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« on: September 19, 2012, 11:34:53 AM »
Based on the excellent results Jeff (JJWMACHINECO) has achieved with burning ‘shades of grey’ into wood…
There are those that have converted imported CO2 laser machines to operate with Mach3 who do not actually have a motorized Z Axis and others who perhaps do not wish to add an encoder to their existing Z Axis. For this scenario I submit the following design idea which is primarily aimed at CO2 lasers with 40 Watts (or more) available output power.
Because the Mach3 internal PWM generator cannot easily be controlled ‘on the fly’ with the S*** commands blended with X Axis movement, or at least not fast enough to be useful, another approach has been considered.
It is perfectly feasible to construct a ‘stand alone - step / direction’ controlled PWM generator the output of which is easily converted to a voltage with a simple DAC (digital to analogue converter). Then by using 2 voltage dividers, one to set the lower voltage at which the wood is just marked and the other to set the higher voltage at which the maximum burn is achieved the laser output power can be fully controlled (via its PSU), with 8 bit resolution, from a lithophane type GCode tool-path.
I threw out the initial idea of connecting an encoder to a stepper motor and then using a standard stepper motor driver in favour of an all electronic solution which will enable ‘on the fly’ adjustment of the minimum and maximum thresholds whilst still maintaining an 8 bit resolution of voltage over a typical 2mm depth of tool-path.
My design may well be a bit theoretical at this stage but so far I have established that a PIC microprocessor, clocked at 20MHz, can be configured within Mach3 as an Axis which can then be driven at a typical feed-rate of 1000mm/min with 127.5 steps/mm and produce a continuously updated PWM output between 0 and 100%.
Referring to my very basic schematic, this PWM is then opto-isolated, to remove the GND constraint and place the 255 step output potential between the maximum and minimum voltage settings of the two potential dividers. A simple DAC is then used to drive the emitter-follower output voltage to the laser PSU.
For the initial testing and for convenience, I am using a partially populated pcb from my RF Laser controller design (mentioned many posts back) purely because the display is useful, at the software design stage, to be able to observe the various settings and values. The final design will be a lot less complicated.
As always, early days yet but it all looks promising.
Tweakie.
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