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Safety Function Response Time (SFRT)

What is the Safety Function Response Time (SFRT)

When planning a safety application, you must determine the correct mechanical positions of the safety equipment involved at the machine, i.e., the safety-related command devices, sensors, and actuators. This position and the physical distance of each safety equipment must fulfill the requirements of your application according to applicable standards and regulations.

To be able to perform safety distance calculations, you must know the safety function response time (SFRT).

The SFRT is the duration that elapses between
  • the receipt of the safety request signal (coming from the safety-related command device or sensor, such as an emergency stop button or a light barrier) in the safety-related input module, and
  • the output of the request signal for the defined safe state of the machine at the safety-related output module. Whether the defined safe state means the standstill of the machine, or for example, a torque-free axis or a limited speed, depends on your application and must be determined in the context of your mandatory risk analysis.

Note
The maximum permissible safety function response time (SFRTmax) depends on the relevant safety function to be implemented.

Note
In the application, the SFRTmax must be determined for each implemented safety function.

The longer the SFRT of the safety system is, the greater must be the minimum distance of the safety command device/sensor from the zone of operation.

SFRT determination

The SFRT of your safety application is composed as follows:

TWCDT is the Total Worst Case Delay Time which is the sum of all processing and transmission times in the respective signal path of the safety system. TWCDT is composed as follows:

TWCDT = t1 + t2 + t3

The partial delay times t1, t2, and t3 are composed as follows:

t1 = worst case input delay time
   = signal processing time in the sensor and in the input safety module
     Refer to the technical data sheet/user manual of the devices involved.

t2 = transmission delay + processing time
   = the transfer time from input module to Safety PLC via bus coupler/Axioline
     + execution processing time in the Safety PLC (see note below)
     + transfer time from Safety PLC to output module via bus coupler/Axioline

t3 = worst case output delay time
   = signal processing time in the safety-related output module
     Refer to the technical data sheet/user manual of the devices involved.

Note
The execution processing time included in t2 is displayed as 'Program Execution Time' in the Safety Cockpit. See topic "Safety PLC Diagnostics out of the Safety Cockpit" for details.

SFRT = TWCDT + Δtmax of the longest output device WD time
For information on Δtmax refer to the next section "Relevant Watchdogs".

Note
The SFRT must be determined for each safety function to be implemented.

SFRTmax = maximum Safety Function Response Time of all SFRTs in your application.

Note
SFRT must be ≤ SFRTmax.

WARNING
Unintended machine operation
  • Verify that each link which is involved in the safety function and programmed in the safety-related application program is taken into account when determining the SFRT.
  • Verify that also the physical properties of the machine (such as the casting down of an axis, etc.) are taken into account in addition to the calculated SFRT.

Relevant Watchdogs

In a PLCnext Technology safety system, various watchdogs (WD) are implemented with which the correct function of the safety-related communication and devices can be monitored. Some watchdogs are device-internal and therefore not parameterizable, some must be parameterized in PLCnext Engineer (for example, Profisafe communication watchdog times).

Depending on the SFRTmax, you must determine the resulting maximum monitoring/watchdog times as an upper limit for each individual safety function. If, for example, the WD of a safety-related output device is set higher than the device actually needs for processing the output signal, a time delta Δt results which must be considered when planning the safety system.

The following watchdogs are relevant: