Plasma torch condition monitoring

Details Plasma torch condition monitoring Plasma torch condition monitoring

1996-09-24

1998-02-10

Paschall, Mark H.

Electric heating

Metal heating

By arc

21912155, 21912151, 21912159, B23K 1000

Patent

active

057171874

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

This invention relates to a method and apparatus for monitoring the condition of a plasma arc torch and is directed in particular towards determining whether a nozzle of the torch has undergone any "axisymmetric" wear and distinguishing between electrode wear and nozzle wear. Plasma arc torches to which the invention is applicable may be used for example for cutting metallic sheets or plates in metal fabrication, or in material spraying or waste destruction systems. The invention will be described hereinafter in relation to a plasma arc cutting torch, but it is to be understood that the application of the invention is not limited to such a cutting torch.


BACKGROUND

Plasma arc cutting processes make use of the heat and momentum of a high velocity plasma jet to sever materials by the dual actions of melting as well as vaporization and material displacement along the jet path. The melting and vaporization of the material relies on the heat from the plasma jet and from an electric arc established between an electrode of the plasma torch and the workpiece (that is, a transferred arc system), or between two electrodes in the torch (that is, a non transferred arc system).
A typical plasma cutting system comprises a plasma cutting torch, power supply, arc igniter and consumables such as plasma and shield gases as well as torch coolant. The plasma torch can be hand held or can be mounted on a contouring machine such as a planar profiling machine, a three axis gantry or an articulated robotic manipulator. The plasma cutting torch includes an electrode (typically the cathode) centered above an orifice in a constricting nozzle. A suitable plasma forming gas flows under pressure around the electrode and through the nozzle orifice towards the workpiece. The arc is constricted by the nozzle and can be further constricted by shielding gas or water. An arc igniter is used to establish a pilot arc between the electrode and the nozzle and subsequently, under the influence of a strong gas flow, this arc transfers to the workpiece (in a transferred arc torch) and the pilot arc is extinguished.
The quality of the cut made with a plasma arc torch (which is determined by factors such as the dimensional accuracy of the cut parts, cut angle (degree of squarenesss of the cut face), sharpness of the bottom and top edges of the part, roughness of the cut face, amount of dross on the bottom of the plate (workpiece), amount of splatter on the top of the plate etc.,) is extremely sensitive to the condition of the torch and in particular to the condition of its nozzle and electrode, which are consumable parts. Presently, an operator usually visually supervises the cutting operation and stops cutting if the quality of the cut deteriorates. Such visual inspection of the cutting process is very cumbersome due to extreme brightness of the plasma arc, presence of metallic fumes, cut parts remaining in the workpiece plate until the cutting is completed for a given plate and often under-water or water-muffler cutting. Alternatively, the torch may be inspected by the operator in an off-line mode, either periodically or after deterioration of the cut quality has been observed. In order to increase the degree of autonomy of a plasma cutting system, increase its reliability and consistency of the cut quality as well as reduce material waste, a method and an apparatus which are suitable for automatically testing and monitoring the condition of the torch are needed.
Condition monitoring is concerned with determining the type and degree of wear of consumable parts of a plasma arc torch, in particular the type and degree of erosion of the nozzle around the orifice and of the degree of erosion of an electron emitting element embedded in the electrode. For the nozzle, the following types of erosion can be distinguished: nozzle around the orifice, axisymmetric wear).
A combination of the above types of erosion often occurs. For example, wear of type (ii) and (iii) occurs after prolonged torch operation. On the other hand, erosion of typ

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patent: 4775774 (1988-10-01), Caneer, Jr.
patent: 5225658 (1993-07-01), Yamaguchi
patent: 5326955 (1994-07-01), Nishi

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