JP3597378B2 - Pressurization control method for electric spot welder - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a pressurization control method in an electric spot welder using an electric motor as a pressurization source.
[0002]
[Prior art]
The electric spot welder is provided with a motion conversion mechanism such as a ball screw mechanism that converts the rotary motion of the electric motor into a linear motion, and is configured to open and close a pair of electrode tips by the electric motor via the motion conversion mechanism. Have been. Then, the work is clamped between the two electrode tips by the closing operation, and in this state, a current is applied between the two electrode tips to perform spot welding of the work.
[0003]
Here, when a work is sandwiched between the two electrode chips, the load on the electric motor increases, and the motor current flowing through the electric motor increases. Therefore, conventionally, a further increase in the motor current is regulated at a predetermined set current value, and the pressure applied to the workpiece is controlled to a set pressure corresponding to the set current value.
[0004]
[Problems to be solved by the invention]
When the applied pressure was actually measured, it was found that the actual applied pressure did not reach the set applied pressure corresponding to the set current value. This is considered to be due to friction loss in the motion conversion mechanism. This friction loss depends on the state immediately before the electrode tip comes into contact with the workpiece and immediately before stopping, and varies depending on the moving speed of the electrode tip until it comes into contact with the workpiece, the elasticity of the workpiece, and the like. The friction loss is minimized when all parts of the motion conversion mechanism are in the kinetic friction state immediately before the stop, and conversely, it is maximized when most of the motion conversion mechanism is in the static friction state. The difference between and minimum is quite large. For this reason, the variation in the deviation between the set pressure and the actual pressure becomes considerably large. In this case, it is conceivable to incorporate a pressure sensor such as a load cell for detecting the pressure and perform feedback control of the motor current so that the pressure becomes the set pressure.However, in this case, cost, maintenance and durability are considered. Be disadvantaged.
[0005]
In view of the above, an object of the present invention is to provide a pressurization control method that can reduce a deviation between a set pressure and an actual pressure as much as possible without using a pressure sensor.
[0006]
[Means for Solving the Problems]
In order to solve the above problems, the present invention is a pressurization control method in an electric spot welder that opens and closes a pair of electrode tips via a motion conversion mechanism using an electric motor, When the motor current flowing through the electric motor increases due to the work being sandwiched, the motor current is regulated to a predetermined set current value and the applied pressure on the work is controlled to the set applied pressure corresponding to the set current value. When the motor current rises to a set current value, a waveform current in which the current value changes in a wave shape is superimposed on the motor current.
[0007]
When the waveform current is superimposed, the output torque of the electric motor changes in a wave-like manner in accordance with the waveform current, and the portion of the motion conversion mechanism which has been in the static friction state shifts to the dynamic friction state due to the change in the output torque. Then, when all parts of the motion conversion mechanism are in the dynamic friction state, the electrode tip stops at the end of the superposition of the waveform current. Therefore, the friction loss in the motion conversion mechanism is minimized, and the deviation between the set pressure and the actual pressure is reduced as much as possible.
[0008]
BEST MODE FOR CARRYING OUT THE INVENTION
Referring to FIG. 1, R is a robot for welding a workpiece W, and a C-shaped welding gun G as a spot welding machine is attached to a wrist Ra at an operating end of the robot R.
[0009]
The welding gun G includes a gun body 2 supported on a gun support bracket 1 fixed to a wrist Ra via a guide bar 1a so as to be capable of performing an equalizing operation in a vertical direction. A C-shaped yoke 3 extending downward is attached to the gun body 2, and a lower tip 4 as a fixed electrode tip is attached to a lower end of the C-shaped yoke 3. Also, an electric motor 5 composed of a servomotor is mounted on the upper surface of the gun body 2, and a rod 7 vertically moved by a motor 5 via a ball screw mechanism 6 described later protrudes below the gun body 2, At the lower end of the rod 7, an upper chip 8, which is a movable electrode tip, is attached to face the lower chip 4.
[0010]
The welding gun G is moved to each hitting position of the workpiece W by the operation of the robot R, and when it arrives at each hitting position, a pressurizing command is transmitted from the robot controller 9 to the gun controller 10, and the gun controller 10 controls the electric motor 5. , The upper chip 8 is moved downward from a predetermined open position. When this comes into contact with the work W by the downward movement of the upper chip 8, the gun body 2 moves up with respect to the gun support bracket 1 by the contact reaction force thereafter, and the lower chip 4 comes into contact with the work W, and the work W is moved up and down. Pressure is applied between the chips 4 and 8. In this state, a current is applied between the two chips 4 and 8 to spot-weld each spot of the work W.
[0011]
