Introduction: The evolution from simple pneumatic devices to sophisticated smart meters has resulted from the need for higher performance, more convenient maintenance and longer life. Smart meters can not only meet these requirements, but also provide more, although at the cost of smart meters are more complex, but once they understand and install smart meters, they not only help reduce the complexity of the process control and information system life cycle, but also improve Performance and reduce costs.

As the old saying goes, if you want to control the process, you must first measure the process. Today, this sentence still works. The old corollary is that only with trustworthy process measurement information will the user be assured of control improvements.

For example, a process requires a minimum of 10 gpm (gallons per minute). If the operator does not trust the accuracy of the flow meter, then the safety will be set at 11 gpm, even if it increases raw material costs and disposal costs. With reliable and accurate intelligent flow meters, operators can confidently reduce their flow to 10.1 gpm, saving money by increasing efficiency.

The smart meter can notify the operator of problems or potential problems in the current operation before the instrument failure occurs, thereby reducing the occurrence of failures. The use of most measuring devices makes it easy to detect significant faults, but only smart meters can detect subtle problems that are the culprit for measuring inaccuracies and are a precursor to faults.

What is a smart meter?

The definition of smart meters has evolved over the past few decades (Table 1). The old-fashioned non-intelligent instrument is a 3-15 psi pneumatic device that performs control operations through an on-site single-cycle controller. The information is usually displayed locally in the form of a meter. The technicians visit the site and manually record the data using pens and white paper. If the data needs to be recorded and analyzed automatically, you need to use a local chart recorder.

The process variable is proportionally converted to 1-5, 4-20, 10, 50mAdc signal output to increase the degree of intelligence for the instrument. Once the data can be remotely transmitted, remote measurement, display and control are possible.

The parallel development of control systems with central processing and I/O introduces new methods that can more efficiently collect the information generated by 4-20 mA meters, convert this information into engineering units, and process and record this information centrally. Loop-powered instruments are also possible. Cn can power multiple transmitters from one current source, usually through analog I/O modules.

The microprocessors are more and more durable and can be installed directly in field instruments. This is a big step forward for smart meters, enabling local data signal processing and creating the first real smart meters because these devices can simulate The 4-20mA signal is converted to a digital signal that is then transmitted over the network. The local microprocessor can also perform some instrument-level tasks, calibration and diagnostics.

Since smart meters can convert process variables and other parameters into digital form, the creation of fieldbus networks is a matter of course. The original fieldbus network uses the existing 4-20mA signal line as the physical transmission medium, and the handheld calibration device can access any position of the 4-20mA current loop. In addition to hand-held devices, some discrete control systems have access to communication functions and digital access to process variables and device diagnostic data.

Many suppliers have their own networks, use existing 4-20mA signals as transmission media, and HART technology emerged in a timely manner to unify various incompatible networks. HART became the supplier's neutral foundation and eventually developed into an excellent multilayer field device communication protocol supported by most instrumentation and control system vendors. Therefore, smart devices are also labeled with HART.

In order to use the digital value of the process variable for real-time control and send more information to the remote host, people have higher performance requirements, and HART can no longer meet. As microprocessors become more and more powerful, in addition to process variables, smart meters can provide more valuable information, which further stimulates the need for more powerful networks.

A digital network dedicated to the connection of instrumentation and automation systems began to appear, among them the Foundation Fieldbus H1 and ProfibusPA. These platforms no longer use 4-20mA output and instead use advanced electronics and software standards to increase speed, improve diagnostics, and bring new features such as embedded local control. These two types of fieldbuses and other fieldbuses were adopted by the standardization organization and became embedded communication options for many smart meter suppliers.

Recently, the emergence of a proven and robust wireless networking technology has led to the development of specific vendors and standards-based solutions that focus on communications between field devices and remote access sites. The HART Communication Foundation has developed backward compatible wireless technologies and obtained IEC62591 standardization approval. The ISASP100.11a committee is also working on the standardization of wireless instrument communications.

Today, the general definition of a smart meter is a device that contains one or more digital network communication options. Because digital communication requires the use of a microprocessor, modern smart meters can also provide many types of other functions.

Smart meter performance

Successful measurements result from the correct installation of suitable instruments in the correct application. In addition to the built-in sensor input, non-intelligent instruments cannot know any other process information. However, the smart meter has a diagnostic function that can detect faults during installation or problems during operation, both of which can affect the measurement quality and/or the reliability of the equipment. Smart meters can also respond to queries or send status information to automation systems and other networked platforms.

Some smart meters have a local HMI for basic setup, calibration, and diagnostics. Picture source: EH

The key to unlocking smart meter information is the operator interface. Cn, This interface can be a local display on the instrument, a local handheld HMI (Human Machine Interface) or a networked HMI. Compared with the local display, HMI can make the communication between technicians or operators and the instrument more smooth.

Multivariable meters that use a variety of built-in sensors to measure multiple process variables simultaneously and communicate using digital or wireless technology are the highest levels of smart meters. For example, a Coriolis meter can measure and/or calculate mass flow, viscosity, density, temperature, and total flow.

Some smart devices equipped with communication protocols such as HART6, WirelessHART or Foundation Fieldbus H1 can exchange process variables directly between similarly configured instruments. These process variables are used to perform additive calculations in the field without the help of an automation system or an additional calculation unit. For example, a vortex flowmeter can interface with a pressure transmitter to generate a corrected energy flow, or two pressure transmitters can be connected to each other to generate a differential pressure value.

The Internet age has brought tremendous connectivity, information management and access options. Although many technologies have been adopted by automation platforms, such as programmable logic controllers, programmable automation controllers, and discrete control systems, protection and safety issues still need to be considered before the key process instruments are connected to Ethernet or other similar networks.

Formally due to security and protection issues, a large part of the installed instruments are still sending process variables to the automation system in the form of old 4-20mA signals, which are then converted and managed in the automation system. However, wireless field networks such as FOUNDATION fieldbus, Profibus and Ethernet/IP, and wireless HART networks increasingly provide digital process variables from the meter directly to the automation platform without the use of 4 -20mA signal and its required I/O infrastructure.

The process variables that define energy usage, environmental reporting, supply chain monitoring, and process unit monitoring are often not part of the real-time control architecture of the automation system—so these variables can be sent directly from the smart meter to the database via the IT access point. Earth simplifies the architecture of automation systems and information systems.

Functions outside of process variables

Smart meters can not only send process variables to multiple hosts, but also many other functions. The smart meter can provide measurement status, linearization, and/or other adjusted raw analog values ​​to one or more engineered process variables while providing meter status data. These engineering process variables and device data communicate digitally through fieldbus or wireless networks and automation systems and/or other networked access points.

Table 2 lists some of the key data points that smart meters currently offer and details their advantages. For example, meter status data can indicate data quality issues or alert. Engineering or maintenance tools can be used locally, or they can be networked using the Control Engineering Network copyright to assess problems and form solutions. Possible remedies include recalibration, configuration changes, or instrument changes.

According to the correlation of the traceability standards, the measurement output of the meter is evaluated according to the calibration standards to ensure the quality of the measurement information. Many smart meters can perform internal self-test diagnostics. For example, a Coriolis flowmeter dynamic test can indicate whether a traceability method is needed for calibration verification. Some smart meters provide National Institute of Standards and Technology (NIST) traceable verification tools to support traceable verification and calibration requirements as specified in ISO 20017.7.

Manufacturers of 4-20mA meters that can design Safety Instrumented Systems (SIS) are increasingly designing, producing, and managing lifecycles according to the IEC 61508 standard. Safety system designers typically follow the IEC61511, ISA, and ANSI 84.01-2004 safety system lifecycle management standards, and some also use IEC 61508 certified instruments with internal diagnostic capabilities.

These certified instruments can help designers achieve the required process safety integrity level (SIL). Smart SIL meters can also interact with status monitoring tools to diagnose information to help complete meter maintenance.

With the popularity of smart meters, the number and complexity of diagnostic information has increased dramatically, putting forward requirements for standards for error identification and diagnostic codes. NAMUR's recommended technical specification, NE107, is part of the standardization of fault diagnostics. It recommends that diagnostic information from smart meters be classified into five categories of standard status signal categories (Table 3).

The NE107-compliant device has an internal self-monitoring and fault self-checking function, which categorizes the diagnostic information and forms a guiding hint. Reduced complexity, reduced training for operators and technicians, improved safety and instrument availability.

Other tools for standardizing smart meter data delivery are standards that use descriptive notation and terminology to define the specific form of each data point. The two main standards currently used for this purpose are Electronic Device Description Language (EDDL) and Field Device Tool (FDT).

As more and more data can be transmitted from the meter, the speed of the meter becomes more and more intelligent, which brings more convenience to the user and simplifies the installation and operation process.

Smart to intelligent transformation
The development of instrumentation from non-intelligent to intelligent instrumentation will not be far behind. Future expert meters have multiple communication channels. Each communication channel has built-in security, much like current Ethernet managed switches. These channels will be assigned IP addresses and supplemented by server technology, making the meter a truly data server.

A high-speed channel is used to transfer process variables to the real-time controller. According to the instrument's communication source, this channel is assigned the highest priority among all channels. Other communication channels will be used to directly connect the instrument to the application, such as process monitoring, equipment construction, environmental monitoring, energy management, equipment management, advanced maintenance, and advanced diagnostics. This direct connection will span real-time control systems, simplifying the entire automation and information system architecture.

Industrial wireless standards beyond WirelessHART and ISA100.11a will continue to evolve to meet the needs of users and are included in NAMURNE124. Users are increasingly accepting wireless technology as a mainstream solution, and smart meter information will increasingly connect with the wireless access point to control the engineering network. The result is a direct evolution to a control or IT network data server.

The annoyance of system designers and end-users for integrating repetitive tasks is forcing mainstream instrument manufacturers and fieldbus protocol organizations to adopt Field Device Integration (FDI) specifications. This FDI specification will consolidate the existing EDDL and FDT specifications and should result in a truly unified field device integration architecture that allows any field device to be connected through virtually any fieldbus network.

As instrumentation is being developed toward safety-relevant recommendations such as NE97 and IEC61508-2, fieldbus-based safety process instrumentation will become more widely used. Safety certified instruments will work in conjunction with fieldbus safety protocols such as CIP Safety, FoundationfieldbusSIS, and PROFIsafe.

The combination of all these developments will bring true smart meters that can easily integrate future automation systems and information systems. For users, this has a strong practical significance, users can use this to achieve the benefits of all smart instruments: better process control, higher efficiency, lower power consumption, less downtime and higher quality.