JPH04336979A - Power tool - Google Patents

Abstract

PURPOSE:To make the fastening torque at a constant value and to improve the torque accuracy of the fastening by providing a control circuit to control the rotation of a motor from the position immediately before screwing perfectly to the position of screwing perfectly, in a constant speed condition. CONSTITUTION:A sensor to detect the condition immediately before the seating of a screw is provided. The rotation of a motor 1 is made at a constant speed by a control means 4 from the position immediately before the screw is seated to the perfect seating. And when the screw is seated, the current feeding to the motor 1 is stopped by the control means 4. Consequently, the shock force when the screw is seated is made constant, the fastening torque is made constant, and the fastening torque accuracy is increased.

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 a removable output shaft 36 that tightens the screws of the electric screwdriver are connected via a speed reduction mechanism B consisting of a plurality of gears, and the power from the motor is transmitted to the output shaft 36. It is controlled by a clutch 31. Here, the clutch 31 is engaged with a clutch plate 32 through which an output shaft 36 is inserted, and a locking portion 33a formed on the front end surface of an internal gear 33 that constitutes a reduction mechanism B and meshes with a planetary gear (not shown). It consists of a ball 34 that is stopped, and a spring 35 that is disposed between the clutch plate 32 and the ball 34 and biases the ball 34 in the direction of locking the ball 34 to the locking portion 33a of the internal gear 33.

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 36 via the speed reduction mechanism B. Now, if the screw is completely tightened and the load torque reaches a predetermined value, at this time the internal gear 33 pushes back the ball 34 against the spring 35 and starts rotating, which causes the reduction mechanism B to idle. As a result, power transmission from the motor to the output shaft 36 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. In addition, this type of clutch has relatively good tightening torque accuracy in a static state, but in a dynamic state in actual use, an impact force that depends on the rotational speed of the output shaft during clutch operation is applied. However, there was a drawback that the tightening torque exceeded the specified torque to be controlled, resulting in poor tightening torque accuracy. Furthermore, since the rotational speed of the output shaft changes due to voltage fluctuations of a power source such as a battery, it is difficult to expect that this type of clutch will improve the tightening torque accuracy.

The present invention has been made in view of the above-mentioned points, and its object is to provide a power tool that can improve the accuracy of tightening torque.

[0006]

[Means for Solving the Problems] In order to achieve the above object, the present invention provides a position detecting means for detecting the position before the screwing member is completely screwed onto the screwed member, and a screwing member. Seating detection means detects when the motor is completely screwed into the member to be screwed, and the motor is rotated at a constant speed at a position before the screw is completely screwed, and when the motor is seated, the supply of current to the motor is stopped. and control means.

[0007]

[Operation] By configuring the present invention as described above,
By controlling the rotation of the motor at a constant speed from the position before the seat is completely screwed in until it is completely screwed in, it is possible to control the impact force at the time of seating to be constant. The tightening torque is made constant.

[0008]

【Example】

(Embodiment 1) 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 section 4 creates a PWM signal according to the current setting value given from the arithmetic processing section 7 and the output of the oscillator 5 that determines the frequency of the PWM signal. Note that 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.

An example of a specific circuit of this constant current control section 4 is shown in FIG. In this specific circuit, the constant current control section 4 includes a waveform shaping circuit 40 that shapes the output of the Hall element 11 into a square wave, and a sawtooth waveform that further converts the square wave output from the waveform shaping circuit 40 into a sawtooth wave. A wave generation circuit 41, a sample hold circuit 42 that samples and holds the peak value of the output of the sawtooth wave generation circuit 41, and a sample hold circuit 42.
an error amplifier 44 that amplifies the error between the output of the output and the set current value (speed setting signal) given from the arithmetic processing section 7 via the D/A converter 6; It is composed of a comparator 46 that performs a comparison with the output of the error amplifier 44.

