Abstract:
An apparatus and a method for controlling the path of oscillatory travel of a device within a two-axis system. A device, such as a camera, is held in a two axis gimbal system having motors for driving the azimuth and pitch axes. A two degree-of-freedom gyroscope is fixed to the case of the device. A first derivative circuit is interposed between a first input port for a loop for controlling device movement with respect to a first axis and a second input port for a loop for controlling device movement with respect to a second axis. Likewise, a second derivative circuit is interposed between the two input ports so that a periodic signal driving with respect to one axis generates a derivative function for driving with respect to the cross-axis. The cross axis signal is scaled to counteract the direct torquing of the gyro rotor that otherwise prevents smooth oscillatory slewing or scanning by the device.

Description:
BACKGROUND  
         [0001]    The present invention relates to apparatus and methods for precision slewing, or pointing, a platform-mounted device, such as a camera. More particularly, this invention pertains to the slewing of a two-axis gimbal-mounted device with mechanical gyro stabilization.  
           [0002]    Two degree-of-freedom gyros have been commonly employed in the prior art to maintain or stabilize the orientations of the axes of platforms, cameras and other devices with respect to the earth or inertial space. Such gyros are characterized by a mass (“rotor”) that rotates is about, and thereby defines, a spin axis. Generally the device to be stabilized is fixed to the case of the gyro so that any deflection of the position of the device with respect to a stabilized axis is sensed as movement of the attached case of the gyro with respect to the stabilized and spinning rotor. This generates a corrective signal that is transmitted to a platform-fixed motor for generating a corrective force with respect to the temporarily-misaligned axis.  
           [0003]    In many cases it is also important to be able to slew a device (such as a camera) about some preferred axes to change its orientation in space. The standard technique for doing this is to precess the gyro spin axis about the appropriate output axis. Pickoffs internal to the gyro detect the precession and coerce the supporting gimbals to follow it thereby changing the orientation of the device in space.  
           [0004]    If the gimbal axes are aligned to the gyro axes, the slewing can be controlled about the desired axis without cross-coupling motion into the other axis. This result is only true in the prior art for steady state rates of slewing. However, in the case of low frequency oscillatory slewing (i.e. “scanning”), because the gyro dynamics includes the inertia of the gyro rotor, the gyro torquers tend to deflect the gyro spin axis about both the azimuth and pitch axes when only one axis is desired to be sinusoidally precessed. As a result, the rotor spin axis is caused to cone in an undesirable elliptical manner.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention addresses the foregoing shortcomings of the prior art by providing, in a first aspect, apparatus for controlling the path of oscillatory travel of a device within a two-axis system in which the device is fixed to a two degree-of-freedom gyroscope. The gyroscope includes a first forcer for applying a torque with respect to a first rotor axis in response to a first signal to precess the rotor about a second, orthogonal rotor axis. It includes a second forcer for applying torque to the rotor with respect to the second rotor axis in response to a second signal. The angular displacement of the rotor from a null position generates a signal for activating motion to position the device within the two-axis system.  
           [0006]    The apparatus of the invention includes at least one cross-axis circuit arranged to receive the first signal and to generate the second signal in response so that the said second signal drives the second forcer to precess the rotor with respect to the first axis to substantially cancel the effect of torque applied by the first forcer with respect to the first axis of the rotor.  
           [0007]    In a second aspect, the invention provides apparatus for substantially nulling the effect of torque applied to precess the spinning rotor of a gyroscope. Such apparatus includes a first forcer for applying a torque with respect to a first axis of the rotor in response to a first signal. A second forcer is provided for applying a torque to the rotor with respect to a second axis in response to a second signal. The second axis is orthogonal to the first axis.  
           [0008]    A cross-axis circuit receives the first signal and generates the second signal in response so that the second signal drives the second forcer to apply torque to the rotor with respect to the first axis by precession to cancel the torque applied to the rotor with respect to the first axis by the first forcer.  
           [0009]    In a third aspect, the invention provides a method for substantially nulling the effect of a first torque applied by a first forcer with respect to a first axis of a spinning gyroscope rotor to precess the rotor with respect to a second, orthogonal, axis of the rotor. Such method comprises the step of applying a second torque to the rotor with respect to the second axis of the rotor to precess the rotor with respect to the first axis to substantially cancel the effect of the torque applied to the rotor with respect to the first axis by the first forcer.  
