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
  • Request
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 Controller Principle
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
  • Request
Follow US
All rights reserved. Reproduction in whole or in part without written permission is prohibited.
Inst Tools > Blog > Control Systems > ON-OFF Controller Principle

ON-OFF Controller Principle

Last updated: November 27, 2018 7:26 pm
Editorial Staff
Control Systems
No Comments
Share
6 Min Read
SHARE

On-Off control is the simplest form of feedback control. An on-off controller simply drives the manipulated variable from fully closed to fully open depending on the position of the controlled variable relative to the setpoint. A common example of on-off control is the temperature control in a domestic heating system. When the temperature is below the thermostat setpoint the heating system is switched on and when the temperature is above the setpoint the heating switches off.

There is, however, a bit of subtlety applied in practical on-off systems. If the heating switches on and off the instant the measured temperature crossed the setpoint then the system would chatter – repeatedly switch on and off at very high frequency. If this happened the boiler wouldn’t last very long! To avoid chattering, practical on-off controllers usually have a deadband around the setpoint. When the measured value lies within this dead-band the controller does nothing – its only when the value moves outside that action is taken. The effect of this is to introduce continuous oscillation in the value of the controlled variable – the large the dead-band the higher the amplitude and lower the frequency.

Example:

On-Off Controller example

The valve in the inflow line to the system is an electrically operated solenoid valve. (Remember an electrically operated solenoid valve has only two operating positions – fully open or fully closed.) Assume that under initial conditions with a demand on the system the level will start to fall and V1 will have to be opened to provide an inflow. This can easily be achieved by mounting a differential pressure switch, P1 at the bottom of the tank to operate when the level falls to L1. When the level is at L1 the liquid will be height h1 above switch.

The pressure at the switch will be P1 = ρgh1.

ρ – the mass density of the liquid
g – the acceleration due to gravity
h1 – the height of the liquid

The resulting switch closure can energize the solenoid valve V1 causing an inflow to the tank. Assuming the valve is correctly sized, this will cause a rise in the level back towards the setpoint.

In order to arrest the rise in level the built in differential feature of the switch can be employed to de-energize the solenoid valve when level L2 is reached. This system will achieve a mean level in the tank about the desired setpoint. This method is known as ON/OFF control. Clearly it is impossible to maintain the system at the setpoint since there must be a difference in the operating levels L1 and L2 as the valve can only be energized or de-energized. It is often counter productive to try to reduce the differential between L1 and L2 to too small a value as this will result in excessive cycling, and hence wear, of the valve. Usual practice is to control with a deadband about the setpoint as shown in Below Figure.

Typical On-Off controller Response

The sinusoidal cycling is typical of on/off control. on/off control can be used to advantage on a sluggish system, i.e., where the periodic time is large. Typical uses in electric heater controls. If fine control is required a simple on/off control system is inadequate.

Following are a few aspects of On-Off Control that you should keep in mind when considering it for commercial application:

  • An Open or Shut Case

As its name implies, On-Off Control assigns the Controller Output (CO) to one of two positions such that the final control element (FCE) is either fully open or fully closed. Unlike intermediate value or PID control, there is no in between. Most industrial processes require greater latitude when it comes to adjusting the CO’s position.

  • Ups and Downs

On-Off Control can result in excessive variability as the controller has so few options for maintaining Set Point. A process equipped with On-Off Control will constantly overshoot its Set Point and cycle as a result. The work demanded of the FCE regularly accelerates the time to failure and increases maintenance costs.

  • Setting Boundaries

Deadband is a range of operation around the Set Point and within which the controller’s action will not change. On-Off Control with Deadband establishes upper and lower boundaries that are acceptable to the control loop’s operation. While Deadband’s “cushion” reduces wear on the FCE, variability remains within the process which can present challenges for other downstream processes.

On off controller deadband

Summary :

  • On/off control – control signal is either 0% or 100%.
  • Control at setpoint not achievable, a deadband must be incorporated.
  • Useful for large, sluggish systems particularly those incorporating electric heaters.
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

Introduction to Modbus – Reading and Writing Data
Basics of 4 to 20 mA analog Signals
Instrumentation Cyber Security Glossary
How Integral Controller Reduces offset error ?
Basic Concepts of the Safety Relay
Proportional Integral Derivative Controllers – PID MCQ
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
210kSubscribersSubscribe
38kFollowersFollow

Categories

Explore More

Learn about PLC, DCS, RTU, SCADA, and PAC
Setpoints and Alarms in Control System
How Process Control Loop Works
What is Safety Control System ?
Two Element Drum Level Control System
Hardwired I/O and Serial I/O – Differences Explained
Industrial Automation in the Mining Industry
Liquid Flow Control Loop Controller Action

Keep Learning

Flexible Input Output Modules

Flexible Input Output Modules

Alarms to Annunciator

Basics of Alarms and Trips

Voting System in PLC

Voting Concept in Package Safety System

PID Controller Types

PID Controllers : Parallel, Ideal & Series

Level Loop Troubleshooting

Identify Faults in PID Control Loop

Instrumentation Engineer in Process Plant Project

Instrumentation Engineer in Process Plant Project

What is Ground

What is Ground, and importance of a Grounding System?

What is Open Telemetry?

What is Open Telemetry? – Principles and Benefits

Learn More

Beam-Detectors-Working-Principle-2

Beam Detectors Working Principle Animation

annunciator-alarm-simulator

Alarm Simulator for Annunciator Logic

Electronic Pressure Sensor with Bourdon Tube and LVDT

Electronic Pressure Sensor with Bourdon Tube and LVDT

Electric Motor Winding Temperature Sensors

Electric Motor Winding Temperature Sensors

Step and Touch Potential

What Is Step and Touch Potential and Reducing Resistance To Ground?

TP Timer in CodeSys

Timers in Codesys – Timer ON, Timer OFF, Pulse Timer

Measurement and Instrumentation Objective Questions

Measurements & Instrumentation Quiz – Set 1

Automatic Lamp Control in Storage Facility

PLC Program for Automatic Lamp Control in Storage Facility

Menu

  • About
  • Privacy Policy
  • Copyright

Quick Links

  • Learn PLC
  • Helping Hand
  • Part Time Job

YouTube Subscribe

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?