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: Two Hand Press Safety Control Circuit
Share
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 > PLC Tutorials > Two Hand Press Safety Control Circuit

Two Hand Press Safety Control Circuit

In this article, we will learn how to program a very common function with a safety PLC, which is the two-hand press safety control circuit.

Last updated: March 6, 2023 1:27 pm
Mahmoud Salama
PLC Tutorials
No Comments
Share
8 Min Read
SHARE

In previous articles, we talked about the importance of safety in industrial processes and we mentioned some common examples of safety control circuits like emergency push buttons and safety door interlock switches.

Contents
Problem DescriptionComponents of Two-Hand Press Control CircuitHow does a Two Hand Press control work?PLC Hardware ConfigurationSafety PLC Programming StepsTwo-Hand Press Code simulationConclusion

In this article, we will show how to code another very common safety function with a safety PLC, which is the two-hand press safety control circuit.

Contents:

  • Problem description
  • Components of two hand press control circuit
  • How does it work 
  • Hardware configurations
  • Programming steps
  • Code simulation
  • Conclusion

Problem Description

In order to provide a high level of safety for a worker of a cutting machine a two-hand press safety control circuit is used to ensure the worker’s hands are out of danger while the machine is running.

The operator of the machine must press the two pushbuttons with both hands for the machine to start running, and it will only keep running as long as he is pressing the push buttons, any time he releases one or both hands, the machine will stop immediately.

Two Hand Control Logic using PLC - Safety Circuits

Components of Two-Hand Press Control Circuit

As per safety standards, you need two separate push buttons installed at distance from each other, this distance is also determined in the standards.

You also need an enabling signal to be true for the Two Hand Press function to work, this enable signal can be a safety signal like an emergency condition or a safety door, or it can also be a normal signal like the presence of a piece to be worked on, or it can be a combination of both. 

How does a Two Hand Press control work?

If pushbutton1 and pushbutton2 are activated within a permitted time called the discrepancy time.  _DISCTIME ≤ 500 ms according to standards_ the output signal will be set to 1 when existing ENABLE = 1. 

If the time difference between the activation of pushbutton1 and pushbutton2 is greater than DISCTIME, the machine will not start and the push buttons must be released and reactivated again within the DISCTIME for the machine to run.

The machine will stop immediately as soon as one of the pushbuttons is released or the enable signal is no longer available, ENABLE = 0. Enable signal Q can be reset to 1 only if the other pushbutton has been released, and if both pushbuttons are then reactivated within the discrepancy time when existing ENABLE = 1. See the following animation.

Two Hand Press Animation
GIF 1 – Operation principle of two-hand press control circuit

PLC Hardware Configuration

Here we are using the Siemens TIA portal for the PLC configuration.

First, define your input tags for the two-hand press function; we will have TwoHandPress1 and TwoHandPress2 as tags for our push buttons. See picture 1.

Two Hand Press PLC
Picture 1 – Define the input tags

Depending on the type of push buttons, we will choose the hardware configuration.

We will assume the push buttons are dual channels, so we will use channels 3,11 for TwoHandPress1 and 4, 12 for TwoHandPress2, we will also set the evaluation as 1oo2 for redundancy, see the next picture.

1oo2 evaluation of the input push buttons
Picture 2 – 1oo2 evaluation of the input push buttons

Now, define a new normal tag in the standard input module for the Presence of the piece to be worked on, we will be called it Piece Presence, see picture 3.

Two Hand Press Safety Control Circuit
Picture 3 – Piece Presence tag

Safety PLC Programming Steps

Now, add TWO_H_EN safety instruction, and make it a multi-instance DB, so it would be stored inside the safety function data block and not in a separate data block. See picture 4.

multi instance DB for the safety instruction
Picture 4 – Choose multi-instance DB for the safety instruction

Next, add your inputs, define a new static output TwoHandPress_Output and also set the DISCTIME as you like, the max is 500ms as per standards. We will set it as 500ms. See picture 5.

