Sensors - December 2002 - IEEE P1451.4’s Plug-and-Play Sensors

It doesn’t fit the earlier version of a smart sensor, but it does deliver benefits that build on, not replace, legacy products and systems.

While the notion of smart sensors typically conjures up images of super-intelligent, DSP-enhanced sensor nodes with sophisticated networking technologies, an emerging IEEE standard that takes a pragmatic, more modest approach to smart sensors promises to have a significant and broad impact. IEEE P1451.4 is a developing standard that brings the fundamental and most important element of intelligence, self-identification, to the prevalent analog sensor. Compared with more traditional smart sensor technologies, the IEEE P1451.4 mechanisms for enabling analog plug-and-play sensors are inexpensive, simple, and compatible with existing measurement systems.

Plug-and-Play Sensors
The draft standard defines the concept of plug-and-play sensors with analog outputs, maintaining compatibility with the large existing base of analog instrumentation and interfaces. More specifically, the standard spells out how an analog sensor can be augmented with an embedded Transducer Electronic Data Sheet (TEDS), which contains technical information that identifies the sensor, specifies the sensor’s analog interface, and describes the sensor’s use. The TEDS resides in the sensor in an inexpensive memory component, typically an EEPROM, which can be accessed over the same wires as the analog signal or on a separate connection.

Transducer Electronic Data Sheets
The heart and soul of the IEEE P1451.4 standard, and all of the IEEE 1451 family of smart transducer interface standards (see “The IEEE 1451 Family of Standards for Smart Transducer Interfaces”), is the TEDS, which contains the critical information required by the system for plug-and-play connectivity. The proposed standard defines a compact yet flexible and extensible data structure for TEDS, which can work with a wide range of sensor types and requirements.

Example A: IEPE Accelerometer
Basic TEDS	Manufacturer ID Model ID Version Letter Serial Number	43 7115 B 0073IF
Standard TEDS (IEPE Accelerometer Subtemplate)	Calibration Date Sensitivity @ Reference Reference Frequency Reference Temperature Measurement Range Electrical Output Quality Factor Temperature Coefficient Direction (X, Y, Z)	Jan. 29, 2000 1.094E+03 mV/g 100.0 Hz 23°C ±50 g ±5 V 300 E-3 -0.48%/°C X
User Area	Sensor Location Calibration Due Date	Strut 3A-p2 April 15, 2002

Example A: Bridge (mV/V) Load Cell

Basic TEDS Manufacturer ID
Model ID
Version Letter
Serial Number
21
19
D
0008451

Standard TEDS
(Bridge Sensor Subtemplate)
Calibration Date
Measurement Range
Electrical Output
Bridge Impedance
Measurement Range
Excitation, Nominal
Excitation, Minimum
Excitation, Maximum
Response Time
Feb. 10, 2001
±100 lb
±2 mV/V
350
±50 g
10 VDC
7 VDC
5 ms
X

Calibration TEDS Calibration Table
[–2.06 mV/V, –100 lb.]
[–1.02 mV/V, –50 lb.]
[0.02 mV/V, 0 lb.]
[0.97 mV/V, +50 lb.]
[1.99 mV/V, +100 lb.]

User Area Sensor Location
R32-1

The first portion of the TEDS, called the basic TEDS, contains the basic identification information, including the manufacturer ID, model number, and serial number of the transducer. Following the basic TEDS is one or more IEEE standard TEDS, which contain the technical information for the transducer. This section typically contains the information 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 information.

The content of these sections will vary according to the type of sensor, and it’s defined by a specific TEDS template, or subtemplate as it’s called in the standard. The IEEE P1451.4 standard includes a number of standard transducer subtemplates for a variety of sensor types, including Integrated Electronics for Piezoelectric (IEPE) accelerometers and microphones, IEPE pressure sensors, Wheatstone bridge sensors, strain gauges, load and force transducers, thermocouples, RTDs, thermistors, LVDTs/RVDTs, resistive sensors, frequency output sensors, and amplified sensors (any type) with voltage or 4–20 mA current outputs. To accommodate specialized parameters and requirements, P1451.4 lets manufacturers define custom subtemplates that can be used instead of, or in addition to, the standard templates provided in the draft.

