GB2054852A - Vibration detector - Google Patents

Abstract

In a system arranged to detect vibrations in a rotary machine, a signal 1 corresponding to vibrations produced by non-rotating parts of the machine is applied to a commutating filter, such as that shown in Fig. 3, together with a commutating signal 10 derived from a rotational part of the machine. The device is particularly suitable for use in detecting vibration levels in rotary systems operating at various outputs. <IMAGE>

Description

SPECIFICATION Vibration detection device The present invention relates to vibration detection devices.

According to the invention there is provided a detection device for detecting vibrations of a rotary system, the device comprising a sensor for monitoring the movement of a non-rotary part of the system, and providing an output signal in response thereto, a band-pass filter having at least one integrating circuit connected to integrate the output signal, the or each integrating circuit having a resistor, switching means and a capacitor connected in series and an operational amplifier in which the capacitor forms the feed-back loop between the output and the inverting input of the operational amplifier, pilot means responsive to the rotary speed of a rotary member of the system to generate a control signal to operate the switching means of the or each integrating circuit at a frequency equal to the frequency of rotation of a rotary member of the system, or a whole multiple of this frequency, whereby to adjust the central frequency of the filter accordingly and means for measuring the amplitude of the output signal from the filter.

According to the invention there is further provided a detection device for detecting vibrations in a rotary system having rotary and non-rotary members, first means for providing an output signal in response to vibrations of a nonrotary member and second means for providing a control signal having a frequency equal to. the rotational speed of the rotary member or a multiple thereof, an integrating circuit for integrating the output signal the integrating circuit comprising a resistor and a capacitor connected in series by switching means, the switching means being switched, in response to the control signal whereby to endow the integrating circuit-with a central pass-band frequency equal to that of the control signal, and means for measuring the amplitude of the output signal from the integrator.

Vibration detection devices embodying the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings in which: Figure 1 is a circuit diagram of an integrating circuit of a first one of the devices; Figure 2 is a diagram of a control signal to be applied to the circuit of Figure 1; Figure 3 is a circuit diagram of a filter circuit in the form of a low-pass filter, of a second one of the devices; Figure 4 is a circuit diagram of a second one of the devices including a filter circuit in the form of a universal filter; Figure 5 is a block diagram of a third detection device for detecting vibrations and providing an output signal from which vibration signals can be filtered; Figure 6 is a circuit diagram of a signal generator of the device of Figure 5;; Figures 7a to 7c illustrate signals occuring at different points in the circuit of Figure 1; Figure 8 is a circuit diagram of an amplitude measurement circuit of Figure 5; and Figures 9a to 9c illustrate signals occuring at different points in the circuit of Figure 8.

Figure 1 shows an integrating circuit (or celi) incorporating an operational amplifier. The circuit has an input terminal 1, an output terminal 2, and an operational amplifier 3. A non-inverting input terminal 3a of the amplifier is connected to earth 4, while an inverting input terminal 3b is connected to the input terminal 1 through a resistor 5. The output terminal 6 of the amplifier 3 is connected to the output terminal 2. A capacitor 7 is connected between the output terminal 6 and the inverted input terminal 3b to provide negative feed-back.

An electrtonic switch 8 interconnects the resistor 5 and the junction 9 between the capacitor 7 and the inverting input terminal 3b.

The switch 8 is responsive to a control signal 10 indicated symbolically by an arrow in Figure 1. The control signal 10 shown in Figure 2 is in binary form. When the signal is a binary "1" the switch 8 is closed for a period 0. When the signal is a binary "0" the switch 8 is open, and the repetition rate of the signal is F.

The signal 10 is a periodic rectangular pulse signal having a cyclic ratio: 0 ~ = OF.

T The time constant of the integrator of Figure 1 is equal to RC/0F, R and C being, respectively, the values of the resistor 5 and of the capacitor 7. By varying the cyclic ratio OF of the signal 10, the time constant of the integrating circuit will be varied accordingly.

The described integrating circuit is particularly suitable for use in a filter with a variable cut-off frequency.

Figure 3 shows a low-pass filter of the first order incorporating the integrating circuit of Figure 1. As shown in Figure 3 a gain stabilisation resistor 11 is connected between the output 6 of the amplifier 3 and the junction 12 between the input resistor 5 and the switch 8. Also a resistor is connected between earth and the non-inverting input of the amplifier 3.

