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A control-based analysis and characterization of a free-piston Stirling engine is presented, and proposed as a lightweight power supply for untethered robots. Typically, such devices are designed from the point of view of a thermodynamic cycle in terms of traditional thermodynamic equations of state. Such equations of state are independent of time and therefore lend little insight when dynamic elements are incorporated into the design. The approach presented here is from a system dynamics and control perspective. Equations of state are replaced by dynamic system modeling elements. Utilizing these dynamic elements, control concepts are applied to evaluate a given configuration and ensure an unstable oscillatory response and therefore transform heat into useful work. A simulation of a commercially available free-piston engine is presented, and standard control design tools are applied to its linearized model. The results show promising potential in utilizing small-scale free-piston Stirling engines as portable power supply for robotic systems.

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This paper proposes a control approach that can provide significant energy savings for the control of pneumatic servo systems. The control methodology is formulated by decoupling the standard four-way spool valve used for pneumatic servo control into two three-way valves, then using the resulting two control degrees of freedom to simultaneously satisfy a performance constraint (which for this paper is based on the sliding mode sliding condition), and an energy-saving dynamic constraint that minimizes cylinder pressures. The control formulation is presented, followed by experimental results that indicate significant energy savings with essentially no compromise in tracking performance relative to control with a standard four-way spool valve.

This paper proposes a variation on a sliding mode control approach that provides significant energy savings for the control of pneumatic servo systems. The control methodology is formulated by first decoupling the standard four-way spool valve used for pneumatic servo control into two three-way valves, then using the resulting two control degrees of freedom to simultaneously satisfy both the sliding mode sliding condition and a dynamic constraint that minimizes airflow. The control formulation is presented, followed by experimental results that indicate significant energetic savings with essentially no compromise in tracking performance relative to a standard four-way spool valve approach. Specifically, relative to standard four-way spool valve pneumatic servo actuator control, the experimental results indicate energy saving of 27 to 45%, depending on the desired tracking frequency.