Plasma Current Control on the Fly (synchronous with motion)

phillc54 wrote:

A little bit OT as well but I used a ACPL-M484-000E as a replacement for a pot to control the spindle speed of my mill using PWM via a parallel port.

thanks Phil, sorry I missed this, not seen or used that chip before, good range on it and it looks interesting for isolated pwm or possibly pwm to analog (with a cap if the output impedance is known, but I'm a little twitchy over the linearity of pwm >>analog

robertspark wrote:

I have a 200A current sensor to hand that I'd like to check the effect on adding this arrangement does not detrimentally affect the 45a cutting current

uk.farnell.com/allegro-microsystems/acs7...726&gross_price=true


that is the plan...

Beefy has used one on his 85 amp hypertherm for quite a few years...you can get prebuilt acs758 modules in 50 amp available too. I was planning on using one with a LM358? signal amplifier so I could read it on a 7i76e analog input...

robertspark wrote:

I have a 200A current sensor to hand that I'd like to check the effect on adding this arrangement does not detrimentally affect the 45a cutting current

Hi Rob,

as you can see in an image of that current sensor it's basically a solid copper bar sized to handle 200 amps. When I soldered mine to my homemade PCB the main thing I wanted was plenty volume of solder on and around the pins to maximise the cross sectional area for current flow, and cause minimal voltage drop. Of course if you can find a ready made module I would assume it's design causes minimal voltage drop at full current (but real life tests never hurt).

If you go the DIY construction method, here's what I did. I was worried about the bulk of heat needed to solder such large copper leads, and the potential risk of damaging the device if heat was on the leads too long. So I bought a massive soldering iron, something that would work great for chasing a robber in your home. I also made a big aluminium u-shaped heatsink that wrapped around the device and sat flat on the leads (used heat sink compound too). I could then gently clamp the pcb down onto the component and the heatsink, and the huge bulk of heat in the giant soldering iron quickly got the leads soldered up, despite the big heatsink attached to the them. Filled up with plenty solder to handle the current.

Ah, hold on, just remembered I missed something in the process. Before soldering the leads, I'd also flattened some copper water pipe (that's what I had lying around at the time LOL), drilled holes in it for the device leads and the cable lug bolts, and soldered that to the wide tracks I'd designed into my PCB. So my "tracks" leading from the device leads could probably handle my suburbs electric supply.

I'm not 100% sure but I have a vague recollection that when I first tested it I measured for voltage drop across the device when cutting at 80 amps and it was negligible. Like yourself that's just the type of thing I'd normally do.

Keith

Anoter less invasive form to measure the current is using a Hall efect sensor like the datasheet attached, there are of diferents sizes and out put configuration
and you only need pass the cable trou gth it, also yo can double the sensitivity making more than one turn

I've actually got one of those types of current sensors in my old Candcnc system.

I did a quick Google to find the price of the DHR-C10 and the very cheapest I could find was $275 and as high as $400 (Australian $).

On the other hand the ACS758 is available on ready made boards with terminals for just $25. These boards have all the accompanying circuitry on too. Wish they'd been available when I did mine.

Keith

Beefy, there are cheaper options. Here is a 125 amp one
www.digikey.com.au/product-detail/en/hon...qlSChpEaAs43EALw_wcB

robertspark wrote:

I was not fussed about changing the amperage on arcs, I was more interested in changing the amperage on internal tight corners.

I was also keen to be able to change the amperage from the gcode file for dimpling and marking type operations. I like to keep my working settings in the gcode file as I can never remember the optimum voltage settings that work best for me,

An internal tight corner IS AN ARC. Thats what G64 tolerances do.
You will be able to sense this by the corner lock method.

I think the 7i76e already supports PWM's if you do something like this with hostmot2

num_pwmgens=1 num_stepgens=4
Ref: http://linuxcnc.org/docs/devel/html/man/man9/hostmot2.9.html

Load it with one and review in halshow to see what pins are published/


 

 

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no. not me / my intent.

Rod wanted to attempt to control the current in lieu of the z axis thc

BenDiesel wrote:

[code][code]​​​so I'm a bit new to this but are you referring to then point when the cut velocity drops under a threshold and plasmac stops using arc voltage to controll z height but instead to use amperage to controll ark voltage? Could you also use this method to set amperage for non serial machines from the materials list?

[/code][/code]

I've actually done some work on this in another way in conjunction with others. The thinking is to implement some of the features of high definition plasma used by the major manufacturers in gcode by using a gcode filter that is applied when you open a file. Plasmac already does this.

This will give us several points in the arc when cutting holes to inject code and also modify the arc radius to account for the widened kerf.