Abstract:
An accelerometer module for measuring acceleration in a stabilized platform system includes a power supply configured to accept an input AC reference signal and to generate a regulated DC signal and a reference signal in phase with the input AC reference signal; an accelerometer configured to receive the regulated DC signal from said power supply and to generate an output signal in response to an external force acting on the accelerometer module and an analog multiplier unit configured to receive the output signal from the accelerometer and the reference signal from the power supply, and to modulate the reference signal with the output signal so as to output a modulated accelerometer signal. A method for servicing a stabilized platform system comprising is also provided.

Description:
BACKGROUND 
     This application generally relates measuring forces on a body, and in particular, an accelerometer used to measure forces in a stabilized platform system. 
     The Raytheon Company M-65 Airborne Tube-launched Optically-tracked Wire-to-command-link (“TOW”) system and the Kollsman Inc. Night Targeting System (similar to the M-65 system) use a pair of accelerometers to sense aircraft movement and assist the operator in tracking targets. These stabilized platform systems may be incorporated in vehicles and aircraft, such as the AH-1W USMC or other AH-I1 Cobra Attack Helicopter. It is estimated that there are at least 400 of these systems (or variants thereof) in use worldwide. 
     The TOW airborne system uses an accelerometer for each axis of its stabilized sight to sense aircraft movement with respect to the line of sight. This is used to correct the line-of-sight position for aircraft motion in order to maximize the probability of hit of the system. The conventional accelerometer module used in these system may utilize to Honeywell International Inc. Part No. 258970 (or equivalent), which interfaces with a mount to become an accelerometer module (e.g., Raytheon Part Nos. 3439031 or 3439030), or equivalents thereof. Two accelerometer modules may be provided, one for measuring acceleration in each of the elevation and azimuth directions. 
     Conventionally, the accelerometer modules outputs are integrated using electronics and this output is used to move the operator&#39;s line-of-sight to maintain the crosshair on a missile target, by correcting or compensating for the movement of the aircraft. The accelerometer modules are connected a 20 VAC, 900 Hz signal and provide a phase referenced AC signal for the motion compensating system. A similar system is disclosed, for example, in U.S. Pat. No. 3,829,659, incorporated herein by reference in its entirety. 
       FIG. 1  shows a block diagram of conventional accelerometer module  100  used for measuring acceleration in a stabilized platform system. Reference signal  110  is input to mechanical accelerometer  130 . Typically, reference signal  110  is a 20 VAC, 900 Hz signal. In response to acceleration  50  applied to accelerator  130 , accelerometer  130  generates an amplitude-modulated signal  160  which is provided to the avionics of the stabilized platform system for further processing. 
     Problems with conventional accelerometer modules  100  are that they are constructed using a mechanical strain gauge type accelerometer  130 , which are very fragile and easily damaged due to severe shock and procuring original equipment manufacturer (OEM) replacement accelerometers may be very difficult and costly, particularly for systems with relatively long service lives. 
     SUMMARY 
     According to an embodiment, an accelerometer module for measuring acceleration of a stabilized platform system includes: a power supply configured to accept an input AC reference signal and to generate a regulated DC signal and a reference signal in phase with the input AC reference signal; an internal accelerometer component configured to receive the regulated DC signal from said power supply and to generate an output signal in response to an external force acting on the accelerometer module; and an analog multiplier configured to receive the output signal from the accelerometer component and the reference signal from the power supply, and to modulate the reference signal with the output signal so as to output a modulated accelerometer signal. 
     According to another aspect, a system for measuring acceleration in a stabilized platform includes: two of the above-described accelerometer modules for measuring acceleration in each of two reference directions of the stabilized platform system. The two reference directions may correspond to the elevation and azimuth axes. 
     According to another embodiment, a method of servicing a stabilized platform system includes: removing an existing accelerometer module from the stabilized platform; and installing the above-described accelerometer module in the stabilized platform using existing procedures. 
     Other features and advantages of one or more embodiments of the present application will seem apparent from the following detailed description, and accompanying drawings, and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present application will now be disclosed, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, in which: 
         FIG. 1  shows a block diagram of the conventional accelerometer module used for measuring acceleration in a stabilized platform system; 
         FIG. 2  shows an exemplary schematic for an accelerometer unit, in accordance with an embodiment, where  FIG. 2A  shows one exemplary embodiment of the power supply shown in  FIG. 2 ; 
         FIG. 3  shows an exemplary system including an accelerometer module installed in a stabilized platform system, in accordance with an embodiment; 
         FIGS. 4(   a ) and ( b ) show an accelerometer module, in accordance with an embodiment; and 
         FIG. 5  shows an exemplary method for servicing a stabilized platform, in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  shows an exemplary schematic for accelerometer module  200 , in accordance with an embodiment. Accelerometer module  200  may be configured for use in a stabilized platform system. 
     In one exemplary embodiment, accelerometer module  200  generally includes power supply  220 , accelerometer  220 , and analog multiplier unit  250 . The various components may be mounted, for example, on a printed circuit board. 
     Power supply  220  is configured to provide well-regulated power for accelerometer  230  as well as power for the analog multiplier unit  350 . In one implementation, input power signal  210  supplied to power supply  220  may be 20 VAC, 900 Hz. 
     Power supply  220  is configured to generate three power signals from input power signal  210 . In particular, power supply  220  provides reference signal  222 , which is substantially in phase with the input power form  210 . In some implementations, input power signal  210  may be the same as input reference signal  110  ( FIG. 1 ). In addition, power supply  220  supplies a DC output signals  224 ,  226 , respectively to both accelerometer  230  and analog multiplier unit  250 . 
       FIG. 2A  shows one exemplary embodiment of power supply  220  shown in  FIG. 2 . Power supply  220  may include transformer  221  configured to step down input power signal  210 , rectifier  223  configured to covert the stepped-down AC voltage to DC voltage, and power regulator  225  configured to provide a regulated DC voltage. 
     In one implementation, transformer  221  may provide reference signal  222 . For example, transformer  221  may have one or more secondary coils including a designated secondary coil which provides reference signal  222  from input power signal  210 . It will be appreciated that other devices could also be used to provide reference signal  222 , such as, for example, an optically coupled isolator (not shown). Ideally, reference signal  222  will have little to no phase shift from input power signal  210 . 
     Providing regulated DC power to accelerometer component  230  and modulator  250  insures that the accuracy requirements will be met. For example, power supply  220  may be configured to supply regulated 5 VDC and 12 VDC power from  224 ,  226  respectively. Rectifier  223  may be a single-phase full wave bridge rectifier. Power regulator  225  may be an integrated circuit (IC), such as, for example, Model No. LT3010/LT3010-5 micropower linear regulator manufactured by Linear Technology. Other voltage outputs may similarly be provided, as desired. 
     With reference to  FIG. 2 , in some implementations, attenuator  240  may also be provided for enabling adjustment of gain of reference signal  244  supplied to the modulator. For example, attenuator  240  may include a trimmer  242 , such as a potentiometer, variable resistor, rheostat, or the like, to adjust the gain. 
     Accelerometer  230  may be a solid-state accelerometer or differential capacitance accelerometer which provides or whose output can provide a time-varying DC signal  232  in response to acceleration  50  applied to the accelerometer module. Unlike conventional strain-gauge and other mechanical type accelerometers  130  ( FIG. 1 ), accelerometer  230  is much more robust. In one implementation, accelerometer  230  may be a Model ADXL322 accelerometer manufactured by Analog Devices, Inc. This accelerometer, in particular, is very rugged and has been rated to survive 10,000 Gs. Further, the accelerometer module embodiment provides addition of level control and gain adjustment for precise alignment. An integrator may also be provided that is configured to provide a low-pass filter for the accelerometer component time-varying DC signal  232 . 
