Building a PID Loop UDT and Level 3 Popup Display for a Distillation Process SCADA System (Ignition Perspective)

In Level 1 and Level 2 of a SCADA display, operators see simplified overviews: tanks, flow lines, equipment status, and essential alarms. But when an operator clicks on a control loop, they should be presented with a Level 3 popup—a detailed faceplate showing:

• Process Variable (PV)
• Setpoint (SP)
• Controller Output (OUT)
• Manual Output (MAN)
• PID tuning constants (Kp, Ki, Kd)
• Modes and Setpoint Source
• Alarms and status messages

To make this scalable, I packaged all loop data into a User Defined Type (UDT). From there, a single popup design can display any loop simply by passing the loop’s tag path as a parameter.

This creates a reusable, clean, and highly maintainable SCADA structure.

Building the PID Loop UDT

The heart of this project is the PID_Loop UDT. This UDT stores everything the Level 3 popup needs.

UDT Type Structure

The PID_Loop UDT includes:

Core Signals

• PV – Process Variable
• SP – Operator Setpoint
• OUT – Controller Output (%)
• MAN – Manual Output (%)

Mode Handling

• ModeRaw – numeric value (0=Off, 1=Manual, 2=Auto, 3=Cascade)
• ModeText – text representation
• SP_SourceRaw – Internal / Remote / Cascade
• SP_Effective – the actual SP used in control (depends on source)

Engineering Limits

• engMin
• engMax

These define the display range used in the SP/PV indicators.

Derived Percent Indicators

• PV_Pct
• SP_Pct

These normalize engineering values to 0–100% so they can be used with bar indicators.

PID Tuning Parameters

• kp, ki, kd
  Operator-editable only in Manual mode.

Simulation Components (Optional)

• Error
• Integral
• PrevError

These allow simple simulation of control loop behavior—perfect for learning PID control visually.

Loop Instance: Loop1_FeedFlow

Inside the project, a real example instance is:

[default]Distillation/Loop1_FeedFlow/Loop1_FeedFlow

This instance inherits the full structure of PID_Loop, making it easy to bind to the Level 3 popup.

Building the PID Loop Popup (Level 3 Display)

When the operator clicks a loop in the Level 2 display, a popup opens showing all loop details. This popup is built using:

• Custom SVG indicators
• Embedded views
• Dynamic bindings from the UDT
• Bi-directional text fields
• Mode-based enable/disable logic

The popup view receives one parameter:

loopPath (string)

This allows the popup to show any loop from Level 2.

Custom Indicators (SP–PV & MAN–OUT)

To achieve a clean, responsive, and scalable design, I built two custom SVG-based indicators.

1. SP–PV Vertical Indicator

This indicator shows:

• SP bar (blue)
• PV bar (green)
• Engineering range (engMin → engMax)

It receives the following params:

spPct
pvPct
engMin
engMax
barHeight

These are bound from the UDT:

SP_Pct → spPct
PV_Pct → pvPct
engMin → engMin
engMax → engMax

This separates the drawing logic from the UDT structure and keeps the component reusable across loops.

MAN–OUT Horizontal Indicator

This shows:

• Manual output (MAN)
• Controller output (OUT)

With intuitive behavior:

• When Mode = Manual → MAN is bright, editable, shown
• When Mode ≠ Manual → MAN is dimmed and blank
• OUT always reflects the real controller output

Parameters:

manPct
outPct
barLength
isManual

Bindings inside the popup:

• manPct ← UDT.MAN
• outPct ← UDT.OUT
• isManual ← (ModeRaw == 1)

This creates a realistic faceplate behavior similar to PLC systems.

Mode, SP Source & Dynamic Enable/Disable Logic

Operator interactions depend entirely on the loop mode:

When Mode = Manual

• Kp, Ki, Kd → enabled
• MAN → editable
• SP → editable only if SP Source = Internal
• OUT → updated based on MAN
• Status strip shows “Manual”

When Mode = Auto / Cascade

• Kp, Ki, Kd → disabled
• MAN → disabled + blank
• SP → disabled unless Internal
• OUT → driven by PID equations
• Status strip shows Auto/Cascade

SP Source Logic

• Internal → operator types SP
• Remote → SP comes from external tag
• Cascade → SP comes from upstream controller

This is fully supported using:

SP_SourceRaw
SP_Effective

A clean and predictable control UI.

PID Simulation (Optional but Useful)

Although a real system would calculate PID in the PLC, this project includes a simple Ignition-based simulation for learning:

OUT = (Error*Kp) + (Integral*Ki) + (Derivative*Kd)

And state tags:

• Integral
• PrevError

This creates a dynamic and realistic visualization of PID tuning effects.

View the Interactive Displays

If you’d like to see the Level 2 and Level 3 screens in full clarity, you can view them directly on my live demo page:

➡️ SCADA Display Portfolio – Michelle Alzola

This page is a simple viewer designed to showcase the UI/UX layout only.
The displays are static and do not represent a functioning SCADA system—they are visual mockups intended for portfolio demonstration purposes.

Conclusion

By combining:

• A consistent PID_Loop UDT
• Cleanly separated custom indicators
• A reusable Level 3 popup with loopPath parameter

We now have a professional-grade PID faceplate—the same structure used in modern PLC systems.

This approach scales beautifully: once the UDT and popup are built, you can add new loops with almost no additional work.

See my UI/UX for SCADA Displays here.