Capacitive sensor

Capacitance sensor, the English name for the capacitive type transducer, is a kind of instrument that reflects the change of other quantity by capacitance. It is mainly composed of upper and lower electrodes, insulators, and substrates. Under the action of pressure, the film generates a certain deformation. The distance between the upper and lower stages changes, resulting in a change in capacitance, but the capacitance does not change linearly with the distance between the electrodes. Measure the circuit to make certain non-linear compensation to the output capacitance. [1]

The capacitive sensor principle generates different capacitances between the finger skin and the chip according to the ridges and valleys of the finger pressed on the acquisition head, and the chip obtains a complete fingerprint by measuring different electromagnetic fields in the space. By this construction principle, the security of fingerprints can be greatly improved. Forgery fingerprints are generally made of insulating materials such as silicone or gelatin, and they cannot be imaged on a capacitive sensor. This makes fake fingerprints useless. However, the capacitor technology chip is expensive and susceptible to interference.

From the perspective of energy conversion, the capacitive converter is a passive converter and needs to convert the measured mechanical quantity into voltage or current and then amplify and process it. The linear displacement, angular displacement, interval, distance, thickness, stretching, compression, expansion, deformation, etc. in mechanical quantities are all closely related to the length; these quantities are all measured by length or length ratio. The correlation of their measurement methods is also very close. In addition, under some conditions, these mechanical quantities change very slowly, and the range of variation is extremely small. If it is required to measure a very small distance or displacement, it must have high resolution. Other sensors are difficult to achieve high resolution requirements. The resolution of differential transducers commonly used in precision measurement is only on the order of 1 to 5 μm; there is a capacitance micrometer whose resolution is 0.1 nm, which is four orders of magnitude higher than the former and the maximum range is 1250. Μm, but also zero hysteresis, so he was favored in precision small displacement measurements.

A capacitive sensor is a sensor that converts a change in measurement (eg, size, pressure, etc.) into a change in capacitance. The measured mechanical quantity, such as displacement, pressure, etc., is converted into a sensor whose capacitance changes. Its sensitive part is a capacitor with variable parameters. The most common form is a capacitor with two parallel electrodes and an air-medium between the poles. Neglecting the edge effect, the capacitance of the plate capacitor is εA/δ, where ε is the dielectric constant of the inter-electrode medium, A is the effective area covered by the two electrodes, and δ is the distance between the two electrodes.

Any of the three parameters δ, A, and ε will cause a change in capacitance and can be used for measurement. Therefore, capacitive sensors can be classified into three types: pole pitch change, area change, and medium change. The pole-pitch type is generally used to measure minute line displacements or pole-pitch changes due to forces, pressures, vibrations, etc. (see capacitive pressure sensors). The area change type is generally used to measure angular displacement or larger line displacement. The medium change type is commonly used for level measurement and determination of temperature, density, and humidity of various media. Since the late 1970s, with the development of integrated circuit technology, capacitive sensors have appeared in packages with miniature measuring instruments. This new type of sensor can greatly reduce the influence of distributed capacitance and overcome its inherent disadvantages. Capacitive sensors are extremely versatile and promising sensors. [2]

Capacitive sensors are generally composed of two parallel electrodes with air as the medium between their two electrodes. Without considering the edge effect, the capacitance can be expressed as C=εS/d, where ε represents the two electrodes. The dielectric constant of the medium (ie, air), S represents the area covered by the two electrodes, and d represents the distance between the two electrodes. The capacitance is affected by these three parameters, and any change in the parameters will cause the capacitance to change. Therefore, the capacitive sensor can be classified into three types of media change types (change of ε and hence of C), change of area (change of S and therefore of C), and change of pole pitch (change of d, therefore, change of C).

Capacitance sensors are also referred to as capacitance level meters. The capacitance detection element operates on the principle of a cylindrical capacitor. The cylindrical capacitor is mainly composed of two coaxial cylindrical plates insulated from each other, and the medium is filled between the two plates. The capacity of the capacitor is C=2∏eL/lnD/d, where ε represents the dielectric constant of the dielectric between the two plates, L represents the length of the mutual overlap between the plates, and D represents the outer cylindrical plate The diameter, d, indicates the diameter of the cylindrical plate inside. Since the three parameters D, d, and e do not change when measured under fixed conditions, the level can be known from the measured capacitance. height.

Capacitive sensors have certain advantages over resistive sensors and inductive sensors, but they are not perfect. They also have disadvantages. Here we integrate the advantages and disadvantages of capacitive sensors:

Advantages: cheap, affordable; high sensitivity, good accuracy; simple structure; applicable in harsh environments; good temperature stability; average effect; good dynamic response; strong overload ability.

