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This paper proposes a new approach to the design of a robot actuator with physically variable stiffness. The proposed approach leverages the dynamic characteristics inherent in a pneumatic actuator, which behaves in essence as a series elastic actuator. By replacing the four-way servovalve used to control a typical pneumatic actuator with a pair of three-way valves, the stiffness of the series elastic component can be modulated independently of the actuator output force. Based on this notion, the authors propose a control approach for the simultaneous control of actuator output force and stiffness. Since the chievable output force and stiffness are coupled and configuration-dependent, the authors also present a control law that provides either maximum or minimum actuator output stiffness for a given displacement and desired force output. The general control and maximum/minimum stiffness approaches are experimentally demonstrated and shown to provide high fidelity control of force and stiffness, and additionally shown to provide a factor of 6 dynamic range in stiffness. [DOI: 10.1115/1.2745850]

This paper proposes a structure and control approach for the energy saving servo control of a pneumatic servo system. The energy saving approach is enabled by supplementing a standard four-way spool valve controlled pneumatic actuator with an additional two-way valve that enables flow between the cylinder chambers. The ''crossflow' valve enables recirculation of pressurized air, and thus enables the extraction of stored energy that would otherwise be exhausted to the atmosphere. A control approach is formulated that supplements, to the extent possible, the mass flow required by a sliding mode controller with the recirculated mass flow provided by the crossflow valve. Following the control formulation, experimental results are presented that indicate energy savings of 25-52%, with essentially no compromise in tracking performance relative to the standard sliding mode control approach (i.e., relative to control via a standard four-way spool valve, without the supplemental flow provided by the crossflow valve). [DOI: 10.1115/1.2718244]

This paper presents a control methodology that enables nonlinear model-based control of pulse width modulated (PWM) pneumatic servo actuators. An averaging approach is developed to describe the equivalent continuous-time dynamics of a PWM controlled nonlinear system, which renders the system, originally discontinuous and possibly non-affine in the input, into an equivalent system that is both continuous and affine in control input (i.e., transforms the system to nonlinear control canonical form). This approach is applied to a pneumatic actuator controlled by a pair of three-way solenoid actuated valves. The pneumatic actuation system is transformed into its averaged equivalent control canonical form, and a sliding mode controller is developed based on the resulting model. The controller is implemented on an experimental system, and the effectiveness of the proposed approach validated by experimental trajectory, tracking. [DOI: 10.1115/1.2232689]

This paper describes the design and control of a new chemomuscle actuation system for robotic systems, especially the mobile systems inspired by biological principles. Developed based on the pneumatic artificial muscle, a chemomuscle actuation system features a high power density, as well as similar characteristics to the biological muscles. Furthermore, by introducing monopropellant (a special type of liquid fuel) as the energy storage media, the chemomuscle system leverages the high energy density of liquid fuel and provides a compact form of high-pressure gas supply with a simple structure. The introduction of monopropellant addresses the limitation of pneumatic supply on mobile devices and thus is expected to facilitate the future application of artificial muscle on biorobotic systems. In this paper, the design of a chemomuscle actuation system is presented, as well as a robust controller design that provides effective control for this highly nonlinear system. To demonstrate the proposed chemomuscle actuation system, an experimental prototype is constructed, on which the proposed control algorithm provides good tracking performance. [ABSTRACT FROM AUTHOR]

Inspired by human motor control theory, stiffness control is highly effective in manipulation and human-interactive tasks. The implementation of stiffness control in robotic systems, however, has largely been limited to closed-loop control, and suffers from multiple issues such as limited frequency range, potential instability, and lack of contribution to energy efficiency. Variable-stiffness actuator represents a better solution, but the current designs are complex, heavy, and bulky. The approach in this paper seeks to address these issues by using pneumatic actuator as a variable series elastic actuator (VSEA), leveraging the compressibility of the working fluid. In this work, a pneumatic actuator is modeled as an elastic element with controllable stiffness and equilibrium point, both of which are functions of air masses in the two chambers. As such, for the implementation of stiffness control in a robotic system, the desired stiffness/equilibrium point can be converted to the desired chamber air masses, and a predictive pressure control approach is developed to control the timing of valve switching to obtain the desired air mass while minimizing control action. Experimental results showed that the new approach in this paper requires less expensive hardware (on-off valve instead of proportional valve), causes less control action in implementation, and provides good control performance by leveraging the inherent dynamics of the actuator.