First, the sensor industry
The national standard GB7665-87 defines the definition of sensors as follows: "A device or equipment that can sense the rules and be measured and converted into usable signals according to certain rules, usually composed of sensitive elements and conversion elements". The sensor is a kind of detection equipment, which can feel the measured information, and can transform the information that is detected and touched into an electrical signal or other required form of information output according to certain rules, so as to meet the requirements of information transmission, processing, storage, display, recording and control. It is the first step to complete automatic detection and automatic control.
2. Classification of sensors
At present, there is no unified classification method for sensors, but there are three commonly used ones:
1. According to the physical quantity classification of sensors, they can be divided into displacement, force, speed, temperature, flow, gas composition and other sensors;
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: the output is a switching sensor ("1" and "0" or "on" and "off"); The output is an analog sensor; A digital sensor whose output is a pulse or code.
3. Static characteristics of the sensor
The static characteristic of the sensor refers to the interconnection between the output and the 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, that is, the static characteristics of the sensor, can be described by an algebraic equation without time variables, or by drawing the 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 a sensor are: linearity, sensitivity, resolution, and hysteresis.
Fourth, the dynamic characteristics of the sensor
The so-called dynamic characteristics refer to the characteristics of the output of the sensor when the input changes. In practice, the dynamic nature of a sensor is often indicated by its response to certain specification input signals. This is because the sensor's response to the canonical input signal is easy to find by experimental methods, and there is a definite relationship between its response to the canonical input signal and its response to the arbitrary input signal, and the latter can often be inferred if the former is known. *There are two commonly used standard input signals: step signal and sinusoidal signal, so the dynamic characteristics of the sensor are also commonly indicated by step echo and frequency echo.
5. Linearity of the sensor
Typically, the practical static characteristic output of the sensor is a curve rather than a straight line. In practice, in order to make the appearance have a uniform scale reading, a fitting line is often used to approximate the characteristic curve of the practice, and the linearity (nonlinear error) is a performance indicator of this approximation.
There are several ways to fit a straight 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 square of the deviation from each point on the characteristic curve is *small as the fitting line, and this fitting line is called the *small squares fitting line.
6. Sensitivity of the sensor
Sensitivity refers to the ratio of the change of output △y to the change of input amount △x of the sensor in steady-state operation.
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 change with the change in the input amount.
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 to 200mV when the displacement changes by 1mm, its sensitivity should be indicated as 200mV/mm.
When the output and input dimensions of the sensor are the same, the sensitivity can be understood as an amplification factor.
Improved sensitivity, high measurement accuracy can be obtained. However, the higher the sensitivity, the narrower the measurement scale, and the worse the stability.
7. The resolution of the sensor
Resolving power refers to the ability of a sensor to perceive a small change that is 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. The output changes only when the input quantity changes beyond the resolution.
Generally, the resolution of the sensor is not the same at each point in the full-scale scale, so the largest change value in the input quantity that can make the output step change in the full-scale scale is often used as an index to measure the resolution. If the above indicators are expressed as a percentage of full scale, they are called resolution.
8. Resistive sensors
A resistive sensor is a device that converts physical quantities such as displacement, deformation, force, acceleration, humidity, temperature, etc. into resistance values. There are mainly resistive strain gauge, piezoresistive type, thermal resistance, thermal, gas sensitive, moisture sensitive and other resistive sensing devices.
9. Resistance strain sensor
The resistive strain gauges in the sensor have a metallic strain effect, i.e., they are mechanically deformed under the action of an external force, so that the resistance value changes accordingly. There are two main types of resistance strain gauges: metal and semiconductor, and metal strain gauges are divided into wire type, foil type and thin film type. Semiconductor strain gauges have the advantages of high sensitivity (usually dozens of times that of wire and foil types) and small transverse effect.
10. Piezoresistive sensor
Piezoresistive sensors are devices made by dispersing resistors on the substrate of semiconductor materials based on the piezoresistive effect of semiconductor materials. The substrate can be directly used as a measurement sensing element, and the dispersed resistance is connected into a bridge form in the substrate. When the substrate is deformed by an external force, the resistance values will change, and the bridge will have a corresponding unbalanced output.
The substrate (or diaphragm) used as piezoresistive sensors are mainly silicon wafers and germanium wafers, and silicon piezoresistive sensors made of silicon wafers for sensitive materials have attracted more and more attention, especially for the application of state piezoresistive sensors to measure pressure and velocity.
11. Thermal resistance sensor
RTD sensors mainly use the resistance value to change with the change of temperature to measure the temperature and temperature-related parameters. This sensor is more suitable for occasions where temperature detection accuracy is required. At present, the most widely used RTD materials are platinum, copper, nickel, etc., which have the characteristics of large temperature coefficient of resistance, good linearity, stable performance, wide application temperature scale, and easy processing. It is used to measure the temperature in the scale of -200°C - +500°C.
12. Hysteresis characteristics of the sensor
The hysteresis characteristic characterizes the degree of inconsistency between the output-one input characteristic curve of the sensor between the forward (input increase) and reverse (input decrease) strokes, and is usually used as the *large difference between the two curves △MAX and the full-scale output F · The percentage of S is indicated.
Hysteresis can be caused by the absorption of energy present in the sensor's internal components.
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