Abstract:
A solid-state electronic avionics display instrument includes a needle-like display mounted within a housing in a manner that emulates the manner in which prior electro-mechanical needle indicator mechanisms have been mounted in such housings so as to indicate a value along an arcuate scale. New aircraft can be fitted with such instruments initially, while existing aircraft can be retrofitted with such instruments to replace existing electro-mechanical mechanism-based avionics instruments.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates generally to avionics instrumentation and, more specifically, to outfitting and/or retrofitting aircraft with electronic instruments.  
         [0003]     2. Description of the Related Art  
         [0004]     Aircraft cockpit flight instrumentation (avionics) indicators may employ a variety of mechanisms, including vertical scale indicator mechanisms, radial dial indicator mechanisms, and pivoting mechanisms, such as those of compasses and attitude gyro indicators.  
         [0005]     Many avionics indicators have long been entirely electro-mechanical, with needle pointers, rotating wheels, bands, spheres and so forth moving over or within a fixed dial or housing. For example, a needle pointer gauge that is used to indicate an operating pressure or temperature commonly has a circular housing, with a scale printed circumferentially along its perimeter, and a pointer needle that pivots about a central axis and alongside the scale to indicate the temperature or pressure. A pilot can, for example, determine at a glance that a measured temperature or pressure is abnormal when the pointer needle is pointing to a point above or below where it normally points during nominal conditions. A pilot can read the specific temperature or pressure by noting the point on the scale to which the pointer needle points.  
         [0006]     Although fully solid-state display-based technologies, such as multi-function display instruments, have begun to supplant electro-mechanical indicators in newer commercial and military aircraft, in many existing aircraft the older, electro-mechanical indicators are still very common. For example, in general aviation (i.e., small private aircraft) pilots and regulatory authorities have been slower to accept changing over to solid-state display-based technologies. This is so in part because general aviation pilots generally are more accustomed to the appearance of mechanical instruments. For example, in older CESSNA CITATION aircraft, electro-mechanical vertical scale indicators are used to indicate fuel flow, inter-turbine temperature, fan speed and turbine speed. It would be desirable to retrofit such older aircraft with solid-state electronic display-based instruments, but retrofitting these aircraft with the latest multi-function display instruments generally is impractical and costly because their form factors differ greatly from those of the original instruments, and it can be difficult to obtain approval from regulatory authorities.  
         [0007]     Accordingly, it can be seen that a need yet exists for a method and apparatus to replace existing electro-mechanical avionics display instruments, while allowing the replacement instrument to be fitted within the existing openings of the aircraft. It is to the provision of such a method and apparatus that the present invention is primarily directed.  
       SUMMARY OF THE INVENTION  
       [0008]     Briefly described, in a first preferred form the present invention comprises a replacement avionics display instrument for use in existing aircraft to replace an instrument having an electro-mechanical display. The replacement avionics display instrument comprises an avionics housing either removed from such an aircraft or constructed to replicate the same. If the housing is one removed from an aircraft, the existing electro-mechanical display is removed. In place of the electro-mechanical display in the original instrument, an electronic display is substituted in the replacement avionics display instrument. In one form, the electronic display can comprise an LED display. In another form, the electronic display can comprise an LCD display. The replacement avionics display further includes electronic circuitry to allow the electronic display to be driven by the electrical input signals formerly (or normally) driving the electro-mechanical display. In other words, the replacement avionics display can be connected to the electrical inputs of the aircraft and installed in the existing opening in the aircraft without requiring any modification to the aircraft. Advantageously, this allows modern instrument technology to be retrofitted into older aircraft in a simple, direct and relatively inexpensive manner.  
         [0009]     As described above, one way to accomplish this is to re-use the old avionics instrument housing. Alternatively, a reasonable facsimile of the housing can be fabricated and used instead of the old housing. Such might be preferable in situations where the old housing is damaged, corroded, etc.  
         [0010]     One ready application for such a hybrid or replacement instrument is to replace electro-mechanical needle indicator instruments. These are particularly attractive targets for such a replacement instrument due to the relatively high cost of repairing or replacing the electro-mechanical needle indicator movements contained within the instrument.  
         [0011]     In another preferred form the invention comprises a solid-state electronic avionics needle scale indicator that includes one or more needle-like displays mounted within a housing in a manner that emulates the manner in which prior electro-mechanical indicator mechanisms have been mounted in such housings so as to indicate a value along a peripheral readable scale. Solid-state displays are not only more reliable than electro-mechanical mechanisms but also can be easier for pilots to read because they are generally clearer and brighter. Electronic circuitry in the housing interfaces the display with input signals received from aircraft sensors, such as for example of the type commonly included in general-aviation aircraft, such as fuel level sensors, inter-turbine temperature sensors, fan speed sensors, altitude, hydraulic pressure, etc.  
         [0012]     In one aspect of the invention, existing electro-mechanical needle indicators that have been installed in such aircraft or that are intended for installation in such aircraft can be replaced with electronic displays emulating the electro-mechanical needle displays. Retrofitting such indicators in this manner or, alternatively, providing new indicators that otherwise appear similar to those that have long been used in general-aviation aircraft but employing electronic LED (or LCD) displays instead of electro-mechanical mechanisms, is economical and promotes pilot confidence and the acceptance of such replacement indicators by regulatory authorities.  
         [0013]     In another form, the present invention comprises a method of retrofitting avionic display instruments, the avionics display instrument being of the type having a housing containing an electro-mechanical movement adapted to be driven by electrical input signals from an aircraft to which the avionics display instrument is mounted. The method includes the steps of removing the avionics display instrument from the aircraft and removing the electro-mechanical movement from the housing of the avionics display instrument. The method also includes the step of mounting an electronic display in the housing in place of the electro-mechanical movement, the solid state electronic display being adapted to be driven by the same electrical input signals as the original avionics display instrument. The method also includes the step of remounting the avionics display instrument in the aircraft. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1A  and  FIG. 1B  are perspective front and rear views of a replacement avionics display instrument for use in new or existing aircraft according to a preferred form of the present invention, here illustrated as having a simulated needle indicator.  
         [0015]      FIG. 2  is a perspective view of the replacement avionics display instrument of  FIGS. 1A and 1B  shown with a housing portion thereof removed.  
         [0016]      FIG. 3  is an exploded perspective view of the replacement avionics display instrument of  FIGS. 1A and 1B .  
         [0017]      FIGS. 4A and 4B  are front and perspective views of a faceplate portion of the replacement avionics display instrument of  FIGS. 1A and 1B .  
         [0018]      FIG. 4C  is a side sectional view of a portion of the faceplate portion of  FIGS. 4A and 4B .  
         [0019]      FIG. 4D  is a pictorial representation of an LED display for the replacement avionics display instrument of  FIGS. 1A and 1B  according to another exemplary embodiment of the present invention.  
         [0020]      FIG. 5  is a schematic illustration depicting electronic circuitry of the replacement avionics display instrument of  FIGS. 1A and 1B . 
     
    
     DETAILED DESCRIPTION  
       [0021]     In the following description, like reference numerals indicate like components to enhance the understanding of the invention through the description of the drawings. Also, although specific features, configurations and arrangements are discussed below, it should be understood that such is done for illustrative purposes only. A person skilled in the relevant art will recognize that other steps, configurations and arrangements are useful without departing from the spirit and scope of the invention.  
         [0022]      FIG. 1A  and  FIG. 1B  are perspective front and rear views of a replacement avionics display instrument  10  for use in existing aircraft according to a preferred form of the present invention, here illustrated as having a faceplate  12  with a simulated needle indicator or LED display  14 . The replacement avionics display instrument  10  utilizes highly reliable solid-state technology to display information to the crew.  
         [0023]     As shown in  FIGS. 1A, 1B , and  3 , the avionics display instrument  10  includes a housing or casing  16 . The casing  16  can be that of an existing instrument or it can be similar to such a housing. As shown more clearly in  FIG. 3 , the casing  16  includes a tubular body  18 , a glass cover  20  for the front of the tubular body, a suitable bezel  22  for retaining the glass cover, and a backplate or rear cap  24 , which together provide an enclosed structure. A rear coupling connector  26  extends from the rear cap  24 . In the depicted example, the bezel  22  has an approximately two-inch diameter, and the tubular body  18  is approximately 1.932 inches long with a wall thickness of about 0.060 inch. The casing  16  itself can have an approximately two-inch diameter and a depth of about 2.75 inches, exclusive of the rear coupling connector  26 , although those skilled in the art will understand that the casing can be any suitable size for use with a cockpit control panel. Preferably, the bezel  22 , tubular body  18 , and rear cap  24  are fabricated from aluminum and are conversion coated and painted black for corrosion protection. Also preferably, the glass cover  20  provides an ITO coating for EMI protection and is epoxied to the bezel. Additionally, the glass cover  20  is preferably transparent or translucent so that the user can easily read the faceplate  12  therebehind. To ensure a good environmental seal, the bezel  22  can be soldered to the tubular body  18 , and the rear cap  24  can incorporate an O-ring seal  28 . Moreover, a resilient gasket  30  is provided at the end of the casing  16  having the glass cover  20 , such as a rubber gasket, surrounding the casing for providing a seal and resilient mounting between the instrument  10  and the instrument panel.  
         [0024]     Preferably, the rear cap  24  is attached to the tubular body  18  by installing a plurality of fasteners, preferably three countersunk screws  32 ,  34 , and  36 , through a plurality of openings  38 ,  40 , and  42 , in the casing  16  and into openings  44  (only one of which is shown in the figures) in the rear cap. Those skilled in the art will understand that various types of fasteners, as well as the number of fasteners, and fastening techniques can be employed without deviating from the scope of the present invention. A rear hex nut  45  is threaded on to a threaded portion of the connector  26  to secure the rear cap  24  the connector. Preferably, lightweight components and materials are used to minimize the overall weight of the instrument  10 . In one embodiment, the estimated weight of each instrument  10  is approximately 0.47 pounds, although those skilled in the art will understand that the weight can vary.  
         [0025]     The generally cylindrical rear coupling connector  26  of the rear cap  24  located on the back of the instrument  10  is part of an electronic subassembly  50  (see  FIGS. 2 and 3 ) and receives input signals from one or more avionics sensors (not shown) when the instrument  10  is installed in an aircraft and communicates these input signals to circuitry in the electronic subassembly  50 . Preferably, the instrument  10 , through the rear coupling connector  26 , can interface directly with aircraft power and with the sensor signals that provide the data to be displayed. Also preferably, the instrument  10  can be self-contained such that no external signal conditioning electronics would be required.  
         [0026]     The rear coupling connector  26  includes a plurality of male connector pins  47  for mating with female connectors in a wiring harness of the instrument control panel. Typically, the rear coupling connector  26  is “keyed” to ensure proper connection of the cabling to the instrument  10 . As shown more clearly in  FIG. 1B , in this regard the rear coupling connector has at least one and preferably a plurality of grooves  48  arranged in a particular configuration for mating with one or more ribs on the instrument panel port (not shown). Preferably, the grooves  48  extend from the distal end of the rear connector  26  to the rear cap  24  or to some point between the distal end and the rear cap. Also preferably, the pins  47  of the rear connector  26  do not extend all the way to the distal end of the rear coupling connector. Thus, the pins  47  are protected from being inadvertently damaged.  
         [0027]     The faceplate  12  is positioned behind the glass cover  20  of the instrument  10  and is described in more detail below with reference to  FIGS. 4A and 4B . The electronics subassembly  50  is positioned behind the faceplate. The electronics subassembly  50  includes an LED circuit card assembly (hereinafter referred to as “CCA”)  52 , a power supply CCA  54 , a system controller CCA  56 , and a connector CCA  58 , all of which are structurally connected to one another by a carrier or support frame  60 , as shown in  FIGS. 2 and 3 . Preferably, the interfaces between the circuit card assemblies are implemented with male/female multi-pin socket connectors to minimize assembly time and facilitate service and repair.  
         [0028]     The carrier  60  is a generally rectangular rigid frame that includes four threaded connectors  62  at or near its corners on each side for connecting the power supply CCA  54  to one side and the system controller CCA  56  to the opposite side. The power supply CCA  54  has openings in its corners, which generally align with the connectors  62  of the carrier  60 . The system control CCA  56  has openings in its corners, which generally align with connectors of the carrier  60  on the opposite side. Thus, the power supply CCA  54  can be connected to one side of the carrier  60  by securing four fasteners  64 , such as screws, through each opening in the power supply CCA  54  and into the connectors  62  of the carrier  60 . Optionally, the connectors  60  can comprise self-locking inserts to prevent unintended loosening of the screws. Similarly, the system controller CCA  56  can be connected to the opposite side of the carrier  60  by securing four fasteners  64 , such as screws, through each opening in the system controller CCA  56  and into the connectors  62  of the carrier.  
         [0029]     Additionally, the carrier  60  includes nubs or indexing pins on its front and rear surfaces for connecting the LED CCA  52  to its front and the connector CCA  58  to its rear. Preferably, two nubs  70  and  72  located on the front surface of the carrier  60  mate with openings  74  and  76  of the LED CCA  52  so as to position the LED CCA relative to the carrier. Two nubs  80  and  82  located on the rear surface of the carrier  60  mate with openings  84  and  86  of the connector CCA  58  to position the connector CCA relative to the carrier.  
         [0030]     Those skilled in the art will understand that the carrier  60  can have various shapes, sizes, and configurations and be within the scope of the present invention. Moreover, various fasteners  64  and fastening techniques can be employed to attach the various circuit card assemblies to the carrier without deviating from the scope of the present invention. In an alternative embodiment, no carrier is used, but instead the circuit card assemblies can be coupled more or less directly to one another with electrical couplings that also serve as the mechanical couplings.  
         [0031]     The LED CCA  52  includes a plate  90  with a display  14  of a plurality of LEDs  92  (light emitting diodes) arrayed adjacent to one another and in the form of an arc in which a selectable length or band can be displayed. Additionally, the LED CCA  52  includes additional LEDs  94  behind lettering and scales on the faceplate  12  so that such scales are visible in low light. Optionally, one or more of the additional LEDs  94  can be an indicator light for indicating an atypical condition. Preferably, an ambient light sensor  96  on the front of the plate  90  of the LED CCA senses the ambient light level and transmits the ambient light level back to the system controller CCA  56 , as described in more detail below.  
         [0032]     In the depicted embodiment, thirty-nine LEDs  92  are used to display the current hydraulic pressure, although those skilled in the art will understand that fewer or more LEDs can be used to accomplish a satisfactory result. The more LEDs that are activated, the longer the band appears. Thus, when the electronics subassembly  50  receives an input signal from an aircraft sensor, such as a sensor for hydraulic pressure, it activates a number of adjacent LEDs  92  in proportion to the hydraulic pressure represented by the input signal. A pair of connector sockets  98  (only one of which is shown in the figures) having male connector pins is located on the back of the plate  90 , which mate with sockets  100  of the system control CCA  56  and power supply CCA having female connector pins.  
         [0033]     Referring now to  FIGS. 4A and 4B , the faceplate  12  includes a generally arced or banana-shaped diffuser lens  110  set in a display disk  111  showing a scale or legend  112 . The diffuser lens  110  provides a sharp transition from light to dark and blends the light emitted from individual LEDs  92  together. As shown in more detail in  FIG. 4C , the diffuser lens  110  includes a base  114 , typically constructed of a clear acrylic, and a film  116  adhered to the base. Preferably, the film  116  is black (or dark) and translucent and has a textured surface, preferably a somewhat nubbly outer surface. The base  114  of the diffuser lens  110  can be the faceplate  12  itself with the translucent black film  116  applied thereon. Alternatively, the diffuser lens  110  can be a separate piece inserted and secured in a complementing aperture in the faceplate  12 . The diffuser lens  110  can be constructed of commercially available lens material, such as that used in front of rear projection televisions. Preferably, the diffuser lens  110  permits viewing at side viewing angles, and is thus readable over viewing angles of +/−60 degrees horizontal and +/−30 degrees vertical. The diffuser lens  110  can be treated to minimize reflected glare.  
