Abstract:
An I/O circuit for measuring temperatures uses multiple cold-junction compensation sensors permanently affixed near the terminals of the terminal block in order to compensate significant temperature variation across the terminals of the I/O module (up to 3° C.) that can substantially affect the accuracy of thermocouple measurements. The use of these multiple sensors is enabled by a compensation system that corrects for the distance between the built-in sensors and the terminals, a multiplexer that accommodates the additional signal burden produced by the sensors, and a compensation system that allows low-cost sensors to be used and calibrated to as little as a single high accuracy sensor. In one embodiment, a third temperature sensor with relatively higher accuracy is used to compensate for lower accuracy of permanently affixed sensors.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to Singapore Patent Application No. 201100201-1 filed on Jan. 12, 2011, the disclosure of which is expressly incorporated herein. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to industrial control systems used for real-time control of industrial processes, and in particular to an input/output (I/O) module for connection to thermocouples to provide for temperature measurement. 
     Industrial control systems are special purpose computer systems used in controlling industrial processes. Under the direction of a stored control program, a programmable logic controller, being part of the industrial control system, reads inputs from one or more I/O modules and writes outputs one or more I/O modules. The inputs are derived from signals obtained from sensors associated with the industrial process and the output signals produce electrical signals to actuators and the like in the industrial process. The inputs and outputs may be binary, that is on or off or analog, providing a value with a continuous range for more complex I/O devices like motor controllers and the like. 
     One form of analog input I/O module receives an input from a thermocouple. As is understood in the art, thermocouples provide a voltage that is proportional to a difference in temperature between two junctions of dissimilar metals per the Seebeck effect. In order to determine a temperature at one junction (“hot junction”), the second junction (“cold junction”) may be held at a standard and known temperature. For practical devices, however, this cold junction is not held at a particular temperature but rather its temperature is measured and used to provide for “cold junction compensation” in which to measure temperatures applied to empirically derived compensation tables that may be used to correct the value of the hot junction. These tables may also be used to correct for inherent nonlinearities in the voltage-to-temperature function of the thermocouple. 
     It is known to provide I/O modules for use with thermocouples with a cold junction compensation circuit providing a precision temperature measuring element, for example, a resistive thermal device (RTD) that may be attached to terminals near the terminals receiving signals from thermocouples to provide a measurement of the temperature in the vicinity of those terminals. This temperature measurement is used for the cold junction compensation by the I/O module through computations typically performed by a processor internal to the I/O module. 
     The cost of these precision temperature references is relatively high and they take up terminal space that could be used for other devices and accordingly a typical I/O module will use one or two cold junction compensation circuits at most. 
     SUMMARY OF THE INVENTION 
     The present inventors have determined that there can be a significant temperature variation across the terminals of the I/O module in use (up to 3° C.) that can substantially affect the accuracy of thermocouple measurements. The present invention addresses this problem by providing multiple built-in auxiliary temperature sensors positioned at a temperature sensing terminals. The use of these multiple sensors is enabled in various embodiments by a compensation system that corrects for the distance between the built-in sensors and the terminals, a multiplexer that accommodates the additional signal burden produced by the sensors, and a compensation system that allows low-cost sensors to be used and calibrated to as little as a single high accuracy sensor. 
     Specifically then the present invention provides an I/O circuit for use with an industrial control system including a terminal block having terminals for releasably receiving leads of at least one first temperature sensor and a multiplicity of second temperatures sensors permanently affixed in thermal communication with the terminals of the terminal block. Input circuitry electrically connects to the terminals and to the second temperature sensors for receiving temperature signals from the first temperature sensors and second temperature sensors, the input circuitry operating to compensate the received temperature signals from the first temperature sensors using temperature signals from the second temperature sensors. 
     It is thus a feature of at least one embodiment of the invention to provide a system that greatly reduces errors caused by unpredictable terminal-to-terminal temperature variation. 
     The terminal block may be attached to a printed circuit board and the multiplicity of second temperature sensors are soldered to the printed circuit board. 
     It is thus a feature of at least one embodiment of the invention to provide a simple method of incorporating multiple compensation sensors into a terminal block system. 
     A housing covering a portion of the terminal block and the multiplicity of second temperature sensors may be covered by at least one of the housing and terminal block. 
     It is thus a feature of at least one embodiment of the invention to provide high accuracy cold junction compensation which is invisible to the user and does not interfere with the use of the terminal block. 
     The I/O circuit may further include terminals for connecting at least one third temperature sensor having a higher accuracy than the multiplicity of second temperature sensors, and the input circuitry may also electrically connect to the terminals for connecting the third temperature sensor for receiving a temperature signal from the third temperature sensor, the input circuitry operating to compensate the multiplicity of second temperature sensors using the temperature signal from the third temperature sensor. 
     It is thus a feature of at least one embodiment of the invention to permit the use of commercially practical low-cost sensors to provide for the multiple temperature sensing points needed for terminal-to-terminal corrections. 
     The multiplicity of second temperature sensors are RTD sensors. 
     It is thus a feature of at least one embodiment of the invention to provide low-cost temperature sensors readily available in integrated circuit packages. 
     The I/O circuit may execute a stored program to use a thermal model to extend the temperature signals of the second temperature sensors to an estimated cold junction temperature at the terminals. 
     It is thus a feature of at least one embodiment of the invention to permit practical mounting of the cold junction compensation sensors at practical distances from the terminals. 
     The invention may include the steps of: (1) determining a temperature at a third location using the third temperature sensor; (2) using at least one second temperature sensor at a second location to determine a temperature difference between the second location and the third location; (3) using a thermal model and the temperature difference to calculate a corrected temperature at the second location; (4) using the corrected temperature at the second location to estimate a cold junction temperature at a terminal for releasably receiving leads of a first temperature sensor; and (5) estimating a first temperature sensor temperature using a first temperature sensor reading and the cold junction temperature estimate. 
     It is thus a feature of at least one embodiment of the invention to synergistically combine high accuracy and low accuracy cold junction compensation to provide for correction of terminal-to-terminal temperature variations 
     The invention may include a multiplexer circuit having inputs connected to at least two temperature sensors and an output connected to the analog to digital converter. 
     It is thus a feature of at least one embodiment of the invention to manage the additional I/O burden and power consumption of the precision power reference presented by multiple cold junction compensators. 
     These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an I/O circuit per the present invention showing optional four thermocouples and an optional cold junction compensation device attached to two terminal blocks of the I/O circuit for temperature measurement; 
         FIG. 2  is a front perspective view of a portion of the I/O circuit of  FIG. 1  showing one terminal block and a housing; 
         FIG. 3  is a rear perspective view the terminal block of  FIG. 2  without the housings showing two cold-junction compensation sensors associated with the terminals holding the thermocouples; 
         FIG. 4  is a block diagram of a multiplexer of the I/O circuit switching among the cold junction compensation temperature sensors; and 
         FIG. 5  is a flow chart of a program executed by the present invention within the I/O circuit or elsewhere; 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , an I/O circuit  10  may provide for processing electronics  12  having connections, for example, to two terminal blocks  14 ,  16 . The terminal blocks  14 ,  16  have screw terminals  28  or similar design to permit releasable attachment to leads of temperature sensors  30  such as thermocouples, resistance temperature detectors (RTD), thermistors, and solid-state temperature sensors of a type used for sensing temperatures as part of an industrial control process. In addition, the screw terminals  28  may releasably receive a removable cold junction compensation (CJC) temperature sensor, typically a highly accurate resistive temperature device of a type known in the art. 
     The processing electronics  12  provide a processor  18  such as a microprocessor communicating with a network interface  20  connecting over an industrial control network to other parts of an industrial control system  24  allowing communication between the I/O circuit  10  and the industrial control system  24 . The processor  18  may access memory  44  holding stored program  45  for implementation of the present invention, the latter as will be described below. The memory  44  may also hold software for the operation of the I/O circuit  10  according to techniques known in the art (not shown). The processor  18  also communicates with interface circuitry  22  providing analog to digital conversion, amplification and filtering of input signals of a type known in the art. The interface circuitry, in turn, communicates via a first multiplexer portion  46 , the latter communicating with a plurality of compensation RTDs  38 , and a prime RTD  39  via connections  51  as will be discussed below. As well, the interface circuitry  22  communicates via a second multiplexer portion  25  communicating with a plurality of thermocouples  30  via connections  34   
     Referring now to  FIGS. 1 and 2 , generally, the processing electronics  12 , terminal blocks  14 ,  16  and other components listed above may be mounted on printed circuit board  52  and interconnected by conductive traces. The printed circuit board  52  may further include connections for power and the like not shown for clarity. 
     The RTDs  38 ,  39  may be mounted on the printed circuit board in close thermal proximity with the terminal blocks terminals  28 . As noted, the RTDs  38 ,  39  provide temperature signals via lines  51  to a first multiplexer portion  46  under control of an external signal from the processor  18  as will be described below. The first multiplexer portion  46  is in turn connected to the interface circuitry such as and ADC  22  to provide temperature signals to the processor  18 . 
     Typically, RTDs  38 ,  39  require a precisely known amount of power from a power supply  42  in order to calculate the resistance of a given RTD, which is used to determine its temperature. The resistance of the RTDs  38 ,  39  may be converted to a temperature using a linear approximation or by curve fitting to a model of a type known in the art as may be implemented by processor  18 . For this reason, the RTDs  38 ,  39  also receive precision power from power supply  42  over lines  51  from first multiplexer portion  46 . The first multiplexer portion  46  receives a power input  48  from power supply  42  and periodically switches this power among connections  51  to RTDs  38 ,  39  under control of the external signal from the processor  18  also driving the first multiplexer portion  46 . 
     Referring now to  FIGS. 1 and 4 , the RTDs  38 ,  39  may be supplied with power from power supply  42  by a switching component such as first multiplexer portion  46 . The first multiplexer portion  46  may comprise an internal switching element such as solid-state single pole multiple throw switch  96  which may have an external switching control  98  from processor  18 . The switching may be done in a round-robin fashion wherein each switch terminal  100  is connected for some predetermined amount of time as controlled by switching control  98  thereby supplying power to each individual RTD  38 ,  39  for that amount of time. Note that he RTDs&#39; outputs  51  may comprise a pair of differential voltage outputs per RTD wherein the differential voltage is measured by ADC  22  across the RTDs  38 ,  39  resistive component (not shown). 
     Similarly, the thermocouple outputs  34  may be connected to a switching component such as second multiplexer portion  25  as shown in  FIG. 1  . As with the first multiplexer portion  46 , the second multiplexer portion  25  may comprise an internal switching element such as solid-state single pole multiple throw switch which may have an external switching control (not shown). The switching may be done in a round-robin fashion synchronously with the second multiplexer portion  25  wherein each switch terminal is connected for some predetermined amount of time as controlled by switching control  98  thereby supplying the respective RTD signal to the interface circuitry ADC  22  for that amount of time. 
     As noted, the first multiplexer portion  46  and the second multiplexer portion  25  may be operated in tandem wherein control signals connected to switch controls may be synchronized. In this manner, only a single analog to digital converter is needed within interface circuitry  22  for multiple RTDs. As shown, the switching control signals may be provided by clock circuitry under control of the microprocessor or by some other control apparatus. 
