Apparatus and method for detection of an open thermocouple in a process control network
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to networks that receive and transmit groups of logic signals from remote locations and, more particularly, to the verification of operation of the thermocouple elements located at remote sites in a control network. 
     2. Description of the Related Art 
     In the automatic control of processes, process control networks are provided to collect status information by at least one central location and to distribute control signals to the devices coupled to the network. Referring to FIG. 1, a portion of such a process control network is shown. The process control network includes a system bus 5 which has at least one central control unit 6 coupled thereto and at least one bus interface module 9 also coupled thereto. The central control unit 6 receives signals from the system bus 5 that typically communicate the status of user devices coupled to the process control network and applies signals to the network that provide the control signals determining the operation of the user devices. The bus interface module 9 applies status signals to the system bus and receives signals from the system bus 5 which are relevant to a subset of network user devices 10 that communicate with the central control unit 6 through bus interface module 9. User devices 10 capable of responding to digital signals, such as motors, lamps and heaters, receive control signals through device interface unit 16 and digital output module 12 from the bus interface unit 9. User devices 10 providing digital status signals in response to contact closures provide status signals to the bus interface module 9 by means of device interface unit 17 and digital input module 13. User devices 10 such as control valves, responding to analog control signals, receive signals from the bus interface module 9 through output analog module 14 and device interface unit 18. User devices 10 such as flowmeters, level detectors, pressure meters, temperature sensors, etc. of the type providing analog status information, provide the status signals to bus interface module 9 through device interface unit 19 and analog input module 15. The device interface units 16, 17, 18 and 19 convert the signals received from and transmitted to the user devices 10 into a format usable by the digital input and output modules and the analog input and output modules, respectively. The digital input and output modules 13 and 12 and the analog input and output modules 15 and 14 couple the signals of the user devices, as transformed by the device interface unit, with the bus interface module 9 by means of local bus 11. 
     The status information applied to the process control network has an accuracy limited by the accuracy of the sensor elements providing signals to a central control unit determined by environmental conditions. One of the important environmental factors in many processes is the temperature. The temperature is typically determined by a thermocouple element. Because of the critical nature of the temperature in the control of processes, failure of a thermocouple can compromise large amounts of processing activity prior to the identification of a failure. In the related art, a thermocouple has been tested by placing a voltage source and a large resistor in series with a thermocouple and checking for continuity. Using this technique, only catastrophic failures were detected and a thermocouple could provide erroneous reading prior to detectable failure. 
     A need has therefore been felt for apparatus and method that can operate continuously to identify the failure of a thermocouple to prevent the distribution of erroneous data. 
     FEATURES OF THE INVENTION 
     It is an object of the present invention to provide an improved process control network for transmission of status and control signals. 
     It is a feature of the present invention to provide a technique for the verification of a thermocouple. 
     It is another feature of the present invention to test periodically a thermocouple to determine when failure has occurred. 
     It is a more particular feature of the present invention to couple periodically a circuit in parallel with thermocouple and compare voltage levels with and without the inserted coupled circuit. 
     SUMMARY OF THE INVENTION 
     The aforementioned and other features are accomplished, according to the present invention, by testing apparatus that continuously monitors the parameters of a thermocouple. Periodically a known voltage source and series resistance are coupled in parallel with the thermocouple element and the associated lead resistance. A high impedance amplifier measures the voltage at an intermediate point in the circuit when the voltage source and series resistance are coupled to the thermocouple and the amplifier measures the voltage of the thermocouple when the voltage source and the series resistance are removed. The circuit components are chosen so that, in general, the voltage levels do not reach equilibrium during the periodic switching operation. By comparing voltage measurements at preselected times in the switching cycle, the parameters of the thermocouple can be monitored and a failing component identified. 
     These and other features of the present invention will be understood upon reading of the following description along with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a block diagram of process control network capable of using the present invention. 
     FIG. 2 is a circuit diagram of the components used in periodically testing a thermocouple according to the present invention. 
     FIG. 3 illustrates the voltage applied to the input terminal of the amplifier A. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     1. Detailed Description of the Figures 
     FIG. 1 has been described with reference to the related art. 
     Referring next to FIG. 2, a circuit diagram illustrates the components associated with a thermocouple that can be used to test periodically whether the thermocouple has failed. A thermocouple TC 1  is coupled between the common voltage terminal and a first terminal of resistor R LEAD . A second terminal of R LEAD  is coupled to a first terminal of resistor R BO , to a first terminal of resistor R 2  and to a first terminal of switch JS 4 . A second terminal of resistor R BO  is coupled through switch JS BO  to a first terminal of voltage source V BO , a second terminal of voltage source V BO  being coupled to the common potential. A second terminal of switch JS 4  is coupled to an input terminal of amplifier A. A second terminal of resistor R 2  is coupled through capacitor C 2  to the common potential and through switch JS 2  to the input terminal of amplifier A. The output terminal of amplifier A is coupled to an analog to digital converter unit 21 and the output of the analog to digital converter unit 21 is applied to comparison unit 22. Comparison unit 22 is also coupled to switches JS BO , JS 4  and JS 2 . 
