FORCE-DISTRIBUTION ANALYSIS FOR REDUNDANT CABLE-DRIVEN PARALLEL ROBOTS UNDER HYBRID JOINT-SPACE INPUT

Cable-driven parallel robots (CDPRs) represent a class of particular parallel robots whose rigid links are replaced by cables, where cable can only generate pull force and cannot be compressed. The force distribution in cables is one of the core problems for redundant CDPRs. The hybrid joint-space control strategy, where the chosen redundant cables are force-controlled, whereas the remaining ones are length-controlled in the joint space, is the main type of control strategy discussed in this paper. Because different cable combinations may lead to different control effects. This study provides the selection criteria for the target force-controlled cable combination in the hybrid-input control strategy. The cable tensions in the space with tension vectors as basis for two redundancies for cable-driven parallel robots were expressed based on the equivalent transformation method of vector space basis. The acceptable cable force errors limit proposed in this paper (CFEL) were defined and calculated based on the cable tensions in the space with tension vectors to find appropriate cable combinations for the force control. To validate the analysis of the force-distribution characteristics, a hybrid-input control trajectory planning strategy was developed using multibody dynamics simulations, based on a suspended cable configuration with layouts including two redundancies, while considering the interference of cable length and cable forces. In addition, a fix-pose simulation case via hybrid-input control strategy was performed to validate accuracy of the proposed calculation method for the CFEL. Finally, we found that cable combinations play an essential role for force control as the force control errors may be significantly magnified in cable combinations with high force-distribution sensitivity characteristics. The simulation results illustrate the significance of the analysis in this paper. What’s more, the concept of CFEL proposed in this paper provides guidance for the design of cable force controllers under the control strategies of hybrid joint-space input.