This paper proposes a Third-Order Smooth Transition Strategy (TOSTS) for Permanent Magnet Synchronous Motors (PMSM) to achieve seamless switching from Image to Frame Ratio(I/F) control to sensor-less sliding mode observer control (SMO). Additionally, the conventional torque-angle method is enhanced by integrating torque monitoring and position monitoring techniques. Simulations in Simulink demonstrate the superior dynamic performance of TOSTS in tightening applications. A preload force test platform was designed and constructed to experimentally validate the proposed TOSTS-enhanced torque-angle method. Repeated experiments with varying tightening parameters and screw types reveal that the preload force accuracy achieved by the proposed method approaches that of traditional sensor-equipped torque-,angle methods (±8% error). This approach facilitates the advancement of sensor-less intelligent assembly systems. This approach facilitates the advancement of sensor-less intelligent assembly systems, providing a foundation for Industry 4.0-ready manufacturing infrastructure