These days, jog functions can generally be carried out while guards are open thanks to the safely limited speed (SLS) function. The respective application will determine the type of increment that can be classified as non-hazardous. It may be helpful to consult EN 349 (Minimum gaps to avoid crushing of parts of the human body) and EN 999 (The positioning of protective equipment in respect of approach speeds of parts of the human body).
Determination of the performance level for the overall circuit
In terms of structure, the jog function with safely limited speed is similar to the safe stop function. The key difference lies in the push buttons used for the jog function and the impact this has on the calculation of the performance level. In EN ISO 13849-1, push buttons (enable switches) are given a B10d of 100 000. The time between two operations (cycles) is the key factor in calculating the MTTFd.
Calculation formula for MTTFd:
The following assumptions are made, based on the application of the component:
B10d = 100 000
hop = 16 h/day
dop = 220 d/year
Calculation MTTFd:
tCycle = 5 s ➔ MTTFd = 0.395 years
tCycle = 3 600 s ➔ MTTFd = 284.1 years
As shown in the example with cyclical operation in 5 s intervals, even in the best case it is only possible
to achieve PL c with a B10d value of 100 000. This demonstrates very clearly that the application range for wearing components has a direct influence on the calculation of the performance level and therefore affects the achievable safety level. The design engineer must therefore look very closely at the application range of his components in the respective application. Even if EN ISO 13849-1 states 100 000 cycles for B10d, there may well be special components with a higher B10d value. If an application uses a push button as an E-STOP command device, it will certainly not be operated constantly at 5 second intervals. The situation is completely different if a push button is used as a command device for cyclic initiation of a machine cycle and
has to trigger a safe stop once released. The values stated in the example may cause a problem if a higher performance level is required.
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| Structure of the Safety Function |
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| The block diagram shows the logical structure of the safety function, consisting of the series alignment of the safety-related subcircuits. |
In terms of structure, the jog function with safely limited speed is similar to the safe stop function. The key difference lies in the push buttons used for the jog function and the impact this has on the calculation of the performance level. In EN ISO 13849-1, push buttons (enable switches) are given a B10d of 100 000. The time between two operations (cycles) is the key factor in calculating the MTTFd.
Calculation formula for MTTFd:
The following assumptions are made, based on the application of the component:
- hop is the mean operating time in hours per day
- dop is the mean operating time in days per year
- tcycle is the mean time between the start of two consecutive cycles of the component (e.g. switching a valve) in seconds per cycle
B10d = 100 000
hop = 16 h/day
dop = 220 d/year
Calculation MTTFd:
tCycle = 5 s ➔ MTTFd = 0.395 years
tCycle = 3 600 s ➔ MTTFd = 284.1 years
As shown in the example with cyclical operation in 5 s intervals, even in the best case it is only possible
to achieve PL c with a B10d value of 100 000. This demonstrates very clearly that the application range for wearing components has a direct influence on the calculation of the performance level and therefore affects the achievable safety level. The design engineer must therefore look very closely at the application range of his components in the respective application. Even if EN ISO 13849-1 states 100 000 cycles for B10d, there may well be special components with a higher B10d value. If an application uses a push button as an E-STOP command device, it will certainly not be operated constantly at 5 second intervals. The situation is completely different if a push button is used as a command device for cyclic initiation of a machine cycle and
has to trigger a safe stop once released. The values stated in the example may cause a problem if a higher performance level is required.
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