Passivity Approach to Pneumatic Actuated Human Interactive Robots

The high power density of fluid-powered actuators can facilitate design of compact and powerful devices. Pneumatic actuators in particular are preferred in human interactive devices due to their properties such as inherent compliance, backdrivability and benign consequences of leakage. A drawback of pneumatic actuators is that the current sources of compressed air are bulky and not suitable for mobile human-centered applications. To address these concerns, research is underway on advanced gas based actuation devices such as chemo-fluidic actuators, dry ice actuators, and mini-HCCI engines. These actuators are ideal for development of powerful and mobile devices for human scale applications. The operation of these devices typically requires direct human interaction between the pneumatic (or gas) actuated system and the human operator. Therefore safety of operation is imperative. One way of investigating interaction stability (and hence safe operation) between multiple systems is by using the framework of Passivity analysis from systems theory.;The objective of the research presented in this dissertation is investigation of passivity characteristics of pneumatic actuators. This passivity analysis is a preliminary step in understanding the feasibility of using gas based actuators in human interactive applications. Passivity analysis requires definition of a storage function to quantify the effect of inputs and outputs on the system dynamics. The nonlinear dynamics of air compression and expansion in a pneumatic actuator are affected by the heat transfer across the walls of the actuator. In this thesis, physics based energy functions are developed and defined to be the storage function for three specific models of heat transfer viz adiabatic, isothermal, and finite heat transfer. For reversible thermodynamic process (adiabatic or isothermal), the storage function is defined as the work that can be extracted from the actuator. The storage function for actuator with finite heat transfer is defined as the maximum work that can be extracted from the pneumatic actuator. It is shown that the solution to this optimization problem provides a storage function similar to exergy of the air in the actuator. The supply rate based on these storage functions corresponds to physically meaningful power interaction between different subsystems, such as the actuator and the inertia load. Both adiabatic and isothermal actuators have two ports for power interaction : mechanical port corresponding to interaction with an inertial load, and fluid port corresponding to interaction with source/sink of compressed air. The adiabatic/isothermal actuator behaves as a two-port nonlinear spring with an active flow input at the fluid port of the actuator. Pneumatic actuator with finite heat transfer to the ambient has an additional port corresponding to the thermal interaction with the ambient. The supply rate to the pneumatic actuator with finite heat transfer illustrates that irrespective of chamber air temperature, heat transfer reduces the ability of the actuator to do work, thus contributing to passive behavior of the actuator.;Energetically passive controller design for pneumatic actuated human power amplifier and tele-operated human scale devices is presented in this thesis. A framework for controlling the active flow variable at the fluid port of the pneumatic actuator to provide passive operation of a pneumatically actuated human power amplifier is presented by assuming the heat transfer model in the actuator to be either adiabatic or isothermal. This framework is then extended to define the framework for achieving co-ordinated tele-operation of multiple pneumatic actuated devices, while again amplifying input human power. The control problem is suitably defined within the proposed framework and a two-stage back-stepping controller is used to achieve the control objective. The Lyapunov function for the actuator controller design is defined based on the energy functions developed for adiabatic and isothermal actuators. The designed controllers are implemented on single-DOF systems and a multi-DOF robotic rescue crawler with pneumatic actuators. Experimental results confirming the efficacy of the proposed controller are provided.;Finally, independent metering of each actuator chamber to improve the operational efficiency of the pneumatic actuators is investigated. In independent metering, two servo valves are used to control the air flow rate to the two chambers of the actuator. The valves are controlled to maintain a low operating pressure in both the chambers, while providing the desired actuator force. This mitigates the losses associated with discharge of high pressure air to atmosphere. Effectiveness of independent metering is evaluated in a single-DOF human power amplifier by assuming the heat transfer model in the actuator to be isothermal. Experimental results show 70% improvement in operation time.