Learning to implement Sequential Function Charts (SFC) for Temperature Control using Unitronics's VisiLogic / UniLogic is a useful skill for PLC programmers working in Process Control. This guide walks through the fundamentals with clear explanations and an illustrative example that you can adapt to a simulator or test bench.
The example uses Unitronics terminology and references controller families such as Jazz 2 and Samba 7". Confirm the exact instructions, data types, firmware requirements, and licensing in the current vendor documentation before choosing hardware or deploying a project.
The Sequential Function Charts (SFC) approach is particularly well-suited for Temperature Control because batch processes, step-by-step operations, state machines, and complex sequential control. This combination allows you to leverage perfect for sequential processes while managing the typical challenges of Temperature Control, including pid tuning and temperature stability.
Throughout this guide, you'll find step-by-step implementation guidance, an illustrative code example, and a verification checklist specific to Process Control. Whether you're programming your first Temperature Control exercise or transitioning from another PLC platform, use the material as a starting point and validate it in your exact VisiLogic / UniLogic version and controller environment.
Unitronics VisiLogic / UniLogic for Temperature Control
VisiLogic / UniLogic is a programming environment associated with Unitronics controller families such as Jazz 2, Samba 7", Vision V350. This guide uses Sequential Function Charts (SFC) terminology from the supplied guide dataset, but controller capabilities and language support can change by model, firmware, software edition, and license.
Verify Before You Start:
- The selected controller supports the required Sequential Function Charts (SFC) constructs
- The project version matches the installed VisiLogic / UniLogic release
- Required communications, motion, safety, and simulation options are licensed
- Firmware and device-description files are compatible with the project
- The vendor manuals used for the design match the exact hardware revision
Application Planning:
For a Temperature Control exercise, map the required inputs and outputs before writing logic. The example considers 4 sensor types, including Thermocouples (K-type, J-type), RTD sensors (PT100, PT1000), Infrared temperature sensors, and 5 actuator types.
Control Equipment for Temperature Control:
- Electric resistance heaters (cartridge, band, strip)
- Steam injection systems
- Thermal fluid (hot oil) systems
- Refrigeration and chiller systems
Controller-family references used in this guide include:
- Jazz 2: Confirm CPU, I/O, memory, communications, and Sequential Function Charts (SFC) support in the current selection guide
- Samba 7": Confirm CPU, I/O, memory, communications, and Sequential Function Charts (SFC) support in the current selection guide
- Vision V350: Confirm CPU, I/O, memory, communications, and Sequential Function Charts (SFC) support in the current selection guide
- Vision V570: Confirm CPU, I/O, memory, communications, and Sequential Function Charts (SFC) support in the current selection guide
Hardware Selection Checklist:
- Count local and remote I/O, including planned expansion
- Measure the required task and communications update rates
- Identify memory, data-retention, diagnostics, and cybersecurity requirements
- Treat safety functions as a separate, standards-led design activity
- Confirm lifecycle status, regional availability, licensing, and support
Source and Validation Note:
This page does not represent a vendor certification or a hardware acceptance test. Use current Unitronics manuals, release notes, and safety documentation as the authority for product-specific behavior. Validate adapted logic in a simulator or isolated test setup before connecting it to equipment.
Investment Considerations:
For Temperature Control projects, compare hardware, software licensing, training, engineering, test equipment, commissioning, spares, and ongoing support. Obtain current pricing and lifecycle information directly from the vendor or an authorized regional supplier.
Understanding Sequential Function Charts (SFC) for Temperature Control
Sequential Function Chart (SFC) is a graphical language for programming sequential processes. It models systems as a series of steps connected by transitions, ideal for batch processes and machine sequences.
Execution Model:
Only active steps execute their actions. Transitions define conditions for moving between steps. Multiple steps can be active simultaneously in parallel branches.