The 3dB cut-off frequency fc of the filter of Figure 3 is OF fc= 2R,C where R, is the value of the resistor 11, and t9, F and C are as already defined hereinbefore with reference to Figures 1 and 2. The cut-off frequency fc of the low-pass filter of Figure 3 is modified by varying the value of OF.

Figure 4 is a circuit diagram of a second order filter using four operational amplifier circuits two of which are similar to that of Figure 1. The filter has a first circuit 1 3 formed by an operational amplifier 1 3a arranged in inverse addition for, receiving at a first input terminal 1 3b the signal to be filtered. Second and third input terminals 1 3c and 13dof the circuit 13 are respectively connected to the outputs, 14a and 1 5a of corresponding integrating circuits 14 and 15; the circuit 15 being formed by an operational amplifier 15b connected as an inverting amplifier.The other integrating circuit 1 6 is connected between the output terminal 1 3e of the addition circuit 13 and the input terminal 14b of the first integrating circuit 14, and it is connected, through its output terminal to the input 1 sic of the amplifier circuit 15.

The switch 8 of each integrating circuit is in the form of a field effect transistor 1 7 having n channels; preferably of the type 2N4091. The transistor 17 of each integrating circuit 14 and 16 has its control electrode 1 7a connected to a corresponding one of two terminals 1 4c, and 16b.

A generator (not shown) arranged to produce rectangular periodic pulses having a controllable cyclic ratio OF is arranged to supply its pulses to the terminals 1 4c and 16d.

The filter of Figure 4 has three output terminals 1 8, 1 9 and 20 electrically equivalent to the terminals, respectively 1 4a, 1 3e and 1 sic hereinbefore described. At each of these output terminals 1 8, 1 9 and 20 there is produced a signal derived from the signal applied at the input terminal 1 3b after being filtered, respectively, at low-band pass, at high-pass and at band-pass. In order to protect the control electrode 1 7a of the field effect transistors 17 against positive voltages, a diode 21 is connected in the reverse sense between each electrode 1 7 and corresponding terminal 1 4c and 166.

In order to ensure the operation of each transistor 17 over the whole of the dynamic range of the input signal applied to each integrating circuit 14 and 1 6, the terminal 1 7b of the transistor 1 7 is connected to the junction 22 between two series connected diodes 23 with each series combination of diodes connected between a pair of earth points and the junction 22 of each combination being connected through a resistor to the control electrode of a corresponding FET.

Thus each of the two integrating circuits, 14 and 1 6 is similar to the circuit of Figure 1 and the components 5 and 7 in the circuits 14 and 1 6 of Figure 4 have the same value as their counterparts in Figure 1. By varying the cyclic ratio of the signal applied to the terminals 1 4c and 1 6b, the cut-off frequencies of the filters are caused to vary at lowpass (output 18), high-pass (output 19) and the central frequency of the pass-band filter (output 20).

The gain stabilisation resistor of the circuits 13 and 14 is formed by the resistor 24 while that of the circuits 15 and 1 6 is formed by the resistor 25.

The described frequency cut-off filters are particularly suitable for use in conjunction with detection devices, or indicators for sensing vibratory levels of rotary systems such rotary machines, operating at various outputs and, or having several rotary members rotating at different speeds. A jet engine for an aircraft is onle example of such a rotary system.

The detection device shown in Figure 5 has a detector 26 in the formed pilot means, which provides an analogue or control signal proportional to the acceleration or to the speed or to the position of a point on a non-rotary part, for example the casing of, a rotary system to be monitored. This signal is fed to the input of a narrow band-pass filter 27 (such as that shown in Figure 4) having a central frequency fc controlled by a control pulse signal 10 provided by a generator 28. The frequency F of the signal 10 is arranged to be a multiple of the frequency of rotation N of a rotary element of the system to be monitored; thus F = pN, where p is a positive number.In order to ensure correct operation of the filter 27 F fc For example F is preferably from 5 to 10 times fcX To this end, the values (R) of the resistor 5 (C) of the capacitor 7 and of fl are suitably chosen as a function of the frequency F of the control signal and of the central frequency of the filter 27. In the case of the circuit of Figure 4, this central frequency fc is equal to OF 2RC where 0, F, R and C are as already hereinbefore defined.

The filtered output signal from the filter 27 is fed to an amplitude measurement circuit 29.

