PWM (Pulse Width
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.
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).
following diagram depicts a typical PWM waveform (as would be seen using an
oscilloscope) and this is essentially a graphical representation of Voltage
(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.
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 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.
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.
to the wise…
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
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.