JP7580201B2 - Screw tightening device and screw tightening method - Google Patents

Description

The present invention relates to a screw tightening device and a screw tightening method, and in particular to a screw tightening device and a screw tightening method that can improve axial force accuracy.

In recent years, in the automobile manufacturing industry, for example, the downsizing required for vehicles has led to a trend of reducing the rigidity of various vehicle components. When fastening such vehicle components with threaded components such as bolt components and nut components, the important control item for screw fastening is the axial force acting on the bolt component, rather than the fastening torque. In other words, by controlling the axial force of the threaded components, it is possible to control, for example, the stress related to the deformation of the component to be fastened.

This axial force is a tensile force acting on an axial member such as a bolt member, and is proportional to the tightening rotation angle of the screw member after the seating surface of the screw member, such as a nut or bolt head, comes into contact with the member to be tightened (hereinafter referred to as the tightening target member).

Traditionally, most screw tightening is done according to a tightening method called the torque angle method. In this torque angle method, the seating surface of the screw comes into contact with the workpiece to be tightened (hereinafter also referred to as seating), and once a certain torque (also referred to as snug torque) is generated, the screw tightens by rotating it a certain rotation angle. However, with this torque angle method, the rotation angle of the screw from seating to the generation of snug torque is unknown.

This is because, in addition to the fact that it is difficult to accurately detect the seating of the screw member by monitoring only the tightening torque of the screw member, the screw member rotation angle from seating of the screw member to the generation of snug torque varies depending on the friction coefficient between the seating surface of the screw member and the member to be tightened. Therefore, the torque angle method cannot accurately manage the tightening rotation angle of the screw member from seating of the screw member. As mentioned above, the axial force of screw tightening is proportional to the tightening rotation angle of the screw member from seating of the screw member, so it is difficult to improve the axial force accuracy of the screw member with the torque angle method.

In contrast, the screw tightening method described in the following Patent Document 1 makes it possible to determine the seating of the screw member, so that the tightening rotation angle of the screw member can be managed to a specified rotation angle, and as a result, the axial force accuracy of the screw member can be improved. This screw tightening method is also called the torque tension method, and specifically, after the snug torque occurs, the screw member is tightened by a specified tightening rotation angle, and the torque increase rate for the screw member rotation angle is calculated from the difference between the torque at that time and the snug torque and the specified tightening rotation angle, and the seating point of the screw member (defined as the virtual seating point) is calculated from this torque increase rate and, for example, the snug torque. This makes it possible to determine the rotation angle of the screw member from the virtual seating point to the generation of the snug torque, so that the screw member tightening rotation angle from the virtual seating point can be set as the rotation angle target value, and as a result, the axial force accuracy of the screw member can be improved.

JP 62-102978 Publication

However, the torque value detected by a torque detector such as a torque transducer is accompanied by noise, and the noise varies in magnitude depending on, for example, the contact state between the seating surface of the screw member and the member to be tightened. In the above-mentioned Patent Document 1, when the screw member is tightened by a specified tightening rotation angle after the snug torque occurs, the virtual seating point is determined by calculating the torque increase rate from the torque increase amount and the rotation angle increase amount at a specific point in time. Therefore, if the detected torque value is noisy, the virtual seating point cannot be accurately determined, and as a result, the axial force accuracy of the screw member cannot be improved.

The present invention was made in consideration of the above problems, and its purpose is to provide a screw tightening device and a screw tightening method that can maximize the axial force accuracy of screw members.