The transducer TEDS can also include a calibration TEDS that specifies an entire calibration table, defining the sensor output over its full operating range. Alternatively, the calibration TEDS can specify the calibrated transfer function through a set of polynomial coefficients.

Finally, the last portion of the TEDS lets you store custom data and information in the sensor. This includes the sensor’s location (coded as an ID), additional maintenance information, or other custom information.

2-Wire EEPROM Interfaces for TEDS
The smart transducer stores TEDS information in a 2-wire memory device, typically an EEPROM. Because some transducers (e.g., low-mass accelerometers) are sensitive to increases in size or weight, the information is encoded in a compact binary format. To minimize the TEDS memory footprint, the standard or manufacturer subtemplates that include the formatting details for each sensor type will reside external to the sensor, in the instrument or sensor software.

For communicating with the TEDS memory device, IEEE P1451.4 uses a simple, low-cost serial transmission protocol that provides power and data on one wire, with a second wire used for ground reference. The protocol, known as the 1-Wire protocol, allows the use of simple, low-cost EEPROMS with two leads. In fact, 1-Wire EEPROMS are commercially available with capacities of 256 bits and 4 Kb of storage, in small TO-92, PR-45, and chip-scale packages.

IEEE P1451.4 sensors use a mixed-mode interface that includes the serial TEDS protocol and the analog sensor signal interface. The standard defines two classes of mixed-mode sensors, allowing analog and digital TEDS data to sequentially share the same two wires or to be available simultaneously through separate wires (see Figure 2).

Class 1 interfaces are primarily intended for constant-current-powered piezoelectric transducers (e.g., 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 devices, generally referred to as IEPE transducers, incorporate internal signal conditioning powered by a constant current sourced by the measurement system on the signal wires. Class 1 transducers take advantage of the de facto analog standard by adding the TEDS with an electronic switch controlled by the direction of the current source (see Figure 3A).

Figure 3. Class 1 transducers incorporate a simple switch circuit to provide the analog signal and Transducer Electronic Data Sheet (TEDS) data on a single pair of wires (A). Class 2 transducers use one or two extra wires for a separate TEDS connection, as shown in the Class 2 bridge sensor above (B).

By reversing the direction of the current, the instrumentation switches the sensor to the digital mode.

Most sensor types will implement a form of the Class 2 interface, which separates the digital TEDS interface from the sensor’s analog output. The transducer’s analog I/O is left unmodified, and the 2-wire TEDS interface is added in parallel to the analog interface. This approach enables the implementation of plug and play with virtually any type of amplified or unamplified sensor or actuator, including thermocouples, RTDs, thermistors, bridge sensors, 4–20 mA current-loop sensors, and electrolytic chemical cells. Figure 3B shows an example implementation of a Class 2 mixed-mode interface with a bridge transducer.

Maximum Compatibility, Simple Adoption
Because IEEE P1451.4 maintains the analog output of the sensor, the standard retains a high degree of compatibility with legacy sensors and instrumentation systems. The analog portion of these sensors and instruments are completely compatible with non-plug-and-play components, minimizing the investment risk of users and developers.

Also, the addition of the TEDS memory component to a sensor is a relatively simple engineering task, often requiring the addition of only a single 2-wire component that, in the case of a Class 2 sensor, doesn’t require any modification of the electrical interface circuitry of the sensor. In fact, the Class 2 interface can actually be implemented in a variety of ways, including placing the TEDS memory chip on the sensor, down the cable from the sensor, or even in the connector of the sensor. The add-on nature of Class 2 interfaces facilitates easy upgrade or retrofit of legacy sensors in the field to plug-and-play sensors.

