Parts counter with conveyor in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of a simple parts counter logic

Given the following PLC ladder logic snippet that increments a counter each time a sensor detects a part, what will be the counter value after 5 parts pass if the sensor input is triggered 5 times?

PLC Programming

Counter C1;
SensorInput: BOOL;

IF SensorInput THEN
  C1.PRE := C1.PRE + 1;
END_IF;

A 1

B 4

C 0

D 5

💡 Hint

Each sensor trigger increments the counter by one.

The counter increments by 1 each time the sensor input is true. After 5 triggers, the counter value is 5.

📝 Syntax

intermediate

Identify the syntax error in the conveyor start logic

Which option contains the correct syntax to start the conveyor motor when the start button is pressed and the emergency stop is not active?

PLC Programming

StartButton: BOOL;
EmergencyStop: BOOL;
ConveyorMotor: BOOL;

// Logic to start conveyor

A IF StartButton AND NOT EmergencyStop THEN ConveyorMotor := TRUE;

B IF StartButton && !EmergencyStop THEN ConveyorMotor = TRUE;

C IF (StartButton AND NOT EmergencyStop) ConveyorMotor := TRUE;

D IF StartButton AND NOT EmergencyStop THEN ConveyorMotor == TRUE;

💡 Hint

Check the correct use of IF statement and assignment operator in PLC structured text.

Option A uses correct PLC structured text syntax: IF condition THEN assignment; Options A, B, and C have syntax errors such as wrong operators or missing THEN.

🔧 Debug

advanced

Debugging a parts count reset issue

The parts counter does not reset when the reset button is pressed. Which option correctly fixes the reset logic?

PLC Programming

Counter C1;
ResetButton: BOOL;

IF ResetButton THEN
  C1.PRE = 0;
END_IF;

A IF ResetButton THEN C1.PRE = 0;

B IF ResetButton THEN C1.PRE := 0;

C IF ResetButton THEN C1.PRE == 0;

D IF ResetButton THEN C1.PRE := FALSE;

💡 Hint

Assignment in PLC structured text uses ':=' not '=' or '=='.

Option B uses the correct assignment operator ':=' to reset the counter. Options A and B use invalid operators, and D assigns a boolean instead of a number.

🚀 Application

advanced

Design logic to stop conveyor after counting 10 parts

Which option correctly implements logic to stop the conveyor motor after the parts counter reaches 10?

PLC Programming

Counter C1;
ConveyorMotor: BOOL;

// Implement stop logic

A IF C1.PRE == 10 THEN ConveyorMotor := TRUE;

B IF C1.PRE > 10 THEN ConveyorMotor = FALSE;

C IF C1.PRE >= 10 THEN ConveyorMotor := FALSE;

D IF C1.PRE < 10 THEN ConveyorMotor := FALSE;

💡 Hint

Stop the motor when the count is 10 or more.

Option C correctly stops the motor when the count is 10 or more. Option C uses wrong assignment operator, C incorrectly starts motor at 10, D stops motor before 10.

🧠 Conceptual

expert

Understanding race conditions in parts counting with conveyor

In a PLC program counting parts on a conveyor, what problem can occur if the sensor input is not properly debounced and the conveyor speed is high?

A The counter may increment multiple times for a single part due to sensor noise.

B The conveyor motor will stop unexpectedly due to overload.

C The PLC will crash due to memory overflow.

D The parts will be physically damaged by the sensor.

💡 Hint

Think about how noisy signals affect counting accuracy.

Without debouncing, sensor noise can cause multiple triggers for one part, leading to overcounting. Other options are unrelated to sensor input noise.

Practice

1. What is the main purpose of a parts counter in a conveyor system?

easy

A. To count the number of parts passing a sensor and stop the conveyor after a set count

B. To increase the speed of the conveyor belt automatically

C. To sort parts by size on the conveyor

D. To clean the conveyor belt during operation

Solution

1. Step 1: Understand the role of the parts counter

   The parts counter uses a sensor to detect each part passing on the conveyor.

2. Step 2: Identify the action after counting

   Once the set number of parts is reached, the conveyor stops to control production.

