KR101759301B1

KR101759301B1 - Control method of electrically-drive tool

Info

Publication number: KR101759301B1
Application number: KR1020160003276A
Authority: KR
Inventors: 오성섭; 이종진; 김준영
Original assignee: 계양전기 주식회사
Priority date: 2016-01-11
Filing date: 2016-01-11
Publication date: 2017-08-01
Also published as: WO2017122866A1

Abstract

Provided is a method for controlling an electrically driven tool having an electric motor disposed therein and fastening nuts by controlling the current supplied to the electric motor, the electrically driven tool controlling method comprising: a step (1) for determining, at the time of seating a nut, whether or not the rotation speed of the electric motor reaches a preset speed (Vt); a step (2-1) for, if the speed (Vt) is not reached, suspending the supply of current, fastening the nut solely by the rotational inertia force of the electric motor and, at the same time, detecting measured torques according to time, and thereby calculating a first torque rate; and a step (3) for, after the rotation speed of the electric motor becomes zero, consecutively providing an impact torque and variably controlling the measured torques with respect to each impact torque according to the first torque rate.

Description

[0001] CONTROL METHOD OF ELECTRICALLY-DRIVE TOOL [0002]

BACKGROUND OF THE INVENTION 1. Field of the Invention [0002] The present invention relates to a control method of an electric power tool, and more particularly, to a control method of a power tool that can accurately fasten a nut in an assembly process of an automobile or the like.

A worker who fastens a bolt, a nut, or the like is likely to be exposed to an injury due to repetitive reaction force generated by the power tool during the work. In recent years, a large part of the assembling line of an automobile assembly factory has been automated. However, some of the zones responsible for fastening bolts and nuts are still in operation by workers.

On the other hand, the fastening of nuts and the like in the automobile industry requires a higher level of fastening quality, especially since it is closely related to safety. At the same time, the fastening process is required to be terminated quickly for high productivity. Therefore, a control program dedicated to nut tightening is built in the power tool.

The process of fastening the nut is as follows. First, the nut is moved toward the workpiece by the power tool while rotating along the thread of the bolt. However, when the nut is brought into contact with the workpiece, the rotation speed of the nut at the time of seating is significantly reduced. Thereafter, the nut is further torqued to tighten it. This is to prevent the nut from loosening in the future. That is, the fastening process is completed only by providing a predetermined target torque to the nut and finally tightening the nut.

Conventionally, the power tool was controlled to rapidly seat the nut while rotating at a high torque with low torque before seating. And, after seating, the power tool is set to 1) either a continuous torque that is not interrupted in the middle or 2) an impact torque that occurs intermittently but provides instantaneous impact. However, the continuous torque has a problem that a relatively large reaction force is generated as compared with the impact torque. On the other hand, the impact torque has a problem that 1) the torque can not be accurately controlled in comparison with the continuous torque, and 2) the energy consumption is relatively high.

On the other hand, when the nut is further tightened after seating, the rotation angle of the nut and the target torque, that is, the joint characteristics, required for completion of the engagement are different due to the difference in friction coefficient depending on the fastening site. At this time, it is not possible to reflect such joint characteristics, and there is a problem in that tightening accuracy is lowered if torque is provided to the nut. However, since the conventional power tool has no way of determining the joint characteristics by itself, the fastening process has been carried out by grasping the joint characteristics according to the fastening part in advance before the fastening process and switching the mode of the power tool accordingly.

The embodiments of the present invention have been devised to solve the above problems and provide a control method of a power tool capable of grasping the characteristics of a joint with respect to a fastening part of a nut or the like and reflecting the result and improving the tightening accuracy do. In addition, it aims at minimizing the energy consumed in the power tool and fastening it at the same time.

Another object of the present invention is to reduce the defective rate of the fastening process in case the power tool can not be properly controlled due to a failure of the fastening member or the like.

In order to solve the above-described problems, an embodiment of the present invention provides a control method for an electric power tool in which an electric motor is disposed, and a current is supplied to control a current supplied to the electric motor, A first step of determining whether the rotation speed reaches a preset speed Vt; A second step of calculating a first torque ratio by detecting a measured torque according to time while stopping supply of current and engaging a nut only with rotational inertia of the electric motor when speed Vt is not reached; And a third step of variably controlling a measured torque for each impact torque in accordance with the first torque rate by sequentially providing an impact torque after the rotational speed of the electric motor becomes zero, Control method.

In the third step, the rate of increase with respect to the maximum value of the measured torque may be controlled to coincide with the first torque rate.

In the third step, the impact torque may be generated by supplying a pulse current to the electric motor.

