Patent Description:
The present disclosure generally relates to coordinate measuring machines and, more particularly, relates to systems and methods for verifying performance of coordinate measuring machine sensors.

Coordinate measuring machines (CMMs) are the gold standard for accurately measuring a wide variety of different types of work pieces/objects. For example, CMMs can measure critical dimensions of aircraft engine components, surgical tools, and machine parts. Precise and accurate measurements help ensure that their underlying systems, such as an aircraft in the case of aircraft components, operate as specified.

Some objects are measured to a fine precision, such as on the micron level. The accuracy of a CMM may depend, in part, on the calibration of the CMM and the accuracy of the measuring device (e.g., optical probe) used for the measurement.

A CMM may use one or more types of sensor, such as tactile sensors, touchless sensors, photographic sensors (e.g., video sensors), to measure a workpiece. Calibrating a CMM may involve causing the CMM to measure a calibration artifact of known dimensions, and take remedial steps (e.g., adjust the CMM and/or determine mathematical data for use in correcting measurement data) to mitigate differences between the measurements and the known dimensions of the artifact.

The ISO <NUM>-<NUM> standard establishes specific procedures for verifying the performance of a CMM that uses multiple probing systems in contacting and non-contacting mode. The standard describes analysis of the quality of the associativity of multiple sensors (e.g., tactile and video sensors) along with their different operating conditions (e.g., the orientation of the sensor as defined by the articulation of a wrist) to assess whether different sensors at different wrist orientations can measure the same artifact and report data on the size, form, and location of that artifact that correlates within some tolerance zone.

For tactile sensors the artifact of choice has traditionally been a calibrated sphere.

For optical sensors, including video sensors, the choice of a sphere as the test artifact would present a unique set of challenges, including the challenge of illumination, e.g. as discussed in <CIT>. The video sensor, for example, operates by detecting edges defined by some contrast in the greyscale analysis of the pixels seen by the sensor's field of view (FOV). In essence, such a sensor probes points by "looking" at the part and choosing the point where the part shows some contrast between black and white. <CIT> and <CIT> are prior art examples of calibrated spheres being used for calibrating vision CMMs.

The invention provides a vision CMM and a test artifact for calibrating the vision CMM according to claim <NUM>, the test artifact including a base configured to rest on a table of the CMM, and a calibrated standard suspended from the base. In one embodiment, the calibrated standard is a sphere or hemisphere, but in other embodiments may have other shapes.

The test artifact also includes a convex background illumination surface suspended relative to the base.

The convex background illumination surface is suspended relative to the base to be positionable such that the calibrated standard is between the background illumination surface and a vision sensor such that light from the background illumination surface creates a silhouette of the calibrated standard as seen from the vision sensor.

In some embodiments, the convex background illumination surface is reflective (and may be a passive reflector), and creates the silhouette by reflecting light towards the vision sensor. As an example, in some embodiments the light is generated by the vision sensor and transmitted towards the convex background illumination surface. In some embodiments, the convex background illumination surface is a light source that produces light.

The convex background illumination surface, in some embodiments, is configured such that its location can be controllably adjusted (i.e., it is moveably positionable) relative to the calibrated standard. Further, in some embodiments the convex background illumination surface is movable with respect to the calibrated standard, without moving the calibrated standard relative to the base.

Alternately, or in addition, some embodiments include a reflective hemisphere positioned between the base and the calibrated standard. For example, in some embodiments the reflective hemisphere in a fixed position between the base and the calibrated standard such that a line normal to the base passes through the reflective hemisphere and the calibrated standard.

The invention further provides a method of illuminating a calibrated standard for use with a CMM having a vision sensor having a field of view according to claim <NUM>. The method includes positioning the calibrated standard with the vision sensor's field of view; positioning a background illuminator such that the calibrated standard is between the background illuminator and the vision sensor; and illuminating background illuminator to create a silhouette of the calibrated standard within the vision sensor's field of view. According to the invention, the background illuminator comprises one of a convex background illumination surface and a concave background illumination surface.

In some embodiments, the calibrated standard and the background illuminator are both suspended from a base, the base configured to rest on a table of the CMM. Moreover, in some embodiments the background illuminator is movably suspended from the base such that the background illuminator is movable relative to the calibrated standard without moving the calibrated standard relative to the base.

In some embodiments, the background illuminator includes a convex background illumination surface, and illuminating the background illuminator includes first illuminating the convex background illumination surface when the convex background illumination surface is in a first position relative to the calibrated standard, and subsequently illuminating the convex background illumination surface when the convex background illumination surface is in a second position relative to the calibrated standard. Further, in some embodiments, the method includes moving the vision sensor relative to the calibrated standard using a movable wrist, such that, after the vision sensor has been moved, the calibrated standard is positioned between the vision sensor and the background illuminator.

Yet another embodiment includes a method of assessing the associativity of a vision sensor used by a given CMM, and a second measuring sensor used by the given CMM. The method includes providing to the given CMM a test artifact, the test artifact having both a calibrated sphere, and a background illuminator comprising one of a convex background illumination surface and a concave background illumination surface.

In some such embodiments, the background illuminator is a passive reflector.

The method includes orienting the test artifact and the vision sensor such that the calibrated sphere is between the vision sensor and the background illuminator so that the background illuminator produces, to the vision sensor, a silhouette of the calibrated sphere. The method then includes measuring the calibrated sphere with the vision sensor to produce a first set of measurements; measuring the same calibrated sphere with the second measuring sensor to produce a second set of measurements; and comparing the first set of measurements to the second set of measurements. The second measuring sensor may be a second vision sensor, or a tactile sensor or a non-contact sensor.

Some embodiments also include re-orienting the test artifact and the vision sensor to one or more additional positions (e.g., a second position, third position, etc.) such that the calibrated sphere is again between the vision sensor and the background illuminator so that the background illuminator produces, to the vision sensor, a silhouette of the calibrated sphere; and measuring the calibrated sphere with the vision sensor for a second time.

