Abstract:
An integrated microelectromechanical system (MEMS) sun sensor includes a filter, microlens, aperture and a folded MEMS optical element combined with an active pixel sensor array to form an integrated spacecraft sun sensor in an integrated sealed package, offering lower power, smaller size and higher performance for use on spinning spacecraft useful in attitude determinations. Multiple like sun sensors can be disposed for increasing the reliability, spatial coverage or spatial resolution for a specific performance requirement.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to the fields of optics and semiconductors. More particularly, the present invention relates to optical microlens sun sensors for sensing solar light.  
         BACKGROUND OF THE INVENTION  
         [0002]    Advanced space missions using nanosatellite constellations are hindered by a lack of miniaturized low power attitude determination and control systems. Replacing large conventional spacecraft attitude sensors with smaller low power sun sensors is desirable for both three axis stabilized spacecraft and spinning spacecraft. A three axis stabilized spacecraft may have a centroid sun sensor having a central detector pointing at the sun with a series of circumferentially disposed detectors surrounding the central detector for determining when the sun is off center of a pointing line of sight when pointing directly towards the sun. An optical lens is used to focus the received sun light onto the central detector. A spinning spacecraft uses a scanning sun sensor that necessary scans a field of view for determining when the sun is in-view by creating an intensity profile where the maximum intensity points to the sun along a line of sight for determining the location of the sun and relating the location of the sun to the spin phase of the spacecraft for attitude determination. The sun sensor on the spinning spacecraft also has multiple detectors for providing an elevation angle relative to a spin intensity bit map profile for indicating the spinning phase and the sun elevation angle. The scanning sun sensor also has a lens for focusing the received sun light onto the sensor. In both the centroid sun sensor and the scanning sun sensor, the received sun light passes directly through a viewing port having a band limited filter, a lens for focusing the light on a detector that may be, as examples, a single pixel, a linear array, a circular array, or a matrix array of photodetectors. These conventional sun sensors are well suited for sun sensing. However, these conventional sun sensors do not sweep the field-of-view for creating a two-dimensional bit map of the sky, which may enhance attitude determination with interpolations.  
           [0003]    In general, sun sensors should have low-mass, low power usage, and low volume with accurate and wide spatial coverage. For nanosatellite and microsatellite applications, smaller and lower power sun sensors can be made using advanced semiconductor processing. Imaging systems have used readout chips with photodiode detectors to collect received light and to channelize bit image information. These imaging systems have also used discrete lenses for focusing the image onto the photodetectors. These imaging systems, such as a imaging camera, have a CMOS active pixel array upon which is focused the received light. The availability of submicron CMOS technology, the maturity of CMOS fabrication methods, and the advent of low noise active pixel sensors have enabled high performance CMOS digital imager developments. The primary advantages of a CMOS based design are random access, lower power usage, digital interfacing, simplicity of operation using a single CMOS compatible power supply, high speed, miniaturization through system integration, on-chip signal processing circuits, and radiation tolerance. However, existing sun sensors are relatively large and consume high power. These and other disadvantages are solved or reduced using the invention.  
         SUMMARY OF THE INVENTION  
         [0004]    An object of the invention is to provide a compact sun sensor.  
           [0005]    Another object of the invention is to provide a compact sun sensor in a sealed package.  
           [0006]    Yet another object of the invention is to provide a compact sun sensor having a microlens for focusing collected sun light.  
           [0007]    Still another object of the invention is to provide a compact sun sensor having a microlens for focusing collected sun light in a sensor housing having a reflective surface for folding an optical path onto an array of photosensitive diodes such as a pixelized array.  
           [0008]    Still a further object of the invention is to provide a compact sun sensor having a microlens for focusing collected sun light in a sensor housing having a reflective surface for folding an optical path onto an array of photosensitive diodes as part of a readout chip for providing a bit image of the received sunlight.  
