Re: Plasma Voltage Servo

Ballendo,
I'm not quite sure what a 4016 Quad switch is, but I suspect it's
some type of solid state IC relay.
The air cylinder makes some sense, but the purpose in having Mariss
design the circuit was to eliminate the need for a Z axis mounted on
a Z axis. I allready have a z axis with about 12" of travel.
conventional torch height controllers are independant of the cnc
system and operate via a dc moter. they also cost about $1800.
Mariss's circuit will allow me to utilize my existing z axis to
control the height. The only problem is that the circuit maintains
approximately a 1/16" tolerance of travel. this is terrific for
cutting, but I would like to utilize the rest of my z travel between
cuts to avoid "crashes" with the work at traverse speeds. My question
is 1. will step and direction signals from 2 different sources
(Mariss's Circuit and the computer)Conflict with each other if they
are wired to the same drive even if they arent giving commands at the
same time? And 2. If they do conflict, could a Double pole double
throw relay at the drive input set up so as to toggle between the
two command sources when ordered to toggle by a spindle moter
command. I probably wouldnt tie it to the plasma on off command
because I would like the torch to lower before the plasma turns on
and raise after it turns off. Again the air system is a nice Idea,
But I'm trying to reduce the need for extra mechanical assemblies if
I can accomplish it all with the same Z axis.
Thanks, Dale

---
In CAD_CAM_EDM_DRO@y..., "ballendo" <ballendo@y...> wrote:

> Dale,
>
> A 4016 quad switch could work, but...
>
> You could use an air cylinder to move the whole z in a large move

to

> provide clearance. This was done on some old multi head machines to
> save the cost of drives for additional z axes. (The slide

containing

> the multiple heads was under computer control, and each spindle had
> an air cylinder to "select" it into cutting position).
>
> As you suggest, this air cylinder(clearance move) could be tied to
> the on/off of the plasma torch.
>
> Hope this helps.
>
> Ballendo
>
> --- In CAD_CAM_EDM_DRO@y..., "aspaguy" <spaguy@l...> wrote:
> > --- In CAD_CAM_EDM_DRO@y..., "aspaguy" <spaguy@l...> wrote:
> > > Mariss and Tom, In the "real world" the theory has been being
> used
> > > for some time in commercial applications, it just controls the
> > > direction of a dc moter by reversing polarity rather than

using

> a
> > > stepper.
> > > Also Mariss, I wonder how hard it would be to add a feature

of

> > the
> > > axis being raised all the way up when the voltage drops to

zero,

> > then
> > > lowers again to the previous height when the plasma is

activated

> > > again. this would be to give more safety room between cuts to
> avoid
> > > tip ups, etc. from damaging the plasma cutter. I would assume
> this
> > > would need to be handled in the software. What would happen if

I

> > were
> > > to wire up both this circuit and a computer output to the same
> > gecko
> > > drive? In theory, they would be not be sending commands at the
> same
> > > time.
> > > Thanks,
> > >
> >
>

Dale

> >
>
>
> > Mariss, or others,
> > It occurred to me after posting the above message that I could
> > probably use a software command such as one normally designated

to

> > turning on and off a spindle to Activate or deactivate a DPDT

relay

> > in order to toggle back and forth between computer control and
> torch
> > voltage control. The two poles of the relay would switch the step
> and
> > direction inputs. What do you think? Is this even neccesary?
> > Thanks, Dale
> >
>
>
> >
> >
>
>
> >
> > > --- In CAD_CAM_EDM_DRO@y..., "mariss92705"
> <mariss92705@y...>
> > > wrote:
> > > > Hi,
> > > >
> > > > The circuit I drew up has a 30Hz bandwidth; any signal
> frequency
> > > > component above that value will average to its DC value. The
> > > trimpot
> > > > would allow you to adjust the torch to any height between 70
> and
> > > 140V.
> > > >
> > > > It would be useful for me to see a 'scope picture of what the
> > > actual
> > > > voltage looks like in operation. You mention a sawtooth
> waveform.
> > > > What is the amplitude and frequency? What is desirable to

servo

> > to?
> > > > Since I lack any practical knowledge on this subject, I would
> > need
> > > > this info to make a sensible comment.
> > > >
> > > > Mariss
> > > >
> > > >
> > > >
> > > > --- In CAD_CAM_EDM_DRO@y..., "ths992001" <thscarince@h...>
> wrote:
> > > > > WRT plasma arc voltage vs torch to work distance:
> > > > >
> > > > > Once I read an Amercan Welding Society paper on this
> subject.
> > > > > According to this article, the arc voltage wavform is
> something
> > > of
> > > > a
> > > > > sawtooth. At the beginning of the cycle the arch starts at
> the
> > > top
> > > > > of the material at a certain voltage. As it rapidly burns
> its
> > > way
> > > > > down the face of the cut, the voltage ramps up to some peak
> at
> > > the
> > > > > bottom surface of the material. At this point, the arc

jumps

> > > back
> > > > to
> > > > > the top surface and the cycle begins again.
> > > > >
> > > > > This means that the average arc voltage may not be an ideal
> > > > > indication of the torch height in all conditions (material,
> > > > > thickness, feed rate, amps, target height...). The author
> > > proposed
> > > > > using the "peak low" (the base of the sawtooth) voltage as
> this
> > > > best
> > > > > represents the actual height under varying conditions.
> > > > >
> > > > > Got an extra op amp for an averaging peak (or valley, in

this

> > > case)
> > > > > detector Mariss? Seriously, his design will probably do

the

> > > trick
> > > > > just fine in the real world.
> > > > >
> > > > > Tom Scarince