WO2015090358A1

WO2015090358A1 - Optical fiber sensor used for oil temperature monitoring

Info

Publication number: WO2015090358A1
Application number: PCT/EP2013/076790
Authority: WO
Inventors: Defeng LANG; Florin Tatar
Original assignee: Aktiebolaget Skf
Priority date: 2013-12-17
Filing date: 2013-12-17
Publication date: 2015-06-25

Abstract

A detector (1) for oil temperature monitoring includes an optical fibre (4) having a first end (6) and a second end (8) having an end face (10). A sensor body (5) has a gap (14) in which a sample (A) of the oil may be received and a reflecting surface (16), the second end of the optical fibre being embedded in the sensor body and having an end face spaced from the reflecting surface across the gap. Light emitted from light source (22) and coupled to the optical fibre by semi-reflective mirror (20) can pass through the sample of oil and be reflected by the reflecting surface back into the optical fibre. By interferometry of the respective signals, the temperature of the oil can be determined.

Description

OPTICAL FIBER SENSOR USED FOR OIL TEMPERATURE MONITORING

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The present invention relates to oil condition monitoring and in particular, to a sensor for detecting the temperature of oil or like substances. The invention also relates to a method of monitoring the temperature of a lubricant.

2. Description of the Related Art

[0002] Optical sensors have been used for oil condition monitoring for determining the presence of debris or otherwise monitor deterioration of a lubricant. Such devices may operate by shining light through a small gap and analysing the transmitted light with a suitable optical sensor. Alternative sensors may make use of scattering of light and may operate over different frequencies including outside of the visible range. Oil condition monitoring may be significant in providing feedback in advance of likely failure of a lubricant system. Action may be taken to perform maintenance or otherwise renew the lubricant.

[0003] The temperature of oil is of considerable concern to many mechanical systems. It can be indicative of the state of the system as a whole and it is also relevant for determining the expected lifetime of the oil. Additionally, other parameters are temperature dependent, such as the saturation level of water in the oil. Oil temperature monitoring is therefore of great importance, in particular for determining a local temperature at a particular critical location. Although reference is made to oil, it will be understood that this can apply to other similar substances and in the following, reference to oil is intended to denote and include measurement of any substance that can be monitored in this manner.

[0004] A sensor has been proposed in co-pending application No PCT/EP2012/075437 by which water in oil can be conveniently detected and whereby calibration of the device is simplified. An alternative arrangement has been proposed in co-pending application No PCT/EP2012/075395 in which use of cost-effective LED's is proposed. The contents of these documents are incorporated herein by reference in their entirety. In both cases, the sensor has a gap for transmission of light between an emitter and receiver through a sample of the oil. The light passing through the oil from the emitter is detected at the receiver and a light signal representative of the light detected is analysed to determine an amount of signal fluctuation. A step change in the signal fluctuation is indicative of saturation of the oil. Another sensor has been proposed US7339657 that uses solid-state source and detector combinations. This may include a source reference channel for temperature compensation purposes.

[0005] It would be desirable to provide a device having a simple and cost effective construction that could provide for temperature monitoring of a lubricant.

BRIEF SUMMARY OF THE INVENTION

[0006] According to the invention there is provided a detector for oil temperature monitoring comprising an optical fibre having a first end and a second end having an end face, a sensor body, having a gap in which a sample of the oil may be received and a reflecting surface, the second end of the optical fibre being embedded in a ferrule portion of the sensor body and having an end face spaced from the reflecting surface across the gap, whereby light emitted from the optical fibre can pass through the sample of oil and be reflected by the reflecting surface back into the optical fibre. As a result of the proposed configuration, the sensor is relatively insensitive to movement of the sensor or vibrations. Optical fibres used in sensing configurations are rather sensitive to bending. As the bending curvature of the fibre changes, so too does the amplitude and phase of a signal passing through the fibre. As a consequence for existing sensing systems, the whole system must be free from geometry change during operation. According to the presently claimed detector, the signal path includes a mirror and the phase shift between a first light signal reflected internally by the fibre and a second light signal reflected by the mirror can be monitored. In this manner, signal changes due to changes in the fibre curvature will affect both signals equally and are eliminated.