     Analog multiplier unit  250  may include an analog 4-quadrant multiplier chip to modulate reference signal  244  with time-varying DC signal  232  output from accelerometer  230 . In one implementation, multiplier  250  may be a Model AD534 internally trimmed integrated circuit (IC) multiplier manufactured by Analog Devices, Inc. The accelerometer module may have an operational response of approximately 0-10 Hz. 
     In one implementation, analog multiplier unit  250  generates amplitude modulation (AM) using a carrier wave having the same frequency as input signal  210 . For example, modulated acceleration signal  260  may have a phase and amplitude, where the phase indicates acceleration polarity and the amplitude (of the envelope) indicates the acceleration magnitude. Other modulation algorithms, such as frequency or phase modulation could also be used. Modulated acceleration signal  260  may be adjusted, via trimming device  252 , to bias the output of the modulator (e.g., 0 G bias). Trimming device  252  may be a potentiometer, trimmer, variable resistor, rheostat, or the like. 
     The modulated acceleration signal  260  may conform to the form, fit, and function of existing electronic hardware and in the stabilized platform system. Downstream in such systems, phase-sensitive de-modulation (and further modulation) of the modulated signal occurs. See, for example, U.S. Pat. No. 3,829,659, mentioned above. 
     For a 900 Hz input signal  210 , reference signals  222 ,  244  will be approximately 900 Hz. While 900 Hz AC voltage is disclosed, it will be appreciated that other military or avionics power forms might also be used, such as, for example, 400 Hz AC. Thus, any mention of 900 Hz or 400 Hz AC voltage is not to be construed as limiting. 
       FIG. 3  shows an exemplary system  300  including an accelerometer module  310  installed in a stabilized platform system  320 , in accordance with an embodiment. 
     Stabilized platform system  320  may be a M-65 Telescopic sight unit or similar unit which utilizes a 20 VAC 900 Hz signal as the power source, as well as the reference for the amplitude modulated output. Accelerometer component  310  may be formed on a printed circuit board, which is installed into housing  330 . To secure accelerometer component  310  into housing  330 , a suitable potting or encapsulating compound may be utilized. In one implementation, accelerometer  310  may be configured to couple to the existing housing in the stabilized platform. 
     As shown in  FIG. 3 , housing  330  may be configured to be mounted on balanced gimbals in stabilized platform system  320 . In one embodiment, housing  330  may be a cylindrical can or box, for example, constructed from aluminum or lightweight alloys so as to reduce weight. Plastics could also be used. Housing  330  may provide an interface with the existing stabilized platform system. 
     Accelerometer module  310  may be adapted for use with the existing hardware. Accordingly, accelerometer module  310  provides a form, fit and function replacement to the conventional accelerometer unit module using a Honeywell International Inc. Part No. 258970 or a substantially equivalent device manufactured and/or distributed by another company. 
     In one implementation, input signals  210  and modulated acceleration signal  260  ( FIG. 2 ) may be transmitted using the existing connector cables used with the conventional accelerator unit. There may be one or more plug-in connections. For example, input power signal  210  may be supplied by connector  340 . Modulated acceleration signal  260  may be output from the accelerometer module  310  via connector  350 . Connectors  340 ,  350  may be pin-type connectors, as known in the art. As such, modulated acceleration signal  260  may appear substantially similar to modulated output signal  160  ( FIG. 1 ) to the stabilized platform system. 
     Two or three accelerometer modules  310  may be installed for measuring acceleration in each of elevation and azimuth (or a x, y, z coordinate frame) on stabilized platforms, such as, for example, Raytheon Part Nos. 3234001-110, -134, or a substantially equivalent device manufactured and/or distributed by another company. In one implementation, two accelerometer modules  310  having different keyed pin-type connectors, one for elevation and one for azimuth, may be provided. Other indicia may be used for identifying the two. As such, service personnel may distinguish between the two, although the inner working of each of accelerometer units  310  may otherwise be similar. 
       FIG. 4(   a ) and ( b ) show an accelerometer module, in accordance with an embodiment.  FIG. 4(   a ) shows a cut-away view of accelerometer module  400 A before encapsulation. In one embodiment, the various elements, including the accelerometer component, may be assembled on printed circuit board  410  which is installed in housing  420 . Connectors  440 ,  450  may be attached to printed circuit board  410 . 
