Abstract:
An industrial process field device is provided. The device includes wireless process field device electronics disposed within the industrial process field device and at least one battery cell disposed within the industrial process field device and operably coupled to the wireless process field device electronics. A circuit is provided that senses an electrical characteristic related to the at least one cell and provides an output to a switch to decouple the at least one cell from the industrial process field device electronics in response to certain conditions. Such conditions include too much current flowing from the at least one cell; the voltage of the at least one cell being too low; or a short circuit being generated or otherwise observed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is based on and claims the benefit of U.S. provisional patent application Ser. No. 60/931,313, filed May 22, 2007, and U.S. provisional patent application Ser. No. 60/927,285, filed May 2, 2007, the contents of which are hereby incorporated by reference in their entireties. 
    
    
     BACKGROUND 
     In many industrial process environments, combustible atmospheres are present in the environmental space surrounding industrial process field devices (also referred to herein as transmitters). A high power spark from connection or disconnection of a battery has a potential to ignite the combustible atmosphere. 
     In many industrial process environments, corrosive dust, liquids or mists are present which can damage electronic circuitry. Electronic circuitry is typically enclosed in a sealed electronic compartment. However, when such compartments are opened to replace a battery and then resealed, there is a potential to contaminate battery contacts or to seal corrosive chemicals inside the electronics compartment causing long term degradation of the electronics. On the other hand, batteries installed outside the transmitter housing are also subject to corrosion. 
     Circuits inside a transmitter typically carry enough electrical energy to spark and ignite a combustible atmosphere under accidental short circuit or fault conditions. Special precautions are thus taken before opening a transmitter electronics compartment. Either the surrounding environment is cleared of combustible vapors, the cable providing power to the transmitter is deenergized, or both. To increase safety, organizations require “hot work permits” and specially trained personnel before a transmitter is opened in an area where combustible atmospheres are sometimes present. 
     While cells and batteries can provide low current levels under ordinary operating conditions, batteries and cells typically produce very high short circuit currents under fault conditions. A typical fault condition is a short circuit in a circuit that is external to the cell or battery. In addition, cells and batteries have a large energy storage capacity or equivalent electrical capacitance C. The high short circuit currents under fault conditions and the large energy storage capacity are typically incompatible with intrinsically safe circuit specifications. It is thus difficult to mount cells or batteries outside from the transmitter housing and run a battery cable through a combustible atmosphere between the transmitter and the battery. Such a battery cable would typically violate intrinsically safe circuit requirements. 
     Similar difficulties are encountered with other types of battery powered industrial process field devices. A method and apparatus are needed for providing improved battery powered industrial process field devices that have a wide range of applicability in industrial process environments. 
     SUMMARY 
     An industrial process field device is provided. The device includes wireless process field device electronics disposed within the industrial process field device and at least one battery cell disposed within the industrial process field device and operably coupled to the wireless process field device electronics. A circuit is provided that senses an electrical characteristic related to at least one cell and provides an output to a switch to decouple at least one cell from the industrial process field device electronics in response to certain conditions. Such conditions include too much current flowing from the at least one cell; the voltage of the at least one cell being too low; or a short circuit being generated or otherwise observed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a pressure transmitter mounted to a pressurized process pipe. 
         FIG. 2  illustrates a cross-sectional view of a pressure transmitter. 
         FIG. 3  illustrates a cross-sectional view of a battery compartment. 
         FIG. 4  is a diagrammatic view of a current limiting circuit in accordance with an embodiment of the present invention. 
         FIG. 5  is a diagrammatic view of another current limiting circuit in accordance with another embodiment of the present invention. 
         FIG. 6  is a diagrammatic view of a short-circuit protection circuit in accordance with another embodiment of the present invention. 
         FIG. 7  is a diagrammatic view of a low-voltage cutoff circuit in accordance with an embodiment of the present invention. 
         FIG. 8  is a diagrammatic view of another low-voltage cutoff circuit in accordance with another embodiment of the present invention. 
