Has anybody out there found a good way to do height following?  That is, to have the z-axis plunge, have a sensor read the material surface and set the height, then maintain that same height above the material as the part is being cut?

What are we talking about here -
Are you asking a simple device to come down, read where the top of the work is, then move z down and cut level -

Or are you asking for a device that comes down, and continuously reads an undulating surface and keeps the cutter a set distance above (or below) it - i.e. cutting a groove of a set depth.

If its the latter of what Jim said then it will not be an easy thiug to do. Just thinking aloud here but it may be ppossible to use a plasma style torch height control. I think they work off a voltage and adjust to keep the torch height constant when materilas warp due to heat. It might be possible to set up some sort of sensor that alters voltage as the material lowers or raises in  comparison to the axis height then feed this to a THC and it will vary the axis.
 As I said just thinking aloud and not sure even if thats how the THC work

Hood

#2 is the correct answer, almost.  We wouldn't be cutting a groove.  Think plasma torch, laser, or in our case waterjet.  This would be especially nice when cutting underwater where you couldn't see the material or the cutting tip (also done with plasma torches).

In general, plasma set-ups do exactly what you suggested.  They plunge to a set voltage and then maintain that voltage.  With plasma, this can be read in the torch itself as the voltage increases as the cutting tip gets farther from the material.

What we'd like to do is use a linear resistor that passes voltage from 0-10 vdc.  The setting for height would be at 5 vdc.  This could be attenuated using a potentiometer to adjust the perceived voltage and to set the start point (that is, plunge to position and let the basic setup pick a height - then turn the knob to adjust up/down slightly to a proper height).

So, the z-axis would plunge to a start point - the point where the feedback voltage is 5 vdc.  The height could be altered manually with a potentiometer, if required.  From there, the control would move the z-axis up/down to maintain the voltage at 5 vdc.

i am working on a plugin at the moment to do exactly that for a Laser ,
let me know directly what voltages u need etc , or exact requirements

i hope to have it tested shortly

Dave 

That's great news!

I really don't care what voltages you pick - we'll work with whatever.  However, we prefer to work with or inside of a 0-24 vdc range.  The sensor we are planning to use is 10 vdc to 24 vdc.

What are you planning on using for the laser?

here's a pic of the type of sensor we use , although it's set in it's holder , it's a linear sensor gives us + - volts out ,dc swing either side of 0v of around 8 - 10v
you can see the shoe that rests on the material being cut , this is spring loaded to keep the tension on the sensor shaft , it has a calibrated range of  + - 1.0mm
with a range or + - 3mm each side of 0v point. we are using 15v + & -  dc  so as to get a negative voltage swing from the sensor , but if your sensors only positive output , then we should be able to set it for a specific voltage for your zero reference point , this then depends on what voltage drift you may have etc .   

we are looking at either a remote triming encoder for fine tuning or just setting the points in mach 3 , i am hoping to get some machine time to start testing it all shortly
to test for voltage drift and repeatability of height control

email me off list
what's  required ? , the full electronics or just a plugin for mach3 , if so what type of connections do you need if any ,  parrallel port , serial etc


Dave
   

We've only just started thinking about this.  If we can buy a package that works (is the one in the picture a production thing or custom made?), we will use what's available.

Otherwise, we'll use a linear encoder and make something similar to your picture.  We don't care if it's +/- 10 vdc or 0-20 vdc, or whatever.  However, 0 vdc to 24 vdc is REALLY easy since every system we've ever done has this already.

Currently, we use two LPT ports for Mach3.  We'll use the Smooth Stepper when it's ready.  For this device, we'll use whatever you recommend that works.

Sure would be nice to be able to set the high, center, and bottom voltages in Mach.  That would eliminate the need for a manual adjustment to get the height right.  Actually, just the middle might work - with a choice of polarity for what raises or lowers the z-axis.  That is, depending on the resistor and wiring, an increasing voltage might mean you are either too close or too far away.  A simple check box could be used to set polarity.

Another possibility would be a variable for response speed if this is to be truly dynamic, or alternately a response distance.  For example, if the sensor reads "too high", the plunge is 1/20" and recheck.  That could prove easier by eliminating the possibility of "hunting" like a badly tuned servo.


BTW - forgot to ask if this could be used for twin heads - nothing's ever simple!!

well one thing is , if it dont work then i won't sell it !.

the height unit shown is a one off , but we will be making similar units , it all depends on whats needed etc to suit the particular machine ,
the unit it's self is fully dynamic in that once you have set the parameters it looks after it's self , but i was looking at incorporating a mini mpg rotary control that could move the zero point over a small range , more as a final trimming pot , but actualy i hope we won't need it , this of course depends on the accuracy of height control required  .

we are in too minds to incorporate the controller electronics via USB  , or via an addon unit to the smoothstepper as done by warp9 etc , by which we hope to have a similar product , available in a few months but suiting servo & encoder system users .

Dave

but suiting servo & encoder system users .

Dave

Just a bit curious about what you are meaning?

Hood