The technical indicators of the sensor are divided into two categories: static indicators and dynamic indicators: static indicators mainly evaluate the performance of the sensor under the static and constant conditions under test, including resolution, repeatability, sensitivity, linearity, return error, threshold, creep, Stability, etc.; dynamic indicators mainly investigate the performance of the sensor being measured under rapidly changing conditions, including frequency response and step response.
1. Resolution and resolution #
Definition: Resolution (ResoluTIon) refers to the smallest change in the measurand that the sensor can detect. Resolution (ResoluTIon) refers to the ratio of resolution to full scale value.
Interpretation 1: Resolution is the most basic indicator of a sensor, which characterizes the sensor’s ability to resolve the measurand. Other technical indicators of the sensor are described in terms of resolution as the smallest unit.
For sensors and instruments with digital display function, the resolution determines the minimum number of digits displayed in the measurement result. For example: the resolution of the electronic digital caliper is 0.01mm, and its indicating error is ±0.02mm.
Interpretation 2: Resolution is an absolute value with units. For example, the resolution of a temperature sensor is 0.1°C, and the resolution of an acceleration sensor is 0.1g.
Interpretation 3: Resolution is a concept related to and very similar to resolution, which both characterize the ability of the sensor to resolve the measurand.
The main difference between the two is that the resolution is a percentage of the resolution capability of the sensor, which is a relative number and has no dimension. For example, the resolution of the above temperature sensor is 0.1°C and the full scale is 500°C, then its resolution is 0.1/500=0.02%.
2. Repeatability #
Definition: The repeatability of a sensor refers to the degree of difference between the measurement results when repeated measurements are performed for the same measurand along the same direction under the same conditions. Also known as repetition error, reproduction error, etc.
Interpretation 1: The repeatability of a sensor must be the degree of difference between multiple measurements obtained under the same conditions. If the measurement conditions change, the comparability between measurement results disappears and cannot be used as a basis for assessing repeatability.
Interpretation 2: The repeatability of the sensor characterizes the dispersion and randomness of the sensor measurement results. The reason for this kind of dispersion and randomness is that there are inevitably various random disturbances inside and outside the sensor, resulting in the final measurement result of the sensor showing the characteristics of random variables.
Interpretation 3: The quantitative expression method of repeatability can use the standard deviation of random variables.
Interpretation 4: In the case of repeated measurements, if the average of all measurement results is used as the final measurement result, higher measurement accuracy can be obtained. Because the standard deviation of the mean is significantly less than the standard deviation of each measurement.
3. Linearity #
Definition: Linearity refers to the degree of deviation of the sensor input and output curve from the ideal straight line.
Interpretation 1: The ideal sensor input-output relationship should be linear, and its input-output curve should be a straight line (the red line in the figure below).
However, the actual sensor has more or less various errors, resulting in the actual input and output curve is not an ideal straight line, but a curve (the green curve in the figure below).
Linearity is the degree of difference between the actual characteristic curve of the sensor and the off-line straight line, also called nonlinearity or nonlinearity error.
Interpretation 2: Because the difference between the actual characteristic curve of the sensor and the ideal straight line is different under the measured conditions of different sizes, it is often the ratio of the maximum difference between the two in the full scale range to the full scale value. Obviously, linearity is also a relative quantity.
Interpretation 3: For general measurement occasions, the ideal straight line of the sensor is unknown and cannot be obtained. For this reason, a compromise method is often adopted, that is, a fitting straight line that is closer to the ideal straight line is calculated directly by using the measurement results of the sensor. The specific calculation methods include the method of connecting the endpoints, the best straight line method, the least square method and so on.
4. Stability #
Definition: Stability is the ability of a sensor to maintain its performance over a period of time.
Interpretation 1: Stability is the main indicator to examine whether the sensor works stably within a certain time range. The factors that lead to sensor instability mainly include factors such as temperature drift and internal stress release. Therefore, it is helpful to increase the stability by adding measures such as temperature compensation and aging treatment.
Interpretation 2: According to the length of the time period, stability can be divided into short-term stability and long-term stability. When the investigation time is too short, the stability is close to the repeatability. Therefore, the stability index mainly examines the long-term stability. The specific time is determined according to the usage environment and requirements.
Interpretation 3: The quantitative expression method of the stability index can use either absolute error or relative error. For example, the stability of a strain gauge force sensor is 0.02%/12h.
5. Sampling frequency #
Definition: The sampling rate refers to the number of measurement results that the sensor can sample per unit time.
Interpretation 1: The sampling frequency reflects the rapid response capability of the sensor, and is the most important one of the dynamic characteristic indicators. For occasions where the measured changes rapidly, the sampling frequency is one of the technical indicators that must be fully considered. According to Shannon’s sampling law, the sampling frequency of the sensor should not be lower than twice the frequency of the measured change.
Interpretation 2: As the frequency of use is different, the accuracy index of the sensor also changes accordingly. In general, the higher the sampling frequency, the lower the measurement accuracy.
The highest accuracy given by the sensor is often the measurement obtained at the lowest sampling speed or even under static conditions. Therefore, two indicators of accuracy and speed must be taken into account when selecting sensors.
