Thickness correction for video extensometer systems and methods

Abstract

To provide systems and methods for correcting for perspective calibration variations of a variable thickness specimen with a single camera extensometer in a video extensometer system.SOLUTION: In some examples, systems and methods disclosed herein compensate for a change between a reference characteristic, such as a calibration plane, and an actual physical characteristic, such as a testing plane associated with a surface of a test specimen, during a testing operation. In some examples, a correction value is applied to an output (e.g., measured dimensions of the imaged test specimen) to compensate for the difference between the reference characteristic and the physical characteristic.SELECTED DRAWING: Figure 1

Description

[Background technology]

[0001] Camera-based vision systems have been implemented as part of materials testing systems for measuring specimen strain. These systems collect one or more images of the specimen under test. These images are synchronized with other signals of interest for the test (e.g., specimen load, machine actuator / crosshead displacement, etc.). The images of the test specimen can be analyzed to identify and track the location of specific specimen features as the test progresses. Changes in the location of such features, such as specimen width, allow for the calculation of local specimen deformation, which in turn allows for the calculation of specimen strain.

[0002] Conventional systems employ cameras or other imaging systems to capture images from which to measure properties of the test sample. However, differences between the reference position and the actual position can lead to distorted readings and inaccurate measurements. Thus, a system that corrects for such errors is desirable. Summary of the Invention

[0003] Disclosed herein are systems and methods for compensating for perspective calibration variations caused by nominally different specimen thicknesses using a single camera extensometer in a video extensometer system. In some examples, the systems and methods compensate for changes during a test run between a reference characteristic, such as a calibration plane, and an actual physical characteristic, such as a test plane associated with the surface of the test specimen. In some examples, a correction value is applied to the output (e.g., the measured dimensions of the imaged test specimen) to compensate for the difference between the reference characteristic and the physical characteristic.

[0004] These and other features and advantages of the present invention will become apparent from the following detailed description taken in conjunction with the appended claims.

[0005] The benefits and advantages of the present invention will become readily apparent to those skilled in the art after reviewing the following detailed description and accompanying drawings. [Brief explanation of the drawings]

[0006] [Figure 1] FIG. 1 is a block diagram of an exemplary extensometer system according to aspects of the present disclosure.

[0007] [Figure 2] FIG. 2 illustrates an exemplary test sample measured in the extensometer system of FIG. 1 according to an embodiment of the present disclosure.

[0008] [Figure 3] FIG. 2 is a block diagram of another view of the exemplary extensometer system of FIG. 1 according to an embodiment of the present disclosure.

[0009] [Figure 4] FIG. 2 is a block diagram of calibration and testing operations in the exemplary extensometer system of FIG. 1 according to an aspect of the present disclosure.

[0010] [Figure 5] FIG. 1 illustrates a captured image of a test sample according to an aspect of the present disclosure.

[0011] [Figure 6] FIG. 2 is a block diagram of an exemplary embodiment of the extensometer system of FIG. 1 according to aspects of the present disclosure.

[0012] [Figure 7] 1 is a flowchart representing example machine-readable instructions for correcting brightness distortion of a test specimen in an extensometer system, according to aspects of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0013] The drawings are not necessarily to scale. Where appropriate, like or identical reference numbers are used to refer to like or identical components.

[0014] The present disclosure describes a system and method for compensating for perspective calibration variations in samples of variable thickness using a single camera extensometer in a video extensometer system.

[0015] Disclosed examples relate to systems and methods for compensating for variations between a reference characteristic, such as a calibration plane, and an actual physical characteristic, such as a test plane, associated with a surface of a test specimen during a test operation. In some examples, one or more predetermined geometric variables or characteristics (e.g., measured, calculated, and / or accessed via a list) of the test specimen (e.g., specimen width, thickness, edge position, curvature, etc.) are compared against observed physical geometric variables to determine differences between the predetermined physical variables and the observed physical variables.

[0016] In some instances, this difference is a correction value that is applied to the output (eg, the measured dimensions of the imaged test sample) to compensate for the difference between the reference characteristic and the physical characteristic.

[0017] In some examples, the reference geometric variable is determined during a calibration process. For optical measurement systems, this is a calibration relationship between the apparent optical size of the geometric variable (e.g., width, thickness, edge position, curvature, and / or strain) measured and / or determined using sample markers of other physical characteristics captured by an image sensor and the value associated with the geometric variable employed by the extensometer measurement system. In some examples, the system continuously or periodically measures the geometric variable throughout the testing process. Thus, changes to the geometric variable, and thus the correction values, can be updated during the testing process.

[0018] Some conventional testing systems employ camera-based vision systems to capture information (e.g., measurements of one or more properties or geometric variables) during the materials testing process (e.g., to determine strain of the test specimen). Such systems may capture multiple images of the test specimen and synchronize these images with other information related to the testing process (e.g., specimen load, machine actuator / crosshead displacement, etc.). The images of the test specimen may then be analyzed via one or more algorithms to identify and / or locate specific features of the test specimen and further track such features as the testing operation progresses. Changes in the absolute and / or relative locations of such features allow local specimen deformation, and thus specimen strain, to be calculated.

[0019] The sample features of interest may consist of markings on the surface of the test sample that are visible to the camera. For example, a processor may analyze the image to determine the location and / or shape of the markings (and their changes) and track the movement of these marks relative to one another during testing. For example, multiple markings may be present on the front surface of the sample, such as pair groupings for determining gage-length-based strain measurements (axial marks, lateral marks, etc.), or pseudo-random speckle patterns used in digital image correlation (DIC) techniques. An alternative set of features that may be of interest for determining lateral sample strain are the edges of the test sample.

[0020] For a single-camera measurement system, a calibration process can be performed on a selected calibration plane positioned a predetermined distance from the image sensor. The calibration process establishes a relationship between one or more characteristics (e.g., size, position, width, etc.) captured by the imaging device and one or more physical characteristics (e.g., determined in physical coordinates) on the calibration plane.

