Soft Pneumatic Actuators (SPAs) have been increasingly used as fingers in robotic hands because of their inherent compliance, cost-effectiveness, and ease of construction. Nonetheless, the efficient modeling and controlling of soft pneumatic actuators is challenging due to inherent hysteresis nonlinearity, uncertainties, and external environmental perturbations. Another challenge in controlling soft mechanisms is the need for bending angle feedback signals from curvature sensors, but integrating curvature sensors into soft mechanisms is difficult and increases manufacturing costs. This paper proposes a simpler approach to controlling soft mechanisms. Instead of using the bending angle feedback signal from the curvature sensor, this study proposes a bending angle control solution through pressure. The analytical models for both the soft finger and pneumatic valves have been constructed. Subsequent bending tests are performed to ascertain the relationship between bending angle and air pressure. This work analyzes the adaptive sliding mode utilizing active rejected control to regulate the position of the soft pneumatic finger. The suggested control approach integrates parametric uncertainty and input constraints to mitigate the effects of system uncertainties. The simulation results reveal modest overshoots and little steady-state errors in the actuator's response; hence, the proposed controller has effectively fulfilled its control function. Comprehending soft material actuators devoid of curvature sensors would facilitate the rapid replication of novel design concepts and enable estimations of their efficacy without reliance on curvature sensors. This will result in more applications and the development of increasingly intricate motion systems.