           [0010]    The preceding and other features of this invention will become further apparent from the detailed description that follows. Such description is accompanied by a set of drawing figures. Numerals of the drawing figures, corresponding to those of the written description, point to the features of the invention with like numerals referring to like features throughout both the written text and the drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a perspective view of a camera device mounted within a two axis gimbal and including a two degree-of-freedom gyro for stabilization;  
         [0012]    [0012]FIG. 2 is a perspective view of a two degree-of-freedom gyroscope;  
         [0013]    [0013]FIG. 3 is a block diagram of the invention for stabilizing a device that is movable about two axes and stabilized by a two degree-of=freedom gyroscope;  
         [0014]    [0014]FIG. 4 is a schematic diagram of a differentiator circuit for use in the invention; and  
         [0015]    [0015]FIG. 5 is a graph of the frequency response of a differentiator circuit in accordance with the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0016]    [0016]FIG. 1 is a perspective view of a system for utilizing the present invention. A camera  10  is mounted for two-degree of freedom movement relative to a fixed frame  12  comprising an azimuth gimbal  14  and a pitch gimbal  15  that contains the camera  10 . Azimuthal movement of the camera  10  with respect to an axis  16  is provided by the pivotal engagement of the azimuth gimbal  14  to the fixed frame  12  while movement with respect to a pitch axis  18  is provided by the pivotal engagement of the pitch gimbal  15  to the azimuth gimbal  14 . Such two-axis gimbal mounting arrangements are well known to those skilled in the art.  
         [0017]    The case of a two degree-of-freedom gyro  20  is fixed to either the camera  10  or to the pitch gimbal  15 . The operational elements of the gyro  20  are illustrated in FIG. 2. A rotor  22  is driven by a motor  24  to define a spin axis  26 . The drive shaft  28  of the motor  24  includes a universal joint  30  that permits deflection of the generally disk-like rotor  22  with respect to orthogonal case-fixed axes  32  and  34 . A first forcer comprising an opposed pair of electromagnets  36 ,  38  is arranged to torque the disk-like rotor  22  with respect to the axis  32  while a second forcer comprising an opposed pair of electromagnets  40 ,  42 , displaced 90 degrees therefrom, torques the rotor  22  with respect to the axis  34 . A pickoff comprising opposed pair of electromagnets  44 ,  46  in combination detect deflection of the rotor  22  with respect to the axis  32  while a pickoff comprising opposed pair of electromagnets  48 ,  50  detect deflection with respect to the axis  34 .  
         [0018]    While the forcers and pickoffs will be designated throughout as comprising opposed pairs of electromagnets, the invention is not limited to forcers and pickoffs of the electromagnetic type. Rather, the invention may incorporate a variety of forcer and pickoff technologies, including, but not limited to pickoffs and forcers of the capacitive type, each of which is well known to those skilled in the art.  
         [0019]    Either of the gyro forcers initiates the slewing process by applying a torquing force with respect to one of the axes  32 ,  34  of the rotor  22 . This torque T causes the spinning rotor  22  to be precessed at a rate dθ/dt with respect to the orthogonal axis. Such precession with respect to the orthogonal axes  32 ,  34  is defined as follows:  
         dθ 34 /dt=T 32 /H  (1)  
         dθ 32 /dt=T 34 /H  (2)  
         [0020]    where H, the angular momentum of the spinning rotor, is defined as  
         H=CN  (3)  
         [0021]    and  
         [0022]    C=polar moment of inertia of rotor  22  about its spin axis; and  
         [0023]    N=spin speed.  
         [0024]    The precession due to the application of torque T 32  about the axis  32  or torque T 34  about the axis  34  is detected by the appropriate pickoffs and input, as an error signal, to one of servos  52 ,  54  for energizing gibmal torquer motors  56 ,  58  respectively. The torquer motors  56 ,  58  rotate the camera  10  about the azimuth and pitch axes  16  and  18 . In operation, the servos  52 ,  54  energize the torquer motors  56 ,  58  rotating the orientation of the case of the gyro  20 , and the position of the pickoffs comprising opposed pairs of electromagnets  44  through  50  until they again assume a null position relative to the new orientation of the rotor  22 . In this way, the position (pointing direction) of the camera  10  is stabilized in in its new orientation in this two axis gimbal system using a two degree of freedom gyro.  
