PID temperature control system in PLC Programming - Mini Project: Build & Apply

PID Temperature Control System

📖 Scenario: You are programming a simple PID temperature control system for a small oven. The oven needs to maintain a target temperature by adjusting the heater power based on the difference between the current temperature and the target temperature.This project will guide you through creating the basic data setup, configuration, core PID control logic, and output display.

🎯 Goal: Build a basic PID temperature control program that reads the current temperature, compares it to a target temperature, calculates the control output using PID logic, and displays the heater power level.

📋 What You'll Learn

Create variables for current temperature and target temperature

Add PID controller parameters: proportional gain, integral gain, derivative gain

Implement the PID control calculation

Output the calculated heater power level

💡 Why This Matters

🌍 Real World

PID temperature control is used in ovens, furnaces, and HVAC systems to keep temperatures steady and safe.

💼 Career

Understanding PID control programming is essential for automation engineers working with industrial control systems.

1

DATA SETUP: Define temperature variables

Create two variables: current_temp and target_temp. Set current_temp to 70 and target_temp to 100.

PLC Programming

# Create variables current_temp and target_temp with values 70 and 100
# Your code here

Hint

Use the assignment operator := to set variable values.

2

CONFIGURATION: Add PID parameters

Add three variables for PID parameters: Kp set to 2.0, Ki set to 0.1, and Kd set to 0.05.

PLC Programming

current_temp := 70;
target_temp := 100;
# Add PID parameters Kp, Ki, and Kd below
# Your code here

Hint

Define each PID parameter as a variable with the given values.

3

CORE LOGIC: Implement PID control calculation

Create variables error, integral, and derivative. Calculate error as target_temp - current_temp. Assume previous_error is 0 and integral is 0 initially. Calculate integral by adding error. Calculate derivative as error - previous_error. Then calculate output as Kp * error + Ki * integral + Kd * derivative. Finally, update previous_error to error.

PLC Programming

current_temp := 70;
target_temp := 100;
Kp := 2.0;
Ki := 0.1;
Kd := 0.05;
# Calculate PID control output below
# Your code here

Hint

Follow the PID formula step by step using the variables given.

4

OUTPUT: Display heater power level

Write a statement to output the heater power level stored in output. Use WriteOutput(output); to display the value.

PLC Programming

current_temp := 70;
target_temp := 100;
Kp := 2.0;
Ki := 0.1;
Kd := 0.05;
error := target_temp - current_temp;
previous_error := 0;
integral := 0;
integral := integral + error;
derivative := error - previous_error;
output := Kp * error + Ki * integral + Kd * derivative;
previous_error := error;
# Display the heater power level
# Your code here

Hint

Use the provided function WriteOutput() to show the output value.

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