Abstract:
A method is disclosed for operating a coordinate measuring machine (CMM) including a workpiece scanning probe. The method provides two different measurement sampling period durations in the scanning probe: a first shorter sampling duration provides a faster measurement having a first accuracy, a second longer sampling duration provides a slower measurement having a second (better) accuracy. The shorter sampling duration may be repeatedly interleaved or alternated with the longer sampling duration to provide sufficient accuracy and response time for motion control purposes during ongoing operation of the CMM. The longer sampling duration may provide high accuracy probe measurements to combine with position coordinate values from encoders located on motion axes of the CMM (outside the scanning probe) to provide high accuracy workpiece measurements at a desired frequency, or upon demand. A probe measurement timing subsystem may determine initiation times of the first and second sampling durations.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 15/192,799, entitled “METHOD FOR OPERATING A COORDINATE MEASURING MACHINE,” filed Jun. 24, 2016, the disclosure of which is hereby incorporated herein by reference in its entirety. 
     
    
     BACKGROUND 
     Technical Field 
       [0002]    This disclosure relates to precision metrology, and more particularly to signals in probes used with coordinate measuring machines. 
       Description of the Related Art 
       [0003]    Coordinate measuring machines (CMMs) can obtain measurements of inspected workpieces. One exemplary prior art CMM described in U.S. Pat. No. 8,438,746 (the &#39;746 patent), which is hereby incorporated herein by reference in its entirety, includes a probe for measuring a workpiece, a movement mechanism for moving the probe, and a controller for controlling the movement. CMMs employing mechanical contact probes are also described in U.S. Pat. Nos. 6,971,183 and 6,487,785, which are hereby incorporated herein by reference in their entirety. A CMM including a surface scanning probe is described in U.S. Pat. No. 7,652,275, which is hereby incorporated herein by reference in its entirety. As disclosed therein, a scanning probe such as a mechanical contact probe or a non-contact optical probe may scan across the workpiece surface. 
         [0004]    In various CMMs which employ scanning probes, measurement synchronization trigger signals trigger measurements from CMM scales or encoders (e.g., linear and rotary scales or encoders) that track an overall position and orientation of the scanning probe (e.g., its base position) in the machine coordinate system, as well as triggering a local surface measurement from the scanning probe. The scanning probe measurement is in a local coordinate system that is referenced to (or measured relative to) the scanning probe base. It is known that there may be a delay or deviation between the time when a measurement synchronization trigger signal latches the CMM scales and the time related to a signal sample period or timing of the scanning probe. The deviation may arise from signal acquisition delays, signal processing delays (including analog to digital conversion) and signal transmission delays, or the like. When such a timing discrepancy exists, the CMM scale measurement data and the scanning probe measuring data cannot be combined into an accurate measurement. The &#39;746 patent, incorporated above, is one prior art reference that describes this problem and various prior art solutions, in detail. As described in the &#39;746 patent, prior art solutions include sending a global trigger signal to all subsystems of a CMM, and/or precisely detecting and calibrating the various delays in the various subsystems, and/or “time stamping” and/or adjusting the measurement data from the various subsystems. The &#39;746 patent also notes that, when the various subsystems include local digital electronics and processing, the limited number of wires available in typical CMM systems may prevent providing a dedicated line or channel for each desired timing signal. Thus, the complexity and/or cost of signal transmission and/or decoding may increase. The &#39;746 patent notes that, even after all of the aforementioned problems are addressed, the phase of local clocks in various digital subsystems may disagree within a clock period. The &#39;746 patent discloses providing a synchronization signal in addition to a measurement trigger signal. The synchronization signal is used to phase-synchronize various local clocks. The trigger signal defines the instant for triggering the measured value acquisition by the various subsystems, which then acquire their measurements in a time-quantified manner. However, shortcomings regarding system retrofit compatibility (e.g., for new probes), limited electrical connections for scanning probes (e.g., at articulated probe connection joints), and with regard to processing options in “smart probes,” remain in the method disclosed in the &#39;746 patent, as well as other prior art methods of measurement synchronization in a CMM. Further improvements and alternatives for CMM scanning probe measurement data synchronization are desired. 
         [0005]    A method is disclosed for operating a coordinate measuring machine (CMM) including a CMM control system, a surface scanning probe that measures a workpiece surface by outputting probe workpiece measurements, and a probe measurement timing subsystem. The method comprises: operating the CMM control system to output measurement synchronization trigger signals at predictable times; operating the probe measurement timing subsystem to determine the predictable times; operating the CMM to define a first probe workpiece measurement sample period that has a first sampling duration that is relatively shorter than a second sampling duration, and that provides a faster type of probe workpiece measurement that has a first level of accuracy; operating the CMM to define a second probe workpiece measurement sample period that has a second sampling duration that is relatively longer than the first sampling duration, and that provides a slower type of probe workpiece measurement that has a second level of accuracy that is better than the first level of accuracy; and operating the CMM to perform of set of measurement operations including the first and second probe workpiece measurement sample periods, the set of measurement operations comprising: 
         [0006]    a) initiating a current instance of the first probe workpiece measurement sample period at a first measurement lead time before a first measurement synchronization trigger signal and within a low-latency time window close to the first measurement synchronization trigger signal, wherein the first measurement synchronization trigger signal occurs at the next predictable time of the measurement synchronization trigger signals; 
         [0007]    b) operating the CMM control system to output the first measurement synchronization trigger signal at the next predictable time and latch a first set of CMM position coordinate values associated with the first measurement synchronization trigger signal; 
         [0008]    c) operating the surface scanning probe to output a current instance of the faster type of probe workpiece measurement associated with the current instance of a first probe workpiece measurement sample period, at a first output time that is associated with the first measurement synchronization trigger signal and that ends within the low-latency time window close to the first measurement synchronization trigger signal; 
         [0009]    d) initiating a current instance of the second probe workpiece measurement sample period at a second measurement time that is defined relative to its corresponding operative measurement synchronization trigger signal, wherein the corresponding operative measurement synchronization trigger signal is one of the first measurement synchronization trigger signal or a second measurement synchronization trigger signal that occurs subsequent to the first measurement synchronization trigger signal, 
         [0010]    e) operating the surface scanning probe to output a current instance of the slower type of probe workpiece measurement associated with the current instance of the second probe workpiece measurement sample period, at a second output time that is associated with the corresponding operative measurement synchronization trigger signal; and 
         [0011]    f) operating the CMM control system to associate the current instance of the slower type of probe workpiece measurement with a properly combinable set of CMM position coordinate values that are determined based at least partially on a set of CMM position coordinate values associated with the corresponding operative measurement synchronization trigger signal. 
