Pressure Sensor Types Explained with Examples

A pressure sensor is a device that can sense pressure signals and convert them into usable output electrical signals according to certain rules. It usually consists of pressure sensitive components and signal processing units. Commonly used in industrial, medical, automotive and other fields. It comes in many different types according to different usage environments.

Working Principle

The working principle of a pressure sensor is based on the force exerted by pressure on an object, and the value of pressure is determined by measuring the magnitude of this force. Common pressure sensors are classified according to their working principles into piezoresistive, capacitive and electromagnetic types.

1) Piezoresistive type

This type of sensor uses changes in the resistance of a metal film to measure pressure. When external pressure is applied, the metal film deforms, thereby changing the resistance value. By measuring the change in resistance, the pressure can be determined.

2) Capacitive type

This type of sensor uses changes in capacitance to measure pressure. When external pressure is applied, the structure of the pressure sensor changes, resulting in a change in capacitance value. By measuring the change in capacitance, the pressure can be determined.

3) Electromagnetic type

This type of sensor uses the principle of electromagnetic induction to measure pressure. When external pressure is applied, the magnetic field changes, causing changes in the induced current. By measuring the change in induced current, the magnitude of the pressure can be determined.

Common Pressure Sensors

1. Piezoresistive pressure sensor

Piezoresistive pressure sensor refers to a sensor made by utilizing the piezoresistive effect of single crystal silicon material and integrated circuit technology. After the single crystal silicon material is subjected to force, the resistivity changes, and then through the measurement circuit, an electrical signal output proportional to the force change can be obtained. That is, the resistance change occurs after the sensitive core is pressurized, and then the resistance change is amplified by the amplification circuit and converted to a standard signal output. It is different from the adhesive strain gauge which needs to indirectly sense the external force through the elastic sensitive element, but directly senses the measured pressure through the silicon diaphragm.

* Structure:

This sensor consists of a sensing element and a support. The sensing element is usually a piezoresistive device made of a conductive polymer or thin film material. When the material is subjected to force, it causes a small change in the resistance value within the conductive polymer. While the support plays the role of maintaining the position of the piezoresistive device and connecting the circuit.

* Work process:

1) The force-bearing object exerts pressure on the sensing element.

2) The resistance value of the sensing element changes slightly accordingly.

3) After circuit processing such as amplification and processing, the electrical signal is converted into a readable physical quantity.

Features:

1) High sensitivity

2) High accuracy

3) Small size

4) Small temperature coefficient

5) Adapt to a wide range of media

6) Stable performance

7) High safety

2. Capacitive pressure sensors

Capacitive pressure sensor is a component that uses a capacitive sensitive element to convert the measured pressure into an electrical output that is related to it. It is characterized by low input energy, high dynamic response, small natural effects, and good environmental adaptability. And it generally uses a circular metal film or metal-plated film as an electrode of the capacitor. When the film deforms due to pressure, the capacitance formed between the film and the fixed electrode changes, and the measurement circuit can output a electric signal related to the voltage. This sensor can be divided into single capacitive pressure sensors and differential capacitive pressure sensors.

1) Single capacitive pressure sensor

It consists of a circular film and fixed electrodes. The membrane deforms under the action of pressure, thereby changing the capacity of the capacitor. Its sensitivity is roughly proportional to the area of the membrane and pressure and inversely proportional to the tension of the membrane and the distance from the membrane to the fixed electrode. Another type of fixed electrode is in the shape of a concave spherical surface, and the diaphragm is a tensioned plane fixed around the periphery. The diaphragm can be made of plastic with a metal layer.

2) Differential capacitive pressure sensor

Its pressure diaphragm electrode is located between two fixed electrodes, forming two capacitors. Under the action of pressure, the capacity of one capacitor increases and the value of the other decreases accordingly, and the measurement result is output by a differential circuit. Its fixed electrode is made by plating a metal layer on the concave curved glass surface. The diaphragm is protected by the concave surface from rupture when overloaded. Differential capacitive pressure sensors have higher sensitivity and better linearity than single capacitive pressure sensors, but they are more difficult to process (especially difficult to ensure symmetry) and cannot isolate the gas or liquid being measured, so they are not suitable for working in areas with corrosion properties or impurities in the fluid.

3. Potentiometric pressure sensor

Potentiometer sensors are a common sensor type whose working principle is based on changes in resistance. The sensor consists of a potentiometer and a measuring circuit. The potentiometer consists of a movable contact and a fixed contact. The movable contact is affected by external force or movement, causing the resistance value of the potentiometer to change. A measurement circuit determines the magnitude of an external force or movement by measuring the resistance of a potentiometer.

This sensor is widely used in mechanical and industrial automation systems, such as pressure sensors, displacement sensors, etc. In the automotive industry, they are used to measure the position of the accelerator pedal, the position of the brake pedal, etc. In medical devices, they are used to detect patients' vital signs such as heart rate, respiration, etc.

4. Piezoelectric pressure sensor

Piezoelectric pressure sensors are precision measurement instruments that use electrical components and other machinery to convert the pressure to be measured into electricity and then perform related measurements. Its sensitive element is made of piezoelectric material. When it is stressed, charges are generated on its surface. After this charge is amplified and impeded by the charge amplifier and measurement circuit, it becomes an electric quantity output proportional to the external force exerted. Its advantages are wide bandwidth, high sensitivity, high signal-to-noise ratio, simple structure, reliable operation and light weight.

The working principle of the piezoelectric pressure sensor is based on the positive piezoelectric effect. The positive piezoelectric effect refers to: when the crystal is acted upon by an external force in a fixed direction, an electric polarization phenomenon occurs internally, and charges with opposite signs are generated on two surfaces at the same time. When the external force is removed, the crystal returns to its uncharged state. If the direction of the external force changes, the polarity of the charge also changes, and meanwhile, the amount of charge generated by the force on the crystal is proportional to the size of the external force. It has many types and models, and can be divided into diaphragm type and piston type according to the form of elastic sensitive components and force-bearing mechanisms.

5. Resonance line pressure sensor

The resonant pressure sensor is a device that uses a resonant element to convert the measured pressure into a frequency signal. It uses itself as a periodic signal output (quasi-digital signal), which can be converted into a digital signal that is easy to interface with a microprocessor using only a simple digital circuit. At the same time, its repeatability, resolution, and stability are good.

* Working principle

Any elastic body has a natural frequency of vibration. When external excitation can overcome the damping force, it may vibrate. Its oscillation frequency is related to the natural frequency, damping characteristics and excitation characteristics of the elastic body. If the excitation frequency is the same as the natural frequency of the elastic body and is just large enough to compensate for the damping loss, the elastic body can continuously oscillate with constant amplitude, and the vibration frequency is the natural frequency. When the measured value changes, it will cause a vibration system, then the equivalent stiffness or equivalent mass changes, causing the natural frequency of the elastomer to change.

* Structure

1) The vibrating element is the core component, called the oscillator or resonator.

2) Commonly used detection methods: magnetoelectric effect, electrostatic effect, piezoelectric effect, piezoresistive effect, and optical effect.

3) Commonly used excitation methods include: piezoelectric excitation, electromagnetic excitation, electrostatic excitation, electrothermal excitation, and optical excitation.

4) The amplifier is used to adjust the amplitude and phase of the signal so that the system can work stably and reliably in a self-excited state.

5) The compensation device mainly compensates for nonlinearity and temperature errors.

* Advantages:

1) The output is a quasi-digital signal, which is easy to process.

2) The system has low power consumption, good stability and good anti-interference performance.

3) The sensor is in a resonant state: the output automatically tracks the input, has good repeatability, and has high sensitivity, resolution, and accuracy.

4) Integral structure, no moving parts, solid mechanical structure and good reliability.

* Disadvantages:

1) Requires higher quality materials.

2) The processing technology is complex, the production cycle is long, and the cost is high.

3) The relationship between its output frequency and the measured value is often nonlinear, and linearization must be performed to ensure good accuracy.

6. Strain gauge pressure sensor

Strain gauge pressure sensor is a sensor that can convert mechanical strain into electrical signal output. It is mainly composed of elastic sensitive components, signal conditioning circuits, output circuits, etc. When subjected to external pressure, the elastic element will produce strain, thereby changing characteristics such as resistance value or capacitance value, thereby outputting an electrical signal related to the pressure. Its working principle is based on the thermistor effect or vertical effect. And its main principle is: when an external load is applied to the surface of a sensitive element, and the element deforms, causing strain inside it. These strains will also cause changes in the resistance value, thereby generating electrical signals in the circuit, thereby measuring external pressure information.

Strain gauge pressure sensors are widely used in many fields such as medical equipment, automobiles, and industrial manufacturing. It can accurately measure the pressure exerted by the outside world, thereby providing corresponding control and detection data. For example, in automobiles, this sensor can be used to measure the air pressure of tires to ensure vehicle stability and safety. In machinery manufacturing, this sensor can be used to measure the mechanical properties of metal materials to evaluate their quality.

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