Abstract:
A hub unit adapted for use in a monitoring system that monitors engine performance parameters of a gas turbine engine. The hub unit includes a housing, at least one signal conditioning circuit board within the housing and adapted to receive the analog sensor outputs from the sensors, and a control circuit board within the housing, connected to the signal conditioning circuit board, and adapted to produce digital data corresponding to analog sensor outputs. The control circuit board and the signal conditioning circuit board each comprise electrical circuit components that define an analog signal processing path and have accuracy and precision characteristics that drift in response to component aging and to changes in the temperature to which the hub unit is subjected. The hub unit performs a continuous calibration scheme to determine and remove errors in the analog signal processing path resulting from the drifts of the electrical circuit components.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention generally relates to electronic equipment, and more particularly to electronic hardware capable of operating within a high temperature environment, such as on or adjacent a gas turbine engine. 
         [0002]    Aircraft gas turbine engines undergo testing during their development, as well as during production and subsequent servicing. Numerous engine performance parameters are typically monitored to assess the performance of an engine, including various temperatures, pressures, flow rates, forces, rotational speeds, etc. As nonlimiting examples, it is typically desirable to monitor engine inlet, compressor and exhaust gas temperatures, pressures within the fan, compressor and turbine sections, fuel and airflow rates, compressor and fan rotor speeds, blade tip clearances, mechanical stresses and part vibrations. Development and flight test aircraft engines may be required to have thousands of sensors to monitor the various parameters of interest. 
         [0003]    Engine testing is typically conducted on a stationary test stand that is often located outdoors. A nonlimiting example of such a test stand  100  is schematically represented in  FIG. 1 . The stand  100  is represented as including a vertical support column  102  mounted to a foundation  104  in the ground, and a head (thrust) frame  106  mounted on the column  102  from which an aircraft engine  108  is mounted for testing. The head frame  106  includes an adapter  110  to which the engine  108  is attached with a pylon  112  that is appropriately configured for the particular engine  108 . 
         [0004]    During engine testing, the engine  108  and its immediate surroundings can reach very high temperatures. For example, temperatures may approach or exceed 260° C. surrounding the engine core beneath the engine cowling (nacelle)  114 , as well as on the head frame  106  and its adapter  110 . While sensors used to monitor the engine  108  have been developed to withstand these temperatures, the electronics used to process the sensor data have been limited to much lower temperatures. For example, typical commercial electronic components are often limited to about 85° C., and even military standard components are typically rated to not higher than 125° C. As such, each sensor typically requires a separate continuous wire or tube to carry its output signal to a remote data acquisition system, which is often located within an enclosed facility equipped with a controlled environment. The facility may be a considerable distance from the engine test stand, for example, 50 meters to in excess of 300 meters. Routing, managing and maintaining the numerous (potentially thousands) of data wires and tubes requires a considerable effort. Consequently, the ability to reduce the length and number of wires and tubes would be helpful and beneficial. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0005]    The present invention provides a hub unit adapted for use in a monitoring system adapted to monitor engine performance parameters of a gas turbine engine operating on a stationary test stand, or during on-wing flight test, or during normal aircraft operation, and particularly a monitoring system comprising sensors mounted on the engine for sensing the engine performance parameters and generating digital sensor outputs. 
         [0006]    According to a first aspect of the invention, the hub unit includes a housing, at least one signal conditioning circuit board within the housing and adapted to receive the analog sensor outputs from the sensors, and a control circuit board within the housing, connected to the signal conditioning circuit board, and adapted to produce digital data corresponding to analog sensor outputs. The control circuit board and the signal conditioning circuit board each comprise electrical circuit components that define an analog signal processing path and have accuracy and precision characteristics that drift in response to component aging and to changes in the temperature to which the hub unit is subjected. The hub unit further includes means for performing a continuous calibration scheme by periodically applying a reference voltage and a zero voltage to the signal conditioning circuit board to determine and remove errors in the analog signal processing path resulting from the drifts of the electrical circuit components of the control circuit board and the signal conditioning circuit board. 
