Sensor static
The static characteristics of a sensor refer to the correlation between the output and input of the sensor for the static input signal. Since the input quantity and the output quantity are independent of time, the relationship between them, i.e., the static characteristics of the sensor, can be described by an algebraic equation without time variables, or by drawing a characteristic curve with the input quantity as the abscissa and the corresponding output quantity as the ordinate. The main parameters that characterize the static characteristics of the sensor are: linearity, sensitivity, hysteresis, repeatability, drift, etc.
1. Linearity: Refers to the degree to which the actual relationship curve between the output and input of the sensor deviates from the fitting straight line. Defined as the ratio of the maximum deviation between the actual characteristic curve and the fitted line over the full scale range to the full-scale output.
2. Sensitivity: Sensitivity is an important indicator of the static characteristics of a sensor. It is defined as the ratio of the increment of the output to the corresponding increment of the input that caused the increment. S is used to indicate sensitivity.
3. Hysteresis: The phenomenon that the input and output characteristic curves of the sensor do not coincide during the change of input quantity from small to large (positive stroke) and input quantity from large to small (reverse stroke). For input signals of the same size, the output signals of the forward and reverse strokes of the sensor are not equal, and this difference is called the hysteresis difference.
4. Repeatability: Repeatability refers to the degree to which the characteristic curves obtained by the sensor are inconsistent when the input quantity changes the full scale in the same direction for several times.
5. Drift: Drift of the sensor refers to the change of the output of the sensor over time while the input amount remains the same, which is called drift. There are two reasons for drift: one is the structural parameters of the sensor itself; The second is the surrounding environment (such as temperature, humidity, etc.).
6. Resolution: When the input of the sensor slowly increases from a non-zero value, the output changes observably after exceeding a certain increment, and this input increment is called the resolution of the sensor, that is, the minimum input increment.
7. Threshold: When the input of the sensor slowly increases from a zero value, the output changes observably after reaching a certain value, and this input value is called the threshold voltage of the sensor.
Sensor dynamics
The so-called dynamic characteristics refer to the characteristics of the output of the sensor when the input changes. In practice, the dynamic characteristics of a sensor are often expressed in terms of its response to some standard input signal. This is because the sensor's response to the standard input signal is easy to find experimentally, and there is a certain relationship between its response to the standard input signal and its response to any input signal. The most commonly used standard input signals are step signal and sinusoidal signal, so the dynamic characteristics of the sensor are also expressed in step response and frequency response.
Linearity
Typically, the actual static characteristic output of the sensor is a curve rather than a straight line. In practice, in order to make the meter have a uniform scale reading, a fitting straight line is often used to approximate the actual characteristic curve, and the linearity (non-linearity error) is a performance index of this approximation.
There are several ways to select a fitting line. For example, the theoretical straight line connected by the zero input and the full-scale output point is used as the fitting line; Or the theoretical straight line with the smallest sum of squares of the deviations from each point on the characteristic curve is used as the fitting line, and this fitting line is called the least squares fitting line.
Sensitivity
Sensitivity refers to the ratio of the change in output △y to the change in input amount △x in the steady-state operation of the sensor.
It is the slope of the output one input characteristic curve. If there is a linear relationship between the output and input of the sensor, then the sensitivity S is a constant. Otherwise, it will vary with the amount of input.
The dimension of sensitivity is the ratio of the dimension of the output to the input quantity. For example, if the output voltage of a displacement sensor changes by 1 mm and the output voltage changes by 200 mV, 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 the magnification.
The increased sensitivity results in high measurement accuracy. However, the higher the sensitivity, the narrower the measurement range and the worse the stability.
Resolution
Resolution refers to the ability of a sensor to perceive the smallest change being measured. 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 cannot distinguish the change of this input. Its output changes only when the input changes more than the resolution.
Generally, the resolution of the sensor is not the same at each point in the full-scale range, so the maximum change in the input quantity that can cause the output to change in step in the full-scale range is often used as a measure of resolution. These metrics are referred to as resolutions as a percentage of full scale. Resolution is negatively correlated with the stability of the sensor.