The Improvement of the End Position Accuracy of the Serial Manipulator with the Auxiliary Constraints

Due to the presence of joint clearance, the end accuracy of the serial robot deteriorates as the number of joints increases. For diverse activities in medical contexts, the robot has different requirements for end position accuracy and operation space. We are intended to trade less operation space for higher local end-point accuracy. Therefore, a kinematic error model that incorporates joint errors via virtual links is established. Then the expression of the auxiliary constraints to serial manipulator’s intermediate joints is discussed. The variation and spatial distribution of the serial manipulator’s errors is investigated under the constraints of various joint points. The simulation results demonstrate that when point constraints are on joints 1, 2, 3, and 4, the mean value of the end position repeatability error is reduced by 41.05%, 67.78%, 92.55%, and 96.25%, respectively, and the repeatability of the end position is improved. The repeatability error of the end position is positively related to its distance from the center. According to the results of physical experiment, the position repeatability error under the constraint of the 2nd and the 3rd joints are decreased by 81.71% and 88.17% respectively, compared to the unconstrained xArm manipulator which confirms the results of the simulated experiments on the trend. Auxiliary point constraints that match the requirements of operating space range and accuracy index, can be designed based on the obtained rules. Thereby, the application range of the serial robot will be extended.