RU2575693C2 - Current diagnostics in double-wire process control circuit - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: group of inventions is referred to process variable transmitter. The invention suggests the process variable transmitter comprised of processor, digital-to-analogue (D/A) converter, control circuit component that receives analogue signal and controls double-wire process control circuit on the basis of voltage generated at resistive element, and diagnostic circuit component that includes analogue comparator, which compares the first signal value specifying analogue signal from D/A converter with the second signal value specifying output signal of the transmitter in order to define whether this output signal of the transmitter contains an error within the range, and in response it displays error indicator to the processor, at that the second value is generated depending on voltage at the resistive element.

EFFECT: accurate method is provided for error detection within the range.

19 cl, 8 dwg

Description

BACKGROUND

The present invention relates to process variable transmitters used in process control and monitoring systems. More specifically, the present invention relates to performing loop current diagnostics in order to detect errors in a transmitter loop current range.

Process parameter transmitters are used to measure process parameters (or process variables) in a process control or monitoring system. Microprocessor-based transmitters often include a sensor, an analog-to-digital converter for converting the output from the sensor into digital form, a microprocessor for aligning the digitized output signal, and an output circuit for transmitting a aligned output signal. Currently, this transfer is usually performed by a process control loop, such as a 4-20 mA control loop, or wirelessly.

Typically, in 4-20 mA process equipment, the control loop is controlled by the loop current controller. The loop current controller adjusts the loop current to reflect the process parameters measured with sensors in the equipment.

SUMMARY OF THE INVENTION

The process parameter transmitter controls the signal in the communication loop. The diagnostic component on the transmitter compares the expected signal level in the communication loop with the actual value to detect errors in the range.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a process parameter transmitter coupled to a host system and sensors in a process.

In FIG. 2 is a flowchart illustrating one embodiment of the operation of the diagnostic component of the loop current shown in FIG. one.

FIG. 3 is a schematic diagram illustrating one embodiment of a loop current control component.

In FIG. 4 is a graph showing one embodiment of a voltage dependence of a digital-to-analog converter on a loop current.

FIG. 5 is a graph showing one embodiment of a dependence of a measured loop voltage on a loop current.

FIG. 6 is a graph showing one illustrative embodiment of a voltage dependence of a digital-to-analog converter and an inverted and scaled measured loop voltage versus loop current.

FIG. 7 is a partially block diagram, a partially schematic diagram of another embodiment of a loop control component.

FIG. 8 is a flowchart illustrating one embodiment of a system operation shown in FIG. 1 and 7.

DETAILED DESCRIPTION

In FIG. 1 is a simplified block diagram of a transmitter 10 in accordance with one embodiment. The transmitter 10 in the embodiment shown in FIG. 1 includes an analog-to-digital (A / D) converter 12, a processor 14, a memory and a clock circuit 16, a digital-to-analog converter 18, a loop control component 20, and a loop current diagnostic component 22. A transmitter 10 is shown coupled to a plurality of different process parameter (PV) sensors 24 and 26. The transmitter 10 may also illustratively be connected to a host system or control room (not shown) via a control circuit 28. The transmitter 10 may be connected to a wireless communication line in addition to the process control loop 28. In one embodiment, the process control loop 28 also provides power to the transmitter 10.

Sensors 24 and 26 are illustrative process parameter sensors that receive input from a process 30, the parameters of which are measured. For example, sensor 24 may be an illustrative thermocouple that measures temperature, and sensor 26 may be either the same or another type of sensor, such as a flow sensor. Other PV sensors may include a variety of sensors, such as pressure sensors, pH sensors, etc. Sensors 24 and 26, by way of illustration, transmit an output signal, which indicates a measured process parameter, to A / D converter 12.

Matching logic is also included (but not shown now) to amplify, linearize and otherwise match the signals provided by sensors 24 and 26. In any case, A / D converter 12 receives signals indicative of process parameters measured by sensors 24 and 26. A / D converter 12 converts the analog signals into digital signals and transfers them to the processor 14.

In one embodiment, the processor 14 is a computer microprocessor or microcontroller that couples the memory and the clock circuit 16 and transmits digital information indicative of the measured process parameters to the D / A converter 18. The D / A converter 18 illustratively converts signals indicative of the process parameters, into the analog signals that are transmitted to the loop control component 20 to control the current (I) in loop 28. Loop control component 20 can transmit information over loop control circuit 28 more in digital format (for example, using the HART protocol), or in analog format (or both), controlling the current (I) through circuit 28. In any case, information related to the measured process parameters is transmitted along circuit 28 of the process control transmitter 10.

In one embodiment, the D / A converter 18 also transmits an input signal to the loop current diagnostic component 22. The signals output by the D / A converter 18 indicate the desired loop current (I). That is, the signal output by the D / A converter 18, by way of illustration, indicates the current (I) of the circuit, which will reflect the value of the measured process parameter. Based on the signal transmitted by the D / A converter 18, the loop control component 20 illustratively controls the circuit 28 so that the current (I) indicates a signal output by the D / A converter 18.

It is useful to determine whether the loop control component 20 controls the loop current (I) in loop 28 accurately, especially when the error in loop current is a range error. In other words, in a 4-20 mA process control loop, the loop current varies from 4 to 20 mA (that is, it changes between the minimum range value and the maximum range value, 4 mA and 20 mA, respectively). However, under certain conditions (for example, when the operating current of the equipment exceeds the permissible current), errors in the range (incorrect readings between 4 mA and 20 mA) may occur. For example, if the current in loop 28 is assumed to be set to 10.0 mA, but actually set to 12.2 mA, it is useful to detect this type of error in the range. This type of error can occur, as an example, when excess current is supplied by the integrated circuit to the printed circuit board of the transmitter 10 of the process parameter or due to current leakage of the printed circuit board. Of course, these are just examples, and errors in the range can also occur for other reasons.

Thus, FIG. 1 shows that transmitter 10 also includes a loop current diagnostic component 22. In the embodiment shown in FIG. 1, the output D / A of the converter 18 is transmitted to the diagnostic component 22, being an indication of the loop control component 20, which indicates the level of the actual current of the loop flowing along the loop 28. FIG. 2 is a flowchart illustrating how the loop current diagnostic component 22 operates in accordance with one embodiment to identify errors in a range in the control loop 28.

Diagnostic component 22 first receives the D / A output of converter 18. This is shown by block 40 in FIG. 2. The diagnostic component 22 also receives an output from the loop control component 20. This is shown by block 42 in FIG. 2. The signal output from the D / A converter 18 and the output signal from the loop control component 20 indicate the required and actual values of the loop current, respectively. Thus, the loop current diagnostic component 22 compares the expected (or required) and actual values of the loop current, as shown by block 44 in FIG. 2. If the two values are close enough, then the loop current control component 20 precisely controls the current in loop 28 based on the output D / A of converter 18. This is indicated by blocks 46 and 48 in FIG. 2.

However, in block 46, it is determined that if the two signals are not close enough, the loop current diagnostic component 22 generates and sends an error indicator 50 to the processor 14 and / or D / A converter 18, giving an alarm. This is shown by block 52 in FIG. 2.

In order to determine whether the actual and expected circuit currents are close enough, the current diagnostic component 22 illustratively compares the two signals to determine whether they are within a predetermined threshold value relative to each other. If so, then they are close enough. Otherwise, they are not close enough, and an error indicator 50 is generated. The specific threshold value may be set empirically or differently and may vary depending on the application based on the particular control loop used or based on other factors. In one embodiment, it may be set to 100 μA.

In order to describe the loop current diagnostic component 22 in more detail, it may be helpful to understand a conventional loop control component. FIG. 3 illustrates a partially block diagram and a partially schematic diagram of a conventional loop control component 20. It can be seen that the loop control component 60 includes resistors 62, 64, 66, 68, and 70, an operational amplifier 72, and a transistor 74.

In accordance with one embodiment, the D / A converter 18 transmits an analog output voltage that varies linearly proportional to the desired loop current in the loop 28. As an example, the D / A converter 18 illustratively transfers 0.25 V at its output when the loop current in loop 28 of 4 mA and 1.25 V when loop current of loop 28 of 20 mA is required. FIG. 4 illustrates this graphically. As can be seen from FIG. 4, the expected loop current varies from 4 mA to 20 mA, and the output voltage D / A of the converter 18 varies linearly from 0.25 V to 1.25 V.

In order to adjust the loop current according to the value given by the output voltage D / A of the converter 18, the loop control component 20 illustratively controls the loop current by measuring the voltage across the precision resistor 70, the resistance of which may be, for example, 49.9 ohms. As can be seen from FIG. 3, the voltage across resistor 70 is negative with respect to ground. In addition, it can be seen that based on the values of the resistors 62, 66, 68 and 70, the voltage across the precision resistor 70 will illustratively vary linearly from -0.20 V to -1.00 V. FIG. 5 shows this graphically. As can be seen from FIG. 5, the loop voltage on the precision resistor 70 varies from -0.20 V to -1.00 V, the actual loop current flowing in the loop 28 varies from 4 mA to 20 mA.

From graphs 4 and 5 it can be seen that when inverting and scaling either the voltage output by the D / A converter 18 (shown in Fig. 4) or the circuit voltage on the resistor 70 (shown in Fig. 5), these two values are very similar. For example, in FIG. 6 is a graph of the voltage outputted by the D / A converter 18 and the loop voltage across the resistor 70 when the loop voltage shown in FIG. 5 is inverted and multiplied by a scale factor of 1.25. Since the voltage output by the D / A converter 18 (shown at 90 in FIG. 6) represents the desired or expected loop current, and since the loop voltage at resistor 70 (shown at 92 in FIG. 6) actually represents the loop current, the errors in the range can be identified by simply comparing the two values indicated in FIG. 6. This is essentially a comparison of the required or expected loop current with the actual loop current.

FIG. 7 illustrates one embodiment of a loop control component 20 and a loop current diagnostic component 22 for performing this type of comparison. It should be noted, of course, that the embodiment shown in FIG. 7 represents only one illustrative embodiment, and a wide variety of other schemes can also be used to compare two values. However, the embodiment shown in FIG. 7 is one of the relatively inexpensive and accurate methods for comparing two values and transmitting a signal to processor 14 and / or D / A converter 18, which indicates when an error has occurred.

As seen in FIG. 7, the loop control component 20 includes some elements that are similar to those shown in FIG. 3, and similar elements are likewise numbered. In addition, it can be seen that the resistors 62 and 70 were replaced by the resistors 94 and 96. The values of the resistors 94 and 96 were chosen to scale the voltage across the resistor 96 created by the current flowing in the circuit 28 by a factor of 1.25 (or any another factor to make it substantially equal in magnitude to the voltage output by the D / A converter 18).

The loop current diagnostic component 22 illustratively includes operational amplifiers 98, 100, and 102. The operational amplifier 98 is configured as an inverter so that the voltage across the resistor 96 is inverted with respect to the ground of the circuit to have the same polarity as the voltage, outputted by the D / A converter 18. It is seen that in the embodiment shown in FIG. 7, the (scaled) voltage across resistor 96 will vary from -0.25 V to -1.25 V. Thus, the output signal of operational amplifier 98 varies from 0.25 V to 1.25 V.

An operational amplifier 100 is connected as a differential operational amplifier. Therefore, he compares the voltage output by the D / A converter 18 (which also varies from 0.25 V to 1.25 V) with the output of the operational amplifier 98. These two values should be essentially the same. If this is not the case, then the loop control component 20 does not accurately control the loop current in loop 28 to reflect the output D / A of converter 18. However, since the two signals received by the operational amplifier 100 may not be identical, but close enough to each other, a comparator 102 is also provided. The comparator 102 compares the output of the operational amplifier 100 (which reflects the difference between its two input signals) with a reference or threshold value. The output signal of the comparator 102, therefore, transmits an error indicator 50 to the processor 14 and / or D / A converter 18 only if the difference between the two signals transmitted to the input of the operational amplifier 100 exceeds the reference value of the input signal of the operational amplifier 102.

