Earlier we have discussed in brief Sequential motor control and now we will discuss another PLC ladder logic example of sequential process control in an automated plant.
In a plant, a large number of processes occur at a time. These processes are predefined as per their requirements and need to be controlled.
In this example, we are going to see how to draw a ladder logic diagram for sequential process control in an automated plant using PLC. One should note that these are the basic examples of PLC ladder logic examples for PLC ladder logic practices.
Table of Contents
Problem Statement
Draw a ladder logic diagram for sequential process control in an automated plant using PLC.
Let us consider that there are 3 process lines in an automated plant. Each line has 2 motors and 1 indicator lamp. Start and stop the system using push buttons and use switches for process control of each line.
Switch 1 will control motors M1 and M2 and an indicator lamp L1, switch 2 will control motors M3 and M4 and an indicator lamp L2, switch 3 will control motors M5 and M6 and an indicator lamp L3.
List of Input and Output Tags
In this example, we have used two push buttons, three switches, six motors, and three indicator lamps. Along with these, memory is used to store the system’s ON/OFF status.
The push buttons and switches indicate the inputs and are kept normally open. Inputs are denoted by the letter I.
The motors and indicator lamps indicate the outputs. Outputs are denoted by the letter Q.
| Address | Description | Type |
| I0.0 | START PUSH BUTTON | INPUT |
| I0.1 | STOP PUSH BUTTON | INPUT |
| I0.2 | SWITCH 1 | INPUT |
| I0.3 | SWITCH 2 | INPUT |
| I0.4 | SWITCH 3 | INPUT |
| M0.0 | SYSTEM ON/OFF STATUS | MEMORY |
| Q0.0 | MOTOR M1 | OUTPUT |
| Q0.1 | MOTOR M2 | OUTPUT |
| Q0.2 | INDICATOR LAMP L1 | OUTPUT |
| Q0.3 | MOTOR M3 | OUTPUT |
| Q0.4 | MOTOR M4 | OUTPUT |
| Q0.5 | INDICATOR LAMP L2 | OUTPUT |
| Q0.6 | MOTOR M5 | OUTPUT |
| Q0.7 | MOTOR M6 | OUTPUT |
| Q1.0 | INDICATOR LAMP L3 | OUTPUT |
Sequential Process Control Ladder Logic Diagram
In this example, we have used Siemens S7-1200 PLC and Siemens TIA portal for programming a ladder logic diagram. The PLC we selected has a work memory of 100 kb, 24 V DC power supply with 24V DC 14 digital inputs, 10 digital outputs, and 2 analog inputs on board.
For a better explanation, we have divided the program into 4 networks. The function of each network is discussed below.
Network 1
Network 1 shows the system’s start and stop function. The push buttons are used to start and stop the system. Output is stored in a memory M0.0 and is latched to the input.
Network 2
Network 2 consists of a switch and 3 outputs i.e. Motor M1, M2, and an indicator lamp L1. System ON/OFF memory is in series with switch 1 and outputs M1, M2, and L1 are connected in parallel to each other.
When switch 1 is pressed, Motor M1, Motor M2, and indicator lamp L1 turn ON.
Network 3
The ladder logic diagram of network 3 is similar to network 2. Only the difference is the switch, motors, and the indicator lamp is replaced by the next numbers.
Network 3 consists of a switch and 3 outputs i.e. Motor M3, M4, and an indicator lamp L2. System ON/OFF memory is in series with switch 2 and outputs M3, M4, and L2 are connected in parallel to each other.
When switch 2 is pressed, Motor M3, Motor M4, and indicator lamp L2 turn ON.
Network 4
The ladder logic diagram of network 4 is also similar to network 2. Only the difference is the switch, motors, and the indicator lamp is replaced by the next numbers.
Network 4 consists of a switch and 3 outputs i.e. Motor M5, M6, and an indicator lamp L3. System ON/OFF memory is in series with switch 3 and outputs M5, M6, and L3 are connected in parallel to each other.
When switch 3 is pressed, Motor M5, Motor M6, and indicator lamp L6 turn ON.
The last 3 networks can be combined to form a single network that shows the automatic lamp control of the warehouse. The ladder logic diagram of the same is shown in the figure below.
The full program cannot be posted in an image. Hence, it is given in the pdf below.
Download sequential process control of an automated plant plc program, plc programming examples, and solutions pdf.
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