PROPORTIONAL-INTEGRAL-DERIVATIVE (PID) CONTROL
Reasons for the widespread use of PID control.
PID control systems offerthree-term functionality that cover treatment to both steady-stateresponses, and therefore, it has provided the most convenientstrategies to a wide number of control problems experienced today.PID was invented at about 1910, and since then, it has developedimmensely (Anget al 2005). In the recent world, advancements in digital technologyhas paved a way for the science of automatic control schemes.Additionally, at least more than 90% of industrial controllers havebeen utilizing PID controllers due to its flexibility in handlingmany industrial challenges. According to Ang et al., PID controllersgive simplicity, clear functionality, and real-time applicability intheir operations. Moreover, there is a considerable amount ofresearch in PID studies and technologists have been in thefore-front of finding the best integration techniques betweensoftware and PID-hardware module systems.
The range of applications that have adopted PID as the preferred control method.
Electric Vehicle Control- PID has been adopted to solve process control in an intelligent manner.
Robotic control systems-Many industries have adopted the use of robotics to enhance their performance, induce automation and improve production. So, PID has been a front-runner in the advancements of robotic technology.
Heating, ventilating and air-conditioning system- In these systems, intelligent PID controllers have shown several advantages such as, improved robustness, self-organizing and self-learning.
The range of methods used to obtain suitable PID parameter settings.
Analytical methods- PID parameters are designed from analytical or algebraic association between a plant model and an objective (Ang et al 2005). Therefore, these results to a suitable and elaborate formula.
Heuristic methods- These methods are adopted from manual tuning and from artificial intelligence (Ang et al 2005). These methods can be formula-based or rule-based for online applications, often with tradeoff design objectives.
Frequency response methods: This method is used to tune the PID controller. Their main objective is stability robustness (Ang et al 2005).
Optimization methods- These are special optimal control methods, whereby PID parameters are arranged using various optimization and computerized heuristics. They are timed-settings methods are mainly offline-based.
Adaptive tuning methods- Basically, these techniques are utilized in automated-online-tuning. These methods can use a blend of the above methods in actual identification (Ang et al 2005).
The particular control problems caused by process dead-time or transport delay.
Dead time introduces an extra additional lag phase in the system. Hence, decreasing the phase and gain margin of the transfer function and thus systems control becomes difficult to operate (Ang et al 2005).
Transport delays also causes a weakening of the closed-looped response because of the negative impact on the phase of the system.
Dead time causes a linear shift that limit the control bandwidth.
At high levels of transport delay, control becomes impossible and thus requires special model mechanism (Ang et al 2005).
The cause and effect of “integrator wind-up”, and methods that have been developed to overcome this problem.
“Integrator wind-up” alsoreferred to as resetwindup defines asituation in the PID feedback controller whereby a big shift inset-point develops. The integral terms accumulate a considerableerror throughout windup and therefore, overshooting as it continuesto increase while the accumulated error is unwound. Overshooting isthe unique problem in this phenomenon. Reset wind-up often occurs asa limitation of physical systems. In the analog error, integralwind-up was more prevalent, however, with contemporary digital andprogrammable control systems, it has become easier to prevent it.With advancement in technology, numerous methods have been developedto overcome this problem:
Setting the controller integral to a preferred point.
Enhancing the set-point in an appropriate gradient.
Deactivating the integral function for the controlled process variable to enter the regulated region.
Stopping the integral term from accumulating above or below pre-set limits
Back-calculating the integral term to contain the control mechanism within reasonable confines.
Ang, K.H., Chong, G.C.Y. and Li,Y. 2005. PID control system analysis, design, and technology. IEEETransactions on Control Systems Technology 13(4):pp. 559-576.