Why PID function block for process control in PLC Programming? - Purpose & Use Cases

The Big Idea

What if your machine could fix its own mistakes before you even notice them?

The Scenario

Imagine you are trying to keep the temperature of a large oven steady by turning a valve manually. You watch the temperature rise and fall, adjusting the valve little by little, hoping to keep it just right.

The Problem

Doing this by hand is slow and tiring. You might overcorrect and cause the temperature to swing wildly. It's easy to make mistakes, and the oven's product quality suffers because the heat isn't steady.

The Solution

A PID function block automatically adjusts the valve to keep the temperature steady. It reacts quickly and smoothly to changes, making sure the oven stays at the perfect temperature without constant human effort.

Before vs After

✗ Before

IF temperature < setpoint THEN open_valve ELSE close_valve

✓ After

PID_Control(setpoint, measured_value, output_valve_position);

What It Enables

It lets machines control themselves precisely and reliably, freeing people to focus on other tasks.

Real Life Example

In a brewery, a PID block keeps the fermentation tank at the right temperature by adjusting cooling valves automatically, ensuring consistent beer quality every batch.

Key Takeaways

Manual control is slow and error-prone.

PID blocks automate smooth, precise adjustments.

This improves product quality and reduces human workload.

Practice

1. What is the main purpose of a PID function block in process control?

easy

A. To keep a process variable close to a desired target value

B. To store data for later analysis

C. To display process values on a screen

D. To manually adjust valves and switches

Solution

1. Step 1: Understand the role of PID blocks

   PID blocks automatically adjust outputs to keep a process variable near a setpoint.

2. Step 2: Compare options to this role

   Only To keep a process variable close to a desired target value describes this automatic control function; others describe unrelated tasks.

3. Final Answer:

   To keep a process variable close to a desired target value -> Option A

4. Quick Check:

   PID controls process variable = keep near target [OK]

Hint: PID blocks control variables automatically, not manually [OK]

Common Mistakes:

• Thinking PID stores data instead of controlling
• Confusing PID with display or manual control
• Assuming PID only measures values

2. Which of the following is the correct way to declare a PID function block instance named pid1 in a PLC program?

easy

A. pid1 : PID;

B. pid1 = PID();

C. PID pid1;

D. pid1 -> PID();

Solution

1. Step 1: Recall PLC function block declaration syntax

   In PLC languages like Structured Text, function blocks are declared with name : Type;.

2. Step 2: Match options to this syntax

   pid1 : PID; matches the correct syntax; others use incorrect assignment or C-like syntax.

3. Final Answer:

   pid1 : PID; -> Option A

4. Quick Check:

   Function block declaration uses colon and semicolon [OK]

Hint: Use colon and semicolon to declare function blocks [OK]

Common Mistakes:

• Using equal sign instead of colon
• Using C or Java style declarations
• Using arrow operator incorrectly

3. Given the following PLC code snippet using a PID block:

pid1.Kp := 2.0;
pid1.Ki := 0.5;
pid1.Kd := 0.1;
pid1.SetPoint := 100;
pid1.ProcessVariable := 90;
pid1();
Output := pid1.ControlOutput;

What is the expected behavior of Output after this code runs once?

medium

A. Output will be negative to decrease the process variable

B. Output will be zero because PID needs multiple cycles

C. Output will be a positive value to increase the process variable

D. Output will equal the SetPoint value

Solution

1. Step 1: Analyze PID inputs and error

   The error is SetPoint (100) minus ProcessVariable (90) = 10, positive error.

2. Step 2: Understand PID output direction

   Positive error means output should increase to raise the process variable, so output is positive.

3. Final Answer:

   Output will be a positive value to increase the process variable -> Option C

4. Quick Check:

   Positive error -> positive output to correct [OK]

Hint: Positive error means output pushes variable up [OK]

Common Mistakes:

• Assuming output is zero on first run
• Confusing output direction with error sign
• Thinking output equals setpoint directly

4. Identify the error in this PID block usage snippet:

pid1.Kp := 1.5;
pid1.Ki := 0.3;
pid1.Kd := 0.05;
pid1.SetPoint := 50;
pid1.ProcessVariable := 55;
Output := pid1.ControlOutput;
pid1();

medium

A. Assigning ProcessVariable before SetPoint

B. Setting Kp, Ki, Kd values too low

C. Missing declaration of pid1

D. Calling the PID block function pid1() after reading output

Solution

1. Step 1: Check order of function call and output read

   The PID block must be called (executed) before reading ControlOutput to update output.

2. Step 2: Identify the mistake in code order

   Here, Output is assigned before pid1() is called, so output is stale or invalid.

3. Final Answer:

   Calling the PID block function pid1() after reading output -> Option D

4. Quick Check:

   Call PID block before reading output [OK]

Hint: Always call PID block before reading output [OK]

Common Mistakes:

• Reading output before calling PID block
• Confusing parameter order
• Ignoring function block declaration

5. You want to tune a PID block to control temperature in a tank. The temperature rises too slowly and overshoots the target. Which tuning change should you make to reduce overshoot and improve response?

hard

A. Decrease Kp and increase Ki

B. Decrease Kp and increase Kd

C. Increase Kp and decrease Kd

D. Increase Ki and Kd equally

Solution

1. Step 1: Understand overshoot and slow rise causes

   High Kp causes overshoot; low Kd means less damping; slow rise means Kp might be too low.

2. Step 2: Choose tuning to reduce overshoot and improve speed

   Decreasing Kp reduces overshoot; increasing Kd adds damping to reduce oscillations and overshoot.

3. Final Answer:

   Decrease Kp and increase Kd -> Option B

4. Quick Check:

   Lower Kp + higher Kd = less overshoot, better control [OK]

Hint: Lower Kp and raise Kd to reduce overshoot [OK]

Common Mistakes:

• Increasing Kp causes more overshoot
• Ignoring derivative gain effect
• Changing Ki without addressing overshoot