Core Advantages for Temperature Control:
- Perfect for sequential processes: Critical for Temperature Control when handling intermediate control logic
- Clear visualization of process flow: Critical for Temperature Control when handling intermediate control logic
- Easy to understand process steps: Critical for Temperature Control when handling intermediate control logic
- Good for batch operations: Critical for Temperature Control when handling intermediate control logic
- Simplifies complex sequences: Critical for Temperature Control when handling intermediate control logic
Why Sequential Function Charts (SFC) Fits Temperature Control:
Temperature Control systems in Process Control typically involve:
- Sensors: RTDs (PT100/PT1000) for high-accuracy measurements, Thermocouples (J, K, T types) for high-temperature applications, Infrared pyrometers for non-contact measurement
- Actuators: SCR (thyristor) power controllers for electric heaters, Solid-state relays for on/off heating control, Proportional control valves for steam or thermal fluid
- Complexity: Intermediate with challenges including Long thermal time constants making tuning difficult
Control Strategies for Temperature Control:
- pid: Standard PID control with proportional, integral, and derivative terms tuned for the thermal process dynamics
- cascade: Master temperature loop outputs to slave heater/cooler control loop for tighter control
- ratio: Maintain temperature ratio between zones for gradient applications
Programming Fundamentals in Sequential Function Charts (SFC):
Steps:
- initialStep: Double-bordered box - starting point of sequence, active on program start
- normalStep: Single-bordered box - becomes active when preceding transition fires
- actions: Associated code that executes while step is active
Transitions:
- condition: Boolean expression that must be TRUE to advance
- firing: Transition fires when preceding step is active AND condition is TRUE
- priority: In selective branches, transitions are evaluated in defined order
ActionQualifiers:
- N: Non-stored - executes while step is active
- S: Set - sets output TRUE on step entry, remains TRUE
- R: Reset - sets output FALSE on step entry
Best Practices for Sequential Function Charts (SFC):
- Start with a clear process flow diagram before implementing SFC
- Use descriptive step names indicating what happens (e.g., Filling, Heating)
- Keep transition conditions simple - complex logic goes in action code
- Implement timeout transitions to prevent stuck sequences
- Always provide a path back to initial step for reset/restart
Common Mistakes to Avoid:
- Forgetting to include stop/abort transitions for emergency handling
- Creating deadlocks where no transition can fire
- Not handling the case where transition conditions never become TRUE
- Using S (Set) actions without corresponding R (Reset) actions
Typical Applications:
1. Bottle filling: Directly applicable to Temperature Control
2. Assembly sequences: Related control patterns
3. Material handling: Related control patterns
4. Batch mixing: Related control patterns
Understanding these fundamentals prepares you to implement effective Sequential Function Charts (SFC) solutions for Temperature Control using Unitronics VisiLogic / UniLogic.
Implementing Temperature Control with Sequential Function Charts (SFC)
Industrial temperature control systems use PLCs to regulate process temperatures in manufacturing, food processing, chemical processing, and other applications. These systems maintain precise temperature setpoints through heating and cooling control while ensuring product quality and energy efficiency.
This walkthrough demonstrates practical implementation using Unitronics VisiLogic / UniLogic and Sequential Function Charts (SFC) programming.
System Requirements:
A typical Temperature Control implementation includes:
Input Devices (Sensors):
1. RTDs (PT100/PT1000) for high-accuracy measurements: Critical for monitoring system state
2. Thermocouples (J, K, T types) for high-temperature applications: Critical for monitoring system state
3. Infrared pyrometers for non-contact measurement: Critical for monitoring system state
4. Thermistors for fast response applications: Critical for monitoring system state
5. Thermal imaging cameras for surface temperature monitoring: Critical for monitoring system state
Output Devices (Actuators):
1. SCR (thyristor) power controllers for electric heaters: Primary control output
2. Solid-state relays for on/off heating control: Supporting control function
3. Proportional control valves for steam or thermal fluid: Supporting control function
4. Solenoid valves for cooling water or refrigerant: Supporting control function
5. Variable frequency drives for cooling fan control: Supporting control function
Control Equipment:
- Electric resistance heaters (cartridge, band, strip)
- Steam injection systems
- Thermal fluid (hot oil) systems
- Refrigeration and chiller systems
Control Strategies for Temperature Control:
- pid: Standard PID control with proportional, integral, and derivative terms tuned for the thermal process dynamics
- cascade: Master temperature loop outputs to slave heater/cooler control loop for tighter control
- ratio: Maintain temperature ratio between zones for gradient applications
Implementation Steps:
Step 1: Characterize thermal system dynamics (time constants, dead time)
In VisiLogic / UniLogic, characterize thermal system dynamics (time constants, dead time).
