Abstract:
An apparatus and method for measuring, collecting, and processing the temperatures of a number of hot junctions in a thermocouple sensor array simultaneously comprises means for measuring the temperature of one or more reference junctions and means for collecting, processing, storing, and transmitting data collected from an array of thermocouple temperature sensors. The measured temperature(s) of the one or more reference junctions is used to correct the measured voltage in each of a plurality of thermocouple circuits to obtain accurate hot junction temperature measurements. The apparatus and method are particularly useful for collecting and processing temperature data from various locations within engines such as turbine, rocket, and internal combustion engines.

Description:
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     The U.S. Government has certain rights to this invention pursuant to Contract Numbers FA9101-09-M-0011 and FA9101-10-C-0018 awarded by the United States Air Force. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method and apparatus for transducing electronic signals from a plurality of analog thermocouple sensors into digital output. The apparatus and method provide for accurate temperature measurement simultaneously from an array of thermocouple sensors and thermal sensor data collection, processing, transmission, and storage. The thermocouple sensors may be attached to an object or substrate to obtain real time temperature measurements with data acquisition rates of 50 per second or more, 100 per second or more, 200 per second or higher. 
     Thermocouples are commonly used for industrial temperature measurement applications. A typical thermocouple consists of a pair of wires that are made of dissimilar metals and are joined at one end. Thermocouples operate based on a principle that, when the ends of two dissimilar metals are joined and the junction is heated, a current flows in the wire loop. If the circuit is broken, an electromotive force (EMF) measured as a voltage, develops in a heated pair of dissimilar metal wires joined at a junction (hot junction). The EMF produced at a thermocouple junction may be measured by a voltmeter, which introduces a reference junction into the circuit containing the hot junction. The net EMF measured in the complete circuit, therefore, depends upon the temperature at the hot junction Tj as well as the temperature of the reference junction Tr. Consequently, the accuracy of the measured temperature at the hot junction Tj can be improved when the temperature of the reference junction is known and used to correct the EMF measured for the complete circuit. This may be accomplished by placing the reference junction into an environment having a known, preferably fixed, temperature. Controlling the temperature of the reference junction, however, is not practical for many applications. The present invention provides a temperature sensor array in which the temperature of one or more reference junctions is measured and the measured temperature is used to improve the accuracy of measured thermocouple temperatures. This eliminated the need for placing the reference junction into an environment having a known and/or fixed temperature. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is an apparatus and method for measuring, collecting, and processing the temperature of a number of hot junctions in a thermocouple sensor array. The apparatus comprises means for measuring the temperature of one or more reference junctions and means for collecting, processing, storing, and transmitting data collected from an array of thermocouple temperature sensors. The measured temperature(s) of the one or more reference junctions is used to correct the measured voltage in each of a plurality of thermocouple circuits to obtain accurate hot junction temperature measurements, Tj. The apparatus and method are useful for simultaneously measuring temperatures at multiple locations and particularly useful for collecting and processing temperature data from various locations within engines such as turbine, rocket, and internal combustion engines. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a prior art circuit used to compensate for reference junction temperature in order to improve the accuracy of hot junction temperature measurement; 
         FIG. 2  is a diagram showing a Smart Transducer Interface Module (STIM) interfaced with a number of thermocouple sensors; 
         FIG. 3  is a diagram showing a thermocouple data acquisition unit including 8 STIMs connected to a single Network Capable Application Processor (NCAP); 
         FIG. 4  is a diagram showing multiple NCAPs communicating with a single server; 
         FIG. 5  is a diagram showing a thermocouple interface circuit; and 
         FIG. 6  is a diagram showing a reference junction temperature measurement circuit. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a circuit used to account for reference junction temperature during the measurement of a hot junction ( 3 ) temperature in a thermocouple sensor. The EMF of the circuit, Erj, is measured using the voltmeter ( 11 ) at points  1  and  5 . The reference junction ( 2 , 4 ) temperature Tr is measured independently ( 12 ) using temperature measuring means such as an ice-point compensated thermocouple, a thermistor, resistance temperature detector (RTD) or a solid-state temperature sensor. The reference junction EMF, E 0   r , is determined based on Tr. A corrected hot junction EMF, E 0   j , is calculated, for example, by using the formula E 0   j =Erj+E 0   r . The temperature of the hot junction is then determined based on the corrected hot junction EMF, E 0   j , using a look-up table or equation. The National Institute of Standards and Testing (NIST) has developed tables and curve-fit equations that enable conversion between measured voltage (EMF) and temperature. The table below shows the equations and coefficients required to make these conversions for B-type thermocouples, which are used for high temperature applications. 
     
