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Part 2: Trends and Developments Enhancing the Power of IR Thermometers The New Generation ofIR ThermometersToday's digital processing and data communications are making IR thermometers easier to use, maintain, and integrate into instrument and control systems. Klaus-Dieter Gruner This is Part 2 of a two-part series on the latest advances in IR thermometers. Last month, we discussed changes in detectors, optics, and electronics. This month, we continue by looking at recent developments in software, data communications, and networking and predict what the future will hold and how the changes will affect the design of measurement instrumentation in general and IR temperature sensors in particular. We'll also identify and examine rapidly emerging trends that will profoundly influence industrial sensing devices. The figures in Part 2 are numbered consecutively from those in Part 1. Data Communications
Data communications standards ensure the successful transfer of data among sensors, computers, and other instrumentation and control devices. The Electronics Industry Association has produced standards for industrial data communications, including RS-232 and RS-485 (the RS designation stands for recommended standard). Duplex communications allow devices to act as transceivers--to act as both transmitters and receivers. In this communications mode, data flow in both directions, allowing verification and control of data reception/transmission. RS-485 multidrop communications are half-duplex systems (i.e., they allow both transmission and reception, but not simultaneously) that meet the requirements of a true multidrop communications network (i.e., they allow multiple nodes, such as devices or sensors, to communicate bidirectionally). The standard supports communications in a master-slave, or polled, mode. Such networks require a host (e.g., a PC or PLC) to initiate data communications. RS-485 is considered a workhorse interface standard, and it is the most widely used serial interface found on sensors, actuators, and PLCs. The standard does not specify a protocol (i.e., a command syntax) but defines only the electrical interface requirements of the transceiver. Responsibility for defining protocols is left up to sensor manufacturers, who typically develop their own proprietary protocol (usually ASCII-based). It's impossible to connect RS-485 devices from different manufacturers to the same network unless they use the same protocol.
With shielded twisted-pair cable, an RS-485 network can operate reliably in the presence of ground differential voltages and noise, supporting devices connected in parallel on a "party line." Each device has a distinct address (numbered 1–32), and it responds only when correctly polled from the host. For years, the effective impedance of RS-485 chips supported the parallel connection of no more than 32 sensors on a 2-wire or 4-wire LAN. In recent years, though, chips have become available with higher impedances--four times (or more) the standard unit load, or SUL, allowing connection of 128 (or more) devices on a single LAN. The ability to network multiple sensors to a computer as part of an instrumentation and control system is the most economical method for acquiring, processing, and archiving data. A multidrop LAN offers reduced installation cost and complexity compared with a nonmultidrop configuration, which relies on multiplexing individual analog signals into a computer. Emerging trends pertaining to Ethernet and IEEE-1451 may alter the popularity of the RS-485 standard. Software Interfaces High-speed digital signal processors (DSPs) embedded in IR sensors allow bidirectional serial communications between the factory floor and the control room. This enables engineers to configure, calibrate, troubleshoot, and upgrade IR thermometers remotely, which is especially valuable when sensors are located in hard-to-reach areas. However, these capabilities cannot be realized without software. Temperature data can be incorporated in custom data acquisition software with other functions that make the sensors easier to install, use, and maintain. Configuration and monitoring programs allow remote configuration of IR sensor operating parameters, such as emissivity, signal processing, and alarming. This functionality helps you fine tune the system as process conditions change. The software lets you monitor multiple sensors simultaneously, graph temperature data for one or two sensors, and archive data for analysis and quality control applications.
Screen 1 shows several options available with digital communications for either networked or stand-alone sensors. These include a program that supports field configuration of the communications (COM) port, 2- or 4-wire LAN connectivity, sensor baud rate, and multipoint addressing. A graphic setup and display program identifies the sensor mode, model, and serial number and configures operating parameters using a PC (see Screen 2). A network setup and display program lets you remotely configure, monitor, display, and archive data from as many as 32 sensors on a LAN (see Screen 3). In addition to off-the-shelf software packages, custom data communications interfaces are frequently written by customers to provide application-specific functions using the protocol command syntax used in the sensors. Emerging trends involving OLE for process control (OPC) software, the rapid proliferation of the Internet and company intranets, and the adoption of smart transducer interfaces via the 1451 standard will strongly influence the development of software used to interface sensors with PCs, other DCSs, and third-party enterprise software. Buses and Digital Communications The industry is increasingly demanding that IR temperature sensor manufacturers and other instrument suppliers provide connectivity solutions for customers who use a wide--and unfortunately incompatible--variety of bus standards and communications protocols. All instrument suppliers provide suitable analog outputs (i.e., 4–20 mA) for interfacing instruments to PLCs and other process instrumentation, and the need for this functionality will continue for years. But more and more customers want to profit from the lower costs associated with integrating sensors in other systems and software using digital data communications. The demand for digital communications challenges instrument manufacturers to offer cost-effective products that do not restrict your interface choices. This isn't an easy undertaking. There are numerous bus structures on the market today (e.g., Profibus, CANBUS, DeviceNet, ControlNet, and Foundation Fieldbus), but no one bus or protocol meets the needs of most users. The proliferation of