In order to achieve the above object, a screw fastening device of the present invention detects a rotation angle of a screw member and a torque of the screw fastening when the screw is fastened, calculates a virtual seating point of a seating surface of the screw member from a torque increase rate of the torque with respect to the rotation angle, and fastens the screw member from the virtual seating point to a rotation angle target value. In this screw fastening device, the torque increase rate is calculated by rotating the screw member by an initial rotation angle increase value when the screw member seats on a member to be fastened and the detected torque value of the fastening torque reaches a snug torque, and dividing the difference between the detected torque value at that time and the snug torque by the initial rotation angle increase value, and the virtual seating point is a seating point calculated based on the torque increase rate, and the rotation angle of the screw member with respect to the member to be fastened after the seating on the seating surface is zero. the virtual seating point is a point at which the fastening torque of the screw member increases linearly with respect to the rotation angle from the seating point, and the fastening torque of the screw member is calculated from the torque increase rate based on the fact that the fastening torque of the screw member increases linearly with respect to the rotation angle from the seating point, and the fastening torque of the screw member is calculated by determining the torque increase rate from the detected torque for each rotation angle specified value which is a preset rotation angle interval, the virtual seating point of the screw member is calculated, the virtual seating point calculation means is configured to approximate a distribution of the virtual seating points calculated by the virtual seating point calculation means with a normal distribution, the virtual seating point setting means is configured to set a virtual seating point corresponding to a mode of the normal distribution of the virtual seating points approximated by the normal distribution approximation means as a most likely virtual seating point, and the fastening torque of the screw member is controlled based on the virtual seating point set by the virtual seating point setting means.
In addition, the mode in a normal distribution is the median and also the mean.

According to this configuration, during the tightening control of the screw member, the virtual seating point is calculated from the torque and the rotation angle for each preset rotation angle specified value, the most likely virtual seating point is set from the mode of the approximate normal distribution of the virtual seating points, and the tightening control of the screw member is performed up to the rotation angle target value based on the virtual seating point, thereby performing the tightening control of the screw member according to the torque tension method. At this time, since the noise on the detected torque value increases and decreases in the vicinity of the torque value to be detected, the influence of the noise is reduced if the frequency (number of data) of the virtual seating point set from the mode of the approximate normal distribution increases. Therefore, as the tightening control of the screw member progresses, the accuracy of the virtual seating point set from the mode of the approximate normal distribution is improved as the most likely virtual seating point, and the tightening control of the screw member is performed up to the rotation angle target value based on this highly accurate virtual seating point, thereby achieving a highly accurate torque tension method, and thereby increasing the axial force accuracy of the screw member as much as possible.

In addition, another configuration of the present invention is characterized in that the virtual seating point setting means sets, instead of the mode of the normal distribution of the virtual seating points, a virtual seating point corresponding to the intersection of a straight line connecting a plurality of virtual seating point data on the side immediately adjacent to the mode but larger than the mode, and a straight line connecting a plurality of virtual seating point data on the side immediately adjacent to the mode but smaller than the mode, as the most likely virtual seating point.

According to this configuration, even if the distribution of the calculated virtual seating points cannot be approximated by a normal distribution, the virtual seating points can be made into, for example, a histogram. By setting the intersection of lines connecting multiple virtual seating points on the large and small sides of the histogram closest to the mode of the histogram as the virtual seating point, as described above, the virtual seating point closest to the mode of the approximate normal distribution can be set as the most likely virtual seating point as the screw tightening control progresses. Therefore, even if the calculation processing capacity of the screw tightening device is not particularly high, screw tightening control in accordance with the torque tension method can be achieved, thereby making it possible to increase the axial force accuracy of the screw member as much as possible.

A further configuration of the present invention is that, when the torque increase rate relative to a preset rotation angle of the screw member is less than a preset torque increase rate specified value, the control means switches to screw member tightening control in which the screw member is rotated by a preset rotation angle after the torque reaches a preset torque specified value.

According to this configuration, after switching of the screw member tightening control, the screw member is rotated by a predetermined rotation angle set in advance after the torque reaches a specified value, for example, the snug torque, and the screw member is tightened according to the torque angle method. The virtual seating point is calculated from the torque increase rate relative to the rotation angle of the screw member, and the smaller the friction coefficient between the seating surface of the screw member and the tightening target member, the smaller the torque increase rate. If this torque increase rate is significantly small, the calculation accuracy of the virtual seating point where the rotation angle of the screw member relative to the tightening target member after seating is set to 0 decreases, and as a result, the axial force accuracy of the screw member by the screw tightening control according to the torque tension method decreases. Therefore, when the torque increase rate relative to the rotation angle of the screw member is less than the torque increase rate specified value, the screw tightening control according to the torque angle method is used to ensure the axial force accuracy of the screw member.

