Patent Application
20230330802
The present invention relates to a flaw grinding system, a flaw grinding method, and a steel-product manufacturing method employing the flaw grinding method, which grind and remove a flaw or defect on the surface of a workpiece such as a steel product.
Steel products such as steel pipes, bar steel, shape steel, and steel ingots (to be referred to as “workpieces” hereinafter) may in some cases have a flaw or defect (to be referred to as “flaw” hereinafter) on their surface due to various factors arising during manufacture. A flaw on the surface of such a workpiece needs to be removed by a rework that involves using a grinder or other tool to grind and level out the flaw with the surroundings of the flaw. Problems with such a rework include: the potential danger associated with the task of the worker manually performing grinding by placing the grinder's rotor against the workpiece; and the working environment where the worker is exposed to vibrations, dust, or other adverse conditions that arise due to grinding. Such a rework also requires extra laborers to perform the rework. A need thus exists for automating the rework.
Unfortunately, steel products serving as workpieces come in a wide variety of types, and in various three-dimensional shapes. Further, the location of a flaw or defect on the surface of such a workpiece tends to vary with the type of the steel product. This means that the steel products are subjected to reworks at various locations and in various attitudes.
With regard to automating a rework on a workpiece, for example, Patent Literature 1 discloses an automatic flaw reworking method and an automatic flaw reworking apparatus. The disclosed method and apparatus involve applying, to the location of a flaw on the surface of the workpiece, a marking that contains information about the nature of the flaw, and detecting the marking and performing information processing to provide an operational command for the automatic flaw reworking apparatus.
Patent Literature 2 discloses an apparatus for automatically grinding a surface flaw on a metallic material. The disclosed apparatus detects the degree of contact of the grinder's grindstone with the metallic material surface, and controls how much cutting is to be done by the grinder's grindstone to a flaw on the metallic material surface.
Further, Patent Literature 3 discloses a robot operation teaching method directed to teaching an operation for a robot that performs marking or rework on a finished product such as a steel pipe or a shape steel. The disclosed method includes: outputting the manufacturing specifications of the finished product from a production management computer; in response to the outputting of the manufacturing specifications, determining, by a control computer, operational information showing location data and task type corresponding to the manufacturing specifications; and in response to the determining of the operational information, teaching, by a robot controller, an operation for the robot.
The techniques disclosed in Patent Literatures 1 and 2 are directed to flat-shaped workpieces such as steel sheets with a flat surface to be ground. This means that if the techniques disclosed in Patent Literatures 1 and 2 are applied to a workpiece having a three-dimensional shape with a curved surface, such as a steel pipe, due to the curved shape of the surface to be ground, the angle of the grindstone with respect to the surface of the workpiece, or the direction of the reaction force to the grinding performed by pressing of the grindstone changes during grinding. This may lead to malfunction of the grinding apparatus due to interference between the grindstone and the workpiece, or faulty grinding such as over-grinding or under-grinding.
The technique disclosed in Patent Literature 3 teaches an operation for the robot by acquiring, from an external computer, information such as the model of a workpiece or the manufacturing specifications for its dimensions. In the actual workpiece rework, however, dimension errors of the workpiece itself, or variations in, for example, the placement location or attitude of the workpiece exist. This may cause problems such as malfunction of the grinding apparatus due to interference between the robot arm or the grindstone and the workpiece, or faulty grinding due to the grindstone not following an intended target trajectory with respect to the surface of the workpiece.
When a worker is to manually perform a rework, the worker visually inspects the actual workpiece for the errors or variations mentioned above, presses the grinder with a suitable pressure against the location of a flaw on the surface of the workpiece, and moves the grinder in such a way as to level out the flaw with the surroundings of the flaw. The worker then stops the pressing of the grinder at a suitable timing, visually judges whether any flaw remains, and determines whether the worker needs to perform re grinding or can now finish the rework. An analysis of such a manual rework reveals that the rework proceeds with the following procedure: (1) measuring the shape and attitude of the actual workpiece based on visual information; (2) pressing the grinder's grindstone against the flaw location; (3) sensing the grinding reaction force/load, and based on the result of the sensing, adjusting the force with which to press the grindstone; and (4) based on visual information, inspecting the surface of the workpiece after grinding of the flaw is performed.
