OPTICAL DISPLACEMENT SENSOR ARRANGEMENT AND METHOD OF OPERATION

Abstract

An optical displacement sensor arrangement including a light source, a light detector, and a reflective moveable member. The reflective moveable member is moveable relative to the light detector. The light source is disposed to direct light onto the reflective moveable member such that the light is reflected by the reflective moveable member. The light detector is arranged to detect the light reflected by the reflective moveable member, wherein the light is indicative of movement of the reflective moveable member. The optical displacement sensor arrangement is arranged to generate measurement data representing movement of the reflective moveable member based on the light detected by the light detector. The optical displacement sensor arrangement is further arranged to determine a change in a signal generated therein indicative of a rupture of the reflective moveable member; and, in response to determining said signal change, change a power level of the light source.

Claims

1. An optical displacement sensor arrangement, comprising: a light source; a light detector; and a reflective moveable member, wherein the reflective moveable member is moveable relative to the light detector; wherein the light source is disposed to direct light onto the reflective moveable member such that the light is reflected by the reflective moveable member; wherein the light detector is arranged to detect the light reflected by the reflective moveable member, wherein said light is indicative of movement of the reflective moveable member; and wherein the optical displacement sensor arrangement is arranged to generate measurement data representing movement of the reflective moveable member based on the light detected by the light detector; the optical displacement sensor arrangement being further arranged to: determine a change in a signal generated therein indicative of a rupture of the reflective moveable member; and in response to determining said signal change, change a power level of the light source.

2. The optical displacement sensor arrangement of claim 1, wherein changing the power level of the light source comprises turning off the light source or reducing the power level to a reduced non-zero power level.

3. The optical displacement sensor arrangement of claim 1, wherein the optical displacement sensor arrangement is arranged to determine a further signal change that is confirmative of a rupture of the reflective moveable member.

4. The optical displacement sensor arrangement of claim 1, wherein the optical displacement sensor arrangement is arranged to monitor the signal over a first time duration wherein the energy density output by the light source during the first time duration does not exceed a safe maximum permissible exposure for the light source, wherein the step of changing the power level of the light source in response to determining the signal change is carried out after the first time duration has elapsed.

5. The optical displacement sensor arrangement of claim 1, wherein the optical displacement sensor arrangement comprises an interferometric arrangement comprising the reflective moveable member and an optical element, wherein the reflective moveable member is moveable with respect to the optical element; wherein the light source is disposed to provide light to the interferometric arrangement such that a first portion of said light propagates via said interferometric arrangement along a first optical path in which the first light portion is reflected by the reflective moveable member, and such that a second portion of said light propagates along a second different optical path via said interferometric arrangement, thereby giving rise to an optical path difference between the first and second optical paths which depends on a distance between the reflective moveable member and the optical element; and wherein the light detector is disposed to detect at least part of an interference pattern generated by said first and second portions of light dependent on said optical path difference.

6. The optical displacement sensor arrangement of claim 1, wherein determining the signal change comprises at least one of: determining that a variation in the signal during a time interval is below a threshold variation value; determining that an amplitude of the signal during a time interval is below a threshold amplitude value; and determining that a parameter of the signal has a value that is outside of a range of values that are possible for the parameter if the moveable member were not ruptured.

7. The optical displacement sensor arrangement of claim 1, wherein determining the signal change comprises determining that a noise contribution in the signal during a time interval is above a threshold noise value.

8. The optical displacement sensor arrangement of claim 1, comprising a plurality of light detectors arranged to detect the light reflected by the reflective moveable member and to generate respective detector signals corresponding to the detected light, wherein the signal comprises the detector signals, and wherein determining the signal change comprises analyzing two or more of the detector signals.

9. The optical displacement sensor arrangement of claim 8, wherein analyzing the detector signals comprises: defining or determining an expected relationship between the two or more detector signals; calculating a corresponding actual relationship between the two or more detector signals; and determining that the actual relationship deviates from the expected relationship by more than a threshold amount.

10. The optical displacement sensor arrangement of claim 9, wherein the expected relationship is a phase relationship or an amplitude relationship.

