PLC Conditional Control Logic

This is PLC Program for Conditional Logic Circuit

Problem Description

• In Industry or plants, there are lots of gear box systems used for different machines/motors.
• For smooth operation of gear box motors, it needs to be lubricated every time because good maintenance work can extend gear box life.
• But problem is that operators often do mistake during machine operation because in every gearbox motor mechanism we need to start lubrication first and then main gear box mechanism should be start.
• So we have to implement a logic to make sure things are properly controlled from a PLC system.

Problem Diagram

Note : For easy discussions, Local/Remote, or any other permissive interlocks are not considered in this example.

Problem Solution

• Here we solve this problem by using simple conditional logic example, in this example there is one gear box motor and we need to provide lubrication before starting it.
• So for lubrication we have lubrication motor (also called as Oil Pump or Auxiliary Lube Oil Pump) and it will provide lubrication oil to the main motor or gear box motor.
• Also we will provide interlock system, so operator cannot start/operate Main motor directly without using proper lubrication otherwise main motor may over heat and it may damage after some runs without proper care.
• Operator has to switch ON oil pump first and then only he can able to operate the main motor.
• By using this logic, we can take care of gear box motor for long time run with proper lubrication.
• Operators start/stop oil pump by using START and STOP push buttons of oil pump.
• Both Oil Pump and Main Motor have separate individual START & STOP push buttons as shown in above diagram.

PLC Inputs List

• Oil Pump StART PB : I0.0
• Oil Pump STOP PB : I0.1
• Main Motor START PB : I0.2
• Main Motor STOP PB : I0.3

PLC Outputs List

• Oil Pump Motor : Q0.0
• Main Motor : Q0.1

PLC Ladder diagram for conditional Control Circuit

Ladder Logic Description

• In this application, we used Siemens S7-1200 PLC and TIA Portal Software for programming. We can also design this logic with relay circuit.
• This circuit is also known as conditional control circuit because second sequence is depended on first condition.
• We will write logic for oil pump in Network 1. Here we will take NO contact of oil pump START PB (I0.0) and also we have to consider one NO contact of oil pump (Q.0) coil for latching START command.
• Put NC contact of oil pump stop PB (I0.1) in series for unlatching the circuit by pressing oil pump STOP PB (I0.1), so operator can stop the oil pump (Q0.0).
• Now write the logic for main motor in Network 2. Here we will take NO contact of main motor START PB (I0.2) and also take one more NO contact of the main motor coil for latching the main motor (Q0.1).
• Put NC contact of main motor stop PB(I0.3) in series for unlatching the circuit by pressing the main motor STOP PB(I0.3), so operator can stop the main motor(Q0.1).
• Put NO contact of oil pump (Q0.0) in series after the main motor START PB(I0.2) for interlocking. So that operator has to start oil pump (Q0.0) and then only he can START the main motor (Q0.1).

Runtime Test Cases

Note : The above PLC Logic provided for basic idea about application of PLC Program for Conditional Control Logic. The Logic is limited and not complete application.

Author : Bhavesh

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