JPH04336980A - Power tool - Google Patents

Abstract

PURPOSE:To improve the fastening torque accuracy without deteriorating the working efficiency. CONSTITUTION:A screw fastening is carried out beforehand to make an operation processor 7 carry out a learning before a screw fastening work is started. The operation processor 7 recognizes the position immediately before the screw seating corresponding to the result of the above learning. The operation processor 7 stops once the rotation of a motor 1 at the position immediately before the screw is seated. After that, the feeding current to the motor 1 is increased gradually until the motor current becomes to a specific current value. And at the time when the current feeding becomes to the specific current value, the current feeding to the motor 1 is stopped.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to tightening torque control for electric tools such as electric screwdrivers for tightening screws.

0002

2. Description of the Related Art An example of an electric tool is an electric screwdriver, which uses a mechanical clutch to control screw tightening torque. An example is shown in FIG. In this electric screwdriver, the motor and the output shaft 30 of the electric screwdriver are connected through a reduction mechanism A consisting of a plurality of gears, and the transmission of power from the motor to the output shaft 30 is controlled by a clutch 31. There is. Here, the clutch 31 includes a clutch plate 32 into which the output shaft 30 is inserted;
A locking portion 33a formed on the front end surface of an internal gear 33 that constitutes the speed reduction mechanism A and meshes with a planetary gear (not shown).
The ball 34 that is locked to the clutch plate 32 and the ball 3
4 and the ball 34 is connected to the internal gear 33.
The spring 35 is biased in the direction of locking the locking portion 33a.

In this electric screwdriver, when the load torque is less than a predetermined value, the ball 34 is locked to the locking portion 33a of the internal gear 33, thereby preventing the internal gear 33 from rotating. Power is transmitted to the output shaft 30 via the speed reduction mechanism A. Now, if the screw is completely tightened and the load torque reaches a predetermined value, the internal gear 33 will push back the ball 34 against the spring 35 and start rotating, causing the reduction mechanism A to idle. As a result, power transmission from the motor to the output shaft 30 is cut off, and the screw is tightened with a predetermined torque. The tightening torque of the screw is adjusted by changing the amount of compression of the spring 35 using the clutch plate 32.

0004

[Problems to be Solved by the Invention] However, in the case of the above-mentioned mechanical clutch, noise and vibration during operation are large;
The problem was that it had a short lifespan. Therefore, as a torque control method to eliminate the above drawbacks, for example, the current supplied to the motor is limited to a predetermined value or less using a circuit, and the tightening torque is limited to a predetermined value or less. One possibility is to limit the torque.

However, with this method, when the head of the screw comes into contact with the member to be screwed (hereinafter referred to as seating), the load increases instantaneously and the motor stops. In addition, extra force is applied to the screw due to the inertial torque of the motor's rotor, gears, etc., and this force causes an uncontrollable error when limiting the torque generated by the motor to a predetermined value or less to achieve the desired tightening force, resulting in extreme In some cases, the threads could be damaged. FIG. 8 shows an actual measurement diagram of the inertia torque generated during seating.

[0006] In principle, inertial torque is proportional to the inertial force and the change in rotational speed (acceleration), and can be reduced by slowing down the rotational speed of the motor, etc., so it is a good idea to reduce the rotational speed of the motor, etc. It will be done. However, in this case, there is a problem in that it takes time to tighten the screws, reducing work efficiency. The present invention has been made in view of the above-mentioned points, and an object thereof is to provide a power tool that can improve tightening torque accuracy without reducing workability.

[0007]

[Means for Solving the Problems] In order to achieve the above object, the present invention provides a motor having DC motor characteristics, a driving means for driving the motor at a constant current, and a screwing member that is completely attached to the screwed member. a position detecting means for detecting a position before the screwed member is completely screwed onto the screwed member;
Control means for gradually increasing the current supplied to the motor until the motor current reaches a predetermined current value, and stopping the supply of current to the motor when the motor current reaches the predetermined current value;
Before starting work, the screwing member is screwed onto the screwed member in advance, the position detecting means is made to perform learning, and the screwing member is completely screwed onto the screwed member according to the learning result. The position detecting means is made to recognize the position in front of the object.

