A Smart Transducer Interface Standard for Sensors and Actuators (Copyright CRC Press) Kang Lee, National Institute of Standards and Technology, USA Abstract A set of common transducer interfaces has been defined in the family of IEEE (Institute of Electrical and Electronics Engineers) 1451 Standards for connecting sensors and actuators to microprocessors, control and field networks, and instrumentation systems. The standards also defined the Transducer Electronic Data Sheet (TEDS), which allows the self-identification of sensors. The interfaces provide standardized methods to facilitate the “plug and play” of sensors to networks and instrumentation systems. The network-independent smart transducer object model defined by IEEE 1451.1 allows sensor manufacturers to support multiple networks and protocols. In order to fulfill the needs of various industries and user requirements, four other standards in the family support various physical interface requirements for connecting sensors and actuators through the IEEE P1451.2, P1451.3, P1451.4, and P1451.5 specifications. In the long run, transducer vendors and users, system integrators, and network providers can all benefit from the IEEE 1451 interface standards and the associated enabling technologies. This paper will discuss the IEEE 1451 family of standards. Keywords: IEEE 1451, NCAP, network capable application processor, sensor interface standard, sensor networking, smart sensor, transducer electronic data sheet, TEDS, wireless sensor. 1. Introduction  Sensors are used in many devices and systems to provide information on the parameters being measured or to identify the states of control. They are good candidates for increased built-in intelligence. Microprocessors can make smart sensors or devices a reality. With this added capability, it is possible for a smart sensor to directly communicate measurements to an instrument or a system. In recent years, the concept of computer networking has gradually migrated into the sensor community. Networking of transducers (sensors or actuators) in a system and communicating transducer information via digital means versus analog cabling facilitates easy distributed measurements and control. In other words, intelligence and control, which were traditionally centralized, are gradually migrating to the sensor level. They can provide flexibility, improve system performance, and ease system installation, upgrade, and maintenance. Thus, the trend in industry is moving toward distributed control with intelligent sensing architecture. These enabling technologies can be applied to aerospace, automotive, industrial automation, military and homeland defenses, manufacturing process control, smart buildings and homes, and smart toys and appliances for consumers. As examples, 1) in order to reduce the number of personnel to run a naval ship from 400 to less than 100 as required by the reduced-manning program, the U.S. Navy needs tens of thousands of networked sensors per vessel to enhance automation, 2) Boeing needs to network hundreds of sensors for monitoring and characterizing airplane performance. Sensors are used across industries and are going global [1]. The sensor market is extremely diverse, and it is expected to grow to $43 billion by 2008. The rapid development and emergence of smart sensor and field network technologies have made the networking of smart transducers a very economical and attractive solution for a broad range of measurement and control applications. However, with the existence of a  multitude of incompatible networks and protocols, the number of sensor interfaces and amount of hardware and software development efforts required to support this variety of networks are enormous for both sensor producers and users alike. The reason is that a sensor interface customized for a particular network will not necessarily work with another network. It seems that a variety of networks will coexist to serve their specific industries. The sensor manufacturers are uncertain of which network(s) to support and are restrained from full-scale smart sensor product development. Hence, this condition has impeded the widespread adoption of the smart sensor and networking technologies despite a great desire to build and use them. Clearly, a sensor interface standard is needed to help alleviate this problem [2]. 2. A Smart Transducer Model  In order to develop a sensor interface standard, a smart transducer model should first be defined. As defined in the IEEE Std 1451.2-1997 [3], a smart transducer is a transducer that provides functions beyond those necessary for generating a correct representation of a sensed or controlled quantity. This functionality typically simplifies the integration of the transducer into applications in a networked environment. Thus, let us consider the functional capability of a smart transducer. A smart transducer should have: integrated intelligence closer to the point of measurement and control, basic computation capability, and capability to communicate data and information in a standardized digital format. Based on this premise, a smart transducer model is shown in Figure 1. It applies