A.3.4 Requirements For Encoder Errors From The Point Of View Of The Application - Festo CAMC-G-S3 Manual

A Technical appendix
A.3.4 Requirements for encoder errors from the point of view of the application
The analysis of the position transmitters on the safety module is based on one of the following basic
principles:
a) Two redundant items of position information are available, which are also analysed redundantly
and separately by two microcontrollers, i.e. also two independent speed signals.
Example: Motor with ENDAT encoder + two incremental position transmitters on the axle.
b) Only one item of position information is "reliably" available. The information is sent along the same
cables and via some identical circuit sections (e.g. differential amplifiers for the input signals) and
analysed redundantly by two microcontrollers on the safety module.
Example: Resolver (SIL2) or SIN/COS encoder with SIL 2/SIL 3.
Case a) The detection of encoder errors and position deviations between position encoders 1 and 2 is
"non-time critical", as monitoring is always guaranteed by the other encoder, even if one of the
encoders fails.
Case b) The detection of position deviations between microcontroller 1 and microcontroller 2 is "non-
time critical", as the monitoring is always guaranteed by the other microcontroller.
In the transmission channel (e.g. cable) and in the shared circuit sections for encoder analysis,
errors must be detected as quickly as possible, within an error response time to be specified.
The detection of the encoder errors is based primarily on the monitoring of analogue signals.
The limits of the signal monitoring have an influence on the resulting accuracy of the monitoring
and on the diagnostic coverage è See Section A.2.3.
Monitoring of the position difference does not need to be more accurate than in other cases of error.
The worst case for an uncontrolled movement of the axle is a fusing of two output semi-conductors in
the output stage of the CMMP-AS-M3. In the worst case, this error leads to an electrical shift of the
axle by up to 180° (== 60° on the shaft with an 8-pin motor).
Note: Of course, the system must be designed in such a way that this error does not lead to an imper­
missible position difference - e.g. through the selection of a suitably low feed or gear ratio.
Please note
There is a risk that the drive will advance if there are multiple errors in the
CMMP-AS-...-M3.
Failure of the motor controller output phase during STO status (simultaneous short
circuit of 2 power semiconductors in different phases) may result in a limited detent
movement of the rotor. The rotation angle / path corresponds to a pole pitch. Ex­
amples:
– Rotary axle, synchronous machine, 8-pin è movement  45° at the motor shaft.
– Linear motor, pole pitch 20 mm è movement  20 mm at the moving part.
264 Festo – GDCP-CAMC-G-S3-EN – 1406NH – English