disparate bus structures, dissimilar hardware, and different software precludes general connectivity or interoperability. A growing number of sensor manufacturers provide serial port connections for data communications, which, in the near-term, offer a solution. Gateways bridge dissimilar networks and facilitate the connection of proprietary sensor protocols and networks to other bus networks, but this is not always the most economical solution. Rapidly emerging trends indicate a decided shift away from the traditional analog and serial port solutions toward more universal connectivity and greater freedom of choice. Ethernet and the Internet The major influence driving these network trends emanates not from the industrial sector of our economy but from the commercial sector--namely, the rapid, exponentially growing digital business community fueled by three interrelated, interconnected technologies: enterprisewide Ethernet; Internet- and TCP/IP-compliant systems and software; and Windows NT. The need to provide data and information from the factory floor to enterprisewide systems and networks is pulling Ethernet onto the factory floor from the front office. TCP/IP-compliant systems provide open interfaces with universal connectivity. Windows NT and Ethernet networks provide an infrastructure to acquire, view, process, and transmit files, graphics, and information over a company's intranet or the Internet. The increased use of industrial PCs on the factory floor reinforces this trend. This in turn influences trends in industrial software. Software and OPC Customers who need to run software on their systems, other than that associated purely with control, strongly advocate the move to open systems. Object-oriented technology, object linking and embedding (OLE), and OPC are at the forefront of this trend. OPC provides increased interoperability and facilitates the exchange of information among different sensors, automation devices, control systems, and production applications running across an entire manufacturing enterprise. With open-network protocols, a user can modify one part of the system without affecting communications to other areas. OPC makes it easier to integrate sensors into different plant information systems by standardizing the interfaces between dissimilar software and hardware devices that run on the NT platform. OPC software drivers allow full interoperability with sensors using third-party OPC-compliant software. The creation of these drivers by sensor manufacturers will help minimize their involvement in software development (at least at the application level), allowing them to focus on sensor and measurement technology. Sensors with OPC drivers allow customers full interoperability without regard to the specific application software (as long as it is OPC compliant). Enterprise software from suppliers such as Wonderware, Intellution, Rockwell, and National Instruments offers OPC-compliant software products. OPC obviates the need to write custom data communications software for sensors or to worry about proprietary protocols. Smart Sensors and Ethernet Smart IR thermometers using digital electronics and DSP chips let you adjust operating parameters remotely using the digital interface as an extended operator panel. DSP chips permit downloading of special temperature measurement algorithms and special signal processing formulas, eliminating the need for additional instrumentation or external processing. But the many incompatible bus protocols prevent adoption of a general interface. Widespread adoption of the 1451 standard could pave the way for general plug-and-play capabilities at the sensor level. IEEE-1451.2 includes an embedded transducer electronic data sheet, or TEDS, containing sensor-specific data characteristics that are part of the sensor (e.g., calibration information). When such smart sensors are connected to Ethernet, the sensor effectively can have a dedicated Web page. Interoperability is one of 1451.2's greatest assets, letting you select those instruments best suited to your performance requirements and not because they connect to a particular network technology. IEEE-1451.2 could find widespread use as it transcends the many disparities among competing field buses. Importantly, 1451.2 allows sensors, actuators, and final control elements to be connected to Ethernet. The objective of IEEE is to provide a standard that defines common data communication interfaces for connecting sensors, microprocessor-based embedded systems, actuators, and other devices in a network-independent manner. If embraced by a critical mass of sensor manufacturers, the standard will benefit customers and suppliers alike by offering sensor-to-network plug-and-play interoperability and interchangeability. Conclusion Analog and digital interface capabilities give you several sensor interface choices. The demand for a traditional 4–20 mA analog output will continue, though perhaps with diminished popularity. Networking is increasingly in demand for measurement, control, and data archival to better support modern production practices. Customers realize that sensors with networking capabilities save time and money when interfacing sensors to computers compared with interfacing multiple sensors to a computer using traditional 4–20 mA analog signals. Sensors with built-in, field-configurable networking capabilities offer customers the flexibility to re-deploy and reconfigure sensors as their measurement and control requirements change. Sensors providing separate analog and digital outputs offer the flexibility of allowing local closed-loop control via the analog output and data archival and networking via a multidrop LAN. Data communications and networking technologies adopted in the commercial sector are being pulled onto the factory floor so that customers can benefit economically from integrating production information and enterprisewide systems and networks. This is causing the Internet, Windows NT, Ethernet, and TCP/IP technologies to exert profound influence on the future direction of industrial sensor data communications and software capabilities. The adoption of 1451 by sensor manufacturers may enable customers to realize true plug-and-play capability among sensors, actuators, software, and systems on the factory floor. These trends confirm the observation that, in many factories and process plants, there is more to measuring temperature than having just a 4–20mA output.
Klaus-Dieter Gruner, Ph.D., is Vice President of Marketing and Alan Young, Ph.D., is Product Manager, Raytek Corp., 1201 Shaffer Rd., PO Box 1820, Santa Cruz, CA 95081-1820; 831-458-1110, fax 831-458-1239, klaus@raytek.com or alany@ raytek.com
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