to both sensors and actuators. The output of a sensor is conditioned and scaled, then converted to a digital format through an analog-to-digital converter. The digitized sensor signal can then be easily processed by a microprocessor using a digital application control algorithm. The output, after being converted to an analog signal via a digital-to-analog converter, can then be used to control an actuator. Any of the measured or calculated parameters can be passed on to any device or host in a network by means of network communication protocol. Figure 1. A smart transducer model  The different modules of the smart transducer model can be grouped into functional units as shown in Figure 2. The transducers and signal conditioning and conversion modules can be grouped into a building block called a Smart Transducer Interface Module (STIM). Likewise, the application algorithm and network communication modules can be combined into a single entity called a Network Capable Application Processor (NCAP). With this functional partitioning, transducer to network interoperability can be achieved in these manners: 1)      STIMs from different sensor manufacturers can “plug and play” with NCAPs from a particular sensor network supplier, 2)      STIMs from a sensor manufacturer can “plug and play” with NCAPs supplied by different sensor or field network vendors, 3)      STIMs from different manufacturers can be interoperable with NCAPs from different field network suppliers. Using this partitioning approach, a migration path is provided to those sensor manufacturers who want to build STIMs with their sensors, but do not intend to become field network providers. Similarly, it applies to those sensor network builders who do not want to become sensor manufacturers. Figure 2. Functional partitioning As technology becomes more advanced and microcontrollers become smaller relative to the size of the transducer, integrated networked smart transducers that are economically feasible to implement will emerge in the marketplace. In this case, all the modules are incorporated into a single unit as shown in Figure 3. Thus, the interface between the STIM and NCAP is not exposed for external access and separation. The only connection to the integrated transducer is through the network connector. The integrated smart transducer approach simplifies the use of transducers by merely plugging the device into a sensor network. Figure 3. An integrated networked smart transducer 3. Networking smart transducers Not until recently have sensors been connected to instruments or computer systems by means of a point-to-point or multiplexing scheme. These techniques involve a large amount of cabling, which is very bulky and costly to implement and maintain. With the emergence of computer networking technology, transducer manufacturers and users alike are finding ways to apply this networking technology to their transducers for monitoring, measurement, and control applications [4]. Networking smart sensors provides the following features and benefits: enable peer-to-peer communication and distributed sensing and control, significantly lower the total system cost by simplified wiring, use prefabricated cables instead of custom laying of cables for ease of installation and maintenance, facilitate expansion and reconfiguration, allow time-stamping of sensor data, enable sharing of sensor measurement and control data, provide Internet connectivity, meaning global or anywhere, access of sensor information.   4. Establishment of the IEEE 1451 Standards As discussed earlier, a smart sensor interface standard is needed in industry. In view of this situation, the Technical Committee on Sensor Technology of the Institute of Electrical and Electronics Engineer (IEEE)’s Instrumentation and Measurement Society sponsored a series of projects for establishing a family of IEEE 1451 Standards [5]. These standards specify a set of common interfaces for connecting transducers to instruments, microprocessors, or field networks. They cover digital, mixed-mode, distributed multi-drop, and wireless interfaces to address the needs of different sectors of industry. A key concept in the IEEE 1451 standards is the Transducer Electronic Data Sheets (TEDS), which contain manufacture-related information about the sensor such as manufacturer name, sensor types, serial number, and calibration data and standardized data format for the TEDS. The TEDS has many benefits: Enable self-identification of sensors or actuators -   A sensor or actuator equipped with the IEEE 1451 TEDS can identify and describe itself to the host or network via the sending of the TEDS. Provide long-term self-documentation - the TEDS in the sensor can be updated and stored with information such as location of the sensor, recalibration date, repair record, and many maintenance-related data. Reduce human error - automatic transfer of TEDS data to the network or system eliminates the entering of sensor parameters by hands which could induce errors due to various conditions. Ease field installation, upgrade, and maintenance of sensors - this helps to reduce life cycle costs because only a less skilled person is needed to perform the task by simply using “plug and play”. IEEE 1451, designated as Standard Transducer Interface for Sensors and Actuators, consists of six document standards.  