SELF-MIXING REFLECTIVE SENSOR
FIELD OF THE INVENTION
The invention generally relates to the field of optoelectronic detection of movement-related parameters. More specifically, the invention relates to measurement of movement-related parameters by means of self-mixing sensors.
BACKGROUND OF THE INVENTION
Application of vertical cavity surface emitting lasers (VCSEL) for reflective sensors is known from US 2006/043278 and WO2006042072. The device comprises a single-mode VCSEL as a light source and a separate photodiode as a photodetector. The application of LEDs as reflective sensors is disclosed in
DE 10 2006 003269 and DE 19 526 249, in which application at least two LEDs and an external photodiode are employed. LED power is modulated to suppress the influences of high ambient light. It is known that laser self-mixing can be used for sensing applications, see G. Giuliani, M. Norgia, S. Donati, T. Bosch "Laser Diode Self-mixing Technique for Sensing Applications" in Journal of Pure and Applied Optics, 6 (2002), pages 283-294. A VCSEL-based self-mixing computer input device is known from US patent 6,707,027 and patent WO2007026293.
An efficient optical reflective sensor must be sensitive to weakly reflected light, even in the presence of high ambient light. Current reflective sensors employ sophisticated light-modulation circuits to suppress influences of high ambient light.
OBJECT AND SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a sensor for speed or displacement which is capable of operation in high ambient light environments and is simplified in design, particularly regarding its circuitry. This object is solved by the subject of the independent claims. Advantageous embodiments and refinements of the invention are defined in the respective dependent claims.
The invention relates to a self-mixing reflective sensor, preferably VCSEL-based, which does not require a separate photodetector nor complicated signal- processing circuitry. A self-mixing reflective sensor according to the invention is particularly suitable for detection of weak or diffuse reflected surfaces in high ambient light environments.
Laser self-mixing occurs if an external reflecting surface is arranged within the optical path of a laser so that an external cavity is obtained. Tuning of the external cavity results in a readjustment of the laser equilibrium conditions and thus in detectable changes in the laser output power. These changes, typically in the form of undulations or oscillations, are repetitive as a function of displacement of the external reflecting surface over a distance of half a laser wavelength. The undulation frequency is proportional to the velocity of the external reflector. However, the measurement principle according to the invention is based on a detection of the self-mixing oscillation amplitude rather than the oscillation frequency of the self-mixing signal.
According to the invention, a laser self-mixing reflective sensor is provided, comprising
-a laser diode with a laser cavity, -a detector for detecting the laser intensity, -circuitry for detecting self-mixing oscillations in the laser intensity due to laser light reflected back into the laser cavity, the circuitry being connected to the detector for detecting the laser intensity, and
-detector circuitry for detecting a signal proportional to the absolute value of the oscillation amplitude. The corresponding method of detecting a displacement-related parameter by means of this self-mixing sensor comprises the steps of:
-generating a laser beam within the laser cavity of a laser, -measuring the laser intensity by means of a detector, -moving a surface within the optical path, the surface having a laterally varying structure which causes a laterally varying reflection of the laser light back along the direction of incidence of the laser beam,
-extracting self-mixing oscillations in the laser intensity due to laser light reflected back from said surface into the laser cavity, using circuitry connected to said detector for detecting the laser intensity,
-determining a signal proportional to the absolute value of the oscillation amplitude of said self-mixing oscillations, and -calculating a displacement-related parameter from the variation of said absolute value of the oscillation amplitude of said self-mixing oscillations.
The self-mixing reflective sensor is a self-aligned interferometric device which is a general advantage over known sensors because the measurement accuracy is nearly independent of the alignment of the laser with respect to the movable surface. Furthermore, a power supply maintaining the injection current to the laser diode at a constant level may be advantageously employed so as to stabilize the emission wavelength.
It is further generally preferred to use single-mode laser diodes. This is advantageous because superposition of self-mixing effects at different wavelengths which may have opposite phases is avoided.
The displacement-related parameter may be the displacement of the surface, an angle of rotation corresponding to the rotating displacement of the surface of a rotatable element or the velocity, i.e. the displacement per unit of time. A coherent reflective sensor according to the invention can thus precisely measure displacement- related parameters such as the angular speed and displacement of rotating gears or wheels.
