected in accordance with at least one of the following criteria:
the speed of measurement,
the relative accuracy of measurement to be obtained, and
the type or material of the object to be measured.
20. A method according to claim 18, in which at least two of the following
parameters can be set:
the sensitivity of an amplifier (37) for the probe signal;
the characteristics of a frequency filter (31, 41) for the probe signal;
the threshold level of a trigger circuit (35, 38, 45, 48) for the probe
signals;
the permissible duration (F1, F2, F3) of the probe signal;
the permissible timing of several interrelated probe-signal pulses (51 to
54);
the selection as to which of several probe signals (51 to 54) is to be
further processed (mode of operation).
21. A method according to claim 20, in which the frequency filter is a
band-pass filter in the form of a switched-capacitance filter having a
microprocessor connection.
22. A method according to claim 20, in which the frequency filter is a
high-pass filter in the form of a switched-capacitance filter having a
microprocessor connection.
23. A method according to claim 18, in which the loading of sets of
parameter values and the setting of said values in the electronic system
takes place automatically in accordance with a program-controlled analysis
of the probe signal obtained in the course of a workpiece-contacting
procedure.
24. A method according to claim 18 in which, for the selection of optimal
parameters, several contacting procedures with different parameters are
performed in a test run.
25. A method according to claim 18, in which the loading and setting of
sets of parameter values in the electronic system takes place
automatically, pursuant to and controlled by a change of probe head.
26. A method according to claim 18, in which, for the selection of optimal
parameters, several contacting procedures with different combinations of
parameters are performed in a test run.
27. A method according to claim 18, in which the loading and setting of
sets of parameter values in the electronic system takes place
automatically, pursuant to and controlled by a change of probe pin.
28. A method according to claim 18, in which the loading and setting of
sets of parameter values in the electronic system takes place
automatically, pursuant to and controlled by a change of probe
combination.
29. A coordinate-measuring machine, comprising:
a computer;
probe means including means for producing a probe signal in the course of a
work-probing procedure;
signal-processing electronics for processing said probe signal, said
electronics being connected to said signal-producing means and comprising
means for setting at least two different parameters determining the manner
and nature of processing said signal;
setting means for digitally setting said parameters to predetermined
values;
one or more data files stored in said computer, each data file containing a
set of values for said at least two different parameters; and
means for calling up from said computer a selected one of said data files
and forwarding the set of values contained therein to said setting means.
30. A coordinate-measuring machine, comprising:
a computer;
probe means with means for mounting exchangeable probe pins of differing
type, and means for producing a probe signal when one of said probe pins
touches a workpiece;
signal-processing means for processing said probe signal, said
signal-processing means being connected to said signal-producing means and
comprising means for setting at least two different parameters determining
the manner and nature of processing said probe signal according to the
type of probe pin mounted to said probe means;
setting means for digitally setting said parameters to predetermined
values;
one or more data files stored in said computer, each data file containing a
set of values for said at least two different parameters; and
means for calling up a selected one of said data files and for forwarding
the set of values contained therein to said setting means.
31. A method for coordinate-measurement on workpieces using a probe head
adapted for interchangeable use of a selected one of a plurality of
associated replaceable probes and being connected to an electronic
signal-processing system for processing signals produced upon probe
contact with a workpiece, wherein the electronic system permits the
setting of at least two different parameters, said method comprising the
steps of:
creating at least two different data files, each data file containing a set
of at least two different parameter values which determine the nature and
manner of processing said signals;
storing said data files;
calling up a selected one of said data files, and loading the set of
parameter values contained therein into said electronic system, thereby
resetting said different parameters to the parameter values contained in
the selected data file; and
using the newly loaded set of parameter values in the processing of one or
more signals produced upon probe contact with the workpiece.
