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This paper presents a comparative study of a sliding-mode controller (SMC) and a cascaded proportional/proportional integral (P/PI) controller with zero-phase error tracking controller (ZPETC) and disturbance observer (DOB). The tests are performed on a new underwater grinding robot. The comparison is based on trajectory tracking performance and robustness to external disturbances, such as those induced by the underwater environment and the grinding process. Experimental results show that for the same design criteria the P/PI controller produces better performance in terms of trajectory tracking error and disturbance rejection. The DOB was shown to be a simple and effective way to further reduce the positioning error.