COntrol strategy design

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 A successful control strategy is tailored to the process objectives and is focused on delivering low variability in the key process variables. · It responds effectively to important disturbances. and delivers consistent control performance over the entire operating range. It is understandable to the operators and is relatively easy to operate. Moreover, it delivers smooth transition between operating modes, accounts for unexpected events such as sensor or valve failure and provides decision making information to the operator.

The control strategy must be developed analytically in order to achieve the best design. Often the design simply defaults to the most commonly used strategy or the strategy that was used the last time. These approaches do not yield the best strategy, particularly where there are multiple, interactive control loops.  

Developing a successful control strategy requires a strong understanding of the process objectives, the operating modes, process and control fundamentals and instrumentation. Accordingly, the development team should be multidisciplinary. A process control specialist is usually best equipped to lead the effort but there needs to be plenty of input from the process engineering, instrumentation and operations groups. 

There are some key concepts that the development team needs to use in defining the best strategy.

• The control strategy needs to be matched to the known process disturbances. The important disturbances should be identified and characterized at the beginning of the control strategy design process. 
• The control strategy needs to be developed analytically (i.e. math is involved) in order to ensure that the controller will work well over the entire operating range. 
• The PID feedback control loop has limitations. It is only effective in responding to relatively slow external disturbances. External disturbances faster than 20 times the process deadtime or time constant are not going to be attenuated significantly by the feedback controller. The slower the dynamics the less capable the feedback controller. 
• Advanced regulatory strategies such as cascade, ratio and feedforward have the potential to substantially improve control response to external disturbances. Implementing these more advanced strategies increases capital and maintenance costs and operator training requirements.
• Process non-linearities will significantly compromise feedback control performance, usually by requiring more conservative controller tuning. The control strategy should minimize non-linearities, particularly for key loops. 
• Non-linearities should be pushed as far down the control strategy as possible. A fast slave loop can deal with a non linear process gain much more effectively (i.e. less impact on the master process) than a slow master loop. Control valve tracking non-linearities (backlash, stiction) will not only compromise loop performance but will also increase process variability via controller induced limit cycles 

Omni Process Solutions has extensive experience in developing best practice control strategies for our clients in the pulp and paper industry. 

Our control strategy development services include the following.

• Process and Control strategy review
•  Data collection and analysis to identify the primary disturbances 
• Development of conceptual control strategy
• Translation of strategy into detailed control logic diagrams for DCS /PLC configuration
• Developing customized process simulation to evaluate and test control strategy performance 
• Factory acceptance checkout of control system logic
• Start-up assistance including logic testing, operator training and loop tuning.