KR20040082680A - Resistance Spot Welding System with a Servo-Gun - Google Patents

Abstract

PURPOSE: A servo-gun type resistance spot welding system is provided to control pressing force and to rapidly perform a process by exchanging a pneumatic gun with a servo gun using a servo motor. CONSTITUTION: A servo-gun type resistance spot welding system controls electrostatic force by exchanging a non-linear welding system with a linear welding system. In order to exchange the non-linear welding system with the linear welding system, a non-linear feedback linear scheme derived from a differential geometrical theory is applied to an inverter type direct current resistance spot electrostatic welding system. A pneumatic gun is exchanged with a servo gun using a servo motor. The servo gun is controlled by using a database according to a welding condition.

Description

Resistance Spot Welding System with a Servo-Gun}

Resistance spot welding is an electrical resistance welding method in which two metal plates are placed between electrodes and pressurized and a large current of several thousand amperes flows in a short time of about 0.3 seconds, causing the metal plate to generate joule heat due to contact resistance to form a nugget. It is a kind.

The welding gun attached to the end of the robot arm in the automatic welding line in the industrial field used a pneumatic gun that generates the pressure to act on the specimen by pneumatic pressure. The characteristic of pneumatic gun is to open / close control type by setting the pressure given to the specimen to a certain value, so it is difficult to change the pressing force during welding process and it is not easy to control the exact pressing force. In addition, the movement distance of the electrode is constant, the interference phenomenon occurs when moving in accordance with the conditions of the workpiece, a lot of down time in the process time is consumed.

On the other hand, current resistance spot welding uses a constant current control method using the thyristors phase control. However, this method does not consider the dynamic resistance characteristics during the short welding time, so many spatters are generated, resulting in deterioration of the welding quality and contamination of the welding. In order to compensate for this, an inverter-type constant current control method has been introduced, but this also has a problem in that the power supplied is not uniform.

In the present invention, by converting the existing pneumatic gun into a servo gun using a servo motor, it is possible to perform the control of the pressing force and the process speed more quickly, and to control the pressing force in real time to minimize the collision occurring when the welding tip and the specimen contact. do. In addition, by applying the nonlinear feedback constant power control method to the inverter power control to improve the welding quality of the resistance spot welding.

In resistance spot welding, three factors that have the greatest impact on welding quality are welding current, pressing force, and welding time. 1 shows the principle of a resistance spot welder. 2 shows a characteristic curve of a typical copper resistance. Dynamic resistance means that the resistance value of the welded specimen changes during welding. In Fig. 2, section I has an insulation resistance of several M due to the surface oxide film of the weld specimen. It becomes about. As the insulation state is destroyed by the electrode pressing force, current starts to flow, and when the temperature rises at the contact point, the oxide film is rapidly destroyed and the copper resistance decreases very quickly. In section II, when the current starts to flow, the ductility of the contact surface increases as the temperature rises, so the volume resistance of the welded specimen increases due to the increase of the contact area and the temperature increase, so the minimum dynamic resistance is at the α point where the opposite effect is balanced. When the temperature rises, the copper resistance starts to increase. In section III, the copper resistance continues to rise with temperature rise, but at the end of this section, partial melting occurs at the contact surface of the weld specimen. In section IV, the temperature of the welded specimen continues to rise to increase the volume resistance, while the increased cross-sectional area through which the current flows due to the increase of the melted portion at the contact surface decreases the resistance. The β point, which is the maximum point of the copper resistance of the equilibrium state due to the opposite effect, is the point at which the temperature starts to stabilize, and then the copper resistance decreases as the melting part increases. In section V, the resistance decreases because the contact surface widens as the molten portion continues to grow in the weld specimen contact.

As such, the resistance value in the process of resistance spot welding changes, so that the powder applied to the molten part during the constant current control is not constant, and thus good welding quality cannot be expected by the current control alone. The present invention solves this problem by using a constant power control method of nonlinear feedback.

1 is the principle of resistance spot welder

2 is a characteristic curve of a typical copper resistance

3 is a configuration of the inverter DC spot welder and the waveform of each part

Figure 4 is a transformer and load modeling of the constant power control system

5 is a welding system control block diagram

6 is a block diagram of a linear system.

7 is a block diagram for controlling the present system

8 is a structure of a servo gun used in the present invention

9 is a pressure graph according to the voltage of the servo gun when the base material is stainless steel

10 is a pressure graph according to the voltage of the servo gun when the base material is aluminum

11 is servo gun control by resistance welding process

<Definitions of terms and abbreviations used in formulas>

: Power command

: Measured power

: Power error (= )

: Primary voltage of transformer

: Transformer secondary voltage

: Voltage across electrode tip

: Transformer secondary current

: Transformer secondary side inductance

Transformer turns ratio ( )

: Proportional (integral) gain constant

Power Amplifier Gain

The present invention largely consists of an inverter welding machine and a servo gun. 3 shows the configuration of the inverter type DC spot welder and the waveforms of the respective parts. In a three-phase commercial power supply, the voltage is first directed by a three-phase rectifier, smoothed by an electrolytic capacitor, and then alternated back to a frequency higher than the commercial frequency using a switching element, and supplied to the transformer primary circuit. The supplied voltage is configured to supply direct current to the load through a full-wave rectifier circuit configured on the secondary side after voltage conversion with a power transformer. At the bottom of the circuit diagram, the waveform of each part is shown schematically.

The present invention adopts a constant power control method in which a nonlinear triangular linearization technique based on differential geometry theory is applied to a system. 4 is an equivalent circuit modeling the transformer and the load of the constant power control system. The quadratic dynamic equation is obtained from this modeling equation (1).

