Analysis of Key Points in Sensor Design
Time:2023-05-06
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The design of a good sensor is the result of experience and technology. A general understanding of a sensor is to convert a physical quantity through a circuit into a description that can be expressed in another intuitive way. In the following text, we will introduce the concept, principle and characteristics of sensors one by one, and then analyze the key points of sensor design.
1. Concept of sensors
A sensor is a detection device that can sense the measured information and transform it into electrical signals or other required forms of information output according to certain rules, in order to meet the requirements of information transmission, processing, storage, display, recording, and control. It is the primary link to achieve automatic detection and control.
The definition of a sensor in the national standard GB7665-87 is: "A device or device that can sense a specified measured object and convert it into a usable signal according to certain laws (mathematical function rules), usually composed of sensitive elements and conversion elements.
2. Working principle of sensors
The classification of sensor working principles: Physical sensors apply physical effects, such as piezoelectric effects, magnetostrictive phenomena, ionization, polarization, thermoelectric, optoelectronic, magnetoelectric, and other effects. Any small changes in the measured signal quantity will be converted into electrical signals. Chemical sensors include sensors that rely on phenomena such as chemical adsorption and electrochemical reactions as causal relationships, and small changes in the measured signal quantity will also be converted into electrical signals. Provide ± 15V power supply to the sensor, and the crystal oscillator in the excitation circuit generates a 400Hz square wave. After passing through the tda2030 power amplifier, an AC excitation power supply is generated. The AC power supply is transmitted from the stationary primary coil to the rotating secondary coil through the energy ring transformer T1. The obtained AC power supply is obtained through the rectification and filtering circuit on the shaft to obtain ± 5V DC power supply, which serves as the working power supply for the operational amplifier AD822; A high-precision stabilized power supply composed of a reference power supply AD589 and a dual operational amplifier AD822 generates a precision DC power supply of ± 4.5V. This power supply serves as both the bridge power supply and the working power supply for amplifiers and V/F converters.
When the elastic shaft is twisted, the mV level strain signal detected by the strain bridge is amplified into a strong signal of 1.5v ± 1v through the instrument amplifier AD620, and then transformed into a frequency signal through the V/F converter LM131. It is transmitted from the rotating primary coil to the stationary secondary coil through the signal ring transformer T2, and then filtered and reshaped by the signal processing circuit on the shell to obtain a frequency signal proportional to the torque borne by the elastic shaft, This signal is at TTL level and can be provided for display by dedicated secondary instruments or frequency meters, or directly sent to a computer for processing. Due to the fact that there is only a gap of a few tenths of a millimeter between the dynamic and static rings of the rotary transformer, and the upper part of the sensor shaft is sealed inside a metal casing, forming an effective shielding, it has strong anti-interference ability. Some sensors cannot be classified into either physical or chemical categories. Most sensors operate based on physical principles. There are many technical problems with chemical sensors, such as reliability issues, the possibility of large-scale production, price issues, etc. By solving these problems, the application of chemical sensors will have a huge growth.
3.Introduction to the characteristics of sensors
3.1 Static characteristics: Refers to the mutual relationship between the output and input of a sensor for static input signals. Since the input and output are independent of time, the relationship between them, that is, the static characteristics of the sensor can be described by a algebraic equation without time variables, or by a characteristic curve drawn with the input as the abscissa and the corresponding output as the ordinate. The main parameters that characterize the static characteristics of sensors include linearity, sensitivity, resolution, and hysteresis.
3.2. Dynamic characteristics: refers to the output characteristics of a sensor when its input changes. In practical work, the dynamic characteristics of sensors are often represented by their response to certain standard input signals. This is because the response of the sensor to the standard input signal is easily obtained through experimental methods, and there is a certain relationship between its response to the standard input signal and its response to any input signal. Often, knowing the former can infer the latter. The most commonly used standard input signals are step signal and sine signal, so the dynamic characteristics of sensors are also commonly represented by step response and frequency response.
3.3. Linearity: Typically, the actual static characteristic output of a sensor is a curve rather than a straight line. In practical work, in order to ensure that the instrument has a uniform scale reading, a fitted straight line is commonly used to approximate the actual characteristic curve, and linearity (nonlinear error) is a performance indicator of this approximation degree. There are multiple methods for selecting fitting straight lines. If the theoretical straight line connecting the zero input and full scale output points is used as the fitting straight line; Alternatively, the theoretical line with the smallest sum of squares of the deviations from each point on the characteristic curve can be used as a fitting line, which is called the least squares fitting line.
3.4 Hysteresis characteristic: Characterizes the degree of inconsistency in the output input characteristic curve between the forward (input increase) and reverse (input decrease) strokes of the sensor. Usually, the maximum difference △ MAX between these two curves and the full scale output F is used? The percentage of S is represented. Hysteresis can be caused by energy absorption in the internal components of the sensor.
3.5 Sensitivity: Sensitivity refers to the ratio of the output change △ y of a sensor to the input change △ x under steady-state operating conditions. It is the slope of the output 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 vary with the input quantity.
4. Key points of sensor design
4.1. Generally, the measured physical quantities are very small, and there is also inherent conversion noise as a physical conversion component of the sensor. For example, when the signal strength of a sensor is 0.1~1uV at a magnification of 1, the background noise signal also has such a large level that it can even be annihilated. How to extract useful signals as much as possible and reduce noise is the primary problem to be solved in sensor design.
4.2. The sensor circuit must be simple and refined. Imagine an amplification circuit with a 3-stage amplification circuit and a 2-stage active filter, which amplifies the signal while also amplifying the noise. If the noise does not deviate significantly from the useful signal spectrum, no matter how the filter amplifies both, the signal-to-noise ratio does not improve. Therefore, the sensor circuit must be refined and concise. If one resistor or capacitor can be saved, it must be removed. This is a problem that many engineers designing sensors tend to overlook. The known situation is that the sensor circuit becomes more complex and becomes a strange circle as the noise problem troubles it.
4.3. Power consumption issues. Sensors are usually located at the front end of subsequent circuits and may require longer lead connections. When the power consumption of the sensor is high, the connection of the leads will introduce all unnecessary noise and power supply noise, making it increasingly difficult to design subsequent circuits. How to reduce power consumption when sufficient is also a significant challenge.
4.4. Selection of components and power circuit. The selection of components must be sufficient, as long as the device specifications are within the required range, the rest is circuit design issues. Power supply is a problem that must be encountered in the design process of sensor circuit. Instead of pursuing an unattainable power supply index, choose an op amp with a good common mode rejection ratio. The most common switching power supply and devices designed with differential amplifier circuit can meet your requirements.
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