PID (Proportional-integral-derivative) control is the most common used closed loop control system in industrial automation. It has applications where a variable parameter need to be controlled continuously. In this article we will discuss what is PID control, it’s applications and how it works?
What is PID Control
To drive bike at a constant speed, the driver needs to change the throttle continuously. He/She takes input from speedometer, predict & analyse the error in speed (error) and change throttle accordingly. PID controller works in similar way. Firstly It compares set-point and measured process variable. Predict future error and change the input to reduce error value to zero.
In technical terms a PID controller is a closed loop control system. It senses the output value and changes the input according to error value. Error value is the difference between set-point and measured process variable.
Everyday example of PID control includes car cruise control, quad-copter flight control etc.
Why we Need PID Controller
Whereas PID controller can control a process within very small process set limits. Therefore PID controller are used to control critical process.
For example, to control water temperature with PID controller. Controller will calculate the rate of change of temperature with time, predict the future error value and will change the heater power accordingly.
Proportional response changes the process according to proportional to error value. It can be calculated by multiplying error with a constant Kp (Proportional Gain Constant)Higher the value of proportional gain constant. Higher will be the change in process based on error value. That can make the system unstable as well.
Whereas lower the value of proportional gain constant. Process will not be responsive to error value. Therefore optimized value for proportional gain constant should be used.
Derivative controller have the capability to predict the future behavior of the error. It’s value depends on the rate of change of error value.
Derivative control helps in decrease in process output if process variable is increasing rapidly. Contribution of derivative control in PID controller is very less because it is very sensitive to noise in the system.
PID Controller Response
Lets consider a scenario where we are using PID controller to control the temperature of a liquid. We will try to find how Kp, Ki and Kd values will affect the controlled parameters.
- When only proportional control is used. Controlled parameter value will vary drastically. If we try to achieve accuracy, variation increases.
- If we integral control along with proportional control is used. It helps in achieving required controlled parameter more accurately and variation is also reduced.
- When PID controller is used. Accuracy is improved and variations are reduced. It helps in controlling more quickly.
PID Controller Tuning
PID controller tuning is required to achieve desired performance from control system. Various types of PID controller tuning methods are developed to find optimized values of P, I and D gain. Now a days most of PID controls are optimized by tuning and loop optimizing software.
Trial and Error Method
In this method P, I and D gain values are optimized with trial and error. To achieve this, first integral gain and derivative gain values are kept zero and proportional gain value is adjusted to achieve system oscillating behavior. After that Integral gain value is adjusted to stop oscillation behavior. Once system is stabilized, derivative gain is adjusted to improve response time.
This method is similar to trial and error method but in this oscillation behavior is different. In this method I and D gains are set to zero and P gain value is adjusted to achieve system oscillating behavior. Once oscillation starts, the critical gain Kc and the period of oscillations Pc are noted. The P, I and D are then adjusted as per below table.
To sum up, PID control algorithms are the simplest way to design a robust control system. Tuning and loop optimizing software are used to improve control system performance. We suggest you to read this article on various types of proximity sensors.
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