Washing machine control sequence in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of the washing machine timer after 5 seconds?

Consider a PLC program controlling a washing machine timer that increments every second. The timer starts at 0 and increments by 1 every second until it reaches 10, then resets to 0. What is the timer value after 5 seconds?

PLC Programming

TIMER = 0
for second in range(5):
    TIMER = (TIMER + 1) % 11
print(TIMER)

A 5

B 4

C 6

D 0

💡 Hint

The timer increases by 1 each second and resets after 10.

The timer starts at 0 and increments by 1 each second. After 5 increments, the timer value is 5.

📝 Syntax

intermediate

Identify the correct ladder logic rung for starting the washing machine motor

Which ladder logic rung correctly starts the motor when the start button is pressed and the door is closed?

A |--[Start_Button]--(Door_Closed)--(Motor_On)--|

B |--[Start_Button]--[Door_Closed]--[Motor_On]--|

C |--(Start_Button)--[Door_Closed]--(Motor_On)--|

D |--[Start_Button]--[Door_Closed]--(Motor_On)--|

💡 Hint

Look for correct use of contacts and coil symbols in ladder logic.

In ladder logic, square brackets [] represent contacts (conditions), and parentheses () represent coils (outputs). Option D correctly uses contacts for Start_Button and Door_Closed and coil for Motor_On.

🔧 Debug

advanced

Why does the washing machine not stop after the wash cycle?

Given this PLC code snippet controlling the wash cycle, why does the motor keep running indefinitely? IF wash_timer < 30 THEN motor = ON ELSE motor = ON END IF

PLC Programming

IF wash_timer < 30 THEN
    motor = ON
ELSE
    motor = ON
END IF

A Because motor variable is not declared

B Because motor is set to ON in both IF and ELSE branches

C Because the ELSE branch is missing

D Because wash_timer never increments

💡 Hint

Check what happens to motor in both conditions.

The motor is set to ON regardless of the wash_timer value, so it never stops.

🚀 Application

advanced

How many steps are in this washing machine control sequence?

A washing machine control sequence has these steps: Fill water, Wash, Rinse, Spin, Drain. How many steps does this sequence have?

A 5

B 4

C 6

D 3

💡 Hint

Count each distinct step listed.

There are 5 distinct steps listed in the sequence.

🧠 Conceptual

expert

What error occurs if the door sensor input is missing in the safety check?

In a PLC program, the washing machine safety check requires the door sensor input to be TRUE before starting. What error will occur if the door sensor input is missing or not connected?

A The door sensor input will default to TRUE

B The program will raise a runtime error due to missing input

C The safety check will always fail, preventing the machine from starting

D The machine will start without safety check

💡 Hint

Think about how missing inputs affect logical conditions.

If the door sensor input is missing, the safety condition checking for TRUE will fail, so the machine will not start.

Practice

1. In a washing machine control sequence, what is the main purpose of using timers?

easy

A. To turn the machine on and off manually

B. To measure water temperature

C. To detect if the door is open

D. To control how long each washing step runs

Solution

1. Step 1: Understand washing machine steps

   The washing machine runs steps like filling, washing, rinsing, and spinning, each needing a set time.

2. Step 2: Role of timers in automation

   Timers control how long each step runs automatically without manual intervention.

3. Final Answer:

   To control how long each washing step runs -> Option D

4. Quick Check:

   Timers = control step duration [OK]

Hint: Timers set step durations automatically [OK]

Common Mistakes:

• Confusing timers with manual switches
• Thinking timers detect door status
• Assuming timers measure temperature

2. Which of the following is the correct way to represent a step transition in a PLC ladder logic for washing machine control?

easy

A. IF timer_done THEN next_step = TRUE

B. FOR step IN sequence DO run()

C. while(timer < max) { step = next }

D. switch(step) { case timer: run }

Solution

1. Step 1: Identify PLC ladder logic style

   PLC ladder logic uses conditions like IF timer_done THEN to move to next step.

2. Step 2: Match correct syntax

   IF timer_done THEN next_step = TRUE uses a simple IF condition to set the next step flag, matching PLC logic style.

3. Final Answer:

   IF timer_done THEN next_step = TRUE -> Option A

4. Quick Check:

   IF timer_done THEN next_step [OK]

Hint: Look for IF condition style for PLC step control [OK]

Common Mistakes:

• Using programming loops not typical in PLC ladder logic
• Confusing switch-case with PLC logic
• Using syntax from other programming languages

3. Given this PLC pseudocode for washing machine steps:

step = 1
if step == 1 and timer_done:
    step = 2
elif step == 2 and timer_done:
    step = 3
elif step == 3 and timer_done:
    step = 4

What will be the value of step after three timer_done events?

medium

A. 1

B. 4

C. 3

D. 2

Solution

1. Step 1: Trace step changes on timer_done events

   Initially step=1. After first timer_done, step=2. After second, step=3. After third, step=4.

2. Step 2: Confirm final step value

   After three timer_done events, step reaches 4.

3. Final Answer:

   4 -> Option B

4. Quick Check:

   3 timer_done events advance step to 4 [OK]

Hint: Count timer_done events to track step increments [OK]

Common Mistakes:

• Stopping count at 3 instead of 4
• Confusing elif conditions
• Ignoring step update after each event

4. Identify the error in this PLC pseudocode snippet controlling washing machine steps:

if step == 1 and timer_done:
    step = 2
if step == 2 and timer_done:
    step = 3
if step == 3 and timer_done:
    step = 4

What problem can occur during execution?

medium

A. Multiple steps may be skipped in one cycle

B. The timer_done flag is never checked

C. Step never changes from 1

D. Syntax error due to missing else

Solution

1. Step 1: Analyze if statements without else

   All if statements run independently, so if timer_done is true, multiple steps can update in one cycle.

2. Step 2: Understand effect on step variable

   Step can jump from 1 to 4 in one cycle, skipping intermediate steps.

3. Final Answer:

   Multiple steps may be skipped in one cycle -> Option A

4. Quick Check:

   Separate ifs cause multiple step changes [OK]

Hint: Use elif to prevent multiple step changes [OK]

Common Mistakes:

• Assuming only one if runs per cycle
• Ignoring need for exclusive conditions
• Thinking timer_done is unchecked

5. You want to add a safety check to the washing machine PLC program to prevent spinning if the door is open. Which logic addition correctly implements this?

hard

A. if door_open and step == 'spin': skip spinning step

B. if step == 'spin' or door_open: start spinning immediately

C. if door_closed and step == 'spin' and timer_done: proceed to next step

D. if door_closed or step != 'spin': stop the machine

Solution

1. Step 1: Understand safety requirement

   Spinning should only happen if the door is closed to avoid accidents.

2. Step 2: Choose logic that checks door status before spinning

   if door_closed and step == 'spin' and timer_done: proceed to next step checks door_closed before allowing spinning step to proceed, ensuring safety.

3. Final Answer:

   if door_closed and step == 'spin' and timer_done: proceed to next step -> Option C

4. Quick Check:

   Door closed required before spin step [OK]

Hint: Check door_closed before spinning step [OK]

Common Mistakes:

• Using OR instead of AND allowing unsafe spin
• Skipping spin step without proper condition
• Stopping machine incorrectly on door status