Current Control and Modelling of an Inspiration Valve
Time: Wed 2021-06-23 14.00 - 14.30
Respondent: Astrid Lindstedt , DCS - Reglerteknik
Opponent: Bianca Harumi Otake
Supervisor: Rodrigo González
Examiner: Associate Professor Cristian Rojas
An analog current servo of an intensive care ventilator is being developed at Getinge in Stockholm. The current servo is used to control the output gas flow of an inspiration valve to a patient's lungs. The ventilator's control system is in a cascade design of two closed-loops, of which the current servo is the controller for the inner loop. The inner-loop control system consists of three parts; a PI-controller, a PWM signal generator, and an actuator of a solenoid. The project aims to investigate if a digital version of the current controller is feasible with comparable performance as the present analog controller. The project involves studying linear mathematical models, nonlinear simulation models, and physic prototypes.
The solenoid model is first modelled as an RL-circuit, secondly modified by adding the term Electro-Motive-Force (EMF), and finally verified by physical measurement data. Different control techniques are tested with the simulation model, such as feed-forward control. A digital controller is then implemented in a processor of a Nucleo board and connected to the inspiration valve hardware with a resulting sampling rate of 8 kHz. The selected hardware components for the implementation are described in detail, and a step-by-step explanation of the software implementation is included in Appendix B. Finally, the model-tested control techniques have been implemented in a physical prototype and discussed.
The developed digital current controller is feasible to achieve similar control performance as the present analog design. However, for configuring the digital current controller into a future digital ventilator's CPU, the sampling rate will need to be adopted to the limited recourses of the CPU executing both the current controller and all other functions of the inspiration valve. The code of the digital current controller will also need to be optimized in future work to compensate for the shared data capacity.