From Dual-Channel Wiring to Acceptance Testing: Commissioning and Validating AGV Servo STO Systems

Introduction: Why STO Commissioning Has Become Critical for Modern AGV Systems

As Automated Guided Vehicles (AGVs) and Autonomous Mobile Robots (AMRs) are increasingly deployed in factories, warehouses, and logistics centers, functional safety has become one of the most important factors during system integration. Unlike traditional conveyor systems, AGVs operate in dynamic environments where humans, machines, and mobile robots share the same workspace.

A Safe Torque Off (STO) function in AGV servo drives provides a reliable method to stop motor torque generation immediately when a safety event occurs, such as emergency stop activation, safety scanner detection, protective door opening, or safety PLC intervention.

However, simply installing servo motors with STO capability does not guarantee a safe AGV system. OEM manufacturers and system integrators must properly design the safety architecture, complete dual-channel wiring, configure parameters, verify diagnostic functions, and perform acceptance testing according to safety requirements.

This article explains the complete AGV servo STO commissioning process, covering:

• How to connect dual-channel STO circuits between safety PLCs and servo drives

• How to test and validate STO functions during AGV commissioning

• Required diagnostic coverage (DC) and MTTFd values for AGV safety circuits

• How STO architecture affects dual-wheel differential drive AGVs

• How STO influences stopping distance in heavy-load AGV applications

1. How to Connect Dual-Channel STO Circuits on AGV Servo Drives

Understanding Dual-Channel STO Architecture in AGV Systems

Modern AGV servo systems typically use dual-channel STO inputs to achieve higher functional safety levels. A dual-channel design means that two independent safety signals must be monitored separately.

Instead of relying on a single emergency stop signal, the safety controller continuously checks two independent channels:

• STO Channel 1 (STO1)

• STO Channel 2 (STO2)

When both channels are interrupted, the servo drive immediately disables motor torque output.

A typical AGV safety architecture includes:

Safety Devices → Safety PLC → STO Inputs → Servo Drives → AGV Motors

Common safety input devices include:

• Emergency stop buttons

• Safety laser scanners

• Safety bumpers

• Safety door switches

• Safety light curtains

The safety PLC evaluates these signals and sends a safe shutdown command to the AGV servo drives.

Typical Dual-Channel STO Wiring Method

For AGV applications, the STO wiring usually follows this structure:

During normal operation:

• STO1 = ON

• STO2 = ON

• Servo drive enables torque output

During a safety event:

• STO1 = OFF

• STO2 = OFF

• Servo drive immediately removes torque generation

The motor itself does not receive a braking command. Instead, the drive electronics block the power stage, preventing electrical torque production.

Why AGV Manufacturers Prefer Dual-Channel STO

AGVs often operate around:

• Factory workers

• Forklifts

• Automated production lines

• Robotic cells

• High-value materials

A single-channel safety circuit has several limitations:

• A broken wire may not be detected

• A failed safety contact may remain unnoticed

• Diagnostic capability is limited

Dual-channel STO improves:

1. Fault Detection Capability

The system can identify:

• Open circuit faults

• Short circuits

• Channel mismatch errors

2. Safety Integrity

A redundant architecture helps AGV manufacturers achieve higher safety performance levels, such as:

• PL d

• PL e

depending on the complete safety design.

3. Easier Certification

For OEM AGV manufacturers, a standardized dual-channel STO architecture simplifies compliance with:

• ISO 13849-1

• IEC 61800-5-2

• IEC 62061

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2. How to Test and Validate STO Function During AGV Commissioning

Why STO Testing Is Critical During AGV Commissioning

For AGV and AMR manufacturers, Safe Torque Off (STO) validation is one of the most important steps before equipment delivery and mass production. Although servo drives with integrated STO functions can quickly remove motor torque during a safety event, the entire safety chain must be verified to ensure reliable operation.

