Patent Application
20220120945
The subject disclosure relates to a feedback sensor for a micro-electromechanical system (MEMS) mirror.
An optical mirror may be used to direct or deflect light in a number of applications (e.g., beamsteering in a lidar sensor, two-dimensional optical scanning). The optical mirror may be tilted as needed using a micro-electromechanical system (MEMS). The orientation of this so-called MEMS mirror may not precisely match a desired orientation that is controlled via the MEMS. That is, real-world conditions such as temperature, shock, and vibration, may prevent a given signal to the MEMS from resulting in the orientation of the optical mirror that is expected based on laboratory conditions. Accordingly, it is desirable to provide a feedback sensor for a MEMS mirror.
In one exemplary embodiment, an optical mirror assembly includes a mirror with a reflective surface and a back surface, opposite the reflective surface. The mirror is tilted around a first axis or a second axis, perpendicular to the first axis. The optical mirror assembly also includes an inverted light emitting device (LED) of a feedback sensor arranged to emit light onto the back surface of the mirror, and four photodiodes of the feedback sensor arranged to receive reflected light resulting from the back surface of the mirror reflecting the light emitted by the inverted LED. Each of the four photodiodes is disposed in a different one of four quadrants defined by the first axis and the second axis and the inverted LED being disposed at a center of the four photo diodes.
In addition to one or more of the features described herein, the optical mirror assembly also includes actuators to tilt the mirror around the first axis or the second axis based on control signals provided to the actuators.
In addition to one or more of the features described herein, each of the actuators is a micro-electro-mechanical system.
In addition to one or more of the features described herein, the optical mirror assembly also includes a controller to provide the control signals to the actuators to orient the mirror with a desired tilt angle around the first axis and a desired tilt angle around the second axis.
In addition to one or more of the features described herein, the controller obtains an indication of intensity of the reflected light received by each of the four photodiodes.
In addition to one or more of the features described herein, the controller determines actual orientation of the mirror including an actual tilt angle around the first axis and an actual tilt angle around the second axis based on the intensity of the reflected light received from the four photodiodes, the intensity of the reflected light received from all of the four photodiodes being used to determine both the actual tilt angle around the first axis and the actual tilt angle around the second axis.
In addition to one or more of the features described herein, the controller determines a correction in orientation as a difference between the actual tilt angle and the desired tilt angle around the first axis and a difference between the actual tilt angle and the desired tilt angle around the second axis.
In addition to one or more of the features described herein, the controller provides correction control signals to the actuators based on the correction in orientation to achieve the desired tilt angle around the first axis and the desired tilt angle around the second axis.
In another exemplary embodiment, a method of assembling an optical mirror assembly includes disposing a mirror with a reflective surface and a back surface, opposite the reflective surface. The mirror is tilted around a first axis or a second axis, perpendicular to the first axis. The method also includes arranging an inverted light emitting device (LED) of a feedback sensor to emit light onto the back surface of the mirror, and arranging four photodiodes of the feedback sensor to receive reflected light resulting from the back surface of the mirror reflecting the light emitted by the inverted LED. Each of the four photodiodes is disposed in a different one of four quadrants defined by the first axis and the second axis and the inverted LED being disposed in a center of the four photodiodes.
In addition to one or more of the features described herein, the method also includes arranging actuators to tilt the mirror around the first axis or the second axis based on control signals provided to the actuators.
In addition to one or more of the features described herein, the method also includes configuring a controller to provide the control signals to the actuators to orient the mirror with a desired tilt angle around the first axis and a desired tilt angle around the second axis.
In addition to one or more of the features described herein, the method also includes coupling the controller to the four photodiodes to obtain an indication of intensity of the reflected light received by each of the four photodiodes.
In addition to one or more of the features described herein, the method also includes configuring the controller to determine actual orientation of the mirror including an actual tilt angle around the first axis and an actual tilt angle around the second axis based on the intensity of the reflected light received from the four photodiodes, wherein the intensity of the reflected light received from all of the four photodiodes is used to determine both the actual tilt angle around the first axis and the actual tilt angle around the second axis.
