Emerson SI-Universal User Manual page 74

P1 Device Type (1x.038): SSI
If P1 SSI Incremental Mode (1x.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. 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. P1 Normalized Position (1x.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 P1 SSI Incremental Mode (1x.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 apparent large change of position, and this 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.
P1 Device Type (1x.038): All other device types
P1 SSI Incremental Mode (1x.047) has no effect.
1x.048
Minimum
Default
Type
Display Format
Coding
P1 Device Type (1x.038): SC SSI, SSI
SSI encoders normally use Gray code data format. However, some encoders use binary format
which may be selected by setting P1 SSI Binary Mode (1x.048) to one.
P1 Device Type (1x.038): All other device types
P1 SSI Binary Mode (1x.048) has no effect.
74
P1 SSI Binary Mode
0
0
1 Bit User Save
None
RW
Maximum
Units
Update Rate
Decimal Places
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