Various embodiments facilitate assessment of the accuracy of a CMM by providing a device that improves the assessment of the CMM's vision sensor, and which is also configured for assessing other measuring sensors of the CMM. This may be useful, for example, not only in testing and calibrating the CMM, but also to assessing the associativity of the CMM's sensors. Various embodiments enable different sensors at different wrist orientations to measure the same artifact and report data on the size, form, and location of that artifact that correlates within some tolerance zone. Some embodiments also facilitate verification of sensor performance.

<FIG> schematically illustrate a coordinate measurement machine <NUM> (hereinafter "CMM <NUM>") that may be configured in accordance with illustrative embodiments.

As known by those in the art, a CMM is a system configured to measure one or more features of a workpiece <NUM>. An illustrative embodiment of a workpiece <NUM> is schematically illustrated in <FIG>, and is included only to illustrate features that a workpiece <NUM> may have. Typically, a workpiece <NUM> has a specified shape with specified dimensions, which may be referred-to collectively as the "geometry" <NUM> of the workpiece <NUM>. A workpiece <NUM> may also have surfaces, such as a flat surface <NUM>, and a curved surface <NUM>. A meeting of two surfaces may create an inside angle <NUM>, an edge <NUM>, or a corner <NUM>. Moreover, each surface may have physical characteristic such as waviness <NUM> and/or surface finish <NUM>, as known in the art. A workpiece <NUM> may also have a cavity <NUM>, which may also be an aperture through the workpiece <NUM>. As known in the art, a cavity <NUM> may have dimensions such as width and depth, which may in turn define an aspect ratio of the cavity <NUM>.

In the illustrative embodiment of <FIG>, the CMM <NUM> includes a base <NUM> having a table <NUM>. The table <NUM> of the CMM <NUM> defines an X-Y plane <NUM> that typically is parallel to the plane of the floor <NUM>, and a Z-axis normal to the X-Y plane, and a corresponding X-Z plane and Y-Z plane. The table <NUM> also defines a boundary of a measuring space <NUM> above the table <NUM>. In some embodiments, the CMM <NUM> includes a probe rack <NUM> configured to hold one or more measuring sensors <NUM>. A movable part of the CMM <NUM> may move to the probe rack <NUM> and place a measuring sensor <NUM> into the probe rack <NUM>, and/or remove another measuring sensor <NUM> from the probe rack <NUM>.

The CMM <NUM> also has movable features (collectively, <NUM>) arranged to move and orient a measuring sensor <NUM> (and in some embodiments, a plurality of such devices) relative to the workpiece <NUM>. As described below, movable features of the CMM <NUM> are configured to move and orient the measuring sensor <NUM>, relative to the workpiece <NUM>, in one dimension (X-axis; Y-axis; or Z-axis), two dimensions (X-Y plane; X-Z plane; or Y-Z plane), or three dimensions (a volume defined by the X-axis, Y-axis, and Z-axis). Accordingly, the CMM <NUM> is configured to measure the location of one or more point on, or features of, the workpiece <NUM>.

The CMM <NUM> of <FIG> is known as a "bridge" CMM. Movable features <NUM> of the bridge CMM <NUM> include a bridge <NUM> movably coupled to the base <NUM> by legs <NUM>. The bridge <NUM> and legs <NUM> are controllably movable relative to the base <NUM> along the Y-axis.

To facilitate motion of the legs relative to the base <NUM>, the legs <NUM> may be coupled to the base <NUM> by one or bearings <NUM>. As known in the art, a bearing may be a roller bearing or an air bearing, to name just a few examples.

The movable features also include a carriage <NUM> movably coupled to the bridge <NUM>. The carriage is configured to controllably move in the X-axis along the bridge <NUM>. The position of the carriage <NUM> along the bridge <NUM> may be determined by a bridge scale <NUM> operably coupled to the bridge <NUM>.

A spindle <NUM> is moveably coupled to the carriage <NUM>. The spindle <NUM> is configured to controllably move in the Z-axis. The position in the Z-axis of the spindle <NUM> may be determined by a spindle scale <NUM> operably coupled to the spindle <NUM>. The measuring sensor <NUM> is operably coupled to the spindle <NUM>. Consequently, the measuring sensor <NUM> is controllably movable in three dimensions relative to a workpiece <NUM> in the measuring space <NUM>.

In some embodiments, the measuring sensor <NUM> is moveably coupled to the spindle <NUM> by an articulated arm <NUM>. For example, the measuring sensor <NUM> may be movably coupled to the arm <NUM> by a movable joint <NUM>. The movable joint <NUM> allows the orientation of the measuring sensor <NUM> to be controllably adjusted relative to the arm <NUM>, to provide to the measuring sensor <NUM> additional degrees of freedom in the X-axis, Y-axis, and/or Z-axis.

In other embodiments, which may be generally referred-to as "gantry" CMMs, the legs <NUM> stand on the floor <NUM>, and the measuring space <NUM> is defined relative to the floor <NUM>.

In yet other embodiments, the measuring sensor <NUM> is fixed to (i.e., not movable relative to) the base <NUM>, and the table <NUM> is movable in one, two or three dimensions relative to the measuring sensor <NUM>. In some coordinate measuring machines, the table <NUM> may also be rotatable in the X-Y plane, or in the Y-Z plane, or in the X-Z plane, or in any other plane that intersects the measurement envelope <NUM>. In such embodiments, the CMM <NUM> moves the workpiece <NUM> relative to the measuring sensor.

In other embodiments, which may be generally referred-to as "horizontal arm" CMMs, the bridge <NUM> is movably coupled to the base <NUM> to extend in the Z-axis, and to be controllably movable along the Y-axis. In such a CMM, the arm <NUM> is controllably extendable in the X-axis, and controllably movable up and down the bridge <NUM> in the Z-axis.

In yet other embodiments, the arm <NUM> is articulated. One end of the arm <NUM> is fixed to the base <NUM>, and a distal end of the arm <NUM> is movable relative to the base <NUM> in one, two or three dimensions relative to a workpiece <NUM> in the measuring space <NUM>.