[0021] Such a calibration process may employ a calibration reference device placed on a calibration plane. The reference device includes predetermined physical characteristics with known geometric dimensions associated with covering some or all of the field of view (FOV) of interest. The calibration process allows an image of the calibration device to be captured and compared to the known calibration device geometry, where a transfer function is established to transform the coordinates of the image from a pixel coordinate system to a real-world physical coordinate system.

[0022] Conventional video extensometer systems track and measure the dimensions and / or relative locations of markings on the surface of a test specimen. During the testing process, image processing algorithms are executed (e.g., via a processor in the video extensometer system) to determine the locations of the markings on the surface of the specimen. Based on the determined locations, the processor can calculate the initial specimen gage length and the instantaneous change in the specimen gage length (e.g., axial and / or lateral strain) from the value(s) at the start of the test specimen. The accuracy with which a video extensometer system can measure the absolute and / or relative position and / or location changes of the markings depends, at least in part, on whether the specimen surface is coplanar with the initial calibration plane. The difference between the position of the measurement plane (corresponding to the surface of the test specimen) and the position of the calibration plane (corresponding to the reference plane) will result in measurement error (e.g., perspective error). A greater deviation between the measurement plane and the reference plane will result in a larger measurement error.

[0023] In some examples, multiple test samples are subjected to a calibration process followed by a testing process, where each sample has a different thickness, resulting in a different distance between the surface of the sample and the imaging device for each sample.

[0024] In a test device employing a symmetrical specimen grip, an increase in thickness z from a first specimen to a second specimen results in the second specimen being closer to the video extensometer by z / 2. If the first specimen is positioned coplanar with the calibration plane, the second specimen will be closer to the imaging device by z / 2, with a consequent decrease in the optical working distance (e.g., relative to the calibrated working distance). As a result, measurements of the second specimen based on images captured by the imaging device will appear larger than the actual physical properties of the second specimen being measured.

[0025] Such perspective errors can be more problematic, for example, in material testing applications where absolute dimensional measurements are desired, as opposed to testing applications where measurements are used to determine proportional (e.g., ratiometric) strain. In determining proportional strain, perspective errors result in similar proportional errors in the initial gauge length measurement and / or strain-displacement measurement. Because strain is calculated as displacement across the gauge length, perspective errors are present in both the numerator and denominator and therefore cancel out.

[0026] However, in situations where absolute dimensions are of interest (e.g., when an extensometer is being used to measure strain displacement, but the initial gage length is measured using a different system or process), perspective errors can become significant. For example, as the working distance of the test specimen increasingly deviates from the initial calibrated working distance, perspective errors also increase. This is especially true when the thickness of the test specimen varies significantly, making it difficult to select a single representative calibration plane that covers all thicknesses.

[0027] Previous systems have attempted to mitigate these issues through various techniques, each with significant drawbacks. One option is a calibration plane positioned at the average or mid-plane of all test planes of interest so that perspective measurement error is optimized across specimens with different thicknesses. Another option is to make physical adjustments to the extensometer's test specimen mounting position to maintain a single working distance equal to the calibration plane to compensate for different test specimens. Yet another option is to use telecentric optics, which are significantly less susceptible to out-of-plane perspective error but are more expensive and have a more limited field of view. In yet another option, multiple cameras can be employed to obtain perspective information from various angles, which can be incorporated into the calibration and specimen measurement process.

[0028] However, existing solutions for mitigating perspective errors encountered in video extensometer systems all have drawbacks. For example, using an average distance relative to a calibration plane and / or using less accurate measurement equipment inevitably results in less accurate measurements. Making manual adjustments to the extensometer mounting position to compensate for different sample thicknesses is time-consuming and requires the operator to remember to consistently make multiple different adjustments for each sample based on its individual thickness. Furthermore, such adjustments are difficult to automate.

[0029] Telecentric optics are large, heavy, expensive, and have a limited field of view (FOV), therefore video extensometer systems utilizing multiple cameras are expensive, complex, and require extensive 3D calibration processes and equipment.

[0030] The disclosed systems and methods mitigate systematic and deterministic errors in video extensometers that arise from deviations in the measurement plane relative to the calibration plane. In some examples, the errors are corrected in real time during the testing process. The systems are implemented in single-camera extensometer systems and configured to automatically identify and adjust for deviations. Advantageously, the disclosed systems and methods avoid the need to move the equipment or sample to test samples of different thicknesses (to maintain a calibrated working distance) and / or recalibrate the video extensometer system for each different working distance resulting from samples of various thicknesses.

[0031] As disclosed herein, a video extensometer system is configured to perform optical width measurements of a test sample. In some examples, the edge of a substantially opaque test sample is measured based on the level of brightness contrast between the test sample and a back screen. For example, the test sample is fixed within a testing machine and positioned in front of an illuminated (e.g., actively or passively illuminated) back screen. An imaging device is positioned to observe the surface of the test sample facing the camera. This surface is located near the focal plane of the imaging device optics (see, e.g., FIG. 3). With this arrangement, the test sample is viewed and imaged by the imaging device as a dark silhouette feature because it is positioned in front of the brightly illuminated back screen (see, e.g., FIG. 5).

[0032] For example, when placed between an illuminated back screen and an imaging device, the test sample's sharply focused dark silhouette is sharp, and edge shapes and features are clearly defined when imaged in front of the illuminated back screen. In some examples, the test sample is made from a material with a higher transparency. Such a translucent test sample can absorb a portion of the light from the light source sufficient to provide a measurable level of contrast between the test sample and the back screen.

[0033] As described herein, material testing systems, including those that apply tension, compression, and / or torsion, include one or more components that incur a displacement and / or load to apply and / or measure stress to a test sample. In some examples, a video extensometer system is used for sample strain testing. This testing can include one or more of collecting high-resolution images, providing these images to an image processor, analyzing the images to identify one or more sample characteristics that correspond to displacement or strain values, and generating an output corresponding to these characteristics. In disclosed examples, the identified characteristics (e.g., width) from one or more collected images are compared to one or more sources, such as a list of thresholds or images collected previously (i.e., before testing). In some examples, the values of the identified characteristics can be applied to one or more algorithms to generate an output corresponding to a displacement or strain value associated with the test sample.

