Capacitive sensors are various types of capacitors used as sensing elements. A conversion device that converts the measured physical quantity or mechanical quantity into a change in capacitance is actually a capacitor with variable parameters. Capacitive sensors are widely used for measurement of displacement, angle, vibration, speed, pressure, composition analysis, and dielectric properties. The most commonly used are parallel plate capacitors or cylindrical capacitors.
Since the late 1970s, with the development of integrated circuit technology, capacitive sensors have been introduced that are packaged with miniature measuring instruments. This new type of sensor greatly reduces the effects of distributed capacitance, and its inherent shortcomings are overcome. Capacitive sensors are a very versatile and promising sensor.
A typical capacitive sensor consists of upper and lower electrodes, an insulator, and a substrate. When the film is subjected to pressure, the film will be deformed to some extent. Therefore, the distance between the upper and lower electrodes changes to some extent, so that the capacitance changes. However, the relationship between the capacitance of the capacitive pressure sensor and the distance between the upper and lower electrodes is a nonlinear relationship. Therefore, the output capacitance is nonlinearly compensated by a measurement circuit having a compensation function.
Classification of capacitive sensors and their advantages and disadvantagesClassification of capacitive sensors
According to the working principle of the sensor, the capacitive sensor can be divided into three types: variable pole distance type, variable area type and variable medium type.
Capacitive sensors can be classified into three types of structural forms depending on the structure of the sensor. They can be divided into linear displacement and angular displacement in the form of displacement, each of which is divided into a flat (circular) plate shape and a cylindrical (cylindrical) shape according to the shape of the sensor plate, although there are spherical and zigzag shapes. Other shapes, but generally rarely used. Among them, the differential type is generally superior to the single-group (single-sided) type sensor, which has the characteristics of high sensitivity, wide linear range and high stability.
Advantages and disadvantages of capacitive sensors
1, advantages
(1) Good temperature stability
The capacitance value of the capacitive sensor is generally independent of the electrode material, which is advantageous for selecting a material with a low temperature coefficient, and the heat generation is extremely small, which has little effect on stability. The resistance sensor has copper loss, which is easy to generate heat and produce zero drift.
(2) Simple structure
The capacitive sensor is simple in structure, easy to manufacture and guarantees high precision. It can be made very compact to achieve some special measurements. It can work in high temperature, strong radiation and strong magnetic fields, and can withstand large temperatures. Change, withstand high pressure, high impact, overload, etc.; can measure ultra-high temperature and low pressure difference, can also measure the magnetic work.
(3) Good dynamic response
Due to the small electrostatic attraction between the electrodes (about 10^(-5)N), the capacitive sensor requires very little energy and can be made very small and thin due to its movable part. Very light, so its natural frequency is very high, dynamic response time is short, can work at a few megahertz frequency, especially suitable for dynamic measurement. Because of its small dielectric loss, it can be powered by higher frequencies, so the system operates at a higher frequency. It can be used to measure parameters that change at high speeds.
(4) Non-contact measurement and high sensitivity
Non-contact measurement of vibration or eccentricity of the rotary shaft, radial clearance of small ball bearings, etc. When non-contact measurement is used, the capacitive sensor has an average effect, which can reduce the influence of the surface roughness of the workpiece on the measurement.
In addition to the above advantages, the capacitive sensor has a small electrostatic attraction between the electrode plates and a minimum input force and input energy, so that extremely low pressure, force, and small acceleration and displacement can be measured. It can be made very sensitive, has high resolution, and can sense displacements of 0.01 μm or less. Due to the small dielectric loss such as air, the zero residue generated when the differential structure is connected to the bridge type is extremely small, thus allowing the circuit to perform high-magnification amplification, so that the instrument has high sensitivity.
1, shortcomings
(1) The output impedance is high and the load capacity is poor.
Regardless of the type of capacitive sensor, the capacitance is very small, generally tens to hundreds of picofarads (pF), so the output impedance of the capacitive sensor is very high. 
~ 
Ω. Due to the high output impedance, the output power is small, the load capacity is poor, and it is susceptible to external disturbances, which may cause instability. In severe cases, it may not work.
(2) The parasitic capacitance has a large influence.
The initial capacitance of a capacitive sensor is small, and the parasitic capacitance of the lead cable capacitor connecting the sensor and the electronic circuit, the stray capacitance of the electronic circuit, and the capacitance formed by the capacitor plate and the surrounding conductor are large. The presence of parasitic capacitance not only reduces measurement sensitivity but also causes nonlinear output. Because the parasitic capacitance is randomly changing. Therefore, the sensor is in an unstable working state. Affects measurement accuracy.
Capacitive sensor measurement conversion circuitA capacitive sensor converts a measured physical quantity into a capacitance change, and a circuit that converts the capacitance into a power amount is called a conversion circuit of the capacitive sensor. At present, bridge circuits, frequency modulation circuits, pulse width adjustment circuits, and operational amplifier circuits are commonly used. Only the bridge circuit and the operational amplifier circuit are introduced here.
First, the bridge circuit
Connect the capacitive sensor to the AC bridge as one or two adjacent arms of the bridge. The other two arms can be resistors, capacitors or inductors, or the two secondary coils of the transformer, as shown in Figure 1.
In the single-arm bridge circuit of Figure 1a, capacitors C1, C2, C3, and Cx form the four arms of the bridge, and CX is the capacitive sensor. When Cx changes, U0≠0 has an output voltage.
In the differential connection bridge circuit of Figure 1 b, the output voltage can be expressed by the following equation:
Since the bridge output voltage is proportional to the power supply voltage, the power supply voltage fluctuation is required to be extremely small, and measures such as stabilization and frequency stabilization are required. Therefore, in practical applications, the output impedance of the AC bridge with the capacitive sensor is very high (generally several mega ohms to tens of mega ohms), and the output voltage amplitude is small, so the signal must be amplified after the high input impedance amplifier is connected. Can be measured.
The block diagram of the system consisting of bridge circuits is shown in Figure 2.
Second, the frequency modulation circuit
The capacitive sensor is connected to the LC resonant tank of the high frequency oscillator as part of the loop. When the measured change changes the sensor capacitance, the oscillator's oscillation frequency changes, ie the oscillator frequency is modulated by the sensor capacitance. The block diagram of its circuit composition is shown in Figure 3.
Frequency of the frequency modulation oscillator:
Features:
? The conversion circuit generates a frequency signal that can be transmitted over long distances without interference.
? With high sensitivity, it can measure displacement changes up to 0.01μm.
? However, the nonlinearity is poor, and it can be compensated by being converted into a voltage signal by a frequency discriminator (frequency voltage conversion).
Third, the operational amplifier circuit
Connect the capacitive sensor to the operational amplifier circuit with open-loop amplification factor A, as the feedback component of the circuit, as shown in Figure 4. In the figure, U is the AC power supply voltage, C is the fixed capacitor, Cx is the sensor capacitance, and Uo is the output signal voltage.
From the working principle of an ideal amplifier:
TPS Flat Cable
AS/NZS 5000.2 approved eletric cable: TPS,building cable export to Australia, New Zealand, Fiji, Papua New Guinea and other Oceania market for more than 10 years, quality assured.
- Standard applied: AS/NZS 5000.2
- U0/U: 450/750V
- Certification: SAA
- fire resistance or Low smoking and Halogen free or other property can be available
Tps Flat Cable,Tps Flat Wires,Tps Flat Electrical Cable,Rubber Electrical Cable
Shenzhen Bendakang Cables Holding Co., Ltd , https://www.bdkcables.com