Conditions for removing intersample ripples in multirate control

• Main Authors: Tangirala, A.K., Shah, S.L., Tongwen Chen
• Format: Conference Proceeding
• Language: English
• Subjects: Chemical processes; Control systems; Distillation equipment; Frequency; MIMO; Pressure measurement; Sampling methods; Switches; Temperature measurement; Time varying systems
• ISSN: 0840-7789; 2576-7046
• DOI: 10.1109/CCECE.1999.804949

Multirate systems arise when signals of interest are sampled at different rates. Measurements from chemical processes are typically available at different sampling rates. For example, composition estimates in a distillation column are available at a much slower rate than flow, temperature and pressure measurements. Multirate systems pose a challenging problem due to several reasons such as increased complexity design, time-varying nature, etc. Systems consisting of fast-rate control moves and slow-sampled outputs are a common scenario in chemical processes and of practical interest. Traditionally, inferential techniques based on secondary measurements have been used to design the fast-rate input moves. Lifting techniques conveniently transform multirate systems to single-rate lifted systems with increased dimensionality. Controllers designed using lifting techniques require that certain causality constraints are satisfied. We show firstly, that intersample ripples can arise in the closed-loop output of a multirate system as a result of non-uniform gains of the discrete lifted system and inverse lifting. Secondly, we present conditions on the controller gains to avoid intersample ripples.