Sensor Diagnostics Improved Process Temperature Transmitter
Description
The present invention relates to a process variable transmitter for monitoring the type of process variable in an industrial process. More specifically, the present invention relates to a process variable transmitter for detecting the temperature of a process.
Background of the invention
The process industry uses process variable transducers to detect process variables associated with materials such as solids, slurries, liquids, vapors, and gases in chemical, pump, petroleum, pharmaceutical, food, and other processing plants. Process variables include pressure, temperature, flow, level, turbidity, density, concentration, chemical composition, and other characteristics. The process fluid temperature transmitter provides an output related to the detected process material temperature. A temperature control loop or other output to the control room can be sent to another process unit so that the process can be monitored and controlled. To monitor the temperature of the process fluid, the transfer includes a temperature sensor such as a thermocouple or a resistive temperature device (RTD).
In temperature measurement applications, it is important to confirm when the temperature measurement is incorrect. This can be caused by broken leads/sensors, excessive line resistance or short circuit conditions. There are often a large number of connection points between the temperature sensor and the process temperature transmitter that can fail or become aging. Some commercial process temperature transmitters verify temperature sensor connection integrity before providing readings.
One such diagnostic technique that has been used in temperature transmitters is based on the collection of multiple measurements. These measurements are sequentially collected by the process temperature transmitter to test each conductor independently. For open circuit sensors and excessive line resistance testing, the process temperature transmitter can apply current pulses independently to each of the temperature sensor leads and measure the resulting voltage. If the measured voltage exceeds the threshold, you can confirm that a fault has occurred. If the lead or temperature sensor element is open, the current pulse drives the measured voltage high enough to exceed the threshold. When the leads are properly connected and the sensor elements are not damaged, a path for current flow is provided and the detected voltage will remain below a threshold indicating the correct connection. For a four-wire RTD, this requires three open sensor measurements to evaluate the integrity of the RTD (one line is always connected to the circuit shared by the current return path).
Wire-wound RTD temperature sensors are susceptible to open circuit due to exposure to vibration. Such vibrations may include large amplitude transient activity, persistent low amplitude activity, or both. In order to test the state of the open circuit sensor, a resistance measurement of the temperature detecting element is obtained. If the ohms measurement is below the threshold set by the software, it is determined that the sensor is disconnected. This process currently requires three measurement points to confirm this state and may take up to 200 milliseconds. Therefore, there is an ongoing need to improve the diagnosis of temperature sensors.
Summary of the invention
The present invention provides diagnostics for process temperature transmitters. In various aspects, the present invention provides methods and apparatus for quickly and accurately determining the state of a temperature sensor, including confirming that the transmitter is properly connected to the temperature sensor. Embodiments of the present invention primarily provide methods and apparatus for simultaneously testing multiple, and preferably all, temperature sensor circuits for open circuit and/or short circuit sensor conditions or other degradation behavior, thereby reducing or potentially minimizing sensor evaluation time. The following diagnosis can be made at each selected temperature interval or at other times before each temperature sensor measurement.
A process temperature transmitter capable of operating with at least one temperature sensor having a plurality of leads. The temperature transmitter includes a measurement circuit operably coupled to the at least one temperature sensor to provide an indication of an electrical parameter of the at least one temperature sensor. A controller is coupled to the measurement circuit to obtain the indication and provide a process temperature output. A current source applies a test current to the plurality of leads simultaneously. A diagnostic circuit measures the voltage response on each lead to provide a diagnostic indication of the temperature sensor.
The picture shows the environment of the process fluid temperature transmitter
Transmitter 12 includes a power module 20, a loop communication device 22, an RTD input terminal 24, a measurement circuit 26, a current source 28, and a controller 30. Transmitter 12 can be coupled to RTD 32 such that transmitter 12 can obtain a voltage measurement from RTD 32 and correlate the measured value to the calculated process fluid temperature. Transmitter 12 then provides the calculated process fluid temperature on two-wire process control loop 16.
The power module 20 is disposed within the transmitter 12 and is connectable to the two-wire process control loop 16. Module 20 properly regulates the power received from circuit 16 for the various components of transmitter 12. With power module 20, transmitter 12 is capable of operating separately based on power received from process control loop 16. Module 20 may include, for example, known electronic components such as DC-DC power conditioning devices. On loop 16 (using analog signal transmission between 4 mA and 20 mA in some embodiments), module 20 operates to limit 4 amps or less of amperage for providing other components within transmitter 12. Additionally, module 20 can be adjusted to prevent electrical noise received from loop 16 from reaching the other components.
Loop communication device 22 can be coupled to two-wire process control loop 16 and configured to communicate over loop 16. Communication device 22 can be of a type generally known in the art. For example, communication device 22 can be suitably selected to provide analog communication, digital communication, or a combination of both. As mentioned above, one such combination of analog and digital communications is known as a high speed addressable remote sensor protocol. One version of the protocol superimposes the digital signal on a 4-20 mA analog signal. With this protocol, the host variable of interest can be provided in one mode (such as analog mode) while the diagnostic signal is provided in another mode. However, the invention may be practiced in pure analog communication as well as in purely digital communication (as provided by FOUNDATIONTM Fieldbus), and the invention may also be implemented using a wireless protocol.
Transmitter 12 also includes an RTD input or patch panel 24. The input or patch panel 24 provides a detachable electrical connection to the RTD 32. Typically, this connection will provide a four-wire connection so that a Kelvin connection to the RTD can be obtained. With Kelvin connections, two wires are used to send current through the RTD and a second pair is used to detect the voltage on the RTD. This reduces errors in resistance measurements as is known in the art.
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