Piezoresistive sensor is a device made by diffusion resistance on a semiconductor material substrate based on the piezoresistive effect of semiconductor material. The substrate can be directly used as a measuring sensor element, and the diffusion resistance is connected in the form of a bridge in the substrate.
Piezoresistive sensor principle
piezoresistance type transducer refers to a sensor made of the piezoresistance effect of single crystal silicon material and integrated circuit technology. After the monocrystalline silicon material receives the force, the resistivity changes, and the electrical signal output proportional to the force change can be obtained through the measurement circuit. Piezoresistive sensors are used for the measurement and control of pressure, tension, pressure difference, and other physical quantities that can be transformed into changes in force (such as liquid level, acceleration, weight, strain, flow, and vacuum).
The piezoresistive sensor is a device made by diffusion resistance on a semiconductor material substrate based on the piezoresistive effect of semiconductor materials. The substrate can be directly used as a measuring sensor element, and the diffusion resistance is connected in the form of a bridge in the substrate. When the substrate is deformed by an external force, the resistance value will change, and the bridge will produce a corresponding unbalanced output. The substrate (or diaphragm) materials used as piezoresistive sensors are mainly silicon wafers and germanium wafers. Silicon piezoresistive sensors made of silicon wafers as sensitive materials have attracted more and more attention, especially for pressure measurement. And speed solid-state piezoresistive sensors are the most common applications.
Piezoresistive sensor application
Piezoresistive sensors are widely used in aerospace, aviation, navigation, petrochemical, power machinery, biomedical engineering, meteorology, geology, seismic surveying and other fields. Pressure is a key parameter in the aerospace and aviation industries, and high accuracy is required for the measurement of static and dynamic pressure, partial pressure and the entire pressure field. Piezoresistive sensors are ideal sensors for this purpose. For example, it is used to measure the airflow pressure distribution of a helicopter wing, test the dynamic distortion of the engine air inlet, the pulsating pressure of the cascade, and the shaking of the wing. In the measurement of the center pressure of aircraft jet engines, a specially designed silicon pressure sensor is used, and its working temperature is above 500°C. A matching silicon pressure sensor with an accuracy of up to 0.05% is used in the air data measurement system of the Boeing airliner. In the reduced size wind tunnel model test, piezoresistive sensors can be densely installed at the entrance of the wind tunnel and the engine intake duct model.
The diameter of a single sensor is only 2.36 mm, the natural frequency is up to 300 kHz, and the nonlinearity and hysteresis are both ±0.22% of the full scale. In biomedicine, piezoresistive sensors are also ideal detection tools. The injection needle type piezoresistive pressure sensor with a diffusion silicon membrane as thin as 10 microns and an outer diameter of only 0.5 mm has been manufactured, and a sensor capable of measuring pressure in the cardiovascular, intracranial, urethra, uterus, and eyeballs. Figure 3 is a block diagram of a sensor used to measure brain 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, as well as the measurement of weapons such as measuring the pressure in the barrel of a gun and launching shock waves. In addition, piezoresistive sensors are widely used in oil well pressure measurement, direction finding while drilling and location detection of underground sealed cable fault points, as well as flow and liquid level measurement. With the further development of microelectronic technology and computers, the application of piezoresistive sensors will also develop rapidly.
What are the typical characteristics of piezoresistive sensors
①The sensitivity coefficient of piezoresistive sensors is 50-100 times larger than that of metal strain-type pressure sensors. Sometimes the output of a piezoresistive sensor can be directly measured without an amplifier.
②Because it is processed by integrated circuit technology, the structure is small in size and light in weight.
③High pressure resolution, it can detect micro pressure as small as blood pressure.
④Good frequency response, it can measure pulsating pressure of tens of kilohertz.
⑤Because the force-sensitive element and detection element of the sensor are made on the same silicon chip, it has reliable operation, high comprehensive accuracy and long service life.
⑥Because of the use of semiconductor material silicon, the sensor is more sensitive to temperature. If temperature compensation is not used, the temperature error is relatively large.
Why does piezoresistive sensor produce temperature error? How to compensate?
There are two types of piezoresistive sensors: one is to use the bulk resistance of semiconductor materials to make a sticky strain gauge to form a semiconductor strain sensor; the other is to use integrated circuit technology to make a diffusion resistor on a semiconductor material substrate , Constitute a sensitive element, called a diffusion piezoresistive sensor.
Piezoresistive sensors are greatly affected by temperature, and will produce zero drift and sensitivity drift, which will result in temperature errors.
In the piezoresistive sensor, the temperature coefficient of the diffusion resistance is large, and the resistance value changes with temperature, which causes the zero drift of the sensor.
The temperature drift of sensor sensitivity is caused by the change of piezoresistive coefficient with temperature. When the temperature rises, the piezoresistive coefficient becomes smaller, and the sensitivity of the sensor decreases. On the contrary, the sensitivity increases.
Zero temperature drift can generally be compensated by the method of series and parallel resistance.
Sensitivity temperature drift is compensated by connecting diodes in series in the power circuit of the bridge.
In addition, the piezoresistive sensor can also connect four diffusion resistors into a full bridge. In order to reduce the influence of temperature, a constant current source can be used for power supply.
It can be seen that the output of the bridge is proportional to the resistance change, that is, it is proportional to the measured value, and also proportional to the power supply current. The output voltage is related to the size and accuracy of the constant current source current, but has nothing to do with the temperature, so the constant current source has a lot of Good temperature compensation.
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