Hello steemians, welcome to the continuation of our ENGINEERING CLASS ON SENSORS, like i clearly stated at the end of our last class, today we are going to be looking at the various types of sensor we know around us.     
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# TYPES OF SENSOR  
The major types of sensor/transducers are based on its field of application and the stimulus we intend to measure. Table 1. Below summarises common types of sensor. 
S/N SENSOR TYPE COMMON SENSORS EMPLOYED 
1. LIGHT - Photodiodes  - Phototransistor - Photo voltaic cell - Photoresistor 

2. FORCE/STRAIN - Strain gauge - Load Cell
 
3. DISPLACEMENT/POSITION/PROXIMITY - Potentiometer  - Capacitive sensor - Linear Variable Differential Transformers (LVDT) - Eddy current proximity sensors - Hall effect sensor 

4. MOTION (VELOCITY, ACCELERATION) - Incremental encoder  - Tachogenerator/tachometer - Pyroelectric sensor 

5. PRESSURE - Diaphragm pressure gauge  - Capsules, bellows, pressure tubes  - Piezoelectric sensors - Tactile sensor - Piezoresistive Sensor 

6. TEMPERATURE - Bimetallic strips - Thermostat - Thermistors - Thermo-diodes and transistors - Thermocouples 

7. ACOUSTIC (SOUND) - Resistive Microphones - Condenser Microphones - Fiber-Optic Microphone - Piezoelectric Microphone Table 1. Transducer/Sensor types 

# LIGHT SENSORS 

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A light sensor is a device that is used to detect light i.e electromagnetic radiation in the spectral range from ultraviolet to far infrared.   

- X-rays are also utilised in industry mainly as inspection tools. They can be sensed by some sophisticated light sensors but major precaution is usually taken as they can be very harmful.  

Most light detectors rely on the interaction of the different photons particles emitted by light with a crystalline lattice of semiconductor materials. Photodiodes and phototransistors are examples. Their operations are based on the photo effect. 

- A photoresistor changes their resistance with light intensity. It is made of a high resistance semiconductor material e.g. cadmium sulfide (CdS). The resistance of the material varies inversely to the amount of light incident upon it.  

- Photovoltaic cells develop a voltage in the presence of light. Copper oxide and selenium are examples of photovoltaic materials. A micro ammeter calibrated in lux is connected across the cells to measure the output current.  

Applications of light sensors include, Camera (Photographic Flash Control, Auto focus), Industry (bar code scanners, brightness control), Automotive (Sunlight detector, headlight dimmer), Communication (fibre optic links) 


# FORCE/STRIAN SENSORS  

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Measurement of force is critical as it is a mainstay of many industrial applications. Many force sensors are also employed in measurement of motion/acceleration and pressure, since, according the newton’s first law; If no net force acts on a body, its acceleration a is zero (law of inertia) and  pressure is a “force” when dealing with fluids (measured when force is distributed over a relatively large area). 

An example is a computer keyboard, on which a key makes a contact only when it is pressed sufficiently hard. The qualitative force sensors are often used for the detection of motion and position. 

- Strain gauge sensors measure force based on the principle of change in electrical resistance. Usually, electrical resistance of a semiconductor or metal changes when the material is mechanically 
deformed. This is called the piezoresistive effect. 

- A spring transducer can also measure a weight/force by measuring the deflection of a spring when a weight is applied. 

- Hydraulic and pneumatic devices can be used to measure force by monitoring the pressure in a cylinder when the force is applied to a piston. 

# DISPLACEMENT/POSITION/PROXIMITY 
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The measurement of displacement and position is very important in our modern world. Displacement is the movement of an object from a point to another for a specific distance or angle. In effect, when an object is referenced to its own previous position rather than to another reference, a displacement is measured. 

Position means the determination of the object’s coordinates (linear or angular) with respect to a particular reference. 

- Proximity sensors sense when an object has moved within a critical distance. That is to say that a proximity sensor is a sort of threshold position detector. A potentiometer is a commonly used device for measuring displacement. Its elementary principle is based on the resistance of a conductor.

- Potentiometers are used in process feedback control systems, automobile throttle controls, elevators, robotics, wood working machinery, printers, liquid level assemblies, etc.  

- The Linear Variable Differential Transformer (LVDT) and Rotary Variable Differential Transformer (RVDT) are another type of transducers that could be used for measuring displacement/position. A LVDT measures linear displacement while a RVDT measures angular displacement. 

- Capacitive devices can be used to measure angular or linear displacement by measuring a change in capacitance through a change in distance between two plates. 

 # MOTION/SPEED (VELOCITY, ACCELERATION) DETECTORS 

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Acceleration is also a dynamic characteristic of an object, since it effectively requires the application of a force (F = Ma). Consequently, velocity, acceleration, position and displacement are closely relate

- A velocity sensor utilizes the principle of magnetic induction and is usually designed very similar to the LVDT. 

- A Tachogenerator utilizes the principle of variable reluctance and can be used to measure angular velocity/acceleration. Its basic structure is made up of a toothed wheel and a wound coil on a ferromagnetic material. 

- Accelerometers sense speed changes by measuring the force produced by the change in the velocity of a known mass (seismic mass). 

# PRESSURE SENSORS 

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Pressure can be defined as a “force” when dealing with fluids (measured when force is distributed over a relatively large area). When we say fluid, we mean something that can flow; gasses and liquids, for a fluid at rest, pressure can be defined as the force exerted perpendicularly on a unit 

- A Capsule is constructed by the combination of two corrugated diaphragms.  A diaphragm is a flexible membrane separating two chambers, bent into regular curved folds/grooves which is fixed around its periphery that expands into one or other chamber as the difference in the pressure in the chambers varies.

- Corrugated designs helps to provide a linear deflection and also increase the member strength. A diaphragm can be used to measure pressure by employing it in a diaphragm pressure gauge. Pressure is measured by the deflection of the diaphragm when subjected to a force.  


## Thanks for joining us today, if this post was helpful, do well to share with other steemians.


# REFERENCES
- Dunn, William C. “Introduction to instrumentation, sensors, and process control”

- Jacob Fraden, “Handbook of modern sensors: physics, designs, and applications / 3rd ed.”

- Kato, H., Kojima, M., Gattoh, M., Okumura,Y., and Morinaga, S. “Photoelectric inclination sensor and its application to the measurement of the shapes of 3-D objects.” IEEE Trans. Instrum. Meas. 40(6), 1021–1026, 1991.

- Barker, M. J. and Colclough, M. S. “A two-dimensional capacitive position transducer with rotation output.” Rev. Sci. Instrum., 68(8), 3238–3240, 1997.

- Peters, R.D. U.S. Patent 5,461,319, 1995. - De Silva, C. W. “Control Sensors and Actuators,” Prentice-Hall, Englewood Cliffs, NJ, 1989.

- Efedua J. Eziashi. “Electronic Instrumentation Principles”, 2004. - Clifford,M. A., “Accelerometers Jump into the Consumer Goods Market, ”Sensors Magazine, Vol. 21, No. 8, August 2004.

- Massa, D. P., “Choosing an Ultrasonic Sensor for Proximity or Distance Measurement,” Sensors Magazine, Vol. 16, No. 2, February 1999.

- Young, W. C., Roark’s Formulas for Stress and Strain, 6th ed., McGraw-Hill.

- Shigley, J., Mechanical Engineering Design, McGraw-Hill, 1963, pp. 284–289.

- Nagy, M. L., C. Spanius, and J. W. Siekkinen, “A User Friendly High Sensitivity Strain Gauge,” Sensors Magazine, Vol. 18, No. 6, June 2001.