A standing wave displacement sensor

Abstract

The application discloses a standing wave displacement sensor and relates to the technical field of sensors.The standing wave excitation unit comprises a standing wave waveguide base, a support and a waveguide rod, two excitation piezoelectric ceramics are symmetrically arranged on the top surface of the waveguide rod, feedback piezoelectric ceramics are arranged on the bottom surface of the waveguide rod, the feedback piezoelectric ceramics are connected with the excitation piezoelectric ceramics through a standing wave closed-loop control unit; the standing wave detection unit comprises at least one sensor orthogonal holder, two eddy current displacement sensors are arranged on each sensor orthogonal holder, and the output ends of the eddy current displacement sensors are connected with a displacement output circuit.The excitation piezoelectric ceramics and the feedback piezoelectric ceramics close-loop excite the waveguide rod, a fixed standing wave is generated to form a displacement scale, the eddy current displacement sensors pick up the amplitudes of the standing waves at various positions on the waveguide rod, multiple eddy current displacement sensors are cooperatively used to identify the direction and range of displacement and expand the range of displacement, and the non-contact sensor output with a large range and high resolution of displacement is realized.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of sensors, and specifically relates to a standing wave displacement sensor, which is a non-contact displacement sensor with sub-micron resolution, meter range and high anti-interference performance. BACKGROUND

[0002] At present, sensors with a meter range mainly include laser interferometers and various grid displacement sensors (magnetic grating, capacitive grating and optical grating). They are widely used in the field of machine tools and other industrial equipment, and some super-precision models even have nanometer resolution and can be applied to the field of semiconductor manufacturing equipment, advanced manufacturing lithography machines and the like. For general industrial applications, such as general machine tools, machining centers and intelligent manufacturing, there is only a demand for meter range and sub-micron resolution, and therefore the above-mentioned sensors do not have advantages in cost, and have relatively high requirements for working environment and more stringent requirements for protection design in application, which also limits their application.

[0003] A laser interferometer mainly includes a laser source, a beam splitter, a corner reflector and a detector, and can achieve a resolution of half a light wave length. In general, a subdivision means is additionally arranged to divide one period of interference fringes into several hundred to several thousand parts, and the resolution can be improved to 0.1 nm. However, it should be noted that the use environment is limited: dust, airflow and vibration in the environment will affect the stability of the light path, and temperature, humidity and air pressure will change the laser wavelength by affecting the refractive index of air, which results in that it can only be used for short-time error calibration of machine tools, and it is difficult to use in machine tool working.

[0004] In actual industrial machines, commonly used are grid structure displacement sensors such as magnetic grating, capacitive grating and optical grating. Their working principles are different but similar to some extent. The resolution of an optical grating displacement sensor can reach microns, and some special models can even reach nanometers; the resolution of a magnetic grating can reach microns; and the resolution of a capacitive grating is close to microns. Except for the capacitive grating, although the optical grating and the magnetic grating have high resolution, they are expensive, and the resolution of the capacitive grating is also defective.

[0005] The grid displacement sensor has a common feature that a large range of periodic structures need to be processed, which accounts for a large part of the cost of such sensors. How to simply and low-costly generate periodic structures in a distance is a direction worth exploring. A standing wave has periodic characteristics, and it is very easy to excite, and with the increase of the standing wave resonance order, the standing wave period will also decrease, and a subdivision circuit can also achieve high-resolution displacement sensing. SUMMARY

[0006] In view of the defects of high cost, complex processing period structure, weak anti-environmental interference ability (temperature / vibration / air flow sensitivity) of existing laser interferometer and grating displacement sensor (grating ruler / magnetic grating ruler), and the difficulty in meeting the demand of low cost and high adaptability displacement measurement in industrial scene, the application proposes and designs a non-contact large range and high resolution displacement sensor based on standing wave and eddy current sensing. Firstly, the piezoelectric ceramic piece excites the waveguide rod in a closed loop to generate a standing wave with fixed spatial period and fixed amplitude on the waveguide rod to form a displacement scale, and then the amplitude of the standing wave at each position on the waveguide rod is picked up by a small size eddy current displacement sensor. In addition, the displacement direction and range expansion are cooperated by multiple small size eddy current displacement sensors. Finally, the large range, high resolution and non-contact sensing output of displacement are realized.

[0007] To solve the above technical problems, one technical solution adopted by the application is:

[0008] A standing wave displacement sensor comprises a standing wave excitation unit and a standing wave detection unit.

