JP2646445B2 - Screw fastening method and device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for tightening a screw by rotating the screw with a motor.

2. Description of the Related Art Screw assembling work is the most basic in assembling work in various industries such as precision equipment such as watches and cameras, electronic equipment such as computers and disk drives.

Previously, this screwing operation was performed by an operator turning a screw using a normal driver. But,
2. Description of the Related Art In recent years, electric drivers that rotate a screw with a motor have been widely used because of difficulty in improving work efficiency.

The most important thing in the screw tightening operation is to perform the screw tightening with an appropriate tightening force that does not easily loosen even when an external force is applied. When screwing by hand,
We learned empirically how much tightening power should be used, and as a skilled person, we were able to perform screw tightening work with extremely appropriate tightening force. However, the work efficiency could not be improved to a certain degree or more.

On the other hand, when an electric screwdriver is used, it can be expected to improve the working efficiency. However, when an electric screwdriver is used to fasten the screw, there is a possibility that the screw is over-tightened and the screw is broken, and furthermore, it is necessary to control the screw tightening force to a constant value.

For this reason, the following various measures are taken in the conventional electric driver.

If the rotation speed of the motor is increased, even a slight shift in timing may cause the screw to be overtightened and destroyed. The larger the moment of inertia of the motor, the easier it is to overtighten the screw. For this reason, the conventional electric driver uses a small motor having a small moment of inertia as much as possible.

On the other hand, a considerably high screw tightening force in the screw tightening operation is required. For this reason, in the conventional electric screwdriver, screw tightening is performed in two stages, the motor is driven at a relatively high speed with a small motor current in the initial stage of screw tightening, and the motor current is increased in the final stage of screw tightening. The control is switched to constant current control so as to obtain a desired large tightening force, and the tightening is performed with a constant torque.

[Problem to be Solved by the Invention] However, in the case of such a conventional electric screwdriver, if the rotation speed of the motor is increased in the initial stage, the screw will be overtightened, and there is a problem that the screw cannot be increased to a certain extent. Was. However, in order to increase work efficiency, it is desirable to rotate the screw as fast as possible in the initial stage before the screw is seated. Therefore, conventionally, adjustment was made based on experimental data based on experience and intuition, and how to rotate the screw at a high speed without excessively tightening the screw was determined. In addition, since the allowable rotation speed differs depending on the type of screw to be tightened, it is necessary to make adjustments in a groping state for each screw, which further reduces the working efficiency.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a screw tightening method and apparatus that can always perform screw tightening with an appropriate tightening force at high speed.

[Means for solving the problem] Conventionally, screw tightening has been performed based on experience and intuition, and it is not a full-scale analysis of the dynamic operation of screw tightening, but basically based on any equation of motion. It was unknown at all whether the screws would be tightened.

The inventor of the present application has performed a dynamic analysis on screw tightening using a motor, and as a result of earnest research efforts, has succeeded in obtaining a basic equation of motion that appropriately expresses a screw tightening operation. That is, the basic equation of motion of the screw tightening operation is as follows.

I · d ² θ / dt ² = −f (θ) + g (i) where I is the total moment of inertia rotated by the motor, θ is the rotation angle of the motor, and f (θ) is the drag torque repelled from the screw. , I is the current value of the motor, and g (i) is the motor torque generated by the motor.

The present invention has been made based on such an equation of motion. The principle of the present invention will be described with reference to FIG.

The screw 1 is tightened by a bit 3 rotated by a motor 2. The rotation angular acceleration detection means 4 constantly detects the rotation angular acceleration d ² θ / dt ² from the rotation state of the motor 2,
The torque detecting means 5 detects a motor torque g (i) generated by the motor 2. The motor drive control unit 6 calculates a motion equation f (θ) obtained by transforming the above motion equation from the rotation angular acceleration d ² θ / dt ² from the rotation angular acceleration detection unit 4 and the motor torque g (i) from the torque detection unit 5. ) = − I · d ² θ / dt ² + g (i), the drag torque f (θ) of the screw, that is, the tightening torque of the screw is calculated, and the drag torque f (θ) becomes the target torque value. The drive of the motor 2 is controlled so as to stop at the time.

[Operation] According to the present invention, since the drive control of the motor is performed based on the equation of motion of screw tightening, the drag torque of the screw can always be accurately grasped. Therefore, if the motor is stopped when the drag torque reaches the set value, the screw can always be tightened with an accurate tightening torque.