         [0012]    In various implementations, a properly combinable set of CMM position coordinate values are latched at a time that approximately coincides with an effective sample time of the of the combined instance (e.g., the current instance) of the second probe workpiece measurement sample period. In some implementations, the effective sample time may be the average time of a plurality of measurement samples included in the current instance of the second probe workpiece measurement sample period, or approximately the average or center of the measurement duration of that sample period. In some implementations, a properly combinable set of CMM position coordinate values are latched at a time that precisely coincides with the effective sample time of the combined instance of the second probe workpiece measurement sample period. In other implementations, a properly combinable set of CMM position coordinate values are latched at a time that only approximately coincides (e.g., within an allowed time difference) with the effective sample time of the combined instance of the second probe workpiece measurement sample period. In general, the allowed difference is small enough that the CMM provides its desired or specified performance and/or accuracy, despite the presence of the allowed difference. 
         [0013]    Various details and alternative implementations consistent with the above summary are described in greater detail below, particularly with reference to the description of  FIGS. 6-9 . 
       BRIEF SUMMARY 
       [0014]    This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
         [0015]    Scanning probes that include local signal processing (which may be characterized as “smart probes”), may include adjustable noise filters and/or measurement averaging or the like, provided in the probe. Related parameters may be adjusted by program instructions that are used to control the CMM and/or the probe. The parameters may be changed (e.g., downloaded to the probe) at any time during the execution of an inspection routine for a workpiece, depending on the required accuracy for measuring a particular feature, for example. When such parameters are changed, the delays or timing deviations outlined previously are inherently affected. In general, it may be desired to retrofit such smart probes onto older host systems that did not anticipate the smart probe features, and therefore lack the capability of adjusting such parameters and/or compensating for such frequently changing timing deviations in a flexible and easy to use manner. Furthermore, host systems may also lack a signal line and/or data transmission protocol which can support a synchronization signal of the type disclosed in the &#39;746 patent, or the like. Therefore, according to principles disclosed herein, it may be desirable to compensate for such timing deviations, including those resulting from intentional parameter changes, in a probe measurement timing subsystem that is easily added to a host CMM. In some embodiments, the probe measurement timing subsystem operations may be divided between circuits internal to the probe and an external circuit connected to the probe. In other embodiments, the probe measurement timing subsystem operations may be implemented entirely in a circuit internal to the probe. A brief summary of such a system and method follows. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a diagram showing various typical components of a CMM; 
           [0017]      FIG. 2  is a block diagram showing various elements of a scanning probe as coupled to a CMM and providing X, Y and Z position signals; 
           [0018]      FIG. 3  is a block diagram showing various elements of a CMM; 
           [0019]      FIG. 4  is a timing diagram showing operations of the CMM of  FIG. 3 ; 
           [0020]      FIG. 5  is a flow diagram showing a method for operating a CMM; 
           [0021]      FIG. 6  is a block diagram showing various elements of a CMM. 
           [0022]      FIG. 7  is a timing diagram showing a first implementation of operations of the CMM of  FIG. 6 ; 
           [0023]      FIG. 8  is a timing diagram showing a second implementation of operations of the CMM of  FIG. 6 ; and 
           [0024]      FIGS. 9A and 9B  are flow diagrams showing a method for operating a CMM. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]      FIG. 1  is a diagram showing various typical components of a CMM  100 . The CMM  100  includes a CMM control system  110 , and a surface scanning probe  120 . The CMM control system  110  includes an operating unit  111 , a motion controller  112  that controls movements of the CMM  100 , and a host computer  113 . The operating unit  111  is coupled to the motion controller  112  and may include joysticks  114  for manually operating the CMM  100 . The host computer  113  is coupled to the motion controller  112  and operates the CMM  100  and processes measurement data for a workpiece W. The host computer  113  includes input means  116  (e.g., a keyboard, etc.) for inputting, for example, measurement conditions, and output means  117  (e.g., a display, printer, etc.) for outputting, for example, measurement results. 
         [0026]    The CMM  100  includes a drive mechanism  170  which is located on a surface plate  180 , and an attachment portion  124  for attaching the scanning probe  120  to the drive mechanism  170 . The drive mechanism  170  includes x-axis, y-axis, and z-axis slide mechanisms  172 ,  171 , and  173 , respectively, for moving the scanning probe  120  three-dimensionally. A stylus  125  attached to the end of the scanning probe  120  includes a contact portion  126 . The stylus  125  is attached to a stylus suspension portion of the scanning probe  120 , which allows the contact portion  126  to freely change its position in three directions when the contact portion  126  moves along a measurement path on the surface of the workpiece W. 
         [0027]      FIG. 2  is a block diagram showing various elements of a surface scanning probe  220  as coupled to a CMM  200  by an attachment portion  224  and providing X, Y and Z position signals. The CMM  200  includes a CMM control system  210 . The scanning probe  220  includes a probe main body  202  which incorporates a stylus suspension portion  207 , a stylus position detection portion  211 , and a probe signal processing and control portion  270 . The stylus suspension portion  207  includes a stylus coupling portion  242  and a stylus motion mechanism  209 . The stylus coupling portion  242  is rigidly coupled to a stylus  226 . The stylus motion mechanism  209  is configured to enable axial motion of the stylus coupling portion  242  and attached stylus  226  along an axial direction, and to enable rotary motion of the stylus coupling portion  242  and attached stylus  226  about a rotation center. In the implementation shown in  FIG. 2 , the surface scanning probe  220  is a contact type surface scanning probe that senses a variable amount of deflection of the stylus  226 . 
