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Interruption in the cabling to a sensor will lead to that Pluto read 0-speed from that sensor.
Such fault must therefore be detected in the application by using two independent sensors that
are automatically cyclically checked with regard to that there is motion in the machine at least a
couple of times per day.
Note: By use of two encoders which are compared with each other, faults in one sensor are
monitored. The encoder can normally be of same type since the same fault in the two sensors
in the same time is unlikely. But to get even higher degree of safety two of different type can be
used in order to achieve diversity. This diversity minimises the risk of common cause failures.
4.4.6 Speed monitoring with one sensor
Overspeed, Safe low speed etc.
With a single sensor normally Category 2/PL c is reached.
However, by monitoring of dynamic behaviour in the application it is possible to reach
Category 3/PL d.
Such monitoring for safe low speed monitoring (SLS) can be:
1) When the motion in the machine is stopped Pluto checks that the sensor also indicates a
stop. Then when the motion is started the program checks that the sensor value changes
from indicating stop to the expected speed.
2) Another solution is to compare the sensor value with for example a feed back from another
system such as frequency converter. The independent source of the speed information
must be proven.
Stand still monitoring, single channel
For stand still monitoring with one sensor Category 2/PL c can be achieved under the
requirement that the sensor application is automatically cyclically tested. The interval is
typically several times a day.
One solution for testing is to read the speed value at every cycle start and stop of the machine
cycle. Every time a motion in a machine is started the PLC program requires a corresponding
reaction from the sensor. At start the program can check that the sensor value changes from
stand still to an expected speed within a certain time. At stop command the program can check
that the speed value decreases down to stand still.
NOTE: Interruption in the cabling to a sensor will lead to that Pluto reads 0-speed. At stand still
monitoring this is loss of safety function if it happens during a stop.
(This is however according to the definition of category 2.)
