Sensors are devices that measure a physical quantity and convert it into a signal which can be read by an observer or by an instrument. The most common types of sensors are those used to measure temperature, light, pressure, force, etc.
The first step in sensor development is the identification of the physical phenomenon to be measured. For example, one might want to measure the temperature of a fluid flowing through a pipe. The next step is the selection of a suitable transducer. A transducer is any device that converts energy from one form to another. In this case, we need a device that will convert the thermal energy of the fluid into some other form of energy such as electrical or optical energy. Once we have selected our transducer, we must design it so that it will produce an output signal whose strength is proportional to the quantity being measured (in this case, temperature).
There are many different types of sensors available on the market today. Some of the more common ones include:
Thermocouples: Thermocouples consist of two wires made from different metals which are joined at one end. When the junction between these two wires is heated, a voltage difference develops across the wires which can be measured using an electronic voltmeter. Thermocouples are commonly used for measuring temperatures in industrial applications where they can withstand high temperatures (up to 2000°C).
Linear Variable Differential Transformer (LVDT): An LVDT consists of three coils arranged in series with a movable core located in the centre coil (the primary coil). When an alternating current is passed through all three coils, a magnetic field is generated which causes the central core to move towards or away from one of the secondary coils depending on its polarity. This movement produces a change in inductance in one of the secondary coils which can be detected and converted into an electrical signal using electronic circuitry. LVDTs are often used for measuring displacements in mechanical systems such as actuators and engines .
Force-Sensitive Resistor (FSR): FSRs are resistors whose resistance decreases when force is applied to them . They contain conductive ink or carbon particles suspended in epoxy which make contact with each other when pressure is applied , thereby reducing resistance . FSRs are frequently used as input devices for human-computer interaction such as touch screens and interactive floor mats .
Piezoelectric Sensors: Piezoelectric sensors generate electricity when they experience mechanical stress . The piezoelectric effect was first discovered in 1880 by French physicist Pierre Curie who found that certain crystals , such as quartz , would produce sparks when rubbed together . Today , piezoelectric materials such as lead zirconate titanate (PZT) ceramics are commonly used for sensing purposes . For example , PZT ceramics are often used as microphones and accelerometers .
Photoresistors: Photoresistors , also known as light-dependent resistors (LDRs) , vary their resistance in response to light intensity . They contain semiconductor materials such as cadmium sulfide or cadmium selenide whose conductivity increases when exposed to light due to promotion of electrons from valence band into conduction band . Photoresistors find applications in automatic lighting control systems and photocopiers .