         [0028]    As shown in  FIG. 2 , the stylus position detection portion  211  includes a light source configuration  217 , a rotary position detection configuration  213 , and an axial position detection configuration  225 . The rotary position detection configuration  213  receives light from the light source configuration  217  and outputs X and Y position signals. The axial position detection configuration  225  receives light from the light source configuration  217  and outputs a Z position signal. The probe signal processing and control portion  270  is configured to receive the X, Y and Z position signals and output signals  220 S to the CMM control system  210  which are indicative of a 3-D position of the stylus coupling portion  242  and/or of the contact portion of the attached stylus  226  as the contact portion moves along a surface of a workpiece W that is being measured. In some implementations, the probe signal processing and control portion  270  may be configured to convert analog X, Y and Z position signals to digital values and average a plurality of samples of X, Y and Z position values in order to provide signals  220 S including probe workpiece measurements to the CMM control system  210 . The probe signal processing and control portion  270  may also be configured to receive commands from the CMM control system  210  for how to process X, Y and Z position signals. 
         [0029]    In some implementations, the stylus position detection portion  211  may be similar to a stylus position detection portion disclosed in U.S. patent application Ser. No. 14/973,431, which is hereby incorporated herein by reference in its entirety. It should be appreciated that stylus detection portion  211  includes optical detection configurations. However, a stylus detection portion employing alternative types of detection configurations may be incorporated in a surface scanning probe suitable for a CMM configured and operated according to the principles disclosed herein. For example, a stylus detection portion may employ electromagnetic deflection sensors (e.g., linear variable differential transformer sensors) or strain gauges. 
         [0030]      FIG. 3  is a block diagram showing various elements of a CMM  300 . The CMM  300  comprises a CMM control system  310 , a surface scanning probe  320  that measures a workpiece surface by outputting probe workpiece measurements  321 , a probe measurement timing subsystem  330 , CMM scales  340  and rotary joint encoders  350 . The CMM control system is operable to output a measurement synchronization trigger signal  311  at predictable times. The probe measurement timing subsystem  330  is operable to determine the predictable times, and to determine a current duration of a probe workpiece measurement sample period during which the surface scanning probe  320  acquires measurement data associated with a single instance of the output probe workpiece measurements  321 . The probe measurement timing subsystem  330  is operable to determine a pre-trigger lead time that is a fraction of the current duration of the probe workpiece measurement sample period. The probe measurement timing subsystem  330  is operable to initiate a current instance of the probe workpiece measurement sample period at the pre-trigger lead time before a next predictable time of the measurement synchronization trigger signal  311 , and determine an associated current instance of the probe workpiece measurement  321 . More specifically, the probe measurement timing subsystem  330  initiates the current instance of the probe workpiece measurement sample period by outputting a pre-trigger signal  331  to the surface scanning probe  320 . The CMM control system  310  is operable to output a current measurement synchronization trigger signal  311  at the next predictable time and latch a current set of CMM position coordinate values  360  associated with the current measurement synchronization trigger signal. Each set of the CMM position coordinate values  360  includes CMM scale values  361  from the CMM scales  340  and rotary joint encoder values  362  from the rotary joint encoders  350 . The surface scanning probe  320  is operable to output the current instance of the probe workpiece measurement  321  at a time associated with the current measurement synchronization trigger signal, such that the CMM control system  310  associates the current instance of the probe workpiece measurement  321  with the current set of CMM position coordinate values  360 . 
         [0031]    If a sample period were to begin at the same time as an instance of the measurement synchronization trigger signals  311 , a corresponding instance of the probe workpiece measurements  321  would include an error component resulting from a distance the surface scanning probe  320  has moved since the beginning of the sample period. Therefore, the configuration of the CMM  300  and the operating methods described herein are especially suitable at mitigating this error component by initiating the current instance of the probe measurement sample period according to the pre-trigger lead time. 
         [0032]    In various implementations, the probe measurement timing subsystem  330  may be located partly or wholly in the surface scanning probe  320 . In some implementations, all or part of the probe measurement timing subsystem  330  may be located proximate to the CMM control system  310 . In some implementations, the probe measurement timing subsystem  330  may be located in an interchangeable card connected to the CMM control system  310 . In some implementations, the interchangeable card may be specifically associated with at least one of an individual surface scanning probe  320 , or a model or type of the surface scanning probe  320 . 
         [0033]      FIG. 4  is a timing diagram  400  showing operations of the CMM  300 . As shown in  FIG. 4 , the CMM control system  310  outputs a signal  310 S including repeated measurement synchronization trigger signals  311  at a trigger period t sync . In some implementations, the trigger period t sync  may be in a range of 200 μs to 1,000 μs. As previously described with respect to  FIG. 3 , the probe measurement timing subsystem  330  initiates the current instance of the probe workpiece measurement sample period (e.g., a sample period  322 A or a sample period  322 B) by outputting pre-trigger signals  331  to the surface scanning probe  320  through a bidirectional signal communication  330 S. The surface scanning probe  320  generates a signal  320 S 1  including analog sample to digital conversion (ADC) triggers  322  during probe workpiece measurement sample periods which are initiated in response to the pre-trigger signals  331 . The surface scanning probe  320  outputs a signal  320 S 2  including probe workpiece measurements  321  to the CMM control system  310  based on data sampled during the workpiece measurement sample periods. The probe measurement timing subsystem  330  is also configured to output data clock signals  332  corresponding to the probe workpiece measurements  321  to the CMM control system  310  via the bidirectional signal communication  330 S. As previously outlined, the probe measurement timing subsystem  330  may reside partly or wholly in the surface scanning probe  320 . In various embodiments, timing or clock signals depicted for the bidirectional signal communication  330 S may originate in a portion of the probe measurement timing subsystem  330  located either inside or outside the surface scanning probe  320 . 
         [0034]    In some implementations, operating the probe measurement timing subsystem  330  to determine the predictable times may comprise inputting the repeated measurement synchronization trigger signals  311  to the probe measurement timing subsystem  330  at the trigger period t sync , and determining a timing of the measurement synchronization trigger signals  311 . In some implementations, operating the probe measurement timing subsystem  330  to initiate a current instance of the probe measurement sample period at the pre-trigger lead time before a next predictable time of the measurement synchronization trigger signals  311  may comprise initiating the current instance of the probe measurement sample period at a time after a previous measurement synchronization trigger signal  311  that corresponds to the pre-trigger lead time before the next predictable time of the measurement synchronization trigger signals  311 . 
