Abstract:
A technique that reduces or eliminates the non-linearities associated with the internal feedback sensor used in a micro-electro-mechanical mirror assembly. Using the relatively linear response of the mirror positioning motor, associated driver electronics, and the mirror itself, a calibration is performed that compensates for the internal feedback sensor non-linearity. An expected position can then be calculated simply by multiplying the gain of the system by the output, due to the good inherent linearity in the system. The calibration will compare measured versus expected position criteria for a predefined set of constant outputs. The data will form a look-up table that will be used to correct for the sensor non-linearities.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates generally to a micro-electro-mechanical (MEM) mirror, and more particularly, to a method of linearizing the mirror&#39;s internal position sensor based on driver linearity.  
           [0003]    2. Description of the Prior Art  
           [0004]    A MEM mirror consists of a small reflective surface with a means for adjusting the angle of the reflective surface relative to the fixed base in which it is mounted. In this system the means for detecting the angle of rotation may be located in close proximity to the mirror itself, possibly contained within the package holding the mirror. It may be implemented for instance using four light emitting diodes (LEDs) surrounding a photo-detector located behind the mirror. This mechanism for detecting rotational position is herein after referred to as the internal feedback sensor.  
           [0005]    The internal feedback sensor used in a MEM mirror has inherent non-linearities. These non-linearities adversely affect the mirror movement control system. These non-linearities are especially troublesome when using feedback to move the mirror from one angular location to another (hereto referred to as seeks), because they change the gain in the middle of a mirror adjustment. Such non-linearities can also change the apparent gain when seen at different angles that affects the ability to hold the mirror substantially still (tracking) as well.  
           [0006]    In view of the foregoing, it would be both desirable and advantageous in a micro-electro-mechanical (MEM) mirror assembly to provide a technique that reduces or eliminates the non-linearities associated with the internal feedback sensor.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention is directed to a technique that reduces or eliminates the non-linearities associated with the internal feedback sensor used in a micro-electromechanical (MEM) mirror assembly. Using the relatively linear response of the mirror positioning motor, the associated driver electronics, and the mirror itself, a calibration is performed that compensates for the internal feedback sensor non-linearity. An expected position can then be calculated simply by multiplying the gain of the system by a given output, due to the good inherent linearity in the system. The calibration will compare measured versus expected position criteria for a predefined set of constant outputs. The data will form a look-up table that will be used to correct for the sensor non-linearities.  
           [0008]    In one aspect of the invention, a technique that reduces or eliminates the non-linearities associated with the internal feedback sensor used in a micro-electromechanical (MEM) mirror assembly is implemented to provide faster settling times with less overshoot and stalls when performing seeks.  
           [0009]    In another aspect of the invention, a technique that reduces or eliminates the non-linearities associated with the internal feedback sensor used in a micro-electromechanical (MEM) mirror assembly is implemented to enhance consistency associated with tracking performance.  
           [0010]    In yet another aspect of the invention, a technique that reduces or eliminates the non-linearities associated with the internal feedback sensor used in a micro-electromechanical (MEM) mirror assembly is implemented in a manner that does not rely on external hardware to perform measurements.  
           [0011]    According to one embodiment, a method of linearizing a micro-electromechanical (MEM) mirror position sensor comprises the steps of providing a MEM mirror, a mirror position sensor having a non-linear response, and a mirror position control system; tabulating mirror position sensor signals in response to a plurality of desired mirror positions; and adjusting a mirror position control system such that the tabulated mirror position sensor signals are adjusted to have a linear relationship to the plurality of desired mirror positions.  
           [0012]    According to another embodiment, a method of linearizing a micro-electromechanical (MEM) mirror position sensor comprises the steps of providing a MEM mirror and a mirror position control loop having a non-linear mirror position sensor; tabulating non-linear mirror position sensor signals in response to a plurality of desired mirror positions; and adjusting the tabulated non-linear mirror position sensor signals such that the mirror position control loop provides a linear response to the non-linear mirror position sensor signals.  
           [0013]    According to yet another embodiment, a micro-electro-mechanical (MEM) mirror positioning system comprises a MEM mirror; a non-linear mirror position sensor; and a controller configured to adjust output signals generated via the non-linear mirror position sensor such that the controller operates to provide a linear mirror position response to the output signals generated via the non-linear mirror position sensor.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    Other aspects, features and advantages of the present invention will be readily appreciated, as the invention becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawing figures wherein:  
         [0015]    [0015]FIG. 1 is a system block diagram illustrating a MEM mirror system; and  
         [0016]    [0016]FIG. 2 is a flow chart depicting a method of linearizing the MEM internal sensor shown in FIG. 1, according to one embodiment of the present invention. 
     
    
       [0017]    While the above-identified drawing figures set forth particular embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    [0018]FIG. 1 is a system block diagram illustrating a micro-electro-mechanical (MEM) mirror system  100 . MEM system  100  can be seen to have a MEM mirror assembly  104  including an internal (local) sensor  106 , as well as a controller  108  that function generally as described in U.S. patent application, entitled Method Of Sampling Local And Remote Feedback In An Optical Wireless Link, docket number TI-33553, filed on May ______ , 2002, by Oettinger et al., assigned to Texas Instruments Incorporated, the assignee of the present application, and that is hereby incorporated by reference in its entirety herein.  
