Three dimentional control of a diode based laser cutter

Master Thesis


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University of Cape Town

Laser cutting is a widely used technology in many areas of industry and research. Conventional laser cutters only offer control of two axes and either cut through a material or rudimentary control of the third dimension is possible by varying the power, pulse rate and travel rate of the laser beam. These rudimentary three-dimensional systems (often called 2.5D laser cutters) do not incorporate any feedback mechanism to control the depth of cut. The idea of measuring distance using diode lasers (and other laser technologies) is a relatively mature technology and is common to various consumer and industrial products. Recently diode lasers have become powerful enough to perform as laser cutters allowing a merger of these technologies. The aim of this project is to verify the concept of using a laser diode to achieve both material processing and distance measurement. This would allow the creation of a full three-dimension laser cutting machine that is capable of accurate material processing in all three dimensions. This would also offer the ability to cut non-homogenous materials, such as timber, which current ‘2.5D’ laser cutters are unable to cut with any accuracy. A gantry system was designed and constructed, which was able to move the laser cutting toolhead in the x-y plane, using stepper motors and a belt-driven drive system. A 2W single emitter laser diode was used for both laser cutting and distance measurement. Optics were designed and assembled that focused the laser onto the workpiece and directed light reflected back from the workpiece onto a photodiode. Laser driver circuitry was constructed to control the DC current of the laser and to modulate the laser power at the high frequencies required for accurate phase shift measurements. A photodetector and phase shift measurement circuit was designed, simulated and constructed. The phase shift circuit amplified the signal from the light reflected off the workpiece and then compared that signal to a reference signal in order to determine the phase shift between the two. An Atmel® ATmega2560 microcontroller was used to control the gantry, laser driver circuitry and to measure the phase shift output of the phase detector circuitry. Software written in MATLAB® was used to command the microcontroller and to interpret the data received from the microcontroller The photo sensor circuit was not sensitive enough to detect the weak signals that were present when the workpiece had a low reflectivity but was able to be tested using reflective tape. On the other hand the laser diode was not powerful enough to cut reflective tape as it absorbs very little energy from the laser. Nevertheless, the same laser diode was used, without changing any configuration other than the workpiece material, to measure distance and to cut materials. Testing of both the materials processing ability and the distance measurement ability were carried out. Many aspects of each of these major functions were tested, individually and together, in order to determine the areas that performed well and those that need more research. In conclusion, this project was able to verify the concept of a three-dimensionally controlled diode powered laser cutter. Future work will be needed before a practical and useful laser cutter can be built but this project should prove a good starting point for any such future work.

Includes bibliographical references