         [0025]    Problems arise when one attempts to slew the camera  10  back and forth in an oscillatory manner at a low frequency. A forcer comprising one of the opposed pairs of electromagnets  36 ,  38  or  40 ,  42  is driven to apply a torque T to tilt the rotor  22  about one of the axes  32  or  34  in a periodic or oscillatory manner at frequency f, the deflection of the rotor  22  is not limited to only the desired precession. Rather, due to the inertia of the rotor  22 , the following reactions are also observed:  
         dθ 32 /dt=AdT 32 /dt  (4)  
         dθ 34 /dt=AdT 34 /dt  (5)  
         [0026]    where  
         [0027]    A=1/Hω nut    
         [0028]    ω nut =nutation frequency (=H/I where I is the moment of inertia of the rotor  22  about a diametrical axis, approximately ½ C for a flat rotor).  
         [0029]    As a result of the undesired presence of the cross-coupling of motions with respect to both of the axes  32  and  34 , detected by the pairs of pickoffs associated with each axis, error signals are generated and transmitted to each of the servos  52  and  54 . The servos  52  and  54  simultaneously drive the torquer motors  56  and  58  to rotate the camera  10  to null the error signals that result from the precession and direct deflections of the rotor  22 . This results in undesired off-axis motion of the camera  10  rather than the strictly back-and-forth scanning that is desired. As the torque is applied in an oscillatory manner at some low frequency f, the camera  10  is subject to an elliptical coning type of motion with the ratio of motion about the cross-axis relative to the driven axis defined by f/f nut , where f nut =ω nut /2π. For example, should it be desired to slew (or scan) about one axis at 20 Hz and the nutation frequency is 500 Hz, an undesired 4 per cent cross-coupling motion occurs on the other axis. Typical operational requirements for cross-coupling are better than −60 dB or 0.1 per cent.  
         [0030]    As disclosed above, the cross-axis precession rate is equal (subject to a scale factor) to the torque T applied, while the rate of deflection about the direct axis is a function of the rate of application dT/dt of the torque. In the invention, undesired motion about the direct axis is nulled to eliminate coning of the gyro spin axis. This is accomplished by generating a signal that directly torques the rotor  22  with respect to the cross axis to thereby cause a counteracting precession torque to act upon the direct axis that cancels out the direct axis angular deflection. As the relationship between the torque T and the rates of direct deflection and precession of the rotor  22  are known (see equations 1, 2, 4 and 5), the signal for directly driving cross-axis motion to cause precession with respect to the direct axis that offsets the direct deflection of the axis is derived from the original drive signal.  
         [0031]    [0031]FIG. 3 is a block diagram for illustrating a two-axis system according to the prior figures (and equations) that incorporates the invention. The discussion that accompanies this figure will make frequent reference to elements of the systems of the prior figures by employing like numerals.  
         [0032]    The invention comprises the addition of complementary cross-axis differentiator circuits  64 ,  66  into a two axis system with a two degree of freedom gyroscope. As shown, periodic currents i 32  and i 34  for energizing the forcers comprising opposed pairs of electromagnets  36 ,  38  and  40 ,  42  to precess the rotor  22  with respect to the axes  34  and  32  respectively are each tapped and input to a differentiator circuit  64 ,  66  then fed to the cross-axis torquer pair. That is, a first signal comprising the gyro torquer current i 32 , in addition to directly driving the gyro comprising opposed pair of electromagnets  36 ,  38 , is directed to drive the cross-axis forcer comprising the opposed pair of electromagnets  40 ,  42  through the differentiator circuit  64 . Likewise, a second signal comprising the gyro torquer current i 34  drives the cross-axis forcer comprising the opposed pair of electromagnets  36 ,  38  through the differentiator circuit  66  in addition to directly driving the gyro forcer comprising the opposed pair of electromagnets  40 ,  42 .  
         [0033]    In operation, precession of the rotor  22  about the axis  34  results from the torquing of the rotor  22  with respect to the axis  32 . This is initiated by inputting current i 32  to the gyro forcer comprising the opposed pair of electromagnets  36 ,  38  to produce a rate dθ 34 /dt. The integration of this rate, represented by a block  59 , tilts the rotor about the axis  34  by an angle Δθ 34 . Such tilting of the rotor  22  offsets the rotor&#39;s spin axis  26  from null with respect to the gyro case. Within an upper feedback loop for controlling slewing with respect azimuth, this offset is detected by gyro pickoff comprising opposed pair of electromagnets  48 ,  50  and a pickoff voltage V 34  is generated. Such pickoff voltage V 34  is amplified, servo compensated by the servo  52  and then applied to the azimuth gimbal motor  56  to cause the camera  10  to slew about azimuth gimbal axis  16  at a rate dφ A /dt. The integration of the slewing rate at an integrator  60  results in the slewing or sweeping of the camera  10  through an angle Δφ A . The slewing angular displacement of the camera  10 , Δφ A , is compared with the precession displacement of the rotor  22 , Δθ 34 , at a difference junction  62  of the servo loop  52 . The slewing motion of the camera  10  is completed at such time as the angular diaplacements of the rotor  22  and the camera  10  are equal to one another. This mode of operation is likewise employed for servoing the pitch axis (note the like feedback loop for controlling slewing with respect to pitch that lies below the just-described feedback loop for controlling slewing with respect to azimuth).  