         [0019]    With continued reference now to FIG. 1, the MEM system  100  also can be seen to include a mirror positioning motor  102 . Controller  108  includes an analog-to-digital converter (ADC)  110 , and a data processor  112  that may be, for example, a digital signal processor (DSP), a micro-controller, a CPU, a computer, a micro-computer, or other like data processing device. Controller  108  also includes a digital-to-analog converter (DAC)  114  and a motor driver  116 .  
         [0020]    [0020]FIG. 2 is a flow chart depicting a method  200  of linearizing the MEM mirror internal feedback sensor  106  shown in FIG. 1, according to one embodiment of the present invention. The internal feedback sensor  106  has inherent non-linearities, as stated herein before. These non-linearities adversely affect the MEM system  100 . As also stated herein before, these non-linearities are especially troublesome when using feedback to perform seeks, because they change the gain in the middle of a mirror movement. They can also change the apparent gain seen at different mirror  204  angles, which affects the tracking performance as well.  
         [0021]    Method  200  is then directed to a technique of calibrating the MEM system  100  to compensate for the non-linearity associated with the internal feedback sensor  106 . The present inventors specifically recognized that the relatively linear response of the mirror positioning motor  102  and controller  108  electronics could be used to perform a calibration procedure in a manner that compensates for the sensor  106  non-linearity. They also recognized that because of the good linearity in the rest of the system  100 , an expected ADC  110  output representing the mirror  104  position could, for example, be calculated simply by multiplying the gain of the MEM system  100  by the DAC  114  input if the sensor was linearized.  
         [0022]    Method  200  shall be understood to apply to a wide variety of internal sensor types. Those skilled in the light sensor art will already be aware of a vast array of position sensors and methods of implementing position sensors; and so detailed descriptions relating to the internal feedback sensor  106  are not set forth herein to preserve brevity and clarity in describing the preferred embodiments.  
         [0023]    With continued reference now to FIG. 2, a calibration procedure  200  according to one embodiment of the present invention looks at “counts” rather than current or voltage i.e. the values in abstract units of “counts” that the processor  112  writes to the DAC  114 , and reads from the ADC  110 . The calibration procedure  200  functions to map the non-linearities of the local sensor  106 .  
         [0024]    Because the rest of the MEM system  100  is linear, it is easy to see that some fixed gain is associated with the DAC  114 , while another fixed gain is associated with the driver  116  that together will cause the mirror  104  to move proportionally to the output of the driver  116  in response to a desired value (e.g. 1,000) that is written to the DAC  114 . The mirror  104  may, for example, move 20 milli-radians in response to the 1000 counts that are written to the DAC  114 . Because the rest of the MEM system  100  is linear, writing a value of 2,000 counts to the DAC  114  will then cause the mirror  104  to move 40 milli-radians; 3,000 counts to the DAC  114  will cause the mirror  104  to move 60 millradians, etc.  
         [0025]    If the local sensor  106  was ideal (having a fixed gain), the foregoing 20 milliradian motion of the mirror  104  would cause the fixed gain through the ADC  110  to generate a count value that could then be further scaled (via another gain) to generate exactly the desired 1,000 count value (1,000 counts to the DAC  114  would provide 1,000 counts from the ADC  210 ). This fixed ratio (1 to 1) would then be valid, regardless of input value, in a completely linear system.  
         [0026]    The present inventors have found, however, that in a real MEM system, the local sensor  106  is not in fact ideal. When the mirror  104  moves 20 milli-radians, a relative count reading might be, for example, 1,000; but when the mirror  104  moves 40 milliradians, the resultant count reading is not the desired 2,000, but instead is some other undesired count value, i.e. 1,500, delivering less than optimal performance in the MEM system  100  control loop. The MEM system  100  must then be compensated in some manner to achieve optimal performance in the control loop.  
         [0027]    Looking again at FIG. 2, calibration procedure  200  operates to create a table of measured ADC  110  inputs for a fixed set of DAC  114  outputs as shown in blocks  202 - 210 . Specifically, processor  112  functions to deliver desired inputs to the DAC  114  as shown in block  202  such that the DAC  114  will generate a set of output signals to driver  116  as shown in block  204 . Driver  116  will then activate the mirror motor  102  to reposition mirror  104  in response to each of the desired DAC  114  inputs as shown in block  206 . The local sensor  106  will sense the re-positioning of the mirror  104  to generate a unique ADC  110  input signal (count) at each of the desired DAC  114  inputs as shown in block  208 . Each of the unique ADC  110  input signals (counts) is then tabularized as shown in block  210 . Finally, an algorithmic software is implemented to perform a lookup operation that corrects for the error in the pending measurement as seen in block  212 . If, for example, a measured ADC  10  input count value during operation of an optical wireless link was found to be 1,500, and it was known by reference to the look-up table that the real count value should have been 2,000, then a correction is made to the DAC  114  input count prior to using the position value of 1,500 for any mirror  104  control operations.  
         [0028]    In view of the above, it can be seen the present invention presents a significant advancement in the art of MEM mirror control techniques. Further, this invention has been described in considerable detail in order to provide those skilled in the optical wireless communication art with the information needed to apply the novel principles and to construct and use such specialized components as are required. In view of the foregoing descriptions, it should be apparent that the present invention represents a significant departure from the prior art in construction and operation. However, while particular embodiments of the present invention have been described herein in detail, it is to be understood that various alterations, modifications and substitutions can be made therein without departing in any way from the spirit and scope of the present invention, as defined in the claims which follow.