JPS6159834B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a ground fault detection device in an automatic spot welding device such as a spot welding robot. It is well known that spot welding robots are used in many welding operations, including automobile body assembly lines, but in recent years, welding transformers have been used to save energy in spot welding and to prevent wear and tear on water-cooled cables for welding current circuits. The use of so-called transformer-incorporated guns, which are integrated with a spot welding head, is increasing.

Previously, welding current was supplied to the spot welding gun attached to the tip of the robot arm using a long water-cooled cable with a large cross-sectional area from a large welding transformer suspended from the ceiling. . Therefore, the robot swings around this thick cable and moves at high speed, and the cable is subjected to sudden twists and bends due to the posture control of the welding gun at each welding point. It has become a consumable item.

On the other hand, with the transformer built-in gun mentioned above, the secondary coil side of the welding transformer is integrated with the welding head with a pressurizing mechanism, so there is no need for a thick water-cooled cable for the welding current circuit, and a large welding current Because the impedance of the circuit through which the welding transformer flows is extremely small, the electric capacity of the welding transformer can be reduced, making it extremely compact and lightweight.

However, a new problem arose when this gun with a built-in transformer was attached to the tip of a welding robot arm to perform welding work.

In other words, in this system, instead of the conventional low-voltage, high-current circuit cable, a high-voltage circuit rated at 200V or 400V is used by a robot, and although the cable eye is narrower and its lifespan is longer, This increases the risk of short circuits between two cable core wires that form a reciprocating circuit, and of dielectric breakdown between cable core wires that are at high potential and robot arms, jigs, etc. that are at ground potential. The damage caused by sparks to peripheral equipment and the impact on the power supply system when sparks occur has become a major threat that cannot be overlooked.

These risks seriously impede the practical application of the transformer-equipped gun, which has many other advantages, but the present invention provides a drastic solution to these problems.

The figure is an electric circuit diagram showing one embodiment of the present invention. A comparative explanation of the prior art and the present invention using the figure will be as follows.

Generally, the gun 1 with a built-in transformer is fixed to the tip of a movable arm of a spot welding robot 2 (not shown), and is sequentially transferred at high speed according to the spot welding position of the workpiece, and the posture of the gun itself is also adjusted according to the shape of the workpiece. It will undergo rapid changes.

Therefore, the movable cable 3 connected to the primary side of the welding transistor is forced to undergo extremely violent twisting and bending movements and gradually wears out, or in particular, the insulator on the outer periphery of the cable is damaged by the robot arm or surrounding jigs. As the cable rubs against devices and repeats the movement of the cable, its insulation is eventually mechanically broken down, resulting in many problems called ground faults, where the cable core comes into contact with surrounding structures at ground potential.

Conventionally, as a safety measure against such ground fault accidents, an earth leakage breaker has been adopted for the power supply side breaker 9, and when a leakage current to the ground is detected,
Measures were taken to immediately cut off the power supply.

However, the thyristor contactor 7, which is currently in practical use as the most standard electronic switch, is water-cooled, and if water scale accumulates on the water-cooled hose and the insulation resistance of the electrical circuit decreases, the earth leakage circuit breaker may malfunction. Troublesome measures have become necessary, such as improving the quality of the cooling water and, in some cases, using special thyristors that insulate the water-cooled part from live parts.

Also, although the operating time of the earth leakage breaker is short,
If a high power supply voltage rated at 200V or 400V is grounded, a large ground fault current will flow, and the risk of sparking damage to robot arms, jigs, etc. cannot be ignored.

In contrast, the present invention installs a step-down isolation transformer 4 between the welding power source 10 and the transformer-built-in gun 1, and connects the electric path including the movable cable 3 to ground via a current detector 5 with a low voltage power source. 6, the above-mentioned problems can be solved.

That is, by installing the isolation transformer 4 and the current detector 5, the potential of the movable cable 3 to the ground can be kept extremely low, reducing the risk of dielectric breakdown. Since the current of the thyristor contact 7 with respect to the ground becomes unrelated to the current detected by the ground current detector 5, a normal commercially available thyristor can be used as is.

On the other hand, the current detector 5 is, for example, 12V DC or
It has a built-in low voltage power supply of about 20V, which is less dangerous, and has the function of detecting the flow of a minute current of about 10 to 20mA and issuing an alarm.

Now, by connecting the current detector 5 described above between a part of the electric path of the movable cable 3, which is at a floating potential due to the installation of the isolation transformer 4, and the earth 6, the potential of the cable 3 is lowered with respect to the earth.
Since the voltage can be maintained at an extremely low value of 12V or 24V, the risk of damage to the insulator that protects the outer periphery of the cable due to voltage is drastically reduced as mentioned above, and the cable can also be kept at a very low value of 12V or 24V for maintenance purposes such as replacing the cable. It is also possible to maintain safety in the event that a human touches the electric circuit in step 3.

In such a state, for example, if the insulation of the flexible cable 3 is mechanically broken and the core wire of the cable comes into contact with a robot ground, jig, etc. that is at ground potential, the built-in current detector 5 An earth fault current Ie begins to flow from the low voltage power supply through the ground. The current detector 5 detects this weak ground fault current Ie that occurs at the initial stage of dielectric breakdown, and issues an alarm such as a relay contact signal.

This alarm is mainly used to notify when it is time to replace the movable cable 3, but it can also be used as a signal to immediately shut off the power breaker 9 and stop the operation of an automatic machine such as a welding robot.

On the other hand, the current detector 5 has the same role as a conventional earth leakage breaker, but the main difference from a general earth leakage breaker is that it has a built-in low voltage power supply. The point is that

This makes it possible to detect dielectric breakdown of the movable cable 3 to earth even in a safe state with the welding power source 10 cut off, so it can be detected when the spot welding equipment such as a robot is operated idle and the movable cable 3 is in a moving state. This makes it possible to reproduce and confirm ground fault phenomena that tend to occur or occur only during a momentary period of movement.

Note that 8 in the figure indicates a timer, which is a device that transmits a spot welding sequence and a welding energization signal to the thyristor contactor 7.

[Brief explanation of the drawing]

The figure is an electrical circuit diagram showing an embodiment of the present invention. 1... Gun with built-in transformer, 2... Spot welding robot, 3... Movable cable, 4... Insulation transformer for step-down, 5... Current detector, 6...
...Earth, 7...Thyristor contactor, 8...
...Timer, 9...Breaker, 10...200V or
400V rated welding power supply.

Claims (1)

[Scope of Claims] 1. In an automatic spot welding device such as a spotting robot that uses a so-called transformer built-in gun that integrates a welding transformer and a spot welding head, a welding power source that uses an electronic switch to control welding current flow is provided. A step-down isolation transformer is installed between the welding transformer and the welding transformer, and between the ground and an electrical circuit including a flexible cable that connects the secondary coil side of the isolation transformer and the primary coil side of the welding transformer. A built-in transformer characterized in that a current detector with a built-in low voltage power supply is connected to the transformer, and an alarm is issued when a ground fault current is detected due to insulation breakdown with respect to earth in the electric circuit including the flexible cable. Ground fault detection device for gun electrical circuit.