3. Final Answer:

   To count the number of parts passing a sensor and stop the conveyor after a set count -> Option A

4. Quick Check:

   Parts counter purpose = Count and stop conveyor [OK]

Hint: Focus on counting and stopping conveyor after set parts [OK]

Common Mistakes:

• Confusing counting with sorting
• Thinking the counter speeds up conveyor
• Assuming it cleans the conveyor

2. Which of the following is the correct way to reset a parts counter in a PLC program?

easy

A. Disconnect the sensor from the PLC

B. Increase the counter value by one

C. Stop the conveyor without changing the counter

D. Set the counter value to zero using a reset instruction

Solution

1. Step 1: Identify how counters reset in PLCs

   Counters are reset by setting their value back to zero using a reset instruction.

2. Step 2: Eliminate incorrect options

   Increasing the counter or stopping conveyor does not reset the count; disconnecting sensor stops detection but not reset.

3. Final Answer:

   Set the counter value to zero using a reset instruction -> Option D

4. Quick Check:

   Reset counter = set to zero [OK]

Hint: Reset means setting count back to zero, not stopping conveyor [OK]

Common Mistakes:

• Confusing reset with increment
• Stopping conveyor does not reset count
• Disconnecting sensor is not a reset

3. Given this PLC ladder logic snippet for a parts counter:

IF Sensor = TRUE THEN
  Counter = Counter + 1
END IF
IF Counter >= 10 THEN
  Conveyor = OFF
END IF

What will be the state of the conveyor after 10 parts pass the sensor?

medium

A. Conveyor remains ON

B. Sensor stops detecting parts

C. Conveyor turns OFF

D. Counter resets to zero

Solution

1. Step 1: Analyze the counter increment

   Each time the sensor is TRUE, the counter increases by 1 until it reaches 10.

2. Step 2: Check conveyor control condition

   When counter is 10 or more, the conveyor is turned OFF.

3. Final Answer:

   Conveyor turns OFF -> Option C

4. Quick Check:

   Counter >= 10 stops conveyor [OK]

Hint: Conveyor stops when count reaches or exceeds limit [OK]

Common Mistakes:

• Thinking conveyor stays ON after 10 parts
• Assuming counter resets automatically
• Believing sensor stops detecting

4. Identify the error in this PLC code snippet for counting parts:

IF Sensor = TRUE THEN
  Counter = Counter + 1
END IF
IF Counter = 10 THEN
  Conveyor = OFF
  Counter = 0
END IF

What problem might occur?

medium

A. Counter resets before conveyor stops, so conveyor never stops

B. Counter never increments because sensor is not checked

C. Conveyor never turns OFF because condition uses '=' instead of '>='

D. Sensor input is not used in the code

Solution

1. Step 1: Review counter reset timing

   The counter resets to zero immediately after reaching 10, possibly before conveyor stops.

2. Step 2: Understand effect on conveyor

   If counter resets too soon, the conveyor OFF condition may not hold, causing conveyor to continue running.

3. Final Answer:

   Counter resets before conveyor stops, so conveyor never stops -> Option A

4. Quick Check:

   Resetting counter too early breaks stop logic [OK]

Hint: Reset counter after conveyor stops, not before [OK]

Common Mistakes:

• Resetting counter immediately after reaching target
• Using '=' instead of '>=' causing missed conditions
• Ignoring sensor input usage

5. You want to modify a parts counter program so the conveyor stops only after counting 50 parts, but also resets the counter automatically after stopping. Which sequence of actions in the PLC program is correct?

hard

A. Reset counter, then check if counter >= 50, then stop conveyor

B. Check if counter >= 50, then stop conveyor, then reset counter

C. Stop conveyor first, then check counter, then reset counter

D. Increment counter only after conveyor stops

Solution

1. Step 1: Set condition to check counter value

   The program must check if the counter has reached or exceeded 50 parts.

2. Step 2: Stop conveyor and then reset counter

   After stopping the conveyor, reset the counter to zero to prepare for next batch.

3. Final Answer:

   Check if counter >= 50, then stop conveyor, then reset counter -> Option B

4. Quick Check:

   Stop then reset after count reached [OK]

Hint: Stop conveyor first, then reset counter after count reached [OK]

Common Mistakes:

• Resetting counter before checking count
• Stopping conveyor without checking count
• Incrementing counter only after stopping conveyor