Abstract:
A charging circuit for field devices is disclosed. The circuit has at least three modes and automatically shifts between the modes depending on voltage of the generator. In a first mode, the charging circuit provides voltage regulation. In a second mode, the charging circuit couples the generator directly to en energy storage device. In a third mode, the charging circuit decouples the generator from the storage device. A field device utilizing the charging circuit is also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   The present application is based on and claims the benefit of U.S. provisional patent application Ser. No. 60/675,647, filed Apr. 28, 2005, the content of which is hereby incorporated by reference in its entirety. 

   BACKGROUND OF THE INVENTION 
   In industrial settings, control systems are used to monitor and control inventories of industrial and chemical processes, and the like. Typically, the control system performs these functions using field devices distributed at key locations in the industrial process and coupled to the control circuitry in the control room by a process control loop. The term “field device” refers to any device that performs a function in a distributed control or process monitoring system, including all devices used in the measurement, control and monitoring of industrial processes. 
   Field devices are used by the process control and measurement industry for a variety of purposes. Usually, such devices have a field-hardened enclosure so that they can be installed outdoors in relatively rugged environments and are able to withstand climatalogical extremes of temperature, humidity, vibration, mechanical shock, etc. These devices also can typically operate on relatively low power. For example, field devices are currently available that receive all of their operating power from a known 4-20 mA loop. 
   Some field devices include a transducer. A transducer is understood to mean either a device that generates an output based on a physical input or that generates a physical output based on an input signal. Typically, a transducer transforms an input into an output having a different form. Types of transducers include various analytical equipment, pressure sensors, thermistors, thermocouples, strain gauges, flow transmitters, positioners, actuators, solenoids, indicator lights, and others. 
   Typically, each field device also includes communication circuitry that is used for communicating with a process control room, or other circuitry, over a process control loop. In some installations, the process control loop is also used to deliver a regulated current and/or voltage to the field device for powering the field device. 
   Traditionally, analog field devices have been connected to the control room by two-wire process control current loops, with each device connected to the control room by a single two-wire control loop. Typically, a voltage differential is maintained between the two wires within a range of voltages from 12-45 volts for analog mode and 9-50 volts for digital mode. Some analog field devices transmit a signal to the control room by modulating the current running through the current loop to a current proportional to the sensed process variable. Other analog field devices can perform an action under the control of the control room by controlling the magnitude of the current through the loop. In addition to, or in the alternative, the process control loop can carry digital signals used for communication with field devices. Digital communication allows a much larger degree of communication than analog communication. Moreover, digital devices also do not require separate wiring for each field device. Field devices that communicate digitally can respond to and communicate selectively with the control room and/or other field devices. Further, such devices can provide additional signaling such as diagnostics and/or alarms. 
   In some installations, wireless technologies have begun to be used to communicate with field devices. Wireless operation simplifies field device wiring and setup. Wireless installations are currently used in which the field device is manufactured to include an internal battery or storage cell that can be potentially charged by a solar cell. One of the challenges for charging circuits that are coupled to photovoltaic solar panels arises due to the widely varying voltage of the panel. At low light levels (less than 5000 lux), small solar panels may only provide 1 to 20 milliwatts. Conversely, under full sun conditions, the same panel may output 1-2 watts. Existing solar charging systems are designed to optimize power output when mounted where they will be illuminated by direct sunlight. If the solar panel must be located in an area which receives no direct sunlight, these existing systems do not operate efficiently and the size and cost of the solar panel must be dramatically increased to generate sufficient power. Providing a charging circuit for wireless field devices that can efficiently store energy from a widely varying energy generator, such as a solar panel, would allow more standardized solar panels or generators to be used for a variety of solar applications. 
   SUMMARY 
   A charging circuit for field devices is disclosed. The circuit has at least three modes and automatically shifts between the modes depending on voltage of the generator. In a first mode, the charging circuit provides voltage regulation. In a second mode, the charging circuit couples the generator directly to an energy storage device. In a third mode, the charging circuit decouples the generator from the storage device. A field device utilizing the charging circuit is also disclosed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1 and 2  are diagrammatic and block diagram views of an exemplary field device with which embodiments of the present invention are useful. 
       FIG. 3  is a block diagram of a wireless field device with which embodiments of the present invention are useful. 
       FIG. 4  is a diagrammatic view of conversion a power module in accordance with an embodiment of the present invention. 
       FIG. 5  is a diagrammatic view of a generator showing various options for electricity generation that can be used in accordance with embodiments of the present invention. 