[0007] In a preferred embodiment of the invention, the gap between the end face and the reflecting surface is less than 1 mm, preferably less than 0.5 mm and most preferably around 0.2 mm. The actual gap may be chosen according to the nature of the oil being treated. Since the signal must pass the gap twice before re-entering the fibre, the gap in the presently claimed configuration may need to be around half of the width of a corresponding gap when the sensor is opposed to the light source. [0008] In a further preferred embodiment, the end face of the fibre is provided with a semi- reflective coating. In this manner, the amount of internal reflection can be carefully regulated and can be made independent of the nature or condition of the oil to be monitored.

[0009] The ferrule may be manufactured of any material, including metals, plastics and the like, in particular those suitable for protecting it mechanically and chemically from external influences. Most preferably, the ferrule is made of ceramic material such as is conventionally used for ferrules for fibre-optic connectors. Preferably, the reflecting surface is connected to the ferrule by an expansion beam whereby a width of the gap varies with change in temperature. The expansion beam may be manufactured of any suitable material that can ensure the desired change in gap width over the temperature range to be measured. Preferably this should be chosen in combination with the light signal frequency such that the gap width will change less than a half wavelength over the measuring range. Examplary materials include ceramics, glasses, epoxies, thermosetting plastics, thermoplastics including e.g. ABS plastic. The expansion beam and ferrule may be manufactured in one piece from a single material or may be manufactured of different materials as required.

[0010] According to a further important aspect of the invention, the optical fibre maybe potted in the ferrule in a rigid fashion to avoid any movement or vibration being transmitted thereto. Preferably, the fibre is potted in the ferrule over a length of at least 5 mm.

[0011] The detector is intended for operation with a suitable light source and may further comprise such a light source coupled to the first end of the optical fibre. Most preferably, the light source is a laser source. The laser source may operate at frequencies ranging from 850 nm to 1750 nm. At these frequencies, good transmission through oil is encountered.

[0012] Any suitable detection arrangement capable of determining phase difference between two light signals may be used to interrogate the sensor. Most preferably, the detector further comprises an interferometer coupled to the first end of the optical fibre and arranged to compare a first light signal reflected internally by the end face with a second light signal reflected by the reflecting surface. The first light signal that is reflected by the end face of the fibre is used in the interferometer as the reference signal. The second light signal that is reflected at the reflecting surface is the measurement signal. Any phase change caused by fibre bending will influence both reference signal and measurement signal in a fixed proportional way. In this manner the phase change caused by fibre bending is zero. Any recorded phase change represents the difference in path length travelled by the second light signal. The detector is thus not restricted to any particular geometry and movement can take place without upsetting the calibration.

[0013] The skilled person will be well aware of appropriate ways in which a light source and interferometer may be coupled into the optical fibre. In one preferred configuration, the light source and the interferometer may be coupled via an optical switch. One such optical switch may be in the form of a semi-reflective mirror or prism. Other similar beam splitters may also be used.

[0014] The invention also relates to a method of temperature monitoring of oil in a mechanical system, comprising positioning an end face of an optical fibre in spaced relation to a reflecting surface,providing a sample of the oil to bridge the gap between the end face and the reflecting surface, passing light through the optical fibre towards the end face, whereby a first portion of the light is internally reflected by the end face as a first light signal and a second portion of the light is transmitted through the oil and reflected by the reflecting surface back into the optical fibre as a second light signal and analysing the first and second light signals to determine a variation in phase difference of the second light signal with respect to the first light signal due to variation in a width of the gap due to changes in oil temperature.

[0015] In one embodiment, the step of analysing the first and second light signals comprises scanning the frequency spectrum to identify a maximum for the second light signal. Use of a scanning optical spectrometer can allow the system to determine the frequency at which the second light signal is strongest.

[0016] Preferably, the step of analysing the first and second light signals comprises determining the phase change of the second light signal with respect to the first light signal and comparing the phase change with predetermined values representative of the phase change at given temperatures. The detector may be calibrated in advance for oil having different temperatures. The values may be stored as look-up tables in a suitable memory and extrapolation between these values maybe used to determine a momentary oil temperature.