In addition to one or more of the features described herein, the method also includes configuring the controller to determine a correction in orientation as a difference between the actual tilt angle and the desired tilt angle around the first axis and a difference between the actual tilt angle and the desired tilt angle around the second axis.
In addition to one or more of the features described herein, the method also includes configuring the controller to provide correction control signals to the actuators based on the correction in orientation to achieve the desired tilt angle around the first axis and the desired tilt angle around the second axis.
In yet another exemplary embodiment, a method of performing closed-loop control of a mirror of an optical mirror assembly includes emitting light, using an inverted light emitting device (LED), onto a back surface of a mirror with a reflective surface opposite the back surface. The mirror tilts around a first axis or a second axis that is perpendicular to the first axis. The method also includes receiving reflected light, using four photodiodes respectively disposed in different ones of four quadrants defined by the first axis and the second axis, the reflected light resulting from the back surface of the mirror reflecting the light emitted by the inverted LED, the inverted LED being disposed in a center of the four photodiodes.
In addition to one or more of the features described herein, the method also includes providing, using a controller, control signals to micro-electro-mechanical actuators to tilt the mirror to a desired tilt angle around the first axis and a desired tilt angle around the second axis according to the control signals.
In addition to one or more of the features described herein, the method also includes the controller obtaining an indication of intensity of the reflected light received by each of the four photodiodes.
In addition to one or more of the features described herein, the method also includes the controller determining a true tilt angle around the first axis using the intensity of the reflected light received by all of the four photodiodes and determining a true tilt angle around the second axis using the intensity of the reflected light received by all of the four photodiodes.
In addition to one or more of the features described herein, the method also includes the controller providing additional control signals to the actuators based on a difference between the actual tilt angle and the desired tilt angle around the first axis and a difference between the actual tilt angle and the desired tilt angle around the second axis to obtain the desired tilt angle around the first axis and the desired tilt angle around the second axis for the mirror.
The above features and advantages, and other features and advantages of the disclosure are readily apparent from the following detailed description when taken in connection with the accompanying drawings.
Other features, advantages and details appear, by way of example only, in the following detailed description, the detailed description referring to the drawings in which:
The following description is merely exemplary in nature and is not intended to limit the present disclosure, its application or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features.
Vehicles (e.g., automobiles, trucks, construction equipment, farm equipment, automated factory equipment) increasingly employ sensors to obtain information about the vehicle and its environment. Information from the sensors facilitates semi-autonomous operation (e.g., lane departure correction, automated steering or braking) and autonomous operation of the vehicle. Exemplary sensors that are typically used to obtain information about the environment of the vehicle include cameras, radio detection and ranging (radar) systems, and light detection and ranging (lidar) systems. A lidar system of a vehicle may be among the devices that employ one or more MEMS mirrors.
Embodiments of the systems and methods detailed herein relate to a feedback sensor for a MEMS mirror. As previously noted, the true orientation (e.g., tilt angle) of a MEMS mirror may differ from the desired orientation that gave rise to a signal to the MEMS, which actuates the movement. A feedback sensor that provides the true orientation of the MEMS mirror facilitates closed-loop control to obtain the desired orientation through adjustments of the signal (e.g., current) to the MEMS, as detailed. The feedback sensor according to exemplary embodiments differs in three substantial ways from prior feedback sensors.
An inverted light emitting diode (LED), which exhibits a different illumination to MEMS mirror tilt angle characteristic as compared with a conventional LED is used. Photodiodes used as detectors in the feedback sensor are arranged differently than in conventional feedback sensors, and the calculation of the MEMS mirror tilt angle differs from the calculation used in conventional feedback sensors due to the different arrangement of photodiodes. The feedback sensor is arranged on a back side of the MEMS mirror (i.e., opposite the side used as an optical mirror). The inverted LED is used to emit light on the back side of the MEMS mirror and the photodiodes are arranged to receive reflections resulting from the emitted light.