Another configuration of the present invention is a method for fastening a screw member, which detects a rotation angle of the screw member and a torque of the screw fastening when the screw member is fastened, calculates a virtual seating point of the seating surface of the screw member from a torque increase rate of the torque with respect to the rotation angle, and fastens the screw member from the virtual seating point to a rotation angle target value, the torque increase rate being calculated by rotating the screw member by an initial rotation angle increase value when the screw member seats on a member to be fastened and a detected torque value of the fastening torque reaches a snug torque, and dividing a difference value between the detected torque value at that time and the snug torque by the initial rotation angle increase value, the virtual seating point being a seating point calculated based on the torque increase rate, and being a point where the rotation angle of the screw member with respect to the member to be fastened after the seating on the seating surface becomes zero, and the virtual seating point being a point where the fastening torque of the screw member increases from the seating point to a target rotation angle The method includes a virtual seating point calculation step in which a control device equipped with a computer system calculates the virtual seating point of the screw member by calculating the torque increase rate from the detected torque for each rotation angle specified value, which is a preset rotation angle interval; a normal distribution approximation step in which the control device approximates the distribution of the virtual seating points calculated in the virtual seating point calculation step with a normal distribution; a virtual seating point setting step in which the control device sets a virtual seating point corresponding to the most frequent value of the normal distribution of the virtual seating points approximated in the normal distribution approximation step as the most likely virtual seating point; and a control step in which the control device controls the tightening of the screw member based on the virtual seating point set in the virtual seating point setting step.

According to this configuration, during the tightening control of the screw member, the virtual seating point is calculated from the torque and the rotation angle for each preset rotation angle specified value, the most likely virtual seating point is set from the mode of the approximate normal distribution of the virtual seating points, and the tightening control of the screw member is performed to the rotation angle target value based on the virtual seating point, thereby performing the tightening control of the screw member according to the torque tension method. At this time, since the noise on the detected torque value increases and decreases in the vicinity of the torque value to be detected, the influence of the noise is reduced by increasing the frequency (number of samples) of the virtual seating point set from the mode of the approximate normal distribution. Therefore, as the tightening control of the screw member progresses, the accuracy of the virtual seating point set from the mode of the approximate normal distribution is improved as the most likely virtual seating point, and the tightening control of the screw member is performed to the rotation angle target value based on this highly accurate virtual seating point, thereby achieving a highly accurate torque tension method, and thereby increasing the axial force accuracy of the screw member as much as possible.

As described above, according to the present invention, the effect of noise in the detected torque value can be reduced as the screw member tightening control progresses, the most likely virtual seating point can be set with high precision, and the axial force precision of the screw member can be increased as much as possible in accordance with the highly accurate torque tension method. As a result, it becomes possible to smoothly promote downsizing of vehicles.

1 is a schematic diagram showing an embodiment of a screw fastening device to which a screw fastening method and a screw fastening device according to the present invention are applied; 2A to 2C are explanatory diagrams of a screw fastening method performed by the screw fastening device of FIG. 1 . FIG. 2 is an explanatory diagram of a torque value detected by the torque detector of FIG. 1 . 3 is an explanatory diagram of a case where slippage of a screw member occurs in the screw fastening method of FIG. 2. FIG. 2 is a flowchart of a calculation process performed by the control device of FIG. 1 . FIG. 5 is an explanatory diagram of the operation of the calculation process in FIG. 4 . FIG. 5 is an explanatory diagram of another example of setting the most probable virtual seating point in the calculation process of FIG. 4 . FIG. 1 is an explanatory diagram of a conventional screw tightening method.

The following is a detailed description of an embodiment of the screw fastening device and screw fastening method of the present invention with reference to the drawings. FIG. 1 is a schematic diagram of the screw fastening device of this embodiment, which is used, for example, in the assembly of a vehicle engine. The components of this screw fastening device are the same or nearly the same as those of conventional screw fastening devices. This screw fastening device is configured with a socket 10 that is fitted, for example, to the head of a bolt member or a nut member to rotate the screw member, an electric motor 12 that rotates the socket 10, a torque detector 14 such as a torque transducer that detects the fastening torque of the screw member by the electric motor 12, and a rotation angle sensor 16 that detects the rotation angle of the screw member, and the electric motor 12 is driven by a control device 18. The fastening torque detected by the torque detector 14 and the rotation angle detected by the rotation angle sensor 16 are read into the control device 18.

