Sensorless point-to-point control for a musculoskeletal tendon-driven manipulator: analysis of a two-DOF planar system with six tendons

Simulation Evaluation for Methods Used to Determine Muscular Internal Force Based on Joint Stiffness Using Muscular Internal Force Feedforward Controller for Musculoskeletal System

This study proposes two novel methods for determining the muscular internal force (MIF) based on joint stiffness, using an MIF feedforward controller for the musculoskeletal system. The controller was developed in a previous study, where we found that it could be applied to achieve any desired end-point position without the use of sensors, by providing the MIF as a feedforward input to individual muscles. However, achieving motion with good response and low stiffness using the system, posed a challenge. Furthermore, the controller was subject to an ill-posed problem, where the input could not be uniquely determined. We propose two methods to improve the control performance of this controller. The first method involves determining a MIF that can independently control the response and stiffness at a desired position, and the second method involves the definition of an arbitrary vector that describes the stiffnesses at the initial and desired positions to uniquely determine the MIF balance at each position. The numerical simulation results reported in this study demonstrate the effectiveness of both proposed methods.

Optimal Muscular Arrangement Using Genetic Algorithm for Musculoskeletal Potential Method with Muscle Viscosity

Muscle contractions (or equivalent mechanical elements) are responsible for joint movement in systems with musculoskeletal structure. Because muscles can only transmit force in the tensile direction in such systems, the internal force exists between the muscles. By utilizing the potential field generated by the internal force, the musculoskeletal potential method makes it possible to control the position without complex real-time calculations or sensory feedback by entering step-inputs of the balanced internal force at the target posture. However, the conditions of convergence to the target posture strongly depend on muscular arrangement. Previous studies have elucidated the mathematical conditions of the muscular arrangement; however, they provide sufficient conditions that must be satisfied by the muscular arrangement to converge to the target posture, which do not necessarily lead to optimal muscular arrangement conditions. This study proposes a method to determine the optimal muscular arrangement of a two-joint six-muscle system, wherein muscle viscosity is considered, that uses a genetic algorithm and an evaluation function considering the motion response time. The effect of the obtained muscular arrangement is verified in a simulation.

Set-Point Control of a Musculoskeletal System Under Gravity by a Combination of Feed-Forward and Feedback Manners Considering Output Limitation of Muscular Forces

This paper proposes a new control method for musculoskeletal systems, which combines a feed-forward input with a feedback input, while considering an output limit. Our previous research proposed a set-point control that used a complementary combination of feedback using a time delay and a muscular internal force feed-forward; it achieved robust and rapid positioning with relatively low muscular contraction forces. However, in that control method, the range of motion of the musculoskeletal system was limited within a horizontal plane. In other words, that system did not consider the effect of gravity. The controller proposed in this paper can achieve the reaching movement of the musculoskeletal system without requiring accurate physical parameters under gravity. Moreover, the input of the proposed method can be prevented from becoming saturated with the output limit. This paper describes the design of the proposed controller and demonstrates the effectiveness of the proposed method based on the results of numerical simulations.

Error Analysis by Kinetics for Parallel-Wire Driven System Using Approximated Inverse Kinematics

Parallel-wire driven systems, which use light flexible wires in place of rigid links, control the position of a target object by controlling their wire lengths. In the kinematics for such a parallel-wire driven system, when the relationship between the end-effector position and the wire lengths is investigated, a fixed point for the wire-contacting point on the winding reel in the actuator (or guide pulley) is often approximated to simplify the calculation. The approximated kinematics however could lead to a number of positioning errors in the positioning control. This study proposes a framework for evaluating these positioning control errors by using approximated inverse kinematics. In view of the system dynamics, this study analyzes the positioning control errors for the control method in the wire-length coordinates. We discuss a case study on a two degrees-of-freedom planar system using three wires.