A complete STO commissioning process confirms that:

• Safety devices correctly trigger STO shutdown

• Safety PLC logic works as designed

• Servo drives respond correctly to STO commands

• Both STO channels operate independently

• The AGV cannot restart unexpectedly after a safety stop

• Stopping performance meets the requirements of the risk assessment

Unlike a standard motor function test, STO testing is a functional safety validation process. It ensures that the AGV can enter a safe state whenever operators, equipment, or the environment are at risk.

AGV STO Commissioning Test Process

A professional AGV STO validation procedure normally includes five major stages:

1. Electrical wiring inspection

2. Servo drive STO configuration verification

3. Safety function activation testing

4. Fault simulation testing

5. Final acceptance documentation

Each stage verifies a different part of the safety architecture.

1. Verify STO Wiring Before Power-Up

Before testing the STO function, engineers should first confirm that the dual-channel safety wiring is correctly installed.

A typical AGV STO circuit includes:

Emergency Stop / Safety Scanner / Safety PLC → STO Channel 1 & STO Channel 2 → Servo Drive → AGV Motor

During wiring inspection, engineers should check:

STO Channel Independence

The two STO channels must remain electrically independent.

Incorrect wiring examples:

• Connecting STO1 and STO2 together

• Sharing one safety output for both channels

• Using a standard PLC output instead of a safety-rated output

These mistakes may prevent the system from achieving the required safety performance level.

Wiring Continuity Check

Before applying power, verify:

• STO1 signal continuity

• STO2 signal continuity

• 24V safety supply voltage

• Ground reference connection

• Connector locking condition

For AGVs operating in industrial environments, engineers should also check:

• Cable protection

• Vibration resistance

• Electromagnetic interference protection

Loose connectors or damaged cables are common causes of STO failure during field operation.

2. Confirm Servo Drive STO Configuration

After wiring verification, the servo drive parameters should be checked.

Modern AGV servo drives usually provide STO-related monitoring functions, including:

• STO input status

• Safety fault detection

• Channel mismatch detection

• Restart prevention

Engineers should confirm:

STO Input Status

When the AGV is in normal operation:

• STO Channel 1 = Active

• STO Channel 2 = Active

The servo drive should allow motor torque output.

When STO is triggered:

• STO Channel 1 = Disabled

• STO Channel 2 = Disabled

The drive should immediately remove torque generation.

Safety Parameter Verification

Depending on the servo drive manufacturer, verify:

• STO enable setting

• Safety input polarity

• Fault response mode

• Automatic restart prevention

• Diagnostic feedback output

Incorrect parameter settings can cause situations where the safety input changes but the motor behavior does not match expectations.

3. Perform Normal Operation Verification Before STO Testing

STO testing should only begin after confirming that the AGV operates normally.

Before safety validation, test:

Motion Control

Verify:

• Forward movement

• Reverse movement

• Turning operation

• Speed regulation

• Position accuracy

For differential drive AGVs:

• Left motor operation

• Right motor operation

• Wheel synchronization

should be checked separately.

Encoder and Feedback Signals

Confirm:

• Encoder communication

• Speed feedback stability

• Position feedback accuracy

Although STO removes torque output, incorrect encoder feedback can affect restart logic and diagnostic functions.

4. Execute STO Trigger Tests

After basic operation is confirmed, engineers should perform actual STO activation tests.

Test 1: Emergency Stop Button Test

Procedure:

1. Run the AGV at normal operating speed

2. Press the emergency stop button

3. Monitor servo drive response

Expected Result:

• Safety PLC removes STO signals

• Servo drive disables torque output

• Motor stops producing driving force

• STO status is displayed

• AGV cannot restart automatically

This verifies the complete emergency stop safety chain.

Test 2: Safety Scanner Trigger Test

AGVs commonly use safety laser scanners to detect obstacles and people.

Procedure:

1. Operate AGV in automatic mode

2. Enter the scanner detection area

3. Trigger the safety zone

Expected Result:

• Safety PLC receives scanner signal

• STO output changes

• Servo drives enter safe state

• Vehicle movement stops according to safety requirements

This test is especially important for warehouse and factory AGVs operating around workers.

