Thermo wire testing circuit

Abstract

A thermo wire testing circuit, comprising: a current source terminal for supplying a test current to a first thermo wire via a first terminal during a test mode; a current drain terminal for receiving the test current from a second thermo wire via a second terminal during the test mode; a reference resistor for generating a reference voltage, which reference resistor is arranged in the current circuit of the test current; and a processing unit coupleable to the first and second terminals and to the reference resistor, and configured to compare a voltage drop caused by said test current between the first and second terminal with the reference voltage.

Claims

1. A thermo wire testing circuit for testing a first and a second thermo wire of a thermocouple, comprising: a current source terminal for supplying a test current to a first thermo wire via a first terminal during a test mode; a current drain terminal for receiving said test current from a second thermo wire via a second terminal during said test mode; a reference resistor for generating a reference voltage, which reference resistor is arranged in the current circuit of said test current; a processing unit coupleable to said first and said second terminals and to said reference resistor, and a switch which is embodied to direct the test current either through the thermos wires and the reference resistor in a first stage or in a second stage through the reference resistor, wherein said processing unit is configured to tap a voltage drop caused by said test current between the first and second terminal and to compare said voltage drop with said reference voltage in the first and second stage, wherein said voltage drop in the first stage consists of a component generated by a thermo voltage caused by the thermoelectric effect and a component generated by the resistive component of the thermos wires; and said voltage drop in the second stage consists of said thermo voltage only to eliminate a contribution of said thermo voltage measured in the second stage to said voltage drop tapped during the first stage to determine an ohmic resistance of the said first and second thermo wire based on the resistance of a reference resistor, and to determine a defect of said first and second thermo wire.

2. The thermo wire testing circuit according to claim 1, wherein: said processing unit is further configured to determine a resistance present between said first and said second terminal, in particular to determine the ohmic resistance of said first and said second thermo wire.

3. A thermo wire testing circuit according to claim 1, wherein: said processing unit is configured to determine a defect of said first and said second thermo wire, e.g. based on said voltage drop caused by said test current.

4. A thermo wire testing circuit according to claim 1, wherein: said reference resistor is arranged to one of the following between said current source terminal and said first terminal, or said reference resistor is arranged between said second terminal and said current drain terminal.

5. The thermo wire testing circuit according to claim 1, wherein: a switch for injecting the test current is arranged according to one of the following between said current source terminal and said first terminal, or said switch is arranged between said second terminal and said current drain terminal.

6. The thermo wire testing circuit according to claim 1, wherein: said processing unit possesses an operational input and is configured to receive said reference voltage at its operational input.

7. The thermo wire testing circuit according to claim 1, wherein: said processing unit possesses a reference voltage terminal for receiving said reference voltage.

8. The thermo wire testing circuit according to claim 1, wherein: said processing unit comprises an Analog-to-Digital-Converter which is operably coupled to said first and said second terminal and configured to compare a voltage between said first and said second terminal to said reference voltage.

9. The thermo wire testing circuit according to claim 1, wherein: said processing unit is configured to determine said voltage between said first and said second terminal during said test mode at times said test current is injected and at times when no test current is injected, e.g. injected from said current source in said first and/or said second terminal.

10. The thermo wire testing circuit according to claim 1, wherein: said processing unit is configured to take into account the resistance of said reference resistor in order to determine a resistance of the said first and said second thermo wire and thus a defect of the thermocouple.

11. The thermo wire testing circuit according to claim 1, wherein: said processing unit is configured to determine a defect of the thermocouple based on the ohmic resistance of the said first and said second thermo wire.

12. The thermo wire testing circuit according to claim 1, wherein: said processing unit is configured to compare said ohmic resistance of said first and said second thermo wire with a threshold value.

13. The thermo wire testing circuit according to claim 1, wherein: said processing unit comprises a regular mode in which no test current is supplied and said processing unit is configured to process the voltage drop between said first and said second terminal in order to determine a thermo voltage and thus a temperature.

14. A method for testing a first and a second thermo wire of a thermocouple, comprising the steps of: generating a test current flowing through said first and said second thermo wire; directing the test current either through the thermos wires and the reference resistor in a first stage or in a second stage through the reference resistor only, tapping a voltage drop caused by said test current between the first and second terminal and comparing said voltage drop across the first and second thermo wire with said reference voltage generated by a reference resistor, wherein said voltage drop in the first stage consists of a component generated by a thermo voltage caused by the thermoelectric effect and a component generated by the resistive component of the thermos wires, and wherein said voltage drop in the second stage consists of said thermo voltage (V.sub.RTC) only eliminating a contribution of said thermo voltage (V.sub.RTC) measured in the second stage to said voltage drop tapped during the first stage; and determining an ohmic resistance of the said first and second thermo wire based on the resistance of a reference resistor (R.sub.ref), and determining a defect of said first and second thermo wire.

15. The method according to claim 14, further comprising the step of: comparing the voltage drop across the first and second thermo wire with a reference voltage generated by said reference resistor when no test current is generated.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is described in more detail in accordance with the following figures:

(2) FIG. 1: shows a first embodiment of a thermo wire test circuit in which the reference voltage generated by the voltage drop across the reference resistor is supplied as a reference voltage at a reference input of the ADC; and

(3) FIG. 2: shows a second embodiment of a thermo wire test circuit in which the reference voltage generated by the voltage drop across the reference resistor is supplied to an operational input of the ADC.

DETAILED DISCUSSION IN CONJUNCTION WITH THE DRAWINGS

(4) FIG. 1 shows a thermo wire testing circuit C coupled to a first and second thermo wire T1, T2 forming a thermocouple TC via a first and second terminal F1, F2. The circuit C may be arranged inside a housing H of a temperature transmitter. The circuit C may also be part of an operating electronics which serves for providing other functions and functionalities of the transmitter.

(5) The circuit C comprises an Analog-to-Digital-Converter ADC which is coupled to the first and second terminal F1, F2 via its operating inputs O1, O2. The reference voltage inputs P1, P2 of the ADC is coupled to a reference resistor R.sub.Ref via a first and a second tap-off F4, F4. The circuit C further comprises a current source terminal S for supplying a test current. The current source may comprise an IDAC which may be integrated in the ADC. The current source S is coupleable to the first and second thermo wire T1, T2 via a switch W1.

(6) Thus, the test current originating from the current source may be set to flow via a first current path in case the switch W1 is in a first position, through the thermo wires T1, T2 and the reference resistor R.sub.Ref. The switch W1 is configured to let the test current flow via a second current path in case the switch is in a second position through the reference resistor only. The test current is in both cases flowing from the current source S terminal to a current drain terminal G, which may be a ground potential. The switch W1 may also have further settings in which e.g. the current source terminal is isolated from the first and second terminal F1, F2 and the reference resistor R.sub.Ref. In contrast to the embodiment as shown in FIG. 1, the reference resistor R.sub.Ref may also be arranged in the current path between the switch W1 and the first terminalinstead of between the second terminal F2 and the current drain terminal G. The current path or current circuit is the way the test current flows from the current drain to the current source.

(7) In a first stage the test current is set e.g. via switch W1 to flow through the thermocouple TC and the reference resistor R.sub.Ref. In this first stage the voltage drop between the terminals F1, F2 is measured by the ADC, e.g. by connecting terminals F1, F2 to operating inputs of the ADC. This voltage drop consists of basically two components, i.e. a component generated by the thermovoltage due to the Seebeck-effect and a component due to the ohmic resistance of the thermo wires which is generated by the test current flowing through the thermo wires T1, T2.

(8) The voltage drop at the operating inputs O1, O2 of the ADC may is compared by the ADC with the reference voltage at a reference voltage input P1, P2 of the ADC. The output signal is thus proportional to the ratio of the signal at the operational input to the signal at the reference input. The output signal may be transmitted to a further part SO of the processing unit which serves for further processing, e.g. for outputting an alarm signal.

(9) In a second stage the test current is set to flow through the reference resistor R.sub.Ref only. During this stage the voltage drop between the first and second terminal F1, F2 is also measured (although no test current being present in the thermo wires) and compared with the voltage drop across the reference resistor R.sub.Ref. However the voltage drop between the first and second terminal F1, F2 is now essentially only due to the Seebeck-effect. Thus the contribution of the thermo voltage to the voltage drop between the first and second terminal F1, F2 during the first stage can be eliminated and the ohmic resistance of the thermocouple may be determined. Of course the order of stages may be reversed or intermediate stages may be added.

(10) As shown in FIG. 1 a single excitation source S may be used to inject the test current into the thermocouple and to perform a precise resistance measurement. The thermocouple TC is considered as a pure resistive sensor, i.e. like a platinum based resistive sensor. Another important benefit of the circuit architecture of FIG. 1 and FIG. 2 is the possibility to remove all internal resistances, i.e. the resistances present in the current path between the current source S and the first terminal F1 and between the second terminal F2 and the current drain G. This architecture allows measuring only the voltage drop produced in the thermocouple connected on terminals F1 and F2. Errors produced by resistance on measurement input F1 and F2 are negligible when the device used to measure on F1 and F2 inputs exhibits high input impedance (as several analog-digital converters available on the market).

(11) In the embodiments according to FIG. 1 and FIG. 2 the thermocouple's measurement cycle is divided in two parts; one normal measurement of the thermocouple's thermo voltage and a new diagnostic phase where additional information are collected to perform an advanced diagnostic. The diagnostic phase may comprise the two stages described above.

(12) During normal operation the current source S is switched off or isolated from the first and second terminal F1, F2 and thus from the thermocouple TC by means of a switch W1. This prevents any excitation current to flow into the thermocouple sensor. Measuring on thermocouple terminals F1, F2 the normal thermo voltage contribution is measured and yield the thermovoltage V.sub.thermo.

(13) During diagnostic phase the excitation current source S is used to inject current into the thermocouple sensor. The voltage Vdiag at the thermocouple's terminals F1, F2 will be measured. This voltage is the sum of two contribution Vres and Vth: the normal thermocouple's thermo voltage Vth and the voltage drop generated by the thermocouple's intrinsic resistance sourced by the excitation current Vres. This last term can be isolated by removing the contribution of the thermo voltage measured during normal operation or during the second stage of the diagnostic phase. After that voltage Vres is calculated, the resistance of the thermocouple insert can be measured with high precision on the base of a ratio-metric principle.

(14) Moreover during this diagnostic phase it will also be possible to detect thermocouple wires breakage. A positive overflow in the voltage measured on the thermocouple terminals F1, F2 will be considered as a consequence of a thermocouple's break damage.

(15) Thus according to this first embodiment a thermo wire testing circuit C may comprise a current source terminal S for supplying a test current to a first thermo wire T1 via a first terminal F1 during a test mode, a current drain terminal G for receiving said test current from a second thermo wire T via a second terminal F2 during said test mode, and a reference resistor R.sub.Ref for generating a reference voltage at a first reference terminal P1 and at a second reference terminal P2, which reference resistor R.sub.Ref is arranged in the current circuit of said test current, and a processing unit ADC and/or SO coupleable or operably connected to said first and second terminals F1, F2 with said reference terminals P1, P2, and configured to measure and compare a voltage drop caused by said test current between the first and second input terminal F1, F2 with a voltage drop caused by the said test current on said reference resistance R.sub.Ref connected between the first and the second reference input P1, P2.

(16) FIG. 2 shows an alternative embodiment to detect a deviation from a normal operating condition of the thermocouple. The ADC is now supplied by a reference voltage independent of the reference voltage caused by the voltage drop across the reference resistor R.sub.Ref. However the same principle as described in accordance with FIG. 1 is applied.

(17) During a first stage of a diagnostic phase a current is injected in the thermo wires by way of a current source S. In order to inject this test current a switch W2 is provided to couple the current source to the first and or second terminal F1, F2 and to the thermo wires T1, T2 coupled thereto. The switch may as well be used to deviate the test current from the thermo wires T1, T2 and inject the current directly in the reference resistor R.sub.Ref.

(18) A multiplexer is provided to selectively couple either the voltage drop between the first and second terminal to the operating inputs O1, O2 of the ADC or the voltage drop across the reference resistor. Thus when a test current is injected the voltage drop between the first and second terminal and across the reference resistor can be coupled to the operational input of the ADC. The ADC then outputs a ratio of the voltage drop Vdiag and the reference voltage, from voltage supply PS, supplied to the ADC or a ratio of the voltage drop Vref across the reference resistor R.sub.Ref and the reference voltage supplied by the voltage supply PS. The reference voltage provided by the voltage supply PS is supplied to reference voltage inputs P1, P2 of the ADC. This reference voltage is thus used for operating the ADC and/or for comparing the voltage input at the operating inputs O1, O2 of the ADC with the reference voltage supplied from the voltage supply PS.

(19) In the second stage of this diagnostic phase the current source is switched opening W2. Measuring the voltage on F1 and F2 terminals the thermo voltage component is measured and can removed from the measurement done in the first stage with the excitation current source active.

(20) The result of the subtraction is then divided by the voltage measured across Rref on the first stage. During all three measurements of the diagnostic phase the ADC reference inputs P1 and P2 are always connected to the voltage source PS to guarantee consistency between the measurements of the diagnostic phase. The measurement of the thermo voltage component according to the embodiment in FIG. 2 is the same as the normal measurement done to measure temperature but nevertheless is necessary in the diagnostic phase. However, the thermo voltage determined during the regular measurement phase in order to determine the temperature may be used during the diagnostic phase.

(21) Thermocouple internal resistance is thus calculated on the base of a pseudo ratio metric method.

(22) The output may then be transmitted to another part SO of the processing unit, such as a register or a microprocessor or a memory unit, in order to eliminate the thermovoltage Vth and to determine the internal resistance Rres of the thermo wires T1, T2.

(23) Thus according to this embodiment a thermo wire testing circuit C may comprise a current source terminal S for supplying a test current to a first thermo wire T1 via a first input terminal F1 during a test mode, a current drain terminal G for receiving said test current from a second thermo wire T via a second input terminal F2 during said test mode, and a reference resistor for generating a voltage drop at terminals F3 and F4, which reference resistor R.sub.Ref is arranged in the current circuit of said test current, and a processing unit ADC, SO coupleable or operably connected to said first and second input terminals F1, F2 and to said resistor reference R.sub.Ref at input terminals F3 and F4 and to a reference voltage PS at reference terminals P1 and P2, and configured to measure and compare a voltage drop caused by said test current between the two said terminals F1, F2, and a voltage drop caused by the said test current between the two said terminals F3, F4 with a voltage drop provided by the said reference voltage PS at the said reference terminals P1 and P2.

(24) The testing circuit C implements a method of improved diagnostics. The diagnostic phase may be executed at a fixed time interval at the beginning of a thermocouple's measurement cycle. At the end of each diagnostic phase the actual information concerning the thermocouple TC absolute resistance R_sens is calculated on the base of the measurement of V_res and V_ref as described before. In a possible alternative embodiment a typical value R_typ for the sensor resistance can be defined on the base of a real reference measurement or estimated. Absolute resistance can be substituted by a relative value:
R.sub.sens/R.sub.typ

(25) As a first step the actual R_sens value is compared with two threshold values R_min and R_max (R_max>Rmin) to ensure that the thermocouple resistance is within a certain range. The thresholds values R_min and R_max can be adjustable according to the application. In case the value is outside this interval, diagnostic information can be generated depending on selectable criteria.

(26) In a second step the incremental change in the thermocouple sensor resistance is used. The actual Rsens value is compared with the previous value of R_sens and an incremental value R_inc is calculated as the difference of the actual R_sens and previous R_sens divided by the actual R_sens

(27) The incremental resistance R_inc is compared with two threshold values R_inc_min and R_inc_max (R_inc_max>R_inc_min) to ensure that the thermocouple resistance change is within a certain range. Thresholds values R_inc_min and R_inc_max can be adjustable according to the application. In case the value is outside this interval, diagnostic information is generated.

(28) Thermocouples used in industrial application are normally based on mineral insulated cables with metallic sheath. Such a rugged construction normally guarantees that the whole thermocouple resistance is not changing very fast due to a change of process temperature. The minimum interval in measurement used in this new embodiment is below one second. In such a short interval the change of thermocouple sensor resistance is not relevant even when a long part of the thermocouple is in contact with the process temperature and the process temperature is not stable.

(29) In every case the minimum interval in measurement is much faster compare to the change of ambient temperature in order to remove the influence of the ambient temperature on the calculation of the incremental value R_inc.

(30) In an alternative embodiment several consecutive incremental values of R_inc can be recorded in order to obtain a profile of resistance increments. Diagnostic information will be issued in this case on the base of the comparison of the measurement profile compared with a predefined profile.