In some embodiments, the measuring sensor <NUM> may be a tactile probe (configured to detect the location of a point on the workpiece <NUM> by contacting a probe tip to the workpiece <NUM>, as known in the art), a non-contact probe (configured to detect the location of a point on the workpiece <NUM> without physically contacting the workpiece <NUM>), such as a capacitive probe or an inductive probe as known in the art, or an optical probe (configured to optically detect the location of a point on the workpiece <NUM>), to name but a few examples.

In some embodiments, the measuring sensor <NUM> is a vision sensor that "sees" the workpiece <NUM>. Such a vision sensor may be a camera having a light sensor (e.g., a charge-coupled device), and one or more lenses, and is capable of focusing on the workpiece <NUM>, or the measurement area <NUM>, and configured to capture and record still images or video images. Such images, and/or pixels within such images, may be analyzed to locate the workpiece <NUM>; determine the placement and/or orientation of the workpiece <NUM>; identify the workpiece <NUM>; and/or measure the workpiece <NUM>, to name but a few examples.

In operation, the CMM <NUM> measures the workpiece <NUM> by moving the measuring sensor <NUM> relative to the workpiece <NUM> to measure the workpiece <NUM>.

Some embodiments of a CMM <NUM> may include one, or more than one, camera <NUM> configured such that the measurement envelope <NUM> is within the field of view of the camera <NUM>. Such a camera <NUM> may be in addition to a measuring sensor <NUM>. The camera <NUM> may be a digital camera configured to capture still images and/or video images of the measurement envelope <NUM>, a workpiece <NUM> on the CMM <NUM>, and/or the environment around the CMM <NUM>. Such images may be color images, black and white images, and/or grayscale image, and the camera <NUM> may output such images as digital data, discrete pixels, or in analog form.

Some embodiments of a CMM <NUM> may also include an environmental sensor <NUM> configured to measure one or more characteristics of the environment <NUM> in which the CMM is placed, and some embodiments may have more than one such environmental sensor <NUM>. For example, an environmental sensor <NUM> may be configured to measure the temperature, pressure, or chemical content of the atmosphere around the CMM <NUM>. An environmental sensor <NUM> may also be a motion sensor, such as an accelerometer or a gyroscope, configured to measure vibrations of the CMM caused, for example, the by motion of people or objects near the CMM <NUM>. An environmental sensor <NUM> may also be a light detector configured to measure ambient light in the environment <NUM>, which ambient light might, for example, interfere with the operation of an optical sensor or vision sensor. In yet another embodiment, an environmental sensor <NUM> may be sound sensor, such as a microphone, configured to detect sound energy in the environment.

Some embodiments of a CMM <NUM> include a control system <NUM> (or "controller" or "control logic") configured to control the CMM <NUM>, and process data acquired by the CMM. <FIG> schematically illustrates an embodiment of a control system <NUM> having several modules in electronic communication over a bus <NUM>.

In general, some or all of the modules may be implemented in one or more integrated circuits, such as an ASIC, a gate array, a microcontroller, or a custom circuit, and at least some of the modules may be implemented in non-transient computer-implemented code capable of being executed on a computer processor <NUM>.

Some embodiments include a computer processor <NUM>, which may be a microprocessor as available from Intel Corporation, or an implementation of a processor core, such as an ARM core, to name but a few examples. The computer processor <NUM> may have on-board, non-transient digital memory (e.g., RAM or ROM) for storing data and/or computer code, including non-transient instructions for implementing some or all of the control system operations and methods. Alternately, or in addition, the computer processor <NUM> may be operably coupled to other non-transient digital memory, such as RAM or ROM, or a programmable non-transient memory circuit for storing such computer code and/or control data. Consequently, some or all of the functions of the controller <NUM> may be implemented in software configured to execute on the computer processor <NUM>.

The control system <NUM> includes a communications interface <NUM> configured to communicate with other parts of the CMM <NUM>, or with external devices, such as computer <NUM> via communications link <NUM>. To that end, communications interface <NUM> may include various communications interfaces, such as an Ethernet connection, a USB port, or a Firewire port, to name but a few examples.

The control system <NUM> also includes a sensor input <NUM> operably coupled to one or more sensors, such as a measuring sensor <NUM>, camera <NUM>, or environmental sensor <NUM>. The sensor input <NUM> is configured to receive electronic signals from sensors, and in some embodiments to digitize such signals, using a digital to analog ("D/A") converter ("DAC"). The sensor input <NUM> is coupled to other modules of the control system <NUM> to provide to such other modules the (digitized) signals received from sensors.

The motion controller <NUM> is configured to cause motion of one or more of the movable features of the CMM <NUM>. For example, under control of the computer processor <NUM>, the motion controller <NUM> may send electrical control signals to one or more motors within the CMM <NUM> to cause movable features of the CMM <NUM> to move a measuring sensor <NUM> to various points within the measuring space <NUM> and take measurements of the workpiece <NUM> at such points. The motion controller <NUM> may control such motion in response to a measurement program stored in memory module <NUM>, or stored in computer <NUM>, or in response to manual control by an operator using manual controller <NUM>, to name but a few examples.

Measurements taken by the CMM <NUM> may be stored in a memory module <NUM>, which includes a non-transient memory. The memory module <NUM> is also configured to store, for example, a specification for a workpiece <NUM> to be measured; a specification for a calibration artifact; an error map; and/or non-transient instructions executable on the computer processor <NUM>, to name but a few examples. Such instructions may include, among other things, instructions for controlling the movable features of the CMM <NUM> for measuring a workpiece <NUM> and/or a calibration artifact; instructions for analyzing measurement data; and/or instructions for correcting measurement data (e.g., with an error map).

The measurement analyzer module <NUM> is configured to process measurement data received from one or more sensors, such as measuring sensor <NUM>, and/or environmental sensor <NUM>. In some embodiments, the measurement analyzer module <NUM> may revise the measurement data, for example by modifying the measurement data using an error map, and/or compare the measurement data to a specification, for example to assess deviation between a workpiece <NUM> and a specification for that workpiece <NUM>. To that end, the measurement analyzer module <NUM> may be a programmed digital signal processor integrated circuit, as known in the art.

Alternately, or in addition, some embodiments couple the CMM <NUM> with an external computer (or "host computer") <NUM>. The host computer <NUM> has a computer processor such as those described above, and non-transient computer memory <NUM>. The memory <NUM> is configured to hold non-transient computer instructions capable of being executed by the processor of external computer <NUM>, and/or to store non-transient data, such as data acquired as a result of the measurements of an object <NUM> on the base <NUM>.

Among other things, the host computer <NUM> may be a desktop computer, a tower computer, or a laptop computer, such as those available from Dell Inc. , or even a tablet computer, such as the iPad™ available from Apple Inc. In addition to the computer memory <NUM>, the host computer <NUM> may include a memory interface <NUM>, such as a USB port or slot for a memory card configured to couple with a non-transient computer readable medium and enable transfer of computer code or data, etc. between the computer <NUM> and the computer readable medium.

The communication link <NUM> between the CMM <NUM> and the host computer <NUM> may be a hardwired connection, such as an Ethernet cable, or a wireless link, such as a Bluetooth link or a Wi-Fi link. The host computer <NUM> may, for example, include software to control the CMM <NUM> during use or calibration, and/or may include software configured to process data acquired during operation of the CMM <NUM>. In addition, the host computer <NUM> may include a user interface configured to allow a user to manually operate the CMM <NUM>. In some embodiments, the CMM and/or the host computer <NUM> may be coupled to one or more other computers, such as server <NUM>, via a network <NUM>. The network <NUM> may be a local area network, or the Internet, to name two examples.

Because their relative positions are determined by the action of the movable features of the CMM <NUM>, the CMM <NUM> may be considered as having knowledge of the relative locations of the table <NUM>, and the workpiece <NUM>. More particularly, the computer processor <NUM> and/or computer <NUM> control and store information about the motions of the movable features. Alternately, or in addition, the movable features of some embodiments include sensors that sense the locations of the table <NUM> and/or measuring sensor <NUM>, and report that data to the computers <NUM> or <NUM>. The information about the motion and positions of the table and/or measuring sensor <NUM> of the CMM <NUM> may be recorded in terms of a onedimensional (e.g., X, Y Z), two-dimensional (e.g., X-Y; X-Z; Y-Z) or three-dimensional (X-Y-Z) coordinate system referenced to a point on the CMM <NUM>.

Some CMMs also include a manual user interface <NUM>. As shown, the manual user interface <NUM> may have controls (e.g., buttons; knobs, etc.) that allow a user to manually operate the CMM <NUM>. Among other things, the interface <NUM> may include controls that enable the user to change the position of the measuring sensor <NUM> relative to the workpiece <NUM>. For example, a user can move the measuring sensor <NUM> in the X-axis using controls <NUM>, in the Y-axis using controls <NUM>, and/or in the Z-axis using controls <NUM>.

If the measuring sensor <NUM> is a vision sensor, or if the CMM <NUM> includes a camera <NUM>, then the user can manually move the sensor <NUM>, camera <NUM>, or change field of view of the vision sensor and/or camera using controls <NUM>. The user may also focus the vision sensor and/or camera <NUM> using control <NUM> (which may be a turnable knob in some embodiments) and capture an image, or control recording of video, using control <NUM>.

As such, the movable features may respond to manual control, or be under control of the computer processor <NUM>, to move the table <NUM> and/or the measuring sensor <NUM> relative to one another. Accordingly, this arrangement is configured to present a workpiece <NUM> to the measuring sensor <NUM> from a variety of angles, and in a variety of positions.

The accuracy of operation of a CMM <NUM> may be characterized by several criteria. For example, the "repeatability" of a CMM <NUM> is a measure of its ability to accurately repeat a measurement of the same workpiece <NUM> under the same conditions (e.g., same measuring instrument; same observer; same measurement procedure; same location and environment of CMM <NUM>, etc.) within a short period of time. The repeatability of a CMM <NUM> may be quantified as the variation of measurements taken by the CMM <NUM> when it repeatedly measures the same characteristic of the same workpiece <NUM>.

The "reproducibility" of a measurement refers to the degree of agreement between measurements of the same workpiece <NUM> when the measurements are performed under different (i.e., non-identical) conditions within a short period of time. A statement of the reproducibility of a measurement includes a specification of the differences between measurements (e.g., a different principle of measurement; different observer; a different method of measurement; a different measuring instrument; different location and environment of CMM <NUM>, etc.). The reproducibility of a measurement may be quantified as the variation between measurements of the same characteristic of the same workpiece <NUM> taken by different measurers using the same CMM <NUM>.

The "associativity" of one measuring sensor <NUM> with another measuring sensor <NUM> refers to the degree of disagreement between their respective measurements of the same workpiece <NUM> under conditions that are the same, except for the use of the difference measuring devices. For example, if a calibrated workpiece <NUM> is measured using a contact probe, and the same calibrated workpiece <NUM> is subsequently measured by a vision sensor within a short period of time, their respective measurements ideally should be identical, but in practice will be different. The degree of associativity between the contact probe and a vision sensor may be quantified by the differences in their respective measurements of the calibrated workpiece <NUM>.

Typically, a CMM <NUM> is calibrated and recalibrated from time to time. The manufacturer of a CMM <NUM>, or the owner or operator, a customer of the owner or operator of the CMM, or a regulatory agency, may specify that the CMM <NUM> be calibrated at certain time intervals. Alternately, or in addition, the operator of the CMM <NUM> may calibrate the CMM <NUM> in response to a change of location, a change of environment <NUM> (e.g., temperature; pressure, etc.), a change to the machine (e.g., replacing, or changing the type of, a measuring sensor <NUM>), and/or a change of a programmed measuring process, to name but a few examples.

Generally, calibration may be described as assessing one or more measures of the accuracy of a CMM <NUM>. Typically, calibrating a CMM <NUM> includes using the CMM <NUM> to measure a calibrated artifact, which is essentially a workpiece having known, highly-accurate dimensions, and comparing the measurements taken by the CMM <NUM> against those known dimensions. A difference between the measurements taken by the CMM <NUM> and the known dimensions of the calibrated artifact represents an inaccuracy.

If the calibration indicates that the CMM <NUM> meets a required specification (e.g., any identified inaccuracy is within a specified tolerance), the person performing the calibration may, for example, provide to the owner or operator a certificate certifying that the CMM <NUM> is calibrated.

If the calibration measurements indicate an inaccuracy, a technician may adjust a portion of the CMM <NUM>, including one or more of its movable features, so that the inaccuracy is reduced or eliminated. Alternately, or in addition, subsequent measurement data produced by the CMM <NUM> may be mathematically adjusted in ways known in the art, to counteract the inaccuracy.

<FIG> is a photograph of a calibrated standard, in the form of a sphere, which may be referred to a calibrated sphere <NUM>, as seen by a vision sensor <NUM> without the benefit of various embodiments. A calibrated standard is a device having well-known dimensions, as known in the art for purposes of calibrating a CMM <NUM>, and for assessing associativity of a plurality of probes.

It should be noted that embodiments herein are illustrated and described in terms of a calibrated sphere <NUM>, but the calibrated sphere <NUM> is only an illustrative embodiment of a calibrated standard. A calibrated standard can have any shape, provided that the shape can produce a silhouette as described above. For example, a calibrated standard may be square, cubic, oval, or oblong, to name but a few examples.

As shown, the calibrated sphere <NUM> is effectively indistinguishable from its background <NUM>, and is therefore not useful for calibrating a CMM, verifying performance of an optical sensor, assessing the associativity of different sensors, or measuring a workpiece <NUM>.

In contrast, <FIG> is a photograph of a calibrated sphere <NUM> against a background <NUM> according to various embodiments, and <FIG> is a schematic illustration of an embodiment of a calibrated sphere <NUM> against a background <NUM> according to various embodiments. As shown in <FIG> and <FIG>, the calibrated sphere <NUM> is easily distinguishable from the background <NUM>. More specifically, to a vision sensor, the edge <NUM> of the calibrated sphere <NUM>, which is the surface <NUM> of the sphere <NUM> seen in profile or silhouette, is visibly distinct from the background <NUM>.

<FIG> schematically illustrate features of portions of an embodiment of a conformance test artifact <NUM>, along with a measuring sensor <NUM>, which in this embodiment is a vision sensor <NUM>. The vision sensor <NUM> operates by detecting edges defined by some contrast in the analysis of the pixels seen by the sensor's field of view (FOV). For example, vision sensor <NUM> probes points by "looking" at the part being measured (e.g., the artifact) and choosing the point where the part shows some (or the sharpest) contrast between the color of calibrated standard and its background. In illustrative embodiments, the image produced by the vision sensor <NUM> is in black and white, or greyscale, or is processed to be in black and white or greyscale. In such embodiments, the contrast appears between a black portion (e.g., the silhouette of calibrated sphere <NUM>) and it's a white portion (e.g., the background). It should be noted that viewing the images in black and white, or grayscale, is not a requirement of the methods and systems described herein. Color images, in which the calibrated sphere <NUM> is visible in contrast to its background, may also be used.

The conformance test artifact <NUM> includes a calibrated standard (in illustrative embodiments, a calibrated sphere <NUM>), which preferably has a lowreflectivity surface, and in preferred embodiments has a matte finish. In some embodiments, the calibrated sphere may be tungsten carbide with a matte finish. This material allows the vision sensor <NUM> to focus on the calibrated sphere <NUM>. This sphere artifact <NUM> is black/dark grey and when it is viewed by the vision sensor <NUM> on its own; the sensor sees a black object against a black field (see, for example, <FIG>).

The calibrated sphere <NUM> is supported by a post <NUM>. The post <NUM> has a length, and may have a variable length in order to facilitate changing to location of the calibrated sphere <NUM>, for example relative to the background illuminator <NUM> and/or a vision sensor <NUM>. Similarly, for the same reason, in some embodiments the post <NUM> is flexible, and may be bent or otherwise have its shape changed.

The conformance test artifact <NUM> also includes a background illuminator <NUM>. In some embodiments, the background illuminator <NUM> is a convex background illumination surface configured to reflect incident light. Among other things, the convex background illumination surface may be the surface of a sphere, the surface of an ellipse, or the surface of a hemisphere, to name but a few examples.

The convex shape of the background illumination surface reduces the risk that light reflected from that surface impinges on a sensor-facing portion of the surface of the calibrated sphere <NUM>. In other embodiments, the background illumination surface may be concave, yet preferably configured not to project light onto a sensor-facing portion the calibrated standard. To that end, the shape of the background illumination surface <NUM> may depend on the shape of the calibrated standard <NUM>.

In other embodiments, the background illuminator <NUM> is a source of light, and is configured to produce light on a hemisphere <NUM> of a calibrated sphere <NUM> that faces away from a vision sensor <NUM>. In preferred embodiments, such an embodiment has a convex light-emitting surface to reduce the risk that light emanating from the background illuminator <NUM> impinges on a sensor-facing portion of the surface of the calibrated standard (e.g., sphere <NUM>). In other embodiments, the light-emitting surface may be concave, yet preferably configured not to project light onto a sensor-facing portion of the calibrated standard. To that end, the shape of the background illuminator <NUM> may depend on the shape of the calibrated standard <NUM>.

The background illuminator <NUM> is supported by an illuminator post <NUM>. The background illuminator post <NUM> has a length, and may have a variable length in order to facilitate changing to location of the background illumination surface <NUM>, for example relative to the calibrated sphere <NUM> and/or a vision sensor <NUM>. Similarly, for the same reason, in some embodiments the background illuminator post <NUM> is flexible, and may be bent or otherwise have its shape changed.

The illuminator post <NUM> may be supported in a post aperture <NUM> in the base <NUM> (which, as described below, may be a carousel in some embodiments). In some embodiments, the base <NUM> includes more than one post aperture <NUM>, and the location of the background illuminator <NUM> may be changed by moving the illuminator post <NUM> from one post aperture <NUM> to another.

Some embodiments include two or more units of background illuminator <NUM>, as schematically illustrated in <FIG> for example. In such embodiments, the posts <NUM> may be disposed in a corresponding number of post apertures <NUM>.

In some embodiments, the calibrated sphere <NUM> may be physically separate from, and independently movable with respect to, the background illuminator <NUM>. This provides to an operator of the CMM <NUM> flexibility in the selection and arrangement of the calibrated sphere <NUM> and the background illuminator <NUM>. One illustrative embodiment of is schematically illustrated in <FIG>, in which the calibrated sphere <NUM> is supported by base <NUM> via post <NUM>, and in which the background illuminator <NUM> is supported by base <NUM> via post <NUM>.

In use, the calibrated sphere <NUM> is positioned, at least in part, within the field of view of vision sensor <NUM>, and preferably between the background illuminator <NUM> and the vision sensor <NUM>. In general, the background illuminator <NUM> may be positioned so that a portion of the background illuminator <NUM> is visible to the vision sensor <NUM> (i.e., within the field of view of the vision sensor <NUM>, and not entirely blocked by the calibrated sphere <NUM>), or the background illuminator <NUM> may be concealed from the vision sensor <NUM> by the calibrated sphere <NUM>.

In embodiments in which the background illuminator <NUM> is a convex background illumination surface, it reflects incident light (<NUM>). Some of the reflected light (e.g., portion <NUM>) impinges on the calibrated sphere on a side opposite (facing away from) the vision sensor <NUM>, while some of the reflected light (e.g., portion <NUM>) impinges on light sensor <NUM> in the vision sensor <NUM>, all resulting in an apparent silhouette at the light sensor <NUM> (such as illustrated in <FIG>, for example). In some embodiments, the convex background illumination surface does not reflect light onto the hemisphere (<NUM>) of the calibrated sphere <NUM> that faces the vision sensor <NUM>, which may distort the image of the calibrated sphere <NUM>. This is a direct consequence of the convex (e.g., spherical) geometry of the convex background illumination surface <NUM>, which reflects light to the sensor from behind the calibrated sphere <NUM> while maintaining the "darkness" of the "sensor facing" hemisphere (<NUM>).

In embodiments in which the background illuminator <NUM> is a light source, the background illuminator produces light, some of which (represented in <FIG> by ray <NUM>) impinges on the calibrated sphere <NUM>, and some of which (represented in <FIG> by ray <NUM>) reaches the vision sensor <NUM> to produce a silhouette of the calibrated sphere <NUM>.

In some embodiments, the vision sensor <NUM> is a video sensor. Also, in some embodiments, the vision sensor <NUM> may include a light source, such as a ring of lights <NUM> (which may be referred to as a "ring light") surrounding a light sensor <NUM>. Such embodiments may include two concentric rings (<NUM>, <NUM>) of lights (<NUM>) surrounding light sensor <NUM>. As such, the light source is associated with the vision sensor such that the light source moves with the light sensor <NUM>. More specifically, in the embodiment of <FIG>, the light source is coupled to the vision sensor. In some embodiments, the CMM's controller can control such a ring light (<NUM>, <NUM>) with the application software to facilitate developing (increasing) contrast on the artifact <NUM> which allows the sensor to detect the edge and probe a point. For example, the controller <NUM> may set or change when a ring light (<NUM> or <NUM>) is illuminated, the intensity of the illumination, and/or the spectrum of the illumination, etc..

<FIG> schematically illustrate another embodiment of a conformance test artifact <NUM> standing on the table <NUM> of a CMM. In this embodiment, the conformance test artifact <NUM> includes a carousel <NUM> mounted to a carousel base <NUM>. The calibrated sphere <NUM> is suspended from the carousel <NUM> by a calibrated sphere post <NUM> and a mounting pad <NUM>, which in this embodiment is removably coupled to the carousel <NUM>.

The background illuminator <NUM> is suspended from the carousel. In this embodiment, the background illuminator <NUM> is at substantially the same height (relative to the carousel) as the calibrated sphere <NUM> but in some embodiments, the height of the background illuminator <NUM>, and/or the height of the calibrated sphere <NUM>, are adjustable via their respective posts <NUM> and <NUM>.

In some embodiments, the carousel <NUM> is movably coupled to the base <NUM>, such that the carousel <NUM> may rotate relative to the base <NUM>, so that the background illuminator <NUM> moves relative to the table <NUM> but the calibrated sphere <NUM> remains substantially stationary relative to the table <NUM>. To that end, some embodiments include a control post <NUM>, as schematically illustrated in <FIG>. The control post <NUM> may serve as a handle to assist a user in turning, moving, or orienting the carousel <NUM>. In other embodiments, the control post <NUM> is a set post configured to mate with the carousel <NUM> and secure the carousel <NUM> in a fixed position relative to the base <NUM>. To change the position of the carousel, an operator loosens or removes the control post <NUM>, moves the carousel to a new position, and tightens or re-installs the set post <NUM>. Among other things, the control post <NUM> may include a set screw. In other embodiments, the control post <NUM> is a peg that fits through an aperture <NUM> in the carousel <NUM>, and mates with a corresponding aperture <NUM> in the base <NUM>.

<FIG> schematically illustrates an embodiment of a conformance test artifact <NUM>, and <FIG> schematically illustrates a cross-section of the conformance test artifact <NUM> along section A-A.

<FIG> is a photograph of an embodiment of the conformance test artifact <NUM> being measured by the vision sensor <NUM>. In this embodiment, the vision sensor <NUM> is suspended from a spindle <NUM> by a movable joint <NUM>, which may be referred to as a "wrist" <NUM>. In <FIG>, the vision sensor is oriented along the X axis.

