Latching and unlatching Circuit.

PLC Program for Latching and Unlatching Circuit

Problem Description:

In many basic conveying or water-filling systems, the operator manually controls a water pump to fill a tank. This requires the operator to:

• Start the pump manually and wait while the tank fills.
• Monitor the tank level continuously.
• Stop the pump manually when the tank reaches the High-Level sensor.

This approach is inefficient and prone to human error.

 

Automation Requirement:

The system should:

• Allow the operator to start the pump manually with a push button.
• Keep the pump latched ON (i.e., running continuously) after the button is released.
• Automatically stop the pump when the tank reaches the High-Level (sensor).
• Prevent the pump from restarting until the operator initiates it again.

 

Example Scenario:

Let’s consider a manual water conveying system where:

• A START pushbutton is used to begin pumping.
• A High-Level float sensor detects when the tank is full.
• A STOP condition is automatically triggered when the tank reaches this level.

Problem Diagram

Problem Solution

In this application, we consider a storage tank that is filled using a water pump. The system is equipped with the following components:

• One high-level sensor to detect when the tank is full.
• A control panel with:
• A START push button (PB) to manually start the pump.
• A STOP push button (PB) to stop the pump manually if needed.

The operation is as follows:

• When the operator presses the START PB, the water pump is energized to begin filling the tank.
• The pump continues to run until the high-level sensor detects that the tank is full.
• Upon detecting the high level, the sensor sends a signal to automatically stop the pump.
• This sequence is implemented using SET and RESET (Latch/Unlatch) instructions in the PLC to maintain the pump state between operator actions and sensor inputs.

Although the actual system includes a manual discharge valve for emptying the tank, it is not included in the logic control for this program.

Note: This same logic can also be implemented using relays in conventional control panels.

 

Program

Here is PLC program for Latching and unlatched circuit for output.

 

Network 1

 

Network 2

List of Inputs and Outputs

Input

Signal Description	Address
START Push Button	I0.0
STOP Push Button	I0.1
High-Level Sensor	I0.2
Low-Level Sensor	I0.3

 

Output

Signal Description	Address
Water Pump	Q0.0

 

Program Description

For this application, we are using a Siemens S7-1200 PLC programmed with TIA Portal software. The same logic can also be implemented using conventional relay-based circuits.

This program demonstrates a latching and unlatching circuit, commonly used in fluid control applications.

Network 1: Latching Logic (SET Instruction)

The START push button (I0.0) is used to start the water pump (Q0.0).

When pressed, it activates the SET instruction, latching the pump output (Q0.0).

A normally open contact of the Low-Level sensor (I0.3) is placed in series with the START PB to ensure the pump only starts if the tank is not already full.

A normally open contact of the Water Pump output (Q0.0) is also included to maintain the latched condition once the START PB is released.

 

Network 2: Unlatching Logic (RESET Instruction)

The High-Level sensor (I0.2) is used to stop the pump automatically when the tank is full.

A normally open contact of I0.2 is used with the RESET instruction to unlatch the output (Q0.0).

For manual stopping, the STOP push button (I0.1) is connected in parallel with the High-Level sensor input, providing an alternative way to reset the pump.

Runtime Test Cases

Inputs Condition	Output	Physical Result
I0.0 = 1 & I0.3 = 1	Q0.0 = 1	Water Pump ON
I0.1 = 1	Q0.0 = 0	Water Pump OFF (Manually)
I0.2 = 1	Q0.0 = 0	Water Pump OFF (Auto-Stop)