Disadvantages: The output is non-linear; the sensitivity of the parasitic capacitance, distributed capacitance, and measurement accuracy are easily affected and unstable; the connection circuit is more complicated. [3]

For these mechanical quantities mentioned above, especially slow-moving or micro-volume measurements, it is generally more appropriate to use capacitive sensors for detection. The main advantage of these sensors is the following:

Large measuring range

Its relative rate of change can exceed 100%;

high sensitivity

If using a ratio transformer bridge measurement, the relative variation can reach 10-7 orders of magnitude;

Dynamic response is fast

Because of its small movable mass and high natural frequency, high frequency characteristics are suitable for both dynamic and static measurements.

Good stability

Because the capacitor plates are mostly metallic materials, the inter-electrode lining materials are mostly inorganic materials such as air, glass, ceramics, quartz, etc.; therefore, they can work for a long time under high temperature, low temperature, strong magnetic field and strong radiation, especially to solve the high temperature and high pressure environment. The detection problem.

The forms of oil contamination are usually metal abrasive particles, oxides, sludge, carbon, moisture, sediments, fuel, and hydrogen, chlorine, heat, electricity, air and other pollution. After the oil pollution, its physical or chemical properties will change. According to the change of the dielectric constant, the overall pollution degree and quality of the used oil can be comprehensively measured.

The sensor can also be connected to the secondary instrument or controller in the control room to detect the moisture content of various low moisture oil products online, continuously and in real time. Direct display, remote control and alarm. Realize data storage, integration, transmission and control functions. The hydraulic and lubrication circulation systems commonly used in large and medium mechanical linkage groups include high-speed rolling mills, strip rolling mill lubricant systems, strip mills and bar-line mill hydraulic drive systems, turbogenerator lubrication systems, and paper mill lubrication systems. Marine machinery lubrication system, fuel tanks.

FW-C1 Capacitive Lubricant Real-time Online Monitoring Sensor

The sensor adopts capacitive measurement method, which can accurately measure the degree of contamination of lubricating oil online, including the degree of oxidation, moisture content, and other mechanical and chemical impurities, so as to accurately determine the quality of lubricating oil and determine whether it is necessary to replace the lubricating oil. The ability to predict equipment failures is a key component in the oil change management that changes the traditional oil change schedule and achieves quality change. The sensor adopts screw connection, small size, light weight, and reliable structure. It is an ideal on-line lubricating oil detection sensor and can be widely applied to the real-time detection of lubricating oil detection quality of various large-scale power machinery, bearings, gear boxes, pumps and steam turbines. in. The sensor can also be connected to a secondary instrument or controller in the control room to implement data storage, integration, transmission and control functions.

FWS-CII Online Capacitance Moisture Sensor

The sensor adopts the capacitive measurement method to detect the moisture content of the hydraulic and lubrication system media of various working machines on line, especially the external water can easily infiltrate into the rolling mill, paper machine, steam turbine and ship machinery inside the machine. Monitor the circulating oil system for leaks, such as water coolers. Monitor the sealing element of the working machine for damage, causing the penetration of external water. Monitor the effect of ambient air humidity on the quality and moisture content of lubricating hydraulic system oils. The accurate determination of the quality of lubricants and the prediction of equipment failures are key components in the management of equipment lubricants. The sensor adopts threaded connection, small size, light weight, reliable structure, high measurement accuracy, stable operation, and strong anti-electromagnetic interference performance. Closed stainless steel housing has good waterproof and dustproof properties. Can be installed directly on the factory site hydraulic lubrication pipe. It is an ideal online moisture detection sensor.

ZCS1100 precision capacitance displacement sensor

The sensor adopts capacitive measurement method, which can detect piezoelectric micro-displacement, vibration table, micro-adjustment of electron microscope, fine adjustment of astronomical telescope lens, and precision micro-displacement measurement. The sensor is a single channel, high performance linear displacement measurement system, innovative capacitive displacement measurement technology, nanometer measurement capability, low cost, and suitable for measuring any conductive target.

ZNXsensor type ultra-precision capacitance displacement sensor

The sensor is a non-contact capacitive displacement sensor; two sensor plates form a parallel plate capacitor, each sensor can be used in two unused measurement ranges; nano resolution; zero hysteresis.

How Capacitive Sensors Replace Mechanical Switches

Traditional mechanical switches have familiar user sensitivity and haptic feedback. For capacitive sensors, these parameters must also be considered and optimized. Different sensors may require unique sensitivity depending on the switch function or the physical location of the switch in the product. Moreover, a set of sensitivity settings may not be suitable for all users. Therefore, users should be allowed to set different sensitivity levels. It would be ideal if they can be selected through the sensitivity control menu. For example, the AD7142 supports these sensitivity requirements, allowing a single 16-bit sensitivity control register to be programmed for each sensor. These registers can also be embedded in the host firmware and provided in the menu display, allowing the user to select different sensitivity levels to meet their specific needs. When the user is not in contact with the sensor, sampling each sensor input wastes battery power. To maximize the efficiency of the battery, the CDC should be able to detect that the user stops touching the sensor and automatically switch to the low power mode. When the sensor is touched again, the IC will automatically re-enter normal operating mode.