The control device 18 is configured with a computer system (not shown) such as a microprocessor. Like well-known computer systems, this computer system is configured with a processing unit with advanced arithmetic processing functions, as well as a storage device for storing programs, and an input/output device for reading sensor signals and communicating with other control devices. An example of a computer system used in the automobile manufacturing industry is a programmable logic controller.

Before describing the screw tightening method performed by the screw tightening device of this embodiment, the screw tightening method using the conventional torque angle method will be described with reference to FIG. 8. In the torque angle method, the screw member is seated on the member to be tightened, and the tightening is completed by rotating the screw member by a predetermined tightening rotation angle φ ₀ after the tightening detection torque value reaches a preset torque specified value, for example, the snug torque ST. The detected torque value is generated from the time when the screw member is seated on the member to be tightened. For example, the straight line shown by the solid line in the figure is the average (intermediate) tightening torque-rotation angle characteristic, and this characteristic changes depending on, for example, the friction coefficient between the seat surface of the screw member and the member to be tightened (hereinafter also referred to as the seat surface between the screw member and the member to be tightened) (naturally, the friction coefficient of the thread also has an effect).

For example, as shown by the dashed line in FIG. 8, when the coefficient of friction between the bearing surfaces is large (high μ in the figure), the rotation angle of the screw member from seating to the generation of snug torque is small. On the other hand, as shown by the dashed line in the figure, when the coefficient of friction between the bearing surfaces is small (low μ in the figure), the rotation angle of the screw member from seating to the generation of snug torque is large. As will be described later, the actual detected torque value is not a simple straight line (curve) as shown in the figure, but is accompanied by fine noise, and the waveform is often disturbed when the screw member is seated, making it difficult to determine at what point in time the screw member is seated. For example, if there is an angle difference Δφ between the rotation angle of the screw member until the generation of snug torque when the screw member is rotated by the above-mentioned predetermined tightening rotation angle φ ₀ to complete the screw tightening, the actual rotation angle of the screw member from seating differs by this angle difference Δφ. Since the axial force of a screw member depends on the tightening rotation angle from when the screw member is seated, if the tightening rotation angle at the time of completion of tightening differs by an angle difference Δφ, the axial forces of both will also differ by that amount. Therefore, it is difficult to improve the axial force accuracy of a screw member when tightening screws using the torque angle method.

2 is an explanatory diagram of the principle of the screw tightening method performed by the screw tightening device of this embodiment, specifically, the screw tightening method by the torque tension method. In this screw tightening method by the torque tension method, the screw member is seated on the member to be tightened, and when the detected torque value of the tightening torque reaches the snug torque ST, the screw member is rotated by the rotation angle increase initial value φ _i . Then, the difference between the detected torque value at that time and the snug torque is divided by the rotation angle increase initial value φ _i to obtain the torque increase rate per unit rotation. As is clear from the figure, the torque value reached when the screw member is rotated by the rotation angle increase initial value φ _i from the snug torque arrival differs depending on whether the friction coefficient between the bearing surfaces is large (high μ) or small (low μ), but the torque increase rate obtained by dividing the difference between the detected torque value at that time and the snug torque by the rotation angle increase initial value φ _i reflects the respective friction coefficients between the bearing surfaces, and is large if the friction coefficient is large, and is small if the friction coefficient is small.

Therefore, by tracing back each tightening torque-rotation angle curve (straight line) with this torque increase rate, it is possible to obtain the intercept of the curve (straight line) where the detected torque value is 0, that is, the rotation angle of the screw member with respect to the tightening target member after seating is 0, and this becomes the virtual seating point. Therefore, in a diagrammatic manner, it is possible to obtain the rotation angle of the screw member from seating of the screw member to the generation of the snug torque from this virtual seating point and, for example, the snug torque. In addition, it is also possible to directly obtain the rotation angle of the screw member from seating of the screw member to the generation of the snug torque by dividing the snug torque by the torque increase rate. Since the axial force of the screw member can be specified by the rotation angle target value φT from seating of the screw member, the rotation angle obtained by subtracting the screw member rotation angle until the generation of the snug torque and the above-mentioned rotation angle increase initial value _φi from this rotation _angle target value _φT becomes the tightening rotation angle of the remaining screw member to be rotated.

However, the torque value detected by the torque detector 14 includes high-frequency noise. FIG. 3 is an explanatory diagram of torque values detected during screw tightening control, and as is clear from the figure, the detected torque value fluctuates with fine noise. The screw tightening control using the torque tension method is based on the torque increase rate at a specific time point, that is, when the screw member is rotated by the initial rotation angle increase value φ _i from the occurrence of the snug torque. Therefore, if the detected torque value at that time point includes noise, the calculated torque increase rate and the virtual seating point will be uncertain due to the influence of the noise. However, as can be inferred from FIG. 3, the noise on the detected torque value only increases and decreases near the true value of the torque value that should be detected, and the detected torque value will be more reliable if the noise is averaged. That is, for example, the more virtual seating points are calculated, and the greater the frequency of their distribution, the more reliable the virtual seating point can be set.

Figure 4 is a schematic diagram of a portion of the actual waveform of the tightening torque-rotation angle curve of Figure 2, for example. The fine waveform in the figure indicates noise riding on the detected torque value. In this torque curve, a temporary large fluctuation that appears in the tightening torque-rotation angle curve with a large friction coefficient (high μ in the figure) represents slippage of the screw member. If the torque increase rate is calculated during the occurrence of this slippage, the tightening torque-rotation angle curve calculated using that torque increase rate will be as shown by the dashed line in the figure. This will be equivalent to the tightening torque-rotation angle curve with a small friction coefficient (low μ) shown by the dashed line in the figure, for example, so the virtual seating point calculated using that torque increase rate will differ from the actual virtual seating point. However, since the slip occurrence rate of the screw member is extremely small, even if a virtual seating point different from the actual virtual seating point is calculated due to the occurrence of slippage, the frequency is extremely small in the normal distribution of the calculated virtual seating points, and it will not be counted as a substantially reliable virtual seating point.

On the other hand, as can be inferred from FIG. 2, the virtual seating point where the rotation angle of the screw member relative to the tightening target member after seating is set to 0 by tracing back the torque increase curve (straight line) based on the torque increase rate varies more and the calculation accuracy decreases as the torque increase rate decreases, that is, as the friction coefficient between the bearing surfaces decreases. In addition, the smaller the friction coefficient between the bearing surfaces, the smaller the rotation angle of the screw member remaining from the occurrence of the snug torque to the target rotation angle value _φT , so the number of virtual seating points calculated during that time is smaller, and the frequency of the most frequent value of the normal distribution of the virtual seating points is also smaller, so the accuracy of the virtual seating point set from the most frequent value is also lower. Therefore, in this embodiment, the torque increase rate at the time of the occurrence of the snug torque is calculated, and if this torque increase rate is less than a preset torque increase rate specified value, the torque tension method is stopped and the screw member is tightened by the torque angle method. The torque angle method provides a higher axial force accuracy for the screw member than the torque tension method, which is performed while the virtual seating point of the screw member is unclear. Therefore, the torque increase rate specified value corresponds to the torque increase rate when the friction coefficient between the bearing surfaces is so small that it is difficult to accurately determine the virtual seating point.

Figure 5 is a flowchart of the calculation process performed by the control device 18 to control the screw tightening of the screw member. This calculation process is executed in the control device 18 for each rotation angle of the screw member, for example, 1 degree. The rotation angle of the screw member detected by the rotation angle sensor 16 such as the encoder, and the torque detected by the torque detector 14 such as a transducer are read into the control device 18 at a very short sampling period and stored in the storage device. In addition, in this flowchart, the calculated data is plotted on a distribution diagram and the plotted data is approximated by a normal distribution, but these processes may actually be performed only inside the calculation processing device and the storage device.

This calculation process begins, for example, when the screw member starts to be tightened. First, in step S1, the detected values of the rotation angle and torque of the screw member are read. Hereinafter, the rotation angle of the screw member is simply referred to as the rotation angle.

Next, the process proceeds to step S2, where it is determined whether or not the control flag F is reset to 0. If the control flag F is reset to 0, the process proceeds to step S3; if not, the process proceeds to step S6.

In step S3, it is determined whether or not snug torque ST is detected in the detected torque value. If snug torque ST is detected, the process proceeds to step S4, and if not, the process returns.