         [0034]    At the same time that the current i 32  is applied to the forcer comprising the opposed pair of electromagnets  36 ,  38 , it is applied to a cross-axis circuit  64 . An output of the cross-axis circuit  64  is received as an input to gyro forcer comprising the opposed pair of electromagnets  40 ,  42 . (Likewise, a second cross-axis circuit  66  is arranged to receive an input current i 34  for slewing the rotor  22  with respect to the axis  32  and to direct the output of the circuit  66  to the forcer comprising the opposed pair of electromagnets  36 ,  38 ).  
         [0035]    It shall be seen that each of the cross-axis circuits  64  and  66  is arranged to provide an output that is the derivative of an input function and a gain such that, when input to the cross-axis gyro forcer, a precession torque will be generated that cancels the undesired deflection of the rotor  22  with respect to the axis of initial application of torque. For example, the output of the cross-axis circuit  64  drives the gyro forcer comprising the opposed pair of electromagnets  40 ,  42  to torque the rotor  22  to precess with respect to the axis  32  by an amount equal and opposite to the deflection of the rotor  22  with respect to the axis  32  by the direct application of the current i 32  to the forcer comprising the opposed pair of electromagnets  36 ,  38 .  
         [0036]    Referring to equations 1, 2, 4 and 5, it can be seen (with respect to the application of the current i 32 ) that, while the rate of precession dθ 34 /dt is a function of the torque T 32 , the rate dθ 32 /dt is a function of the rate of torquing dT 32 /dt. In the invention, the undesired deflection of the rotor  22  with respect to the direct axis (as opposed to the axis about which precession takes place) is overcome by directly applying a function to the cross-axis gyro torquer pair that generates a precession torque −dT 32 /dt. Such a cross-axis circuit  64  or  66  is characterized by a LaPlace transfer function of the form  
           T ( s )= s /(2 π·f   nut )  (6)  
         [0037]    which defines a derivative function. When a periodic oscillatory signal of the form sin 2πft, for example, directly torques the rotor  22 , the output of the cross-axis circuit is of the form (f/f nut )cos 2πft. Likewise, when a periodic oscillatory signal of the form cos 2πft directly torques the rotor  22 , the output of the cross-axis circuit is of the form −(f/f nut ) sin 2πft. In either case, the output from the cross-axis circuit generates a precession torque that is equal and opposite to the undesired direct torquing of the rotor.  
         [0038]    While the above transfer function for the cross-axis differentiator circuit will produce the desired offsetting torquing signal for nulling cross-axis motion, it provides a derivative gain that results in noisy operation due to the increase of gain with frequency. For this reason, better performance, coupled with the desired cancellation of cross-axis motion, is provided by a circuit arrangement in which high frequency gain is rolled off to prevent noise degradation. Such a differentiator circuit for use in the invention is illustrated by the schematic diagram of FIG. 4. The circuit  68  (used in cross-axis circuits  64  and  66 ) comprises an operational amplifier  70  that includes a feedback capacitor C f  in parallel with a feedback resistor R. An input capacitor C completes the circuit. Such a circuit is characterized by a transfer function of the form T(s)=V o /V in =ks/(s+2πkf nut ) with RC=½πf nut  and k=C/C f .  
         [0039]    [0039]FIG. 5 is a graph of the frequency response of the transfer function of the circuit  68 . As can be seen, at lower frequencies, the gain is s/2πf nut  as desired. At higher frequencies, gain reaches and maintains the constant value k, thereby avoiding the noise problem at high frequencies.  
         [0040]    While the invention has been described with reference to its presently-preferred embodiment, it is not limited thereto. Rather, this invention is limited only insofar as it is defined by the following set of patent claims and includes within its scope all equivalents thereof.