[0034] Video processing employing extensometers can include an external machine vision imaging device connected to a processing system or computing platform and / or video processing hardware, and can use software and / or hardware to convert data from the camera into an electrical signal, or can have a software interface compatible with the materials testing system.

[0035] As disclosed herein, an imaging device employing a camera-based image capture (e.g., vision or video) system is implemented in a materials testing system to measure strain in a test specimen. Such a system collects multiple images of the specimen under test (i.e., during the testing process). These images are synchronized with other signals of interest for the test (e.g., specimen load, machine actuator and / or crosshead displacement, etc.). The specimen images are analyzed by an algorithm (e.g., in real time and / or post-test) to identify and track the location of specific specimen characteristics as the test progresses. For example, changes in the location, size, shape, etc. of such characteristics allow for calculation of the deformation of the test specimen, which in turn allows for analysis and calculation of specimen strain.

[0036] Characteristics such as specimen width can be captured via an imaging device, and the captured image can be transmitted to a processing system. Image analysis can be performed by the extensometer system (e.g., via the processing system) to determine the first, i.e., initial, position and / or location of the specimen width(s) in order to track changes in the width(s) as the test progresses.

[0037] An image processing algorithm then determines the edges of the sample, calculates the width of the sample, and tracks the change in sample width (ie, lateral strain) compared to the initial width at the start of the test.

[0038] Thus, the systems and methods disclosed herein compensate for changes during a test operation between a reference characteristic, such as a calibration plane, and an actual physical characteristic, such as a test plane associated with the surface of a test specimen. In some examples, a correction value is applied to an output (e.g., a measured dimension of an imaged test specimen) to compensate for the difference between the reference characteristic and the physical characteristic.

[0039] In the disclosed example, a system for correcting thickness [claim to be added by MHM after final review]

[0040] Referring now to the figures, Figure 1 illustrates an exemplary extensometer system 10 for measuring changes in one or more properties of a test specimen 16 undergoing mechanical property testing. This exemplary extensometer system 10 may be connected to, for example, a testing system 33 capable of mechanically testing the test specimen 16. The extensometer system 10 may measure and / or calculate changes in the test specimen 16 undergoing, for example, a compressive strength test, a tensile strength test, a shear strength test, a flexural strength test, a flexural strength test, a tear strength test, a peel strength test (e.g., adhesive bond strength), a torsional strength test, and / or any other compressive and / or tensile test. Additionally or alternatively, the material extensometer system 10 may perform dynamic testing.

[0041] According to the disclosed example, the extensometer system 10 may include a test system 33 that manipulates and tests the test specimen 16, and / or a computing or processing system 32 communicatively coupled to the test system 33, a light source, and / or an imaging device, as further shown in Figure 6. The test system 33 applies a load to the test specimen 16 and measures mechanical properties of the test, such as the displacement of the test specimen 16 and / or the force applied to the test specimen 16.

[0042] The extensometer system 10 includes a remote and / or integrated light source 14 (e.g., an LED array) and / or a reflective back screen 18 that illuminates the test specimen 16. The extensometer system 10 includes a processing system 32 (see also FIG. 6 ) and a camera or imaging device 12. In some examples, the light source 14 and imaging device 12 are configured to transmit and receive in infrared (IR) wavelengths, although other illumination sources and / or wavelengths are equally applicable. In some examples, one or both of the light source 14 or imaging device 12 include one or more filters (e.g., polarizing filters), one or more lenses, etc. In some examples, a calibration routine (e.g., a two-dimensional calibration routine) is performed to identify one or more characteristics of the test specimen 16, one or more markers 20 (including the pattern of markers), and / or other parameters of the test specimen 16 that may be used.

[0043] In some examples, the back screen 18 is configured to reflect light from the light source 14 back toward the imaging device 12. For example, the surface of the back screen 18 can be configured with properties that enhance reflection and / or direct reflected light toward the imaging device. Properties can include the shape of the back screen 18 (e.g., a parabolic configuration) and / or treatments that enhance reflection (e.g., application of corner cube reflectors, reflective materials, etc.). Additionally or alternatively, the filter 30 can be disposed and / or applied to the surface to increase the amount of reflection and / or direct reflected light in desired directions and / or direct reflected light of desired wavelengths. In some examples, the filter 30 is configured as a collimating filter to provide as much reflected light as possible toward the imaging device 12 and away from other nearby components.

[0044] In the disclosed example, the computing device 32 can be used to configure the testing system 33, control the testing system 33, and / or receive measurement data (e.g., transducer measurements such as force and displacement) and / or test results (e.g., peak force, break displacement, etc.) from the testing system 33 for processing, display, reporting, and / or any other desired purpose. The extensometer system 10 connects to the testing system 33 and software using standard interfaces, including Ethernet, analog encoder, or SPI. This allows the device to be plugged into and used with existing systems without the need for dedicated integration software or hardware. The extensometer system 10 provides real-time axial and transverse encoder information or analog information to the materials testing machine 33. The real-time video extensometer 10 and materials testing machine 33 exchange real-time test data, including extension / strain data, with the external computer 32, which can be configured, via wired and / or wireless communication channels. The extensometer system 10 provides measurements and / or calculations of stretch / strain data captured from a test specimen 16 undergoing testing in a materials testing machine 33 and further provides the stress data and stretch / strain data to a processor 32 .

[0045] As disclosed herein, the captured image is input from the imaging device to a processor 32. The processor uses one or more algorithms and / or look-up tables to calculate multi-axial stretch / strain values for the test specimen 16 (i.e., the change or percentage change in target distance calculated by image monitoring of markers 20 affixed to the test specimen 16). After calculation, this data can be stored in memory or output to a network and / or one or more display devices, I / O devices, etc. (see also FIG. 6).

[0046] FIG. 2 illustrates an exemplary test specimen 16 that may be measured in the extensometer system 10 of FIG. 1. For example, one or more markings are affixed to a surface 28 that faces the light source 14 and imaging device 12. The grippers 26 are configured to be placed within the grips of a testing system 33 (see also FIG. 6) and apply a force to the test specimen 16. For example, a cross-member loader applies a force to the test specimen 16 while the grips grip or are otherwise connected to the testing system 33. A force applicator, such as a motor, moves the crosshead relative to the frame, applying a force to the test specimen 16 as indicated by the double-headed arrow 34. The force 34 that moves the grippers 26 apart can stretch the test specimen 16, causing the markings to move from a first position 20A to a second position 20B. Additionally or alternatively, the marking may change shape or size, which can also be measured by processing system 32 by viewing the captured image. Force 34 can also move the edge of the test specimen from first position 22A to second position 22B. For example, in the first or initial position, the edge has width 24A and decreases to width 24B upon application of force 34.