In step S4, the rotation angle at which the snug torque ST is detected is set as the rotation angle reference value, and then the process proceeds to step S5.

In step S5, the torque increase rate for the rotation angle from the rotation angle and torque stored in the storage device until snug torque detection is calculated, and then the process proceeds to step S6.

In step S6, it is determined whether the torque increase rate calculated in step S5 is equal to or greater than a preset torque increase rate threshold value. If the torque increase rate is equal to or greater than the torque increase rate threshold value, the process proceeds to step S7. If not, that is, if the torque increase rate is less than the torque increase rate threshold value, the process proceeds to step S19. The torque increase rate threshold value is the torque increase rate when the friction coefficient between the bearing surfaces is so small that it is difficult to accurately determine the virtual seating point.

In step S7, the control flag F is set to 1 and then the process proceeds to step S8.

In step S8, it is determined whether the rotation angle has increased from the rotation angle reference value by more than the preset rotation angle increase initial value (φ _i ), and if the rotation angle has increased by more than the rotation angle increase initial value, the process proceeds to step S9, and if not, the process returns.

In step S9, the current rotation angle value and previous rotation angle value, as well as the current torque value and previous torque value are set. Specifically, the currently detected torque value is set as the current torque value. In addition, the rotation angle reference value is set to 0, and the rotation angle from there to the currently detected rotation angle is set as the current rotation angle value. In addition, the rotation angle of the screw member that is earlier than the current rotation angle (current rotation angle value) by the rotation angle increase initial value is set as the previous rotation angle value, and the torque value detected at the rotation angle of the previous rotation angle value is set as the previous torque value.

Next, the process proceeds to step S10, where the rotation angle difference values between the current rotation angle and all rotation angles (= previous rotation angle values) that were previously set as the previous rotation angle value between the previous rotation angle value and the above-mentioned rotation angle reference value, including the current previous rotation angle value, are calculated, and then the process proceeds to step S11.

In step S11, the torque difference value between the current torque value and all torque values (= previous torque values) previously set as the previous torque value between the current previous torque value and the snug torque is calculated, and then the process proceeds to step S12.

In step S12, the virtual seating point is calculated from all the corresponding rotation angle difference values and torque difference values calculated in steps S10 and S11, and then the process proceeds to step S13. Specifically, all the rotation angle difference values are divided by the corresponding torque difference values to obtain the inverse ratio of the torque increase rate (slope) relative to the rotation angle, and the virtual seating point is obtained by multiplying the inverse ratio of the torque increase rate by the current torque value and subtracting the result from the current rotation angle value. Note that, depending on the computing power of the control device 18, a limit may be set on the number of virtual seating points calculated for each computation process (however, there is no limit on the number of data plotted in the distribution diagram described below).

In step S13, the virtual seating points determined in step S12 are plotted on a distribution map, and then the process proceeds to step S14.

In step S14, the data of the virtual seating points plotted on the distribution diagram in step S13 is approximated by a normal distribution, and then the process proceeds to step S15. This product distribution approximation of the virtual seating point data can be performed by the computer system using a well-known application.

In step S15, the most likely virtual seating point in the approximate normal distribution is calculated and set according to individual calculation processing not shown, and then the process proceeds to step S16. In this example, it is assumed that a large number of calculated virtual seating points can be approximated by a normal distribution, so the virtual seating point corresponding to the most frequent value of this normal distribution is set as the most likely virtual seating point. As is well known, in a normal distribution, the most frequent value is the median and the mean value.

In step S16, it is determined whether or not the virtual seating point and the rotation angle increase amount of the current rotation angle value set in step S15 have reached the rotation angle target value ( _φT ). If the rotation angle increase amount has reached the rotation angle target value, the process proceeds to step S17, and if not, the process returns.

In step S17, the screw member tightening control according to the torque tension method is terminated and then the process proceeds to step S18.

In step S18, the control flag F is reset to 0 before returning.

Meanwhile, in step S19, screw tightening control is performed using the torque angle method according to a calculation process not shown, and then the process proceeds to step S20.

In step S20, it is determined whether the screw member has been tightened a predetermined rotation angle from the rotation angle reference value (snug torque generating rotation angle), and if the screw member has been tightened a predetermined rotation angle from the rotation angle reference value, the process returns to normal, and if not, the process proceeds to step S19.

