Capacitive sensor: the device and the principle of operation. Capacitive sensors: application

Modern industry, and human everyday human activity, too, can not be imagined without a different kind of electronic devices. They help us in everything, and some technological operations without them will not be possible at all. To such "assistants" should be attributed and a capacitive sensor.

This is what transducers are called, manufactured according to the parametric type. Measurement of a certain volume by such devices is carried out due to fluctuations in the capacitive resistance when some important parameters change. Simply put, the change in capacitor capacitance is evaluated under the influence of some external factors.

Principle of operation of the capacitive sensor

That's what capacitive sensors are. The principle of their work is not so complicated, but to understand it you need to know something. To begin with, let us recall the principle of determining the capacitance of a capacitor. This action is expressed using the following formula:

C = εεₒS / δ.

This expression is known to many from the school course of physics, but it would not hurt to refresh the memory and remember what each of the variables implies:

• S is the area of the capacitor plate.
• Ε is the relative permeability of the dielectric material used in the construction of the capacitor.
• Εₒ - so in physics it is customary to designate the dielectric constant of a vacuum.
• Δ - this can be either the thickness of the dielectric plate or the distance between several layers of the material.

Thus, it follows from the above formula that it is easy to change the capacitance of the capacitor. It is enough to somehow act on the area of the plate of dielectric material, the distance between the plates or directly on the permeability of the material used in the production. Accordingly, the choice of a specific value depends solely on the list of tasks that the designers have placed in front of the device.

Thus, you can even make a capacitive sensor yourself, since from a constructive point of view it is an ordinary flat or cylindrical capacitor, one of whose plates is constantly experiencing controlled movement in space, which leads to a change in capacitance. It should be remembered that the above formula is correct only if you completely neglect edge effects. We'll talk about this in the final part of this article.

One should know that such electronic devices are intensively used for measuring angular and linear movements of objects, calculating the dimensions, applied work, humidity, concentration of the active substance and other characteristics. With regard to the constructive side of the issue, these instruments are manufactured by plane-parallel, in cylindrical bodies, with pin electrodes, with a spacer of dielectric material and without it at all.

Here's how the capacitive sensors function. The principle of operation of some of them needs to be known especially in detail. Within the framework of this article, we give some formulas that may be useful to you.

Formulas for describing the principle of operation of certain types of sensors

A level sensor with a possible change in the area of dielectric plates can be fairly easily described using the following equation:

C = εεₒaX / δ.

The "X" in this case is the length of the overlap of the electrodes used. Accordingly, "a" denotes the width of the plates of the condenser itself. It should be noted that such instruments have found their application in various fields of industry, where they are used to accurately measure the angular quantities. The capacity of the converter is then found by the following expression:

C = εεₒ (r₂- r₁) / 2δ * (φₒ-φ).

In order to accurately measure the sensitivity, a somewhat different formula should be used:

K = εεₒ (r₂- r₁) / 2δ.

Let's see what is meant by those variables that are part of these equations:

• R₁ is the internal radius of the condenser plate;
• R₂ is the outer radius of the same plate;
• Φ is the current (current) value of the overlap angle;
• Φₒ is the initial value of the overlap angle.

Finally, we will analyze a mathematical expression that describes the operation principle of a capacitive meter with a variable air gap:

C = εεₒS / (δₒ-X).

It is not difficult to guess that δₒ is the primary gap, while the same letter X denotes the amount of displacement of the plate. Note! Since the static characteristics are not strictly linear, usually a level sensor of this type is used to measure extremely small displacements, the magnitude of which does not exceed 0.1δ. Naturally, these devices are in great demand in precision engineering, where even a smaller error can lead to serious enough problems.

Where can they be used?

Areas of their possible application are extremely diverse. So, in almost all industries, operations that are controlled by these devices can be found. They are used to control the filling of various tanks, and their contents can be liquid, granular or gaseous (gas sensor).

Their prevalence in industry and ordinary human production is higher the more reliable and simpler the design of such devices. For the totality of these signs, they are so good that they can be used even in incredibly aggressive conditions of oil tanker holds.

