1.6 Configuring Intelligent Devices

Transcription

1 1.6 Configuring Intelligent Devices J. BERGE (2003) DESIGN FEATURE RECOMMENDATIONS Use permanently connected communications infrastructure. For the HART Field Communications Protocol, use a handheld with large, user-friendly screen. Use device configuration templates. Use a fieldbus tool that requires no proprietary files from the host manufacturer. Be mindful of revisions when configuring fieldbus devices. COSTS A HART handheld, such as Smar International s HPC301, costs approximately U.S. $1000. A FOUNDATION fieldbus host, such as the SYSTEM302, starts at U.S. $4000. INTRODUCTION Intelligent devices include smart instruments that have both an analog 4- to 20-mA signal with simultaneous digital communication, such as HART (Volume 3, Section 4.11), and fieldbus devices that are completely digital, such as FOUNDATION fieldbus (Volume 3, Section 4.12). Intelligence in these devices, along with networking and the right tools, can also be put to good use to improve maintenance practices. Both FOUNDATION fieldbus and HART protocols are specifically designed for the configuration of field instruments and are therefore the most commonly used in the process industries. They both have special parameters for the specific purpose of device configuration, and both use the concept of device description (DD) files to inform a host device how to communicate these parameters with the device. This is a unique characteristic of these protocols that makes them highly suitable for instrumentation and control. In the HART protocol, device parameter configuration is based on commands for reading and writing. There are three classes of commands: universal, common-practice, and specific. All HART devices support the universal commands, and most also support several common-practice commands. Together, the universal and common-practice commands cover most functions needed in a device. This enables a host to perform most device functions without access to the DD. Essentially, all devices also have a number of specific commands to access unique functionality. Either a DD or a special driver is required to communicate the special commands. For FOUNDATION fieldbus device configuration parameters are arranged in a resource block and in transducer blocks. There are also function blocks, but these are used for control strategy building rather than device configuration. This distinction makes it easy to draw the line between device and strategy configuration. The block parameters are accessed using sophisticated communication services. A host needs to be loaded with the DD to be able to communicate with a device. In addition, to allow configuration of the device off-line, in advance, the capabilities file (CF) is required. Range setting is perhaps the most important setting of a device based on a 4- to 20-mA signal such as used by a HART instrument. Range sets the scale for the 4- to 20-mA signal. Because FOUNDATION fieldbus devices communicate in floatingpoint mode and engineering units, there usually is no need to set a range. Calibration shall not be confused with range setting, since they are in fact different operations. For example, for a transmitter range setting means configuring the measured values at which the output shall be 4 and 20 ma, respectively. Calibration, on the other hand, means adjusting the reading from the sensor to match the correct value from a standard (Section 1.8). Range setting does not correct the sensor reading; it only affects the output scale. To calibrate a transmitter, you must always apply a known input. Therefore, calibration cannot be done remotely. The expression remote calibration is often used erroneously to mean remote range setting. Some confusion exists in the marketplace as to what calibration is. This is a legacy from the era of analog devices in which calibration and range setting was done at the same time using the same set of potentiometers. For the HART protocol, calibration is usually called trim to distinguish it from range setting. FOUNDATION fieldbus avoids this confusion by calling range setting scaling. Still it is quite common in HART devices to change the range instead of calibrating the device when the sensor reading is wrong. The result is a correct 4- to 20-mA output, but any digital reading in the display or host will be wrong, which may lead to confusion. 93

2 94 General Considerations CONFIGURATION TOOLS Operation Engineering Maintenance Business FOUNDATION fieldbus devices are permanently connected via a network to a host in a convenient central location. HART devices, on the other hand, normally operate only using 4 to 20 ma, and a hand-held HART communicator is connected and communicates only when required. A FOUNDATION fieldbus host is therefore predominantly a stationary desktop computer, whereas a HART communicator is usually a portable device. However, portable interfaces for laptops exist for both HART and FOUNDATION fieldbus operation. The handheld can be connected at any convenient point along the 4- to 20-mA wire as long as it is on the instrument side of the stipulated resistor (see Figure 1.6a). This connection is often done at the device itself, giving on-line access to information, configuration, remote monitoring, calibration and range setting, diagnostics, and maintenance. The handheld can be either a dedicated text-based HART device or a HART interface for an organizer with a graphical user interface (GUI). Rapid development in the area of personal organizers has brought about tools that incorporate displays larger than three inches across and are capable of sophisticated graphics with a Windows-like appearance. They are also are very fast (see Figure 1.6b). Fieldbus instruments are connected on the H1 field-level nework that ties in with a linking device on the HSE hostlevel network where the host computers with the configuration tool is (Figure 1.6c). This is a permanent connection where the network used for control, monitoring, and operation is also used for configuration and diagnostics, etc. H1 Fieldbus Devices HSE Fieldbus H1 Fieldbus Devices FIG. 1.6c FOUNDATION fieldbus system architecture. Router + Power Supply FIG. 1.6a HART handheld connection. Al Resistor Handheld Terminal FIG. 1.6d FOUNDATION fieldbus linking device. (Courtesy of Smar International.) FIG. 1.6b HART pocket Configurator. (Courtesy of Smar International.) Linking device is typically one of many functions integrated in a single device. Usually the same device is responsible for powering the field instruments, etc. A linking device typically has several H1 ports and can be connected in a redundant scheme for high availability (Figure 1.6d). For FOUNDATION fieldbus interoperability with the host is achieved through DD. By installing the device support files for a device, the host computer has the information required to communicate with the device and to allow the user to prepare a configuration in advance, even without being connected to the device. The files usually can be obtained from the manufacturer s site on the Internet. To guarantee interoperability, an open host uses only standard files to support any device. However, some tools may require proprietary files for each device to work, in which case a short list of approved devices applies.

3 1.6 Configuring Intelligent Devices 95 DEVICE CONFIGURATION HART devices are typically configured on line only, using a handheld unit communicating directly with the device. However, some handhelds also support off-line configuration for later download. Although HART transmitters can operate in a purely digital mode, the 4- to 20-mA output is almost always used to deliver the process variable to the central controller. Setting the range of the transmitter is therefore required. The primary variable output settings are the most important part of the configuration. The lower range value (LRV) is the measured value at which the transmitter output will be 4 ma, and the upper range value (URV) is the input value at which the output will be 20 ma; i.e., 0 and 100% of range, respectively. Although LRV and URV are the proper terms, most call them zero and span instead. But span is really the difference between the URV and the LRV. The engineering unit can also be selected. Range setting can be accomplished from the handheld by simply keying in the desired range values, regardless of input. This can even be done remotely and stored as an offline configuration. Another method is to apply an input and (by pressing a button on the handheld or on the transmitter itself) informing the device that the applied input is to be the LRV or URV, thus setting the range (see Figure 1.6e). The latter method is often used for pressure transmitters that are installed with impulse lines that add hydrostatic pressure. Pressing the button elevates or suppresses the zero, ensuring that the output is 4 ma when appropriate. If the transmitter has a noninteractive zero and span, the URV will be pushed by the equivalent amount, leaving the span unaffected. For example, if the range of a pressure transmitter in a level application starts off as 0 to 5.48 kpa, and applied rerange is done with 1.86 kpa input, the new range becomes 1.86 to 7.34 kpa. However, when a rerange is applied for the URV, this does not affect the zero; i.e., the span is changed instead. The damping is a first-order lag filter time constant. The transfer function is used to select linear or square root extraction for differential-pressure flowmeters, and possibly for other options such as a freely configurable lookup table or square root of third or fifth power for open-channel flow measurements. Square root extraction is often done in the central control system, but it is in fact better to do it in the transmitter, as this results in less A/D and D/A conversion error. Fail-safe mode can be set as upscale or downscale as per the NAMUR NE-43 standard. Downscale means that the output current will be set to 3.6 ma in case the internal diagnostics detect a fault. Upscale means the output will be set to 21 ma. Ideally, the receiving controller should have an input module that can interpret the failure signal and use this to shut down the control loop. It is also possible to review the write protection status of the device if the device has been write protected using a jumper or other solution. When performing range setting or calibration, the user needs to know the sensor limits and is therefore usually prompted with this information, which typically can be reviewed at any time (see Figure 1.6f). HART device information includes lower range limit (LRL), upper range limit (URL), and the minimum span. The minimum span is the smallest permitted absolute difference between the URL and LRL for the sensor. Many configuration options are device specific in HART devices. For example, for a temperature transmitter, the sensor type and wiring has to be configured (see Figure 1.6g). For a valve positioner, the actuator type has to be configured. In most FOUNDATION system installations, the device configuration is created off-line for all devices in advance and downloaded after installation. Moreover, the device configuration is typically accomplished with the network and control FIG. 1.6f HART transmitter sensor limits. FIG. 1.6e HART transmitter range setting. FIG. 1.6g Sensor and wiring selection is device specific.

