• A sensor is often defined as a device that receives and responds to a signal or stimulus.

• A sensor is a device that detects and responds to some type of input from the physical environment.

• Different types of Sensors in our homes, offices, cars etc. working to make our lives easier by turning on the

lights by detecting our presence, adjusting the room temperature, detect smoke or fire, make us delicious coffee,

open garage doors as soon as our car is near the door and many other tasks.
Sensors (which give inputs to the Computers), the Computers (the brains of the system) and the mechanics (the

outputs of the system like engines and motors) are equally important in building a successful automated system.
Sensor as an input device which provides an output (signal) with respect to a specific physical quantity (input).

The term “input device” in the definition of a Sensor means that it is part of a bigger system which provides input to

a main control system (like a Processor or a Microcontroller).

It is a device that converts signals from one energy domain to electrical domain.
Acoustic: Wave amplitude, phase, polarization Spectrum, Wave velocity, Other.

Biological: Biomass (types, concentration, states), Other

Chemical: Components (identities, concentration, states) Other.

Electric: Charge, current, Potential, voltage, Electric field (amplitude, phase, polarization, spectrum), Conductivity, Permittivity, Other.

Magnetic: Magnetic field (amplitude, phase, polarization, spectrum) Magnetic flux, Permeability, Other

Optical: Wave amplitude, phase, polarization, spectrum, Wave velocity, Refractive index, Emissivity reflectivity, absorption, Other.

Mechanical: Position (linear, angular) Acceleration, Force, Stress, pressure, Strain, Mass, density, Moment, torque Speed of flow, rate

of mass transport, Shape, roughness, orientation, Stiffness, compliance, Viscosity, Crystallinity, structural integrity, Other.

Radiation: Type, Energy, Intensity, Other.

Thermal: Temperature, Flux, Specific heat, Thermal conductivity, Other.

• When we say “electrical,” we mean a signal which can be channelled, amplified, and modified by electronic

devices.

• The sensor’s output signal may be in the form of voltage, current, or charge.

• These may be further described in terms of amplitude, frequency, phase, or digital code. This set of characteristics

is called the output signal format.

• Any sensor is an energy converter (energy transfer from the object of measurement to the sensor). The process of
sensing is a particular case of information transfer, and any transmission of information requires transmission of
energy.

• The term sensor should be distinguished from the transducer. The latter is a converter of one type of energy into
another, whereas the former converts any type of energy into electrical.

• An example of a transducer is a loudspeaker that converts an electrical signal into a variable magnetic field and,
subsequently, into acoustic waves. This is nothing to do with perception or sensing.

• Transducers may be used as actuators in various systems. An actuator may be described as opposite to a sensor—
it converts the electrical signal into generally nonelectrical energy. For example, an electric motor is an actuator
—it converts electric energy into mechanical action.

A sensor does not function by itself; it is always a part of a larger system that may incorporate many other
detectors, signal conditioners, signal processors, memory devices, data recorders, and actuators.
• Transducers may be parts of complex sensors. For example, a chemical sensor may have a part, which converts the

energy of a chemical reaction into heat (transducer) and another part, a thermopile, which converts heat into an

electrical signal.

• Hence a chemical sensor is a complex sensor; it is comprised of a nonelectrical transducer and a simple (direct) sensor

converting heat to electricity.

• Many sensors incorporate at least one direct-type sensor and a number of transducers.

• Direct sensors are those that employ certain physical effects to make a direct energy conversion into an electrical signal

generation or modification. Examples of such physical effects are photoeffect and Seebeck effect.
There are two types of sensors

Direct and Complex.

A direct sensor converts a stimulus into an electrical signal or modifies an electrical signal by using an appropriate physical

effect,

A complex sensor in addition needs one or more transducers of energy before a direct sensor can be employed to generate an

electrical output.
A sensor is always a part of a larger system that may incorporate many other detectors, signal conditioners,

signal processors, memory devices, data recorders, and actuators.

The sensor’s place in a device is either intrinsic or extrinsic. It may be positioned at the input of a device to

perceive the outside effects and to signal the system about variations in the outside stimuli.

It may be an internal part of a device that monitors the devices’ own state to cause the appropriate performance.

A sensor is always a part of some kind of a data acquisition system. Often, such a system may be a part of a

larger control system that includes various feedback mechanisms.

Depending on the complexity of the system, the total number of sensors may vary from as little as one

Anesthetic vapor delivery system

It is intended to control the level of anesthetic drugs delivered to a patient by means of inhalation during surgical

procedures. The system employs several active and passive sensors.

The microwave, ultrasonic, and infrared range measuring sensors are incorporated into the

emergency braking advanced advisory system to illuminate the break lamps even before the

driver brakes hard in an emergency.

The sensor’s input signals (stimuli) may have almost any conceivable physical or chemical

nature (e.g., light, temperature, pressure, vibration, displacement, position, velocity, ion

concentration, etc).

Some sensors are specifically designed to be very selective in a particular range of input

stimulus and be quite immune to signals outside the desirable limits.

Sensor Classification

• 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.

• 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. For example, a thermistor is a temperature sensitive resistor
• Depending on the selected reference, sensors can be classified into absolute and relative.