11. The optical displacement sensor arrangement of claim 1, wherein the optical displacement sensor arrangement comprises a plurality of interferometric arrangements, each interferometric arrangement comprising a first optical element and a second optical element, wherein the first optical element comprises a respective portion of the reflective moveable member such that the first optical element is moveable relative to the second optical element; wherein the light source is disposed to provide light to the interferometric arrangements such that for each interferometric arrangement, a first portion of said light propagates via said interferometric arrangement along a first optical path in which the first light portion is reflected by the first optical element, and such that a second portion of said light propagates along a second different optical path via said interferometric arrangement, thereby giving rise to an optical path difference between the first and second optical paths which depends on a distance between the first optical element and the second optical element; wherein the optical displacement sensor arrangement comprises a respective light detector for each interferometric arrangement, wherein each light detector is disposed to detect at least part of an interference pattern generated by said first and second portions of light dependent on said optical path difference for the respective interferometric arrangement.

12. The optical displacement sensor arrangement of claim 11, wherein a respective optical path length between the first and second optical elements for light propagating from the light source to one of the light detectors is different for each interferometric arrangement.

13. The optical displacement sensor arrangement of claim 8, wherein determining the signal change comprises at least one of: determining that a phase offset between two of the detector signals deviates from an expected phase offset by more than a threshold amount; and determining that an amplitude difference between two of the detector signals deviates from an expected amplitude difference by more than a threshold amount.

14. The optical displacement sensor arrangement of claim 1, wherein the reflective moveable member comprises a membrane.

15. The optical displacement sensor arrangement of claim 1, wherein the optical displacement sensor arrangement comprises an optical microphone.

16. The optical displacement sensor arrangement of claim 1, comprising an electrically conductive path on the reflective moveable member, wherein the signal change is caused by the electrically conductive path breaking.

17. The optical displacement sensor arrangement of claim 1, wherein the optical displacement sensor arrangement comprises a pair of capacitor plates, wherein one of the capacitor plates is disposed on the reflective moveable member, and wherein the signal change is caused by a rupture of the capacitor plate on the reflective moveable member.

18. The optical displacement sensor arrangement of claim 1, wherein, the optical displacement sensor arrangement comprises an additional light detector in an interior of the optical displacement sensor arrangement, and wherein optical displacement sensor arrangement is provided with at least one of the following features: i) the additional light detector is arranged to detect light entering the interior of the optical displacement sensor arrangement via a hole in the moveable member in the event that the moveable member is ruptured; and ii) the optical displacement sensor arrangement comprises an additional light source arranged to direct a monitoring beam onto the reflective moveable member, such that the monitoring beam is reflected onto the additional detector.

19. The optical displacement sensor arrangement of claim 1, comprising an aperture, wherein the reflective moveable member is arranged to close the aperture.

20. A method of operating an optical displacement sensor arrangement, wherein the optical displacement sensor arrangement comprises a light source; a light detector; and a reflective moveable member, wherein the reflective moveable member is moveable relative to the light detector, the method comprising: generating light from the light source and directing the light onto the reflective moveable member; the reflective moveable member reflecting the light; the light detector detecting the light reflected by the reflective moveable member, wherein said light is indicative of movement of the reflective moveable member; generating measurement data representing movement of the reflective moveable member based on the light detected by the light detector; determining a change in a signal indicative of a rupture of the reflective moveable member; and in response to determining said signal change, changing a power level of the light source.

21. An optical displacement sensor arrangement comprising a closed optical sensing system, the closed optical sensing system comprising: a light source; a light detector; and a reflective moveable member, wherein the reflective moveable member is moveable relative to the light detector; wherein the light source is disposed to direct light onto the reflective moveable member such that the light is reflected by the reflective moveable member; wherein the light detector is arranged to detect the light reflected by the reflective moveable member, wherein said light is indicative of movement of the reflective moveable member; and wherein the closed optical sensing system is arranged to generate measurement data representing movement of the reflective moveable member based on the light detected by the light detector; the optical displacement sensor arrangement being further arranged to: determine a change in a signal generated therein indicative of a rupture of the reflective moveable member; and in response to determining said signal change, change a power level of the light source thereby reducing or preventing leakage of the light from the closed optical sensing system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0131] Certain preferred examples will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0132] FIG. 1 shows a first example of an optical displacement sensor arrangement in accordance with the present disclosure;

[0133] FIG. 2 shows a graph of two optical detector signals generated by the optical displacement sensor arrangement of FIG. 1;

[0134] FIG. 3 shows a flowchart of a method of operation of the optical displacement sensor arrangement of FIG. 1;

[0135] FIG. 4 shows an illustrative example of a photo detector signal generated by the optical displacement sensor arrangement of FIG. 1;

[0136] FIG. 5A shows a second example of an optical displacement sensor arrangement in accordance with the present disclosure;

[0137] FIG. 5B shows a plan view of the membrane of the optical displacement sensor arrangement of FIG. 5A;

[0138] FIG. 6 shows a third example of an optical displacement sensor arrangement in accordance with the present disclosure;

[0139] FIG. 7A shows a fourth example of an optical displacement sensor arrangement in accordance with the present disclosure;

[0140] FIG. 7B shows the optical displacement sensor arrangement of FIG. 7A in the event of a membrane rupture; and

[0141] FIG. 8 shows a fifth example of an optical displacement sensor arrangement in accordance with the present disclosure.