The magnitude of the pulse current which is generated first among the pulse currents can be variably controlled by reflecting the total number of generation of pulse currents and the duration thereof.

The pulse current is a square wave current, and the rate of increase of each square wave current with respect to each initial value may be a constant constant.

And a fourth step of variably controlling and correcting the increase rate when the increase rate is inconsistent with the first torque rate.

And (2-2), in the first step, when the speed Vt is reached, a step 2-2 of supplying a brake current to the electric motor to provide a continuous torque gradually increasing so that a target torque is applied to the nut.

If the instantaneous rate of change with respect to the rotational speed of the electric motor is equal to or greater than D in the step 2-2, the supply of the brake current is stopped, and the torque is detected with time while the nut is fastened only by the rotary inertia force of the electric motor And a second step of calculating a second torque ratio.

The method of claim 1 or 2, further comprising: after the step of 2-3, providing an impact torque sequentially after the rotational speed of the electric motor becomes zero, wherein an increase rate of the measured torque with respect to a maximum value of each impact torque is matched with the second torque rate (Step 2-4).

As described above, according to the present invention, various effects including the following can be expected. However, the present invention does not necessarily achieve the following effects.

If the rotational speed of the electric motor does not reach the speed Vt at the time of seating, the first torque ratio is calculated by detecting the measured torque with time only by the rotary inertia force of the electric motor after the nut is seated, It is possible to improve the tightening accuracy by matching the rate of increase with respect to the maximum value of the measured torque with respect to each impact torque provided to the nut to the first torque rate. At this time, if the increase rate is inconsistent with the first torque ratio, it can be corrected and the tightening accuracy can be further improved.

Further, when the rotational speed of the electric motor reaches the speed Vt at the time of seating, a brake current is supplied to the electric motor to control the target torque to be applied to the nut, so that the engaging energy can be minimized. At this time, if the power tool is not properly controlled due to the failure of the fastening member such as a nut, the method of providing the torque is changed to the impact torque, and the defect rate of the fastening process can be reduced. In this case, however, the second torque ratio can be calculated based only on the rotational inertia of the electric motor, and the rate of increase with respect to the maximum value of the measured torque for each impact torque can be controlled as a reference. As a result, the fastening accuracy can be improved.

At the same time, when the impact torque is provided by the power tool, the impact torque can be quickly reached to the target torque, thereby improving the productivity.

1 is a flowchart showing a control method of a power tool according to an embodiment of the present invention;
2 is a graph showing a measured torque according to a rotation angle;
3 is a graph showing measured torque and pulse current with time;

Hereinafter, specific embodiments of the present invention will be described in detail with reference to the drawings.

1 is a flowchart showing a method of controlling an electric power tool according to an embodiment of the present invention. 1, a method of controlling an electric power tool includes a first step (s10) of judging 1) a second step (s20) of calculating a first torque ratio (a), 3) A third step (s30), (4) a fourth step (s40) of correcting, 5) a second step (s25) of providing a continuous torque, 6) a second step of calculating a second torque rate ), And 7) second-fourth step (s27) for controlling the second torque ratio to coincide with the second torque ratio.

The control method according to an embodiment can be applied to a power tool using an electric motor as a means for providing torque. At this time, the power tool further includes a current control unit dedicated to control of the current, so that the tightening accuracy can be improved. The power tool may further include a speed sensor for detecting the rotational speed of the electric motor, a torque sensor for detecting a measurement torque actually applied to the nut, an encoder for detecting a rotation angle of the nut after being seated, and the like.

The term 'sitting' means that as the nut advances while rotating along the thread of the bolt, especially when the head of the nut comes into contact with the workpiece and the rotation speed of the nut begins to decrease due to the increase of the coefficient of friction, It refers to the placement status. On the other hand, the fastening process is completed only after the nut is further tightened after being seated.

The power tool suddenly raises the rotational speed of the electric motor from zero for seating. That is, a low current is instantaneously supplied to the electric motor in the stopped state, and the time required for seating can be shortened.

On the other hand, the operator can calculate the fastening energy required for fastening according to the type of the nut and the fastening parts of each kind. However, since the fastening energy depends on the fastening method and its path, it is necessary to optimize the fastening method and the like in order to minimize the fastening energy. For example, by using the rotational inertia force of the electric motor, the consumption of the engagement energy can be reduced accordingly. This is a method of controlling the rotation speed of the electric motor by controlling the electric current supplied to the electric motor.