         [0034]     The scale or legend  112  can be painted or printed on the faceplate  12 , etched into the faceplate, or applied as a decal or sticker to the faceplate. Those skilled in the art will understand that various methods and techniques may be used to create the scale  112  and still be within the scope of the present invention. In one embodiment, white indicia are applied on a black background so as to provide an easy-to-read legend in daylight mode. The scale or legend  112  can be backlit with green LEDs at night to appear green on a black background for better visibility. Optionally, the faceplate  12  can include a generally rectangular-shaped aperture  118  for viewing an optional secondary, digital display  120  of the LED CCA  52  (see  FIG. 3 ).  
         [0035]     The LEDs  92  of the LED CCA  52  appear through the generally banana-shaped diffuser lens  110  in the faceplate. In one embodiment, the LEDs  92  in the linear array sequentially light to create a continuous light bar from the minimum reading on the instrument  10  to the current, sensed value. Alternatively, the LEDs  92  can be operated so that only one or a few LEDs are illuminated at any one time, to more closely approximate the look of a conventional needle. In still another embodiment, the LEDs  92  can be arranged as shown in  FIG. 4D  so as to more look more like a conventional, mechanical needle. The resolution of data that may be presented can be limited by the minimum physical spacing that can be achieved between adjacent LEDs  92 . In general, this resolution is sufficient to meet the requirements in many applications. The LED arc presentation provides the quick-look indication of current status that is required for most references to the indicator by the crew.  
         [0036]     In the depicted embodiment, the LEDs  92  preferably are green, bright, and moderately narrow band emitters having a peak output between about 520 and about 530 nanometers (nm). Also preferably, this energy is filtered to remove the “tail” of the output band beyond 600 nm, to allow compatibility with Generation III Night Vision devices. Preferably, the indicia of the scale  112  are white for daytime use, but are transilluminated for nighttime use with filtered green lighting.  
         [0037]     In addition to the LED display  14 , which is the primary indicating means, the instrument  10  can optionally include, as secondary indicating means, a digital display  120  that numerically displays the same quantity represented by LED display  14  simultaneously with the lighting of the LEDs  92 . The digital display  120  can include a three digit, seven segments per digit, readout of the current sensed value. In embodiments of the invention in which the digital display  120  is included, a pilot can choose to read either type of display according to his or her preference. Many pilots are accustomed to instruments that are similar in appearance and function to the primary means, and therefore may prefer to read the LED display  14 . This digital display  120  may be useful in instances when a crewmember, for examples, wants to evaluate a specific indication in detail. Alternatively, other arrangements can be employed and still be within the scope of the present invention.  
         [0038]     Although the seven-segment LED  120  is intended for the display of numerals, the LED display can also show non-numeric characters. For example, if the instrument  10  is reading a level in a reservoir, then the LED  120  can show “FUL” if the reservoir is full, and “ADD” if the reservoir is not full and if fluid should be added. In one embodiment, readings between the minimum and the “ADD” level result in the display of the legend “ADD” in the seven-segment LED  120 . Sensor readings between the “ADD” level and the “FULL” level result in the seven-segment display  120  being blank. Sensor readings above the “FULL” level will result in the legend “FUL” being displayed in the seven-segment display  120 . Additionally, because reservoir indicators generally display the “ADD” and “FUL” legends associated with two different scales (ramp up and ramp down), preferably, the instrument  10  includes a discrete signal as an input that defines whether the ramp is up or down. This discrete signal may be generated by a position-sensing switch located at the ramp or by a manual switch located on the instrument control panel.  
         [0039]     Referring now to  FIG. 5 , the electronic circuitry of the replacement avionics display instrument  10  is depicted in a generally functional manner. The connector CCA  58  can be a motherboard having connector sockets that provide an interface between the connector CCA, the power supply CCA  54 , and the wiring harness in the instrument control panel. Preferably, the connector CCA  58  includes a set of configuration resistors  130  that define the identity of the gauge to the system control CCA  56  in order to determine which interface to use. Also preferably, there are no active components on the connector CCA  58 .  
