Patent Application
20150276498
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 to 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 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.
In using a thermocouple in an industrial process, it is important to establish that the thermocouple remains connected; otherwise a disconnected thermocouple may be interpreted as an erroneous temperature value. For this purpose, it is known to provide a small bias current (e.g. 25 nA) through the thermocouple wire to establish continuity and hence the presence of the thermocouple. The voltage change provided by the Seebeck effect may be small and this bias current is selected so that the resistance of the thermocouple and the voltage drop caused by the bias current is minor compared to the Seebeck effect voltage.
However, thermocouple wire typically has a high resistivity. Further, the thermocouple may be positioned some distance from the input module. As a result, a voltage drop is generated along the thermocouple wire due to the bias current and the resistance of the thermocouple wire. Although the bias current is kept to a minimal value (e.g. 25 nA), the magnitude of the voltage potential generated from the bias current conducted in the thermocouple wire may be sufficient to introduce error in the temperature measurement. Thus, it would be desirable to provide a control system to compensate for the voltage potential generated by the bias current.
The subject matter disclosed herein describes an interface circuit for an PO module in an industrial controller to compensate for a voltage potential generated by a bias current in a thermocouple. During a calibration routine, the interface circuit supplies two known bias currents to a thermocouple and measures a voltage generated across the thermocouple as a result of each bias current. The measured voltages and known current values are used to determine the resistance value of the thermocouple leads. Using two known bias currents provides for an accurate measurement of the resistance value of the thermocouple leads when the thermocouple is generating a voltage corresponding to the measured temperature. Either the I/O module or the industrial controller may determine a voltage resulting from the bias current applied to the thermocouple during operation as a function of the measured resistance and compensate the voltage measured at the thermocouple leads to accurately determine the measured temperature. Accordingly, the present invention dynamically measures the resistance of the thermocouple wire to provide an accurate offset voltage that can be used to remove the voltage of the bias current in the thermocouple measurement.
According to one embodiment of the invention, an interface circuit for use with a temperature sensor connected to an industrial controller includes a first terminal, a second terminal, and a resistance measuring circuit. The first terminal is configured to releasably receive a first lead of the temperature sensor, and the second terminal is configured to releasably receive a second lead of the temperature sensor. The temperature sensor is of a type to generate a voltage as a function of a measured temperature. The resistance measuring circuit is operatively connected to at least one of the first terminal and the second terminal and configured to measure a resistance of the first and second leads when the temperature sensor is generating the voltage.
According to another embodiment of the invention, a module for use with an industrial controller and configured to be connected to a temperature sensor includes a first terminal, a second terminal, a switch, and a controller. The first terminal is configured to releasably receive a first lead of the temperature sensor, and the second terminal is configured to releasably receive a second lead of the temperature sensor. The temperature sensor is of a type to generate a voltage as a function of a measured temperature. The switch is configured to receive a control signal and to selectively provide either a first bias current or a second bias current to either the first or the second terminal as a function of the control signal. The controller is configured to generate the control signal for the switch and to receive a signal corresponding to a measured voltage present between the first terminal and the second terminal.
According to yet another embodiment of the invention, a method of measuring temperature with a module of an industrial controller is disclosed. The temperature is measured with a temperature sensor having a first lead and a second lead, where the first lead is connected to a first terminal of the module and the second lead is connected to a second terminal of the module. The temperature sensor generates a signal voltage as a function of the temperature. A first bias current is supplied to the first lead of the temperature sensor, and a first voltage is measured between the first terminal and the second terminal. A second bias current is supplied to the first lead of the temperature sensor, and a second voltage is measured between the first terminal and the second terminal, where the second bias current has a different amplitude than the first bias current. A resistance value of the first lead and the second lead is determined as a function of the first and the second measured voltages, and a compensation voltage is subtracted from the signal voltage, where the compensation voltage corresponds to a magnitude of a bias current applied to the first terminal when measuring temperature multiplied by the resistance value of the first lead and the second lead from the signal voltage.
These and other advantages and features of the invention will become apparent to those skilled in the art from the detailed description and the accompanying drawings. It should be understood, however, that the detailed description and accompanying drawings, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.
Various exemplary embodiments of the subject matter disclosed herein are illustrated in the accompanying drawings in which like reference numerals represent like parts throughout, and in which:
In describing the various embodiments of the invention which are illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, it is not intended, that the invention be limited to the specific terms so selected and it is understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. For example, the word “connected,” “attached,” or terms similar thereto are often used. They are not limited to direct connection but include connection through other elements where such connection is recognized as being equivalent by those skilled in the art.
