A pressure transmitter is an industrial automation instrument that converts pressure signals into standard electrical signals (such as 4-20mA DC, 0-10V DC, or digital signals). Its technical principles can be summarized into four core links: pressure sensing, signal conversion, signal amplification and processing, output and transmission, as follows:
1、 Pressure sensing
The core of a pressure transmitter is the sensor, which is responsible for directly sensing changes in external pressure. Common types of sensors include:
Piezoresistive sensor: utilizing the piezoresistive effect of semiconductor or metal materials. When pressure is applied to the sensor diaphragm, the diaphragm undergoes deformation, causing a change in the internal resistance value. Convert resistance changes into voltage signals through a Wheatstone bridge.
Capacitive sensor: composed of two parallel electrode plates. Pressure causes a change in the capacitance value between the diaphragm electrode and the fixed electrode, which is then converted into an electrical signal.
Piezoelectric sensor: Utilizing the characteristic of piezoelectric materials (such as quartz crystals) generating charges under pressure, it outputs an electrical signal proportional to the pressure. This type of sensor is suitable for dynamic pressure measurement.
Strain gauge sensor: Resistance strain gauges are sensitive devices that convert strain changes on the measured component into an electrical signal. It is one of the main components of a piezoresistive strain transmitter. The most commonly used types of resistance strain gauges are metal resistance strain gauges and semiconductor strain gauges. There are two types of metal resistance strain gauges: wire strain gauges and metal foil strain gauges. Usually, strain gauges are tightly adhered to a mechanically strained substrate using a special adhesive. When the substrate undergoes stress changes, the resistance strain gauges also deform, causing a change in the resistance value of the strain gauges and a change in the voltage applied to the resistance.
2、 Signal conversion
The raw signal output by sensors is often weak and does not have a linear relationship. Therefore, it is necessary to convert these signals into initial electrical signals proportional to pressure changes through signal processing circuits. This process may involve resistance voltage conversion, capacitance voltage conversion, etc.
3、 Signal amplification and processing
The initially generated signal needs to be amplified and linearized by a signal processing circuit to ensure that the signal can be correctly recognized by the external control system. The processing steps usually include:
Signal amplification: Use operational amplifiers to amplify weak millivolt level signals to volt level for subsequent processing.
Temperature compensation: Sensors are easily affected by temperature and need to compensate for temperature drift through built-in temperature sensors or algorithms to ensure output stability.
Linearization correction: The sensor output may have nonlinear characteristics and needs to be linearly corrected through digital or analog circuits to ensure that the output signal has a strict linear relationship with pressure changes.
4、 Output and transmission
The processed signal will be transmitted to the monitoring or control system in the form of a standard signal. Common output signals include:
4-20mA DC current signal: the most commonly used standard signal in industrial automation, with advantages such as strong anti-interference ability and long transmission distance.
0-10V DC voltage signal: suitable for short distance transmission and occasions requiring voltage signals.
Digital signals: such as HART, Modbus and other protocols, support remote configuration, diagnosis and calibration, and achieve intelligent management.