SMART SENSORS
Certain types of transducers are classified as smart sensors. They contain a Smart Transducer Interface Module (STIM), which in turn, contains an onboard EEPROM memory IC, called TEDS, Transducer Electronic Data Sheet (See Figure 12.01). The TEDS electronically stores information regarding the transducer’s characteristics and parameters such as type of device, manufacturer, model number, serial number, calibration date, sensitivity, reference frequency, and other data. The TEDS also stores the calibration coefficients and frequency response for a transducer in terms of a table or an algorithm.

Not all sensor manufacturers provide this feature, but those that do are encouraged to follow an evolving standard, IEEE 1451.0. One part of the standard, 1451.4, defines the TEDS format, channel identification formats, electrical interface, and read and write functions for accessing the TEDS and transducer data. The specification also defines the data set, that is, the number of samples acquired for one command that varies from 0 to 65,535 samples per set.

The standard does not specify requirements for signal conditioning, signal conversion, or how applications can use the TEDS data. However, the signal conditioners and other interface hardware used with these smart sensors must provide for an option or include circuits for TEDS communications under the direction of a software module specifically intended for this purpose. Another special module that works with TEDS is usually part of the data acquisition system’s application software package that collects, stores, and displays the measured variables. The hardware automatically scales the readings and sets the range according to the data stored in the sensor. This is typically done for each TEDS-associated channel.

TEDS capability was originally intended for piezoelectric sensors such as accelerometers and pressure sensors, but it now includes all common analog sensors and actuators, such as MEMS (micro-electromechanical sensors), accelerometers, pressure transducers, and temperature sensors with two-wire and mixed-mode (analog and digital) input/output.

For two-wire analog sensors and actuators (called Class 1), the output signal is generally coupled to the signal conditioner or driver while the TEDS data are enabled and read out with a dc bias voltage applied to the same two wires (See Figure 12.02). The transducers with mixed-mode capability (called Class 2) also communicate digitally with the TEDS memory (See Figure 12.03). The analog part is the signal representing the measured variable, and the digital interface communicates with the embedded EEPROM. The TEDS architecture contains standard templates for common transducers, and custom sub-templates that manufacturers can use for defining special parameters and custom requirements.

The TEDS file may be contained onboard the sensor in the EEPROM, or off board in a reserved file in the data acquisition system. A number of transducer manufacturers plan to launch an Internet-accessible site where they list sensors that presently do not contain a TEDS memory IC. Users may enter the serial number of their sensor and obtain an equivalent TEDS specification to download. This lets application software programs work with both older sensors and newer TEDS-equipped sensors.

Networks
The original intent of the TEDS concept is to handle arrays of smart sensors and actuators over various types of networks. Different types of control networks now exist for both analog and digital communications, but most are intrinsically incompatible with one another. And not all transducer manufacturers are either able or willing to provide a unique sensor for each kind of network bus. As a consequence, they are increasingly turning to digital networks controlled with microprocessors to develop a universally accepted interface that is more economical for them to interconnect systems, networks, and instruments. The guiding document is the evolving IEEE 1451 standard, which also allows the new devices to be compatible with older and existing systems.

System Architecture
The IEEE has partitioned the P1451 standard into five major parts. Each part addresses a different facet or interface of the project that enables sensor manufacturers to design and build new sensors compatible with all networks. Each part is defined as follows:

Part 1. Network-Capable Application Processor (NCAP) Information Model
The purpose of this part is to define an object model for smart transducers that connect to a network, and specify a software interface to work with the transducer’s components. The major components of this model include the NCAP block, function block, and transducer block. The NCAP processor connects the network to the transducer modules, and each different network requires a different NCAP physical interface. The smart transducer object model interfaces to the NCAP processor and to the transducer block. The interface to the transducer block contains details of the transducer hardware in a program model, and the interface to the NCAP block contains details describing the network protocols.

The NCAP’s primary purpose is to communicate between the STIM and a particular network. The NCAP also computes calibration corrections and converts between values in metric units and values coming from the STIM’s DACs or going to the ADCs.

Part 2. Transducer to Microprocessor Communications Protocols and
Transducer Electronic Data Sheet (TEDS) Format
Part 2 defines the details of a TEDS, including the format for storing data in a small (256-bit to 4 kb) EEPROM and the interface between the NCAP processor and the transducers. The TEDS contains the information needed by the software to convert the sensor values such as volts or resistance to physical units, such as force in pounds or acceleration in gs. TEDS is also part of the Smart Transducer Interface Module. STIM includes the ADCs, DACs, digital I/O, and triggers that connect to the transducers.

Part 3. Digital Communications and TEDS Formats for Distributed Multidrop Systems
This part is intended to define the TEDS format and the standard for the interface between multiple transducers in a multidrop network. Some of the issues dealing with a multidrop system include automatically identifying the transducer when it connects to the bus, and how quickly the system recovers after a short power dropout.

Part 4. Mixed-Mode Communication Protocols and TEDS Formats
Mixed-mode communications deals with the issue of using two-wire I/O, which shares both signal and digital interfaces vs. a multi-wire system where signal and digital communications are handled on separate wires or ports. Small analog sensors contain TEDS that let the sensors interface on the network through the same two wires they use for signals. The digital TEDS data uses the same two wires.

Part 5. Wireless Communication Protocols and TEDS Formats
At the time of this writing, the IEEE P1451 standards committee study group had defined the details of the interfaces for wireless communications, but not the modules. The intent of the standard is to separate the physical layers from the upper layers of the protocol stack. Significant progress on the specification is expected in 2004.

TEDS Composition
The data structure for the TEDS works well with a variety of sensor types. Regardless of the sensor’s principle of operation, the structure contains three major subdivisions; the Basic TEDS; the Standard TEDS (subtemplate), and the User Area (See Figure 12.04, Tables 1, 2, and 3).

The Basic TEDS contains the manufacturer’s identification, model identification, version letter, and the serial number, which are common to all types of sensors. The Standard TEDS, however, contains data unique to the sensor’s principle of operation. For instance, a piezoelectric accelerometer differs from a strain gage load cell, so the Standard TEDS EEPROM contains some common and some unique information. Both sensor TEDS list Calibration Date, Measurement Range, and Electrical Output, but they differ in as many as 12 other parameters. The accelerometer has 6 parameters that the strain gage lacks, and the strain gage has 6 parameters the accelerometer lacks. The User Area contains information such as Sensor Location, Calibration Due Date, and Calibration Table.

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