       
         
               
             
               
               
               
             
               
             
               
               
               
             
           
               
                   
               
             
             
               
                 a. Coefficients to compute the temperature of a B-type thermocouple 
               
               
                 junction based on the measured (corrected) EMF (voltage) 
               
               
                 t 90  = d 0  + d 1 E + d 2 E 2  + . . . d n E n   
               
             
          
           
               
                   
                 Coefficients for voltage range 
                 Coefficients for voltage range 
               
               
                   
                 0.291 mV to 2.431 mV (Temper- 
                 2.431 mV to 13.820 mV (Temper- 
               
               
                   
                 ature range 250° C. to 700° C.) 
                 ature range 700° C. to 1820° C.) 
               
               
                   
               
               
                 d 0  = 
                   9.8423321E+01 
                   2.1315071E+02 
               
               
                 d 1  = 
                   6.9971500E+02 
                   2.8510504E+02 
               
               
                 d 2  = 
                 −8.4765304E+02 
                 −5.2742887E+01 
               
               
                 d 3  = 
                   1.0052644E+03 
                   9.9160804E+00 
               
               
                 d 4  = 
                 −8.3345952E+02 
                 −1.2965303E+00 
               
               
                 d 5  = 
                   4.5508542E+02 
                   1.1195870E−01 
               
               
                 d 6  = 
                 −1.5523037E+02 
                 −6.0625199E−03 
               
               
                 d 7  = 
                   2.9886750E+01 
                   1.8661696E−04 
               
               
                 d 8  = 
                 −2.4742860E+00 
                 −2.4878585E−06 
               
               
                   
               
             
          
           
               
                 b. Coefficients to compute the EMF (voltage) generated by a B-type 
               
               
                 thermocouple based on the junction temperature. 
               
               
                   
               
               
                 
                   
                     
                       
                         E 
                         = 
                         
                           
                             ∑ 
                             
                               i 
                               = 
                               0 
                             
                             n 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             
                               
                                 c 
                                 i 
                               
                               ⁡ 
                               
                                 ( 
                                 
                                   t 
                                   90 
                                 
                                 ) 
                               
                             
                             i 
                           
                         
                       
                     
                   
                 
               
             
          
           
               
                   
                 Coefficients for temperature range 
                 Coefficients for temperature range 
               
               
                   
                 0.0° C. to 630.615° C. 
                 630.615° C. to 1820° C. 
               
               
                   
               
               
                 C 0  = 
                   0.000000000000E+00 
                 −0.389381686210E+01 
               
               
                 C 1  = 
                 −0.246508183460E−03 
                   0.285717474700E−01 
               
               
                 C 2  = 
                   0.590404211710E−05 
                 −0.848851047850E−04 
               
               
                 C 3  = 
                 −0.132579316360E−08 
                   0.157852801640E−06 
               
               
                 C 4  = 
                   0.156682919010E−11 
                 −0.168353448640E−09 
               
               
                 C 5  = 
                 −0.169445292400E−14 
                   0.111097940130E−12 
               
               
                 C 6  = 
                   0.629903470940E−18 
                 −0.445154310330E−16 
               
               
                 C 7  = 
                   
                   0.989756408210E−20 
               
               
                 C 8  = 
                   
                 −0.937913302890E−24 
               
               
                   
               
             
          
         
       
     
