Pressure sensors are one of the most commonly used sensors in industrial practice and are widely used in a variety of industrial self-control environments involving water conservancy and hydropower, railway transportation, intelligent construction, production self-control, aerospace, military, petrochemical, oil well, power, ship, machine tool, pipeline and many other industries. The following is a brief introduction to some common sensor principles and their applications.

There are many types of force sensors, such as resistance strain gauge pressure sensors, semiconductor strain gauge pressure sensors, piezoresistive pressure sensors, inductive pressure sensors, capacitive pressure sensors, resonant pressure sensors and capacitive acceleration sensors. However, the most widely used is the piezoresistive pressure sensor, which has a very low price, high accuracy and good linearity. Here we introduce these sensors.

Piezoresistive pressure sensor principle:

Piezoresistive pressure sensors are sensors made using the piezoresistive effect of single crystal silicon material and integrated circuit technology. Piezoresistive sensors are commonly used for the measurement and control of pressure, tension, differential pressure and other physical quantities that can be transformed into changes in force (e.g. liquid level, acceleration, weight, strain, flow, vacuum).

Piezoresistive effect

When a force is applied to a silicon crystal, the lattice of the crystal is deformed, causing carriers to scatter from one energy valley to another, causing a change in carrier mobility and perturbing the average amount of carriers in the longitudinal and transverse directions, thus causing a change in the resistivity of the silicon. This variation varies with the orientation of the crystal, so the piezoresistive effect of silicon is related to the orientation of the crystal. The piezoresistive effect of silicon differs from that of metal strain gauges in that the change in resistance with pressure in the former depends mainly on the change in resistivity, while the change in resistance in the latter depends mainly on the change in geometry (strain), and the sensitivity of the former is 50 to 100 times greater than that of the latter.

Piezoresistive pressure sensor construction

Piezoresistive pressure sensors use an integrated process to integrate a resistive strip on a single crystal silicon diaphragm to make a silicon piezoresistive chip, and the periphery of this chip is fixed and encapsulated within the housing, leading to the electrode leads. The piezoresistive pressure sensor is also known as a solid state pressure sensor. It differs from the paste strain gauge in that the external force is felt indirectly through the elastic sensitive element, but directly through the silicon diaphragm. One side of the silicon diaphragm is a high pressure chamber connected to the measured pressure and the other side is a low pressure chamber connected to the atmosphere. The silicon diaphragm is generally designed as a circular shape with a solid peripheral support and a diameter to thickness ratio of approximately 20 to 60. 4 P impurity resistance strips are diffused in the circular silicon diaphragm (type N) and connected to form a full bridge, two of which are located in the compressive stress zone and the other two in the tensile stress zone, symmetrical with respect to the centre of the diaphragm. The silicon pillar-shaped sensitive element is also made by diffusing resistance strips in a certain direction on the surface of the silicon pillar, and the two resistance strips under tensile stress form a full bridge with the other two resistance strips under compressive stress.

Development of piezoresistive pressure sensors

The piezoresistive effect of silicon was studied in detail by C.S. Smith in 1954 and since then pressure sensors have been made from silicon. The early silicon pressure sensors were of the semiconductor strain gauge type. Later on, N-type silicon wafers were diffused with P-type impurities to form resistive strips, which were connected to a bridge and made into a chip. This chip still needs to be attached to an elastic element in order to be sensitive to changes in pressure. Sensors that use this chip as a sensitive element are called diffusion pressure sensors. Since the 1970s, an integrated silicon cup type diffusion pressure sensor with a fixed support resistor and silicon diaphragm has been made. It not only overcomes the inherent defects of the sticky sheet structure, but also enables the integration of resistance strips, compensation circuits and signal adjustment circuits on a silicon sheet, or even the integration of a microprocessor with the sensor to make an intelligent sensor.

The advantages of this new sensor are.

① high frequency response (for example, some products have an inherent frequency of 1.5 MHz or more), suitable for dynamic measurement.

(ii) small size (e.g. some products have an external diameter of up to 0.25 mm), suitable for miniaturisation;

③High accuracy, up to 0.1 to 0.01%;

④High sensitivity, many times higher than that of metal strain gauges, for some applications without amplifiers;

⑤ No moving parts, high reliability, able to work in harsh environments such as vibration, shock, corrosion and strong interference. The disadvantages are the high temperature effect (sometimes requiring temperature compensation), the complexity of the process and the high cost.

Disadvantages are:

① Poor temperature characteristics, because the piezoresistive pressure sensor is made of semiconductor materials, subject to the influence of temperature, therefore, when used in environments with large temperature changes, temperature compensation must be carried out.

