ifm
System Manual
ecomat
mobile
PDM360NG (CR1080, CR1081, CR9042) Target V01
2011-07-28
Glossary of Terms
445
Demand rate rd
The demand rate r
d
is the frequency of
demands to a safety-related reaction of an
SRP/CS per time unit.
Diagnosis
During the diagnosis, the "state of health" of
the device is checked. It is to be found out if
and what faults are given in the device.
Depending on the device, the inputs and
outputs can also be monitored for their correct
function.
- wire break,
- short circuit,
- value outside range.
For diagnosis, configuration and log data can
be used, created during the "normal" operation
of the device.
The correct start of the system components is
monitored during the initialisation and start
phase. Errors are recorded in the log file.
For further diagnosis, self-tests can also be
carried out.
Diagnostic coverage
D
iagnostic
C
overage
Diagnostic coverage is the measure of the
effectiveness of diagnostics as the ratio
between the failure rate of detected dangerous
failures and the failure rate of total dangerous
failures:
Formula: DC = failure rate detected dangerous failures /
total dangerous failures
Designation
Range
none
DC < 60 %
low
60 % < DC < 90 %
medium
90 % < DC < 99 %
high
99 % < DC
Table: Diagnostic coverage DC
An accuracy of 5 % is assumed for the limit
values shown in the table.
Diagnostic coverage can be determined for the
whole safety-related system or for only parts of
the safety-related system.
Dither
Dither is a component of the PWM signals to
control hydraulic valves. It has shown for
electromagnetic drives of hydraulic valves that
it is much easier for controlling the valves if the
control signal (PWM pulse) is superimposed by
a certain frequency of the PWM frequency.
This dither frequency must be an integer part
of the PWM frequency.
Diversity
In technology diversity is a strategy to increase
failure safety.
The systems are designed redundantly,
however different implementations are used
intentionally and not any individual systems of
the same design. It is assumed that systems of
the same performance, however of different
implementation, are sensitive or insensitive to
different interference and will therefore not fail
simultaneously.
The actual implementation may vary according
to the application and the requested safety:
use of components of several
manufacturers,
use of different protocols to control
devices,
use of totally different technologies, for
example an electrical and a pneumatic
controller,
use of different measuring methods
(current, voltage),
two channels with reverse value
progression:
channel A: 0...100 %
channel B: 100...0 %
DRAM
DRAM =
D
ynamic
R
andom
A
ccess
M
emory
Technology for an electronic memory module
with random access (Random Access
Memory, RAM). The memory element is a
capacitor which is either charged or
discharged. It becomes accessible via a
switching transistor and is either read or
overwritten with new contents. The memory
contents are volatile: the stored information is
lost in case of lacking operating voltage or too
late restart.