         [0007]    According to a second aspect of the invention, in addition to certain aspects recited above, the hub unit may further include means on the signal conditioning circuit board for multiplexing a plurality of the analog sensor outputs generated by the sensors to produce an individual multiplexed analog output, and at least one amplifier with adjustable gain for scaling the analog sensor outputs of the individual multiplexed analog output to produce an individual conditioned multiplexed analog output from which the corresponding digital data are produced. The amplifier and the adjustable gain thereof are controlled by the control circuit board. 
         [0008]    A technical effect of the invention is the ability of the hub unit to operate at high temperatures, for example, higher temperatures than possible with more temperature-sensitive hardware of the type conventionally used to process digital data. As such, data processing can be performed at a location remote from the high temperature environment being monitored. On the other hand, the hub unit and particularly its control and signal conditioning circuit boards can be specially adapted for high temperature operation, preferably without the use of active cooling. Furthermore, the continuous calibration scheme removes errors that would otherwise exist in the analog signal processing path as a result of the accuracy and precision characteristics of the electrical circuit components of the control circuit board and the signal conditioning circuit board tending to drift due to component aging and the high temperature environment of the hub unit. In accordance with the second aspect of the invention, the multiplexing capability can reduce the number of wires or cables necessary to transmit data to the remotely-located distributor unit. 
         [0009]    Other aspects and advantages of this invention will be better appreciated from the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic representation of a test stand for a gas turbine engine. 
           [0011]      FIG. 2  is a block diagram representing tiered units of a monitoring system adapted for monitoring performance parameters of a gas turbine engine operating while mounted on a test stand, such as of the type represented in  FIG. 1 . 
           [0012]      FIG. 3  is a block diagram representing certain components of the monitoring system of  FIG. 2 , including details of a processor control board of the monitoring system. 
           [0013]      FIG. 4  is a block diagram representing an analog signal conditioning board of the monitoring system of  FIG. 2 . 
           [0014]      FIG. 5  schematically represents a voltage reference device for use with a collector computer of the monitoring system of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]      FIG. 2  is a block diagram representing various units of a monitoring system  10  adapted for monitoring performance parameters of a gas turbine engine while the engine is mounted and operating on a stationary test stand, for example, the test stand  100  represented in  FIG. 1 . The system  10  can also be used to monitor an engine during on-wing flight tests, as well as during normal aircraft operation. While the monitoring system  10  is particularly well suited for monitoring a gas turbine engine, and for convenience will be described in reference to the engine  108  and its stand  100  represented in  FIG. 1 , the use of the system  10  is not limited to such applications. Instead, the system  10  is more broadly applicable to a wide variety of situations in which there is a desire or need to monitor performance parameters of an apparatus operating in an environment subjected to elevated temperatures. 
         [0016]    As represented in  FIG. 2 , the system  10  is generally identified as having units  12 ,  14 ,  16  and  18  that are located in four environments  34 ,  36 ,  38  and  40  with respect to the gas turbine engine  108 . The first unit  12  comprises an array of sensors  20  appropriately located in and around the engine  108  to monitor performance parameters of the engine  108  for the purpose of assessing the performance of the engine. Any number of sensors  20  may be employed by the system  10 , and the sensors  20  may be of various types, for example, to monitor temperatures, pressures, flow rates, forces, rotational speeds, etc., of the engine  108 , as was previously discussed in reference to  FIG. 1 . Certain types of sensors  20  are typically employed in large numbers during the monitoring of engine operation, including thermocouples, resistance temperature detectors (RTDs), and pressure transducers. Because the sensors  20  are located to directly detect the parameters of interest, the unit (array)  12  of sensors  20  is indicated in  FIG. 2  as being located in a “high temperature engine environment”  34 , where maximum temperatures exceeding 200° C. are often encountered by the system  10  and may reach as high as 260° C. or more. Suitable sensors  20  for use in the system  10  are commercially available and commonly used for monitoring gas turbine engine parameters, and therefore will not be discussed in any detail here. The particular output signals generated by the sensors  20  will depend on the type of sensors  20  used, though in most cases the signals will be analog signals that must be digitized in order for their data to be used by computer processing equipment to assess the performance of the engine. 
