Sensor lec4
Introduction:
A sensor is a device that converts a physical phenomenon into an electrical signal.
As such, sensors represent part of the interface between the physical world and the
world of electrical devices, such as computers. The other part of this interface is
represented by actuators, which convert electrical signals into physical p
All sensors may be of two kinds: passive and active. A passive sensor does not
need any additional energy source and directly generates an electric signal in
response to an external stimulus; that is, the input stimulus energy is converted by
the sensor into the output signal. The examples are a thermocouple, a photodiode,
and a piezoelectric sensor. Most of passive sensors are direct sensors as we defined
them earlier. The active sensors require external power for their operation, which is
called an excitation signal. That signal is modified by the sensor to produce the
output signal.
The active sensors sometimes are called parametric because their own properties
change in response to an external effect and these properties can be subsequently
converted into electric signals.
Sensor Performance Characteristics Definitions
The following are some of the more important sensor characteristics:
Transfer Function
The transfer function shows the functional relationship between physical input
signal and electrical output signal. Usually, this relationship is represented as a
graph showing the relationship between the input and output signal.
Sensitivity
The sensitivity is defined in terms of the relationship between input physical signal
and output electrical signal. It is generally the ratio between a small change in
electrical signal to a small change in physical signal.
Span or Dynamic Range
The range of input physical signals that may be converted to electrical signals by
the sensor is the dynamic range or span. Signals outside of this range are expected
to cause unacceptably large inaccuracy. This span or dynamic range is usually
specified by the sensor supplier as the range over which other performance
characteristics described in the data sheets are expected to apply.
Noise
All sensors produce some output noise in addition to the output signal. In some
cases, the noise of the sensor is less than the noise of the next element in the
electronics, or less than the fluctuations in the physical signal, in which case it is
not important.
Resolution
The resolution of a sensor is defined as the minimum detectable signal fluctuation.
Since fluctuations are temporal phenomena, there is some relationship between the
timescale for the fluctuation and the minimum detectable amplitude.
Therefore, the definition of resolution must include some information about the
nature of the measurement being carried out.
Bandwidth
All sensors have finite response times to an instantaneous change in physical
signal. In addition, many sensors have decay times, which would represent the time
after a step change in physical signal for the sensor output to decay to its original
value.
Selection of Sensors
1 THE DESIGN PROCESS
The design of a technical system involves making choices on the basis of criteria
(from a list of requirements), availability of parts and materials, financial
resources, and time. These aspects play a significant role when designing a
measurement system. Blanchard and Fabrycky (1998) distinguish six major phases
of the design process:
(a) conceptual design;
(b) preliminary design;
(c) detail design and development;
(d) production/construction;
(e) operational use/maintenance;
(f) retirement
sensor selection is a crucial activity in the systems design process, as it will make a
great impact on the production of the measurement instrument and the performance
during its entire lifetime and may even have consequences related to disposal.
Design methods have evolved over time, from purely intuitive (as in art) to formal
(managerial). The process of sensor selection is somewhere in between: it is an act
of engineering, in which the design is supported by advanced tools for simulating
system behavior based on scientific knowledge. The basic attitude is (still) the use
of know-how contained in the minds of people and acquired through experience.
2 THE REQUIREMENTS
Sensor selection means meeting requirements. Unfortunately, these requirements
are often not known precisely or in detail, in particular when the designer and the
user are different persons. The first task of the designer is to get as much
information as possible about the future applications of the measurement
instrument, all possible conditions of operation, the environmental factors, and the
specifications with respect to quality, physical dimensions, and costs.
The list of demands should be exhaustive. Even when not all items are relevant,
they must be indicated as such. This will leave more room to the designer, and will
minimize the risk of being forced to start all over again at a later date. Rework is
an expensive process and should be avoided where possible by reducing errors as
early as possible in the systems engineering life cycle process. The requirements
list should be made in such a way as to enable unambiguous comparison with the
final specifications of the designed instrument. Once the designer has a complete
idea about the future use of the instrument, the phase of the conceptual design can
start.
Before thinking about sensors, the measurement principle has to be considered
first. For the instrumentation of each measurement principle, the designer has a
multitude of sensing methods at his or her disposal. For realization of a particular
sensor method, the designer has to choose the optimal sensor type out of a vast
collection of sensors offered by numerous sensor manufacturers.
3 SELECTING THE MEASUREMENT
PRINCIPLE
The design process is illustrated by using an example of a measurement for a
single, static quantity: the amount of fluid in a container. The first question to be
answered in this case is in what units the amount should be expressed – volume or
mass. It is important to have a sound understanding of the physics involved and the
circumstances of the situation. These may influence the final selection of the
sensor.
Figure 1 shows the various measurement principles that could be used in this case:
A: the tank placed on a balance, to measure its total weight;
B: a pressure gauge on the bottom of the tank;
C: a gauging-rule from top to bottom with electronic readout;
D: level detector on the bottom, measuring the column height;
E: level detector from the top of the tank, measuring the height of the empty part.
Obviously, many more principles can be used to measure a quantity that is related
to the amount of fluid in the tank.
In the conceptive phase of the design, as many principles as possible should be
considered, even unusual or unorthodox ones. On the basis of the list of demands
and not as a ‘hunch’, it should be possible to find a suitable candidate principle
from this list, or at least delete many of the principles, on the basis of arguments.
For instance, where the fluid must remain in the tank during measurement,
principles based on volume or mass flow are excluded. If the tank contains a
chemically aggressive fluid, a noncontact measurement principle is preferred,
placing principles B, C, and D lower on the list, and so on. Also, method A can
possibly be eliminated because of its high costs for large containers.
In this way, the conceptual design ends up with a set of principles having related
pros and cons, ranked according to the prospects of success.
4 SELECTING THE SENSING METHOD
After having specified a list of candidate principles, the next step is to find a
suitable sensing method for each of them. In the example in Figure 1, we will
further investigate principle E, a level detector placed at the top of the tank.
Again, a list of the various possible sensor methods is made.
This may be
1. a float, connected to an electronic readout system;
2. an optical time-of-flight measurement;
3. an optical range measurement;
4. an electromagnetic distance measurement (radar);
5. an acoustic time-of-flight (ToF) measurement;
and so on. As in the conceptual phase, these methods are evaluated using the list
of demands, so not only the characteristics of the sensing method but also the
properties of the measurement object (liquid level) and the environment should be
taken into account. For the tank system, the acoustic ToF method could have an
excellent chance because of being contact-free; or just the contrary, for instance,
because of possible high temperatures.
In this phase, it is also important to consider methods to reduce such environmental
factors (see Article 16, Reduction of Influence Factors, Volume 1). Anyhow,
this phase ends up with a list of candidate sensing methods with merits and
demerits with respect to the requirements.
5 SENSOR SELECTION
The final step is the selection of the components thatmake up the sensing system.
Here, a decision has to be made between a commercially available system and the
development of a dedicated system. The major criteria are costs and time: both are
often underestimated when a new development is considered.
In this phase of the selection process, sensor specifications become important.
Sensor providers publish specifications in data sheets or on the Internet. However,
the accessibility of such data is still poor, making this phase of the selection
process critical and time consuming, in particular for nonspecialists in the sensor
field.
Computer-aided sensor selection programs are under development and are partly
realized, but up to now their use has been limited. A prerequisite for a general,
successful tool facilitating sensor selection is the continuous availability of sensor
data on the Internet, in a more standard format, and regularly updated.
Obviously, the example of the level sensor is greatly simplified here. Usually, the
selection process is not that straightforward. Since the sensor is often just one
element in the design of a complex technical system, close and frequent interaction
with other design disciplines is necessary
