The role of a torch height control system for mechanized cutting systems is often underestimated. A good THC (torch height control) is absolutely essential to obtain a good quality of cut. It is a known fact that by far the best possible control method is to detect the cutoff voltage. The cutting voltage is directly proportional to the height between the torch and the work piece; the greater the distance, the greater the voltage and vice versa. Therefore, one can easily take the cutoff voltage as a control parameter and use it to control the separation distance. This method of control is often referred to as AVC (Arc Voltage Sensing). Arc voltage sensing is not without its challenges. The torch is subjected to a high frequency pilot arc, which creates plasma. This HF can easily return to the sensitive voltage measurement circuit and damage/causes the system to malfunction. It is absolutely essential to isolate the measurement circuit from the HF plasma and yet be able to measure the voltage accurately. Highly sensitive arc voltage control circuits can detect voltage variations of less than 0.5 V. Accurate measurements and isolation can be achieved through the use of Hall sensors.
Piercing thick metals has an adverse effect on the life of the torch consumables. Therefore, drilling must be carefully controlled to maximize the life of the consumables. A proper drilling sequence is explained below with schematics:
I) When a command is received from the CNC to start cutting, the torch height controller moves the torch down to touch the workpiece to determine the zero level. The board can be detected via multiple mechanisms, the most commonly used being ohmic detection/torque detection/a combination of both. Once the plate has been detected, the torch now moves up.
ii) The torch continues to move up to a programmed pierce height. Upon reaching this height, the plasma arc strikes and the plate begins to pierce. While drilling is taking place, a large amount of molten metal (depending on the thickness of the plate being drilled) jumps off the plate and forms a puddle. The torch must be shielded from this molten metal, so the torch continues to move upwards to a programmed “jump into molten pool” height.
iii) The torch now moves down to the programmed cut height. The THC unit now gives a signal to the CNC to start the move
iv) The torch now continues to move at the same cut height. The arc voltage is constantly fed back to the control unit, which maintains a constant arc voltage through an appropriate PID control loop.
The above speech more than demonstrates the role of a good THC for a good plasma cut.
