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, it can be divided into sensors such as displacement, force, speed, temperature, flow, and gas composition.
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.
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. Because the input and output are independent of 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 can be used as the abscissa, and the corresponding output is 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.
Dynamic characteristics of the sensor The so-called dynamic characteristic is the characteristic of the output of the sensor when the input changes. In practice, the dynamic characteristics of the sensor are often represented by its response to certain standard input signals. This is because the response of the sensor to the standard input signal is easily experimentally determined, and its response 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.
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 line is usually used to approximate the actual characteristic curve, and 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 fitting straight line; or the theoretical straight line with the smallest square of the deviation of each point on the characteristic curve is used as the fitting straight line, and the fitting straight line is called the least squares method. Straight line.
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.
Increased sensitivity for higher measurement accuracy. However, the higher the sensitivity, the narrower the measurement range and the worse the stability.
Sensor Resolution The resolution is the ability of a sensor to sense 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 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 commonly used as an index for measuring the resolution. If the above indicators are expressed as a percentage of full scale, they are called resolution.
Resistive Sensors Resistive sensors are devices that convert 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.
Resistance strain sensor The strain gauge in the sensor has a strain effect of the metal, that is, mechanical deformation under 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.
Piezoresistive Sensors Piezoresistive sensors are devices that are fabricated by diffusion resistance on a substrate of a semiconductor material based on the piezoresistive effect of the semiconductor material. The substrate can be directly used as a measuring sensing 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 made of silicon wafer and tantalum. The silicon piezoresistive sensor made of sensitive material is getting more and more attention, especially in measuring pressure. And speed solid state piezoresistive sensors are most commonly used.
Thermistor sensor Thermistor sensor is mainly used to measure temperature and temperature-related parameters by changing the resistance value with temperature. This type of sensor is suitable when the temperature detection accuracy is relatively high. At present, a wide range of 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, and easy processing. It is used to measure temperatures in the range of -200 ° C ~ +500 ° C.
The hysteresis characteristic hysteresis characteristic of the sensor characterizes the output of the sensor between the forward (increase in input) and reverse (increase in input) strokes - the degree to which the input characteristic curve is inconsistent, usually using the maximum difference between the two curves The percentage of ΔMAX to the full-scale output FS indicates that hysteresis can be caused by the absorption of energy from the internal components of the sensor.
Sensor selection Sensors vary widely, and sensors with different operating principles can be used for the same type of measurement. Therefore, the most suitable sensor should be selected as needed.
(1) Measurement conditions If the sensor is misselected, the reliability of the system will be reduced. To this end, from the overall consideration of the system, to clearly understand the purpose of use and the need to use sensors, never use unsuitable sensors and unnecessary sensors. The measurement conditions are listed as follows: measurement purpose, selection of measurement quantity, measurement range, bandwidth of input signal, required accuracy, time required for measurement, and frequency of occurrence of over-input.
(2) Sensor performance When selecting a sensor, the following performance of the sensor should be considered, namely accuracy, stability, response speed, analog signal or digital signal, output quantity and level, influence of the characteristics of the measured object, calibration cycle, Loss of protection.
(3) Sensor usage conditions The conditions under which the sensor is used are the location where the setting is made, the environment (humidity, temperature, vibration, etc.), the time of measurement, the distance between the signal transmission with the display, the connection with the peripherals, and the power supply capacity. .