CN1006130B

CN1006130B - Constant Current Control Method and Device for Spot Welding and Convex Seam Welding

Info

Publication number: CN1006130B
Application number: CN86101966.0A
Authority: CN
Inventors: 鲍力立; 李祥天; 储智; 薛大黎
Original assignee: Second Automotive Works
Current assignee: Dongfeng Motor Corp
Priority date: 1986-03-21
Filing date: 1986-03-21
Publication date: 1989-12-20
Also published as: CN86101966A

Abstract

A constant-current control method and its controller for the welding current of spot welding, projection welding and seam welding features that the trigger angle of SCR module on main power circuit is controlled by microprocessor, and the welding current is controlled by microprocessor.

Description

Constant-current control method and device for spot welding and projection welding

A method and apparatus for controlling welding current constancy during spot, projection and seam welding.

The present invention relates to a method and a device for controlling resistance welding.

In the processes of spot welding, projection welding and seam welding, keeping the welding current constant is an important factor for ensuring the welding quality. In the prior art (for example, japanese patent No. J58044981, japanese welding technology 1979, 27 th volume 10 th phase, etc.), what is used to control the welding current to be constant is a so-called voltage zero point control method, i.e., the on-time of the thyristor assembly on the welding machine main power circuit, i.e., the firing angle α of the thyristor assembly, is controlled with the zero crossing point of the grid voltage as the starting point. The microprocessor for control stores in advance a current control curve relating to the grid voltage zero-crossing control, i.e., a relationship curve between a current relative effective value and a firing angle alpha from the voltage zero-crossing, and inputs a set current value and an actually detected local cycle (or half cycle)) The value of the welding current can be determined by a microprocessor according to the firing angle a of the cycle (or half cycle)_nCalculating the ideal trigger angle a of the next cycle (or half cycle)_n+1And sending an instruction for triggering the silicon controlled rectifier component to realize constant current control of the welding current. The constant current control method has the defect that a current control curve related to the zero-crossing control of the voltage of the power grid is greatly influenced by the power factor angle phi of a welding loop, so that the controller has to detect the power factor angle phi in real time to ensure the control precision. Theoretically, the current control curves are virtually infinite as a function of the angle phi, and in practice, a sufficiently dense family of curves has to be stored in the microprocessor. Obviously, this greatly increases the complexity of the constant current control device and its production cost. At the same time, since the angle phi corresponding to the current control curve on which the microprocessor is based for real-time control must be greater than the actual power factor angle phi' of the welding circuit (which is the best but generally not possible), otherwise control errors and even damage to the welding machine may occur. Under the condition that phi is larger than phi', the capacity of the welding machine cannot be fully utilized, so that a user is forced to select a welding machine with higher power, and waste is caused.

It is an object of the present invention to provide a method and apparatus for controlling welding current constancy that overcomes the above-mentioned disadvantages and drawbacks of the prior art.

The invention adopts a welding current zero control method for spot welding, projection welding or seam welding. The basic idea is to control the trigger angle beta of the silicon controlled rectifier assembly by a microprocessor with the zero point of the falling edge of the welding current as the starting point. In the control method, a current control curve stored in a microprocessor in advance is less influenced by a power factor angle phi of a welding loop, and when constant current control is realized, satisfactory control accuracy can be obtained by only predicting and classifying power factor angle ranges of various welding machines in advance and storing a few control curves (such as 3-5) into the control microprocessor on the basis. And the power factor angle phi required by the existing voltage zero control method and corresponding to the control curve is required to be larger than or equal to the limit of the actual power factor angle phi' of the welding loop, and the limitation does not exist.

The following is a description of the drawings relating to the invention:

fig. 1 shows the line voltage u and the welding current i as a function of time t.

Fig. 2 is a block diagram of a constant current control device using the method of the present invention.

Fig. 3 is a circuit block diagram of the current zero pulse discriminator in fig. 2.

Fig. 4 is a circuit diagram of the current zero pulse generator of fig. 2.

As one mode for carrying out the present invention, the operation principle of the constant current control device shown in fig. 2 will be described below. The differential signal of the welding current detected by the current sensor 1 and the Rogowski coil is sent to the microprocessor 4 through the integrator 2 and the analog-to-digital converter 3, and the microprocessor 4 performs sampling and calculation to obtain the effective value of the actual current. The required constant current value is set by the keyboard 5 and memorized in the RAM 8, and the current control curve is stored in the ROM 7. The microprocessor calculates the trigger angle of the next cycle SCR according to the data and the preselected current control curve, and outputs the trigger angle to the timer 9 and the trigger circuit 10 to realize the real-time control of the welding current, wherein the starting time of the timer 9 is controlled by the current zero pulse output by the current zero pulse generator 11 (and the pulse discriminator 12).