On the other hand, it is desirable that the speed of the electric motor reaching the constant speed is kept constant until the time when the nut is seated. That is, the control method of the power tool according to an embodiment is a method of controlling the rotational speed of the electric motor. At this time, a relatively low torque is constantly applied to the nut, but the nut can be quickly seated by the high-speed rotation of the electric motor.

This method can accurately predict when the seat is seated, and can shorten the time required for seating by the electric motor rotating at a high speed. As a result, the productivity of the fastening process can be improved.

The first step s10 of judging is whether or not the rotational speed of the electric motor reaches the predetermined speed Vt at the time of seating. At this time, the speed Vt is an optimized speed to further tighten the nut after the seat is seated, and to finally apply the target torque to the nut. However, the speed Vt may vary depending on the type of the fastening member, the characteristics of the fastening surface, the size of the target torque, and the like.

In general, since the seating is normally performed, the rotational speed of the electric motor can reach the speed Vt at the time of seating. However, if there is a defect in the thread of the bolt or the nut, the contact surface of the nut, etc., the rotation speed does not reach the speed Vt.

First, if the rotational speed of the electric motor does not reach the speed Vt in the first step s10 to judge, the second-first step s20 is performed. This is because, if the rotational speed does not reach the speed Vt at the time of seating, the fastening method using the rotational inertia force of the electric motor can not provide the target torque to the nut, resulting in poor fastening.

FIG. 2 is a graph showing the measured torque according to the rotation angle, and FIG. 3 is a graph showing the measured torque and the pulse current with time. Referring to FIGS. 2 and 3, the second-first step (s20) immediately stops supplying the electric current to the electric motor, and sets the rotational speed thereof to zero. However, after a certain period of time elapses due to the action of the rotational inertia force, the rotational speed of the electric motor becomes zero. At this time, the nut is further tightened only by the rotational inertia force of the electric motor.

After seating, the tightening of the nut is affected by the joint properties. The joint characteristics are specified by the International Organization for Standardization. Specifically, the hard joint assumes that the head of the nut rotates more than 30 degrees after being seated, while the soft joint assumes that the joint is normally engaged when the angle further rotates within 360 degrees or 720 degrees. However, the target torque is set in advance regardless of such joint characteristics.

That is, the joint characteristics depend on the friction coefficient between the fastening surface and the nut to be fastened in contact with the fastening surface. Such a joint characteristic can be grasped when the current supply to the electric motor is interrupted and only the rotational inertia acts on the nut. Specifically, the joint characteristic is obtained by a value (m1 / d1) obtained by detecting the rotation angle d1 of the nut and the measurement torque m1, respectively, and then dividing the measured torque by the rotation angle. Here, the maximum value of the measured torque is used.

Theoretically, the joint properties remain constant until the fastening process for each fastening site is completed. That is, the fastening process after tightening is a linear region. Therefore, the points shown in Fig. 2 are placed on a straight line. However, the actual joint characteristics may be variable due to changes in the friction coefficient and the like as the rotation angle of the nut increases.

The first torque ratio [theta] can be calculated by detecting the measured torque over time through the second-first step (s20). Referring to Fig. 3, the measured torque by only the rotational inertia force increases linearly. As a result, when the rate of increase m1-ma of the measured torque with respect to the elapsed time t1-ta is found, the first torque rate a corresponding to the slope can be calculated. At this time, the first torque ratio reflects the characteristics of the joint, so that the power tool can judge whether the joint is a hard joint or a soft joint.

Then, in the third step s30, the nut is partially tightened only by the rotational inertia force of the electric motor, and when the rotational speed of the electric motor becomes zero, the impact torque is thereafter sequentially provided. Here, the impact torque is a single value preset by the current control, and the measured torque is a set of successive values in which the impact torque applied to the nut is actually detected through the torque sensor. At this time, it is preferable to match the maximum value of the measured torque with the impact torque.

At this time, the power tool variably controls the measurement torque for each impact torque in accordance with the first torque ratio (a). That is, the first torque ratio (a) may provide a reference to at least one or more measurement torque that sequentially increases so that the target torque is applied to the nut. Preferably, the rate of increase (b) with respect to the maximum value of the measured torque for each impact torque is controlled to coincide with the first torque rate (a). In this way, tightening the nut by reflecting the joint characteristics improves the tightening accuracy.

Specifically, the impact torque is generated by supplying an intermittent pulse current to the electric motor. At this time, the power tool variably controls the magnitude of the pulse current generated first in the pulse current by reflecting the total number of generation of the pulse current and its duration. To do this, you can first calculate the residual energy required to apply the target torque to the nut using the joint properties. This is because the fastening process after being seated is subject to linearity.

