Safety control

This article explains programmable controllers for use in safety-critical systems. Safety control as the Swiss name for a device for monitoring the ability of train drivers to act is described in the article Safety driving circuit.

Safety control in a control cabinet

Sensor module on a machine

A safety controller is usually a programmable logic controller with special design elements on the part of the inputs, processing and outputs in order to be able to guarantee the required level of safety and availability when used in safety-critical systems .

The programmable safety controls have their beginnings with the safety controls PSS, a registered trademark of the company Pilz GmbH & Co KG , in the 1990s (at that time still designed as press controls), in which they were approved by the professional association . Before that, control tasks were largely taken over by electromechanical relays . Like these relay circuits, modern safety controls have a multi-channel structure, but require less space and / or higher functionality and flexibility.

Product history

Safety controls arose from the requirement to be able to interconnect safety by programming, similar to a programmable logic controller (PLC). In the beginning there were central, later decentralized systems followed in connection with safe bus systems . The programming initially followed the same scheme as with a PLC, only the instruction set was reduced to a few languages ​​such as IL (Instruction List) or LD (Ladder Logic / Ladder diagram) . These measures were used for safety in order to be able to minimize errors in program creation by restricting the programming options. The first systems focused on processing the safety functions. Although it was possible to program the safety controller for standard automation from the start , this was only used to a very limited extent in practice. Today, safety controls are widespread that can be programmed more flexibly by freely placing and “wiring” instances from a given library of certified function blocks.

Function and structure

Safety controls differ only slightly in their function from controls for standard automation . At its core, a safety controller consists of two programmable logic controllers that process an application program in parallel, use the same process image of the inputs / outputs and continuously synchronize each other. Cross comparisons, tests of the input / output level, determination of a common valid result etc. are multi-layered, complex processes that such systems represent internally in a complex manner. Except for specific features such as B. the use of clock signals for cross-circuit detection, modern systems behave like other programmable logic controllers . The control functions are saved as a program. The controls are programmed using the associated software .

Structure of a safe control system:

• two separate channels
• diverse structure with different hardware
• constant testing of inputs and outputs
• constant comparison of user data
• Voltage, current and time monitoring
• safe shutdown in the event of a fault or danger

use

The controls are used to automate individual machines, but also spatially extended systems, examples:

• Securing the danger area of ​​machine tools
• Process control in the chemical industry
• Line equipment in rail transport

Web links

Commons : Security control  - collection of pictures, videos and audio files

Individual evidence

1. ↑ European Patent EP0808482B1
2. ↑ Page no longer available , search in web archives: Security Compendium, Chapter 5.3 Security controls today, p. 155
3. ↑ Page no longer available , search in web archives: Security Compendium, Chapter 5.3 Security controls today, p. 155ff.
4. ↑ Page no longer available , search in web archives: Security Compendium, Chapter 5.3 Security controls today, p. 155