Accordingly, the inventors of the present application have investigated on the possibility of, in performing a rework involving grinding a flaw on the surface of a workpiece having a three-dimensional shape, reducing the occurrence of faulty grinding such as over-grinding or under-grinding through automatic execution of a procedure described below. The inventors have thus arrived at the present invention. The procedure includes: (1) measuring the shape and attitude of the actual workpiece with a camera or a shape sensor; (2) detecting the location of the flaw on the workpiece; (3) generating an operation of pressing the grinder's grindstone against the location of the flaw; (4) measuring and controlling the grinding reaction force/load on the grinder; and (5) inspecting the surface of the workpiece after grinding of the flaw is performed, and determining whether re-grinding is required.
The present invention has been made in view of the above-mentioned circumstances. Accordingly, it is an object of the present invention to provide a flaw grinding system, a flaw grinding method, and a steel-product manufacturing method employing the flaw grinding method that enable automation of the task of grinding a flaw on the surface of a workpiece having a three-dimensional shape, while also reducing the occurrence of faulty grinding such as over-grinding or under-grinding.
The means for accomplishing the above-mentioned object is presented as follows.
The term “flaw” mentioned above is meant to include surface defects that cause no change in the shape of the surface of the workpiece.
The flaw grinding system, the flaw grinding method, and the steel-product manufacturing method employing the flaw grinding method according to the present invention enable automation of the task of grinding a flaw on the surface of a workpiece having a three-dimensional shape, while also reducing the occurrence of faulty grinding such as over-grinding or under-grinding.
Referring now to the drawings, a flaw grinding system, a flaw grinding method, and a steel-product manufacturing method employing the flaw grinding method according to an embodiment of the present invention are described in detail.
As used in the following description of the embodiment, the term “flaw” is meant to include not only flaws such as protruding flaws due to deposition of foreign matter or other causes and recessed flaws due to surface loss or other causes, but also surface defects or other defects that do not cause changes in the shape of the surface of a workpiece. The expression “grind a flaw” is used to mean removing a flaw to level out the surface of a workpiece. For a protruding flaw, this means grinding the flaw to level out the surface of the workpiece. For a recessed flaw, this means grinding the surroundings of the flaw to level out the surface of the workpiece.
As illustrated in
The camera 32 is installed, together with the laser rangefinder 31, in a manner that allows the camera 32 to be actuated by the uniaxial actuator 33. By capturing an image of the surface of the workpiece W, the camera, together with the laser rangefinder 31 or instead of the laser rangefinder 31, performs a flaw detection step of detecting the location of the flaw Wf on the workpiece W, and an inspection step of, after the flaw Wf is ground by the grinder 10, inspecting the surface of the workpiece W at the location of the flaw Wf. If no inspection step is to be performed, or if the flaw detection step is to be performed by the laser rangefinder 31, the flaw grinding system 1 may not have to include the camera 32.
The respective operations of the grinder 10 and the grinding robot 21 are controlled by a grinding-tool control PC (an example of a grinding-tool control apparatus) 20, which is implemented by a computer. The respective operations of the laser rangefinder 31, the camera 32, and the uniaxial actuator 33 are controlled by a shape measurement/flaw detection PC (an example of a shape measurement apparatus/flaw detection apparatus) 40, which is implemented by a general-purpose personal computer.
As described above, while the flaw Wf is ground with the grinder 10, the reaction-force meter 13 measures a grinding reaction force exerted on the grinder 10 from the workpiece W. As illustrated in
Specifically, as illustrated in
Alternatively, as illustrated in
As illustrated in
Alternatively, as illustrated in
A still another exemplary method for detecting the flaw Wf on the workpiece W is to attach a flaw detector such as an eddy-current flaw detector or ultrasonic flaw detector to the tip of the arm of the grinding robot 21. In this case, the flaw Wf can be detected by, based on the three-dimensional shape and the attitude of the workpiece W measured with the laser rangefinder 31, moving the flaw detector along the surface of the workpiece W.
