With the advancement of automation technology, in industrial equipment, in addition to liquid column pressure gauges and elastic pressure gauges, pressure transmitters and sensors that can convert pressure into electrical signals are now more commonly used. So how do these pressure transmitters and sensors convert pressure signals into electrical signals? What are the characteristics of different conversion methods? Today, we've compiled the measurement principles of several common pressure sensors to help you understand them better.
I. Piezoelectric pressure sensor
Piezoelectric pressure sensors are precision measuring instruments based on the piezoelectric effect. They use electrical components and other mechanical elements to convert the pressure to be measured into an electrical quantity for measurement. Examples include many pressure transmitters and pressure sensors. Piezoelectric sensors cannot be used for static measurements because the charge generated by an external force can only be preserved when the circuit has infinitely high input resistance. However, this is not the case in reality. Therefore, piezoelectric sensors can only be used for dynamic measurements. Their main piezoelectric materials are ammonium dihydrogen phosphate, sodium potassium tartrate, and quartz. The piezoelectric effect was discovered in quartz.
When stress changes, the change in electric field is very small, and other piezoelectric crystals replace quartz. Sodium potassium tartrate has a large piezoelectric coefficient and sensitivity, but it can only be used in environments with low humidity and temperature. Ammonium dihydrogen phosphate is a synthetic crystal that can be used in environments with high humidity and high temperature, so its applications are very wide. With technological advancements, the piezoelectric effect has also been applied to polycrystalline materials. Examples include piezoelectric ceramics, such as magnesium niobate piezoelectric ceramics, niobate-based piezoelectric ceramics, and barium titanate piezoelectric ceramics.
Sensors operating on the piezoelectric effect are electromechanical conversion and self-generating sensors. Their sensing element is made of a piezoelectric material. When this material is subjected to an external force, a charge forms on its surface. This charge is amplified by a charge amplifier, a measuring circuit, and impedance transformation, converting it into an electrical output proportional to the applied force. It is used to measure force and non-electrical physical quantities that can be converted into force, such as:
Acceleration and pressure. It has many advantages: light weight, reliable operation, simple structure, high signal-to-noise ratio, high sensitivity, and wide signal-to-frequency bandwidth, etc. However, it also has some disadvantages: some voltage materials are susceptible to moisture, so a series of moisture-proof measures are required; and the output current response is relatively poor, so a charge amplifier or a high input impedance circuit is needed to compensate for this shortcoming and allow the instrument to work better.
II. Piezoresistive Pressure Sensor
Piezoresistive pressure sensors are primarily based on the piezoresistive effect. The piezoresistive effect describes the change in resistance of a material under mechanical stress. Unlike the piezoelectric effect, the piezoresistive effect only produces a change in impedance and does not generate electrical charge.
Most metallic and semiconductor materials have been found to exhibit piezoresistive effects. The piezoresistive effect in semiconductors is significantly greater than in metals. Since silicon is the primary material in modern integrated circuits, the application of piezoresistive components made from silicon has become highly significant. The resistance change in silicon comes not only from stress-related geometric deformation but also from the material's own stress-related resistance, making its magnitude factor hundreds of times greater than that of metals. The resistance change in N-type silicon is primarily due to the redistribution of carriers between the conduction band valleys with different mobilities caused by the displacement of its three conduction band valley pairs, thus altering the electron mobility in different flow directions. Secondly, it is due to changes in the effective mass associated with the change in conduction band valley shape. In P-type silicon, this phenomenon becomes more complex and also leads to changes in effective mass and hole conversion.
Piezoresistive pressure sensors are typically connected to a Wheatstone bridge via leads. Normally, when no external pressure is applied to the sensing element, the bridge is in a balanced state (referred to as zero). When pressure is applied to the sensor, the chip resistance changes, and the bridge becomes unbalanced. If a constant current or voltage power supply is applied to the bridge, it will output a voltage signal corresponding to the pressure. Thus, the change in sensor resistance is converted into a pressure signal output by the bridge. The bridge detects the change in resistance, amplifies it, and then converts it into a corresponding current signal through voltage-to-current conversion. This current signal is compensated by a nonlinear correction loop, resulting in a standard output signal of 4–20 mA that is linearly related to the input voltage.
To reduce the impact of temperature changes on the core resistance and improve measurement accuracy, pressure sensors employ temperature compensation measures to maintain high levels of technical indicators such as zero drift, sensitivity, linearity, and stability.
III. Capacitive Pressure Sensor
A capacitive pressure sensor is a pressure sensor that uses capacitance as the sensing element to convert the measured pressure into a change in capacitance. This type of pressure sensor typically uses a circular metal diaphragm or a metal-plated diaphragm as one electrode of a capacitor. When the diaphragm senses pressure and deforms, the capacitance between the diaphragm and the fixed electrode changes. The measuring circuit then outputs an electrical signal that is proportional to the voltage. Capacitive pressure sensors belong to the electrode spacing variation type of capacitive sensors and can be divided into single-capacitor pressure sensors and differential-capacitor pressure sensors.
A single-capacitor pressure sensor consists of a circular diaphragm and a fixed electrode. The diaphragm deforms under pressure, thus changing the capacitance. Its sensitivity is approximately proportional to the diaphragm area and pressure, and inversely proportional to the diaphragm tension and the distance from the diaphragm to the fixed electrode. Another type has a concave spherical fixed electrode and a diaphragm with a fixed periphery and tension plane. The diaphragm can be made by plating plastic with a metal layer. This type is suitable for measuring low pressure and has high overload capacity. A single-capacitor pressure sensor for measuring high pressure can also be made using a diaphragm with a piston-driven moving electrode. This type reduces the direct pressure-bearing area of the diaphragm, allowing for a thinner diaphragm and improved sensitivity. It is also integrated with various compensation and protection components and amplification circuitry to improve anti-interference capabilities. This sensor is suitable for measuring dynamic high pressure and telemetry of aircraft. Single-capacitor pressure sensors also include microphone-type and stethoscope-type types.