       FIG. 6  is a more detailed block diagram of a charging circuit in accordance with an embodiment of the present invention. 
       FIG. 7  is a diagrammatic view of generator voltage versus time illustrating the various charging circuit modes in accordance with embodiments of the present invention. 
       FIG. 8  illustrates energy conversion module  38  in accordance with another embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   While embodiments of the present invention will generally be described with respect to field devices that communicate wirelessly, those skilled in the art will recognize that embodiments of the present invention can be practiced with any field device that requires additional electricity than that otherwise available to it. A wireless field device may need to derive all of its operating power from a solar panel, or other form of generator, and thus would reap significant benefits from embodiments of the present invention. However, even a wired field device that requires more power than available to it through its wired connection, could derive additional power via embodiments of the present invention. 
     FIGS. 1 and 2  are diagrammatic and block diagram views of an exemplary wired field device with which embodiments of the present invention are useful. Process control or monitoring system  10  includes a control room or control system  12  that couples to one or more field devices  14  over a two-wire process control loop  16 . Examples of process control loop  16  include analog 4-20 mA communication, hybrid protocols which include both analog and digital communication, such as the Highway Addressable Remote Transducer (HART®) standard, as well as all-digital protocols such as the FOUNDATION™ Fieldbus standard. Generally, process control loop protocols can both power the field device and allow communication between the field device and other devices. 
   In this example, field device  14  includes circuitry  18  coupled to actuator/transducer  20  and to process control loop  16  via terminal board  21  in housing  23 . Field device  14  is illustrated as a process variable (PV) generator in that it couples to a process and senses an aspect, such as temperature, pressure, pH, flow, et cetera of the process and provides an indication thereof. Other examples of field devices include valves, actuators, controllers, and displays. 
   Generally, field devices are characterized by their ability to operate in the “field” which may expose them to environmental stresses, such as temperature, humidity and pressure. In addition to environmental stresses, field devices must often withstand exposure to corrosive, hazardous and/or even explosive atmospheres. Further, such devices must also operate in the presence of vibration and/or electromagnetic interference. 
     FIG. 3  is a block diagram of a wireless field device with which embodiments of the present invention are particularly useful. Field device  34  includes power conversion module  38 , controller  35 , wireless communications module  32 , and actuator/transducer  20 . Conversion module  38  can be any device that is able to convert potential energy into electrical energy. Accordingly, conversion module  38  can include a photvoltaic solar panel and associated charging circuit coupled to an energy storage device, such as a battery. Conversion module  38  can be any device, known or later developed, that translates potential energy into electricity for use by field device  34 . For example, module  38  can employ known techniques to generate electricity from thermal potential energy, wind energy, pressurized gas, or other forms of potential energy. Conversion module  38  can provide power for wireless communications module  32  alone, other portions of field device  34 , or may even wholly power field device  34 . 
   Wireless communications module  32  is coupled to controller  35  and interacts with external wireless devices via antenna  26  based upon commands and/or data from controller  35 . Wireless communications module  32  can communicate process-related information as well as device-related information. Depending upon the application, wireless communication module  32  may be adapted to communicate in accordance with any suitable wireless communication protocol including, but not limited to: wireless networking technologies (such as IEEE 802.11b wireless access points and wireless networking devices built by Linksys of Irvine, Calif.), cellular or digital networking technologies (such as Microburst® by Aeris Communications Inc. of San Jose, Calif.), ultra wide band, free space optics, Global System for Mobile Communications (GSM), General Packet Radio Service (GPRS), Code Division Multiple Access (CDMA), spread spectrum technology, infrared communications techniques, SMS (Short Messaging Service/text messaging), wireless networking technologies in accordance with IEEE 802.15.4, or any other suitable wireless technology. Further, known data collision technology can be employed such that multiple units can coexist within wireless operating rage of one another. Such collision prevention can include using a number of different radio-frequency channels and/or spread spectrum techniques. 
   Wireless communications module  32  can also include transceivers for a plurality of wireless communications methods. For example, primary wireless communication could be performed using relatively long distance communication methods, such as GSM or GPRS, while a secondary, or additional communication method could be provided for technicians, or operators near the unit, using for example, IEEE 802.11b or Bluetooth. 
