In today's rapidly developing technology, sensors play a crucial role as the "senses" of electronic devices. They can perceive various information such as temperature, pressure, and light, and convert it into electrical signals for devices to interpret. Among numerous sensors, piezoresistive sensors have become an indispensable part of the technology field due to their unique principles and wide applications.
Piezoresistive effect: the core principle of sensor operation
The working principle of a piezoresistive sensor is based on the piezoresistive effect of semiconductor materials. When a solid is subjected to a force, its resistivity changes, which in turn causes a change in resistance. This effect is called the piezoresistive effect. In semiconductor materials, this effect is particularly pronounced. Taking single crystal silicon material as an example, when it is subjected to force, its resistivity will change. By measuring the circuit, an electrical signal output proportional to the force change can be obtained.
Specifically, when force is applied to a silicon crystal, the crystal lattice undergoes deformation, causing charge carriers to scatter from one energy valley to another, resulting in a change in the mobility of charge carriers and disturbing the average longitudinal and transverse amounts of charge carriers, thereby altering the resistivity of silicon. This change varies with the orientation of the crystal, therefore the piezoresistive effect of silicon is related to the orientation of the crystal. Unlike metal strain gauges, the change in resistance of semiconductor materials mainly depends on the change in resistivity rather than the change in geometric dimensions, and the sensitivity of the former is 50-100 times higher than that of the latter.
Structural composition: the key to achieving piezoresistive effect
Piezoresistive sensors typically consist of an inductor and a support body. The inductor can be a varistor, strain gauge, or capacitor, while the support is used to bear pressure. In practical applications, piezoresistive pressure sensors are a common type. It uses a single crystal silicon wafer as the elastic element, and utilizes integrated circuit technology to diffuse a set of equivalent resistors on the single crystal silicon wafer, and connect the resistors into a bridge circuit. A monocrystalline silicon wafer is placed inside the sensor chamber, with one side of the silicon diaphragm being a high-pressure chamber connected to the measured pressure and the other side being a low-pressure chamber connected to the atmosphere. Silicon film is generally designed as a circular shape with fixed support around the periphery, with a diameter to thickness ratio of about 20-60. Four P-impurity resistor strips are locally diffused in a circular silicon film and connected to a full bridge, with two located in the compressive stress region and the other two in the tensile stress region, symmetrically relative to the center of the film.
Working process: Conversion from stress to electrical signals
When the sensor is in a pressure medium, the medium pressure acts on the corrugated diaphragm, and the silicone oil is compressed to sense the pressure of the diaphragm to the semiconductor core. After being compressed, the resistance value of the silicon film changes, and the resistance signal is led out through the lead wire. After this change is amplified by an amplifier and calibrated with standard pressure, pressure detection can be performed.
For example, when a piezoresistive pressure sensor is connected to a Wheatstone bridge, the sensitive chip is not subjected to external pressure, and the bridge is in a balanced state (known as the zero position). When the sensor is under pressure, the chip resistance changes and the bridge loses balance. If a constant current or voltage power supply is added to the bridge, the bridge will output an electrical signal corresponding to the pressure, so that the resistance change of the sensor is converted into a pressure signal output through the bridge.
Performance characteristics: advantages and challenges coexist
Piezoresistive sensors have many advantages. Firstly, it has high sensitivity and can accurately perceive small pressure changes. For example, in a scaled down wind tunnel model test, piezoresistive sensors can be densely installed at the wind tunnel inlet and engine intake duct model, with a single sensor diameter of only 2.36 millimeters, a natural frequency of up to 300 kHz, and nonlinearity and hysteresis of ± 0.22% of the full range. Secondly, the piezoresistive sensor has a small volume and is easy to integrate, making it convenient to embed into various small devices. In addition, it also has the characteristics of fast response speed, good linearity, and strong environmental adaptability.
However, there are also some challenges with piezoresistive sensors. For example, the issue of cross sensitivity is related to both the piezoresistive coefficient, elastic modulus, and temperature coefficient of the sensor strain gauge itself, as well as the supply voltage of the bridge. Therefore, when strain and temperature act on the sensor simultaneously, the output of the sensor is not a simple superposition of the output generated when strain and temperature act alone. There is also an interaction between thermodynamic and mechanical quantities, which is reflected as cross sensitivity, and its magnitude reflects the degree of this interaction.
Application Fields: Wide and Important
Piezoresistive sensors have a wide range of applications in multiple fields. In industrial production, piezoresistive sensors are used to monitor fluid pressure, ensuring the stability and safety of the production process. For example, in chemical production, by monitoring the pressure changes inside the reaction vessel in real time and adjusting process parameters in a timely manner, safety accidents can be avoided.
In the aerospace field, piezoresistive sensors are used to measure engine thrust, providing important data support for aircraft performance evaluation and safe flight. At the same time, pressure resistance sensors are also used inside spacecraft to monitor cabin air pressure, ensuring the comfort and safety of passengers and crew members.
In robot technology, piezoresistive sensors can be used to measure the gripping and collision forces of robots, providing precise force feedback to improve the accuracy and safety of the production process.
In the field of medical equipment, piezoresistive sensors are widely used in medical devices such as blood pressure monitors and ventilators to achieve accurate monitoring and recording of patients' vital signs. For example, in a blood pressure monitor, a resistive sensor can sense real-time changes in blood pressure and transmit the data to a display for medical staff to refer to.
In the field of automobile manufacturing, piezoresistive sensors are used to monitor the pressure changes of brake pads, ensuring the normal operation and driving safety of the brake system. In the suspension system, piezoresistive sensors are used to monitor the vibration of the vehicle body, provide data support for the adjustment of the suspension system, and improve ride comfort.
Piezoresistive sensors have become an indispensable part of the technology field due to their unique piezoresistive effect principle, sophisticated structural design, efficient working process, superior performance characteristics, and wide range of applications. With the continuous advancement and innovation of technology, the application fields of piezoresistive sensors will be further expanded, and their performance and stability will also be further improved. In the future, piezoresistive sensors will bring us more surprises and convenience, playing a more important role in the development of technology.
Source: Sensor Expert Network