Unitronics Structured Text for Motor Control: Complete Programming Guide

Advanced Structured Text techniques for Motor Control in Unitronics's VisiLogic / UniLogic can improve code organization, diagnostics, and reuse when they are applied deliberately. This guide explores patterns that go beyond a basic implementation and explains how to evaluate their tradeoffs.

Available language features and libraries depend on the controller family, firmware, installed options, and VisiLogic / UniLogic version. Confirm each feature in current vendor documentation and create a minimal compile-and-run test before incorporating it into a larger project.

Advanced Motor Control implementations leverage sophisticated techniques including multi-sensor fusion algorithms, coordinated multi-actuator control, and intelligent handling of soft start implementation. When implemented using Structured Text, these capabilities are achieved through complex calculations patterns that exploit Unitronics-specific optimizations.

This guide examines custom function blocks, data structures, advanced Structured Text patterns, and version-dependent VisiLogic / UniLogic features. For each technique, assess readability, scan-time cost, failure behavior, portability, and how the design will be tested and maintained.

Unitronics VisiLogic / UniLogic for Motor Control

VisiLogic / UniLogic is a programming environment associated with Unitronics controller families such as Jazz 2, Samba 7", Vision V350. This guide uses Structured Text 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 Structured Text 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 Motor Control exercise, map the required inputs and outputs before writing logic. The example considers 5 sensor types, including Current sensors, Vibration sensors, Temperature sensors, and 5 actuator types.

Control Equipment for Motor Control:

Motor control centers (MCCs)

AC induction motors (NEMA/IEC frame)

Synchronous motors for high efficiency

DC motors for precise speed control

Controller-family references used in this guide include:

Jazz 2: Confirm CPU, I/O, memory, communications, and Structured Text support in the current selection guide

Samba 7": Confirm CPU, I/O, memory, communications, and Structured Text support in the current selection guide

Vision V350: Confirm CPU, I/O, memory, communications, and Structured Text support in the current selection guide

Vision V570: Confirm CPU, I/O, memory, communications, and Structured Text 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 Motor 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 Structured Text for Motor Control

Structured Text (ST) is a high-level, text-based programming language defined in IEC 61131-3. It resembles Pascal and provides powerful constructs for complex algorithms, calculations, and data manipulation.

Execution Model:

Code executes sequentially from top to bottom within each program unit. Variables maintain state between scan cycles unless explicitly reset.

Core Advantages for Motor Control:

Powerful for complex logic: Critical for Motor Control when handling beginner to intermediate control logic

Excellent code reusability: Critical for Motor Control when handling beginner to intermediate control logic

Compact code representation: Critical for Motor Control when handling beginner to intermediate control logic

Good for algorithms and calculations: Critical for Motor Control when handling beginner to intermediate control logic

Familiar to software developers: Critical for Motor Control when handling beginner to intermediate control logic

Why Structured Text Fits Motor Control:

Motor Control systems in Industrial Manufacturing typically involve:

Sensors: Current transformers for motor current monitoring, RTD or thermocouple for motor winding temperature, Vibration sensors for bearing monitoring

Actuators: Contactors for direct-on-line starting, Soft starters for reduced voltage starting, Variable frequency drives for speed control

Complexity: Beginner to Intermediate with challenges including Managing starting current within supply limits

Programming Fundamentals in Structured Text:

Variables:
- declaration: VAR / VAR_INPUT / VAR_OUTPUT / VAR_IN_OUT / VAR_GLOBAL sections
- initialization: Variables can be initialized at declaration: Counter : INT := 0;
- constants: VAR CONSTANT section for read-only values

Operators:
- arithmetic: + - * / MOD (modulo)
- comparison: = <> < > <= >=
- logical: AND OR XOR NOT

ControlStructures:
- if: IF condition THEN statements; ELSIF condition THEN statements; ELSE statements; END_IF;
- case: CASE selector OF value1: statements; value2: statements; ELSE statements; END_CASE;
- for: FOR index := start TO end BY step DO statements; END_FOR;

Best Practices for Structured Text:

Use meaningful variable names with consistent naming conventions

Initialize all variables at declaration to prevent undefined behavior

Use enumerated types for state machines instead of magic numbers

Break complex expressions into intermediate variables for readability

Use functions for reusable calculations and function blocks for stateful operations

Common Mistakes to Avoid:

Using = instead of := for assignment (= is comparison)

Forgetting semicolons at end of statements

Integer division truncation - use REAL for decimal results

Infinite loops from incorrect WHILE/REPEAT conditions

Typical Applications:

1. PID control: Directly applicable to Motor Control
2. Recipe management: Related control patterns
3. Statistical calculations: Related control patterns
4. Data logging: Related control patterns

Understanding these fundamentals prepares you to implement effective Structured Text solutions for Motor Control using Unitronics VisiLogic / UniLogic.