As shown in FIG. 2, the electric motor 5 includes a motor housing 5a erected on the gun body 2, a stator 5b fixed in the motor housing 5a, and a hollow rotor 5c pivotally supported in the motor housing 5a. The rod 7 is inserted into the rotor 5c through the guide sleeve 2a in the gun body 2. The upper part of the rod 7 is formed hollow, and a screw 6a fixed to the upper end of the rotor 5c is inserted into the hollow part of the rod 7, and a number of balls screwed to the screw 6a are built into the upper end of the rod 7. The nut 6b is fixed, and the screw 6a and the nut 6b constitute a ball screw mechanism 6 as a motion converting mechanism for converting the rotation of the rotor 5c into the vertical movement of the rod 7.
[0012]
At the upper end of the motor housing 5a, an encoder 11 having an input shaft 11a connected to the rotor 5c is mounted via a stay 11b so that the rotation angle of the rotor 5c can be detected by the encoder 11. Then, a signal from the encoder 11 is input to the gun controller 10, the position of the upper chip 8 is determined from the rotation angle of the rotor 5c and the pitch of the ball screw mechanism 6, and after the spot welding, the position of the upper chip 8 is set to a predetermined value. The electric motor 5 is moved upward until the position is matched.
[0013]
When the work W is sandwiched between the lower chip 4 and the upper chip 8 as described above due to the downward movement of the upper chip 8 by the electric motor 5, the load on the electric motor 5 increases, and the motor current flowing through the electric motor 5 decreases. To rise. Then, when the motor current rises to the set current value set in accordance with the required set pressure, the gun controller 10 regulates the further increase in the motor current, and presses the work W with the set pressure. I have.
[0014]
However, as shown in FIG. 3, even if the motor current is maintained at the set current value Is, the actual applied pressure becomes lower than the set applied pressure Ps corresponding to the set current value Is due to friction loss in the ball screw mechanism 6. . Here, when a wave-like current a whose current value changes in a wave-like manner is superimposed on the motor current, the actual pressing force approaches the set pressing force Ps. This is because the output torque of the electric motor 5 changes in a wave-like manner due to the superposition of the waveform current a, and the portion (ball) of the ball screw mechanism 6 in the static friction state shifts to the dynamic friction state due to the change in the output torque, and the friction loss Is to decrease. In FIG. 3, the superimposition timing of the waveform current a is intentionally delayed in order to easily confirm the effect. However, in actual spot welding, when the motor current rises to the set current value Is, a virtual line The waveform current a is immediately superimposed as shown in FIG.
[0015]
Note that if the frequency of the waveform current a is too small or too large, the transition from the static friction state to the dynamic friction state does not work well, so the frequency is set to 5 to 100 Hz, preferably 20 to 60 Hz. In FIG. 3, the frequency is 40 Hz.
[0016]
Also, when the amplitude of the waveform current a is too small, the transition from the static friction state to the kinetic friction state does not work well. On the other hand, when the amplitude is too large, the durability of the ball or screw of the ball screw mechanism 6 is deteriorated due to wear. The amplitude of the waveform current a is set so that its half value is 3 to 50%, preferably 10 to 20% of the set current value Is. In FIG. 3, the half value of the amplitude is set to 15% of the set current value Is.
[0017]
If the number of superimposed cycles of the waveform current a is too small, the superposition of the waveform current a ends before the transition from the static friction state to the dynamic friction state is completed. On the other hand, if the number of superimposed cycles of the waveform current a is too large, a fretting corrosion phenomenon of the ball of the ball screw mechanism 6 is caused, and partial wear of the ball occurs. Therefore, the number of superimposed cycles of the waveform current a is 〜１０ to 10 cycles, preferably 1 to 4 cycles. In FIG. 3, the number of superposition cycles is set to 1.5.
[0018]
Although the waveform current a is a sine wave in FIG. 3, it may be a triangular wave or a rectangular wave. It is also possible to change the frequency, amplitude and waveform of the waveform current a during the superposition of the waveform current a. For example, the amplitude may be gradually reduced.
[0019]
【The invention's effect】
As is apparent from the above description, according to the present invention, the deviation between the set pressure and the actual pressure can be reduced as much as possible without performing the feedback control using the pressure sensor, and the cost, maintenance, and durability can be reduced. It is advantageous in terms of.
[Brief description of the drawings]
FIG. 1 is a side view of a robot equipped with a spot welding machine. FIG. 2 is an enlarged sectional view taken along the line II-II of FIG. 1. FIG. 3 is a graph showing a relationship between a motor current and an actual pressing force.
G welding gun (electric spot welding machine)
4,8 Electrode tip 5 Electric motor 6 Ball screw mechanism a Wave current

Claims (1)

A pressurization control method in an electric spot welder in which a pair of electrode tips is opened and closed by an electric motor via a motion conversion mechanism,
When the work is sandwiched between a pair of electrode tips and the motor current flowing through the electric motor rises, the motor current is regulated to a predetermined set current value to further prevent the increase of the motor current, and the pressure applied to the work is reduced to the set current value. For controlling to the corresponding set pressure,
When the motor current rises to the set current value, a waveform current whose current value changes in a wave shape is superimposed on the motor current,
A pressurization control method in an electric spot welder, characterized in that:

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