JPH0451309B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a screw tightening device that detects reaction torque when tightening a screw, controls the screw tightening torque based on the detected output, and determines whether the tightening condition is good or bad. It is.

Conventional structure and problems In conventional screw tightening devices that use a DC motor, the screw tightening torque varies widely due to the influence of inertia torque generated in rotating parts such as the DC motor and the bit of the screwdriver. There is a problem, and as a means to solve this problem,
No. 55-120986 discloses a screw tightening device that switches and controls the applied pressure of a DC motor in two stages. This method simply applies a high voltage to the motor in the second stage, so the problem is that the motor torque increases rapidly and the tightening accuracy is poor. There was a problem that when the torque value at the initial stage when the screw is embedded in the tightening material is large, it cannot be used because the voltage applied to the DC motor in the first stage must be lower than the voltage applied in the second stage. . Furthermore, for the same reason, the DC motor cannot be rotated at a high speed until the screws are seated, so there is a drawback that the time required to tighten the screws becomes longer.

Purpose of the Invention The present invention solves the above conventional problems, and converts the actual screw tightening torque from DC motor current into analog to digital, converts it into torque through calculation, and monitors the value. However, by controlling the DC voltage to the DC motor with a programmable DC power supply and by keeping the voltage applied to the DC motor at the rated voltage until just before the screw is seated, there is less variation in tightening torque, and the screw can be tightened in a short time. The present invention provides a screw tightening device that can perform tightening.

Composition of the Invention The present invention includes a DC motor, a current detector that detects the current flowing through the DC motor, an analog-to-digital conversion circuit that converts the signal output of the current detector into a digital value, and a target value of screw tightening torque. a setting switch for setting, a programmable DC power supply whose DC voltage output can be varied by a digital input signal, a pre-seating detection means for detecting the timing immediately before the screw is seated, an output of the analog-to-digital conversion circuit and the setting switch. a first comparison circuit that compares the target value of the DC motor with a value converted into a current value of the DC motor and outputs a matching signal;
a second comparison circuit that compares the output of the digital conversion circuit and a value that is a fraction of the target value of the setting switch with a value converted to the current value of the DC motor, and outputs a matching signal; After receiving the screw tightening start signal and until receiving the output of the pre-seating detecting means, a first step is to drive the DC motor at the rated voltage by the programmable DC power supply, and the output of the pre-seating detecting means is a second step of rapidly stopping the DC motor when the DC motor is received;
Until the output of the second comparator circuit is received, the programmable DC power supply is set to a voltage that generates a torque that is slightly larger than a torque that is a fraction of the target value, and the DC motor is operated. The third step of driving is to compare the output of the analog-to-digital conversion circuit, and if the value is larger than the value stored in the analog-to-digital conversion circuit, the maximum value is updated and stored as the output value of the analog-to-digital conversion circuit. storage means is provided, the current output of the analog-to-digital conversion circuit is compared with the content of the maximum value storage means;
a fourth step of driving the DC motor by keeping the voltage of the programmable DC power supply constant while the voltage is equal or small and gradually increasing the voltage while the voltage is high; It is comprised of a torque control circuit that controls the screw tightening torque according to the fifth step of reducing the voltage of the programmable DC power supply to zero and stopping the DC motor.

According to the above configuration, first, in the first step, the pre-seating detection means detects the timing immediately before the screw is seated, and the screw from the start of screw tightening to the position immediately before the screw is seated is detected as a material to be tightened. Since the rated voltage can be applied to the DC motor while it is being driven in, the screw tightening control will not stop even if a torque larger than the target torque is generated during the time of penetration, and high-speed rotation can be achieved, reducing the screw tightening time. Can be shortened. In addition, while detecting the seating of the screw in the third step and while controlling the final screw tightening torque in the fourth step, an application that causes the screw to rotate slowly against the reaction force of tightening is applied. Since the DC motor is controlled by voltage, the influence of motor inertia is almost eliminated, making it possible to achieve high-precision screw tightening with little variation.