[0047] Based on the captured images, the processing system 33 is configured to perform stretching / straining in the measurement process. For example, to detect stretching / straining in the test specimen 16, the processing system 33 monitors the images provided via the imaging device 12. When the processing system 33 identifies a change in the relative position between two or more of the markers and / or edges of the test specimen 16 (e.g., compared to an initial location at the start of crosshead movement), the processing system 33 measures the amount of change and calculates the amount of stretching and / or strain in the test specimen 16. As disclosed herein, the markers are configured to reflect light from a light source to the camera, while the back screen reflects the light to create a dark silhouette for edge analysis.

[0048] As disclosed herein, the video extensometer system 10 is configured to perform optical width measurements on an opaque test specimen 16. The imaging device 12 is positioned to observe a surface 28 of the test specimen 16 facing the imaging device 12. The surface 28 is located near the focal plane of the imaging device optics (see, for example, FIG. 3). With this arrangement, the test specimen 16 is viewed and imaged by the imaging device 12 as a dark silhouette feature, as it is positioned in front of a brightly illuminated back screen 18.

[0049] As shown in Figure 3, the video extensometer system 10 is arranged to measure one or both of axial strain (based on changes in markers 20 and / or marker patterns on the front surface 28 of the test specimen 16) and transverse strain (calculated from changes in the width of the specimen 16). The components of the video extensometer system 10 are shown in a top perspective view in Figure 3, where the location of each component relative to the other components is schematic. As shown, the components include an imaging device 12 (e.g., a video camera) configured to capture one or more images of the test specimen 16 during physical testing (e.g., at regular intervals, continuously, and / or based on one or more thresholds associated with time, force, or other suitable test characteristic).

[0050] One or more light sources 14 emit light 36 to illuminate a front surface 28 of the test sample 16 and a screen 18 positioned toward the back surface of the test sample 16 opposite the light sources 14. In some examples, the light source(s) 14 are positioned to direct light off-axis (e.g., in an upward, sideways, and / or downward direction as shown from the top view perspective of FIG. 3 ) and are angled to illuminate the front surface 28 of the test sample 16 and / or the back screen 18.

[0051] As shown, a passive (i.e., no active illumination source) back screen 18 is positioned behind the test specimen 16 and is designed to be reflective and of a suitable size to provide a uniformly lit background for the video extensometer imaging device 12. As shown in FIG. 3, light 36 incident on the back screen 18 is reflected back as light 40 directed toward the imaging device 12. In some examples, an actively illuminated back screen is used, the brightness level of which can be adjusted by the processing system 32. As shown, the imaging device 12 and test specimen 16 are positioned at a focal distance 39, which can remain stationary, be predetermined, and / or vary during the testing process. Light from the back screen 18 creates a dark silhouette of the test specimen 16, allowing the imaging device 12 to capture an image of the edge 22 and any changes to the edge during the testing process.

[0052] The test specimen 16 is positioned between the imaging device 12 and the back screen 18. The test specimen 16 features suitable markings 20 on a forward-facing surface 28 of the test specimen 16. Analysis of one or more images associated with the video extensometer system 10 is performed via a processing system 32 to implement identification algorithms that allow both the markings 20 on the test specimen 16 and the edges 22 of the test specimen to be continuously tracked and measured during the testing process.

[0053] For example, when placed between the illuminated back screen 18 and the imaging device 12, the test sample 16 will stand out as a sharply focused, dark silhouette when imaged in front of the illuminated back screen 18, revealing the shape and characteristics of the edge 22. However, the perceived position of the test sample 16 may be offset by the test sample's variable thickness, making it difficult to perform highly accurate measurements. For example, even after a calibration process, the calibration plane is determined according to the thickness of the sample used to calibrate the system 10. When another test sample having a different thickness is subjected to such a calibrated system, the reference plane (e.g., the calibration plane) will be offset based on the thickness, resulting in imaging errors.

[0054] 4, the video extensometer system 10 is configured to measure one or more properties of a test specimen 17 having a width W. For example, the one or more properties may include one or more of the shape or position of markings 21, the edge position of the test specimen 17, or the width of the test specimen 17.

[0055] When surface 29 of test specimen 17 is positioned on calibration plane 52, the perceived dimensions of one or more properties by the extensometer will be optically accurate and faithfully reflect the physical properties of test specimen 17. However, as shown in FIG. 4 , when test specimen 17 is displaced from calibration plane 52 by an amount dz, the perceived dimensions of test specimen 17 relative to the video extensometer will contain an error such that width W is observed as W0. In other words, due to the thickness T of test specimen 17 and the offset of surface 29 relative to calibration plane 52, the actual physical dimension W will be inaccurately captured by imaging device 12.

[0056] For example, during the calibration process, distance Z cal is determined as the distance between the aperture of imaging device 12 and calibration plane 52. When a test process is to be performed on test sample 17, the distance between surface 29 of test sample 17 and calibration plane 52 is measured and defined by dz. However, the width W of test sample 17 is determined by subtracting the thickness T from the distance Z by the distance dz.cal , resulting in a displacement from , which will appear magnified in the captured image. Thus, imaging device 12 will perceive the width of test sample 17 as W0 instead of its true physical width W. As disclosed herein, the true physical width W of the test sample can be calculated by correcting for the displacement as provided in Equation 1 below: Formula 1: w=w0(z cal +δz) / z cal =w0(1+δz / z cal ), Or, correction coefficient c=1+δz / z cal Using w=cw0 If dz is negative (e.g., if the measurement plane 54 is closer to the video extensometer system 10 than the calibration plane 52), each measured property is magnified. To recover the true physical specimen width, this is reflected by a correction factor less than 1.