     The NIST ITS-90 database has coefficients for all common thermocouple types, and coefficients can be generated for any other custom designed thermocouple. 
     An example of a basic data acquisition unit interfaced with a number of thermocouple sensors is shown in  FIG. 2 . The Smart Transducer Interface Module (STIM) ( 21 ) is electrically coupled to the thermocouple(s) (transducer(s)) ( 22 ) and converts analog signals from the thermocouples into digital sensor data using analog to digital converters ( 23 ). The STIM may be configured to interface with a single or multiple thermocouples ( 23 ) (transducers) and stores a local copy of a Transducer Electronic Datasheet (TEDS) for each thermocouple including, for example, a template ID identifying the thermocouple and thermocouple type, calibration information, user specified information, and technical specifications. A STIM processor ( 24 ) may comprise a soft “virtual” processor comprising programmable software in a Field Programmable Gate Array (FPGA) or a combination of a physical processor and a FPGA. A soft STIM may be reconfigured to interface with more or fewer thermocouples via changes to software without changes to the hardware. The temperature measurements are triggered to occur simultaneously, or within a window of time that is less than 1000 nanoseconds long, and preferably less than 500 nanoseconds long, and more preferably less than 200 nanoseconds long. 
     A thermocouple data acquisition unit designed for 64 thermocouples and comprising 8 STIMs ( 21 ) is shown in  FIG. 3 . All of the STIMs may share a common reference junction ( 12 ) as shown in the figure or various combinations of STIMs ( 21 ) and reference junctions ( 12 ) may be used. A single reference junction ( 12 ) is preferred to minimize the number of devices used to measure reference junction temperature. 
     Each of the STIMs is in digital electronic communication with a Network Capable Application Processor (NCAP) ( 31 ). The NCAP ( 31 ) receives digital data from STIMs ( 21 ) that are attached to it, applies a synchronized time stamp, assembles the data into a packet, and transmits data, for example via Ethernet, to a server ( 41 ) that receive the data packets, parses them, and stores them in a database. The database may also store local copies of TEDS data from all of the STIMs ( 21 ) to enable storage of raw thermocouple data in digitized form and processing of digitized raw data. A NCAP ( 31 ) and the STIMs ( 21 ) connected to the NCAP ( 31 ) are preferably located on the same FPGA board but may be on separate boards. A single board may contain more than one NCAP, with each NCAP being connected to more than one STIM ( 21 ). In one embodiment, a FPGA incorporates multiple software-defined processors that perform the functions of the STIM processors ( 24 ) and NCAP ( 31 ) and the NCAP ( 31 ) also functions as an IEEE-1588 client clock. 
     The server ( 41 ) may act as an interface for managing the system and may receive data from multiple NCAPs ( 31 ) as shown in  FIG. 4 . The server ( 41 ) is optimally configured to include a discovery that enables the automatic creation of a map of NCAPs ( 31 ), STIMs ( 21 ), and thermocouples ( 22 ) connected to the server ( 41 ) with the thermocouple scanner system preferably operating according to LXI Standard (LAN eXtensions for Instrumentation) or IEEE 1451 standards for data collection. The thermocouple scanner system may be integrated with a high speed sensor data transfer interface as described in U.S. application Ser. No. 12/496,471, filed 1 Jul. 2009. 
     Thermocouple Interface Circuit: 
     A STIM circuit may, for example, comprise a low power, low noise, 24-bit Σ-Δ analog to digital converter with three differential analog inputs. The output data rate may be software-programmable and varied from 4.17 Hz to 470 Hz. An exemplary thermocouple interface circuit is shown in  FIG. 5 . In this example, the thermocouple voltage is routed to the +IN pin ( 50 ) of the instrumentation amplifier ( 51 ), a gain of 25 is applied as determined by the resistor ( 52 ) between the RG pins of the instrumentation amp, the amplified signal is offset by the voltage on the reference pin, the output voltage is digitized by AIN 1  of the 24-bit ADC ( 55 ), and the offset reference voltage is digitized by AIN 2 . 
     Reference Junction Temperature Measurement Circuit: 
     Reference junction temperature measurement is performed, for example, using a 1000Ω platinum resistance thermal detector (RTD) ( 61 ) ( FIG. 6 ). In this example of operating the circuit, the ADC produces a constant current of 1 mA on IOUT 1  ( 67 ); the current goes through a voltage divider circuit ( 62 ) that consists of: a platinum RTD ( 61 ) with a nominal (0° C.) resistance of 1 kΩ and a linear temperature response and fixed and variable resistors ( 63 , 64 ) that are used to balance the divider at 0° C.; ADC input  1  measures the voltage drop ( 65 ) across the platinum RTD ( 61 ); the ADC REFIN ( 66 ) measures the voltage drop across the balance resistors; the resistors ( 63 , 64 ) in the circuit are selected so that the ADC directly measures R/R 0 ; and the reference junction temperature is determined using characteristics of the RTD ( 61 ). 
     The circuit is designed to perform open thermocouple detection using the 1 mA current analog output on the analog to digital converter IOUT 1 . If the thermocouple is open, then this will cause VDD to be read at the input of the instrumentation amplifier. If the thermocouple is intact, then the 1 MΩ and 4.7 kΩ resistors act as a voltage divider, and an appropriate voltage is read at the input of the instrumentation amplifier. 
     Specific embodiments of the invention have been used for illustrative purposes. The invention is not intended to be limited to the specific embodiments described herein. Modifications may be made to the described embodiments without departing from the invention. Specific values for voltages, resistances, currents, and frequencies are provided in the examples but the invention is not limited to the specific values in the examples.