In this embodiment, the waveform shaping circuit 40 is constructed using a comparator 40a and a hysteresis amplifier 40b. In addition, the sawtooth wave generation circuit 4
Reference numeral 1 denotes a reset pulse generation circuit 41a that generates the reset pulse shown in FIG. 8(c) after the rise of the square wave, a switch SW1 which is controlled to turn on and off by the output of the reset pulse generation circuit 41a, and a resistor RT. and a capacitor CT. Furthermore, the sample and hold circuit 42 includes a sample and hold pulse generation circuit 42a that generates the sample and hold pulse shown in FIG. a capacitor CH that is charged by the output of the sawtooth wave generation circuit 41 when the switch SW2 is turned on, and a hold circuit 42b that holds the peak value of the capacitor CH.
It is composed of Note that the set current value from the arithmetic processing section 7 is input to the error amplifier 44 via the buffer 43.

In this constant current control section 4, the output of the Hall element 11 shown in FIG.
), and from this square wave, the sawtooth wave generation circuit 41 creates the sawtooth wave shown in FIG. The rotation speed is detected. Note that the output of the sample and hold circuit 42 is as shown in FIG. 8(f). Then, the error amplifier 44 detects the error between the output of the sample hold circuit 42 and the speed setting signal given from the arithmetic processing section 7, and the comparator 46 operates as shown in FIG. 8(h) in accordance with the detected error. Create the PWM signal shown below. Note that FIG. 8(g) shows input waveforms to each input terminal of the comparator 46.

[0013] The arithmetic processing unit 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. Further, a control command is given to the arithmetic processing unit 7 via the switch interface 13 from an operation switch 18 which is a switch for setting an operation mode.

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 9(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. 9(c) when the output of the differentiating circuit 21 exceeds the set value.

Furthermore, in this embodiment, a sensor 50 is provided to detect the state before the screw is seated. FIGS. 4 and 5 show an example of the detection rod 51, which protrudes from the tip of the pit 30 of the electric screwdriver A by the thread length and can move forward and backward, and the detection rod 51 is urged forward. and a limit switch 53 that is pressed by the rear end of the detection rod 51 that retreats when the screw is in a tightened state where it is difficult to seat the screw. Furthermore, Figure 1
Although not shown, the output of the limit switch 53 is input to the arithmetic processing section 7.

In addition to this sensor 50, as shown in FIGS. 6 and 7, a sensor 50' composed of a light emitting diode 54 and a phototransistor 55 can also be used. In this sensor 50', a light emitting diode 54 is emitted from the screwed surface of a screwed member 57 onto which a screw 56 is screwed.
The distance to the screw tightening surface is calculated by the reflection of the light, and the state before the screw is seated is detected. This sensor 5
When 0' is used, the arithmetic control unit 7 calculates the distance and determines when the screw is about to be seated.

When performing screw tightening work using the electric screwdriver of this embodiment, a desired tightening torque is set in advance using the volume 10. Then, when you start tightening the screws,
First, at the start of screw tightening shown in FIG.
As shown in 0(a), screw tightening is performed at high speed rotation. The high speed rotation period is shown by A in FIG. Note that the screw tightening at this time can be performed at the maximum rotational speed of the electric screwdriver.

When the above-mentioned sensor 50 detects the state shown in FIG. 11(b) just before seating, a brake is suddenly applied to the rotation of the motor 1 to reduce the speed. The period during which the motor 1 is braked is indicated by ``b'' in FIG. Thereafter, the speed is controlled to tighten the screws with a predetermined tightening torque. Note that the speed at this time is set as follows. In other words, how many revolutions per unit time (for example, rpm) does the arithmetic processing unit 7 of this embodiment perform?
The result data obtained through experiments to determine the tightening torque is stored in the ROM as a table, and therefore the result of calculating the rotation speed according to the set torque is
When the screw is about to be seated, it is given to the constant current control section 4 as a speed setting signal. Note that this speed setting signal may be applied as long as it is before deceleration.