[0009] The standing wave excitation unit comprises a standing wave waveguide base, supports symmetrically arranged on the top surface of the standing wave waveguide base, and a waveguide rod fixedly arranged at the top end of the two supports. Two excitation piezoelectric ceramics are symmetrically arranged on the top surface of the waveguide rod, and two feedback piezoelectric ceramics are arranged below the excitation piezoelectric ceramics on the bottom surface of the waveguide rod. The feedback piezoelectric ceramics and the excitation piezoelectric ceramics are connected through a standing wave closed loop control unit.

[0010] The standing wave detection unit comprises at least one sensor orthogonal holder arranged above the waveguide rod, two eddy current displacement sensors fixedly arranged on each sensor orthogonal holder and parallel arranged, and the signal output of the two eddy current displacement sensors maintains a phase difference of 90° in space phase. The output end of the eddy current displacement sensor is connected to a displacement output circuit.

[0011] The excitation piezoelectric ceramic and the feedback piezoelectric ceramic excite the waveguide rod in a closed loop to generate a standing wave with fixed spatial period and fixed amplitude on the waveguide rod to form a displacement scale, the eddy current displacement sensor picks up the amplitude of the standing wave at each position on the waveguide rod, multiple eddy current displacement sensors cooperate to expand the displacement direction and range, and realize the large range, high resolution and non-contact sensing output of displacement.

[0012] Further, the standing wave closed loop control unit comprises a charge amplifier connected to the feedback piezoelectric ceramic output end, a phase detector and an automatic gain controller connected to the charge amplifier output end respectively, the phase detector output end is connected to the voltage controlled oscillator input end through an integrator, the voltage controlled oscillator output end is connected to the phase detector input end and the multiplier input end respectively, the automatic gain controller output end is connected to the multiplier input end, and the multiplier output end is connected to the input end of the power amplifier and the input end of the excitation piezoelectric ceramic through the power amplifier.

[0013] Further, the standing wave excitation process comprises the following steps:

[0014] S11, excitation feedback signal generation: the feedback piezoelectric ceramic detects the vibration frequency and amplitude of the waveguide rod, and the output signal is amplified by the charge amplifier and used for frequency tracking control and amplitude control;

[0015] S12, phase control: the phase detector converts the phase difference between the output signal of the charge amplifier and the carrier signal output by the voltage controlled oscillator into an actual phase voltage, and inputs the difference between the reference phase voltage into the integrator, and the output signal of the integrator controls the voltage controlled oscillator to generate a carrier signal with a suitable frequency, so that the vibration frequency of the waveguide rod is kept at the resonance point of the standing wave generation;

[0016] S13, standing wave amplitude control: the feedback signal output by the charge amplifier is subjected to RMS operation, and the RMS value is taken as the standing wave amplitude signal, and the automatic gain controller is used to keep the RMS value stable, and the output of the automatic gain controller is taken as the baseband signal of the excitation amplitude, which is modulated into the carrier signal output by the voltage controlled oscillator through the multiplier to form a modulated signal;

[0017] S14, excitation piezoelectric ceramic excitation: the modulated signal is subjected to power amplification to obtain a high-output-power excitation voltage, which drives the excitation piezoelectric ceramic to work and generates a standing wave with fixed spatial period and amplitude on the waveguide rod.

[0018] Further, the displacement output circuit comprises phase-sensitive detection modules connected to the output ends of the two eddy current displacement sensors respectively, a comparator and a resolver decoder connected to the output ends of the phase-sensitive detection modules, a reversible counter connected to the output end of the comparator, and the output end of the resolver decoder is connected to the input end of the low-pass filter through a signal amplifier.

[0019] Further, the step of converting the standing wave into displacement output is as follows:

[0020] S21, amplitude detection: the eddy current displacement sensor outputs the real-time vibration time sequence signal of a certain position on the standing wave, and the excitation signal of the standing wave is taken as the reference signal, which is demodulated by the phase-sensitive detection module to obtain the amplitude of the position;

[0021] S22, cross-cycle range expansion: the output amplitude signals of the two eddy current displacement sensors after phase-sensitive detection are shaped by the comparators to obtain two columns of square waves with a spatial phase difference of 90°, and the reversible counter counts the spatial period of the two columns of square waves to realize the period expansion of the range;

[0022] S23, displacement subdivision in standing wave period: the rotating variable decoder converts the sine signal and the cosine signal obtained by the two eddy current displacement sensors along the standing wave direction into a phase output, and improves its sensitivity through a signal amplifier and a low-pass filter.

[0023] Further, the -3dB bandwidth of the eddy current displacement sensor is greater than twice the excitation frequency of the piezoelectric ceramic and is not less than 100kHz.

[0024] Further, the specific demodulation process of the phase-sensitive detection circuit is: the output signal of the eddy current displacement sensor is multiplied by the reference signal, and then divided by the amplitude of the excitation feedback signal after the effective value operation to perform proportional measurement to improve the signal-to-noise ratio, and then low-pass filtering is performed to obtain the amplitude of the position vibration time sequence.

