Piezoresistive pressure sensors are constructed using the piezoresistive effect of single crystal silicon. Using single crystal silicon wafer as the elastic element, a process of using integrated circuits on the single crystal silicon film, a group of equivalent resistances are diffused in the specific direction of the single crystal silicon, and the resistance is connected into a bridge circuit, and the single crystal silicon wafer is placed In the sensor cavity. When the pressure changes, the single crystal silicon is strained, so that the strain resistor directly diffused on it produces a change proportional to the measured pressure, and then the bridge circuit obtains the corresponding voltage output signal.
Piezoresistive pressure sensor application
Piezoresistive sensors are ideal sensors for this purpose. For example, it is used to measure the air pressure distribution of helicopter wings, to test the dynamic distortion of the engine inlet, the pulsating pressure of the cascade, and the jitter of the wings. In the measurement of the center pressure of an aircraft jet engine, a specially designed silicon pressure sensor is used, and its operating temperature is above 500°C. In the Boeing passenger plane's atmospheric data measurement system, a matching silicon pressure sensor with an accuracy of up to 0.05% is used. In the reduced-size wind tunnel model test, piezoresistive sensors can be densely installed at the entrance of the wind tunnel and in the engine intake duct model. The diameter of a single sensor is only 2.36 mm, the natural frequency is as high as 300 kHz, and the nonlinearity and hysteresis are ±0.22% of the full range. In biomedicine, piezoresistive sensors are also ideal detection tools.
An injection needle piezoresistive pressure sensor with a diffused silicon membrane as thin as 10 microns and an outer diameter of only 0.5 mm and a sensor that can measure pressure in the cardiovascular, intracranial, urethral, uterine, and intraocular balls have been made. Fig. 3 is a structural diagram of a sensor for measuring cerebral pressure. Piezoresistive sensors are also effectively used in the measurement of explosion pressure and shock waves, vacuum measurement, monitoring and control of the performance of automobile engines, and the measurement of weapons such as the measurement of the pressure in the gun barrel and the launch of shock waves. In addition, piezoresistive sensors are widely used in oil well pressure measurement, direction-while-drilling and location detection of underground sealed cable faults, and flow and liquid level measurement. With the further development of microelectronic technology and computers, the application of piezoresistive sensors will also develop rapidly.
Working principle of piezoresistive pressure sensor:
When the sensor is in a pressure medium, the pressure of the medium acts on the corrugated diaphragm, the silicone oil in it is pressurized, and the silicone oil senses the pressure of the diaphragm to the semiconductor core. After being pressed, the resistance value changes, and the resistance signal is led out through the lead. The stainless steel corrugated diaphragm shell senses the pressure and protects the core, so the piezoresistive pressure sensor can sense the pressure signal in a corrosive medium.
The piezoresistive pressure sensor is generally connected to the Wheatstone bridge through the lead wire. Normally, the sensitive core has no external pressure, and the bridge is in a balanced state (called zero position). When the sensor pressure is changed, the chip resistance changes, and the bridge will lose its balance. If you add a constant current or voltage power supply to the bridge, the bridge will output a voltage signal corresponding to the pressure, so that the resistance change of the sensor is converted into a pressure signal output by the bridge. It appears in most pressure sensors that the method of manufacturing integrated circuits forms four resistance strips with equal resistance values, and connects and engraves them to form a Wheatstone bridge. The Wheatstone bridge adopts constant current power supply, so that the output of the bridge is not affected by temperature. The Wheatstone bridge detects the change in resistance value, passes through the differential unidirectional amplifier, the output amplifier is amplified, and then undergoes the conversion of voltage and current It is transformed into a corresponding current signal, which is compensated by a non-linear correction loop, that is, a standard output signal of 4-20 mA whose input voltage is in a linear corresponding relationship is generated.
In order to reduce the influence of temperature changes on the core resistance value and improve the measurement accuracy, pressure sensors adopt temperature compensation measures to maintain a high level of technical indicators such as zero drift, sensitivity, linearity, stability and so on.