Test 3: Protective Device Test

For AGVs equipped with:

• Safety bumpers

• Safety doors

• Access switches

perform additional STO validation.

Example:

When a safety bumper is pressed:

Expected behavior:

• AGV motion stops

• Servo torque is removed

• Safety alarm appears

• Restart requires manual reset

5. Perform STO Channel Fault Simulation Tests

A professional STO acceptance test should not only test normal activation but also verify fault detection.

STO Channel 1 Failure Test

Procedure:

Disconnect or interrupt STO Channel 1.

Expected Result:

The system should:

• Detect channel mismatch

• Generate safety fault

• Prevent AGV movement

STO Channel 2 Failure Test

Repeat the same procedure for Channel 2.

Expected result:

• Fault detected

• Servo drive remains disabled

• No unsafe movement occurs

Short Circuit Detection Test

Where applicable, test:

• Channel-to-channel short circuit

• Channel-to-power short circuit

• Ground fault conditions

The safety system should identify abnormal conditions and enter a safe state.

6. Verify STO Restart Behavior

Unexpected restart after STO release is a major safety risk.

Therefore, restart behavior must be tested carefully.

After STO activation:

1. Remove the safety trigger

2. Reset the safety PLC

3. Enable the AGV control system

4. Send a new motion command

The AGV should remain stationary until a complete restart sequence is completed.

Why Automatic Restart Is Dangerous

If an AGV automatically restarts after STO recovery:

Possible risks include:

• Unexpected vehicle movement

• Operator injury

• Collision with equipment

• Material damage

For most industrial AGV applications, manual reset or controlled restart is required.

7. Measure STO Response Time and Stopping Distance

STO validation is not complete without evaluating stopping performance.

When STO is activated:

The total stopping process includes:

1. Safety device detection time

2. Safety PLC processing time

3. STO signal transmission time

4. Servo drive torque removal time

5. Mechanical stopping time

Factors Affecting AGV Stopping Distance

Stopping distance depends on:

• AGV total weight

• Payload weight

• Travel speed

• Wheel diameter

• Floor friction

• Battery voltage condition

• Brake response time

For heavy-duty AGVs, STO is often combined with:

• Electromagnetic brakes

• Mechanical holding brakes

• Controlled deceleration functions

8. Conduct Final STO Acceptance Testing and Documentation

Before AGV delivery, manufacturers should create a complete STO validation report.

The document should include:

Electrical Documentation

• STO wiring diagram

• Safety PLC configuration

• Servo drive safety parameters

Test Records

• Emergency stop test results

• Scanner trigger test results

• STO channel fault test results

• Restart verification results

Safety Information

• Servo drive STO specifications

• PFHd data

• MTTFd value

• Diagnostic coverage information

Best Practices for AGV OEM Manufacturers

To improve STO reliability during production and field operation:

Use Servo Motors With Integrated STO Functions

Integrated servo solutions reduce:

• External wiring

• Control cabinet complexity

• Commissioning time

They are especially suitable for:

• Compact AMRs

• Warehouse robots

• Mobile inspection robots

• Industrial AGVs

Include STO Testing in Every Production Cycle

For mass-produced AGVs, STO validation should become part of the standard factory acceptance test (FAT).

Recommended checks:

• STO input verification

• Safety communication test

• Emergency stop test

• Fault diagnosis test

Conclusion

Testing and validating STO functions during AGV commissioning is essential for ensuring safe and reliable operation. A complete STO verification process should cover electrical inspection, servo drive configuration, safety trigger testing, fault simulation, restart prevention, and stopping distance measurement.

For AGV and AMR manufacturers, properly commissioned STO systems provide:

• Higher operator safety

• More reliable machine operation

• Easier compliance with safety standards

• Faster customer acceptance

By integrating STO testing into the AGV commissioning workflow, OEM manufacturers can deliver safer autonomous vehicles with predictable and repeatable safety performance.