The period during which the motor 1 is rotated at a constant speed from the state before the seat is seated is indicated by period C in FIG. This constant speed control is performed until the seating state is detected by the seating detection circuit 14. When the screw is seated, the final speed state is maintained for a certain short period of time, and then the supply of current to the motor 1 is stopped to complete screw tightening. A flowchart summarizing the above operations is shown in FIG.

In the case of this embodiment, the torque generated by the electric screwdriver is kept constant by making the rotational speed at the final seating constant and the current stop time after seating constant, thereby achieving constant torque control. There is. By the way, the impact force generated when a rotating body comes to a stop is determined by the magnitude of the inertia of the rotating body and the magnitude of the acceleration until the rotating body stops.

[0023] Here, the impact force is T, the inertial force is J, and the acceleration is dθ2 /dt2 (where θ is the rotation position (angle)
), the simplified formula is as follows. T=J·dθ2 /dt2 Therefore, the greater the inertia force, the greater the impact force, and the greater the speed (the greater the acceleration), the greater the impact force.

[0024] In the electric screwdriver of this embodiment, since the inertia of the rotating parts (gear, chuck, motor rotor, etc.) of the electric screwdriver is constant, the impact force changes depending on the rotational speed when the driver is seated. Occur. Therefore, when tightening the screw loosely, the rotational speed should be slow, and when tightening the screw strongly, the rotational speed should be increased. FIG. 10 shows how the tightening torque changes when the rotational speed is varied.

Therefore, the arithmetic processing unit 7 of this embodiment stores in the ROM a table of data obtained through experiments to determine the number of rotations per unit time (for example, rpm) and the amount of tightening torque. Therefore, the result of calculating the rotational speed according to the set torque is given to the constant current control section 4 as a speed setting signal when the screw is about to be seated. Therefore, as in this embodiment, a constant tightening torque can be obtained by controlling the rotational speed from just before the seat is seated until the seat is seated.

Further, in the case of this embodiment, since screw tightening can be performed at high speed up to a position just before the screw is seated, there is an advantage that the working time is not increased and the workability is good. (Embodiment 2) FIG. 13 shows another embodiment of the present invention. The configuration of this embodiment will be explained below.

In this embodiment as well, an electric screwdriver is used as an example of the power tool, and a DC brushless motor is used as the motor 1 having DC motor characteristics. It is driven by a power conversion section 2 consisting of a circuit. The signal processing circuit 3 drives and controls the FETs of the power converter 2 according to the output of the Hall element 11 that detects the position of the rotor.

The constant current control section 4 uses the current setting value given from the arithmetic processing section 7 or the speed setting volume 16.
The PWM signal is generated according to the current setting value given by the oscillator 5 and the output of the oscillator 5 which determines the frequency of the PWM signal. However, this constant current control section 4 differs from the first embodiment in that it controls the rotational speed of the motor 1 according to the motor current detected by the current detection circuit 12 from the voltage across the detection resistor 9. 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.

[0029] The arithmetic processing unit 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 inputted to the arithmetic processing section 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 the arithmetic processing section 7 controls switching of the setting switch 17 in accordance with the operation of the switch for selecting automatic speed setting and manual speed setting.

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. The seating detection circuit 14 determines that the screw is seated,
The seating state is determined by utilizing the phenomenon that the rotation of the motor stops and the motor current increases when the screw is seated, and the specific structure is the same as that of the first embodiment described above.

The operation of the electric screwdriver of this embodiment will be explained below. In the electric screwdriver of this embodiment, before starting a screw tightening operation, the operation mode is first set to learning mode, and several screws are tightened to learn and memorize the length 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.

[0032] Screw tightening in the 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, the output of the Hall element 11 is converted into the number of rotations of the output shaft of the electric screwdriver, and this converted value is sent to the arithmetic processing section 7. counter counts.

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 rotation from the count data up to seating. Note that the position of the seating arm is not limited to one rotation.