         [0035]    As shown in  FIG. 4 , the surface scanning probe  320  acquires measurement data associated with a single instance of the output probe workpiece measurements during a probe workpiece measurement sample period t samp . In some implementations, the probe measurement timing subsystem may be operated to determine a pre-trigger lead time t lead  that is approximately one half of the current duration of the probe workpiece measurement sample period t samp . This results in a measurement synchronization trigger signal  311  which is approximately centered in a sample period (e.g., the sample period  322 A or the sample period  322 B). 
         [0036]    The pre-trigger lead time t lead  may be determined as follows. The probe measurement timing subsystem  330  may initiate the current instance of the probe workpiece measurement sample period by outputting a pre-trigger signal  331  to the surface scanning probe  320  at a pre-trigger lead time t lead  before the next predictable time of the measurement synchronization trigger signal  311 . During a single probe workpiece measurement sample period (e.g., the sample period  322 A or the sample period  322 B) the surface scanning probe  320  may acquire n samples at a sample timing interval t cyc . In the implementation shown in  FIG. 4 , n is 8. The surface scanning probe  320  may begin an instance of a probe workpiece measurement sample period with a total system latency t lat  after an instance of the pre-trigger signals  331 . The pre-trigger lead time t lead  may then be determined by the expression: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0037]    In some implementations, the sample timing interval t cyc  may be in a range of 5 μs to 7 μs and the total system latency t lat  may be in a range of 1 μs to 2 μs. The pre-trigger lead time t lead  may be in a range of 1 μs to 200 μs. 
         [0038]    In the implementation shown in  FIG. 4 , the control system  310  receives the probe workpiece measurements  321  with a data delay t datdelay  after the corresponding measurement synchronization signal  311 . The surface scanning probe  320  begins outputting the probe workpiece measurements at a time corresponding to a delay t delay  after an instance of the pre-trigger signals  331 . Each instance of the pre-trigger signals  331  corresponds to a trigger width t trigwid  which is the width of the instances of the pre-trigger signals  331 . The surface scanning probe  320  outputs the probe workpiece measurements  321  to the CMM control system  310  over a transmission time t id . The data delay t then t datdelay  may be determined by the expression: 
         [0000]        t   datdelay   =t   trigwid   +t   delay   +t   id   −t   lead   Eq. (2).
 
         [0039]    In some implementations, the trigger width t trigwid  may be in a range of 200 ns to 300 ns, the delay t delay  may be in a range of 5 μs to 350 μs and the transmission time t id  may be in a range of 25 μs to 35 μs. 
         [0040]      FIG. 5  is a flow diagram  500  showing a method for operating a CMM. The CMM includes a CMM control system, a surface scanning probe that measures a workpiece surface by outputting probe workpiece measurements, and a probe measurement timing subsystem. 
         [0041]    At a block  510 , the CMM control system is operated to output a measurement synchronization trigger signal at predictable times. 
         [0042]    At a block  520 , the probe measurement timing subsystem is operated to determine the predictable times, and to determine a current duration of a probe workpiece measurement sample period during which the scanning probe acquires measurement data associated with a single one of the output probe workpiece measurements. 
         [0043]    At a block  530 , the probe measurement timing subsystem is operated to determine a pre-trigger lead time that is a fraction of the current duration of the probe workpiece measurement sample period. 
         [0044]    At a block  540 , the probe measurement timing subsystem is operated to initiate a current instance of the probe workpiece measurement sample period at the pre-trigger lead time before a next predictable time of the measurement synchronization trigger signal, and determine an associated current instance of the probe workpiece measurement. 
         [0045]    At a block  550 , the CMM control system is operated to output a current measurement synchronization trigger signal at the next predictable time and latch a current set of CMM position coordinate values associated with the current measurement synchronization trigger signal. 
         [0046]    At a block  560 , the scanning probe is operated to output the current instance of the probe workpiece measurement at a time associated with the current measurement synchronization trigger signal, such that the CMM control system associates the current instance of the probe workpiece measurement with the current set of CMM position coordinate values. 
         [0047]      FIG. 6  is a block diagram showing various elements of a CMM  600 . The CMM  600  comprises a CMM control system  610 , a surface scanning probe  620  that measures a workpiece surface by outputting probe workpiece measurements, a probe measurement timing subsystem  630 , a match timing subsystem  615  (which may be optional in some implementations), CMM scales  640 , and rotary joint encoders  650 . The CMM control system  610  is operable to output a measurement synchronization trigger signal  611  at predictable times. In various implementations, coincident with or approximately coincident with outputting a current measurement synchronization trigger signal  611 , the CMM control system  610  may latch a current set of CMM position coordinate values  660  (e.g., reflecting the motion control position of the various axes of the CMM) associated with that current measurement synchronization trigger signal  611 . For example, each set of the CMM position coordinate values  660  includes CMM scale values  661  from the CMM scales  640  and rotary joint encoder values  662  from the rotary joint encoders  650 . To determine a workpiece measurement, the CMM position coordinate values  660  are combined with displacement or deflection data that characterize the displacement coordinates of the contact portion  126  of the stylus  125 . Such data is referred to herein as probe workpiece measurements  621  and/or  621 ′. The match timing subsystem  615  may play a role in combining the CMM position coordinate values  660  and the probe workpiece measurements  621 ′, as described further below. 
         [0048]    In various implementations, as described in greater detail below, the probe workpiece measurements  621  may be acquired relatively faster and indicate the stylus deflection coordinates with less accuracy and/or more noise using a first sample period, and the probe workpiece measurements  621 ′ may be acquired relatively slower and indicate the stylus deflection coordinates with better accuracy and/or less noise using a second sample period. 
         [0049]    It should be appreciated that as used herein the term “sample period” may sometimes refer to the duration of a sample period, and/or may sometimes refer more globally to additional characteristics of the sample period, for example including the set of sampling operations and/or signal processing performed during the sample period. 