[0025] Further, the model of the influence of the resolution of the eddy current displacement sensor on the lateral displacement resolution is as follows:

[0026] The standing wave expression is:

[0027] ;

[0028] Wherein, is the spatial period of the standing wave, i.e. the wavelength, is the angular frequency of the standing wave, is the amplitude of the standing wave, and the time phase component in the equation is only left after the spatial phase component, i.e. the longitudinal amplitude and the relationship between the lateral displacement

[0029] ;

[0030] Therefore, the limitation of the resolution of the eddy current displacement sensor on the lateral displacement resolution is:

[0031] ;

[0032] Wherein, represents the resolution of the lateral displacement sensor, represents the resolution of the eddy current displacement sensor, and due to the characteristics of the rotating variable decoder, the value range of the spatial phase is kept at-45°~+45°, and correspondingly, the case where the resolution of the eddy current displacement sensor has the greatest influence on the lateral displacement resolution is: ​

[0033] ;

[0034] The method for reducing the influence of the resolution of the eddy current displacement sensor on the lateral displacement resolution is to reduce the standing wave wavelength and / or increase the standing wave amplitude.

[0035] Compared with the prior art, the present application has the following advantages:

[0036] 1. The present application uses a closed-loop phase-locked excitation of the waveguide rod by exciting piezoelectric ceramics and feedback piezoelectric ceramics, locks the excitation frequency at the standing wave resonance point by using a feedback loop, and maintains a stable standing wave to form a spatial periodic displacement scale. A high-bandwidth, orthogonally arranged eddy current displacement sensor is used to non-contact pick up the standing wave amplitude signal, combined with a phase-sensitive detection technology to eliminate time vibration interference, output a sine / cosine spatial sequence with a spatial phase difference of 90°, realize standing wave period counting, and the range can be extended to meters. Combined with signal subdivision technology, the displacement is finally output in a large range, high resolution, and non-contact sensing, which breaks through the inherent range limitation of the eddy current.

[0037] 2. The standing wave displacement sensor of the present application eliminates the need for a precise etching process, and the manufacturing cost of the waveguide rod is only 5-10% of that of a traditional grid sensor. It is not sensitive to temperature drift, mechanical vibration and surface roughness (experimental noise <20mVpp), and does not require a laser interferometer constant temperature / dust-free environment. By increasing the standing wave mode order (such as a six-order mode of 13.6kHz) to shorten the wavelength, the resolution is further improved; at the same time, the flexible waveguide structure can support higher order modes.

[0038] 3. The standing wave displacement sensor of the present application can be applied to general machine tool displacement monitoring, intelligent manufacturing platform positioning, semiconductor equipment coarse adjustment, and other industrial fields that require compatibility of large range (meter level) and high resolution (sub-micron level), and has a wide application prospect. BRIEF DESCRIPTION OF DRAWINGS

[0039] Figure 1 is a schematic diagram of the three-dimensional structure of the standing wave displacement sensor of the present application;

[0040] Figure 2 is a structure diagram of the excitation principle and frequency tracking amplitude control loop of the standing wave of the present application;

[0041] Figure 3 is a principle diagram of standing wave amplitude detection and lateral displacement conversion of the present application;

[0042] Figure 4 is a range expansion principle and period subdivision principle diagram of the standing wave amplitude to lateral displacement of the present application;

[0043] Figure 5 is a counting and direction discrimination principle diagram of the standing wave displacement sensor of the present application;

[0044] Figure 6 The first twelve order standing wave frequency points and the mode schematic diagram obtained from the simulation experiment;

[0045] Figure 7 The structure schematic diagram of the standing wave displacement sensor principle prototype used for the feasibility experiment of the application;

[0046] Figure 8 The sin output and cos output results of the orthogonal double crystal probe obtained from the feasibility experiment;

[0047] Figure 9 The voltage noise and the corresponding displacement noise results of the sin output and cos output respectively at the 0° phase and ±45° phase positions of the standing wave. DETAILED DESCRIPTION

[0048] The preferred embodiments of the present application will be described in detail below with reference to the drawings, so that the advantages and features of the present application can be more easily understood by those skilled in the art, and the scope of protection of the present application can be more clearly defined.

[0049] It should be noted that when a component is referred to as being "mounted on" another component, it can be directly on the other component or there can be a middle component. When a component is referred to as being "disposed on" another component, it can be directly disposed on the other component or there can be a middle component. When a component is referred to as being "fixed on" another component, it can be directly fixed on the other component or there can be a middle component.

[0050] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present application belongs. The terminology used in the description of the present application herein only for the purpose of describing the specific embodiments and is not intended to limit the present application. The term "or / and" used herein includes any and all combinations of one or more relevant listed items.

