Today we are going to look at what sensors are and the parameters to look for when selecting a sensor.
A sensor is an equipment or item which is widely used in industries for various purposes.
Actually, a sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument.
For an example a thermometer shows its reading as per the expansion and contraction of mercury on the calibrated glass tube.
Relationship between the independent and dependent parameters in sensors can be graphed as,
Any physical property of a material can be used to produce a sensor if that property changes in response to some excitation.
Some of the widely used sensors can be listed as below,
• Resistive
• Inductive
• Capacitive
• Piezoelectric
• Photoresistive
• Elastic
• Thermal
When selecting a sensor there are certain parameters that should be checked so that the selected sensor will be able to deliver the requirement.
The parameters are listed as follows,
• Range
• Span
• Error
• Accuracy
• Sensitivity
• Hysteresis Error
• Non-Linear Error
• Repeatability/Reproducibility
• Stability
• Resolution
• Output Impedance
• Dead band/Time
Range
The range of a sensor defines the limits between which the input can vary.
Ex: (i) 0:100 C (ii) -50:100 C (iii) 100:1000 C
Span
The span is difference between maximum value of the range and the minimum value of the range.
Ex (i) 100 C (ii) 150 C (iii) 900 C
Error
Error is the difference between the result of the measurement and the true value of the quantity.
Error= measured value- true value
Accuracy
Accuracy is the extent to which the value indicated by a measurement system might be wrong. Accuracy express as a percentage of full range
output.
Sensitivity
The sensitivity is the relationship indicating how much output you get per unit input.
Hysteresis
Sensor can give different outputs from the same value of quantity being measured according whether that value has been reached by a continuously increasing change or a continuously decreasing change
Non- linearity error
For many sensors a linear relationship between the input and output is assumed over the working range but actually it is nonlinear thus Nonlinear error is defined as the maximum difference from the straight
line
Stability
The stability of a sensor is its ability to give the same output when used to measure a constant input over a period of time.
Resolution
The input varies continuously over the range, the output signal for some sensors may change in small steps.
The resolution is the smallest change in the input value that will produce an observable change in the output.
For an example the specification of a strain gauge pressure transducer/Sensor is as follows,
• Ranges : 70 to 1000kpa
• Supply voltage :10V dc
• Full range output: 40mV
• Non-linear and hysteresis :±0.5% full range output
• Temperature range: -54°C to +120 °C when operating
• Thermal zero shift : 0.030% full range output /°C
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