FIG. 8 is a flowchart illustrating the operation of the system shown in FIG. 1 and 7, in accordance with one embodiment. FIG. 8 begins with a processor 14 outputting a signal indicating a process parameter to the D / A converter 18. This is shown by block 120 in FIG. 8. The D / A converter 18 then performs a digital-to-analog conversion and outputs the analog voltage of the D / A converter to the loop control component 20 and to the diagnostic component 22. This is shown by block 122 in FIG. 8.

The loop control component 20 then controls the loop current in loop 28 based on the voltage across resistor 96. This is shown by block 124 in FIG. 8. The loop control component 20 also, due to the values of the resistors, scales the loop voltage at the resistor 96 and transfers it to the loop current diagnostic component 22. The loop current diagnostic component 22 inverts the scaled voltage and compares it with the voltage output by the D / A converter 18. This is displayed by blocks 126 and 128 in FIG. 8. The loop current diagnostic component 22 determines whether the compared voltages are close enough (using the operational amplifier 100 and the comparator 102). This is shown by block 130 in FIG. 8. If the two voltage values are close enough, the system simply monitors the output signal D / A of the converter 18 and the current of the circuit in circuit 28. This is displayed by block 132.

However, if it is determined in block 130 that the two compared voltages are not close enough to each other, then the loop current diagnostic component 22 transmits an error indicator 50 to the processor 14 and / or D / A converter 18. This is shown by block 134 in FIG. 8. The processor 14 can then perform any number of error operations, as shown by block 136. For example, the processor 14 can perform numerous tasks, such as resetting the D / A converter 18, to check if an error actually occurs. The processor 14 may also give an alarm or perform additional diagnostics. The processor 14 may also perform other necessary operations in response to receiving the error indicator 50 from the diagnostic component 22 of the loop current.

It should be understood that, although the description relates to illustrated embodiments, various changes may be made. For example, the functions performed by the loop current diagnostic component 22 and the loop control component 20 may be performed by one component, or the functions may be distributed differently between these components (or among other components in the transmitter 10). Similarly, along with given values for resistances, voltages, and currents, other values can also be used. The values given are merely examples. In addition, while certain components (operational amplifiers, resistive elements, resistors, etc.) are defined in FIG. 7, they are defined as an example only. Also, the function of scaling and inverting both the loop voltage and the D / A voltage of the converter and their comparison can be carried out in many different ways, with different circuits, except those shown in FIG. 7.

In addition, although the above description contains examples of process parameters that can be measured, a wide variety of other process parameters that can be measured and processed are essentially the same. Examples of such other process parameters include pressure, level, flow rate or flow rate, etc. In addition, although the embodiment described herein is described in the context of a two-wire transmitter, the present invention can be easily applied to a four-wire transmitter or any other type the transmitter.

Although the present invention has been described with reference to illustrative embodiments, those skilled in the art will understand that changes can be made in form and detail without departing from the spirit and scope of the present invention.

Claims (19)

