Vibration monitoring system for an aircraft engine

A monitoring system for detecting and indicating typical operating conditions, particularly vibration of an aircraft engine, wherein at least two measuring feelers such as accelerometers are exposed to the same conditions, the output signals from said feeler are compared with each other and logical means detect normal conditions when the output signals are equal, system defect conditions when the output signals differ from each other and alarm conditions when the output signals both exceed a safety limit. Such a system greatly increases the reliability and at the same time avoids false alarms.

BACKGROUND OF THE INVENTION 
Vibration monitoring systems for aircraft engines usually comprise at least 
one measuring feeler, particularly an accelerometer connected to a 
measuring and detecting system. Each of the feelers is mounted at a 
particular place of the engine in order to measure one particularly 
interesting vibration, and each of the feelers is connected to an 
individual measuring system the output of which is operating a display 
unit. This display unit usually comprises a meter continuously indicating 
the level of vibration and an alarm lamp and/or bell for indicating alarm 
conditions to the crew. Such a system has a number of drawbacks. Whenever 
a feeler or a measuring or indicating unit is defectuous a proper 
monitoring of at least one typical vibration of the engine is no longer 
obtained. On the other hand if the defect of the feeler and/or measuring 
system results in an increase of the value indicated an alarm bell or lamp 
is operated, this forcing the crew to take measures, for instance to shut 
down the engine. There is no possibility to check the operating condition 
of the feeler and/or measuring and display system and thereby to 
distinguish between real alarm conditions in the engine and a defect of 
the feeler and/or measuring and display unit. 
SUMMARY OF THE INVENTION 
This invention relates to a monitoring system of the type explained above 
but of which the reliability and the information available for an operator 
or the crew is substantially improved. When used as a vibration monitoring 
system for an aircraft engine, it substantially comprises at least two 
measuring feelers responsive to vibration, such feelers having similar 
characteristics and being exposed to the same vibration, means for 
producing measuring signals corresponding to the output at each of said 
feelers, comparator means for comparing said measuring signals and logic 
means interconnected with indicating means for detecting and indicating a 
normal condition when said measuring signals are substantially equal, a 
defect condition when said measuring signals differ from each other and an 
alarm condition when said measuring signals exceed a safety limit. The 
reliability of the system is substantially increased by the use of at 
least two measuring feelers mounted on the same support and thus exposed 
to the same vibration or other quantity to be detected, and the 
information is appreciably improved by the display of three distinct 
conditions, namely the normal operating condition, a defect condition 
indicating that the measuring system is at least partly inoperative and an 
alarm condition which is only operative when there is a confirmation from 
both feelers and measuring systems respectively that the value measured 
exceeds a safety limit. 
Preferably the measuring signals from each of the feelers and measuring 
systems respectively may individually be selected and displayed, this 
further improving the information available for the crew. If, as an 
example, the one feeler and measuring channel is defectuous and thus has 
no output signal at all while the other feeler and measuring channel has 
an output signal exceeding said safety limit, the crew or operator still 
is free to take this situation as an alarm condition and to take measures. 
The measuring system may itself include internal checking circuits 
connected with a display unit for indicating a defect condition.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1a schematically shows an engine 1 of an aircraft. It is assumed that 
this engine has two coaxial shafts and it comprises two tachometers well 
known in the art schematically shown at N.sub.1 and N.sub.2 in the 
drawing. On a common mounting pad fixed to the engine 1 and schematically 
shown at 2 in FIG. 1 are fixed two similar vibration feelers, preferably 
accelerometers A and B connected each to the input of an amplifier 3a and 
3b respectively, which are parts of a broad-band channel module 4. This 
module 4 further includes an electronic switch or selector 5, the inputs 
of which are connected to the outputs of amplifier 3a and 3b respectively 
and by which such outputs may alternatively be connected to the input of a 
broad-band filter 6 as indicated by the solid-state changeover switch in 
selector 5. Selector 5 is controlled at inputs A and B by signals form a 
clock generator 7 shown in FIG. 1b, the control signals being 
schematically shown on the conductors connecting inputs A and B of 
selector 5 with outputs A and B respectively of generator 7. The output of 
broad-band filter 6 is connected to the input of an integrator 8 of which 
the output is rectified in a rectifier 9 and may be applied to a display 
instrument 10 when a selector switch 10' is in its normal position as 
shown in FIG. 1a. 