DESCRIPTION OF EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows a side view of the mechanical part of the screw tightening device.
In FIG. 1, 1 is an electric screwdriver for tightening screws incorporating a DC motor and a reduction gear, and 2 is an output shaft of the electric screwdriver. The electric screwdriver 1 is fixed to a bracket 3, and the bracket 3 is attached to a plate 4.
and is further attached to the upper slide block 5. The air cylinder 6 is fixed to a bracket 7, and an upper slide block 5 is attached to the middle part of the rod 8.
is fixed by two nuts 9-a and 9-b. Reference numeral 10 denotes a slide shaft into which the upper slide block 5 is slidably fitted. 11 is a joint connecting the electric screwdriver 1 and the bit 12. Reference numeral 13 denotes a bit guide, and a catcher 14 for holding supplied screws is fixed at the lower end. A bit guide bracket 15 is fixed to a lower slide block 16 which is slidably fitted to the slide shaft 10. Further, the lower slide block 16 is supported at the lower end of the rod 8 of the air cylinder 6 by a nut 9-c. Nuts 9-b
A spring 17 is provided on the rod 8 between the lower slide block 16 and the lower slide block 16.
is always applied to the nut 9-c. A bracket 18 supports the slide shaft 10 and is fixed to the chassis 19 together with the bracket 7. 2
0 is the lowering limit stopper of the upper slide block 5. 21 is a lowering limit stopper of the lower slide block 16.

FIG. 2 is a front view of the mechanism section with the electric screwdriver portion of FIG. 1 removed, and shows the state in which the limit switch 22 is attached. Figure 3 shows the relationship between the tip of the screw fastener and the screw fastener (base material). The limit switch 22 is fixed to the bracket 7 by a switch bracket 23, and its position can be adjusted up and down. The structure is such that when the upper slide block 5 descends to a predetermined position, the limit switch 22 is turned on. The position of this limit switch 22 is adjusted so that it is turned on just before the screw is seated.

The operation of the screw tightening device configured as described above will be explained below.

When the air cylinder 6 descends while the electric screwdriver 1 is rotating, the upper slide block 5 fixed to the rod 8 also begins to descend, and the electric screwdriver 1 attached thereto via the plate 4 and bracket 3 descends. . Also, lower slide block 16
The nut 9-c at the lower end of the rod 8 is also tightened by the spring 18.
Since the bit guide bracket 15, the bit guide 13, and the catcher 14 also descend at the same time. Catcher 1 as shown in Figure 3.
4 reaches the fastener 25, the rotational force of the electric screwdriver 1 is transmitted to the screw 24 via the output shaft 2, the joint 11, and the bit 12, and is tightened by the fastener 25. The rotational force of the electric screwdriver 1 is a tightening torque, and the relationship between the tightening torque _TQ and the current of the DC motor of the electric screwdriver 1 is as shown in FIG. 4 and can be expressed by equation (1).

T=K ₁ I _M +K ₂ (1) (K ₁ and K ₂ are constants) Therefore, by measuring the motor current I _M , the torque T can be calculated.

Next, the configuration of the control circuit of the embodiment will be explained with reference to FIG. The part surrounded by broken lines shows a block diagram of the control device of the present invention.

Connector 26a to DC motor 1 of electric screwdriver
Supply a current I _M through. Motor current I _M is detected by current detector 27 . The current detector 27 uses, for example, a small series resistor to measure the voltage drop across the resistor, or uses a coil and a Hall element to output a voltage proportional to the motor current I _M. The output of the current detector 27 is amplified by the preamplifier 28, and the low-pass filter circuit 29 blocks noise and high frequency vibration components generated from the motor 1 of the electric screwdriver.
The signal is input to an analog-to-digital conversion circuit 30 (hereinafter abbreviated as A-D circuit). The A-D conversion circuit 30 converts the motor current value into a digital signal, and the motor current value is input to the microcomputer 31. The microcomputer 31 (hereinafter abbreviated as microcomputer)
It has ROM (read-only memory), RAM (random access memory), and input/output interface circuits, and is equipped with arithmetic functions.

