PRIMARY SENSOR. Characteristic and types. Definition of a sensor. Def. 1. (Oxford dictionary) A device giving a signal for the detection or measurement of a physical property to which it responds. Def. 2.
Characteristic and types
Attributes which can be used to classify sensors:
. working principle
. properties (attributes of the characteristic)
Heisenberg (1927): ”The momentum and position of a particle can not both be precisely determined at the same time.”
Measuring activity disturbs the physical process (loading effect).
That is the difference between the measured value and the true value.
error = measured value - true value
They are repeated at every measurement, e.g. reading offset or bias. Such
errors can be corrected by calibration.
They are caused by several parameters and change in time in an
unpredictable fashion. They can be quantified by mean errors, standard
Measurements with small deviation
Measurements with small errors, i.e. small bias and high precision.
A sensor should represent a physical variable as fast and as accurately as possible.
A sensor is represented by its characteristic.
Ideally, the sensor characteristic is a straight line
Accuracy : Error measurement
Sensitivity: change in output for unit change in input
Resolution: the smallest change in the signal that can be detected and accurately indicated by a sensor.
Linearity: the closeness of the calibration curve to a straight line.
Drift: the deviation from the null reading of the sensor when the value is kept constant for a long time.
Hysteresis: the indicated value depends on direction of the test (increasing and decreasing)
Repeatability (precision): the maximum deviation from the average of repeated measurements of the same static variable.
Dynamic Characteristics: A sensor may have some transient characteristic. The sensor can be tested by a step response where the sensor output is recorded for a sudden change of the physical variable.
The rise time, delay time, peak time, settling time, percentage overshoot should be as small as possible.
V01= K vref sin θ
V02= K vref sin θ
Strain (ε) = ΔL/L
1 μstrain = 106 strain
GF = (ΔR/R)/(ΔL/L)
Furthermore, a permanently-polarized material such as quartz (SiO2) or barium titanate (BaTiO3) will produce an electric field when the material changes dimensions as a result of an imposed mechanical force.
These materials are piezoelectric, and this phenomenon is known as the piezoelectric effect.