32. A method for coordinate-measurement on workpieces using a probe head
adapted for interchangeable use of a selected one of a plurality of
associated replaceable probes and being connected to an electronic
signal-processing system for processing signals produced upon probe
contact with a workpiece, wherein the electronic system permits the
resetting of at least two different parameters, said method comprising the
steps of:
creating at least two different data files, each data file containing a set
of at least two different parameter values which determine the nature and
manner of processing said signals according to a selected one of several
different measurement tasks;
storing said data files;
selecting one of the different measurement tasks;
calling up the selected one of said data files and loading the set of
parameter values contained therein into said electronic system, thereby
resetting said different parameters to the parameter values contained in
said selected data file; and
processing said signals during performance of the selected measurement task
using the newly loaded set of parameter values. |
|
 |
|
 |
Claims |
|
|
|
Description |
|
|
BACKGROUND OF THE INVENTION
The present invention relates to a coordinate measurement machine having a
probe head with associated replaceably interchangeable probes and an
electronic system for processing signals produced in the course of a
workpiece-probing procedure, wherein the electronic system permits
parameters to be set for determining the nature and manner of signal
processing.
Such a coordinate-measurement machine is sold by applicants' assignee under
the designation MC. This machine has a displaceable probe head with a
so-called switching type probe which, pursuant to its workpiece-contacting
procedure, produces two successive signals; the first or so-called piezo
signal is produced by piezoelectric sensors in the probe pin at the
instant of contact with a workpiece. In the course of further probe-head
movement, the probe pin is deflected and a second, so-called contact
signal, is produced; this second signal is produced when a resiliently
mounted carrier for the probe pin is deflected from its position of rest,
thereby opening contacts of an electromechanical switch integrated in the
mounting of the probe-pin carrier.
The signals of this switching-type probe head have heretofore been
evaluated substantially in accordance with a fixed scheme. There is only a
limited possibility of adapting the evaluation to changed surrounding
conditions. For example, by turning a screw on the housing of the probe
head, the operator can adjust the sensitivity of the amplifier for the
piezo signal. In automatic CNC programs, the sensitivity of the evaluation
is determined by this adjustment, even though it is not optimal for all
probe configurations that are replaced or interchanged in the course of a
given measurement, or for all measurement tasks. Thus, to adapt a given
coordinate measuring machine to different fields of use, as for example,
for use in manufacture under rough surrounding conditions, special circuit
boards are provided which contain filters for a probe signal which is
optimally adapted to this use. If new machines are developed, the
components of the evaluation electronics must also be adapted. In this
way, inventory-stocking cost increases with each new variant of the
machine.
Furthermore, despite these measures, there are cases or measurement tasks
in which electronic circuitry within the probe head does not supply an
unequivocal, i.e., a valid, work-contact signal, as for example, when soft
materials are to be contacted at a relatively slow speed of workpiece
contact.
International Patent Application, Publication No. WO 87/01798, describes a
coordinate-measurement machine with replaceable probe heads of different
types. In that case, the replaceable probe heads are individually coded,
so that upon substituting one probe head for another, the code of the
probe carried by the newly substituted probe head is electronically
recognized and the newly substituted probe is connected to its correct
interface, there being several interfaces between which switching can be
effected. The interfaces contain electronic circuitry for further
processing the probe-head signals. In this connection, however, switching
is effected between complete, hard-wired interfaces, and there is no
disclosure of any way to adapt probe-signal processing of an interchanged
probe head to different measurement tasks or surrounding conditions.
EP 0,501,680 describes an electronic system for processing the signals of a
probe head which has a switch circuit for monitoring internal resistance
of the mechanical contact which opens to produce the probe signal. The
circuit automatically adapts the contact resistance, which becomes greater
as a result of aging or use, to a fixed trigger level for the signal.
However, there is no provision for externally controlled adjustment of the
trigger threshold together with other parameters.
EP 0,388,993 describes an electronic system which automatically recognizes
the introduction of probe heads of different type. This circuit sets up
window comparators or their trigger level for the probe signal in case of
high travel speeds, and as a result, the circuit becomes less sensitive.
This publication also fails to disclose an adjustment which is switchably
controlled from the outside, along with other parameters.
BRIEF STATEMENT OF THE INVENTION
The object of the present invention is, in a coordinate measurement machine
of the character indicated, to provide electronic circuitry for processing
of probe-head signals in such manner that they are, with the simplest
possible means and at all times, optimally adapted to the measurement task
in question.
The invention achieves this object by providing an electronic system (20)
for processing signals generated during the probing procedure, wherein the
electronic system (20) permits parameters to be set for determining the
manner and nature of signal processing. The parameters can be set
digitally, and at least one set of values of different parameters is
stored in a date file (*.ST3) which can be called up in accordance with
the selected measurement task.
