...how important fast risk times of RFID sensors are...
...and how does the risk time affect the distance to the danger point?
The standard EN ISO 13855:2010 describes the arrangement of protective devices with regard to the approach speed of human bodies. The minimum distance of a guard without guard locking to the first dangerous movement is therefore calculated as shown in the following diagram.
S= K * T
S= Distance to danger point
K= Constant = 1600 mm/s
T= Total time to standstill
Example calculations of the risk time
In the following calculations we would like to show you an example of how the risk time of RFID safety sensors can affect the distance to the danger point (without series connection).
Example
RFID safety sensor SAFIX 3 or process guard locking HOLDX R
Switch-off delay (toff) actuator to OSSD output:
max. 75 ms
Example
Standard RFID safety sensor or process guard locking
Switch-off delay (toff) actuator to OSSD output:
max. 260 ms
S = 1600 mm/s * 75 ms S = 120 mm
S = 1600 mm/s * 260 ms S = 416 mm
The risk time of 75 ms alone results in a safety distance of 120 mm. For very precise calculation, you must add the risk time of the evaluation unit (e.g. safety controller) and the overrun to standstill.
The calculation shows that by using a risk time of 260 ms, a safety distance of 412 mm is already maintained. This is 292 mm more than in the previous calculation. This can be an important factor for the design of a machine.
If safety sensors are connected in series, the risk time of switching off the inputs must also be taken into account in the calculation. In the following example, 24 sensors are connected in series.
Example
RFID safety sensor SAFIX 3 or process guard locking HOLDX R
1 x Switch-off delay (toff) actuator to OSSD output: max. 75 ms
23 x Switch-off delay (toff) inputs max. 3ms
Example
Standard RFID safety sensors or process guard locking devices
1 x Switch-off delay (toff) actuator to OSSD output: max. 260 ms.
23 x switch-off delay (toff) inputs max. 20 ms
T = 75 ms + 23 * 3 ms T = 144 ms
S = 1600 mm/s * 144 ms S = 230,4 mm
T = 260 ms + 23 * 20 ms T = 720 ms
S = 1600 mm/s * 720 ms S = 1152 mm
For correct calculation, you must also add the risk time of the evaluation unit (e.g. safety controller) and the overrun to standstill.