[0051] Stationary wave is a special wave phenomenon formed by superposition of two waves with same frequency, amplitude and propagation speed but opposite directions. It is often found at fixed boundary (such as string, pipe) or interface of medium, with unique spatial distribution and temporal vibration characteristics. When incident wave meets obstacle or boundary, reflection occurs, and reflected wave meets incident wave and interferes. If two waves meet coherent condition (same frequency, consistent vibration direction, fixed phase difference), temporal phase and spatial phase split, forming stationary wave, in which spatial phase presents as periodic structure on waveguide. Spatial phase can be demodulated from stationary wave vibration by phase-sensitive detection method, and then multi-probe structure can be used for direction discrimination and range expansion, reflecting displacement change along waveguide direction. The stationary wave displacement sensor of the present application is designed based on the above characteristics of stationary wave.

[0052] Referring to the drawings Figure 1 A stationary wave displacement sensor, which can be divided into two parts according to different working purposes, including stationary wave excitation unit and stationary wave detection unit.

[0053] Specifically, the stationary wave excitation unit includes stationary wave waveguide base 1, supports 2 symmetrically arranged on the top surface of the stationary wave waveguide base 1, waveguide rods 3 fixedly arranged at the top ends of the two supports 2, two excitation piezoelectric ceramics 4 symmetrically arranged on the top surface of the waveguide rod 3, and feedback piezoelectric ceramics 5 arranged below the excitation piezoelectric ceramics 4 on the bottom surface of the waveguide rod 3. The waveguide rod 3 is used as the base of stationary wave generation, and a periodic structure is formed thereon by stationary wave; the excitation piezoelectric ceramic 4 provides continuous energy for the maintenance of stationary wave shape; the feedback piezoelectric ceramic 5 is used to pick up excitation and feedback control to keep the excitation frequency at the resonance point when stationary wave is formed, and to control the amplitude of stationary wave to keep at a certain specific value.

[0054] The feedback piezoelectric ceramic 5 is connected with the excitation piezoelectric ceramic 4 through a stationary wave closed-loop control unit. As shown in Figure 2 The stationary wave closed-loop control unit includes a charge amplifier connected to the output end of the feedback piezoelectric ceramic 5, a phase detector and an automatic gain controller connected to the output end of the charge amplifier, respectively, the output end of the phase detector is connected with the input end of the voltage-controlled oscillator through the integrator, the output end of the voltage-controlled oscillator is connected with the input end of the phase detector and the input end of the multiplier, respectively, the output end of the automatic gain controller is connected with the input end of the multiplier, and the output end of the multiplier is connected with the input end of the excitation piezoelectric ceramic through the power amplifier.

[0055] The excitation process of stationary wave includes the following steps:

[0056] S11, excitation feedback signal generation: the feedback piezoelectric ceramic 5 detects the vibration frequency and amplitude of the waveguide rod 3, and the output signal is amplified by a charge amplifier and used for frequency tracking control and amplitude control. As a detection device, the piezoelectric ceramic can detect displacement, force, vibration and other parameters. Here, the piezoelectric effect of the piezoelectric ceramic is used (when the piezoelectric material is subjected to external force, the lattice structure is deformed, resulting in separation of positive and negative charge centers inside, and equal and opposite charges are generated on the surface. Through the capacitance effect, the output voltage is accumulated). In this application, compared with a voltage amplifier, a charge amplifier has a higher signal-to-noise ratio. The feedback piezoelectric ceramic 5 generates a periodically changing charge due to the vibration of the waveguide rod 3. The charge amplifier provides a very small equivalent load impedance, which lowers the piezoelectric ceramic output potential to almost 0V. At this time, all the charges generated in the feedback piezoelectric ceramic 5 flow into the feedback capacitor of the charge amplifier and generate the output voltage of the charge amplifier. The amplification factor of the charge amplifier can be simply written as: wherein is the equivalent capacitance of the feedback piezoelectric ceramic 5, is the feedback capacitance of the charge amplifier.

[0057] S12, phase control: the phase detector converts the phase difference between the output signal of the charge amplifier and the carrier signal output by the voltage-controlled oscillator (VCO) into an actual phase voltage, and inputs the difference between the reference phase voltage into the integrator. Due to the characteristics of the integrator, when its output remains at a certain stable value, the input is zero, that is, the actual phase voltage is equal to the reference phase voltage, and the difference between the two (the input of the integrator) is zero. In the feedback control loop (standing wave closed-loop control unit) for tracking frequency, the change of the actual phase voltage is caused by the fluctuation of the vibration frequency of the waveguide rod 3, and the output signal of the integrator can control the VCO to generate a suitable frequency to keep the vibration frequency of the waveguide rod 3 at the resonance point of the standing wave.

