The present invention relates generally to plasma cutting systems and, more particularly, to a controller for use with such systems.
Plasma cutting is a process in which an electric arc is used for cutting a workpiece. Plasma cutters typically include a power source, an air supply, and a torch. The torch, or plasma torch, is used to create and maintain the plasma arc that performs the cutting. A plasma cutting power source receives an input voltage from a transmission power receptacle or generator and provides output power to a pair of output terminals, one of which is connected to an electrode and the other of which is connected to the workpiece. An air supply is used with most plasma cutters to carry and propel the arc to the workpiece and help cool the torch.
There are multiple ways of initiating this cutting process, for example contact starting or high frequency or high voltage starting. Generally, in contact start plasma cutters, a movable or fixed electrode or consumable serves as a cathode and a fixed or movable nozzle or tip serves as an anode. In some units, the air supply is used to force a separation of the electrode and tip to create an initial or pilot arc. In others, mechanical or electromechanical means serve to separate the contacts and generate the pilot arc. In either case, once the pilot arc is established, air is forced past the pilot arc whereby it is heated and ionized to form a plasma jet that is forced out of the torch through the opening in the nozzle. The air aids in extending the arc to the workpiece forming a cutting arc and initiating the cutting process.
Other systems utilize a high frequency starter to initiate the pilot and cutting arc, and still others systems employ high voltage to initiate the pilot and cutting arc. In any of the arrangements, the spaced relationship or the range of movement of the cathodic component and the anodic component require precise design and maintenance to generate the pilot arc and maintain the cutting arc.
Regardless of which system is used to generate the pilot arc, some systems provide for multiple operating modes. One such mode is an expanded metal cutting mode. Expanded metal mode (EMM) allows an operator to perform many individual plasma cuts with a single trigger activation. Such a mode allows an operator to quickly and efficiently cut “broken metal”, such as grates, mesh, screen, chain, chain-link fencing, or any metallic material separated by an air gap. During expanded metal cutting mode, the plasma torch attempts to maintain an arc in the presence of an air gap in the workpiece. To accomplish this, when a controller predicts that the cutting arc is about to collapse due to an air gap in the workpiece, the pilot arc circuit is enabled such that, as the cutting arc collapses, a pilot arc is formed. The pilot arc is maintained internally within the plasma torch to sustain an arc during the gaps in the workpiece. Once the gap in the workpiece is traversed, the cutting arc is reestablished to cut the workpiece. This process is repeated until the expanded metal cutting operation is completed and the trigger is released.
While an expanded metal cutting mode allows an operator to efficiently cut expanded metal, this mode increases wear experienced by the consumable components of the plasma torch. Although this wear is normal while in the expanded metal cutting mode, when an operator desires to perform regular solid metal cutting, operating the plasma torch in expanded metal mode exposes the components of the plasma torch to unnecessary wear. That is, in solid metal mode, the plasma torch only needs to generate a pilot arc upon actuation of the torch trigger. At the end of this process, the arc is allowed to fully collapse. Therefore, performing solid metal mode cutting while in expanded metal mode can result in excess pilot arc generation.
Expanded metal cutting can also somewhat reduce the cutting power available at the end of a cutting process. This is particularly true with systems that rely on output current error detection or systems that rely on monitoring changes in output current to predict that an imminent cutting arc collapse. Specifically, since the “error” identified by these prediction systems will be high at the end of a cut, full power may not be available to ensure a clean finish on the cut.
Nevertheless, while resulting in unnecessary wear and less accurate cuts, some operators may inadvertently operate in expanded metal mode while cutting solid metal. One attempt to resolve this problem has been to provide a physical switch on the power supply requiring an operator to mechanically switch the plasma cutting system from one mode to another. However, in dynamic work environments, operators might be required to cut several different materials in a generally consecutive manner. Such an operation would require an operator to repeatedly change the operating mode of the plasma torch. Not only would such a requirement detrimentally affect process efficiency by requiring the operator to repeatedly stop a particular cutting process, travel to the power source to effectuate the change of cutting mode, and return to the cutting process, there would be no guarantee that the operator would not forget to switch the control or would simply ignore it and continue to cut solid metal in the expanded metal mode.
It would therefore be desirable to design a plasma cutting system that automatically controls the plasma torch cutter between various modes.