I. Core Working Principle of Pressure Sensors
A pressure sensor is an indispensable key component in modern industrial automation and intelligent devices; it can convert physical pressure signals into measurable electrical signals. Understanding its working principle is crucial for correctly selecting and using pressure sensors.
The working principle of pressure sensors is mainly based on the following physical effects:
1. Piezoresistive effect: When a semiconductor material is subjected to pressure, its resistivity changes. By diffusing resistors on a silicon wafer to form a Wheatstone bridge, when pressure acts on the silicon diaphragm, the bridge produces an unbalanced voltage output whose magnitude is proportional to the pressure.
2. Piezoelectric effect: Some crystalline materials (such as quartz and ceramics) generate an electric charge when subjected to pressure, and the amount of charge is proportional to the applied pressure. This effect can work without an external power supply and is suitable for dynamic pressure measurement.
3. Capacitance variation principle: For a capacitor composed of two parallel plates, when pressure causes the distance between the plates to change, the capacitance changes accordingly. The magnitude of the pressure can be determined by measuring the change in capacitance.
4. Electromagnetic induction principle: Utilizing the property that pressure causes changes in the magnetic permeability of ferromagnetic materials, pressure is detected by measuring changes in the inductance of a coil.
5. Optical principle: Pressure is detected by measuring changes in optical signals caused by pressure-induced fiber deformation; it is suitable for harsh environments.
II. Types and Structural Characteristics of Pressure Sensors
According to different measurement principles and application scenarios, pressure sensors can be divided into many types:
1. Strain Gauge Pressure Sensor
Structure: elastic element (usually a metal or silicon diaphragm), strain gauges (metal or semiconductor), and a Wheatstone bridge circuit
Working process: pressure→elastic element deforms→strain gauge resistance changes→bridge outputs unbalanced voltage
Features: high accuracy, good stability, and relatively large temperature influence
2. Piezoelectric Pressure Sensor
Core materials: quartz crystal, piezoelectric ceramics (PZT), etc.
Signal output: generates an electric charge proportional to the pressure
Advantages: good high-frequency response, suitable for dynamic measurement, no external power supply required
Limitations: not suitable for static pressure measurement, high output impedance requiring a special amplifier
3. Capacitive Pressure Sensor
Typical structure: a fixed plate and a movable plate (pressure-sensitive diaphragm) form a parallel-plate capacitor
Sensitivity: inversely proportional to the square of the initial gap, so a small-gap design can increase sensitivity
Advantages: low power consumption, good temperature characteristics, suitable for micro-pressure measurement
Applications: air pressure measurement, medical equipment, consumer electronic products
4. MEMS Pressure Sensor
Manufacturing process: using micro-electromechanical systems technology to integrate mechanical structures and circuits on a silicon wafer
Features: small size, light weight, low cost, suitable for mass production
Typical applications: automotive TPMS, smartphone altimeters, and pressure control in home appliances
III. Key Performance Parameters of Pressure Sensors
When selecting a pressure sensor, the following core parameters need attention:
1. Measurement range (span): the minimum to maximum pressure value the sensor can measure, usually expressed in units such as Pa, kPa, MPa or psi.
2. Accuracy: indicates the deviation between the measurement result and the true value, usually expressed as a percentage of full scale (e.g.±0.5%FS).
3. Sensitivity: the ratio of the change in output signal to the change in input pressure, reflecting the sensor's ability to respond to pressure changes.
4. Response time: the time from a pressure change until the output signal reaches 90% of its stable value, reflecting dynamic characteristics.
5. Operating temperature range: the ambient temperature range in which the sensor can work normally; exceeding the range may cause performance degradation or damage.
6. Long-term stability: the ability of the sensor's performance parameters to remain unchanged over time under specified operating conditions.
7. Media compatibility: the chemical compatibility between the materials of the sensor's contact parts and the measured medium, to avoid corrosion or contamination.
8. Output signal type: common ones include analog (4-20mA, 0-5V, 0-10v), digital (RS485, I2C, SPI) and wireless output, etc.
IV. Pressure Sensor Selection Guide
Selecting a suitable pressure sensor requires considering the following factors:
1. Measured medium: gas, liquid or special media (corrosive, high-viscosity, etc.), which determines the material selection.
2. Pressure type:
gauge pressure (relative to atmospheric pressure);
absolute pressure (relative to vacuum);
differential pressure (the difference between two pressures);
sealed reference pressure (relative to a fixed reference pressure)
3. Environmental conditions: temperature, humidity, vibration, electromagnetic interference, etc., which affect the choice of the sensor's protection rating.
4. Accuracy requirements: generally 0.5%-1%FS for industrial applications, and 0.1%FS or higher for high-precision applications.
5. Output interface: choose analog output, digital output or wireless transmission according to system requirements.
6. Installation method: threaded installation, flange installation, embedded installation, etc.; space constraints need to be considered.
7. Certification requirements: depending on the industry application, explosion-proof certification (ATEX) or medical certification (FDA) may be required.
