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Sensors are advanced devices that are often used to sense and react to electrical or optical signals. A sensor translates the physical characteristic into a signal which can be calculated electrically. For example, the mercury in the glass thermometer increases and decreases the liquid to translate the calculated temperature which can be viewed on the regulated glass tube.
A sensor is a device that senses and reacts to some type of input from the physical environment. Types of input are as follows: heat, light, moisture, motion, pressure or any other environmental phenomena. The result is generally a signal that is translated to human-readable display at the sensor location or sent electronically over a network for reading or further processing.
Properties of a Good Sensor
- It is sensitive to the measured property.
- It is sensitive to any other property likely to be encountered in its application.
- It does not influence the measured property.
A sensor’s sensitivity specifies how much the sensor’s output varies when the input quantity being measured varies. For example, if the mercury in the thermometer moves 1 cm when the temperature varies by 1 C, the sensitivity is 1cm/C.
Examples of Sensors
The input is temperature in a mercury-based glass thermometer. The liquid contained increases and decreases in response, causing the level to be higher or lower on the marked gauge, which is human-readable.
An oxygen sensor in a car’s emission control system senses the gasoline / oxygen ratio, usually through a chemical reaction that produces a voltage. A computer in the engine reads the voltage and, if the mixture is not ideal, readjusts the balance.
Motion sensors in various systems including home security lights, automatic doors and bathroom fixtures typically transmit some type of energy like microwaves, ultrasonic waves or light beams and senses when the flow of energy is hampered by something entering its path.
A photosensor senses the presence of visible light, infrared transmission (IR), and / or ultraviolet (UV) energy.
Sensor Selection Criteria
- Accuracy – measurements are expected to be accurate
- Calibration – readings changes with time, so it should be calibrated
- Cost – should be within budget
- Environmental condition – check for temperature or humidity limits
- Range – limits of measurement
- Repeatability – variable readings are repeated under the same environment
- Resolution – smallest increment sensed
- Primary input quantity
- Transduction principles – fundamental criteria followed for an efficient approach
- Material and Technology – usually chosen by development engineering group
- Temperature – Thermistors, IC (Integrated Circuits)
- Pressure – Fiber optic, LVDT (Linear Variable Differential Transformer)
- Flow – Electromagnetic, Thermal mass
- Level Sensors – Differential pressure, Thermal displacement
- Proximity and displacement – Photoelectric, Ultrasonic
- Biosensors – Resonant mirror, Surface plasmon resonance
- Image – Charge coupled devices, CMOS (Complementary Metal-Oxide-Semiconductor)
- Gas and chemical – Semiconductor, Conductance
- Acceleration – Gyroscopes, Accelerometers
- Others – Moisture, Speed sensor
- Industrial process control, measurement and automation
- Non-industrial use – Aircraft, Automobiles
- Power or Energy Supply Requirement
- Active Sensor – requires power supply like LiDAR (Light Detection and Ranging), Photoconductive cell
- Passive Sensor – does not require power supply like Radiometers, Film photography
Sensor Group Classification
- Accelerometers – are based on the Micro Electro Mechanical sensor technology. They are utilized for patient monitoring like pacemakers and vehicle dynamic systems.
- Biosensors – are based on the electrochemical technology. They are utilized for food testing, medical care device, water testing and biological warfare agent detection.
- Image Sensors – are based on the CMOS technology. They are utilized in consumer electronics, biometrics, traffic and security surveillance and PC imaging.
- Motion Detectors – are based on the Infrared, Ultrasonic and Microwave / Radar Technology. They are utilised in video games and simulations, light activation and security detection.