Why PID Control Loop Tuning And Optimization Is Essential?

Since the beginning of the 20th century the field of process automation and process control optimization has become increasingly important in all manufacturing industry worldwide. Moreover, after 2010 the new revolution has been started under the name Industry 4.0: Increased automation and digital transformation. The main goal of it is to use advanced information and technologies in order to increase the level of automation, diagnose and reduce the appearance of faults and to optimize the performance of a process and/or equipment.

By observing only chemical and petrochemical huge industry pool we can easily state that the primary control layer is the backbone in the overall process control hierarchy. It supports all advanced control and complex optimization applications which are the financial foundation of any industry. Studies of the chemical and other manufacturing industries reveal that often the modern, complex advanced control applications receive primary focus and attention and often the underlying bottom-layer (primary control) issues are somewhat neglected because of lesser emphasis on the profit itself. If we go even deeper, we will realize that the heart and the soul of a primary control layer is an PID control loop. Any manufacturing plant can have anywhere from few tens to several thousands of PID control loops. Amazingly, but even if the PID control algorithm is considered as an old-fashion it is still unclear and misunderstood by many. More than several international studies have shown that each manufacturing plant no matter big or small, still faces with at least:

• 20 % of PID control loops that are not designed well.
• 30 % of PID control loops that has got valve problems.
• 15 % of PID control loops that are not properly activated.
• 10 % of PID control loops that have instrumentation problems.
• 30 % of PID control loops that are not tuned at all.
• 30 % of PID control loops that are in manual mode.
• 40 % of PID control loops that are oscillatory.

Figure 1 shows typical process control issue in any manufacturing plant.

Overall, the studies show that between 65 to 85 % of PID control loops can be improved. If PID control philosophy is understood clearly, it could provide tremendous benefits, control improvements and a financial rise. Imagine how much money was spent on the instrumentation (smart sensors), wiring and communication, DCS/PLC systems, design of SCADA/HMI graphical interfaces and control logic, PID control loops configuration, final control elements (in 90 % this is a valve) and engineering man-hours. By adding all these numbers together, it can easily be realized that if a PID control loop does not work optimally or if it is kept in manual mode for a long period will produce nothing but a wasted investment! Even if all PID control loops currently perform optimally, after a year or two the things will certainly change because of:

• Change in the operating conditions
• Changes in market or economic conditions
• Changes in plant capacity
• Stronger impact of disturbances
• New equipment or pipe installation
• Different product grades and properties
• Process equipment fouling
• Valve or sensor issues

On the other hand, many PID control loops are not fully tuned and optimized due to lack of awareness of the potential and the resulting benefits.  If PID control loop does not perform optimally, overall process will behave too sluggish or oscillatory.  This can result with product quality reduction, off-spec products and other.  Many times, in order to prevent unstable plant or an equipment operation, the operators run the plant conservatively in its so-called “the comfort zone”.  Due to possible plant instability the optimal performance lies far away from the economical, process and/or equipment constraint limits resulting in the lost profit.  Also, prior to any advanced process control (APC) project, base-level PID control loop tuning and optimization is a critical pre-requisite step.  Unless base-level PID control loops are well tuned and optimized, advanced process control (APC) algorithms will not work well.  

With all stated facts above billions of $ are lost worldwide because of inefficient (not optimal) plant operation, the lack of automation and the inability to move the plant in direction of maximum efficiency and profit.  Therefore, no matter how large or small the plant is, PID control loop tuning and optimization is an essential prerequisite for achieving any plant benefits.

There are plenty of methods, tools & theories which are available for tuning and optimization of PID control loops, however finding the optimal PID control loop tuning parameters is still a tricky task.  Nowadays still, many processes suffer from typical problems such as: 

• Continuous process interactions
• Heat and mass balance integration
• Slow and complex recycle process dynamics
• Tight process control demands of sensitive processes
• Different operating conditions and plant transitions
• Different process constraints
• Frequent issues with catalyst or pipe fouling
• Process and equipment nonlinearities and drifts
• Wrongly sized valves and their mechanical issues

Often, still widely used old-fashion Trial-and-Error, Ziegler Nichols, IMC, Lambda and other similar empirical PID tuning methods rely on time consuming and large step-test changes by forcing the PID control loop to manual mode and producing either too aggressive or sluggish PID control loop behavior. Therefore, all these methods will never give optimal PID control loop tuning parameter values required for tight and crisp PID control in auto nor in cascade mode when typical and frequent SP changes and/or large unmeasured process disturbances are present. The reasons why these methods are still used are the absence of engineering knowledge and understanding, unavailability of robust process control software tools for robust PID tuning and optimization without conducting serious plant step-tests.

Therefore, PiControl Solutions has invented the PITOPS, the only system identification, PID or APC (advanced process control) control loop tuning and optimization software tool in the world which:

• Operates completely in the time domain.
• Does system identification based on the open-loop data (step changes of PID OP), closed-loop data (step changes of PID SP), completely oscillatory and non-steady-state closed-loop data, and stepless closed-loop data.
• Does multivariable system identification using closed-or-open loop data.
• Does system identification based on the ultra-short duration data (1/5th of data).
• Does control valve stiction identification.
• Does unmeasured disturbance pattern identification.
• Does identification of process or equipment nonlinearities.
• Does need data preconditioning (resample, high noise, missing data, outliers).
• Possesses all commercially available PLC/DCS PID algorithms.
• Does master and slave control loop optimization (based on complex SP trajectory).
• Does mathematical calculation of feedforward parameters (gain, deadtime, lead/lag).
• Does inferential control loop design.
• Does deadtime compensator control loop design (Smith predictor).
• Does PID control loop optimization based on the SP change (step or ramp), different disturbances (step, pulse, ramp, sine), control valve stiction, PID OP rate of change, over-or-undersized control valve.

To conclude, the process control continues to be one of the most fascinating and growing areas with tremendous future prospects related to economy, safety and process stability.

PiControl Solutions is the only process control and automation company in the world which can perform PID control loop tuning and optimization project completely remotely. Nowadays, every country uses high speed and reliable internet connection and with a help of medium to high resolution cheap web-cameras or even over widely used remote meeting and screen sharing applications it is possible to do complete PID control loop tuning and optimization project. Over this low cost online/remote approach huge travel and accommodation costs can be saved and human health and safety can be kept on the high level.