[0058] S13, standing wave amplitude control: in theory, when the excitation voltage is stable, the standing wave amplitude generated by the excitation is stable, which belongs to the open-loop control of amplitude. Although the open-loop control method has simple structure and fast control response, it inevitably cannot have the ability to suppress impact interference such as vibration. Here, a closed-loop control logic is used. First, the effective value of the feedback signal output by the charge amplifier is calculated, and the effective value is used as the standing wave amplitude signal. By using the characteristics of the automatic gain controller, the effective value is kept stable through the automatic gain controller. The output of the automatic gain controller is used as the baseband signal of the excitation amplitude, which is modulated into the carrier signal output by the voltage-controlled oscillator through the multiplier to form the modulated signal.

[0059] S14, piezoelectric ceramic excitation: in order to form a stable standing wave on the waveguide rod 3, the device needs to have a certain power output capacity, and although the ordinary operational amplifier has good precision, its output power is mostly only 0.001~0.1W / channel, which is far from enough to form a stable standing wave. At this time, the multiplier has obtained complete excitation signals (including phase and frequency and amplitude), and only needs to use power amplifiers (to obtain high output power), transformers (to improve excitation voltage), and passive band-pass filters (to reduce transformer distortion) to drive. That is, the modulated signal is subjected to the action of the power amplifier to obtain high output power and high excitation voltage, and the excitation voltage drives the excitation piezoelectric ceramic 4 to work, generating a spatially periodic fixed, amplitude fixed standing wave on the waveguide rod 3.

[0060] As shown in Figure 1 , the standing wave detection unit includes at least one sensor orthogonal holder 6 arranged above the waveguide rod 3, and two eddy current displacement sensors 7 are fixedly arranged on each sensor orthogonal holder 6 in parallel and the signal outputs thereof are kept at a phase difference of 90° in space. The output ends of the eddy current displacement sensors 7 are connected to a phase-sensitive detection circuit. The excitation piezoelectric ceramic 4 excites the waveguide rod 3 in a closed loop, generating a spatially periodic fixed, amplitude fixed standing wave on the waveguide rod 3 to form a displacement scale. The eddy current displacement sensors 7 pick up the amplitudes of the standing waves at various positions on the waveguide rod 3, and multiple eddy current displacement sensors 7 cooperate to expand the displacement direction and range, realizing a large range, high resolution, and non-contact sensing output of displacement.

[0061] As shown in Figure 3 , this part of the body is a high-bandwidth eddy current displacement sensor 7, which converts the time sequence of vibration at a certain position on the waveguide rod 3 into an electrical signal through the high-bandwidth advantage of the eddy current displacement sensor 7, and at the same time, a phase-sensitive detection circuit is used to demodulate the electrical signal reflecting the vibration at the point into an electrical signal reflecting the amplitude at the point. The specific demodulation process of the phase-sensitive detection circuit is: the output signal of the eddy current displacement sensor is multiplied by the reference signal, and then divided by the amplitude of the excitation feedback signal (the result of the output of the charge amplifier in the excitation closed-loop control circuit and the effective value operation) for proportional measurement to improve the signal-to-noise ratio, and then low-pass filtering is performed to obtain the amplitude of the vibration time sequence at the position. In addition, the sensor orthogonal holder 6 can be used to arrange two eddy current displacement sensors in parallel while keeping a phase difference of 90° in space. Then, the range expansion structure form of the grid sensor and the induction synchronizer is used to extend the range to tens of centimeters or even several meters. The displacement output circuit includes a phase-sensitive detection module connected to the output ends of the two eddy current displacement sensors, a comparator and a resolver decoder connected to the output end of the phase-sensitive detection module, and a reversible counter connected to the output end of the comparator. The output end of the resolver decoder is connected to the input end of the signal amplifier and the low-pass filter through the signal amplifier.

[0062] AsFigure 4 The step of converting the standing wave into displacement output is as follows:

[0063] S21, amplitude detection: the eddy current displacement sensor 7 outputs the real-time vibration time sequence signal of a certain position on the standing wave, and obtains the amplitude of the position after demodulation by the phase-sensitive detection module with the excitation signal of the standing wave as the reference signal. In this step, it needs to be noted that the -3dB bandwidth of the eddy current displacement sensor needs to be at least greater than twice the excitation frequency of the piezoelectric ceramic, and at present, the bandwidth needs to reach more than 100 kHz.

