## 4 Experimental Validation
The highest point and lowest point were measured at 24.25 mm and 21.33 mm, respectively, making a height difference of 2.92 mm. The significant unevenness of the top surface was mainly due to the inconsistent printing velocity along the spiral tool path when the robot carrying the optical head kept accelerating or decelerating in both X and Y directions. The inconsistent velocity resulted in inconsistent laser energy density input to the melt pool. Figure 10b shows the sample fabricated with the proposed adaptive controller enabled. The adaptively controlled sample had a more even surface than the uncontrolled counterparts. The lowest and highest points were 22.90 mm and 24.15 mm, respectively, and the 1.25 mm height difference was less than half that of the uncontrolled sample. As a result of the adaptive closed-loop control, the geometric accuracy of the thin-walled part could be improved. The inconsistent  
Figure 9: Time plots of the MPW and laser voltage signals for Experiment 2 (solid semicylinder without the profile tool path, 316 L stainless steel). (**a**) The data without control; (**b**) The data with conventional PID control; (**c**) The data with the proposed adaptive control. energy density due to velocity inconsistency was compensated for by the controlled laser voltage. Figure 10c,d shows the time plots of the MPW data and laser voltage signal in Experiment 3. When the MPW rose due to higher energy density (and slower absolute speed), the laser signal was reduced, attempting to lower the MPW, and vice versa. The stabilizing effect of the proposed controller can be observed from the MPW plot in Figure 10d, which has considerably smaller fluctuation than that in Figure 10c. Similar to Experiments 1 and 2, Experiment 3 also demonstrated the capability of the proposed controller in reducing heat accumulation. This capability was more important for thin-walled hollow parts than solid parts since thin-walled parts had smaller cross-sections and hence poorer heat conduction rate. The MPW of the uncontrolled sample grew to nearly 140 pixels in Figure 10c, whereas the MPW of the adaptively controlled sample remained in the narrow range of 95-105 pixels in Figure 10d throughout the process. The reduced heat accumulation due to the relative consistency of the MPW also contributed to the better accuracy of the controlled sample. The above experiments demonstrated the effectiveness of the proposed data-driven adaptive control strategy in enhancing the laser-based DED system's performance. In general, the proposed adaptive control method could produce a better result than the uncontrolled and conventional PID controlled processes in terms of MPW stabilization and geometric accuracy improvement. More importantly, the proposed  
Figure 10: Results of Experiment 3 (thin-walled hollow pipes with a spiral tool path, LPW-35N nickel alloy). (**a**) The sample fabricated without control; (**b**) The sample fabricated with adaptive control; (**c**) The MPW and laser voltage plots for the uncontrolled sample; (**d**) The MPW and laser voltage plots for the adaptively controlled sample. adaptive control method could eliminate the costly and inefficient controller tuning procedures used in the conventional PID method. A different set of controller parameters needed to be obtained by system identification and trial-and-error experiments in conventional PID whenever the process conditions were changed. In comparison, the proposed data-driven adaptive controller could be used to fabricate parts with any shapes, materials, tool paths, or process parameters. Experiment-based system identification and manual tweaking of the controller was not required even when the DED process conditions were changed for different parts. The controller parameters could be optimized and updated automatically on-the-fly during the DED process without human intervention, and the reduced complexity in controller implementation could pave the way to broader adoption of closed-loop DED systems by industry end-users.