The design of a good sensor is the result of experience plus technology. It is generally understood that a sensor converts a physical quantity through a circuit into a description that can be expressed in another intuitively expressible physical quantity. In the following, we will introduce the concept and principle characteristics of the sensor one by one, and then analyze the key points of the sensor design.
1, the concept of the sensor
The sensor is a detecting device that can sense the measured information and can transform the sensed information into an electrical signal or other required form of information output to meet the information transmission, processing, storage and display. Requirements for recording, control and control. It is the first step in achieving automatic detection and automatic control.
2, the working principle of the sensor
Classification of sensor operating principles Physical sensors use physical effects such as piezoelectric effects, magnetostriction, ionization, polarization, thermoelectric, optoelectronic, magnetoelectric and other effects. Minor changes in the measured semaphore will be converted into electrical signals. Chemical sensors include those that cause a causal relationship between chemical adsorption, electrochemical reactions, etc., and small changes in the measured signal volume are also converted into electrical signals. Providing ±15V power to the sensor, the crystal oscillator in the excitation circuit generates a 400Hz square wave, and the AC excitation power source is generated by the tda2030 power amplifier, and is transmitted from the stationary primary coil to the rotating secondary coil through the energy toroidal transformer T1. The AC power supply obtains a ±5V DC power supply through the rectifying and filtering circuit on the shaft. The power supply is used as the operating power supply of the operational amplifier AD822. The high-precision regulated power supply consisting of the reference power supply AD589 and the dual op amp AD822 generates a precision DC of ±4.5V. Power supply, which acts as both a bridge power supply and an operating power source for amplifiers and V/F converters.
When the elastic shaft is twisted, the strain signal of the mV level obtained by the strain bridge is amplified by the instrumentation amplifier AD620 into a strong signal of 1.5v±1v, and then converted into a frequency signal by the V/F converter, and rotated by the signal toroidal transformer T2. The primary coil is transmitted to the stationary secondary coil, and then filtered and shaped by the signal processing circuit on the outer casing to obtain a frequency signal proportional to the torque received by the elastic bearing. The signal is TTL level and can be provided to the dedicated secondary The meter or frequency meter display can also be sent directly to the computer for processing. Because the rotary transformer has only a few millimeters of clearance between the static and dynamic rings, and the upper part of the sensor shaft is sealed inside the metal casing to form an effective shielding, it has strong anti-interference ability. Some sensors can neither be classified into physical classes nor classified into chemical classes. Most sensors operate on the basis of physical principles. There are many technical problems in chemical sensors, such as reliability problems, the possibility of mass production, and price issues. These problems have been solved, and the application of chemical sensors will grow tremendously.
3, the characteristics of the sensor
1. Static characteristics: It refers to the static input signal, the relationship between the output of the sensor and the input quantity. Because the input and output are independent of time at this 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.
2. Dynamic characteristics: refers to the characteristics 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.
3. Linearity: Under normal circumstances, 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.
4. Hysteresis characteristic: It characterizes the degree of inconsistency between the output-input characteristic curve of the sensor in the forward (increase input) and reverse (increase input) strokes. Usually, the maximum difference ΔMAX between the two curves is used. With full scale output F? The percentage of S is expressed. Hysteresis can be caused by the absorption of energy from the internal components of the sensor.
5. Sensitivity: Sensitivity refers to the ratio of the output change Δy to the input quantity 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.
4, sensor design points
1. Generally, the measured physical quantity is very small, and usually has conversion noise inherent to the physical conversion element of the sensor. For example, the signal strength of the sensor at 1 magnification is 0.1~1uV, and the background noise signal at this time has such a large level, even annihilating it. How to take out the useful signal as much as possible and reduce the noise is the primary problem in sensor design.
2, the sensor circuit must be simple and refined. Imagine an amplifying loop with a 2-stage active filter with a 2-stage active filter that amplifies the signal and amplifies the noise. If the noise does not significantly deviate from the spectrum of the wanted signal, the filter is amplified at the same time. The signal to noise ratio has not improved. Therefore, the sensor circuit must be refined and simple. To save 1 resistor or capacitor, you must remove it. This is a problem that many engineers who design sensors are easy to ignore. It is known that the sensor circuit is troubled by the problem of noise, and the more complicated the circuit is modified, the more complicated it becomes.
3. Power consumption issues. The sensor is usually at the front end of the subsequent circuit and may require a long lead connection. When the sensor consumes a lot of power, the connection of the leads will introduce all the unnecessary noise and power supply noise, making subsequent circuits more difficult to design. In the case of sufficient use, how to reduce power consumption is also a small test.
4. Selection of components and power supply circuit. The selection of components must be sufficient, as long as the device specifications are within the required range, and the rest is the circuit design problem. The power supply is a difficult problem in the sensor circuit design process. Don't pursue the power supply index that cannot be achieved. Choose an op amp with a good common mode rejection ratio. Use a differential amplifier circuit to design the most common switching power supply. The device will meet your requirements.
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