Today’s state-of-the-art technology stop functions to have a drive-integrated solution. This solution reduces the space requirement in the control cabinet and also the amount of wiring necessary, as additional external components required in the past, such as contactors, are now superfluous. Even additional components to monitor standstill or speed are now surplus to requirements. Servo amplifiers with integrated safety functions in
accordance with EN 61800-5-2 are now available, providing much simpler solutions, even for complex safety requirements. The standard EN 61800-5-2 divides safety functions into stop functions and miscellaneous safety functions. The description is only rudimentary and allows a great deal of freedom in how it is implemented and interpreted. This is particularly evident with the stop functions, which are among the most complex of safety functions. The implementation method can vary greatly, but so too can the external behavior of the safety
functions.
When the safety functions are operated in practice, subsequent effects can often be attributed to the poor quality of the sensor signals or to the actual behavior of an electrical drive in general. Poorly tuned control loops and EMC are frequently the cause of restricted availability of safe drive axes. One example of this is the definition of standstill: On a closed loop system, zero speed is more of a theoretical value. Depending on the quality of the control loops, some jitter may be observed around the zero position; if the limit value was set to zero, this would immediately trigger a reaction on account of a limit value violation. The safety function would
shut the drive down safely – at the expense of system availability. In this case it helps to define a standstill threshold > 0, where the permitted speed is still non-hazardous. An alternative is to define a position window, from which the motor may not deviate. In this case, even the slightest movements would not lead to a limit value violation.
To guarantee the security of the manufacturing and production process as well as the safety of personnel, safety functions may also be permanently active, without the requirement of the plant remaining in a special operating mode. Several components and their respective interfaces must be considered in order to implement the safety functions; the whole safety chain must be considered when calculating the required safety integrity.
It is not mandatory for the safety functions listed in EN 61800-5-2 to be implemented using drive integrated safety. An external solution may also be used.
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