         FIG. 9  is a diagrammatic view of another circuit in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Field devices, such as transmitters, used in industrial processes can be installed in the field on pipelines, tanks and other industrial process equipment. Transmitters sense process variables such as process pressure, process flow, process fluid temperature, process fluid conductivity, process fluid pH and other process variables. Other types of industrial process field devices include valves, actuators, field controllers, data displays and communication equipment such as industrial field network bridges. 
     Some industrial process field devices have no cabled connection to electrical power and rely on an internal battery for power. Wireless transmitters transmit outputs representing the process variables over a wireless communication channel to control or monitor equipment that is remote from the wireless transmitters. The control or monitoring equipment is typically located in a control room. The wireless transmitter typically includes an antenna used for wireless transmission with a control room antenna or other wireless network device such as a gateway. Use of wireless communication avoids connecting a communication or power cable between the transmitter and the control room. 
     Wireless transmitters typically include electronic circuits that require only small amounts of power for operation. The amount of power required is so low that small replaceable primary cells could be considered to provide the power. There are, however, difficult problems associated with the industrial process environment that limit general, widespread use of internal batteries in that environment. 
       FIG. 1  is a diagram of process control system  10  which includes a pressure transmitter  12  connected to a pressurized process pipe  16 . Pressure transmitter  12  is coupled to a two-wire process control loop  18  which operates in accordance with a desired protocol such as the HART® standard, a 4-20 milliamperes analog standard or other known process control communication standard. Two-wire process control loop  18  runs between pressure transmitter  12  and a remotely located control room  20 . In an embodiment in which loop  18  operates in accordance with the HART® protocol, loop  18  can carry a current I which is representative of a sensed process pressure and which also provides all of the energization for pressure transmitter  12 . In some applications, there are disadvantages to use of the two-wire process control loop  18  to energize the transmitter  12 . In such applications, the wired process control loop  18  is not used, and transmitter  12  is instead energized by a cell or battery and uses wireless communication as described below. 
       FIG. 2  illustrates an embodiment of an industrial process field device  200  that senses pressure. The industrial process field device  200  comprises a housing  202 . The housing  202  comprises a wall  204  between an electronics compartment  206  and a battery compartment  208 . The wall  204  includes a feedthrough opening  207 . In this embodiment, a first housing cover  210  is generally round and has threads that screw into threads on the housing  202  to enclose industrial process instrument or field device electronics  212  in the electronics compartment  206 . A second housing cover  214  is generally round and has threads that screw into threads on the housing  202  to enclose a battery assembly  216  in the battery compartment  208 . Industrial process field device electronics  212  generally comprise electronic circuits that are energy limited and that have temperature, reliability and vibration resistant characteristics that make them suitable for use in monitoring and controlling industrial processes in process plants such as chemical plants, petroleum refineries and the like. The field device electronics may include communication circuitry for communicating wirelessly with a process control loop. 
     The housing  202  has threads  218  that thread onto threads  220  of a pressure sensor housing  222 . The pressure sensor housing  222  encloses a pressure sensor  224  and sensor circuitry  226 . Electrical leads  228  from sensor circuitry  226  connect to the industrial process field device electronics  212 . In one embodiment, the housing  202  and the housing covers  210 ,  214  comprise metal die castings. 
     In this embodiment, a feedthrough connector  230  is mounted to the feedthrough wall  204 . The feedthrough connector  230  includes sealed electrical connections  232  that preferably extend through the feedthrough wall  204 . The feedthrough connector  230  includes power connectors  234  and  236  that connect power to industrial process field device electronics  212 . The wall  204  and the feedthrough connector  230  seal the feedthrough opening between the battery compartment  208  and the electronics compartment  206 . The battery compartment  208  can be opened in a corrosive process environment and the corrosive process environment is blocked from leaking into the electronics compartment by the feedthrough wall  204  and the feedthrough connector  230 . In one embodiment, the feedthrough connector  230  comprises an injection molded plastic part that includes metal electrical connections that are molded into the plastic. 
