What is a sensor
Sensors are a device that measures the physical quantity or produces a signal relating to the quantity being measured and the physical quantities are temperature, pressure, light, current, weight… it represents the part of the interface between the physical world and the world of electrical devices, such as computers
Types of sensors
- Motion sensor
- Chemical sensor
- Magnetic sensor
- Smart sensor
- Gas sensor
- Temperature sensor
- Vibration sensor
- Position sensor
- Light sensor
- Velocity sensor
- Flow sensor
- Proximity/range sensor
- Tactile sensor
- Vision system
What are the performance characteristics of a sensor
Transfer function – it shows the functional relationship between the physical input signal and electric output signal
Sensitivity – it is the relationship between the input physical signal and output electrical signal it can be described as the ratio between the small change in electrical signal to a small change in the physical signal
Dynamic range – it is the range of input physical signal that is converted into electrical signals by the sensor and the signals outside of this range could cause inaccuracy
Uncertainty – it is the largest expected error between the actual and measured output signals
Precision – it can be considered as the reliability of the instrument. The ability of an instrument to reproduce a certain set of readings within a given accuracy
Stability – it is the ability of a sensor to give the same output when used to measure a constant input over a period of time.
Repeatability – the ability of the sensor to give the same output for repeated applications of the same input value
Classification of sensors
- Based on signal characteristics – analog and digital
- Based on power supply – active and passive
- It is also classified according to the subject of measurement - acoustic, biological, chemical, electric, mechanical, optical, radiation, thermal…
Its output will be proportional to the variable being measured and the output changes in a continuous way.
Its output will be in digital form, a sequence of on/off signals spells out a number whose value is related to the size of the variable being measured. They are known for their accuracy and precision and does not need any converters when interfaced with a computer monitoring system.
This type of sensor needs an external source of excitation, resistor based sensors such as thermistors, RTD, and strain gauges are an example of active sensors, current will be passed through them and the resistance value is determined by measuring the corresponding voltage. It is a modulator so it can deliver more energy to the next stage than it draws from the measurand.
The passive sensor doesn’t need an excitation system and it generates its own electrical output signal. Thermocouples and photodiodes which generates thermoelectric voltages and photocurrent are examples of passive sensors. Passive sensors are also called as self-generating sensors.
Position sensors are used to detect the position of an object with reference to some reference point. Potentiometers and encoders are examples of position sensors. While selecting position sensors we must consider certain things like the size of the displacement, linear or angular, the material of the measuring object. Encoders are widely used as position detectors. Potentiometers can be used for linear or rotary displacements, by converting displacement into the potential difference.
Tacho-generator is a velocity sensor it is an electromechanical generator which is capable of producing electrical power from mechanical energy mostly by the turning of a generator. Tacho-generators are mostly used to measure the speed of electric motors and engines
Photodiodes are light sensors which are a p-n junction diode connected in reverse bias conditions. Reverse bias junction has high resistance. Incoming lights can excite the electrons which are bounded in the crystal lattice and it can generate free electron-hole pairs in the junction, because of this the resistance drops and current increases proportional to the intensity of the light. A phototransistor is another type of light sensor when light falls on the collector-base junction reducing the junction resistance and rises the collector current which is sufficient enough for detection its capability to convert the light energy to electrical energy is called as quantum efficiency.
Flow sensors are devices which can measure the fluid flow without any discrimination in the nature of the fluid and transforms it into signals. Ultrasonic sensors are an example for flow sensor it will measure fluid velocity by passing high-frequency sound waves through the fluid. its operation is by measuring transmission time difference of an ultrasonic beam passed through a homogeneous fluid contained in a pipe both upstream and downstream location
The proximity switch is a non-contact switch is a proximity sensor which works on the principle of inducing changes in an electromagnetic field. Proximity sensors can detect an object which is close to it without any physical contact. Most commonly used proximity switches are inductive proximity switch and capacitive proximity switch.
These sensors can measure the parameters of contact between the sensor and an object. Types of tactile sensors are optical, piezoelectric, resistive, capacitive and magnetic.
Machine vision is the acquisition of the image data followed by the processing and interpretation of these data by a computer for some useful application. The operation of the machine vision is divided into three they are Image acquisition and digitization, image processing and analysis, interpretation