[0057] If the video extensometer system 10 is calibrated in a known, fixed plane relative to the centerline 50 of the imaging device 12, and the sample is fixed via one or more grips to ensure that the sample centerline remains on the centerline 50, the offset of the sample surface on the measurement plane 54 caused by variations in nominal sample thickness can be calculated to be one-half the sample thickness. Therefore, a compensation factor c can be calculated for the particular sample being tested prior to each test using sample thickness information entered by the operator into the materials testing software. The compensation factor c may be employed by the processor 202 to calculate the dimension of interest, and the output from the processor represents the corrected measurement. Thus, perspective errors due to non-identical test and calibration planes have been eliminated.

[0058] 4, the calibration plane 52 is coplanar with the center of the thickness T. In some examples, the measured distance Z between the imaging device 12 and the surface 29 of the test sample 17 mea is the calibration distance Z calminus one-half the thickness T.

[0059] As shown in the illustrated example, if the grips of the testing system maintain a central plane for loading the test specimen, the correction distance dz corresponds to one-half the thickness T. However, if the central plane is variable (e.g., one or more of the front or rear faces of the loader and / or grips are fixed), the system may adjust the correction distance dz according to known parameters (e.g., the offset distance relative to the fixed plane and / or the thickness of the test specimen) and / or the measurement distance Z. mea The difference may be determined by employing one or more sensors to determine the corrected distance dz, etc.

[0060] For example, system 10 may employ sensor 60 to measure one or more characteristics of the configuration of test system 10. For example, sensor 60 may measure distance Z between imaging device 12 and surface 29 of test sample 17. mea One or more technologies (e.g., infrared (IR) light, light emitting diode (LED) output, ultrasonic sensors, structured light imaging, time-of-flight calculations, laser-based sensors, etc.) may be employed to sense . The results may be transmitted from sensor 60 to a computing device (e.g., via an interface to computing device 32) for analysis. The computing device may then generate a correction factor c based on the difference between calibration plane 52 and measurement plane 54 corresponding to the correction distance dz.

[0061] In the illustrated example, imaging device 12 is a single-view camera having a single optical axis 50. In some examples, two or more imaging devices may be employed, which may be aligned and / or positioned at different viewing angles of test sample 17. By employing a stereo imaging configuration, measurement variables related to perspective and / or depth of multiple dimensions of test sample 17 may also be used to further calibrate and / or measure properties of test sample 17.

[0062] In some examples, the measurements and / or positions of the one or more edges are provided in pixel coordinates captured by the imaging device 12. Additionally or alternatively, the measurements and / or positions of the one or more edges are provided in other standard coordinate systems / units, such as meters. In such examples, a calibration process may be implemented to determine the absolute and / or relative placement and / or dimensions of the test specimen within the test system prior to measurements, and similar coordinate systems / units may be employed during the testing process.

[0063] While FIG. 5 shows a captured image of test sample 16, the following description is equally applicable to other test samples, such as test sample 17. As shown, when imaged in front of an illuminated back screen 18, test sample 16 stands out as a sharply focused, dark silhouette, highlighting the shape and characteristics of edge 22. If the testing process were limited to comparing multiple images over time, perceived error due to the offset between the measurement and calibration planes would have only a limited impact. However, to capture absolute measurements of the imaged test sample, the offset distance between the measurement and calibration planes is corrected for by the systems and methods disclosed herein.

[0064] FIG. 6 is a block diagram of the exemplary extensometer system 10 of FIG. 1. As shown in FIG. 1, the extensometer system 10 includes a testing system 33 and a computing device 32. The exemplary computing device 32 may be a general-purpose computer, a laptop computer, a tablet computer, a mobile device, a server, an all-in-one computer, and / or any other type of computing device. The computing device 32 of FIG. 6 includes a processor 202, which may be a general-purpose central processing unit (CPU). In some examples, the processor 202 may include one or more dedicated processing devices, such as an FPGA, a RISC processor with an ARM core, an image processing device, a digital signal processor, and / or a system-on-chip (SoC). The processor 202 executes machine-readable instructions 204, which may be stored locally at the processor (e.g., in an internal cache or on the SoC), in random access memory 206 (or other volatile memory), in read-only memory 208 (or other non-volatile memory, such as flash memory), and / or in a mass storage device 210. The exemplary mass storage device 210 may be a hard drive, a solid-state storage drive, a hybrid drive, a RAID array, and / or any other mass data storage device. The bus 212 enables communication between the processor 202, the RAM 206, the ROM 208, the mass storage device 210, the network interface 214, and / or the input / output interface 216.

[0065] An example network interface 214 includes hardware, firmware, and / or software that connects computing device 201 to a communications network 218, such as the Internet. For example, network interface 214 may include IEEE 202.X compliant wireless and / or wired communications hardware for sending and / or receiving communications.

[0066] 6 includes hardware, firmware, and / or software that couples one or more input / output devices 220 to the processor 202 to provide input to and / or output from the processor 202. For example, the I / O interface 216 may include an image processing device that interfaces with a display device, a Universal Serial Bus port that interfaces with one or more USB-compliant devices, FireWire®, Fieldbus, and / or any other type of interface. The example extensometer system 10 includes a display device 224 (e.g., an LCD screen) coupled to the I / O interface 216. Other example I / O device(s) 220 may include a keyboard, keypad, mouse, trackball, pointing device, microphone, audio speaker, display device, optical media drive, multi-touch touchscreen, gesture recognition interface, magnetic media drive, and / or any other type of input and / or output device.

[0067] Computing device 32 can access non-transitory machine-readable medium 222 via I / O interface 216 and / or I / O device(s) 220. Examples of machine-readable medium 222 in Figure 6 include optical disks (e.g., compact discs (CDs), digital versatile / video discs (DVDs), Blu-ray® discs, etc.), magnetic media (e.g., floppy disks), portable storage media (e.g., portable flash drives, Secure Digital (SD) cards, etc.), and / or any other type of removable and / or installed machine-readable medium.