[0008]

[Operation] As described above, the present invention temporarily stops or decelerates the rotation of the motor at a position before it is completely screwed, and then gradually increases the current supplied to the motor until the motor current reaches a predetermined current value. By doing so, the influence of inertial torque due to rotation is reduced and the tightening torque accuracy is improved.Also, by controlling the motor in this way, the motor can be operated at high speed until it reaches the position just before it is completely screwed. Since it can be rotated, work efficiency is not reduced.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a circuit configuration of an embodiment of the present invention. In this embodiment as well, an electric screwdriver is used as an example of the electric tool. In this electric driver, a DC brushless motor is used as the motor 1 having DC motor characteristics. However, a DC brush motor may also be used. This motor 1 is driven by a power converter 2 consisting of a bridge circuit configured using six FETs.

The signal processing circuit 3 detects the FET of the power converter 2 according to the output of the Hall element 11 that detects the position of the rotor.
The drive signal output from the signal processing circuit 3 is created based on the PWM signal given from the PWM type constant current control section 4. The constant current control unit 4 creates a PWM signal according to the current setting value given by the arithmetic processing unit 7 or the current setting value given by the speed setting volume 16, and the output of the oscillator 5 that determines the frequency of the PWM signal. It is something to do. Data corresponding to the motor current detected by the current detection circuit 12 from the voltage across the detection resistor 9 is input to the constant current control section 4, and the motor speed can be controlled to a set speed by feedback control. The setting switch 17 provided on the input side of the constant current control section 4 selects which current setting value of the arithmetic processing section 7 or the speed setting volume 16 is to be used. Further, the command data of the current value of the arithmetic processing section 7 is converted into an analog value by the D/A converter 6 and is given to the constant current control section 4. In the case of this embodiment, the power conversion section 2, signal processing section 3, and constant current control section 4 constitute a constant current driving means for driving the motor 1 at a constant current.

[0011] The arithmetic processing section 7 is composed of a CPU having a built-in ROM and RAM, and an I/O interface, and controls the entire operation sequence of the electric screwdriver. The arithmetic processing unit 7 is supplied with torque setting data for determining the tightening torque from the torque setting volume 10 via the A/D converter 8. In addition, a control command is given to the arithmetic processing unit 7 via the switch interface 13 from an operation switch 18 that includes a switch for selecting between automatic speed setting and manual speed setting and a switch for setting an operation mode. Note that, depending on the operation of the switch for selecting automatic speed setting and manual speed setting,
controls the switching of the setting switch 17.

The frequency divider 15 divides the frequency of the output of the Hall element 11 by the gear ratio of the electric driver, and the output of the frequency divider 15 is input to a counter included in the arithmetic processing section 7. That is, the frequency divider 15 converts the output of the Hall element 11 into the number of rotations of the output shaft of the electric screwdriver, and this converted value is provided to the arithmetic processing section 7. In the case of this embodiment, since the motor 1 is a brushless motor, the rotation of the motor 1 can be detected using the existing Hall element 11 in this type of brushless motor, but in the case of a brush motor. It is necessary to provide a speed sensor or a position detection sensor.

By the way, in the case of this embodiment, the frequency divider 15
Although the number of rotations of the output shaft of the electric screwdriver is determined using , there is no particular problem in detecting the number of rotations directly from the output shaft of the electric screwdriver. For example, a method such as attaching a magnet to the output shaft and detecting the number of rotations of the output shaft using a Hall element can be considered. However, in this case, it is difficult to attach the magnet and the Hall element, and the cost is high.

