Lecture 8 - Sensors Flashcards
What is a sensor?
- A sensor is a device that detects and responds to some type of input from the physical environment.
- The input can be light, heat, motion, moisture, pressure or any number of other environmental phenomena.
- # The output is generally a signal that is converted to a human-readable display at the sensor location or transmitted electronically over a network for reading or further processing.Can also be known as:
A device which provides a usable output in response to a specified measurand.
A sensor acquires a physical quantity and converts it into a signal suitable for processing (e.g. optical, electrical, mechanical).
Nowadays common sensors convert measurement of physical phenomena into an electrical signal
What is a transducer?
Active element of a sensor is called a transducer.
A device which converts one form of energy to another.
Sensors detect by receiving a signal from a transducer, then responding to that signal by converting it into an output that can easily be read and understood.
The output can be analog or digital.
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A transducer converts one form of energy to another, but does not quantify the conversion, e.g. a light bulb converts electrical energy into light and heat; however, it does not quantify how much light or heat. Similarly, battery converts chemical energy into electrical energy.
If the purpose of a device is to quantify an energy level, it is a sensor.
When input is a physical quantity and output electrical reading → Sensor
When input is electrical and output a physical quantity → Actuator
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Transducer can be further classified into Sensors, which monitors a system and Actuators, which impose an action on the system.
What are some real life examples?
A thermometer senses and converts temperature into a readable output, thus it is
a sensor. This output can be direct or indirect.
A mercury thermometer that uses a level of mercury against a fixed scale is a direct output.
A digital thermometer is an indirect output using a thermocouple and then quantification.
Perhaps one of the first sensors used by man was a compass, which is essentially a magnetic sensor. The compass needle, which is steel, aligns itself with the earth’s magnetic field lines and points toward the north.
How are sensors categorised?
Classification based on physical phenomena
– Mechanical: strain gage, displacement, velocity, accelerometer, tilt meter, viscometer, pressure, etc.
– Thermal: thermo-couple measuring temperature, heat
– Optical: camera, infrared sensor
– Electro-magnetics: voltage, current, frequency phase
– Chemical quantities: moisture, pH value
– Others …
Classification based on measuring mechanism
– Resistance sensing, capacitance sensing, inductance sensing, piezoelectricity, etc.
What are the conversion methods of sensors?
*Physical
–thermo-electric, thermo-elastic, thermo-magnetic, thermo-optic
–photo-electric, photo-elastic, photo-magnetic,
–electro-elastic, electro-magnetic
–magneto-electric
*Chemical
–chemical transport, physical transformation, electro-chemical
*Biological
–biological transformation, physical transformation
What are the measuring quantities?
Acoustic - Wave (amplitude, phase, polarization), Spectrum, Wave Velocity
Biological & Chemical - Fluid Concentrations (Gas or Liquid)
Electric - Charge, Voltage, Current, Electric Field (amplitude, phase, polarization), Conductivity, Permittivity
Magnetic - Magnetic Field (amplitude, phase, polarization), Flux,Permeability
Optical - Refractive Index, Reflectivity, Absorption
Thermal -Temperature, Flux, Specific Heat, Thermal Conductivity
Mechanical - Position, Velocity, Acceleration, Force, Strain, Stress, Pressure, Torque
What are some physical principles examples?
Amperes’s Law
– A current carrying conductor in a magnetic field experiences a force (e.g. galvanometer)
* Curie-Weiss Law
– There is a transition temperature at which ferromagnetic materials exhibit paramagnetic behavior
* Faraday’s Law of Induction
– A coil resist a change in magnetic field by generating an opposing voltage/current (e.g. transformer)
* Photoconductive Effect
– When light strikes certain semiconductor materials, the resistance of the material decreases (e.g. photoresistor)
What are the 4 types of sensors?
- Active
– send signal into environment and measure interaction of signal with environment/ target
– e.g. radar, sonar - Passive
– record signals already present in environment
– e.g. video cameras
– A passive infrared sensor (PIR motion sensor) is an electronic sensor that measures infrared (IR) light radiating from objects in its field of view. - Analog
The output of these sensors is analog and to use with digital hardware an A/D converter is required. Example, the temperature sensor (specifically LM35) is an analog sensor. - Digital
The output of these sensors is digital data that you can read via serial or parallel communication buses (as UART, SPI, I2C, etc). The typical format for the data is demonstrated exactly in the sensor’s datasheet.
What are the 8 specifications of sensors?
- Accuracy: Error between the result of a measurement and the true value being measured.
- Resolution: The smallest increment of measure that a device can make.
- Sensitivity: The ratio between the change in the output signal to a small change in input physical signal. Slope of the input-output fit line.
- Repeatability/Precision: The ability of the sensor to output the same value for the same input over several trials.
- Dynamic Range: the ratio of maximum recordable input amplitude to minimum input amplitude, i.e. D.R. = 20 log (Max. Input Ampl./Min. Input Ampl.) dB
- Linearity: the deviation of the output from a best-fit straight line for a given range of the sensor
- Transfer Function (Frequency Response): The relationship between physical input signal and electrical output signal, which may constitute a complete description of the sensor characteristics.
- Bandwidth: the frequency range between the lower and upper cutoff frequencies, within which the sensor transfer function is constant gain or linear.
- Noise: Random fluctuation in the value of input that causes random fluctuation in the output value.
What is accuracy?
- Accuracy is the capacity of a measuring instrument to give RESULTS close to the TRUE VALUE of the measured quantity
- Accuracy is measured by the absolute and relative errors:
Absolute Error = Result – True Value
Relative Error = Absolute Error / True Value
What are the 8 attributes of sensors?
- Operating Principle: Embedded technologies that make sensors function, such as electro-optics, electromagnetic, piezoelectricity, active and passive ultraviolet.
- Dimension of Variables: The number of dimensions of physical variables.
- Size: The physical volume of sensors.
- Data Format: The measuring feature of data in time; continuous or discrete/analog or digital.
- Intelligence: Capabilities of on-board data processing and decision- making.
- Active versus Passive Sensors: Capability of generating vs. just receiving signals.
- Physical Contact: The way sensors observe the disturbance in environment.
- Environmental durability: The sensor robustness under its operation conditions
Why do we need to calibrate sensors?
- The relationship between the physical measurement variable(X) and the signal variable (S) may need calibration.
- A sensor or instrument is calibrated by applying a number of KNOWN physical inputs and recording the response of the system.
Addtional inputs - What is interfering inputs (Y)?
Those that the sensor to respond as the linear superposition with the measurand variable X
Linear superposition assumption: S(aX+bY)=aS(X)+bS(Y)
Addtional inputs - What is modifying inputs (Z)?
Those that change the behaviour of the sensor and, hence, the calibration curve, eg Temperature is a typical modifying input
What is MEMS technology?
What is MEMS?
– Acronym for Microelectromechanical Systems
– “MEMS is the name given to the practice of making and combining miniaturized mechanical and electrical components.”
Synonym to:
– Micromachines (in Japan)
– Microsystems technology (in Europe)
Leverage on existing IC-based fabrication techniques (but now extend to other non IC techniques)
– Potential for low cost through batch fabrication
– Thousands of MEMS devices (scale from ~ 0.2 µm to 1 mm) could be made simultaneously on a single silicon wafer
* Co-location of sensing, computing, actuating, control, communication & power on a
small chip-size device
* High spatial functionality and fast response speed
– Very high precision in manufacture
– miniaturized components improve response speed and reduce power consumption
