JPH04346004A - Displacement measuring apparatus - Google Patents

Abstract

PURPOSE:To make it possible to perform measurement highly accurately at a low cost in the broad range from zero to fracture when the strain or the moving distance of a substance is measured by using an image sensing device such as a CCD camera and the like in a material test and the like. CONSTITUTION:An image sensing device 2 such as a CCD camera and a line sensor is attached to an actuator 3 which is a driving mechanism. The image sensing device is driven in parallel with the measuring surface of a test piece 1 of a substance which is to become an object. The image data are outputted into an image processing part (board) 5. The actuator 3 receives the moving command from a movement control device 4 and reports the position data on the other hand. An operating device (personal computer EWS) 6 operates the strain or the displacement based on the position data and the image data and outputs the measured result 7.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a displacement measuring device for measuring distortion or moving distance of an object during material testing, and more particularly to a displacement measuring device using an imaging device such as a CCD camera.

0002

2. Description of the Related Art When a material is used as a structural member, among its physical properties, mechanical properties such as elastic modulus and elongation are important. In order to determine this mechanical property, we measure the load (stress) and elongation (strain) of the test piece during a tensile test, and investigate the relationship between stress and strain, such as elastic modulus, tensile strength, and elongation at break. . In addition, since fatigue strength must be taken into account when subjected to repeated loads, fatigue strength may be determined experimentally by conducting a fatigue test. For example, in the case of a metal material, the magnitude of the stress that does not break even if the stress is repeated 1×10 7 times is determined and used as a measure of the strength characteristics (fatigue limit) of the material. Therefore, highly accurate measurement of strain and stress is required. Here, if the load is P, the cross-sectional area is A, the actual length is L, and the original length is Lo, then stress σ = P/A, Amount of elongation δ=L−L
o, strain ε=δ/Lo. In addition, the longitudinal elastic modulus is E=ε
/σ, Poisson's ratio is ν=lateral strain/longitudinal strain. Elongation is the amount of elongation with the length after breakage being the actual length, and the distance between the gauges used to measure the length is specified by the Japanese Industrial Standards along with the specimen shape. Conventionally, strain in such material tests has been measured using a photoelastic method, a moiré method, or the like, which uses strain gauges or interference fringes. Among these, strain gauges have the advantage of being low cost and capable of measuring strains from a minute range to about 10%, but cannot measure large strains that may lead to breakage. This is because the gauges are in contact with each other, so if the strain is repeated more than 1x106 times as in a fatigue test, the characteristics of the gauge itself and the adhesive that fixes the gauge and the object to be measured deteriorate. This is because the measured value may fluctuate or the gauge may peel off. On the other hand, among the methods using interference fringes, the moiré method, which uses grid points on the surface and utilizes interference with the reference grid in the camera, cannot measure minute distortions, but can measure large distortions of about 0.1%. When slip bands occur on the surface of a metal material, resulting in unevenness, the lattice points become distorted and no stripes are formed. In addition, in order to attach a grid to the surface of the test piece, polishing is required as a pretreatment of the surface.
It takes a lot of time to prepare for the experiment. The photoelastic method, which is another method that utilizes interference fringes, has extremely high accuracy within the elastic range, but has the limitation that its application must literally be within the elastic limit. Therefore, in recent years, CC
A method of measuring deformation using image processing technology using a D-camera has been devised, but in order to measure minute deformations, the resolution of the CCD camera, that is, the number of pixels, is required, the cost is high, and image analysis is difficult. The amount of processing is also large. On the other hand, if the amount of deformation increases, it will move out of the measurement range of the CCD camera, making it difficult to measure large deformations that may lead to breakage.

0003

[Problems to be Solved by the Invention] As described above, among the conventional displacement measuring devices, contact type ones such as strain gauges are
Because it needs to be bonded to the test object, the gauge measurement device itself is affected by the test environment such as temperature and atmosphere, and by distortion.
When subjected to large strain, the part to which the measured object is attached, such as adhesive, may peel or shift, making measurement impossible. Also, when subjected to repeated deformation, the gauge itself will become fatigued and its characteristics will change, and the adhesive may The agent also deteriorates due to repeated deformation, resulting in peeling and a decrease in adhesive strength. On the other hand, in the method using interference fringes, if the surface condition differs from the initial state, the interference fringes become unclear, making it impossible to measure a wide range including large deformations. In the method using a CCD camera, it is possible to measure minute displacements with just one camera by enlarging the screen, but the measurement range becomes smaller, and if the measurement range is widened, the resolution of displacement decreases. By increasing the number of pixels in the CCD light receiving section, the measurement range can be expanded without reducing the resolution, but the CCD
Increasing the D resolution significantly increases cost. For example, there are commercially available devices with 1024×1024 pixels, but the resolution is only 1/1024=0.1%. Increasing the resolution also increases the amount of processing required for image analysis, which takes time and makes real-time measurement difficult. Furthermore, as the amount of processing data increases, it becomes necessary to increase the capacity of the image memory.

