Adaptive Drilling Method Based on Equivalent Voltage of Ultrasonic Vibration Device

Abstract

The present invention discloses an adaptive hole-making method based on the equivalent voltage of an ultrasonic vibration device, belonging to the field of ultrasonic vibration-assisted cutting machining. In the present invention, the host computer performs filtering and Fourier transform processing on the original signal to obtain the frequency, phase, and amplitude characteristics of the signal; and establishes an electromechanical mapping model. Based on this theoretical basis and combined with the extracted signal characteristics, the equivalent voltage is calculated, which is used to characterize the real-time axial force during the machining process and serves as the input signal of the adaptive controller. The adaptive controller, based on the system process model identified offline and the reference signal reflecting the machining strategy, derives the model reference adaptive control law based on the Lyapunov stability theory, calculates the adjustment amount of the excitation voltage in real time, and applies the updated excitation signal to the ultrasonic device through an ultrasonic generator to achieve closed-loop control. The stability of the ultrasonic vibration-assisted drilling process is improved, and the effect of suppressing hole-making defects and improving machining quality is achieved.

Description

Technical Field

[0001] The present invention belongs to the field of ultrasonic vibration assisted drilling, and particularly relates to an adaptive hole-making method based on the equivalent voltage of an ultrasonic vibration device. Background Art

[0002] With the continuous development of equipment in the aerospace field, the requirements for high strength, high stiffness and light weight of materials are constantly increasing. New materials such as CFRP and titanium alloy are widely used in the aerospace field. In order to ensure good anti-fatigue performance, conductivity, monitoring and disassembly and maintenance convenience between structures, mechanical connection methods are often used to connect composite material structural parts. And hole processing for assembly is an essential part of assembly connection. To ensure the reliability of bolt connection or riveting, it is necessary to ensure that the hole-making process can achieve non-damage and high precision. However, most new materials are difficult-to-machine materials, and traditional drilling processes are difficult to meet the requirements of precision and efficiency.

[0003] As a new type of composite machining process, ultrasonic vibration assisted machining refers to adding a certain ultrasonic vibration on the basis of the tool movement in the traditional machining process. According to different process requirements, the vibration direction can be divided into unidirectional vibrations such as longitudinal vibration and torsional vibration, and composite vibrations such as bending vibration and longitudinal-torsional vibration. A large number of studies have proved the superiority of ultrasonic vibration assisted process in difficult-to-machine materials. And the realization of ultrasonic vibration assisted machining only depends on the corresponding device. In theory, precise cutting can be achieved on ordinary machine tools, reducing the production cost.

[0004] At present, due to factors such as piezoelectric hysteresis nonlinearity, tool wear during drilling, machine tool vibration, and workpiece deformation in the ultrasonic vibration assisted drilling process, without active control, problems such as large axial force fluctuations and unstable amplitude output of the ultrasonic device will occur, seriously affecting the hole-making quality. In terms of control, the control of the drilling process itself is not a research hotspot in the machining process control. The current control is mainly based on machining parameters such as feed speed for constant axial force or torque control. Since the control system and the machining machine tool are two independent parts, this method requires additional development of the servo system, and the control accuracy will also be affected by the response speed of the machine tool. And the control methods and phased control strategies for ultrasonic vibration assisted drilling based on vibration parameters are relatively lacking. It has become an urgent problem to study a drilling process control method that does not rely on external sensors and can adaptively adjust the ultrasonic amplitude of the device according to a predetermined machining strategy. Summary of the Invention

[0005] Object of the Invention: To provide an adaptive hole-making method based on the equivalent voltage of an ultrasonic vibration device, which can sense the axial force in the machining process without sensors, and by controlling the device amplitude, make the axial force change in the machining process approach the reference trend, thereby improving the stability of the drilling process and the hole-making quality.

[0006] Technical solution: To achieve the above-mentioned invention purpose, the present invention adopts the following technical solutions:

[0007] An adaptive hole-making method based on the equivalent voltage of an ultrasonic vibration device, which is characterized by including the following processes:

[0008] The hole-making system includes an ultrasonic vibration-assisted drilling device, an ultrasonic generator, a signal acquisition circuit, a data acquisition card, a host computer, and a numerical control machine tool; among them, the ultrasonic vibration-assisted drilling device includes a piezoelectric transducer and a horn, which are installed on the electric spindle of the numerical control machine tool to achieve basic rotation and feeding functions; by applying an alternating voltage to the piezoelectric transducer, ultrasonic vibration is applied on the basis of ordinary drilling to achieve ultrasonic vibration-assisted drilling. Within a certain excitation voltage amplitude range, the excitation amplitude is positively correlated with the voltage; the ultrasonic generator converts the received digital excitation signal through D / A conversion and amplifies the signal amplitude through a voltage amplifier, so as to excite the ultrasonic device to generate vibration; during the signal acquisition process, during processing, the signal acquisition circuit collects the voltage and current signals of the ultrasonic device in real time, performs A / D conversion through the data acquisition card, and transmits the converted digital signal to the host computer;

[0009] In the host computer, the digital signal first removes the process noise through median filtering, and then extracts the amplitude, frequency, and phase frequency domain characteristics through Fourier transform, and calculates the equivalent voltage; the reference input is formulated by a pre-determined optimal drilling strategy, and its value reflects the expected change trend of the axial force during the three drilling stages of drilling-in - drilling - drilling-out, which is used as the control target of the control system. The adaptive control rate is designed based on the transfer function of the ultrasonic vibration-assisted drilling process obtained by off-line identification. Based on this, the real-time equivalent voltage signal is input, and the updated excitation signal is calculated to achieve feedback control. The specific steps are as follows:

[0010] Step 1: The signal acquisition circuit extracts the additional voltage and current generated inside the device during the ultrasonic vibration-assisted drilling process due to external load changes. After performing A / D conversion through the data acquisition card and transmitting the converted digital signal to the host computer, the current I and voltage V are obtained through median filtering and fast Fourier transform.