The current status of their development are: 1)   IEEE P1451.0**, Common Functions, Communication Protocols, and Transducer Electronic Data Sheet (TEDS) Formats-- In progress. 2)  IEEE Std 1451.1-1999, Network Capable Application Processor (NCAP) Information Model for Smart Transducers [6]-- Published standard. 3)   IEEE Std 1451.2-1997, Transducer to Microprocessor Communication Protocols and Transducer Electronic Data Sheet (TEDS) Formats  -- Published standard. 4)   IEEE P1451.3, Digital Communication and Transducer Electronic Data Sheet (TEDS) Formats for Distributed Multidrop Systems -- balloted and is awaiting IEEE approval in September 2003. 5)  IEEE P1451.4, Mixed-mode Communication Protocols and Transducer Electronic Data Sheet (TEDS) Formats-- balloted and is expected to be submitted to IEEE for approval in December 2003 6)   IEEE P1451.5, Wireless Communication and Transducer Electronic Data Sheet (TEDS) Formats --In progress 5. Goals of IEEE 1451 The goals of the IEEE 1451 standards are to: Develop network-independent and vendor-independent transducer interfaces, Define TEDS and standardized data formats, Support general transducer data, control, timing, configuration, and calibration models, Allow transducers to be installed, upgraded, replaced and moved with minimum effort by simple “plug and play”, Eliminate error prone, manual entering of data and system configuration steps, Ease the connection of sensors and actuators by wireline or wireless means.   6. The IEEE 1451 Standards  6.1 The IEEE 1451 Smart Transducer Model The IEEE 1451 smart transducer model parallels the smart transducer model discussed in Figure 2. In addition, the IEEE 1451 model includes the TEDS. The model for each of the IEEE 1451.X standards is discussed in the following. 6.2 IEEE P1451.0 Common Functionality Several standards in the IEEE 1451 family share certain characteristics, but there is no common set of functions, communications protocols, and TEDS formats that facilitate interoperability among these standards. The IEEE P1451.0 standard provides that commonality and simplifies the creation of future standards with different physical layers that will facilitate interoperability in the family. This project defines a set of common functionality for the family of IEEE P1451 smart transducer interface standards. This functionality is independent of the physical communications media. It includes the basic functions required to control and manage smart transducers, common communications protocols, and media-independent Transducer Electronic Data Sheet formats. The block diagram for IEEE P1451.0 is shown in Figure 4. P1451.0 defines functional characteristics, but it does not define any physical interface. Figure 4. The block diagram for IEEE P1451.0 6.3 IEEE 1451.1 Smart Transducer Information Model The IEEE 1451.1 Standard defines a common object model for the components of a networked smart transducer and the software interface specifications to these components [7]. Some of the components are the NCAP block, function block, and transducer block.  The networked smart transducer object model provides two interfaces. 1)      The interface to the transducer block, which encapsulates the details of the transducer hardware implementation within a simple programming model. This makes the sensor or actuator hardware interface look like an input/output (I/O)-driver. 2)      The interface to the NCAP block and ports encapsulate the details of the different network protocol implementations behind a small set of communications methods.  Application-specific behavior is modeled by function blocks. To produce the desired behavior, the function blocks communicate with other blocks both on and off the smart transducer. This common network-independent application model has the following two advantages: 1)      Establishment of a high degree of interoperability between sensors/actuators and networks, thus enabling “plug and play” capability. 2)      Simplification of the support of multiple sensor/actuator control network protocols. A conceptual view of IEEE 1451.1 NCAP is shown in Figure 5, which uses the idea of a “backplane” or “card cage” to explain the functionality of the NCAP. The NCAP centralizes all system and communications facilities. Network communication can be viewed as port through the NCAP and communication interfaces support both client-server and publish-subscribe communication models. Client-server is a tightly coupled, point-to-point communication model, where a specific object, the client, communicates in a one-to-one fashion with a specific server object, the server. On the other hand, the publish-subscribe communication model provides a loosely coupled mechanism for network communications between objects, where the sending object, the publisher object, does not need to be aware of the receiving objects, the subscriber objects. The loosely coupled, publish-subscribe model is used for one-to-many and many-to-many