DESCRIPTION
The Ectron Model 491 is a precision frequency monitor that
provides high-speed control and protection for a variety
of mechanical devices. Designed specifically for gas
turbine engines, the OSM (Overspeed Monitor) can be
applied to reciprocating engines and other mechanical
systems as well. Input frequency signals are normally
from magnetic or optical sensors that detect the
movement of the machine’s operation which usually is
the speed of rotation of an engine. The millisecond
response of the Model 491 is mandatory for many
rotating devices such as gas turbine engines.
Safety Features
The design of the OSM incorporates a number of
features to enhance its reliability and assure safe
operation. All internal systems incorporate high-reliability
components. For instance, internal semiconductor devices
are rated from -55°C to +125°C. In addition,
fail-safe circuitry is incorporated such that if there
is component failure, that failure must almost always
result in a fail-safe condition. The mean time before
a non-fail-safe condition exceeds 3,500,000 hours at 85°C.
Certifications include CE and NEC. The design will meet
CSA Class 1, Div 2 Groups A, B, C, and D. The design will
also meet ATEX Cat 3, Zone 2 hazardous area.
Several safety checks are incorporated in the OSM. Some
of these safety checks include:
- Checks of set point memory, processor RAM, and processor
program memory are performed at startup and continually
while the OSM is operating.
- The processor’s crystal oscillator is monitored
while in operation and compared to a second oscillator.
When the two differ, a shut down is initiated.
- A hardware watchdog continuously monitors firmware operation.
- Speed at last overspeed event, and highest speed since
last overspeed event are stored in non-volatile memory
for forensic analysis of overspeed events.
- Asserting the test digital input performs a controlled
engine shut down which can be used to test shut down
and measure the margin between full speed and the
overspeed trip set point.
- Asserting the reset input while the engine is running
reduces the speed indicated on the 4 mA to 20 mA
output by 2:1. This can serve as a check that the overspeed
monitor is operating normally.
Intrinsic Safety
The input of the Model 491 is not intrinsically safe.
Therefore, for those systems where this safety requirement
is mandatory, a suitable intrinsically safe barrier must be
provided between the sensor and the input to the Model 491.
Input Signal
The acceptable frequency range is from 150 Hz to
25,000 Hz. The typical signal is generated from a
magnetic sensor operating in close proximity to a cogged
wheel attached to the shaft of an engine. The magnetic
sensor can be active or passive. The Model 491 provides
a 9 V signal to power an active sensor.
The wave shape of the input signal can be sine, square,
or pulse, any of which will provide proper operation.
Amplitude can be from 0.25 V up to 50 V peak.
The input to the OSM is differential so either lead can
be used as signal low. Shielding is recommended for the
input signal leads to take advantage of the excellent
performance of this monitor particularly where electrical
noise is present. The input signal can be floating off
ground up to 50 V dc or peak ac.
On some gas turbine engines, the input frequency sensor
is mounted on the engine where it detects the blades moving
past. Under some conditions the blades flutter sending a
frequency-modulated signal to the OSM. The algorithm used
by the OSM’s processor uses an averaging technique to
minimize the effects of this frequency variation. Eleven
waveforms are used in this algorithm such that the monitored
result is still able to accurately determine the engine’s
rpm.
Output 4 mA to 20 mA Signal
This signal from the OSM is both highly accurate (0.1%)
and fast responding so it can serve as an on-line check of
the control system of an engine. For grounding considerations,
note that the output common of the 4-20 mA signal is connected
to input power common. A separate common lead should be used
for the 4-20 mA signal so that power current fluctuation will
not mix with the signal current.
Relays
The three programmable relays typically are used as follows:
- Overspeed relay normally used to produce
an engine shutdown. This relay has form C contacts.
- Low speed relay is used to monitor startup condition plus
it includes open probe detection. This relay has form C contacts.
- Status relay monitors the condition of the OSM.
This relay has form A contacts.
Programming
User programming is via the RS-232 port using a PC running
Windows 9x, NT, 2000, or XP using the HyperTerminal program
distributed by Microsoft.
The following four set points are programmable:
- Frequency in Hz equivalent to 100% engine speed settable
from 3,000 Hz to 20,000 Hz.
- Overspeed trip point in % of 100% frequency (typically
102% to 110% of maximum engine speed).
- Low speed and open probe detection set point in % of
100% frequency (typically 15%).
- Status indicator set point in % of 100% frequency (typically 98%).
Response Time
The response time for an overspeed event is frequency
dependent (see response time in the specification section).
For example, for an input frequency of 1,000 Hz the
shut down relay will actuate in a maximum of 10 milliseconds.
For a 10,000 Hz signal the actuation time is 6 milliseconds,
maximum. Consequently, if minimum response time is required
use the highest frequency possible. Perhaps the engine drives
a gear train and the pick-off devices can be used on these
higher speed components. Or, preferably the cogged wheel
driving the frequency sensor can be designed with a greater
number of teeth. Note that any irregularity in the spacing
of these teeth will result in “noise” to the OSM much like
the flutter in the blades of the engine as described above.
Mounting and Ruggedness
When high vibration and shock conditions can occur, the DIN
mount may not be sufficiently strong and the flange mount
should be used. The Model 491 OSM can withstand considerable
shock and vibration while continuing to operate reliably.
Internal components are virtually all surface mount. The
aluminum case is a solid extrusion and the interior is
encapsulated using an electronic potting compound. A void
is included within the potting compound to allow for
expansion. Similar Ectron 300 and 400 series products have
exhibited high reliability on off-shore drilling rigs,
vehicle crash testing, and space-bound launch vehicles.
Another OSM model operates successfully mounted on the
sleds in close proximity to large gas turbine engines.
Power
The power supply used in the Model 491 is designed to
function properly when connected to available power in
the range from 18 V dc to 32 V dc.
Considerable noise and variation in power voltage is
allowed without affecting the OSM.
Model 491 with DIN-rail mount
Model 491 Dimensions
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