We are aware that too high of a PID gain can cause excessive oscillations and even instability. The ultimate gain for processes with no steady state on PID horizon is usually much higher than our comfort level.
Many of the most important process variables, such as vessel and column composition, pressure and temperature, do not reach a steady state in the time frame of PID action. Batch composition, pH and temperature and, of course, level have no steady state.
The PID is by far the most prevalent controller in the process industry. Here we step back for a view of the basics of the proportional, integral, and derivative modes. These PID controller modes have distinct advantages and disadvantages and consequences if one mode dominates.
Older Distributed Control Systems (DCS) and analog controllers tended to have different tuning setting units and methods of implementing integral and derivative action. A lack of understanding of the difference between the old and new PID features and tuning settings can lead to poor and even unstable control when migrating...
For advanced regulatory control, when do I use Model Predictive Control (MPC) instead of PID control? There are many PID techniques for dealing with batch operations, abnormal operation, startups, and transitions. Feedforward, ratio control, and override control are a main stay for PID control. If desired, PID dead time compensators...
For basic control, when do I use Fuzzy Logic Control (FLC) and Model Predictive Control (MPC) instead of PID control? The automation and integration of FLC and MPC into distributed control systems makes these a more viable choice. Surprisingly, there may be less decisions and expertise required than for PID control.
In part 5 we finish with a list of my foremost best practices. These practices build on the essential concepts given in Part 3. These practices offer simple fixes in the automation system design. Major improvements in the mechanical design are also introduced.
In part 4 we start a list of best practices. The guidance is the result of decades of experience in plants by industry experts Michel Ruel and Jacques Smuts. The practices are insightful and apply to almost every control loop. The series will conclude next week with my offering.
In part 2 we evaluate a misleading statement about the amount of derivative to use and provide some better guidance. We take a look at how mechanical and process design and operating conditions affect the need for derivative action.
The mechanical, piping, and process design determines the steady state and integrating process gains and the process deadtimes and lags. The process engineer usually sets the project basis for the control system in the development of the Process Flow Diagram (PFD) and in the writing of the operating and process descriptions.