Understanding Pulse width, rate etc

[DaveDoesIT]

Hi,

I have an older ULS system and I can set

Pulse Width between 0 and 1400

Pulse Rate between 0 and 39,000

I have tried many different Rate and Width settings but cannot seem to make sense of them. Can someone please give me an idea of the correlation between these settings.

So far I have deduced that **sometimes** the higher the Width and lower the Rate the higher the intensity but it is not linear and at some point the intensity falls off instead of increasing. Example 700 Width and 350 Rate it marks anodized Aluminum. At 1400 Width and 100 Rate is does not.

I'd really like to find a user's manual for this.

Dave

[der_fisherman]

I guess you understand what a pulse or a frequency are, well the "width" is the relationship of the "on" time to the "off" time, or duty cycle. A signal that is 50% on and 50% off is then "middle of the road" so to say. Increasing the "on" time will change the percentages, you could for example got to 75% on with 25% off (it will ALWAYS add up to 100% by the way!), or the other way. Some circuits are able to go from 100% on, to 100% off, though most go only from 5% to 95% or thereabouts.
The "frequency" at which this happens can be changed completely independently of the pulse width, it just then happens faster or slower, but the percentage of on time to off time remains the same....!
You will find a better, if somewhat complicated explanation at:-
http://en.wikipedia.org/wiki/Pulse-width_modulation
I hope this helps.
Andy

[DaveDoesIT]

Originally Posted by der_fisherman I guess you understand what a pulse or a frequency are, well the "width" is the relationship of the "on" time to the "off"

Hi Andy, thanks for the reply, I should have stated I understand the concept of PWM etc but thanks for the wiki link.

It is trying to understand what combinations of Rate and Width I should use to get meaningful intensity levels from the laser. The confusing thing is as you state, the frequency can be changed and I assume that is what they refer to as Rate, but the PWM mark/space ratio remains the same. It is that aspect that has me baffled.

If intensity was to be throttled back, surely just reducing the "on" time for the PWM would do that. Why though do I have a combination or "on" time and frequency. What benefits are there to increasing or decreasing frequency as opposed to reducing "on" time to change intensity?

I assume it is being done as some materials behave (cut/burn etc) better at higher or lower frequencies than others. I am trying to figure out what goes where to do what.

Dave

[der_fisherman]

Let us think of a small 12 volt DC motor, one with field magnets for example as most have.
Also, let us forget frequency for the moment.
If you want to control the speed of such a motor and used only a potentiometer controlling a voltage, you would find out that the low speed control to be mostly unusable. Why is that, well the reason is called "Stiction" in the trade, what it means is that to actually get the motor to even start, you may need 6 volts to overcome this "stickyness" (for want of a better word) and then it promptly runs at more than half speed.
You can slow it down, and find out that it needs say only 3 volts to run at its lowest speed.
The next problem is that the torque is also very low and the slightest extra load and the motor will stop and probably overheat!!
Here is where PWM comes in because it ALWAYS supplies the FULL 12 VOLTS, but at the beginning, only for a short time. This allows the motor to start and to run smoothly & slowly, with much better torque characteristics, eg a small extra load will not stop it....
But the "AVERAGE" power given to the motor is still low, so it runs slowly.
As we increase the on time (still ignoring frequency!), the motor gets more and more power and runs faster and faster.
With motors, if the frequency is too low, they make strange "strangled" noises, so generally the frequency used is made high enough that the motor is smooth in response and is not mechanically loud.....and left there.
You have an engraver I believe, and I am not going to pretend that I have ever used one, but by varying the frequency, you change the up and down speed (I would guess) and by changing the on to off ratio, you change (increase and decrease) the energy you are putting into it!!
I hope this helps you a bit more.....
By the way, set it to a middle frequency and test it on various metals at different ratios, make a table of your findings. Then try at a lower frequency and then a higher one and compare all the results.
I am sure that softer metals will need less energy than hard ones for example.....
regards
Andy

[MacGyver]

In a nutshell...

Consider the pulse width the power you're using to cut with. 100% pulse width on a 40W laser is the same as using 40W, 25% pulse width is the same as using a 10W laser. Even though it is truly on/off, the time between on/off is so fast that you can consider the power as being averaged.

Pulse rate is how often that averaged pulse fires. The pulse rate should be taken into account when determining the speed at which your head will be moving. Let's say you chose a 0.25 pps (pulse per second) rate and the gantry was moving at 1 ips (inch per second)... your cutline would be dashed, with 0.25" of cut followed by 0.75" of no cut, repeating. Although these are extreme values, it is actually quite useful in some circumstances such as cutting items from paper that you want left attached to the sheet (think perforations). Metals may want a high instantaneous heat pulse to make the actual cut, but a fairly low pulse rate to avoid heating surrounding metal. Some plastics will curl on the edge if average heat is too high, so you adjust accordingly. YMMV...

[DaveDoesIT]

Originally Posted by der_fisherman by varying the frequency, you change the up and down speed (I would guess) and by changing the on to off ratio, you change (increase and decrease) the energy you are putting into it!!

Hi Guys,

Thanks for taking the time. I have since done some evaluation and while I am still perplexed I am following, if not understanding what is going on with this.

The Rate in this instance is also tied in with acceleration and deceleration. At various combinations of Width:Rate it burns a series of dots and depending on the Width setting, acceleration and deceleration towards corners is added or removed.

It is a complicated thing to understand and I have just spent 3 hours testing different combinations and can see no real trends. Suffice to say I will just trial an error for different materials to build a database of what works best.

Dave

[MacGyver]

The pulse rate should be adjusted with your speed ramps... as the gantry moves faster, your pulses should increase by the same percentage. If the machine doesn't do this automatically, you're going to see widely variable vector cutting quality.