Inst ToolsInst ToolsInst Tools
  • Courses
  • Automation
    • PLC
    • Control System
    • Safety System
    • Communication
    • Fire & Gas System
  • Instrumentation
    • Design
    • Pressure
    • Temperature
    • Flow
    • Level
    • Vibration
    • Analyzer
    • Control Valve
    • Switch
    • Calibration
    • Erection & Commissioning
  • Interview
    • Instrumentation
    • Electrical
    • Electronics
    • Practical
  • Q&A
    • Instrumentation
    • Control System
    • Electrical
    • Electronics
    • Analog Electronics
    • Digital Electronics
    • Power Electronics
    • Microprocessor
Search
  • Books
  • Software
  • Projects
  • Process
  • Tools
  • Basics
  • Formula
  • Power Plant
  • Root Cause Analysis
  • Electrical Basics
  • Animation
  • Standards
  • 4-20 mA Course
  • Siemens PLC Course
Reading: On/off Control Theory
Share
Notification Show More
Font ResizerAa
Inst ToolsInst Tools
Font ResizerAa
  • Courses
  • Design
  • PLC
  • Interview
  • Control System
Search
  • Courses
  • Automation
    • PLC
    • Control System
    • Safety System
    • Communication
    • Fire & Gas System
  • Instrumentation
    • Design
    • Pressure
    • Temperature
    • Flow
    • Level
    • Vibration
    • Analyzer
    • Control Valve
    • Switch
    • Calibration
    • Erection & Commissioning
  • Interview
    • Instrumentation
    • Electrical
    • Electronics
    • Practical
  • Q&A
    • Instrumentation
    • Control System
    • Electrical
    • Electronics
    • Analog Electronics
    • Digital Electronics
    • Power Electronics
    • Microprocessor
Follow US
All rights reserved. Reproduction in whole or in part without written permission is prohibited.
Inst Tools > Blog > Control Systems > On/off Control Theory

On/off Control Theory

Last updated: November 10, 2019 2:23 pm
Editorial Staff
Control Systems
No Comments
Share
4 Min Read
SHARE

Once while working as an instrument technician in an aluminum foundry, a mechanic asked me what it was that I did.

I began to explain my job, which was essentially to calibrate, maintain, troubleshoot, document, and modify (as needed) all automatic control systems in the facility.

The mechanic seemed puzzled as I explained the task of “tuning” loop controllers, especially those controllers used to maintain the temperature of large, gas-fired industrial furnaces holding many tons of molten metal. “Why does a controller have to be ‘tuned’?” he asked.

“All a controller does is turn the burner on when the metal’s too cold, and turn it off when it becomes too hot!”

In its most basic form, the mechanic’s/engineers’s assessment of the control system was correct: to turn the burner on when the process variable (molten metal temperature) drops below setpoint, and turn it off when it rises above setpoint.

However, the actual algorithm is much more complex than that, finely adjusting the burner intensity according to the amount of error between PV and SP, the amount of time the error has accumulated, and the rate-of-change of the error over time.

In his casual observation of the furnace controllers, though, he had noticed nothing more than the full-on/full-off action of the controller.

The technical term for a control algorithm that merely checks for the process variable exceeding or falling below setpoint is on/off control. In colloquial terms, it is known as bang-bang control, since the manipulated variable output of the controller rapidly switches between fully “on” and fully “off” with no intermediate state.

Control systems this crude usually provide very imprecise control of the process variable. Consider our example of the shell-and-tube heat exchanger, if we were to implement simple on/off control :

Note : To be precise, this form of on/off control is known as differential gap because there are two setpoints with a gap in between. While on/off control is possible with a single setpoint (FCE on when below setpoint and off when above), it is usually not practical due to the frequent cycling of the final control element.

ON OFF Control Principle

As you can see, the degree of control is rather poor. The process variable “cycles” between the upper and lower setpoints (USP and LSP) without ever stabilizing at the setpoint, because that would require the steam valve to be position somewhere between fully closed and fully open.

This simple control algorithm may be adequate for temperature control in a house, but not for a sensitive chemical process! Can you imagine what it would be like if an automobile’s cruise control system relied on this algorithm?

Not only is the lack of precision a problem, but the frequent cycling of the final control element may contribute to premature failure due to mechanical wear.

In the heat exchanger scenario, thermal cycling (hot-cold-hot-cold) will cause metal fatigue in the tubes, resulting in a shortened service life.

Furthermore, every excursion of the process variable above setpoint is wasted energy, because the process fluid is being heated to a greater temperature than what is necessary.

Clearly, the only practical answer to this dilemma is a control algorithm able to proportion the final control element rather than just operate it at zero or full effect (the control valve fully closed or fully open). This, in its simplest form, is called proportional control.

Don't Miss Our Updates
Be the first to get exclusive content straight to your email.
We promise not to spam you. You can unsubscribe at any time.
Invalid email address
You've successfully subscribed !

Continue Reading

What is Instrumentation Control System ?
Does Really a Safety Barrier is Important? – PLC DCS Systems
Industrial Automation Pre-Engineering Design Documents – Project & Process
Zener Diode Barrier Principle
4-20 mA Loop Splitter
What is NAMUR OPEN ARCHITECTURE?
Share This Article
Facebook Whatsapp Whatsapp LinkedIn Copy Link
Share
Leave a Comment

Leave a Reply Cancel reply

Your email address will not be published. Required fields are marked *

Stay Connected

128.3kFollowersLike
69.1kFollowersFollow
208kSubscribersSubscribe
38kFollowersFollow

Categories

Explore More

I&C Engineer Roles & Responsibilities – Instrumentation Design
What is ERP in Industrial Automation? – Enterprise Resource Planning
How to Solve Electrical Ground Loop Problems?
Grounding or Earthing Scheme in DCS or PLC Systems
Ziegler-Nichols Closed-Loop Method (Ultimate Gain)
How does a Redundant Controller comes in line when the Main Controller fails?
What is a Transducer ?
Technical Evaluation for DCS and ESD – Distributed Control System

Keep Learning

Create a Project in LabVIEW

How to Create a Project in LabVIEW?

Alarms to Annunciator

Basics of Alarms and Trips

VFD Commissioning and Testing Procedure

VFD Commissioning and Testing Procedure (Variable Frequency Drive)

Demo of a SCADA Software – Level Control System

Ziegler-Nichols closed-loop Tuning - 1

Heuristic PID Tuning Method

Problem in Water Level Control System

Problem in Water Level Control System

How to Train Your Maintenance Team for Industrial Automation Systems

How to Train Your Maintenance Team for Industrial Automation Systems?

Compare DCS, PLC, RTU

Difference between DCS, PLC, and RTU ?

Learn More

quadruple block valve

What is Safety Instrumented Functions ?

AI Projects for Engineering Students - Artificial Intelligence

AI Projects for Engineering Students – Artificial Intelligence

baffle-and-nozzle assembly

Pneumatic Sensing Elements

Sensors and Transducers Classification

Sensors and Transducers Classification

DeviceNet

What is DeviceNet?

S7 1200 PLC Program

Siemens S7 1200 PLC configuration in TIA Portal

Three Phase To Single Phase Wiring Connections

Single Phase Power Wiring Schemes

Switch High level alarm occurs

Basics of Switches & its applications

Follow US
All rights reserved. Reproduction in whole or in part without written permission is prohibited.
Welcome Back!

Sign in to your account

Username or Email Address
Password

Lost your password?