         [0017]    The remaining primary units  14 ,  16  and  18  of the system  10  are identified in  FIG. 2  as located in environments  36 ,  38  and  40  where lower temperatures are likely to occur. A first of these units will be referred to as a hub unit  14 , with which the sensors  20  directly communicate through any suitable wires, tubes, or other appropriate connectors commonly employed with the particular type of sensors  20  used. The hub unit  14  represented in  FIG. 2  will typically be one of a number of hub units  14  that may be used in the system  10 , depending on the number of sensors  20  and the number of sensors  20  each hub unit  14  can manage. Similar to the environment  34  for the sensors  20 , the environment  36  for the hub unit  14  is identified as a “high temperature engine environment”  34 , in that the hub units  14  are adapted to be located in close proximity to the engine  108 , for example, within about three meters of the engine  108 , such as the engine core environment beneath the engine cowling  114 . In addition to locations directly on the engine  108  and beneath its cowling  114 , other locations may include adjacent locations on the head frame  106  or adapter  110  of the stand  100 , where very high temperatures are still likely to be encountered by the system  10 . For example, temperatures beneath the cowling  114  and adjacent locations on the head frame  106  or adapter  110  often exceed 125° C., and can reach much higher temperatures, for example, higher than 200° C. and potentially as high as 260° C. or more. Consequently, electronic components of the hub units  14  must be capable of withstanding significantly higher temperatures than is possible with conventional electronic components and even military standard components. 
         [0018]    In contrast, the environments  38  and  40  for the remaining two units  16  and  18  of the system  10 , referred to as a collector unit  16  and a distributor unit  18 , are identified as a “near-engine environment”  38  and a “low temperature environment”  40 . The former is designated as such because the collector unit  16  is adapted to be located in proximity to the engine  108  but not as close to the engine core as the hub units  14 . For example, the collector unit  16  may be located within the engine fan case environment or on the stand  100 , such as on the head (thrust) frame  106 , at distances of about three to ten meters from the core of the engine  108 . At these locations, temperatures will usually exceed 55° C., but are significantly less than 260° C. and typically less than 125° C. Consequently, electronic components of the collector unit  16  must typically be capable of withstanding high temperatures, though not as high as the hub units  14 . In some situations, military standard components rated up to 125° C. may be used, and possibly conventional electronic components rated up to 85° C. 
         [0019]    On the other hand, the low temperature environment  40  of the distributor unit  18  permits the use of conventional electronic components rated at no more than 85° C. The environment  40  is designated as “low temperature” in that the distributor unit  18  can be and preferably is located in a controlled-temperature environment, for example, an enclosed facility that is near the test stand  100  and is stabilized with air-conditioning to maintain a temperature of less than 55° C. For on-wing engine operation, the environment  40  may be within the aircraft. The distributor unit  18  preferably has the most processing power of the system  10 , and therefore will typically comprise one or more computer servers, personal computers, and/or other processing equipment adapted for data processing, collectively represented by a distributor computer  42  in  FIG. 2 . In addition to a real-time calibration functionality discussed below, the distributor computer  42  may also provide the capability of engineering unit conversion, system configuration, and database functionality. Suitable equipment for the distributor computer  42  are likely to be relatively sensitive to temperature, and therefore benefit from being housed at roughly room temperature. The low temperature environment  40  is typically remotely located from the engine test stand  100 , for example, in excess of fifty meters. 
