Self-discharging reserve power units and related methods

Self-discharging reserve power units and related methods are described. A self-discharging reserve power unit comprises an electrical energy storage component to provide power to a process control device. The electric energy storage component is coupled to an energy discharge component and a controller, which causes the discharge component to discharge energy from the electrical energy storage component following completion of an operation by the process control device.

FIELD OF THE DISCLOSURE

This disclosure relates generally to reserve power units and, more particularly, to self-discharging reserve power units.

BACKGROUND

Reserve power units (RPUs) are used to provide backup power in the event of power loss and are typically implemented using generators and/or electrical energy storage devices. The energy storage devices generally include batteries or capacitors, and are often used to provide power to perform safety functions or operations within a process control system, such as moving a valve or other process control component to a safe shutdown position (e.g., a failsafe position). Many energy storage devices are either rechargeable and/or easily replaceable.

The function(s) or operation(s) performed using the power from the energy storage device may not require all the energy in the energy storage device. In some applications, the presence of remaining energy in the energy storage device of an RPU is problematic. Many RPUs currently in use do not control or fully discharge remaining energy in the energy storage device and, thus, are not suitable for use in certain applications or environments.

SUMMARY

An example apparatus comprises an electrical energy storage component or device to supply power to a process control device, a discharge component operatively coupled to the electrical energy storage device, and a controller to, in response to a loss of power, cause the discharge component to discharge energy from the electrical energy storage component following completion of an operation by the process control device.

An example method comprises detecting a loss of power to a process control device, providing the process control device with power from an electrical energy storage component, interrupting a control signal to the process control device in response to the loss of power, and discharging remaining energy in the electrical energy storage component following completion of an operation by the process control device.

Another example method comprises supplying energy to a process control device via an electrical energy storage device and discharging remaining energy from the electrical energy storage device after the process control device completes an operation

DETAILED DESCRIPTION

The example apparatus described herein involves a reserve power unit (RPU) located between a process control device (e.g., an actuator and valve assembly) and an electrical power source. The power source provides electrical power to the process control device when the process control system is operating normally. The example RPU is also positioned between the process control device and a signal(s) to the process control device from a control system. In response to a power loss, either intentional or unintentional, the RPU disrupts or interrupts the control signal(s) and provides power to the process control device via an energy storage device(s) or component(s) (e.g., capacitors, batteries, etc.) disposed within the RPU. The interruption of the control signal(s) may cause the process control device to move toward a predetermined or failsafe position (e.g., fully open or fully close a valve). The energy storage device of the example apparatus is sized to enable the RPU to provide power to the process control device for a time period at least long enough for the process control device to move to the failsafe position. The example RPU then discharges any remaining energy in the energy storage device to facilitate safe maintenance of the process control device and/or the RPU.

In some examples, the discharge of energy from the energy storage device requires a predetermined maximum amount of time to complete. However, the actual completion time may vary depending on the position of the process control device at the time of a power loss. In some examples, the apparatus may be disposed in an explosion proof housing and/or operating in a potentially hazardous environment. Waiting the predetermined maximum amount of time before opening the explosion proof housing to perform maintenance assures service personnel that any energy in the energy storage device of the RPU has been substantially fully discharged and eliminates risk that a spark may be generated while servicing the process control device and/or RPU. In other examples, the RPU may receive a feedback signal (e.g., digital output, analog output, digital communications signal) from the process control device when the process control device has completed the move to the failsafe position. Upon receiving such a signal, the RPU may begin discharging.

The discharge operation of the example apparatus described herein controls power dissipation of one or more solid state switches with a pulse-width modulated (PWM) signal. The example RPU is configured so that the discharge of energy does not exceed a predetermined rate to prevent a maximum operating temperature of the switches from being exceeded. A duty cycle of the PWM signal affects the time required to complete the discharge of the energy storage device and also ensures that the discharge rate will not overheat the components of the RPU. An appropriate duty cycle is determined by certain factors including the voltage remaining in the energy storage device and characteristics of a heat sink (e.g., thermal resistance, size, etc.) that is to facilitate the dissipation of energy.

FIG. 1is a schematic block diagram of an example apparatus100in accordance with the teachings herein. The example apparatus100is depicted as an RPU100that provides reserve electrical power to a process control device102in the event of a loss of power from a primary power source104. The RPU100provides power to the process control device102via an electrical energy storage device106. The energy storage device106may be implemented using one or more components such as, for example, capacitors, batteries, etc., or a combination thereof. Additionally, the energy storage device106is designed to store sufficient electrical energy to provide power to the process control device102for at least an amount of time to allow the process control device102to complete an operation following a disruption or loss of power from the primary power source104. For example, the process control device102may be a fluid valve and actuator assembly and the RPU100may provide power to the actuator for an amount of time sufficient for the actuator to move the fluid valve to a failsafe position, regardless of the initial position of the fluid valve.