     The battery assembly  216  is electrically connected to the power connectors  234  and  236 . The battery assembly  216  comprises an energy limiter  240  and at least one cell  242 . The energy limiter can comprise a picofuse, a fuse or an electronic circuit that limits energy. In one embodiment, the series energy limiter  240  limits energy to an intrinsic safety level at an electrical connection  244  between the battery assembly  216  and the power connectors  234  and  236 . The connection  244  comprises an intrinsically safe circuit. An intrinsically safe circuit is a circuit in which no spark or thermal effect that is produced under test conditions (which include normal operations and specified fault conditions) is capable of causing ignition of a given explosive atmosphere surrounding the connection  244 . The current limit of the series current limiter  240  is calculable for a particular combustible gas (such as methane) and particular circuit characteristics (such as circuit capacitance and inductance) using known methods. In one embodiment, redundant intrinsic safety protection is provided by using two energy limiters instead of a single energy limiter  240 . 
     A seal  250  surrounds the electrical connection  244  between the battery assembly  216  and the power connectors  234  and  236 . The seal  250  preferably comprises an O-ring seal that is mounted to the battery assembly  216  and that slidingly engages the feedthrough connector  230  when the battery assembly  216  is removed or installed. The cover  214  preferably engages the battery assembly  216  along a circular contact ring  252  to provide mechanical support that is especially useful in high vibration environments. 
       FIG. 3  illustrates a cross-sectional view of a battery compartment  302  of an industrial process field device according to one embodiment of the invention. A feedthrough wall portion of a field device housing  304  separates the battery compartment  302  from an electronics compartment. A feedthrough connector  306  is sealed to the feedthrough wall portion of instrument housing  304 . Alternatively, the feedthrough connector could be integrally formed with a portion of the housing. The feedthrough connector  306  preferably comprises a plastic resin shroud and includes a sealed electrical power connector  308 . The contacts of the electrical power connector are recessed in a shroud  307  to meet the IP20 per IEC 529 standard. The electrical power connector  308  is mounted to a circuit card assembly  310  in the electronics compartment. The circuit card assembly  310  is sealed to the feedthrough connector  306  around the electrical power connector  308 . In one embodiment, the feedthrough connector is potted with an appropriate material. 
     In this embodiment, a battery assembly  320  is placed in the battery compartment  302 . The battery assembly  320  comprises a housing base  322  and a housing cap  324  that enclose a cell  326  (or cells  326 ) and a series current limiter  328  (or multiple series current limiters  328 ). The housing base includes a molded connector body  323 . 
     In one embodiment, the molded connector body  323  comprises a protruding plug that protrudes from the battery housing base  322 . Electrical battery contacts  332  are recessed in the molded connector body  323  and are protected from mechanical damage during handling. The shroud  307  comprises a protruding socket that protrudes from the feedthrough connector  306 . The power connector  308  comprises two pins that are recessed in the shroud  307  and are protected from mechanical damage. The engagement of the connection is a sliding connection that is easily put together or taken apart in the field after the cover  340  is removed. 
     Other intrinsic safety protection devices such as voltage limiting diodes can also be included in the battery assembly  320 . The cell  326  and the series current limiter  328  are preferably mounted to a printed wiring board  330 . The battery assembly includes electrical contacts  332  that electrically connect to the power connector  308  at an electrical connection  334 . The series current limiter  328  limits energy to an intrinsic safety level at the electrical connection  334 . A seal  336  surrounds the electrical connection  334 . 
     In one embodiment, the molded connector body  323  has an external taper, and the connector on the shroud  307  has an internal taper, and there is a tapered fit between the molded connector body  323  and the connector on the shroud  307 . The tapered fit is a tight fit so that vibration does not cause relative motion between the shroud  307  and the connector body  323 . The tapered fit allows the battery contacts  332  to be prealigned with the power connector  308  while sliding the molded connector body  323  into the shroud  307 . This prealignment prevents bending or other damage to the power connector  308 . 
     A cover  340  preferably includes threads  344  for screwing the cover  340  to the instrument housing  304 . A seal  342  seals the cover  340  to the housing  340 . Inside the cover  340 , a thrust bearing assembly preferably includes a thrust bearing plate  346  and a resilient compression ring  348 . The thrust bearing plate  346  presses against a top surface  350  of the battery assembly housing  320 . The thrust bearing plate  346  rotates relative to the top surface  350  as the cover  340  is screwed on. The compression ring  348  is compressed as the cover  340  is screwed on. The cover  340  is removably attachable to the housing  340  and presses the battery assembly  320  toward the power connector  308 . The screw down cover  340  presses down on the battery assembly  320  to maintain battery assembly connection during extreme vibration. In one embodiment the thrust bearing plate  346  includes a protruding ring  347  that has a tapered fit to a ring  325  on the housing cap  324 . The tapered fit eliminates relative motion between the thrust bearing plate  346  and the housing cap  324  during vibration. 