           [0009]    The invention is directed to an integrated microelectro-mechanical system (MEMS) sun sensor having a microlens, a folded optical element, and an active pixel sensor array. When used in spinning spacecraft, the scanning sun sensor scans the sky during the rotation of the spacecraft to sweep out a two-dimensional intensity bitmap image of the sky. This image is divided into pixels in azimuth and elevation over the sensor field-of-view during one revolution of the spacecraft. This two-dimensional bit map offers additional information for accurately interpolating the position of the sun through data processing.  
           [0010]    The advantages of this MEMS sun sensor includes lower power usage, smaller size, higher performance, and compatibility with planar semiconductor fabrication techniques. The sun sensor can be integrated as an ultra low power CMOS device with an on-board processor in a radiation tolerant hermetic package. Preferably, the MEMS sun sensor is micro-optical element used to form an image of the sun on the active pixel linear array formed by silicon micromachining MEMS techniques. The imaging pixels of the pixel array are formed in the integrated readout circuit. Folded optics are combined with analog and digital readout circuitry formed in an integrated circuit substrate. An integrated microcontroller provides sun sensor data processing and control, and spacecraft interfacing for communicating data. The sun sensor uses low power, has low-mass and low volume by integrating together the readout chip and the data processor chip as a critical spacecraft subsystem for effective use in nanosatellites and microsatellites, where spacecraft resources are at a premium. The power, weight and size savings of resources enable the use of multiple and redundant fault-tolerant sun sensor implementations on large spacecraft where more resources are available. The sun sensor is preferably fabricated using commercial CMOS processes and batch fabrication methods that benefit from mass production for reducing manufacturing costs. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1A is a top view of the MEMS Sun Sensor.  
         [0012]    [0012]FIG. 1B is a side view of the MEMS Sun Sensor.  
         [0013]    [0013]FIG. 2 is a schematic of an active pixel readout channel.  
         [0014]    [0014]FIG. 3 is a block diagram of an active pixel readout chip.  
         [0015]    [0015]FIG. 4 is a block diagram of the MEMS Sun Sensor data processor.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0016]    An embodiment of the invention is described with reference to the figures using reference designations as shown in the figures. Referring to FIGS. 1A and 1B, an integrated microelectro-mechanical system (MEMS) sun sensor is packaged using a ceramic package substrate  10  and a ceramic package lid  12  for forming a ceramic package cavity  14  that is hermetically sealed. The substrate  10  and lid  12  form a sun sensor package that can be mounted on a spinning spacecraft. The sun sensor package includes a front-end optical bandpass filter  16  disposed in a sun sensor viewing aperture  18  within the ceramic substrate  10 . The filter  16  is an optically neutral density and band limiting filter.  
         [0017]    A sun sensor data processor chip  20  is an integrated chip packaged with a sun sensor readout integrated chip  22 . The readout chip  22  and the data processor chip  20  are bonded to the substrate  10  within the cavity  14 . Sun sensor folded optics  24  are disposed on the sun sensor readout chip  22  that is bonded to the substrate. Conventional wire bonding techniques are used to interconnect the readout chip  22  to the processor chip  20 , and used for package external connections. For example, the processor chip  20  includes bonding pads, such as pad  28 , with wires, such as wire  30 , for connecting the processor chip  20  to the package having boding pads, such as ceramic package pad  32  that is extended as an interconnection package pin. The readout chip  22  also has bonding pads, such as pad  34 , and bonding wires, such as wire  36 , that are used for example to interconnect the readout chip  22  to the processor chip  20 . The readout chip  22  and processor chip  20  are placed in the hermetically sealed ceramic package consisting of the substrate  10  and lid  12 .  
         [0018]    The folded optics  24  include a microlens  38  that is formed on a sun sensor microlens substrate  40 , an optical mask  42  having an optical mask aperture  44 , and a MEMS sun sensor housing  46  having a reflective mirror  50 . The microlens  38  may have a 4.0 mm focal length. The microlens  38  abuts the optical mask  42  having the optical mask aperture  44  through which passes sunlight within the field of view of the viewing aperture  18  into the housing  46 . The optical mask aperture  44  may be 200.0 um in diameter. The microlens  38  is used to fill the entrance aperture diameter and shorten the overall focal length, that may be 4.0 mm, while maintaining the desired sun image spot size, that may be 50.0 um. The optical neutral density and band limiting filter  16  attenuates the intensity of collected solar light and filters received photons to a wavelength of 600.0 nm for reducing diffraction. With a 200.0 um aperture  44  and a 600.0 nm band limiting filter  16 , the diffraction limited spot size is less than 36.0 um in diameter at the focal plane.  
