Emerson SI-Universal User Manual page 114

2x.047
Minimum
Default
Type
Display Format
Coding
P2 Device type (2x.038): SSI
If P2 SSI Incremental Mode (2x.047) = 0 the complete absolute position is read at each sample.
Care should be taken when using this mode as some unwanted effects can occur when the
encoder passes through the boundary between its maximum position and zero.
When in this mode, the encoder can be used for motor control provided at least 6 bits of turns
information are provided by the encoder otherwise an over speed trip will be produced as the
position passes over the maximum position to zero boundary. P2 Normalized Position (2x.058) can
be used for position control over this boundary provided the normalized turns bits are set up so that
the normalized positions do not contain turns information that is not available from the encoder. As
the SSI format does not include any error checking, it is not possible to detect if the position data
has been corrupted by noise.
The benefit of using the absolute position directly from an SSI encoder, is that even if the encoder
communications are disturbed by noise and position errors occur, the position will always recover
the correct position after the disturbance has ended.
If P2 SSI Incremental Mode (2x.047) = 1 the absolute position is only taken from the encoder during
Initialization. The change of position over each sample is then accumulated to determine the
position. This method always gives 16 bits of turns information that can always be used without
jumps in position whatever value is used as the turns bits for normalization. If noise corrupts the
data from an SSI encoder, it is possible to have an apparently large change in position which can
result in the turns information becoming and remaining, corrupted until the encoder is re-initialized.
If an SSI encoder is used but is not powered from the drive, and the encoder is powered up after
the drive, it is possible that the first change of position detected could be large enough to cause the
problems described above. This can be avoided if the encoder interface is initialized with Initialise
Position Feedback (1x.075) after the encoder has powered up.
If the encoder includes a power supply alarm bit, the power supply monitor should be enabled. This
will ensure that the drive remains tripped until the encoder is powered up and the action of resetting
the trip will re initialize the encoder interface.
P2 Device type (2x.038): All other device types
P2 SSI Incremental Mode (2x.047) has no effect.
114
P2 SSI Incremental Mode
0
0
16 Bit User Save
None
RW
Maximum
Units
Update Rate
Decimal Places
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