     Embodiments of the present invention generally include circuitry that senses one or more electrical conditions relative to the battery pack of a field device, and quickly ameliorates such conditions before they generate a fault and/or hazard. 
       FIG. 4  is a diagrammatic view of a current limiting circuit in accordance with an embodiment of the present invention. Circuit  400  includes one or more battery cell(s)  442  coupled to fuse  440 . Battery cell(s)  442  shares a common ground with voltage reference  404  via connection  402 . Additionally, battery cell(s)  442  and voltage reference  404  share a common ground with industrial process field device wireless electronics  412 . As set forth above, one or more battery cells  442  can comprise a battery pack that is replaceable in the field in a hazardous location without the need for declassifying the area or obtaining a hot work permit. Cell(s)  442  are placed in the terminal block side of the field device, and thus replacement of cell(s)  442  does not expose electronics  412  to the surrounding environment. Fuse  440  limits energy discharge from cell(s)  442 . However, due to potentially high inrush currents into electronics  412 , fuse  440  can inadvertently be blown when there is no real fault. This type of failure, when there is no real fault, would generally cause poor customer satisfaction with the overall product. Accordingly, the embodiment illustrated in  FIG. 4  includes additional circuitry to protect fuse  440  from blowing due to transient current spikes that present no hazard. Circuit  400  includes operational amplifier  406  that has its inverting input  408  coupled to sense resistor  410 . The non-inverting input  412  of operational amplifier  406  is coupled to output  414  of voltage reference  404 . The voltage drop across sense resistor  410  is proportional to the current discharge from cell(s)  442  into wireless electronics  412 . This voltage drop across sense resistor  410  is then compared with a fixed voltage provided by output  414  of voltage reference  404 . Operational amplifier  406  provides output  416  that is coupled to metal oxide semiconductor field effect transistor (MOSFET)  418  to limit the current that wireless electronics  412  can consume to a value below the rated fuse current of fuse  440 . While the embodiment illustrated in  FIG. 4  includes a specific voltage reference  404 , those skilled in the art will appreciate that voltage reference  404  can be replaced with a simple voltage divider comprising a pair of resistors coupled in series between the output of fuse  440  and the common ground. The connection between the resistors is then also coupled to the non-inverting input  412  of operational amplifier  406 . 
       FIG. 5  is a diagrammatic view of another current limiting circuit in accordance with another embodiment of the present invention. Circuit  500  bears many similarities to circuit  400  (described with respect to  FIG. 4 ) and like components are numbered similarly. Circuit  500  is different than circuit  400  in that sense resistor  410  has been eliminated. In essence, circuit  500  employs fuse  540  as the low-resistance sense resistor. Fuse  540  can be characterized and can be used as the sensing element in the comparator circuit in the same manner as that described above with respect to sense resistor  410 . One benefit of the elimination of the sense resistor, in circuit  500 , is that not only does the current limiting circuit protect fuse  540 , it also reduces the amount of resistance between cell(s)  552  and wireless electronics  512 , to just the resistance of fuse  540  alone. Depending on the wireless device, peak currents out of cell(s)  552  can be hundreds of milliamps for short durations of time. To avoid an additional voltage drops (and therefore system inefficiencies), the least amount of resistance (either in the fuse or in the sense resistor) is the best. 
     The current limiting embodiments described above with respect to  FIGS. 4 and 5  can be augmented with additional short-circuit protection.  FIG. 6  is a diagrammatic view of a circuit  600  providing such short-circuit protection. Circuit  600  includes transistor  602  having a first leg  604  coupled to a first side of fuse  640 , a second leg  606  coupled to the opposite side of fuse  640 , and a third leg  608  coupled to the input of MOSFET  610 . With circuit  600 , in the event that a customer accidentally shorts the output of the battery pack, the fast acting analog circuitry, such as transistor  602  and sense resistor or fuse  640 , act to protect the fuse and battery(s). 