[0017] The detector may also be used to identify the presence of water in oil, as described in co-pending application No entitled "Optical fiber sensor used for oil conditioning monitoring" filed on the same date as the present application. By analysing the signals and determining a relative change in amplitude between the first and second signals, the condition of the oil in the gap can be determined. As will be understood, moisture in the oil will cause absorption of light, leading to a decrease in signal strength of the second light signal. This will be particularly strong at certain frequencies where absorption is at a maximum. The sensor may also be used for detecting the saturation point. It has been observed that a significant change in signal characteristic is to be observed at the point at which free water appears in the oil. Below the saturation level, the amplitude of the second light signal is relatively stable and only steadily decreases with increasing absorbed water content. As the amount of water approaches saturation, the second light signal becomes highly unstable and may appear noisy. Without wishing to be bound by theory, it is believed that bubbles of free water are formed within the oil in a manner similar to cavitation or boiling of a liquid. As these bubbles pass the sensor, they disturb the signal, effectively leading to greater absorption of the light and a lower second light signal. A significant advantage of the above effect is that the detector can be easily calibrated in-situ to the saturation level, without requiring knowledge of either the oil or sensor characteristics. Additionally, the sensor can provide real-time results with negligible delay in identifying the presence of free water in the oil.

[0018] Although light across a range of frequencies may be used to carry out the invention, preferably the light comprises infra red light in the range 850 nm to 1750 nm. These frequencies allow good transmission through oil and also include frequencies at which water in oil can be detected.

[0019] The process is preferably carried out using a controller, which may be any appropriate processing device such as a computer or dedicated microprocessor. In addition to other control tasks, the controller is preferably arranged to determine the phase difference between the first and second light signals and compare this with values in a memory. In particular the controller may carry out signal analysis, sampling and filtering as described above.

[0020] The skilled person will understand that the sensor of the present invention may be implemented in a number of different situations where the temperature of oil or like substances is to be monitored. Preferably, the optical sensor is located in an oil supply line to a mechanical system. The mechanical system may be a motor, a gear, a bearing, a journal, a cam or a complex system comprising one or more of the above.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The features and advantages of the invention will be appreciated upon reference to the following drawing of an exemplary embodiment, in which:

[0022] Figure 1 shows a schematic view of a system according to the present invention; and

[0023] Figure 2 shows a plot of the light signals received by the spectroanalyzer. DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0024] Figure 1 shows a schematic view of a detector 1 for oil condition monitoring according to the invention. The detector 1 comprises a sensor body 2 and an optical fibre 4. The optical fibre 4 has a first end 6 and a second end 8 having a semi-reflective end face 10. This may be achieved with an appropriate semi-reflective mirror coating. The second end 8 is embedded in a channel 12 through a ferrule portion 3 of the sensor body 2 such that the end face 10 is coincident with a gap 14 extending through the sensor body 2. At an opposite side of the gap 14 from the channel 12, facing the end face 10 of the optical fibre 4 is a reflecting surface 16. The reflecting surface 16 is separated from the ferrule portion 3 by an expansion beam 5.

[0025] At its second end 8, the optical fibre 4 is coupled through a semi-reflective mirror 20 to a laser source 22 and a spectroanalyzer 24. The semi-reflective mirror 20 acts as an optical switch between the laser source 22 and the spectroanalyzer 24 as described further in detail below.

[0026] In use, the sensor body 2 is located within a mechanical system (not shown) such that oil A is received in the gap 14. Light L from the laser source 22 is coupled into the fibre 4 and guided through the optical fibre 4 to exit at the end face 10. A portion of the light L is reflected internally by the semi-reflective surface of the end face 10 and returns through the optical fibre as first light signal SI . The remainder of the light L passes into and through the oil A in the gap 14 and impinges on the reflecting surface 16, which reflects it back across the gap 14 and into the second end 8 of the optical fibre 4 as second light signal S2. [0027] The first and second light signals SI, S2 are transmitted through the optical fibre 4 and the semi-reflective mirror 20 to the spectroanalyzer 24. The spectroanalyzer 24 is operated to scan the frequency spectrum and determine the frequency at which the signal S2 is at a maximum. In general, once determined, this frequency will remain relatively stable for a given configuration. It will be understood that the system may also operate at a single frequency, e.g. with a single frequency light source and without a spectral analyser.