         [0050]    It will be understood that relatively less accurate position or deflection measurements (e.g., using the relatively faster probe workpiece measurements  621 ) may be sufficient for servo control, wherein fast acquisition and response time may also be of value for high speed motion control (e.g., to decelerate quickly and avoid “overtravel” damage when the stylus  125  contacts a workpiece. In contrast, relatively more accurate position or deflection measurements (e.g., using the relatively slower probe workpiece measurements  621 ′), may be desirable for determining a workpiece surface location with higher accuracy and/or lower noise. For example, the relatively slower probe workpiece measurements  621 ′ may combine more samples of the sensed stylus deflection, using filter or averaging, in order to improve measurement accuracy and/or meaningful resolution. 
         [0051]    As previously indicated, the CMM control system  610  is operable to output a measurement synchronization trigger signal  611  at predictable times. For example, the predictable times may be associated with a fixed operating frequency of a motion control cycle, and/or measurement cycle, and/or the like. The probe measurement timing subsystem  630  is operable to determine the predictable times. 
         [0052]    As described in greater detail below with respect to  FIG. 7 , the probe measurement timing subsystem  630  is further operable to determine a duration of a first probe workpiece measurement sample period during which the surface scanning probe  620  acquires measurement data associated with a first instance of the output probe workpiece measurements  621 , and to determine a duration of a second probe workpiece measurement sample period during which the surface scanning probe acquires measurement data associated with a second instance of the output probe workpiece measurements  621 ′, which includes more samples than the first probe workpiece measurement sample period. The probe measurement timing subsystem  630  is also operable to determine a first measurement lead time t lead1  that, in some implementations, is desirably larger than a data transmission time between the surface scanning probe  620  and the CMM control system  610  for the output probe workpiece measurements  621 . The probe measurement timing subsystem  630  is operable to initiate a current instance of the first probe workpiece measurement sample period at the first measurement lead time t lead1  before a next predictable time of the measurement synchronization trigger signal  611 . 
         [0053]    In various implementations, the probe measurement timing subsystem  630  may be located partly or wholly in the surface scanning probe  620 . In some implementations, all or part of the probe measurement timing subsystem  630  may be located proximate to or in the CMM control system  610 . In some implementations, the probe measurement timing subsystem  630  may be located in an interchangeable card connected to the CMM control system  610 . In some implementations, the interchangeable card may be specifically associated with at least one of an individual surface scanning probe  620 , or a model or type of the surface scanning probe  620 . 
         [0054]    The surface scanning probe  620  is operable to output the first instance of the probe workpiece measurements  621  at a first time associated with the current measurement synchronization trigger signal  611 , such that the CMM control system  610  associates the current instance of the first probe workpiece measurements with the current set of CMM position coordinate values  660 . The probe measurement timing subsystem  630  is operable to initiate a current instance of the second probe workpiece measurement sample period at a second measurement time. The surface scanning probe  620  is operable to output the second instance of the probe workpiece measurements  621 ′ at a second time associated with the current measurement synchronization trigger signal  611 , such that the CMM control system  610  associates the current instance of the second probe workpiece measurements with the current set of CMM position coordinate values  660 . As previously indicated, the current instance of the second probe workpiece measurements  621 ′ and the associated current set of CMM position coordinate values  660 , may be combined (e.g., in the CMM control system  610 ) to determine a high accuracy workpiece surface location measurement. In some implementations, the timing of the second probe workpiece measurements  621 ′ and the associated current set of CMM position coordinate values  660  may be different. In such implementations, the match timing subsystem  615  may determine the timing difference and provide an adjustment value for the CMM position coordinate values  660 , such that they are properly combinable corresponding to the same instant in time, as described in greater detail below with respect to  FIG. 7 . 
         [0055]    In some implementations, such as that shown in  FIG. 8 , the first probe workpiece measurement sample period may take place within the second probe workpiece measurement sample period, and the current instance of the first probe workpiece measurements  621  may share at least one common sample with the current instance of the second probe workpiece measurements  621 ′. 
         [0056]    In various implementations, the match timing subsystem  615  may be located partly or wholly in the surface scanning probe  620 , or proximate to or in the CMM control system  610 . In some implementations, the match timing subsystem  615  may be located in an interchangeable card connected to the CMM control system  610 . In some implementations, the interchangeable card may be specifically associated with at least one of an individual surface scanning probe  620 , or a model or type of the surface scanning probe  620 . In some implementations, the timing of the second probe workpiece measurements  621 ′ and the associated current set of CMM position coordinate values  660  may be sufficiently close, or identical, in which case the match timing subsystem  615  may be omitted, or optional. 
         [0057]      FIG. 7  is a timing diagram  700  showing a first implementation of operations of the CMM  600 .  FIG. 7  shows various signals numbered 7XX some of which may be understood by analogy to implementations of signals numbered 6XX in  FIG. 6 , except as otherwise described or implied below. 
         [0058]    As shown in  FIG. 7 , a signal  710 S (e.g., on a signal line) includes repeated measurement synchronization trigger signals  711  at a trigger period t sync . (e.g., as output by the CMM control system  610 , as previously described with respect to  FIG. 6 ). In some implementations, the trigger period t sync  may be in a range of 200 μs to 1,000 μs, although these values are exemplary only and not limiting. 
         [0059]    As previously outlined with reference to  FIG. 6 , the probe measurement timing subsystem  630  may initiate a current instance of the first probe workpiece measurement sample period (e.g., a sample period  722 A or a sample period  7226 ) by outputting first probe sample period trigger signals  731  to the surface scanning probe  620  through a bidirectional signal communication  730 S. In various implementations the timing of the first probe sample period trigger signals  731  is determined such that it initiates a current instance of the first probe workpiece measurement sample period at a first measurement lead time t lead1  before an associated (e.g., next) measurement synchronization trigger signal  711  and within a low-latency time window LLW close to the associated measurement synchronization trigger signal  711 . In general, the low-latency time window LLW is defined such that it is small enough and close enough to the associated measurement synchronization trigger signal  711  such that the CMM provides its desired or specified performance and/or accuracy, despite the limited difference that it may allow between the effective sample time of the first probe workpiece measurement sample period (e.g.,  722 A) and the time of the associated measurement synchronization trigger signal  711 . 
         [0060]    The probe measurement timing subsystem  630  may also initiate a current instance of the second probe workpiece measurement sample period (e.g., a sample period  722 A′ or a sample period  7226 ′) by outputting second probe sample period trigger signals  731 ′ through the bidirectional signal communication  730 S. 