     FIG. 4  is a diagrammatic view of conversion module  38  in accordance with an embodiment of the present invention. Conversion module  38  includes electricity generator  100  coupled to charging circuit  102  which, in turn, is coupled to energy storage device  104 . Charging circuit  102  provides a power output  106  for utilization by a field device. Generator  100 , as illustrated in  FIG. 5 , can include one or more individual generator modules. For example, generator  100  can include photovoltaic panel  110 , wind-based generator  112 , compressed-gas based generator  114 , thermal generator  116 , vibration-based generator  117 , or any combination thereof. Conversion module  38  may be embodied within a field device, or disposed externally to a field device and electrically coupled to the field device to provide power to the field device. Energy storage device  104 , coupled to charging circuit  102 , can be any suitable device that is able to store electrical energy for any useable period of time. For example, storage device  104  may be a rechargeable battery, such as a gel cell lead-acid battery, or any suitable type of capacitor, such as a super capacitor. 
     FIG. 6  is a more detailed block diagram of charging circuit  102  in accordance with an embodiment of the present invention. Charging circuit  102  includes a plurality of conductors  120  that couple to generator module  100 . Charging circuit  102  includes measurement module  122  that is coupled to conductors  120  and is adapted to provide an indication of whether a voltage present across conductors  120  exceeds first and/or second voltage thresholds. Measurement circuit  122  can be any suitable device that is able to provide a signal in response to a voltage magnitude measured across conductors  120 . Measurement module  122  can include an analog-to-digital converter, a comparator circuit, a source of one or more reference potentials, or any combination thereof. Measurement module  122  provides for operation of charging circuit  102  in at least three modes. In a first mode, measurement circuit  122  sets both outputs  124  and  126  to a low or disengaged state. Accordingly, neither bypass  128  nor cutout  130  are engaged. Accordingly, energy from generator  100  flows through conductors  120  into voltage regulator  132 , which provides linear voltage regulation to storage device  104 . When measurement circuit  122  determines that the voltage across conductors  120  has fallen below a first threshold (bypass threshold), measurement circuit  122  engages bypass  128  to effectively couple conductors  120  to storage device  104  without passing through voltage regulator  132 . In this mode, the entire charging circuit  102  is designed to consume less than 200 microwatts. This provides highly efficient operation in conditions where electrical output from the generator is diminished, such as a solar panel or photovoltaic cell operating in shade. 
   When the voltage measured across conductors  120 , by measurement circuit  122 , falls below a second, lower, threshold (cutout threshold), measurement circuit  122  distinguishes bypass  128  via line  124  and instead engages cutout  130  through line  126  to complexly decouple storage device  104  from the charging circuit. In this mode, for example, when a solar panel is operating at night, circuitry  102  functions to prevent storage device  104  from discharging back through the generator  100 . 
     FIG. 7  is a diagrammatic view of generator voltage versus time illustrating the various charging circuit modes in accordance with embodiments of the present invention. At time t 0 , the generator voltage is V initial  and since V initial  exceeds bypass threshold  140 , the charging circuit operates in linear mode. In this mode, the charging circuit provides a regulated voltage output to the storage device. At time t 1 , the voltage from the generator crosses bypass threshold  140  and charging circuit  102  enters “direct” mode. In this mode, the charging circuit directly couples the generator to the storage device while operating on as little energy as possible. For example, the circuitry of charging circuit  102  is designed to consume less than 200 microwatts of power in this mode. Finally, at time t 2 , the voltage of the generator crosses cutoff threshold  142  and charging circuit  102  enters disconnect mode. In this mode, the storage device is completely decoupled from the generator. This ensures that the storage device does not discharge back through the generator. 
     FIG. 8  illustrates energy conversion module  38  in accordance with another embodiment of the present invention. The embodiment illustrated in  FIG. 8  is particularly appropriate where storage device  104  is a gel cell lead acid battery. Such batteries can be damaged by overcharging. In order to address this potential problem, temperature sensor  146  is thermally coupled to battery  104 . Sensor  146  is electrically coupled to charging circuit  102  such that charging circuit  102  can limit the charge voltage to a safe float value regardless of ambient temperature.  FIG. 8  also illustrates optional battery protection circuitry  148  (illustrated in phantom) within charging circuit  102 . Battery protection circuitry  148  can include any circuitry that helps extend battery life and/or diagnose any faults in battery  104 . For example, battery life can be reduced if the battery is subjected to short circuits, or if the battery voltage is allowed to drop too low. Accordingly, battery protection circuitry  148  can include circuitry that is able to detect when the battery voltage is threatening to drop too low, and will inhibit any further draw of electricity from the battery. Additionally, battery protection circuitry  148  can include current limiting circuitry, or circuitry that is able to measure the amount of current drawn from battery  104  and inhibit, or reduce such current if it becomes excessive. 
   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.