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Smart Sensors & IEEE 1451

What is IEEE 1451?
The IEEE has defined IEEE 1451, a family of Smart Transducer Interface standards that permit systems to automatically identify sensors and obtain their calibration and operating parameters. In March of 2006 the IEEE updated the portion dealing with analog sensors, IEEE 1451.4, bringing the benefits of automatic configuration to systems using single-signal analog sensors.

Connecting a sensor to a data acquisition system involves more than plugging in the wires. In order for the system to interpret the sensor data correctly, it must have access to a variety of parameters. These include the range, scale, and measurement units for the sensor, bandwidth, calibration details, and more. Manual efforts to enter this data into the system are prone to error as well as being time consuming. The IEEE 1451 standards were developed to automate this process by incorporating the necessary data as part of the sensor itself. Having the sensor carry its own data allows the data acquisition system to automatically configure itself correctly.

Six Parts of IEEE 1451
IEEE 1451 comprises six parts, as shown in the table. The first parts define the commands, operations, protocols, and behaviors governing smart sensors. The remaining sections define the physical interfaces to be used with different sensor configurations. The standard for wireless communication to a smart sensor is still under development.

What is TEDS?
One of the key elements in the 1451 standards is the use of a Transducer Electronic Data Sheet (TEDS) that carries the sensor's data. The TEDS for a given sensor can have as many as four sections, containing different types of data. The basic TEDS data simply identifies the device. The manufacturer, sensor model number, version, and a unique serial number are all encoded in a 64-bit block. The data acquisition system uses this data to uniquely identify each device that it connects to in order to manage communications to the devices.

The second section of TEDS data includes detailed information on and control of the sensor itself. The field definitions vary with the type of sensor, but in general these include provisions for reading or writing operating parameters, scale factors, and basic calibration data. An additional calibration parameter set, such as tables, polynomial curve parameters, or frequency response tables, is available as an option in the third section.

The final section of the TEDS data is user defined. Here, system operators can insert information into the sensor such as due dates for sensor recalibration, descriptions of sensor placement, and other maintenance information that needs to remain resident in the sensor.

A healthy number of vendors offer IEEE 1451-compatible sensors. In addition,-many have joined with National Instruments in promoting interoperability in sensors and data acquisition platforms under the company's Sensors Plug&Play program. National Instrument's LabView application development software supports the 1451 standard, simplifying system setup and control of systems that use compatible sensors.

Templates Minimize MEMORY NEEDS
In order to pack sensor data into a minimum amount of memory space, the TEDS follow templates that define the format and content of this data for a variety of sensor types. Standard templates have been defined for:

  • Accelerometer and force transducers
  • Charge amplifiers with attached accelerometer or force transducer
  • Capacitive microphones and microphones with built-in preamplifiers
  • Microphone preamplifiers with attached microphones
  • High-level voltage and current-loop output sensors
  • Resistive sensors
  • Bridge sensors
  • Strain gages
  • Thermocouples, thermistors, and resistance temperature detectors (RTDs)
  • Potentiometric voltage dividers
  • Both AC linear and rotary variable differential transformer (LVDT/RVDT) sensors

The templates for high-level voltage and current-loop sensors have been broadly defined, allowing their use by virtually any sensor with an analog output. In addition, vendors can create custom templates for sensors not already defined, using a standardized template description language (TDL) that defines for the data acquisition system how to interpret the TEDS data.

It is the standard templates, however, that provide users with the greatest benefit. An IEEE 1451-compatible sensor allows the data acquisition system to automatically download all the essential parameters needed for scaling and interpreting data from the sensor. The result is considerable savings in setup time as well as reduction in errors when configuring the data acquisition system to work with a sensor. If the sensors use standard templates, the system can use a single set of software with a variety of different vendors' products reducing development time and eliminating the need to reprogram when switching vendors.

For analog and mixed-mode transducers, the revised 1451.4 standard defines an interface for retrieving the digital TEDS information that is backward-compatible with legacy sensors. This compatibility ensures that 1451-compatible data acquisition systems can utilize both TEDS and legacy sensors without system modification.

There are two types of interface, shown in the figure on the opening page, both of which utilize the 1-Wire protocol from Maxim/Dallas Semiconductor for retrieving the digital TEDS data. Class 1 interfaces are used with constant-current powered sensors such as integrated electronic piezoelectric (IEPE) sensors. The interface adds digital communication in parallel with the analog sensor, allowing both types of information to share a single wire (and return) connection to the data acquisition system. This is accomplished by defining the digital signal as a negative voltage and using diodes to isolate the analog sensor or digital TEDS sections, depending on the signal line's polarity. A switch in the data acquisition system disconnects the sensor's current source and connects a negative bias voltage to the wire when communicating with the digital TEDS section.

Class 2 interfaces are used with devices that cannot tolerate an interruption in the power coming from the data acquisition system, such as a bridge-type sensor. In these devices, the interface simply adds a dedicated digital signal line and return to the sensor for communicating with the TEDS section. The digital line uses positive-voltage signaling, and both the analog and the digital signals are continuously available.

The definition of IEEE 1451.4 allows data acquisition systems to use both IEEE 1451.4-compatible and legacy sensors without modification. In addition, it simplifies the task of upgrading a sensor design to become TEDS-compatible. In Class 1 devices, the addition of two diodes and a serial EEPROM containing the TEDS data are all that is required. Class 2 devices simply need the EEPROM and two additional wires. EEPROMs as small as 256 bits are available with the 1-Wire interface for upgrading the sensor.

Fany systems have a large installed base of legacy sensors that are not outfitted with TEDS information. However, the benefits of automatic configuration and calibration need not wait until the legacy devices are replaced. National Instruments has developed the Virtual TEDS program to bring those benefits to the installed base.

Virtual TEDS takes advantage of the fact that the data about a sensor does not have to physically reside on the sensor. Instead, that data can be located in a database on a local or networked computer to which the data acquisition system is attached. Rather than query the sensor for its TEDS data, the system queries the database.

Systems can acquire the needed TEDS data in one of two ways. Users can download the templates for their sensors from the manufacturer, or can use the central clearinghouse that National Instruments offers. Once the system has access to a database, it can then download the TEDS data over the network, and proceed as though the sensor itself had provided the information.

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