Process control systems, like those used in chemical and petroleum processes, typically include one or more process controllers communicatively coupled to at least one host or operator workstation and to one or more field devices via analog, digital, or combined analog/digital communication links.
A process controller (sometimes referred to as a “controller”), which is typically located within the plant environment, receives signals (sometimes referred to as “control inputs”) indicative of process measurements and uses the information carried by these signals to implement control routines that cause the controller to generate control signals (sometimes referred to as “control outputs”) based on the control inputs and the internal logic of the control routines. The controllers send the generated control signals over buses or other communication links to control operation of field devices. In some instances, the controllers may coordinate with control routines implemented by smart field devices, such as Highway Addressable Remote Transmitter (HART®), Wireless HART®, and FOUNDATION® Fieldbus (sometimes just called “Fieldbus”) field devices.
The field devices, which may be, for example, valves, valve positioners, switches, and transmitters (e.g., including temperature, pressure, level, or flow rate sensors), are located within the plant environment and generally perform physical or process control functions. For example, a valve may open or close in response to a control output received from a controller, or may transmit to a controller a measurement of a process parameter so that the controller can utilize the measurement as a control input. Smart field devices, such as field devices conforming to the Fieldbus protocol, may also perform control calculations, alarming functions, and other control functions commonly implemented within a process controller. Field devices may be configured to communicate with controllers and/or other field devices according to various communication protocols. For example, a plant may include traditional analog 4-20 mA field devices, HART® field devices, or Fieldbus field devices.
Traditional analog 4-20 mA field devices communicate with a controller via a two-wire communication link (sometimes called a “loop” or “current loop”) configured to carry an analog 4-20 mA DC signal indicative of a measurement or control command. For example, a level transmitter may sense a tank level and transmit via the loop a current signal corresponding to that measurement (e.g., a 4 mA signal for 0% full, a 12 mA signal for 50% full, and a 20 mA signal for 100% full). The controller receives the current signal, determines the tank level measurement based on the current signal, and takes some action based on the tank level measurement (e.g., opening or closing an inlet valve). Analog 4-20 mA field devices typically come in two varieties: four-wire field devices and two-wire field devices. A four-wire field device typically relies on a first set of wires (i.e., the loop) for communication, and a second set of wires for power. A two-wire field device relies on the loop for both communication and power. These two-wire field devices may be called “loop powered” field devices.
Process plants often implement traditional 4-20 mA systems due to the simplicity and effectiveness of the design. Unfortunately, traditional 4-20 mA current loops only transmit one process signal at a time. Thus, a set-up including a control valve and a flow transmitter on a pipe carrying material may require three separate current loops: one for carrying a 4-20 mA signal indicative of a control command for the valve (e.g., to move the valve to 60% open); a second for carrying, to the controller, a 4-20 mA signal indicative of the valve's actual position (e.g., so the controller knows the degree to which the valve has responded to control commands); and a third for carrying, to the controller, a 4-20 mA signal indicative of a measured flow (e.g., so the controller knows how a change in valve position has affected the flow). As a result, a traditional 4-20 mA set-up in a plant having a large number of field devices may require extensive wiring, which can be costly and can lead to complexity when setting up and maintaining the communication system.
More recently, the process control industry has moved to implement digital communications within the process control environment. For example, the HART® protocol uses the loop DC magnitude to send and receive analog signals, but also superimposes an AC digital carrier signal on the DC signal to enable two-way field communication with smart field instruments. As another example, the Fieldbus protocol provides all-digital communications on a two-wire bus (sometimes called a “trunk,” “segment,” or “Fieldbus segment”). This two-wire Fieldbus segment can be coupled to multiple field devices to provide power to the multiple field devices (via a DC voltage available on the segment) and to enable communication by the field devices (via an AC digital communication signal superimposed on the DC power supply voltage).
These digital communication protocols generally enable more field devices to be connected to a particular communication link, support more and faster communication between the field devices and the controller, and/or allow field devices to send more and different types of information (such as information pertaining to the status and configuration of the field device itself) to the process controller. Furthermore, these standard digital protocols enable field devices made by different manufacturers to be used together within the same process control network.
Regardless of the communication protocol utilized, field devices may require on-site setup, configuration, testing, and maintenance. For example, before a field device can be installed at a particular location at a process control plant, the field device may need to be programmed and may then need to be tested before and after the field device is installed. Field devices that are already installed may also need to be regularly checked for maintenance reasons or, for example, when a fault is detected and the field device needs to be diagnosed for service or repair. Generally speaking, configuration and testing of field devices are performed on location using a handheld maintenance tool, such as a portable testing device (“PTD”). Because many field devices are installed in remote, hard-to-reach locations, it is more convenient for a user to test the installed devices in such remote locations using a PTD rather than using a full configuration and testing device, which can be heavy, bulky, and non-portable, generally requiring the installed field device to be transported to the site of the diagnostic device.