The generator is shown in more detail in Figure 6. As shown the generator includes a sensor 30 for sensing the rotational frequency of a rotary member of the system to be monitored, a differential amplifier 31 receiving the signal from the sensor 30, a SCHMITT trigger 32 for shaping the curved input signal 33 (see Figure 7a) produced from the amplifier 31 into a rectangular signal 34 (see Figure 7b) and a monostable circuit for transforming the signal 34 into a calibrated signal 35 (see Figure 36) having the same frequency F as the signals 33 and 34.

The sensor 30 is of the variable inductance type and includes a wheel of ferro-magnetic material 37 mounted on a rotary member 38 of the system to be monitored, to be coaxially with the axis of rotation of this rotary member. The wheel 37 has a plurality of equiangularly spaced projections 37a about its periphery. These projections are arranged to pass close to the adjacent end of a boss of ferro-magnetic material 39a carrying a winding 39. The two terminals 39b, 39c of the winding 39 are connected to respective inputs 318 and 31 b of the differential amplifier 31. Any parasitic signals of common mode are eliminated by a differential amplifier. The monostable circuit 35 provides a signal 36 in which the value 0 (the duration of each rectangular pulse) is constant and independent of the frequency F = 1/T of this signal 36.

The measuring circuit 29 of Figure 5 is shown in more detail in Figure 8. This circuit includes a peak detector for detecting the peak of the filtered signal 40 supplied by the filter 27. The signal 40 is substantially sinusoidal if the band-pass at the filter 27 is narrow and if there exists, in the signal 41 suppiied by the detector 26, a component having the frequency OF fc = = K 27rRC where K is a constant.

The circuit Figure 9 includes a detector circuit 42 for receiving, at its input terminal 42a the filtered signal 40 and a storage capacitor 43 for storing the peak value of the filtered signal. The capacitor 43 is connected between earth 4 and the emitter 44a of a transistor 44 forming part of the detector circuit 42. A discharge circuit 45 is connected to effect the discharge of the capacitor 43. An output amplifier circuit 46 has an input terminal 46a connected to the junction 43a between the capacitor 43 and the emitter 44a.

The detector circuit 42 includes an operational amplifier having an output feeding the base of the transistor 44 through a resistor 47. The base emitter circuit of the transistor 44 controls the charging of the capacitor 43. The collector 44b of the transistor 44 is connected to a voltage source + V. The input 42a is connected to the non-inverted input 47a of the operational amplifier 47.

The discharge circuit 45 includes a resistor 48 connected, between the junction 43a and earth 4, in series with the emitter-collector circuit of a transistor 49. The base 49a of the transistor 49 is connected to the output terminal of the circuit 28 to receive the signal 36. The output terminal of the circuit 28 is connected to a control terminal 45a of the circuit 45. A resistor 50 is connected between the base 49a and the control terminal 45a and a resistor 51 is connected between the base 49a, and earth 4. The transistor 49 acts as a switch connecting the resistor 48 to earth 4 and thus isolating the resistor 48, from earth according to whether the signal 36 is a binary "1" or "O".

The circuit 46 includes an operational amplifier 52 connected as a follower amplifier of the noninverting type having a high input impedance and acting as a current amplifier for a galvanometer 55 connected in the negative feed-back path of the amplifier 52 in series with the resistor 54, a resistor 53 connects the input 52a of the amplifier to earth 4. In operation, when a periodic disturbance in the frequency N (for example an imbalance) appears in the rotary system being monitored, the analogue signal 41 will have a component at this frequency N. The filter 27 will eliminate all other frequency components of the signal 41. The detected component provided at the output terminal of this filter will be a pure sine wave of frequency N, having a value which can be measured by the device 29.The device 29 measures the peak amplitude of the signal 41 but since this signal 41 is a pure sine wave, its peak value will be proportional to the effective value and so the scale of the galvanometer is graduated in effective values so that the effective value can be read directly off the galvanometer. The voltage appearing at the junction 43a of the circuit 29 (Figure 9) is not constant but includes a certain residual undulation known as hum of amplitude "a". The frequency of the hum is independent of N (the frequency of the filtered signal 40) owing to the commutation by transistor 49 as will be explained in more detail hereinafter.

