1 Working mechanism of eddy current switch The eddy current switch is similar to the conventional eddy current sensor. It also contains a high-frequency excitation coil, which is operated with a large-amplitude AC current with a stable frequency and stable amplitude. There is also a coil for the detection coil, without external excitation. The source, whose function is to induce a eddy current magnetic signal. The probes of the eddy current switch are constituted by the two coils. Under the excitation of the excitation coil, a magnetic field is generated around the probe. When no metal object enters the magnetic field, the amplitude of the signal obtained by the detection coil is the largest. Once a metal body enters the magnetic field, an eddy current is formed in the metal body, and the excitation signal in the probe is reacted, and the amplitude of the signal obtained by the detection coil will be Will be greatly reduced. This process can be equivalent to the modulation process of the high-frequency signal, and its principle block diagram includes a high-frequency oscillator, a threshold comparator and a switch output circuit. In the figure, Vdc represents a DC power supply and RS represents a pull-up resistor.
In short, the eddy current switch uses the eddy current effect to control the start-up and stop-motion states of the high-frequency oscillator, thereby converting the metal body position signal into 0 (low level of 0Vdc) and 1 (high level of Vdc). The amount of switching. At present, eddy current switches mostly use NPN or PNP transistors to acquire square wave pulses.
In order to study the interaction characteristics of the structural parameters and electromagnetic parameters of the eddy current switch, it is necessary to first obtain its equivalent circuit model. For the convenience of processing, the eddy current formed by the conductor to be tested coupled with the excitation coil can be regarded as a single-turn current loop, which is referred to as an eddy current short-circuit loop, an equivalent resistance and an equivalent inductance of the eddy-short loop; M is an excitation coil and a eddy current The mutual inductance between the shorting rings; the angular frequency of the high frequency excitation power supply.
(3) According to the Biot-Shavar-Laplace theorem, the magnetic induction intensity generated by the NåŒ current-carrying wire on the shaft can be calculated by integral calculation to obtain the magnetic induction intensity Bx generated at the x-point of the entire flat coil on the axis. The expression is: Bx=NI12(r12-r11)h1(x h1)1nr12 r212 (x h1)2r11 r211 (x h1)2-x1nr12 r212 x2r11 r211 x2(4) Analytical formula (2)-(4) Mutual inductance M, eddy current intensity I2 and magnetic induction intensity Bx are affected by the structural parameters of the eddy current switch and also by the detection gap. In order to obtain the best performance of the eddy current switch, the detection gap and the eddy current switch model must be properly selected.
The structural dimensions of the metal body to be tested, including its tooth height l, tooth width b, tooth thickness h and tooth pitch of adjacent teeth w. inner radius ra of the eddy current shorting ring, outer radius rb and axial thickness hS of the vortex ring, It can be estimated by the following method: ra04d12(5)rb07d12(6)hS=2(7) where 2 and respectively are the resistivity and magnetic permeability of the metal body to be tested.
The analytical formulas (5) and (6) show that the inner radius ra and the outer radius rb of the eddy current shorting ring depend only on the probe size r12, regardless of the material of the test object.
As shown in the analytical formula (7), the axial thickness hS of the vortex ring depends on the metal material and the excitation power frequency.
The relationship between the frequency f of the pulse signal reflecting the speed of the gear and the speed of the gear line v is obtained as follows: v=Df (8) where D is the tooth spacing size, which can be expressed as the tooth width b of the gear and the tooth The sum of the distance w, namely: D=bw(9)2 Test system The typical parameters of the NPN type eddy current switch selected in this paper are: the probe body diameter is 18mm, the response frequency is 2kHz, the rated detection gap is 12mm, and the 24V DC power supply is used. powered by. The vortex switch is used to measure the linear velocity of the gear. The block diagram of the entire inspection system is shown in Figure 2. It includes several parts of the gear drive mechanism, eddy current switch, subsequent processing system and computer system. The subsequent processing system is mainly composed of a receiving circuit and an optical isolating circuit of the eddy current switch. In the figure, RS is a pull-up resistor, RL is a current limiting resistor of the optical isolation circuit, RT is a pull-up resistor of the optical isolation circuit, and 6N137 is an optical coupler.
