Re: Plasma Voltage Servo

Thanks Chris, That was a good thought provoking answer. I wonder how
Marises circuit will react when the voltage drops to zero as a result
of the plasma shutting off. I also wonder what, if any voltage will
be present when the computer turns the torch back on but before the
arc is actually struck. I think the approach of building the circuit
and seeing how a stepper will react as an experiment is probably the
best route. Shoot, maybe the drop in voltage to zero can trigger an
up command in an additional circuit and whatever voltage is present
before the arc is struck can trigger a down command untill the arc
strikes and commences maintaining a proper cut height. Now all I have
to do is Kidnapp Mariss and have his brain cloned. I suppose I should
start with building the circuit and learning the actions &
parameters. I'll leave planning the kidnap/clone till later. Thanks
again Chris, .... and Mariss (insert evil laugh here)
Dale



--
- In CAD_CAM_EDM_DRO@y..., ccs@m... wrote:

>
> > 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.
>
> Dale,
>
> Since I haven't seen an answer to your question (or missed it if

there

> was one) I'll take a stab at it.
>
> There are ways to combine signals from two sources, but you would

have

> to think out carefully how you want to combine them.
>
> The torch height control is easy to deal with since it is based on
> feedback - as long as the torch is too low, it will be generating up
> commands, and the reverse. So it can probably deal with being

handed

> control in any position the computer leaves the torch at.
>
> But the computer height control is trickier - since the computer has
> no way of knowing where the arc control has left the torch head.

One

> method would be to add an encoder so the computer knows. Another
> would be to make assumptions about where the height control leaves

the

> z axis. A nifty approach might be to use the computer to change the
> desired voltage which the height control is trying to achieve
> (possibly even to an unachieveable value, if experiment shows this
> will drive the head to the plus or minus limit) and control the

height

> that way.
>
> Whatever you do, you will probably need switches and logic gates to
> implement smart limits - ie, the max height limit should prohibit
> further movement up, but allow movement back down down. OTOH, if

your

> stepper doesn't need to be too strong you can probably just turn

down

> the current and let it stall against physical stops without hurting
> anything.
>
> Mode switching is a possibility, but you should not use a relay for
> this. Relay contacts, like switch contacts, can 'bounce' when they
> are actuated - that is, they can connect and disconnet a few times
> before settling out to the desired state. This happens faster than
> you can see, but electronics are fast enough to possibly

misinterpret

> this bouncing as step pulses. So you should use integrated circuit
> multiplexors or logic gates to do mode switching.
>
> Actually, you might be able to tolerate some bounce-induced false
> steps since the torch height control will work to recover from any
> height error they induce.
>
> What you really need to do is sit down with a sheet of paper and
> devise the 'rules' for controlling the z axis - when should it be in
> arc voltage mode, when should the computer move it, and which

control

> has priority when they overlap. Once that is figured out, a circuit
> to implement these rules is relatively simple. One of the things

you

> will want to do in preparation for this is to build Mariss's height
> control circuit and see what it does when you turn off the torch, if
> you turn it on a long ways from the work, and possibly what happens

if

> you set the desired voltage level unreasonably high or too low - ie,
> could the computer simply control that when it needs to move the

head.

>
> Chris Stratton
>
> -------------------------------------------------
> Christopher C. Stratton
> Engineer, Instrument Maker, and Horn Player
> ccs@m... 617 628 1062
> http://web.mit.edu/~stratton/www/brassbuild.html