Embodiment FIG. 2 shows a screw fastening device according to an embodiment of the present invention.

In this embodiment, for example, a DC motor 12 is used. A bit 14 is attached to the tip of the motor 12, and the screw 10 is rotated and tightened by the bit 14.

At the rear of the motor 12, for example, an encoder 16 is attached to detect the rotation angle θ. The encoder 16
For example, the grating engraved on a rotating plate attached to the rotating shaft of the motor 12 is optically read, and a pulse signal corresponding to the rotating angle θ of the motor 12 is generated.

The rotation angle signal from the encoder 16 is differentiated at the time t by the differentiating circuit 18, and the rotation angular velocity dθ / dt (rotational angular velocity ω)
Is required. Further, the rotation angular velocity signal is differentiated at time t by the differentiating circuit 20, and the rotation angular acceleration d ² θ / dt ² is obtained.

Rotational angular acceleration d ² θ / dt obtained by differentiating circuit 20
² is multiplied by a moment of inertia I by a multiplying unit 22, and I
・ D ² θ / dt ² is required. The moment of inertia I is
The moment of inertia is the total moment of inertia rotated by the motor, and more precisely includes the moment of inertia of the bit 14 and the screw 10 in addition to the moment of inertia of the motor 12. However, since the moment of inertia of a member having a small diameter is negligibly small, the moment of inertia I may actually be used as the moment of inertia of the motor 12. Therefore, screw 10 or bit
The inertia moment I is the same even if the 14 types are different.

The multiplication unit 24 calculates the motor torque g (i) generated by the motor 12.
Is calculated. That is, in the case of the DC motor 12, since it is known that the motor torque g (i) is obtained by multiplying the motor current i by the torque constant k, the torque i of the motor 12 is reduced by the current i supplied to the motor 12. The constant k is multiplied to obtain ki which is the motor torque g (i).

The subtraction unit 26 subtracts I · d ² θ / dt ² from the multiplication unit 22 from k · i, which is the motor torque g (i) from the multiplication unit 24, to obtain a drag torque f (θ) f (θ). = −I · d ² θ / dt ² + k · i.

The comparison unit 28 calculates the drag torque f obtained by the subtraction unit 26.
(Θ) is compared with the set drag torque value f ₀ . The drag torque value f ₀ is determined by a torque setting unit 29 constituted by a variable resistor.
Can be set freely from outside.

The comparing unit 28 changes the drag torque f (θ) to the drag torque value f _0.
Is detected, the detection signal is output to the motor current switch 30 immediately. The drag torque value
It is desirable that f ₀ be a value slightly lower than the target torque value F ₀ defined as the tightening torque at the time of final screw tightening. This is because the motor 12 cannot actually be stopped in 0 seconds or a sufficiently short time.

Motor current change-over switch 30 is switched and a motor current output from the constant speed control unit 34 for rotating the motor 12 in the direction of tightening the screw 10, the reverse and braking current i _B, the motor driving unit 32 To supply. Constant speed rotation control unit 34
Is for outputting a current for rotating the motor 12 at a constant speed at the designated rotational angular speed ω _0. The motor current during the constant speed rotation control is limited by the maximum motor current i _{R so} that unnecessary tightening torque is not generated. I have to. First, the constant speed rotation control unit
The motor current i _R from 34 is supplied to the motor drive unit 32 to rotate the motor 12. When the comparison unit 28 detects that the drag torque f (θ) has reached the set value f ₀ , the motor drive unit 32 The brake current i _B is supplied to apply a brake to the motor 12.

The motor drive unit 32 drives the motor 12 with the current from the motor current changeover switch 30 and has a rotational angular velocity dθ / d
When t becomes zero, the supply of current to the motor 12 is stopped.

Next, the operation of this embodiment will be described with reference to FIG. 2A is a time chart of the motor current i, FIG. 2B is a time chart of the rotational angular velocity ω (= dθ / dt) of the motor 12, and FIG. f
6 is a time chart of (θ).

Initially, the screw head 10a of the screw 10 is not seated, the motor 12 is rotated at a constant speed controlled by the constant-speed rotation control unit 34, it is rotated at a rotational angular velocity omega _0. At this time, screw 10
Is also not seated, so its drag torque f (θ)
Is an almost constant low value because it is caused only by the frictional force between the external thread and the internal thread.