(One)

In addition, the power equation of the welding part and the dynamic equation of the power control part are as follows (2), (3), (4).

(2)

(3)

(4)

Looking at the block diagram for system control Since is a function of time, it is difficult to express in the Laplace domain. Therefore, tip end resistance Can be regarded as a parameter with a slower dynamic response characteristic than the welding current or welding voltage variable.

(5)

(6)

From these, a block diagram for system control as shown in FIG. 5 can be obtained. Looking at equations (1)-(6) or as can be seen in Figure 5, the system is a non-linear system. Therefore, controller gain tuning is not only difficult, but also has a disadvantage in that dynamic characteristics are different for each operating point, and response characteristics are difficult to predict.

In the present invention, the nonlinear feedback linearization technique based on the differential branch theory is applied to this system to make and control a complete linear system. From equation (2), the following can be obtained.

(7)

Where new input Is defined as

(8)

Equation (7) is as follows.

(9)

As can be seen from equation (9), this system is transformed into a complete linear system. A block diagram for controlling this system is shown in FIG. 6 is a complete linear system, so the entire system operates linearly. PI gain can also be easily tuned given the range of R and L in FIG. The actual control is as follows. Is given by Eq. (9) or Fig. 6 as follows.

10

From equations (8) and (10), the following equations can be obtained.

(11)

From equations (4) and (11), the following can be obtained.

(12)

Since the power amplifier gain is K, the output of the D / A converter is It can be seen that. The entire block diagram for control is in FIG. Input to the transformer in equation (12) Is the normal PI multiplied by the constants N and L and the welding voltage Divided into Therefore, welding voltage at the beginning of welding When the output power is smaller than the normal PI, the welding voltage is gradually increased over time. Decreases and the PI output returns to normal. As a result, the excellent transient properties, the welding power is introduced in a short time and the energization time for welding is reduced.

8 shows the structure of a servo gun of a portion of the invention. Servo gun in the present invention is configured in one bracket by separating the drive system using a mechanical component such as a servo motor, a timing fleece, a screw and a linear guide, a power transmission unit and a power supply system such as a welding trace and a gun arm required for welding. It is equipped with a separate control system according to welding conditions.

In the present invention, the conventional resistance spot welding system using a pneumatic gun is replaced with a servo gun using a servo motor for linear rotation. At this time, the LM Guide serves to hold the axis of the linear motion. The torque command by the motor generates the pressing force by the timing belt and the ball screw, which converts the rotational motion into the linear motion.The mechanical characteristics for transmitting torque are disturbed by the frictional force between the ball screw and the LM Guide. The bending phenomenon of the electrode occurs. On the other hand, the welding setting conditions according to each specimen and the welding current to the database is used by the user to select according to the welding conditions. Here, the welding environment refers to the pressurization time, the duration of power, the magnitude of the welding power, the number of points, and the like. 9 and 10 are graphs showing the pressing force of the servo gun according to the voltage which is the pressing force command value in the torque control system controlled by the open loop when the base materials are stainless and aluminum, respectively. It can be seen that the servo gun pressing force of the present invention is roughly proportional to the voltage which is the pressing force command value. In other words, it can be seen that the pressing force of the servo gun can be adjusted by selecting the voltage value according to the welding condition.

Looking at the welding process of the servo gun, the servo gun can be classified into an approach-pressurization-welding-maintenance step as shown in FIG. 11 according to the movement of the welding tip. In the 'approach' step, the electrode moves to the site to be welded, and accurate position control is required at this time. Servo gun welder of the present invention is accurate in position control can be almost completely accessible to the base material can reduce the deformation of the base material by the impact. In the 'pressing' step, the welded specimen is brought into metal contact and serves to bring the resistance to a desired level. The 'welding' step is the most important part in determining the welding quality during the actual welding process in which melting occurs while the set welding current flows during the welding period. In the 'maintenance' step, the metal recombination occurs to maintain the pressure to cool slowly. The 'rest' phase is where the pressing force is removed and the servo gun moves toward the next welding target.

The present invention applies a nonlinear feedback linearization technique based on differential geometry theory to an inverter type resistance spot welding system to convert the nonlinear welding system to linear to perform electrostatic power control. The constant power control method allows for more precise control than the constant current control method in consideration of the dynamic resistance in the resistance spot welding, thereby reducing spatter and obtaining excellent welding quality.

The present invention replaces the existing pneumatic gun with a servo gun using a servo motor to enable high speed welding, control the position of the electrode in real time, and adjust the pressing force to be easily performed by the user. In addition, due to the light contact and pressure of the electrode to the welded object, there is less deformation of the welded object, thereby improving welding quality. The servo gun can arbitrarily adjust the electrode opening, and can adjust the arbitrary pressing force. In addition, due to the light contact of the electrode, the impact sound of the electrode to the welded object is reduced, which is superior to the existing in the durability and abrasion of the welding gun. The existing pneumatic gun has an interference phenomenon with the workpiece due to a certain distance. In this case, the working distance can be changed, thereby making it easier to access the work object and increasing the flexibility of the work. It does not use compressed air, so there is no exhaust sound of compressed air, and there is no fine oil scattered when exhausting compressed air, which improves the working environment. Finally, the pneumatic piping equipment and its construction are not necessary, so the amount of facility investment is reduced.

Claims (2)

In the constant power control of resistance spot welding system Nonlinear feedback linearization technique based on differential geometry theory is applied to inverter DC resistance spot constant power welding system to convert nonlinear welding system to linear to perform electrostatic power control. In resistance spot electrostatic welding system Replaces the existing pneumatic gun with a servo gun using a servo motor, and controls the servo gun according to the welding conditions by using the welding condition database.