[0064] S22, cross-cycle range expansion: according to the working principle of the magnetic grating, the capacitive grating and the inductive synchronizer, if the space period is to be counted, a 90-degree electrical angle is needed, and the measurement of the lateral displacement is realized through the linkage of multiple probes. The output amplitude signals of the two eddy current displacement sensors 7 after phase-sensitive detection are respectively shaped by the comparators to obtain two columns of square waves with a spatial phase difference of 90°, and the reversible counter counts the spatial periods of the two columns of square waves reversibly to realize the cycle expansion of the range. As shown in the figure, Figure 5 When the standing wave detection unit (orthogonal eddy current displacement sensor probe) moves to the right, every time the cosine probe output is at the rising edge, the sine probe output is at the low level, at which time the addition counting can be performed. Conversely, when the standing wave detection unit moves to the left, every time the cosine probe output is at the rising edge, the sine probe output is at the high level, at which time the subtraction counting can be performed. For example, the function can be simply realized by using 74HC191. Thus, the cycle expansion of the range can be realized.

[0065] S23, displacement subdivision within the standing wave period: within a standing wave period, the displacement to be measured is actually a spatial phase signal, and the displacement is in a proportional linear relationship with the phase angle. Because the sine signal and the cosine signal along the standing wave direction can be obtained simultaneously by the standing wave detection unit, the phase output is converted from the sine signal and the cosine signal by the resolver decoder, and the sensitivity is improved by the signal amplifier and the low-pass filter. The subdivision method of this part directly determines the resolution of the final displacement sensor.

[0066] The model of the influence of the eddy current displacement sensor resolution on the lateral displacement resolution (transmission effect) is as follows:

[0067] The expression of the standing wave is:

[0068] ;

[0069] wherein, is the spatial period of the standing wave, i.e. the wavelength, is the angular frequency of the standing wave, is the amplitude of standing wave, the time phase component in the equation is only left the spatial phase component, i.e. the longitudinal amplitude after multiplied with the reference signal and demodulated by the way of phase sensitive detection and the relationship of transverse displacement

[0070]

[0071] Therefore, the resolution of eddy current displacement sensor limits the resolution of transverse displacement as follows:

[0072]

[0073] wherein, represents the resolution of transverse displacement, represents the resolution of eddy current displacement sensor. Since the two eddy current displacement sensors 7 are arranged orthogonally with a phase difference of 90° in space, the range of spatial phase is kept at -45°~+45° due to the characteristics of resolver decoder when discussing the influence of the resolution of eddy current displacement sensor 7 on the resolution of transverse displacement. Correspondingly, the case that the resolution of eddy current displacement sensor has the greatest influence on the resolution of transverse displacement is:

[0074]

[0075] Therefore, the method to reduce the influence of the resolution of eddy current displacement sensor on the resolution of transverse displacement is to reduce the wavelength of standing wave and / or increase the amplitude of standing wave. In the structure of standing wave displacement sensor in the embodiment, these two points have been considered in the design, i.e. to increase the excitation power to obtain a greater amplitude and to use a higher standing wave modal resonance frequency. For example, using an eddy current displacement sensor with a resolution of , the standing wave amplitude , and the period , the resolution limited by the eddy current displacement sensor is . That is to say, the large-range displacement sensor has an extremely high upper limit of resolution.

[0076] Feasibility experiment:

[0077] Different from the prototype design, the experiment uses a bimorph probe instead of an eddy current displacement sensor to pick up vibration. However, there is a problem. Unlike the eddy current displacement sensor, the bimorph probe picks up vibration signals by contact measurement. In the case of static measurement, the noise and resolution performance are very excellent, but if in the case of dynamic measurement, the surface roughness and other noise signals of the waveguide rod 3 will greatly mix into the output signal of the bimorph probe, resulting in poor noise and resolution performance. Although the experiment uses a bimorph probe as a longitudinal vibration sensing element, it can also reflect some performance indicators of the designed prototype and verify whether the prototype design is reasonable and feasible.​​​​

[0078] In the feasibility experiment, the following work was mainly carried out: standing wave frequency point simulation, principle prototype establishment, single cycle resolution test.

[0079] The standing wave is formed on the waveguide rod 3, which is a special interference phenomenon formed by superimposing two rows of coherent waves with the same frequency, amplitude and propagation speed but opposite directions. In essence, it is a kind of resonance state. Using COMSOL software, resonant modes conforming to the characteristics of standing waves can be easily found under various characteristic modes of the waveguide rod 3. As shown in Figure 6 , the first 12 standing wave modes conforming to the characteristics of standing waves are shown, and their frequencies are distributed from 700Hz to 50kHz. There are also many resonant modes that do not conform to the characteristics of standing waves between these frequency points. Since there are too many modes, they are not shown one by one.

