PID temperature control system in PLC Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the PID controller variable.

PLC Programming

PID_Controller [1];

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

Atemperature

BtempPID

CcontrolVar

DpidTemp

Attempts:

3 left

💡 Hint

Common Mistakes

Using generic names like 'temperature' which do not specify the PID controller.

Using names that do not include 'PID' which can cause confusion.

The PID controller variable is named tempPID to clearly represent the temperature control.

2fill in blank

medium

Complete the code to set the proportional gain (Kp) of the PID controller.

PLC Programming

tempPID.Kp = [1];

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

A0.5

B5

C0

D50

Attempts:

3 left

💡 Hint

Common Mistakes

Setting Kp to zero disables proportional control.

Using very large values can cause instability.

The proportional gain (Kp) is set to 0.5 to provide moderate response to error.

3fill in blank

hard

Fix the error in the code to correctly update the integral term of the PID controller.

PLC Programming

tempPID.I = tempPID.I + ([1] * error);

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

AtempPID.Kd

BtempPID.Kp

CtempPID.Ki

Derror

Attempts:

3 left

💡 Hint

Common Mistakes

Using Kp or Kd instead of Ki for the integral term.

Multiplying by the wrong variable.

The integral term uses the integral gain Ki multiplied by the error to accumulate over time.

4fill in blank

hard

Fill both blanks to calculate the derivative term and update the previous error.

PLC Programming

tempPID.D = tempPID.Kd * (error - [1]); 
prevError = [2];

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

AprevError

Berror

CtempPID.I

DtempPID.D

Attempts:

3 left

💡 Hint

Common Mistakes

Using integral or derivative variables instead of previous error.

Not updating previous error correctly.

The derivative term is calculated using the difference between current error and prevError. Then prevError is updated to the current error.

5fill in blank

hard

Fill all three blanks to compute the PID output and apply it to the heater control.

PLC Programming

pidOutput = [1] + [2] + [3];
heaterPower = pidOutput;
applyPower(heaterPower);

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

AtempPID.P

BtempPID.I

CtempPID.D

Derror

Attempts:

3 left

💡 Hint

Common Mistakes

Including error directly instead of PID terms.

Omitting one of the PID terms.

The PID output is the sum of proportional (P), integral (I), and derivative (D) terms. This output controls the heater power.

Practice

1. What is the main purpose of a PID controller in a temperature control system?

easy

A. To increase the temperature without feedback

B. To keep the temperature steady by adjusting the output based on error

C. To turn the heater on and off randomly

D. To measure the temperature only

Solution

1. Step 1: Understand PID controller role

   A PID controller adjusts output to reduce the difference between desired and actual temperature.

2. Step 2: Identify correct purpose

   Only To keep the temperature steady by adjusting the output based on error describes this feedback control to keep temperature steady.

3. Final Answer:

   To keep the temperature steady by adjusting the output based on error -> Option B

4. Quick Check:

   PID controls temperature by error adjustment = D [OK]

Hint: PID controls by error correction, not just measuring [OK]

Common Mistakes:

• Confusing measurement with control
• Thinking PID turns heater randomly
• Ignoring feedback in control

2. Which of the following is the correct way to represent the proportional term in a PID control formula in PLC programming?

easy

A. P = error / Kp

B. P = Kp - error

C. P = Kp + error

D. P = Kp * error

Solution

1. Step 1: Recall proportional term formula

   The proportional term multiplies the proportional gain (Kp) by the error value.

2. Step 2: Match formula to options

   Only P = Kp * error correctly shows multiplication of Kp and error.

3. Final Answer:

   P = Kp * error -> Option D

4. Quick Check:

   Proportional term = Kp * error [OK]

Hint: Proportional term multiplies gain by error [OK]

Common Mistakes:

• Dividing error by gain instead of multiplying
• Adding or subtracting gain and error
• Confusing proportional with integral or derivative

3. Given the following PLC code snippet for a PID controller output calculation:

error = setpoint - temperature
P = Kp * error
I = I + Ki * error
D = Kd * (error - last_error)
output = P + I + D
last_error = error
print(output)

If setpoint=100, temperature=90, Kp=2, Ki=0.5, Kd=1, I=5, and last_error=8, what will be printed?

medium

A. 32.0

B. 30.0

C. 20.0

D. 15.5

Solution

1. Step 1: Calculate error and PID terms

   error = 100 - 90 = 10
   P = 2 * 10 = 20
   I = 5 + 0.5 * 10 = 10
   D = 1 * (10 - 8) = 2

2. Step 2: Calculate output and print

   output = P + I + D = 20 + 10 + 2 = 32
   print(output) will print 32.0

3. Final Answer:

   32.0 -> Option A

4. Quick Check:

   20 + 10 + 2 = 32.0 [OK]

Hint: Calculate each PID term stepwise, then sum [OK]

Common Mistakes:

• Mixing up error and last_error
• Forgetting to update integral term
• Adding terms incorrectly

4. Identify the error in this PLC PID control snippet:

error = setpoint - temperature
I = I + Ki * error
D = Kd * (error - last_error)
output = Kp * error + I + D
last_error = error
if output < 0:
    output = 0
print(output)

Assuming all variables are initialized, what is the main problem?

medium

A. Integral term I is not initialized before use

B. Derivative term D uses wrong error difference

C. Output is not limited to maximum value

D. last_error is updated before output calculation

Solution

1. Step 1: Review output limiting logic

   The code limits output only if it is less than 0, setting output to 0. But it does not limit output above a maximum safe value.

2. Step 2: Check other parts for errors

   Integral term I is updated correctly assuming initialized.
   Derivative term uses (error - last_error), which is correct.
   last_error is updated after output calculation, which is correct.

3. Final Answer:

   Output is not limited to maximum value -> Option C

4. Quick Check:

   Output upper limit missing = A [OK]

Hint: Check if output has both lower and upper limits [OK]

Common Mistakes:

• Assuming only lower limit is enough
• Updating last_error too early
• Confusing derivative calculation

5. You want to improve the stability of a PID temperature control system that oscillates around the setpoint. Which tuning change is most likely to reduce oscillations?

hard

A. Increase the derivative gain (Kd)

B. Increase the integral gain (Ki)

C. Decrease the proportional gain (Kp)

D. Remove the derivative term

Solution

1. Step 1: Understand causes of oscillation

   High proportional gain can cause oscillations. Integral gain affects steady-state error. Derivative gain helps dampen oscillations.

2. Step 2: Choose tuning to reduce oscillations

   Increasing derivative gain adds damping, reducing oscillations and improving stability.

3. Final Answer:

   Increase the derivative gain (Kd) -> Option A

4. Quick Check:

   More derivative gain reduces oscillations = A [OK]

Hint: Increase derivative gain to damp oscillations [OK]

Common Mistakes:

• Decreasing Kp too much causing slow response
• Increasing Ki causing more oscillations
• Removing derivative term losing damping effect