         [0020]      FIG. 2  schematically represents the hub unit  14  as comprising a processor control board  22  and one or more analog signal conditioning boards  24 . These boards  22  and  24  are preferably enclosed in a housing  44 , schematically represented in  FIG. 2  as completely surrounding and enclosing the boards  22  and  24 . The processor control board  22  and analog signal conditioning boards  24  operate together to convert the analog output signals of the sensors  20  to digital data that can be processed by the distributor unit  18 . According to certain preferred aspects of the invention, the processor control board  22  and analog signal conditioning boards  24  also combine to perform additional processes to ensure the integrity of the analog output signals received from the sensors  20  prior to their analog-digital conversion. As will be explained in more detail below, one such additional process is to provide a continuous calibration feature that detects any drift in the accuracy and precision characteristics of the electronic components of the analog signal conditioning boards  24  and the processor control board  22  that can result from component aging and variations in temperature, such as the extreme temperature changes to which the hub unit  14  is subjected. The calibration feature produces calibration data that can be used by the distributor computer  42  to perform real-time corrections of the digital data acquired from the hub unit  14  via the collector unit  16 , and more particularly a collector computer  26  of the unit  16 . Another preferred process is to multiplex the analog output signals of multiple sensors  20  into multiplexed analog outputs, thereby reducing the number of connections required to transmit the digital data to the collector unit  16  over, for example, a serial data connector such as an RS-485 serial communications cable. Still another preferred process is to interleave the multiplexed analog outputs of one group (bank) of sensors  20  with the multiplexed analog outputs of other banks of sensors  20 , so that the individual analog output signals of the multiplexed analog outputs more quickly “settle” between the sets of outputs. These and other aspects of the hub unit  14  will be discussed in further detail below. 
         [0021]    The collector hub  16  is schematically represented in  FIG. 2  as comprising the collector computer  26 , a power supply  28 , and a temperature-controlled environment  30  that contains a system voltage reference device  32 , as will be explained in more detail below. The primary function of the power supply  28  is to supply power to the electronic components of the system  10 , including the sensors  20  (as may be required) and the electronic components housed in the hub unit  14 . A preferred power supply  28  is a dual topology design with a switching regulator front end and linear regulator back end. The power supply  28  may be configured to generate multiple independently-regulated voltages for each hub unit  14  to increase system fault tolerance and decrease noise coupling. The collector computer  26  receives the digital data from the hub unit  14 , as well as any additional hub units  14  contained in the system  10 , prior to forwarding the digital data to the distributor computer  42  of the distributor unit  18 . The collector computer  26  is preferably configured to have a logging capability for synchronizing the flow of the digital data to the distributor computer  42 , for example, utilizing inter-range instrumentation group (IRIG) time codes or a Network Time Protocol (NTP). More particularly, the collector computer  26  preferably operates as an intelligent switch for the incoming digital data from multiple hub units  14  by accurately time stamping multiple streams of digital data coming from the hub units  14 , packing the data into frames, and then transmitting the data to the distributor computer  42 , for example, over a fiber-based Ethernet connection. Suitable components for time stamping multiple data streams and packing data into frames are well known in the art, and therefore will not be discussed in any detail here. The use of a fiber optic cable for the data connection between the collector computer  26  and the distributor computer  42  is preferred for the purpose of reducing the susceptibility of the transmission to lightening, which is desirable since the transmission cable will typically be exposed to an outdoor environment as a result of being routed between the test stand  100  and the remote facility housing the distributor unit  18 . The collector computer  26 , power supply  28  and controlled environment  30  may all be enclosed within a suitable protective housing (not shown) that protects these components from direct exposure to the elements. 
         [0022]    Notably, because of multiplexing at the level of the hub units  14  and synchronization at the level of the collector unit  16 , the digital data can be supplied to the distributor unit  18  over a single Ethernet connection, which is in stark contrast to the typical thousands of cables and tubes previously required to transmit sensor output to a remote data acquisition system of the prior art. 
