PID, the term stands for proportional integral derivative. A PID controller typically makes use of a closed-loop feedback control device that helps to control and regulate a variety of process variables including pressure, temperature, speed, flow, etc. A PID controller is used in factories and industries at a relatively large scale to control various activities. The first PID controller was invented by an American inventor and entrepreneur, Elmer Ambrose Sperry Sr. in 1911. Automatic PID controllers were introduced for industrial and commercial use in the mid of 1950s.
Structure and Block Diagram of a PID Controller
The structure of a PID controller typically comprises three controls, namely proportional, integral, and derivative control. These three controllers of a PID controller are used to generate a combined output that is used to form a control strategy that further manages the operation of a device and manipulates the physical parameters such as temperature, pressure, etc. The PID block of the controller transfers the output to the process block. The process or the plant block contains the main control devices such as actuators, controller valves, etc. A feedback block takes the current input signal and compares it with the reference signal or the setpoint to produce an error signal. This error signal is then fed to the PID algorithm. The combined response or the controlled signal is then fed to the control devices installed in the plant. A PID controller does not necessarily consist of both the three control variables. The combination of PI and PD controllers is quite often used to control various processes at an industry or a plant.
Working of a PID Controller
A PID controller typically works on the principle of adjusting or tuning the proportional, integral, or derivative terms. The difference between these variables is evaluated and part of the input signal is fed back to the device as a feedback control signal. This value is generally known as the correction factor. For instance, if an air conditioner does not generate enough cool air, then the controller evaluates the correction factor, i.e., the difference between the room temperature and the desired temperature and feeds it back to the device. The internal mechanism of the device then reduces the temperature until a minimum difference is obtained between the desired and the resultant value. Similarly, in the case of an oven if sufficient heat is not generated the temperature is adjusted and the heat is increased. There are generally three steps to tune the physical paraments such as temperature, flow, and pressure as per the requirement. These include proportional tuning, integral tuning, and derivative tuning.
This type of tuning involves adjusting or correcting the desired signal proportional to the current error signal e(t). The error signal is the difference between the setpoint or the target value and the actual value. The current error signal is further multiplied with the pre-calculated proportional constant to obtain the output. If the error value is equal to zero, the controller output value is also zero. Here, the desired value rarely gets achieved as it never reaches the steady-state condition. The control devices that make use of proportional tuning or the p controllers, provide a stable operation but tend to maintain a steady-state error. The proportional constant is directly comparable to the speed of response of the device. This means that when the proportional constant Kc is increased, the speed of response increases accordingly. If this type of controller is not used in combination with the other type of controller, it requires manual reset and proper biasing.
The ‘P’ controllers or the controllers that make use of proportional tuning are susceptible to suffer a limitation that there always exists a steady-state error, which is an offset between the setpoint and the feedback process variable. This type of limitation can be eliminated by employing the ‘I’ controllers. I-controllers make use of integral tuning to generate the output signal to manipulate various physical parameters. This type of tuning tends to integrate the error signal with respect to time until a minimum or zero value of the error is achieved. In case of a negative error value, the integral control tends to decrease the output. The speed of response and the stability of the system; however, get affected during the process. A PI controller is susceptible to undergo integral wind up conditions due to the non-linearities present in the plant. Under this condition, the integral output continues to increase even at a zero error state. To overcome this disadvantage, the ‘I’ controller output is limited to a certain range. The integral gain of the ‘I’ controllers, usually denoted by Ki, is indirectly proportional to the speed of response of the system. This means that if the integral gain is increased, the speed of response of the system gets reduced by a comparable value. Similarly, the integral gain is directly proportional to the steady-state error. This means that the steady-state error gets reduced by reducing the integral gain of the controller and vice versa. PI controllers are typically used in applications where the high response time of the system is not primarily important.
The ‘D’ controllers or derivative tuning is normally used to overcome certain limitations of the ‘I’ tuning. For instance, an ‘I’ controller does not have the ability to predict the future behaviour of the error value. Such devices tend to act normal even when the set fixed point or the desired value is changed. The output of the derivative controllers is equal to the rate of change of the error signal with respect to time multiplied by the derivative constant. The derivative tuning helps in improving the system response and decreasing the settling time of the output. The derivative gain Kd is directly proportional to the responding speed of the system. This means that an increase in the derivative gain of the controller tends to increase the response time of the system.
Types of a PID Controller
On the basis of the type of control system used, the PID controllers can be broadly classified into three categories, namely ON/OFF, proportional, and standard type controllers.
