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Fail-safe output modules of
safety control systems and
electronic safety switchgear
connect test pulses to their
outputs for diagnostic purposes.
On the one hand, these test
pulses help detect cross circuits
or to check the function of the
outputs relative to their
deactivation efficiency.
Depending on the manufacturer,
these test pulses have varying
pulse widths of up to several
milliseconds. For example, a
controller manufacturer
deactivates their outputs for a
period of several milliseconds in
the event of an ON signal. In the
event of an OFF signal, the
outputs are switched on for up
to 4 ms to check whether they
can be deactivated safely if a
safety function request is made.
How does a solenoid valve react
to these test pulses?
If a solenoid valve is connected
to a failsafe output, the test
pulses often cause the LED on
the solenoid valve to flicker at
the same speed as the pulses
and a clicking can be heard in
the solenoid valve. That clearly
shows that these test pulses
have an effect on the solenoid
valve. Many modern solenoid
valves consist of a magnetic
system, which actuates a pilot
valve via an armature, which in
turn actuates the main part,
which then controls the
actuators. Even if the switching
times for activation or
deactivation, which are listed in
the technical data, are far higher
than the duration of the test
pulses, the armature reacts
much earlier. In some solenoid
valves, this occurs with blackout
times of just 0.1 ms.
Does this result in accidental
deactivation of a solenoid valve
in the event of an ON signal?
This reaction in the armature
generally indicates a reduction
of the holding force for the
armature. In turn, this means
that unfavourable vibration-
shock conditions on the machine
could result in an unplanned
deactivation of the pilot valve
and thus of the power valve.
Does this result in accidental
activation of the solenoid valve
in the event of an OFF signal?
Although these positive test
pulses of several milliseconds
cause the LED on the solenoid
valve to flicker at the same speed
as the test pulses, it is extremely
rare for it to cause the solenoid
valve to switch.
In some solenoid valves, the
armature already reacts after
just 0.4 ms. This means that the
armature in the solenoid system,
which controls the pilot valve of
the named solenoid valves,
moves. This reaction in the
magnetic system generally
indicates a reduction of the
break-away force for the
armature. In turn, this means
that unfavourable vibration-
shock conditions could result in
an unplanned activation of the
pilot valve, and thus of the
power valve.
Does my controller still comply
with the EC Machinery
Directive?
As long as the basic safety and
health protection requirements
from the EC Machinery Directive
are complied with, it is in
compliance with the EC
Machinery Directive. If we
assume that in SRP/CS, the
deactivation of the solenoid
valves represents the safe status
of the function, hazards still will
not result.
Summary:
All measurements at Festo were
performed at worst case
conditions. That means in the
event of deactivation with
minimal pressure and minimal
output voltage. As the pressure
and output voltage values
approach the upper limits, the
How test pulses affect solenoid valves