Implementing Motor Control with Structured Text

Motor control systems use PLCs to start, stop, and regulate electric motors in industrial applications. These systems provide protection, speed control, and coordination for motors ranging from fractional horsepower to thousands of horsepower.

This walkthrough demonstrates practical implementation using Unitronics VisiLogic / UniLogic and Structured Text programming.

System Requirements:

A typical Motor Control implementation includes:

Input Devices (Sensors):
1. Current transformers for motor current monitoring: Critical for monitoring system state
2. RTD or thermocouple for motor winding temperature: Critical for monitoring system state
3. Vibration sensors for bearing monitoring: Critical for monitoring system state
4. Speed encoders or tachometers: Critical for monitoring system state
5. Torque sensors for load monitoring: Critical for monitoring system state

Output Devices (Actuators):
1. Contactors for direct-on-line starting: Primary control output
2. Soft starters for reduced voltage starting: Supporting control function
3. Variable frequency drives for speed control: Supporting control function
4. Brakes (mechanical or dynamic): Supporting control function
5. Starters (star-delta, autotransformer): Supporting control function

Control Equipment:

Motor control centers (MCCs)

AC induction motors (NEMA/IEC frame)

Synchronous motors for high efficiency

DC motors for precise speed control

Control Strategies for Motor Control:

1. Primary Control: Industrial motor control using PLCs for start/stop, speed control, and protection of electric motors.
2. Safety Interlocks: Preventing Soft start implementation
3. Error Recovery: Handling Overload protection

Implementation Steps:

Step 1: Calculate motor starting current and verify supply capacity

In VisiLogic / UniLogic, calculate motor starting current and verify supply capacity.

Step 2: Select starting method based on motor size and load requirements

In VisiLogic / UniLogic, select starting method based on motor size and load requirements.

Step 3: Configure motor protection with correct thermal curve

In VisiLogic / UniLogic, configure motor protection with correct thermal curve.

Step 4: Implement control logic for start/stop with proper interlocks

In VisiLogic / UniLogic, implement control logic for start/stop with proper interlocks.

Step 5: Add speed control loop if VFD is used

In VisiLogic / UniLogic, add speed control loop if vfd is used.

Step 6: Configure acceleration and deceleration ramps

In VisiLogic / UniLogic, configure acceleration and deceleration ramps.

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. Managing starting current within supply limits

Solution: Structured Text addresses this through Powerful for complex logic.

2. Coordinating acceleration with driven load requirements

Solution: Structured Text addresses this through Excellent code reusability.

3. Protecting motors from frequent starting (thermal cycling)

Solution: Structured Text addresses this through Compact code representation.

4. Handling regenerative energy during deceleration

Solution: Structured Text addresses this through Good for algorithms and calculations.

Safety Considerations:

Proper machine guarding for rotating equipment

Emergency stop functionality with safe torque off

Lockout/tagout provisions for maintenance

Arc flash protection and PPE requirements

Proper grounding and bonding

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 Structured Text Example for Motor Control

Illustrative Structured Text example for Motor 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 - Motor Control Control *)
(* Structured Text Implementation for Industrial Manufacturing *)
(* Unitronics projects use IDE-managed tag names rather than raw memory a *)

PROGRAM PRG_MOTOR_CONTROL_Control

VAR
    (* State Machine Variables *)
    eState : E_MOTOR_CONTROL_States := IDLE;
    bEnable : BOOL := FALSE;
    bFaultActive : BOOL := FALSE;

    (* Timers *)
    tonDebounce : TON;
    tonProcessTimeout : TON;
    tonFeedbackCheck : TON;

    (* Counters *)
    ctuCycleCounter : CTU;

    (* Process Variables *)
    rCurrentsensors : REAL := 0.0;
    rMotorstarters : REAL := 0.0;
    rSetpoint : REAL := 100.0; (* Illustrative value; replace with a reviewed requirement *)
END_VAR

VAR CONSTANT
    (* Industrial Manufacturing Process Parameters *)
    C_DEBOUNCE_TIME : TIME := T#500MS;
    C_PROCESS_TIMEOUT : TIME := T#30S; (* Illustrative value; verify for the process *)
    C_BATCH_SIZE : INT := 50; (* Illustrative value; replace with a reviewed requirement *)
END_VAR

(* Input Conditioning *)
tonDebounce(IN := bStartButton, PT := C_DEBOUNCE_TIME);
bEnable := tonDebounce.Q AND NOT bEmergencyStop AND bSafetyOK;

(* Main State Machine - Pattern: State machines are commonly implemented  *)
CASE eState OF
    IDLE:
        rMotorstarters := 0.0;
        ctuCycleCounter(RESET := TRUE);
        IF bEnable AND rCurrentsensors > 0.0 THEN
            eState := STARTING;
        END_IF;