Piezoresistive pressure sensor applications

Piezoresistive sensors are widely used in various fields such as aerospace, aviation, navigation, petrochemical, power machinery, biomedical engineering, meteorology, geology and seismic measurement. In the aerospace and aviation industries pressure is a key parameter and high accuracy is required for the measurement of static and dynamic pressure, local pressure and the entire pressure field. Piezoresistive sensors are ideal for this purpose. For example, they are used to measure the airflow pressure distribution in the wings of helicopters, to test dynamic distortions in engine inlets, pulsating pressures in leaf grilles and jitter in wings. For the measurement of centre pressure in aircraft jet engines, specially designed silicon pressure sensors are used, which operate at temperatures of over 500°C.

A matching silicon pressure sensor with an accuracy of up to 0.05% is used in the atmospheric data measurement system of the Boeing airliner. The piezoresistive sensor can be installed densely at the wind tunnel inlet and in the engine inlet duct model in a reduced size wind tunnel model test. With a single sensor diameter of only 2.36 mm and an intrinsic frequency of up to 300 kHz, the non-linearity and hysteresis are ±0.22% of the full range. In biomedical applications, piezoresistive sensors are also ideal for detection.

Piezoresistive pressure sensors of the injector needle type have been developed with diffused silicon membranes as thin as 10 µm and an external diameter of only 0.5 mm, as well as sensors capable of measuring cardiovascular, intracranial, urethral, uterine and intraocular pressures. Figure 3 shows the construction of a sensor for measuring brain pressure. Piezoresistive sensors are also effectively used in explosive pressure and shock wave measurements, vacuum measurements, monitoring and controlling the performance of automotive engines and in weaponry such as measuring the pressure in the bore of a gun, firing shock waves and so on. In addition, piezoresistive sensors are widely used in oil well pressure measurements, in the detection of fault points in underground sealed cables for directional and level measurement with drilling, and in flow and level measurements. With the further development of microelectronics and computers, the application of piezoresistive sensors will also develop rapidly.

Advantages of piezoresistive pressure sensors: 1:

1. high frequency response, f0 up to 1.5M; 2. small size and low power consumption

3. high sensitivity, good accuracy, can measure to 0.1% accuracy; 4. no moving parts (sensitive element and conversion element in one).

Disadvantages of piezoresistive pressure sensors:

1. poor temperature characteristics; 2. complex process.

Principle of operation of pressure sensors:

Semiconductor piezoresistive

Semiconductor piezoelectric impedance diffusion pressure sensors are formed on the surface of a thin sheet of semiconductor deformation pressure, through the external force (pressure) to deform the sheet and produce a piezoelectric impedance effect, so that the change in impedance into an electrical signal.

Electrostatic capacitive pressure sensors

The electrostatic capacity type pressure sensor forms a capacitance between a fixed pole of glass and a movable pole of silicon, and converts the change in electrostatic capacity generated by the deformation of the movable pole by an external force (pressure) into an electrical signal. (The E8Y's principle of operation is the electrostatic capacity method; other models use the semiconductor method)

Principle of operation of the piezoresistive air inlet pressure sensor

The strain resistors R1, R2, R3 and R4, which form a Wheatstone bridge, are bonded to a silicon diaphragm. The silicon diaphragm is deformed by the absolute pressure in the manifold.

Principles and applications of strain gauge pressure sensors

There are many different types of force transducers, such as resistive strain gauge pressure transducers, semiconductor strain gauge pressure transducers, piezoresistive pressure transducers, inductive pressure transducers, capacitive pressure transducers, resonant pressure transducers and capacitive acceleration transducers. However, the most widely used is the piezoresistive pressure sensor, which has a very low price, high accuracy and good linearity. In the following we focus on this type of sensor.

When learning about piezoresistive force sensors, we first meet the element of resistance strain gauges. A resistive strain gauge is a sensitive device that converts the strain change in the measured part into an electrical signal. It is one of the main components of a piezoresistive strain transducer. The two most used types of resistance strain gauges are metal resistance strain gauges and semiconductor strain gauges. There are two types of metal strain gauges: wire strain gauges and metal foil strain gauges. The strain gauges are usually bonded tightly to a mechanical strain-generating substrate by means of a special adhesive. When the substrate is subjected to stress changes, the strain gauges are deformed together, resulting in a change in the resistance of the strain gauges and thus in the voltage applied to the resistance. These strain gauges, which usually have a small change in resistance when stressed, are usually formed into strain bridges and amplified by subsequent instrumentation amplifiers, which are then transmitted to the processing circuit (usually A/D conversion and CPU) for display or actuation.

The internal structure of a metal resistance strain gauge

The strain gauge is made up of a base material, a strain gage