2. UNIT 4: SMART SENSORS AND APPLICATIONS
CONTENTS: Introduction, Primary Sensors, Excitation,
Amplification, Filters, Converters, Compensation, Information
Coding/Processing, Data Communication, Standards for Smart
Sensor Interface, the Automation. Sensors Applications:
Introduction, On-board Automobile Sensors (Automotive Sensors),
Home Appliance Sensors, Aerospace Sensors, Sensors for
Manufacturing, Sensors for environmental Monitoring.
3. Introduction
A sensor producing an electrical output when combined with interface electronic
circuits is said to be an intelligent sensor if the interfacing circuits can perform (a)
ranging. (b) calibration, (c) decision making for communication and utilization of
data.
Both sensors and actuators are used as intelligent components of instrumentation
systems. In fact they are used as field devices. The block diagram of one such
intelligent equipment is shown. The simplified version with facilities of processing
that can be incorporated.
4.
5. Primary Sensors :
Existing sensors of all kinds with a cascaded block for providing electrical output
in the form of voltage or current can be adapted to an integrated processing
system but the system can then be hardly called a smart sensor.
External stimuli such as strain/stress, thermal/optical agitation, and
electric/magnetic field change the behaviour of materials at atomic/molecular
level or in crystalline state. This concept is utilized in designing a primary sensing
element for particular stimulus or a specific physical variable so that, in response
to this, the considered material yields a maximized output and its response to
other stimuli is minimized
6. Excitation :
Although excitation is a generalized term used for supply to the primary sensors,
when necessary this also means the supply for the entire chip including the
processing units. This supply may be required to provide different output to
different stages of the system. In the thermocouple form of sensors, no excitation to
the sensors is needed while for resistive bridge, an extremely stable supply is
required. In stages of electronic processing units, ac supply or else pulsed form
supply may be required for phase sensitive detection in the processor unit.
7. Amplification :
Considering the output of the sensor to be generally small, amplification is essential
in all smart sensors. If the gain requirement is very high, noise becomes a problem.
However, stage-wise approach with adequate compensation realizes the
requirement, the design and layout being critical as well.
8. Filters :
Analog filters are often resorted to although filters are necessary at conversion
stages, mainly because the digital type, consume large real time processing power.
9. Conversion
Conversion is the stage of internal interfacing between the continuous and the
discrete processing units. The conversion, in most of the situations, does not have
one-to-one correspondence. Often. controlled conversion through software is
provided with range selection and so on. Data conversion from analog amplitude to
frequency is often done for convenience of signal transmission, internally or
externally, and/or for subsequent digital conversion. Voltage-controlled oscillators
are used for these purposes.
One such converter is a multivibrator shown
10. COMPENSATION
Compensation is an attempt to counter all sorts of nonideality in the primary sensor
characteristics as well as environment of measurement. The commonly encountered
sensor defects are:
(a) nonlinearity, (f) interference.
(b) noise,
(c) response time,
(d) drift,
(e) cross sensitivity, and
11. INFORMATION CODING/PROCESSING
Signal from a sensor is processed providing correction, compensation, linearization,
freedom from cross-sensitivity and drift, and so on. Smart sensors are generally multi-
sensor systems and a number of signals are available for either display or further
processing subsequently to be connected to the 'communication bus. the state of the
process in the form of a processed signal through sensor and signal processing
systems, is first received by the information coding system.
12. DATA COMMUNICATION
Data communication is essential in smart transmitters where the sensor outputs are
communicated with the host through bus-system. Coded data are processed for
communication by a software processor and a suitable interface system
communicates between the processor and the bus.
13. Standards for smart sensor interface
The ultimate goal of the standards is to provide the means for achieving transducer-
to-network interchangeability and interoperability. The objectives are to define a set
of common communication interfaces for connecting transducers to microprocessor-
based systems, instruments, and field networks in a network-independent
environment. Figure shows a scheme of communication using IEEE 1451. Here, NCAP
(Network Capable Application Processor) information model is intended for defining a
common object model for the components of the smart transducer working in
networked mode and also to develop the software interface specifications for them.
Such an object model provides two interfaces (i) to the transducer block with details of
transducer hardware implementation and simple programming model this resembles
an I/O driver and (ii) to the NCAP block and ports with details of different network
protocol implementation schemes, this is IEEE P 1451.1
14. The IEEE P 1451.2 provides the transducer-to-microprocessor communication protocols and
transducer electronic data sheet (TEDS) formats. It also provides the digital interface and
communication protocols between the transducers and microprocessors.
IEEE P 1451.3 provides digital communication and TEDS formats for distributed multiloop systems.
This is basically intended to develop a standard digital interface for multiple physically
isolated/separated transducers in multidrop configuration.
IEEE P 1451.4 provides mixed mode communication protocols and also the TEDS formats. This is
intended to develop bidirectional communication of digital TEDS in addition to an interface for mixed
mode transducers.
15. The Automation:
In modern control systems, signal communication standards have been of tremendous
significance. The first signalling standard (IEC Technology Committee TC-65, 1971,
namely IEC 381-1) established was 4-20 mA. In 1981, work on International standards for
PLC; in 1985, for fieldbus; and in 1987, functional safety for programmable electronic
systems started but proprietary standards still continue to exist.
Hierarchical structure of control of large complex processes has specific advantages.
distributed control structure reduces the cost significantly by eliminating the need for
long transmission lines between the controller and the sensors and actuators. A typical
scheme of such a structure is shown in Fig. 7.20. By connecting field-located devices
with a serial bus and the fieldbus, cabling costs can be reduced further.
16.
17. APPLICATIONS :
On-board Automobile Sensors (Automotive Sensors),
Home Appliance Sensors
Aerospace Sensors
Sensors for Manufacturing
Sensors for environmental Monitoring.
