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.
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