    STARTING:
        (* Ramp up output - Gradual start *)
        rMotorstarters := MIN(rMotorstarters + 5.0, rSetpoint);
        IF rMotorstarters >= rSetpoint THEN
            eState := RUNNING;
        END_IF;

    RUNNING:
        (* Motor Control active - Motor control systems use PLCs to start, stop, and *)
        tonProcessTimeout(IN := TRUE, PT := C_PROCESS_TIMEOUT);
        ctuCycleCounter(CU := bCyclePulse, PV := C_BATCH_SIZE);

        IF ctuCycleCounter.Q THEN
            eState := COMPLETE;
        ELSIF tonProcessTimeout.Q THEN
            bFaultActive := TRUE;
            eState := FAULT;
        END_IF;

    COMPLETE:
        rMotorstarters := 0.0;
        (* Log production data - Data logging uses UniLogic's Data Samplers — configured triggers (time-based or event-based) write structured records to Data Tables or SD card in CSV format. Exported files can be pushed via FTP or email. For cloud integration, UniCloud provides managed data ingestion. Simpler VisiLogic projects use HMI-triggered SD writes via custom ladder code. *)
        eState := IDLE;

    FAULT:
        rMotorstarters := 0.0;
        (* Alarm handling uses UniLogic's built-in Alarm Manager — configure alarm conditions in tables with severity, message text, and logging behaviour, and the engine handles detection, acknowledgement, history, and HMI display without custom code. VisiLogic uses a simpler alarm approach via HMI event handlers. *)
        IF bFaultReset AND NOT bEmergencyStop THEN
            bFaultActive := FALSE;
            eState := IDLE;
        END_IF;
END_CASE;

(* Safety Override - Always executes *)
IF bEmergencyStop OR NOT bSafetyOK THEN
    rMotorstarters := 0.0;
    eState := FAULT;
    bFaultActive := TRUE;
END_IF;

END_PROGRAM

Code Explanation:

1.Enumerated state machine (State machines are commonly implemented in ladder with step-counter registers or in ST using CASE structures with named state constants. UniLogic's HMI graphical bindings make state-to-screen visualisation straightforward — a single state variable drives both logic and operator screen transitions. SFC is not a primary language on Unitronics; state logic is typically CASE or ladder.) for clear Motor Control sequence control
2.Constants use clearly marked illustrative values that must be replaced with reviewed project requirements
3.Input conditioning with debounce timer prevents false triggers in industrial environment
4.STARTING state implements soft-start ramp - prevents mechanical shock
5.Process timeout detection flags a possible stuck condition for investigation
6.The final override illustrates a software permissive only; it is not a safety-rated function and must not replace a validated safety system

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
✓Structured Text: Use meaningful variable names with consistent naming conventions
✓Structured Text: Initialize all variables at declaration to prevent undefined behavior
✓Structured Text: Use enumerated types for state machines instead of magic numbers
✓Motor Control: Verify motor running with current or speed feedback, not just contactor status
✓Motor Control: Implement minimum off time between starts for motor cooling
✓Motor Control: Add phase loss and phase reversal protection
✓Debug with VisiLogic / UniLogic: Use the built-in simulator to reproduce issues before hardware visit
✓Safety: Proper machine guarding for rotating equipment
✓Use a compatible simulator or isolated test rig to test Motor Control logic before deployment

Common Pitfalls to Avoid

⚠Structured Text: Using = instead of := for assignment (= is comparison)
⚠Structured Text: Forgetting semicolons at end of statements
⚠Structured Text: Integer division truncation - use REAL for decimal results
⚠Unitronics common error: VisiLogic-to-UniLogic migration issues — not all projects convert cleanly
⚠Motor Control: Managing starting current within supply limits
⚠Motor Control: Coordinating acceleration with driven load requirements
⚠Neglecting to validate Current transformers for motor current monitoring leads to control errors
⚠Insufficient comments make Structured Text programs unmaintainable over time

Related Certifications

🏆Unitronics Certified Integrator

🏆UniLogic Developer Training

🏆Advanced Unitronics Programming Certification

Applying Structured Text to Motor 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 beginner to intermediate Motor 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

Structured Text Foundation:

Structured Text (ST) is a high-level, text-based programming language defined in IEC 61131-3. It resembles Pascal and provides powerful constructs for...

Project duration depends on scope, reviews, hardware availability, software and firmware versions, testing, commissioning, and site constraints. Remember: Verify motor running with current or speed feedback, not just contactor status

For further learning, explore related topics including Recipe management, Fan systems, and Unitronics platform-specific features for Motor Control optimization.