Sensor - Intro, working mechanism, latest Flashcards
Sensors Definition
A sensor transforms a stimulus into a measurable signal, which can subsequently be converted into an observable output for further processing.
Sensors System Design
Sensor Classifications, 4 types? and examples of each
- Mechanical: strain gage, displacement, accelerometer, force, pressure, etc
- Thermal: thermal couple
- Optical: camera, infrared sensor
- Chemical: moisture sensor, pH sensor
Resistor sensor equation
Capacitance sensor equation
Inductance sensor equation
Classification based on sensor placement 2 types?
- Contact sensors.
- Non-contact sensors
Passive sensor definition
Passive sensor: response to its environment so does not require a power source
Active sensor definition
Active sensor: requires a power source to emit a signal that is scatter back to the sensor
What are the Sensor Specifications, 10 types?
- Sensitivity
- Linearity
- Hysteresis
- Environmental Effects
- Drift
- Accuracy
- Precision (Repeatability)
- Threshold
- Range
- Resolution
Calibration
The calibration procedure involves the comparison of the output from the sensor with a known measurement system.
Sensitivity
- Sensitivity is a measure of the change in output of a sensor for a unit change in input.
- Sensitivity can be obtained from the gradient of a graph of output as a function of input
Linearity
A sensor is linear if the output is linearly proportional to the input i.e. corresponding values of input and output lie on a straight line and sensitivity is constant across the input range
* This is considered desirable!
* The sensitivity of a non-linear sensor varies across part or all of the input range
Hysteresis
For a given input, the output from the sensor varies depending on whether the input stimulus is increasing or decreasing at the time
* Hysteresis is normally expressed as a percentage of the full scale output reading
Environmental Effects? 3 things
Sensitivity drift defines the amount by which the sensitivity of a sensor varies with environmental conditions.
In general, the output of a sensor depends not only on the input but on environmental conditions, such as
* Ambient temperature
* Atmospheric pressure
* Relative humidity etc.
Accuracy
Precision (Repeatability)
Accuracy is a measure of how close the measured value (the output from the system) is to the ‘true’ value (the input)
Repeatability is a measure of the similarity of output values when the same input is applied repetitively (it is NOT an indication of accuracy!)
What are the two types of Measurement errors?
System errors: imperfect design of the measurement setup and the approximation.
Random errors: variations due to uncontrolled variables. Can be reduced by averaging.
Threshold
Range or span
Resolution
- Threshold is the smallest input that will produce a detectable change in the output of a sensor
- The range of a sensor defines the minimum and maximum values of a quantity that the sensor is designed to measure
- Resolution is the smallest change in input that will produce a detectable change in the output of a sensor at a particular point in its active range
Temperature Sensors - three mechanisms to gain or lose heat
- Conduction - The transfer of heat within a substance, or between substances through molecular interaction
- Convection - Heat transfer within a fluid by the movement of heated molecules from one place to another
- Radiation – heat transfer in the form of electromagnetic waves, generally in the infra-red region of the electromagnetic spectrum
3 types of Contact Temperature Sensors?
Thermocouples - It utilise the thermoelectric effect where a small voltage is generated at the join between two metals. It have quick response times, and they can operate over the widest temperature range.
Thermistor - It contain a material in which a large changes in electrical resistance is produced by a small changes in temperature (High accuracy).
Resistance thermometer - RTDs materials have a positive temperature coefficient (the resistance of a metal increases with temperature). RTDs typically have higher thermal mass, and therefore they usually respond slower to temperature changes than thermocouple temperature sensors
Non-Contact Temperature Sensors
Thermographic camera - It contains a sensor capable of non- contact detecting infrared radiation emitted from a surface.
Contact Vs non-contact Temperature Sensor
What is a force sensor?
A Force sensor is defined as a transducer that converts an input mechanical load, weight, tension, compression or pressure into an electrical output signal.
Examples
* Piezoelectric force sensor
* Strain Gauge
* Resistive force sensor
Piezoelectric force sensor?
Strain Gauge?
- Piezoelectric materials generate an electrical potential (output voltage) when subjected to a force.
- The electrical resistance of a length of wire varies in direct proportion to the change in any strain applied to it.
