Messages

331

General Mach Discussion / Re: Ignore Z

« on: August 18, 2011, 08:00:23 PM »

The CB option is DoOemButton(252) same as the setting screen button

Hope that helps, (;-) TP

Just got around to trying this.  Worked great.  Thanks for the tip.

332

General Mach Discussion / Re: Height Control for Router Table

« on: August 17, 2011, 08:53:16 AM »

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.

333

General Mach Discussion / Ignore Z

« on: August 16, 2011, 04:35:09 PM »

Is there a way to tell Mach to ignore all Z moves?  For instance, if I wanted to run a program but just cut air with no Z moves.  Rather than generating a separate program with no Z moves, can I just do it in Mach with a setting?

Thanks

334

General Mach Discussion / Re: Height Control for Router Table

« on: August 14, 2011, 05:20:18 PM »

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.

335

General Mach Discussion / Re: Height Control for Router Table

« on: August 14, 2011, 12:15:35 PM »

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.

336

General Mach Discussion / Re: Height Control for Router Table

« on: August 14, 2011, 12:00:33 PM »

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.

337

General Mach Discussion / Re: Height Control for Router Table

« on: August 13, 2011, 10:00:10 PM »

RRC , it is very easy to get the MACHINE to operate outside of the preset PID range. IF you use a feed screw that is too fast it is possible to outrun the pid because is not adjustable in most THCs and most pids are range specific to tuning SO if the PID is not use adjustable you MUST engineer the feedsytem to OPERATE in the boundaries of the set PID.

If the PID is programmed properly, it adjusts the feed rate to meet conditions.  You're confusing signal conditioning with PID loops.  For instance, the Thermal Dynamics THC on their controller running a Koike machine with an Ultra-cut 400 plasma will run the Z at 100IPM.  The PID controller has control of that.  If the gap suddenly widens, the Z can go as high 100IPM to close the gap.  The actual feed rate depends on the amount of correction needed.

338

General Mach Discussion / Re: Height Control for Router Table

« on: August 13, 2011, 09:53:03 PM »

The pid is used as PART of the sensoring of the arc voltages. IT tries to average out the input stream and make a smooth control output signal not just filter out high /low.

That's not PID, That's signal conditioning.  Our THC does not have a PID loop built in, but it does have a signal conditioner...and it's as stable as the day is long.  A PID loop is used to dynamically change motion parameters based on conditions, such as temperature, flow, voltage, etc.  They are used when a simple on/off won't work.  That's why the higher end THC's have PID control loops.  They control the Z axis directly and based on conditions adjust the feed rate accordingly.  That's why a THC with a PID control loop can respond at a much faster feed rate that one of these $500 THC's that use the Mach3 THC commands.

339

General Mach Discussion / Re: Height Control for Router Table

« on: August 13, 2011, 08:02:26 PM »

THE thcup/down is only the fairly high speed digital INPUT to control the Z thru MACH.

THC UP/DOWN is the only input to control Z through Mach...Unless you're talking about a THC that controls the Z independent of Mach.

 IF those signals are erratic then the Z will be erratic. The PID filtering takes place in the DTHC.

You're talking about signal conditioning, not PID control.  If the tip voltage is all over the place, then the height will be erratic.

Being that Mach3 is NOT a real time controller your on the fly height correction calculations are not going to work well. Yes you can use #vars for Zheight control but ONCE the moves are loading into the buffer then that is it.  IF you try to calculate the variable at the point the #var is loaded into the buffer you will find you cannot comp something that has not happened yet. the next problem is the only way you have to update those vars are thru either the brains or the macropump. At a MAX of 10 updates per sec you will fall FAR short of the speed to do it well.

I'm not talking about doing the math inside Mach.  I agree that this is way to slow.  The purpose of a PID loop is to attempt to look ahead.  If the there is a large difference between the set voltage and the tip voltage, then the loop moves the Z at a faster feed rate than if there were a smaller difference in an attempt to "catch up".  As the two voltages get closer the PID loop slows the feed rate.  It works very well, but it only work if the THC is controlling the Z axis motor drive directly.

The ONLY access you have at that point IS the THC functions up/down to comp the surface heights based on what the DTHC sees. AND it is NOT value based it is like playing ping pong slapping  the ball back and forth. THe Z at that point will not even use the motor vel/accel parameters it just basically slaps the motor wide open in the direction it is to move and lets the DTHC deal with it being correct in height. Woops overshot slap it back in the other direction.  

This is exactly how the low end THC's do it.  Just turn your THC feed rate up to about 25 and watch the Z axis bounce.  If these $500 THC's have a PID loop, then it's not working.  They simply filter the tip voltage to remove any noise and compare that to a set voltage.  If there is a difference, it triggers the THC UP or DOWN commands in Mach.  If there were some sort of PID control, then we would be able to run the THC feed rate at 100% as the PID would adjust feed rate and prevent over shooting.  As it stands, to reduce over shoot, we have to run the THC feed rate at 5-10%.  even that doesn't eliminate it.  It just reduces it to an acceptable level.

That is where the PID has to step in. Same basic princible as a servo motor drive.

How exactly does the PID step in?  We're talking about a system with ON/OFF UP/DOWN inputs.  You can not vary the feed rate based on conditions, which is the purpose of a PID loop.  The only possible way this could happen is if the THC were to pulse the THC inputs sort of like a DC speed control, but then I doubt Mach would be able to handle that.

Which THC are you running?  I have a Sound Logic and a CandCNC on the bench and I can tell you that neither of them have build in PID control. I know this because (1) They operate through Mach's THC commands and (2) Turning the THC feed rate past 15 turns the Z into a pogo stick.

340

General Mach Discussion / Re: Height Control for Router Table

« on: August 13, 2011, 05:47:09 PM »

"That is exactly what I need, but if it does see a chip I don't want it pulling Z up.... I suppose using a slow feedrate for compensation may work alright... Any suggestions?"

Then you don't need a THC.  You want to read the material height, do some math and apply a height offset to the Z to compensate for changes in material thickness.  A THC controls the Z axis.  Sounds like that's not what you're after. 

The only way this could work is if you wanted to take say....a consistent 1/4" off the top of the material.  Then you could adjust the THC to run the Z at a constant - 0.25...Which would be only as accurate as you ultrasonic sensor.

PID loops have been around forever, but there's more to an industrial controller that the PID loop.  It's all relative.  When you drop $200k on a brand new Koike MasterGraph, $30K for a controller and $10K for a THC doesn't sound so bad.