A system is a collection of components that work together or are interconnected and behave in a predetermined way to form a coherent whole. Many times, systems do not perform as we would like. For example, we set the temperature of water to the desired value in the bathroom but someone else’s using water in the kitchen or elsewhere can change the temperature of this water.
Therefore, we need to control the system. There are a lot of methods for controlling the system manually and automatically.
The most well-known automatic control methods are logic control, on-off control, and PID control. In this article, we will talk generally about automatic control systems, but especially the PID control systems.
Logic control is used in routine applications such as washing machines, traffic lamps, and elevators. For example, the washing machine has a sequence of commands to do. Manufacturers code information such as how many minutes the clothes will be kept in water and how many minutes the squeezing process in the washing machine. The washing machine uses a timer for determining which processes must start and It does these sequentially with the data it receives from the timer.
The on-off controller has a simple working method that switches on when the error is positive and switches off when the error is zero or negative. We can use on off controller in the oven. In Figure 5, on-off controller principle is sketched with a time vs temperature plot stacked with the respective time vs Power Output plot.
PID controller plays an important role in the industry. Most industrial systems like flow, temperature, pressure, and level can be controlled by PID. It has 3 parts. These are proportional, integral, and derivative. PID can decrease the error value (difference between the set point and the measured value) by using these parts. These 3 parts contribute to the different abilities of PID.
The proportional control term is PID control's essential and first term. Why proportional control term is most important for PID control? It is because the proportional control term determines the ratio of controller output response to the error signal. For example, if the error term has a magnitude of 8, and a proportional gain of 2, the controller would produce a proportional response of 16. Also increasing the proportional gain will increase the speed of the control system response. However, if the proportional gain is too large, the process variable will begin to oscillate. If proportional gain is raised more, the system will become unstable or oscillate and out of control.
Proportional term is given by P=Kp x error and controller output is Pout=P
PID control's integral term is more significant for a sensitive system like surgeon robots. Why PID control’s integral term is more important for a sensitive system? It is because the integral control term calculates the sum of all of the error values therefore integral response will increase or decrease until the error will be zero. Also a steady-state error must be equal to 0 because the integral term makes the error 0. However high values may be an integral windup so we should limit the controller output. Moreover, if the error is high, the controller output will be high so the system response is changed quickly. On the other hand, if the error is small, the controller output will be small so the system response is changed slowly.
The integral term is given by the I=KI x ∫error and the PI controller output is Pout=P+I.
If integral gain increases, system response gets faster but overshoot will be high. There is a difference between increasing integral and proportional gains.İncreasing proportional gain causes oscillations however integral gain stabilizes on the set-point after a few oscillations.
The derivative control term predicts what next error will happen. For this, it calculates the error's derivative by using past error values. PID control's derivative control term may a little dangerous for a noisy system. If the system feedback signal is noisy or if the control loop rate is slow, the derivative response can make the system unstable. However, if you use carefully PID control's derivative control terms, it will very useful for PID control.
The Derivative term is given by D=KD x(en-en-1) and the PID controller output is Pout=P+I+D.