It is also possible to control the total number of times of generation of the pulse current within a preset range by dividing the remaining energy into several impact torques. Further, due to the characteristics of the impact torque, the duration of the pulse current is relatively short. At this time, the sustain time can be constantly controlled for all the pulse currents. Therefore, the magnitude of the pulse current for generating the initial impact torque can be variably controlled by reflecting the remaining energy, the total number of generation of the pulse current, and the duration thereof.

Referring again to FIG. 3, the maximum value of the measured torque for the initially supplied pulse current is m2. At this time, in Fig. 2, the nut is rotated by a predetermined angle by the pulse current initially supplied, and the power tool can detect the accumulated angle d2 through the encoder. However, from the maximum values (m3 to mf) of the measured torque for the pulse current supplied immediately thereafter, the slope of the first torque rate is constantly increased. At this time, d3-d2, d4-d3, etc., which are the rates of increase of the rotation angle with respect to the nut, will also be constant.

Alternatively, however, the maximum value of the measured torque for the initially supplied pulse current may be increased constantly from the value of m2 to the slope of the first torque rate. This is because the magnitude of the initially supplied pulse current can be variably controlled as described above.

In one embodiment, the pulse current is preferably a square wave current. That is, the electric motor is supplied with a square wave current having the same initial value and final value. At this time, it is preferable that the rate of increase with respect to each initial value of the adjacent square wave current is a constant constant. As a result, the rate of increase b with respect to the maximum value of the measured torque for each impact torque can be controlled to coincide with the first torque rate a. Because the impact torque is proportional to the magnitude of the supplied energy, the magnitude of the energy is proportional to the magnitude of the square wave current under the condition that the duration of each square wave current is constant.

The control method of the power tool according to an embodiment of the present invention includes a fourth step of variably controlling the rate of increase with respect to the initial value of each square wave current when the rate of increase b is inconsistent with the first torque rate a s40. That is, if the maximum value of each detected torque to be detected is in excess of each impact torque set in advance according to the first torque ratio, the initial value of the square wave current can be corrected.

Specifically, the maximum value of the measured torque to be detected and the preset impact torque are compared with each other, and the error is calculated. The result is determined to be one of three things: 1) match, 2) less than, and 3) greater. In the case of 2) and 3), since the impact torque due to the square wave current in which the actual first torque ratio is reflected is not applied to the nut, the magnitude of the square wave current supplied immediately after that is corrected by the error.

At this time, a proportional expression is used for the correction for the error. As described above, the fastening process after being seated is theoretically a linear region in which the friction coefficient and the like are unchanged by the first torque ratio. As a result, for example, if the error occurring between the impact torque and the maximum value of the measured torque is minus 10%, the initial value of the next square wave current is 10% to the previously calculated increase rate in accordance with the first torque rate Can be calculated. Therefore, the maximum value of the measurement torque actually applied to the immediately following nut becomes closer to the predetermined impact torque (the maximum value of the expected measurement torque). As a result, the tightening accuracy can be improved. Further, when providing an impact torque in the electric power tool, the impact torque can be quickly reached to the target torque, thereby improving the productivity.

The power tool according to an embodiment continuously corrects whether the magnitude of the impact torque set in advance and the maximum value of the detected torque match with each other, corrects the magnitude of the impact torque when the magnitude of the impact torque is equal to or less than the predetermined value, When the torque is reached, the supply of the square-wave current is cut off through the current control unit to complete the fastening process.

Alternatively, when the rotational speed of the electric motor reaches the speed Vt in the first step s10 to be judged, the second-2 step (s25) is performed. Step 2-2 (s25) is a step of supplying a brake current to the electric motor to provide a continuous torque that gradually increases so that the target torque is applied to the nut. At this time, the continuous torque is generated by the rotational inertia force of the electric motor by the speed Vt, and the speed Vt is gradually decelerated. On the other hand, the deceleration rate may be constant or variable.

The brake current is continuously supplied to the electric motor only after the seating time, until the target torque is applied to the nut. However, it is preferable that the brake current is controlled so that the magnitude of the brake current becomes zero as soon as the target torque is applied to the nut. At this time, the speed Vt is moderately decelerated by the brake current, and becomes zero as soon as the target torque is applied to the nut. That is, the electric motor stops. As a result, the engaging process is normally finished when the maximum value of the measured torque coincides with the target torque.

Therefore, the control method of the electric power tool including the second step (s25) of placing the nut on the target and then applying the target torque to the nut is a method of controlling only the rotational speed of the electric motor, can do. This is a result of precisely controlling the rotational speed of the electric motor while using the rotational inertia force of the electric motor.