DETAILED DESCRIPTION OF THE DRAWINGS

[0142] The optical displacement sensor arrangement of FIG. 1 is an optical microphone 100. The optical microphone 100 comprises a base 102 with ASIC (application-specific integrated circuit) chip 104 mounted thereon, a micro-electromechanical systems (MEMS) component 106 mounted on the ASIC chip 104 via a spacer 108, and an enclosure 110 mounted over the base 102 to define an acoustic cavity 112.

[0143] A laser light source 114 and two photo detectors 116, 118 are mounted on the ASIC chip 104. The MEMS component 106 comprises a reflective membrane 120 and a transparent substrate 122 with two diffraction gratings 124, 126 fabricated on the substrate 122. The substrate 122 has a stepped profile, so that the diffraction gratings 124, 126 have a height offset relative to each other.

[0144] One side of the membrane 120 faces the exterior 128 of the optical microphone 100. The other side of the membrane 120 faces the substrate 122 and is in fluid communication with the acoustic cavity 112 via holes 130 in the substrate 122 and the spacer 108. When an acoustic wave impinges on the membrane 120, the membrane 120 vibrates, causing the separation between the membrane 120 and the diffraction gratings 124, 126 to vary.

[0145] During normal operation, the laser 114 directs light 132 onto the diffraction gratings 124, 126. Each diffraction grating 124, 126 together with the membrane 120 defines an interferometric arrangement. For each interferometric arrangement, a first portion 134, 136 of the light is transmitted and diffracted by the respective diffraction grating 124, 126. The first portion of light 134, 136 then impinges on the membrane 120 and is reflected back through the respective diffraction grating 124, 126. A second portion 138, 140 of the light for each interferometric arrangement is diffracted and reflected back by the respective diffraction grating 124, 126. For each interferometric arrangement, both portions of light 134, 136, 138, 140 impinge on a respective one of the detectors 116, 118, where they interfere. In this example, the first diffraction order for each diffraction grating 124, 126 is detected by the respective photo detector 116, 118. The amplitude of the light detected by each photo detector 116, 118 depends on the difference in optical path length for the first and second portions of light 134, 136, 138, 140, which depends on the separation between the membrane 120 and the respective diffraction grating 124, 126.

[0146] Each photo detector 116, 118 generates a signal which is communicated to and processed by the ASIC chip 104 to determine the membrane displacement (although the signals could be processed remotely from the optical microphone 100 instead). In accordance with the present disclosure, the signals also allow a detection as to whether the membrane 120 has ruptured as will be explained in greater detail below.

[0147] FIG. 2 shows how the amplitude of the photo detector signals varies with membrane displacement. The amplitude is expressed as a relative diffraction efficiency, which indicates the proportion of light that is diffracted into the first diffraction order detected by the respective photo detector. The solid and dotted lines 200, 202 show the photo detector signal amplitudes for each of the interferometric arrangements, which vary sinusoidally as the membrane position varies.

[0148] The separation of the membrane 120 relative to the diffraction gratings 124, 126, and thus the time-varying membrane displacement, can be calculated based on the sinusoidal relationship shown in FIG. 2. The operating range of each interferometric arrangement corresponds to the substantially linear portion 204, 206 of each line 200, 202, as this is where there is greatest sensitivity in the output signal responsive to a change in membrane displacement.

[0149] The height offset of the diffraction gratings 124, 126 introduces a relative phase offset in the photo detector signals. In this example, the phase offset is 90, but in other examples other offsets may be used. For example, three diffraction gratings may be used with three respective detectors (or sets of detectors), wherein the diffraction gratings are arranged to introduce respective phase offsets of 0, 120 and 240 to the detector signals.

[0150] The phase offsets provide a different working point for each interferometric arrangement which extends the operating range of the optical microphone 100. The phase offsets are also used to detect a rupture of the membrane 120 in a method in accordance with the disclosure, as described with reference to FIG. 3.

