Automatic gas control for a plasma arc torch

Abstract

A method and apparatus for controlling a gas supply to a plasma arc torch uses a proportional control solenoid valve positioned adjacent the torch to manipulate the gas flow to the torch, thereby extending electrode life during arc transfer and shutdown. Swirl ring design can be simplified and gas supply and distribution systems become less complicated. The invention also allows manipulation of shield gas flow to reduce divot formation when making interior cuts. The system can be controlled with a digital signal processor utilizing a feedback loop from a sensor.

1 . A method for piercing a workpiece with a plasma arc torch comprising the steps of: 
 establishing a plasma gas flow to a plasma chamber in the plasma arc torch, the plasma chamber defined at least in part by an electrode and a nozzle;    initiating a plasma arc in the plasma chamber between the electrode and the nozzle;    transferring the plasma arc to the workpiece; and    adjusting a flow of the shield gas based on cut characteristics of the workpiece, without significantly adjusting the plasma gas flow.    
   
   
       2 . The method of  claim 1  further comprising the step of increasing a current flow to the torch to pierce the workpiece.  
   
   
       3 . The method of  claim 1  wherein the adjusting step includes increasing the shield gas flow during piercing to compensate for an increased workpiece thickness.  
   
   
       4 . The method of  claim 1  wherein the cut characteristics include at least one of a workpiece thickness, a type of processes being used, a current level of the torch, a material type of the workpiece, or the cut shape and geometry.  
   
   
       5 . The method of  claim 1  wherein adjustment of the shield gas flow is accomplished using a programmable control valve located adjacent the torch.  
   
   
       6 . The method of  claim 1  wherein the programmable control valve is directly coupled to a body of the plasma arc torch.  
   
   
       7 . The method of  claim 5  wherein the programmable control valve is a proportional solenoid control valve.  
   
   
       8 . A plasma arc torch for cutting a workpiece, the plasma torch having a plasma gas source to supply a plasma chamber and a shield gas source to supply a shield gas to pass through a space between a nozzle and a shield, such that an electrical current passing between an electrode and a nozzle produces a plasma arc that exits the torch through a nozzle exit orifice, the plasma torch comprising: 
 a means for sensing a parameter in a first fluid line that supplies at least one of a plasma gas from the plasma gas source or a shield gas from the shield gas source; and    a means for controlling a flow of the first gas based on the sensed parameter using a programmable control valve disposed in the first fluid line adjacent the plasma torch.    
   
   
       9 . The plasma arc torch of  claim 8  wherein the programmable control valve is directly coupled to the plasma arc torch.  
   
   
       10 . The plasma arc torch of  claim 8  further comprising a plasma gas vent valve that includes at least one of a programmable control valve or a plurality of on-off solenoid valves.  
   
   
       11 . A method for control of a gas flow to a plasma arc torch including a plasma chamber disposed within a torch body comprising: 
 providing a first fluid line for supplying a first gas to the torch;    positioning a programmable control valve in the first fluid line adjacent the torch to control a flow of the first gas; and    manipulating the programmable control valve thereby 
 a) controlling the flow of the first gas to the torch during operation of the torch; and  
 b) compensating for a volume in the first fluid line between the programmable control valve and the torch.  
   
   
   
       12 . The method of  claim 11  wherein the programmable control valve is directly coupled to a torch body of the plasma arc torch.  
   
   
       13 . The method of  claim 11  further comprising the step of adjusting a ramp down time of the flowing first gas to prolong a life of a plasma arc torch electrode based on a cut duration or a current level of the electrode.  
   
   
       14 . The method of  claim 13  wherein the adjusting step includes increasing the ramp down time based on an increased cut duration or an increased current level of the electrode.  
   
   
       15 . The method of  claim 13  wherein the adjusting step includes decreasing the ramp down time based on a reduced cut duration of the electrode.  
   
   
       16 . The method of  claim 11  wherein the programmable control valve controls the flow of the first gas a plurality of times during a ramp cycle of the first gas.  
   
   
       17 . The method of  claim 11  wherein the first gas is a plasma gas and at least a portion of the plasma gas is vented to the atmosphere through a vent valve.  
   
   
       18 . The method of  claim 17  wherein the vent valve includes two or more on-off solenoid valves mounted in parallel.  
   
   
       19 . The method of  claim 17  wherein the vent valve includes a programmable control valve.  
   
   
       20 . The method of  claim 11  wherein the first gas comprises at least one of oxygen, nitrogen, hydrogen, methane, helium, or argon.