Immediately before sitting, the rotational angular velocity ω tends to gradually decrease, and as the motor current i increases for constant-speed rotation control, the drag torque f (θ) increases. However, since the motor current i of the motor drive unit 32 is limited by the current limit value i _R, and seated rotational angular velocity ω becomes rapidly lowered, drag torque f (theta) increases sharply.

When the drag torque f (θ) reaches a preset drag torque value f ₀ , the motor current switch 30 is switched to the brake current i _B by a signal from the comparison unit 28. Then, the motor drive unit 32 supplies the brake current i _B in the reverse direction to the motor 12 to apply a sudden brake.

When the motor 12 stops and the rotational angular velocity ω output from the differentiating circuit 18 becomes zero, the motor drive unit 32 immediately turns off the brake current as shown in FIG.

Since the motor 12 also rotates during the braking period of the motor 12, the drag torque f (θ) rises beyond the set value f ₀ and stops almost at the tightening target torque value F ₀ .

As described above, according to the present embodiment, the motor is driven and controlled while detecting the drag torque of the screw. Therefore, if the motor is suddenly stopped when the drag torque reaches a set value, a desired value can be obtained. Screws can be tightened with tightening torque.

Conventionally, in order to prevent the screw from being overtightened, it was necessary to use a motor having a small moment of inertia and to keep the screw in a range where it is not overtightened until seating. However, according to the present embodiment, screw tightening is always performed while accurately grasping the drag torque, so that proper screw tightening can be performed even when the moment of inertia is increased and the rotational angular velocity of the motor is increased. This has a number of advantages:

Generally, a motor having a small inertia moment and a large motor torque is expensive, while a motor having a large inertia moment is inexpensive. Therefore, according to this embodiment, an inexpensive motor having a large inertia moment can be used.

Conventionally, the screw is tightened to minimize the influence of the moment of inertia.In contrast, in the present embodiment, the screw is tightened by using the force of the moment of inertia. Screw can be tightened.

Further, by using a motor having a large moment of inertia, it is possible to avoid a situation in which the screw stops spontaneously before the drag torque of the screw reaches the set value and the screw cannot be tightened with a desired torque.

Further, according to the present embodiment, screw tightening can be performed at a high speed from the initial stage of screw tightening. Usually, in the screw tightening operation, the angle at which the screw is rotated to the seating position is overwhelmingly larger than the tightening angle after the seating operation. In addition, the driving of the motor until seating does not affect the tightening torque at all. Therefore, in order to shorten the total time of the screw tightening operation, it is more effective to shorten the time until the seating than the screw tightening time after sitting. However, conventionally, high-speed rotation could not be performed until the seat was seated in order to prevent the screw from being overtightened. However, according to the present embodiment, the screw can be fastened from the beginning without considering excessive screw tightening, so that the entire time of the screw tightening operation can be greatly reduced.

According to the present embodiment, screw tightening can be performed at an angular velocity 3 to 5 times or more as compared with the related art, and the screw tightening time is reduced to 3 to 5 times.
It was able to shorten more.

The present invention is not limited to the above embodiment, and various modifications are possible.

In the above embodiment, the motor current was set to a constant value until the drag torque reached the set value.However, at the start of driving the motor, a large current was supplied to start the motor as quickly as possible, and after the speed became constant, the current value was reduced. You may make it.

Further, according to the present invention, the seating of the screw can be reliably detected by monitoring the drag torque f (θ). Therefore, after seating, the current of the motor is turned off to make g (i) = 0,
The screw may be tightened only by the inertia force up to the set torque, and stopped by the brake means when the set torque value is reached. After the screw is seated, the drag torque f
Before (θ) reaches the set torque value, the motor is stopped or the control for reducing the rotational angular velocity is performed, and thereafter, −I · d ² at a rotational angular velocity that does not cause excessive tightening due to inertia.
While maintaining the state of θ / dt ² ≒ 0, screw tightening proceeds with the generated torque g (i) of the motor, and the drag torque f (θ)
The screw may be tightened by stopping the motor when the motor reaches the set torque value.

Further, since the motor torque g (i) can be detected even if the motor current value is not constant, complicated motor control such as constant-speed rotation control may be performed. Further, a plurality of set values of the drag torque may be prepared in a stepwise manner and compared, and the motor control may be changed in each step so that the motor is stopped at the torque to be finally tightened.