To measure the angular speed and/or displacement of rotating gears, the beam is directed in one embodiment onto the top surface of the gear teeth, so that the intensity of the laser beam reflected back along the incident laser beam varies between an incidence of the laser beam onto the teeth and into gaps between the teeth of the gear wheel. This effect can be strongly enhanced if the divergent beam of the laser diode is focused on the tips of the teeth, preferably with an integrated lens. If the focal depth is shorter than the height of the teeth, there will be a strong increase of the intensity of the reflected light if a tip of a gear wheel tooth passes through the laser beam. This strong increase can be easily detected by evaluating the absolute value of the amplitude or a related parameter such as the RMS-value or the envelope of the oscillating signal.
Alternatively, the movable surface may have a periodically varying reflectivity, so that the portion of the laser beam reflected back periodically switches its intensity due to reflection from structures having a lower and higher reflectance. For example, the surface whose movement or displacement is to be determined may be equipped with a structured film having alternating zones of different reflectance. The detector circuitry for detecting a signal proportional to the absolute value of the oscillation amplitude can be designed in a simple and straightforward manner.
In particular, in contrast to known self-mixing range or speed sensors which require advanced frequency detection circuits, a simple envelope or root mean square detector (RMS detector) circuitry is sufficient to quantify the reflectance of remote surfaces. An envelope detector circuit may be set up by simply connecting a half-wave or full-wave rectifier element to a low-pass filter.
As compared with current transductive gear/wheel sensors, a self-mixing optical gear/wheel sensor according to the invention offers the unique advantage of a simple structure, a high resolution, and large tolerances to temperature and position variations.
As an interferometric sensing device, the self-mixing reflective sensor according to the invention offers a greater sensitivity than conventional incoherent reflective sensors because incoherent high ambient light does not contribute to the self- mixing signals at all. Reflected light of 10 of intensity relative to the incident laser beam can be readily detected with a signal-to-noise ratio beyond 20 dB. The focus size of laser diodes, particularly of single-mode VCSELs is typically much less than 1 mm, so that a high spatial resolution can be achieved.
Furthermore, evaluation of the measured signals and calculation of the displacement-related parameter may be kept simple as well. To this end, the laser self- mixing reflective sensor may further comprise a threshold detector circuit connected to the detector circuitry for detecting a signal proportional to the absolute value of the oscillation amplitude, so that the threshold detector circuit outputs a signal which changes in level if the absolute value of the self-mixing oscillation or a parameter proportional thereto exceeds a predetermined threshold. In this way, the signal proportional to the absolute value of the oscillation amplitude of the self-mixing oscillations is compared with a threshold level, and a signal with a first and a second signal level is generated.
The circuit generates the first signal level if the signal proportional to the absolute value of the oscillation amplitude is lower than the threshold, and generates the second signal level if the signal proportional to the absolute value of the oscillation amplitude is higher than the threshold. According to a preferred embodiment of the invention, a VCSEL, preferably a single-mode VCSEL is used as the diode laser. This laser allows a very compact design of the sensor, particularly if the VCSEL includes a vertically integrated photodiode as a detector for the laser intensity.
However, an edge emitting laser diode may also be used. Independent of the type of laser diode used, the laser diode may comprise an integrated photodiode. This photodiode may advantageously be used as a detector for detecting the laser intensity. It is further possible to detect the laser intensity via the driving current through the laser diode. However, detecting the laser intensity by means of a monitoring photodiode is preferred, particularly due to the high sensitivity of this arrangement. Alternatively or additionally to an integrated monitoring photodiode, an external photodiode may be used as well.
The measurement of a displacement-related parameter such as a displacement or movement of the surface is particularly accurate and straightforward if a movable surface arranged within the optical path of the laser beam is used, which has a periodically varying reflectance for a reflection back along the direction of incidence of the laser beam. The varying reflectance may be due to a variation of the texture and to the shape and local spatial position of the surface with respect to the incident laser beam. Due to the laterally varying structure of the movable surface, the intensity of the light reflected back into the laser cavity varies accordingly. This causes a variation of the strength, i.e. the absolute value of the amplitude of the self-mixing oscillations. An increase of the intensity of the light reflected back thus causes a burst in the oscillating signal.
Due to the presence of laser speckle, typical digitalized photocurrent signals comprise bursts of short pulses and long intervals between pulse bursts which correspond to high and low reflective zones of the remote surface, respectively. The speckle-related amplitude variation superpositioned on the self-mixing oscillations causes an additional random amplitude modulation. If the speckle effect is strong, the bursts in this case are series of oscillation pulses having random lengths.