As mentioned above, the output torque of electric screwdriver 1
Since T _Q and motor current I _M have the relationship shown in equation (1), the microcomputer can know the value of the screw tightening torque by monitoring the output voltage v of the low-pass filter circuit 29. Low pass filter circuit 2 below
The output voltage of No. 9 is referred to as torque voltage v. Also A-D
The conversion starts at the rising edge of the ADST signal from the microcomputer 31, and when the AD conversion is completed, the ADEND signal is output.

On the other hand, 32 is a programmable DC power supply that can vary the effective value of the DC voltage output, and is used to vary the applied voltage V _M of the motor 1 of the electric driver, and 33 and 34 are setting switches that can be used to tighten the screws. Torque target value This is for the operator to set the torque constant of the motor as a digital value. The air cylinder 6 is controlled by a microcomputer 31 via a relay 34. Also, as a control signal from the outside via the connector 26c.
START, STOP, and MEAS are input to the microcomputer 31. Note that 26a, 26b, and 26c indicate connectors.

Next, the operation of the present invention will be explained in detail with reference to FIG. 6.

When the power is turned on, the microcomputer 31 inputs the torque voltage v via the A-D conversion circuit 30. When a START signal is received from an operator or other automatic machine to signal the start of screw tightening, the microcomputer 31
stores the torque voltage v ₀ at this time. The stored value of v ₀ is the value when the motor current is zero.

Next, while turning on the air cylinder 6,
Set the programmable DC power supply 32 to the rated voltage V _R , rotate the motor of the electric driver 1 at high speed,
Indicates completion of screw tightening to external device 35
Turn off the END signal.

The motor rotates, and at timing _t1 , its speed becomes stable and the motor current becomes constant. Continuing,
After timing _t2 when the screw starts to sink into the object to be fastened, the limit switch MEAS signal indicating that the screw is about to be seated is outputted from the limit switch ₂₂ at timing t3. Then, the microcomputer 31 controls the programmable DC power supply 3.
2 output V _M is set to zero, and both terminals of the motor are short-circuited to bring it to a rapid stop. The screws are still tightened during this time. t ₄ after a certain period of time T ₁ from timing t ₃
The output voltage of the programmable DC power supply 32 at the time of
Set V _M to a set value v _t of approximately 1/2 to 1/4 of the target torque described below.
Voltage that can generate greater torque
Set to _VS. The motor then tightens the screw at a low rotation speed. Eventually, at timing _t5 , the screw is seated and the torque voltage v increases rapidly. The microcomputer 31 supplies the A/D conversion start signal ADST to the A/D conversion circuit 30 in short cycles from timing _t5 , and constantly monitors this torque voltage v. By the way, the DC motor current for screw tightening
Since I _M and the output torque T _Q have a relationship as shown in equation (1) above, the torque T when tightening the screw is calculated from the torque voltage v based on the value set by the setting switch B shown at 34. _Q can be calculated. Therefore, this torque voltage v is approximately 1/2 to 1/4 of the set tightening torque v _t
At time t ₆ when the time t 6 is exceeded, the microcomputer 31 linearly increases the output V _M of the programmable DC power supply 32 at fixed time intervals (ΔT). Since it is already seated, the screws are gradually tightened at low speed. The microcomputer 31 monitors the output of the A-D conversion circuit 30 until the torque output reaches a predetermined tightening torque value v _a . However, in Figure 6, the timing is t ₇
As in ~ _t8 , _t9 ~ _t10 , when the same voltage is applied to the motor to tighten the screws, the torque required for screw tightening may temporarily decrease and then increase again. The reason is thought to be that after the screw is seated, the frictional force at the interface between the base material to be screwed and the screw head differs depending on the angle at which the screw rotates. In this case, the microcomputer 31 temporarily stops increasing the output V _M of the programmable DC power supply 32 linearly over time and makes it a constant voltage (at times t ₇ and t ₉ ). And the torque voltage v so far
The maximum value of , that is, the value of v ₁ at timing t ₇ and the value of v ₂ at timing t ₉ is stored. After that, the value of the torque voltage v is continuously monitored and the stored maximum value is
From timing _t8 or _t10 when the torque voltage v exceeds the value of _v1 or _v2 , the output of the programmable DC power supply 32 is increased again in proportion to time. Thereafter, at timing _t11 when the value of the torque voltage v reaches the target tightening torque v _a , the microcomputer 31 reduces the output V _M of the programmable DC power supply 32 to zero and stops applying voltage to the motor.