Further, in the above embodiment, the brake was applied by flowing a current in the reverse direction to the motor immediately before the drag torque f (θ) of the screw reached the target torque value. The brake may be controlled to brake the motor. Further, for example, a clutch may be provided between the rotating shaft of the motor and the bit, and instead of applying the brake, the bit may be separated from the motor so that the torque of the motor is not transmitted. Since the bit has a smaller moment of inertia than the motor, when the bit is disconnected from the motor, it stops immediately, as in the case of applying the brake.

Also, the reference drag torque curve is compared with the generated drag torque f (θ), and the motor current is continuously controlled so that the motor stops when it finally reaches the target torque value. Is also good.

In the above embodiment, the rotation angle of the motor is detected by the encoder, but may be detected by other means. In addition, the rotational angular velocity is directly detected using a tachometer or the like,
The rotation angular acceleration may be obtained by differentiating the difference. further,
The rotation angular acceleration may be directly detected from the motor.

Further, in the above embodiment, the calculation for control is performed using an analog circuit, but the same calculation may be performed using a digital circuit using a microcomputer or the like.

Further, although a DC motor is used in the above embodiment, another type of motor such as an AC motor may be used.

[Effects of the Invention] As described above, according to the present invention, screw tightening can always be performed at a high speed with an appropriate tightening force.

[Brief description of the drawings]

FIG. 1 is a view for explaining the principle of the present invention, FIG. 2 is a block diagram of a screw tightening device according to an embodiment of the present invention, and FIG. 3 is a time chart showing the operation of the screw tightening device. In the figure, 1 ... screw 2 ... motor 3 ... bit 4 ... rotational angular acceleration detecting means 5 ... torque detecting means 6 ... motor drive control means 10 ... screw 10a ... screw head 12 ... motor 14 Bit 16 Encoder 18, 20 Differential circuit 22, 24 Multiplication unit 26 Subtraction unit 28 Comparison unit 29 Torque setting unit 30 Motor current switch 32 Motor drive unit 34 ... Constant speed rotation control unit

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Saburo Kojima 4-51-15 Izumi, Suginami-ku, Tokyo Inside Japan Technat Co., Ltd. (72) Eizo Maeda 2-10-1, Toranomon, Minato-ku, Tokyo Japan Mining (56) References JP-A-63-212425 (JP, A) JP-A-59-93270 (JP, A) JP-A-58-59770 (JP, A) Jpn. U)

Claims (4)

(57) [Claims] 1. A screw tightening method for tightening a screw by rotating the screw by means of a motor, wherein a moment of inertia is I, a rotation angle of the motor is θ, and a current value of the motor is i, a motor torque generated by the motor. g (i) and the rotational angular acceleration d ² θ / dt ^{2 of the} motor
, The drag torque f (θ) of the screw is calculated based on the equation of motion f (θ) = − I · d ² θ / dt ² + g (i), and the drag torque f ( a screw driving method, wherein the motor is drive-controlled so that θ) becomes a preset target torque value. 2. The screw tightening method according to claim 1, wherein the brake is applied to the motor immediately before the drag torque f (θ) of the screw reaches the target torque value, and the drag torque of the screw is stopped when the motor is stopped. Wherein the screw is tightened so that the target torque value is obtained. 3. A screw tightening device for tightening a screw by rotating the screw by a motor, wherein: a rotation angular acceleration detecting means for detecting a rotation angular acceleration d ² θ / dt ² of the motor; A torque detecting means for detecting a motor torque g (i) generated by the motor; and a rotational angular acceleration d ² θ / dt ² from the rotational angular acceleration detecting means.
And from the motor torque g (i) from the torque detecting means, a drag torque f (θ) of the screw is calculated based on the equation of motion f (θ) = − I · d ² θ / dt ² + g (i), And a motor drive control means for controlling the drive of the motor such that the drag torque f (θ) at the time of stopping the motor has a preset target torque value. 4. The screw tightening device according to claim 3, further comprising a brake means for braking the motor, wherein the brake means controls the motor by the brake means immediately before the screw drag torque f (θ) reaches the target torque value. When the brake is applied and the motor stops, the drag torque f of the screw
A screw tightening device, wherein the screw is tightened so that (θ) becomes the target torque value.