3. Required Diagnostic Coverage (DC) and MTTFd Values for AGV STO Safety Circuits

Understanding Safety Parameters in AGV Applications

When designing AGV safety systems, engineers often evaluate:

• Diagnostic Coverage (DC)

• Mean Time To Dangerous Failure (MTTFd)

• Performance Level (PL)

• Safety Integrity Level (SIL)

These parameters determine whether the safety circuit can reliably detect dangerous failures.

Diagnostic Coverage (DC) Requirements

Diagnostic Coverage describes how effectively a safety system detects failures.

Typical categories:

For AGV servo STO systems, medium or high diagnostic coverage is generally preferred.

Examples of diagnostic functions include:

• STO channel monitoring

• Cross-channel comparison

• Drive fault feedback

• Safety PLC diagnostics

MTTFd Requirements for AGV Servo Systems

MTTFd represents the expected time before a dangerous failure occurs.

For AGV applications:

Higher MTTFd values improve:

• System reliability

• Safety classification

• Maintenance intervals

Servo drives with integrated STO functions typically provide safety data including:

• MTTFd value

• DC rating

• PFHd value

OEM AGV manufacturers should request these values from servo motor suppliers during component selection.

How Integrated Servo Motors Help AGV Safety Design

Integrated DC servo motors and BLDC servo motors with STO functions simplify safety architecture by combining:

• Motor

• Encoder

• Servo controller

• Safety input interface

Advantages include:

• Reduced wiring complexity

• Faster commissioning

• Smaller control cabinet

• Easier certification

For compact AGVs and AMRs, integrated servo solutions are becoming increasingly popular because they reduce system integration time.

4. How STO Architecture Affects Left and Right Wheel Synchronization in AGVs

Why STO Architecture Matters for Differential Drive AGVs

Most industrial AGVs and AMRs use a differential drive architecture, where two independent servo motors control the left and right drive wheels. By adjusting the speed difference between these two motors, the vehicle can move forward, reverse, turn, and rotate in place.

A typical differential drive AGV includes:

• Left-side servo motor

• Right-side servo motor

• Independent servo drives

• Encoder feedback systems

• Safety PLC

• STO safety inputs

In normal operation, both motors work together through motion control algorithms. However, when a safety event occurs, the STO architecture determines how quickly and safely both motors enter a torque-off state.

An improperly designed STO system can create uneven wheel behavior, resulting in:

• Unexpected AGV rotation

• Longer stopping distance

• Payload instability

• Reduced safety performance

Therefore, STO design must consider the synchronization of both left and right wheel drives.

Understanding Differential Drive AGV Motion Control

How Left and Right Motors Control AGV Movement

A differential drive AGV changes direction by controlling the speed relationship between two wheels.

Examples:

Straight Movement

Both motors rotate at the same speed:

• Left motor = 100 RPM

• Right motor = 100 RPM

Result:

• AGV moves straight forward

Turning Right

The left wheel rotates faster:

• Left motor = 120 RPM

• Right motor = 80 RPM

Result:

• AGV turns right smoothly

Rotating in Place

The motors rotate in opposite directions:

• Left motor = Forward

• Right motor = Reverse

Result:

• AGV rotates around its center axis

Because both motors directly affect vehicle stability, safety functions such as STO must shut down both drive systems in a coordinated manner.

How STO Works in Dual-Motor AGV Systems

Typical Dual-Channel STO Architecture

A standard AGV safety architecture uses a safety PLC to control both servo drives.

Example:

Safety Devices
(E-Stop / Scanner / Bumper)

          ↓

Safety PLC

          ↓

STO Channel 1  ───── Left Servo Drive
             └───── Right Servo Drive

STO Channel 2  ───── Left Servo Drive
             └───── Right Servo Drive

          ↓

Left Motor + Right Motor

When a safety event occurs:

1. Safety PLC detects danger

2. Both STO channels are interrupted

3. Left servo drive removes torque

4. Right servo drive removes torque

5. AGV enters a safe state

The goal is to ensure both wheels stop generating driving force at almost the same time.

Why Both Wheels Must Enter STO Simultaneously

1. Prevent Unexpected AGV Rotation

If only one motor enters STO while the other continues producing torque, the AGV may rotate unexpectedly.

