Stiffness Analysis and Control of a Stewart Platform-Based Manipulator With Decoupled Sensor–Actuator Locations for Ultrahigh Accuracy Positioning Under Large External Loads

Abstract

Robot frame compliance has a large negative effect on the global accuracy of the system when large external forces/torques are exerted. This phenomenon is particularly problematic in applications where the robot is required to achieve ultrahigh (micron level) accuracy under very large external loads, e.g., in biomechanical testing and high precision machining. To ensure the positioning accuracy of the robot in these applications, the authors proposed a novel Stewart platform-based manipulator with decoupled sensor–actuator locations. The unique mechanism has the sensor locations fully decoupled from the actuator locations for the purpose of passively compensating for the load frame compliance, as a result improving the effective stiffness of the manipulator in six degrees of freedom (6DOF). In this paper, the stiffness of the proposed manipulator is quantified via a simplified method, which combines both an analytical model (robot kinematics error model) and a numerical model [finite element analysis (FEA) model] in the analysis. This method can be used to design systems with specific stiffness requirements. In the control aspect, the noncollocated positions of the sensors and actuators lead to a suboptimal control structure, which is addressed in the paper using a simple Jacobian-based decoupling method under both kinematics- and dynamics-based control. Simulation results demonstrate that the proposed manipulator configuration has an effective stiffness that is increased by a factor of greater than 15 compared to a general design. Experimental results show that the Jacobian-based decoupling method effectively increases the dynamic tracking performance of the manipulator by 25% on average over a conventional method.

Adaptive velocity-based six degree of freedom load control for real-time unconstrained biomechanical testing

“…An adaptive velocity-based load control algorithm was implemented on a parallel robot (Ding et al, 2011(Ding et al, , 2014) and demonstrated on an ovine FSU. This algorithm builds upon the velocitybased methods of Goertzen and Kawchuk (2009) and improves upon previous implementations of load control (Goertzen and Kawchuk, 2009;Walker and Dickey, 2007;Bell et al, 2013;Bennett and Kelly, 2013), particularly in its ability to test at physiological rates.…”

“…Displacements and rotations of the specimen were directly measured by six linear optical encoders with a resolution of 0:5 μm (B366784180185 LDM54, MicroE Systems, USA) that were positioned independently to the loading frame and load cell. This configuration eliminated system compliance from the measurement of specimen behaviour, as detailed in Ding et al (2011Ding et al ( , 2014. Forces and moments were measured by a six axis load cell (MC3A-6-1000, AMTI, USA) having a maximum axial compressive force capacity of 4450 N and 56.5 N m of axial torque.…”

“…This study specifically aims to build on the work of Goertzen and Kawchuk (2009) and develops an adaptive stiffness velocity-based six degree of freedom (6DOF) unconstrained load control method in an effort to increase the loading rate Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jbiomech www.JBiomech.com and command complexity of dynamic testing beyond that which has previously been possible. The algorithm is applied to a hexapod robot (Ding et al, 2011(Ding et al, , 2014 (Fig. 1, Table 1) but is similarly applicable to other parallel or serial robots.…”

Consistent modeling resolves asymmetry in stiffness matrices

“…Several researchers have recognized that loading can have a significant effect on a stiffness analysis [19,20], which in turn can affect stability [21] and accuracy [22,23]. It also means that stiffness properties can be controlled in redundantly actuated manipulators [24,25].…”

Thrust Bearing Stiffness Testing Based on Multidimensional Loading Device

“…The properties of material vary according to the loading order and loading value, that's to say the different loading orders and values can result in the coupling effect [5,6,7]. A novel Stewart platform-based manipulator was analyzed for applications where ultrahigh accurate positioning is required under large external loads by Ding [8]. A multidimensional loading material testing machine was proposed and multidimensional loading and standard tension material tests were established by Guo [9][10][11].…”