Pleated Pneumatic Artificial Muscles Research Articles

A Pneumatic Artificial Muscle Manufactured Out of Self-Healing Polymers That Can Repair Macroscopic Damages

Pneumatic muscles are mostly constructed out of flexible membranes and are implemented in a wide range of robotic applications. In these soft materials, perforations are often caused by overpressuring of the muscles and wear. In many cases, this leads to limited numbers of actuation cycles. In this letter, pleated pneumatic artificial muscles were developed, whose membrane is constructed out of self-healing polymer. This allows for the complete healing of microscopic and macroscopic damages through a mild heat treatment. Two prototypes were designed, manufactured, and experimentally characterized. The self-healing was validated by repairing large realistic cuts in the muscle membrane completely. Even when repeatedly damaging and healing the muscles, the mechanical properties were recovered in a reversible and repeatable fashion through the self-healing procedure.

Modeling the Peano fluidic muscle and the effects of its material properties on its static and dynamic behavior

The promise of wearable assistive robotics cannot be realized without the development of actuators that mimic the behavior and form of biological muscles. Planar fluidic muscles known as Peano muscles or pouch motors have the potential to provide the high force and compliance of McKibben pneumatic artificial muscles with the low threshold pressure of pleated pneumatic artificial muscles. Yet they do so in a soft and slim form that can be discreetly distributed over the human body. This work is an investigation into the empirical modeling of the Peano muscle, the effect of its material on its performance, and its capabilities and limitations. We discovered that the Peano muscle could provide responsive and discreet actuation of soft and rigid bodies requiring strains between 15% and 30%. Ideally, they are made of non-viscoelastic materials with high tensile and low bending stiffnesses. While Sarosi et al’s empirical model accurately captures its static behavior with an root mean square error of 10.2 N, their dynamic model overestimates oscillation frequency and damping. We propose that the Peano muscle be modeled by a parallel ideal contractile unit and viscoelastic element, both in series with another viscoelastic element.

Pleated Pneumatic Artificial Muscle-Based Actuator System as a Torque Source for Compliant Lower Limb Exoskeletons

Powered lower limb exoskeletons require high-performance actuator systems, capable of producing zero to high-assistive torque and at the same time yielding to human interaction torques. Such variable impedance can be achieved by the means of compliant actuators. Because of their intrinsic compliance and high force-to-weight ratio pneumatic muscles are a viable option. However, previous pneumatic muscle powered exoskeleton designs either used them as a position source or failed to meet the high-dynamic torque requirements when using them as a torque source. This paper contributes to the improvement of pneumatic muscle-based actuator systems as a torque source for exoskeleton-type robots. The use of pleated pneumatic artificial muscles in a novel actuator system design allows for a higher torque range in a larger range of motion. Performance evaluation results are given for a 1 DOF test setup and a powered knee exoskeleton. The proposed torque controller achieves the dynamic torques required for zero to full assistance at moderate walking speeds.

Third–Generation Pleated Pneumatic Artificial Muscles for Robotic Applications: Development and Comparison with McKibben Muscle

This paper introduces the third generation of Pleated Pneumatic Artificial Muscles (PPAM), which has been developed to simplify the production over the first and second prototype. This type of artificial muscle was developed to overcome dry friction and material deformation, which is present in the widely used McKibben muscle. The essence of the PPAM is its pleated membrane structure which enables the muscle to work at low pressures and at large contractions. In order to validate the new PPAM generation, it has been compared with the mathematical model and the previous generation. The new production process and the use of new materials introduce improvements such as 55% reduction in the actuator’s weight, a higher reliability, a 75% reduction in the production time and PPAMs can now be produced in all sizes from 4 to 50 cm. This opens the possibility to commercialize this type of muscles so others can implement it. Furthermore, a comparison with experiments between PPAM and Festo McKibben muscles is discussed. Small PPAMs present similar force ranges and larger contractions than commercially available McKibben-like muscles. The use of series arrangements of PPAMs allows for large strokes and relatively small diameters at the same time and, since PPAM 3.0 is much more lightweight than the commong McKibben models made by Festo, it presents better force-to-mass and energy to mass ratios than Festo models.

