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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.

  • Were going to explain why PID Controllers are used in industrial processes

  • instead of simple ON/OFF Controllers.

  • Well illustrate how Controller settings called Proportional, Integral and Derivative

  • affect different processes under control.

  • Well 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.

  • Want to learn PLC programming in an easy-to-understand format?

  • and take your career to the next level?

  • Head on over to realpars.com

This video is brought to you by us,

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B1 中級 美國腔

PID控制(PID Controller Explained)

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    詹允崴 發佈於 2024 年 03 月 26 日
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