The solution of the invention has various advantages: Since the combination
of parameters which is optimal for each desired measurement task is stored
and the parameters or components can be digitally set in the electronic
system, these parameters can be adjusted simply and rapidly. This is
particularly advantageous in the case of automatic measurement runs. For
example, parameter-data files associated with various replaceable probe
configurations, or probe-head data files associated with various
replaceable probe heads, are contained in computer-accessible storage,
such that upon substitution of a newly installed probe configuration or
probe head, the set of parameters appropriate for the particular use of
the newly substituted probe configuration or probe head can be
automatically computer-loaded into the electronic system. For example, the
association of the data files with a newly substituted probe pin, may
occur by having the operator, upon calibration of the probe pin, select
the corresponding data file from a computer display, to then connect the
selected data file to the address of the probe pin.
The system is furthermore very user-friendly since a large number of
different parameters need not be individually set by the operator, but he
can operate with a few data files which have been predefined; for example,
on the work side for given measurement tasks or probes, the optimal value
combination of the parameters can have been already established in the
data files. Thus, also by way of example, different data files can be
provided, on the one hand for the most-precise possible measurement with
relatively low travel and contacting speeds and, on the other hand, for
less-precise measurement using high travel and contacting speeds. For
these two cases, different signal forms of contact signal are produced,
and in each case, either different parameters for signal evaluation are
evaluated optimally, or the contact signal is evaluated in a different
mode of operation.
The setting of parameters specific to each selected probe configuration, or
the setting of parameters specific to each selected type of probe head,
can be effected completely automatically if, for example, an identifiable
code is associated with the probe configuration to be substituted or with
the probe head to be substituted. The selection or pre-selection of the
correct parameter-data files, and also (if desired) the subsequent setting
of stored parameter-data values in the electronic system, can then be
effected under code-identified control.
Furthermore, it is possible to automate the selection of the most suitable
parameter-data files and possibly the setting of individual parameters,
referred to the workpiece; thus, in a trial run, the signals of the probe
head may be recorded (i.e., during the trial run) and evaluated by a
suitable program module or firmware module of a corresponding processor
and processors. This can be done, for example, by having the program
module or firmware module analyze the course of the signal with respect to
frequency, amplitude, succession in time, etc.; and, from such an
analysis, the program module or the firmware module can develop and
propose the most suitable combination of parameters, either as a proposal
to the operator or for automatically adopting and storing this most
suitable combination. Another possibility is so to program the module that
the surface of the workpiece is contacted several times with different
parameter settings, so that then a selection can be made for the setting
which gives best results. In both cases, the parameter combination which
is optimal with respect to the specific measurement task is selected, and
it will be understood that measurement of a given type of workpiece is
part of the measurement task.
Not only can the sensitivity of the setting of the probe signal be changed,
for example via various levels of trigger circuits, but an entire series
of stored further parameters such as the characteristics of frequency
filters, or signal durations, or time intervals between individual
probe-signal pulses, can also be changed. Thus, the probe signal can be
reliably generated even under poor surrounding conditions and for unusual
measurement tasks.
BRIEF DESCRIPTION OF THE DRAWINGS
Other advantages of the invention will become evident from the following
description of a preferred embodiment, in conjunction with the
accompanying drawings, in which:
FIG. 1 is a basic diagram of important parts of a probe system and of an
associated electronic system, shown connected to relevant parts of a
coordinate-measurement machine;
FIG. 2 is a block diagram of the electronic system 20 of FIG. 1 for
processing probe signals;
FIG. 3 is a simplified diagram to show the course of signals in logic part
40 of the electronic system of FIG. 2;
FIG. 4 is a computer-displayed menu list of the different parameters which
can be set in the electronic system 20 of FIGS. 1 and 2;
FIG. 5 is a view in elevation of a "standard" interchangeable probe pin,
for which the invention provides one or more different associated
parameter-data files;
FIG. 5A is a similar view for a so-called "long probe" that is also
interchangeable in place of the standard probe of FIG. 5 and for which the
invention provides one or more different associated parameter-data files;
FIG. 5B is a similar view for another interchangeable probe device, of
so-called "heavy-probe" variety, and for which the invention provides one
or more different associated parameter-data files;
FIG. 5C is a further similar view for another interchangeable probe device,
of so-called "light-probe" variety, and for which the invention provides
one or more different associated parameter-data files; and
FIG. 6 is another computer-displayed menu for set-up selection pursuant to
the invention, providing for operator selection of different
parameter-data files, for selective association with various of the
interchangeable probe devices, illustratively represented by the probes of
FIGS. 5, 5A, 5B and 5C.