         [0023]      FIG. 3  is a block diagram representing the processor control board  22 , some of its components, and its connection to the analog signal conditioning boards  24  and the collector unit  16 . The processor control board  22  is represented as being equipped with a microprocessor  46  adapted to run from a program stored in ROM (read-only memory)  48 , such as an EEPROM (electrically-erasable programmable read only memory), and uses RAM (random access memory)  50  to store the digital data generated from the sensors  20 , as well as any variables used in calculations performed by the control board  22 . The microprocessor  46  preferably performs a gain setting function associated with the signal conditioning boards  24  (discussed below), controls/selects which individual or blocks of signal channels of the sensors  20  are read, the timing of the data acquisition, error sensing, analog-to-digital conversion, execution of any built-in test (BIT) modes, sensor adaptation (based on the types of sensors  20 ), and the collection, formatting and transfer of the digital data to the collector computer  26 . As indicated in  FIG. 3 , the input/output (I/O) functions of the board  22  are preferably directed in the form of memory mapped I/O operations. As also seen in  FIG. 3 , the processor control board  22  also transmits zero and full-scale control outputs to the analog signal conditioning boards  24 , as well as directly communicates with the collector computer  26 . As will be discussed in reference to the conditioning boards  24  and  FIG. 4 , the zero and full-scale control outputs transmitted by the control board  22  are part of a continuous calibration scheme that periodically applies a zero voltage and reference voltage to detect and compensate for any drift in the accuracy and precision characteristics of the electronic components of the conditioning boards  24  resulting from variations in temperature and component aging. 
         [0024]    As previously noted, the hub unit  14  is intended to operate at temperatures greater than 125° C., and preferably as high as at least 200° C. In preferred embodiments, the microprocessor  46 , ROM  48 , RAM  48  and passive components mounted to the control board  22  are capable of operating at temperatures above 200° C. To achieve this capability, the microprocessor  46 , ROM  48  and RAM  48  are preferably implemented with silicon-on-insulator (SOI) substrates and processing technology. As known in the art, SOI substrates typically comprise a thin epitaxial layer on an insulator. The substrate is typically formed by oxidizing one or both bonding surfaces of a pair of semiconductor (e.g., silicon) wafers prior to bonding the wafers. Most typically, a single silicon dioxide layer is grown on an epitaxial layer formed on a silicon wafer. After bonding the wafers, all but the insulator and epitaxial layer (and optionally the silicon layer of the second wafer) are etched away, such that the silicon dioxide layer forms an insulator that electrically isolates the epitaxial layer. A commercial example of a solid-state microprocessor implemented on an SOI substrate using SOI processing technology is the HT83C51 microprocessor commercially available from Honeywell. Commercial examples of RAM components implemented on SOI substrates include the HT6256 256 Kbit SRAM component available from Honeywell, and commercial examples of ROM components implemented on SOI substrates include ROM components from Twilight Technology Inc. 
         [0025]    The substrate on which the electronic components of the processor control board  22  are mounted is also preferably capable of withstanding temperatures of at least 260° C. A preferred high-temperature substrate material is commercially available from Rogers Corporation under the name RO4003C, which is a glass-reinforced hydrocarbon/ceramic laminate. Furthermore, the components are preferably attached with high melting point solders, a notable but nonlimiting example of which is 92.5Pb-5Sn-2.5Ag, which has a melting range of about 287 to about 296° C. To reduce thermal stresses resulting from thermal expansion and contraction of the board, the microprocessor  46 , ROM  48 , RAM  50  and other components on the board  22  are preferably through-hole components having one or more metal leads (sticks) that are inserted into through-holes (typically plated through-holes) in the substrate and then soldered to the substrate. Other approaches to reducing thermal stresses include the use of high-temperature, thermally-conductive potting materials to minimize thermal gradients, increase thermal time constants and damp vibrations, and limiting the number of metallized vias that are susceptible to breaking due to board delamination and expansion/contraction. Notably, the metal leads of the through-hole components are believed to promote the structural integrity of the vias in which they are placed. 