     Referring now to  FIG. 1 , in one embodiment, the second multiplexer portion  25  may receive output signals from temperature sensors shown as thermocouples  30  via associated terminals  28  and electrical conductors  34 . The thermocouples may have a single lead tied to ground  43  with the other lead  34  connected to the second multiplexer portion  25 . Alternatively, the thermocouples&#39; outputs  34  may comprise a pair of differential voltage outputs per thermocouple wherein the differential voltage is measured across the thermocouple  30  leads (not shown). In addition, interface circuitry ADC  22  may receive CJC  33  output  36  as a single lead (i.e. single-ended) or may comprise a pair of differential voltage outputs  36 . The first multiplexer portion  46  may, alternatively, also receive signals from RTDs  38 ,  39  via electrical conductors  51 . 
     It will be understood that in an alternative embodiment electrical conductors  34  and  51  which conduct the thermal signals may interface directly with a multiple input interface circuitry  22  without first multiplexer portion  46 , or may interface directly to one or more inputs of processor  18  having the necessary processing circuitry such as an A/D converter. 
     Referring now to  FIG. 2 , a housing  50  may support and/or enclose printed circuit board  52  and processing electronics  12  and a portion of terminal block  16 . In addition, the housing  50  may generally provide a planar base  47  that may be mounted by means of a DIN rail or the like (not shown) so that the base  47  abuts a vertical planar surface of a similar cabinet structure (not shown) or may provide a module or component fitting within a programmable logic controller or the like. 
     The terminal block  16  may consist of a linear array of screw terminals that receive electrical conductors such as leads from one or more thermocouples  30   a  and  30   b . As depicted, temperature sensors such as thermocouples  30   a ,  30   b  may be connected to terminal block  16  at locations  64  and  66 , for example, to measure the temperatures at a remote locations  60  and  62 . In this example, thermocouple  30   a  having leads  68 ,  70  may be connected to terminal block  16  at terminals  72 ,  74  by inserting the thermocouple leads  68 ,  70  into terminals  72 ,  74  respectively and then tightening screws  76 ,  78  to ensure the leads  68 ,  70  are releasably attached. Similarly, for thermocouple  30   b , the leads are inserted into the terminals at location  66  and the associated screws are tightened. 
     The terminal block  16  may have terminals that are configured for receiving CJC  33 . The CJC  33  may measure the temperature at the terminals  32  to which it is connected, or may measure the temperature at different terminals such as terminal  35  to which they communicate by means of a temperature conductive pigtail  54  as illustrated in  FIG. 2 . The location, whether at terminals  32  or  35 , may be entered to the processor  18  by an input device (not shown), for example, communicating with the industrial control system  24  (and thus to the processor  18 ) over the network interface  20 , or the information about the terminal  32  of CJC  33  may be pre-programmed or otherwise accounted for by information stored in the processor memory  44 . 
     Turning now to  FIG. 3  but also referring to  FIG. 2 , the printed circuit board  52  holding the processing electronics  12  described above may also support one or more compensation RTDs  38   a ,  38   b  and prime RTD  39 . Compensation RTDs  38   b  and  38   a  are in thermal proximity to the thermocouple cold junctions  30   a ,  30   b  respectively at locations  64  and  66 , while prime RTD  39  is in thermal proximity to CJC  33 . 
     The compensation RTDs  38   a ,  38   b  are utilized in the measurement of the temperature of their respective proximal thermocouple cold junctions. For example, compensation RTD  38   a  may be proximate to screw terminals  86 ,  84  (of thermocouple  30   b ) and compensation RTD  38   b  may be proximate to screw terminals  76 ,  78  (of thermocouple  30   a ). Thus, compensation RTD  38   a  may be used to help determine the temperature of the cold junction for thermocouple  30   b  and compensation RTD  38   b  may be used to help determine the temperature of cold junction for thermocouple  30   a . In addition, prime RTD  39  may be used to help determine the temperature of both thermocouples  30   a ,  30   b  cold junctions as will be explained below. 
     Referring primarily to  FIG. 5  but also to  FIGS. 1 ,  3  and  4 , a method of determining the temperature of a thermocouple is described. To measure the temperature of a thermocouple, for example  30   b , the processor  18  may execute a stored program  45  in memory  44  to read the CJC  33  and determine its cold junction temperature per process block  102 . 