According to the invention, the bursts can be detected with a detector circuit so as to determine the displacement-related parameter. Again, the circuitry for this detector can be held simple, involving e.g. the above-mentioned threshold detector. Particularly if the surface has a periodical structure, the number of bursts directly corresponds to the displacement of the surface at the point of incidence of the laser beam. Likewise, the duration between bursts, e.g. the duration between the onsets of the bursts corresponds to the velocity of the surface at the point of incidence. To calculate a displacement or velocity, the number of bursts or the period of consecutive bursts may be determined, respectively. To determine the amount of displacement, a simple counter circuit counting the number of bursts may thus be included.
As a coherent sensing device, self-mixing reflective sensors according to the invention are suitable for remote detection of weak or diffuse reflective surfaces in high ambient light environments. The sensors are valuable for detecting weak or diffuse reflective surfaces in high ambient light environments. Gear or wheel rotation detectors based on self-mixing reflective sensors can be widely used in industrial and technical applications such as automotive sensors. Typical applications include gear sensors for automatic transmission, wheel speed sensors for anti-lock brake systems (ABS), steering wheel sensors for electronic stability programs (ESP) and sensors for global vehicle chassis control, e.g. in the form of a ball joint angle sensor or wishbone position sensor, and headlamp leveling.
The self-mixing reflective sensor or a multitude of such sensors may also be used in a lane departure warning system in vehicles by detecting e.g. road markings.
As compared with current self-mixing range or speed sensors which require complicated frequency detection circuits, self-mixing reflective sensors can be made with much simpler circuits such as envelope or RMS detectors.
In contrast to widely used transductive sensors (Hall effect sensors, GMR sensors, capacitive sensors), self-mixing gear/wheel sensors also offer unique advantages of compact size (chip level integration), low cost (integrated photodiode, optical encoder), improved resolution (sub-mm focus size), greater sensitivity (typical signal-to- noise ratio beyond 20 dB), low power consumption (typically in the milliwatt range), and broad dynamic range, because the inventive sensor works well at both low and high angular speeds and has large tolerances to temperature and position variations as the sensor is substantially insensitive to air gap and temperature variations.
BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a schematic setup of a self-mixing reflective sensor,
Fig. 2 shows a self-mixing reflective sensor set up as a gear rotation sensor, Fig. 3 shows a self-mixing reflective sensor set up as a wheel rotation sensor.
DESCRIPTION OF EMBODIMENTS
Fig. 1 shows a schematic setup of a self-mixing reflective sensor 1 according to the invention.
The sensor 1 is based on the principle of generating a laser beam within the laser cavity of a laser diode and measuring the laser intensity by means of a photo detector during movement of the surface of a movable element which is placed within the optical path of the laser beam. The surface is furnished with a laterally varying structure which causes a laterally varying reflection back along the direction of incidence of the laser beam. The movement of the reflecting surface causes self-mixing oscillations in the laser intensity due to laser light reflected back from the surface into the laser cavity. The oscillations are extracted by using appropriate circuitry connected to the detector for detecting the laser intensity.
A signal proportional to the absolute value of the oscillation amplitude of the self-mixing oscillations is determined and a displacement- related parameter is calculated therefrom. Instead of utilizing sophisticated signal sampling and data-processing electronics to detect the modulation frequency of the photocurrent, a simple envelope and/or RMS detector circuitry is employed to quantify the absolute modulation amplitude of the photocurrent. This value is correlated with the local reflectivity of the surface at the point of incidence of the laser beam.
In the embodiment shown in Fig. 1, a single-mode VCSEL 3 is used as a laser diode. The beam 9 of the VCSEL 3 is focused on the surface of a moving element 11. The surface of this element 11 has a periodically varying reflectance for a reflection back along the direction of incidence of the laser beam. Specifically, regions 13 having a higher reflectance and regions 15 having a relatively lower reflectance alternate along the direction 12 of movement, indicated by an arrow. Due to this varying reflectance, the beam reflected back into the laser cavity has a higher intensity if the laser beam is incident on a region 13 as compared to an incidence on a region 15 of the surface. Without limiting the example of the embodiment shown in Fig. 1, the typical optical power of a VCSEL for reflective sensing applications is preferably kept below 1 mW.
The VCSEL injection current is kept at a constant level.
