Exploiting Natural Characteristics of Pneumatic Servo-Actuation Through Multi-Input Control
This dissertation presents the research on four advanced topics concerning the pneumatic servo-actuation through multi-input control. The first topic is Nonlinear Model Based Pulse Width Modulated Control of Pneumatic Servo Systems. In this work, an averaging approach, which describes the equivalent continuous-time dynamics of a PWM controlled nonlinear system, is developed and 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. Experimental tracking performance demonstrated the effectiveness of the proposed approach. The second topic is Simultaneous Force and Stiffness Control of a Pneumatic Actuator. This work involves the design of a robotic 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. Based on this notion, a control approach is developed for the simultaneous control of stiffness and output force, as well as a control law for the specific case of force control of maximum or minimum stiffness. The third topic is On the Enhanced Passivity of Pneumatically-Actuated Impedance-Type Haptic Interfaces. This work proposed an approach to achieving a stable, high-stiffness surface in a haptic interface by leveraging the open-loop properties of pneumatic actuators. By utilizing the open-loop component of actuator stiffness as a primary component of stiffness simulation in a haptic interface, the system requires a comparatively small component of simulated stiffness from closed-loop control of the actuator. The enhancement of the stability range is demonstrated by both a stability analysis and experimental results. The last topic is Energy Saving in Pneumatic Servo Control Utilizing Inter-Chamber Cross-Flow. The energy saving is achieved by supplementing the standard configuration of a four-way valve controlled pneumatic system with an additional two-way valve that enables direct flow between the cylinder chambers. The inter-chamber crossflow enables the recirculation of the pressured air, thus saves the energy in the compressed air that would otherwise be exhausted to the atmosphere. A control approach is presented to utilize the crossflow, to the extent possible, to supplement the mass flow required by a sliding mode controller with the recirculated mass flow provided by the crossflow valve. Experimental results indicate an energy saving of 25% to 52%, with essentially no sacrifice in the tracking performance.
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