     Housing  420  may be generally cylindrical having central recessed portion  422 . For example, housing  20  may be a CNC machined block of metal, although, steel has been found to provide a good weight match with existing hardware requirements. 
     The printed circuit board  410  is inserted into recessed portion  422  (a portion thereof is shown “cut-away” in the figure). Once the printed circuit board  410  is inserted into recessed portion  422 , there may be a mechanical adjustment procedure in which the accelerometer component is aligned with the reference axes of housing  420 . Alignment holes  428  may be provided at various locations on housing  420  to facilitate alignment using an alignment fixture (not shown). 
     After alignment, the printed circuit board  410  may be secured to the housing  420  via fasteners  424 . Fasteners  424  may be screws, although it will be appreciated that other fastening elements and/or methods could also be used. Potting material  430  may then be molded or otherwise formed over the exposed portion of the printed circuit board  410  to encapsulate the printed circuit board  410  within the housing. 
       FIG. 4(   b ) shows encapsulated accelerometer module  400 B before installation. In one embodiment, encapsulated accelerometer module  400 B may have a nominal diameter D of approximately 1.5-1.6 inches and a height H of about 0.6-0.7 inch. 
       FIG. 5  shows an exemplary method  500  for servicing a stabilized platform, in accordance with an embodiment. 
     Beginning in step  510 , a stabilized platform system may be returned for servicing. For example, the stabilized platform system may be a Raytheon Company M-65 Airborne TOW system or the Kollsman Inc. Night Targeting System. As noted above, these systems may be incorporated in an aircraft, such as the AH-1W USMC Cobra or other type of Attack Helicopter. Other systems which utilized accelerometer units may similarly be serviced or refurbished. 
     One or more service centers may be provided, which include specialized equipment and personnel for serving these systems. The services centers may be located on military bases, government installation, and/or on premises that are owned or operated by defense contractors. Moreover, these service centers may provide other services and repairs for the vehicles or aircraft which utilize the stabilized platform systems. 
     In step  520 , the existing accelerometer unit is removed and subsequently disposed of. As will be appreciated, various fasteners, power and/or output cables, connectors, wire-harnesses, etc. may need to be released and/or removed to provide access to the accelerometer unit. Typically these system uses a strain-gauge and other mechanical type accelerometers, such as, for example, Raytheon Part Nos. 32349031 or 3439030, or equivalents thereof. As noted above, the stabilized platform system may include two such accelerometers, one for measuring acceleration in each of the elevation and azimuth directions. The removed accelerometer units may be discarded or dismantled for recycling the components thereof. 
     Next, in step  530 , one or more accelerometer units, according to an embodiment of the present application, is installed in the stabilized platform system. The accelerometer unit may be configured to couple to an existing mount such as the Raytheon Part Nos. 32349031 or 3439030, or equivalents thereof. This may require reassembling the various fasteners, cables, connectors, wire-harnesses, etc. to retain the accelerometer unit. The input power and output cables may be coupled with the acceleration unit. In one implementation, the accelerometer unit may have different keyed pin connectors, one for elevation and one for azimuth. 
     Finally, in step  540 , the stabilized platform system is returned to duty. Other systems which utilized accelerometer units may similarly be serviced or refurbished. 
     Accordingly, the proposed accelerometer module provides a form, fit and functional replacement to the conventionally known and potentially obsolete accelerometers used in currently fielded stabilized platform systems. 
     While this disclosure has been described in connection with what is presently considered to be the most practical embodiments, it is to be understood that it is capable of further modifications and is not to be limited to the disclosed embodiments, and this application is intended to cover any variations, uses, equivalent arrangements or adaptations of the disclosure following, in general, the principles of the disclosure and including such departures from the present disclosure as come within known or customary practice in the art to which the application pertains, and as may be applied to the essential features hereinbefore set forth and followed in the spirit and scope of the appended claims.