More Efficient System Setup and Management
As the technology for interoperable plug-and-play sensors becomes available, more users are experiencing the benefits of plug-and-play measurement and automation systems, including:

The most direct impact of plug-and-play sensors is quicker, more automated system setup. Without this technology, setting up and configuring a measurement system involves manually entering multiple sensor measurement parameters for each channel. For applications involving hundreds or thousands of sensors, this becomes a time-consuming and expensive process. Accurate entry of the data is critical because a data entry error or a sensor connected to the wrong input channel can lead to incorrect test data. Plug-and-play sensors eliminate the manual process, automatically uploading needed information into the measurement system and checking that each sensor is connected to the correct channel.

While these benefits are most obvious for large, sensor-intensive testing applications, the benefits of plug and play apply to any and all applications that involve an analog sensor-to-instrument interface. Whether it’s a simple digital meter readout or a networked, intelligent sensor node requiring more autonomous configuration and operation, IEEE P1451.4 provides simple, low-cost technology that simplifies sensor connectivity and use.

The Outlook for Plug-and-Play Sensors
Work on the IEEE P1451.4 specification is winding down, and a balloted and ratified standard is expected in early 2003. But because much of the standard is based on commercially available products, the standard already enjoys a high level of commercial adoption in the industry.

IEPE accelerometers and microphones, along with compatible signal conditioners, have long been available with the IEEE P1451.4 Class 1 shared-wire interface. Additionally, manufacturers of Class 2 sensors—such as load cells, pressure sensors, displacement transducers, and temperature sensors—have committed to manufacturing sensors with the standard’s TEDS technology.

The IEEE P1451.4 plug-and-play sensor concept appears to be one of those rare technologies whose strength and value come from its simplicity and focus. Although it doesn’t fit many of the typical definitions of a smart sensor, it does provide real, tangible benefits in a way that builds on, not replaces, existing systems and technologies.

The IEEE 1451 Family of Standards for Smart Transducer Interfaces

In 1994, IEEE and the National Institute of Standards and Technology (NIST) organized and kicked off the first working group meeting to address the need for standardization in the world of smart-sensor technology. The first meeting led to the development of the family of IEEE 1451 smart-transducer interface standards, whose overriding goal was to promote the adoption of smart-transducer technology. The IEEE 1451 standards seek to simplify smart-transducer (sensor and actuator) connectivity through the development of standard interfaces.

Recognizing that smart sensors will inevitably take a number of different forms with different levels of integration, the IEEE 1451 standards define a set of complementary interfaces designed to work together or independently (see Figure 4).

Figure 4. The IEEE 1451 family of smart transducer interfaces defines a number of interface alternatives for smart sensors and actuators.

In addition to the IEEE P1451.4 draft standard for analog, mixed-mode transducers, described in this article, the IEEE 1451 family now includes three alternative interface options, all in varying stages of development:

IEEE 1451.2. The IEEE Std 1451.2-1997 Transducer to Microprocessor Communication Protocol and TEDS Formats defines a digital point-to-point interface to connect a smart transducer module with digital output to a microprocessor-based network adapter. Although the standard has enjoyed limited commercial success, the general concept of the Transducer Electronic Data Sheet (TEDS) introduced by this standard has been adopted widely in various forms. A working group has reconvened this year to investigate revisions of the specification, including an alternative physical layer.

IEEE P1451.3. The IEEE P1451.3 draft standard defines a multidrop transducer network for distributed sensors. The standard will allow multiple, time-synchronized, high-bandwidth sensor data channels to occupy a single transmission medium that is based on Home Phoneline Networking Alliance (HPNA) technology. As of the publication of this issue, the specification is being balloted.

IEEE P1451.5. The newest member of the 1451 family, IEEE P1451.5 was kicked off only this year and will address the need for a standardized wireless interface.

In addition to the transducer interfaces, the IEEE 1451 family includes the definition of a common-object model, with interface specifications, of a networked smart transducer. This definition is specified in the IEEE Std 1451.1-1999, Network Capable Application Processor (NCAP) Information Model specification.

All of the current standards, including IEEE P1451.4, are designed to be complementary; they can be used independently or together. For example, an IEEE P1451.4 mixed-mode sensor could plug into an IEEE P1451.3 transducer bus interface module that digitizes the transducer signal and communicates the data over the IEEE P1451.3 sensor network.