I guess the engineer that designed and built the unit did not know what he was talking about then. Shame to waste all that time on education. I'll let him know that he should not waste all his time working on such silly things that could never be possible.

But then again MAYBE that is why it works  so good.

You guys are funny,

(;-) TP

I have no experience with THC at all, just trying to piece the information from the two of you together. The possibility does exist that the unit has PID and Mach3 can't utilize it. I'm not saying that is or isn't so, I have no idea. I don't run torch or plasma and am trying to learn if it would be applicable in my application.

Could you recommend such a unit for me to research?

Thanks, Chris

I guess the engineer that designed and built the unit did not know what he was talking about then. Shame to waste all that time on education. I'll let him know that he should not waste all his time working on such silly things that could never be possible.

But then again MAYBE that is why it works  so good.

You guys are funny,

(;-) TP

Just because it works well doesn't mean it has an internal PID controller.  Sargon nailed the definition of PID to a T.  PID control of the Z axis through the Mach THC commands is not possible.  PID is not necessary for a THC to work well.  It's simply a matter conditioning the incoming signal and tuning the THC feed rate to reduce over-shoot to an acceptable level. 

Get your hands on high end THC and controller like a TD, Hypertherm or Burny and you'll see the difference.  The TD THC can track corrugated material at 150IPM.  Maybe faster, but the material we were cutting called for 150IPM.

Could you recommend such a unit for me to research?

If I understood correctly, what you're trying to do is read the TOM and make Z offset adjustments on the fly to maintain accurate Z cutting depths even though TOM height may change.  If you use a THC, you will lose control of the Z axis while the program is running.  Maybe a better approach would be to read the TOM and use a brain or macropump to adjust the tool length offset on the fly.  Never done that, so I can't tell you how to do it.

You could use Peter Homanns MODIO to get the analog signal from the sensor into Mach.

We tried that with a Modio and the overall system responce (CB) was too slow at 10hzs.The reason I suggested the THC functions is that the LPT inputs sample at kernal speeds.

The second probem was getting the correction data to the Gcode buffer fast enough. You can run 1:1 on the buffer filling BUT now the machine was limited in overal performance and  speed because of waiting on the buffer.

To run a full buffer with a lot of look ahead (best for smoothness) you lost the ability to do the comp because the buffer loaded BEFORE the machine got to the point you needed to measue and apply the comp. ( catch 22).

The last option we tested was to take a linear encoder and use it to measure the height and use the quad output to drive the stepper drive directly that way it was geared to the surface measurements. With a modern digital servo drive running the Z this worked fairly well . BUT you had to swapaxis back and forth to have full control of Z and this made it VERY clunky to code and use. ALSO you woul dloose control of the positional state of Z each time and needed to refhome each time.

I still think the THC method could be made to work. WHen TOMc and Art developed the function Along with MUXing the ports it could be classed as a work of genius's in the DIY field of CNC.
BUT it would have to be fully investigated and tested. The nice part withte hTHC metod is you never loose postional control of the Z no matter how much the DTHC slapps it around(;-) MACH always knows where it SHOULD be.  SO Gcoding is never effected and it runs on the fly.

You mileage may vary based on local conditions, (;-) TP

If Z is at say...0 and if you move up say .25" using the THC UP input, does Mach offset the axis up .25 or just make a rapid move to .25?  If it does an offset, it should work for what he's trying to do.  If it just makes a move, then the next Z move in the G-code would just move it back where it was.

Thanks for the responses, it has been incredibly helpful. It seems the only sure-fire way of making a robust dynamic height control would be to send the pulses into a PLC and apply the THC there, delaying all signals equally. A PID loop could also be implemented in the PLC, and wouldn't be too difficult. The result would be a kick-a$$ dynamic height controller.

To me, it looks like there are too many barriers in Mach3 for this application (i.e. no velocity/acceleration control for THC, macros way, way too slow, applying offset directly to motor causes lost position, 1:1 buffer results in buffer under-runs, etc...). While I think it's likely possible, it doesn't look simple at all, while the PLC approach is straight forward and extremely robust.

Thanks,
Chris

Hi Chris

Though I agree with a lot (but not all) of what's been said here I think your conclusions are perhaps unneccessarily pessimistic about THC with Mach.

I've built several DTHCs which work just fine with (and without) Mach and I've thus far not seen the need to add PID control. I guess I'll have to try it to see if there's any appreciable difference in performance, but frankly I'm not holding my breath.

I think that sometimes we can get a tad carried away with theory and forget the real world. I can accept (maybe) that PID could be usefull in plasma THC if there were DRAMATIC and SUDDEN changes in the level of the metal but I'd argue that in 99% of cases this just isn't the case. THCs do NOT need to "rocket about" at great speed. Watch one doing it's thing and you'll see that more often than not they just wander up and down gently over a few mm or so.

That said, I think the success you'll have with your particular requirement, however you implement a height control, will depend ultimately on whether your sensor device has the response speed, resolution and accuracy commensurate with the cut quality you require.

Just my 2 whatnots.

Cheers

Ian

Thanks for the responses, it has been incredibly helpful. It seems the only sure-fire way of making a robust dynamic height control would be to send the pulses into a PLC and apply the THC there, delaying all signals equally. A PID loop could also be implemented in the PLC, and wouldn't be too difficult. The result would be a kick-a$$ dynamic height controller.

That is the best way to control torch height, but keep in mind in that application, Mach3 has no control of the Z axis during feed rate moves.  Mach only makes Z clearance moves.  All of the high end THC's use an internal PID controller.  It's the only way to get a high feed rate with out bouncing the Z like a pogo stick, but as Stirling said, that sort of speed in a THC is not needed most of the time. 

While I think it's likely possible, it doesn't look simple at all, while the PLC approach is straight forward and extremely robust.

In a THC application it is fairly straight forward.  In your application, you'll be controlling the Z axis drive with the PLC, which means you'll need to send the Z step and direction pulses from Mach to the PLC, modify them and send them out to the drive.  Keeping the PLC and Mach in sync might prove to be a challenge.

You keep talking about a THC, but that's not really what you're doing.

Just a noted point(;-) if You are in a commercial cutting business to use theTHC and plasma it is imperitive that it control the z down to a few thou  for control of the cuting process. The plasma arc is very sensitve to changes in the arc length. Loose control of that process and you will not be competitive in cut quality. Today everyone thinks plasma should be as precise as a milled profile. SO yes there is a Need for fast thc responce. Cutting coragated sheets it is a must or you end up  dragging the head or a very sloppy cut. DOing it on thin sheets requires you to CUT fast and close. Try it with diamond plate. It probably is the worst at crashing torch heads. Remember cut quality come with very close arc length so Fast cutting at .050" height can be very tricky.

Now to balance out that in some cutting you could turn the thc off and never know it wasn't working.

If you are talking hobby plasma or plasma art? After you have cut with a high speed thc you will not want to go back.The quality of cut is very noticable and the level of cleanup is greatly reduced. It lowers the cost of the end product.

With a slow thc you can actually WATCH the torch lagging behind the process or minor almost not noticable pogo effect. That will leave a minor scalloping to the cut.

SO I guess it all depends on what you require out of the process. (;-)

(;-) TP