         [0019]    A microlens assembly may consist of the microlens  38 , the sun sensor microlens substrate  40 , the optical mask  42  having the optical mask aperture  44 . The microlens assembly  38 ,  40 ,  42  and  44  is supported in the folded optics MEMS sun sensor housing  46  having housing baffles  48  and a folded optics mirror  50 . Sun light passes along a sun light path  52  defining the field of view into a housing optics recess  54  in which is disposed the microlens  38  and the microlens substrate  40 . The focused received sun light passes through the optical mask aperture  44  into and through a folded optical cavity  56  within the folded optics housing  46  to then be reflected by the optical mirror  50  to illuminate a linear pixel array  58  of the read out chip  22 . The microlens  38  and the reflective mirror  50  create a 36.0 um spot size image of the sun on the linear pixel array  58  that may include N active pixels, for example 340 pixels, that may be aligned in N rows, for example a single row. The reflection of the sun light by the mirror  50  effectively folds the sun light path  52  down on to the linear pixel array  58  that may be made of optically sensitive pixelized photodiodes.  
         [0020]    Micromachined microlens technology is used to fabricate the microlens  38  in a sequence of masking steps, each followed by successive Argon ion beam milling operations, to create a course lens profile of desired geometry. Subsequently, high temperature annealing is performed in an inert gas to cause mass transport smoothing of the microlens surface. Accurately shaped apertures greater than 300.0 um may be created in this manner. The compact, planar, optical design of the sun sensor formed by using microlenses and MEMS technology, enables miniature folded optics and sensing within a single semiconductor package  10  and  12 . The MEMS optical housing  24  provides a rigid mount for the microlens  38 , folding mirror  50 , and housing baffles  48  for the absorption of scattered photons. The housing  24  provides enclosed contamination protection of the optical cavity  56 .  
         [0021]    The sun sensor housing  46  is fabricated from a single crystal silicon substrate in order to provide a stress free match to the silicon wafer of the readout chip  22  over a wide range of temperatures, for example, between −50 C. to +80 C., which can be expected near exterior surfaces of a spacecraft. The internal baffles  48  formed on interior surfaces of the silicon sensor housing  24  can be microtextured to absorb visible-to-near infrared light with high efficiency of greater than 96% for eliminating the need for extra internal antireflective surface coatings, which may otherwise out-gas and contaminate other optical surfaces within the sealed cavity. Antireflective black silicon interior surfaces of the housing  46  can be fabricated using reactive ion etching (RIE) or pulsed laser irradiation of silicon surfaces in a halogen atmosphere.  
         [0022]    The housing  46  is formed by micromachining a three-dimensional block of silicon using a combination of anisotropic etching and laser micromachining. Anisotropic etching using liquid KOH solution produces an inverted pyramidal pit with the (111) side walls at a 54.7° angle with the surface. Bulk anisotropic etching is used to fabricate the gross internal housing cavity  56  of the sun sensor with one angled wall use to form the folding reflective mirror  50 . This reflective mirror  50 , that is, the angled wall, of the housing  46  forming an enclosing surface of the housing cavity  56 , can be left uncoated with the etched silicon reflective received light, or can be coated with a thin layer of metal for improving the reflectivity of the angled wall. Fine features of the housing  46 , such as the recess  54  and light baffles  48  can be created using laser micromachining. A laser assisted chemical etcher can be computer controlled for three-dimensional cutting for removing silicon at rates of about 50,000 cubic microns per minute. The final microstructuring of the surface to black silicon is performed using either laser processing or RIE to produce black silicon baffles  48 .  