4 96 General Considerations FIG. 1.6h Pay attention to revisions when creating device configurations. strategy configuration, all in the same tool. Thus, when configuring devices in a FOUNDATION system, the first step is to create the devices on the networks and give the devices a physical device tag (PD_TAG). Fieldbus devices are developing very rapidly. New versions are constantly being released, and many users already have several versions around the plant and in the store, this on top of a variety of brands of the same kind of device. When inserting new devices, it is important to specify the revisions to be used (see Figure 1.6h). Usually, the latest revision is the default. There is a risk that you will accidentally download a device configuration to the wrong model or wrong version device. A good tool prevents this and thus avoids the many headaches that could result. Fieldbus devices do not require any range, because all values are communicated as floating-point values in engineering units. Ranges are typically used only in PID function blocks, or possibly in AI function blocks, to cater for conversions in inferred measurements, such as converting differential pressure to flow within specified ranges. That is, scaling is done as part of the control strategy configuration, not as part of the device configuration. Every fieldbus device, H1 as well as HSE, needs to have one resource block. Really, the only parameter that must be configured is the mode. The MODE_BLK parameter target shall be set to automatic. A fieldbus transducer block is required, in conjunction with every sensor and actuator, to act as an interface between the device and the control strategy. By parameterizing the transducer blocks, the device can be set up for the proper sensor or actuator type, such as HART devices. Similarly the transducer block also contains information about sensor limits. Indeed, there is a transducer scale range indication in the transducer block, but it is essentially a reflection of a setting done in the associated I/O function block. The MODE_BLK parameter target shall be set to automatic. important dates, and model numbers. The user can configure some of these parameters to be pertinent to the application. These parameters do not affect the operation of the device. In addition to the sensor information such as range limits, HART device data such as sensor serial number, final assembly number, message, device tag, descriptor, date, manufacturer, device type, software and hardware revisions, and circuit board serial number are provided. For HART devices, the tag can use up to eight characters. The descriptor and message are 16 and 32 characters in length and can be configured as annotations describing the application and remind technicians of special precautions when servicing a device (see Figure 1.6i). The date has no specific purpose but may be used to store when calibration or maintenance was last performed or is scheduled. Manufacturer, model, and version information can also be accessed (see Figure 1.6j). HART devices such as pressure transmitters usually provide additional information about the materials of construction for the parts wetted by the process, e.g., the sensor isolating diaphragm, O-ring, flange and remote seal parts, and so on. The resource block found in any FOUNDATION fieldbus device contains, among other things, identification information for the device. The tag descriptor parameter may be used to describe the application of the device. Manufacturer, model, and version information can also be accessed (see Figure 1.6k). FIG. 1.6i Application information from a HART device. IDENTIFICATION Information for identifying the device is very helpful during commissioning and maintenance. During the commissioning stage, it is useful for further assurance that connection has been made to the correct device. During maintenance, it is helpful for retrieving serial numbers, special instructions, FIG. 1.6j Detail attributes of a HART device.