After some waiting time from time _t11 , at timing _t12 , the microcomputer 31 turns on the END signal indicating the completion of screw tightening, turns off the signal _S1 , and turns off the air cylinder 6. As already mentioned, the value of v ₀ is the value when the torque is zero. Therefore, during the timing t ₆ to _{t 11} of monitoring the torque voltage v, the microcomputer 31 determines whether the value of (v−v ₀ ) is v _t or v _a
By determining whether or not the difference is equal to , it is also possible to cancel out errors due to drift caused by temperature changes in the current detector 27 and the preamplifier 28 .

After timing _t12 , the microcomputer 31 inputs the torque voltage v via the AD converter 30 in the same manner as in the state immediately after the power is turned on. and again
Wait for the START signal to turn ON.

By the way, since the rated voltage is applied to the motor from the time of starting to just before seating, it is possible to fasten screws at high speed, and even if the torque is large when the screws penetrate into the tightening material, the screws can be tightened without any problems. Furthermore, since the motor is rotated at high speed until just before the screw is seated, the time required for the screw to sink into the material can be shortened. Then, the motor is stopped just before the screws are seated, and then the microcomputer controls the programmable DC power supply to tighten the screws at low speed. When the target tightening torque is reached, the voltage applied to the motor is reduced to zero. . Since screw tightening is completed at low speed, the influence of motor inertia is eliminated, allowing accurate screw tightening with less variation in screw tightening torque.

FIG. 7 shows a schematic flowchart of the microcomputer for operating as described above. Note that the timing t ₃ was detected by the external MEAS signal, but the microcomputer 31 uses a timer to detect the timing from t ₀ .
Similar control is possible by managing the time up to _t3 .

On the other hand, the torque constant of the motor is set by the setting switch B shown at 34, but since it is a constant value until the motor fails, the ROM of the microcomputer
By storing this torque constant in the memory, the present invention can be carried out without using the setting switch B.

Effects of the Invention According to the present invention, there is provided a means for converting the signal output of a current detector that detects the current flowing through the DC motor of an electric screwdriver during screw tightening from analog to digital, and calculating a tightening torque from the converted value, and a DC voltage output. a programmable DC power supply that can be varied by a digital input signal, a pre-seating detection means that detects the timing just before the screw is seated, and a target tightening torque set by the signal output value of the current detector during tightening and the setting switch. A first comparison circuit compares the value with the converted value of the DC motor current and outputs a matching signal, and compares the value of the signal output of the current detector during tightening with the value of a fraction of the target value torque. A second comparator circuit converts the current into motor current and outputs a matching signal, and the programmable circuit receives the screw tightening start signal from the external device and receives the output of the pre-seating detection means. a first step of driving the DC motor at a rated voltage by a DC power source; a second step of rapidly stopping the DC motor upon receiving the output of the pre-seating detection means; an output of the second comparison circuit; A third step of driving the DC motor by setting the programmable DC power supply to a voltage that generates a torque that is slightly larger than a torque that is a fraction of the target value until the current a fourth step of gradually increasing the voltage of the programmable DC power supply when the output of the detector has increased from the previous maximum value, and driving the DC motor with a constant voltage while the voltage is equal to or smaller than the first voltage; When receiving the output of the comparison circuit, the torque control circuit controls the screw tightening torque in a fifth step of reducing the voltage of the programmable DC power supply to zero and stopping the DC motor.