         [0061]    The surface scanning probe  620  generates a signal  720 S 1  including analog sample to digital conversion (ADC) triggers  722  (which have corresponding ADC measurements  722 ) during first probe workpiece measurement sample periods which are initiated in response to the first probe sample period trigger signals  731 , and analog sample to digital conversion (ADC) triggers  722 ′ (which have corresponding ADC measurements  722 ′) during second probe workpiece measurement sample periods which are initiated in response to the second probe sample period trigger signals  731 ′. The surface scanning probe  620  outputs a signal  720 S 2  to the CMM control system  610  including a first instance of the output probe workpiece measurements  721  (based on data sampled during the first workpiece measurement sample period  722 A) at a first output time FOT associated with a corresponding (e.g., next) measurement synchronization trigger signal  711 . In various implementations, the first output time FOT ends within the low-latency time window LLW close to the corresponding (e.g., next) measurement synchronization trigger signal. In general, the low-latency time window LLW is defined such that it is small enough and close enough to the corresponding measurement synchronization trigger signal  711  such that the CMM provides its desired or specified performance (e.g., motion control performance) and/or accuracy, despite the limited difference that it may allow between the availability of the output probe workpiece measurements  721  (e.g., at the end of the first output time FOT, in a position register of the CMM control system  610 ) and the time of the associated measurement synchronization trigger signal  711 . 
         [0062]    The signal  720 S 2  output from the surface scanning probe  620  to the CMM control system  610  may also include a second instance of the output probe workpiece measurements  721 ′ (based on data sampled during the second workpiece measurement sample period  722 A′), at a second output time SOT associated with a corresponding operative measurement synchronization trigger signal (e.g.,  711 , or  711 ′). 
         [0063]    In one implementation illustrated in  FIG. 7 , the corresponding operative measurement synchronization trigger signal is the measurement synchronization trigger signal  711 . The timing of the second probe sample period trigger signal  731 ′ is determined such that it initiates a current instance of the second probe workpiece measurement sample period at a second measurement time that occurs after the first probe sample period trigger signal  731 , and after the first output time FOT, and after the corresponding operative measurement synchronization trigger signal  711 . The second output time SOT occurs after the first output time FOT. In this implementation the second workpiece measurement sample period  722 A′ has an effective sample time EST approximately in the middle of its set of individual measurement samples, which has a time difference t match  relative to the corresponding operative measurement synchronization trigger signal  711 . In order to overcome this time difference t match  and provide a properly combinable set of CMM position coordinate values for the output probe workpiece measurement  721 ′ corresponding to the current instance of the second probe workpiece measurement sample period, the properly combinable set of CMM position coordinate values are determined corresponding to a time that is approximately the same as the effective sample time EST of the current instance of the second probe workpiece measurement sample period  722 A′. In one implementation the properly combinable set of CMM position coordinate values are a set of CMM position coordinate values extrapolated from the set of CMM position coordinate values latched at the time of the corresponding operative measurement synchronization trigger signal  711 . The extrapolation is based on the time difference t match  and a characterization of the rate of change of the CMM position coordinate values over a time period before the first measurement synchronization trigger signal, according to known principles. For example, with reference to  FIG. 6 , the timing difference t match  may be determined and recorded in the match timing subsystem  615  according to previously outlined principles. The match timing subsystem  615  may further record and analyze a set of the previous CMM position coordinate values  660 , latched at known times by previous measurement synchronization trigger signals  711 , and determine a current velocity or rate of change of the CMM position coordinate values  660 . Based on the rate of change and the time difference t match  a properly combinable set of CMM position coordinate values may be determined by extrapolating the value of the set of CMM position coordinate values latched at the time of the corresponding operative measurement synchronization trigger signal  711  to a time that is approximately the same as the effective sample time EST of the current instance of the second probe workpiece measurement sample period  722 A′, according to known extrapolation methods. 
         [0064]    Alternatively, in another implementation that is illustrated in  FIG. 7 , which provides a properly combinable set of CMM position coordinate values for the output probe workpiece measurement  721 ′ corresponding to the current instance of the second probe workpiece measurement sample period, the properly combinable set of CMM position coordinate values are determined by providing a corresponding operative measurement synchronization trigger signal that is a second measurement synchronization trigger signal  711 ′ (shown in dashed outline). In one implementation, the second probe workpiece measurement sample period  722 A′ may be initiated at a second measurement time (e.g., by the second probe sample period trigger signal  731 ′) that is defined relative to the second measurement synchronization trigger signal  711 ′. In another implementation, the match timing subsystem  615  may be used or operated to generate the second measurement synchronization trigger signal  711 ′, which is analogous to the measurement synchronization trigger signal  711 , but which is primarily or only used to latch a properly combinable current set of CMM position coordinate values  660  at a time coincident with the effective sample time EST associated with a sample period  722 A′ and corresponding second probe workpiece measurement  721 ′. In either implementation, a properly combinable set of CMM position coordinate values are obtained, which corresponds to a time that is approximately the same as the effective sample time EST of the current instance of the second probe workpiece measurement sample period  722 A′. 
         [0065]    The probe measurement timing subsystem  630  is also configured to output data clock signals  732  and  732 ′ corresponding to the first instance of the output probe workpiece measurements  721  and the second instance of the output probe workpiece measurements  721 ′ to the CMM control system  610  via the bidirectional signal communication  730 S. As previously outlined, the probe measurement timing subsystem  630  may reside partly or wholly in the surface scanning probe  620 . In various embodiments, timing or clock signals depicted for the bidirectional signal communication  730 S may originate in a portion of the probe measurement timing subsystem  630  located either inside or outside the surface scanning probe  620 . 
