Plasma anti dive

Problems and questions
1

hi, I can’t find much information on the anti dive feature. Could someone school me on it or point me in the direction of who to talk to. Thanks

2

The term ‘anti-dive’ in the context of CNC plasma table control usually pertains to prevention of the Z axis motion going negative (down) while the torch arc is ON and the XY motion transitions from cutting metal to not cutting metal. That’s my definition.

Anti-dive is used in conjunction with a Torch Height Controller (THC).

There are at least 2 conditions while cutting which anti-dive can come into play.

  1. when XY motion is decelerating from normal feedrate, stopped (even for 1 millisecond to make a tight turn), or accelerating to normal feedrate. In these cases ARC OK signal is usually not lost since some arc to metal contact is still occurring. However, because of the slower to no speed of the arc, more metal is burning away than at normal feedrate. Thus the arc envelope is larger, and thus the arc voltage is spiking, and if the THC is doing its job, then it senses the higher voltage and assumes the torch should be lowered to correct for it. Hence the ‘dive’ when a height correction is not needed.

  2. when the torch is cutting and moves off the metal. In this case, ARC OK of the THC should detect that (eventually) and halt any dive, but some dive could still occur while the ARC OK circuit is detecting transition.

In the first case, the anti-dive is usually handled by the CNC controller MCU or a custom circuit working with the MCU.

In the second case, it could be either the THC or the controller MCU that handles it.

In both cases, the gcode can also be a factor to mitigate anti-dive. Sheetcam has Path Rules that trigger when starting, ending, and cornering during a cut. Thus rules can be written to disable the THC at these trigger points, provided your controller has such ‘control’ over the THC.

I hope this helps,
Lou

1 Like
3

Thanks Lou, my controller doesn’t have anti dive yet. Masso is going to release it one day, i mainly use path rules to turn on and off thc for slow areas

Do you know what the anti dive percentage is representing?
Is it a percentage of voltage or speed ?

4

then you are managing anti-dive well already. well done.

without the benefit/capability of turning the THC off and on via gcode in slow feedrate conditions, I believe anti-dive trigger is a combination of both speed (deceleration) and ARC [not] OK signal (aka arc on pilot).

I built an anti-dive circuit for my controller and THC, it triggers based on feedrate deceleration, when actual feedrate < 94% of programmed value, and releases when feedrate > 95% of programmed value during acceleration. It works by blocking/unblocking the THC STEP signal to the Z motor. It also allows for dynamic THC ON/OFF via gcode command, again by blocking/unblocking the THC STEP signal. It’s built to work with an autonomous THC such as Proma THC SD.

I found during testing that my THC moves Z 0.1mm / 1volt deviation of arc voltage. I also discovered that there is a significant difference (improvement) in activating anti-dive at speed threshold of <94% of feedrate vs. waiting until <84%.

You can read about my trials and tribulations here: Proma THC SD or 150 discussion - (warning: bring coffee)
and here: GRBL Plasma w THC, Torch Touchoff, and Rotary - #40 by bLouChip (read prior posts also)
The ant-dive circuit diagram is available in those threads also, and gcode described in the comments of the scpost.

5

A plasma cutter is a constant current device. To maintain a constant current (amperage) torch voltage must vary based on cutting speed and torch height. Where torch height and cut velocity is held constant, there is a linear relationship between torch height and torch voltage so torch voltage is an easily measured process control variable. That is the function of the THC. Measure voltage and adjust torch height.

Reputable plasma manufacturers publish ideal cut heights and velocities for different materials and thicknesses that are set in gcode. But the problem is that the laws of physics (eg. centripetal Accelleration) say that an object must slow down to negotiate a corner. This causes several issues.

  1. The kerf gets wider
  2. The torch voltage increases
  3. If under THC control, the THC will respond to the increase in voltage by lowering the torch (rapidly) which can cause a crash into the material.

Velocity antidive needs to monitor the torch velocity and if it falls below a predetermined threshold, disable the THC.

This is just one instance where internal THC control by the motion controller has a significant advantage over an external THC hardware circuit because the motion controller sees and knows everything while the external circuit is groping in the dark.

And this is why we think Linuxcnc is by far the best affordable motion controller for plasma. With Linuxcnc, you just need to measure the torch voltage 1000 times a second and leave it to the motion controller and its sophisticated pid routines to manage torch height.