As shown in <FIG>, light from the vision sensor <NUM> impinges the convex background illumination surface of a background illuminator <NUM>. As explained above, some light reflected from the convex background illumination surface is absorbed by the calibrated sphere <NUM>, while some light reflected from the convex background illumination surface reaches light sensor <NUM> in the vision sensor <NUM> to form a silhouette of calibrated sphere <NUM>.

In some embodiments, if the calibrated sphere <NUM> is not positioned between the vision sensor <NUM> and the background illuminator <NUM>, the background illuminator <NUM> may be moved by adjusting the carousel <NUM> as described above, so that the calibrated sphere <NUM> is positioned between the vision sensor <NUM> and the background illuminator <NUM>.

<FIG> is a photograph of an embodiment of the conformance test artifact <NUM> being measured by the vision sensor <NUM> configured at a different angle than the angle in <FIG>. In this embodiment, the vision sensor <NUM> is directly above the calibrated sphere <NUM> (i.e., in the Z axis). Some light reflected from the convex vertical reflector <NUM> is absorbed by the calibrated sphere <NUM>, while some light reflected from the convex vertical reflector <NUM> reaches light sensor <NUM> in the vision sensor <NUM> to form a silhouette of calibrated sphere <NUM>.

<FIG> is a flow chart illustrating a method of using illustrative embodiments of a conformance test artifact <NUM> to calibrate a CMM <NUM> using a vision sensor <NUM>. In addition to its other qualities, the calibrated sphere <NUM> has precise, known dimensions, as known in the art of calibration artifacts.

At step <NUM>, the method includes orienting the vision sensor <NUM> relative to the calibrated sphere <NUM> so that the calibrated sphere <NUM> is within the field of view of the vision sensor <NUM>. This may include moving the vision sensor <NUM> relative to the calibrated sphere <NUM>, and/or moving the calibrated sphere <NUM> relative to the vision sensor <NUM>, for example by adjusting the length or shape of the calibrated sphere post <NUM>. In preferred embodiments, no light from the background illuminator <NUM> impinges on the side <NUM> of the calibrated sphere <NUM> facing the vision sensor <NUM>. To that end, in preferred embodiments, the calibrated sphere <NUM> is between the background illuminator <NUM> and the vision sensor <NUM>.

Step <NUM> includes capturing one or more still images, or video images, of the calibrated sphere <NUM>.

In some embodiments, step <NUM> includes assessing the contrast between the calibrated sphere <NUM> and its background. For example, step <NUM> may include assessing the sharpness of the distinction in the silhouette of the calibrated sphere <NUM>.

In some embodiments, step <NUM> includes adjusting one or more aspects of the conformance test artifact <NUM> and or the CMM <NUM> to improve the image captured by the vision sensor <NUM>. For example, some embodiments increase the contrast by adjusting the illumination of the background illuminator <NUM>, e.g., by controllably adjusting the lights <NUM> in embodiments in which the background illuminator is a passive reflector, or the light output of the background illuminator <NUM> itself in embodiments in which the background illuminator <NUM> is an active light source. Alternately, or in addition, some embodiments change the location of the vision sensor <NUM>, or the angle at which the vision sensor <NUM> looks at the calibrated sphere <NUM>.

At step <NUM>, the method uses the image of the calibrated sphere <NUM> captured by the vision sensor <NUM> to compare the measurements of the calibrated sphere <NUM> to the known dimensions of the calibrated sphere <NUM>, and to compare any discrepancy between the measurements and the known dimensions to a tolerance specified in a specification for the CMM <NUM> and/or the vision sensor <NUM> and/or the workpiece <NUM> to be measured.

At step <NUM>, the method optionally includes making adjustments to the CMM, using methods known in the art.

Optionally, after step <NUM>, the method may repeat the previous steps using the same conformance test artifact <NUM>. For example, some embodiments repeat some or all of the steps after re-orienting the test artifact <NUM>, relative to the vision sensor <NUM>, to a second position (e.g., a second wrist angle; see for example a first such position in <FIG>, and a second such position in <FIG>).

<FIG> is a flow chart illustrating a method of using illustrative embodiments of a conformance test artifact <NUM> to test associativity of a plurality of sensors.

Step <NUM> includes calibrating the CMM <NUM> in ways known in the art, or as described above. In preferred embodiments, the following steps are performed using a conformance test artifact <NUM> that is not the same artifact used for in the calibration step <NUM>.

After the CMM <NUM> has been calibrated, step <NUM> of the method includes orienting the vision sensor <NUM> relative to the calibrated sphere <NUM> so that the calibrated sphere <NUM> is within the field of view of the vision sensor <NUM>. This may include moving the vision sensor <NUM> relative to the calibrated sphere <NUM>, and/or moving the calibrated sphere <NUM> relative to the vision sensor <NUM>, for example by adjusting the length or shape of the calibrated sphere post <NUM>. In preferred embodiments, no light from the background illuminator <NUM> impinges on the side <NUM> of the calibrated sphere <NUM> facing the vision sensor <NUM>. To that end, in preferred embodiments, the calibrated sphere <NUM> is between the background illuminator <NUM> and the vision sensor <NUM>.

At step <NUM>, the method captures one or more images of the calibrated sphere <NUM> using the vision sensor <NUM>.

At step <NUM>, the method assesses the captured image of the calibrated sphere <NUM> using methods known in the art to produce a first set of associativity measurements.

Some embodiments also assess the contrast between the calibrated sphere <NUM> and its background, which may include assessing the sharpness of the distinction in the silhouette of the calibrated sphere <NUM>. Such embodiments include step <NUM>, which includes adjusting one or more aspects of the conformance test artifact <NUM> and or the CMM <NUM> to improve the image captured by the vision sensor <NUM>. For example, some embodiments increase the contrast by adjusting the illumination of the background illuminator <NUM>, e.g., by controllably adjusting the lights <NUM> in embodiments in which the background illuminator is a passive reflector, or the light output of the background illuminator <NUM> itself in embodiments in which the background illuminator <NUM> is an active light source. Alternately, or in addition, some embodiments change the location of the vision sensor <NUM>, or the angle at which the vision sensor <NUM> looks at the calibrated sphere <NUM>.