Therefore, in the first step, by the pre-seating detection means that detects the timing immediately before the screw is seated,
Since the rated voltage can be applied to the DC motor while the screw is embedded in the tightening material from the start of screw tightening to the position just before the screw is seated, a torque greater than the target torque is generated when the screw is embedded. The screw tightening control does not stop even when the screw is tightened, and the screw tightening time can be shortened because the screw can be rotated at high speed. In addition, while detecting the seating of the screw in the third step and while controlling the final screw tightening torque in the fourth step, an application that causes the screw to rotate slowly against the reaction force of tightening is applied. Since the DC motor is controlled by voltage, the influence of motor inertia is almost eliminated, making it possible to achieve high-precision screw tightening with little variation.

[Brief explanation of the drawing]

Figure 1 is a side view of the mechanical part of a screw tightening device according to an embodiment of the present invention, Figure 2 is a front view of the mechanism with the electric driver part removed from Figure 1, and Figure 3 is the tip of the screw tightening device. 4 is a diagram showing the relationship between motor current and torque, FIG. 5 is a control circuit block diagram of the same embodiment, and FIG. 6 is an operation timing diagram of the same example.
FIGS. 7a and 7b are flowcharts of the same microcomputer. 1...Electric screwdriver, 22...Limit switch,
27...Current detector, 28...Pre-stage amplifier, 29...Low pass filter circuit, 30...A-D conversion circuit, 3
1...Microcomputer, 32...Programmable DC power supply, 33...Setting switch.

Claims (1)

[Claims] 1. A screw tightening device that controls the tightening torque of a screw by controlling the current of a DC motor, which includes a current detector that detects the current flowing through the DC motor, and a high frequency component of the signal output of the current detector that is removed. an analog-to-digital conversion circuit that converts the output of the low-pass filter circuit into a digital value, a setting switch that sets the target value of screw tightening torque as a digital value, and a DC voltage output that converts the DC voltage output into a digital input signal. a programmable DC power source that can be varied by a programmable DC power supply, a pre-seating detection means that detects the timing immediately before the screw is seated, and a value obtained by converting the output of the analog-to-digital conversion circuit and the target value of the setting switch into a current value of the DC motor. a first comparison circuit that compares and outputs a matching signal; and a fraction of the output of the analog-to-digital conversion circuit and the target value of the setting switch.
a second comparison circuit that compares the value of the current value with a value converted into a current value of the DC motor and outputs a matching signal, receives a screw tightening start signal from an external device, and receives the output of the pre-seating detection means. A first step of driving the DC motor at a rated voltage by the programmable DC power supply; a second step of rapidly stopping the DC motor upon receiving the output of the pre-seating detection means; Until the output of the second comparator circuit is received, the DC motor is driven by setting the programmable DC power supply to a voltage that generates a torque that is slightly larger than a torque that is a fraction of the target value. The third step is to compare the output of the analog-to-digital conversion circuit, and if the value is larger than the stored value, store the maximum value updated with the output value of the analog-to-digital conversion circuit. means for comparing the current output of the analog-to-digital conversion circuit with the contents of the maximum value storage means, and setting the voltage of the programmable DC power supply to a constant voltage while the current output is equal to or smaller than the current output of the analog-to-digital conversion circuit; a fourth step of gradually increasing the voltage to drive the DC motor; and a fifth step of reducing the voltage of the programmable DC power supply to zero to stop the DC motor upon receiving the output of the first comparison circuit; A screw tightening device characterized by being equipped with a torque control circuit for controlling screw tightening torque.