Intelligent Pressure Transmitter is an industrial automation instrument that integrates pressure detection, signal processing, and intelligent communication. It is widely used for pressure measurement and control in fields such as petroleum, chemical, and power. Its working principle can be divided into four core links: pressure sensing, signal conversion, intelligent processing, and data transmission, as follows:
1, Pressure perception: converting physical pressure into mechanical displacement
The core of an intelligent pressure transmitter is a pressure sensor, which converts the pressure signal of the measured medium (liquid, gas, or vapor) into measurable mechanical displacement or physical quantity changes.
Common sensor types:
Capacitive sensor: The most common type, consisting of a measuring diaphragm and a fixed electrode. When pressure is applied to the membrane, it undergoes slight deformation, resulting in a change in the capacitance value between the membrane and the fixed electrode (the greater the pressure, the smaller the spacing, and the larger the capacitance).
Piezoresistive sensor: Utilizing the piezoresistive effect of semiconductor materials, pressure changes the resistance value of the chip's internal resistance, which is converted into a voltage signal through a Wheatstone bridge.
Inductive/vibrating wire sensor: indirectly reflects the magnitude of pressure through changes in inductance or vibration frequency caused by pressure.
2, Signal conversion: converting physical quantities into electrical signals
The raw signals output by sensors (such as small changes in capacitance, resistance, and voltage) need to be converted into standard electrical signals (such as 4-20mA DC current or 0-5V DC voltage) through signal conditioning circuits:
Excitation and detection: The circuit provides a stable excitation power supply (such as constant voltage or constant current) for the sensor, while detecting changes in physical quantities of the sensor (such as changes in capacitance).
Amplification and filtering: The original signal is usually weak (in the millivolt range) and needs to be amplified by an operational amplifier to filter out environmental noise (such as temperature interference and electromagnetic interference).
Analog to digital conversion (A/D conversion): converts amplified analog electrical signals into digital signals for subsequent intelligent chip processing.
3, Intelligent processing: digital computing and compensation
The "intelligence" of intelligent pressure transmitters is reflected in the digital processing of data by the microprocessor (MCU), and the core functions include:
Nonlinear compensation: The output signal of the sensor may have a nonlinear relationship with the actual pressure. The microprocessor corrects the deviation through a preset calibration curve (such as polynomial fitting) to improve measurement accuracy.
Temperature compensation: Temperature changes can affect sensor performance (such as membrane elasticity coefficient, resistance temperature coefficient). The processor detects the ambient temperature in real-time through a built-in temperature sensor and automatically corrects errors caused by temperature.
Range adjustment: Supports remote setting of measurement range through software (such as HART protocol communication), without the need for mechanical adjustment, and can flexibly adapt to different scenarios (such as adjusting from 0-1MPa to 0-5MPa).
Fault diagnosis: Real time monitoring of sensor and circuit status (such as disconnection, overload), and outputting alarm signals when abnormalities occur (such as current signal jumping to 22mA).
4, Data Transmission: Standardized Signal and Communication
Intelligent transmitters support both analog signal output and digital communication, balancing compatibility with traditional systems and intelligent requirements
Analog signal output: The processed digital signal is restored to a 4-20mA standard current signal (or 0-10V voltage) through D/A conversion, and directly connected to traditional control systems such as PLC and DCS (4mA corresponds to the lower range limit, 20mA corresponds to the upper range limit).
Digital communication: Bidirectional data transmission is achieved through industrial bus protocols such as HART, Profibus, FF fieldbus, which can remotely read real-time pressure values, equipment parameters (such as range and accuracy), or modify settings (such as calibration and alarm thresholds).
For example, the HART protocol adopts a superposition method of "analog signal+digital signal" (superimposing high-frequency digital signal on 4-20mA current), which retains traditional analog transmission and supports digital communication.