     Embodiments of the present invention described thus far address the potential problems of excess current blowing the fuse and generating an undesirable experience of blowing the fuse when no real fault exists. Additionally, short-circuit protection has also been described. One additional concern for batteries that power industrial process field devices, is that as the battery voltage level becomes too low, the battery itself can overheat, and/or otherwise become damaged or do damage itself. 
     One battery technology that can be employed in battery packs for wireless process field transmitters is lithium-thionyl chloride (Li—SO—Cl 2 )primary cells. This particular chemistry of primary cells offers ultra-high power density over a wide operating temperature range and ultra-low self discharge. These characteristics enable wireless devices to operate in the standard −40 to 85° Celsius operating range for several years. 
     To further extend battery life of wireless industrial process transmitters, multiple primary cells are used in a series, parallel, or combination series/parallel configuration. In doing so, additional precautions need to be taken to ensure that the cells are never operated in an unstable state. A fully-charged primary cell voltage is nominally around 3.6 volts. A mostly discharged primary cell should not be discharged below 2 volts per manufacturer recommendation. In the case of two series primary cells, one of the cells could deplete faster than the other, and this could potentially lead to an unstable condition where the higher voltage cells force current through the depleted cell causing the depleted cell voltage to drop below the manufacturer recommendation (2 volts) per cell lower limit. This could potentially lead to cell rupture. 
     In accordance with an embodiment of the present invention, an industrial process transmitter includes a battery pack that is replaceable in hazardous conditions without the need for declassifying the area or obtaining a hot work permit, wherein the battery pack includes a low voltage cutoff capability. Preferably, low-power electronics are used to cut off the voltage between the battery pack and the industrial process transmitter electronics in certain situations. 
       FIG. 7  is a diagrammatic view of a circuit for cutting off, or otherwise de-coupling, one or more battery cells from industrial process field device electronics in certain situations. Circuit  700  bears some similarities to circuits  400  and  500 , and like components are numbered similarly. As illustrated in  FIG. 7 , one or more battery cells  742  are operably coupled to wireless electronics  712  through MOSFET  718 . operational amplifier  706  has an inverting input coupled to node  730  in the divider network comprising resistors  732  and  734 . The resistor divider network formed by resistors  732  and  734  scales down the voltage from battery cell(s)  742 , and this scaled voltage is then compared against a fixed voltage (reference voltage  704 ) with the use of a low power operational amplifier  706 , or suitable a suitable comparator. The output of operational amplifier  706  is connected to a switch, such as MOSFET  718 , which disconnects battery cell(s)  742  from electronics  712  when the voltage becomes too low. 
       FIG. 8  is a diagrammatic view of a circuit for cutting off a low-voltage battery cell(s) from industrial process field electronics when the battery voltage level is too low. Circuit  800  includes a voltage detector  802  coupled across the positive and negative polarity lines of battery cell(s)  842 . Output  804  of voltage detector  802  is coupled to and controls a switch, such as MOSFET  818 , which selectively couples wireless electronics  812  to battery cell(s)  842 . Voltage detectors, such as detector  802 , are integrated circuits that are typically used in microprocessor or microcontroller applications. They are used for situations when the voltage rails dip (also known as “brown out”) below recommended operating conditions. A reset is then applied to the microprocessor or microcontroller and the reset line is then held until the battery voltage rises. Applying this “brown out” concept to embodiments of the present invention, voltage detector  802  is used in such a way as to disconnect cell(s)  842  when the battery voltage becomes too low. Voltage detectors can come with either a predetermined (fixed) detection voltage, or in with adjustable detection voltage.  FIG. 8  illustrates how the voltage detector is used in conjunction with the battery pack. The battery voltage is applied to the voltage detector IC. When the battery voltage dips below the IC detection voltage, the output will disconnect the wireless electronics circuitry from the battery pack via switch  818 . 