         [0026]    With the above-noted high temperature capabilities, the control board  22  can be contained within the hub unit housing  44 , preferably without the need for an active cooling system dedicated to maintaining the temperature of the board  22  below 125° C. as would be required by conventional electronics. The term “active cooling” is used herein to mean cooling systems that are specifically designed to transfer heat from the board  22  and out of the hub unit housing  44  by conduction, convection, and/or radiation. 
         [0027]      FIG. 4  is a block diagram representing two analog signal conditioning boards  24  and their connection to the processor control board  22  of  FIG. 3 . The analog signal conditioning boards  24  are combined with the processor control board  22  within the housing  44  of the hub unit  14 , and as such are also required to operate at high temperatures in the harsh environment of the engine  108 . The high temperature operation of the hub unit  14  and its conditioning boards  24  enables the sensors  20 , including thermocouples, RTDs, and pressure transducers, to be terminated directly on the engine  108  and their outputs conditioned prior to the A/D (analog-to-digital) conversion performed by the processor control board  22 . Furthermore, the hardware of the conditioning boards  24  preferably incorporates the previously-noted continuous calibration, multiplexing and interleaving features. 
         [0028]    As note above, the continuous calibration scheme performed on the conditioning boards  24  produces calibration data that can be used by the distributor computer  42  to perform real-time corrections of the digital data acquired from the hub unit  14 . The continuous calibration scheme preferably compensates for all passive and active components on the conditioning boards  24  and processor control board  22  that may significantly affect signal accuracy. The need for a continuous calibration feature arises because, at the system level, discrete components are not currently available that do not exhibit drift over the foreseeable operating range of the hub unit  14 , for example, about −55° C. to above 200° C. In preferred embodiments of the invention, the continuous calibration scheme provides for zero and full-scale data to be continuously collected, while any drifting of the acquired data over time and temperature is automatically compensated. 
         [0029]    The continuous calibration feature relies in part on the system voltage reference device  32 , represented in  FIG. 2  as located in the controlled environment  30  of the collector unit  16  remote from the hub unit  14 . Though a location with the collector unit  16  is believed to be preferred, it is foreseeable that other locations could be found suitable for system voltage reference device  32 . The controlled environment  30  is schematically represented in greater detail in  FIG. 5  as comprising the voltage reference device  32  enclosed within a housing  52 , which further contains a heating element  54 , copper plate  56  and thermal RTV potting material  58  that achieve uniform heating of the voltage reference device  32 . The temperature of the reference device  32  can be regulated to any suitable level, for example, about 55° C. to about 125° C. The reference device  32  generates highly-precise zero and full-scale reference voltages, which are then transmitted over dedicated differential links to the conditioning boards  24 . 
         [0030]    Temperature-induced drifting in the accuracy and precision of the electrical circuit components of the conditioning boards  24  are captured and recorded along with the analog output signals of the sensors  20  during A/D conversion. During each cycle in which analog output signals are read from the sensors  20 , the processor control board  22  causes the highly-precise zero volt and reference voltage signals of the reference device  32  to be transmitted through all analog signal processing paths (channels) defined by the electronic components of each conditioning board  24 . The zero volt and reference voltage signals are then used to correct the digitalized sensor data, in that any change in the output voltage from the previous calibration reading is attributed to board-level component drift and transmitted as calibration data to the distributor computer  42 , which digitally corrects the digitalized sensor data before further use of the data. In practice, the zero and full-scale reference signals may be applied several times per second. Accuracies over time, temperature and distance on the order of having an accuracy on the order of about +/−20 ppm (parts per million) and less have been achieved in the analog signal processing path with the continuous calibration feature described above. 