Alternatively, the detection of the flaw Wf by the shape measurement/flaw detection PC 40 may be combined with a judgement made by the worker. For example, an image of the surface of the workpiece W captured with the camera 32 is displayed on the screen of the shape measurement/flaw detection PC 40, and this is visually checked by the worker. If the flaw Wf is confirmed to exist, the flaw Wf may be detected by the worker operating the shape measurement/flaw detection PC 40 and specifying the location of the flaw Wf on the image.
As described above, for each individual workpiece W, the three-dimensional shape and the attitude of the workpiece W are measured with the laser rangefinder 31 to enable real-time generation of a machining trajectory based on information about the actual workpiece W. This configuration obviates the need to prepare 3D data of the workpiece W in advance and, in comparison to generating a machining trajectory from 3D data, allows various variations present in the actual workpiece W to be automatically reflected on the machining trajectory.
As for the detection of the flaw Wf as well, directly measuring the flaw Wf present on the actual workpiece W makes it possible to detect not only the location of the flaw Wf on the workpiece W, but also information such as the size, angle, and depth of the flaw Wf, and further, information about the surface of the workpiece W in the surroundings of the flaw Wf.
With the respective operations of the grinder 10 and the grinding robot 21 controlled by the grinding-tool control PC 20, the flaw Wf is ground for a predetermined time or a predetermined amount, and then an inspection step is performed. This specifically involves, after performing grinding of the flaw Wf, inspecting the surface of the workpiece W with the laser rangefinder 31 or the camera 32 to check whether the flaw Wf remains. Similar to the flaw detection step described above, the inspection step may be performed remotely by analyzing the shape data 31D of the workpiece W measured with the laser rangefinder 31 or the image data 32D of the surface of the workpiece W captured with the camera 32. The inspection step may be performed directly by use of an eddy-current flaw detector, an ultrasonic flaw detector, or other such flaw detector that is attached to the tip of the arm of the grinding robot 21.
The inspection of the surface of the workpiece W subsequent to grinding of the flaw Wf may be performed by using the same apparatus (i.e., the laser rangefinder 31 or the camera 32) used for the detection of the flaw Wf prior to grinding of the flaw Wf, or may be performed by use of a different apparatus. Using the same apparatus makes it possible to reduce the number of components of the flaw grinding system 1. The flaw Wf remaining on the workpiece W after grinding of the flaw Wf is performed is more difficult to detect than is the flaw Wf present before grinding of the flaw Wf is performed. Even in such cases, using a different apparatus, for example, using an apparatus with a higher accuracy than that used for detection of the flaw Wf prior to performing grinding of the flaw Wf allows for more accurate inspection of whether the flaw Wf remains on the surface of the workpiece W after grinding of the flaw Wf is performed.
If the flaw Wf remains, grinding is performed again. If no flaw Wf remains, grinding is ended.
Further, a steel-product manufacturing method according to the embodiment includes the step of grinding a flaw on the surface of a steel product serving as a workpiece by use of the flaw grinding method described above.
Reference is now made to the flaw grinding system, the flaw grinding method, and the steel-product manufacturing method employing the flaw grinding method according to a specific example of the present invention. According to the present example, a steel pipe with a diameter of 50 mm to 600 mm serves as the workpiece W, and the flaw Wf on its surface is removed by grinding.
As illustrated in
While the laser rangefinder 31 is moved in the x-direction at a velocity V (mm/s) with the uniaxial actuator 33 operated by the shape measurement/flaw detection PC40 (see
Δx=V/fs (1)
The velocity V and the sampling frequency fs of the laser rangefinder 31 are set so that the measurement pitch Δx is within a range of 0.1 mm to 5.0 mm. The velocity V of the laser rangefinder 31 is selected as appropriate within a range of 10 mm/s to 1000 mm/s in accordance with the characteristics of the laser rangefinder 31 and the characteristics of the uniaxial actuator 33. This is because too high a velocity V of the laser rangefinder 31 leads to reduced measurement accuracy due to vibration of the laser rangefinder 31 during operation, whereas too low a velocity V leads to extended measurement time. The sampling frequency fs of the laser rangefinder 31 is set to be within a range of 10 Hz to 10 kHz, based on the relationship between the velocity V of the laser rangefinder 31 and the measurement pitch Δx in the x-direction. The operation of the laser rangefinder 31 is set so that the measurement pitch Δy in the y-direction is within a range of 0.1 mm to 5.0 mm.