[0080] According to the expression of the influence of the longitudinal amplitude detection resolution on the transverse displacement resolution proposed in the foregoing, it can be seen that the standing wave wavelength is directly related to the transverse displacement resolution, so increasing the excitation frequency to higher order standing wave modes to shorten the standing wave wavelength is very effective for improving the transverse displacement resolution. However, as shown in the results of the mode shapes in Figure 6 , for the waveguide base 1 designed at present, when the selected mode is too high, the waveguide rod support structure (support 2) has the problem of insufficient stiffness, which is particularly evident in the ninth and above standing wave modes, while it performs well in the first six standing wave modes. The sixth mode is finally selected as the experimental object in the feasibility experiment. The cause of the insufficient stiffness of the support structure is that the stiffness of the waveguide rod 3 itself is too high, that is, the insufficient stiffness difference between the waveguide rod support structure and the waveguide rod leads to the difficulty of practical application of high-order standing wave modes. From another angle, it is indirectly proved that a flexible wire can be used instead of the waveguide rod as a waveguide structure to form higher order periodic standing waves thereon.

[0081] The standing wave displacement sensor principle prototype feasibility experiment verification platform is shown in Figure 7 , and the prototype mainly includes the following components: waveguide base, piezoelectric bimorph probe, piezoelectric bimorph probe orthogonal holder, excitation piezoelectric ceramic, feedback piezoelectric ceramic, and sliding guide rail. The principle prototype is installed on an air floating isolation table (natural frequency 2Hz) for experiments. In the full range of 150mm, the orthogonal bimorph probe corresponds to the sin output and the cos output, as shown in Figure 8 . The demodulated standing wave output signal peak-to-peak value is 16.5Vpp, and the full range voltage output noise is controlled within 20mVpp. Figure 9As shown, part (a) and (b) respectively show the voltage noise and corresponding displacement noise of the sin output and cos output at the standing wave-45° phase position, part (c) and (d) respectively show the voltage noise and corresponding displacement noise of the sin output and cos output at the standing wave 0° phase position, part (e) and (f) respectively show the voltage noise and corresponding displacement noise of the sin output and cos output at the standing wave 45° phase position. According to the results, the resolutions of the three phase positions of-45°, 0° and 45° are respectively reached 、 、 , all corresponding to a bandwidth of 16 Hz.

[0082] The experimental results show that the structure sensor and the design scheme have very good resolution and very good range scalability. In addition, the method can theoretically reduce the output displacement drift caused by temperature drift. When applied to machining centers, the method simplifies the structure of the displacement sensor, provides higher environmental adaptability, long-term stability, measurement result repeatability, improved anti-interference and wider application possibilities of the displacement sensor.

[0083] The technical features of the above-described embodiments can be combined in any manner. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described, however, as long as the combinations of the technical features do not contradict, they should be considered within the scope of the present disclosure.

[0084] The above-described embodiments are merely examples of the present disclosure, and do not limit the patent scope of the present disclosure. Any equivalent structure or equivalent process transformation based on the content of the specification and drawings, or direct or indirect application in other related technical fields, is also included in the patent protection scope of the present disclosure.

Claims

1. A standing wave displacement sensor, characterized by: The standing wave excitation unit and the standing wave detection unit are included. The standing wave excitation unit comprises a standing wave waveguide base, supports symmetrically arranged on both sides of the top surface of the standing wave waveguide base, waveguide rods fixedly arranged at the top ends of the two supports, two excitation piezoelectric ceramics symmetrically arranged on both sides of the top surface of the waveguide rod, and feedback piezoelectric ceramics arranged on both sides of the bottom surface of the waveguide rod below the excitation piezoelectric ceramics, wherein the feedback piezoelectric ceramics and the excitation piezoelectric ceramics are connected through a standing wave closed-loop control unit. The standing wave detection unit comprises at least one sensor orthogonal holder arranged above the waveguide rod, two eddy current displacement sensors fixedly arranged on each sensor orthogonal holder in parallel and having a phase difference of 90° in spatial phase, and a displacement output circuit connected to the output end of the eddy current displacement sensor. The excitation piezoelectric ceramic and the feedback piezoelectric ceramic excite the waveguide rod in a closed loop, generate a standing wave with fixed spatial period and amplitude on the waveguide rod to form a displacement scale, the eddy current displacement sensor picks up the amplitude of the standing wave at each position on the waveguide rod, and multiple eddy current displacement sensors cooperate to expand the displacement scale and realize large-range, high-resolution and non-contact sensing output.

2. The standing wave displacement sensor of claim 1, wherein: The standing wave closed-loop control unit comprises a charge amplifier connected to the output end of the feedback piezoelectric ceramic, a phase detector and an automatic gain controller connected to the output end of the charge amplifier, an integrator connected between the output end of the phase detector and the input end of a voltage-controlled oscillator, the output end of the voltage-controlled oscillator connected to the input end of the phase detector and the input end of a multiplier, the output end of the automatic gain controller connected to the input end of the multiplier, and the output end of the multiplier connected to the input end of the excitation piezoelectric ceramic through a power amplifier.