When a user, such as a service technician, performs maintenance testing and/or communications with a field device, the PTD is typically communicatively connected to a communication link (e.g., a current loop or Fieldbus segment) or directly to a field device (e.g., via communication terminals of the field device). The PTD initially attempts to communicate with the field device, such as by sending and/or receiving digital communication signals along the loop or segment. If the current loop or segment is in proper operating condition, the communications signals may be sent and/or received without problem. However, if the loop, segment, or field device contains an electrical fault, such as a short or a break, communications may be impeded, and it may be necessary to diagnose the loop, segment, and/or field device to identify the fault.
When such a fault is identified, a technician might need to use a variety of other tools to test the field device and/or communication link. As an example, the technician may need to carry a multimeter to diagnose the actual signals transmitted or received by the field device. The multimeter is necessary because traditional PTDs are incapable of accurately analyzing the electrical characteristics of signals sent or received by a field device. As another example, the technician may need to use a portable power supply to power an isolated field device. The technician may need to power an isolated field device, for example, when the field device loses power due to a plant-wide power outage or due to an issue with a local power supply. As another example, the technician may simply need to take a field device offline for troubleshooting in order to avoid negatively effecting other field devices and the rest of the process control system. The technician may also need to carry a multimeter to measure the current available on a segment or loop, etc. Each of these tools can take up a fair amount of space, and may be inconvenient for a technician to carry in the field. To address this problem with carrying multiple tools, manufacturers have developed PTDs that include a power supply for providing power to a HART loop. Unfortunately, these powered PTDs are typically incapable of providing power to Fieldbus field devices. Further, typical portable power supplies and powered PTDs often fail to comply with Intrinsic Safety (IS) standards, and thus cannot be safely used in hazardous areas (e.g., an environments or atmospheres that are potentially explosive due to explosive gas or dust).
If a field device is located in a hazardous area, the technician may need to verify that each of his or her tools operates in an intrinsically safe manner. When in a hazardous area, a technician's tools may need to comply with IS standards to ensure safe operation. Generally speaking, IS standards require that plant personnel analyze all equipment attached to a loop or segment (including any PTDs or other tools that will be attached to the loop or segment) to verify that all attached equipment will operate in a safe manner in a hazardous environment. More particularly, IS standards impose restrictions on electrical equipment and wiring in hazardous environments to ensure that the electrical equipment and wiring does not ignite an explosion. To comply with IS standards, electrical equipment generally needs to be designed with two core concepts in mind: energy limitation and fault tolerance.
The first IS concept dictates that an IS device be designed such that the total amount of energy available in the device be below a threshold sufficient to ignite an explosive atmosphere. The energy can be electrical (e.g., in the form of a spark) or thermal (e.g., in the form of a hot surface). While IS standards can be complex, they generally require that any voltage within a circuit be less than 29 V; that any current within a circuit be under 300 mA; and that the power associated with any circuit or circuit component be under 1.3 W. A circuit having electrical characteristics exceeding these thresholds may pose an explosion risk due to arcing or heat.
The second IS concept dictates that that an IS device be designed in a fault tolerant manner, such that it maintains safe energy levels even after experiencing multiple failures. In short, IS standards reflect a philosophy that circuit faults are inevitable and that energy levels of the circuit must be limited to safe levels when these circuit faults occur.
Generally speaking, portable power supplies and powered PTDs are not IS compliant and thus cannot be used in hazardous areas because: (i) portable power supplies and powered PTDs are typically designed such that one or more components may exceed energy levels sufficient to risk igniting an explosive atmosphere, and/or (ii) the portable power supplies and powered PTDs are vulnerable to component failures that would result in the portable power supplies or powered PTDs exceeding energy levels sufficient to risk igniting the explosive atmosphere.
For example, a typical portable power supply may generate a voltage across its terminals sufficient to risk an explosion in a hazardous environment (e.g., above 29 V). Even when designed to supply a voltage of under 29 V, a typical portable power supply does not include fail-safe mechanisms guaranteed to prevent the supplied voltage or current from spiking. Consequently, when in a hazardous environment, technicians needing to provide power to a field device generally must uninstall the field device and transport the field device to a safe area where it can be powered and tested.