The output of integrator 8 is also connected to the one input of each of 
two similar narrow-band filters or tracking filters 11 and 12 of a 
narrow-band channel module 13. Two similar narrow-band channels 13 are 
provided, each being associated with one of tachometers N1 and N2 
respectively. The tachometer signals are transmitted to the input pulse 
formers 14 through transformers 15. The shaped pulses of which the 
frequency corresponds to the frequency of the tachometer signals and is 
directly proportional to the speed of the engine shafts respectively are 
applied to frequency multipliers 16 of the phase-locked loop type. The 
multiplied output frequency of each multiplier is applied to the inputs of 
two similar monostable circuits 17 determining the pulse duration. The 
pulses at the outputs of circuits 17 are transmitted to the similar 
tracking filters 11 and 12. These tracking filters are well known in the 
art and their output signals practically only depend on the amplitude of 
the basic frequency of the input signal from the integrator 8, that is, on 
the frequency equal to the 1/revolution speed N1 of the first shaft. All 
other frequencies which are not related to 1/revolution rotor unbalance N1 
like higher harmonic frequencies, noise frequencies and airframe 
vibrations are practically filtered out and without effect whereby the 
sensitivity of the measurement is substantially improved. The output 
signal from filter 11 is applied to an amplifier 18 and a rectifier 19 and 
from there is transmitted on one hand to the selector switch 10' and on 
the other hand to the alarm module shown in FIG. 1b. The output of each 
tracking filter 11 and 12 respectively is connected to inputs of a phase 
comparator 20, a signal being produced at the output of this phase 
comparator 20 whenever the phases of the input signals differ from each 
other by an appreciable amount. 
A narrow-band channel 13 is associated with each shaft and tachometer 
N.sub.1 and N.sub.2 respectively, and the output of the phase comparators 
20 of each of these channels is applied to an input of an OR circuit 21. 
Each of the frequency multipliers 16 has a phase comparator in its 
phase-lock loop indicated by .psi. in the drawing. This phase comparators 
have outputs similar to the output of phase comparators 20, whereby a 
signal is transmitted to the OR circuit whenever the input signal from 
pulse former 14 and the signal from the phase-lock loop of the frequency 
multiplier are no longer in phase, that is, when there is no clear phase 
relation between such signals. 
This is a signal indicating that the frequency multiplier is not properly 
locked on its input frequency and that, therefore, the whole narrow-band 
channel is not synchronised at the correct frequency. The same applies 
when the phase of the output signals from the tracking filters 11 and 12 
are not the same, this being only possible when one of such filters does 
not properly operate. In any such case the OR circuit 21 will produce an 
output signal at its output which will be transmitted to a 
system-inoperative indication as described later. 
The analogous signals from rectifiers 19 are transmitted to level detectors 
22 of the indicator unit shown in FIG. 1b. When the levels of the input 
signals change from a normal level to a level exceeding a safety level, 
the output signals from level detectors change and whenever the one of 
such output signals changes the output signal of an OR circuit 23 also 
changes. The output from OR circuit 23 is applied to the one input of 
another OR circuit 24 of which the output is connected to the reset inputs 
R of counters 25 and 26. The clock or counting input of counter 25 is 
connected to the clock signal output A of the clock generator 7 while the 
counting input of counter 26 is connected to clock signal output B. Each 
of the counters 25 and 26 is followed by a selector switch schematically 
shown at 27 and 28 respectively, for selecting the counting capacity of 
the counter which may for instance be selected at 2, 4 or 8. The output of 
each counter is connected via the selector switch to a latch input L of 
the counter. The outputs of both counters 26 and 27 and of their output 
selector switches 27 and 28 respectively are also connected to an AND gate 
29 of which the output controls a transistor 30 at the input of an alarm 
or warning unit 31. 
The indicating unit (Fig. 1b) has a control switch 32 which is usually in 
an off position as shown and may be thrown either to a reset position R 
wherein resetting pulses are transmitted to the OR circuit 24 and to a 
bistable circuit 33 or to a test position T for testing the indicating 
unit. The output of bistable circuit 33 is connected to an input of OR 
circuit 23. 
The indicating unit (FIG. 1b) further comprises a sample and hold circuit 
34 in which the levels at the inputs of level detectors 22 are 
alternatively sampled and memorized under control of the clock signals A 
and B from the clock generator 7. The output of this sample and hold 
circuit 34 is connected to an input of an OR circuit 35 of which the 
output controls a display lamp 36 "SYSTEM INOPERATIVE" through a delay 
circuit 37 and a blocking input of AND gate 29. 
The output signal of rectifier 9 of the broad-band channel 4 is also 
transmitted to a level detector 38 the output of which transmits a signal 
to OR circuit 35 when the level at the input of detector 37 is below a 
predetermined operating limit indicating that no signal is present. 
OPERATION 
Since transducers A and B are fixed on a common support to which all 
important vibrations of the engine 1 are transmitted and since such 
transducers are similar, the same output signals are normally transmitted 
from both to amplifiers 3A and 3B. Switch 5 is sampling alternatively each 
one of the signals for a time interval for instance of one second. The 
sampled signals are transmitted through circuits 6, 8 and 9 and the 
overall level of the vibrations and noise are indicated at instrument 10 
thus giving a first coarse information on the health conditions of the 
engine. 