DETAILED DESCRIPTION
In the basic diagram shown in FIG. 1, 10 designates the probe head of a
coordinate measurement machine (CMM), which will be understood to have
multiple-coordinate drives for controlled displacement and positioning of
probe head 10 within the displacement volume of the CMM machine. The probe
head 10 has a cylindrical housing 11 within which a movable carrier 12a,
12b for a probe pin 15 is seated in its rest position. Seating for the
rest portion is via three cylindrical pins 13a, 13b, and 13c (not shown),
arranged 120.degree. apart, on pairs of balls 14a, 14b and 14c (not shown)
in housing 11; the drawing shows only the visible two of the three
cylindrical pins and only visible balls of two of the three pairs of
balls. A spring 9 resiliently loads the movable carrier 12a, 12b into the
three-point support formed by the three pairs of balls. In the course of a
workpiece-contacting procedure, probe pin (15) contact with the workpiece
causes the carrier 12a, 12b to lift off at least one of the pin/ball-pair
engagements. In the at-rest condition shown, the three pairs of balls
(14a-c) and the three cylindrical pins form a normally closed electrical
switch in an electric circuit, and this switch is opened upon contact with
the workpiece.
The resiliently movable probe-pin carrier comprises two separable parts
12a, 12b in coaxially nested relation; and between these two parts are
three piezoelectric sensors 18a, b, and c, also 120.degree. apart. Any of
these sensors will produce an electric signal, in the event of even the
slightest contact the probe ball 16. The probe head 10 which has thus far
been described is known per se and is sold by applicants' assignee under
the name "switching probe head" (ST) for use on the assignee's coordinate
measurement machines.
A signal line k from the electromechanical switch formed by the cylindrical
pins 13 and pairs of balls 14 is connected to a probe-head electronic
system 20 which may be located on one of the displaceable measurement
carriages of the coordinate measurement machine, which need not be shown
in further detail. Also connected to this probe-head electronic system is
a signal line S, for code reader 62 which recognizes from a code generator
61 associated with probe head 10, the kind or class of probe head to which
the currently installed probe head 10 belongs.
Signals generated by any one or more of the three sensitive piezoelectric
sensors 18a, 18b, 18c are fed to the electronic system 20, via separate
lines having separate preamplifier stages 17a, 17b and 17c; these
preamplifier stages will be understood to be contained within the
probe-head housing 11. Outputs of the preamplifiers are ungrounded to
provide push-pull signals to electronic system 20 via pairs of signal
lines a, b and c, for differential measurement-value transmission to the
probe-head electronic system 20.
The probe-head electronic system 20 is connected, in its turn, for instance
via a so-called CAN-bus, i.e., a high-speed bidirectional, serial two-wire
bus, to control means 21 of the coordinate measurement machine. Via this
bus, two microprocessors in the probe-head electronic system 20 and in the
control means 21 can communicate with each other in both directions. The
control means 21 will also be understood to receive measurement signals
from linear-measurement systems of measurement carriages of the CMM and to
control the associated CMM drives; this part of the electronic system 20
need not be discussed in detail, it being sufficient only to establish
that connections exist for probe signals processed or delivered by the
probe-head electronic system 20. In FIG. 1, connections are provided by
two signal lines designated NIM and MECHK. The signal on signal line NIM
to the control means 21 is derived, in a manner described below, primarily
from the piezoelectric sensors 14; this derived signal represents the
actual contact signal and serves to retain (i.e., freeze) counter readings
of the multiple-coordinate linear measurement systems of the CMM. On the
other hand, the signal on signal line MECHK to the control means 21 is
obtained in a manner described below, from any opening of the switch
contacts 13/14 and serves to confirm the validity of the contacting
process and/or to report a workpiece-contact deflection of the probe pin
or a collision, and to stop or switch the drives of the CMM, so that the
probe head can move freely again.