In some embodiments, step <NUM> includes re-orienting the test artifact <NUM>, relative to the vision sensor, to a second position (e.g., a second wrist angle; see for example a first such position in <FIG>, and a second such position in <FIG>), and measuring the calibrated standard with the vision sensor for a second time.

At step <NUM>, the method measures the calibrated sphere <NUM> with the second measuring sensor <NUM>, to produce a second set of associativity measurements. The calibrated sphere <NUM> is the same one measured by the vision sensor <NUM> in step <NUM>.

The second measuring sensor <NUM> may be another vision sensor, or may be a tactile probe, non-contact probe, or optical probe, to name but a few examples. In preferred embodiments, the only difference in the CMM <NUM> between steps <NUM> and <NUM> is the change of measuring sensor <NUM>. In some embodiments, step <NUM> includes exchanging the vision sensor <NUM> for the second measuring sensor <NUM> by placing the vision sensor <NUM> on the probe rack <NUM> and replacing it with a second measuring sensor <NUM> from the probe rack <NUM>. In some embodiments, step <NUM> is performed before step <NUM>.

Then at step <NUM>, the method uses the first set of measurements and the second set measurements to assess the associativity of the vision sensor <NUM> and the second sensor <NUM>, using methods known in the art. For example, step <NUM> may include comparing measurements of the calibrated sphere <NUM> taken by the vision sensor to measurements of the same calibrated sphere <NUM> taken by the second measuring sensor <NUM> to assess the differences between such measurements.

Alternately, in some embodiments, at step <NUM> the method measures the calibrated sphere <NUM> a second time with the same vision sensor <NUM>, but from a different angle (see, for example, <FIG> and <FIG>), to produce second set of associativity measurements. Indeed, in some embodiments, the step <NUM> measures the calibrated sphere <NUM> more than two times (e.g., three times; four times; five times). In such embodiments, step <NUM> assesses the associativity of the vision sensor <NUM> to itself. The associativity of a measuring sensor <NUM> to itself may be referred-to as the sensor's "self-associativity.

As part of assessing the associativity, some embodiments compare the associativity to a standard. For example, a specified standard for associativity may arise in a specification for a CMM <NUM>, or in a specification for a workpiece <NUM>, or in a specification for a measurement to be performed on a workpiece <NUM>. In some embodiments if the associativity does not meet the specified standard, the method may include remedial action, such as changing one or more sensors <NUM>, or calibrating and/or adjusting a sensor <NUM> or other part of the CMM <NUM>.

The following is a list of some reference numbers used herein:.

Definitions. As used in this description and any accompanying claims, the following terms shall have the meanings indicated, unless the context otherwise requires:
A "silhouette" is an image of an object such that the periphery of the object is optically distinguishable from its background. For example, a silhouette of a dark sphere against a light background would appear as a circle, with the outer edge of the circle revealing the periphery of the sphere.

An object has a surface that is a "passive reflector" if the object's surface reflects incident light, but the object does not generate light.

A "calibrated sphere" is a sphere having fixed, known dimensions. A calibrated sphere may be useful, for example, in calibrating a CMM.

Various embodiments may be implemented in part in any conventional computer programming language. For example, some embodiments may be implemented in a procedural programming language (e.g., "C"), or in an object oriented programming language (e.g., "C++"). Other embodiments of the invention may be implemented as a pre-configured, standalong hardware element and/or as preprogrammed hardware elements (e.g., application specific integrated circuits (ASICs), programmable gate arrays (e.g., FPGAs), and digital signal processor integrated circuits (DSPs), or other related components.

The disclosed apparatus and methods (e.g., see the various flow charts described above) may be implemented in part as a computer program product for use with a computer system. Such implementation may include a series of computer instructions fixed either on a tangible, nontransitory medium, such as a computer readable medium. The series of computer instructions can embody all or part of the functionality previously described herein with respect to the system. For example, some embodiments may be implemented in part by a processor (e.g., a microprocessor integrated circuit; digital signal processor integrated circuit) executing, or controlled by, instructions stored in a memory. The memory may be random access memory (RAM), read-only memory (ROM), flash memory or any other memory, or combination thereof, suitable for storing control software or other instructions and data.

Those skilled in the art should appreciate that such computer instructions can be written in a number of programming languages for use with many computer architectures or operating systems. Furthermore, such instructions may be stored in any memory device, such as semiconductor, magnetic, flash, optical or other memory devices, and may be transmitted using any communications technology, such as optical, infrared, microwave, or other transmission technologies.

Among other ways, such a computer program product may be distributed as a removable medium with accompanying printed or electronic documentation (e.g., shrink wrapped software), preloaded with a computer system (e.g., on system ROM or fixed disk), or distributed from a server or electronic bulletin board over the network (e.g., the Internet or World Wide Web). In fact, some embodiments may be implemented in part in a software-as-a-service model ("SAAS") or cloud computing model. Of course, some embodiments of the invention may be implemented as a combination of both software (e.g., a computer program product) and hardware. Still other embodiments of the invention are implemented as entirely hardware.

Claim 1:
A vision CMM (<NUM>) and a test artifact (<NUM>), the test artifact (<NUM>) being configured for calibrating the vision CMM (<NUM>), the test artifact comprising:
a base (<NUM>) configured to rest on a table of the CMM (<NUM>);
a calibrated standard (<NUM>) suspended from the base (<NUM>);
a convex background illumination surface (<NUM>);
the vision CMM (<NUM>) comprising a vision sensor (<NUM>), the vision CMM (<NUM>) being configured to cause the vision sensor (<NUM>) capturing a photograph of the calibrated standard (<NUM>) and to cause assessing the contrast between the calibrated standard (<NUM>) and its background;
wherein the convex background illumination surface (<NUM>) being suspended relative to the base (<NUM>) to be positionable such that the calibrated standard is between the background illumination surface (<NUM>) and the vision sensor (<NUM>) such that light from the background illumination surface (<NUM>) creates a silhouette of the calibrated standard (<NUM>) as seen from the vision sensor (<NUM>).