Abstract
This dissertation presents control architectures using sliding mode control theory that enable significant energetic savings in closed-loop controlled servo actuation. Two approaches were developed, one based on switching between active and passive modes and one in the form of a unified model-based controller. If the single 4-way valve used in typical pneumatic system is replaced with two 3-way valves, then dissipative forces can be imposed on the load without the use of supply power. The first approach was achieved by developing two control modes; active mode, which utilizes the airflow from the air supply, and a passive mode, which utilizes exclusive control of the flow from the cylinder to the exhaust. The active mode is patterned after a standard pneumatic servo system and as such couples the two 3-way valves to effectively behave as a single 4-way valve. The passive mode utilizes each 3-way valve as if it were a 2-way valve modulating the flow resistance between each side of the cylinder and atmosphere. A switching law is developed to switch between active and passive modes of operation based on the current pressure states in the cylinder and the desired actuation force. The second approach takes advantage of the additional control degree of freedom (i.e., the modified servoactuator is a two-input, single output system instead of single input single output in standard active control). This additional degree-of-freedom is utilized to minimize the average cylinder pressure in addition to satisfying the sliding condition, which in turn minimizes the airflow utilized to track a desired trajectory. The net effect is a controller that only maintains the necessary output impedance required to track a given command. Experiments comparing both proposed approaches to standard active sliding mode control demonstrate that the power consumption of a pneumatic servo system is reduced significantly with essentially no sacrifice in tracking performance. The maximum energy saving occurs somewhere between the friction domination of low frequency motion and the large actuator demands required to overcome inertial forces at higher frequencies.