Two-Hand Safety Controls
Picture 5 – Add your inputs and output to your instruction

Add the TwoHandPress_Output to your motor control, see picture 6.

two-hand operation using PLC
Picture 6 – Motor control network

As you can see from the picture, the motor will not start unless there is no emergency active and the safety door is closed and unless the TwoHandPress_Output is set to true, which will not happen unless the two push buttons are pressed at the same or within 500ms.

Two-Hand Press Code simulation

If only one button is pressed, there will be no output, see picture 7.

PLC Program for 2 Hand Control
Picture 7 – Only one push button is pressed, and No output

If both hands were pressed, but in a time range greater than DISCTIME, also there will be no output. See picture 8.

Two-hand control panel
Picture 8 – Both hands are pressed, but still no output

Now, we will press the two hands at the same time. We still notice that even when the two hands were pressed within the DISCTIME, we still have no output, because ENABLE signal is still false because still no piece presence.

You can see from picture 6 that the ENABLE will only be true if there is no emergency active and the safety door is closed and the piece to be worked on is present inside the machine at its required place. See picture 9.

Plc programming examples two hand press logic
Picture 9 – No piece presence, No output

Now, when the piece is in place, and the two hands are pressed within the allowed time, the motor will start working. See picture 10.

Two Hand Ladder Logic Diagram
Picture 10 – All conditions are met, and the output is ON

At any time if one hand was released, the motor will immediately stop. See picture 11.

Siemens Safety PLC Example Program
Picture 11 – Output is OFF when one hand is released

Download the Two Hand Press TIA Portal Logic and the Safety PLC example code.

Conclusion

Two-hand press is one of the most commonly used safety functions, using a safety PLC made coding and implementation of its functionality fairly easy due to the pre-defined TWO-H-EN instruction, using the safety PLC ensures that all used safety functions are employing all safety standards recommendations.

Unlike the ESTOP and SFDOOR, the TWO-H-EN safety function doesn’t require acknowledgment of the safety condition.

If you liked this article, then please subscribe to our YouTube Channel for Instrumentation, Electrical, PLC, and SCADA video tutorials.

You can also follow us on Facebook and Twitter to receive daily updates.

Read Next:

  • PLC Feedback Monitoring
  • Safety Interlock PLC Program
  • Industrial Automation Solution
  • Automation Project Investment
  • Motor Feedback Fail Logic in PLC
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

Rockwell PLCs for Large Scale Applications
How to Choose the Right Industrial Automation Solution?
Single-Acting Cylinder Control using Proximity Sensor
PLC Programming for Sequential Batch Mixing System
Always ON and OFF Bit using Ladder Logic and Statement List
Hardware Troubleshooting Steps for PLC Automation Systems
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

Omron PLC Online Training Course
Introduction to PLC – Advantages of PLC
Automate Batch Mixing with Repeated Cycles in Omron PLC
How to Operate a Group of Induction Motors using PLC Logic?
Various Communication Protocols in PLC
How Modbus is used in Industrial Networks?
Automatic Packing of Bottles Programming
How to use Converters in PLC

Keep Learning

PID Simulator

PID Simulator Download

SCADA

Top 100+ SCADA Questions

FIFO and LIFO Sequences in PLC

What are FIFO and LIFO Sequences in PLC?

Use of Comments in SCL Language

Use of Comments in SCL Language

DeMorgan's Theorems

DeMorgan’s Theorems using Ladder Diagram

Transistor Output Wiring

PLC Output Types

Preventive Maintenance of Variable Frequency Drive

Preventive Maintenance of Variable Frequency Drive (VFD)

Surge Protection Device

What is a Surge Protection Device? – Principle, Types, Advantages

Learn More

Electrical Machines Objective Questions

Electrical Machines MCQ Series 16

5 Tips to Pass CFSE Exam

5 Tips to Pass CFSE Exam

On-Off Controller example

ON-OFF Controller Principle

PLC Control Panel

Inside the PLC Control Panel: How Much Do You Know?

Electrical Machines Questions and Answers

DC Motors Characteristics Quiz

Renewable Energy Projects

200 Renewable Energy Projects for Students and Engineers

WirelessHART Device Terminals

WirelessHART Network Gateway Device

Electrical Machines Questions and Answers

Induction Motor Phasor Diagram

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?