Referring now to
The industrial controller 12 may communicate with a human machine interface (HMI) 19 including, for example, a display 20 and a keyboard 22 or the like, for outputting information to a user and receiving instructions from a user. It is contemplated that the HMI 19 may include other devices, either separately connectable or integrated into a single chassis, including, but not limited to, a keyboard, touchpad, mouse, trackball, or a touch-screen display device. The HMI 19 may further include a memory device, processor, communication ports and other hardware components according to the system requirements. It is further contemplated that multiple display devices and/or multiple input devices may be distributed about the controlled machine or process and connected to one or more processing devices. The HMI 19 may be used to display operating parameters and/or conditions of the controlled machine or process, receive commands from the operator, or change and/or load a control program or configuration parameters.
The industrial controller 12 is further configured to communicate with one or more I/O modules 24a, 24b that may provide signals to actuators on the industrial machine or process or receive signals from sensors on that industrial machine or process. In the illustrated example, the I/O module 24a provides connections to leads 23 of a thermocouple 26 having a hot junction 32, the latter generating a Seebeck effect voltage according to the temperature of the hot junction 32 as is understood in the art. The I/O module 24a also includes a controller 25 that may be a microprocessor, logic circuit, or combination thereof and that may be configured to execute a firmware program stored in memory in the I/O module. The I/O module 24a further includes an interface circuit 27 configured to be connected to the thermocouple leads and which will be discussed in more detail below.
Referring next to
The interface circuit 27 also includes a resistance measuring circuit configured to measure the resistance of the thermocouple leads 23 connected to the terminals 30a, 30b. According to the embodiment illustrated in
A switch module 42 is provided to selectively connect the first terminal 30a to one of two voltage sources. The switch module 42 receives a control signal 43 to selectively control operation. It is contemplated that the switch module 42 may include a solenoid and relay, where the relay is in a first position when the solenoid is energized and a second position when the relay is de-energized, and the solenoid may be energized/de-energized responsive to the control signal 43. Optionally, the switch module 42 may be a sold-state device including one or more transistors that establish a first conduction path in a first mode and a second conduction path in a second mode, and the operating mode is selected responsive to the control signal 43. Still other configurations of the switch module 42 may be utilized to select between two modes as a function of the control signal 43 without deviating from the scope of the invention.
Referring now to
A bias current may be provided through cold junction terminal 30a by a current source. According to the illustrated embodiment, the current source is an operational amplifier 36 whose output is connected to cold junction terminal 30a and has its inverting input connected to the cold junction terminal 30b and also to a junction of two different precision resistors 38, 40. The non-inverting input of the operational amplifier 36 may be connected to a ground reference. The ends of these precision resistors 38, 40 opposite the inverting terminal of the operational amplifier 36 may be connected to a switch module 42. The switch module 42 is illustrated as a single pole dual throw switch with the end of each of the precision resistors 38, 40 being connected to one of the throws of the switch and the pole of the switch being connected to a precision negative voltage source 44. The position of the pole of the switch module 42 may be toggled between throws by a control signal 43 generated by the controller 25. As discussed above, it is contemplated that the switch module 42 may include a solenoid and relay, where the relay is in a first position when the solenoid is energized and a second position when the relay is de-energized, and the solenoid may be energized/de-energized responsive to the control signal 43. Optionally, the switch module 42 may be a sold-state device including one or more transistors that establish a first conduction path in a first mode and a second conduction path in a second mode, and the operating mode is selected responsive to the control signal 43. Still other configurations of the switch module 42 may be utilized to select between two modes as a function of the control signal 43 without deviating from the scope of the invention.
In operation, the interface circuit 27 is used to measure the resistance of the thermocouple leads 23 and to provide the measurement to the controller 25. The controller 25, in turn, may compensate the voltage present at the cold terminals 30a, 30b to subtract a voltage generated by the bias current to improve the accuracy of the temperature measurement. Optionally, the interface circuit 27 may be configured to compensate the voltage present at the cold terminals 30a, 30b prior to providing the measurement to the controller 25. Referring next, to
As shown in steps 102-106, the I/O module 24a provides the bias current during normal operation of the industrial controller 12 to detect that the thermocouple is connected and has not failed. The I/O module 24a also includes a circuit to detect if the thermocouple is not connected or if the connection between the two metals at the hot junction 32 has failed, creating an open circuit. At step 102, the I/O module 24a measures the bias current provided to the thermocouple 26. At step 104, the I/O module 24a determines whether the bias current is zero. If no bias current is flowing through the thermocouple 26, either the thermocouple 26 has become disconnected from the cold terminals 30a, 30b or a failure has occurred. The I/O module 24a may set an internal status flag at step 106 which may be, for example, transmitted to the industrial controller 12 such that a message may be posted on the HMI 19 or an interlock condition initiated in the control program 18 to prevent further execution until the thermocouple 26 has been repaired or replaced. Referring to
If, at step 104, the I/O module 24a determines that continuity exists in the thermocouple circuit, the controller 25 continues execution by determining whether a calibration routine has been executed, as shown in step 108. It is contemplated that the calibration routine may be run a single time during a commissioning procedure for the I/O module 24a, upon power up of the I/O module 24a, or at a periodic interval to detect temperature dependent effects on resistance in the thermocouple 26. Upon completion of a calibration routine, an internal status flag may be stored in a memory device of the I/O module 24a. The status flag may be reset, for example at the periodic interval, if desired, or upon loss of power. If the calibration routine has been executed, determination of the measured temperature continues at step 114 as discussed below. If the calibration routine has not been executed, the controller 25 executes steps 110 and 112 to determine a resistance value of the thermocouple leads 23.
As shown in step 110, determination of the resistance value of the thermocouple leads 23 begins by taking at least two measurements of the voltage present at the cold terminals 30a, 30b under two different operating conditions in which two different bias currents are provided to the thermocouple 26. It is contemplated that one of the bias currents may be the normal operational bias current and a second bias current may be a calibration bias current. Optionally, two calibration bias currents may be established under the two different operating conditions. With reference again to
With reference again to
Referring also to
After running the calibration routine, temperature measurement continues at step 114. The voltage potential, Vjunction, at the cold junction terminals 30a, 30b, is measured by the A/D converter 34. The A/D converter 34 generates a digital signal corresponding to the voltage potential, Vjunction, which is readable by the controller 25. At step 116, the junction voltage potential, Vjunction, is compensated for the voltage generated by the operational bias current. According to one embodiment of the invention, an offset value may be stored in the A/D converter 34 and subtracted from the junction voltage potential, Vjunction. The offset value is a compensation voltage, Vjunction, which corresponds to the magnitude of voltage generated in the thermocouple leads 23 due to the operational bias current applied and of the resistance value of the thermocouple leads 23. This compensated voltage corresponds to a temperature voltage, Vt, generated at the hot junction 32 of the thermocouple 26. The compensation voltage, Vcomp, may be determined according to the following formula:
V
comp
=R
thermocouple leads
*I
operational bias (1)
Optionally, the A/D converter 34 may provide a digital signal corresponding to the junction temperature to the controller 25 and the controller may compensate the digital signal by subtracting an offset value corresponding to the magnitude of the compensation voltage.
At step 118, the controller 25 determines the temperature at the hot terminal 32 of the thermocouple 26. The temperature may be determined, for example, based on a table stored in memory in which the table includes a list of temperatures corresponding to the digital value. Optionally, configuration parameters may be stored in memory setting for example, a temperature corresponding to a minimum digital value and a maximum digital value expected from the A/D converter 34 and the controller 25 interpolates between the minimum and maximum digital values based on the measured value. According to still other options, the controller 25 in the I/O module may transmit the digital value to another microcontroller, such as a processor module in the industrial controller 12 where the temperature is determined. It is further contemplated that still other methods of determining the temperature from the compensated voltage, Vcomp, may be implemented without deviating from the scope of the invention.
Historically, the operational bias current has been kept to a minimal value (e.g. 25 nA) to similarly keep the magnitude of the voltage generated by the bias current on the thermocouple leads to a minimum. However, generating a bias current of such small value requires precise current control, and small variations in the amplitude due, for example, to electrical noise or other disturbances result in large percentage error in the bias current. Because the I/O module 24a determines the compensation voltage, Vcomp, and subtracts it from the junction voltage, Vjunction, providing an accurate voltage from which temperature may be determined, a higher magnitude operational bias current may be used, making it more immune to electrical noise or other disturbances. In addition, the higher magnitude operational bias current permits more rapid detection of failures in the thermocouple 26. It is contemplated that the magnitude of the operational bias current may be greater than 25 nA and in a range between, for example, 25 nA-1 μA. According to one embodiment of the invention, the operational bias current is about 150 nA.
According to another aspect of the invention, the precision resistors 38, 40 and voltage reference 44 and/or current source may also be configured to generate a compensation bias current having a relatively higher current. The compensation bias current may be at least one order, and preferably more than one order, of magnitude higher than the operational bias current. The compensation bias current may be, for example, at least 10 μA and preferably about 100 μA. The increased magnitude of the compensation bias current makes it again less susceptible to disturbance and also generates two measurement points for the VI plot, as shown in
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 dc-multiplexer are used synonymously, being simply a matter of context.
It should be understood that the invention is not limited in its application to the details of construction and arrangements of the components set forth herein. The invention is capable of other embodiments and of being practiced or carried out in various ways. Variations and modifications of the foregoing are within the scope of the present invention. It also being understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text and/or drawings. All of these different combinations constitute various alternative aspects of the present invention. The embodiments described herein explain the best modes known for practicing the invention and will enable others skilled in the art to utilize the invention
This application claims priority to U.S. provisional application Ser. No. 61/971,207, filed on Mar. 27, 2014, the entire content of which is hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61971207 | Mar 2014 | US |
