Design and Manufacturing Considerations of a Constant-Force Mechanism for Low Force Regimes

The evolution of rapid prototyping and additive manufacturing technologies has triggered the return of constant-force mechanisms (CFMs) among trending research topics over the past decade. Moreover, CFMs represent functional, cost-efficient alternatives to more complicated force sensing setups widely used in precision manipulation systems. Delivering essentially constant force response over a prescribed displacement range solely due to their mechanical structure, CFMs lend themselves towards a damage-free interaction between actuators and their environment. Mechanical overload protection may be granted to robot end-effectors, with CFMs compensating for absent or costly force feedback. Targeting low forces such as those typically required in the biomedical sector has proven problematic due to the increased relative impact of friction during operation. This effort proposes a low force device which also acts as a safety release mechanism. Intended for oropharyngeal swabbing, the design employs standard mechanical components and 3D printed fixtures, thusly distinguishing itself from compliant mechanism counterparts, more sensitive to manufacturing tolerances.This paper encompasses the mathematical modeling of a curved surface which engages rollers, in turn connected to compression springs. The tailored use of bearings and contact surface materials reduces the impact of friction on the response. The formulation of the curve is also valid for forces far above the goal and may be used as a design guide for parabolic rolling. After numerically validating the mathematical model, a prototype was manufactured and tested. The repeated testing of the physical setup exhibited a maximum deviation of within +/- 13% from the 1 N target force. The findings are subsequently leveraged towards a case study of a hand-held throat swabbing device, manufactured and successfully tested within patient-acceptable swabbing force ranges.The presented procedure is helpful as a design guide for creating constant force mechanisms targeting low forces, manufactured from 3D printed parts and off-the-shelf mechanical components.