Batch process control in PLC Programming - Step-by-Step Execution

Concept Flow - Batch process control

Start Batch

↓

Initialize Variables

↓

Check Step 1 Condition

No→Wait or Error

Yes↓

Execute Step 1

↓

Check Step 2 Condition

No→Wait or Error

Yes↓

Execute Step 2

↓

Check Final Step Condition

Yes↓

Complete Batch

↓

Reset or Stop

The batch process control runs steps in order, checking conditions before each step, executing steps if conditions are met, and completing the batch at the end.

Execution Sample

PLC Programming

START_BATCH := TRUE;
STEP := 0;
IF START_BATCH THEN
  STEP := STEP + 1;
  IF STEP = 1 THEN
    EXECUTE_STEP1();
  END_IF;
END_IF;

This code starts a batch, increments the step, and executes step 1 if the batch started.

Execution Table

Step	START_BATCH	STEP	Condition (STEP=1)	Action	Output
1	TRUE	0	FALSE	STEP incremented to 1	No step executed yet
2	TRUE	1	TRUE	EXECUTE_STEP1() called	Step 1 executed
3	TRUE	1	TRUE	Batch continues	Waiting for next step
4	FALSE	1	TRUE	No increment	Batch stopped
Exit	FALSE	1	N/A	Batch stopped	Process ends

💡 Batch stops when START_BATCH is FALSE, no further steps executed.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
START_BATCH	FALSE	TRUE	TRUE	FALSE
STEP	0	1	1	1

Key Moments - 2 Insights

Why does STEP only increment when START_BATCH is TRUE?

Because the condition IF START_BATCH THEN controls the increment, as shown in execution_table row 1 where STEP increments only if START_BATCH is TRUE.

What happens if the condition STEP=1 is false?

No step executes, the program waits or handles error, as seen in execution_table row 1 where condition is FALSE and no step runs.

Visual Quiz - 3 Questions

Look at the execution_table at Step 2, what is the value of STEP?

A 0

B 1

C 2

D Undefined

💡 Hint

Check the STEP column in execution_table row 2.

At which step does the batch stop according to the execution_table?

A Step 4

B Step 2

C Exit row

D Step 1

💡 Hint

Look at the exit_note and the last row in execution_table.

If START_BATCH remains TRUE, what happens to STEP in the next cycle?

A STEP increments by 1

B STEP resets to 0

C STEP stays the same

D Batch stops

💡 Hint

Refer to the condition and action in execution_table row 1.

Concept Snapshot

Batch process control runs steps in order.
Each step runs only if its condition is true.
START_BATCH triggers the process.
STEP tracks current step number.
Process stops when START_BATCH is false.

Full Transcript

Batch process control is a way to run a series of steps one after another. The process starts when a signal called START_BATCH is true. A variable STEP keeps track of which step is running. Each step only runs if its condition is true. If the condition is false, the process waits or stops. When all steps finish, the batch completes and resets or stops. This method ensures the process runs safely and in order.

Practice

1. What is the main purpose of batch process control in PLC programming?

easy

A. To randomly execute tasks without order

B. To run tasks step-by-step automatically in a specific order

C. To manually start each machine separately

D. To only monitor machine temperatures

Solution

1. Step 1: Understand batch process control

   Batch process control means running tasks automatically one after another in a set order.

2. Step 2: Compare options to definition

   Only To run tasks step-by-step automatically in a specific order describes running tasks step-by-step automatically, which matches batch control.

3. Final Answer:

   To run tasks step-by-step automatically in a specific order -> Option B

4. Quick Check:

   Batch control = step-by-step automatic tasks [OK]

Hint: Batch control means automatic steps in order [OK]

Common Mistakes:

• Thinking batch control is manual
• Confusing batch control with random task execution
• Assuming batch control only monitors data

2. Which of the following is the correct way to check if a batch step is complete in a PLC ladder logic?

easy

A. WHILE StepDone = FALSE THEN StartNextStep

B. FOR StepDone = TRUE DO StartNextStep

C. IF StepDone = TRUE THEN StartNextStep

D. StepDone == FALSE; StartNextStep

Solution

1. Step 1: Identify correct syntax for condition check

   In PLC ladder logic or structured text, checking a boolean flag uses IF condition THEN action.