     Next, in process block  104 , the compensation RTD  38   a  for thermocouple  30   b  is read along with the prime sensor RTD  39  nearby CJC  33 . This reading process may include clocking of the first and second multiplexer portion  25 ,  46  appropriately until all necessary readings have been obtained. 
     At process block  106  a temperature difference is determined between compensation RTD  38   a  and prime RTD  39 . The temperature difference is used to correct the temperature measured by CJC  33  to yield the estimated cold junction temperature of thermocouple  30   b  at location  66  (similarly, the temperature difference between compensation RTD  38   b  and prime RTD  39  is used with CJC  33  to estimate the cold junction temperature of thermocouple  30   a  at location  64 ). By measuring the temperature difference between RTDs this eliminates any common temperature offset error between the respective RTDs which may improve accuracy. 
     However, compensation RTDs  38  and prime RTD  39  may also be calibrated to achieve the measurement accuracy required for a given industrial process. For example, in one embodiment, all of the RTDs  38 ,  39  may be calibrated by taking at least one calibrating measurement at the factory. Alternatively or in addition, RTDs  38 ,  39  may also be calibrated after manufacture through the use of the more accurate CJC  33 . Either calibration yields a model to map RTD readings to temperatures. For example, the model for a given RTD could comprise a single temperature coefficient to map the measured resistance of the RTDs to a temperature. 
     At process block  108  the thermocouples are read and their respective cold junction temperatures (determined in block  106 ) are used to compensate the thermocouple readings to produce an the compensated thermocouple temperature. Finally, the compensated thermocouple temperature signals may be transmitted to the industrial control system  24  for use in controlling equipment or the like at process block  110 . 
     Generally an empirically derived thermal model may be determined by tabulating readings at a variety of different ambient temperatures and in different applications. Multiple temperature measurements may then be averaged or otherwise statistically analyzed to produce model parameters that function to correct the temperature readings or difference temperature estimates of the compensation RTDs and prime RTD. In one embodiment, a simple thermal model such as a multiplicative factor k may be multiplied by the RTD. In another embodiment the thermal model may comprise a multiplicative factor k and an offset C. In either embodiment, the model parameters k and C or other parameters may be unique to a given installation, a given hardware configuration, or may vary with temperature. In yet another embodiment the thermal model may provide a lookup table that may be accessed by some value such as the temperature or reading difference of compensation RTD  38  and a prime RTD  39 . 
     Certain terminology is used herein for purposes of reference only, and thus is not intended to be limiting. For example, terms such as “upper”, “lower”, “above”, and “below” refer to directions in the drawings to which reference is made. Terms such as “front”, “back”, “rear”, “bottom” and “side”, describe the orientation of portions of the component within a consistent but arbitrary frame of reference which is made clear by reference to the text and the associated drawings describing the component under discussion. Such terminology may include the words specifically mentioned above, derivatives thereof, and words of similar import. Similarly, the terms “first”, “second” and other such numerical terms referring to structures do not imply a sequence or order unless clearly indicated by the context. 
     When introducing elements or features of the present disclosure and the exemplary embodiments, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of such elements or features. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements or features other than those specifically noted. It is further to be understood that the method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
     References to “a microprocessor” and “a processor” or “the microprocessor” and “the processor,” can be understood to include one or more microprocessors that can communicate in a stand-alone and/or a distributed environment(s), and can thus be configured to communicate via wired or wireless communications with other processors, where such one or more processor can be configured to operate on one or more processor-controlled devices that can be similar or different devices. Furthermore, references to memory, unless otherwise specified, can include one or more processor-readable and accessible memory elements and/or components that can be internal to the processor-controlled device, external to the processor-controlled device, and can be accessed via a wired or wireless network. The terms multiplexer and de-multiplexer are used synonymously, being simply a matter of context. 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. All of the publications described herein, including patents and non-patent publications, are hereby incorporated herein by reference in their entireties.