Patent Application
20070152645
The present invention relates generally to industrial process transmitters, and more particularly, to power management in such transmitters.
Industrial process transmitters are devices that can be coupled to industrial process equipment and/or conduits and are adapted to measure process parameters, such as pressure, mass flow, flow rate, temperature, and the like. Frequently, such transmitters draw power from a two-wire loop that carries an energy limited loop current, which varies within a range of 4-20 mA. When the current is low (such as 4 mA), a majority of the power available to the transmitter from the loop is used by circuitry within the transmitter to sense a process variable and to generate a process variable output representative of the sensed process variable.
In some configurations, transmitters can utilize primary and secondary process measurements, using multiple sensors or field devices. For instance, to make a mass flow measurement of gas or steam through a pipe, a flowmeter can be used to measure flow rate, and a second sensor can be used to measure the line pressure, for example.
Power delivery to the sensor or field device performing such secondary process measurements contributes to the overall current and power consumption of the system. At low current levels (such as 4 mA), very little power (typically 1 to 2 milliwatts) is available for powering accessory loads and for communicating with feature modules.
An industrial process transmitter is provided which includes a loop current control to couple to a two-wire process control loop and adapted to control a loop current level based upon a process variable. Power from the loop is provided to primary circuitry of the process transmitter at a quiescent current level. A databus is configured to couple to secondary circuitry of the transmitter. A secondary current control circuit dynamically limits current delivered to secondary circuitry.
In general, industrial process devices contain circuitry for measuring a process parameter and for communicating, for example, with a communications network, such as a 4-20 mA two-wire process control loop. Such transmitter circuitry requires a quiescent current (typically less than 4 mA) for standard operation. Embodiments of the present invention employ a current limiter to limit current provided to secondary circuitry, such as secondary measurement circuitry, sensors, operator interfaces, and the like. The secondary circuitry is coupled to primary circuitry of the transmitter through a databus such as that described in Nelson et al. U.S. Pat. No. 6,765,968 which is incorporated herein by reference. In one embodiment, the current limiter may be used in conjunction with circuitry which provides power-up energization for the secondary circuitry, even when the loop current is at a minimum (such as 4 mA). As used herein, the term primary circuitry refers to sensor and other circuitry contained within a sealed electronics housing of a transmitter, (such as electronics housing 110 in
The transmitter 102 is a two-wire modular differential pressure transmitter, shown in an exploded view. The transmitter 102 is a two-wire transmitter in the sense that it is an electronic transmitter that uses two wires for signal transmission and power. For example, two-wire process control loops can use 4-20 mA signaling techniques and digital communication techniques, such as HART®, Fieldbus, Profibus, and other communication protocols. The modular differential pressure transmitter 102 is only one example of a suitable process monitoring and control device and is not intended to suggest any limitation as to the scope of use or functionality of the invention.
Transmitter 102 includes a feature module 108, an electronics housing 110, and a process coupling 112. The process coupling 112 can be attached to a pipe or conduit of an industrial process, such as pipe 114, with flange 116 and flange adapter unions 118 shown in phantom.
The transmitter electronics housing 110 is sealed to the pressure sensing module 106 and encloses electronic circuitry (shown in
In the example of
Field wiring 106 from a process monitoring and control center 104 connects to a two-wire output interface of the transmitter 102. The field wiring 106 carries a 4-20 mA current and is used for powering and communication with transmitter 102.
The current required for powering the transmitter circuitry and for communicating with the monitoring and control center can be referred to as quiescent current. In one embodiment, the quiescent current must be less than 3.6 mA. A standard established by NAMUR (Normenarbeitsgemeinshaft für Mess- und Regeltechnik der chemischen Industrie) specifies that to indicate an alarm low condition for the transmitter 102, the current on the 4-20 mA loop should decrease to 3.6 mA. Since field devices adapted for Highway Addressable Remote Transmitter (HART®)-based communications use approximately ±0.5 mA for signaling on the two-wire loop 106, 3.1 mA of current is allocated to the transmitter circuitry for the quiescent current budget.
