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Using a piezoelectric unimorph vibrator with constraint-tuning modified-mode (CTMM) mechanism, a novel design of a thin-disc ultrasonic actuator was developed to drive an optical sled. The theoretical estimation of in-plane wave propagation on a thin disc is introduced to explain the novel actuating mechanism, via a modal expansion technique and modal participation factors. Furthermore, the approximate wave propagations could be illustrated by the practical estimated wave equations in this study. Applying four screws at the exact distribution of angles on the thin-disc vibrator, the actuating mechanism of ultrasonic modified modes is generated and propagated. The in-plane vibration modes could be tuned by these desired screw constraints on the piezoelectric vibrator. The ultrasonic actuator offers the output force to drive an optical sled by friction contact in bilateral motions. To implement the equilibrium structure force in bilateral directions, natural and forced analysis as well as impedance comparison of FEM software ANSYS are also introduced into the constrained design. Hence, there are two various modified modes chosen at the different resonant frequencies within the electromechanical coupling of piezoelectric material to pursue more efficiency in energy conversion. Experimental results have demonstrated consistency with the approximate theoretical approach of the in-plane wave propagation and simulations based on the concept of constrained-tuning modified modes.