In a differential capacitive pressure sensor, the pressure-bearing diaphragm electrode is located between two fixed electrodes, forming two capacitors. Under pressure, the capacitance of one capacitor increases while the capacitance of the other decreases accordingly, and the measurement result is output by the differential circuit. Its fixed electrodes are made by depositing a metal layer on a concave glass surface. Under overload, the diaphragm is protected from rupture by the concave surface. Differential capacitive pressure sensors have higher sensitivity and better linearity than single-capacitive sensors, but they are more difficult to manufacture (especially difficult to ensure symmetry), and cannot achieve isolation between the measured gas or liquid, therefore they are not suitable for operation in corrosive or impure fluids.
IV. Electromagnetic Pressure Sensor
This refers to a variety of sensors that utilize electromagnetic principles, including inductive pressure sensors, Hall effect pressure sensors, and eddy current pressure sensors.
Inductive pressure sensor
The working principle of an inductive pressure sensor is based on the fact that different magnetic materials and permeabilities cause a change in the air gap size when pressure is applied to the diaphragm. This change in air gap affects the inductance of the coil, and the processing circuit converts this change in inductance into a corresponding signal output, thus achieving the purpose of pressure measurement. This type of pressure sensor can be divided into two types according to the change in the magnetic circuit: variable reluctance and variable permeability. The advantages of inductive pressure sensors are high sensitivity and a large measurement range; the disadvantage is that they cannot be used in high-frequency dynamic environments.
The main components of a variable reluctance pressure sensor are an iron core and a diaphragm. The air gap between them forms a magnetic circuit. When pressure is applied, the size of the air gap changes, meaning the reluctance changes. If a certain voltage is applied to the iron core coil, the current will change with the change in the air gap, thus measuring the pressure.
In applications with high magnetic flux density, the permeability of ferromagnetic materials becomes unstable. In such cases, a variable permeability pressure sensor can be used for measurement. This sensor replaces the iron core with a movable magnetic element. Changes in pressure cause the magnetic element to move, thus altering the permeability, from which the pressure value is derived.
Hall pressure sensor
Hall pressure sensors are based on the Hall effect of certain semiconductor materials. The Hall effect refers to the phenomenon where, when a solid conductor is placed in a magnetic field and a current flows through it, the charge carriers within the conductor are deflected to one side by the Lorentz force, thus generating a voltage (Hall voltage). The electric field force induced by the voltage balances the Lorentz force. The polarity of the Hall voltage confirms that the current inside the conductor is caused by the movement of negatively charged particles (free electrons).
When a magnetic field perpendicular to the direction of current is applied to a conductor, electrons in the conductor are pulled together by the Lorentz force, creating an electric field in the direction of electron concentration. This electric field then exerts an electric force on subsequent electrons, balancing the Lorentz force caused by the magnetic field, allowing them to pass through smoothly without being deflected. This is known as the Hall effect. The resulting built-in voltage is called the Hall voltage.
When the magnetic field is an alternating magnetic field, the Hall electromotive force (EMF) is also an alternating EMF of the same frequency. The time to build up the Hall EMF is extremely short, hence its high response frequency. Ideal Hall elements require materials with high resistivity and carrier mobility to obtain a large Hall EMF. Commonly used Hall element materials are mostly semiconductors, including N-type silicon (Si), indium antimonide (InSb), indium arsenide (InAs), germanium (Ge), gallium arsenide (GaAs), and multilayer semiconductor structures. N-type silicon has good Hall coefficient, temperature stability, and linearity, while gallium arsenide has low temperature drift and is currently widely used.
Eddy current pressure sensor
Pressure sensors based on the eddy current effect. The eddy current effect is generated by a moving magnetic field intersecting a metallic conductor, or by a moving metallic conductor perpendicularly intersecting a magnetic field. In short, it is caused by electromagnetic induction. This action produces a current circulating within the conductor.
Eddy current characteristics give eddy current detection features such as zero-frequency response, so eddy current pressure sensors can be used for static force detection.
V. Vibrating Wire Pressure Sensor
Vibrating wire pressure sensors are frequency-sensitive sensors. This type of frequency measurement offers high accuracy because time and frequency are accurately measurable physical parameters, and the influence of factors such as cable resistance, inductance, and capacitance can be ignored during frequency signal transmission. Furthermore, vibrating wire pressure sensors possess strong anti-interference capabilities, low zero-point drift, good temperature characteristics, simple structure, high resolution, and stable performance. They are also convenient for data transmission, processing, and storage, facilitating instrument digitization. Therefore, vibrating wire pressure sensors can be considered one of the directions for the development of sensing technology.
The sensing element of a vibrating wire pressure sensor is a taut steel wire. The natural frequency of the sensing element is related to the magnitude of the tension force. The length of the wire is fixed, and the change in the wire's vibration frequency can be used to measure the magnitude of the tension force; that is, the input is a force signal, and the output is a frequency signal. A vibrating wire pressure sensor consists of two parts: the lower component is mainly the assembly of the sensing element, and the upper component is an aluminum shell containing an electronic module and a wiring terminal, housed in two small compartments. This design ensures that wiring does not affect the airtightness of the electronic module compartment.
Vibrating wire pressure sensors can be selected as either current output type or frequency output type. In operation, the vibrating wire vibrates continuously at its resonant frequency. When the measured pressure changes, the frequency changes, and this frequency signal can be converted into a 4~20mA current signal by a converter.
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