The present invention was devised in view of these problems, and aims to provide a displacement measuring device that is low cost, highly accurate, and capable of measuring strain over a wide range from zero to fracture. The purpose is

[0005]

[Means for Solving the Problems] Means for solving the above problems in the present invention includes a measuring section in which an imaging device is attached to a drive mechanism, and a moving mechanism that causes the measuring section itself to follow the distortion or displacement of an object. The displacement measuring device is equipped with a means for detecting the amount of movement of the measuring section itself and the position of the imaging device within its measurement range, and measures the distortion or displacement of those objects. It is something.

[0006]

[Operation] The present invention is a displacement measuring device that uses an imaging device such as a CCD camera and is capable of measuring a wide range with high precision. Therefore, in the present invention, the imaging device is attached to a drive mechanism, and the measuring section itself is made to follow the distortion or displacement of the object by the moving mechanism, so that the amount of movement of the measuring section itself and the imaging device within its measurement range are The position of the object is detected separately, and the distortion or displacement of the object is measured from these values.

[0007]

Embodiments Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a configuration diagram showing an embodiment of the measuring section of the present invention. In the figure, 1 is a test piece of a target object, and 2 is an imaging device such as a CCD camera or a line sensor. The imaging device 2 is attached to an actuator 3, which is its driving mechanism, and is driven parallel to the measurement surface of the test piece 1. There is also a method of setting the number of imaging devices 2 to be the same as the number between the gauge points 1a set in advance by marking lines or indentations on the measurement surface of the test piece 1, or by moving a plurality of imaging devices to adjust the number of gauge points 1a. There is also a way to measure. The actuator 3 receives a movement command from the movement control device 4 of the movement mechanism of the present invention, and reports position data. The imaging device 2 outputs image data to an image processing unit 5, and based on the measurement data and the position data of the actuator, the calculation device 6 calculates distortion or displacement, and outputs a measurement result 7. An actuator movement command is sent to the movement control device 4.

FIG. 2 shows the image processing unit 5 from the above imaging device.
FIG. 2 is an explanatory diagram of image data output to. In the figure, image data 21 includes a linear mark 2 having a desired thickness.
2 is given. The mark 22 has a center of gravity 23,
Usually in the form of a "line" or "indentation", the brightness differs between the part where the mark is present and the part where the mark is not present, and the imaging device 2 detects the mark position of the gauge point based on this difference in brightness. The image processing unit 5 determines whether the mark 22 is within the measurement range, moves the imaging device 2 if the mark 22 is not within the measurement range, and moves the mark 22 if the mark 22 is within the measurement range.
Calculate the position of. The distance between the center of gravity position of the mark 22 and the reference position value is the amount of movement of the center of gravity 23, and is expressed in units of pixels. The calculation device 6 adds the position calculation value of the imaging device 2 and the movement amount of the actuator 3 to obtain the deformation amount of the test piece 1, and outputs it together with the load (stress) at that time.

FIG. 3 is a flowchart showing an example of measurement processing according to the present invention using image data of the above-mentioned apparatus. Hereinafter, embodiments of the present invention will be described in more detail with reference to the drawings. First, the actuator 3 receives a movement command from the movement control device 4 based on the result of image processing and moves to the initial position. As for the movement mechanism, a two-axis stage with a movement accuracy of about 0.005mm using a pulse motor is used, since the displacement is several tens of mm in a standard tensile test of metal materials. Although the resolution is lower, if a single axis with a movement precision of about 0.2 mm is used, it is possible to measure up to about 5 m. When measurement is started, the imaging device 2 needs to measure two gauge points. For this purpose, two imaging devices may be used to measure only the amount of movement of one of the gauge points, or one imaging device may be used to move between two points using an actuator. Specifically, 512×51
When using a 2-pixel CCD camera, the size of the mark is finite, so if you use part of the screen so that it does not protrude from the screen and use the remaining screen for measurement, the number of effective pixels can be reduced, for example in one axis direction. If the number of pixels is 256, the amount of deformation within the screen should be within the minimum positioning accuracy of the stage, and the measurement limit D for one pixel is D = 0.005 for metal materials.
/256≒2×10−5 mm, and even for large size D=0.2/256≒8×10−4 mm. Since the standard gauge distance for metal materials is around 50 mm, these values are converted into distortion when using a stage.
The distortion resolution per pixel is εD=2×10-5/
50≒4×10-7% (≒4×10-3μSTRAIN
), and similarly for large size εD≒1.6×10-5% (
≒1.6×10−1 μSTRAIN). here,
If the number of pixels of the CCD camera differs, the resolution will also change, but the amount of change is proportional to the number of pixels as described above. Note that the camera can measure not only a CCD but also an image pickup tube such as a vidicon, and if only uniaxial deformation is to be detected, a line sensor or the like may be used.