         [0066]    In the implementation shown in  FIG. 7 , the sample periods  722 A and  7226  include just one sample, whereas the sample periods  722 A′ and  7226 ′ include eight samples. It should be appreciated that each number of samples is exemplary only, and not limiting. For example, in some implementations, a first probe workpiece measurement sample period may include more than one sample. In any case, relatively less accurate stylus position or deflection measurements (e.g., using the relatively faster or shorter sample periods  722 A and  7226 , which include relatively fewer samples) may be sufficient for servo control, wherein fast acquisition and response time may also be of value for high speed motion control (e.g., to decelerate quickly and avoid “overtravel” damage when the stylus  125  contacts a workpiece. In contrast, a CMM control system may subsequently or additionally rely on a second probe workpiece measurement sample period for relatively more accurate stylus position or deflection measurements (e.g., using the relatively slower or longer sample periods  722 A′ and  7226 ′, which include relatively more samples), which may be desirable for determining a workpiece surface location with higher accuracy and/or lower noise. For example, the relatively slower or longer the sample periods  722 A′ and  7226 ′ may provide more samples of the sensed stylus deflection, which may be filtered or averaged, in order to improve measurement accuracy and/or meaningful resolution. 
         [0067]    Regarding operating the probe measurement timing subsystem  630  to determine the predictable times, related operations may comprise inputting the repeated measurement synchronization trigger signals  711  to the probe measurement timing subsystem  630  at the trigger period t sync , and determining a timing of the measurement synchronization trigger signals  711 . In some implementations, operating the probe measurement timing subsystem  630  to initiate a current instance of the probe measurement sample period at the first measurement lead time t lead1  before a next predictable time of the measurement synchronization trigger signals  711  may comprise initiating the current instance of the probe measurement sample period at a time after a previous measurement synchronization trigger signal  711  that corresponds to the first measurement lead time t lead1  before the next predictable time of the measurement synchronization trigger signals  711 . 
         [0068]    As outlined with respect to  FIG. 6 , the first measurement lead time t lead1  is larger than a transmission time t id  between the surface scanning probe and the CMM control system. In some embodiments, it may be advantageous if the first measurement lead time t lead1  t is as small as possible to avoid latency errors, subject to the constraint that it should be long enough to allow enough time to transmit the data of the first probe workpiece measurement sample period to the CMM control system  610  so that the data is ready for use in the CMM control system (e.g., stored in a probe deflection data register) at the time of the associated measurement synchronization trigger signal  711 . 
         [0069]    Each instance of the first probe sample period trigger signals  731  corresponds to a trigger width t trigwid1  which is the width of the instances of the first probe sample period trigger signals  731 . Each instance of the second probe sample period trigger signals  731 ′ corresponds to a trigger width t trigwid2  which is the width of the instances of the second probe sample period trigger signals  731 ′. In some implementations, it may be desirable for the trigger width t trigwid1  and the trigger width t trigwid2  to have different values such that the surface scanning probe  620  may recognize whether to output the first instance of the probe workpiece measurements  721  or the second instance of the probe workpiece measurements  721 ′. 
         [0070]    It will be appreciated that, according to the teachings previously outlined with reference to  FIG. 4 , the timing difference between the measurement synchronization trigger signal  311  and the effective sample time that corresponds to the sample period  322 A (and the associated probe workpiece measurement  321 ) are negligible. Therefore, the operations outlined above in relation to the match timing subsystem  615  are not needed, and the match timing subsystem  615  is not needed to provide a high accuracy workpiece location measurement in that implementation. The same is true for the implementation shown in  FIG. 8 . 
         [0071]      FIG. 8  is a timing diagram  800  showing a second implementation of operations of the CMM  600 .  FIG. 8  shows various signals numbered 8XX which may be understood to correspond to implementations of signals numbered 6XX in  FIG. 6  and/or 7XX in  FIG. 7 . The implementation shown in  FIG. 8  differs from that shown in  FIG. 7 , in that in  FIG. 8  the first workpiece measurement sample period  822 A is shorter than and included within the duration of the second workpiece measurement sample period  822 B, and a first sample set of analog sample to digital conversion (ADC) triggers  822  included in the first workpiece measurement sample period  822 A comprises at least one individual measurement sample that is shared with a second sample set of ADC triggers  822 ′ included in the second workpiece measurement sample period  822 B. It will be appreciated that the timing and duration of the first workpiece measurement sample period  822 A (e.g., as initiated by the first probe sample period trigger signal  831  with the first lead time t lead1 ) is such that each sample included in the first sample set of ADC triggers  822  occurs within the low-latency time window LLW close to the associated or corresponding operative measurement synchronization trigger signal  811 , according to previously outlined principles. It will be appreciated that the timing of the second probe sample period trigger signal  831 ′ initiates a current instance of the second probe workpiece measurement sample period  822 B at a second measurement time that occurs at a second lead time t lead2  before the corresponding operative measurement synchronization trigger signal  811 , and before the first measurement lead time t lead1 , wherein the second lead time t lead2  is determined such that the effective sample time EST of the current instance of the second probe workpiece measurement sample period  822 B approximately coincides with the corresponding operative measurement synchronization trigger signal  811 . Thus, a set of CMM position coordinate values latched by the corresponding operative measurement synchronization trigger signal  811  are a properly combinable set of CMM position coordinate values of the output probe workpiece measurement  821 ′ corresponding to the current instance of the second probe workpiece measurement sample period  822 B. 
         [0072]    To further describe  FIG. 8 , in the illustrated implementation the CMM control system  610  outputs a signal  810 S including repeated measurement synchronization trigger signals  811  at the trigger period t sync . The probe measurement timing subsystem  630  initiates a current instance of the second probe workpiece measurement sample period (e.g., a sample period  822 B) by outputting second probe sample period trigger signals  831 ′ through the bidirectional signal communication  830 S at a second measurement time t lead2  before a next predictable time of the measurement synchronization trigger signals  811 . The probe measurement timing subsystem  630  initiates a current instance of the first probe workpiece measurement sample period (e.g., a sample period  822 A) by outputting first probe sample period trigger signals  831  to the surface scanning probe  620  through a bidirectional signal communication  830 S. The surface scanning probe  620  generates a signal  820 S 1  including analog sample to digital conversion (ADC) triggers  822  (which have corresponding ADC measurements  822 ) during the first probe workpiece measurement sample period which are initiated in response to the trigger signal  831 ′. At least one analog sample to digital conversion (ADC) trigger  822 ′ occurring within the first probe workpiece measurement sample period  822 A, may designate at least one of the corresponding ADC measurements to be included as a sample associated with both the first and second probe workpiece measurement sample periods  822 A and  8226 . 
