I. The Concept of Sensors
A sensor is a detection device that can sense the information being measured and transform the sensed information into an electrical signal or other required form of information output according to a certain rule, so as 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.
Generally, a sensor is understood to be a physical quantity that is converted into a physical quantity that can be described by another intuitive and expressible physical quantity through circuitry.
II. Principle and Characteristics of Sensors
Static characteristics
This refers to the relationship between the sensor's output and input quantities for a static input signal. Because both the input and output quantities are independent of time, their relationship—that is, 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 quantity on the x-axis and the corresponding output quantity on the y-axis. The main parameters characterizing the sensor's static characteristics include linearity, sensitivity, resolution, and hysteresis.
Dynamic characteristics
Dynamic characteristics refer to the output characteristics of a sensor in response to changes in input. 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. The most commonly used standard input signals are step signals and sinusoidal signals, so the dynamic characteristics of sensors are often represented by step response and frequency response.
linearity
Typically, the actual static characteristic output of a sensor is a curve rather than a straight line. In practical applications, to ensure uniformly calibrated readings, a fitted straight line is often used to approximate the actual characteristic curve. Linearity (non-linear error) is a performance indicator of this approximation. There are several methods for selecting the fitted straight line. For example, the theoretical straight line connecting the zero input and full-scale output points can be used as the fitted straight line; or the theoretical straight line that minimizes the sum of the squares of the deviations from the characteristic curve can be used as the fitted straight line. This fitted straight line is called the least squares fitted straight line.
Hysteresis characteristics
Hysteresis characterizes the degree of inconsistency between the sensor's output-input characteristic curves during the forward (increased input) and reverse (decreased input) strokes, and is usually expressed as a percentage of the maximum difference between these two curves relative to the full-scale output. Hysteresis can be caused by energy absorption by internal sensor components.
Sensitivity
Sensitivity refers to the ratio of the change in output to the change in input of a sensor under steady-state operating conditions. It is the slope of the output-input characteristic curve. If the sensor's output and input have a linear relationship, then the sensitivity S is a constant. Otherwise, it will vary with the input.
III. Key Design Considerations for Sensors
Extract the useful signal and reduce noise
The physical quantities measured are typically very small and usually contain inherent conversion noise, which is present in the physical conversion elements of a sensor. For example, the signal strength of a sensor at a magnification of 1 is 0.1~1µV, and the background noise at this level is also quite high, even capable of silencing it. How to extract the useful signal as much as possible and suppress noise is the primary problem to be solved in sensor design.
Sensor circuits must be simple and concise.
Imagine an amplifier circuit with three stages of amplification and two stages of active filters. While amplifying the signal, it also amplifies the noise. If the noise doesn't significantly deviate from the useful signal's spectrum, then regardless of filtering, the simultaneous amplification of both results in no improvement in the signal-to-noise ratio. Therefore, sensor circuits must be streamlined and simple. Any resistor or capacitor that can be omitted should be removed. This is a point often overlooked by engineers designing sensors. It's known that sensor circuits become increasingly complex with each modification due to noise issues, creating a vicious cycle.
Power consumption issue
Sensors are typically located at the front end of subsequent circuits, which may require long lead connections. When the sensor consumes a lot of power, these lead connections introduce unwanted noise and power supply noise, making the design of subsequent circuits increasingly difficult. Reducing power consumption while still meeting minimum requirements is also a significant challenge.
Component selection and power supply circuit
When selecting components, it's best to choose enough. As long as the component specifications are within the required range, the rest is a matter of circuit design. Power supply is a challenge that will inevitably be encountered in the sensor circuit design process. Don't pursue unattainable power supply specifications. Instead, choose an op-amp with a good common-mode rejection ratio and use a differential amplifier circuit design. The most common switching power supply and components may be sufficient to meet your requirements.