       FIG. 9  is a diagrammatic view of one exemplary circuit in accordance with the embodiments of the present invention. Circuit  900  includes current-limiting circuitry in accordance with similar embodiments of the present invention set forth above, as well as low-voltage and short-circuit protection. Accordingly, those skilled in the art will recognize that various aspects and embodiments of the present invention can be combined in various combinations as desired. Circuit  900  includes a pair of terminals  902 ,  904  that are coupleable to a battery, such as one or more cells set forth with respect to previous embodiments. The output of circuit  900  is a pair of terminals  906 ,  908  that couple to the terminal block of an industrial process field device. As illustrated in  FIG. 9 , all or a portion of the circuitry for circuit  900  may be designed in accordance with intrinsic safety requirements. For example, the components within region  912  of dashed line  910  can comply with intrinsic safety requirements, for example, having minimum safety spacings of 0.020 inches between circuit traces and being disposed under a suitable coating or potting. 
       FIG. 9  illustrates voltage detector U 2  having an input that is operably coupled to terminal  902  through resistor R 4 . The ground terminal of voltage detector U 2  is coupled to ground terminal  904 . The output pin ( 1 ) of detector U 2  is coupled to switch Q 3  through resistor R 9 . Resistor R 9  preferably has a resistance of 0 ohms, or extremely low resistance, such that the output from voltage detector U 2  is basically coupled directly to Q 3 . R 5  preferably has a value of 10 megohms. Detector U 2  functions to detect the battery voltage across terminals  902  and  904 . If the battery voltage is at or below a threshold, determined by the voltage detector, voltage detector U 2  will disengage the entire circuit by causing Q 3  to open. 
     Circuit  900  also includes current limiting circuitry based upon the utilization of operational amplifier U 1  and switch Q 2 . As illustrated in  FIG. 9 , the non-inverting input of U 1  is coupled to a resistor/divider network comprising R 7  and R 6 . The output of U 1  is operably coupled, through R 3 , to switch Q 2 . In this regard, the inverting input of U 1  is coupled to terminal  902  through R 8 . U 1  compares the inputs presented at the inverting, and non-inverting inputs, and generates an output to Q 2  that can disengage Q 2  in such as way as to protect fuse F 1  from slight non-fault transients. 
       FIG. 9  also illustrates circuit  900  having short-circuit protection with transistor Q 1  coupled to opposite sides of fuse F 1 , and also coupled to switch Q 2 . In this way, if battery terminals  902  and  904  are short-circuited, fast acting analog circuitry such as transistor Q 1 , cause MOSFET Q 2  to open, thereby preventing damage, or other undesirable conditions. 
     The specific circuit illustrated in  FIG. 9  is provided as merely one example of an embodiment of the present invention. Embodiments of the present invention are not to be limited to the specific combination of features set forth in  FIG. 9 , or the exact arrangements thereof. That said, the actual component values used for the various elements of circuit  900  are as follows:
         R 1 —10 k   R 2 —10 k   R 3 —100 k   R 5 —10 megohms   R 6 —10 megohms   R 7 —604 k   R 8 —604 k   R 9 —0   R 10 —10 megohms   C 1 —100 pF   C 2 —1000 pF   C 3 —0.010 μFarads   C 4 —4.7 μFarads   C 5 —0.010 μFarads   C 6 —1.0 μFarads   F 1 — 1/16 amp fuse   Q 1 —part number MMBT3906 LT1   Q 2 —SI 9407AEY   Q 3 —SI 4850 EY   U 1 —TLV 2401 I   U 2 —NCM 302 LSN 40       

     In the embodiments described above, the battery assembly arrangement is useful with a wide variety of industrial process field devices including process control network bridges, data displays, field control valves, mechanical actuators, field controllers and transmitters. In one embodiment, the electrical cells comprise primary cells. In some embodiments, the battery assembly comprises two cells connected in series with a fuse. In one embodiment, the battery assembly is designed to have an open circuit voltage of at least 3 volts. In other embodiments, particularly single cell embodiments, open circuit voltages lower than 3 volts are used. 
     In one embodiment, removal of the battery is interlocked with actuation of a series switch in the battery assembly that disconnects the cells such that the power connections are not energized when the battery assembly is removed. When the switch is “on,” the battery assembly cannot be disconnected. The switch can also be used to switch off the battery assembly during shipment. 
     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.