         [0023]    The microlens  38  fills the entrance aperture diameter  44 , shortens the overall optical length, while maintaining angular precision. The optical neutral density and band limiting filter  16  is added to reduced the solar intensity and to limit the accepted photons to a wavelength of 600 nm. This filter  16  prevents saturation of the pixelized photodiodes during an imaging integration interval and reduces diffraction at the focal plane. The sun sensor can be aligned and calibrated with standard HeNe laser sources. With intensity thresholding, the folding optics create an image that can be kept below a 50.0 um spot size. The microlens  38  focuses and passes abundant sunlight to the detector  58  with limited diffraction due to the miniature aperture  44 . With a 200.00 um aperture  44  and a 600.0 nm band limiting filter  16 , the calculated Airy pattern is 18.0 um.  
         [0024]    Referring to FIGS. 1A, 1B, and  2 , and particularly to FIG. 2, an active pixel readout channel is used to process a photodiode signal PD from a respective optically sensitive pixel photodiode. The array  58  may be formed using CMOS photodiodes. The photodiodes may be reversed biased photodiodes that convert sun light photons into electron charges as the photodiode signal PD. An integration amplifier  62  using an integration capacitor  64  and an integration switch  66  integrates photodiode current PD for creating an integration voltage at the output of the integration amplifier  62 . The integration switch  66  is closed as controlled by a clear signal CLR defining an end of an integration period. Closing the integration switch  66  resets the integration amplifier  62 , and opening the integration switch  62  begins the integration period. At the end of an integration period, the integration switch  66  is held open and a sample and hold switch  70  is closed for transferring the integration output voltage onto a sample and hold capacitor  76  through line  72  that is connected to a comparator  74 , and onto a sample and hold reset switch  78  while the sample and hold reset switch  78  is held open. The transfer switch  70  is opened to hold the voltage on the sample and hold capacitor  76 . The clear switch  66  is then closed and then opened to reset the integration amplifier  62  to being another image integration cycle. The capacitor  76  holds the prior integration voltage until the sample and hold reset switch  78  is closed. The sampled integration voltage is compared by the comparator  74  to a reference provided by a digital to analog converter (DAC)  80 , to determine the presence of integrated photons when the comparator output equals a one bit, or to determine the absence of integrated photons when the comparator equals a zero bit. The output of the comparator  74  sets the flip flop  82  when high indicating the presence of the sun in the field of view for those illuminated photodiodes in the array  58 . The output of the comparator  74  is low when the sun is not in the field of view. The holding register flip flop  82  maintains a low output from a prior FF RESET initialization. The comparator  74  sets the flip flop  82  in the presence of the sun. The flip flop  82  also has an IN input and a SHIFT input to the flip flop  82  for shifting data through the flip flop  82  during a digital bit stream read-out of the photodiode array  58 . These digital read-outs occur such that the prior comparator results of an image integration period are read-out during the current image integration cycle.  
         [0025]    Referring to FIGS. 1A, 1B,  2  and  3 , and particularly to FIG. 3, the active pixel readout chip  22  includes a sun sensor state machine  82 , input shift registers  86 , holding registers  88  and a readout array  90 . The readout array  90  includes daisy chain connected active pixels readout channels  92   a,    92   b,  through  92   n.  The output of one flip flop  82  of one readout array channel, such as channel  92   a  is connected to the D input of another flip flop in another readout channel, such as channel  92   b,  in the chain of flip flops  82  of the chain readout channels,  92   a  through  92   n.  The SHIFT signal is used to shift the bits of all of the flip flops  82  effectively forming a serial shift register. The plurality of readout channels  92   a  through  92   n  are connected in series for forming the shift register. This shift register made of the flip flops  82  of the readout channels  92   a  through  9   n , shifts the linear bit map through the readout chip output to the data processor  20 . The state machine  82  controls the shifting of the linear bit map image. As a scanning sun sensor, on a spinning spacecraft, the sensor field-of-view sweeps the sky, successively shifting out azimuthal segments of the sky as individual linear bit map images ultimately providing a composite two dimensional image reference. This composite image is composed of azimuthal sun angle information, provided by each successive serial digital read-out and solar elevation angle information imaged during the presence or absence of the sun in each pixel of the read-out bit stream. This information is referenced to the spin phase of the spinning spacecraft and is well suited for solar attitude determinations in spinning spacecraft.  