communications. A function block containing application code or control algorithm is “plugged” in as needed. Physical transducers are mapped into the NCAP using transducer block objects via the hardware interface, for example, the IEEE 1451.2 interface. Figure 5. Conceptual view of IEEE 1451.1 The IEEE 1451 logical interfaces are illustrated in Figure 6. The transducer logical interface specification defines how the transducers communicate with the NCAP block object via the transducer block. The network protocol logical interface specification defines how the NCAP block object communicates with any network protocol via the ports. Figure 6. IEEE 1451 logical interface 6.4 IEEE 1451.2 Transducer-to-Microprocessor Interface The IEEE 1451.2 standard defines a TEDS, its data format, and the digital interface and communication protocols between the STIM and NCAP [8]. A block diagram and detailed system diagram of IEEE 1451 are shown in Figures 7 and 8, respectively. The STIM contains the transducer(s) and the TEDS, which is stored in a nonvolatile memory attached to a transducer. The TEDS contains fields that describe the type, attributes, operation, and calibration of the transducer. The mandatory requirement for the TEDS is only 179 bytes. The rest of the TEDS specification is optional. A transducer integrated with the TEDS provides a very unique feature that makes possible the self-describing of transducers to the system or network. Since the manufacture-related data in the TEDS always goes with the transducer, and this information is electronically transferred to a NCAP or host, human errors associated with manual entering of sensor parameters into the host are eliminated. Because of this distinctive feature of the TEDS, upgrading transducers with higher accuracy and enhanced capability or replacing transducers for maintenance purpose is simply considered “plug and play.” Eight different types of TEDS are defined in the standard. Two of them are mandatory and six are optional. They are listed in Table 1. The TEDS are divided into two categories. The first category of TEDS contains data in a machine readable form which is intended to be used by the NCAP. The second category of TEDS contains data in a human readable form. The human readable TEDS may be represented in multiple languages using different encoding for each language. The Meta TEDS contains the data that describes the whole  STIM. It contains the revision of the standard, the version number of the TEDS, the number of channels in the STIM and the worst case timing required to access these channels. This information will allow the NCAP to access the channel information. In addition, the Meta TEDS includes the channel groupings that describe the relationships between channels. Each transducer is represented by a channel. Each channel in the STIM contains a Channel TEDS. The Channel TEDS lists the actual timing parameters for each individual channel. It also lists the type of transducer, the format of the data word being output by the channel, the physical units, the upper and lower range limits, the uncertainty or accuracy, whether or not a calibration TEDS is provided and where the calibration is to be performed The Calibration TEDS contains all the necessary information for the sensor data to be converted from the analog-to-digital converter raw output into the physical units specified in the Channel TEDS. If actuators are included in the STIM, it also contains the parameters that convert data in the physical units into the proper output format to drive the actuators. It also contains the calibration interval and last calibration date and time. This allows the system to determine when a calibration is needed. A general calibration algorithm is specified in the standard. The Generic Extension TEDS is provided to allow industry groups to provide additional TEDS in a machine readable format. The Meta Identification TEDS is human readable data that the system can retrieve from the STIM for display purposes. This TEDS contains fields for the manufacturer’s name, the model number and serial number of the STIM, and a date code. The Channel Identification TEDS is similar to the Meta Identification TEDS. When transducers from different manufacturers are built into a STIM, this information will be very useful for the identification of channels. The Channel Identification TEDS provides information about each channel, whereas the Meta Identification TEDS provides information for the STIM. The Calibration Identification TEDS provides details of the calibration in the STIM. This information includes who performed the calibration and what standards were used. The End User Application Specific TEDS is not defined in detail by the standard. It allows the user to insert information such as installation location, time it was installed, or any other desired text. The STIM module can contain a combination of sensors and actuators of up to 255 channels, signal conditioning/processing, analog-to-digital converter (A/D), digital-to-analog converter (D/A), and digital logics to support the transducer independent interface (TII). Currently, the P1451.2 working group