5 1.6 Configuring Intelligent Devices 97 FIG. 1.6k Fieldbus device identification from resource block. FIG. 1.6m Zero calibration of HART transmitter. FIG. 1.6l Pressure transmitter materials of construction. Fieldbus devices such as pressure transmitters usually provide additional information about the materials of construction for the parts wetted by the process, e.g., the sensor isolating diaphragm, O-ring, flange and remote seal parts, and so on (see Figure 1.6l). If any changes are ever made to the device parts, it is important to update this information. CALIBRATION When the sensor reading differs from the actual applied input, the sensor has to be calibrated. The correct reading is entered from the maintenance tool, and the device then performs the necessary adjustment. A special case of sensor calibration is zeroing, which by definition is done with a zero value applied. This is accomplished, for example, by venting in the case of a pressure transmitter; hence, no value has to be entered (see Figure 1.6.m). Nonzero calibration is usually done with a precision source applied. Calibration is usually done in two points, known as the low and high calibration points, respectively. For most transmitters, calibration of these two points is noninteractive. There is a limit to how close the two calibration points can be, and the distance is referred to as the minimum span. Output converters also need to be calibrated. This is usually done by first forcing an output at one end of the scale and then FIG. 1.6n Loop current calibration. comparing the actual output against a standard. The actual reading is entered into the device that makes the necessary correction. Control valve positioners usually calibrate their position-sensing sensor themselves by automatically stroking the valve over its entire travel. Since HART devices rely on 4 to 20 ma, this current loop can also be calibrated. However, it is rarely done because, being totally electronic, this part of the device rarely experiences any drift at all. For a HART transmitter, this means that a fixed output current is generated. The technician checks the current against a standard and keys it into a device that makes the necessary correction (see Figure 1.6n). For HART output devices, a signal is injected, and the device is informed of the true current. Sensor or actuation calibration for FOUNDATION fieldbus devices is done from the associated transducer block. The standard transducer block also includes several parameters for storing information about the last calibration. This includes two parameters for keeping the values of the two calibration points. This is useful when determining if a device has been calibrated at points that are suitable for the operating range. Additional information that can be stored includes calibration date, location, the method used, and who performed the calibration. It is good practice to update this information at each calibration and to display it along with useful sensor limit information (Figure 1.6o).

6 98 General Considerations FIG. 1.6o Calibration-related information. FIG. 1.6q Using loop test to simulate process variable from a handheld. FIG. 1.6p Monitoring of dynamic transmitter variables from a HART tool. MONITORING Configuration tools for HART and FOUNDATION fieldbus systems are generally designed in different ways, not only in terms of the way they look, but also the way they work. In HART tools, menus are usually arranged according to the function performed, whereas device configuration in fieldbus tools is arranged according to the resource and transducer blocks. Thus, in a HART device, there generally is a screen on which the transmitter variables can be monitored on line (see Figure 1.6p). SIMULATION At the time of commissioning, it is common to check that all indicators, recorders, and computer screens show the correct values, that alarm trips are working properly, and so on. This is particularly important for HART devices, because there is a chance that the range set in the device does not match that set in the central controller, and any difference would result in operational problems. To verify that all ranges are consistent, it is helpful to use the simulation function found in HART and fieldbus devices. When performing simulation in a HART transmitter, the output current is manipulated independently of the applied FIG. 1.6r Basic HART diagnostics. input (see Figure 1.6q). Generally, the handheld allows the current to be set slightly above and below the 4- to 20-mA range to simulate fault conditions. Simulation is primarily used to test the control strategy. Therefore, simulation for FOUNDATION fieldbus devices is done from the input and output function blocks. DIAGNOSTICS Many device diagnostics can be performed from a configuration tool, but only if communication is established. If there is no functional communication, troubleshooting has to rely on traditional means. When it comes to diagnostics, one of the major differences in the application of HART and FOUN- DATION fieldbus becomes most evident. While fieldbus devices are constantly communicating, making it possible to monitor device conditions continuously and instantly detect faults, communication with HART devices is typically carried out to confirm a problem only after the failure has already been detected. Although applications with continuous HART communications do exist, they are rare. To fully benefit from intelligence in field devices, the engineering tool should continuously communicate with the instruments. Generic diagnostic information is communicated in every exchanged HART message, giving the device the opportunity to inform the user of any failure. Such error message includes general malfunction, exceeded limits, and a message that more detailed status report is available (see Figure 1.6r).

7 1.6 Configuring Intelligent Devices 99 FIG. 1.6s Advanced diagnostic self-test. For some devices, a more thorough diagnostic self-test routine can be invoked (see Figure 1.6s). Every FOUNDATION block, including function blocks, contains a block error parameter with some basic condition for the block itself, including configuration errors, I/O or memory failure, and others. In addition, the block mode parameter is useful for spotting problems, which often results in the actual mode being different from the target mode. Yet another good source of diagnostic information is the status associated with the function block inputs and outputs, and also with some of the parameters contained in transducer blocks and function blocks (see Figure 1.6t). FIG. 1.6t Abnormal status and actual mode are useful for tracing problems. Diagnostic information for sensors and actuators is kept in their respective transducer blocks. The primary source for this information is the transducer error parameter that displays a single error at the time. Transducer blocks are always customized and typically contain extensive manufacturerspecific parameters related to diagnostics. Reference 1. Berge, J., Fieldbuses for Process Control Engineering, Operation and Maintenance, ISA, Research Triangle Park, NC, 2002.