Optical Sensor for Estimating Focal Distance and Cut Width During Laser Cutting

Summary: In this thesis, a new fiber-based optical sensor for sensing distance during laser cutting is introduced. This sensor works based upon different angles of the reflected pilot laser beam in order to gauge the workpiece displacement. During this investigation, the aim was to assess the optimal sensor resolution and measurement range.

In the setup there is a sensor box which receives the reflected beam from the workpiece in different ways. It consists of a shielded case, two photodiodes and a tapered fiber. This fiber is cut and polished at 42° angle and adjusted in front of two photodiodes inside the box. The total internal reflection law defines the intensity of the light that each photodiode captures. Any displacement of the workpiece changes the reflected beam's focal point and angle and consequently, changes photodiodes's output voltages. The fraction of these two voltages defines the measurement function of the sensor. In this study, two different methods for a capturing reflected beam have been examined in order to compare to each other. Additionally this difference gives us opportunity to sensing the gap as well as the distance.

In the first method, the reflected beam from the workpiece is focused on a screen. A fiber is adjusted behind the screen to transmit the captured light to the sensor box. The results of experiments in this method indicate that there is a strong correlation between the optical signals and the workpiece displacement. In the first step of this method, a mirror is used to simulate a fully reflective workpiece. Our study shows that although in 4mm workpiece movement range, the photodiodes have considerable voltages, the effective range of high resolution measurement is in a range of 2mm. We observed a 25μm resolution for 100% direct reflection workpiece and zero cancellation of electronic noise or lateral laser profile fluctuations via the mean value calculation. By increasing the number of data sampling from one to 1000, the resolution is observed to be 10μm. Moreover, increasing the distance between the screen and the fiber from 0.5mm to 1.5mm improves the measurement range, although simultaneously decreases the resolution. Selecting a suitable measurement function and finding a tradeoff between the resolution and the measurement range depends upon the application of the sensor. In the second step of this method, a piece of steel sheet is used to simulate a partial diffuser workpiece. For this workpiece, we observed 205μm resolution while there is zero cancellation of electronic noise or lateral laser profile fluctuations via mean value calculation. Like previous step, increasing the number of sampling, improved the resolution to 120μm.

It was observed, for different workpiece by increasing the number of data sampling from one to 1000, the resolution of the sensor goes up as well. Increasing the number of the samples for mean value calculation, cancels the electronic noise and lateral laser profile fluctuation effect, partially. In addition, using 400μm fiber exhibits better sensing specification in comparison to the 105μm fiber. One of the possible reasons might be catching less light due to the smaller NA and diameter. To compensate this low intensity, the gain is increased and consequently the noise effects steps up. The high power laser source might improve the measurement function.

In the second method, the screen was removed and the fiber was placed at the focal point of the backward reflected beam. This experiment did not fulfill our expectations and has thrown up many questions in need of further investigation. The measurement function in this method has low gradient and measurement range. It is recommended that further research be undertaken in this area.

The second method can be improved by recalculating and changing the angle of the fiber inside the sensor box. Moreover, we suggest using 1mm fiber with higher quality and NA. Locating precisely a blocker in front of the fiber to completely eliminate middle part of the reflected beam might be another improvement for this sensor. Finally, some important limitations need to be considered: First, fiber bending can change the angle of the light significantly. Second, dirt on the fiber end can dramatically decrease the efficiency of the sensor. We suggest that all of these limitations and suggestions be studied in the future.

To sum up, fiber-based optical sensor with diffusion screen can be employed for distance sensing in material processing and especially in laser cutting. The statistical relation between the optical signals and the workpiece indicates that, by further optimization, higher sensor resolution can be achieved. In some applications such as scanner based remote laser welding, the angle of incidence changes during the process. In order to implement this new method, we should make sure the measurement function is independent of the angle of incidence; otherwise the angular changing should be measured and be considered in the calculation.