is considering an update to the standard to include a popular serial interface, such as RS232, in addition to the TII for connecting sensors and actuators. Table 1. Different types of TEDS Figure 7. Block diagram of IEEE 1451 Figure 8. Detailed system block diagram of IEEE 1451 smart transducer interface   6.5 IEEE P14513 Distributed Multidrop Systems The EEE P1451.3 defines a transducer bus for connecting transducer modules to a NCAP in a distributed multidrop fashion. A block diagram is shown in Figure 9. The physical interface for the transducer bus is based on Home Phoneline Networking Alliance (HomePNA) specification. Both power and data run on a twisted pair of wires. Multiple transducer modules, called Transducer Bus Interface Modules (TBIM), can be connected to a NCAP via the bus. Each TBIM contains transducers, signal conditioning / processing, A/D, D/A, and digital logics to support the bus and can accommodate large arrays of transducers for synchronized access at up to 128 Mbps with HomePNA 3.0 and up to 240 Mbps with extensions. The TEDS is defined in the eXtensible Markup  Language (XML). Figure 9. Block diagram of IEEE P1451.3 6.6 IEEE P1451.4 Mixed-Mode Transducer Interface The IEEE P1451.4 defines a mixed-mode transducer interface (MMI), which is used for connecting transducer modules, Mixed-mode Transducers (MMT), to an instrument, a computer, or a NCAP. The block diagram of the system is shown in Figure 10. The physical transducer interface  is based on the Maxim/Dallas Semiconductor’s one-wire protocol, but it also supports up to 4 wires for bridge-type sensors. It is a simple, low-cost connectivity for analog sensors with a very small TEDS - 64 bits mandatory and 256 bits optional. The mixed-mode interface supports a digital interface for reading and writing the TEDS by the instrument or NCAP. After the TEDS transaction is completed, the interface switches into analog mode, where the analog sensor signal is sent straight to the instrument and NCAP which is equipped with A/D to read the sensor data. Figure 10. Block diagram of IEEE P1451.4 6.7 IEEE P1451.5 Wireless Transducer Interface Wireless communication is emerging, and low-cost wireless technology is on the horizon. Wireless communication links could replace the costly cabling for sensor connectivity. It also could greatly reduce sensor installation cost. Industry would like to apply the wireless technology for sensors, however, there is a need to solve the interoperability problem among wireless sensors, equipment, and data. In response to this need,  the IEEE P1451.5 working group is working to define a wireless sensor communication interface standard that will leverage existing wireless communication technologies and protocols [9]. A block diagram of IEEE P1451.5 is shown in Figure 11. The working  group seeks to define the wireless message formats, data/control model, security model, and TEDS that are scalable to meet the needs of low-cost to sophisticated sensor or device manufacturers. It allows for a minimum of 64 sensors per access point. Intrinsic safety is not required but the standard would allow for it. The physical communication protocol(s) being considered by the working group are: 1) IEEE 802.11 (WiFi), 2) IEEE 802.15.1 (Bluetooth), 3) IEEE 802.15.4 (WPAN-LR), and 4) ultra wideband. 6.8 IEEE 1451 Family Figure 12 summarizes the family of IEEE 1451 Standards. Each of the IEEE P1451.X is designed to work with each other. However, they can also stand on their own. For example, IEEE 1451.1 can work without any IEEE 1451.X hardware interface. Likewise, IEEE1451.X can also be used without IEEE 1451.1, but software with similar functionality should provide sensor data/information to each network. Figure 11. Block diagram of IEEE P1451.5 wireless transducer Figure 12. Family of IEEE P1451 Standards 6.9 Benefits of IEEE 1451 IEEE 1451 defines a set of common transducer interfaces, which will help to l ower the cost of designing smart sensors and actuators because designers would only have to design to a single set of standardized digital interfaces. Thus, the overall cost to make networked sensors will decrease. Incorporating the TEDS with the sensors will enable self-description of sensors and actuators, eliminating error-prone, manual configuration. 6.9.1 Sensor manufacturers Sensor manufacturers can benefit from the standard because they only have to design a single standard physical interface. Standard calibration specification and data format can help to design and develop multi-level products based on TEDS with a minimum effort. 6.9.2 Application software developers Applications can benefit from the standard as well because standard transducer models for control and data can support and facilitate distributed measurement and control applications. The standard also provides support for multiple languages - good for international developers. 6.9.3 System integrators Sensor system integrators can benefit from IEEE 1451 because sensor systems become easier to install, maintain, modify, and upgrade. Quick and efficient transducer replacement results by simple “plug and play.” It can also provide a means to store installation details in the TEDS. Self-documenting of hardware and software is done via the TEDS. Best of all is the ability to choose sensors and networks based on merit. 