Passive Gravity Compensation for Robot Manipulator Joints Using a Compression Spring–Cam Mechanism: Modeling, Design, and Experimental Validation

Gravitational torque at manipulator joints persists even at rest and thus determines the continuous torque rating of the actuator. This study developed and validated a passive gravity-compensation joint wherein a cam plate extends a compression-spring link with an offset pin joint. Spring torque was derived by virtual work for an arbitrary cam profile, and the pitch curve was distinguished from the machined surface by a normal-offset correction omitted under the point-contact assumption. Because each harmonic coefficient of the Fourier-parameterized profile contributes to curvature with a weight proportional to the square of its order, raising the order trades curvature for accuracy: over the converged orders, the minimum curvature radius (the largest follower admitted without undercutting) fell by 20–34% while the residual fell 4.5- to 6.9-fold. Attainable accuracy was governed not by stiffness but by whether the springs can store the gravitational energy within their allowable stroke. A 5 kg prototype reduced the peak joint torque by 73.2% on the hysteresis-removed basis but by 20.8% in actuator demand, which includes friction. The measured spring torque matched the model within 5%; half the shortfall from simulation stemmed from a load 11% above design, and the gap between bases from sliding friction.