         [0031]    As part of the calibration scheme, the conditioning boards  24  also provide for multiplexing of multiple signal channels from the sensors  20 , enabling each conditioning board  24  to condition multiple sensor signals through a fewer number of circuit paths, for example, two as represented in  FIG. 4 . Signals from multiple sensors  20  are represented in  FIG. 4  as passing through multiplexors  60  to generate multiplexed analog outputs, thereby reducing the number of connections required to transmit the digital data to the collector unit  16  over a serial data connector. Within each circuit path, the multiplexed analog outputs are conditioned with an instrumentation operational amplifier  62 . Each amplifier  62  is represented in  FIG. 4  as incorporating active gain changes  64  controlled by the processor control board  22 , which enables each conditioning board  24  to be used to scale many different sensor types with different voltage outputs to a set output voltage prior to A/D conversion. 
         [0032]    As further evidenced from  FIG. 4 , switches  68  can be used to interleave the multiplexed analog outputs of one bank (group) of sensors  20  along one circuit path on the board  24  with the multiplexed analog outputs of another bank of sensors  20  on another circuit path of the same board  24  to increase system throughput. As one series of multiplexed analog outputs from one bank of sensors  20  is output to the A/D converter of the processor control board  22 , sensor outputs on other banks of sensors  20  are at various stages of settling. Once the series of multiplexed analog outputs from the first bank of sensors  20  has been read by the A/D converter, the next bank can be chosen while signals of the first bank begin to settle on a different sensor  20 . This feature allows a higher system level throughput to be implemented with slower, but higher-temperature capable circuit components of the processor control board  22  and conditioning boards  24 . 
         [0033]    The conditioning board  24  depicted in  FIG. 4  is further represented as incorporating dynamic, dual-time constant filtering  66  on the amplifier outputs of each operational amplifier  62  and controlled by the processor control board  22 . This feature further enables rapid settling when switching between the circuit paths over which the multiplexed analog outputs are transmitted, while still providing a high level of low pass filtering to reduce electrical noise of a sensor output signal present in the engine test environment. Dynamic filtering can be achieved by, for example, removing a resistor from an RC circuit, allowing a rapid output change from one channel voltage to another, then switching the resistor back into the circuit to minimize sensor noise and ripple, improving the analog data quality presented to the A/D converter (ADC). 
         [0034]    Notably, each conditioning board  24  is preferably able to accommodate both positive and negative input voltages, for example, in the event that the sensors  20  include thermocouples and pressure transducers that can output negative voltages. Additionally, because the conditioning boards  24  are located in the high temperature environment of the hub unit  14 , “cold junction” compensation conventionally performed on thermocouples board can be “hot junction” compensation since thermocouples among the sensors  20  may be at a lower temperature than the thermocouple wire-to-reference junction measured by the conditioning board  24 . For this reason, the instrumentation operational amplifiers  62  are preferably capable of differential voltages and scales these ±voltages to a positive-only voltage range necessary for A/D conversion. 
         [0035]    As with the processor control board  22 , at least some of the circuit components of the analog signal conditioning boards  24  are preferably implemented with SOI technology to allow operation of the boards  24  at temperatures of at least 200° C., enabling the entire hub unit  14  to operate at such elevated temperatures. As a result, the hub unit  14  and its control and conditioning boards  22  and  24  overcome prior limitations of data acquisition systems that have necessitated that each individual sensor output must be transmitted by wire or tube to a remote location a considerable distance from an engine under test. Such restrictions have resulted in long wires and tubes routed from engines to the data acquisition systems, incurring additional expense, introducing additional sources of error, and necessitating a considerable amount of man-hours to install and debug. In contrast, the hub unit  14  can be placed directly on the head frame  106 , its adapter  110 , or even directly on the engine  108 , for example, under the cowling  114 , resulting in a relatively short distance (for example, less than three meters) between the sensors  20  and their terminations on the hub unit  14 . 
         [0036]    While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. For example, the physical configuration of the units  12 ,  14 ,  16  and  18  and the components could differ from that shown, and materials and processes other than those noted could be used. Therefore, the scope of the invention is to be limited only by the following claims.