1. A process parameter transmitter comprising:
a processor receiving an input signal indicative of a measured process parameter, and outputting a digital signal indicative of an input signal;
a digital-to-analog (D / A) converter that receives a digital signal and converts it into an analog signal;
a loop control component receiving an analog signal and controlling a two-wire process control loop based on a voltage generated on a resistive element connected in series with the two-wire process control loop to provide a transmitter output signal indicative of an analog signal, the transmitter output signal varying between the first signal level and second signal level;
and a circuit diagnostic component including an analog comparator that compares a first signal value indicative of an analog signal from a D / A converter with a second signal value indicative of a transmitter output signal to determine if the transmitter output signal contains an error in a range, and in response, an output error indicator to the processor,
moreover, the second value is generated depending on the voltage on the resistive element. 2. The process parameter transmitter according to claim 1, wherein the loop control component controls the current in the two-wire process control loop as an output signal of the transmitter based on the voltage of the resistive element. 3. The process parameter transmitter according to claim 2, wherein the analog signal output by the D / A converter comprises an analog voltage and wherein the analog comparator compares the analog voltage as the first signal value with the voltage across the resistive element as the second signal value. 4. The process parameter transmitter of claim 3, wherein the loop control component includes at least one additional resistive element, the resistive element and at least one additional resistive element have values that scale either the voltage across the resistive element, or the analog voltage output by the D / A converter, so that when the current in the two-wire process control loop accurately indicates the analog signal output by the D / A converter, the voltage on the resistive element ente has a value equal to the value of the analog voltage output by the D / A converter. 5. The process parameter transmitter of claim 4, wherein the diagnostic component of the circuit includes an inverter that inverts one of the voltage across the resistive element and the analog voltage output by the D / A converter, such that the analog voltage output by the D / A converter, varies from the maximum value of the range to the minimum value of the range, and the voltage on the resistive element, when it accurately reflects the analog voltage output by the D / A converter, changes so as to have the same value as and analog voltage output by the D / A converter. 6. The process parameter transmitter according to claim 1, in which the diagnostic component of the circuit compares the first signal value and the second signal value to determine whether the difference between them is within the limits of the analog threshold value, and if not, gives an error indicator. 7. The process parameter transmitter according to claim 1, wherein the processor performs additional diagnostics in response to receiving an error indicator. 8. The process parameter transmitter of claim 1, wherein the processor performs a verification operation to verify whether an error has occurred in response to receiving an error indicator. 9. The process parameter transmitter of claim 1, wherein the processor acknowledges the alarm in response to receiving an error indicator. 10. The process parameter transmitter of claim 1, wherein the two-wire process control loop varies between 4 mA as a first signal level and 20 mA as a second signal level. 11. A method for identifying errors output using a process parameter transmitter, including:
generating a digital signal associated with the measured process parameter;
measuring a process parameter using a process parameter sensor;
generating an analog signal associated with the measured process parameter using a digital-to-analog (D / A) converter;
controlling a two-wire process control loop to carry an analog output of a transmitter indicative of an analog signal from a D / A converter based on a voltage generated on a resistive element connected in series with a two-wire process control loop, the analog output signal varying from a maximum value range to the minimum range value;
comparing, in the transmitter of the process parameter using an analog comparator, a first signal value indicating an analog output signal of a transmitter with a second signal level indicating an analog signal from a D / A converter to detect range errors in an analog output signal;
moreover, the second value is generated depending on the voltage on the resistive element. 12. The method of claim 11, further comprising:
processing at least one of the analog signal or the analog output signal of the transmitter such that the first and second signal values are equal when the analog output signal of the transmitter accurately indicates the analog input signal. 13. The method of claim 11, wherein the processing comprises inverting at the transmitter a process parameter of at least one of the analog input signal or the analog output signal. 14. The method according to p. 13, in which the two-wire process control loop contains a 4-20 mA control loop that carries a current that varies in the range between 4 mA and 20 mA, while controlling the communication loop:
receiving, as an analog signal, an analog voltage output by a D / A converter indicating a sensor signal, and
current control based on the analog voltage output by the D / A converter and based on the voltage across the resistive element in the control loop. 15. The method according to p. 12, in which the analog voltage output by the D / A converter and the analog voltage on the resistive element in the control loop are scaled so that when the work is done properly, they have the same value. 16. The method according to p. 15, in which one of the analog voltage output by the D / A converter and the analog voltage on the resistive element in the control loop is inverted on the transmitter of the process parameter so that when the work is done properly, they have one and the same the same value, within the analog threshold difference. 17. A process parameter transmitter, comprising:
a processor that outputs a digital sensor signal indicating a value of the sensor input signal;
a digital-to-analog (D / A) converter that receives a digital sensor signal and transmits an analog sensor voltage indicating a digital sensor signal;
a loop control component that controls the current in the two-wire process control loop so that the current varies between the maximum range current and the minimum range current based on the sensor analog voltage, the loop control component adjusts the current in the two-wire process control loop based on the control voltage across the resistive an element in a two-wire process control loop; and
an analog circuit that scales and inverts at least one of the control voltage and the analog voltage of the sensor so that when working properly they have the same amplitude and includes an analog comparator that compares the control voltage and the analog voltage sensor, and displays an error indicator if they differ by more than a threshold value. 18. The process parameter transmitter of claim 17, wherein the analog circuitry inverts one of the control voltage and the analog voltage of the sensor so that when operating properly, they have the same polarity. 19. The process parameter transmitter of claim 18, wherein the two-wire process control loop includes a 4-20 mA control loop.

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