Patent Application
20080291009
The present invention relates to the field devices of the type used in monitoring or controlling industrial processes. More specifically, the present invention relates to powering such field devices.
Field devices are used in industrial processes to monitor process variables of the process, or control such process variables. Examples of process variables include pressure, temperature, flow rate, level, etc. Typically, the field devices are located in the “field”, away from a central location. For example, such field devices may be scattered throughout an oil refinery or the like. Field devices are used to transmit the measured process variables back to the control room while controllers are used to control the process from the control room.
Typically, communication between the control room and the field device occurs over a two wire process control loop. Example control loops include 4-20 mA process control loops in which a current level through the loop is used to represent a process variable, control loops operating in accordance with the HART® communications protocol, FieldBus protocol, the ProfiBus protocol, etc. In many configurations, the field device is also powered with electrical power received over the two wire process control loop.
Although field devices typically communicate using process control loops, there are situations in which wireless communications is desirable. For example, wireless communication techniques reduce, or eliminate, the wiring which is needed to connect to field devices. In such wireless configuration, there may be no process control loop available to power the field device. Therefore, an alternate power source must be used. For example, in some configurations, the field device is powered using stored power, for example, from a battery, obtained through solar cells, or other techniques. In such configurations, it is desirable to reduce the power consumption required by the field device. In other instances, the device is connected to a wired process control loop but further includes a wireless transceiver for sending or receiving communications. In such configurations, it is useful to conserve power in the device.
A field device for use in monitoring or controlling an industrial process includes two wire process control loop electrical connections and field device electrical circuitry a power-on input. Digital communications monitoring circuitry couples to the electrical connections and provides a signal to the power-on input of the field device electrical circuitry in response to a digital signal on the two wire process control loop. A method of controlling operation of electrical circuitry in a field device of the type used in monitoring or controlling an industrial process is also provided. The method includes monitoring signals carried on a two wire process control loop and detecting a communications signal. Power to field device electrical circuitry is provided in response to detected digital communications signal on the two wire process control loop.
Field device 102 communicates with another location, such as process control room 110 over a wireless communication link 112. Wireless communication link 112 extends between antenna 114 of field device 102 and antenna 116 of control room 110. Antenna 116 couples to process control equipment 120 in control room 110.
Although field device 102 is illustrated as including a wireless communication link, there are some installations in which it is also desirable for field device 102 to communicate over a two wire process control loop such as process control loop 126. In the example of
As described below in greater detail, as communications over two wire process control loop 126 is only used in some instances, the present invention reduces power consumption by field device 102 by powering down circuitry, two wire process control loop communication circuitry, when not in use. Digital communication monitoring circuitry is used to monitor process control loop 126 and is used to power up the circuitry when digital communications are detected.
A power source 150 is provided and configured to provide electrical power to electrical circuitry of field device 102. Example, power source 150 can comprise a battery or other power storage and/or generation device. The power source can be self contained and periodically recharged or replaced, or can couple to a charging apparatus such as solar cells, or other source of power. A switch 154 is shown as coupling power source 150 through process loop communication circuitry 156. Process control loop circuitry 156 is used for communication over process control 126. Communication circuitry 156 can operate in accordance with any communication protocol including, for example, the HART® communication protocol, the FieldBus protocol, the ProfiBus protocol, or others. Communication circuitry 156 includes a power-on input. The power-on input can be a “wake up” input causing circuitry 156 to begin normal operations. In such a configuration, circuit 156 can be permanently coupled to power source 150 and simply draw more power when it receives a wake up signal. Similarly, power-on input can comprise the power input which is provided to circuitry 156. In the configuration of
Circuitry 162 monitors digital communications on loop 126. For example, this filter can be used to eliminate any spurious signals that can enter into the electronics. The comparator 182 compares the filtered HART® signal to a known reference voltage. This provides a square wave output to low pass filter 184. The square wave output is a function of the incoming digital communication signal. In order to avoid false interrupts, the square wave signal is filtered again using low pass filter 184. This helps to ensure that the incoming data transmission is actually a digital communication message and not simply random noise. As a final stage, the filtered square wave output is then compared to a known voltage with comparator 186. The output of comparator 186 is then provided to control the power used by process control loop communication circuitry 156. In another example configuration, the output 160 from digital communication monitoring circuitry is provided as an interrupt 185 to microprocessor 140. Microprocessor 140 is then configured to control the power used by process control loop communication circuitry 156. In such a configuration, the microprocessor 140 can cause circuitry 156 to enter a low power mode. For example, such a configuration can be used whereby communication circuitry 156 is taken offline while the microprocessor 140 assembles the communication data for transmission over processor control loop 126.
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. Although the illustrations set forth herein show providing power to communication circuitry, the present invention can be used to control power or operation of any circuitry within the field device 102 including digital circuitry, analog circuitry, or a combination. Such field device electrical circuitry can comprise, for example, a microprocessor, or other digital control circuitry.
