Sensors Plug&Play – The New Standard for Automated Sensor Measurements

The IEEE 1451.4 standard reduces the time and challenges associated with sensor configuration. The standard establishes a universally accepted method of giving sensors plug-and-play capability, similar to the plug-and-play capability of a USB mouse and your computer. IEEE 1451.4 defines a mechanism for adding self-describing behavior to sensors with an analog signal interface. This mixed-mode interface combines the traditional analog sensor signal with a low-cost serial digital link to access a transducer electronic data sheet (TEDS) embedded in the sensor. To extend the benefits of Sensors Plug&Play to traditional analog sensors, Virtual TEDS provides the same transducer electronic data sheet in electronic file format. With TEDS, the sensor identifies and describes itself to the data acquisition system to which it is connected.

The addition of plug-and-play capabilities to analog sensors delivers real benefits to users and developers:

• Quicker system setup
• Improved diagnostics
• Reduced downtime for sensor repair and replacement
• Improved asset management
• Automated use of calibration data

IEEE 1451.4 Standard

The IEEE 1451.4 standard, Mixed-Mode Interface for Smart Transducers, defines a mechanism for adding self-identification technology to traditional analog-mode sensors and actuators. Developed jointly by sensor manufacturers, instrumentation and software suppliers, and users, IEEE 1451.4 defines the concept of a mixed-mode transducer that supplies both an analog and digital interface. The analog electrical interface provides a signal reflecting the physical phenomenon (such as temperature, pressure, and force) in the traditional manner.

An IEEE 1451.4 smart TEDS sensor, however, also provides a digital interface for communicating with an embedded memory device within the transducer. This memory contains the binary TEDS information that identifies and describes the sensor or actuator. The TEDS contains information such as manufacturer, sensor model number, serial number, measurement range, sensitivity, and calibration information.

Figure 1. Smart TEDS Sensor with Embedded TEDS EEPROM

Historically, when you set up and configured a measurement system, you had to manually enter important sensor parameters, such as the range, sensitivity, and scale factors, in order for the software to properly convert and interpret the sensor data. Now, a system outfitted with smart TEDS sensors can automate this configuration step, while also increasing the general integrity and reliability of the system.

While other smart sensor technologies also deliver plug-and-play operation, IEEE 1451.4 is unique because it maintains the analog output of the sensor. Therefore, smart TEDS sensors are compatible with legacy systems that include traditional analog interfaces. And the simplicity of IEEE 1451.4 implementations has the very significant and pragmatic advantage of easy retrofit of legacy sensors. The two main components of the IEEE 1451.4 standard are the standardized data sheets and the mixed-mode interface.

TEDS (Transducer Electronic Data Sheet)

The heart of the IEEE 1451.4 standard is the definition of the TEDS, the information structure that contains the critical sensor information to enable plug-and-play operation. The TEDS, which typically resides in an EEPROM embedded in the sensor, is accessed by the measurement system via a simple low-cost serial interface.

IEEE 1451.4 defines the TEDS structure to be very compact yet flexible and extensible enough to handle a wide range of sensor types and requirements. The TEDS information is divided into several key sections. The first portion of the TEDS, the basic TEDS, contains the required sensor identification information, including manufacturer, model number, and serial number of the sensor. The basic TEDS may be followed by a standard TEDS that contains the specific ‘data sheet’ information for the sensors – typically the data needed to properly configure the electrical interface and convert the measurement data into engineering units. Typical TEDS parameters include measurement range, electrical output range, sensitivity, power requirements, and calibration data. The standard TEDS section describes everything needed to make a measurement using the sensors.

The IEEE standard specifies a collection of standard TEDS formats, defined as templates, for different sensor types. The templates provide the means for the measurement system to convert the binary data stored on a smart TEDS sensor EEPROM (or Virtual TEDS file) into meaningful specifications for that sensor. The collection of IEEE standard templates include IEPE (constant-current powered) accelerometers and microphones, IEPE pressure sensors, Wheatstone bridge sensors, strain gauges, load and force transducers, thermocouples, RTDs, thermistors, LVDT/RVDT, resistive sensors, and amplified sensors (any type) with voltage or current outputs. Within the standard, manufacturers also can define custom subtemplates that can be used instead of, or in addition to, the standard templates to accommodate specialized parameters and requirements.

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Table 1. Structure of the IEEE 1451.4 TEDS, with Example Templates for an IEPE Accelerometers and a Bridge-Based Load Cell

Finally, the last portion of the TEDS is available for users to store custom data and information resident in the sensor. This is a very useful feature for storing sensor location (coded as an ID), additional maintenance information, or other custom information resident in the sensor.

IEEE 1451.4 Mixed-Mode Interface

A smart TEDS sensor, as defined by IEEE 1451.4, includes a mixed-mode interface that accommodates both an analog signal (for the measurement signal) and a serial digital channel (for accessing the digital TEDS information). There are two types of mixed-mode interfaces defined in the standard – Class I and Class II.

Class I interfaces are primarily intended for constant-current powered piezoelectric transducers (such as accelerometers and microphones) and define a scheme for sequentially switching between analog mode and digital TEDS mode on a single pair of transducer wires. Constant-current powered transducers, generally referred to as integrated electronic piezoelectric (IEPE) transducers, incorporate internal signal conditioning powered by a constant current that is sourced by the measurement system on the signal wires. Class I transducers take advantage of this de facto analog standard by adding the TEDS with a switch controlled by the direction of the current source, as diagrammed in Figure 2. By reversing the direction of the current, the instrumentation system switches the sensor into digital TEDS mode.