Figures 9a to 9c show, the signal 40, as it is shaped by the operation of the base-emitter of the transistor 44, and sampled by the capacitor 43 associated with the discharge circuit 45, in synchronism with the signal 36. As shown in Figure 1 or, the signal 36 has the same frequency as the signal 40. However in practice, the frequency (F) of the signal 36 is clearly higher than that (N) of the signal 40; nevertheless if the value of the resistor 48 is not too low, then the action of the circuit 45, between two adjacent positive peaks 40a of the signal 40, is the same for the two signals 36 of different frequency (F) but of the same cyclical ratio OF. The time constant of the discharge capacitor 43 and circuit 45 is inversely proportional to the frequency N of the signal 40 the amplitude of which is to be measured.It follows that the amplitude "A" is independent of N. The detector 26 can detect acceleration, speed, or the position of a point relative to the casing of the rotary system. This detector includes a sensor and a differential amplifier for receiving the signal detected by the sensor to amplify this signal and to eliminate parasitic signals generated by the common mode.

In a modification, the filter of Figure 4 takes the form of a unit having more than two integrating circuits connected in series with the operational amplifier 1 3a provided with negative feed-back. In this case, the amplifier 1 sub acts as an addition circuit and each supplementary integrating circuit is connected, through its output, to a unique input of one or the other of the addition circuits according to its range in the sequence, thus the third integrating circuit is connected to the second addition circuit 15, the fourth integrating circuit is connected to the first addition circuit 1 3 and so on in sequence.

The described filter circuits detect and filter a frequency disturbance N in the rotary system and at least one of the parameters R, C, 0, and p is controlled so that fc = N. To detect harmonics of this disturbance, at the frequencies 2N,3N,...

etc. the frequency fc will need to be controlled to the values 2N, ..... . etc.

Claims (9)

1. A detection device for detecting vibrations of a rotary system, the device comprising a sensor for monitoring the movement of a non-rotary part of the system, and providing an output signal in response thereto, a band-pass filter having at least one integrating circuit connected to integrate the output signal, the or each integrating circuit having a resistor, switching means and a capacitor connected in series and an operational amplifier in which the capacitor forms the feed-back loop between the output and the inverting input of the operational amplifier, pilot means responsive to the rotary speed of a rotary member of the system to generate a control signal to operate the switching means of the or each integrating circuit at a frequency equal to the frequency of rotation of a rotary member of the system, or a whole multiple of this frequency, whereby to adjust the central frequency of the filter accordingly and means for measuring the amplitude of the output signal from the filter.

2. A device according to claim 1, wherein the measuring means is a peak detector, comprising a storage capacitor for receiving the output signal from the filter, and a discharge circuit for discharging the storage capacitor, the discharge circuit including a discharge resistor connected in series with a switch, the switch being operated in response to said control signal.

3. A device according to claim 1 or to claim 2, wherein the filter comprises two integrating circuits connected in series, and including an operational amplifier arranged as an inverting adding circuit, wherein the second input of the adding circuit is connected to the output of a first integrating circuit, through the intermediary of an inverting circuit, and the output of the second integrating circuit is connected, to a first input of the adding circuit, the arrangement being such that a band-pass filter exists between a third input of the adding circuit and the output of the first integrating circuit.

4. A device according to claim 1 or to claim 2, wherein the filter comprises at least three integrating circuits connected in series and including an adding circuit amplifier, and an inverting amplifier arranged as an adding circuit, wherein each alternate integrating circuit in the series has its output connected to an input of the inverting amplifier and each intervening integrating circuit in the series, and the inverting amplifier has its output connected to said input of the adding circuit amplifier.

5. A detection device for detecting vibrations in a rotary system having rotary and non-rotary members, first means for providing an output signal in response to vibrations of a non-rotary member and second means for providing a control signal having a frequency equal to the rotational speed of the rotary member or a multiple thereof, an integrating circuit for integrating the output signal, the integrating circuit comprising a resistor and a capacitor connected in series by switching means, the switching means being switched, in response to the control signal whereby to endow the integrating circuit with a central pass-band frequency equal to that of the control signal, and means for measuring the amplitude of the output signal from the integrator.

6. A detection device including an integrating circuit substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.

7. A detection device including a filter circuit substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.

8. A detection device including a filter circuit substantially as hereinbefore described with reference to Figure 4 of the accompanying drawings.

9. A detection device for detecting vibrations of a rotary member as hereinbefore described with reference to Figures 5 to 9 of the accompanying drawings.