[0151] FIG. 3 shows a flowchart 300 of a method of monitoring for a rupture of the membrane 120. The optical microphone 100 is initially operated under normal conditions, as indicated in box 302. While the optical microphone 100 is under normal operations, the photo detector signals are continuously monitored to detect a rupture of the membrane.

[0152] The signals are monitored over a rolling time interval t of 0.33 ms. The maximum signal amplitude S.sub.max and the minimum signal amplitude S.sub.min during the rolling interval period t are determined, as shown for two example times t.sub.1 and t.sub.2 for an illustrative photo detector signal 400 depicted in FIG. 4.

[0153] FIG. 4 illustrates the determination of S.sub.max and S.sub.min at time t.sub.1 when there is significant variation in the signal 400 (corresponding to membrane movement) and at time t.sub.2 when there the signal 400 is substantially flat. A substantially flat signal may indicate that the membrane 120 simply isn't moving because the microphone surroundings are quiet. However, it may also indicate that the membrane 120 has ruptured. For example, if the membrane 120 is partially or completely missing, there may be little or no light from the laser 114 that is being reflected back to the photo detectors 116, 118 by the membrane 120 and/or there may be a substantially flat signal due to constant ambient light entering the optical microphone 100 through a hole in the membrane 120.

[0154] It can be seen that during the time interval t immediately preceding t.sub.1, there is a large difference between the values of S.sub.max and S.sub.min because there is significant variation in the signal, whereas at t.sub.2, S.sub.max and S.sub.min are very close in value because the signal is substantially flat. For illustrative purposes, the variations in the photo detector signal are not shown to scale in FIG. 4 and it is to be understood that in a practical example, there may typically be many more vibrations visible in the time interval t.

[0155] Referring again to FIG. 3, the difference between the maximum signal amplitude S.sub.max and the minimum signal amplitude S.sub.min for the rolling time interval t is continuously calculated and compared with a threshold, as shown in box 304. In this example, the threshold is 50% of the maximum signal measured during calibration.

[0156] If the difference is above the threshold for both of the detector signals, this indicates movement of the membrane 120 and normal operation of the optical microphone 100 continues. If the difference is below the threshold for either or both of the detector signals, this indicates that the signal is substantially flat and that the membrane 120 may have ruptured. In that case, the method proceeds to a second test step as shown in box 306.

[0157] In the second test step, the phase relationship between the signals from the two photo detectors 124, 126 is calculated. As discussed above, in this example, the photo detector signals have a phase offset of 90 owing to the height offsets of the diffraction gratings 124, 126. The signals therefore have an expected phase relationship of a constant phase difference of 90 as the signals vary. If the signal is flat merely because the surroundings of the optical microphone 100 are quiet and the membrane 120 is not moving, this phase relationship will still be observed. However, if the membrane 120 is ruptured such that part or all of the surface that reflects the laser light 132 is missing, the detector signals do not exhibit a constant phase difference of 90. How far the actual phase relationship deviates from the expected phase relationship typically depends on how much, if any, of the membrane 120 is still intact following a rupture. A rupture of the membrane 120 is indicated if the phase relationship changes by more than a threshold amount (e.g. determined during calibration).

[0158] If the determined phase relationship indicates that the membrane 120 is ruptured, the laser 114 is turned off, as shown in box 308. In the case of a potential membrane rupture, laser light 132 could propagate outside of the optical microphone 100. The laser 114 is therefore shut off straightaway upon detection of a potential rupture. However, there is the possibility that the detection of the membrane rupture was a false positive, so after a delay the laser 114 is turned on again at a lower power to check for a false positive, as shown in box 306. For example, the second test may be repeated. The laser 114 is only turned on again for a short time, so that even if laser light 132 is leaking out of the optical microphone 100, the duration and power level is low enough that it cannot exceed the maximum permitted exposure for the laser 114. The second test could be repeated multiple times, with the laser 114 being turned on only for a short time for each check.

[0159] If it is determined that the rupture detection was a false positive, the laser 114 is switched back on at normal power and normal operation continues. Otherwise, if it was not a false positive, the laser 114 is turned off and operation of the optical microphone 100 does not resume, as shown at box 312.

