Static Characteristics of Sensors
The static characteristic of the sensor refers to the relationship between the output and the input of the sensor for the static input signal. Because both input and output are independent of time at this time, the relationship between them is that the static characteristics of the sensor can be described by an algebraic equation without time variables, or by using input as abscissa and output as longitudinal coordinates. The main parameters that characterize the static characteristics of the sensor are linearity, sensitivity, hysteresis, repeatability, drift and so on.
(1) Linearity: refers to the degree to which the actual relationship curve between sensor output and input deviates from the fitting line. It is defined as the ratio of the maximum deviation between the actual characteristic curve and the fitting straight line in the full range to the output value of the full range.
(2) Sensitivity: Sensitivity is an important indicator of static characteristics of sensors. It is defined as the ratio of the increment of output to the corresponding increment of input that causes the increment. Sensitivity is expressed by S.
(3) Hysteresis: The phenomenon that the input-output characteristic curve does not coincide with the output characteristic curve becomes hysteresis when the input of the sensor changes from small to large (positive stroke) and from large to small (reverse stroke). For the input signal of the same size, the positive and negative stroke output signals of the sensor are different in size. This difference is called hysteresis difference.
(4) Repeatability: Repeatability refers to the degree of inconsistency in the characteristic curve of the sensor when the input varies continuously and repeatedly over the whole range in the same direction.
(5) Drift: Sensor drift refers to the change of sensor output over time when the input is constant. This phenomenon is called drift. There are two reasons for the drift: one is the sensor’s own structural parameters; the other is the surrounding environment (such as temperature, humidity, etc.).
Dynamic characteristics of sensors
The so-called dynamic characteristics refer to the output characteristics of the sensor when the input changes. In practical work, the dynamic characteristics of the sensor are often expressed by its response to some standard input signals. This is because the response of the sensor to the standard input signal can be easily obtained by experimental method, and there is a certain relationship between the response of the sensor to the standard input signal and its response to any input signal. The latter can be inferred by knowing the former. The most commonly used standard input signals are step signal and sinusoidal signal, so the dynamic characteristics of the sensor are often expressed by step response and frequency response.
Usually, the actual static characteristic output of the sensor is a curve rather than a straight line. In practice, in order to make the instrument have uniform calibration reading, a fitting straight line is often used to approximate the actual characteristic curve. Linearity (non-linear error) is a performance index of this approximation degree.
There are many ways to select the fitting line. If the theoretical straight line connected with zero input and full range output points is used as the fitting line, or the theoretical straight line with the least square deviation of each point on the characteristic curve is used as the fitting line, the fitting line is called the least square fitting line.
Sensitivity of Sensors
Sensitivity refers to the ratio of output change (y) to input change (x) of the sensor under steady-state operation.
It is the slope of the output-input characteristic curve. If there is a linear relationship between the output and input of the sensor, the sensitivity S is a constant. Otherwise, it will vary with the input.
The dimension of sensitivity is the dimension ratio of output to input. For example, if the output voltage of a displacement sensor changes to 200 mV when the displacement changes 1 mm, its sensitivity should be expressed as 200 mV/mm.
When the output and input dimensions of the sensor are the same, the sensitivity can be understood as an amplification factor.
Higher measurement accuracy can be obtained by improving sensitivity. However, the higher the sensitivity, the narrower the measurement range and the worse the stability.
Resolution of sensor
Resolution refers to the ability of the sensor to sense the smallest change in the measured value. That is, if the input changes slowly from a non-zero value. When the input change value does not exceed a certain value, the output of the sensor will not change, that is, the sensor can not distinguish the change of the input. Only when the change of input exceeds the resolution will the output change.
Generally, the resolution of the sensor varies from point to point in the full range, so the maximum change value of the input which can make the output step change in the full range is often used as the index to measure the resolution. If the above indicators are expressed as percentage of full range, they are called resolution. The resolution is negatively correlated with the stability of the sensor.