Re: [CAD_CAM_EDM_DRO] Re: Servos drivers + THC

Thanks again Tom,

I had just finished reading the Gecko manual when I read about the THC and
I remembered the manual mentioning the 5V reference used for the encoder's
output. When the steps fall out of sync with the command input, they produce
a slight voltage change from the 5V reference, making the amplifier force
the servo into correction.

This made me think that perhaps a THC could implement the torch's own
voltage as a kind of encoder. The voltage would be fractionalised to
something reasonable, such that, like an encoder, a change in height
produced a change in the reference voltage being sent back to the
amplifier - rather than up and down commands to the software.

So, say the torch moves slightly too high, the THC's reference 5V would
move up slight, causing the amplifier to try and correct for mistake.

But anyway! Just ramblings from someone who regularly traps his fingers in
things!

I have a copy of the Mach2 demo on my PC now and I prefer to use Corel
also, so perhaps I'm not too far off. SheetCam looks like my next goal.

What are you using for your plasma source? I was thinking of a Hypertherm
Powermax 600 of 1000. They seem cheaper than Miller and ESAB don't produce
many single phase machine here in England.

I have a lot of spare time to think about my design while I'm waiting for
our garage to be knocked down and rebuilt. Any picture of your table would
be great!

Best wishes,
John

----- Original Message -----
From: "caudlet" <thom@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Wednesday, November 17, 2004 12:59 AM
Subject: [CAD_CAM_EDM_DRO] 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
>
>
>
>
>
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