However, given that the current in the two-wire loop varies from 4 mA (minimum) to 20 mA (maximum), conventional transmitters discard up to 82% of their available power when the loop current is at a maximum. Specifically, the ratio of power consumed by the transmitter (given the quiescent current requirements of 3.1 mA plus 0.5 mA communication current) to available power (when the two-wire loop current is at its maximum of 20 mA) can be calculated as follows:
Embodiments of the present invention are adapted to limit the current provided to secondary circuitry to a current level that is within quiescent current budget. For example, current on the two-wire loop 106 in excess of the quiescent current can be provided to the secondary circuitry for use in powering secondary circuit loads and in communicating with the secondary circuitry.
IsecondaryMax=ILoop−IPrimary−ISignalingOverhead (2).
During operation, the databus current limit circuitry 332 limits the available current provided over databus 133. This limiting function is based upon the voltage of summing node 320 and a fixed minimum current level which can be conservatively provided to the databus. The voltage of summing node 320 is controlled based upon shunt control circuitry 316 in accordance with the requirements set forth above such that the total current provided to secondary circuitry 168 does not exceed a desired current budget.
Current limiting circuit 332 diverts some or all of the excess current (in excess of the quiescent current needs of the primary circuitry 206 and any additional overhead such as required for signaling) from the process control loop 106 to the secondary circuitry 168. The excess or secondary current provides power to the secondary circuitry 168 for taking measurements, displaying data, or performing other functions, depending on the specific implementation. More or less current is available to the bus 133 depending on the unused or excess current output of the transmitter 102. The secondary bus current can be managed to enable the secondary circuitry 168 to provide faster updates under certain loop current conditions (such as when the loop current is greater than 4 mA). Conversely, the bus current can be managed to provide less current to the bus 133, when the loop current is low. In some instances, the low current delivery to the bus 133 reduces the frequency with which the secondary circuitry 168 takes measurements. If the transmitter 102 is adapted for HART®-based communications, the shunt control 316 can increase or decrease the excess current to the bus 133 or to the transmitter circuitry 206 based on the HART® signal. For example, a portion of the HART® signal can be diverted to supplement either the quiescent current level or the excess current level, as needed.
With the present invention, the voltage regulator provides one example of a power connection which provides power to primary circuitry of the process transmitter which is derived from the loop current. However, any type of power connection can be used and the invention is not limited to the disclosed voltage regulator. The secondary current control circuit is configured to dynamically limit the current delivered to secondary circuitry. In other words, the current limit is not set to a fixed value but is variable. In general, the secondary current control has an adjustable input which is used to dynamically limit the current which can be delivered to the secondary circuitry. The current can be limited based upon the excess current which is related to the loop current and the quiescent current level drawn by primary circuitry. The loop current can be inferred based upon operation of the transmitter or can be measured directly by using analog or digital circuitry. The quiescent current level can also be inferred based upon transmitter operation, can be measured directly using analog or digital circuitry or can be estimated using a fixed value. The operation of the secondary circuitry can be changed based upon the available current. For example, if the secondary circuitry is measuring a process variable or performing a calculation, the update rate or the clock of the secondary circuitry can be controlled based upon the available current. In general, the performance or functionality of the secondary circuitry can adaptively change based upon the available current. The current limiting circuitry also provides electrical isolation between the secondary circuitry and the primary circuitry. For example, if the secondary circuitry fails, such as develops a short circuit which increases current draw, the current limiting circuit will prevent this increased current draw from negatively affecting the primary circuitry.
Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. The secondary circuitry of the present invention can be any appropriate secondary circuitry including local displays such as LCD circuitry, measurement circuitry adapted to monitor a secondary process parameter or process variable, a local operator interface adapted to receive inputs from an operator, etc. Another example secondary circuit comprises includes secondary communication circuitry adapted to communicate with the field device over a communications bus. In one configuration, when communication occurs using the HART® communication protocol, the current provided to the secondary circuitry is limited dynamically by plus and minus 0.25 mA during respective positive and negative portions of the HART® transmit signal such that 3.35 mA quiescent current can be accommodated, instead of 3.1 mA, and still meet NAMUR alarm level low (3.6 ma) conditions on the loop.