According to this calculation process, when the rotation angle increases by the initial value of the rotation angle increase amount from the occurrence of the snug torque, for example, for each of the rotation angle specified values set to 1 deg., the virtual seating points are calculated based on all the difference values between all the rotation angles set in the previous rotation angle value in the past and the current rotation angle value, and all the difference values between all the torque values set in the previous torque value in the past and the current torque value. The calculated virtual seating points are plotted on a distribution diagram and further approximated by a normal distribution. As shown in FIG. 6, for example, the data of the virtual seating points approximated by this normal distribution gradually increases with the progress of the screw tightening control, and the fluctuation of the value of the virtual seating point, which varies due to the influence of noise on the torque value, also converges with the accumulation of the number of data (frequency), with the influence of noise being reduced. Therefore, the more the number of data of the calculated virtual seating points increases, the more likely the virtual seating point set from the most frequent value of the normal distribution increases. As a result, in the above calculation process, the virtual seating point when the rotation angle target value is achieved is the most probable virtual seating point, and the axial force accuracy of the screw member rotated by the rotation angle target value from this most probable virtual seating point is the highest.

In the above explanation, it is assumed that the control device has high computational processing power and that the calculated large number of virtual seating point data are approximated by a normal distribution. However, if the computational processing power of the control device is not so high, it may be difficult to approximate the large number of virtual seating point data by a normal distribution. In such a case, for example, the large number of virtual seating point data may be represented in a histogram, and as shown in FIG. 7, the intersection point between a straight line connecting multiple virtual seating point data on the side with a value larger than the mode and a straight line connecting multiple virtual seating point data on the side with a value smaller than the mode may be set as the most likely virtual seating point, close to the mode of the histogram. The well-known least squares method, for example, may be used to set the straight line connecting the virtual seating point data.

In addition, as described above, in the above calculation process, the torque increase rate when snug torque occurs (detection) is calculated, and if the torque increase rate is less than a preset torque increase rate specified value, the torque tension method is switched to the torque angle method to control the screw tightening. The axial force accuracy of the screw member is higher when the screw member is tightened using the torque angle method than when the screw member is tightened using the torque tension method while the virtual seating point is unclear.

In this way, according to the screw tightening device of this embodiment, during the tightening control of the screw member, the virtual seating point is calculated from the torque and rotation angle for each preset rotation angle specified value, the most likely virtual seating point is set from the mode of the approximate normal distribution of the virtual seating points, and the tightening control of the screw member is performed up to the rotation angle target value based on the virtual seating point, thereby performing the tightening control of the screw member according to the torque tension method. At this time, since the noise on the detected torque value increases and decreases in the vicinity of the torque value to be detected, the influence of the noise is reduced if the frequency (number of data) of the virtual seating point set from the mode of the approximate normal distribution increases. Therefore, as the screw member tightening control progresses, the accuracy of the virtual seating point set from the mode of the approximate normal distribution is improved as the most likely virtual seating point, and the tightening control of the screw member is performed up to the rotation angle target value based on this highly accurate virtual seating point, thereby achieving a highly accurate torque tension method, and thereby increasing the axial force accuracy of the screw member as much as possible.

Even if the distribution of the calculated virtual seating points cannot be approximated by a normal distribution, the virtual seating points can be made into a histogram, for example. By setting the intersection of lines connecting multiple virtual seating points on the large and small sides of the most frequent value of this histogram as the virtual seating point, as described above, the virtual seating point closest to the most frequent value of the approximate normal distribution can be set as the most likely virtual seating point as the screw tightening control progresses. Therefore, even if the calculation processing capacity of the screw tightening device is not particularly high, screw tightening control in accordance with the torque tension method can be achieved, thereby making it possible to increase the axial force accuracy of the screw member as much as possible.

In addition, if the torque increase rate relative to the rotation angle of the screw member is less than the torque increase rate specified value, the screw member tightening control is switched, and after the screw member tightening control is switched, the screw member is rotated a predetermined rotation angle set in advance after the torque reaches a specified value, for example, snug torque, in accordance with the above-mentioned torque angle method. This makes it possible to increase the axial force accuracy of the screw member as much as possible, even if it is not as high as the original torque tension method.