[0028] The first light signal SI is used as the reference signal. The second light signal S2 is compared to the first light signal SI in terms of its phase shift. Any phase changes caused by the fibre 4 bending will influence both signals SI and S2 in a fixed proportional way. In this way phase changes caused by fibre bending can be ignored. The actual phase difference between the first and second light signals SI, S2 therefore represents the additional path length due to the second light signal crossing twice the gap 14. Any temperature increase in the oil A surrounding the sensor body 2, causes the expansion beam 5 to expand and consequently the gap 14 to widen. As a result, an additional lag is applied to the second light signal S2 with respect to the first light signal S 1. This lag can be identified by the spectroanalyzer 24 and compared with preset values representative of calibration temperature measurements.

[0029] Figure 2 shows a plot of the light signals S 1 and S2. In an initial condition at a first temperature Tl, the signals S I and S2 have a phase difference Φ1. In a subsequent measurement, signal S2' has a phase difference Φ2. The spectroanalyzer 24 evaluates the change in phase difference. Values for the phase difference may be provided in advance and stored in a look-up table in an appropriate memory (not shown). The measured value of Φ2 can then be compared with the precalibrated value to determine the temperature T2 of the oil.

[0030] Thus, the invention has been described by reference to the embodiment discussed above. It will be recognized that this embodiment is susceptible to various modifications and alternative forms well known to those of skill in the art without departing from the spirit and scope of the invention. In particular, for implementation in a mechanical system, the detection cell may be located in an oil supply line whereby a portion of the oil supply passes through the gap. Furthermore, the analysis of the signals may take place on a personal computer or a dedicated controller or microprocessor which may be located in-situ or remotely. Accordingly, although specific embodiments have been described, these are examples only and are not limiting upon the scope of the invention.

Claims

1. A detector for oil temperature monitoring comprising: an optical fibre having a first end and a second end having an end face; a sensor body, having a gap in which a sample of the oil may be received and a reflecting surface, the second end of the optical fibre being embedded in a ferrule portion of the sensor body and having an end face spaced from the reflecting surface across the gap, whereby light emitted from the optical fibre can pass through the sample of oil and be reflected by the reflecting surface back into the optical fibre.

2. The detector of claim 1, wherein the gap between the end face and the reflecting surface is less than 1 mm, preferably less than 0.5 mm and most preferably around 0.2 mm.

3. The detector of claim 1 or claim 2, wherein the end face is provided with a semi- reflective coating.

4. The detector according to any preceding claim, wherein the reflecting surface is connected to the ferrule by an expansion beam whereby a width of the gap varies with change in temperature.

5. The detector according to any preceding claim, wherein the optical fibre is potted in the ferrule over a length of at least 5 mm.

6. The detector according to any preceding claim, further comprising a light source coupled to the first end of the optical fibre.

7. The detector according to any preceding claim, further comprising an interferometer coupled to the first end of the optical fibre and arranged to compare a first light signal reflected internally by the end face with a second light signal reflected by the reflecting surface.

8. The detector according to claim 6 and claim 7, wherein the light source and the

interferometer are coupled via an optical switch.

9. The detector according to any of claims 6 to 8, wherein the light source is an infra-red laser source.

10. A method of temperature monitoring of oil in a mechanical system, comprising: positioning an end face of an optical fibre in spaced relation to a reflecting surface; providing a sample of the oil to bridge the gap between the end face and the reflecting surface; passing light through the optical fibre towards the end face, whereby a first portion of the light is internally reflected by the end face as a first light signal and a second portion of the light is transmitted through the oil and reflected by the reflecting surface back into the optical fibre as a second light signal; and analysing the first and second light signals to determine a variation in phase difference of the second light signal with respect to the first light signal due to variation in a width of the gap due to changes in oil temperature.

1 1. The method according to claim 10, wherein the step of analysing the first and second light signals includes scanning the frequency spectrum to identify a maximum value for the second light signal.

12. The method according to claims 10 or 11 , wherein analysing the first and second light signals comprises determining the phase difference of the second light signal with respect to the first light signal and comparing the phase difference with predetermined values representative of the phase difference for a given temperature. -i lls. The method according to any of claims 10 to 12, wherein the light comprises infra-red light in the range 850 nm to 1750 nm.

14. The method according the any of claims 10 to 13, wherein analysing the first and second light signals further comprises comparing a change in amplitude of the second light signal with respect to the first light signal and determining a condition of the oil based on the attenuation of the second light signal.

15. The detector according to any of claims 1 to 9, further comprising a controller adapted to analyse the light signals according to the method of any of claims 10 to 13.