During normal operation, when the primary power source104is providing energy to the process control device102, a charger108disposed within the RPU100is charging and/or maintaining the charge of the energy storage device106to a substantially fully charged condition. The charger108may be implemented using circuitry tailored to most effectively and efficiently charge the energy storage device106. For example, the charger108may function as a variable current source if the energy storage device106is implemented using multiple, series-connected large value capacitors, commonly known as super-capacitors. In that case, the charger108may provide current that may be varied by a controller110so that the charging current is decreased as the energy storage device106approaches the fully charged condition. In this manner, the temperature of the energy storage device106may be controlled and/or the possibility of overcharging the energy storage device106is substantially eliminated.

As depicted inFIG. 1, the charger108receives electrical power from the primary power source104via a power router112that is controlled by the controller110as described in greater detail below. If the energy storage device106contains multiple components or devices, a charge balance circuit114may also be interposed between the charger108and the energy storage device106to ensure that each of the components of the energy storage device106is substantially equally charged. For example, if the energy storage device106contains multiple capacitors, the charge balance circuit114ensures that each of the capacitors is charged to substantially the same voltage.

During normal operation, the controller110causes the power router112to route the power provided by the primary power source104to the charger108and the process control device102. Additionally, the controller110causes a communication switch116to communicatively couple one or more communication lines118to the process control device102. The communication lines118may convey commands, messages, data, etc. between a control system and the process control device102. Thus, during normal operation, the RPU100functions transparently (i.e., acts as a pass-through device) with the respect to the power and communication signals associated with the process control device102.

As shown inFIG. 1, the example RPU100includes an internal power supply120that provides power to the controller110and numerous other circuits, devices, etc. making up the functional blocks of the example RPU100. A more detailed description of the manner in which the power provided by the power supply120is distributed within the RPU100is provided below in connection with the description of the detailed schematics depicted inFIGS. 2 and 3. During normal operation, the power supply120derives the power it provides to the devices of the RPU100from the primary power source104. Typically, but not necessarily, during normal operation, the power supply120steps down (e.g., using a buck converter, a linear regulator, etc.) the voltage of the primary power source104to a lower voltage or multiple, different lower voltages for use by the various circuits within the RPU100.

As is also shown inFIG. 1, the controller110is operatively coupled to a voltage monitor122to monitor one or more voltages associated with the example RPU100. For example, the voltage monitor122may provide signals corresponding to a voltage of the primary power source104, a power supply voltage provided to the process control device102, a voltage of the energy storage device106and/or any other voltages that may be used to control or affect the operation of the RPU100.

In the event of a power loss at the primary power source104, the power supply120continues to receive electrical energy from the energy storage device106. In this manner, as described in more detail below, the power supply120can continue to supply power to the circuitry within the RPU100for a period of time sufficient to enable the process control device102to complete an operation such as, for example, moving to a shutdown or failsafe position (e.g., a fully open or fully closed position). In response to detecting a power failure at the primary power source104via the voltage monitor122, the controller110causes the power router112to enable (e.g., close) a connection between the energy storage device106and the process control device102. Thus, in response to the detected power failure, the power router112routes the power from the energy storage device106to the process control device102to allow continued operation of the process control device102. Additionally, in response to the detected power failure, the controller110causes the power router112to disable (e.g., open) a connection between the primary power source104and the charger108, thereby disabling the charger108and preventing further charging of the energy storage device106and prevent back-feeding of the primary power source via the charge balance circuitry114. Still further, in response to the detected power failure, the controller110causes the communications switch116to open to prevent the signals on the communications lines118from reaching the process control device102. The loss of the signals on the communications lines118, in turn, causes the process control device102to enter a power failure mode and to begin moving toward a predetermined (e.g., failsafe) position.

In response to the detection of the power failure at the primary power source104, the controller110also performs a controlled discharge of the energy storage device106via a main discharge circuit124and a near-zero discharge circuit126. The controlled discharge of the energy storage device106may begin after a predetermined amount of time following the detection of the power failure, be initiated by the process control device102, or may begin immediately following the detection of the power failure, depending on the needs of a particular application. The controlled discharge is initiated and supervised by the controller110to enable the process control device102to complete an operation such as, for example, the movement to a failsafe position before the remaining energy in the energy storage device106falls below a threshold amount that prevents further movement of the process control device102.

