Re: Servos drivers + THC

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "John Heritage"
<john.heritage@v...> wrote:

> Thanks for the reply Thomas,
>
> Eerily, I was just about to email you! :^)
>
> I noticed the THC folder on Master5 earlier - that I think you

created

> right? If I was using a servo for the THC, would I need an encoder

to derive

> positional feedback for the system? If the THC is controlled by the

arc

> voltage, then doesn't the loop become closed by the torch's arc

voltage? Is

> the THC board a complete system in it's self, or would I need to

feed it's

> output into an amplifier?
>
> Many thanks!
> John
>
>
>
>

John, Based on the post from the other Tom:

Obviously there is more than one "Thomas" on the list and while the
post is hopefully a satire it needs a few smilee faces to impart the
funny parts. ;-)

The THC I created is designed to work with MACH2 as the "logic" of
the circuit. It assumes that you have full control from MACH2 of
your Z axis with a stepper or step to servo controller. For the
motor you listed, the 79 buck solution would be perfect. The encoder
from the servo is fed into the motor controller and used for
postiional comparison.

When there is a valid arc, the THC circuit measures the gap voltage
which is porportional to the gap distance. The higher the gap the
greater the voltage. It outputs simple logic signals of UP or DOWN
based on comparing a fraction of the gap voltage to a reference. The
circuit has three "windows". The UP is active when the voltage is
higher than the front panel setting. Conversely the DOWN is active
when the voltage is lower than the setting and there is a "sweet
spot" of about 3% of the voltage where neither is active and the unit
is real close to the setting.

In looking at existing controls they have to have lots of internal
logic because the controller software has no feedback as to where the
tip really is. With the software (MACH2) reading the pins and
controlling the motor it KNOWS where the tip is at any given point
and keeps track of all the ups and downs as you cut. In fact you can
read the position on the Z DRO. As a result, when you tell it to
lift the head 1" above the material it knows where that is and also
knows that the next cut sequence starts at a .2" pierce and a .14 cut
height. Only if the material is uneven or warped do you need to do a
reference move to re-zero the tip. Using it with a good CAM software
like SheetCAM that gives you control over the pierce, lead-in,
cutting speed and offsets you can do some reliable cutting with
increased tip life

By making the controller software part of the equation we could
easily add in things like min/max excursion limits, anti-dive on
sharp angles, programmed response speed and other settings.

So this is a long but concise answer to your servo question but it
gives the logic behind tying the THC to MACH2. BTW, even though the
schematic and floating pictures are on the Master5 list the THC only
works with MACH2 or later.

There exists stand alone controls for THC. The good ones are
expensive.

If it sounds like I am sold on MACH2, SheetCAM, Gecko's the THC and
using Corel as my drawing package (gasp!) then it's because I am. I
use my machine on a daily basis and my time and the amount of scrap
directly affects wheather I eat Steak or hot dogs. Through
(sometimes bitter) trial and error I have found a working formula.

Welcome from the Torchheads!

TOM C
North Texas Torchhead