6.9.4 End users End users can benefit from a standard interface because sensors will be easy to use by simple “plug and play”. Based on the information provided in the TEDS, software can automatically provide the physical units, readings with significant digits as defined in the TEDS, installation details such as instruction, identification, and location of the sensor. 6.9.5 “Plug and play” of sensors IEEE 1451 enables “Plug and Play” of transducers to a network as illustrated in Figure 13. In this example, IEEE P1451.4-compatible transducers from different companies are shown to work with a sensor network. IEEE 1451 also enables “Plug and Play” of transducers to a data acquisition system/instrumentation system as shown in Figure 14. In this example, various IEEE P1451.4-compatible transducers such as an accelerometer, a thermistor, a load cell, and a linear variable differential transformer (LVDT) are shown to work with a LabView-based system***.  Figure13. IEEE 1451 enables “Plug and Play” of transducers to a network Figure 14. IEEE 1451 enables “Plug and Play” of transducers to data acquisition/instrumentation system 7.0 Example application of IEEE 1451.2 IEEE 1451-based sensor network consisting of sensors, STIM, and NCAP are designed and built into a cabinet  as shown in Figure 15. There were a total of four STIM and NCAP network nodes as pointed to in Figure 15. Thermistor sensors were used for temperature measurements. They were calibrated in the laboratory to generate IEEE 1451.2-compliance calibration TEDS for all four STIMs and NCAPs. The thermistors were mounted on the spindle motor housing, bearing, and axis drive motors of a 3-axis vertical machining center, which is shown in Figure 16. Since each NCAP has a built-in micro web server, a custom web page was constructed using the web tool provided with the NCAP. Thus remote monitoring of the machine thermal condition was easily achieved via the Ethernet network and the Internet using a readily available common web browser. The daily trend chart of the temperature of the spindle motor (top trace) and the temperature of the Z-axis drive motor (bottom trace) in the machine is shown in Figure 17. The temperature rise tracks the working of the machine during the day and the temperature fall indicates the machine is cooling off after the machine shop is closed. Figure 15. NCAP-based condition monitoring system Figure 16. 3-axis vertical machining center Figure 17. Temperature trend chart 8. Application of IEEE 1451-based Sensor Network A distributed measurement and control system can be easily implemented based on the IEEE 1451 standards [10]. An application model of IEEE 1451 is shown in Figure 18. Three NCAP/STIMs are used to illustrate the distributed control, remote sensing or monitoring, and remote actuating.  In the first scenario, a sensor and actuator are connected to the STIM of NCAP #1, and an application software running in the NCAP can perform a locally distributed control function, such as maintaining a constant temperature for a bath. The NCAP reports measurement data, process information, and control status to a remote monitoring station or host. It frees the host from the processor-intensive, closed-loop control operation. In the second scenario, only sensors are connected to NCAP #2, which can perform remote process or condition monitoring functions, such as monitoring the vibration level of a set of bearings in a turbine. In the third scenario, based on the broadcast data received from NCAP #2, NCAP #3 activates an alarm when the vibration level of the bearings exceeds a critical set point. As illustrated in these examples, IEEE 1451-based sensor network can easily facilitate peer-to-peer communications and distributed control functions. Figure 18. Application model of IEEE 1451 9. Summary   The IEEE 1451 smart transducer interface standards are defined to allow a transducer manufacturer to build transducers of various performance capabilities that are interoperable within a networking system. The IEEE 1451 family of standards has provided the common interface and enabling technology for the connectivity of transducers to microprocessors,  field networks, and instrumentation systems using wired and wireless means. The standardized TEDS allows the self-description of sensors which turns out to be a very valuable tool for condition-based maintenance. The expanding Internet market has created a good opportunity for sensor and network manufacturers to exploit web-based and smart sensor technologies. As a result, users will greatly benefit from many innovations and new applications. 10. Acknowledgments The author sincerely thanks the IEEE 1451 working groups for the use of the materials in this paper. Through its program in Smart Machine Tools, the Manufacturing Engineering Laboratory of the National Institute of Standards and Technology has contributed to the development of the IEEE 1451 standards. 11. 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