To control the main discharge circuit124, the controller110may provide a pulse-width modulated (PWM) signal to control one or more power switches that periodically shunt the energy storage device106to a ground potential, thereby dissipating the energy stored in the energy storage device106. The duty cycle of the PWM signal may be varied in accordance with a voltage of the energy storage device106measured via the voltage monitor122to control a maximum power dissipation and, thus, temperature of the main discharge circuit124. For example, the duty cycle of the PWM signal may be increased as the voltage of the energy storage device106decreases. To facilitate the removal of heat from the main discharge circuit124, various components of the main discharge circuit124may be thermally coupled to a housing128of the RPU100. The housing128may be composed of metal(s) and/or any other material. Thus, the housing128, in addition to forming a protective covering for the circuitry of the RPU100, may also function as a heat sink for some or all of the main discharge circuitry124and any other circuitry in the RPU100.

When the main discharge circuit124is functioning, a negative voltage converter130provides a negative voltage to the near-zero discharge circuit126to disable the near-zero discharge circuit126, thereby preventing the near-zero discharge circuit126from shunting energy stored in the energy storage device106to a ground potential. As the main discharge circuit124dissipates the energy stored in the energy storage device106, the voltage provided to the power supply120by the energy stored in the energy storage device106continues to decrease. While the voltage provided to the power supply120by the energy storage device106exceeds the voltage needed by the controller110for proper operation of the controller110, the power supply120uses a buck regulator to provide power to the controller110. However, when the voltage of the energy storage device106is no longer sufficient to enable the power supply120to use the buck regulator to provide power to the controller110, a boost circuit within the power supply120becomes active and continues to supply power to the controller110as the voltage at the energy storage device106continues to decrease. In this manner, the dual operating modes (i.e., buck/boost) of the power supply120enable the controller110to continue controlling the discharge of the remaining energy in the energy storage device106via the main discharge circuit124. In one example, the power supply120may continue to operate and provide sufficient power to the controller110for a voltage as low as, for example, 150 millivolts at the energy storage device106.

When the voltage of the energy storage device106falls below a low threshold at which the power supply120can no longer operate in a boost mode to provide sufficient power to the controller110, the controller110becomes inoperative, which disables the main discharge circuit124and prevents the main discharge circuit124from dissipating any remaining energy in the energy storage device106. Also, when the power supply120becomes inoperative, the negative voltage converter130no longer provides a negative disabling voltage to the near-zero discharge circuit126, which enables the near-zero discharge circuit126to shunt the remaining energy in the energy storage device106to a ground potential. As shown in more detail inFIG. 3-3, the near-zero discharge circuit126includes one or more normally closed switches that function to shunt the energy storage device106to a ground potential in the absence of power being provided to the near-zero discharge circuit126via the negative voltage converter130.

Other examples may use other methods to discharge the remaining energy. A particular example may not require the use of a boost circuit to maintain a minimum voltage requirement of the controller. Instead, an example apparatus may discharge any power remaining at this point through the use of one or more resistors. In this method, the controller would be operative to prevent the discharge as opposed to causing the discharge. Additionally, the resistor(s) and heat sink may need to be re-sized appropriately.

Thus, in response to a power failure at the primary power source104, the example RPU100enables the process control device102to complete, for example, movement to a failsafe position and then performs a controlled discharge of the energy storage device106after a maximum predetermined amount of time has elapsed. As such, service personnel, for example, can be assured that after waiting the maximum predetermined amount of time following a failure or removal of the primary power source104, an explosion proof container surrounding the RPU100and/or process control device102can be opened and the internal components of the RPU100or the process control device102can be serviced without risk of generating a spark or any other potentially harmful electrical event.

As shown inFIG. 1, the example RPU100also includes a manual override circuit132. The manual override circuit132may include a switch external to the housing128that enables a person to select an RPU override mode. When the RPU override mode is selected or enabled, the RPU100functionality described above is bypassed and the process control device102operates as if it is directly coupled to the primary power source104and the communication lines118. As a result, if the primary power source104fails or is otherwise removed, the process control device102does not receive any power from the energy storage device106and the process control device102may remain in the position it was in at the time of the power failure (i.e., may not be in a failsafe position).

The example RPU100also includes a status indicator134, which may be mounted external to the housing128to facilitate viewing by a person. In this example, the status indicator134is a light controlled by the controller110to provide different blink patterns to indicate the operational status or mode of the RPU100and/or the energy storage device106. Other examples may have a different status indicator134such as multiple light emitting diodes, a digital display, etc. The status indicator134of this example provides a different blink pattern for each of the modes of the RPU100including charging, discharging (i.e., when the energy storage device106is providing power to the process control device102), normal, override and discharged. For example, the status indicator134may blink in a slow steady manner to indicate the RPU100is charging, the status indicator134may periodically blink twice rapidly followed by a pause to indicate discharge operation, the status indicator134may periodically blink once rapidly followed by a pause to indicate normal operation (i.e., the process control device102is receiving power from the primary power source104via the RPU100), the status indicator134may provide a steady continuous light to indicate that the RPU100is in manual override mode, and the status indicator134may remain unlit to indicate that the RPU100is completely discharged.

