Non-contact capacitive sensors. Encyclopedia of radio electronics and electrical engineering

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Capacitive sensors respond to a wide variety of substances - solid and liquid, metals and dielectrics. They are used, for example, for non-contact control of filling tanks with liquids and bulk materials, positioning and counting various objects, and protection of objects. The proposed article describes the principle of operation of non-contact sensors, provides diagrams suitable for practical implementation and use. relative permittivity of the environment. A typical sensor with a sensitive surface diameter of 1 mm fixes a "standard target" (term according to [2]) at a distance of 60 mm.

The sensitive element of a non-contact capacitive sensor is a capacitor with plates deployed in one plane, as shown in Fig. 1.

Depending on the presence or absence of a foreign object, the average permittivity of the surrounding plate of the medium changes and, consequently, the capacitance of the capacitor. The latter serves as a frequency-setting element of the oscillator. The threshold device present in the sensor monitors the amplitude or frequency of oscillations, when they change, actuating the actuating unit.

In many capacitive sensors, the oscillator frequency is chosen to be several megahertz. Generators are built on discrete transistors, the number of which reaches five. However, a generator that is sufficiently sensitive to changes in capacitance and operates at frequencies of hundreds of kilohertz can be built on just one middle-class op amp.

The classical scheme of the generator of rectangular pulses on the op-amp, shown in fig. 2.

Its detailed description and calculation are given in [4]. If the op-amp DA1 is ideal, the oscillation frequency is inversely proportional to the capacitance of the capacitor C1 (sensing element of the sensor), and their amplitude is unchanged. In fact, with a decrease in capacitance and an increase in frequency, there comes a moment when, due to the inertia inherent in a real op-amp, the conditions for self-excitation of the generator cease to be fulfilled and the oscillations break down.

It remains to ensure that the generator works in the presence of a foreign object in the sensitive zone, and when it is removed (which is equivalent to a decrease in the capacitance of the capacitor), it no longer exists. This mode has certain advantages over the known ones, when the generator operates continuously [5, 6], or only in the absence of a foreign object [7, 8].

The idea was tested by simulating a generator using the ELECTRONIC WORKBENCH program. From the library of standard program elements, the OS HA2502 was chosen for the model. The resistor values ​​were: R1 - 330 kOhm, R2 - 1 kOhm, R3 - 2 kOhm. Oscillations gently arose and broke down when the capacitance of the capacitor C1 changed from 11 to 12 pF, and vice versa. With a high degree of confidence, it can be argued that this is sufficient for the reliable operation of the capacitive sensor. Subsequently, the conclusion was confirmed by testing real structures.

The sensitive element of the sensor was made of one-sided foil-coated insulating material, on which two rectangular sections of foil 70x50 mm in size were left, adjacent to each other with short sides with a gap of 2 mm. The capacitance of the "unwrapped capacitor" formed in this way is approximately 5 pF. The length of the wires connecting the capacitor plates to the generator must be minimal, not more than 50 mm.

A practical circuit of the generator on one of the two op amps of the KR157UD2 chip is shown in fig. 3.

Since the microcircuit is powered from a single source, a bias equal to half the supply voltage is applied to the non-inverting input of the op-amp using a resistive divider R3R4. The frequency-setting circuit is formed by a resistor R2 and a capacitance of the sensing element E1. Resistor R1 serves to protect the input of the op-amp from interference and interference that can disable the op-amp.

It should be noted the important role of the capacitor C1, which corrects the frequency response of the op-amp. The "working point" of the generator on the slope of the frequency response depends on the capacitance of this capacitor. Two options were tested: C1=12 pF, R5=180 kOhm (frequency 200 kHz) and C1=6,8 pF, R5=1 MΩ (frequency 500 kHz). In both cases, by adjusting the resistor R2, it was possible to achieve that the generator was excited when a foreign object approached the sensitive element. Adjustment should preferably be done with a long screwdriver made of insulating material.

During the tests, the sensor "felt" a human hand or a tank of water at a distance of several centimeters. At a shorter distance, it was possible to find a wooden block, an empty glass jar, and even a student's eraser.

The generator circuit on the K1407UD1 chip is shown in fig. four.

Its properties are approximately the same as those discussed above. Since the applied op-amp does not have pins for connecting correction circuits, its performance is degraded with the help of feedback through the R3C1 circuit. In addition, like the resistor R1 in the previous device (see Fig. 3), the resistor R3 protects the input of the op-amp from interference. The operating frequency of the generator is approximately 100 kHz.

On fig. 5 shows a diagram of a contactless sensor on a KR157DA1 microcircuit [9].

In contrast to the previously considered ones (see Fig. 3 and 4), an additional OS was not required in the sensor generator, since the own bandwidth of the op-amp DA1.1 is quite narrow. However, in order to achieve reliable operation, the R6C1 circuit had to be introduced. Resistor R1 - protective.

The oscillation frequency of the generator on the op-amp DA1.1 is 20 kHz at R5=10 kOhm and 80 kHz at R5=100 kOhm. In the absence of an object in the sensitive area, the generator does not work, the HL1 LED does not light up. The latter makes the device more economical compared, for example, with that described in [8]. From the second output of the DA1.2 detector, the load of which is the R7C2 circuit, the signal is fed to the input of the threshold device - op-amp DA1.3. At its output (pin 7 of the DA1 chip), when the sensor is triggered, the low voltage level is replaced by a high one.

In the absence of an external object, generators of capacitive sensors, including the one under consideration, sometimes give out short-term "flashes" of oscillations that follow at a frequency of 100 Hz. This is probably the result of network interference. The duty cycle of the "flashes" is quite high, and the R7C2 inertial circuit weakens them, preventing them from reaching the trigger level of DA1.3.

As the test showed, the dimensions of the sensing element E1 indicated earlier can be reduced. For example, the device on the K1407UD1 chip (see Fig. 4) also operated with plate sizes of 30x6 mm, and in order to maintain the constant time constant of the feedback circuit, the value of the variable resistor R5 had to be increased to 560 kOhm. The sensitivity of the sensor remained quite satisfactory.

It was possible to increase the size of the sensitive zone by pushing the capacitor plates apart or completely removing the one that is connected to the common wire. In the latter case, the role of the remote lining passes to the most common wire and the elements connected to it. After appropriate tuning with a tuning resistor R5, the generator was excited when approaching the remaining lining of the hand at a distance of 100 mm or a wooden block - by 30 mm. However, the amplitude of "flashes" with a frequency of 100 Hz increased noticeably.

Literature

1. TURCK Proximity Sensors. Catalog of contactless sensors (switches) by TURCK (Germany).
2. BALLUFF Sensor Technik. Catalog of contactless sensors (switches) from BALLUFF (Germany).
3. GOST R 50030.5.2-99 (IEC 60947-5-2) Low-voltage distribution and control equipment. Part 5.2. Apparatus and switching elements of control circuits. Contactless sensors.
4. Frolkin V., Popov L. Impulse devices. - M.: Soviet radio, 1980.
5. Nechaev I. Capacitive relay. - Radio, 1988, No. 1, p. 33.
6. Nechaev I. Capacitive relay. - Radio, 1992, No. 9, p. 48.
7. Alarm device when approaching an object. - Radio, 1999, No. 5, p. 40.
8. Moskvin A. Watchdog with capacitive sensor. - Radio, 2001, No. 8, p. 35, 36.
9. Ataev D., Bolotnikov V. Analog integrated circuits for household equipment. Directory. - M.: PKF "Printing", 1992.

Author: A. Moskvin, Yekaterinburg

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