Woodward AtlasPC, Hardware Manual

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AtlasPC Digital Control, Vol. I Manual 85586V1
28 Woodward
Speed Sensor Inputs
The MPU and proximity probe inputs are read and the speed is provided to the
application program. A derivative output is also provided. The speed sensor
inputs are filtered by the SmartCore board, and the filter time constant is
selectable at 8 milliseconds or 16 milliseconds. Eight milliseconds should be
acceptable for most turbine applications, while 16 milliseconds may be necessary
for very slow speed applications. The speed range is selected in the application
software and determines the maximum speed that the board will detect. The
control output of the software will detect a minimum speed of one fiftieth of the
speed range. This allows detection of failed speed sensors to help prevent
overspeed due to slow update times at very low speeds. The monitor output of
the GAP block will read down to 0.5 Hz, irrespective of the speed range. An
application may use any combination of accepted MPU and proximity probes,
and any combination of speed ranges.
The SmartCore board uses speed sensing probes mounted on a gear connected
or coupled to the turbine’s rotor to sense turbine rotor speed. Any of the board’s
speed channels accept passive magnetic pickup units (MPUs) or proximity
probes. It is not recommended that gears mounted on an auxiliary shaft coupled
to the rotor be used to sense speed. Auxiliary shafts tend to turn more slowly
than the rotor (reducing speed sensing resolution) and have coupling gear
backlash, resulting in less than optimum speed control. For safety purposes, it is
also not recommended that the speed sensing device sense speed from a gear
coupled to a generator or mechanical drive side of a system’s rotor coupling.
Atlas
53
54
55
56
Passive
MPU
+
GND
(isol)
Figure 5-4. Wiring Example–MPU Interface to the SmartCore Board
A proximity probe may be used to sense very low speeds. With a proximity
probe, speed can be sensed down to 0.5 Hz. When interfacing to open collector
type proximity probes, a pull-up resistor is required between the supplied
proximity probe voltage and the proximity probe input to the SmartCore board.