From the group of shape data points, the location of the flaw Wf is detected by the above-mentioned method illustrated in
First, multiple target grinding points pg are set based on a group of shape data points representing a set of measured points pm present within the grinding range Wa. The target grinding points pg are points through which the trajectory T0 (T1) of the grindstone 11 is to pass in grinding the flaw Wf on the surface of the workpiece W as the grindstone 11 is moved while being pressed against the workpiece W. As the direction in which to move the grindstone 11 during grinding of the flaw Wf (to be referred to as “grinding direction” hereinafter), which represents one condition of a grinding process, either the x-direction or the y-direction can be selected. In the present example, the grindstone 11 is moved in the y-direction.
Then, as illustrated in
Of the measured points pm present within the grinding range Wa, the point with the smallest coordinate values in the x- and y-directions is defined as a measurement origin p0. Then, with reference to the measurement origin p0, the location Lxi (mm), Lyi (mm) of a grinding origin pg0 in the x- and y-directions is set. The grinding origin pg0 represents the location at which to start grinding by the grindstone 11. The location Lxi, Lyi of the grinding origin pg0 in the x- and y-directions is set to be less than or equal to approximately 1/10 of the lengths Lx and Ly of the grinding range Wa in the x- and y-directions.
Further, the intervals ΔLx (mm) and ΔLy (mm) between target grinding points pg, in the x- and y-directions are set. The interval between target grinding points pg in the grinding direction (ΔLy in the present example) is set in accordance with, for example, the radius of curvature of the workpiece W in the grinding direction. The above-mentioned interval is preferably set within a range of 0.5 mm to 20 mm. Making the interval between multiple trajectories T0 (T1) (ΔLx, in the present example) less than the width of grinding by the grindstone 11 helps to prevent the flaw Wf from remaining unground. The number N of target grinding points pg in the x-direction, and the number M of target grinding points pg in the y-direction are respectively given by Equations (2) and (3) below.
N=(Lx−Lxi)/ΔLx (2)
M=(Ly−Lyi)/ΔLy (3)
The number of target grinding points pg, which is determined in accordance with the size of the flaw Wf, the radius of curvature of the workpiece W in the grinding direction, or other factors, is set within a range of 10 to 100. The above range is set because an excessively large number of target grinding points pg may lead to increased computational load on the grinding-tool control PC and consequently increased cycle time. The above range is set also because an excessively small number of target grinding points pg may result in the grinder 10 experiencing, as the grinder 10 moves between target grinding points pg, a greater grinding reaction force from the workpiece W than is expected, which may lead to unstable control of the grinding reaction force. The location xg (mm), yg (mm) of each individual target grinding point pg in the x- and y-directions is thus determined as described above.
Reference is now made to
Reference is now made to
[Math. 2]
L
k=√{square root over ((xg−xk)2+(yg−yk)2+(zg−zk))}k=1˜42 (5)
The three points thus selected are defined as pp1(xp1, yp1, zp1), pp2(xp2, yp2, zp2), and pp3 (xp3, yp3, zp3). Then, the x-, y-, and z-direction components p (mm), q (mm), and r (mm) of the normal vector n in a virtual plane A in which these three points exist are respectively given as Equations (6) to (8) below.
p=(yp2−yp1)(zp3−zp1)−(yp3−yp1)(zp3−zp1) (6)
q=(zp2−zp1)(xp3−xp1)−(zp3−zp1)(xp3−xp1) (7)
r=(xp2−xp1)(yp3−yp1)−(xp3−xp1)(yp3−yp1) (8)
Subsequently, an inter-target-grinding-point vector f(af, bf, cf), which is a directional vector between target grinding points pg, is determined. The x-, y-, and z-direction components af (mm), bf (mm), and cf (mm) of the inter-target-grinding-point vector f that points from the target grinding point pg to the next target grinding point pg+1(xg+1, yg+1, zg+1) are respectively given as Equations (9) to (11) below.
a
f
=x
g+1
−x
g (9)
b
f
=y
g+1
−y
g (10)
c
f
=z
g+1
−z
g (11)
Then, the normal vector n in the virtual plane A, and the inter-target-grinding-point vector f are non-dimensionalized into a non-dimensionalized normal vector N and a non-dimensionalized inter-target-grinding-point vector F. The x-, y-, and z-direction components P, Q, and R of the non-dimensionalized normal vector N, and the x-, y-, and z-direction components Af, Bf, and Cf of the non-dimensionalized inter-target-grinding-point vector F are respectively given as Equations (12) to (17) below.