         [0073]    The surface scanning probe  620  outputs a signal  820 S 2  including a first instance of the output probe workpiece measurements  821  and a second instance of the output probe workpiece measurements  821 ′ to the CMM control system  610  based on data sampled during the first and second workpiece measurement sample periods, respectively. The probe measurement timing subsystem  630  is also configured to output data clock signals  832  and  832 ′ corresponding to the probe workpiece measurements  821  and  821 ′ to the CMM control system  610  via the bidirectional signal communication  830 S. As previously outlined, the probe measurement timing subsystem  630  may reside partly or wholly in the surface scanning probe  620 . In various embodiments, timing or clock signals depicted for the bidirectional signal communication  830 S may originate in a portion of the probe measurement timing subsystem  630  located either inside or outside the surface scanning probe  620 . 
         [0074]    In some implementations which are similar to that shown in  FIG. 8 , a first probe workpiece measurement sample period may alternatively include more than one sample and share more than one common sample with the second probe workpiece measurement sample period. 
         [0075]    In some implementations, operating the probe measurement timing subsystem  630  to determine the second measurement lead time t lead2  may comprise determining a second measurement lead time t lead2  that is approximately one half of the current duration of the second probe workpiece measurement sample period. 
         [0076]    In some implementations, the CMM control system  610  may output repeated measurement synchronization trigger signals  811  at the trigger period t sync , and operating the probe measurement timing subsystem to determine the predictable times may comprise inputting the repeated measurement synchronization trigger signals  811  to the probe measurement timing subsystem  630  at the trigger period t sync , and determining a timing of the repeated measurement synchronization trigger signals  811 . 
         [0077]    In some implementations, operating the probe measurement timing subsystem  630  to initiate a current instance of the second probe measurement sample period  822 B at the second measurement lead time t lead2  before the next predictable time of the measurement synchronization trigger signal  811  may comprise initiating the current instance of the second probe measurement sample period  822 B at a time after a previous measurement synchronization trigger signal  811  that corresponds to the second measurement lead time t lead2  before the next predictable time of the measurement synchronization trigger signal  811 . 
         [0078]    The second measurement lead time t lead2  may be determined in a similar manner to the pre-trigger lead time t lead  described with respect to  FIG. 4 . During a single second probe workpiece measurement sample period (e.g., second probe measurement sample period  822 B) the surface scanning probe  620  may acquire n samples at a sample timing interval t cyc . In the implementation shown in  FIG. 8 , n is 16. The surface scanning probe  620  may begin an instance of a second probe workpiece measurement sample period with a total system latency t eat  after an instance of the second probe sample period trigger signals  831 . The second measurement lead time t lead2  may then be determined by the expression: 
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         [0079]      FIG. 9A  and  FIG. 9B  show a flow diagram  900  showing the generally described operations of a method for operating a CMM according to principles outlined with reference to various implementations above. The CMM includes a CMM control system, a surface scanning probe that measures a workpiece surface by outputting probe workpiece measurements, and a probe measurement timing subsystem. 
         [0080]    As shown in  FIG. 9A , at a block  910 , the CMM control system is operated to output a measurement synchronization trigger signal at predictable times. 
         [0081]    At a block  920 , the CMM control system is operated to output measurement synchronization trigger signals at predictable times. 
         [0082]    At a block  930 , the CMM is operated to define a first probe workpiece measurement sample period that has a first sampling duration that is relatively shorter than a second sampling duration, and that provides a faster type of probe workpiece measurement that has a first level of accuracy. 
         [0083]    At a block  940 , the CMM is operated to define a second probe workpiece measurement sample period that has a second sampling duration that is relatively longer than the first sampling duration, and that provides a slower type of probe workpiece measurement that has a second level of accuracy that is better than the first level of accuracy. 
         [0084]    The block  940  continues to a block A which is continued in  FIG. 9B . 
         [0085]    As shown in  FIG. 9B , at a block  950 , the CMM is operated to perform of set of measurement operations including the first and second probe workpiece measurement sample periods, the set of measurement operations comprising: 
         [0086]    a) initiating a current instance of the first probe workpiece measurement sample period at a first measurement lead time before a first measurement synchronization trigger signal and within a low-latency time window close to the first measurement synchronization trigger signal, wherein the first measurement synchronization trigger signal occurs at the next predictable time of the measurement synchronization trigger signals; 
         [0087]    b) operating the CMM control system to output the first measurement synchronization trigger signal at the next predictable time and latch a first set of CMM position coordinate values associated with the first measurement synchronization trigger signal; 
         [0088]    c) operating the surface scanning probe to output a current instance of the faster type of probe workpiece measurement associated with the current instance of a first probe workpiece measurement sample period, at a first output time that is associated with the first measurement synchronization trigger signal and that ends within the low-latency time window close to the first measurement synchronization trigger signal; 
         [0089]    d) initiating a current instance of the second probe workpiece measurement sample period at a second measurement time that is defined relative to its corresponding operative measurement synchronization trigger signal, wherein the corresponding operative measurement synchronization trigger signal is one of the first measurement synchronization trigger signal or a second measurement synchronization trigger signal that occurs subsequent to the first measurement synchronization trigger signal, 
         [0090]    e) operating the surface scanning probe to output a current instance of the slower type of probe workpiece measurement associated with the current instance of the second probe workpiece measurement sample period, at a second output time that is associated with the corresponding operative measurement synchronization trigger signal; and 
         [0091]    f) operating the CMM control system to associate the current instance of the slower type of probe workpiece measurement with a properly combinable set of CMM position coordinate values that are determined based at least partially on a set of CMM position coordinate values associated with the corresponding operative measurement synchronization trigger signal. 
         [0092]    While preferred implementations of the present disclosure have been illustrated and described, numerous variations in the illustrated and described arrangements of features and sequences of operations will be apparent to one skilled in the art based on this disclosure. Various alternative forms may be used to implement the principles disclosed herein. In addition, the various implementations described above can be combined to provide further implementations. All of the U.S. patents and U.S. patent applications referred to in this specification are incorporated herein by reference, in their entirety. Aspects of the implementations can be modified, if necessary to employ concepts of the various patents and applications to provide yet further implementations. 
         [0093]    These and other changes can be made to the implementations in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific implementations disclosed in the specification and the claims, but should be construed to include all possible implementations along with the full scope of equivalents to which such claims are entitled.