• An absolute sensor detects a stimulus in reference to an absolute physical scale that is independent of the measurement

conditions, whereas a relative sensor produces a signal that relates to some special case.

• An example of an absolute sensor is a thermistor, a temperature-sensitive resistor. Its electrical resistance directly relates

to the absolute temperature scale of Kelvin.

• Another very popular temperature sensor thermocouple is a relative sensor. It produces an electric voltage, which is a

function of a temperature gradient across the thermocouple wires. Thus, a thermocouple output signal cannot be related

to any particular temperature without referencing to a known baseline.

• Another example of the absolute and relative sensors is a pressure sensor. An absolute pressure sensor produces signal in

reference to vacuum – an absolute zero on a pressure scale. A relative pressure sensor produces signal with respect to a

selected baseline that is not zero pressure, for example, to the atmospheric pressure.
Units of Measurements
Sensor Characteristics

Transfer Function

The main objective of sensing is to determine a value of the input stimulus (s) from the value of the sensor output
signal (S).
The transfer function represents the relation between stimulus s and response electrical signal S produced by the
sensor
S=f(s).

In the measurement system an inverse transfer function f-1(S), which will be denoted F(S), is employed to obtain
the value of the stimulus s.
The value of S that becomes known during the measurement is just a number (voltage, current, digital count, etc.)
that represents the value of stimulus s.
The simplest linear transfer function: S=A+Bs
where A is the intercept (i.e., the output signal at zero input signal) and B is the slope, which is sometimes called
sensitivity. S is one of the characteristics of the output electric signal used by the data acquisition devices as the
sensor’s output. It may be amplitude, frequency, or phase, depending on the sensor properties.
 Logarithmic transfer function for nonlinear sensor’s response:

Exponential function:

Power function:

where A, B are parameters and k is the power factor.

Figure 1a illustrates the transfer function of a thermo-anemometer (a sensor that measures mass flow of gas). In general, it can
be modeled by a square root function f(s) of the input airflow rate. The output of the sensor can be in volts or in digital counts
from an analog-to-digital (A/D) converter. After the output count n = f(s) is measured, it must be translated back to the flow
rate. The monotonic square root function f(s) has parabola F(n) as its inverse. This parabola is shown in Fig. 1b illustrating the
relation between the output counts (or volts) and the input flow rate. Graphically, the inverse function can be obtained by
mirror reflection with respect to the bisector of the right angle formed by x and y-axes.
Calibration

If sensor’s manufacturer tolerances and tolerances of the interface (signal conditioning) circuit are broader than the

required system accuracy, a calibration of the sensor or a combination of a sensor and an interface circuit is required to

minimize errors. The purpose of the calibration then is to find the unknown coefficients (parameters) of the inverted

transfer function so that the fully defined function can be employed during the measurement process to compute any

stimulus in the desirable range, not only at the points used during the calibration but anywhere in-between.

Calibration of a sensor can be done in several possible ways

1. Calculation of the transfer function or its approximation to fit the selected calibration points

2. Adjustment of the data acquisition system to modify the measured data by making them to fit into a normalized or

“ideal” transfer function.

3. Modification (trimming) of the sensor’ properties to fit the predetermined transfer function.

4. Creating a sensor-specific reference device with matching properties at particular calibrating points.
Span (Full-Scale Full Scale Input)

• A dynamic range of stimuli that may be converted by a sensor is called a span or an input full scale (FS). It represents the

highest possible input value, which can be applied to the sensor without causing unacceptably large inaccuracy.

• For the sensors with a very broad and nonlinear response characteristic, a dynamic range of the input stimuli is often

expressed in decibels, which is a logarithmic measure of ratios of either power or force (voltage).
Full-Scale Output

• Full-scale output (FSO) is the algebraic difference between the electrical output signals measured with maximum input

stimulus and the lowest input stimulus applied.

• This must include all deviations from the ideal transfer function.

Accuracy

• A very important characteristic of a sensor is accuracy, which really means inaccuracy.

• Inaccuracy is measured as a highest deviation of a value represented by the sensor from the ideal or true value of a

stimulus at its input.

• The deviation can be described as a difference between the value, which is computed from the output voltage, and the

actual input value.

• A real function rarely coincides with the ideal. Because of the material variations workmanship, design errors, manufacturing

tolerances, and other limitations, it is possible to have a large family of real transfer functions, even when sensors are tested

under presumably identical conditions.

• The accuracy rating includes a combined effect of part-to-part variations, hysteresis, dead band, calibration, and repeatability

errors. The specified accuracy limits generally are used in the worst-case analysis to determine the worst possible

performance of the system.

• To improve accuracy, the number of the error-contributing factors should be reduced. This can be achieved by not relaying

on the manufacturer’s tolerances, but calibrating each sensor individually under selected conditions.
Calibration Error

• Calibration error is inaccuracy permitted by a manufacturer when a sensor is calibrated in the factory.

• This error is of a systematic nature, meaning that it is added to all possible real transfer functions.

• It shifts the accuracy of transduction for each stimulus point by a constant.

• This error is not necessarily uniform over the range and may change depending on the type of error in calibration
Hysteresis

• A hysteresis error is a deviation of the sensor’s output at a specified point of the input signal when it is approached from

the opposite directions.

• Typical causes for hysteresis are geometry of design, friction, and structural changes in the materials.