2. Step 2: Evaluate options

   IF StepDone = TRUE THEN StartNextStep uses IF StepDone = TRUE THEN StartNextStep, which is correct syntax. Others use incorrect or invalid syntax.

3. Final Answer:

   IF StepDone = TRUE THEN StartNextStep -> Option C

4. Quick Check:

   IF condition THEN action is correct [OK]

Hint: Use IF condition THEN action for step checks [OK]

Common Mistakes:

• Using FOR or WHILE loops incorrectly in ladder logic
• Using wrong comparison operators
• Confusing assignment with comparison

3. Given the following PLC pseudo-code for batch control:

Step1Done = FALSE
Step2Done = FALSE

IF StartBatch THEN
  Step1Done = TRUE
END_IF

IF Step1Done THEN
  Step2Done = TRUE
END_IF

Output = Step2Done

What is the value of Output after StartBatch is TRUE?

medium

A. FALSE

B. Error

C. Undefined

D. TRUE

Solution

1. Step 1: Trace code when StartBatch is TRUE

   When StartBatch is TRUE, Step1Done is set to TRUE.

2. Step 2: Check Step2Done assignment

   Since Step1Done is TRUE, Step2Done is set to TRUE.

3. Step 3: Determine Output value

   Output is assigned Step2Done, which is TRUE.

4. Final Answer:

   TRUE -> Option D

5. Quick Check:

   StartBatch TRUE leads to Output TRUE [OK]

Hint: Follow flags step-by-step to find output [OK]

Common Mistakes:

• Assuming Output stays FALSE
• Not updating Step2Done after Step1Done
• Confusing variable assignments

4. In a batch process, the following code snippet is intended to start Step 2 only after Step 1 completes:

IF Step1Done = TRUE THEN
  StartStep2 = TRUE
ELSE
  StartStep2 = FALSE
END_IF

IF StartStep2 = TRUE THEN
  Step2Done = TRUE
END_IF

However, Step 2 never starts. What is the likely problem?

medium

A. Step1Done is never set to TRUE

B. StartStep2 is assigned inside ELSE incorrectly

C. Step2Done should be set before StartStep2

D. Missing a delay between steps

Solution

1. Step 1: Analyze condition for starting Step 2

   Step 2 starts only if Step1Done = TRUE, so Step1Done must be TRUE first.

2. Step 2: Identify why Step 2 never starts

   If Step1Done never becomes TRUE, StartStep2 stays FALSE, so Step2Done never sets TRUE.

3. Final Answer:

   Step1Done is never set to TRUE -> Option A

4. Quick Check:

   Step1Done TRUE needed to start Step 2 [OK]

Hint: Check if previous step flag is set TRUE [OK]

Common Mistakes:

• Assuming StartStep2 triggers Step2Done without Step1Done
• Ignoring initial Step1Done value
• Thinking delay is needed when flag is missing

5. You want to design a batch process with three steps: Mix, Heat, and Cool. Each step must start only after the previous step finishes. Which approach best ensures correct order and safety?

hard

A. Use boolean flags like MixDone, HeatDone, CoolDone to trigger next steps sequentially

B. Start all steps at once and monitor temperatures to stop if unsafe

C. Use a timer to run each step for fixed time regardless of completion

D. Manually start each step using operator input

Solution

1. Step 1: Understand batch step dependencies

   Each step must wait for the previous step to finish to maintain order and safety.

2. Step 2: Evaluate options for controlling step order

   Use boolean flags like MixDone, HeatDone, CoolDone to trigger next steps sequentially uses flags to signal completion and trigger next step, ensuring correct sequence and safety.

3. Final Answer:

   Use boolean flags like MixDone, HeatDone, CoolDone to trigger next steps sequentially -> Option A

4. Quick Check:

   Flags ensure step order and safety [OK]

Hint: Use done flags to trigger next step safely [OK]

Common Mistakes:

• Starting all steps simultaneously
• Relying only on timers without completion checks
• Manual starts risking wrong order