Referring now further to
[Math. 9]
{right arrow over (S)}={right arrow over (tF)}+{right arrow over (sN)} (18)
cos θ=Af·As+Bf·Bs+Cf·Cs (19)
cos α=As·P+Bs·Q+Cs·R (20)
In the above equations, t and s are real numbers, As, Bs, and Cs, are the x-, y-, and z-direction components of the grindstone attitude vector S, a is an angle α(°) made by the grindstone attitude vector S and the non-dimensionalized normal vector N.
The angle α(°) made by the grindstone attitude vector S and the non-dimensionalized normal vector N is given as Equation (21) below.
Equations (18) to (21) are combined into a system of equations represented by Equation (22). Solving the system of equations represented by Equation (22) gives the respective values of the x-, y-, and z-direction components As, B3, and Cs of the grindstone attitude vector S.
Based on the locations of individual target grinding points pg and the grindstone attitude vector S that have been determined as described above, the trajectory T0 of the grindstone 11 is generated. The grindstone 11 is then moved along the trajectory T0 while being pressed against the workpiece W to thereby grind the flaw Wf.
An angle grinder is used as the grinder 10 in the present example. A rotation speed 11R of the grindstone 11 of the grinder 10 is set at a suitable value within a range of 1000 rpm to 15000 rpm in accordance with the properties of the grindstone 11, the material of the workpiece W, or other factors. A feed rate 11V of the grindstone 11 is preferably set to be within a range of 50 mm/min to 2000 mm/min.
The magnitude of a grinding reaction force Fr, which is generated in a direction perpendicular to the feed direction of the grindstone 11 as the grindstone 11 is pressed against the workpiece W, is measured by the reaction-force meter 13 (see
It is also possible to correct the trajectory T0 of the grindstone it not by the above-mentioned method in which the magnitude of the grinding reaction force Fr generated perpendicularly to the feed direction of the grindstone 11 is made to approach a target value, but instead by another method in which a value such the direction of the grinding reaction force Fr generated during grinding of the flaw Wf, the counter moment acting on the arm of the grinding robot 21, or the electric current load on the grinder 10 is made to approach a target value.
Although the example described above is directed to a case where the workpiece W is a steel pipe with a diameter of 50 mm to 600 mm and the flaw Wf on the surface of the workpiece W is removed by grinding, this is not intended to limit the application of the flaw grinding system, the flaw grinding method, and the steel-material manufacturing method according to the present invention to the above-mentioned case. For example, the present invention is also applicable to cases where the workpiece W is an H-shaped steel with a web length of 200 mm to 1000 mm, a steel sheet pile with an effective width of 400 mm to 900 mm, or other steel product.
The example described above employs the reaction-force meter (grinding-reaction-force measurement apparatus) 13 that measures the grinding reaction force exerted on the grindstone 11 from the workpiece W. It is to be noted, however, that even in the absence of such a grinding-reaction-force measurement apparatus, the flaw grinding system, the flaw grinding method, and the steel-material manufacturing method according to the present invention provide the above-mentioned effect, that is, enabling automation of the task of grinding a flaw on the surface of a workpiece having a three-dimensional shape, while also reducing the occurrence of faulty grinding such as over-grinding or under-grinding.
According to the example described above, employing the flaw grinding system and the flaw grinding method according to the present invention to grind a flaw on the surface of a steel pipe has made it possible to reassign approximately 50% of workers otherwise required for flaw reworking to other tasks. This in turn has made it possible to relieve the workers from problems including: the potential danger associated with the task of manually performing grinding by placing the grinder's rotor against the workpiece; and the working environment where the workers are exposed to vibrations, dust, or other adverse conditions that arise due to grinding.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2020-172981 | Oct 2020 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2021/027912 | 7/28/2021 | WO |
