This paper presents a model-based approach to developing easily configurable and customizable and safe-to-use UCAADS. The acronym UCAADS stands for user-centric automation and assistive devices and systems, which include automation tools used by individuals and care-providing institutions for purposes of improving their quality of life and quality of care. The UCAADS model underlying the development process of such devices incorporates workflow elements with goals, objectives, methods, and selections elements, and is executable. This unique combination enables the device behavior, user actions, and user-device interactions to be specified in terms of a model for requirement capture and design assessment purposes early in the development process. The workflow elements in the device model become its implementation when the required programs and other resources become available. Reusable models, resource components, tools, and runtime environment needed to support this process are provided by UCAADS simulation environment and embedded workflow framework also described in this paper.