The current zero pulse generator 11 and the pulse discriminator 12 in the device are used for diagnosing the working state of the silicon controlled rectifier assembly SCR in the constant current control device in real time.

As shown in figures 1 and 2, the voltage u across a pair of anti-parallel SCR components SCR on the main circuit of the welder_scI.e. the zero point of the falling edge of the welding current. By monitoring the time interval T of the zero crossing of the welding current, whether the SCR has the phenomena of short circuit, open circuit or single-side conduction or not can be known in time, so that the welding failure or/and the burning of a welding machine, a welding tool and a welded part can be avoided. In fact, the adjacent current is zeroThe time interval T of the point pulses, during steady state operation of the system, should be equal to the half period T' of the alternating current. If T ≧ T' + [ delta ] T₁Or T.ltoreq.T' - Δ T₂（Δt₁And Δ t₂For setting time difference), it indicates that there is an open circuit or non-open triggering (single-side conduction) phenomenon in the thyristor assembly, if no zero signal is detected, it indicates that there is at least one thyristor short circuit.

Because the voltage u is added at the two ends of the SCR_scThe zero crossing point of (1) is the zero point of the welding current falling edge, so that the current zero pulse generator 11 has the function of converting the voltage u_scConverted into a current zero pulse and the zero pulse discriminator is used to discriminate the time interval T of adjacent current zero pulses. For the former purpose, a zero-voltage comparator U with positive feedback is provided in the circuit of the pulse generator 11₁Differentiator U₂And a photoelectric isolation circuit U₃And U₄. The input terminals 13 and 14 receive terminal voltage signals from the SCR assembly SCR, and the output terminal 15 can obtain a current zero pulse. In the circuit diagram of the discrimination pulse, T over-width discriminator U is included₅Over-narrow discriminator U₆And OR gate U₇AND gate U₈. A current zero pulse signal is sent from the terminal 15, if T is in accordance with the set range, 16 and 18 are high level, and then an AND gate U is opened₈Normal current zero pulse output is obtained at terminal 21, while 17 and 19 assume a low level and terminal 20 does not issue an alarm signal. If T is too wide, 16 is low, and the AND gate U is blocked₈Terminal 21 has no output and at the same time 17 is high, terminal 20 sends out an alarm signal. If T is too narrow, 18 is at low level, 19 is at high level, no output is provided at terminal 21, and an alarm is given at terminal 20.

The method has the advantages that the complexity of the constant-current control device of the welding machine in spot welding, projection welding and seam welding can be greatly reduced, so that the cost can be reduced by about one fourth. Meanwhile, the constant-current control method is beneficial to fully utilizing the capacity of the welding machine, exerting the capacity of the welding machine and reducing equipment investment and electric energy loss. In addition, the method can be used for diagnosing the silicon controlled components on the main power circuit in real time, thereby being beneficial to ensuring the welding quality and avoiding various losses caused by short circuit and open circuit of the silicon controlled components.

Claims

1. A constant current control method for welding current of spot welding, projection welding and seam welding is characterized in that a microprocessor is used for storing a current control curve in advance, and the constant current control of the welding current is realized by controlling a trigger angle of a silicon controlled rectifier assembly on a welding machine main power circuit.

2. A constant current control device according to the method of claim 1, comprising a current sensor, an integrator, an analog-to-digital converter, a microprocessor, a keyboard, a display, a rom, a ram, a timer, a trigger circuit and a pulse discriminator, wherein a current zero pulse generator (11) is connected in parallel across the thyristor to convert the voltage across the thyristor into a current zero pulse.

3. A constant current control device according to claim 2, wherein the current zero pulse generator (11) comprises a zero voltage comparator (u 1) with positive feedback, a differentiator (u 2) and opto-electronic isolation circuits (u 3) and (u 4).

4. The constant current control device according to claim 3, wherein the current zero pulse discriminator (12) is composed of an over-wide discriminator (u 5), an over-narrow discriminator (u 6), an OR gate (u 7) and an AND gate (u 8).