The VCSEL 3 comprises a vertically integrated photodiode 5 which is used as a photodetector. The electric signal of the photodiode 5 is fed to a circuit 17 for detecting self-mixing oscillations in the laser intensity. The electric signal is shown as a diagram within circuit 17. Due to interference of the generated beam with the reflected beam having a time-dependent phase shift caused by the movement of the surface of element 11, the laser intensity and hence the electric signal of photodiode 5 oscillates with a period 22 and amplitude 23 around a median value 18. Furthermore, the oscillating signal is superpositioned by a random oscillation caused by the speckle effect.
During time intervals 24 and 26, the laser beam hits regions 13 having a higher reflectance, whereas a region 15 having a relatively lower reflectance is scanned during the intermediate time interval 25. As the absolute value of the oscillation amplitude is proportional to the intensity of the reflected beam, the oscillation amplitude is higher in time intervals 24 and 26. Accordingly, bursts of the oscillation signal are observed if the laser beam sweeps across regions 13. As shown in the diagram, these bursts may have the shape of a random series of pulses due to the superposition of the speckle effect.
In the example shown in Fig. 1 , the signal is further evaluated by an envelope detector circuit 19 as detector circuitry for detecting a signal proportional to the absolute value of the oscillation amplitude. The envelope signal 28 shown as a diagram within envelope detector circuit 19 reflects the course of the absolute amplitude values. A RMS detector circuit may be used alternatively because the RMS signal corresponds to the absolute value of the self-mixing amplitude. Furthermore, the sensor 1 comprises a threshold detector circuit 21 connected to the envelope detector circuit 19. The threshold detector circuit 21, such as an analog-to-digital converter or comparator, compares the envelope signal with a threshold level shown as solid line 30 in the diagram of the envelope signal 28.
A signal 32 is output, which changes between two levels 34, 35 if the envelope signal 28 exceeds threshold 30. Accordingly, due to the superpositioned speckle effect, the threshold detector circuit 21 outputs bursts in the form of a series of switching pulses, while the beam 9 sweeps across regions 13 and the output remains at a constant level during a sweep across regions 15.
The output of threshold detector circuit 21 is further evaluated by a circuit 37. Specifically, the velocity of the surface can be determined by determining the period or frequency of the bursts. Moreover, by counting the bursts within a time interval, the displacement of the surface within this time interval can be easily calculated.
Fig. 2 shows an application of the self-mixing reflective sensor 1 as a gear-wheel rotation sensor. In this embodiment, a coherent self-mixing reflective sensor 1 is used to measure the angular speed and/or displacement of a gear wheel 40 rotating about its axis 41. The VCSEL beam 9 is focused on the tips of the gear teeth 42, which introduces significant modulations to the VCSEL photocurrent, similarly as shown in Fig. 1.
The VCSEL beam focal depth is preferably designed to be much shorter than the height of the gear teeth. Since the optical feedback strength drops rapidly as the remote surface moves out of focus, the VCSEL photocurrent modulation due to reflection of the base of the gear tooth become insignificant. Bursts of the self-mixing oscillation are thus detected, each burst corresponding to a tooth passing through the laser beam 9.
However, even if the lens 9 has a longer focal depth than the height of the teeth, the reflection back along the incident beam will vary considerably, because the light is deflected away from the direction of incidence at the sloped side walls of the teeth.
The photocurrent signal is evaluated analogously as described with reference to Fig. 1, using circuitry 44 comprising an envelope detector or RMS detector circuit and a comparator or ADC circuit. This sensor may be used, for example, as a sensor in an automatic transmission system.
Fig. 3 shows a self-mixing reflective wheel sensor according to the invention. The surface of a rotatable element such as a shaft or wheel 45 is optically encoded with alternating bright and dark areas 13, 15. As for the embodiment shown in Fig. 1, the reflectivity of the different areas is correlated with the modulation amplitude of the VCSEL photocurrent.
By evaluating the amplitude, the rotation velocity and/or rotation angle can thus be detected to correspond to the circumferential velocity and displacement of the optically encoded surface, respectively. The optically encoded surface may be provided simply by attaching a structured film 46 to the shaft or wheel 45.
Although preferred embodiments of the present invention have been illustrated in the accompanying drawings and elucidated in the foregoing description, it will be understood that the invention is not limited to the embodiments disclosed and that numerous modifications can be conceived without departing from the scope of the invention as set out in the following claims.