THC Velocity lock and void lock

rodw wrote:

I don't think Linuxcnc has any problem reacting fast enough at 1 ms servo thread to manage any cutting condition, That lets the THC respond as fast as motion parameters change so the THC can be stopped  more or less instantly (as far as Linuxcnc's view of the world.)

Following corrugated iron is well within plasmac's capabilities, its just that the maximum velocity of the Z axis determines the maximum XY velocity because the z axis must be able to follow the contour of the material at the cutting speed. The magic of plasmac, linuxcnc and external offsets will do the rest.

But yes, you are on the right track. There are many cut parameters that could be fine tuned if the system knows the material thickness. I guess it can make a guess at the material thickness based on the cut velocity but with a larger machine that has consumables to cut at 8-9 different amperages and do not communicate the amperage to the plasma controller its probably not possible.

I'll take your comments on board as a vote to include material thickness in the cut files.


Rod,

I think you may have mis-understood what I was saying.

At no point was I ever saying Plasmac could not react fast enough or was not capable of cutting corrugated. On the contrary, my point was that for Void Lock to activate, if the THC (including the mechanics too) is indeed fast enough to correct the voltage (drive the torch down) when passing over a void, you won't get the necessary voltage "spike" that causes the THC to lock. I also theorised that potentially, a fast THC reaction could even cause the torch to dive and crash into the material because Void Lock may not happen. So dampening the THC reaction on slower cuts could be a potential solution.

Of course when I'm talking about fast THC reaction, the assumption is the mechanics of the Z are physically capable of carrying out the THC moves. Without that a fast THC reaction in software is meaningless.

As for your last paragraph my thoughts are that XY feedrate is the main factor as opposed to material thickness. Yes feedrates generally go down as material gets thicker but for any given thickness, feedrates can vary based on nozzle size and corresponding amperage. And the system always knows the commanded feedrate so no guessing.

Keith
 

A wrote:

I don't see any point in moving average. You already know what the target voltage is either from the Auto Volts calculation or from the Cut Volts from the material file. The target voltage is the one you should be trying to maintain so that should be the base for any calculations.

Regarding the image you show, my last testing of kerf crossing didn't show up any dips in the arc voltage like that plot.
Crikey that was over a year ago, time flies when your having fun...


 

    dvdt = (tvolts - avgvolts)/fperiod;       // Calculate change in volts for this servo cycle.

rodw wrote:

The reason why your plot differs to what I shared is that you are plotting arc voltage not dv/dt
This is what it looks like in halscope

The moving average is not used directly. Instead its used as to calculate dv/dt
dvdt = (tvolts - avgvolts)/fperiod; // Calculate change in volts for this servo cycle.
where torch volts = tvolts and avgvolts = moving average

Clearly some people saw there was a point in calculating a moving average and dv/dt because they registered a patent over the technique which they milked for 20 years. It provides a much more sensitive indicator to a void crossing event.

phillc54 wrote:

So how is it a more sensitive indicator than using the target voltage?

Testing has demonstrated that the absolute values of measured arc voltage slopes generated during a kerf crossing tend to be at least an order of magnitude greater than those generated due to workpiece warpage. For example, kerf crossing slopes can range from about 100 volts/sec to about 1000 volts/sec; whereas, warpage slopes are generally much less than 100 volts/sec. In FIG. 3, the kerf crossing spike slope is about 720 volts/sec. The warpage slope is about 14 volts/sec. Accordingly, kerf crossings and similar discontinuities can be reliably detected and accommodated readily with little chance of improperly overriding standoff control.

 A natural complement to any time series interpretation, a moving average can smooth out the noise of random outliers and emphasize long-term trends.

phillc54 wrote:

6000 is a heck of a lot more than HT's 1000 upper value...

Edit: Oops I submitted too quickly...

From the look of your plot it would be better to use the arc voltage plot then the calculated dv/dt plot.

Kerf Crossing:

Typically there are 3 ways that the CNCs deal with this. Do not mistake this with holes, see my "notes" for hole cutting.

One:
Same as you do, they take the cutting speed and reference that to the arc voltages rate of change. As your thickness range can be 1-150mm the speed has a significant effect and has to be used in the equation (like seems you do). This is the best way to do it and can with the right settings makes it nice. I think there is a patent about this so not everybody does it this way.

Two:
Simple but not so good. You have 2 voltage values that you set Kerf detect ON and Kerf detect OFF. One triggers the torch to lock the height and disable AVC and the other releases the lock. Lets assume your set arc voltage is 100V KD_ON 20 and KD_OFF 15. If the arc voltage reaches 120V (100+120), the height will lock. It will stay locked until it goes below 115 (100+15).
Very simple and avoids collision but is not nearly as good as option 1.

Three:
CAM handles it. So typically you don't cross kerfs intentionally so that it would matter (normally you just cross the kerf in the end and it does really affect cut quality). But for example in corner loops, multipass bevel etc. you might cross the kerf. 99% of the times you do this on a CAM so it will put height locks in those sections where the kerf will be crossed. CNC just has to lock/unlock based on G-codes.