Error model and verification of three degrees of freedom parallel mechanical leg on hexapod drilling robot

In the process of large passenger aircraft manufacturing and assembly, a lot of drilling and milling work needs to be done.Due to the limitation of the structure, the traditional drilling and milling machines can’t be used in the drilling and milling work of large passenger aircraft.The hexapod robot with motorized spindle on its trunk is very suitable for the drilling and milling work.The hexapod robot can crawl on the aircraft’s exterior surface through suckers on its foot.So the hexapod robot can do the drilling and milling work by the motorized spindle on its trunk.For the hexapod drilling robot, a new type of 3-DOF (3 degrees of freedom) mechanical leg based on (U+UPS)P+UPS parallel mechanism is presented (U, P and S represent universal pair, prismatic pair and spherical joint, respectively).The decoupling performance of the mechanical leg’s motions is very good, and its 2 rotational motions are completely decoupled.Its control algorithm is very simple, so it can be real-time controlled very easily.In order to improve the accuracy of the hexapod drilling robot, the error modeling and evaluation method of the mechanical leg is established.The prototype is manufactured by precision design, and the error calibration experiment is done.First, using the vector chain method, the error vector constraint equation is established and the error propagation model is obtained.Using the error model, the mechanical leg’s output errors with a given set of error sources can be calculated.Also, the mechanical leg’s tolerance allocation with a given set of output errors can be calculated too.In order to evaluate the error transfer performance of the mechanical leg, a set of error sensitivity evaluation indices are defined, and the distribution of error sensitivity evaluation indices in the mechanical leg’s workspace is drawn.It reveals that the error transfer characteristics of the mechanical leg are good in the central area of the workspace.Based on the set of indices, the error sensitivity of the mechanical leg is evaluated.Based on the comprehensive consideration of various performance indices of the mechanical leg, the structure parameters are designed by Monte Carlo method.The parameters are as follows: the distance between the universal joints which are connected to the fixed platform is 200 mm, the length of the middle connecting rod is 70 mm, the length of the end connecting rod is 50 mm, and the distance between the middle connecting rod and the main branch’s universal joint is 450 mm.Based on this set of structural parameters, the mechanical leg’s prototype is manufactured.Finally, using high precision robot calibration system, the error calibration experiment of mechanical leg is done, and the error characteristics of the mechanical leg’s prototype are obtained.Experimental results indicate that the deviations between the measured and the theoretical position error are less than 0.003 mm, the deviations between the measured and the theoretical attitude error are less than 0.05o, and the deviations between the measured and the theoretical error sensitivity evaluation index are less than 0.05.So, the error of mechanical leg’s prototype is in reasonable range, and the structure and parameters of the mechanical leg have been proved to be reasonable.