Abstract:
A large profile, high speed laser micrometer is formed from a light source unit comprised of a plurality of emitter modules that combine to emit a laser sheet and a detector array comprised of a plurality of detector modules. The laser micrometer also includes a data processing unit. Each of the emitter modules is aligned with a corresponding detector module such that an object passing between the light source unit and the detector array can be measured to an accuracy of at least 4/100ths of an inch.

Description:
FIELD  
         [0001]    The invention relates to a large format, high speed laser micrometer.  
         BACKGROUND  
         [0002]    A laser micrometer provides dimensional information about objects placed in the path of a sheet of laser light which is detected by, or scanned across, a detector. The width of the “shadow” created on the detector provides a dimension of the object.  
           [0003]    Existing “laser micrometers” are typically designed for small objects, with a maximum dimension of 6 inches. However, laser micrometers offer accuracy better than 1/1000 th  of an inch, at a scan rate of up to a thousand samples per second. A “sample” refers to a readout of the complete detector array providing one or more measurements of the object(s) within the micrometer.  
           [0004]    In a laser micrometer, a laser light sheet is usually formed using either static refractive lens elements or a rotating mirror to scan the beam. The limitation on maximum size is a limitation on the size of these light sheet forming elements.  
           [0005]    Alternative systems, known as “light curtains” are usually used in applications such as safety monitoring for preventing access to hazardous or secured areas. However, a laser curtain can be adapted to provide measurements of approximately ⅛ th  of an inch accuracy and resolution. The scan rate from a light curtain is typically 100-200 samples per second.  
           [0006]    In a light curtain system, that light sheet is typically a series of independent light beams emitted from a linear array of light emitting diodes (LEDs) spaced at the desired measurement resolution. An array of matching photodiode detectors completes the system. The light curtain design is limited in resolution by the physical spacing between the LEDs. The maximum scan rate is limited by the need to strobe the LEDs in segments to avoid crosstalk arising from adjacent photodiodes “seeing” the wrong LED. The maximum scan rate is also reduced as the size of the light curtain increase due to the large amount of data produced and the limitations of the typical interface and data encoding scheme.  
           [0007]    Therefore, there is a need for a large format, high speed laser micrometer that is capable of scanning large objects with a high scan rate and high degree of accuracy.  
           [0008]    It is an object of this invention to provide a large format, high speed laser micrometer to scan large objects with a high scan rate and a high degree of accuracy. It is an additional object of this invention to provide a parabolic mirror assembly for forming a collimated laser light sheet.  
         SUMMARY  
         [0009]    A large profile, high speed laser micrometer is formed from a light source unit comprised of a plurality of emitter modules that combine to emit a laser sheet and a detector array comprised of a plurality of detector modules. The laser micrometer also includes a data processing unit. Each of the emitter modules is aligned with a corresponding detector module such that an object passing between the light source unit and the detector array can be measured to an accuracy of at least 4/100ths of an inch.  
           [0010]    Preferably, each of the emitter modules is comprised of two or more laser line generators arranged in an overlapping stair-step fashion to prevent gaps in the laser sheet emitted by the emitter module. Each of the detector modules is comprised of two or more linear CIS detectors, equal to the number of laser line generators, arranged in an overlapping stair-step fashion corresponding to said laser line generators.  
           [0011]    Alternatively, the number of data processing units is equal to a fraction of the number of the detector modules such that each data processing unit provides data processing for a number of detector modules located adjacent to one another. Preferably, this fraction is one-third. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The invention itself both as to organization and method of operation, as well as additional objects and advantages thereof, will become readily apparent from the following detailed description when read in connection with the accompanying drawings:  
         [0013]    [0013]FIG. 1 is a top view of a large format laser micrometer;  
         [0014]    [0014]FIG. 2 is a front view of an emitter module;  
         [0015]    [0015]FIG. 3 is a front view of a detector module; and  
         [0016]    [0016]FIG. 4 is a plan view of the laser sheet mirror optics. 
     
    
     DETAILED DESCRIPTION  
       [0017]    The large format laser micrometer  10  shown in FIG. 1 is defined by a light source unit  20  and a detector array  30 . The light source unit  20  is connected to a power supply (not shown) for activating and deactivating the light source unit  20 . The detector array  30  is connected to a central processing unit (CPU)  34  (not shown) for receiving and interpreting data received from the detector array  30 . The CPU  34  may be a dedicated hardware unit provided with a summary display of the micrometer output, a personal computer (PC) or a proprietary system. More generally, any system that interprets and presents the results, preferably while providing control options to the user, will suffice.  
         [0018]    The light source unit  20  is comprised of a number of emitter modules  40  as shown in FIG. 2. Each emitter module  40  has several laser line generators  42  (three in FIG. 2) along with the associated optics for laser line formation (shown in FIG. 4 and discussed below). The laser line generators  42  are arranged in a stair-step configuration with a slight overlap to eliminate any gaps between the laser line generators. Each laser line generator  42  forms a sheet of light equal in width to the laser line generator  42 . The overall result is a laser sheet with an effective width equal to that of the emitter module  40 .  
         [0019]    The laser optics are shown in more detail in FIG. 4. A laser  70  is passed through an aspherical lens  72  (Kodak Part # LG-11) to collect the light from the laser diode and create a two-dimensional fan-shaped light sheet. This light sheet is reflected off a flat mirror  74  to expand the width of the light sheet without increasing the size required by the optical path. The light sheet is then reflected by parabolic mirror  76  to create a straight, planar light sheet.  
         [0020]    In normal operation, each emitter module  40  will be either on (emitting light) or off. However, a pulsed mode of operation should also be provided to allow for alignment and adjustment of the detector array  30  at a lower (less than saturated) signal level.  
         [0021]    The detector array  30  is comprised of a number of detector modules  50  as shown in FIG. 3. Each detector module  50  is comprised of a number of linear CIS detectors  52  arranged in a stair-step configuration to match the laser line generators  42  in the corresponding emitter module  40 . An optical filter covers the CIS detectors to prevent signal interference from ambient or stray light sources.  
         [0022]    The detector array  30  also includes one or more data processing units  32 . As shown in FIG. 1, one data processing unit  32  is connected to three detector modules  50 . The data processing units  32  are used to receive, interpret, and transmit the signals from the detector modules  50  to the CPU  34 .  
         [0023]    An object  16  passing between the light source unit  20  and the detector array  30  causes an interruption in the path of the laser light incident on the detectors. The resulting transition in the detector is recorded by the data processing unit  32  and passed to the CPU  34  (not shown). The CPU  34  then interprets the transition data and reports it to the user, either in a raw form, or as a calculated measurement of size, whichever is required.  
         [0024]    While each detector module  50  may include its own data processing unit  32 , it is preferable to have more than one detector module  50  coupled to a data processing unit  32 , to reduce cost and system bandwidth requirements. In one configuration, there are two “slave” detector modules coupled to a “master” detector module, one to either side. The “master” detector module houses the data processing unit  32 , which receives detector signals from the “master” unit and the two adjoining “slaves”.  
         [0025]    While more “slaves” can be connected to one “master”, there will be a threshold based on the available data bandwidth for transmitting signals. If the number of “slaves” is too large, there will be signal loss at the data processing unit and gaps or errors will result. In a similar vein, while a separate data processing unit  32  could be used for all detector modules  50 , the data bandwidth requirements for the CPU  34  make this configuration unsuitable for a detector array  30  with a large number of detector modules  50 . The described array using one “master” with two “slaves” represents a balanced approach that should work with the majority of detector array configurations.  
         [0026]    The data processing unit  32  is the interface between the detector array  20  and the CPU  34 . The data processing unit  32  receives timing signals and commands from the CPU  34  and transmits transition data and gray-scale “video” (if required) back.  
         [0027]    During the scanning process, the data processing unit  32  receives an analog pixel data stream from each detector module  50  simultaneously. The analog data is then converted to 8-bit digital data and a threshold comparison is made. The threshold comparison creates a serial bit stream representing ON or OFF pixels. Spurious pixels are removed according to user-defined parameters and the final data is sent to the CPU  34 .  
         [0028]    Threshold comparison is a digital operation comparing the digitized pixel amplitude to a value defined in the configuration of each detector. The design provides a saturated detector signal when the laser is incident on the detector and a &lt;20% background signal when the laser is interrupted. The optical filter mitigates the effects of ambient lighting on the signal.  
         [0029]    The output from the threshold comparison results in a HI or LO logic level clocked for each detector connected to the data processing unit  32  with the same pixel clock. This is encoded as a 24-bit value when there is a transition on any detector. The most significant 12 bits contain the pixel count where the transition occurred, and the next 9 bits report the HI or LO state of each detector pixel at that instant. The least significant 3 bits are used for error control as described below.  
         [0030]    With a standard detector at 200 dots per inch (dpi) resolution, the detector resolution is 0.005″ per pixel. The practical limit on the resolution is determined by the collimation quality of the laser light sheet and ambient or spurious lighting effects on edge definition. It may be preferable for the data processing unit to ignore every other pixel to reduce the number of spurious transitions and the data bandwidth requirements. The result is an effective resolution of 0.01″.  
         [0031]    The data processing unit  32  requires some logic to account for conditions that produce a large number of transitions in a single scan line. For example, if a sharp edge of the object being measure is coincident with the longitudinal axis of one of the detectors it would result in a gray edge, a series of pixels rapidly exchanging between ON and OFF states, producing numerous transitions reported from the data processor. This could result in an overload of the data buffers and a consequential loss of data from the scan line and subsequent scan lines.  
         [0032]    A second potential problem is excessive or false triggering resulting from interference from dust and other small particulates. Again, the repeated random transitions could overload the data buffers and result in a loss of subsequent data.  
         [0033]    The solution is to provide for a user-defined value for transitions below which the transition should be ignored. For example, setting the value to one means that single pixel transitions are ignored i.e. a neighboring pixel must also undergo a transition at the same time for the transition to be recorded and the transition data transmitted.  
         [0034]    The data buffer problem must also be considered in the context of available bandwidth both to and from the data processing unit  32 . If each detector module  50  has a data processing unit  32 , bandwidth to the data processing unit  32  is not a problem, however, bandwidth from the data processing unit  32  (to the CPU  34 ) becomes a larger factor. The combination of “master” and “slave” detector modules alleviates the situation, however, too many “slaves” and not enough “masters” creates the opposite scenario, in that bandwidth to the data processing unit  32  is now at a premium, and bandwidth from is not a concern.  
         [0035]    The limitations on the system, therefore, lie in the bandwidth capabilities of the data processing unit  32  and CPU  34 . One “master” and two “slaves” is presented herein as a example that provides efficient data handling capabilities. Obviously, systems with a higher bandwidth can use a larger “slave” to “master” ratio, which will permit a larger array, fewer “masters” to reduce cost, or both.  
         [0036]    Accordingly, while this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to this description. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the scope of the invention.