The working principle of the whole test system is as follows: the drive mechanism drags the gear to rotate at a high speed, and the eddy current switch obtains a pulse signal reflecting the speed of the gear speed, and the signal is transmitted to the computer system through the subsequent processing system to perform level conversion and electrical isolation, and the acquisition is performed. The frequency of the pulse signal is obtained, and the gear line speed is calculated by the formula (8).
The whole detection system frame eddy current switch receiving circuit NPN type eddy current switch and pull-up resistor RS receiving circuit When the transistor is static, the output voltage is the power supply voltage Vdc, which is similar to TTL logic on the circuit and can be compatible with it.
If the transistor is saturated, the output turns to a low level of 0Vdc, and vice versa, from zero to a positive voltage.
Since the collector of the transistor and the power supply subsequently processed by the eddy current switch can be independent of each other after the pull-up resistor shaft is connected, it can obtain a level signal different from the power supply voltage of the eddy current switch, and can also reduce the cable length and the pulse frequency. Bad effects.
(2) Optical isolation circuit Optical coupler (optical coupler) is called optical isolator or photoelectric coupler, referred to as optical è—•. It is a device that transmits electrical signals by means of light. The illuminator and the photoreceptor are usually packaged in the same package. When the input terminal energizes the signal, the illuminator emits light, and the light receiver receives the light to generate a photocurrent, which flows out from the output end, thereby realizing the electro-optical-electrical conversion.
The characteristics of the diaphragm are: the input and output are completely electrically isolated, the anti-interference ability is strong, the service life is long, and the transmission efficiency is high. It is widely used in many interface circuits such as level shifting, signal isolation, interstage isolation, switching circuits, long-distance signal transmission, pulse amplification, instrumentation, communication equipment and microcomputers.
The main technical parameters of the aperture include LED forward voltage drop VF, forward current IF, current transfer ratio CTR, and collector-emitter saturation voltage drop VCE(sat).
6N137 optical isolation is a very common form of high-speed signal isolation.
Due to its simple circuit and superior performance, it is often used in digital isolation circuits or data transmission circuits.
(3) Test conditions: The interval between two adjacent teeth is 32mm, the tooth thickness h is 4mm, the tooth height l is 40mm, the tooth width b is 14mm, and the actual detection gap of the test system is 8mm (less than its rated value). The gear material is hard aluminum and the DC power supply voltage is 240V.
3 test results eddy current switch in different pull-up resistors (011k, ​​38k, 10k and 100k), its output signal frequency f and gear line speed v relationship 8 (13) four test curves are respectively shown, the slope obtained by fitting curve fv Compared with the theoretical value of 32, the relative errors are: 0625, 09375, 03125 and 15625. It can be seen that the relative error when the pull-up resistor is 10k is the smallest.
Therefore, under the premise of ensuring the normal operation of the eddy current switch, appropriately reducing the pull-up resistor can effectively improve the linearity of the relationship between the output signal frequency f and the gear line speed v. However, if the pull-up resistor is too small, it will increase the burden on the power supply. In addition, the resistance value is too small, and the power requirement is also higher, which needs to be weighed.
Indicates the output waveform of the eddy current switch when the test system is not connected to the pull-up resistor. When the other parameters are the same, the pull-up resistor has a significant effect on the output waveform quality of the eddy current switch. In particular, when the rotational speed of the rotating disk is continuously increased, the degree of distortion of the output waveform is also more serious, that is, the output waveform will be changed by the square wave sawtooth triangular wave, which is a significant change of the waveform.
Channel 1 indicates the output waveform of the eddy current switch when the test system is not connected to the pull-up resistor, and channel 2 indicates the output waveform of the eddy current switch when the pull-up resistor 10k is connected. Comparing the waveforms of channels 1 and 2, it can be seen that when the other parameters are the same, after the pull-up resistor 10k is connected, the output quality of the eddy current switch is much higher than that of the pull-up resistor (the waveform is still a square wave).
4 Conclusions Because the detection gap, the width of the measured body and the pull-up resistance affect the output waveform quality of the eddy current switch, when constructing the gear speed test system, it is necessary to determine the optimal detection gap according to the specific model of the eddy current switch. Pull-up resistor values ​​ensure the accuracy and reliability of the test system.
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