This paper presents new results to compute the time-delay margin of the hot-dip galvanizing control system. Compared to previous works on this issue, this paper considers a mathematical model of the plant with two inputs and one output. The inputs are used to regulate the output, which represents the Zinc mass coating of steel strip. To achieve this objective, a multivariable PI controller is used, this controller is tuned applying the well known Ziegler and Nichols method, and then the maximum time-delay is computed in order to guarantee the stability property of the close loop control system. The work bases its results on a transformation of the time-delay operator and it is performed in order to get a two variable polynomial; after this, to obtain the robust stability property, a result based on the Hurwitz matrix is applied.

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Introduction Working at night can lead to misalignment between the timing of the biological clock and the work/sleep schedule, resulting in sleepiness, inattention, and impaired performance during the night shift and poor quality, shortened sleep during the day. These adverse effects are reported to be worse in older workers due to their decreased ability to sleep during the daytime. Although numerous quantitative studies have examined these associations, there are few qualitative studies to describe the sleep patterns and experiences from older night workers’ perspectives. Methods Four virtual focus groups were conducted with 12 nurses aged 50-65, working regular 8-hr night shifts, to learn about their sleep patterns and experiences, and factors affecting their sleep and non-sleep activities before, between, and after consecutive night shifts. Two facilitators and two research assistants reviewed and coded the focus group transcripts for consistent themes among participants. Results Eight common themes were identified: (1) sleep patterns are self-selected based on family and childcare responsibilities; (2) sleep timing switches between nights and days from before- to between- to after-consecutive night shifts; (3) frequent extended (24+ h) wake before and after consecutive night shifts; (4) difficulty maintaining long and sound sleep during daytime; (5) family, social, health, and environmental disturbances interrupt daytime sleep; (6) more satisfaction with sleep quantity and quality in participants with split sleep than those with one episode; (7) sleep is considered less important compared to childcare, family, and social activities; (8) better on-shift alertness and performance when the main sleep episode or a nap immediately precedes night shift. Conclusion Sleep patterns and experiences reported by older night shift nurses are helpful in understanding their priorities and challenges to obtaining adequate sleep. This information is critical for the future design and implementation of feasible and acceptable interventions to improve sleep in this occupational group. Support (If Any) This study was supported by NIH grant R01 AG044416.

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