The operation in the learning mode 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 reduces the rotation of the motor 1 to a constant low speed state at a position detected by the position detecting means before the screw is seated, and after a short time when the screw is seated, the motor 1 is stopped.
It also functions as a control means for stopping the supply of current to.

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. 11(a), screw tightening is performed at high speed rotation as shown in FIG. 10(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.

[0036] Then, when the screw is tightened just before seating, which was determined in the learning mode shown in Fig. 11(b), the rotation speed of the motor 1 is suddenly braked to reduce the rotational speed of the motor 1. The period during which the motor 1 is braked is indicated by ``b'' in FIG. Here, it is determined that the screw is tightened before seating, by storing the data before seating in a subtraction counter built in the arithmetic processing unit 7.
The output of the frequency divider 15 is inputted as a subtraction pulse, and when the value of the counter becomes zero, an interrupt is used to notify the arithmetic processing section 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.

After the deceleration, the motor 1 is rotated at a constant rotation speed according to the setting of the tightening torque in the same manner as described above. In the case of this embodiment as well, the arithmetic processing unit 7 stores in the ROM a table of data obtained through experiments to determine the number of rotations per unit time (for example, rpm) and the amount of tightening torque. Therefore, the result of calculating the rotational speed according to the set torque is given to the constant current control section 4 as a speed setting signal when the screw is about to be seated. When this speed setting signal is given, the constant current control unit 4 keeps the rotational speed of the motor 1 constant by feeding back data corresponding to the motor current detected by the current detection circuit 12 from the voltage across the detection resistor 9. It works like this. This constant speed rotation period is indicated by C in FIG.

When an output indicating that the screw is seated is obtained from the seating detection circuit 14, the current corresponding to the set current value is maintained for a predetermined short period of time, and then the supply of current to the motor 1 is stopped. do. This completes screw tightening. 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 case of this embodiment, unlike the first embodiment, there is no need for the sensor 50 for detecting the state before the screw is seated, so there is an advantage that the structure can be simplified.

[0040]

Effects of the Invention As described above, the present invention includes a position detecting means for detecting the position before the screwing member is completely screwed onto the screwed member; It is equipped with a seating detection means for detecting that the seat is screwed, and a control means that makes the motor rotate at a constant speed at a position before the seat is completely screwed, and stops supplying current to the motor when the seat is seated. , the rotation of the motor can be controlled at a constant speed from the position before it is completely screwed in until it is completely screwed in, and the impact force when seating is kept constant. The torque becomes constant and the tightening torque accuracy can be improved.

[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 a constant current control section same as the above.

FIG. 3 is a circuit diagram showing a specific configuration of the seating detection circuit same as above.

FIG. 4 is an explanatory diagram showing a sensor that detects the state of the screw just before it is seated.

FIG. 5 is an explanatory diagram showing a specific configuration of a sensor.

FIG. 6 is an explanatory diagram showing a sensor that detects the state of another screw before it is seated.

FIG. 7 is an explanatory diagram showing a specific configuration same as the above.

FIG. 8 is an explanatory diagram of the operation of a constant current control section.

FIG. 9 is an explanatory diagram of the operation of the seating detection circuit.

FIG. 10 is an explanatory diagram of the operation of the electric screwdriver.

FIG. 11 is an explanatory diagram showing a screw tightening state in the same operation process as above.

FIG. 12 is a flowchart summarizing operations.

FIG. 13 is a block diagram showing a circuit configuration of another embodiment of the present invention.

FIG. 14 is a flowchart summarizing the operations same as above.

FIG. 15 is a sectional view showing the structure of a conventional mechanical clutch.

FIG. 16 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 circuit 50 sensor

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. Seating detection means for detecting that the screwing member is completely screwed onto the screwed member, and a seating detection means that sets the motor rotation at a constant speed at a position before the screwing member is completely screwed on, and supplies current to the motor when the screwing member is seated. A power tool comprising: a control means for stopping the power tool.