First, the definition of the sensor
The national standard GB7665-87 defines the sensor as: "A device or device that can be measured and converted into a usable signal according to a certain rule, usually composed of a sensitive component and a conversion component." The sensor is a detecting device that can sense the measured information and can transform the detected information into an electrical signal or other required form of information output according to a certain law to meet the information transmission, processing, storage, Requirements for display, recording, and control. It is the first step in achieving automatic detection and automatic control.
Second, the classification of sensors
At present, there is no unified classification method for sensors, but the following three types are commonly used:
1, according to the physical quantity of the sensor, can be divided into displacement, force, speed, temperature, flow, gas composition and other sensors
2, according to the working principle of the sensor, can be divided into resistance, capacitance, inductance, voltage, Hall, photoelectric, grating, thermocouple and other sensors.
3. According to the nature of the sensor output signal, it can be divided into: switch type sensor whose output is switch quantity ("1" and "0" or "on" and "off"); the output is analog type sensor; the output is pulse or The digital sensor of the code.
About the classification of sensors:
1. According to the measured physical quantity: such as: force, pressure, displacement, temperature, angle sensor, etc.;
2. According to the working principle of the sensor: such as: strain gauge sensor, piezoelectric sensor, piezoresistive sensor, inductive sensor, capacitive sensor, photoelectric sensor, etc.;
3. According to the way the sensor converts energy:
(1) Energy conversion type: such as: piezoelectric type, thermocouple, photoelectric sensor, etc.;
(2) Energy control type: such as: resistive, inductive, Hall-type sensors and thermistors, photoresistors, humidity-sensitive resistors, etc.;
4. According to the working mechanism of the sensor:
Structure type: such as: inductive, capacitive sensor, etc.;
(2) Physical properties: such as piezoelectric, photoelectric, various semiconductor sensors, etc.;
5. According to the form of the sensor output signal:
(1) Analog: The sensor output is an analog voltage;
(2) Digital: The sensor output is a digital quantity, such as an encoder type sensor.
Third, the static characteristics of the sensor
The static characteristics of the sensor are related to the static input signal, the output of the sensor and the input. Since the input and output are independent of time at this time, the relationship between them, that is, the static characteristics of the sensor, can be an algebraic equation without a time variable, or the input amount is used as the abscissa, and the corresponding output is made. The characteristic curve drawn on the ordinate is used to describe. The main parameters characterizing the static characteristics of the sensor are: linearity, sensitivity, resolution and hysteresis.
Fourth, the dynamic characteristics of the sensor
The so-called dynamic characteristic refers to the characteristic 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 certain standard input signals. This is because the sensor's response to a standard input signal is easily experimentally determined, and its relationship to the standard input signal has a certain relationship with its response to any input signal, and it is often known that the former can presume the latter. The most common standard input signals are step signals and sinusoidal signals, so the dynamic characteristics of the sensor are also commonly expressed by step response and frequency response.
Fifth, the linearity of the sensor
Typically, the actual static characteristic output of the sensor is a bar 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 commonly used to represent the actual characteristic curve, and the linearity (non-linearity error) is a performance index of this approximation.
There are several ways to select a fitted line. For example, 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 square of the deviation from each point on the characteristic curve is used as the fitted straight line, and the fitted straight line is called the least squares method. Straight line.
Sixth, the sensitivity of the sensor
Sensitivity refers to the ratio of the output change Δy to the input change Δx under steady-state operation.
It is the slope of the output-input characteristic curve. Sensitivity S is a constant if there is a linear relationship between the output and the input of the sensor. Otherwise, it will change as the amount of input changes.
The dimension of sensitivity is the ratio of the dimensions of the output and the input. For example, for a displacement sensor, when the displacement changes by 1 mm and the output voltage changes to 200 mV, the sensitivity should be expressed as 200 mV/mm.
When the output of the sensor and the amount of input are the same, the sensitivity can be understood as a magnification.
Increase the sensitivity and get higher measurement accuracy. However, the higher the sensitivity, the narrower the measurement range and the worse the stability.
Seven, resistive sensor
A resistive sensor is a device that converts measured physical quantities such as displacement, deformation, force, acceleration, humidity, temperature, etc. into resistance values. There are mainly resistive strain sensors such as resistance strain type, piezoresistive type, thermal resistance, heat sensitive, gas sensitive and humidity sensitive.
Eight, the resolution of the sensor
Resolution is the ability of a sensor to feel the smallest change being measured. That is, if the input varies slowly from some non-zero value. When the input change value does not exceed a certain value, the output of the sensor does not change, that is, the change in the sensor's input amount is not resolved.
The output changes only when the input changes more than the resolution.
Usually, the resolution of the sensor at different points in the full-scale range is not the same. Therefore, the maximum change value of the input amount that can make the step change of the output in the full-scale range is used as an index for measuring the resolution. If the above indicators are expressed as a percentage of full scale, they are called resolution.
Nine, resistance strain sensor
The strain gauge in the sensor has a strain effect of the metal, that is, mechanical deformation under the action of an external force, so that the resistance value changes accordingly. There are two types of resistance strain gauges: metal and semiconductor. Metal strain gauges are available in wire, foil and film. Semiconductor strain gauges have the advantages of high sensitivity (usually dozens of times of silk and foil) and small lateral effects.
Ten, piezoresistive sensor
Piezoresistive sensors are devices that are fabricated by diffusion resistance on a substrate of a semiconductor material in accordance with the piezoresistive effect of the semiconductor material. The substrate can be directly used as a measuring sensor element, and the diffusion resistor is connected in the form of a bridge in the substrate. When the substrate is deformed by an external force, the resistance values ​​will change, and the bridge will produce a corresponding unbalanced output.
The substrate (or diaphragm) used as a piezoresistive sensor is mainly a silicon wafer and a silicon wafer, and the silicon piezoresistive sensor made of a sensitive material is more and more popular, especially in measuring pressure. And speed solid state piezoresistive sensors are most commonly used.
XI. Thermal resistance sensor
Thermistor sensor mainly measures the temperature and temperature-related parameters by using the characteristic that the resistance value changes with temperature. This type of sensor is suitable when the temperature detection accuracy is relatively high. At present, the most widely used thermal resistance materials are platinum, copper, nickel, etc., which have the characteristics of large temperature coefficient of resistance, good linearity, stable performance, wide temperature range of use, and easy processing. It is used to measure temperatures in the range of -200 ° C to +500 ° C.
Twelve, sensor hysteresis characteristics
The hysteresis characteristic characterizes the degree of inconsistency between the forward (increase in input) and reverse (increase in input) stroke-input characteristic curves, usually using the maximum difference ΔMAX between the two curves and full The percentage of the range output F·S is expressed.
Hysteresis can be caused by the absorption of energy from the internal components of the sensor.
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