PDA

View Full Version : Output power issue - 70% perform a better cut than 100% of laser power



Glauco Lins
09-26-2012, 11:08 AM
I am wondering if you guys could help me with a problem I am now facing.
It is related to the laser power.

Long story short, my machine is only capable to make clean cuts at 70% of power. (it was 75% about two months ago)
If I keep the same speed and use 100% of power, the machine is incapable of making a clean cut.

The material I am using is a 3mm MDF.

Note: Mirror and table alignment are correct.
My biggest worry is that the machine cannot cut at lower power than 70%, which mean it will stop work in the next few months...

What I think it is strange it the fact when I output the project at 100% power, the cut is worst than if the project is output at 70%.


Speed 10mm/s and Power 100% = Don't cut
Speed 10mm/s and Power 70% = Clean cut



Noticed the speeds are the same?

According to people I have asked in Brazil, they have never seen this issue.
Even if the laser tube is old, they said the 100% power must have better cut than 70%.

I made two videos on youtube showing this problem, one with a 50mm focal distance lens and other with a 40mm focal distance.
They are in portuguese, but I have added english captions.


http://www.youtube.com/watch?v=FWyVVEesXSo
http://www.youtube.com/watch?v=8pTY_33anxQ



Hope you help me identify the problems!

John Johnson 2
09-26-2012, 1:04 PM
I understand how 70% can cut better than 100%. Let me explain to you by using an analogy. If you have speakers capable of working perfectly at 70watt and you put 100watt of music into them then they sound terrible but if you put 70watt into them they sound good. Now lets say those speakers worked perfectly at 100watt when you bought them, but now due to age, the cones don't respond fast enough to be driven in and out fully at 100watt they now start to sound distorted at that power level. So, you turn the volume down only to output 70watt and the speaker sounds good because it doesn't have to travel as far in and out.

I think that the same sort of thing is happening to the tube, or the power supply, the tube may not be able to cope with the power any longer or the power supply is distorting its output at high power settings. I don't know of a way to determine whether it is the tube or the power supply that is faulty without using sophisticated testgear.

Glauco Lins
09-26-2012, 3:58 PM
I understand your analogy, but I don't think it applies to this case, because there is no mechanics in the tube.

The tube can lose potency over time. Maybe today my tube is no longer a 60W, but a 40W one...
In this case it should output 40W when I requested 100% of its potency.

If your analogy for this case were correct, an old light bulb should respond as you described.
Lets say if you apply 90V to an old 50W lamp, it should be brighter than if you applied 110V.
Which is not case... The lamp will always be brighter at higher input power.

However my belief is on a defect in the power supply, on a capacitor more precisely.
A capacitor has a charge curve, which is not linear.
For instance, if you apply 100V to it, it might take 10ms to fully charge and 1ms to discharge.

Lets say that my capacitor is defective, and it is now taking 12ms to fully charge at 100V.
At a power output of 100%, my power supply will work at an output of higher frequency, not necessarily on a higher voltage.

Lets assume that at 100%, each cycle takes 11ms (to charge and discharge the capacitor).
This means that each second my laser tube is activated 83 times at 100V. (in other words, 100V at 83Hz)
Since my charging time takes longer than the cycle, I will have a poor performance.
In this case my tube is being activated 83 times per second at, lets say, 80V.

From that principle, a 70% cycle will take about 15.71ms.
This means my tube will be activated 63 times per second at 100V. (in other words, 100V at 63Hz)
Since the cycle time is longer than the charging time of my capacitor, indeed it will be activated 63 times per second at 100V.

However this is just a wild wild guess from a computer engineer...
I don't know if the power supply modulates frequency or it indeed varies the output voltage.
If it is the second case, my hypothesis is wrong.

That's why I am hoping that someone else has faced this problem... (and solved it!)

Rodne Gold
09-27-2012, 2:00 AM
Running a glass tube at 100% power severely shortens it's life ... you can normally see the slight discolouration of the electrodes in the tube meaning its maybe be overdriven.
The speaker analogy holds , overpowering the speaker will give sound , but it will be distorted , turning down the volume restores it to sweet sound. If overdriving the speaker has in some way damaged it , then the problem gets worse.
It would actually help if you listed your machine and the type and power of tube it has..
Anyway , tubes are cheap , buy another and swap it out and keep the old one for spares...

