Non-contact capacitive sensor with a quartz resonator. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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For non-contact capacitive sensors used in burglar alarm devices to control the approach of an object to a protected area, rectangular pulse generators on operational amplifiers, assembled according to the classical scheme [1], are often used. Among the shortcomings of such generators, it should be noted, first of all, the low stability of the frequency of the self-oscillator set by the RC circuit, which leads to the unreliability of the device.

Attempts to increase the sensitivity of the device, as noted in this article, cause interference (“flashes”) or false positives from network interference, which causes periodic false positives without approaching the object sensor or, conversely, failure to operate when an object approaches the sensor .

These shortcomings can be eliminated if a quartz resonator is connected in series with the capacitive sensor, which, being excited at the frequency of the series resonance, compensates for the reactive component of the complex resistance of the capacitive sensor, facilitating the conversion of changes in the electrical capacitance of the sensor into active resistance [2]. Such a device is called a quartz dielcometer.

Fig. 1

In the proximity sensor described below, assembled according to the scheme in Fig. 1, a commercially available evacuated quartz resonator ZQ1 of series resonance at a frequency fpe3 = 300 kHz is connected in series with the capacitive sensor Sd. The resonator has the following equivalent electrical parameters: inductance - 21,7 H; capacitance - 0,013 pF; resistance - 90 Ohm; interelectrode capacitance - 6,5 pF; quality factor - about 455000.

It should be noted that most self-oscillators operate at a frequency that does not coincide with the frequency of the series resonance of the quartz resonator. For example, a known capacitive three-point is excited at a higher frequency. This leads to the fact that the quality factor of the resonator decreases, reducing the frequency stability of the oscillator. The closest to the resonant frequency of the series resonance is provided by the bridge oscillator, which therefore has maximum frequency stability.

In order to increase the sensitivity and stability of the non-contact capacitive proximity meter, described in detail in [1], it is advisable to use a quartz dielcometer.

For experiments, a sensitive element (sensor) with a diameter of 60 mm, similar to that used in the device mentioned in [1], was made of foil-coated getinax. The capacitance of the sensor in free space (without closely spaced objects), measured by a high-frequency device E7-9, turned out to be 2,51 pF. With such a sensor and the above quartz resonator, the equivalent electrical resistance of the series resonator-sensor circuit is 1160 ohms.

When approaching the sensor of any object - a hand, for example, the capacitance of the sensor increases, and the equivalent active resistance of the circuit decreases. If the capacitance is increased by 1 pF, then the equivalent electrical resistance will become 732 ohms, i.e., it will decrease by 428 ohms.

Thus, the sensitivity of the dielcometer to a change in the capacitance of the sensor is 428 Ohm/pF.

As a secondary converter in the meter, a bridge oscillator based on one transistor is used, powered by a galvanic cell with a voltage of 1,5 V.

The device consists of a measuring bridge, a voltage amplifier on a transistor VT1, a detector on diodes VD1, VD2 and a proximity indicator, which is a microammeter RA1. Two arms of the measuring bridge are represented by halves of the winding L1 of a high-frequency transformer. The third arm - measuring - consists of a quartz resonator ZQ1 and a capacitive sensor SD1, and the fourth - exemplary - of resistors R1 and R2.

The output voltage of the measuring bridge through the capacitor C1 is connected to the base of the amplifying transistor VT1. The winding L2 together with the capacitor C3 form a parallel oscillatory circuit, which must be tuned to the series resonance frequency of the quartz resonator of 300 kHz by selecting the capacitor C3. At this frequency, the circuit has maximum resistance, providing the maximum gain of the transistor VT1 and favoring the excitation of oscillations at the fundamental frequency of the quartz resonator.

The amplified output voltage is fed to the input of the measuring bridge as an OS signal, creating conditions for the excitation of self-oscillations at the frequency of series resonance, and to the input of the detector made on the diodes VD1 and VD2 according to the doubling scheme. The detected voltage causes the arrow of the PA1 microammeter to deviate.

In the initial state (when there are no objects in the sensitivity zone of the sensor), there are no self-oscillations and there is no voltage at the output of the detector, since the resistance of the measuring arm of the bridge is greater than the resistance of the exemplary one, which is set by the tuning resistor R2. If the active resistance of the measuring and exemplary arms of the bridge is equal, there are also no self-oscillations.

The approach of an object to a capacitive sensor causes an increase in its capacitance, and hence a decrease in the equivalent resistance. When the resistance of the measuring arm of the bridge becomes less than the exemplary one, self-oscillations will occur, which will be noted by the microammeter. The trimming resistor R2 regulates the sensitivity of the device, or, in other words, sets the distance to an approaching object that causes self-oscillations.

The device can reliably fix the approach to the hand sensor at a distance of 10 cm (the needle of the microammeter deviates by 10 divisions). The sensitivity of the device can be increased by increasing the size of the sensor, the supply voltage, the transformation ratio of the high-frequency transformer, as well as reducing the resistance of resistors R3 and R4.

An M283K microammeter with a maximum needle deflection current of 100 μA (100 divisions) was used as an indicator. In the experiments, the sensitivity was set such that when the sensor capacitance changed by 1 pF, the microammeter needle deviated to the full scale, which corresponds to a change in the equivalent active resistance of the resonator-sensor circuit from 1160 up to 732 ohms, i.e., 428 ohms (linear scale). Therefore, one division of the M283K microammeter scale corresponded to a change in resistance by 4,3 ohms and capacitance by 0,01 pF.

The sensitivity of the device can be increased to 0,001 pF per division of the microammeter. This excludes network interference.

Fig. 2

With a supply voltage of 1 5 V, the current consumption is 0,5 mA. The KT315B transistor can be replaced with KT368B or KT342B. The high-frequency transformer is wound on a K 10x6x2 ring of M3000NM ferrite. To increase the quality factor of the oscillatory circuit L2C3, a gap 0,9 ... 1,1 mm wide is cut in the ring, as shown in fig. 2 using an abrasive disc used in dental practice. The gap greatly facilitates the winding of the transformer coils. Winding L1 contains 50 turns with a tap from the middle, and L2 - 75 turns. Both of them are made in bulk with PELSHO wire with a diameter of 0,15 mm

Capacitors - ceramic KM series. Capacitor C3 is selected within 750...900 pF to provide a resonant frequency of 300 kHz.

Literature:

1. Moskvin A. Contactless capacitive sensors. - Radio, 2002, No. 10. p. 38, 39.
2. Savchenko V., Gribova L. Quartz resonator converts non-electric quantities into electrical ones. - Radio, 2004, No. 2, p. 34-36.

Author: V. Savchenko, L. Gribova, Ivanovo; Publication: radioradar.net

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