Knowledge is power, and in today's world, it is the key to opening up doors of opportunities and advancement. In this paper, learn more about a PID controller's structure and working. PID controllers are among the most widely used controllers in the industrial process for process control. 95% of the closed-loop operations of the industrial automation sector use PID controllers. Hence, being in the know about PID controllers is of essence.

What is a PID controller?

Before getting into its nitty-gritties, let's first understand what a PID controller really is. PID full form in electrical and electronics is Proportional-Integral-Derivative controller. A PID controller, definition-wise, is the control loop mechanism that "continuously calculates an error value e(t) as the difference between a desired setpoint (SP) and a measured process variable (PV) and applies a correction based on proportional, integral and derivative terms denoted P, I, D respectively hence the name. In other words, this is a versatile feedback compensator structure for the control of temperature, flow, pressure, speed, and so forth process variables. It is easily understandable, quite practical and the most accurate and stable controller. What makes it a simple yet sophisticated device is that it is fundamentally easy enough for any engineer to understand as the root concepts are differentiation and integration but it also performs the complex tasks of capturing the behavior of a system and predicting its future behavior. Working principle Principle of working of a PID controller is that the gains of the proportional, integral and derivative terms can be adjusted or "tuned" separately. From the difference between these three values, a correction factor is calculated and applied to the input given. In order to get the values to meet the current requirement. The three steps into which the work can be divided are: PID controller block diagram Reference or Setpoint Proportional Action - P Integral Action - I Derivative Action - D Plant Operation or Structure Actual Response or Output Feedback Path Summing Points

Function of a PID controller

PID controller works for the purpose of delivering feedback to track a setpoint. An example could be forcing the thermostat to turn on and off according to the preset temperature. It is important to understand that PID controllers are best used in systems with relatively small masses and those showing quick reactions to changes in energy added in the process. This automatically compensates the quantity of energy available or mass to be controlled due to frequent changes in setpoint and hence is recommended for the systems in which the load changes quite frequently. How does a PID controller work? PID controllers work in a closed-loop system, operating the output such that there is zero error between the process variable and setpoint/desired output. Closed-loop system: brief overview First, let's consider how the closed-loop system functions with a view to understanding the working of the PID controller well enough. The output of a PID controller is the control input to the plant and it is equal to the feedback error which is calculated in the time domain by the following equation:

ν(t)=Kpe(t)+Ki∫e(t)dt+Kdede/dt

Here, e(t) is the tracking error, i.e., the difference between the desired output, r, and the actual output, y. The actions taken by the PID controller broadly depend upon this value. On receiving this value, the controller computes both the derivative and the integral of this error w.r.t. time. The control signal, u, is fed to the plant, and the new output, y, is obtained. This new output y is then fed back and compared with the reference to find the new error signal e. The controller takes this new error signal and computes an updated control input. And this procedure continues indefinitely. In the continuous time domain, the transfer function of a PID controller is found by taking the Laplace transform:

Kp +Kis+Kd*s = Kd*s2 + Kp*s + Kis

P, I, and D Terms First, let's understand what the proportional, integral, and derivative terms - that is, Kp, Ki, and Kd - mean. Kp is the proportional gain and hence represents the P term. Increase in Kp increases the control signal for a particular amount of error proportionally. Also, it will decrease the steady-state error but not eliminate it. The term I stands for the integral term Ki. In the event of the build-up of steady error that is sustained, increasing the value of I forces the error to go down. The disadvantage of this action is that it tends to make the system sluggish and oscillatory because when the error signal changes sign, it takes some time for the integrator to readjust accordingly. The D term is the derivative term of the controller, Kd. It allows the controller to anticipate error. As derivative control increases so does the error, that is, the signal can become large if the error begins to slope upward even if the magnitude of the error itself is relatively small.

P-Controller

Proportional or P-controller decreases rise time, increases overshoot and decreases steady-state error. It gives an output proportional to the current error value e(t). It compares desired or set point with actual value or feedback process value. The resultant error is multiplied with a proportional constant to get the output. If error is zero, output will be zero (though practically this is never the case). When used alone the P-controller requires biasing or manual reset. This is because it never reaches the steady-state condition. Though it provides stable operation, it always maintains some value of steady state error however minimal. The response speed is directly proportional to Kp.

I-Controller

Because of the limitation of the P-controller for elimination of a steady-state error, there is some offset between the process variable and setpoint. Now, this is the point where the I-controller actually acts. It integrates the error over the time period until the value of error becomes zero, holds the value to the final control device at which the error becomes zero. When the error values are negative, the amount of integral control decreases inversely. This is done in a parallel manner to dampening response speeds and the stability of the system as well. Response speed is enhanced by a reduction in Ki. For that reason, PI controller finds major application where high-speed response is not considered to be a major fact. 

D-controller

While the I-controller predicts the error, D-controller can forecast the future behavior of error. Its output depends on the derivative of the error about time multiplied by the derivative constant Kd. In the D-controller, the settling time of output is reduced. It also increases the stability of the system because it compensates for the phase lag generated by the I-controller. Derivative gain Kd is directly proportional to the speed of response. Tuning Methods "Tuning" is a terminology used for the process of finding the range of values of the interacting parameters for which optimal performance can be obtained. This, depending on the process, calls for different tuning settings to better suit the various facets of the situation the PID controller is being used in. For example, in a furnace, airflow will vary; high temperatures change fluid density and viscosity steadily, and barometric pressure does not stay the same over a period of time. All of the above must, therefore, be considered when choosing the PID settings - namely, the gain applied to the correction factor known as "reset" and the time used in the integral and derivative calculations called "rate."

