Stress estimation using unknown input observer

The hypothalamic-pituitary-adrenal (HPA) axis is the body´s primary stress management system. The role of the HPA axis is to maintain bodily functions in the presence of physical and mental stressors. This is accomplished by controlling the body´s cortisol level. A non-linear mathematical model of the HPA axis from the literature is used to construct a stressor estimator. The mathematical model of the HPA axis is in the state-space format containing an unknown input that models the stressor acting on the body. The controlled input variable models external treatment. The unknown input observer (UIO) is constructed to estimate the unknown input modeling the stressor and its mathematical analysis is provided. Availability of the stressor estimate can be employed in the design of effective treatment strategies for stress related diseases. Simulations studies illustrate the effectiveness of the proposed observer-based stressor estimator.