Pneumatic Muscle Research Articles

Modeling the Static Force of a Festo Pneumatic Muscle Actuator: A New Approach and a Comparison to Existing Models

In this paper, a new approach for modeling the static force characteristic of Festo pneumatic muscle actuators (PMAs) will be presented. The model is physically motivated and therefore gives a deeper understanding of the Festo PMA. After introducing the new model, it will be validated through a comparison to a measured force map of a Festo DMSP-10-250 and a DMSP-20-300, respectively. It will be shown that the error between the new model and the measured data is below 4.4% for the DMSP-10-250 and below 2.35% for the DMSP-20-300. In addition, the quality of the presented model will be compared to the quality of existing models by comparing the maximum error. It can be seen that the newly introduced model is closer to the measured force characteristic of a Festo PMA than any existing model.

Efficient Structure-Based Models for the McKibben Contraction Pneumatic Muscle Actuator: The Full Description of the Behaviour of the Contraction PMA

To clarify the advantages of using soft robots in all aspects of life, the effective behaviour of the pneumatic muscle actuator (PMA) must be known. In this work, the performances of the PMA are explained and modelled with three formulas. The first formula describes the pulling force of the actuator based on the structure parameters; furthermore, the formula presented is the generalised contraction force for wholly-pneumatic muscle actuators. The second important model is the length formula, which is modified to our previous work to fit different actuator structures. Based on these two models, the stiffness of the actuator is formulated to illustrate its variability at different air pressure amounts. In addition, these formulas will make the selection of proper actuators for any robot arm structure easier using the knowledge gained from their performance. On the other hand, the desired behaviour of this type of actuator will be predefined and controlled.

Hysteresis modeling and trajectory tracking control of the pneumatic muscle actuator using modified Prandtl–Ishlinskii model

The pneumatic muscle actuator (PMA) has attracted extensive attentions from both industrial and academic fields due to its high power/weight ratio and significant compliance. However, on the one hand the inherent hysteresis nonlinearity has high influence on the accuracy of trajectory tracking control of PMA, and on the other hand a system actuated by PMAs is at a high cost. This paper presents a modified Prandtl–Ishlinskii (MPI) model for the asymmetric hysteresis characterization and compensation of the PMA using fast switching valves. By using the Levenberg–Marquardt (L-M) method, the parameters in the MPI model are identified. To compensate the nonlinear length/pressure hysteresis and reduce the cost, a cascade control scheme using fast switching valves based on the inverse MPI model is developed to realize the trajectory tracking control. The experimental result shows the effectiveness of the proposed control scheme on compensating the asymmetric length/pressure hysteresis of the PMA.

Active disturbance rejection tracking control for a nonlinear pneumatic muscle system

This paper presents active disturbance rejection tracking control for a nonlinear pneumatic muscle system. Unmodeled dynamics, varying parameters and external disturbances are merged into generalized disturbances in the pneumatic muscle system. An error-feedback controller is designed to guarantee tracking precision based on an extended state observer. Some experimental results are given to show the effectiveness of developed techniques in this paper.

A nested pneumatic muscle arrangement for amplified stroke and force behavior

This article presents a compact nested architecture to amplify the displacement and forces of pneumatic artificial muscles for potential use in human assistive devices and other robotic applications. The nested architecture consists of several levels in series, and each level is made up of contracting pneumatic muscles, passive force transfer members, and additively manufactured interconnects. The stroke obtained from the nested pneumatic artificial muscle architecture is not always beneficial and is limited by the length-dependent behavior of pneumatic artificial muscles and other practical manufacturing constraints such as the size of the interconnects. Thus, this article studies the effect of the pneumatic artificial muscle length on its stroke using a modified constrained volume maximization formulation, which predicts the actual shape of the deformed pneumatic artificial muscle, and models additional stiffness due to membrane bending. Using this formulation, a framework is presented to optimally design the number of nested levels and individual actuators in each level to obtain a required stroke. Such a system is designed to actuate the human elbow by an angle of 80°, where almost 40% contraction is obtained using custom-manufactured pneumatic artificial muscles inherently capable of contracting upto 17% of its length. The framework can be used to amplify the stroke and forces of any pneumatic artificial muscle actuator and adapt it to different application requirements.

Design of a Variable Stiffness Soft Dexterous Gripper.

