 ## Sensor definition, classification and hysteresis

The hysteresis characteristic indicates the degree of inconsistency between the output - input characteristic curve of the sensor between the forward (input increase) and reverse (input decrease) stroke. It is usually expressed as the percentage of the large difference between the two curves △ Max and the full scale output f · s.

Hysteresis can be caused by the absorption of energy in the sensor's internal components.

I. definition of sensor

the national standard gb7665-87 defines the sensor as "a device or device that can sense the specified measured signal and convert it into available signal according to certain rules, usually composed of sensitive elements and conversion elements". Sensor is a kind of detection device, which can sense the measured information, and can transform the detected information into electrical signal or other required 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 realize automatic detection and control.

II. Classification of sensors

At present, there is no unified classification method for sensors, but there are three commonly used methods:

1. According to the physical quantity of the sensor, it 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 switch type sensor whose output is the switching value ("1" and "0" or "on" and "off"); the analog type sensor whose output is the analog type sensor; and the digital type sensor whose output is the pulse or code.

III. static characteristics of the sensor

The static characteristic of   sensor refers to the relationship between the output and input of the sensor for the static input signal. Because the input and output 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 variable, or by the characteristic curve drawn by taking the input as the abscissa and the corresponding output as the ordinate. The main parameters that characterize the static characteristics of the sensor are linearity, sensitivity, resolution and hysteresis.

IV. dynamic characteristics of the sensor

the so-called dynamic characteristic refers to the output characteristic of the sensor when the input changes. In practice, the dynamic characteristics of the sensor are usually represented by its response to some standard input signals. This is because the response of the sensor to the standard input signal is easy to obtain by experimental method, and there is a certain relationship between its response to the standard input signal and its response to any input signal, often knowing that the former can infer the latter. The commonly used standard input signals are step signal and sine signal, so the dynamic characteristics of the sensor are also commonly expressed by step response and frequency response.

V. linearity of sensor

In general, the actual static characteristic output of the sensor is a curve rather than a straight line. In practical work, in order to make the instrument have even scale reading, a fitting line is often used to approximately represent the actual characteristic curve, and the linearity (nonlinear error) is a performance index of this approximation degree.

There are many ways to choose the fitting line. For example, the theoretical straight line connecting the zero input and the full scale output point is regarded as the fitting straight line; or the theoretical straight line whose square sum of the deviation from each point on the characteristic curve is small is regarded as the fitting straight line, which is called the small two multiplication fitting straight line.

Vi. sensitivity of sensor

sensitivity refers to the ratio of output change △ y to input change △ x under steady-state operation

It is the slope of the output input characteristic curve. If there is a linear relationship between the output and the input of the sensor, the sensitivity s is a constant. Otherwise, it will change with the amount of input.

The dimension of sensitivity is the dimension ratio of output and input. For example, when the displacement of a displacement sensor changes by 1mm and the output voltage changes by 200mV, its sensitivity should be expressed as 200mV / mm.

When the output and input dimensions of the sensor are the same, the sensitivity can be understood as the magnification.

High accuracy can be obtained by improving the sensitivity. But the higher the sensitivity, the narrower the measurement range and the worse the stability.

VII. Resolution of sensor

Resolution is the ability of a sensor to sense small changes that may be 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 can not distinguish the change of this input value. Only when the change of the input exceeds the resolution, the output will change.

generally, the resolution of each point in the full range of the sensor is not the same, so the large change value in the input which can make the output produce step change in the full range is commonly used as the index to measure the resolution. If the above indicators are expressed as percentage of full scale, they are called resolution.

VIII. Resistance sensor

Resistance sensor is a kind of device which can convert physical quantities such as displacement, deformation, force, acceleration, humidity, temperature, etc. to resistance value. It mainly includes resistance strain type, piezoresistance type, thermal resistance, thermal sensitivity, gas sensitivity, humidity sensitivity and other resistance sensor parts.

IX. resistance strain sensor

The resistance strain gauge in the sensor has the strain effect of metal, that is, the mechanical deformation is produced under the action of external force, so that the resistance value changes accordingly. There are two types of resistance strain gauge: metal and semiconductor. Metal strain gauge can be divided into wire type, foil type and film type. The semiconductor strain gauge has the advantages of high sensitivity (usually dozens of times of wire type and foil type) and small transverse effect.

X. piezoresistive sensor

Piezoresistive sensor is a device which is based on the piezoresistive effect of semiconductor materials and diffused on the substrate of semiconductor materials. The substrate can be used as a sensing element directly, and the diffusion resistance is connected into a bridge form in the substrate. When the substrate is deformed by external force, each resistance value will change, and the bridge will produce corresponding unbalanced output.

Silicon and germanium are the main substrate (or diaphragm) materials used as piezoresistive sensors. Silicon piezoresistive sensors made of silicon as sensitive materials are paid more and more attention, especially the solid piezoresistive sensors which measure pressure and velocity are widely used.

Xi. Thermal resistance sensor

The thermistor sensor mainly uses the characteristic that the resistance changes with the temperature to measure the temperature and the parameters related to the temperature. This kind of sensor is suitable for the occasion of high temperature detection accuracy. At present, platinum, copper and nickel are widely used as thermal resistance materials. They have the characteristics of large temperature coefficient, good linearity, stable performance, wide temperature range and easy processing. It is used to measure the temperature in the range of - 200 ℃ ~ + 500 ℃.

Xii. Hysteresis characteristics of the sensor

The hysteresis characteristic indicates the degree of inconsistency between the output - input characteristic curve of the sensor between the forward (input increase) and reverse (input decrease) stroke. It is usually expressed as the percentage of the large difference between the two curves △ Max and the full scale output f · s.

Hysteresis can be caused by the absorption of energy in the sensor's internal components.