[0160] FIG. 5A shows an optical microphone 500 with a different mechanism for detecting a membrane rupture. The optical microphone has the same readout features as the optical microphone 100 of FIG. 1, including a MEMS component 502 supporting a membrane 504, except that it only has one diffraction grating 506 forming an interferometric arrangement with the membrane 504, and it has two photo detectors 508 that each detect one diffraction order (the 1.sup.st and 1.sup.st orders). The optical readout is carried out by an ASIC chip 510 and works on the same principles of interferometry as described above in relation to each interferometric arrangement in FIG. 1, although other types of optical readout could be used instead.

[0161] In this example, a rupture of the membrane 504 is identified by detecting a break in a conductive path on the membrane 504. FIG. 5B shows an example of how the conductive path may be implemented. The membrane 504 is held by a peripheral support 512, which is part of the MEMS component 502 shown in FIG. 5A. A conductive path 514 is deposited over a central region of surface of the membrane 504 using photolithography, and two wires 516 are formed by wire bonding to connect the conductive path 514 to the ASIC chip 510, as shown in FIG. 5A. A small current is used to monitor the resistance of the conductive path 514 and thus to monitor the integrity of the path 514. If the conductive path 514 is broken, this indicates that the membrane 504 is ruptured at least in the central region where the laser light impinges. When the ASIC chip 510 detects a change in the resistance of the conductive path 514, this indicates that the membrane 504 is likely to be ruptured and the ASIC chip 510 switches off the laser 518 in response.

[0162] FIG. 6 shows an optical microphone 600 with another different mechanism for detecting a membrane rupture. The features relating to the optical readout are the same as in the example of FIG. 5A.

[0163] In this example, a respective conductive layer 602, 604 is deposited on the membrane 606 and the substrate 608 supporting the diffraction grating 610, leaving a gap 612 in the centre of the conductive layers 602. 604 where the laser light 614 impinges. Each conductive layer 602, 604 is connected by wire bonding 616 to the ASIC chip 618, which measures the capacitance of the conductive layers 602, 604. If the membrane 606 is ruptured, removing a portion of the conductive layer 602 deposited thereon, the capacitance of the two conductive layers 602, 604 changes. If the change in capacitance exceeds a defined threshold, the ASIC chip 608 determines that the membrane 606 has ruptured and shuts off the laser 620.

[0164] FIG. 7A shows an optical microphone 700 with another different mechanism for detecting a membrane rupture. The features relating to the optical readout are the same as in the example of FIG. 5A.

[0165] The optical microphone 700 comprises two additional light sources 702 positioned on the enclosure 704 and arranged to direct light 706 onto the outward facing surface of the membrane 708. In this example, the additional light sources 702 are LEDs. There are also two additional photo detectors 710 inside the optical microphone 700 on the ASIC chip 712.

[0166] When the membrane 708 is intact, the light 706 from the LEDs 702 is reflected by the outer surface of the membrane 708 and does not enter the interior 714 of the optical microphone 700. However, if the membrane 708 is ruptured, as shown in FIG. 7B, the LED light 706 propagates into the optical microphone 706 and impinges on the additional photo detectors 710. The output signal from the additional photo detectors 710 is provided to the ASIC chip 712. When the ASIC chip 712 detects a significant increase in the output signal, it determines that the LED light 706 is impinging on at least one of the additional photo detectors 710 due to a membrane rupture and shuts off the laser 716.

[0167] In a variation on this example, the additional photo detectors 170 are provided, but there are no LEDs attached to the enclosure 704. Instead, the additional photo detectors 710 detect ambient light from the optical microphone surroundings that propagates into the optical microphone interior 714 in the event that the membrane 708 is ruptured.

[0168] FIG. 8 shows an optical microphone 800 with another different mechanism for detecting a membrane rupture. The features relating to the optical readout are the same as in the example of FIG. 5A.

[0169] In this example, two additional laser light sources 802 are provided, as well as two additional photo detectors 804. The additional lasers 802 each direct a low-power monitoring beam 806 onto the membrane 808. As long as the membrane 808 is intact, it reflects the monitoring beams 806 back onto the additional photo detectors 804. However, if the membrane 808 is ruptured, one or both of the monitoring beams 806 are not be reflected back onto the additional photo detectors 804. The output signal from the additional photo detectors 804 is provided to the ASIC chip 810. When the ASIC chip 810 detects a significant decrease in the output signal, it determines that at least one of the monitoring beams 806 is no longer being reflected back onto the additional photo detectors 804 due to a membrane rupture and shuts off the laser 812.

[0170] It will be appreciated by those skilled in the art that the disclosure has been illustrated by describing one or more specific aspects thereof, but is not limited to these aspects; many variations and modifications are possible within the scope of the accompanying claims.