The image processing section 5 receives position data of marks and the like as an analog signal from the imaging device 2, and converts this signal into a position value. For example, when a CCD camera is used as the imaging device 2, a signal from the CCD camera is input to an image processing board, and the intensity of light on each CCD element is digitized. As for the resolution of this light intensity, if the difference in brightness between the part where the mark is present and the part where the mark is not present is more than a predetermined value, the usual 8 bits (256 steps) are sufficient for detection. In the mark position calculation process, data on the light intensity of each CCD element is sent to the image processing unit 5 to the calculation device 6 such as a personal computer or EWS.
, and first performs binarization as preprocessing to distinguish marked parts from non-marked parts. Next, the position of the center of gravity of the mark is determined from the position and area of the binarized mark portion, and this is taken as the position of the mark. For example, if the center of gravity of the mark is shifted by one pixel, the amount of distortion at that time is 4 x 10-3μSTRAIN, or 1.6
×10−1 μSTRAIN. That is, by detecting the center of gravity, even if the mark is slightly deformed, its position can be detected. Note that if pattern recognition is performed using an imaging device, even if there is no mark, it is possible to use scratches on the surface of the test piece or patterns of the material's structure (crystals) in place of the mark. Furthermore, if the mark protrudes from the measurement range, the image processing unit 5 sends a command to the movement control device 4 so that the mark appears within the measurement range. This command may be issued directly from the conversion device 6 such as a personal computer, or may be issued via a D/A conversion board. To determine whether a mark is within the measurement range, count the number of elements with the light intensity corresponding to the mark during binarization processing, and if the number is the same as the area of the mark, it is within the measurement range. If not, it is outside the measurement range. According to this method, the only image processing required to detect the presence or absence of a mark is binarization, resulting in faster processing. Note that since the size of the mark is finite, it is of course necessary to set the magnification so that the mark does not protrude from the screen, taking into consideration the resolution and field of view of the CCD camera.

The movement control device 4 drives the actuator 3 upon receiving a command to move the actuator from the image processing section 5 . The driving direction is set by the calculation device 6 so that if the differential value of the previous movement amount is positive, the test piece 1 is extended, and if it is negative, the test piece 1 is moved in the direction of contraction.
The movement control device 4 is often set with the actuator 3, which is convenient.

The following effects are evident in this embodiment.

(1) When measuring the deformation or strain of a material,
High precision and wide range measurement (wide measurement range relative to resolution) becomes possible.

(2) Pretreatment for the tensile test requires only marking marks, punch holes, hardness meter indentations, etc. on the test piece, and conventional complicated methods such as attaching gauges and baking checkered stripes are sufficient. No pre-treatment required.

(3) Better resolution than when using a strain gauge or Moiré method.

(4) It is possible to measure both minute displacements and large deformations.

(5) Measurement of displacement and strain is possible in real time and automated.

(6) Imaging devices and actuators can be selected depending on resolution and magnitude of deformation.

(7) The cost is lower, the calculation speed is faster, and the responsiveness is better than when a high-resolution CCD camera and an image processing device are combined.

(8) Since it is non-contact, measurement is possible even if the test piece is repeatedly deformed, and measurement is possible even at high temperatures and from outside the corrosion chamber.

(9) It is also possible to measure the three-dimensional movement of an object.

[0022]

As described above, according to the present invention, it is possible to provide a displacement measuring device capable of measuring strain over a wide range from zero to breakage at low cost and with high accuracy.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of a measuring section of the present invention.

FIG. 2 is an explanatory diagram of the amount of deformation according to the present invention.

FIG. 3 is a flowchart of measurement processing according to the present invention.

[Explanation of symbols]

1... Test piece, 2... Imaging device, 3... Actuator, 4...
Movement control device, 5... Image processing section, 6... Arithmetic device, 7... Measurement result, 21... Image data, 22... Mark, 23... Gauge point.

Claims (1)

[Claims] 1. A displacement measuring device comprising a measuring section in which an imaging device is attached to a drive mechanism, and a moving mechanism that causes the measuring section itself to follow the distortion or displacement of an object, the displacement measuring device being able to measure the amount of movement of the measuring section itself and its displacement. 1. A displacement measuring device comprising means for detecting the position of an imaging device within a measurement range, and measuring distortion or displacement of an object from these values.