Nonlinear Controller Design for Asymmetric Actuators

This paper deals with the development of a nonlinear control strategy for asymmetric actuators. In asymmetric actuators, the generated force/torque is not symmetric in push and pull. Examples of asymmetric actuators are thrusters, shape memory alloy wires, and in general, cables and long rods that cannot be used for compression forces. These actuators are called unidirectional. More complex asymmetric actuators are those that generate uneven push and pull forces. Examples of these actuators are double acting hydraulic and pneumatic cylinders in which the area of the piston is not the same in forward and reverse directions. Since in the existing control techniques the actuator is assumed to be symmetric, the application of asymmetric actuators requires new control schemes. In this paper, we propose a nonlinear control method capable of producing any biased input that can be used by asymmetric actuators. The controller is based on quadratic coupling terms with which positive, negative, and any biased input can be generated. We study the stability of the system along with a method to select the controller gains for tuning the output of the controller to match the actuator type. The normal form method and perturbation techniques are used to study the controller design.