Before the 1990s, machine safety was brute-force: contactors, relays, and hardwired emergency stops that simply killed power to everything. The problem? Killing power instantly on a high-inertia axis doesn't stop it — it just stops controlling it. The result was often a dangerous, uncontrolled coast-down, not a safe stop.

The 1998 release of IEC 61508 — Functional Safety of Electrical/Electronic/Programmable Electronic Safety-related Systems — changed the paradigm entirely. Instead of removing power, modern safety architecture requires the system to transition to a known, controlled, pre-defined safe state for any single fault condition.

This article — informed by a February 2026 technical piece in Control Engineering authored by Yaskawa America's own Product Support Engineering team — walks through the full spectrum of safety integrated functions available in modern servo drives, with a specific look at what Yaskawa's Sigma-X series and ASM-X Advanced Safety Module bring to the table.

The Standards Landscape: IEC 61800-5-2 and Beyond

The key standard governing drive-based safety is IEC 61800-5-2, which defines the safety functions, performance levels (PL), and Safety Integrity Levels (SIL) applicable to adjustable speed electrical power drive systems. Alongside it, EN ISO 13849-1 provides the framework for safety-related parts of control systems, categorizing them from Category 1 through Category 4 / PLe — the highest performance level.

Yaskawa's ASM-X is certified to SIL 3 / PLe, the top tier for safety performance, supporting ISO 13849-1 Category 4 machine builds — all without external safety relays.

For networked safety, fieldbus protocols such as Functional Safety over EtherCAT (FsOE), PROFIsafe, and CIP Safety allow safety function signals to travel over the same industrial Ethernet infrastructure already used for motion control — dramatically reducing wiring complexity and cabinet footprint.

Key Terms: HWBB and SRIS

Before stepping through the function library, two core concepts underpin how all Yaskawa safety functions operate:

HWBB (Hard-Wired Base Block) is the drive state in which servo drive current output is physically shut off, preventing servomotor movement. Multiple safety functions use HWBB differently depending on parameter configuration.

SRIS (Safety Request Input Signal) is the trigger that activates or deactivates a safety function. It can arrive as a hardwired digital signal from a sensor — such as a guard door interlock or light curtain — or over a fieldbus like FsOE. The behavior on SRIS activation depends on which safety function is configured and how parameters are set.

The Complete Safety Function Library

Modern servo drives compliant with IEC 61800-5-2 support a layered set of safety functions, progressing from basic stopping through active motion monitoring. Here is the full set relevant to Yaskawa Sigma-X applications:

Yaskawa Sigma-X + ASM-X: Safety That Scales

Yaskawa has been a pioneer in servo drive safety since the 2000s, first integrating STO into their Sigma-5 SERVOPACK series and progressively adding SS1, SS2, and SLS through safety module option cards. With the Sigma-X (Σ-X) series launched in 2021 and the ASM-X Advanced Safety Module released in January 2025, Yaskawa has brought the full IEC 61800-5-2 safety function library to a single, side-mounted module — with no external safety relays required.

The Sigma-X SERVOPACK pairs with Yaskawa's SXG-series functional safety servo motors. Two stocked examples include the SGMXG-20AWA61A2 (1.8kW, 200V, no-battery absolute encoder) and the SGMXG-20DWA61A2 (1.8kW, 400V, absolute encoder). Both feature the absolute encoder feedback required for SLP and SOS position-monitoring safety functions.

Networked Safety: FsOE, PROFIsafe, and CIP Safety

One of the most significant developments in functional safety over the past decade has been the migration from hardwired safety I/O to safety-rated fieldbus protocols. Where legacy systems required dedicated safety cable runs for every STO circuit, modern networked safety allows SRIS signals to travel over the same EtherCAT, PROFINET, or EtherNet/IP infrastructure that carries all other machine control traffic.

FsOE (Functional Safety over EtherCAT) is TÜV-certified and particularly well-suited to robotics applications where minimizing cable count inside a robot arm is critical. PROFIsafe over PROFINET integrates natively with Siemens safety PLCs. CIP Safety over EtherNet/IP serves Allen-Bradley and Rockwell ecosystems.

Yaskawa's ASM-X supports FsOE, enabling full software-configured safety function activation without additional safety relay hardware — a major reduction in BOM cost, panel footprint, and commissioning time on new machine builds.

A Brief History of Drive-Based Safety

Application Examples: Matching the Function to the Hazard

Guard Door / Light Curtain → STO

The classic STO use case: a sensor on a guard door or safety light curtain activates SRIS when the door opens or a person breaks the beam. The drive immediately enters HWBB, removing torque from the servomotor. Once the door closes and SRIS deactivates, the drive can restart without cycling power — minimizing downtime. Any Yaskawa Sigma-X SERVOPACK supports STO natively without the ASM-X module.

Collaborative Robot / Manual Loading Zone → SLS

When an operator enters a monitored zone for manual part loading, the system activates SLS. Rather than stopping the robot entirely, the axis continues at a safely limited speed — often a "creep speed" of 250 mm/s or less per ISO/TS 15066. If actual speed exceeds the SLS threshold, STO activates immediately. This allows production to continue at reduced throughput during manual operations rather than forcing a full stop. The SGMXG SXG servo motors with absolute encoders provide the speed feedback resolution SLS requires.

Vertical Z-Axis / Gantry → SBC + STO

On vertical axes with gravity loads, STO alone is insufficient — the axis would freefall without motor torque. SBC is always paired with STO in these applications, activating a fail-safe holding brake simultaneously with HWBB. The Yaskawa SGM7P motors with integrated brake are purpose-built for exactly this configuration. Safe Brake Test (SBT) can be scheduled to automatically verify brake performance without manual intervention.

Servo Press / Forming Machine → SLP + SS2 / SOS

In press applications where the tooling must never exceed a defined stroke limit, SLP monitors absolute axis position and triggers a stop if limits are approached. SS2 followed by SOS allows the axis to be held precisely at position with drive power maintained, enabling production restart without re-homing. Absolute encoder feedback — standard on the SGMXG-20DWA61A2 and other SXG motors — is a prerequisite for SLP and SOS operation.

Why Source Yaskawa Through Automation Distribution?

As an authorized Yaskawa distributor, Automation Distribution provides the full Sigma-X SERVOPACK and ASM-X product line with the technical support depth to match. Our engineering team works directly with machine builders, OEMs, and system integrators to select the right SERVOPACK model, power range, safety module configuration, and fieldbus option for each application — not just process a part number.

We stock Yaskawa servo products for fast fulfillment, and our team can support everything from initial safety architecture design to drive commissioning documentation requirements under IEC 62061 and ISO 13849.

Content informed by Control Engineering, February 2026. Technical details courtesy Yaskawa America Inc. Automation Distribution is an authorized Yaskawa distributor.