Basic knowledge of sensors
I. Definition of a Sensor
The national standard GB7665-87 defines a sensor as: "A device or apparatus that can sense a specified measurand and convert it into a usable signal according to a certain rule, usually composed of a sensing element and a conversion element." A sensor is a detection device that can sense the information being measured and convert that information into an electrical signal or other required form of information output according to a certain rule, to meet the requirements of information transmission, processing, storage, display, recording, and control. It is the primary link in realizing automatic detection and automatic control.
II. Classification of Sensors
There is currently no unified classification method for sensors, but the following three are commonly used:
1. Based on the physical quantities measured by the sensor, sensors can be classified into displacement, force, velocity, temperature, flow rate, and gas composition sensors, etc.
2. According to the working principle of the sensor, it can be classified into resistance, capacitance, inductance, voltage, Hall effect, photoelectric, grating, thermocouple, and other types of sensors;
3. According to the nature of the sensor output signal, it can be classified into: switch type sensors with output as switching quantity ("1" and "0" or "on" and "off"); analog type sensors; and digital type sensors with output as pulse or code.
III. Static Characteristics of Sensors
The static characteristics of a sensor refer to the relationship between the sensor's output and input signals when the input signal is static. Because both the input and output are independent of time, their relationship—the sensor's static characteristics—can be described by an algebraic equation without a time variable, or by a characteristic curve plotted with the input signal on the x-axis and the corresponding output signal on the y-axis. The main parameters characterizing the static characteristics of a sensor include linearity, sensitivity, resolution, and hysteresis.
IV. Dynamic Characteristics of the Sensor
Dynamic characteristics refer to the output characteristics of a sensor when the input changes. In practical applications, the dynamic characteristics of a sensor are often represented by its response to certain standard input signals. This is because the sensor's response to standard input signals is easily obtained experimentally, and there is a certain relationship between its response to standard input signals and its response to arbitrary input signals; often, knowing the former allows us to infer the latter. Commonly used standard input signals include step signals and sinusoidal signals, so the dynamic characteristics of sensors are often represented by step response and frequency response.
V. Sensor linearity
Typically, the actual static characteristic output of a sensor is a curve rather than a straight line. In practical applications, to ensure that the instrument has a uniformly calibrated reading, a fitted straight line is often used to approximate the actual characteristic curve. Linearity (non-linear error) is a performance indicator of the degree of this approximation.
There are several methods for selecting the fitting line. For example, the theoretical line connecting the zero input and full-scale output points can be used as the fitting line; or the theoretical line whose sum of the squares of the deviations from the characteristic curve is the smallest can be used as the fitting line, which is called the least squares fitting line.
VI. Sensor Sensitivity
Sensitivity refers to the ratio of the change in output Δy to the change in input Δx of a sensor under steady-state operating conditions.
It is the slope of the output-input characteristic curve. If there is a linear relationship between the sensor's output and input, then the sensitivity S is a constant. Otherwise, it will vary with the input.
Sensitivity is measured as the ratio of the dimensions of the output and input quantities. For example, if a displacement sensor changes its output voltage by 200mV when the displacement changes by 1mm, its sensitivity should be expressed as 200mV/mm.
When the dimensions of the sensor's output and input quantities are the same, sensitivity can be understood as the amplification factor.
Increasing sensitivity can lead to higher measurement accuracy. However, the higher the sensitivity, the narrower the measurement range, and the worse the stability often becomes.
VII. Sensor Resolution
Resolution refers to the ability of a sensor to detect the smallest change in the measured quantity. In other words, if the input quantity changes slowly from a non-zero value, the sensor's output will not change if the change in input does not exceed a certain value; that is, the sensor cannot distinguish this change in input quantity. Only when the change in input quantity exceeds the resolution will its output change.
Typically, the resolution of a sensor varies across its full-scale range. Therefore, the maximum change in the input quantity that causes a step change in the output within the full-scale range is commonly used as an indicator of resolution. This indicator, expressed as a percentage of the full-scale range, is called resolution.
8. Resistive Sensors
A resistive sensor is a device that converts physical quantities such as displacement, deformation, force, acceleration, humidity, and temperature into resistance values. The main types of resistive sensors include strain gauge, piezoresistive, resistance temperature detectors (RTDs), thermistors, gas sensors, and humidity sensors.
IX. Resistance Strain Gauge Sensor
Resistance strain gauges in sensors exhibit the strain effect of metals, meaning they undergo mechanical deformation under external force, resulting in a corresponding change in resistance. Resistance strain gauges are mainly classified into two types: metallic and semiconductor. Metallic strain gauges are further divided into wire, foil, and thin-film types. Semiconductor strain gauges offer advantages such as high sensitivity (typically tens of times higher than wire and foil strain gauges) and low transverse effect.
10. Piezoresistive Sensors
A piezoresistive sensor is a device made by spreading resistors on a semiconductor substrate based on the piezoresistive effect of semiconductor materials. The substrate can be directly used as the sensing element, and the spreading resistors are connected in a Wheatstone bridge configuration within the substrate. When the substrate is subjected to external force and deforms, the resistance values will change, and the Wheatstone bridge will produce a corresponding unbalanced output.
The substrate (or diaphragm) materials used in piezoresistive sensors are mainly silicon and germanium wafers. Silicon piezoresistive sensors, made with silicon wafers as the sensing material, are receiving increasing attention, especially in applications such as pressure and velocity piezoresistive sensors.
XI. Resistance Temperature Detector (RTD) Sensor
Resistance temperature detectors (RTDs) primarily utilize the characteristic that resistance changes with temperature to measure temperature and temperature-related parameters. These sensors are suitable for applications requiring high temperature detection accuracy. Currently, widely used RTD materials include platinum, copper, and nickel, which possess advantages such as a large temperature coefficient of resistance, good linearity, stable performance, wide operating temperature range, and ease of fabrication. They are used to measure temperatures within the range of -200℃ to +500℃.
12. Hysteresis Characteristics of Sensors
Hysteresis characterizes the degree of inconsistency between the output-input characteristic curves of a sensor during the forward (increased input) and reverse (decreased input) strokes. It is usually expressed as a percentage of the maximum difference ΔMAX between these two curves and the full-scale output F·S.
Hysteresis can be caused by energy absorption within the sensor's internal components.