Safe and Compliant Guidance by a Powered Knee Exoskeleton for Robot-Assisted Rehabilitation of Gait

In the research field of robot-assisted gait rehabilitation there is increased focus on the improvement of physical human–robot interaction by means of high-performance actuator technologies and dedicated control strategies. In this context we propose a combination of lightweight, intrinsically compliant, high-torque actuators (pleated pneumatic artificial muscles) with safe and adaptable guidance along a target trajectory by means of proxy-based sliding mode control. We developed a powered knee exoskeleton (KNEXO) to evaluate these concepts. In addition to the trajectory-based controller a torque controller was implemented with a view to minimizing the interaction during unassisted walking. First, various treadmill walking experiments were performed with unimpaired subjects wearing KNEXO to evaluate the performance of the proposed controllers. Test results confirm the ability of KNEXO to display low actuator torques in unassisted mode and to provide safe, adaptable guidance in assisted mode. Subsequently, a multiple sclerosis patient participated in a series of pilot experiments. Provided there was some patient-specific controller tuning KNEXO was found to effectively support and compliantly guide the subject's knee.

Successful Preliminary Walking Experiments on a Transtibial Amputee Fitted with a Powered Prosthesis

This paper presents the results of preliminary walking experiments on a transtibial amputee wearing a powered prosthesis. The prosthesis prototype serves as a proof-of-concept implementation for investigating the potential of pleated pneumatic artificial muscles to power a transtibial prosthesis. The device is equipped with pleated pneumatic artificial muscles, and tethered to a laboratory pressure source. The prosthesis is capable of providing the amputee with 100% of the required push-off torque and it can adapt its joint stiffness to the walking speed. This study supports the hypothesis that a powered transtibial prosthesis with adaptable stiffness might be beneficial to the amputee.

Design and Control of a Lower Limb Exoskeleton for Robot-Assisted Gait Training

Robot-assisted rehabilitation of gait still faces many challenges, one of which is improving physical human-robot interaction. The use of pleated pneumatic artificial muscles to power a step rehabilitation robot has the potential to meet this challenge. This paper reports on the development of a gait rehabilitation exoskeleton with a knee joint powered by pleated pneumatic artificial muscles. It is intended as a platform for the evaluation of design and control concepts in view of improved physical human-robot interaction. The design was focused on the optimal dimensioning of the actuator configuration. Safety being the most important prerequisite, a proxy-based sliding mode controller (PSMC) was implemented as it combines accurate tracking during normal operation with a smooth, slow and safe recovery from large position errors. Treadmill walking experiments of a healthy subject wearing the powered exoskeleton show the potential of PSMC as a safe robot-in-charge control strategy for robot-assisted gait training.

Design and Control of a Lower Limb Exoskeleton for Robot‐Assisted Gait Training

Robot‐assisted rehabilitation of gait still faces many challenges, one of which is improving physical human‐robot interaction. The use of pleated pneumatic artificial muscles to power a step rehabilitation robot has the potential to meet this challenge. This paper reports on the development of a gait rehabilitation exoskeleton with a knee joint powered by pleated pneumatic artificial muscles. It is intended as a platform for the evaluation of design and control concepts in view of improved physical human‐robot interaction. The design was focused on the optimal dimensioning of the actuator configuration. Safety being the most important prerequisite, a proxy‐based sliding mode controller (PSMC) was implemented as it combines accurate tracking during normal operation with a smooth, slow and safe recovery from large position errors. Treadmill walking experiments of a healthy subject wearing the powered exoskeleton show the potential of PSMC as a safe robot‐in‐charge control strategy for robot‐assisted gait training.

