Practical Implementation of Resistance Feedback Measurement for Position Control of a Flexible Smart Inhaler Nozzle

Many “smart materials” have the capacity to be used simultaneously as both an actuator and sensor. For example, SMA actuator wires can be heated by Joule heating to induce contraction; at the same time, the resistance across the SMA wire can be measured to give the user some indication of the strain in the wire. This multi-functional capability enables the design of applications requiring extremely light-weight and streamlined embedded sensors and actuators. One such “smart structure” application is the flexible nozzle used in the Smart Inhaler system under development at North Carolina State University. The Smart Inhaler allows a doctor to control the locations within the pulmonary system that are medicated by controlling the location at which medication is injected into an inhaled airflow. This can reduce the amount of healthy tissue that is exposed to potentially toxic medications, such as those used to treat lung cancer. However, the practical challenge of injecting medication into a flow without disturbing the flow requires a highly controllable yet non-obstructive nozzle. This paper presents a scheme that correlates the resistance measurement across an SMA actuator wire to the wire strain and the resulting deformation of the flexible nozzle. The relationship between resistance and nozzle deformation is nonlinear and hysteretic; however, the repeatability of the relationship allows the user to calibrate the feedback measurement. This enables the wire to be used as both position sensor and positioning actuator. The results represent the first experiments that exploit the multi-functional capabilities of SMA wires in the context of a practical embedded sensor and actuator application.