         [0026]    The readout chip input includes the DAC  80  having reference REF data for each of the active pixels, and hence, for each of the DACs  80  for normalizing the response of all the active pixels of the array  58  so that all of the active pixels of the array  58  provide the same level of detection sensitivity and rejection of background light sources. The shift register  86  indicates to the state machine  82  when new data has been received, including new DAC REF reference data. The shift register  86  holds a DAC value for a respective one of the channels  92   a  through  92   n . The DAC value is then stored in one of the hold registers  88 , and communicated to a respective DACs  80 . The next received DAC value is shifted in the shift register  86  and then again loaded into another one of the hold registers  82  and again communicated to another respective one of the DACs  80 . Each of the hold registers  88  will be loaded in turn with all the DAC values of the DACs  80  for all of the channels  92   a  through  92   n . The state machine  82  is used to control the operation of the storing the DAC reference values in the hold registers  88  and for shifting out the bit map through the readout chip output as well as controlling, clearing and setting the integration switch  66 , the sample and hold switch  70 , and sample and hold reset switch  78 , and the flip flop  82  FF Reset and Shift controls for synchronized detection of the sun image and for serial readout of the image bit map.  
         [0027]    Referring to all of the Figures, and particularly to FIG. 4, the sun sensor data processor  20  includes a microcode PROM  100  and a crystal clock generator  102 . Interface circuits, such as a spacecraft interface  104  are used for transceiving spacecraft I/O signals. A level shifter  106 , including a clock signal for operation with a conventrional microcontroller  108  that may further be integrated with a serial port  110 , data registers  112 , and user I/O  114  for communication to the readout chip inputs and outputs. The readout chip  22  and processor chip  20  require power lines, ground lines, address pins, a clock input and bidirectional serial data bus connections for spacecraft I/O to a spacecraft through the spacecraft interface  104 . The microcontroller  108  may be for example a conventional  8051  microcontroller implementing the command, control, calibration, and sun centroid interpolation algorithms on the raw bit map data provided by the readout chip  22 . Focal plane data processing is provided by the data processor  20  having a conventional  8051  microcontroller  108 . The  8051  microcontroller  108  is an 8-bit embedded controller. The  8051  microcontroller  108  can access program instructions from the PROM and provides interface control for serial communications as well as a necessary system interval timer. The data processor  20  also contains voltage level shifting  106  required to interface to the readout chip  22  as well as to communicate with the spacecraft systems through the spacecraft interface  104 . With a power supply operating the microcontroller in an ultra low power mode at 0.5 volts, the total required power may be approximately 4.0 mW when operating at 4.0 MHz.  
         [0028]    The microcode PROM  100  stores computer programs for communicating the bit map to the microcontroller  108  from the readout chip  20 , for setting the comparator DAC calibration values in the readout chip  20 , and for performing an algorithmic determination of the spin phase synchronized solar azimuth and elevation angles. The data processor  20  reads the readout chip outputs, stores bit map slices, and interpolates the position of the sun by providing the sun elevation angle and sun azimuth angle to the spacecraft. The serial port  110  is used for communications to the spacecraft through the level shifters  106  and the spacecraft interface  104 . Communications from the data processor  20  to the readout chip  22  is through the programmed user I/O for serial communication with the readout chip  22 . The communications to the readout chip  20  includes DAC reference calibration information. The shift register  86  can also be used for communicating adaptive control data to the sun sensor state machine  82 . The data registers  112  are used as scratch pad memory during execution of the microcode of the PROM  100  for calculating the spin phase synchronized azimuth and elevation angles of the sun.  
