Design, analysis, and experimental validation of an active constant-force system based on a low-stiffness mechanism

In low-gravity suspension simulation experiments, the partial gravitational forces of tested objects are balanced by the constant vertical forces on cables generated by constant-force systems. To improve system robustness against external payload disturbance, such systems usually employ low-stiffness mechanisms. The schematic diagram of our proposed low-stiffness mechanism is derived from an energy approach, which is especially preferable when the low-stiffness mechanism comprises two kinds of elastic components. The mechanism uses a combination of an axially arranged torsion bar and a group of radially arranged springs. While the former exhibits high energy density and generates major output force, the latter offers a negative stiffness to shape the output force curve so that it resembles a constant one. The mechanism has a comparatively smaller overall size, lower stiffness, and wider adjustable force range. The low-stiffness mechanism is used to form an active constant-force system. The system, as well as its dynamic model and controller, are also detailed in this paper. Experimental results demonstrate that the active constant-force system can be robustly controlled by a proportional-derivative controller with incomplete derivation to generate a high-accuracy dynamic force.