In accordance with an exemplary embodiment,
The controller 120 may obtain information from the lidar system 110 and/or additional sensors 130 to control aspects of the operation of the vehicle 100. The information may facilitate semi-autonomous or autonomous operation, for example. The controller 120 may additionally interact with the optical mirror assembly 200 and participate in the control and/or closed-loop control of the MEMS mirror 210 as further discussed. The controller 120 may include processing circuitry that may include an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that executes one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality.
A controller 230 is shown to provide a signal 205 to each of the actuators 220 (i.e., MEMS). The signal 205 may be a current, for example, that is applied through a coil 320, as discussed with reference to
As previously noted, the use of the inverted LED 410 is one of the distinguishing features of the feedback sensor 400 according to one or more embodiments. The inverted LED 410 exhibits different angular intensity (i.e., a different illumination-to-MEMS mirror tilt angle characteristic) as compared with a conventional LED. Specifically, as the tilt angle of the MEMS mirror 210 (around either the x or y axes) goes from −20 degrees through 0 degrees to 20 degrees, the percentage of illumination from a conventional LED decreases while the percentage of illumination from an inverted LED 410 increases. The decrease for the conventional LED is less than the increase for the inverted LED 410. The resultant, which is a product of percentage illumination and photodiode reflectance, is non-linear for the conventional LED and approximately linear for the inverted LED 410. These differences lead to a lower signal-to-noise ratio (SNR) for the conventional LED as compared to the inverted LED 410.
According to prior feedback devices, a pair of light detectors are placed on each of the axes x and y. Thus, a tilt of the MEMS mirror 210 around the x axis would result in the MEMS mirror 210 being closer to one of the light detectors on the y axis than to the other light detector on they axis. Consequently, the two light detectors on they axis would receive different intensities of reflections. Similarly, a tilt of the MEMS mirror 210 around they axis would result in the two light detectors on the x axis receiving different intensities of reflections.
As previously noted, the four photodiodes 420a, 420b, 420c, 420d not being on the tilt axes x and y of the MEMS mirror 210 is another distinguishing feature of the feedback sensor 400 according to one or more embodiments. Each of the photodiodes 420 is disposed in a different quadrant defined by the x axis and the y axis. As a result of the off-axis arrangement, all four photodiodes 420 contribute to a determination of the tilt angle 211 of the MEMS mirror 210 around either the x and all four photodiodes 420 also contribute to the determination of the tilt angle 212 of the MEMS mirror 210 around they axis. When the analog detected intensity of the reflected light 425 at each photodiode 420 is digitized, the accuracy of the feedback sensor 400 according to one or more embodiments is improved through the use of four photodiodes 420 to determine tilt angles 211, 212 of the MEMS mirror 210 rather than two.
The controller 230, the controller 120, or a combination may be used to process the intensity of reflected light 425 detected by each photodiode 420 and perform closed-loop control of the orientation of the MEMS mirror 210. The intensity of the reflected light 425 received at each of the photodiodes 420a, 420b, 420c, and 420d is respectively designated as PDa, PDb, PDc, and PDd. To determine the tilt angle 211 of the MEMS mirror 210 around the x axis, a difference D1 is determined as:
D1=(PDc+PDd)−(PDa+PDb) [EQ. 1]
To determine the tilt angle 212 of the MEMS mirror 210 around they axis, a difference D2 is determined as:
D2=(PDb+PDd)−(PDa+PDc) [EQ. 2]
The difference D1 is mapped to a tilt angle 211 and the difference D2 is mapped to a tilt angle 212. This mapping, which is based on a prior calibration, represents the true orientation of the MEMS mirror 210, which may or may not match the desired orientation based on the signals 205 provided to the actuators 220.
While the above disclosure has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from its scope. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the disclosure without departing from the essential scope thereof. Therefore, it is intended that the present disclosure not be limited to the particular embodiments disclosed, but will include all embodiments falling within the scope thereof.
