Standoff control method and apparatus for plasma cutting machine

A standoff control apparatus for a plasma cutting machine comprising:

• a voltage detector for detecting the arc voltage between a workpiece to be cut and one of (a) an electrode of a torch and (b) a nozzle surrounding said electrode,
• a standoff correction computing device for subjecting the thus detected voltage outputted from said voltage detector and a reference voltage to a comparison so as to output a standoff correction signal,
• wherein said standoff correction computing device has a reference voltage computation setter for outputting a reference voltage between said workpiece to be cut and said one of (a) said electrode and (b) said nozzle with respect to a predetermined standoff.
• an error computing device for calculating a deviation between said detected voltage and said reference voltage;
• and
• a correction quantity computing device for outputting a vertical repositioning speed signal of said torch in accordance with the degree of said evaluation;
• wherein said standoff correction computing device has an electrode consumption/standoff detector which receives said detected voltage so as to output, to said error computing device, a standoff signal obtained by correcting for the degree of increase in the voltage due to the consumption of said electrode.

A standoff control apparatus for a plasma cutting machine comprising:

• a voltage detector for detecting the arc voltage between a workpiece to be cut and one of (a) an electrode of a torch and (b) a nozzle surrounding said electrode,
• a standoff correction computing device for subjecting the thus detected voltage outputted from said voltage detector and a reference voltage to a comparison so as to output a standoff correction signal,
• wherein said standoff correction computing device has a reference voltage computation setter for outputting a reference voltage between said workpiece to be cut and said one of (a) said electrode and (b) said nozzle with respect to a predetermined standoff,
• an error computing device for calculating a deviation between said detected voltage and said reference voltage;
• and
• a correction quantity computing device for outputting a vertical repositioning speed signal of said torch in accordance with the degree of said deviation;
• wherein said reference voltage computation setter outputs, as a reference voltage, a voltage obtained by adding the degree of change of said arc voltage, which has taken place due to the deviation of a cutting speed from a reference cutting speed, to an arc voltage determined in accordance with said reference cutting speed for said torch.

In a standoff control method for a plasma cutting machine in which the arc voltage between a workpiece to be cut and one of (a) an electrode of a torch and (b) a nozzle surrounding said electrode is detected so as to control the standoff between said torch and said workpiece to be cut to a predetermined value in accordance with the detected voltage, the improvement comprising the steps of:

• detecting a voltage drop between said nozzle and said workpiece to be cut which is indicative of the generation of a double-arc during an operation of said torch;
• continuing said operation of said torch under a safety control by positioning said torch away from said workpiece to be cut by a predetermined quantity until said double-arc disappears;
• and
• cancelling said safety control and making a restoration to a normal torch control after the voltage drop between said nozzle and said workpiece to be cut has been restored to an original level.