Characteristics, Debugging and Anti-interference Measures of Non-contact Capacitive Sensor


In the process of industrial production, it is often necessary to detect and control the position of objects, such as various position control on automatic production line and driving position control, which are inseparable from position detection. People hope that the position detection is non-contact. When the measured object is a conductor, using capacitance to detect the position is a better non-contact detection method.

1 principle of non-contact capacitance sensor composed of NE556

1.1 overall composition block diagram of sensor

The composition of the sensor is shown in Figure 1. CX is the working capacitor, which is a parallel plate detection capacitor composed of two metal plate electrodes. It is installed at a certain position of the line through which the metal conductor passes, and it can feel the metal conductive object close to it. When a metal conductor approaches it, its capacitance will increase. CREF is a reference capacitor. Its structure is exactly the same as CX, but it does not feel metal conductors.

Fig. 1 block diagram of capacitance sensor

The detection of the sensor is completed by converting the electric capacity of working capacitor CX and reference capacitor CREF into frequency signals respectively, and then comparing the capacitance difference between the two capacitors by counter and comparator. Firstly, the capacitance values of Cx and CREF are converted into two counting pulses by two 555 multivibrator in NE556, and then the two counting pulses are counted by their respective counters, and the comparator compares the two counting values. When a metal object passes through the working capacitor CX, the capacitance of the working capacitor increases, the counting pulse frequency output to the counter 1 decreases, and the count value of the counter 1 is smaller than that of the counter 2, so that the comparator outputs a "1" signal to promote the action of the relevant circuit, so as to complete the detection.

1.2 detection probe composed of NE556

1.2.1 circuit diagram and working process there are two 555 time base circuits in NE556. Because they are packaged in one chip, they can obtain better consistency. In addition, NE556 is a bipolar circuit with large output driving capacity, output current of 200mA and working frequency range of 01001hz 500KHz. As shown in Figure 2, the two 555 circuits inside the NE556 are connected into an indirect feedback non steady state self-excited multivibrator. The two oscillation circuits are symmetrical, that is, R11 = R21, R12 = R22, C11 = C21, CX = CREF. CX is connected to the first 555 circuit from its pins 2 and 6, so that the square wave frequency of its output Vo1 changes with the change of Cx. CREF connects the second 555 circuit from pins 8 and 12 of NE556 to output the square wave signal of VO2. When no conductor approaches CX, CX = CREF, the square wave frequencies output by Vo1 and VO2 are equal. After passing through the following counter and comparator, the detection output signal is low level "0". When a conductor approaches CX, the square wave frequency output on CX, CREF and Vo1 will be lower than the square wave frequency output by VO2. After passing through the counter and comparator, the detection output signal will be high level "1", so as to obtain the position detection signal for position control by the position control circuit.

Fig. 2 detection probe circuit composed of NE556

1.2.2 circuit working waveform diagram

Fig. 3 shows the working waveform diagram of the detection circuit, in which the waveform diagrams are drawn when there is no metal conductor close to Cx. The timing cycle described in the figure refers to the time cycle for controlling the comparator for comparison. Generally, an appropriate timing cycle can be selected according to the square wave frequency output by 555 circuit, the number of bits of the counter used and the nature of interference, so as to overcome the field interference and ensure that the counter does not overflow. Generally, it can be 20ms (the purpose is to make the count value caused by power frequency interference only "1") , the interference can be eliminated by removing the lowest bit of the count value).

Fig. 3 output waveform of probe

The initial phases of Vo1 and VO2 shall be consistent during the operation of the detection probe, as shown in Figure 3. In the circuit, this can be achieved by pressing the switch K, which controls the forced reset end of the two 555 circuits in NE556. When it is pressed, the output of the two 555 circuits must be low level "0".

2 sensor installation, commissioning and anti-interference measures

Because the capacitance used by the sensor is a parallel metal plate, the power frequency interference signal will be introduced on site, so it should be installed in a place with low power frequency interference as far as possible. After the test, the two metal plate electrodes of Cx can be installed in parallel, and at the same time, an open metal box is grounded and shielded outside the two metal plates, as long as the distance that can allow the metal object to pass through the middle of the two metal plates is reserved, as shown in figure 4.

Figure 4cx structure diagram

During installation, the connecting length between the two plates of Cx and NE556 shall be as short as possible, and the connecting wires between them shall be shielded. During commissioning, the external resistance of each 555 circuit shall be adjusted to make the frequency and duty cycle of their output square wave waveform consistent. Main anti-interference measures in the sensor:

â‘  The sensitivity of the sensor can also be adjusted by setting the comparator to discard the low part and only compare the high part of the count value when comparing the output value of the counter;

â‘¡ CREF reference capacitors are used, which can introduce interference and cancel each other at the same time;

â‘¢ NE556 dual time base circuit is adopted to make the internal two 555 circuits have good consistency;

â‘£ Because the driving ability of NE556 is very large, and its output signal is frequency signal, the anti-interference ability of the circuit itself in the signal transmission part is very strong.

3 circuit characteristics

Compared with other proximity switches, because the sensor uses reference capacitors and working capacitors, their structures are similar, and they need to be installed in similar positions and work in the same environment, the sensor is not afraid of dust, humidity, high temperature and other environments. Once adjusted and tested, it does not need maintenance, reliable operation, strong anti-interference ability and low cost. It is suitable for position control in mining and metallurgical industries.

4 Conclusion

The sensor introduced in this paper uses a parallel plate capacitance detection element as a proximity switch, which is suitable for use in harsh environments. It converts the change of capacitance into the change of frequency, which greatly improves its anti-interference ability. From its working principle, with appropriate changes to the circuit, it can also be used as an instrument for accurately measuring capacitance, which has good popularization and application value.

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