Step 2: Select appropriate sensor type and placement for representative measurement
In VisiLogic / UniLogic, select appropriate sensor type and placement for representative measurement.
Step 3: Size heating and cooling capacity for worst-case load conditions
In VisiLogic / UniLogic, size heating and cooling capacity for worst-case load conditions.
Step 4: Implement PID control with appropriate sample time (typically 10x faster than process time constant)
In VisiLogic / UniLogic, implement pid control with appropriate sample time (typically 10x faster than process time constant).
Step 5: Add output limiting and anti-windup for safe operation
In VisiLogic / UniLogic, add output limiting and anti-windup for safe operation.
Step 6: Program ramp/soak profiles if required
In VisiLogic / UniLogic, program ramp/soak profiles if required.
Unitronics Function Design:
Function block design in Unitronics uses user-defined FBs in UniLogic (more limited in VisiLogic). Extensive vendor-provided helper FBs cover common tasks (PID, motion, communication, HMI utilities). OEM machine builders typically maintain private FB libraries for their common machine patterns, though code reuse is less mature than in mainstream PLC ecosystems.
Common Challenges and Solutions:
1. Long thermal time constants making tuning difficult
- Solution: Sequential Function Charts (SFC) addresses this through Perfect for sequential processes.
2. Transport delay (dead time) causing instability
- Solution: Sequential Function Charts (SFC) addresses this through Clear visualization of process flow.
3. Non-linear response at different temperature ranges
- Solution: Sequential Function Charts (SFC) addresses this through Easy to understand process steps.
4. Sensor placement affecting measurement accuracy
- Solution: Sequential Function Charts (SFC) addresses this through Good for batch operations.
Safety Considerations:
- Independent high-limit safety thermostats (redundant to PLC)
- Watchdog timers for heater control validity
- Safe-state definition on controller failure (heaters off)
- Thermal fuse backup for runaway conditions
- Proper ventilation for combustible atmospheres
Performance Metrics:
- Task and I/O timing: Record minimum, average, and maximum values under a defined test load
- Accuracy: Define an acceptable tolerance and compare it with calibrated reference measurements
- Throughput: Count completed cycles over a fixed interval and record rejected or incomplete cycles
- Fault response: Measure detection, safe-state, alarm, and recovery behavior for each test case
- Resource use: Record memory, communications load, and diagnostic-buffer behavior
Unitronics Diagnostic Tools:
UniLogic (current) and VisiLogic (legacy) integrated debuggers with breakpoints,Built-in simulator covering PLC logic, HMI screens, alarms, recipes, and data tables,Web visualisation for UniStream — remote HMI viewing without additional software,SD card logging with PC-side export tools for offline trend analysis,Modbus RTU/TCP transaction logging built into the IDE,Controller status monitor — CPU load, scan time, memory usage,HMI event logger capturing operator actions for audit purposes,CAN bus diagnostic tools for CANopen-equipped models,Remote support tool — Unitronics' own screen-sharing for technical support,User community forum with active troubleshooting discussions
Use the monitoring and diagnostic functions available in your VisiLogic / UniLogic version, and record the software, firmware, hardware, workload, and test procedure with every result.
Unitronics Sequential Function Charts (SFC) Example for Temperature Control
Illustrative Sequential Function Charts (SFC) example for Temperature Control using Unitronics terminology. Adapt the syntax to your VisiLogic / UniLogic release, compile it, and verify it in an isolated test environment before use on equipment.