Example:

Before STO:

• Left motor: 200 RPM

• Right motor: 200 RPM

Safety event occurs:

• Left motor: STO activated

• Right motor: Still running at 200 RPM

Result:

• Right wheel continues pushing

• AGV rotates around the stopped wheel

This can create collision risks, especially in narrow factory aisles.

2. Maintain Vehicle Stability Under Heavy Loads

Heavy-duty AGVs often transport:

• Automotive components

• Pallets

• Raw materials

• Industrial equipment

When one wheel loses torque before the other:

Possible consequences:

• Load shifting

• Uneven braking force

• Vehicle deviation from path

• Increased stopping distance

For high-payload AGVs, synchronized STO response is especially important.

3. Avoid Uneven Mechanical Stress

Asymmetric torque removal can create additional stress on:

• Gearboxes

• Wheel couplings

• Drive shafts

• Mechanical frames

A synchronized STO design reduces mechanical shock during emergency stops.

STO Implementation Methods for Left and Right Servo Drives

Method 1: Common Dual-Channel STO Control

This is the most common approach for industrial AGVs.

Both servo drives receive the same STO signals.

Example:

Advantages:

• Simple wiring

• Easy safety validation

• Consistent shutdown timing

Suitable for:

• Warehouse AGVs

• Factory transport robots

• Indoor AMRs

Method 2: Individual STO Monitoring

For higher-performance AGVs, each motor drive may have independent safety monitoring.

Example:

Safety PLC

 ↓

Left STO Monitoring → Left Servo Drive

Right STO Monitoring → Right Servo Drive

Advantages:

• Individual fault diagnosis

• Better troubleshooting

• Higher diagnostic capability

Suitable for:

• Heavy-load AGVs

• Outdoor mobile robots

• Safety-critical applications

How Servo Drive Response Time Affects Wheel Synchronization

Even when both motors receive STO commands simultaneously, the actual torque removal time may vary.

Factors include:

• Servo drive hardware design

• Internal processing time

• Communication delay

• Motor inertia

• Load condition

For example:

Left servo STO response:

• 5 ms

Right servo STO response:

• 8 ms

The 3 ms difference may seem small, but at high AGV speeds it can affect stopping behavior.

Best Practices to Improve STO Synchronization

1. Use Matched Servo Motors and Drives

For differential drive AGVs, selecting identical servo systems helps maintain:

• Similar response time

• Similar torque characteristics

• Consistent diagnostics

Examples:

• Same motor series

• Same encoder type

• Same STO-rated servo drive

2. Use Hardware-Based STO Instead of Standard Communication

Although some AGVs use communication-based safety functions, hardware STO inputs are often preferred for emergency safety functions.

Advantages:

• Faster response

• Independent from software communication

• Easier safety validation

3. Synchronize Brake Control With STO

For heavy AGVs, STO alone may not provide enough stopping force.

A common safety strategy:

Normal stop:

→ Servo controlled deceleration

Emergency stop:

→ Servo torque removal + mechanical brake activation

The brake system should activate consistently on both wheels.

4. Validate Left and Right Stopping Performance

During commissioning, engineers should measure:

• STO trigger time

• Left wheel stopping time

• Right wheel stopping time

• Vehicle stopping distance

• Vehicle direction deviation

The AGV should stop without:

• Excessive rotation

• Side drift

• Load instability

STO and AGV Safety Standards

When designing STO systems, AGV manufacturers commonly consider:

IEC 61800-5-2

Defines safety-related functions for adjustable speed drives, including:

• Safe Torque Off (STO)

• Safe Stop functions

ISO 13849-1

Used for evaluating:

• Performance Level (PL)

• Safety circuit reliability

• Diagnostic coverage

IEC 62061

Provides safety lifecycle requirements for machinery control systems.

Special Considerations for Heavy-Duty Differential Drive AGVs

For AGVs carrying large payloads, additional safety factors should be evaluated.

High Inertia Loads

Heavy loads continue moving after torque removal.

