Control equipment for resistance welding machine

• US 5,844,193 A
• Filed: 05/26/1995
• Issued: 12/01/1998
• Est. Priority Date: 05/27/1994
• Status: Expired due to Fees

• Current Assignee: Toshiba It&Control Systems Corporation (Toshiba Corporation)
• Original Assignee: Kabushiki Kaisha Toshiba (Toshiba Corporation)
• Inventors: Souma, Saburo, Ambo, Tatsuaki, Okado, Chihiro, Nomura, Yoshihito
• Primary Examiner(s): Shaw, Clifford C.
• Application Number:
  US08/452,338
• Time in Patent Office: 1,285 Days
• Field of Search: 219/110, 219/108, 219/109, 219/114, 219/117.1
• US Class Current: 219/110
• CPC Class Codes: B23K 11/24 Electric supply or control circuits therefor
• CPC Class Codes: B23K 11/257 the measured parameter being an electrical current
• CPC Class Codes: H02M 7/4803 with means for reducing DC component from AC output voltage
• CPC Class Codes: H02M 7/53871 with automatic control of output voltage or current

Abstract

A control equipment for a resistance welding machine for welding a material by the Joule heat generated in the material by applying an electric power to the material, including an inverter for converting a DC voltage to an AC voltage by pulse width modulation control, a transformer having a primary winding to which the AC voltage is applied and having a secondary winding which supplies an AC current to the material, and a control unit for generating an AC current reference with a rectangular waveform of a specified frequency, comparing the AC current reference with an output current of the inverter to obtain an error, and for PWM controlling the inverter so that the error becomes zero.

Claims

1. A control system for a resistance welding machine for welding a material by heat generated in said material by electric power supplied to said material, comprising:

a pulse width modulation controller;

an inverter connected with and controlled by the pulse width modulation controller to convert a DC voltage at the inverter input to a pulse width modulated AC voltage at the inverter output;

a transformer having a primary winding connected to the inverter output receiving the pulse width modulated AC voltage and having a secondary winding connected to supply said electric power to said material;

a control circuit generating an AC reference current having a rectangular waveform of a specified frequency, said control circuit further comprising;

a level detecting and judging portion detecting a reference signal indicating magnetic flux density of said transformer and judging whether or not said magnetic flux density exceeds a specified value by judging whether or not the reference signal exceeds a specified value and providing a judgement signal output when said reference signal exceeds said specified value, and

a frequency correction portion connected to receive and to respond to said judgement signal to prematurely invert a polarity of a portion of the rectangular waveform of said AC reference current to increase the frequency of the reference current;

a current comparator controller connected to receive said AC reference current and a current present at the transformer and comparing said AC reference current with said current present at the transformer to obtain a transient control signal; and

said pulse width modulation controller having an input connected to the current comparator controller and receiving the transient control signal to control the pulse width modulation of said inverter to increase the inverter AC voltage in the presence of said transient control signal.

2. The control system for a resistance welding machine according to claim 1, wherein said level detecting and judging portion further includes:

a reactor simulating said transformer flux density connected in parallel with said primary winding of said transformer; and

level detection judgement circuitry connected to receive a current flowing in said reactor as said reference signal and detecting and judging that said transformer magnetic flux density exceeds said specified value so as to generate said judgement signal when said current flowing in said reactor exceeds a specified current value.

3. A control system for a resistance welding machine for welding a material by heat generated in said material by electric power supplied to said material, comprising:

a pulse width modulation controller;

an inverter connected with and controlled by the pulse width modulation controller to convert a DC voltage at the inverter input to a pulse width modulated AC voltage at the inverter output;

a transformer having a primary winding connected to the inverter output receiving the pulse width modulated AC voltage and having a secondary winding connected to supply said electric power to said material;

a control circuit generating an AC reference current having a rectangular waveform of a specified frequency, said control circuit further comprising;

a conduction control portion generating a conduction instruction for a preset time after a welding starting instruction is given, said AC reference current being generated in response to said conduction instruction being received at an input of the control portion, and

an initial condition control portion judging a polarity of residual magnetic flux of said transformer at the time of completion of said preset time of said conduction instruction and using the judged polarity to determine an initial polarity of the rectangular waveform of said AC reference current to be initially generated by a next condition instruction;

a current comparator controller connected to receive said AC reference current and a current present at the transformer and comparing said AC reference current with said current present at the transformer to obtain a transient control signal; and

said pulse width modulated controller having an input connected to the current comparator controller and receiving the transient control signal to control the pulse width modulation of said inverter to increase the inverter AC voltage in the presence of the transient control signal.

4. The control system for a resistance welding machine according to claim 3, wherein:

said initial conduction control portion judges said polarity of said residual magnetic flux from a polarity of said rectangular waveform of the AC reference current at the completion of said conductive instruction and a conduction time in a final conduction cycle.