     Referring next to FIG. 3A, the waveform of the voltage applied to amplifier A is shown. Prior to time S 1 , an equilibrium value of the thermocouple 20 output voltage will be applied to amplifier A. At time S 1 , the voltage source V BO  and associated resistance R BO  are coupled to the thermocouple 20 and the voltage applied to amplifier A will rise. At time S 2 , the voltage source V BO  and the series resistance will be removed from the circuit and the voltage applied to amplifier A will approach the equilibrium value established prior to time S 1 . At time S 3 , the switch states at time S 1  are replicated and the source voltage V BO  and resistance R BO  are coupled to the thermocouple. 
     2. Operation of the Preferred Embodiment 
     The operation of the testing apparatus can be understood by reference to FIG. 2. When switches JS BO  and JS 4  are open and switch JS 2  is closed, the high input impedance amplifier, which is typically an analog to digital converter unit in an analog input module 15, measures the voltage generated by the thermocouple. When switches JS BO  and JS 4  are closed and switch JS 2  is open, the amplifier A measures a voltage 
     
         V.sub.A=VIN =V.sub.TC +R.sub.LEAD *(V.sub.TC -V.sub.BO)/(R.sub.LEAD +R.sub.BO)                                                (1) 
    
     from which R LEAD  can be determined, R BO  and V BO  being known and V A  being a measured quantity. V TC  is measured again with switches JS BO  and JS 4  open and switch JS 2  closed. The magnitude of the disturbance caused by measuring the voltage involving both the thermocouple voltage and the reference voltage on the thermocouple can then be determined. This correction that is determined by the measurements is used to compensate for the disturbance of the thermocouple voltage before the quantity is used by the system. 
     Rather than measure an equilibrium parameter (voltage) value as described above, the preferred embodiment couples the source voltage V BO  and series resistance R BO  periodically and the voltage measurements are performed at preselected periods in the cycle. Referring once again to FIG. 3, the time interval T 1  illustrates a voltage level when switch JS BO  and switch JS 4  are open and switch JS 2  is closed and sufficient time has elapsed for transient conditions to have ended. At time S 1 , the switch positions are reversed and the voltage applied to amplifier A, V A  =V IN , approaches the value given by equation 1. The time interval T 2  provides a measurement that is a function of the thermocouple parameters and the known circuit parameters. At time S 2 , the switch states are reversed and the voltage applied to amplifier A approaches the equilibrium value measured during period T 1 . At time S 3 , the cycle started at time S 1  is repeated. The voltages determined intervals T 1 , T 2  and T 3  are measured by an integrating type analog to digital converter (21). The integration takes place over preselected portions of the switching cycle represented symbolically by periods T 2  and T 3 . Included in these measurements is an indicia of the quality of the thermocouple. For example, as the thermocouple resistance R LEAD   increases or becomes an open circuit, the magnitude of the voltage measured during T 2  becomes larger. Also, the difference between the voltage measured during T 2  and T 3  will change, the charge on capacitor C 2  decaying slowly or being trapped thereon. The comparison of the voltage measured during T 2  and T 3  is performed in comparison unit 22 by techniques known in the prior art. Comparison unit 22 can also include the logic apparatus to determine an appropriate response to results of a comparison of the two voltage levels, i.e., a functioning thermocouple, a suspect or failing thermocouple and a failed thermocouple. 
     Because the temperature (measured by a thermocouple) can be crucial importance, the apparatus of the present invention is adapted to have a cycle that is 250 milliseconds in length. In this manner, a failed thermocouple can be identified before the process control network, operating in response to questionable data, can institute potentially disastrous responses. It will be clear that the switches JS BO , JS 4  and JS 2  as well as the analog to digital converter 21 must be coordinated (or synchronized) to insure that measurements of the voltages during the intervals T 2  and T 3  occur at the preselected portions of the cycle. In the preferred embodiment, this coordination is accomplished by the comparison unit 22. Other coordination apparatus can be used with the same result. It will also be clear that amplifier A can be included as a part of the other apparatus, such as analog to digital converter 21. 
     The foregoing description is included to illustrate the operation of the preferred embodiment and is not meant to limit the scope of the invention. The scope of the invention is to be limited only by the following claims. From the foregoing description, many variations will be apparent to those skilled in the art that would yet be encompassed by the spirit and scope of the invention.