John Johnson 2
09-27-2012, 5:37 AM
If it is the power supply then it may be a problem occuring a high frequency. The power supplies are switch mode so it could be more than a single capacitor that is faulty. With regards to the tube having no mechanical parts that is why I said that it is merely an analogy. The same sort of thing happens to rechargeable batteries over time. They become less able to hold their charge for long periods...again, this serves as an analogy. A battery may have a capacity of 2700mAh new but over time deteriorates to being only able to hold 1000mAh. Force charging the battery to 2700mAH does not help and in fact further damages the battery because the battery chemistry is already damaged to a degree where it can only hold 1000mAh. So, even though batteries don't have mechanical components they still deteriorate in performance over time and force charging them to expected capacity does more damage than good...again, remember that these are merely analogies used to explain why i think that you may have better performace at 70% than at 100%.

From a sound point of view the same happens when you overdrive the input to an amplifier. Lets say your amplifier expects to get an input appropriate for a set of headphones and you now go and put a small amplifier ahead the main input amplifier stage, the main amplifier is overdriven and the excess "sound" amplitude gets clipped off which causes terrible distortion. The clipping occurs because the amplifier tries to protect itself and simply discards the excess input. Again, no mechanical parts are involved but overloading the input creates a problem. Now, if the input stage of the main amplifier goes faulty and clips at a lower lever than normal, then the same occurs with a normal input. You can only determine where the problem is occuring by putting a signal generator on the input stage and then using a scope to compare the output wave through the various stages of the amp.

Ross Moshinsky
09-27-2012, 8:27 AM
The Chinese machines aren't properly tweaked. A quality stereo system will make 10 the max level where the sound still sounds good. A crap one will make 10 the loudest level possible with no regard to sound quality.

85% probably should be your 100%.

Glauco Lins
09-27-2012, 9:02 AM
Hello Rodne, good morning!
I have one just like yours: 900 x 600 60w Shenhui laser.
However I don't know specific details about controller, power supply or tube.

I have contacted the manufacturer about this issue, they said they have never seen something like it.
Their belief is the tube is damaged.

You seem to have lots of machines...
One time I had electric arches forming at the hot terminal of the laser tube.
Do you think this might be related to the issue?

After that I had to use rubber isolation in the laser tube compartment.
You can see that in the first video, about 3m15s.
It is black EVA rubber, about 1cm thick, I have it under and at the left side of the laser tube.

Without the rubber protecting the machine body, arches forms all the time.
If I pay close attention, I may yet see very small discharges happening in the rubber each time the laser starts firing.

Thanks.

Greg Facer
09-27-2012, 11:36 AM
That arcing is the sign of a tube that isn't working properly. I had the same problem with a RECI tube and they advised me to destroy it and they would send another (that part is a long ongoing story, but their intentions were good). If it's not a RECI and/or not under any warranty, then you might just need to buy a new tube.

Also, on my machine, it didn't ship with an ammeter to tell what power was going through the laser. As previous posters have mentioned, the tuning might be off on the chinese machines, and the only way to confirm is a ammeter hooked up on the low voltage side. I bought one off ebay to use once I have a tube to use it with. You would also need to research the proper amps for your particular tube.

However, if the tubes aren't under warranty, you can always run the machine at what works for now and plan for the purchases later, just make sure your machine is well grounded so you don't zap your control electronics or yourself with the stray arcing.

Glauco Lins
09-27-2012, 1:12 PM
So I assume the arcing may have not to do with the power supply, correct?

I am already speaking with manufacturer to order a new tube.
I probably will upgrade my machine for something around 120W, because I need to cut things faster...
I think the investment will be lower than buy a second machine to work two jobs at the same time.
---------

Since you mentioned another tube type, let me ask you...
My machine has a CO2 tube. Can you explain me what is a RECI tube and what are the differences between them?

This was my first laser machine, bought it about 3 years ago. And the company who sold it to me has a very poor customer service.
I had so much headaches with it...

Just to you have an idea, I already almost disassembled it twice to fix table levels and learn how to properly adjust the laser focus.
Half of its lifetime work was just to make it run better, because the company who sold it had no support...

My next upgrade will be re-do the air and water tubing, as soon I get some idle time.

Rodne Gold
09-27-2012, 1:27 PM
3 years is good for a glass tube , even if you dont use it , it leaks gas over time..
Reci tube will have a RECI label, google reci laser...