Washing machine control sequence in PLC Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start the washing machine when the start button is pressed.

PLC Programming

IF start_button = [1] THEN
    motor := TRUE;
END_IF;

Drag options to blanks, or click blank then click option'

AFALSE

B1

CON

DTRUE

Attempts:

3 left

💡 Hint

Common Mistakes

Using FALSE instead of TRUE for the start button condition.

Using numeric values like 1 instead of boolean TRUE.

The start button must be TRUE (pressed) to start the motor.

2fill in blank

medium

Complete the code to stop the motor when the stop button is pressed.

PLC Programming

IF stop_button = [1] THEN
    motor := FALSE;
END_IF;

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ATRUE

BFALSE

COFF

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Checking for FALSE instead of TRUE on the stop button.

Using numeric values instead of boolean.

The motor stops when the stop button is pressed, which means stop_button is TRUE.

3fill in blank

hard

Fix the error in the code to correctly check if the water level is high.

PLC Programming

IF water_level [1] high_level THEN
    valve := FALSE;
END_IF;

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A!=

B=

C==

D=>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of '=' for comparison.

Using '!=' or '=>' which changes logic.

In PLC programming, '=' is used to compare equality, ':=' is assignment.

4fill in blank

hard

Fill both blanks to create a loop that runs the motor for 30 seconds.

PLC Programming

timer := [1];
IF timer [2] 30 THEN
    motor := FALSE;
END_IF;

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Astart_timer()

B>

C<=

Dstop_timer()

Attempts:

3 left

💡 Hint

Common Mistakes

Using stop_timer() instead of start_timer().

Using '<=' instead of '>' in the condition.

The timer is started with start_timer(), and the motor stops when timer > 30 seconds.

5fill in blank

hard

Fill all three blanks to create a dictionary that maps cycle names to their durations in minutes.

PLC Programming

cycles := {"wash": [1], "rinse": [2], "spin": [3];

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A30

B10

C5

D60

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up the durations for each cycle.

Using string values instead of numbers.

The wash cycle lasts 30 minutes, rinse 10 minutes, and spin 5 minutes.

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