[0068] The extensometer system 10 further includes a test system 33 coupled to the computing device 32. In the example of Figure 6, the test system 33 is coupled to the computing device via an I / O interface 216, such as a USB port, a Thunderbolt port, a FireWire (IEEE 1394) port, and / or any other type of serial or parallel data port. In some examples, the test system 33 is coupled to the network interface 214 and / or the I / O interface 216 via a wired or wireless connection (e.g., Ethernet, Wi-Fi, etc.), either directly or via a network 218.

[0069] Test system 33 includes a frame 228, load cells 230, displacement transducers 232, a cross-member loader 234, a material fixture 236, and a control processor 238. Frame 228 provides rigid structural support for the other components of test system 33 that perform the test. Load cells 230 measure the force applied to the material under test by cross-member loader 234 via grips 236. Cross-member loader 234 applies the force to the material under test, while material fixtures 236 (also referred to as grips) grip or otherwise couple the material under test to cross-member loader 234. An exemplary cross-member loader 234 includes a motor 242 (or other actuator) and a crosshead 244. As used herein, "crosshead" refers to the component of a materials testing system that applies directional (axial) and / or rotational forces to a sample. The materials testing system can have one or more crossheads, and the crosshead(s) can be positioned in any suitable position and / or orientation in the materials testing system. Crosshead 244 couples material fixture 236 to frame 228, and motor 242 moves the crosshead relative to the frame to position material fixture 236 and / or apply force to the material under test. Exemplary actuators that can be used to provide force and / or movement of the components of extensometer system 10 include electric motors, pneumatic actuators, hydraulic actuators, piezoelectric actuators, relays, and / or switches.

[0070] Although the exemplary test system 33 uses a motor 242, such as a servo motor or a direct drive linear motor, other systems may use different types of actuators, such as hydraulic actuators, pneumatic actuators, and / or any other type of actuator, based on the requirements of the system.

[0071] Exemplary grips 236 include compression platens, jaws, or other types of fixtures depending on the mechanical property and / or material being tested. Grips 236 can be manually configured, controlled via manual input, and / or automatically controlled by control processor 238. Crosshead 244 and grips 236 are operator accessible components.

[0072] The extensometer system 10 may further include one or more control panels 250 that include one or more mode switches 252. The mode switches 252 may include buttons, switches, and / or other input devices located on the operator control panel. For example, the mode switches 252 may include a button that controls the motors 242 to jog (e.g., position) the crosshead 244 to a particular position on the frame 228, a switch (e.g., a foot switch) that controls the grip actuators 246 to open or close the pneumatic grips 248, and / or any other input device that controls the operation of the test system 33.

[0073] The example control processor 238 communicates with the computing device 32, for example, receiving test parameters from the computing device 32 and / or reporting measurements and / or other results to the computing device 32. For example, the control processor 238 may include one or more communication or I / O interfaces that allow communication with the computing device 32. The control processor 238 may control the cross member loader 234 to increase or decrease the applied force, may control the fixture(s) 236 to grip or release the material under test, and / or may receive measurements from the displacement transducers 232, load cells 230, and / or other transducers.

[0074] The example control processor 238 is configured to implement the stretch / strain measurement process as the test specimen 16 is undergoing testing in the testing system 33. For example, to detect stretch / strain in the test specimen 16, the control processor 238 monitors images provided via the imaging device 12. When the control processor 238 identifies a change in the location and / or position of the edge 22 of the test specimen 16 (e.g., compared to an initial location at the start of travel of the crosshead 244), the control processor 238 measures the amount of change and calculates the amount of stretch and / or strain in the test specimen 16. For example, real-time video provided by the imaging device 12 captures the absolute position of the edge 22 and monitors their relative movement over several images to calculate the stretch / strain in real time. Stress and strain data are exchanged between the real-time video extensometer 10, the testing system 33, and the processing system 32 and are typically organized and displayed via the display device 224.

[0075] 7 is a representative flowchart of exemplary machine-readable instructions 300 that may be executed by the processing system 32 of FIGS. 1 and 6 to compensate for variable thickness of a test specimen in an extensometer system. In block 302, a calibration distance between an imaging device (e.g., imaging device 12) and a calibration plane associated with a test system (e.g., test system 10) is determined in a processor (e.g., processor 202, 238).

[0076] At block 304, a thickness of a test sample (e.g., test sample 16, 17) is received, such as from a remote computing device via I / O interface 216 and / or from user input via I / O device(s) 220. At block 306, a correction factor is calculated based in part on the calibration distance and a correction distance corresponding to the difference between the measurement plane and the calibration plane.

[0077] At block 308, one or more of markings on the surface of the test sample or a silhouette of the test sample are imaged to measure one or more properties of the test sample. In some examples, the surface of the test sample corresponds to a measurement plane, and the measurement plane is offset from the calibration plane by a measurement distance. At block 310, one or more corrected properties are generated by applying a correction factor to the measurements of the one or more properties of the test sample.

[0078] In some examples, the system is configured to compensate for angular offset of the imaging device relative to the test sample (and / or test platform, load structure, etc.). For example, the imaging device may be positioned at an angle relative to the orientation of the test sample, which may produce a similarly offset image. The system may adjust the image orientation by a corresponding, opposite angular offset to render an image without the perceived angle. This may be achieved by positioning the imaging device by a known angular offset and applying the corresponding angular value to the image.

[0079] In some examples, the system calculates the angular offset, such as via one or more sensors. After receiving the test sample image, a property (e.g., width) associated with one or more positions (e.g., one or more horizontal positions) along the test sample is measured, and an error value for the property is calculated based on the angle. The measurement may be adjusted based on the error value to provide a corrected measurement of the property(ies).

[0080] The methods and systems can be implemented in hardware, software, and / or a combination of hardware and software. The methods and / or systems can be implemented centrally in at least one computing system, or distributed, with different elements distributed across several interconnected computing systems. Any kind of computing system or other apparatus adapted to perform the methods described herein is suitable. A typical combination of hardware and software can include a general-purpose computing system, along with a program or other code that, when loaded and executed, controls the computing system to perform the methods described herein. Another typical embodiment can include an application-specific integrated circuit or chip. Some embodiments can include a non-transitory machine-readable (e.g., computer-readable) medium (e.g., a flash drive, an optical disk, a magnetic storage disk, etc.) that stores one or more lines of code executable by a machine, thereby causing the machine to perform a process as described herein. As used herein, the term "non-transitory machine-readable medium" is defined to include all types of machine-readable storage media and to exclude propagating signals.