The screw tightening device and screw tightening method according to the embodiment have been described above, but the present invention is not limited to the configuration described in the above embodiment, and various modifications are possible within the scope of the gist of the present invention. For example, the rotation angle and torque value used to calculate the virtual seating point are not limited to those at the above timings.

REFERENCE SIGNS LIST 10 Socket 12 Electric motor 14 Torque detector 16 Rotation angle sensor 18 Control device

Claims (4)

1. A screw fastening device which detects a rotation angle of a screw member and a torque of the screw fastening during screw fastening, calculates a virtual seating point of a seating surface of the screw member from a torque increase rate of the torque with respect to the rotation angle, and fastens the screw member from the virtual seating point to a target rotation angle value,
the torque increase rate is calculated by rotating the screw member by an initial rotation angle increase value when the screw member is seated on a member to be fastened and a detected torque value of the fastening torque reaches a snug torque, and dividing a difference between the detected torque value at that time and the snug torque by the initial rotation angle increase value;
the virtual seating point is a seating point calculated based on the torque increase rate, and is a point where a rotation angle of the screw fastener with respect to a target member after the screw fastener has been seated on the seating surface becomes zero, and the virtual seating point is calculated from the torque increase rate on the basis that the fastening torque of the screw fastener increases linearly with respect to the rotation angle from the seating point,
a virtual seating point calculation means for calculating the virtual seating point of the screw member by obtaining the torque increase rate from the detected torque for each rotation angle specified value, which is a preset rotation angle interval;
a normal distribution approximation means for approximating the distribution of the virtual seating points calculated by the virtual seating point calculation means with a normal distribution;
a virtual seating point setting means for setting a virtual seating point corresponding to a mode of the normal distribution of the virtual seating points approximated by the normal distribution approximation means as the most likely virtual seating point;
and a control means for controlling the tightening of the screw member based on the virtual seating point set by the virtual seating point setting means. The screw tightening device according to claim 1, characterized in that the virtual seating point setting means sets, instead of the mode of the normal distribution of the virtual seating points, a virtual seating point corresponding to the intersection of a straight line connecting a plurality of virtual seating point data on the side of the mode that is greater than the mode and a straight line connecting a plurality of virtual seating point data on the side of the mode that is smaller than the mode, as the most likely virtual seating point. The screw tightening device according to claim 1 or 2, characterized in that, when the torque increase rate relative to the predetermined rotation angle of the screw member is less than a predetermined torque increase rate specified value, the control means switches to a screw member tightening control in which the screw member is rotated by a predetermined rotation angle after the torque reaches a predetermined torque specified value. 1. A method for tightening a screw, comprising: detecting a rotation angle of a screw member during tightening and a torque of the screw tightening; determining a virtual seating point of a seating surface of the screw member from a torque increase rate of the torque with respect to the rotation angle; and tightening the screw member from the virtual seating point to a target rotation angle value,
the torque increase rate is calculated by rotating the screw member by an initial rotation angle increase value when the screw member is seated on a member to be fastened and a detected torque value of the fastening torque reaches a snug torque, and dividing a difference between the detected torque value at that time and the snug torque by the initial rotation angle increase value;
the virtual seating point is a seating point calculated based on the torque increase rate, and is a point where a rotation angle of the screw fastener with respect to a target member after the screw fastener has been seated on the seating surface becomes zero, and the virtual seating point is calculated from the torque increase rate on the basis that the fastening torque of the screw fastener increases linearly with respect to the rotation angle from the seating point,
a virtual seating point calculation step in which a control device having a computer system mounted thereon calculates the virtual seating point of the screw member by determining the torque increase rate from the detected torque for each rotation angle specified value, which is a preset rotation angle interval;
a normal distribution approximation step of approximating the distribution of the virtual seating points calculated in the virtual seating point calculation step with a normal distribution by the control device;
a virtual seating point setting step of setting a virtual seating point corresponding to a mode of a normal distribution of the virtual seating points approximated in the normal distribution approximation step as a most probable virtual seating point;
a control step in which the control device controls the tightening of the screw fastener based on the virtual seating point set in the virtual seating point setting step.

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