The example controller110ofFIG. 1may be implemented by hardware, software, firmware and/or any combination of hardware, software and/or firmware. Thus, for example, the example controller110could be implemented by one or more analog or digital circuit(s), logic circuits, programmable processor(s), application specific integrated circuit(s) (ASIC(s)), programmable logic device(s) (PLD(s)) and/or field programmable logic device(s) (FPLD(s)). When reading any of the apparatus or method claims of this patent to cover a purely software and/or firmware implementation, the example controller110is hereby expressly defined to include a tangible computer readable storage device or storage disk such as a memory, a digital versatile disk (DVD), a compact disk (CD), a Blu-ray disk, etc. storing the software and/or firmware. Further still, the example controller110ofFIG. 1may include one or more elements, processes and/or devices and/or may include more than one of any or all of the elements, processes and devices.

Additionally, the example controller110ofFIG. 1may communicate with one or more of the functional components (e.g., the voltage monitor122, the main discharge circuit124, the power router112, etc.) using any type of wired connection (e.g., a databus, a USB connection, etc.) or a wireless communication mechanism (e.g., radio frequency, infrared, etc.) using any past, present or future communication protocol (e.g., Bluetooth, USB 2.0, USB 3.0, etc.). Further, one or more functional components ofFIG. 1may communicate with each other using such wired connections or wireless communication mechanisms.

Turning toFIGS. 2-1, 2-2, 2-3, 2-4, 3-1, 3-2 and 3-3, more detailed schematic diagrams illustrating one manner in which the circuitry of the example RPU100ofFIG. 1may be implemented are provided. The communication switch116is implemented in the example ofFIG. 2-1using opto-isolators U1and U2coupled to a transistor Q3, which is controlled via the processor U5. Similarly, the power router112, shown inFIG. 2-2may be implemented using opto-isolators U13and U14coupled to field-effect transistors (FETs) Q8, Q9, Q11, Q20and Q21. The opto-isolators U13and U14may be controlled by transistors Q4and Q5, which are coupled to a control signal provided by a processor U5, shown inFIG. 2-4. The programmable processor U5may be used to implement the controller110.

As shown inFIG. 2-3, the power supply120may be implemented using a buck converter that is controlled via a controller U3. The power supply120also includes a boost converter that is controlled via a separate controller U6. The outputs of the buck and boost converters are joined via a diode D14and a resistor R28to provide a sufficient operating voltage for the processor U5. During the controlled discharge, the voltage of the energy storage device106may range from a voltage greater than the voltage needed by the processor U5to a voltage that is substantially lower than the voltage needed by the processor U5for proper operation.

Block300ofFIG. 2-1is depicted inFIGS. 3-1-3-3as a more detailed schematic diagram of circuitry that may be used to implement the charger108, the charge balance circuit114, the energy storage device106, the near-zero discharge circuit126and the main discharge circuit124. As shown inFIG. 3-1, the charger108may include a buck converter controller U7that receives feedback from a current monitor U8such that the charger108functions as a current source to charge the energy storage device106. The charging current provided by the charger108is controllably varied by changing the reference voltage at the inverting terminal of an operational amplifier U11A, shown inFIG. 3-2. This reference voltage is varied in accordance with a varying voltage provided by the processor U5(FIG. 2-4) via a digital-to-analog converter (DAC) signal that is provided to a buffer implemented with an operational amplifier U11B, shown inFIG. 3-1. The charging current provided to the energy storage device106, shown inFIG. 3-3, may be decreased as the voltage of the energy storage device106approaches a fully charged condition to prevent overheating and/or overcharging of the energy storage device106.

The charge balance circuit114may be implemented using a resistor divider including equal value resistors R45, R46, R47, R48, and R49. These resistors R45-R49provide equal portions of the total voltage of the energy storage device106to respective individual capacitors C31, C32, C33, C34and C35making up the energy storage device106via respective buffers formed with the operational amplifiers U9A, U9B, U10A and U10B.

