Zhenkun Lin

Position control of a single pneumatic artificial muscle with hysteresis compensation based on modified Prandtl–Ishlinskii model

BACKGROUND High-performance position control of pneumatic artificial muscles is limited by their inherent nonlinearity and hysteresis. OBJECTIVE This study aims to model the length/pressure hysteresis of a single pneumatic artificial muscle and to realize its accurate position tracking control with forward hysteresis compensation. METHODS The classical Prandtl-Ishlinskii model is widely used in hysteresis modelling and compensation. But it is only effective for symmetric hysteresis. Therefore, a modified Prandtl-Ishlinskii model is built to characterize the asymmetric length/pressure hysteresis of a single pneumatic artificial muscle, by replacing the classical play operators with two more flexible elementary operators to independently describe the ascending branch and descending branch of hysteresis loops. On the basis, a position tracking controller, which is composed of cascade forward hysteresis compensation and simple proportional pressure controller, is designed for the pneumatic artificial muscle. RESULTS Experiment results show that the MPI model can reproduce the length/pressure hysteresis of the pneumatic artificial muscle, and the proposed controller for the pneumatic artificial muscle can track the reference position signals with high accuracy. CONCLUSION By modelling the length/pressure hysteresis with the modified Prandtl-Ishlinskii model and using its inversion for compensation, precise position control of a single pneumatic artificial muscle is achieved.

Design and control of a pneumatic-driven biomimetic knee joint for biped robot

This paper presents the design and control of a biomimetic knee joint for biped robots. The robotic knee joint driven by pneumatic artificial muscles is designed by imitating the structure of the human knee. It has desirable characteristics similar with human knee joint including joint compliance, changeable instantaneous center of rotation, as well as large range of joint motion. In order to model and compensate the length/pressure hysteresis of pneumatic artificial muscles in the position control, a novel method termed as direct inverse hysteresis modeling approach is introduced. Other than deriving from the forward hysteresis model, the inversion of the length/pressure hysteresis is directly modeled by a modified Prandtl-Ishlinskii model which has two newly-designed play operators. Then a cascade position controller with forward hysteresis compensation is proposed for the knee joint. The mechanical structure and control scheme of the knee joint are validated by experiments.

Development of a passive dynamic walking robot based on mechanical structural parameters optimization

Passive dynamic walking robot can walk with low energy consumption and exhibits human-like natural gait. However, because the walking performance greatly or fully depends on the mechanical structural parameters, their walking stability is quite low compared to active walking robot. In other words, proper mechanical parameters are one of the key factors to achieve stable walking for a passive dynamic walking robot. In this paper, parametric mechanical structural parameters were used to fulfill the parameters optimization process and optimal mechanical structural parameters were obtained based on the global stability analysis with cell-mapping method by numerical simulation. A passive dynamic biped walking robot prototype with hip joint, knee joints, ankle joints and an upper body was developed based on the optimization result, both the simulation and experiments results proved that the optimization result is reasonable.

BIPED ROBOT DESIGN WITH VARIABLE ANKLE STIFFNESS

Human lower limbs have particular flexibility. Both the efficiency of bipedal walking and the ability to protect actuators with low energy loss are worthy references for the design of bipedal robots. This paper proposes a design for a biped robot with joints of variable stiffness. The robot has three degrees of freedom in the sagittal plane in each leg. The hips and knees are driven directly by the motor, while the ankles are passive joints with adjustable stiffness. After a comprehensive investigation, a variable stiffness mechanism was introduced based on lever principles, and driven by a motor that can realize real-time adjustment. Simulations verified the necessity of variable stiffness joints in the robot. The variable stiffness joint can absorb the ground impact on each joint, reduce the energy loss of the motor, and improve the efficiency of movement.

Control Strategy Research for a Biped Walking Robot with Flexible Ankle Joints

The traditional zero moment point (ZMP) control method is not suitable for underactuated biped robots. In this article, based on the computed torque method, the virtual constraint method and feedback linearization method are introduced into robot control to develop a joint control strategy for a biped robot with flexible ankle joints. Meanwhile, the traditional spline curve planning method of ankle joint and hip joint trajectory is no longer applicable to the biped robot. In this article, the time-invariant gait planning and gait stability of the robot are studied based on the principle of periodic walking. The effectiveness of the control strategy is verified by an experiment on the biped walking robot with flexible ankle joints.

The Design of Pipe Cutting Tools for Remote Handling in Maintenance Manipulator for Tokamak

The article describes the remote cutting tools designed for pipe cutting at Tokamak. Because of the special working environment, there are some design requirements for the cutting tools: the cutting swarf should be removed effectively, the quality of cut necessary for re-welding, and the cutting tools should be compact to fit into the maintenance manipulator for remote handling. The cutting tool consists of two parts: the cutting part and the sealing part. The two parts of cutting tool are designed to cut pipes and retain all cutting debris, respectively. A remote computer is used to input the commands and display data. During the cutting operation, the programmable controller has been used for controlling the cutting tool. The commands through the controller to motor drivers, implement the desired action.

Position control of a bio-inspired semi-active joint with direct inverse hysteresis modeling and compensation:

Accurate position control of pneumatic artificial muscle actuated systems has always been difficult due to their inherent nonlinear hysteresis characteristics. This article designs a bio-inspired semi-active robotic joint that is synergistically driven by a pneumatic artificial muscle and an extension spring by imitating the mechanism of energy storage and return in animals and then studies its position tracking control with hysteresis compensation. To simplify the derivation of the inverse hysteresis model which functions as hysteresis compensator, a novel approach termed as direct inverse hysteresis modeling is adopted. With this method, the inversion of the asymmetric angle–pressure hysteresis of the semi-active joint is modeled directly from experimental measurements by a modified Prandtl–Ishlinskii model. On the basis, a closed-loop position tracking controller composed of forward hysteresis compensation and conventional proportional–integral–derivative control is designed for the joint. Experimental results indicate that the proposed controller can track the reference position signals with high accuracy.