The on/off control is one of the simplest types of devices used to adjust or tune the temperature of a particular area. Here, the controller tends to switch the device to the ON state, once the output signal crosses a particular desired fixed point. One of the most commonly used on/off control devices is a limit controller. A limit controller makes use of a latching relay for its operation. The relay is required to be operated manually. The temperature or the other variable parameter is set to a particular value. The device tends to maintain the off state if the output or the resultant signal falls behind the target value. Once the threshold or the set point is achieved, the device gets turned on.
Proportional control is basically designed to overcome and remove the cycling limitation of an on/off control. This type of control prohibits the controller to exceed a particular pre-decided threshold value. The device tends to achieve the desired point to maintain the physical parameter value at a constant level; however, it does not exceed the fixed threshold point. For instance, a heater that makes use of a proportional control tends to maintain the temperature at a constant value and avoids overheating. When the temperature of the heater tries to surpass the threshold value, the controller gets activated. The supply gets cut off and the temperature of the device is reduced.
Standard Type PID Controller
The standard type PID controllers make use of a combination of proportional control with integral and derivative control. The merged control assists the processing unit and helps the device to automatically induce manipulations in the system and modify the variables. The integral and derivative modifications, in this case, are expressed in terms of time-based units. These controllers are also referred through their reciprocals, RATE & RESET, respectively. These types of controllers provide a more accurate and stable control as compared to the other controllers.
Real-Time PID Controllers
The real-time PID controllers are most commonly used for commercial and practical applications. The control devices that make use of real-time PID controllers, provide different choices for the SOLO and the twin loop control. The stand-alone type of PID controllers tends to provide various configurations of the fixed point to produce a number of autonomous alarms. These types of standalone controllers mainly include the PID controllers provided by Honeywell, autotune controllers from OMEGA, Siemens, and ABB controllers, temperature controllers from Yokogawa, and various others. The arrangement of the PID blocks is usually done with the help of PLCs or PACs. Here, each PAC or PLC includes a PID block within the algorithm or the software programming. In many industrial applications, PLCs are used as PID controllers.
Applications of a PID Controller
A PID controller device is typically used in industries to control or adjust complex physical parameters of the environment such as temperature and pressure. It can also be used to maintain these parameters at a constant value. Some of the most common uses of a PID controller are listed below:
1. A PID controller is an integral part of the temperature control system of an industry. It is typically used to control the temperature of the premises. Here, the input of the PID controller is obtained from a temperature sensor and the output is fed to a fan or a heater. The fan or the heater; therefore, acts as a control element. The speed of the fan or the temperature of the heater gets adjusted according to the feedback signal.
2. Most manufacturing industries make use of huge furnaces to melt and heat different elements. The temperature of such furnaces is required to be monitored periodically. Also, one must be able to control and vary the temperature to maintain the temperature of the furnace at the desired constant value. For this purpose generally, a PID controller is employed.
3. A PID controller is mostly used as a maximum power point tracking charge controller or MPPT charge controller. The V-I characteristics of a photovoltaic cell generally depend on two parameters, namely the irradiance and the range of the temperature. This is the reason why the values of current and the operating voltage is frequently required to be varied as per the weather conditions. The tracking of the highest power point of a photovoltaic cell is a complicated task. A PID controller is usually employed to perform the task of maintaining the stable value of current and voltage by evaluating the MPPT and giving a constant value of current and voltage for every change in weather.
4. A PID controller is most commonly used in power converters.
5. Various research, development, and testing organizations such as chemical, pharmaceutical, and manufacturing industries make use of the PID controllers to maintain the humidity and temperature of a particular area at a constant level.
6. PID controllers are also used in pH, flow, and speed control devices.
Advantages of a PID Controller
A PID controller is advantageous in numerous ways such as:
1. Most of the modern devices equipped with PID controllers are inexpensive.
2. The tuning and operation of PID controllers do not require much experience. Hence, an unskilled person can also operate such devices.
3. PID controllers are process independent.
4. They provide precise control of the setpoint, thereby allowing the user to fix the temperature or pressure value to a particular constant value.
5. PID controllers do not require frequent maintenance.
6. The devices and systems equipped with PID controllers have significantly improved responsiveness.
Disadvantages of a PID Controller
Certain limitations or disadvantages of a PID controller include:
1. PID controllers can be unstable if they are not tuned properly.
2. These types of controllers are susceptible to derivative noise amplification.
3. The controller may sustain oscillations around the operating point and cause problems.
4. The continuous variation in the load tends to affect the dynamic performance of the system.
5. Repeated tuning of the controller may reduce the life span and cause wear and tear.