What type of arc length control forGTAW?

By drg1701 Date 10-25-2001 19:17

Hello,

This is my first post here. I am currrently an engineering student and came across this message board while doing research for our senior project. I thought maybe you guys could provide some useful input or at least point me in the right direction.

Our senior design project involves designing a system to control the distance between the workpiece and the welding tip. Our goal is to produce something that makes better welds and less scrap parts than the current system being used. The current system uses a measurement of voltage drop across the arc to control arc length. The current system is very transient and tends to exaggerate "rises" and "sags" from previous weld passes. For example if the torch passes a "sag" it will follow the sag resulting in increased penetration and possibly "blow through". Also, the arc length measurement is influenced by other variables such as weld gas composition, pool emissions and weld gas flow.

It has been suggested that we develop a system that uses a distance measurement in place of the voltage drop measurement. Have you heard of this beeing done? Can you give the name of any companies doing this? Do you know of any other arc length control systems that might work?

BTW This a circumferencial welder that is being used to weld 1.5-4" diameter 304L stainless steel tubes. They are butt welding a J-groove joint. Also, they are doing 4-6 passes on the weld which seems strange to me, but i am no welding expert. If their is any other info. that you need to help understand the problem better let me know.

Thanks in advance

By - Date 10-25-2001 20:14

I have no expertize in this area, but just a thought that may allow you to think about the problem a bit differently:

Usually GTAW is performed with a constant current (CC) power source. If you are however performing machine welds, it may be possible to use a constant voltage (CV) power source and do the arc length control on the current measurement. Any small changes in distance, as found with little areas that are a little higher will result in a higher current that would in any case tend burn this away. Conversely, deeper areas would result in lower currents, that would tend to result in less melting in these areas. Your distance sensing will therefore have to be done such that it ignores these small spikes and dips, and only reacts on a longer term change in current.

Just a thought?

Regards
Niekie

By RonG

Date 10-25-2001 20:47

Short Arc, Globular, Spray or Spray pulse?

From your discription it sounds like Short Arc because the wire must have a suface to short to and if any thing that dosent make good current is in the way or it incounters a hole the wire just keeps going untill it makes a circut then burns off to start over.

Spray Arc would be easer to work with I should think.

By CHGuilford (****)

Date 10-25-2001 20:56

Are we talking GTAW (Tig) or GMAW (wirefeed) here?
CHG

By Lawrence (*****)

Date 10-26-2001 15:08

Another good question,

In my experience all arc length control for semi-automatic GTAW or Micro-Plasma is either based on Adaptive feedback of the arc voltage, call it AVC (arc voltage control) or manual jogging of the vertical axis (lets call it the Z axis). Sciaky Corp. out of Chicago is not only responsible for the world's largest electron beam welding machine but also is top pioneer in Semi-automatic GTAW welding technology on both the hardware and software side. Using Sciaky equipment we have refurbished knife-edge airseals with lands as thin as 0.010 (ten thousandths) inch. The precise control required calls for both a skilled operator and AVC.

Liburdi/Dimetrics also are great pioneers in GTAW technology applied to automated stuff. Dimetrics are especially qualified to speak to pipe and tube projects. I believe Liburdi, who are experts in power sources, ate Dimetrics or visa versa. With all the takeovers in the industry its hard to keep up anymore. At the welding show a couple years ago they had a demonstration of a welding arc that could talk! No kidding, the thing as I understand was an arc struck on a titanium workpiece, the arc length controlled by adaptive feedback (AVC). The arc itself or the power source I should say was Asymmetric and the frequency (very high indeed) was controlled with such amazing dexterity that the sound of it was actually modulated into voice recordings and music……. Did anybody else see this? Can I get a witness!

Back to topic. The elusive key and secret to keeping your tungsten from splashing down is prolly what you're after. The Sciaky W2000 computer system runs off a garden variety PC and they might share some of there less proprietary information with a student. Perhaps much of it is published now since they have been doing this for many years.

Do you have equipment your doing trials with now? Are you looking for tips to help bend it to your will? Tell us more about what your working with. The Process control variables are also key. With the best equipment comes the need for very tight control on things like Tungsten tip prep, consistency of included angle and finish. This is one place where Lanthanum electrodes dominate. Also consistency of prep is key along with programming of your powersources. Are you adjusting your amperage between fusion-root-and wirefeed passes? Is it necessary? Are you oscillating? Lots of things to consider. But once you have discovered and considered them all, you will have a smooth operation.

http://www.dimetrics.com/dimetrics.htm

http://www.Sciaky-online.com/

Anyhow if any of this is a help that's great, if not keep honing your questions and I bet somebody here will be able to help.

Good luck

Lawrence Bower
Education Development
United Airlines