         [0040]     The power supply CCA  54  connects directly into the connector CCA  58  through male/female multi-pin connector sockets and is responsible for protecting, filtering, and converting the input from a power supply to lower voltages. Transient clamping and EMI filtering  132  provide protection for MIL-STD-461E testing. Hold-up circuitry  134  provides the required hold-up during power interrupts. A DC to DC converter  136  converts input from a power supply, typically from a 28V DC power supply  138  (of the aircraft), to lower voltages of 5V, −5V, 3.3V, and 1.5V DC. In one embodiment, the estimated power used by the entire gauge is approximately 5 W at maximum brightness. Because power dissipation is low (approximately less than five watts at maximum brightness), the use of a fan to provide cooling air to the indicators is obviated.  
         [0041]     The system control CCA  56  connects directly into the connector CCA  58  through male/female multi-pin connector sockets and is responsible for interfacing with the hydraulic pressure transmitters, resistance temperature detectors, and fluid level transducers. Preferably, these inputs are filtered by an EMI filter and protector  142 , amplified and then converted into a digital value by an amplifier and an Analog/Digital Converter (ADC)  144 . In one embodiment, a field-programmable gate array (FPGA)  146  reads the ADC value and then calculates the appropriate pressure, temperature, or reservoir level. The appropriate calculation to be performed can be determined by the configuration resistors  130  on the connector CCA  58 . Once the value for the digital display  120  has been calculated, the FPGA  146  can use an internal look-up table to determine the number of LEDs  92  to illuminate in the arc. The FPGA  146  transmits both the number of LEDs  92  and the digital display  120  value to the LED CCA  52  through a serial interface. The brightness of the LEDs  92  in the arc, as well as the digital display  120 , is independently controlled by the system control FPGA  146 , which reads the NVIS switch and the ambient light level sensed by the ambient light sensor  96 . Preferably, there is no software on the system control CCA  56 . All of the processing functionality can be provided through the firmware loaded into the FPGA  146 . Those skilled in the art will understand that the configurations of the power supply CCA  54 , the system control CCA  56 , and the connector CCA  58  are exemplary and that the configurations and types may vary and still be within the scope of the present invention.  
         [0042]     The LED CCA  52  connects directly into the system control CCA  56  through male/female multi-pin sockets and receives control signals, power, and outputs status information. Preferably, the LED CCA  52  uses a serial interface from the system control FPGA  146  to determine which LEDs  92  to illuminate in the arc, as well as what segments to display in the digital display  120 . LED driver circuitry  166 , which is located on the back of the LED CCA  52 , reads the serial interface and turns on the LEDs  92  accordingly. The brightness values of the digital display  120  and the arc of LEDs  92  are individually controllable by the system control FPGA  146 . The FPGA  146  receives the ambient light level back sensed by the ambient light sensor  96  on the front of the LED CCA  52 . This light level is used to slowly adjust the brightness of the indicator such that it is readable in direct sunlight but also not too bright in the shade. The FPGA  146  also controls the LEDs  94  behind the faceplate lettering such that the legends are visible in low light. The LED CCA  52  also transmits status information back to the system control FPGA  146  to indicate if any LEDs  92  and  94  have failed to illuminate.  
         [0043]     A DDS waveform generator  148  receives commands from the system FPGA  146  and outputs a sine wave of a desired frequency. An amplifier  150  then scales the sine wave up into a larger AC signal. The EMI filtering and protection  152  reduces electromagnetic emissions and avoids susceptibility problems. The resulting signal  154  created by this chain of blocks excites or stimulates the aircraft sensor.  