// Unitronics VisiLogic / UniLogic - Temperature Control Control
// Sequential Function Charts (SFC) Implementation for Process Control
// Unitronics projects use IDE-managed tag names rather than ra
// ============================================
// Variable Declarations
// ============================================
VAR
bEnable : BOOL := FALSE;
bEmergencyStop : BOOL := FALSE;
rThermocouplesKtypeJtype : REAL;
rHeatingelements : REAL;
END_VAR
// ============================================
// Input Conditioning - RTDs (PT100/PT1000) for high-accuracy measurements
// ============================================
// Standard input processing
IF rThermocouplesKtypeJtype > 0.0 THEN
bEnable := TRUE;
END_IF;
// ============================================
// Safety Interlock - Independent high-limit safety thermostats (redundant to PLC)
// ============================================
IF bEmergencyStop THEN
rHeatingelements := 0.0;
bEnable := FALSE;
END_IF;
// ============================================
// Main Temperature Control Control Logic
// ============================================
IF bEnable AND NOT bEmergencyStop THEN
// Industrial temperature control systems use PLCs to regulate
rHeatingelements := rThermocouplesKtypeJtype * 1.0; (* Illustrative scaling only *)
// Process monitoring
// Add specific control logic here
ELSE
rHeatingelements := 0.0;
END_IF;Code Explanation:
- 1.Sequential Function Charts (SFC) structure organized for a Temperature Control training example
- 2.Input conditioning handles RTDs (PT100/PT1000) for high-accuracy measurements signals
- 3.Safety interlock ensures Independent high-limit safety thermostats (redundant to PLC) always takes priority
- 4.Main control implements Industrial temperature control systems u
- 5.Adapt the scan-cycle assumptions to the selected Jazz 2 task configuration and verify them by measurement
Best Practices
- ✓Follow Unitronics naming conventions: Unitronics projects use IDE-managed tag names rather than raw memory addressing.
- ✓Unitronics function design: Function block design in Unitronics uses user-defined FBs in UniLogic (more limi
- ✓Data organization: Unitronics uses its own tag database concept rather than IEC-standard data block
- ✓Sequential Function Charts (SFC): Start with a clear process flow diagram before implementing SFC
- ✓Sequential Function Charts (SFC): Use descriptive step names indicating what happens (e.g., Filling, Heating)
- ✓Sequential Function Charts (SFC): Keep transition conditions simple - complex logic goes in action code
- ✓Temperature Control: Sample at 1/10 of the process time constant minimum
- ✓Temperature Control: Use derivative on PV, not error, for temperature control
- ✓Temperature Control: Start with conservative tuning and tighten gradually
- ✓Debug with VisiLogic / UniLogic: Use the built-in simulator to reproduce issues before hardware visit
- ✓Safety: Independent high-limit safety thermostats (redundant to PLC)
- ✓Use a compatible simulator or isolated test rig to test Temperature Control logic before deployment
Common Pitfalls to Avoid
- ⚠Sequential Function Charts (SFC): Forgetting to include stop/abort transitions for emergency handling
- ⚠Sequential Function Charts (SFC): Creating deadlocks where no transition can fire
- ⚠Sequential Function Charts (SFC): Not handling the case where transition conditions never become TRUE
- ⚠Unitronics common error: VisiLogic-to-UniLogic migration issues — not all projects convert cleanly
- ⚠Temperature Control: Long thermal time constants making tuning difficult
- ⚠Temperature Control: Transport delay (dead time) causing instability
- ⚠Neglecting to validate RTDs (PT100/PT1000) for high-accuracy measurements leads to control errors
- ⚠Insufficient comments make Sequential Function Charts (SFC) programs unmaintainable over time
Related Certifications
Applying Sequential Function Charts (SFC) to Temperature Control using Unitronics VisiLogic / UniLogic requires understanding the platform, the process, and the project's acceptance criteria. This guide has covered implementation structure, an illustrative code example, verification practices, and common pitfalls for a intermediate Temperature Control exercise.
Use the practices outlined here to create a design that can be reviewed and tested. Define performance targets in the project requirements and confirm them with repeatable measurements.
Next Steps:
1. Check Sources: Read the current VisiLogic / UniLogic help, controller manual, release notes, and relevant standards
2. Practice Safely: Adapt the example in a simulator or isolated training setup
3. Review: Have the I/O map, state behavior, faults, and recovery steps reviewed
4. Test: Record normal, boundary, fault, restart, and communications test results
Sequential Function Charts (SFC) Foundation:
Sequential Function Chart (SFC) is a graphical language for programming sequential processes. It models systems as a series of steps connected by tran...
Project duration depends on scope, reviews, hardware availability, software and firmware versions, testing, commissioning, and site constraints. Remember: Sample at 1/10 of the process time constant minimum
For further learning, explore related topics including Assembly sequences, Plastic molding machines, and Unitronics platform-specific features for Temperature Control optimization.