[0081] As used herein, the terms "circuit" and "circuitry" refer to physical electronic components (i.e., hardware) and any software and / or firmware ("code") that can comprise, be executed by, and / or otherwise be associated with hardware. As used herein, for example, a particular processor and memory can include a first "circuit" when executing a first one or more lines of code, and a second "circuit" when executing a second one or more lines of code. As used herein, "and / or" means any one or more of the items in the list connected by "and / or." As an example, "x and / or y" means any element of the triplet {(x), (y), (x, y)}. In other words, "x and / or y" means "one or both of x and y." As another example, "x, y, and / or z" means any element of the seven-element set {(x), (y), (z), (x,y), (x,z), (y,z), (x,y,z)}. In other words, "x, y, and / or z" means "one or more of x, y, and z." As used herein, the term "exemplary" means serving as a non-limiting example, instance, or illustration. As used herein, the term "for example" begins a list of one or more non-limiting examples, instances, or illustrations. As used herein, circuitry is "operable" to perform a function whenever it includes the hardware and code (if either is necessary) necessary to perform that function, regardless of whether implementation of that function is disabled or enabled (e.g., by a user-configurable setting, factory trim, etc.).

[0082] Although the present methods and / or systems have been described with reference to certain specific embodiments, those skilled in the art will recognize that various modifications and equivalents may be substituted without departing from the scope of the present methods and / or systems. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present disclosure without departing from the scope of the present disclosure. For example, the example systems, blocks, and / or components disclosed may be combined, divided, rearranged, and / or otherwise modified. Therefore, the present methods and / or systems are not limited to the particular embodiments disclosed. Instead, the present methods and / or systems include all embodiments falling within the scope of the appended claims, both literally and under the doctrine of equivalents. The inventions disclosed herein include the following: [Aspect 1] 1. A method for correcting for variable thickness of a test specimen, the method comprising: determining, via a processing system, a calibration distance between the imaging device and a calibration plane associated with the test system; receiving a thickness of the test specimen via the interface; calculating, via the processing system, a correction factor based in part on the calibration distance and a correction distance corresponding to a difference between the measurement plane and the calibration plane; imaging one or more of a marking on a surface of the test sample or a silhouette of the test sample via the imaging device to measure one or more properties of the test sample, wherein the surface of the test sample corresponds to the measurement plane, and the measurement plane is offset from the calibration plane by a measurement distance; applying, via the processor, a correction factor to the measurements of the one or more properties of the test sample to generate one or more corrected properties; A method comprising: [Aspect 2] 2. The method of claim 1, further comprising: calculating, via the processing system, the measured distance between the imaging device and the surface of the test specimen as equal to the sum of the calibration distance and an offset of the calibration plane relative to a centerline of the test system, minus one-half the thickness. [Aspect 3] 2. The method of claim 1, further comprising calculating, via the processing system, the measured distance between the imaging device and the surface of the test sample as equal to the calibration distance minus one-half the thickness. [Aspect 4] 2. The method of claim 1, wherein the calibration plane is coplanar with the center of thickness. [Aspect 5] 2. The method of claim 1, wherein the correction distance corresponds to one-half of the thickness. [Aspect 6] sensing the measured distance between the imaging device and the surface of the test specimen via a sensor; transmitting the distance measurements from the sensor to the processor via the interface; generating, via the processor, the correction factor based on a difference between the calibration plane and the measurement plane corresponding to the measurement distance; 2. The method of embodiment 1, further comprising: [Aspect 7] 7. The method of embodiment 6, wherein the sensor is an infrared sensor, a light emitting diode sensor, an ultrasonic sensor, or a laser-based sensor. [Aspect 8] 2. The method of claim 1, wherein the one or more characteristics include one or more of a shape or position of the marking, an edge position of the test sample, or a width of the test sample. [Aspect 9] 2. The method of embodiment 1, wherein the correction factor is expressed in one of millimeters, inches, or pixels. [Aspect 10] 2. The method of claim 1, wherein the imaging device is a single-view camera. [Aspect 11] 1. A system for compensating for variable thickness of a test specimen, the system comprising: a test system for securing a test sample; a screen that provides illumination to silhouette the test sample; an imaging device positioned on an opposite side of the screen from the test sample and configured to capture an image of the test sample; 1. A processing system comprising: determining a calibration distance between an imaging device and a calibration plane associated with the test system; receiving a thickness of a test sample; calculating a correction factor based in part on the calibration distance and a correction distance corresponding to a difference between the measurement plane and the calibration plane; imaging one or more of a marking on a surface of the test sample or a silhouette of the test sample to measure one or more properties of the test sample, wherein the surface of the test sample corresponds to the measurement plane, and the measurement plane is offset from the calibration plane by a measurement distance; applying a correction factor to the measurements of the one or more properties of the test sample to generate one or more corrected properties; a processing system that performs the steps of: Including, the system. [Aspect 12] 12. The system of claim 11, wherein the test sample is positioned between an illuminated screen and the imaging device. [Aspect 13] The system of aspect 11, wherein the interface is one of a user interface or a network interface that communicates with a computing device or a sensor. [Aspect 14] 12. The system of claim 11, wherein the processor is located on a remote computing platform that communicates with one or more of the test system or the imaging device. [Aspect 15] 12. The system of claim 11, wherein the processor is integrated with one of the imaging device or the test system. [Aspect 16] 12. The system of claim 11, wherein the imaging device is a single-view camera. [Aspect 17] The system of aspect 11, wherein the imaging device is two or more cameras. [Aspect 18] 12. The system of claim 11, wherein the imaging device is configured to capture polarized or infrared light reflected from the screen or the test sample, and the screen reflects light to create a dark silhouette of the test sample for edge analysis. [Aspect 19] 1. A system for compensating for variable thickness of a test specimen, comprising: receiving an image from an imaging device of a test specimen during a testing process, the imaging device being positioned on an opposite side of a reflective screen from the test specimen; determining a calibration distance between the imaging device and a calibration plane associated with the test system; receiving a thickness of a test sample; calculating a correction factor based in part on the calibration distance and a correction distance corresponding to a difference between the measurement plane and the calibration plane; imaging one or more of a marking on a surface of the test sample or a silhouette of the test sample to measure one or more properties of the test sample, wherein the surface of the test sample corresponds to the measurement plane, and the measurement plane is offset from the calibration plane by a measurement distance; applying a correction factor to the measurements of the one or more properties of the test sample to generate one or more corrected properties; 11. A system including a processing system configured to: [Aspect 20] The system of aspect 1, further comprising a sensor that measures the measurement distance between the imaging device and the surface of the test sample, and the processor configured to generate the correction coefficient based on a difference between the calibration plane and the measurement plane corresponding to the measurement distance.

