It is impossible to imagine modern industry, and the everyday human activities of a person, without all kinds of electronic devices. They help us in everything, and some technological operations without them will not succeed at all. These "assistants" should also include a capacitive sensor.
So called converters made by parametric type. A certain volume is measured by such devices due to fluctuations in capacitance when some important parameters change. Simply put, the change in the
capacitance of a capacitor under the influence of some external factors is estimated.
The principle of operation of a 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, we 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 physics course, but it would not hurt to refresh 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 design of the capacitor.
- εₒ - so in physics it is customary to denote the dielectric constant of a vacuum.
- δ - this may indicate either the thickness of the dielectric plate, or the distance between several layers of material.
Thus, it follows from the above formula that it is easy to change the capacitance of a capacitor. It is enough to somehow act on the area of the plate of the dielectric material, on 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 designers set for the device.
Thus, you can even make a capacitive sensor with your own hands, since from a structural point of view it is an ordinary flat or cylindrical capacitor, one of the plates of which constantly experiences controlled movement in space, which leads to a change in capacitance. It should be remembered that the above formula is true only if you completely neglect the edge effects. We will talk about this in the final part of our article.
You should be aware that electronic devices of this kind are intensively used to measure the angular and linear movements of objects, to calculate dimensions, applied work, humidity, active substance concentration and other characteristics. As for the constructive side of the issue, the aforementioned instrumentation devices are made plane-parallel, in cylindrical cases, with pin electrodes, with a gasket made of dielectric material and without it at all.
Here's how capacitive sensors work. The principle of operation of some of them needs to be known in detail. As part of this article, we will provide several 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 the dielectric plates can be quite easily described using the following equation:
C = εεₒaX / δ.
Under "X" in this case refers to the length of the overlap of the electrodes used. Accordingly, “a” denotes the width of the plates of the capacitor itself. It should be noted that such devices have found their application in various fields of industry, where they are used for the most accurate measurement of angular values. The capacity of the Converter in this case is found by the following expression:
C = εεₒ (r₂- r₁) / 2δ * (φₒ-φ).
In order to accurately measure sensitivity, a slightly different formula should be used:
K = εεₒ (r₂- r₁) / 2δ.
Let's look at what is meant by those variables that are part of these equations:
- r₁ is the internal radius of the capacitor plate;
- r₂ is the outer radius of the same plate;
- φ is the currently measured (current) value of the overlap angle;
- φₒ is the initial value of the overlap angle.
Finally, we analyze the mathematical expression that describes the principle of operation of a capacitive meter with a variable air gap:
C = εεₒS / (δₒ-X).
It is easy to guess that δₒ is understood as the primary gap, and the letter X denotes the amount of movement of the plate. Note! Since the static characteristics are purely non-linear, usually a level sensor of this type is used to measure extremely small displacements, the magnitude of which does not exceed 0.1δₒ. Naturally, these devices are extremely popular in precision engineering, where even a smaller error can lead to quite serious problems.
Where can they be used?
The areas of their possible application are extremely diverse. So, in almost all industries, you can find operations that are controlled precisely by these devices. 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 activities is the higher, the more reliable and simpler the design of such devices. By the totality of these features, they are so good that they can be used even in the incredibly aggressive conditions of oil tanker holds.
In addition, a capacitive sensor can be used as a limit switch on a conveyor line or machine tool in a production workshop. It is also necessary for the most accurate positioning of various mechanisms.
Proximity sensors
But at present, proximity sensors, which are made according to exactly the same principle, are in special demand. The range of their use is even wider. This is due to the penny cost of devices and the ability to work in almost all types of industry. However, there are typical industries where devices of this type are the most popular:
- Control over filling liquid in transparent containers made 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 made of transparent materials. The operation of such an instrument as a capacitive fuel sensor is based on the same principle.
- To control hazardous areas where winding wire breakage is possible.
- Control of similar places where the conveyor belt may be damaged.
- Piece control of the type of product manufactured (recalculation of cans, bottles, packaging).
It is not surprising that these electronic devices are the most common type of sensors in precision engineering, energy and many other industries.
Inclinometers
Devices that have become relatively common only in recent years are small-sized capacitive inclinometers that provide the transmission of 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 devices: platform leveling systems, determining the amount of deflection and technical deformation of various kinds of support beams, as well as precise control of the slope of automobile and railway tracks even at the stage of their construction.
In addition, with the help of such devices, the roll of heavy vehicles and other vehicles, lifts and industrial excavators is determined, and the degree of angular displacement in relation to agricultural and industrial machines of especially large size is also determined.
Capacitive fuel level sensors in the oil industry are very important. They are even used on supertankers, which transport tens and hundreds of thousands of tons of refined petroleum products in one trip. These devices are extremely effective even in conditions of formation of extremely plentiful condensate and a high degree of dustiness of the production room (the same gas sensor).
They find their application in measuring 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.
