At present, there is no unified classification method for sensors, but the following three are commonly used:
1. According to the physical quantity of the sensor, it can be divided into sensors such as displacement, force, speed, temperature, flow rate, gas composition and so on.
2. According to the working principle of the sensor, it can be divided into resistance, capacitance, inductance, voltage, Hall, photoelectric, grating thermocouple and other sensors.
3. According to the nature of the output signal of the sensor, it can be divided into: switching sensors with outputs of switching values â€‹â€‹("1" and "0" or "on" and "off"); outputs with analog sensors; outputs with pulse or Coded digital sensor.
Static characteristics of the sensor
The static characteristics of the sensor refer to the static input signal, and there is a mutual relationship between the output of the sensor and the input. Because the input and output are not related to time at this time, the relationship between them, that is, the static characteristics of the sensor can be algebraic equations without time variables, or the input is used as the abscissa, and the corresponding output is used as Describe the characteristic curve drawn on the ordinate. The main parameters that characterize the static characteristics of the sensor are: linearity, sensitivity, resolution and hysteresis.
The so-called dynamic characteristics refer to the characteristics of the sensor's output when its input changes. In actual work, the dynamic characteristics of the sensor are often expressed by its response to certain standard input signals. This is because the response of the sensor 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, and the latter can often be estimated 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 also commonly expressed by step response and frequency response.
Normally, the actual static characteristic output of the sensor is a curve rather than a straight line. In actual work, 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 linearity (non-linear error) is a performance index of this approximation. There are many ways to select the fitted straight line. If the theoretical straight line connecting the zero input and the full-scale output point is used as the fitted straight line; or the theoretical straight line with the smallest sum of square deviations from each point on the characteristic curve is used as the fitted straight line, this fitted straight line is called the least square Close the line.
Sensitivity refers to the ratio of the output change â–³ y to the input change â–³ 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 change with the input quantity.
The dimension of sensitivity is the ratio of the dimension of output and input. For example, for a displacement sensor, when the displacement changes by 1mm, the output voltage changes to 200mV, then its sensitivity should be expressed as 200mV / mm.
When the dimensions of the sensor output and input are the same, the sensitivity can be understood as the magnification.
Increase the sensitivity, you can get a higher measurement accuracy. But the higher the sensitivity, the narrower the measurement range and the worse the stability.
Resolution refers to the ability of the sensor to sense the smallest change that can be measured. That is, if the input value 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 in the input amount. Only when the input changes beyond the resolution, the output will change.
Generally, the resolution of each point of the sensor in the full-scale range is not the same, so the maximum change in the input value that can cause a step change in the output value in the full-scale range is commonly used as an index to measure the resolution. If the above index is expressed as a percentage of full scale, it is called resolution.
A resistive sensor is a device that converts physical quantities such as displacement, deformation, force, acceleration, humidity, temperature, etc. into resistance values. There are resistive sensor devices such as resistance strain type, piezoresistive type, thermal resistance, thermal, gas, and humidity.
Resistance strain sensor
The resistance strain gauge in the sensor has the effect of metal strain, that is, mechanical deformation under the action of external force, so that the resistance value changes accordingly. There are two types of resistance strain gauges: metal and semiconductor. Metal strain gauges are divided into wire type, foil type, and film type. Semiconductor strain gauges have the advantages of high sensitivity (usually dozens of times of wire and foil) and small lateral effects.
A piezoresistive sensor is a device made by spreading resistance on a substrate of semiconductor material according to the piezoresistive effect of semiconductor material. The substrate can be directly used as a measurement sensor element, and the diffusion resistance is connected in the form of an electric bridge in the substrate. When the substrate is deformed by external force, the resistance value will change, and the bridge will produce a corresponding unbalanced output.
The substrate (or diaphragm) materials used as piezoresistive sensors are mainly silicon wafers and germanium wafers. Silicon piezoresistive sensors made of silicon wafers as sensitive materials are receiving more and more attention, especially for measuring pressure And speed solid-state piezoresistive sensors are most commonly used.
Thermal resistance sensor
Thermal resistance sensors mainly use the characteristic that the resistance value changes with temperature to measure temperature and temperature-related parameters. This kind of sensor is more suitable in the occasions where the temperature detection accuracy is relatively high. At present, the more extensive thermal resistance materials are platinum, copper, nickel, etc. They have the characteristics of large temperature coefficient of resistance, good linearity, stable performance, wide use temperature range, and easy processing. Used to measure the temperature in the range of -200 â„ƒ ~ +500 â„ƒ.
The hysteresis characteristic characterizes the output of the sensor between the forward (increasing input) and reverse (decreasing input) strokes-the degree to which an input characteristic curve is inconsistent, usually the maximum difference between the two curves The percentage of the range output FS indicates that the hysteresis can be caused by the absorption of energy present in the sensor's internal components.
Selection of sensors
Sensors vary widely, and even sensors of different working principles can be used for the same amount of measurement. Therefore, the most suitable sensor should be selected as needed.
(1) Measurement conditions
If the sensor is selected by mistake, it will reduce the reliability of the system. For this reason, the overall consideration of the system should be used to clarify the purpose of use and the necessity of using sensors. Never use unsuitable sensors or unnecessary sensors. The measurement conditions are listed as follows, namely the measurement purpose, the selection of the measurement quantity, the measurement range, the bandwidth of the input signal, the required accuracy, the time required for the measurement, and the frequency of occurrence of the input.
(2) Sensor performance
When selecting a sensor, the following properties of the sensor must be considered, namely accuracy, stability, response speed, analog or digital signal, output and its level, the influence of the characteristics of the measured object, calibration cycle, over-input protection.
(3) Operating conditions of the sensor
The operating conditions of the sensor are the setting place, environment (humidity, temperature, vibration, etc.), the measured time, the signal transmission distance between the display, the connection method with the peripherals, and the power supply capacity.
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