Two-wire process control loop current diagnostics

• US 9,020,768 B2
• Filed: 08/16/2011
• Issued: 04/28/2015
• Est. Priority Date: 08/16/2011
• Status: Active Grant

• Current Assignee: Rosemount Incorporated (Emerson Electric Company)
• Original Assignee: Rosemount Incorporated (Emerson Electric Company)
• Inventors: Arntson, Douglas W., Rud, Jason H.
• Primary Examiner(s): LE, TOAN M
• Application Number:
  US13/210,662
• Publication Number:
  US 20130046490A1
• Time in Patent Office: 1,351 Days
• Field of Search: 702/58, 702 33- 35, 702/47, 702/51, 702/53, 702/57, 702 64- 65, 702/81, 702/84, 702/108, 702113-114, 702/116, 702/127, 702/130, 702/133, 702/138, 702182-183, 702/185, 702188-189, 73/11.6, 73/13.5, 73/15.7, 73/15.9, 700/6, 700 9- 10, 700 79- 80, 700108-109, 700/174, 700/177, 307/24, 307/31, 307/82, 340/3.1, 340 342- 344, 341/108, 341/110, 341116-117, 341/126, 341/131, 341/142, 341/144, 341/155
• US Class Current: 702/58
• CPC Class Codes: G08C 25/00 Arrangements for preventing or correcting errors; Monitoring arrangements

Abstract

A process variable transmitter controls a signal on a communication loop. A diagnostic component on the transmitter compares an expected signal level on the communication loop with an actual value to detect on-scale errors.

Claims

1. A process variable transmitter, comprising:

a processor receiving an input signal indicative of a sensed process variable and outputting a digital signal indicative of the input signal;

a digital-to-analog (D/A) converter receiving the digital signal and converting it to an analog signal;

a loop control component receiving the analog signal and controlling a two-wire process control loop based upon a voltage generated across a resistive element coupled in series with the two-wire process control loop to provide a transmitter output signal indicative of the analog signal, the transmitter output signal varying, on-scale, between a first signal level and a second signal level; and

a loop diagnostic component including an analog comparator which compares a first signal value indicative of the analog signal from the D/A converter with a second signal value indicative of the transmitter output signal to determine whether the transmitter output signal includes an on-scale error and responsively outputting an error indicator to the processor;

wherein the second value is generated as a function of the voltage across the resistive element.

2. The process variable transmitter of claim 1 wherein the loop control component regulates current on the two-wire process control loop, as the transmitter output signal, based on the voltage across the resistive element.

3. The process variable transmitter of claim 2 wherein the analog signal output by the D/A converter comprises an analog voltage and wherein the analog comparator compares that analog voltage, as the first signal value with the voltage across the resistive element, as the second signal value.

4. The process variable transmitter of claim 3 wherein the loop control component includes at least one additional resistive element, wherein the resistive element and the 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 on the two-wire process control loop accurately indicates the analog signal output by the D/A converter, the voltage across the resistive element has a magnitude substantially equal to a magnitude of the analog voltage output by the D/A converter.

5. The process variable transmitter of claim 4 wherein the loop diagnostic component 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 as the analog voltage output by the D/A converter varies from a scale maximum value to a scale minimum value, the voltage across the resistive element, when it is accurately reflecting the analog voltage output by the D/A converter, varies to have a same value as the analog voltage output by the D/A converter.

6. The process variable transmitter of claim 1 wherein the loop diagnostic component compares the first signal value and the second signal value to determine whether a difference between them is within an analog threshold value, and if not, outputs the error indicator.

7. The process variable transmitter of claim 1 wherein the processor performs additional diagnostics in response to receiving the error indicator.

8. The process variable transmitter of claim 1 wherein the processor performs a verification operation to verify an error has occurred in response to receiving the error indicator.

9. The process variable transmitter of claim 1 wherein the processor asserts an alarm in response to receiving the error indicator.

10. The process variable transmitter of claim 1 wherein the two-wire process control loop varies, on-scale, between 4 milliamps, as the first signal level, and 20 milliamps, as the second signal level.

11. A method of identifying errors output by a process variable transmitter, comprising:

generating a digital signal related to the sensed process variable;

sensing a process variable using a process variable sensor;

generating an analog signal related to the sensed process variable using a digital-to-analog (D/A) converter;

controlling a two-wire process control loop to carry a transmitter analog output signal indicative of an analog signal from the D/A converter based upon a voltage generated across a resistive element coupled in series with the two-wire process control loop, the analog output signal varying, on-scale, between a scale maximum value and a scale minimum value;

comparing, in the process variable transmitter using an analog comparator, a first signal value indicative of the transmitter analog output signal with a second signal level indicative of the analog signal from the D/A converter to detect on-scale errors in the analog output signal;

wherein the second value is generated as a function of the voltage across the resistive element.

12. The method of claim 11 and further comprising:

processing at least one of the analog signal and the transmitter analog output signal so that the first and second signal values are substantially the same, when the transmitter analog output signal is accurately indicative of the analog input signal.

13. The method of claim 12 wherein processing comprises inverting, on the process variable transmitter, at least one of the transmitter analog signal and the transmitter analog output signal.

14. The method of claim 13 wherein the two-wire process control loop comprises a 4-20 milliamp control loop that carries a current that varies, on scale, between 4 and 20 milliamps, and wherein controlling the communication loop comprises:

receiving, as the analog signal, an analog voltage output by the digital-to-analog (D/A) converter indicative of the sensor signal; and

controlling the current based on the analog voltage output by the D/A converter and based on a voltage across a resistive element in the control loop.

15. The method of claim 14 wherein the analog voltage output by the D/A converter and the analog voltage across the resistive element in the control loop are scaled so, when operating correctly, they have substantially a same magnitude.

16. The method of claim 15 wherein one of the analog voltage output by the D/A converter and the analog voltage across the resistive element in the control loop are inverted, on the process variable transmitter, so, when operating correctly, they have a same value, within an analog threshold difference.

17. A process variable transmitter, comprising:

a processor that outputs a digital sensor signal indicative of a value of a sensor input signal;

a digital-to-analog (D/A) converter that receives the digital sensor signal and provides an analog sensor voltage indicative of the digital sensor signal;

a loop control component that controls current on a two-wire process control loop to vary, on-scale, between a scale maximum current and a scale minimum current, based on the analog sensor voltage, the loop control component regulating the current on the two-wire process control loop based on a regulation voltage across a resistive element in the two-wire process control loop; and

an analog circuit that scales and inverts at least one of the regulation voltage and the analog sensor voltage so, when operating correctly, they have substantially a same amplitude, and including an analog comparator that compares the regulation voltage and the analog sensor voltage and outputs an error indicator if they differ by more than a threshold amount.

18. The process variable transmitter of claim 17 wherein the two-wire process control loop comprises a 4-20 milliamp control loop.