Most sensors
put out some sort of analog signal a voltage, current, or resistance
that varies in a fixed relationship to a physical parameter. Instrumentation
that converts the signal to engineering units must know a few
things about what parameters to expect from the sensor. That's
because full-scale range, sensitivity, connection schemes, and
other factors all vary greatly from one kind of sensor to another.
The end result is that it takes some effort to match up sensors
with monitoring electronics. This configuration process may be
no big deal when there are only a few sensors involved. But it
can be problematic when gangs of sensors get wired into multichannel
measurement systems.
1.1 Sensor
A sensor is a device that measures a physical attribute or a physical
event. It outputs a functional reading of that measurement as
an electrical, optical or digital signal. That signal is data
that can be transformed by other devices into information. The
information can be used by either intelligent devices or monitoring
individuals to make intelligent decisions and maintain or change
a course of action.
1.2 Smart Sensors
A smart sensor is simply one that handles its own acquisition
and conversion of data into a calibrated result in the units of
the physical attribute being measured. For example, a traditional
thermocouple simply provided an analog voltage output. The voltmeter
was responsible for taking this voltage and transforming it into
a meaningful temperature measurement through a set of fairly complex
algorithms as well as an analog to digital acquisition. A smart
sensor would do all that internally and simply provide a temperature
number as data. Smart sensors do not make judgments on the data
collected unless that data goes out of range for the sensor.
IEEE 1451.4
defines a relatively simple, straightforward mechanism for adding
smart, plug and play capabilities to traditional analog sensors.
Without adding any new hardware to the system, these plug and
play sensors can bring real, immediate benefits in ease of use
and productivity to any measurement and automation system that
uses sensors. Additionally, IEEE 1451.4 defines a standard framework
for sensor description, embodied specifically in the TEDS, which
can scale from today's traditional analog sensors to tomorrow's
smart networked sensors.
Two factors have promoted widespread adoption of plug-and-play
sensors: the IEEE P1451.4 Smart Transducer Interface draft standard
and the Internet. IEEE P1451.4 is a proposed standard for self-describing
analog sensors using standardized Transducer Electronic Data Sheets
(TEDS). The Internet can bring the plug-and-play concept to legacy
sensors and systems via distribution of so-called virtual TEDS.
The next generation of measurement and automation systems will
use these concepts to become even more automated, robust, and
smarter.
The 1451.4
standard calls for a mixed-mode interface. This is essentially
the digital and analog signals being transferred back and forth
between the signal conditioning hardware and the sensor, making
it more compatible with the legacy sensors in place. The first
three versions called for sending only digital data back to the
computer. The next version of the standard being drafted now is
P1451.5, which adds wireless capability to sensors. Right now,
the P1451.4 is still a proposed standard which is expected to
be ratified and issued by soon.
Plug-and-play
sensors address the labor involved in connection and configuration.
Based on open industry standards, plug-and-play sensors incorporate
ways of automatically identifying themselves. Benefits include
quicker, more automated system setup; better diagnostics; less
downtime for sensor repair and replacement; and an easier time
keeping track of sensors themselves as well as the data they generate.
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.
