Emerson unidrive m, Installation And Operating Manual, Page: 28 / 88

28
SI-Safety Installation & Operating Manual
Issue Number: 1
This in turn yields the following characteristic curve for the recording field:
Figure 3-13 Failure curve velocity measuring
• The error rate can be optimized through an appropriate choice of sensor resolution
for each given application.
• For applications with limited resolution and/or time variance on the scanning signal,
the functional capability of the monitoring function employed can be improved
through the use of an average value filter. This average value filter 'smoothes'
digital interference components from the sensors. However, this is achieved at the
expense of slower response times on the system as a whole.
• The filter time constant can be adjusted between 0 and 64 ms in multiples of 8. To
establish the response time of the complete system, the filter times need to be
added to the response times as detailed in
SI-Safety module response times on
page 44.
• For a technical safety assessment of the sensor sub system, the manufacturer's
data (MTTFd, FIT figures etc.) need to be incorporated.
• If the manufacturer requires specific diagnostics to ensure the stated key technical
safety indicators, this must be checked with reference to the specific sensor in
accordance with Table 3-6 on page 27 'Specific diagnostic measures for position
and speed sensors'. If in doubt, clarification will be required from the manufacturer.
There is a minimum rate of acceleration that the module can detect which is determined
by the encoder resolution. The module calculates acceleration from the rate of change
of speed and if this rate of change is low, then there will be no change detected between
a single clock update. In this situation the result of the acceleration calculation will be
either zero or fluctuating.
For this reason, acceleration should not be used as the only parameter for a safety
function block. It can be used in conjunction with speed parameters in speed related
functions to provide a means of detecting large acceleration rates.
NOTE
Error curve for velocity measurement
Frequency or pulses / second
Er
ror in %
Safety instructions
WARNING