Figure 2. Class I Two-Wire Interface for IEPE Sensors

Most sensor types implement a form of the Class II interface, which requires additional wires for digital TEDS communication. The analog input/output of the transducer is left unmodified, and the 2-wire TEDS interface is added in parallel to the analog interface. Using this approach, you can implement TEDS on virtually any type of amplified or unamplified sensor or actuator, such as thermocouples, RTDs, thermistors, bridge sensors, electrolytic chemical cells, and 4 to 20 mA current loop sensors. In fact, with the ‘add-on’ approach of Class II, it is very easy to retrofit existing sensors, with a variety of packaging options. Figure 3 illustrates an example implementation of a Class II mixed-mode interface with a bridge transducer.

Figure 3. Class II Multiwire Interface, Shown with Bridge Sensor

The digital portion of the mixed-mode interface (Class I or Class II) is based on the 1-Wire protocol from Maxim/Dallas Semiconductor. This is a very simple, low-cost, master-slave serial communication protocol, requiring that a single master device (the measurement system) supply power and initiate each transaction with each node according to a defined transaction-timing sequence, on a single wire and return. Commercially available 1-Wire EEPROMs provide low-cost, 2-wire solutions for adding TEDS to sensors. Maxim/Dallas Semiconductor manufacturers two such EEPROMs – DS2430 (256 b) and DS2433 (4 Kb).

Virtual TEDS

National Instruments and sensor vendors worldwide have recognized the implicit benefit of the TEDS concept, which provides a standardized description language for sensors. The TEDS data structure can describe any analog sensor, regardless of whether the TEDS is physically located on an EEPROM embedded in the sensor.

A Virtual TEDS file is stored on a local computer or a Web-accessible database instead of on an EEPROM. Thus, the huge installed base of legacy, analog sensors can realize the benefits of TEDS without being retrofitted with an embedded EEPROM. Virtual TEDS files are also valuable in applications where sensor operating conditions prevent the use of EEPROMs (or any electronics) in the sensor.

Figure 4. Virtual TEDS Accessible via a Web Interface

Sensors Plug&Play Hardware for Smart TEDS Sensors

Virtual TEDS and smart TEDS sensors are rapidly being deployed into a variety of test and measurement applications, with many vendors currently offering smart TEDS sensors. National Instruments provides both hardware and software to read and write Virtual TEDS and smart TEDS information.

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Figure 5. Components of a Sensors Plug&Play System

Sensors Plug&Play Software

Leading sensor manufacturers and National Instruments use a common TEDS software platform to ensure interoperability when writing to and reading from IEEE 1451.4 compliant devices. Any data acquisition and signal conditioning products that display the Sensors Plug&Play logo, illustrated in Figure 6, are compatible with any other Sensors Plug&Play products.

Figure 6. Logo for All Sensors Plug&Play Products, Representing Compatibility and Interoperability

The National Instruments flagship software for test and measurement systems – NI-DAQmx measurement services and NI LabVIEW graphical development software – have been upgraded to use TEDS technology to achieve compatibility with Sensors Plug&Play devices. NI-DAQmx driver software is fully compatible with IEEE 1451.4-compliant virtual and smart TEDS sensors. TEDS reader functionality is built into Measurement & Automation Explorer (MAX), DAQ Assistant, and the TEDS library for LabVIEW. Because of this functionality, you can automatically acquire sensor and TEDS-related information from one or more virtual or smart TEDS sensors with the click of a mouse.


NI-DAQmx Measurement Services Software

While the TEDS standard defines communication between the module and the sensors, NI-DAQmx provides the interface to read the data from a sensor and automatically scale the data into engineering units, eliminating the manual entry of sensor parameters from paper data sheets. Once the sensors are connected to Sensors Plug&Play hardware, the user can load the data from the smart TEDS sensor either through a single button in MAX or using a function call in the application development environment. Figure 7 illustrates the MAX interface for scanning and configuring both virtual and smart TEDS sensors..

Figure 7. NI-DAQmx TEDS Configuration for SCXI-1314T

While MAX provides a way to interactively scan and load smart TEDS sensors into computer memory, the NI-DAQmx functions in LabVIEW provide the means to programmatically read, write, and reprogram TEDS data. The functions available in LabVIEW also provide for the TEDS data stream to be retrieved as a bit stream. The ability to reprogram TEDS from LabVIEW is useful for updating calibration information or to digitally store user-defined information, including the physical location of the sensor..

Figure 8. NI-DAQmx TEDS Subpalette

 

 

TEDS Library for LabVIEW

While the NI-DAQmx TEDS subpalette (Figure 8) provides the basic calls needed to programmatically configure a measurement and automation system, the separate TEDS subpalette provides additional tools for TEDS manipulation and Virtual TEDS creation. Once the bit stream from the smart TEDS sensor is read into LabVIEW, the user has the ability to perform a wide variety of TEDS-specific operations, such as receive specific properties, parse the bit stream, and create a new TEDS. Figure 9 shows the top level TEDS palette and some of the functions available.

Figure 9. The VI Palette of the TEDS Library for LabVIEW

Using NI-DAQmx and the TEDS Library for LabVIEW functions, you can easily create an application to read the TEDS information of a specific sensor, parse the bit stream based on the appropriate sensor template, and display the TEDS information in tabular format. This application is illustrated in Figure 10.

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Figure 10. TEDS LabVIEW Example

Summary

IEEE 1451.4 defines a relatively simple, straightforward mechanism for adding smart, plug-and-play capabilities to traditional analog sensors. Sensors Plug&Play hardware, software, and sensors offer substantial improvements in ease of use, accuracy, and reliability for systems measuring just a few sensors to high-channel-count systems measuring thousands of sensors. With the approval of the standard, many vendors including National Instruments are building Sensors Plug&Play products for both Virtual TEDS and smart TEDS sensors.
Related Links:
TEDS Example Programs
Smart Sensors

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