Manual Tuning

Manual tuning proceeds by first setting the reset time to maximum and the rate to zero. The gain is then slowly increased until the loop oscillates at constant amplitude. Once this value is achieved, the gain is set to half this value and the reset is adjusted so it corrects for any offset within an acceptable period. The major disadvantage is that this process is too complex, challenging and requires a lot of calculations and effort. Tuning Heuristics Their methods, first described by Ziegler and Nichols in 1942, involve measuring the time delay in response and the time to achieve a new value of the output. Then the period of a steady-state oscillation is established. One of the problems of this method is that it sometimes yields a response considered too aggressive with respect to overshoot and oscillation. It can also be very time-consuming.

Automatic Tuning

Auto-tuning is favored so as to get rid of the shortcomings brought about by the above two methods. DA's all PID controllers have auto-tuning featured in them. In other words, the PID controller "learns" to determine the right PID settings. It can track a disturbance or set point change by monitoring both the amount of delay and the rate at which the change occurs. This can be used to handle imprecision and non-linearity of complex control situations.

Types of PID Controllers

Depending upon the system to be controlled, controllers can be divided into three basic types:

On/Off Controller

The simplest kind of temperature control device is an on/off PID controller. It is a binary device, with no middle state, and toggles its output only when the temperature crosses the setpoint. Limit controllers are a special class of on/off controllers. They function to stop a process if a set value of temperature exceeds. Proportional control was devised to eliminate the cycling inherent in on/off control. The proportioning action decreases average power to the heater while the temperature is approaching the setpoint, and it does this in a way that slows down the heater so that it will not overshoot the setpoint but will approach the setpoint and maintain a stable temperature. This proportioning action can be accomplished by turning the output on and off for short time intervals. This "time proportioning" varies the ratio of "on" time to "off" time to control the temperature. Proportional Controller

Standard PID Controller

This type of PID controller will combine proportional control through integral & derivative control to automatically help the unit to compensate changes within the system. These changes, integral & derivative are stated in time-based units. The terms of PID need to be tuned separately otherwise to a specific system through the method of trial as well as error. The controllers are also commonly known by their reciprocals, RATE & RESET correspondingly. These controllers will provide the most accurate and stable control of the three types of controllers. Standard PID Controller

Applications of PID Controller

PID controllers benefit almost any process control application universally. Thus, they are considered to be in use in several industries and fields. However, the best PID controller application has to be temperature control where the controller makes use of an input of a temperature sensor and its output can be allied to the element such as a fan, heater, and furnace among others. Thus, the same concept can be applied to metal furnaces. In heat treatment for metals, such as in "Ramp and Soak" sequences, precision is required to achieve the right metallurgical properties. Since the metal in the furnace is usually massive, its temperature does not change even when huge heat is applied. The feature makes the PV signal fairly stable, allowing the Derivative period efficiently to correct fault without extreme changes either to FCE or to CO. Drying solvents from painted surfaces Drying or evaporating of solvents from freshly painted surfaces need to be carried out at specific temperatures. Too high temperature conditions may cause damage to substrates while too low conditions may result in product damage and poor finishing. The same goes for the curing of rubber or leather. pH control Since some processes require the pH values to be strictly within the range, PID controllers can also be used for this kind of application. The behavior may change depending on the existing operating range. The dynamics of pH are generally slow as the acidity increases. Baking The proper temperatures must be provided in the industrial oven so that the required catalysts take their action and the required reactions take place accordingly. Automatic Machine Control PID controllers also play a major role in the automatic machine control system. General processes they are used in include production counting, cutting, etc. Automobile Industry PID controllers see a great demand in the automobile industry and thus are widely used, for example, car speed measurement, automatic driving or self-driving, etc. MPPT Charge controller The current and the operating voltage of the photovoltaic cells would vary constantly according to weather conditions. It is, therefore, of grave importance for an efficient photovoltaic system to trace the highest power point constantly. The Maximum Power Point Tracking could be done with the help of a PID controller where fixed voltage and current points were given to the PID as inputs. Power Electronics PID controllers find their prime application in converters, which is one of the major applications of power electronics. Other than that, the usage can also be seen in inverters, voltage stabilizers, etc. Other than that, the list goes on with plastic processing & testing machines, scientific laboratory and testing equipment's like BOD, COD, Incubator, sterilizer's, etc.

Why investment in PID controllers?

Advantages

P Controller stabilizes the gain but produces a constant steady-state error. I Controller reduces or eliminates the steady-state error. D Controller reduces the rate of change of error. PID controller provides very high efficiency, steady-state output than normal on/off controllers. As the PID controller works in three modes, error minimizing and data validation are easily possible.

Disadvantages

Some PID controlling systems are complex in design and expensive The only other disadvantage is the tuning methodology These are the disadvantages of PID controllers found generally in the market. DA, at all times, tries to deliver innovative and affordable and user-friendly products for various industrial applications. Having firstly introduced PID temperature controllers in 2003, DA is now a pioneer in PID-based technology, with solutions perfected over the years. The main advantage of our range of PID controllers is that they come inbuilt with modifications to better serve the exact situation they work in. Let us take an example of the plastics industry, where we have PID controllers that have built-in ampere indication, so there is no need for separate ampere meters. Other strong points in our controllers are that we have a wide range available with the purpose of countering PID controllers' downside, which can be very expensive. We have economically viable models and also premium ones with additional features. These controllers will provide sophisticated yet minimalistic designs, affordable prices, and the auto/self-tuning methodology, making them a premier choice when choosing where to buy from.