The Ectron Model 820 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 820 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 5,500,000 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 820 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 820.
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 820 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 Windows PC running LabVIEW or 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 820 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 820 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.
- Fast response, 4 ms typical
- Rugged package to endure high vibration and shock
- Outputs include 3 relays plus 4-20 mA
- Sophisticated algorithm determines shut-down
- Programmable via RS-232
- Non-failsafe reliability of 5,500,000 hours at 85°C