Understanding Functional Safety: Ensuring Reliable Industrial Systems

Did you know that most machines not only operate but also stop safely thanks to advanced control systems? Functional Safety is more than just a set of standards; it’s the result of thoughtful engineering that enables machines to anticipate and react to critical situations. Defined in the standard IEC 61508, it refers to a system’s ability to perform protective functions that minimize the risk of failure leading to hazards. Mechanisms like STO (Safe Torque Off), SS1 (Safe Stop 1), and SLS (Safe Limited Speed) described in IEC 61800-5-2 exemplify technologies that make working with machines safer and more predictable. This article explores how these solutions impact the design and operation of industrial systems.

What Does Functional Safety Mean?

Functional Safety focuses on the reliable operation of industrial automation systems responsible for protection in emergency situations. Its goal is not only to eliminate hazards but also to mitigate the effects of those that cannot be fully excluded. In practice, this means designing systems that can detect problems and respond predictably. Examples include functions like STO, which immediately disconnects torque, or SS1, which ensures a controlled stop of the machine. This minimizes risk to both people and equipment.

Why is Functional Safety Important?

Imagine a system that cannot detect a failure or does not react quickly enough. The consequences can be severe—from accidents involving operators to equipment damage and costly downtimes. Functional Safety addresses these threats by predicting and managing risks at every stage—from machine design through operation to maintenance. Functions like SLS, which monitors and limits machine speed, or SS2, which maintains a stable position after stopping, help avoid critical situations and ensure production continuity.

Categories of Safe Stopping

The standard IEC 61800-5-2 (a harmonized standard that helps meet essential requirements) groups safety functions like STO, SS1, and SS2 under categories of safe stopping. Each function corresponds to a different machine stopping scenario:

1. Category 0 (STO):
   Immediate power disconnection of the drive without controlled braking. This approach is quick but does not protect mechanisms that may be damaged by abrupt stopping. Typical use includes emergency stop button activation.
2. Category 1 (SS1):
   Controlled stopping involving active braking of the machine, followed by STO. Used where safety requires slowing down movement, such as in production lines with delicate elements.
3. Category 2 (SS2):
   Controlled stopping with torque maintenance after braking. Essential in systems requiring position stabilization, like industrial elevators or machines with suspended elements.

This clear division allows designers and industrial automation integrators to tailor stopping methods to the machine’s specifics and safety requirements, avoiding hazards for operators and potential mechanical damage.

Functional Safety: Monitoring and Control Functions

Monitoring and control functions are another aspect of Functional Safety where machine stopping isn’t enough, and precise movement management is essential. These functions allow machines to operate under strictly defined conditions, ensuring maximum protection for operators and processes.

• SLS (Safe Limited Speed) – Safe speed limitation allows continuous monitoring of machine movement speed and immediate response if the set limit is exceeded. This solution is ideal for industrial robots, where the machine must operate slower during human collaboration.
• SOS (Safe Operating Stop) – Safe operational stop allows the machine to halt in a ready state for immediate resumption of work. This ensures processes requiring continuity, like assembly lines, function without unnecessary downtimes.
• SLP (Safe Limited Position) – Safe position limitation controls the machine’s movement range to avoid collisions. This solution is indispensable in transport systems where movement occurs in limited space.
• SBC (Safe Brake Control) – Safe brake control adds an additional layer of protection in applications where drive stopping requires support from mechanical braking systems. It’s particularly useful in cranes or large conveyors, where mass and inertia pose challenges.

These functions enable machines to operate predictably, tailored to application specifics, making work not only more efficient but, most importantly, safer.

Practical Aspects of Implementing Safety Functions

Implementing safety functions in industrial systems requires understanding both standards and the specifics of a given application. Here are the key steps:

• Risk Analysis
  Start by asking: “What can go wrong?” Each stage of machine operation must be analyzed for potential hazards. For example, in a production line with an industrial robot, an uncontrolled movement during service mode could be a threat. The selection of functions like SLS or SOS depends on the results of this analysis.
• Component Selection
  Once the hazards to be eliminated are identified, proceed to select appropriate components. For instance:
  • STO requires a safety controller and compatible drives that implement this function.
  • SLS requires speed sensors and monitoring systems integrated with inverters.
• Testing and Validation
  Even the best design requires verification. Testing should include failure simulations, such as activating E-STOP and checking if all functions (e.g., STO or SS1) operate as intended. It’s not just about compliance with the standard but practical safety for people and processes.

Diagnostic and Reliability Requirements

Standards like ISO 13849-1 and IEC 61508 emphasize the importance of reliability in safety systems. Why is this crucial? Any failure in a safety system can lead not only to downtime but, most importantly, to life or health threats.

• Performance Levels (PL)
  The standard ISO 13849-1 introduces performance levels (PL) from PLa to PLe, which define how reliable a system must be depending on the risk. For example, SLS in collaborative robots often requires PLd, meaning it must withstand single failures.
• Safety System Diagnostics
  Safety functions like SBT (Safe Brake Test) are crucial in systems where brake reliability plays a key role. An example? Cranes, where regular tests prevent brake mechanism failures.
• System Lifecycle (SIL)
  IEC 61508 describes the SIL (Safety Integrity Level) concept, which applies to the entire system lifecycle. For instance, a system with SIL2 must be designed so that its failure probability is extremely low, but it allows relatively simpler diagnostic procedures compared to SIL3.

Example of Safety Function Application

Scenario: Industrial Robot in a Production Line

In a production facility, a robot is used that collaborates with humans in teach mode. What safety functions are required?

• STO (Safe Torque Off):
  When an operator enters the robot’s work area, the STOP function immediately disconnects the drive’s torque, eliminating the risk of uncontrolled movement.
• SLS (Safe Limited Speed):
  During programming (teach mode), the robot arm’s speed is limited to 250 mm/s, in accordance with the harmonized standard with the Machinery Directive ISO 10218-1. SLS is implemented using encoders and safety controllers that continuously monitor movement.
• SOS (Safe Operating Stop):
  During tool changes on the robot arm, SOS allows stopping movement but keeping the machine in a ready state. This ensures a smooth and quick process without requiring a full system reset.

Effect:

Thanks to the integration of these functions, the production line meets safety standards, and operators can work close to the machine without accident risk.

FAQ: Functional Safety