The role of a torch height control system for mechanized cutting systems is often understated. A good THC (Torch Height Control) is absolutely critical to good cut quality. It is a known fact that by far the best possible control method is by sensing the cutting voltage. The cutting voltage is directly proportional to the height between the torch and the workpiece; higher the distance, higher the voltage and vice-versa. So one can easily take the cutting voltage as a control parameter and use the same for controlling the stand-off distance. This method of control is frequently referred to as AVC (arc voltage sensing). Sensing the arc voltage is not devoid of challenges. The torch is subjected to a high frequency pilot arc, which creates plasma. This HF can easily traverse back to the sensitive voltage measuring circuitry and damage/cause malfunctioning of the system. It is absolutely essential to isolate the measuring circuitry from the plasma HF and yet be able to measure the voltage accurately. Highly sensitive arc voltage control circuitry can detect voltage variations less than 0.5V. Accurate measurements as well as isolation can be achieved through use of Hall sensors.
Piercing through thick metals has an adverse effect on the life of the torch consumables. Hence, piercing needs to be carefully controlled in order to maximize the life of consumables. A proper piercing 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 ascertain zero level. See Fig. a. Plate can be sensed through multiple mechanisms, most commonly used is ohmic sensing/ motor torque sensing/ a combination of both. Once the plate has been sensed, the torch now moves upwards (see Fig. b)
ii) The torch continues to move upwards to a programmed piercing height. On reaching this height, the plasma arc strikes and the plate begins to get pierced (fig c). While piercing takes place, a lot of molten metal (depending on the thickness of the plate being pierced) jumps out of the plate and forms a pool. The torch needs to be protected from this molten metal,so the torch continues to move upward to a programmed “molten pool jump” height. (fig. d)
iii) The torch now moves downwards to the programmed cutting height. The THC unit now gives a signal to the CNC to start movement
iv) The torch now continues to move at the same cutting height. Arc voltage is constantly fed back to the control unit, which maintains a constant arc voltage through an appropriate PID control loop.
See schematic below.
The above discourse amply demonstrates the role of a good THC for a good cut with plasma.