The main function of a sensor is to serve as a transducer, transforming a physical or chemical occurrence such as heat, into a quantifiable electrical signal, typically a variation in voltage, current, or frequency. The design of nearly all sensors relies on modifying basic electrical properties.
Physical Principles in Sensor
Design
The operation of a
sensor is typically based on the correlation between a physical change and one
of the three fundamental circuit parameters: resistance, capacitance, or
inductance.
1. Resistive Principle
Resistance refers to
the opposition against the flow of electric current within a material or
component, measured in ohms (Ω). It can be determined using two primary
equations:
Ohm's Law,
R = V/I
where resistance is
equal to voltage divided by current) for components that adhere to it, or
The Resistivity Formula,
R = ρ(L/A)
Defines resistance as
resistivity times length divided by cross-sectional area to calculate a
material's inherent resistance.
Resistive sensors
function based on the concept that the electrical resistance (R) of a substance
varies in response to an external physical stimulus. Any
physical condition that alters, the material's capacity to
conduct electricity can thus be estimated by computing the resistance or any of the related
variables.
2. Capacitive Principle
A capacitor resists the
flow of alternating current, and this opposition is called capacitive
reactance. As the frequency of the AC signal increases, the capacitive
reactance decreases. Conversely, a lower frequency results in higher reactance. Capacitive sensors
measure changes in capacitance (C) (Capacitance is determined by the
distance between two conductive plates and the material (dielectric) placed
between them.). Physical
input, such as distance, or displacement, causes a
change in the geometry or the nature of the dielectric. Since the electrical
charge stored changes with capacitance, this results in a voltage change.
3. Inductive Principle
Inductance is the
property of an electric conductor that causes an electromotive force to be
generated by a change in the current flowing. Inductive sensors
detect variations in inductance (L) found within a coil. As the physical input
(such as position or displacement) alters the magnetic flux through the coil,
the inductance of the coil changes. Devices like Linear Variable Differential
Transformers (LVDTs) utilize this concept for precise position measurements.
Key Transduction Effects
A number of physical
effects are employed for specialized sensor functions. These include:
1. Piezoelectric
Effect
The piezoelectric effect refers to the phenomenon where specific materials produce an electric charge when they experience mechanical stress or pressure, which is referred to as the direct effect. Some crystalline substances, such as quartz, demonstrate the Piezoelectric Effect. When the materials undergo mechanical stress, they produce a direct and proportional electrical charge on their surface. This principle enables the straightforward transformation of mechanical energy into an electrical signal, which is applied in accelerometers, vibration sensors, and other pressure transducers.
2. Hall
Effect
The generation of a voltage across an electrical conductor when a magnetic field is applied perpendicular to the direction of current flow. The Hall Effect takes place when a conductor carrying current is exposed to a magnetic field that is perpendicular to the flow of current. This generates a measurable voltage across the conductor, which is perpendicular to both the current and the magnetic field. The Hall voltage is directly related to the intensity of the magnetic field. Hall Effect sensors gauge magnetic field strength to ascertain the location of a magnet and measure rotational speed without the need for physical contact.
Knowledge of the underlying principles is the foundation for selecting and designing appropriate sensors.
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