         [0029]    Multiple sun sensors, such as three sun sensor packages, may be mounted with 1.0° overlap between sensor package Fields-of-View, (FOV), resulting in an overall 100°×0.1° full angle FOV. The microcontroller  108  has serial packet protocol interface as part of the processor chip  20  that communicates calibrated individual sun sensor azimuth and elevation angle information directly to the spacecraft data handling system. Due to the simplicity of interconnecting multiple sun sensor chips in a serial daisy chain manner, in principle more sun sensor chips may be arrayed and addressed either to improve spatial resolution, or to improve spatial coverage, or to improve reliability through redundancy. For example, six packages could be arrayed with individual offset angles to create a 180°×0.1° FOV sun sensor. The sun sensor may have a sun centroid accuracy of 0.5°+/−0.1°, with a FOV of greater than 180° elevation×360° azimuth at spin rate between one RPM and 100.0 RPM, with power of less than 100.0 mW, with a mass of 30 gm, a volume of 10.0 cc, and a DC power supply of 3.3 V.  
         [0030]    The sun sensor configuration is compact with suitable pixel readout rates for creating continuous scan images. The sun sensor is preferably used on a spinning spacecraft so that the sun sensor sweeps out the entire sky each revolution. For spin periods between one RPM and a hundred RPM, the maximum 0.1° integration period will range from 16.7 ms to 167.0 us respectively. For detecting light at 600.0 nm light and 800.0 nm light, 10 um 2  detectors, with 600.0 nm filter, the maximum integrated number of electrons at the detector output will range from 2.75×10 10  to 2.75×10 8  for 1 RPM and 100 RPM integration periods respectively. In practice, these signal values are large to allow integration for a fixed 100.0 us period, accumulating greater than 1×10 8  electrons, for variable spin periods between one RPM and 100.0 RPM. At spin rates over 100.0 RPM however, the data processor  20  will limit the throughput to one set of angular determinations per ten spins. Accuracy of the sun sensor is determined by the optical performance and interpolation algorithm used to process raw sun image data. Five pixels are used to image the entire solar disk, from low earth orbit. This raw image data is then used as an input to a centroid algorithm to determine the location of the sun in 0.5° increments of azimuth and elevation with +/−0.1 degree accuracy. Each MEMS sun sensor package has a 34°×0.1° FOV. Multiples sun sensors can be used to cover a particular FOV requirement. For an exemplar requirement of greater than 100° FOV, three sun sensing elements are used to form a composite sun sensor with slight overlaps of the respective FOVs creating a sensor with 100.5°×0.1° FOV. As the spacecraft rotates, successive slices of the sky are imaged, providing a complete FOV of 100.5°×360° for each spacecraft revolution. Also, stacks of sun sensors may be arrayed to form staring arrays for spacecraft that are three-axis stabilized. Each sun sensor package may provide a 34°×0.1° FOV. For example, a staring sun sensor application with a required 34°×3.4° FOV would require a stacked array of thirty-four individual sun sensors with 0.1° rotational offsets to provide the required FOV.  
         [0031]    The operation of the sun sensor is based on a miniature scanning sensor array. The sun sensor relies on rotation of a spinning spacecraft to sweep out a bitmap image of the sky. The image is divided into 0.1° pixels in azimuth and elevation over the sensor FOV during one revolution of the spacecraft. A microcontroller sequences focal plane operations and performs a sun centroid algorithm on the acquired raw bitmap data to report the location of the sun in 0.5° azimuth and elevation steps with +/−0.1° accuracy. The sun sensor has lower mass, lower power usage, and lower volume with suitable accuracy and spatial coverage than prior art. The sun sensor can be used as part of a critical spacecraft subsystem used effectively in nanosatellites and microsatellites, where spacecraft resources are at a premium. The analog and digital circuitry of the readout chip  22  may use a 0.5 um radiation tolerant commercial process, while data processor digital circuitry  20  may use a 0.35 um radiation tolerant process to mitigate space radiation effects. These processes provide a sun sensor that is insensitive to a total ionizing dose and single event effects found in the near earth space environment. The sun sensor may also be used to provide redundant fault-tolerant sun sensors in implementations on large spacecraft where more resources are available. Placement of individual sun sensor packages in linear arrays, stacks and/or artful decoration of a host spacecraft can provide enhanced reliability, improved spatial resolution and improved spatial coverage of the sun sensor in a particular application. Those skilled in the art can make enhancements, improvements, and modifications to the invention, and these enhancements, improvements, and modifications may nonetheless fall within the spirit and scope of the following claims.