This article presents the design of a variable stiffness, soft, three-fingered dexterous gripper. The gripper uses two designs of McKibben muscles. Extensor muscles that increase in length when pressurized are used to form the fingers of the gripper. Contractor muscles that decrease in length when pressurized are then used to apply forces to the fingers through tendons, which cause flexion and extension of the fingers. The two types of muscles are arranged to act antagonistically and this means that by raising the pressure in all of the pneumatic muscles, the stiffness of the system can be increased without a resulting change in finger position. The article presents the design of the gripper, some basic kinematics to describe its function, and then experimental results demonstrating the ability to adjust the bending stiffness of the gripper's fingers. It has been demonstrated that the fingers' bending stiffness can be increased by more than 150%. The article concludes by demonstrating that the fingers can be closed loop position controlled and are able to track step and sinusoidal inputs.

Presentation of textile pneumatic muscle prototypes applied in an upper limb active suit experimental model

This paper presents some prototypes of textile pneumatic muscles developed at the Department of Mechanical and Aerospace Engineering (DIMEAS) of the Politecnico di Torino. As an application, they are intended to be included in an ‘active suit’ useful for upper limb rehabilitation. They were realized with textile and soft materials in order to guarantee suitable wearability, confort, low weight, small dimensions as required to actuators used in active suits. This paper illustrated the experimental tests carried on these pneumatic muscles prototypes, a theoretical model from literature here applied to these muscles in order to calculate their stiffness, a preliminary active suit prototype for upper limbs rehabilitation purpose. The results obtained, both in terms of contraction and force developed with these prototypes, are highly satisfactory and the preliminary prototype of active suit realized for upper limbs works properly.

HUmanoid Robotic Leg via pneumatic muscle actuators: implementation and control

In this article, a HUmanoid Robotic Leg (HURL) via the utilization of pneumatic muscle actuators (PMAs) is presented. PMAs are a pneumatic form of actuation possessing crucial attributes for the implementation of a design that mimics the motion characteristics of a human ankle. HURL acts as a feasibility study in the conceptual goal of developing a 10 degree-of-freedom (DoF) lower-limb humanoid for compliance and postural control, while serving as a knowledge basis for its future alternative use in prosthetic robotics. HURL’s design properties are described in detail, while its 2-DoF motion capabilities (dorsiflexion–plantar flexion, eversion–inversion) are experimentally evaluated via an advanced nonlinear PID-based control algorithm.

Neural network fuzzy sliding mode control of pneumatic muscle actuators

The pneumatic muscle actuator (PMA) is one of the most promising actuators especially for the applications that require greater proximity between the humans and the robots. Fast and precise control of PMA, however, is difficult to achieve due to the compressibility of the air and the elasticity of the PMA. In order to achieve accurate and consistent tracking performance of a one axis PMA actuated manipulator over considerably wide ranges of frequency and stroke, an intelligent adaptive control algorithm is developed in this paper. The adaptive learning is enabled by a neural network in which the control gains to a fuzzy sliding mode controller (FSMC) and an integrator are adjusted to minimize the tracking error. Experimental results show that the proposed control strategy achieves fast, accurate and consistent performance tracking sinusoidal reference trajectories up to 1 Hz in frequency and close to the extreme stroke of the PMA actuated manipulator with the compressed air regulated to 4 bar. Results also show that the proposed control strategy, with a more aggressive learning for the control gain to the FSMC, achieves satisfactory performance tracking a trapezoidal reference trajectory.

Design and Dynamic Model of a Frog-inspired Swimming Robot Powered by Pneumatic Muscles

Pneumatic muscles with similar characteristics to biological muscles have been widely used in robots, and thus are promising drivers for frog inspired robots. However, the application and nonlinearity of the pneumatic system limit the advance. On the basis of the swimming mechanism of the frog, a frog-inspired robot based on pneumatic muscles is developed. To realize the independent tasks by the robot, a pneumatic system with internal chambers, micro air pump, and valves is implemented. The micro pump is used to maintain the pressure difference between the source and exhaust chambers. The pneumatic muscles are controlled by high-speed switch valves which can reduce the robot cost, volume, and mass. A dynamic model of the pneumatic system is established for the simulation to estimate the system, including the chamber, muscle, and pneumatic circuit models. The robot design is verified by the robot swimming experiments and the dynamic model is verified through the experiments and simulations of the pneumatic system. The simulation results are compared to analyze the functions of the source pressure, internal volume of the muscle, and circuit flow rate which is proved the main factor that limits the response of muscle pressure. The proposed research provides the application of the pneumatic muscles in the frog inspired robot and the pneumatic model to study muscle controller.