A Study on the Bandwidth Characteristics of Pleated Pneumatic Artificial Muscles

Pleated pneumatic artificial muscles have interesting properties that can be of considerable significance in robotics and automation. With a view to the potential use of pleated pneumatic artificial muscles as actuators for a fatigue test bench (high forces and small displacements), the bandwidth characteristics of a muscle-valve system were investigated. Bandwidth is commonly used for linear systems, as the Bode plot is independent of the amplitude of the input signal. However, due to the non-linear behaviour of pleated pneumatic artificial muscles, the system's gain becomes dependent on the amplitude of the input sine wave. As a result, only one Bode plot is insufficient to clearly describe or identify a non-linear system. In this study, the bandwidth of a muscle-valve system was assessed from two perspectives: a varying amplitude and a varying offset of the input sine wave. A brief introduction to pneumatic artificial muscles is given. The concept of pleated pneumatic artificial muscles is explained. Furthermore, the different test methods and experimental results are presented.

Treadmill walking of the pneumatic biped Lucy: Walking at different speeds and step-lengths

Actuators with adaptable compliance are gaining interest in the field of legged robotics due to their capability to store motion energy and to exploit the natural dynamics of the system to reduce energy consumption while walking and running. To perform research on compliant actuators we have built the planar biped Lucy. The robot has six actuated joints, the ankle, knee and hip of both legs with each joint powered by two pleated pneumatic artificial muscles in an antagonistic setup. This makes it possible to control both the torque and the stiffness of the joint. Such compliant actuators are used in passive walkers to overcome friction when walking over level ground and to improve stability. Typically, this kind of robots is only designed to walk with a constant walking speed and step-length, determined by the mechanical design of the mechanism and the properties of the ground. In this paper, we show that by an appropriate control, the robot Lucy is able to walk at different speeds and step-lengths and that adding and releasing weights does not affect the stability of the robot. To perform these experiments, an automated treadmill was built

Objective locomotion parameters based inverted pendulum trajectory generator

This paper describes a new trajectory generator for Lucy, a bipedal walking robot actuated by pleated pneumatic artificial muscles. The strategy is based on the inverted pendulum approximation, which models the robot as a single point mass. The trajectory generator allows the step length, intermediate foot lift and velocity to be chosen for each step while keeping the Zero Moment Point (ZMP) in the ankle point during the single support phase and it provides a smooth transition of the ZMP from the rear ankle point to the front ankle point during the double support phase. The path of the ZMP is discussed for a complete multibody model instead of a single point mass. The proposed trajectory generator has been tested in the robot and the results are presented and discussed.

Development of a compliance controller to reduce energy consumption for bipedal robots

In this paper a strategy is proposed to combine active trajectory tracking for bipedal robots with exploiting the natural dynamics by simultaneously controlling the torque and stiffness of a compliant actuator. The goal of this research is to preserve the versatility of actively controlled humanoids, while reducing their energy consumption. The biped Lucy, powered by pleated pneumatic artificial muscles, has been built and controlled and is able to walk up to a speed of 0.15 m/s. The pressures inside the muscles are controlled by a joint trajectory tracking controller to track the desired joint trajectories calculated by a trajectory generator. However, the actuators are set to a fixed stiffness value. In this paper a compliance controller is presented to reduce the energy consumption by controlling the stiffness. A mathematical formulation has been developed to find an optimal stiffness setting depending on the desired trajectory and physical properties of the system and the proposed strategy has been validated on a pendulum structure powered by artificial muscles. This strategy has not been implemented on the real robot because the walking speed of the robot is currently too slow to benefit already from compliance control.

A biomechatronical transtibial prosthesis powered by pleated pneumatic artificial muscles

Due to its high power-to-weight ratio, a pleated pneumatic artificial muscle (PPAM) offers an interesting alternative actuation source for robotic devices. Its inherent compliant behaviour excites another broad field of interest: assistive clinical devices such as powered exoskeletons and prosthetics. In this paper, the design of a pneumatically powered transtibial prosthetic device is presented. A first prototype has been built and provides a preliminary test bed for control algorithm development and testing with able-bodied subjects in laboratory conditions. The characteristics and working principle of a PPAM are described. The design specifications and the mechanical model of the prosthesis are discussed. The mechanical design and the control structure are outlined. Furthermore, some initial walking trials with an able-bodied subject wearing the prosthesis prototype are presented and discussed.