Consider:

• Brake torque

• Wheel friction

• Floor condition

• Speed limitation

Inclined Operation

AGVs operating on slopes require additional attention.

Without proper braking:

• Vehicle may roll backward

• Load stability may decrease

Recommended solutions:

• Servo motors with electromagnetic brakes

• STO combined with mechanical holding brakes

Outdoor AGV Applications

Outdoor AGVs may experience:

• Uneven surfaces

• Temperature changes

• Water and dust exposure

Recommended motor features:

• IP65/IP67 protection

• High vibration resistance

• Stable STO operation under harsh environments

Conclusion

The STO architecture directly affects the safety and stability of left and right wheel synchronization in differential drive AGVs. Because both wheels determine vehicle movement and direction, both servo drives must enter the STO state reliably and simultaneously.

A well-designed AGV STO system should provide:

• Dual-channel safety control

• Synchronized torque removal

• Reliable fault detection

• Controlled restart behavior

• Verified stopping performance

For AGV and AMR manufacturers, selecting servo motors with integrated STO functionality simplifies safety design while improving system reliability, commissioning efficiency, and compliance with international functional safety requirements.

5. How STO Influences Sliding Distance of Heavy-Duty AGVs After Torque Removal

STO Does Not Mean Immediate Mechanical Stop

A common misunderstanding is that STO instantly stops the AGV.

Actually:

STO removes motor torque generation.

The AGV may continue moving because of:

• Vehicle inertia

• Payload weight

• Floor friction

• Wheel diameter

• Speed

• Slope angle

This remaining movement is called sliding distance.

Factors Affecting AGV Sliding Distance After STO

1. Total Vehicle Mass

A heavier AGV requires a longer stopping distance.

Example:

A 500 kg AGV carrying a 1000 kg load has significantly higher inertia than an unloaded vehicle.

2. Travel Speed

Stopping distance increases approximately with the square of velocity.

Higher speed requires:

• Longer safety distance

• Earlier scanner detection

3. Wheel and Floor Conditions

Factors include:

• Tire material

• Floor cleanliness

• Surface friction coefficient

Wet or dusty floors increase sliding distance.

4. Mechanical Brake Response Time

For heavy-duty AGVs:

STO is often combined with:

• Spring-applied brakes

• Electromagnetic brakes

Brake delay directly affects stopping performance.

How AGV Engineers Validate STO Stopping Distance

During acceptance testing:

Measure:

1. AGV running speed

2. STO trigger position

3. Motor torque removal time

4. Final stopping position

The measured distance must meet the safety requirements defined by:

• Risk assessment

• Safety scanner configuration

• AGV operating environment

Additional Considerations for AGV STO System Acceptance

Software Configuration Verification

Before delivery, engineers should verify:

• STO parameters

• Servo drive firmware

• Safety PLC logic

• Communication status

• Fault handling strategy

Environmental Reliability Testing

AGVs often operate continuously.

STO systems should be tested under:

• Maximum payload

• Maximum speed

• Temperature variation

• Long operation cycles

Documentation Required for OEM Delivery

A professional AGV STO acceptance package should include:

• Electrical schematic

• STO wiring diagram

• Safety validation report

• Test records

• Servo drive safety data

• Risk assessment documents

Conclusion: Building Reliable AGV Servo STO Systems from Wiring to Validation

A reliable AGV safety system requires much more than selecting servo motors with STO functionality. The complete process includes correct dual-channel wiring, safety PLC integration, functional testing, diagnostic verification, and stopping-distance validation.

For AGV and AMR manufacturers, servo motors with integrated STO functions provide significant advantages:

• Simplified safety architecture

• Reduced wiring

• Faster commissioning

• Higher reliability

• Easier compliance with international safety standards

By following a structured STO commissioning and acceptance process, OEM manufacturers can build safer AGVs with predictable stopping behavior and long-term operational reliability.

FAQs for AGV Servo STO Testing and Validation

FAQ 1: What is STO testing in AGV servo systems?