Key benefits of capacitive sensors
It should be noted that a capacitive sensor has a large number of advantages when compared with similar devices, which are made according to slightly different principles. Let's list the main advantages of these instrumentation:
- They are extremely simple to manufacture. In addition, the simplest and cheapest materials can be used in their production. Even capacitive fuel level sensors used at important facilities of the oil industry have extremely modest dimensions and have the lowest possible level of electrical energy consumption. With all these characteristics, they have an excellent level of sensitivity, which is often unattainable for more expensive devices.
- In principle, you can make a capacitive sensor yourself, using any more or less reliable and high-quality industrial capacitor as its basis.
- They have no contacts (one current collector is very rarely used), which is extremely beneficial for work in conditions of high dust and humidity in the room.
- The service life is extremely long, the device manages to “recapture" its low cost many times. Accordingly, a capacitive sensor (the price of which is in the range of 1200-1700 rubles) is an extremely advantageous acquisition.
- To move the moving part of the device, surprisingly little effort is required.
- The device is very easily combined with almost all categories of equipment that is only used in industrial activities.
Negative points
Unfortunately, each capacitive sensor has certain disadvantages, which to one degree or another complicate the widespread use of this type of equipment. We list them in more detail:
- The conversion factor (i.e. transmission) is relatively low.
- The small size and simplicity of the design contribute to the fact that quite high demands are placed on the quality of the shielding of devices.
- A good capacitive level sensor (and other similar measuring instruments) can only work efficiently at a frequency that far exceeds the standard value of 50 Hz.
Important notes
However, everything is not so bad. Many manufacturers achieve excellent sensor shielding performance by making minimal changes to their design. As for the frequency of use, in practice they show excellent results at a widely used industry value of 400 Hz.
We have already talked about the fidelity of the basic formula only if the boundary effect is ignored. But it’s 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 offset by simply using a protective ring. In this case, the boundaries of the effect can be transferred far beyond the limits of the used plates.
Once again, we note that the same pressure sensors are remarkable for their simplicity, which makes it possible to create wonderfully stable, durable and cheap designs. If you correctly select the geometric dimensions of the dielectric used, then the materials used in the production of such a capacitor need not be particularly worried.
Thus, having correctly selected the metal grade for the manufacture of the sensor case, one can almost neglect even strong temperature fluctuations, which could lead to a change in the capacity of the device and inadequacy of its readings. Of course, this does not at all eliminate the need to isolate pressure sensors and other similar indicators as carefully as possible from aggressive environmental factors. Despite their simplicity, high humidity and high levels of radiation can adversely affect the reliability of the device.
Sensor Classification
The methods of their production used in industry make it possible to divide all produced types of sensors into two large groups: single-capacitance and two-capacitance. The latter variety is divided into differential and semi-differential. Let's consider them in more detail.
Single capacitance device. In this case, the capacitive sensor circuits are simple to the extreme, since their main part is the most ordinary capacitor with variable capacitance. Unfortunately, even slightly elevated humidity and temperature have a very noticeable effect on the accuracy of the readings. Because of this, various sensor failures often occur. In order to level the magnitude of such errors, it is necessary to use differentiated designs.
Two-capacitive sensor. Actually, he is such a differentiated structure. Very often you can find a capacitive level sensor, made exactly in this way. These devices are spared from the main disadvantages of the previous model, but have their own weaknesses. Their most significant drawback 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 stray capacitances be suppressed.
However, in this case, it is easy to ignore the rather complex circuits of capacitive sensors , since in return you get an extremely accurate and sensitive device.
Sensor Design Specifics
In many cases (from a design point of view) the creation of such devices is quite problematic. This is especially true when you want 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 the materials used in the production. Most often, these difficulties are faced by manufacturers of dual-capacitance sensors.
In general, the specificity of this type of measuring instruments is that they can be represented as a dimensionless ratio of two physical quantities (capacities) that have an exact physical expression and meaning. So they can be safely called "attitude sensors." The advantage of these devices (a huge plus!) Is that they generally may not have any standard measures in their design, which greatly improves their reliability in really extreme situations and conditions.
Characteristics of linear displacement sensors
All non-electric quantities that often need to be controlled in an industrial environment are extremely diverse and multifaceted. A significant part of the measures that are subject to strict control are angular and even linear movements of various kinds of surfaces in space. If you use a capacitor with an absolutely uniform electric field in the working gap, 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 movements, while with the second variety you can even notice such movements of the body in space, the magnitude of which is only a few microns!
Sensors for detecting angular movements
In general, in design and purpose, they are almost completely identical to the type we just examined. 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 multi-sectional, with the possibility of changing the area of the capacitor plates.
To achieve this, the first electrode is attached to the movable shaft, during rotation of which it changes its position relative to the second, which provides a change in the overlap area of the dielectric plates in the capacitor. Naturally, this changes the capacitance.
conclusions
So we examined the main characteristics of devices of this level, learned about the areas of their application, design features, the principle of operation and possible technical solutions.
As you can 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 a long service life even in difficult environmental conditions.All this is possible due to the fact that, from a structural 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 figure it out by looking again at the mathematical formulas, which in general outline reflect the principles of operation of instrumentation.