Overview of the Lucy Project: Dynamic Stabilization of a Biped Powered by Pneumatic Artificial Muscles

This paper gives an overview of the Lucy project. What is special is that the biped is not actuated with the classical electrical drives, but with pleated pneumatic artificial muscles. In an antagonistic setup of such muscles both the torque and the compliance are controllable. From human walking there is evidence that joint compliance plays an important role in energy-efficient walking and running. To be able to walk at different walking speeds and step lengths, a trajectory generator and joint trajectory tracking controller are combined. The first generates dynamically stable trajectories based on the objective locomotion parameters which can be changed from step to step. The joint trajectory tracking unit controls the pressure inside the muscles so the desired motion is followed. It is based on a computed torque model and takes the torque–angle relation of the antagonistic muscle setup into account. With this strategy the robot is able to walk at a speed up to 0.15 m/s. A compliance controller is developed to reduce the energy consumption by combining active trajectory control with the exploitation of the natural dynamics. A mathematical formulation was developed to find an optimal compliance setting depending on the desired trajectory and physical properties of the system. This strategy is experimentally evaluated on a single pendulum structure and not implemented on the real robot because the walking speed of the robot is currently too slow. At the end a discussion is given about the pros and cons of building a pneumatic biped, and the control architecture used.

Trajectory Planning for the Walking Biped “Lucy”

A real-time joint trajectory generator for planar walking bipeds is proposed. In the near future this trajectory planner will be implemented on the robot “Lucy”, which is actuated by pleated pneumatic artificial muscles. The trajectory planner generates dynamically stable motion patterns by using a set of objective locomotion parameters as its input, and by tuning and exploiting the natural upper body dynamics. The latter can be determined and manipulated by using the angular momentum equation. Basically, trajectories for hip and swing foot motion are generated, which guarantee that the objective locomotion parameters attain certain prescribed values. Additionally, the hip trajectories are slightly modified such that the upper body motion is steered naturally, meaning that it requires practically no actuation. This has the advantage that the upper body actuation hardly influences the position of the Zero Moment Point. The effectiveness of the strategy developed is demonstrated by simulation results. A first simulation is performed under the assumption of perfect tracking by the controllers of the different actuators. This allows one to verify the effectiveness of the trajectory planner and to evaluate the postural stability. A second simulation is performed while taking the control architecture of the real robot into account. In order to have a more realistic simulation the proposed control architecture is evaluated with a full hybrid dynamic simulation model of the biped “Lucy”. This simulator combines the dynamical behaviour of the robot with the thermodynamical effects that take place in the muscle-valves actuation system. The observed hardware limitations of the real robot and expected model errors are taken into account in order to give a realistic qualitative evaluation of the control performance and to test the robustness.

Controlling a bipedal walking robot actuated by pleated pneumatic artificial muscles

This paper reports on the control structure of the pneumatic biped Lucy. The robot is actuated with pleated pneumatic artificial muscles, which have interesting characteristics that can be exploited for legged locomotion. They have a high power to weight ratio, an adaptable compliance and they can absorb impact effects.The discussion of the control architecture focuses on the joint trajectory generator and the joint trajectory tracking controller. The trajectory generator calculates trajectories represented by polynomials based on objective locomotion parameters, which are average forward speed, step length, step height and intermediate foot lift. The joint trajectory tracking controller is divided in three parts: a computed torque module, a delta-p unit and a bang-bang pressure controller. The control design is formulated for the single support and double support phase, where specifically the trajectory generator and the computed torque differs for these two phases.The first results of the incorporation of this control architecture in the real biped Lucy are given. Several essential graphs, such as pressure courses, are discussed and the effectiveness of the proposed algorithm is shown by the small deviations between desired and actual attained objective locomotion parameters.