FIG. 3-1shows the main discharge circuit124includes three FETs Q19, Q22A and Q22B and the voltage provided by the energy storage device106(labeled CapPos). The gate terminals of the FETs Q19, Q22A and Q22B are connected to the PWM signal provided by the processor U5. Thus, during operation of the main discharge circuit124, the PWM signal causes the FETs Q19, Q22A and Q22B to periodically turn on and off to shunt the energy stored in the energy storage device106though the resistor R60and the transistors Q19, Q22A and Q22B to a ground potential. Although three FETs are shown as implementing the main discharge circuit124, fewer or more FETs and/or any other types of transistors or switches may be used instead to achieve the same or similar results.

The near zero-discharge circuit126, shown inFIG. 3-3may be implemented using FETs Q14, Q15, Q16, Q17and Q18, which are connected to shunt across respective ones of the capacitors C31, C32, C33, C34and C35to a ground potential. The FETs Q14-Q18function as normally closed switches that, when energized, open (i.e., do not conduct) in response to the presence of the negative voltage provided by the negative voltage converter130(FIG. 2-4). However, as described above, when the primary power source104fails or is otherwise not available, the power supply120continues to operate by drawing energy from the energy storage device106for a certain amount of time. However, when the voltage of the energy storage device106falls below a threshold, the power supply120no longer functions, which causes the negative voltage converter130to no longer provide the negative voltage to the near zero-discharge circuit126, shown inFIG. 3-3. The loss of the negative voltage causes the FETs Q14-18of the near zero-discharge circuit126to revert to their normally closed (i.e., conducting) states, thereby shunting any remaining energy in the capacitors C31-C35to ground potential and, thus, dissipating the remaining energy in the process.

FIG. 4depicts an example method400implemented by the example apparatus100. Block402represents the process control device102in a normal operating mode, in which the energy storage device106is charging, the process control device102is operating on external power (i.e., the primary power source104) and the control signal(s) from the communication lines118are enabled (i.e., connected to the process control device102). The voltage monitor122monitors the voltage of the primary power source104provided to the RPU100and compares this voltage to a threshold (block404). If the voltage is above the threshold, the example apparatus100and process control device102continue operating normally (i.e., control returns to block402). If the voltage of the primary power source104falls below the threshold, it is indicative that the primary power source104has failed, at which time the power router124causes the process control device102to operate on power provided by the energy storage device106(block406). The controller110then disables a connection between the primary power source104and the charger108(block408). Additionally, the controller110disables a connection between the communication lines118and the process control device102via the communications switch116(block410), thereby activating a loss of signal function in the process control device102and causing the process control device102to move to a failsafe position. The controller110waits for the process control device102to complete the movement, which may include, for example, waiting a predetermined amount of time sufficient for the process control device102to complete the movement or receiving a notification that the movement has completed (block412). The controller110then begins discharging remaining energy from the energy storage device106(block414). In the example method400, discharging the remaining energy from the energy storage component106may include one or more steps (e.g., discharging via the main discharge circuit124, discharging via the near-zero discharge circuit126). After the remaining energy in the energy storage device106is discharged, the controller110is no longer operative because it is not receiving power from the power supply120and the RPU100is fully discharged (block416).

In this example, at least a portion of the method represented by the flowchart inFIG. 4may be implemented using machine readable instructions that comprise a program for execution by a processor such as the processor U5shown in connection withFIG. 2-4. The program may be embodied in software stored on a tangible computer readable storage medium such as a CD-ROM, a floppy disk, a hard drive, a digital versatile disk (DVD), a Blu-ray disk, or a memory associated with the processor U5, but the entire program and/or parts thereof could alternatively be executed by a device other than the processor U5and/or embodied in firmware or dedicated hardware. Further, although the example program is described with reference to the flowchart illustrated inFIG. 4, many other methods of implementing the example apparatus100described herein may alternatively be used. For example, the order of execution of the blocks may be changed, and/or some of the blocks described may be changed, eliminated, or combined.

As mentioned above, at least a portion of the example method ofFIG. 4may be implemented using coded instructions (e.g., computer and/or machine readable instructions) stored on a tangible computer readable storage medium such as a hard disk drive, a flash memory, a read-only memory (ROM), a compact disk (CD), a digital versatile disk (DVD), a cache, a random-access memory (RAM) and/or any other storage device or storage disk in which information is stored for any duration (e.g., for extended time periods, permanently, for brief instances, for temporarily buffering, and/or for caching of the information). As used herein, the term tangible computer readable storage medium is expressly defined to include any type of computer readable storage device and/or storage disk and to exclude propagating signals and to exclude transmission media. As used herein, “tangible computer readable storage medium” and “tangible machine readable storage medium” are used interchangeably. As used herein, the term computer readable medium is expressly defined to include any type of computer readable storage device and/or storage disk and to exclude propagating signals and to exclude transmission media.