Microprocessor controlled welding apparatus

A method of controlling a welding apparatus, comprising the steps of:

• sensing a first welding sample using a circuit that produces a signal indicative of welding current or voltage;
• sensing a second welding sample at a later time using the circuit that produces a signal indicative of welding current or voltage;
• computing a first derivative parameter using a processor by subtracting the first welding sample from the second welding sample;
• computing a difference parameter using a processor by subtracting the second welding sample from a predetermined control parameter;
• looking up an adjustment parameter from a table of predetermined parameters where the table of predetermined parameters contains a predetermined parameter corresponding to each of a range of first derivative parameters and to each of a range of difference parameters;
• and,
• adjusting elements controlling welding current or voltage, responsive to the adjustment parameter obtained from the table.

A microprocessor-controlled welding apparatus, comprising:

• a three-phase transformer, the three-phase transformer having at least one primary winding and having three secondary windings, the primary winding being adapted for coupling to a source of alternating current ("AC") electrical energy;
• at least three solid state switching devices, each solid state switching device being connected between a secondary winding of the three-phase transformer and a first common welding lead, each switching device being responsive to a gating signal to rectify an AC signal applied to its associated secondary winding by conducting direct current during a portion of an AC phase after the gating signal fires the switching device, the switching device being mutually cooperable to generate a welding signal between the first common welding lead and a second welding lead coupled to the secondary windings of the three-phase transformer, the welding signal having a direct current voltage with an average magnitude which is determined by the portion of the AC phase that the switching devices are fired by the gating signal;
• a sensor coupled to the first common welding lead for directly sensing the welding signal;
• an input/output controller, the input/output controller being coupled to the sensor, the input/output controller being coupled to the switching devices;
• a memory;
• a microprocessor, the microprocessor being coupled to the input/output controller and to the memory, the microprocessor being adapted to lead welding data signals from the sensor through the input/output controller, the microprocessor, in accordance with a program stored in memory, being operable to compare the welding data signals from the sensor with control data in memory, the microprocessor being operable to signal the switching devices and to provide gating signals in accordance with a program stored in memory which fire the switching devices for a portion of the AC phase so that the welding signal has a direct current voltage which is determined by the microprocessor;
• the microprocessor being operable to read a first welding data signal and to record a first welding data parameter in memory based upon the first welding data signal;
• the microprocessor being operable to read a second welding data signal at a later point in time, the microprocessor being operable to record a second welding data parameter in memory based upon the second welding data signal;
• the microprocessor being operable to compute an error parameter indicating the extent to which the sensed welding signal fails to conform with program control by subtracting the second welding data parameter from a control parameter selected by the microprocessor from control data in memory;
• the microprocessor being operative to compute a first derivative parameter indicating the rate of change of the welding signal by subtracting the first welding data parameter from the second welding data parameter;
• the microprocessor being operative to look up a memory location in a table stored in memory containing a plurality of predetermined gating parameters stored in memory locations, the table being adapted to quickly permit the microprocessor to determine a proper gating signal for the switching devices, the memory location in the table being determined by both the error parameter and the first derivative parameter, the memory location containing a predetermined gating parameter representative of gating control to be applied to the switching devices based upon the extent to which the sensed welding signal fails to conform with program control and the rate of change of the welding signal;
• and,
• the microprocessor being operative to signal the switching device and to provide firing signals determined from the predetermined gating parameter obtained from the table.