MOTION GENERATION AND CONTROL FOR THE PNEUMATIC BIPED "LUCY"

This paper reports on the control structure of the pneumatic biped "Lucy." The robot is actuated with pleated pneumatic artificial muscles which have interesting characteristics that can be exploited for legged machines. They have a high power-to-weight ratio, an adaptable compliance and they can reduce impact effects. The current control architecture focuses on the trajectory generator and the tracking controller, which is divided into a computed torque controller, a delta-p unit, a PI position controller and a pressure bang-bang controller. The trajectory generator provides polynomial joint trajectories while the computed torque, combined with the delta-p unit, calculates the required muscle pressure levels. The PI and bang-bang controller work at a pressure level to cope with modeling errors and to set the pressures in each muscle. The control design is divided into single support and double support, where specifically the computed torque differs for these two phases. The proposed control architecture is evaluated with a full hybrid dynamic simulation model of the biped. This simulator combines the dynamical behavior of the robot with the thermodynamical effects that take place in the muscle-valves system. The observed hardware limitations of the real robot and expected model errors are taken into account in order to give a realistic qualitative evaluation of the control performance and to test the robustness. A preliminary implementation of the presented controller on the real biped, representing a walking motion of the robot while both feet are in the air, is discussed. This first implementation shows already promising results concerning tracking performance of the proposed control architecture. It confirms that the pneumatic tracking system can be used for a dynamic application such as a biped walking robot.

Control architecture for the pneumatically actuated dynamic walking biped “Lucy”

This paper reports on the control structure of the biped Lucy. The robot is actuated with pleated pneumatic artificial muscles which have interesting characteristics that can be exploited for legged machines. They have a high power to weight ratio, an adaptable compliance and they reduce impact effects. The considered control structure can be divided into two parts: a real-time joint trajectory generator and a trajectory tracking controller. The latter consists of a computed torque controller, a delta- p unit, a PI controller and a bang–bang pressure controller. The trajectory generator provides polynomial joint trajectories while the computed torque in combination with the delta- p unit calculates the required muscle pressure levels. The PI and bang-bang controller work at pressure level to cope with modelling errors and to set the pressures in each muscle. The proposed control architecture is evaluated with a full hybrid dynamic simulation model of the biped. This simulator combines the dynamical behaviour of the robot with the thermodynamical effects that take place in the muscle-valves system. The observed hardware limitations of the real robot and expected model errors are taken into account in order to give a realistic qualitative evaluation of the control performance and to test its robustness. At this stage a controller is designed for and evaluated during the single support phase only. In the double support phase the robot is over-actuated and consequently other control strategies are necessary. Additionally, preliminary results of a walking motion of the real robot with both feet in the air are given. These show promising results concerning tracking performance of the proposed control architecture. The experimental results confirm that the pneumatic tracking system can be used for a dynamic application such as a biped walking robot.

The Pneumatic Biped ?Lucy? Actuated with Pleated Pneumatic Artificial Muscles

This paper reports on the bipedal robot Lucy which is actuated by pleated pneumatic artificial muscles. This novel actuator is very suitable to be used in machines which move by means of legs. Besides its high power to weight ratio the actuator has an adaptable passive behavior, meaning the stiffness of the actuator can be changed on-line. This allows to change the natural frequency of the system while controlling angular joint positions. The main control concept intended for Lucy is joint trajectory control while selecting appropriate actuator compliance characteristics in order to reduce control efforts and energy consumption which is of great importance towards the autonomy of legged robots. Presently Lucy has made her first steps with the implementation of basic control strategies.

The Concept and Design of Pleated Pneumatic Artificial Muscles

This paper describes the design of a new type of Pneumatic Artificial Muscle (PAM), namely the Pleated Pneumatic Artificial Muscle (PPAM). It was developed as an improvement with regard to existing types of PAM, e.g. the McKibben muscle. Its principle characteristic is its pleated membrane. It can inflate without material stretching and friction and has practically no stress in the direction perpendicular to its axis of symmetry. Besides these it is extremely strong and yet very lightweight and it has a large stroke compared to most other designs. A general introduction on PAMs is given together with a short discussion and motivation for this new design. The concept of the PPAM is explained and a mathematical model is derived. This model proves its principle of operation. From the model, several characteristics, such as developed force, maximum contraction, diameter, volume and membrane tensile stress, are obtained. Material choices and dimensions of a typical PPAM are next discussed and its measured values of static force and diameter are compared to the model predicted values. The agreement between both is found to be very good.