In addition, the capacitive sensor can be used as a limit switch on the conveyor line or the machine tool of the production hall. It is also necessary for the most accurate positioning of various mechanisms.

Proximity sensors

But at the present time , proximity sensors are of particular demand , which are executed in exactly the same way. The range of their use is even wider. This is due to the penny value of devices and the ability to work in virtually all types of industry. However, there are typical industries where devices of this type are the most in demand:

• Control over the filling of liquid transparent containers of plastic or glass.
• A similar function is performed by them in the production of food products (including children's), where the finished goods are packaged in containers of transparent materials. On the same principle, the work of such instrumentation as a capacitive fuel sensor is based.
• To control dangerous areas where the winding wire is broken.
• Control of similar places where the conveyor belt can be damaged.
• Piece control of the produced product type (recalculation of cans, bottles, packages).

Not surprisingly, these electronic devices are the most common in precision engineering, energy and many other industries a variety of sensors.

Inclinometers

Instruments that have become relatively common only in recent years are small capacitive inclinometers that provide an electrical output signal whose magnitude is directly proportional to the angle of inclination of the sensor used.

The most common main areas of use of these instruments are platform leveling systems, determining the amount of deflection and technical deformation of various support beams, as well as the most accurate control of the slope of road and railroad tracks at the stage of their construction.

In addition, with the help of such devices, the roll of heavy vehicles and other vehicles, hoists and industrial excavators is determined, and the degree of angular displacement in relation to agricultural and industrial machinery of extra large size is determined.

Capacitive fuel level sensors in the oil industry are very important. They are used even on supertankers, which for one flight transport tens and hundreds of thousands of tons of processed oil products. These devices are extremely effective even in conditions of the formation of extremely abundant condensate and a high degree of dustiness in the production room (the same gas sensor).

They find their application in measuring the magnitude of the absolute and relative pressure levels, as well as the thickness of the dielectric material, which is extremely important in almost all industries where really powerful capacitors are used.

The main advantages of capacitive sensors

It should be noted that the capacitive sensor has a lot of advantages when compared with similar devices that are performed according to slightly different principles. Let's list the main advantages of these instruments:

• In manufacturing, they are extremely simple. In addition, the simplest and cheapest materials can be used in their production. Even capacitive fuel level sensors used in important oil industry facilities have extremely modest dimensions and have the lowest possible level of electrical energy consumption. With all these characteristics, they are distinguished by an excellent level of sensitivity, which is often unattainable for more expensive devices.
• In principle, you can make a capacitive sensor yourself, using as its basis any more or less reliable and high-quality industrial capacitor.
• They do not have contacts (very rarely one single current collector is used), which is extremely beneficial for work in conditions of high dust content and humidity in the room.
• The service life is extremely long, the device repeatedly manages to "repulse" its low cost. Accordingly, the capacitive sensor (whose price is in the range of 1200-1700 rubles) is an extremely profitable acquisition.
• To move the moving part of the device, it takes surprisingly little effort.
• The device is very easy to combine with almost all categories of equipment that is only used in industrial activities.

Negative moments

Unfortunately, each capacitive sensor has certain drawbacks, which to some extent hamper the widespread use of this type of equipment. Let us list them in more detail:

• The conversion factor (i.e. transmission) is relatively low.
• The small size and simplicity of design contribute to the fact that there are put forward rather high requirements to the quality of screening devices.
• A good capacitive level sensor (and other similar measuring devices) can operate efficiently only at a frequency much higher than the standard value of 50 Hz.

Important notes

However, everything is not so bad. Many manufacturers achieve excellent sensor screening characteristics by making minimal changes to their design. As for frequency of use, in practice they show excellent results at a value widely used in industry at 400 Hz.

We have already talked about the validity of the basic formula only if the edge effect is ignored. But it is useful to know that it can really have a negative effect only if the distance between the dielectric plates is comparable to their own dimensions. In addition, the negative effect can be largely leveled by simply using a protective ring. In this case, the boundaries of the influence of the effect can be transferred far beyond the used plates.