Claims

1. 1. A method for correcting for variable thickness of a test specimen, the method comprising: determining, via a processing system, a calibration distance between the imaging device and a calibration plane associated with the test system; receiving, via an interface, a thickness of a test sample, the thickness corresponding to a distance between a first surface and a second surface of the test sample, the test sample being positioned such that the first surface is a first distance from the imaging device and the second surface is a second distance from the imaging device; calculating, via the processing system, a correction factor based in part on the calibration distance and a correction distance corresponding to a difference between a measurement plane and the calibration plane, the correction distance corresponding to one-half of the thickness; imaging one or more of a marking on the first surface of the test sample or a silhouette of the test sample via the imaging device to measure one or more properties of the test sample, wherein the first surface of the test sample corresponds to the measurement plane, and the measurement plane is offset from the calibration plane by the correction distance; applying, via the processing system, a correction factor to the measurements of the one or more properties of the test sample to generate one or more corrected properties; A method comprising:

2. 10. The method of claim 1, further comprising: calculating, via the processing system, a measured distance between the imaging device and the first surface of the test specimen as equal to the sum of the calibration distance and an offset of the calibration plane relative to a centerline of the test system, minus one-half the thickness.

3. 10. The method of claim 1, further comprising calculating, via the processing system, a measured distance between the imaging device and the first surface of the test specimen as equal to the calibrated distance minus one-half the thickness.

4. The method of claim 1 , wherein the calibration plane is coplanar with the center of thickness.

5. sensing a measured distance between the imaging device and the first surface of the test specimen via a sensor; transmitting the distance measurements from the sensor to the processing system via the interface; generating, via the processing system, the correction factor based on a difference between the calibration plane and the measurement plane corresponding to the measurement distance; The method of claim 1 further comprising:

6. The method of claim 5 , wherein the sensor is an infrared sensor, a light emitting diode sensor, an ultrasonic sensor, or a laser-based sensor.

7. The method of claim 1 , wherein the one or more characteristics include one or more of the shape or position of the marking, an edge position of the test specimen, or a width of the test specimen.

8. The method of claim 1 , wherein the correction factor is expressed in one of millimeters, inches, or pixels.

9. The method of claim 1 , wherein the imaging device is a single-view camera.

10. 1. A system for compensating for variable thickness of a test specimen, the system comprising: a test system for securing a test sample; an imaging device positioned on an opposite side of the illuminated screen from the test sample and configured to capture an image of the test sample; 1. A processing system comprising: determining a calibration distance between an imaging device and a calibration plane associated with the test system; receiving a thickness of a test sample, the thickness corresponding to a distance between a first surface and a second surface of the test sample, the test sample being positioned such that the first surface is a first distance from the imaging device and the second surface is a second distance from the imaging device; calculating a correction factor based in part on the calibration distance and a correction distance corresponding to a difference between the measurement plane and the calibration plane, the correction distance corresponding to one-half of the thickness; imaging one or more of a marking on the first surface of the test sample or a silhouette of the test sample formed on an illuminated screen to measure one or more properties of the test sample, wherein the first surface of the test sample corresponds to the measurement plane, and the measurement plane is offset from the calibration plane by the correction distance; applying a correction factor to the measurements of the one or more properties of the test sample to generate one or more corrected properties; a processing system that performs the steps of: Including, the system.

11. The system of claim 10 , wherein the test sample is positioned between an illuminated screen and the imaging device.

12. The system of claim 10 further comprising a user interface or a network interface that communicates with a computing device or a sensor.

13. The system of claim 10 , wherein the processing system is located on a remote computing platform that communicates with one or more of the test system or the imaging device.

14. The system of claim 10 , wherein the processing system is integrated with one of the imaging device or the test system.

15. The system of claim 10 , wherein the imaging device is a single-view camera.

16. The system of claim 10 , wherein the imaging devices are two or more cameras.

17. 11. The system of claim 10, wherein the imaging device is configured to capture polarized or infrared light reflected from the screen or the test sample, the screen reflecting light to create a dark silhouette of the test sample for edge analysis.

18. 1. A system for compensating for variable thickness of a test specimen, comprising: receiving an image from an imaging device of a test specimen during a testing process, the imaging device being positioned on an opposite side of a reflective screen from the test specimen; determining a calibration distance between the imaging device and a calibration plane associated with a test system to which the test sample is fixed; receiving a thickness of a test sample, the thickness corresponding to a distance between a first surface and a second surface of the test sample, the test sample being positioned such that the first surface is a first distance from the imaging device and the second surface is a second distance from the imaging device; calculating a correction factor based in part on the calibration distance and a correction distance corresponding to a difference between the measurement plane and the calibration plane, the correction distance corresponding to one-half of the thickness; imaging one or more of a marking on the first surface of the test sample or a silhouette of the test sample to measure one or more properties of the test sample, wherein the first surface of the test sample corresponds to the measurement plane, and the measurement plane is offset from the calibration plane by the correction distance; applying a correction factor to the measurements of the one or more properties of the test sample to generate one or more corrected properties; 11. A system including a processing system configured to:

19. 20. The system of claim 18, further comprising a sensor that measures a measurement distance between the imaging device and the first surface of the test specimen, wherein the processing system is configured to generate the correction factor based on a difference between the calibration plane and the measurement plane corresponding to the measurement distance.

Images