Answer:

STO testing in AGV servo systems is the process of verifying that the Safe Torque Off function works correctly when a safety event occurs. It confirms that the servo drive can safely remove motor torque after receiving signals from safety devices such as emergency stops, safety scanners, and safety PLCs.

During commissioning, engineers test STO wiring, drive response, fault detection, restart prevention, and stopping performance to ensure the AGV operates safely.

FAQ 2: Why is STO validation important during AGV commissioning?

Answer:

STO validation is essential because installing an STO-enabled servo motor does not automatically guarantee a safe AGV system. The complete safety chain must be tested, including safety sensors, PLC logic, STO inputs, servo drives, and motor response.

Proper STO validation helps prevent:

• Unexpected AGV movement

• Safety circuit failures

• Incorrect emergency stopping behavior

• Certification issues during customer acceptance

FAQ 3: How do you test dual-channel STO inputs on an AGV servo drive?

Answer:

Dual-channel STO testing requires checking both STO1 and STO2 inputs independently.

A typical test procedure includes:

1. Verify STO1 and STO2 wiring

2. Activate emergency stop or safety scanner

3. Confirm both STO channels switch to a safe state

4. Check servo drive STO status feedback

5. Verify the motor cannot generate torque

Engineers should also disconnect each STO channel separately to confirm fault detection capability.

FAQ 4: What happens when STO is activated on an AGV servo motor?

Answer:

When STO is activated, the servo drive immediately disables the power stage, preventing the motor from producing torque.

The AGV motor will:

• Stop generating driving force

• Enter a safe torque-off condition

• Report STO status or safety fault

However, STO does not actively brake the vehicle. The AGV may continue moving due to inertia, payload weight, and floor conditions.

FAQ 5: How does STO affect left and right wheel synchronization in differential drive AGVs?

Answer:

For differential drive AGVs, both left and right servo motors should receive STO commands simultaneously.

If one wheel enters STO before the other:

• The AGV may rotate unexpectedly

• Vehicle stability may decrease

• Heavy loads may shift

• Stopping distance may increase

A synchronized dual-channel STO architecture ensures both drive motors remove torque at nearly the same time.

FAQ 6: Can an AGV automatically restart after STO is released?

Answer:

Generally, AGVs should not automatically restart after STO activation.

A safe restart sequence normally requires:

1. Remove the safety trigger

2. Reset the safety PLC

3. Enable the AGV control system

4. Send a new movement command

Preventing automatic restart reduces the risk of unexpected vehicle movement near operators.

FAQ 7: What tests should be included in an AGV STO acceptance test?

Answer:

A complete AGV STO acceptance test should include:

• Emergency stop test

• Safety scanner activation test

• STO channel failure test

• Restart prevention test

• Servo drive fault diagnosis test

• Stopping distance measurement

These tests verify that the AGV safety system meets design requirements before delivery.

FAQ 8: Does STO immediately stop an AGV?

Answer:

No. STO removes motor torque but does not physically stop the vehicle instantly.

The actual stopping distance depends on:

• AGV weight

• Payload mass

• Travel speed

• Wheel friction

• Brake response time

• Floor conditions

For heavy-duty AGVs, STO is often combined with mechanical brakes or controlled deceleration functions.

FAQ 9: What safety parameters should be checked for AGV STO systems?

Answer:

Important safety parameters include:

• Diagnostic Coverage (DC)

• Mean Time To Dangerous Failure (MTTFd)

• Probability of Dangerous Failure per Hour (PFHd)

• Performance Level (PL)

• Safety Integrity Level (SIL)

These values help determine whether the AGV safety system meets functional safety requirements.

FAQ 10: Why do AGV manufacturers choose servo motors with integrated STO functions?

Answer:

Integrated STO servo motors simplify AGV safety design by combining motor control and safety functions in one compact solution.

Benefits include:

• Reduced wiring complexity

• Smaller control cabinets

• Faster commissioning

• Easier safety validation

• Improved system reliability

They are widely used in:

• AGVs

• AMRs

• Warehouse robots

• Smart factory automation systems