Once again, we note that the same pressure sensors are distinguished by a remarkable simplicity, which makes it possible to create on the marvel stable, durable and cheap designs. If you correctly choose the geometric dimensions of the dielectric used, you can not worry about the materials used in the production of such a capacitor.

Thus, correctly selecting the metal mark for manufacturing the sensor housing, it is possible to neglect practically even the strong temperature fluctuations that could lead to a change in the capacitance of the device and the inadequacy of its readings. Of course, this does not eliminate the need to carefully isolate the pressure sensors and other similar indicators from aggressive environmental factors. Despite their simplicity, high humidity and high radiation levels can negatively affect the reliability of the device.

Classification of sensors

The methods of production used in industry make it possible to divide all produced types of sensors into two large groups: one-sample and two-sampling. The latter variety is divided into differential and semi-differential. We will examine them in more detail.

A single-tool device. In this case, the capacitive sensor circuits are simple to the extreme, since the main part is the most common capacitor with variable capacitance. Unfortunately, even slightly increased humidity and temperature have a very tangible effect on the accuracy of the readings. Because of this, there are often various sensor malfunctions. To level the magnitude of such errors, we have to use differentiated constructions.

Two-sensor sensor. Actually, he is such a differentiated structure. Very often you can find a capacitive level sensor, manufactured exactly according to this scheme. These devices are free from the main drawbacks of the previous model, but have their own weaknesses. The most significant disadvantage is the need to use two or three shielded wires between the device itself and the surface, since only in this way can the so-called parasitic capacitances be suppressed.

However, it's easy not to pay attention to rather complicated schemes of capacitive sensors in this case , because in return you get an extremely accurate and sensitive instrument.

Specificity of designing sensors

In many cases (from the design point of view) the creation of such devices is quite problematic. This is especially true when it is required to create a sensor with a variable level of capacitance. However, practice shows that many problems are almost completely solved by accurate calibration and high characteristics of materials used in production. Most often, these difficulties are faced by producers of two-sensor sensors.

In general, the specificity of this type of measuring instruments is that they can be represented in the form of a dimensionless ratio of two physical quantities (capacitances) that have a precise physical expression and meaning. So they can be called "proximity sensors". The advantage of these devices (their huge plus!) Is that they generally may not have any reference measures in their design, which improves their reliability in really extreme situations and conditions.

Characteristics of linear displacement sensors

All non-electrical quantities, which are often required to be controlled in industrial conditions, are extremely diverse and multifaceted. A significant part of measures that are subject to strict control, are angular and even linear displacements of various kinds of surfaces in space. If we use a capacitor that has an absolutely uniform electric field in the working gap, then it is not so difficult to make electronic sensors of the following two types:

• In which the area of the electrodes will be variable.
• Those that have a variable gap between these electrodes.

It is easy to understand that the first type is most suitable for fixing really large displacements, while with the help of the second type one can even notice such movements of the body in space, the magnitude of which is only a few microns!

Angular displacement sensors

In general, by design and purpose, they are almost completely identical to the type just considered. The similarity is manifested in the fact that sensors with a variable electrode area should also be used for large measurements, and with a variable distance between the electrodes themselves for small ones. As a rule, such devices are made multisection, with the possibility of changing the area of the capacitor plates.

To achieve this, the first electrode is attached to the movable shaft, upon rotation of which it changes its position relative to the second, which ensures a change in the overlapping area of the dielectric plates in the capacitor. Naturally, at the same time the capacitance change is fixed.

conclusions

So we examined the main characteristics of devices of this level, learned about the areas of their application, the features of the design, the principle of operation and possible technical solutions. As you could understand from the article, the prevalence of capacitive sensors and their extremely high popularity are based on the very attractive price of such devices and the long service life even in difficult environments.

All this is possible due to the fact that, from a constructive point of view, all these meters are just standard capacitors, which are characterized by a somewhat unusual way of using them. However, you yourself can find this out by looking again at mathematical formulas that in general terms reflect the principles of the operation of instrumentation.