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Author Topic: The Laser Project.  (Read 835555 times)

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Offline Tweakie.CNC

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Re: The Laser Project.
« Reply #810 on: September 22, 2013, 02:41:44 AM »
An example of testing and early failures…

I built a small test rig which gives off similar RF emissions to an operating laser tube so I could test run the unit without using the laser. After a couple of hours the clock stopped. Cutting the clock apart revealed a long plastic shaving caught up in the gear train and although the mechanism was beautifully constructed the Chinese manufacturers of this device seem to have absolutely no idea of quality control. Cleaning, removing the debris then gluing it all back together seemed to have resolved the issue and the 24 hour test then continued without fault.
Prototype #1 is now off on it’s travels for further testing with professionally manufactured laser machines whilst prototype #2 is now running 24 / 7 until destruction.

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.

Offline Tweakie.CNC

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Re: The Laser Project.
« Reply #811 on: November 17, 2013, 03:10:03 AM »
PWM (Pulse Width Modulation)

The following is just a brief summary of PWM and how it is manipulated to control the output power of CO2 lasers. Where appropriate, I have included the full title of the various acronyms solely for the benefit of those who are not yet familiar with these abbreviations.


PWM is essentially a stream of rectangular, digital, pulses comprising of three separate components – Amplitude, Duty Cycle and Frequency.

Amplitude is never a variable and it is generally fixed at 5Volts but for some, later, equipment this may be at the more recent TTL (Transistor – Transistor Logic) standard of 3.3Volts.

Duty Cycle is usually expressed as a percentage and it is basically the ON / OFF time of the switching signal. For example; with a 50% Duty Cycle the laser output is on for half of the time period and off for the remaining half of the same time period. However, as the variable component in the Duty Cycle is the Pulse Width then Duty Cycle can, on occasion, also be expressed in terms of time – typically in uS (microseconds).

Frequency is the number of times per second the Duty Cycle is repeated. This component is commonly termed the PRF (Pulse Repetition Frequency) and is generally expressed in kHz (kilohertz).The frequencies most commonly used are between 5kHz and 20kHz with 20kHz (20,000Hz) being considered to be the upper operating frequency limit for most laser Power Supply Units (PSU).


The following diagram depicts a typical PWM waveform (as would be seen using an oscilloscope) and this is essentially a graphical representation of Voltage versus Time.
As most (not all) laser PSU’s operate from an Active Low signal, in this and the following examples, the area above the graph (as shown by the shaded portions) represents the laser ON time.


As the speed of light is (essentially) a constant, in the above example, we are unable to change the 12.5 kHz PRF without it changing the 80uS time period of one complete cycle and visa versa because Frequency and Period are reciprocals of each other but there are two distinctly different ways in which the laser ON time and thus the apparent laser output power can be controlled by manipulating the PWM.

Consider the following example where we initially have a 25% Duty Cycle and we wish to double this power output setting to 50%. We could either double the pulse width from 50uS to 100uS or we could keep the pulse width at 50uS and double the PRF from 5kHz to 10kHz. Both these solutions will result in a 50% Duty Cycle.




Quite obviously there are an almost infinite number of combinations of Pulse Width and Frequency which we could use to achieve a 50% Duty Cycle but, depending on the material being worked, each combination may produce different results in the finished product. Bit of an abnormality perhaps but there are reasons for this behaviour, possibly to be described at a later date.

Some words to the wise…
There are constraints on the manipulation of PWM (to avoid a situation where the safe internal photon density of the tube is not exceeded) but provided the Duty Cycle is never greater than 95% and the Frequency does not exceed 20kHz then almost anything goes.
Commercial laser machines generally incorporate safe settings limits within their software / firmware to prevent the user from accidentally over-driving the tube and for this reason it should be considered most unwise to override or change any password protected settings.

As with everything, there will always be exceptions. For example, my DC excited laser can be operated at 100% PWM which is essentially CW (Continuous Wave) mode but only because it has a preset ‘maximum tube current’ control. My RF excited laser does not have this option so 95% PWM is its maximum safe limit. With both lasers my highest operating Frequency is still regarded as being 20kHz.

Tweakie.
 
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.
Re: The Laser Project.
« Reply #812 on: November 17, 2013, 07:33:19 AM »
Have to start calling you "Professor Tweakie"  ;D

Nice Job, thanks for taking the time, sharing and teaching.
Re: The Laser Project.
« Reply #813 on: November 18, 2013, 08:09:22 AM »
Thanks Tweakie.  Very informative and understandable (even to me  ;D).
John Champlain

Offline Tweakie.CNC

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Re: The Laser Project.
« Reply #814 on: November 19, 2013, 08:02:34 AM »
Thanks guys.  :)

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.

Offline Tweakie.CNC

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Re: The Laser Project.
« Reply #815 on: December 21, 2013, 07:04:23 AM »
Knowing how we all like gadgets I thought you may be interested in this little device I put together last week. It is essentially a basic (and more robust) version of the commercially available ‘home energy monitor’ but it’s my hobby to always build as much of my own stuff as I can so thought I would share the experience.

Simple and quick to construct using a combined voltage / current meter display (bought via the bay from China at about £7 inc. postage) 13 Amp plug, socket and cable plus a project box enclosure.

Its purpose is just to get an idea of the combined electricity running cost of the laser, air supply, cooler, extractor etc. under average / normal operational conditions for different types of job.

The first picture shows the readings I get with my Koi air pump, cooling system and laser running at low power during an engraving job. My compressor (only used for cutting thick stuff) and ducted fume extraction system were each measured separately.

As Watts = Volts x Amps my machine, during the average engraving job, uses (237 x 1.7) 402.9 Watts which is 0.4029 kW per hour of job time. Adding to this the figures obtained from the fume extraction system and using the price per kW hour from my energy provider I can get a pretty good idea of the machine running cost (at least as far as the electricity usage is concerned).

I don’t intend for the gadget to be permanently left in circuit, although it may be handy to monitor any possible voltage drop on my supply at peak usage times, it is just used to establish the average electricity usage for different types of jobs.

The second picture shows the readings I get when cutting thin acrylic.

Tweakie.
« Last Edit: December 22, 2013, 01:45:52 AM by Tweakie.CNC »
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.
Re: The Laser Project.
« Reply #816 on: December 28, 2013, 12:31:30 PM »
Would there be any way to connect this to a computer to monitor voltage over time? For some reason our electric into our home seems to fluctuate a lot. It also goes of about once a week, very aggravating, resetting all the clocks drives me nuts! About the time I'm cutting a job on the CNC and the power goes out for a couple seconds, I won't be happy. I'm just thinking of having a time line to show the power company.

Offline Tweakie.CNC

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Re: The Laser Project.
« Reply #817 on: December 29, 2013, 02:04:31 AM »
Hi Jammer,

I think one of the the commercially available 'home energy monitors' would probably be a more suitable device for your particular application. (typically, something like this perhaps  http://www.amazon.com/Weanas-Energy-Voltage-Electricity-Monitor/dp/B00DTMQ1S6/ref=sr_1_1?ie=UTF8&qid=1387628051&sr=8-1&keywords=watt+amp+meter ).

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.
Re: The Laser Project.
« Reply #818 on: December 29, 2013, 08:42:24 AM »
I use the "TED 5000" and love it, was a little struggle with getting it set up.

http://www.theenergydetective.com/

Offline Tweakie.CNC

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Re: The Laser Project.
« Reply #819 on: December 30, 2013, 07:01:24 AM »
Excellent piece of kit Craig, although a bit pricey for my budget I would love one of those.  ;)

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.