Zero-Free-Length Elastic System for Gravity Compensation of Parallel Mechanisms With Delta and Delta-Like Architectures

Passive gravity compensation can effectively reduce the load of actuators, but existing methods are commonly complex in terms of mechanisms and elastic systems with zero-free-length (ZFL) property. A compensation strategy using all joint information is further developed in this paper, as well as the involved elastic system. This approach with considerable compensation effect is simple in principle, and the ZFL property is precisely and completely realized by commercial springs. Compared with the previous method, the process of spring selection is also greatly simplified. Possible arrangements with approximate compensation are also calculated and discussed to achieve a compromise between the compensation effect and the mechanism complexity. The elastic system for Delta-like is implemented and calibrated as an illustration. The results show that the stiffness error rate is reduced from 23.53% to 3.92% and the compensation effect is improved by 63.36% for full compensation and 24.01% for approximate compensation, respectively.