Then, in step 2-2 (s25), if the instantaneous rate of change with respect to the rotational speed of the electric motor is equal to or greater than D, step 2-3 is performed. This reflects the instantaneous rate of change in the rotational speed of the electric motor due to defects present in the threads of the bolt or nut. Here, D is generally a value exceeding 0, and D may be a relatively large value when the rotation speed rapidly changes.

Specifically, when the instantaneous change rate is D or more, the supply of the brake current is stopped, and the nut is fastened only by the rotational inertia force of the electric motor. At this time, the rotational speed becomes zero after a certain time elapses due to the rotational inertia force.

As a result, the second torque ratio can be calculated by detecting the measured torque according to the time only by the rotational inertia force. Here, the second torque ratio is also the same as the first torque ratio (a) in theory, which is a joint characteristic. However, the second torque ratio may be slightly different from the first torque ratio in view of the change in friction coefficient and the like depending on the degree of engagement.

Then, after the rotational speed of the electric motor becomes zero, the power tool proceeds to Step 2-4 (s27) of sequentially providing the impact torque. That is, the manner of providing the torque applied to the nut is switched to the impact torque generated intermittently. This is because the fastening method for controlling the rotational speed of the electric motor can not finally provide the target torque to the nut when the rotational speed thereof varies from a predetermined value. That is, even if the rotational speed of the electric motor suddenly changes, the electric motor can not follow the predetermined rotational speed even if the brake current is controlled.

Further, in step 2-4 (s27), the rate of increase with respect to the maximum value of the measured torque for each impact torque is controlled to coincide with the second torque rate. The detailed reason is the same as that described above, and the detailed description will be omitted. Further, a square wave current is used to provide an impact torque, and the control method thereof is the same as that already described.

In addition, if the maximum value of the measured torque is in excess of the predetermined magnitude of the impact torque, the increment of the square wave current with respect to the initial value is variably controlled and corrected. The correction control is also the same as described above, and a detailed description thereof will be omitted. The control method of the power tool according to one embodiment can minimize the clamping energy. Further, when the power tool can not be properly controlled, it is possible to prevent a failure in the fastening process.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the invention.

s10: First step to judge
s20: Step 2-1 for calculating the first torque ratio
s30: The third step of variable control
s40: Fourth step of correction
s25: Step 2-2 to provide continuous torque
s26: Step 2-3 of calculating the second torque ratio
s27: Step 2-4 to control to match the second torque rate

Claims

A control method for an electric power tool in which an electric motor is disposed, and a current supplied thereto is controlled to fasten a nut,
A first step of determining whether or not the rotational speed of the electric motor reaches a preset speed Vt at the time when the nut is seated;
A second step of calculating a first torque ratio by detecting a measured torque over time while stopping the supply of current and engaging the nut only with the rotary inertia force of the electric motor when the speed Vt is not reached; And
And a third step of variably controlling a measured torque for each impact torque in accordance with the first torque ratio by successively providing an impact torque after the rotational speed of the electric motor becomes zero, Way.

2. The method according to claim 1, wherein in the third step,
And controls the rate of increase with respect to the maximum value of the measured torque to coincide with the first torque rate.

2. The method according to claim 1, wherein in the third step,
Wherein the impact torque is generated by supplying a pulse current to the electric motor.

The method of claim 3,
And controlling the magnitude of the pulse current that is generated first among the pulse currents to be variable by reflecting the total number of generation of pulse currents and the duration thereof.

The method of claim 3,
Wherein the pulse current is a square wave current and the rate of increase with respect to each initial value of the square wave current is a constant constant.

6. The method of claim 5,
And a fourth step of variably controlling and correcting the increase rate when the increase rate does not match the first torque rate.

2. The method according to claim 1, wherein, in the first step,
And a second step (2-2) of supplying a brake current to the electric motor when the speed Vt is reached, thereby providing a continuous torque gradually increasing so that a target torque is applied to the nut.

8. The method according to claim 7, wherein, in step 2-2,
If the instantaneous rate of change with respect to the rotational speed of the electric motor is equal to or greater than D, the supply of the brake current is stopped and the measured torque is detected with time while the nut is fastened only by the rotary inertia force of the electric motor, Further comprising the step (2).

9. The method as claimed in claim 8,
(2-4) controlling the rate of increase with respect to the maximum value of the measured torque with respect to each impact torque to coincide with the second torque rate while sequentially providing the impact torque after the rotational speed of the electric motor becomes zero. Further comprising the steps of: