Abstract:
An alignment system for multiple objects, such as gain generator modules for a distributed laser resonator. The present invention allows for banks or rows of objects, such as gain generator modules, to be maintained in alignment with one another under closed loop control. In particular, one module of every bank or row is designated as the reference or master module, for example, at one end of the bank. Each of the remaining modules or objects are designated as slave modules and carried by a positioning module. The positioning modules are used to control the x and y movements of the slave modules relative to the master or reference module under closed loop. An iris or target with an aperture is mounted on each of the slave modules. A laser source mounted on the master or reference module is dithered relative to the targets in the x and y directions. An optical system, for example, simple telescope at the base of each target images the laser onto a detector. An error signal is created by synchronous detection, which, in turn, is applied to the positioning modules to align each of the slave modules relative to the master or reference module by closed loop control without any extra extensive weight penalties on the aircraft platform.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to commonly owned co-pending application entitled “High Resolution Positioner” by P. Livingston, filed on even date. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an alignment system and more particularly to an alignment system for multiple objects which includes a plurality of positioning modules for carrying the multiple objects. The positioning modules are controlled as a function of the displacement of each object relative to one of the objects, such that the alignment of all the objects relative to each other is uniform. 
     2. Description of the Prior Art 
     Various laser weapons are known in the art. Such laser weapons are known to be mounted on ground based platforms as well as aircraft based platforms. 
     Because of its size, the gain generator may be formed in modules. The proposed Airborne Laser Demonstrator (ABL) laser consists of 14 gain generator modules, adapted to be arranged in two banks of 7 each. Each module weighs in the neighborhood of about 5,000 pounds. On airborne platforms, the banks of the gain generator modules are adapted to be oriented parallel to the airplane roll axis. 
     During flight it is known that the aircraft floor upon which the modules are seated sags causing the gain generator modules to fall out of optical alignment with one another. Such misalignment of the gain generator modules is known to cause serious degradation of the performance of the laser weapon. Unfortunately, the alignment of the component parts of such lasers must be relatively precisely maintained. 
     Conventionally this problem has been addressed by disposing the gain modules on a re-enforced platform or re-enforcing the floor of the aircraft on which the modules are seated. Unfortunately, either solution adds additional weight to the aircraft which is highly undesirable. Thus, there is a need for a system for maintaining the alignment of a plurality of objects, such as gain generator modules, that is relatively light weight and maintains the units in optical alignment both during flight and on the ground. 
     SUMMARY OF THE INVENTION 
     Briefly, the present invention relates to an alignment system for multiple objects, such as gain generator modules of a distributed laser. The present invention allows for banks or rows of objects, such as gain generator modules, to be maintained in alignment with one another under closed loop control. In particular, one module of every bank or row is designated as the reference or master module, for example, at one end of the bank. Each of the remaining modules or objects are designated as slave modules and carried by a positioning module. The positioning modules are used to control the x and y movements of the slave modules relative to the master or reference module under closed loop. An iris or target with an aperture is mounted on each of the slave modules. A laser source mounted on the master or reference module is dithered relative to the targets in the x and y directions. An optical system, for example, simple telescope at the base of each target images the laser onto a detector. An error signal is created by synchronous detection, which, in turn, is applied to the positioning modules to align each of the slave modules relative to the master or reference module by closed loop control without any extra extensive weight penalties on the aircraft platform. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     These and other advantages of the present invention will be readily understood with reference to the following specification and attached drawing wherein: 
     FIG. 1 is a block diagram illustrating the multiple unit alignment system in accordance with the present invention. 
     FIG. 2 is a simplified schematic diagram of an exemplary positioning module in accordance with the present invention. 
     FIG. 3 is a block diagram of a laser source and dither control system for use with the present invention. 
     FIGS.  4   a ,  4   b  and  4   c  are exemplary graphical illustrations which illustrate the aperture diameter for the target apertures as a function of the power intercept for the present invention. 
     FIG. 5 is a schematic diagram of the control system for a positioning module in accordance with the present invention. 
     FIG.  6   a  is a graphical illustration of the input signals to the multipliers illustrated in FIG.  5 . 
     FIG.  6   b  is a graphical illustration of the error characteristic available at the output of the integrator in FIG.  5 . 
    
    
     DETAILED DESCRIPTION 
     The present invention relates to a multiple unit alignment system. As shown and described herein, the alignment system is shown in an application for aligning a plurality of gain modules which form a portion of a cylindrical resonator laser weapon. However, it should be clear to those of ordinary skill in the art that the principles of the present invention are applicable to virtually any types of objects which must be aligned relative to each other. 
     Referring to FIG. 1, one or more banks  20  of a plurality, for example, seven (7), gain modules  22 ,  24 ,  26 ,  28 ,  30 ,  32  and  34 , is adapted to be mounted on an aircraft platform, parallel with the aircraft roll axis. One end module, for example, the module  22 , is designated as a reference or master module. The balance of the modules  24 ,  26 ,  28 ,  30 ,  32  and  34  are designated as the slave modules. As will be discussed in more detail below, the system in accordance with the present invention aligns the slave modules  24 ,  26 ,  28 ,  30 ,  32  and  34  relative to the reference module  22  in an x-y direction so that the bank  20  acts as a rigid block allowing end benches (not shown) to be aligned to them. 
     In accordance with an aspect of the invention, each of the slave modules  24 - 34  is mounted on a positioning module  38 . As will be discussed in more detail below, the positioning module  38  is used to provide x and y positioning of each of the slave modules  24 - 34  under closed loop control. The positioning modules  38  position each of the slave modules  24 - 34  as a function of the x y displacement of each of the slave modules  24 - 34  relative to the reference module  22 . More particularly, each of the modules  22 - 34  is aligned to a basic reference line generated by a laser beam  36 . 
     A laser source  40 , mounted on the reference or master module  22 , is used to generate the laser beam  36  which forms the reference line. 
     An iris or target  42  with an aperture  44  is mounted on each of the slave modules  24 - 34 . The aperture dimensions are selected to decrease successively with each module proceeding from the first slave module  24  to the last module  34  in order to intercept a fixed percentage of the laser beam  36  passing through it. An optical system, for example, a simple telescope  46 , may be mounted at the base of each target  42 , a small distance therefrom. The telescope  46  is used to image an illuminated ring surrounding the aperture  44  and the iris  42  arising from the intercepted laser power onto a detector  47  (FIG.  5 ), for example, a simple silicon detector. As will be discussed in more detail below, the intercepted laser power is synchronously detected and used to generate an error signal which, in turn, is used to drive the positioning module  38  for each of particular slave modules  24 - 34  as a function of the x-y position of the slave module  24 - 34  relative to the x-y position of the reference module  22 . 
     In accordance with an important aspect of the invention, the laser beam  36  is dithered in orthogonal directions (i.e. in the x and y directions), traveling perhaps ±30% of its radius. If a target aperture  44  is centered on the beam  36 , the detected time varying electrical signal will contain only even harmonics of the sinusoidal dither frequency. However, if the aperture  44  is displaced from the beam centroid as a result of motion of a slave module  24 - 36 , the electrical signal will have a temporal component at the fundamental dither frequency but with a phase that is either in phase with the dither or a 180° out of phase depending on whether the aperture  44  is displaced to the left or the right or up or down. As will be discussed in more detail below, the left and right and up and down motions are distinguishable because dither frequencies in the orthogonal directions are distinct. Thus, a synchronous detector will not be sensitive to the wrong frequencies as long as the dither frequencies are not multiples of one another. 
     An exemplary positioning module  38  is illustrated in FIG.  2 . As shown, the positioning module  38  is shown as a pneumatic system; however, other types of positioning modules, such as hydraulic, electrical and electronic modules are contemplated. As discussed above, one positioning module  38  is provided for each of the slave modules  24 - 34 . The positioning module  38  includes an air accumulator  48  and a plurality of air bladders  50 ,  52  and  54 , connected to the air accumulator  48  by is way of a pair of servo valves  56  and  58 . The air bladders  50  and  52  are oriented vertically as shown and control the x translation of the various slave modules  24 - 34  while the air bladder  54  is horizontally oriented and is used to control the y translation of the reference modules  24 - 34 . The air accumulator  48  for all of the positioning modules  30  may be connected to a common compressor (not shown). 
     The air bladders  50 ,  52  and  54  are under the control of the accumulator  48  that is maintained at a roughly constant pressure by the common compressor. The servo valves  56  and  58  operate such that the rate of pressure change in a bladder  50 ,  52  and  54  is proportional to an error signal, discussed below in order to form a first order system. The servo valve  56  is connected by a suitable air line  60  to the accumulator  48  and to each of the bladders  50  and  52  by a pair of air line  62  and  64 . Similarly, the servo valve  58  is connected to the air bladder  54  by an air line  66  and to the accumulator  48  by way of an air line  68 . Each of the servo valves  56  and  58  is also connected to a vent port  70  and  72 , respectively. The position of the servo valves  56  and  58  either causes air pressure from the air accumulator  48  to be provided to the air bladders  50 ,  52 ,  54  or to be vented. 
     The servo valve  56  operates differentially by reducing the pressure on one bladder  50  while increasing the pressure on the other side. For example as shown in FIG. 2, the servo valve  56  is positioned such that the bladder  50  is being vented while the bladder  52  is increasing in air pressure. Similarly the servo valve  58  causes air pressure to either be vented or increased relative to the bladder  54  for causing y translation of the reference module  24 - 34 . In the position shown in FIG. 2, the air from the air accumulator  48  as being used to increase the air pressure in the bladder  54 . 
     The laser source  40  is illustrated in FIG.  3 . The laser source  40  includes a small relatively low wattage laser, for example, a HeNe laser or a small solid state laser having a few milliwatts of output power. The laser source  40  also includes a y dither mirror  74  and an x dither mirror  76 . The y dither mirror  74  and x dither mirror  76  may be formed as voice coil operated piston mirrors or may be mirrors driven by PZT actuators  78  and  80 . The laser source  42  may also include a pentaprism  82  and a telescope  84 , for example, a 3.3× magnification telescope to produce a 1.0 cm collimated beam as shown. 
     The beam is dithered in orthogonal directions by way of the x-dither and y-dither mirrors  80  and  78 . Each of the mirrors  78  and  80  may have a maximum excursion or ±0.9 mm if the beams are to be dithered with a 30% beam radius excursion. If greater excursions are needed, additional mirrors can be added to make the reflections from the dither mirrors  78  and  80  an obtuse angle since the ratio of the beam displacement to the piston stroke is the cotangent of the incidence angle. The telescope  84  may be used to expand the beam diameter to 1 cm for the purpose of improving the performance of the last module  34  in the line. 
     FIGS.  4   a - 4   c  illustrate how the reflected power varies with the decreasing aperture sizes to intercept the fixed percentage of the laser beam passing through the target aperture  44 . As seen in FIGS.  4   a - 4   c , the aperture diameters are selected relative to the beam e folding diameter. Thus, for a nominal 1 cm diameter launched beam, the first aperture may be 0.709 cm. The three curves shown in FIG.  4   c  represent 15%, 10% and 5% of the intercepted power. An optimal trade-off between finite hole size for the last target and small initial diameter is to scrape off 10% of the transmitted power. As shown in FIG. 4, even though the power of the last detector is down by 20 dB, the system will still perform satisfactorily. 
     A control circuit for the servo valves  56  and  58  as well as the x dither position mirror  80  and a y dither position mirror  78  is illustrated in FIG.  5  and generally identified with the reference numeral  86 . The control circuit  86  includes the detector  47  mounted at the base of the targets  42  as discussed above. The output of the detector  47  is applied to a gain amplifier  90 . Each gain amplifier  90  has a gain G such that the output of the upstream integrator will be the same for the target hole displacement. After the detector signal is amplified, it is multiplied by sinusoid signal from a respective sinewave generators  92  and  94  by way of a pair of multipliers  96  and  98  for synchronous detection and forms a product signal. An exemplary product signal at the output of the multipliers  96  and  98  is illustrated in FIG.  6   a  at the first target for 15%, 10% and 5% scraped power for a target hole displacement of 0.2 beam diameters. The sinewave generators  92  and  94  are used to control the dither motion of the x dither piston mirror  80  and the y dither piston mirror  78 , respectively. The x and y sinewave generators  92  and  94  may be common to all modules thus requiring only 2 generators for all modulators. The product signal is applied to a pair of integrators  98  and  100  for the x and y channels. As shown, the integrators  98  and each include an input resistor R and a feedback capacitor C. The integrator time constant is the product of the resistor R and the capacitance C. The output of the integrators  98  and  100  provide error signals which are applied to servo amplifiers  102  and  104  for each of the servo valves  58  and  56 , respectively. An exemplary error signal, available at the output of the integrators  98  and  100 , is illustrated in FIG.  6   b . In each case, the target hole  44  is shown displaced by 0.2 beam diameters. Although there is a strong fundamental component to the signal, the rounded tops of the waveforms relative to the bottoms indicates the presence of waveform second harmonics. The hole centered on the beam waveform which show small wiggles about subnominal DC level but with half the period of a dither signal. The error characteristic illustrated in FIG.  6   b  demonstrates a significant capture of about ±2.2 beam radii. For the first target, this amounts to a displacement of about ±1.1 cm. For the last target the number is much less because the beam is small at that point. 
     A detailed development of the error signal is provided below. In particular, under most circumstances, the far-field spot of a laser beam having a good single TEM 00  beam profile closely approximates a Gaussian. Assuming an e-folding radius of r=2λ/πD, the total power P at an aperture  44  having a diameter d is provided below. 
     The displacement of the Gaussian beam from the aperture center has two components; one is a bias δ that measures the beam displacement from the aperture center in units of beam radius r 0  and the other is an AC term with a dither amplitude η. Let θ=ωt represent the dimensionless dither time given the angular frequency ω. Defining A=δ−ηcos θ, the total power passed through an aperture is provided by equation (1):            P   t          (   θ   )       =       P     π                   r   0   2              ∫       ∫   d                            2          r   _                 -       (       r     r   0       +   A     )     2                                        
     The exponent is expanded and the coordinates of integration are transformed to polar coordinates. To simplify the Fourier-Bessel expansion, the x and y axis definitions are interchanged. Following the transformation, the exponent is expanded carrying the cosine term in a Fourier series as shown by equation (2):                  P   t          (   θ   )       =         P                        -     A   2             π                   r   r0   2              ∫       ∫   0     2      π                            ϕ            ∫   0     d   /   2            r                      r                            -       r   2       r   0   2         -     2        r     r   0          A                 cos                 ϕ                                 =     2      P                        -     A   2                ∫   0     d   /     2   r0              u                      u                          -     u   2                I   0          (     2      A        r     r   0         )                                          
     Most of the action is in the exponential and the integral is a scaling factor that is only weakly dependent on A. For A is &lt;&lt;1 (the usual case for a servo system near null) and for a beam comparable to or smaller than the aperture  44 , a modified Bessel function can be replaced with unity. By expanding the exponential shown in a product of a Fourier-Bessel expansion equation (3) results:               -     A   2         =              -     δ   2       -       η   2     2                ∑   m                         I   m          (     -       η   2     2       )          cos                 m2                 θ                     ∑   m            I        (     2      δη     )          cos                 n                 θ                                    
     After carrying out the integration in equation (2) following substitution of equation (1) for the Bessel function and replacing the exponential A 2  with equation (3), the total received power is provided by equation (4):            P   t          (   θ   )       =     P                          -     δ   2       -       η   2     2                ∑   m              I   m          (     -       η   2     2       )          cos                 m2                 θ          ∑   m            I        (     2      δη     )          cos                 n                     θ        (     1   -          -     d     2   r0             )       2                                      
     where P represents the power collected by the detector 
     The factor following the product of the sums accounts for the power occluded by the aperture. 
     A synchronous detection of P t  results in an error characteristic. If w (δ) represents the error characteristic, then ω is the average power of a dither cycle and is given by equation (5):                W        (   δ   )       =                  1     2      π              ∫   0     2      π                            θcos                   θ                     P   t          (   θ   )                         =                  P        (     1   -            d   2       4        r   0   2             )                   -     δ   2       -       η   2     2                ∑   m            I   m          (     -       η   2     2       )                                      ∑   n              I   n          (     2      δη     )              ∫   0     2      π                                θ         2      π          cos                 2      m                 θcos                 n                 θcos                 θ                                        
     The integral over the trigonometric functions is resolved using orthogonality of the trigonometric functions over a complete period. Thus, the error characteristics is given by equation (6):                W        (   δ   )       =       1     2      π              ∫   0     2      π                            θcos                   θ                     P   t          (   θ   )            P   ·     (     1   -            d   2       4        r   0   2             )                         =              -     δ   2       -       η   2     2                ∑       n   =   1     ,   3   ,   5   ,   …                I   n          (     2      δη     )            {         1   4            I       n   +   1     2            (     -       η   2     2       )         +       1   4            I       n   -   1     2            (     -       η   2     2       )           }                                        
     As mentioned above, a deviation from the beam centroid is δ measured in r 0  units. Since the modified Bessel function proceeding the brackets is an odd function of its argument, ω goes through zero and δ=0 and retains the sign ± of the deviation which allows the system to work with very high precision limited only by the dither mirror resolution. The exponential containing δ rolls off the curve for large values of δ forming a classical sigmoidal curve. 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. Thus, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described above. follows: