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In this video, we're going to talk about the PID Controller

and its transformation from a single station device

to what it has evolved into today.

We’re going to explain why PID Controllers are used in industrial processes

instead of simple ON/OFF Controllers.

We’ll illustrate how Controller settings called Proportional, Integral and Derivative

affect different processes under control.

We’ll also provide an overview of the very important activity called Controller Tuning.

Let’s start with a discussion about home temperature control

as it’s familiar to lots of people.

This house has a furnace that distributes heat throughout,

and a wall-mounted controller called a thermostat.

The thermostat has a sensor that measures the house temperature

and compares that measurement to an adjustable setpoint.

If the room temperature is below the setpoint, the furnace is turned ON.

When the room temperature increases above the setpoint, the furnace turns OFF.

This type of control is referred to as ON/OFF or Bang-Bang Control.

Here’s a plot of what the room temperature does over a period of time

as the furnace turns ON and OFF.

As you can see, the temperature is not exactly held

at the setpoint of seventy degrees Fahrenheit,

but cycles above and below the setpoint.

ON/OFF control may be ok for your house,

but it is not ok for industrial processes or motion control.

Let’s look at an example of tank level control to explain why.

The Valve fills the tank as the pump drains it.

If the valve is operated with ON/OFF control,

the water will fluctuate around the 50% setpoint.

For our purpose, let’s say the fluctuation is plus or minus ten percent.

In most industrial applications,

this fluctuation around the setpoint is not acceptable.

OK, well, what if it’s possible to throttle the valve

and place it in any position between ON and OFF?

Now we can move on to talking about a PID Controller.

P stands for Proportional, I stands for Integral, and D stands for Derivative.

Because every process responds differently,

the PID controller determines how much and how quickly correction is applied

by using varying amounts of *Proportional, Integral, and Derivative* action.

Each block contributes a unique signal

that is added together to create the controller output signal.

Let’s look at how a PID Controller fits into a feedback control loop.

The Controller is responsible for ensuring that the Process

remains as close to the desired value as possible regardless of various disruptions.

The controller ****compares the Transmitter Process Variable,

or PV signal, and the Setpoint.

Based on that comparison,

the controller produces an output signal to operate the Final Control Element.

This PID Controller output is capable of operating the Final Control Element

over its entire 100% range.

Most modern PID Controllers are part of a PLC or DCS

and are created in the program control logic using block commands.

Before PLCs came along,

a PID controller was a stand-alone device responsible for controlling one loop.

A control room would have dozens or hundreds of stand-alone controllers

mounted on a panel.

There are still many stand-alone PID controllers

being manufactured and used today.

OK, let’s get back and talk about

what each of the P, I, and D components of the PID controller does.

Remember earlier we said that the PID Controller

is responsible for ensuring that the Process remains

as close to the setpoint as possible regardless of various disruptions.

Let’s refer to the difference between the Process Variable

and the Setpoint as the Error signal.

*The proportional block* creates an output signal proportional

to the magnitude of the Error Signal.

Unfortunately, the closer you get to the setpoint, the less it pushes.

Eventually, the process just runs continuously close to the setpoint,

but not quite there.

That’s when Integral jumps in.

The *integral block* creates an output proportional

to the duration and magnitude of the Error Signal.

The longer the error and the greater the amount, the larger the integral output.

As long as an Error exists, Integral action will continue.

The *derivative block* creates an output signal proportional

to the rate of change of the error signal.

The faster the error changes, the larger the derivative output.

Derivative control looks ahead to see what the error will be in the future

and contributes to the controller output accordingly.

That brings us to a term called Controller Tuning.

We said earlier that every process responds differently

and that the PID controller determines how much and how quickly correction is applied

by adjusting *Proportional, Integral, and Derivative* action.

Controller Tuning involves correctly setting the controller P, I, and D values

for specific process requirements.

Interestingly, the correct settings achieved by Controller Tuning

can differ vastly between processes because of specific requirements.

For example, after the controller has been tuned,

a setpoint bump of one percent in a tank level control

produces a quarter-wave damped response.

This type of response may be suitable in a tank-level process

but could be disastrous in a motion control process.

There are many different manual methods for tuning a controller

that involves observing the process response

after inflicting controller setpoint changes.

One method involves increasing the amount of setpoint change

and repeating the procedure

until the process enters a state of steady-state oscillation.

This method of tuning produces adequate results

but is often impractical in many applications.

For example, how practical is it to force the fluid level in a large tank

to reach a steady-state oscillation?

Most process controllers, PLC, and DCS loop controllers sold today

have Autotuning capability.

The PID controller learns how the process responds to a change in setpoint,

and suggested PID settings.

Regardless of whether the initial PID parameters are derived

from manual or auto-tuning methods,

additional tweaking is often required by seasoned automation professionals

to get the response desired.

That should do it for this video.

If you want to learn more about PID control

you might want to watch our other two videos called

*“What are PID Tuning Parameters?”*

and *“How to Tune a PID Controller.”*

You can find the links to these videos in the description.

Ok,… let's review:

An ON/OFF or Bang-Bang controller

has only two output conditions and switches abruptly between these two conditions.

In a PID Controller,

P stands for Proportional, I stands for Integral, and D stands for Derivative.

The PID Controller is responsible for ensuring that the Process

remains as close to the desired value as possible regardless of various disruptions.

The PID controller determines how much and how quickly correction is applied

by using varying amounts of P, I, and D action.

*The proportional block* creates an output signal

proportional to the magnitude of the Error Signal.

The *integral block* creates an output

proportional to the duration and magnitude of the Error Signal.

The *derivative block* creates an output signal

proportional to the rate of change of the error signal.

Controller Tuning involves correctly setting the controller P, I, and D values

for specific process requirements either manually or automatically.

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