The seating detection circuit 14 determines whether the screw is seated, and determines the seating state by utilizing the phenomenon that rotation of the motor stops when the screw seats, and the motor current increases. . A specific circuit of the seating detection circuit 14 is shown in FIG. This seating detection circuit 14 includes an integrating circuit 20
, a buffer 23, a differentiating circuit 21, and a comparator 22. After the input is once smoothed by the integrating circuit 20, the rate of increase in current is detected by the differentiating circuit 21. Figure 5 (a
) indicates the motor current, and the output waveform diagram of the differentiating circuit 21 obtained by differentiating the motor current is shown in FIG. 2(b). The comparator 22 generates the output shown in FIG. 5(c) when the output of the differentiating circuit 21 exceeds the set value.

In the electric screwdriver of this embodiment, before starting screw tightening work, the operation mode is first set to learning mode, and several screws are tightened to learn and memorize the lengths of the screws. At this time, the learning mode is set using the operation switch 18, and at this time, the desired tightening torque value is set using the torque setting volume 10. Screw tightening in learning mode is performed at a slow speed. That is, at this time, the arithmetic processing section 7 gives the constant current control section 4 a command to set a current value that slows down the rotation of the motor 1.

When the motor 1 is rotated, the rotation of the motor 1 at that time is detected by the Hall element 11.
Convert the output of 1 to the number of rotations of the output shaft of the electric screwdriver,
The counter of the arithmetic processing unit 7 counts this converted value. When the output of the seating detection circuit 14 is obtained, that is, when the screw is seated, the count value of the number of rotations of the output shaft is stored in the RAM of the arithmetic processing section 7. At the same time, the calculation processing unit 7 also stores in the RAM the data obtained by subtracting the value for one revolution from the count data until seating. Note that the position of the seating arm is not limited to one rotation.

The operation in the learning mode described above is performed in the same way for several screws, and the arithmetic processing section 7 is made to recognize the optimum number of rotations for tightening the screws and the number of rotations before seating. This completes the learning mode operation and prepares for the working mode. In other words, in the case of this embodiment, the position detection means for detecting the position of the screw seating arm is composed of the arithmetic processing section 7, the detection resistor 9, the Hall element 11, the seating detection circuit 14, and the frequency divider 15. be. Note that the arithmetic processing unit 7 temporarily stops the rotation of the motor 1 at the position detected by the position detecting means before the screw is seated, and then gradually increases the current supplied to the motor 1 until the motor current reaches a predetermined current value. It also functions as a control means for increasing the current value and stopping the supply of current to the motor 1 when it reaches a predetermined current value.

When actually starting the screw tightening work,
Set to work mode using the operation switch 18. Tighten this screw as follows. First, at the start of screw tightening shown in FIG. 4(a), screw tightening is performed at high speed rotation as shown in FIG. 3(a). The high-speed rotation period is indicated by A in the figure. Note that the screw tightening at this time can be performed at the maximum rotational speed of the electric screwdriver. However, if the manual setting mode is selected, the speed can be set using the speed setting volume 16.

Further, FIG. 4, which shows the state of screw tightening during the operation process, shows the case where the screw 24 is tightened into the screw hole of the member 25 to be screwed, and numeral 30 in the figure indicates a pit that is detachably attached by a chuck. Then, Fig. 4(b
), the rotation of the motor 1 is suddenly braked when the screw is tightened just before seating, as determined in the learning mode described above. The period during which the brake is applied to the motor 1 is indicated by ``b'' in FIG. At this time, even in the manual setting mode, the setting switch 17 is switched so that the output of the arithmetic processing section 7 is input to the constant current control section 4.

[0020]Here, to determine whether the screw has been tightened before seating, the arithmetic processing section 7 stores the data before seating in a built-in subtraction counter, and inputs the output of the frequency divider 15 as a subtraction pulse. Then, when the value of the counter becomes zero, an interrupt is used to notify the arithmetic processing unit 7 that the screw has been tightened just before seating. It goes without saying that this determination also takes into consideration the number of rotations of the output shaft of the electric screwdriver due to inertia.