Adaptive Impedance Control of Parallel Ankle Rehabilitation Robot

Robots are being increasingly used by physical therapists to carry out rehabilitation treatments owing to their ability of providing repetitive, controlled, and autonomous training sessions. Enhanced treatment outcomes can be achieved by encouraging patients' active participation besides robotic assistance. Advanced control strategies are required to be designed and implemented for the rehabilitation robots in order to persuade patients to contribute actively during the treatments. In this paper, an adaptive impedance control approach is developed and implemented on a parallel ankle rehabilitation robot. The ankle robot was designed based on a parallel mechanism and actuated using four pneumatic muscle actuators (PMAs) to provide three rotational degrees-of-freedom (DOFs) to the ankle joint. The proposed controller adapts the parallel robot's impedance according to the patients' active participation to provide customized robotic assistance. In order to evaluate performance of the proposed controller, experiments were conducted with stroke patients. It is demonstrated from the experimental results that the robotic assistance decreases as a result of patients' active participation. Similarly, increased robotics assistance is recorded in response to decrease in patient's participation in the rehabilitation process. This work will aid in the further development of customized robot-assisted physical therapy of ankle joint impairment.

Design of Nonlinear Predictive Control for Pneumatic Muscle Actuator Based on Echo State Gaussian Process

Recently, the application of Pneumatic Muscle Actuators (PMAs) for driving rehabilitation robots has become a matter of great concern. A traditional control algorithm, such as PID, cannot achieve satisfactory high-precision performance in trajectory tracking problem for PMAs, due to PMAs’ features of nonlinear effects, slow response time, time-varying parameters. In this study we proposed a nonlinear predictive control strategy SNN-ESGP, which is comprised of a novel model called echo state Gaussian process (ESGP) that is suitable for modeling nonlinear unknown systems as well as measuring their uncertainties, and a single neural network (SNN) serves as the controller of the system. To analyze the system convergence, we utilize the gradient descent algorithm and deduce the iterative rules of the weights of SNN. The stability of the closed-loop system is analyzed by using Lyapunov theorem. Finally, studies of physical experiments illustrate the validity of the control strategy with high-precision trajectory performance.

Multi-Modal Active Vibration Control of a Lightweight Stress-Ribbon Footbridge Based on Subspace Identification

Experimental identification of the modal state-space model for Active Vibration Control (AVC) is proposed based on Subspace Identification Method (SIM). A stress-ribbon bridge built at the Technische Universität Berlin was taken as research object. A real-time control system with pneumatic muscle actuators and inertial sensors was set up for AVC of the bridge. For the purpose of AVC, the feedback control design requires a modal state-space model, which describes the multi-modal characteristics of resonance modes of the bridge and the input/output relationship of the controlled system. Analytically, a state-space model of the bridge can be derived from Euler-Lagrange mechanism. However this is computationally expensive and the determination of damping ratios is also inconvenient. Comparably, SIM offers an attractive alternative due to simple and general parametrisation for Multiple-Input Multiple-Output (MIMO) systems. The identified state-space model from SIM can easily be transformed into a reduced modal state-space model for the AVC controller design. The goodness of the identified state-space model and the truncated modal state-space model was investigated in the stabilization diagram of SIM and with model fit indexes. The identified modal parameters of the footbridge from SIM were compared with that of previous free-vibration tests and the analytical model. Implementation of a Delayed Modal Velocity Feedback Control (DMVFC) based on the obtained modal state-space model was carried out for the full-scale lightweight bridge. The effectiveness of the proposed experimental identification method was shown in case of vertical resonance modes in vibration control experiments.

Pneumatic Muscle Actuated Rehabilitation Equipment of the Upper Limb Joints

Rehabilitation equipment of the upper limb joints holds a key role in passive physical therapy. Within this framework, the paper presents two such pieces of equipment developed for the rehabilitation of elbow and of wrist and knuckles, respectively. The presented and discussed equipment is actuated by pneumatic muscles, its benefits being a low cost, simple and robust construction, as well as short response time to commands.

A Novel Geometric Formula for Predicting Contractile Force in McKibben Pneumatic Muscles

Several analytical models exist in the literature for predicting the behavior of braided pneumatic muscles (McKibben muscles). Such models take into consideration the various variables and parameters that are related to the muscle geometry, material properties, and the loads applied to the system, and propose various relationships between these variables. Owing to the complexity of the muscle structure, in several cases, instead of a physical model, empirical or experimental models are used, which generally have limited validity for specific muscle types, i.e., they are only valid for a restricted range of operating parameters. This study proposes a new analytical formula based on the geometry of a pneumatic muscle studied in the rest and work phases and a simple experimental method to obtain corrective factors useful to design muscles. A mathematical formula can thus be obtained that allows one to deduce the measurements of interest in the system as a function of the specific parameters and permits one to interpret in qualitative terms the behavior of the muscle at each moment for various values of pressure, contraction, and applied load and to identify any critical situations. This model can therefore be a very useful design tool because it allows one to adapt the muscle geometry based on the required forces and contractions for different applications that are compatible with the muscle structure on which the model is based. This paper also presents a method for evaluating the efficiency of the muscles, useful to better use them in different applications.