3. The standing wave displacement sensor of claim 2, wherein: The excitation process of the standing wave comprises the following steps: S11, excitation feedback signal generation: the feedback piezoelectric ceramic detects the vibration frequency and amplitude of the waveguide rod, and outputs a signal for frequency tracking control and amplitude control after being amplified by a charge amplifier; S12, phase control: the phase detector converts the phase difference between the output signal of the charge amplifier and the carrier signal output by the voltage-controlled oscillator into an actual phase voltage, inputs the difference between the actual phase voltage and a reference phase voltage into the integrator, and the output signal of the integrator controls the voltage-controlled oscillator to generate a carrier signal with a suitable frequency, so that the vibration frequency of the waveguide rod is kept at the resonance point of the standing wave; S13, standing wave amplitude control: the feedback signal output by the charge amplifier is subjected to a root mean square operation, and the root mean square value is taken as the standing wave amplitude signal, which is kept stable through the automatic gain controller, and the output of the automatic gain controller is taken as the baseband signal of the excitation amplitude, which is modulated into the carrier signal output by the voltage-controlled oscillator through the multiplier to form a modulated signal; S14, excitation of the excitation piezoelectric ceramic: the modulated signal is subjected to power amplification through the power amplifier to obtain a high-output-power excitation voltage, which drives the excitation piezoelectric ceramic to work and generates a standing wave with fixed spatial period and amplitude on the waveguide rod.

4. The standing wave displacement sensor of claim 3, wherein: The displacement output circuit comprises phase-sensitive detection modules connected to the output ends of the two eddy current displacement sensors respectively, a comparator and a resolver decoder connected to the output ends of the phase-sensitive detection modules, and a reversible counter connected to the output end of the comparator, and the output end of the resolver decoder is connected to the input end of a signal amplifier and a low-pass filter.

5. The standing wave displacement sensor of claim 4, wherein: The step of converting the standing wave into displacement output is as follows: S21, amplitude detection: the eddy current displacement sensor outputs the real-time vibration time sequence signal of a position on the standing wave, and obtains the amplitude of the position by taking the excitation signal of the standing wave as a reference signal, demodulating through a phase-sensitive detection module; S22, cross-cycle range expansion: the output amplitude signals of the two eddy current displacement sensors after phase-sensitive detection are shaped by comparators to obtain two columns of square waves with a spatial phase difference of 90°, and a reversible counter reversibly counts the spatial cycles of the two columns of square waves to realize cycle expansion of the range; S23, displacement subdivision in the standing wave cycle: a resolver decoder converts the sine signal and the cosine signal obtained by the two eddy current displacement sensors along the standing wave direction into phase outputs, and improves the sensitivity through a signal amplifier and a low-pass filter.

6. The standing wave displacement sensor of claim 5, wherein: The -3dB bandwidth of the eddy current displacement sensor is greater than twice the excitation frequency of the excitation piezoelectric ceramic, and is not less than 100 kHz.

7. The standing wave displacement sensor of claim 5, wherein: The specific demodulation process of the phase-sensitive detection circuit is: the output signal of the eddy current displacement sensor is multiplied by the reference signal, and then divided by the amplitude of the excitation feedback signal after the effective value operation to perform proportional measurement to improve the signal-to-noise ratio, and then low-pass filtering is performed to obtain the amplitude of the vibration time sequence of the position.

8. The standing wave displacement sensor of any one of claims 5-7, wherein, The model of the influence of the resolution of the eddy current displacement sensor on the lateral displacement resolution is as follows: The expression of the standing wave is: ； wherein, is the spatial period of the standing wave, i.e. the wavelength, is the temporal angular frequency of the standing wave, is the amplitude of the standing wave, the temporal phase component in the equation is only left over as a spatial phase component, i.e. the longitudinal amplitude and the transversal displacement after multiplication with a reference signal and demodulation by means of phase-sensitive detection, ； Therefore, the limitation of the resolution of the eddy current displacement sensor on the lateral displacement resolution is: ； wherein represents the resolution of the lateral displacement sensor, represents the resolution of the eddy current displacement sensor, due to the characteristics of the resolver decoder, the range of the spatial phase value is kept at -45°~+45°, and correspondingly, the case that the resolution of the eddy current displacement sensor has the greatest impact on the lateral displacement resolution is: ； Therefore, the method for reducing the influence of the resolution of the eddy current displacement sensor on the lateral displacement resolution is: reducing the wavelength of the standing wave and / or increasing the amplitude of the standing wave.

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