Control means 21 is connected to the computer 22 of the coordinate
measurement machine, for example, via a slower parallel, so-called
IEEE-bus. Via this bus, the control means receives such data from the
computer as are necessary for the course of the measurement and forwards
to the computer, inter alia, measurement values supplied by the
length-measurement systems of the CMM.
FIG. 2 provides more detail of circuitry of the probe-head electronic
system 20, wherein the signal-line pairs a, b, and c, are fed to three
input amplifiers 27a, b and c, on the circuit board of the probe-head
electronic system 20. Outputs of these amplifiers are in each case
connected to a rectifier 28a, 28b and 28c and at the same time to a
multiplexer (MUX) 58 as well as to a summation component 30. The summation
component 30 combines the three signals produced by the piezoelectric
sensors 18a-c to develop a sum signal, of amplitude relatively independent
of the direction of workpiece-contacting. This sum signal is fed via a
switch 33 to a high-pass filter 31. This high-pass filter is a
microprocessor-controlled switched-capacitance filter having a
characteristic for which the lower-limit frequency can be digitally
displaced by a microprocessor 23 which is also carried on the same circuit
board; in FIG. 2, a heavily shaded arrow 31' will be understood to
symbolize such microprocessor control of the frequency characteristic of
the high-pass filter 31. The high-pass filter 31 can thus be adapted
digitally to the frequency of the acoustic signal produced by the first
probe contact with the workpiece. Such a filter is commercially available
from MAXIM, Inc., 120 St. Gabriel Drive, Sunnyvale, Calif., USA under the
name MAX 261.
The high-pass filter 31 is followed by a rectifier 32 which rectifies the
filtered alternating voltage signal (acoustic signal). The rectified
signal is fed to two comparators 35 and 38 having trigger thresholds
controlled by digital/analog converters 34 and 37, respectively. As
indicated by heavily shaded arrows 34', 37' on the D/A converters 34 and
37, trigger thresholds of comparators 35 and 38 are also set via the
microprocessor 23.
In order to be able to recognize contacting processes as clearly as
possible,the trigger threshold or level of comparator 35 is lower than
that of comparator 38. Accordingly, the ascending flank of the probe
signal produces, at the output of the comparators 35 and 38, two
rectangular signals, following each other in time, on the signal lines
NIM1 and NIM2. The two signals are fed, via a flip-flop 36, 39 for each
signal, to a circuit 40, designated as "LOGIC", the function of which will
be described with reference to FIG. 3.
Since the trigger levels of comparators 35 and 38 can be set independently
of each other, both the sensitivity of probe-signal recognition and the
distance in time between the two rectangular signals on the signal lines
NIM1 and NIM2 can be set via the microprocessor 23.
The outputs of the three rectifiers 28a, 28b and 28c are fed to a summation
component 29 which is followed by a band-pass filter 41. This band-pass
filter is constructed in exactly the same manner as the high-pass filter
31 and is adjustable with respect to its filter characteristic, i.e., the
position of the two cutoff frequencies, and also digitally by the
microprocessor 23. The band-pass filter 41 is followed by an amplifier 42
of adjustable amplification which compensates (a) for changes in signal
intensity in the course of shifting the band-pass filter or (b) for signal
quality when selecting other cutoff frequencies. Again, heavily shaded
arrows 41' and 42' in FIG. 2 are symbolic of filter and amplifier settings
controlled by microprocessor 23.
Since the band-pass filter 41 is set to relatively low frequencies, the
output of amplifier 42 is a signal which characterizes relatively slow
change of forces acting on the piezoelectric sensors 18a-c and thus on the
probe ball 16. If this signal exceeds a level which is compared in
comparator 45 with the output signal of another digital/analog converter
44, then the comparator 45 produces a so-called "measurement force
signal", designated MESK, which indicates that the force acting on the
probe ball has reached a value such as typically occurs upon application
of the probe ball 16 on a workpiece. This signal MESK is also fed via a
flip-flop 46 to the logic circuit 40.
The switch signal k derived from the electromechanical switch contacts
13/14 is fed on the circuit board of the probe-head electronic system 20
directly to a comparator 48 the level of which is also adjustable, again
via a digital/analog converter 47, having a microprocessor (23) controlled
adjustment suggested at 47'. A displacement of the trigger threshold is
used to automatically compensate for changes in resistance of the contact
point, due to aging or burning which otherwise would in an extreme case
falsely indicate a continuously open contact. For this purpose, signal
line k is connected via measurement point A3 with an analog input of the
microprocessor which cyclically interrogates a residual-voltage-dependent
offset voltage on the signal line k and adjusts the trigger threshold of
comparator 48 accordingly, via digital/analog computer 47. For the purpose
of acknowledgment, the trigger input of comparator 48 is also connected
via a measurement point A7 to an analog input of the microprocessor 23.
