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The human-centered robotic systems demand safe and robust controllers in many applications. This paper proposes an adaptive proxy-based sliding mode control approach for a class of typical second-order nonlinear systems. A new PID-type virtual coupling is designed between a virtual proxy and the physical object. Considering the unknown bound of lumped disturbances, an adaptation law is applied to online adjust the gain of a sign function which ensures the proxy to track the reference accurately. By using the Lyapunov theorem, the closed-loop system stability is proved. Both simulations and experiments are conducted to verify the proposed method based on a real-world pneumatic muscle actuator control platform. The results show that the proposed adaptive proxy-based sliding mode control approach presents better tracking accuracy, safety, and robustness than the conventional PID control and sliding mode control. [ABSTRACT FROM AUTHOR]

This paper presents the trajectory tracking control of a multi-section continuum and compliant manipulator with large workspace. Fluidic muscles are utilized as actuators in this manipulator and inertial sensors used to measure its shape in configuration space. The robot's large workspace signifies the effects of the inertial, gravitational, and applied external forces on the trajectory tracking control of the robot end-effector. In this study, a model-based sliding-surface controller is proposed by deriving a simplified Pseudo-Rigid Body (PRB) dynamic model, which utilizes modified constant curvature assumptions to describe the couplings between the robot sections and actuators. Simplification steps are proposed for a PRB dynamic model by eliminating negligible dynamic effects to achieve a computationally efficient implementation with real-time control performance. The unmodeled dynamics and the error of model simplifications are considered as system uncertainties and an integral sliding surface with a variable sliding gain is proposed to overcome these uncertain dynamics. The sliding gain considers changes in the upper bounds of uncertainties. The closed-loop system stability in trajectory tracking is mathematically proved based on the Lyapunov stability theorem. Experiments throughout the manipulator's large workspace in challenging configurations including sudden release of unknown loads at the robot tip also highlight the potential of our contributed controller. [ABSTRACT FROM AUTHOR]

This paper presents a nonlinear adaptive backstepping algorithm for position control of an anthropomorphic robotic hand. The contraction force of the pneumatic artificial muscle (PAM) actuators has been modeled based on an empirical approach and the overall finger is represented as a nonlinear second order system, taking into account the system uncertainty caused by hysteresis phenomenon in PAM actuators. Adaptive backstepping controller has been developed by formulating the estimator of the system uncertainty. A cascade control system is developed by combining a conventional PID control, as an inner loop controller, with the adaptive backstepping position control as the outer loop of the controller. Finally, a simulation test is conducted to evaluate the performance of the proposed controller. [ABSTRACT FROM AUTHOR]

Abstract: This paper describes the development of an active vibration control system for a light and flexible stress ribbon footbridge. The 13 m span carbon fiber reinforced plastic (CFRP) stress ribbon bridge was built in the laboratory of the Department of Civil and Structural Engineering, Berlin Institute of Technology. Its lightness and flexibility result in high vibration sensitivity. To reduce pedestrian-induced vibrations, very light pneumatic muscle actuators are placed at handrail level, introducing control forces. First, a reduced discretized analytical model is derived for the stress ribbon bridge. To verify the analytical prediction, experiments without feedback control are conducted. Based on this model, a delayed velocity feedback control strategy is designed. To handle the nonlinearities of the muscle actuator, a subsidiary force control is implemented. Then the control performance from numerical simulation is verified by experiments under free vibration. As a result, analytical analyses agree well with experimental results. It is demonstrated that handrail-introduced forces can efficiently control the first mode response. [Copyright &y& Elsevier]

Abstract: Pneumatic muscle actuators have the highest power/weight ratio and power/volume ratio among actuators of all types. Therefore, they can be used not only in rehabilitation engineering, but also as actuators in robots, including industrial robots and therapy robots. It is difficult to achieve excellent control performance using classical control methods because the compressibility of gas and the nonlinear elasticity of bladder containers cause parameter variations. An adaptive self-organizing fuzzy sliding mode control (ASOFSMC) is developed in this study. Its fuzzy sliding surface can help reduce the number of fuzzy rule. The self-organizing learning mechanism is employed to modify fuzzy rules on-line. The model-matching technique is then adopted to adjust the scaling factors. Finally, the Lyapunov theory is employed to prove the stability of the ASOFSMC. Experimental results show that this control strategy can attain excellent control performance. [Copyright &y& Elsevier]

Abstract: An empirical and theoretical study is conducted on a special actuator termed “pneumatic muscle” (PM) being used in a force control system framework. Such an actuator has similarities to biological systems and has many advantages (extremely high power/weight, power/volume and power/energy ratios). However, due to its inherent nonlinearities, this actuator suffers from poor position and force control. The study described here accomplishes three main goals. (1) A force control system is developed within an open and closed loop framework to emulate how biological systems work in an agonist–antagonist framework. (2) The PM used in the study has such strength that it excites the frame dynamics. This undesired dynamic response is then effectively cancelled using an impedance model control scheme. (3) The PM is demonstrated to both change length yet still produce force in a controlled manner. [Copyright &y& Elsevier]

Abstract: An evaluation of controller design is performed on an actuator, which resembles a biological system (pneumatic muscle actuator). The procedures studied herein do not require any presumptions of linearity or stationarity. A comparison across various controllers was obtained by considering a pneumatic muscle system being used in a force control sense over a full cycle of operation. Such an actuator has many advantages (extremely high power/weight, power/volume and power/energy ratios). A number of power and energy related measures were assessed including: (1) a workloop metric to specify the external work output of the actuator produced on the environment, (2) a power ellipsoid method to examine efficiency over one cycle of operation, and (3) other key power and energy related criteria, e.g. when peak power varies as a function of the velocity produced. [Copyright &y& Elsevier]