As already explained the signals from integrator 8 are also transmitted to 
the tracking filters 11 and 12 of both narrow-band channels 13, and such 
channels will produce a signal at the output of the tracking filters and 
of the amplifiers 18 and rectifiers 19 respectively which is indicative of 
the level or amplitude of the 1/revolution frequency equal to the rotating 
speed of the first and second shaft respectively. Under normal conditions 
the level detectors 22 of the indicating unit are inoperative, and 
consequently counters 25 and 26 are maintained in reset condition through 
OR circuits 23 and 24. No operating signals are thus transmitted from the 
AND gate 29 to the transistor 30 and to the alarm unit 31, this unit being 
thus inoperative. Assuming that the frequency multipliers 16 and the 
tracking filters 11 and 12 are all properly operating, no signals are 
transmitted therefrom through OR circuits 21 and 35. The display lamp 36 
is inoperative and gate 29 is operative. Since the signals at the inputs 
of sample and hold circuit 34 are equal the output of this circuit is also 
inoperative. 
If the signals from transducers A and B are not equal, the sample and hold 
circuit 34 will transmit an output signal to OR circuit 35 and through 
delay circuit 37 to lamp 36 which indicates that the system is 
inoperative. At the same time AND gate 29 is blocked in order not to 
indicate any alarm condition. 
If any of circuits 11, 12 or 16 of the narrow-band channels is inoperative, 
a signal is transmitted therefrom through OR circuits 21 and 35 and the 
lamp 36 is energized to show that the system is inoperative. Again the 
alarm system is inhibited by blocking of gate 29. 
If the total signal in the broad-band channel 4 as rectified by rectifier 9 
is below an operating limit, level detector 38 transmits a signal through 
OR circuit 35 to lamp 36 whereby the operator or crew is warned that 
something is wrong but there is not an alarm condition. No alarm is 
released due to the fact that gate 29 has been inhibited. Meter 10 
indicates the total vibration level in the broad-band channel thereby 
allowing to the operator or crew to judge the urgency and possibly the 
reason of the trouble. By throwing switch 11 to the two other possible 
positions, the signal level in narrow-band channels may also be checked. 
When an excessively high signal appears in one of the narrow-band channels 
13 and thus at the input of one of level detectors 22, an operating signal 
is transmitted from this level detector 22 through OR circuits 23 and 24 
to the reset inputs of counters 25 and 26. These counters are now ready 
for counting whenever the excess signal from the faulty channel appears 
but not when the normal signal from the other channel appears. Therefore, 
one of the counters will start to count until it has arrived at the count 
set by its selector 27 or 28 respectively. When this count is complete the 
count is latched by the output signal which is also transmitted to AND 
gate 29. However, no output signal appears at this gate since one input 
only is operative, and also since the gate is blocked from the sample and 
hold circuit which detects a difference between signals in channels A and 
B. The "SYSTEM INOPERATIVE" light 36 is again energized indicating that in 
all probability the monitoring system is faulty because both measuring 
channels should deliver the same information. 
If an excessively high signal appears in both narrow-band channels 13 and 
consequently both level detectors 22 are operative and emit an operating 
output signal, an operating signal will continuously be applied to the 
reset inputs of both counters 25 and 26 such that both counters are 
released for counting clock pulses from generator 7. When the count 
adjusted by the switches 27 and 28 is reached in both counters, output 
signals will be applied to both inputs of AND gate 29 and an alarm signal 
is transmitted to the alarm unit 31 whereby an acoustical and/or visual 
alarm is energized. The operator now knows that a real alarm condition 
occurs and that measures should be taken to deal therewith. It is seen 
that according to the position of selector switches 27 and 28 the alarm 
condition is transmitted from OR circuit 24 to the alarm unit 31 with a 
delay of 4, 8 or 16 seconds if it is assumed that the clock frequency is 1 
Hz. This delay is very desirable in order to prevent accidental alarms by 
transitory disturbments in the engine. 
The alarm unit may be tested by throwing switch 32 to its test position T 
in which case a signal is transmitted through bistable circuit 33 and OR 
circuits 23 and 24 for releasing the counters 25 and 26. Within the 
adjusted delay of 4, 8 or 16 seconds the alarm will work. In this manner 
the readiness for service of the alarm circuits may be tested. After this 
test, the circuits may be reset by throwing switch 32 to the reset 
position R. 
Although the invention has been described above with reference to a 
monitoring system for an engine having two shafts, a similar system may be 
provided for an engine having three shafts, in which case three 
narrow-band channels and level detectors 22 have to be provided. 
Usually one complete separate monitoring system is provided for each engine 
of an aircraft or any other plant such as a power station or the like 
where similar engines have to be monitored. However, one single monitoring 
system as shown might also be used for monitoring more than one engine, 
the system being periodically switched over from one engine or other unit 
to be monitored to another, for instance for the duration of one minute.