The output of comparator 48 delivers a signal designated MECHK, which
characterizes the instant of time when probe 15 forces its carrier 12a/12b
to lift off the three-point bearing 13/14. And this output signal is also
fed to the logic circuit 40 via a flip-flop 49.
Information with regard to parameters to be set, i.e., the values for
limiting frequencies of the high-pass filter 31 and of the band-pass
filter 41, as well as the trigger threshold levels of comparators 35, 38,
45 and 48, are received by the microprocessor 23 via a CAN-bus 24 from
control means 21 of the CMM or indirectly via the IEEE-bus from computer
22. The parameters to be set are stored by the microprocessor in an EEprom
25, so that set values are retained, even in the event of a power failure
or a shut-down of the machine.
In the logic circuit 40, which can be constructed in the manner of a
programmable logic array, or from discrete NAND gates, or the function of
which can be developed in firmware of microprocessor 23, time windows F1,
F2, and F3 are set within which, in each case, a signal NIM1 generated by
the first and slightest workpiece contact must be followed by signals
NIM2, MESK and MECHK, if the first workpiece contact is to be recognized
as valid. For this purpose, (a) the pulse duration of the signal NIM1 is
set within the logic circuit 40 to a relatively short value, typically 100
.mu.sec; (b) a window F2, i.e., the pulse duration of the signal NIM2, is
set to an average value of about 15 msec; and (c) another time window F3,
i.e., the pulse duration of the signal MESK, is set to a longer value,
typically 200 msec. And a mode switch 55 enables selection of different
modes of operation for control by microprocessor 23.
First Mode of Operation
A "valid" workpiece-contact event is recognized only if all four signals
NIM1, NIM2, MESK and MECHK have developed such that (a) the rectangular
pulse 52 on signal line NIM2 has occurred during at least a portion of the
rectangular pulse 51 on signal line NIM1, (b) the rectangular pulse 53 on
signal line MESK has occurred during at least a portion of the pulse 52 on
signal line NIM2, and (c) the rectangular pulse 54 on signal line MECHK
has occurred during at least a portion of the rectangular pulse 53 on
signal line MESK, i.e., within windows limits F1, F2 and F3 set by the
microprocessor. In this first mode of operation, the time of contact is
generated with utmost precision and dependability since the multiple
evaluation effectively suppresses disturbances and minimizes dependence
upon the pulse 51 on signal line NIM1; the pulse 51 is produced from a
very small acoustic output, but it reliably represents the instant of very
first contact between probe ball 16 and the workpiece.
Second Mode of Operation
For an evaluation of the instant of workpiece contact pursuant to this
second mode, only the three signals NIM2, MESK and MECHK are used, and the
signal NIM1 is dispensed with. The contacting is recognized as valid when
the two signals 53 and 54 on signal lines MESK and MECHK follow the pulse
52 on signal line NIM2 within the time windows F2 and F3, described for
operation of the first mode. By dispensing with the very first probe pulse
51 obtained with low trigger threshold, the instant of contact is,
admittedly, not as precisely established; but, on the other hand, the
circuit is less susceptible to disturbance since noise pulses of low
signal level are kept away from the contact signal.
Third Mode of Operation
Only the pulses 53 and 54 on the two signal lines MESK and MECHK are
evaluated for the contact signal. The pulse 54 must follow the pulse 53
within window F3. With this third mode of operation, the application
pressure of the probe ball 16 on the workpiece generates the contact
signal, which provides very high security against noise. The instant of
workpiece contact determined thereby is, however, even later than in the
case of the second mode of operation, although, to be sure, the exact
contact time can be determined by extrapolation for given contact speeds.
The use of this third mode of operation is recommended in particular when
environmental influences, for example, a high noise level, could result in
intensive noise signals on the signal lines a, b and c connected to the
piezo elements, or in the case of dirty or soft workpieces which do not
provide a sufficiently strong p | | |