Abstract: Pneumatic muscle actuators are effective devices to generate tension forces by converting pneumatic energy into mechanical energy. They are currently being used in parachute systems for soft-landing and steering control applications. Therefore, the simulation of their nonlinear structural dynamic behavior is necessary for a complete evaluation of these parachute systems. In this paper, the working principles of pneumatic muscle actuators are reviewed; the application of pneumatic muscle actuators in parachute systems for soft-landing and steering control is described; and a new finite element model for pneumatic muscle actuators is presented. Geometrically nonlinear anisotropic membrane elements are used in this model to simulate the nonlinear structural dynamic behavior of pneumatic muscle actuators, which is different from previous approaches. A quasi-static pneumatic muscle actuator model is analyzed for validation and two dynamic applications of pneumatic muscle actuators in parachute systems are also presented. [Copyright &y& Elsevier]

Pneumatic Muscle Actuators (PMAs) have recently been used as control devices in parachute soft landing systems and parachute steering control systems. In this paper, a special cable element is developed to model the mechanical behavior of PMAs. The new element provides relationships between the PMA internal pressure, fiber bias angle, PMA radius and length, and resultant axial force, based on the kinematic assumption of inextensible PMA fibers. The principle of virtual work and total Lagrange formulation are used to derive the element internal force vector and tangent stiffness matrix. The PMA element is implemented in a geometrically nonlinear, transient finite element program for simulating the structural dynamics of airdrop systems. Several numerical examples are given to validate the mechanical behavior of the new element. Two large-scale application problems are also presented to demonstrate the capabilities of the new element for simulating PMAs used in airdrop systems. [Copyright &y& Elsevier]

Two flexible pneumatic actuators, McKibben and straight fiber, were analyzed. The muscles were modeled by finite element method. The numerical models take into account the non-linearity of the expanding rubber inner tube, and of the mechanism for transferring load to the braided cords surrounding the tube. Because of its own fabrication, McKibben muscle model may capture the initial backslash between rubber tube and cords.Despite of the McKibben specimen, a straight fiber muscle prototype was fabricated on purpose in the laboratory. In order to validate the numerical models, numerous experimental tests were carried out. The numerical results showed a good agreement with the data measured experimentally in terms of muscle pull force and of its deformed shape. Finally, the models of two muscles similar in terms of input energy allow their performances to be compared. [Copyright &y& Elsevier]

[Display omitted] • Development of a modular pneumatic muscle actuator using novel end-fittings and clamps. • End-fittings result in a lightweight and cost-effective pneumatic muscle of shorter form factor with > 30 % contraction. • Developed muscle produced a higher force and contraction at a lower actuation pressure. • Developed muscle with bladder of smaller volume and higher tensile modulus had higher accuracy and lower hysteresis error. Pneumatic muscle actuators (PMA) are a class of soft actuators known for their high power to weight ratio and inherent compliance. The pneumatic muscle's inherent properties make them very favorable for assistive applications (e.g., medical exoskeletons). This study presents a novel end-fitting design that makes the developed pneumatic muscle actuator lightweight, cost-effective, and modular, thus simplifying the process of assembly and maintenance. The pneumatic muscle actuator assembled using the novel end fittings achieves a shorter overall length without compromising its contraction. The pneumatic muscle actuator has been assembled using a commercial bladder and a braided sleeve alongside a pair of 3D printed novel end-fittings. The paper also details the developed actuator's characterization for force and deflection parameters at various operating pressures. A total of four muscle actuators of varying diameters with constant actuation length (100 mm) were developed and tested to showcase the effect of size on the muscle actuator's behavior. The study presented here also involved comparing three mathematical models developed for pneumatic muscles in order to find a model which closely resembles the developed muscle actuator. Finally, the developed pneumatic muscle actuator's behavior is compared with a commercially available muscle to determine the efficacy of the developed muscle's design. The tests showed that the muscle using a bladder of smaller volume but higher tensile modulus had a higher accuracy and stable performance. As the muscle is intended for medical applications, it was also put through an endurance test with realistic loading and pressure conditions, which revealed very promising results. [ABSTRACT FROM AUTHOR]

• A continuum and compliant robotic manipulator with large workspace is proposed. • Festo fluidic muscles are utilized to achieve large scale movements with strength. • Bouc–Wen model is modified to describe the asymmetric hysteresis of utilized muscles. • Manipulator is modeled based on constant curvature method and concentrated masses. • Design and dynamic model are validated experimentally on the implemented manipulator. This paper focuses on the design and dynamic modeling of a novel multi-section continuum and compliant robotic manipulator with large workspace. In large scales, many of the design techniques and actuator types which have proved advantageous in creating continuum robots at smaller scales are not applicable. Fluidic muscles, a class of pneumatic muscle actuators (PMAs) with large scale movements, high actuation forces and compliant nature, are utilized in the design of the continuum manipulator. However, these actuators are capable of applying axial contracting forces and cannot produce bending movements. Moreover, their asymmetric hysteresis nonlinearity causes difficulties in the accurate control procedure. These drawbacks are addressed in the design and presented model of the manipulator. The presented model is based on constant curvature method and concentrated masses. The Bouc–Wen hysteretic function is modified to describe the asymmetric force/length hysteresis of the utilized PMAs in this model. Experimental results show that the proposed model has a proper performance to characterize the asymmetric hysteresis loop of the actuators. Finally, the design and dynamic model of the robot are validated experimentally on the implemented manipulator. [ABSTRACT FROM AUTHOR]