The period during which the rotation of the motor 1 is stopped in this manner is shown by C in FIG. Motor 1 is then restarted and
Tighten the screws completely as shown in Figure 5(c). After this restart, the current of the motor 1 is slowly increased until it reaches a predetermined current value, and screw tightening is performed. Note that it is preferable to tighten the screws with constant acceleration rotation at least until the person is seated.

Here, the above-mentioned current setting value is given to the constant current control section 4 by the arithmetic processing section 7, and the constant current control section 4 detects from the output of the current detection circuit 12 that the motor current has reached the current setting value. . After the current value corresponding to the set current value is maintained for a predetermined short time, the supply of current to the motor 1 is stopped, and the screw tightening is completed. In this way, by finally making the motor current constant, the torque generated by the motor is kept constant, and constant torque control is realized.

In the above embodiment, a case has been described in which the motor 1 is completely stopped before the seat is seated, but the same effect can be expected even if the rotation speed of the motor 1 is reduced to a state on the verge of stopping.

[0024]

Effects of the Invention As described above, the present invention provides a motor having DC motor characteristics, a driving means for driving the motor at a constant current, and a position where a screwing member is completely screwed onto a member to be screwed. The rotation of the motor is temporarily stopped or decelerated at a position before the screwed member is completely screwed onto the screwed member, and then the motor is gradually rotated until the motor current reaches a predetermined current value. control means for increasing the current supplied to the motor and stopping the supply of current to the motor when a predetermined current value is reached, and for screwing the screwing member to the screwed member in advance before starting work The position detecting means is made to perform learning based on the learning result, and the position detecting means is made to recognize the position before the screwed member is completely screwed onto the member to be screwed. The rotation of the motor is temporarily stopped or decelerated at a position just before the rotation, and then the current supplied to the motor is gradually increased until the motor current reaches a predetermined current value, so the influence of inertial torque due to rotation can be reduced.
Therefore, the tightening torque accuracy can be improved. Furthermore, by controlling the motor in this manner, it is possible to rotate the motor at high speed up to a position just before the screw is completely screwed in, without reducing work efficiency. Moreover, since the circuit limits the current supplied to the motor to below a predetermined value and limits the tightening torque to below a predetermined value, it does not generate noise or vibration unlike mechanical clutches.
Moreover, it can be made lightweight. Furthermore, before starting the work, the screwing member is screwed onto the screwed member in advance to cause the position detecting means to perform learning, and the screwing member is completely screwed onto the screwed member according to the learning result. If the position detecting means is made to recognize the position before being fastened, it is possible to easily adapt to a change in the target screwed member, and to easily perform control suitable for the screwed member.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a circuit configuration of an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a specific configuration of the seating detection circuit same as above.

FIG. 3 is an explanatory diagram of the same operation as above.

FIG. 4 is an explanatory diagram showing a screw tightening state in the same operation process as above.

FIG. 5 is an explanatory diagram of the operation of the seating detection circuit.

FIG. 6 is a flowchart summarizing operations.

FIG. 7 is a sectional view showing the structure of a conventional mechanical clutch.

FIG. 8 is an explanatory diagram of impact torque generated due to inertia when a person is seated.

[Explanation of symbols]

1 Motor 2 Power conversion section 3 Signal processing section 4 Constant current control section 7 Arithmetic processing unit 9 Detection resistor 11 Hall element 12 Current detection circuit 14 Seating detection circuit 15 Frequency divider

Claims (1)

[Claims] 1. A motor having DC motor characteristics, a driving means for driving the motor at a constant current, and a position detecting means for detecting the position before the screwed member is completely screwed onto the screwed member. Temporarily stop or decelerate the rotation of the motor at a position before the screwing member is completely screwed onto the screwed member,
Control means for gradually increasing the current supplied to the motor until the motor current reaches a predetermined current value, and stopping the supply of current to the motor when the motor current reaches the predetermined current value;
Before starting work, the screwing member is screwed onto the screwed member in advance, the position detecting means is made to perform learning, and the screwing member is completely screwed onto the screwed member according to the learning result. A power tool characterized by having a position detecting means recognize a position in front of the power tool.