Microcomputer
Synchronization (Microfreeze):
The
Microfreeze can also be called as transparent PCHandshake. This
feature is particularly useful when interfacing data directly to a
microcomputer where speed of operation is much higher. In this
case the encoder position data is continuously updated at full
speed. The data are frozen for 100 µs ± 10% after a delay of 10
µs from either transition edge of data transfer command. The
microcomputer can read stable data during these 100 µs and it
automatically unlatches.
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Software
Filtering:
The
problem of synchronizing BCD data to a PLC can also be addressed
by software filtering. Software filtering is usually done in one
of the following two ways:
1.
A window is created around the last correct reading based on the
known operating speed of the encoder. If new position is outside
of this window, the data is rejected.
2.
Three samples of position data are taken, of which two must agree
before data is accepted.
Either
of these approaches will increase the scan time of the PLC. Since
scan time is an important factor in system speed and resolution,
the software approach is usually not a viable approach.
Notes:
1) The synchronization problem does not exist when using Gray Code
absolute encoders because only one bit changes state at a time.
2)
The synchronization process described above does not result in
faster machine operation. The system resolution and permissible
encoder speed will still be limited by the PLC scan time. As a
rule of thumb, a PLC with 16.67 ms (One AC cycle) scan time will
permit V resolution at 10 RPM (The rule of 10: 1: 1).
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