         [0044]     An aircraft cockpit having any number of conventional (electro-mechanical mechanism-based) indicator instruments can be retrofitted with electronic display-based instruments  10 . Each such existing electro-mechanical indicator is removed from its opening in the cockpit control panel and replaced in the same opening with an electronic display-based indicator instrument  10  of the present invention. An indicator instrument  10  of the present invention can fit in the same panel opening as a conventional electro-mechanical indicator because they have the same form factor. As described above, the indicator instrument  10  that replaces the electro-mechanical indicator may even re-use the same housing. Pilots and regulatory authorities will be comfortable with and reassured by the presence of the familiar form factor and appearance of indicator instrument  10 . The operation of the LED display  14  emulates that of electro-mechanical needle indicator mechanisms because the length of the band displayed indicates the quantity measured by the corresponding avionics sensor, but pilots will notice and appreciate that LED and LCD displays  14  and  120  of the instrument  10  are clearer, brighter and thus easier to read than conventional electro-mechanical indicators.  
         [0045]     To assemble the instrument  10 , the electronic subassembly  50  is assembled by connecting the system control CCA  56  and the power supply CCA  54  to the carrier  60 . Each of the system control CCA  56  and the power supply CCA  54  are secured to the carrier  60  with four fasteners  64 . The connector CCA  58  is connected to the system control CCA  56  and the power supply CCA  54  by mating male multi-pin connector sockets of the system control CCA and the power supply CCA with the female multi-pin connector sockets of the connector CCA. In so doing, the nubs  80  and  82  on the rear side of the carrier  60  fit snugly within the openings  84  an  86  in the connector CCA  58 . The LED CCA  52  is secured to the front of the carrier  60  such that the nubs  70  and  72  on the front side of the carrier fit snugly within the holes  74  and  76  of the LED CCA.  
         [0046]     The faceplate  12  is stacked on the front of the electronics subassembly  50  and the glass cover  20  is stacked on the front of the faceplate. The glass cover  20 , faceplate  12 , and the electronics subassembly  50  are slid into the tubular body  18  until the glass cover contacts and is retained by the bezel  22 . The o-ring seal  28  is placed against rear of the tubular body  18 , and the rear cap  24  is secured thereto by inserting the rear cap into the tubular body and securing three fasteners  32 ,  34 , and  36  through the openings  44  in the tubular body and into the openings  38   40 , and  42  in the rear cap  24 . A rear nut  45  can be secured to the threaded boss  46  to further secure the connection.  
         [0047]     In an example of retrofitting an existing electro-mechanical mechanism-based indicator, one removes the existing electro-mechanical assembly and any mounting hardware from the housing and replaces them with the electronic subassembly and its associated backplate and mounting hardware. In embodiments of the invention in which the physical structures and configurations of the removed and replaced structures differ from those described above with regard to the illustrated embodiment, the retrofitting method is essentially the same with accommodations made for such differences.  
         [0048]     Different instruments or indicator types can all utilize substantially the same mechanical and electrical components with a few exceptions. Preferably, each type of indicator instrument (e.g., hydraulic pressure, hydraulic temperature, fluid level, fluid level for utility, and hydraulic pressure for utility) has a unique faceplate  12  and a unique connector CCA  58  with configuration settings. Thus, each type of indicator instrument  10  can have a different scale printed with indicia suitable for whatever quantity the instrument is intended to display, such as hydraulic temperature, hydraulic temperature, and so forth. However, each of the various indicator types can use the same arrangement of LED arc and seven-segment digital LED data presentation. Also preferably, each type of indicator instrument  10  has a unique keyed rear connector (while maintaining the same type and number of pins) so that the instrument cannot be plugged into an incorrect cable assembly in the control panel. Utilizing common components and subassemblies for various indicators contributes to minimizing recurring unit cost by helping to maintain quantities of components and circuit card assemblies at economical procurement and manufacturing levels.  
         [0049]     Although the present invention has been described in terms of a hydraulic pressure instrument, persons skilled in the art to which the invention relates will readily be capable of designing suitable electronic subassemblies for any of the conventional indicator types, such as hydraulic temperature, fluid level, fluid level for utility, hydraulic pressure for utility, and so forth.  
         [0050]     It will be apparent to those skilled in the art that various modifications and variations can be made to this invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided that they come within the scope of the appended claims and their equivalents.