Reducing the metabolic cost of walking with an ankle exoskeleton: interaction between actuation timing and power

BackgroundPowered ankle-foot exoskeletons can reduce the metabolic cost of human walking to below normal levels, but optimal assistance properties remain unclear. The purpose of this study was to test the effects of different assistance timing and power characteristics in an experiment with a tethered ankle-foot exoskeleton.MethodsTen healthy female subjects walked on a treadmill with bilateral ankle-foot exoskeletons in 10 different assistance conditions. Artificial pneumatic muscles assisted plantarflexion during ankle push-off using one of four actuation onset timings (36, 42, 48 and 54% of the stride) and three power levels (average positive exoskeleton power over a stride, summed for both legs, of 0.2, 0.4 and 0.5 W∙kg−1). We compared metabolic rate, kinematics and electromyography (EMG) between conditions.ResultsOptimal assistance was achieved with an onset of 42% stride and average power of 0.4 W∙kg−1, leading to 21% reduction in metabolic cost compared to walking with the exoskeleton deactivated and 12% reduction compared to normal walking without the exoskeleton. With suboptimal timing or power, the exoskeleton still reduced metabolic cost, but substantially less so. The relationship between timing, power and metabolic rate was well-characterized by a two-dimensional quadratic function. The assistive mechanisms leading to these improvements included reducing muscular activity in the ankle plantarflexors and assisting leg swing initiation.ConclusionsThese results emphasize the importance of optimizing exoskeleton actuation properties when assisting or augmenting human locomotion. Our optimal assistance onset timing and average power levels could be used for other exoskeletons to improve assistance and resulting benefits.

Pneumatic Muscle-Actuated Adjustable Compliant Gripper System for Assembly Operations

The aim of this paper is to present and discuss an innovative, constructive solution for a gripper system that can be attached to an industrial robot for assembly operations. The construction of this gripper system is based on a linear pneumatic muscle used as the actuator and the transmission of motion by gear-and-rack mechanisms. Air compressibility renders pneumatic muscle behaviour inherently compliant; this favours automated assembly applications as it allows the correction of inevitable lateral and angular misalignments in mating operations. Therefore, the jamming of a peg-like object can be avoided when this is introduced into a hole with tight clearance. In addition to the construction of the gripper system, this paper discusses its actuation, as well other characteristics.

Design of a Pneumatic Muscle Based Continuum Robot With Embedded Tendons

Continuum robots have attracted increasing focus in recent years due to their intrinsic compliance that allows for dexterous and safe movements. However, the inherent compliance in such systems reduces the structural stiffness, and therefore leads to the issue of reduced positioning accuracy. This paper presents the design of a continuum robot employing tendon embedded pneumatic muscles. The pneumatic muscles are used to achieve large-scale movements for preliminary positioning, while the tendons are used for fine adjustment of position. Such hybrid actuation offers the potential to improve the accuracy of the robotic system, while maintaining large displacement capabilities. A three-dimensional dynamic model of the robot is presented using a mass-damper-spring-based network, in which elastic deformation, actuating forces, and external forces are taken into account. The design and dynamic model of the robot are then validated experimentally with the help of an electromagnetic tracking system.

Development of Giacometti Arm With Balloon Body

As part of our research on developing different types of Giacometti robots, the potential of a very long, very light, and very simple robot arm with a balloon body is discussed in this paper. Although this robot arm is not suitable for precise positioning, rapid motion, and high load capacity, which are the aspects most conventional robots focus on, it is designed for very specific purposes, such as inspection using a small camera at its tip and is designed to be essentially safe even if it falls down or hits an object. This robot arm is realized using helium-filled balloon bodies and thin pneumatic muscles. The arm achieves self-weight compensation, and the possibility of designing a very long arm is confirmed theoretically and experimentally. A prototype of a 7-m-long cantilever arm is designed, developed, and tested.

Adaptive Servomechanism of Pneumatic Muscle Actuators With Uncertainties

Precise control of a pneumatic muscle actuator is a challenging problem, due to its nonlinear and time-varying characteristics. Specifically, it can be modeled as parallel connection of a nonlinear friction force, a nonlinear spring, and a nonlinear contractile element. It is difficult to identify the coefficients associated with these elements within a certain precision as they change along the course of continuous use. In this paper, we introduce a novel adaptive servomechanism controller and rigorously prove that the trajectory tracking can be achieved with a desirable error bound. Moreover, an experiment platform is set up to demonstrate the effectiveness of the proposed controller in real applications. In particular, the proposed adaptive controller significantly improves the tracking accuracy and achieves better performance than nonadaptive and PID controllers and other existing controllers in the literature.