In order to save more power, a complete shutdown mode should also be included. In this case, turning off the sensor turns off the entire IC. In portable products, disabling the sensor switch is usually accomplished by setting a mechanical switch or selecting the blocking mode from the control menu.

The use of capacitive sensors instead of traditional mechanical switches has the added benefit of making the manufacturing and assembly process easier. Traditional mechanical switches require each switch to be manually inserted into a dedicated hole in the plastic housing, and a single capacitive sensor board containing all of these switches can be placed in one step, underneath the plastic housing. A sensor plate mounting hole with a positioning notch and some glue is sufficient to complete the sensor board installation and position calibration.

Electromagnetic noise emitted by the main processor board may be coupled into capacitive sensors and sensor traces, causing unpredictable sensor action and degrading performance. However, simple methods can also help reduce the impact of electromagnetic interference (EMI) on the sensor. To the lowest. First, the CDC should be mounted on the sensor board, which will minimize the length of the sensor traces, thereby reducing the chance that EMI is coupled into the traces. Second, using a four-layer sensor board with a solid ground plane provides additional EMI shielding for the sensor. If these two methods cannot effectively isolate the EMI noise from the sensor, a grounded metal shield can also be placed over the sensor board cavity.

The capacitive sensor electric field is also coupled to the conductive surface of the product's metal shell or conductive metal coating, resulting in unpredictable sensor action. This creates mechanical constraints that require the edge of the capacitive sensor to maintain a certain distance from the edge of the metal surface. Moreover, the sensitivity of the capacitive sensor is also related to the thickness of the plastic just above the sensor. If the plastic is too thick, the flux power lines will not be able to effectively pass through the plastic, making the sensor performance unreliable. Typically, the distance between the housing and the sensor should be greater than 1.0 mm, and the thickness of the plastic should be less than 4.0 mm, keeping the sensitivity within a suitable range.

There are several ways to measure the capacitance. However, only the arithmetic capacitance method is suitable for automatic online measurement. In the application, there are more DC charging and discharging methods and AC methods. From the signal processing point of view, there is no essential difference between the charge and discharge method and the exchange method. [4]

The charge and discharge method signal processing flow is shown in Figure 1.

Figure 1 signal processing flowchart of charge and discharge method

The signal processing flow of AC method is shown in Figure 2.

Figure 2 signal processing flow of AC method

Therefore, the two circuits can be unified. Signal flow chart shown in Figure 3

Figure 3 unified measurement conversion circuit

Phase-controlled rectifier circuit shifts the spectrum of the input noise signal. The low frequency noise components are moved to the high frequency band, and the high frequency noise components are shifted to the low frequency band.

The phase-controlled rectified output signal is sent to the low-pass filter for processing. The high frequency components in the output noise signal will be filtered out. Therefore, the low-frequency noise in the input signal of the phase-controlled rectifier circuit will not affect the final measurement result. Noise near (2n-1)fc will be shifted out to low frequencies, affecting the final measurement results.

In order to make the measurement circuit have a higher resolution, the signal input to the phase-controlled rectifier circuit should have a large amplitude and a high signal-to-noise ratio.

The pre-amplifier circuit not only amplifies the signal but also introduces noise. The noise introduced by the amplifier circuit is determined by the amplifier circuit itself. After the signal passes through the primary processing circuit, it will add a fixed amount of noise. Therefore, when the output signal amplitude is constant, the signal-to-noise ratio of the signal is proportional to the average value of the signal.

In the charge-discharge method, when the square wave is applied, the AC-amplified output is a narrow pulse and the signal duty cycle is very low. Therefore, the signal to noise ratio is also very low. Second, to amplify the pulse signal to a larger bandwidth, the noise on both sides of the higher harmonics will also be moved to the lower frequency band by the phased rectifier, increasing the low frequency noise.

The AC method uses a single frequency sine signal as stimulus. The average value of the signal is large, so a higher output signal-to-noise ratio can be obtained. At the same time, since the processed signal is a single-frequency sinusoidal signal, a narrowband bandpass amplifier can be used to reduce the noise introduced by the amplifier and further output the signal to noise ratio of the signal.

The alternating current measurement conversion circuit can obtain higher resolution. The circuit structure is not more complicated than the charge-discharge method. Therefore, the AC excitation signal is selected to constitute the measurement system.