[0011] The signal acquisition circuit consists of three resistors R m1 , R m2 , R m3 ; among them, R m3 is connected in series with the piezoelectric transducer inside the ultrasonic vibration device, and R m1 and R m2 are connected in series to form another branch, and are connected to R m3The branches formed in series with the piezoelectric transducer are connected in parallel; the voltage and current signals across the piezoelectric transducer usually exceed the range of the data acquisition card. To achieve acquisition, since the current signal flowing through the piezoelectric transducer is equal to the current signal of R m3 the current is calculated by collecting the voltage across R m3 ; R m1 and R m2 have a resistance ratio of K. By dividing the voltage across R m2 , the voltage signal across R m1 is measured. Combining Kirchhoff's voltage law, the voltage signal across the piezoelectric transducer is calculated based on the resistance ratio, thereby realizing the acquisition of current and voltage signals;

[0012] Step 2: Calculate the equivalent voltage U eq based on the current and voltage signals. The model expression is:

[0013]

[0014]

[0015]

[0016] U eq = U′0 - U′ (4)

[0017] where R m1 and R m2 are the resistance values of the signal acquisition circuit, are the voltage signals across resistors R m1 and R m3 respectively, is the voltage across the piezoelectric transducer, is the overall voltage of the signal acquisition circuit branch, are all in the form of phasors. U′ is the voltage of the transducer after excitation decoupling, and U′0 is the static transducer voltage value. The equivalent voltage obtained after calculations (1)-(4) is used to characterize the changing trend of the axial force value during the drilling process;

[0018] Step 3: Input the equivalent voltage U eq into the model reference adaptive controller and compare it with the reference equivalent voltage output by the reference model to obtain the real-time error e0. The calculation method is:

[0019] e0(t) = y p (t) - y M (t) (5)

[0020] y M (s) = G M (s)r p(s) (6)

[0021] where y p (t) is the equivalent voltage U calculated from the measured electrical signal at time t during the processing eq , y M (t) is the reference equivalent voltage calculated by the reference model according to the reference input at time t, r p (s) is the reference input, G M (s) is the designed transfer function of the reference model;

[0022] Step 4: Design a model reference adaptive control law based on the error e0 of the equivalent voltage; Since the signal type is a digital signal, a discrete form needs to be adopted when designing the control law. For the excitation signal value u(k) of the k-th sequence, the calculation method is as follows:

[0023]

[0024]

[0025] where Γ is the adaptive rate adjustment parameter and k1 is the controller adjustment parameter, and their values are determined according to the results of numerical simulation; ω(k) are the adaptive rate and the system input-output vector inside the control system under the k-th sequence respectively, and ω(k) is composed as shown in the following formula:

[0026] ω T (k) = [r(k), v1 T (k), y p (k), v2 T (k)] (9)

[0027] where r(k) and y p (k) are the reference input and the measured equivalent voltage U under the k-th sequence respectively eq , v1(k) and v2(k) are two state variables in the model reference adaptive controller under the k-th sequence, and their calculation formulas are as follows:

[0028] v1(k) = Λv1(k - 1) + bu(k - 1) (10)

[0029] v2(k) = Λv2(k - 1) + by p (k - 1) (11)

[0030] where Λ and b are the model parameters identified by the system.

[0031] Step 5: The ultrasonic generator amplifies the excitation signal u(k) given by the host computer control system and applies it to the ultrasonic device, so that the amplitude makes an adaptive adjustment.

[0032] Beneficial effects: Compared with the prior art, the present invention has the following advantages:

[0033] 1. An equivalent voltage model is established based on the electromechanical model, and the real-time characterization of the axial force during the drilling process is realized based on the internal electrical signals of the ultrasonic device without relying on external sensors;

[0034] 2. The established control system can perform adaptive control for any formulated processing strategy;

[0035] 3. The established digital control method has strong scalability and can be applied to different devices and working conditions after recalibration. Description of the Drawings

[0036] Figure 1 is a schematic diagram of the overall control system of the present invention;

[0037] Figure 2 is a schematic diagram of the signal acquisition circuit;

[0038] Figure 3 is a schematic diagram of the system process model;

[0039] Figure 4 is a structural diagram of the ultrasonic vibration assisted drilling device;

[0040] Figure 5 is a schematic diagram of the control method. Detailed Embodiments

[0041] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings. It should be noted that the following examples are only for illustrating the technical idea of the present invention and cannot be used to limit the protection scope of the present invention. Any modifications made on the basis of the technical solution according to the technical idea proposed by the present invention fall within the protection scope of the present invention.

[0042] As Figure 1 shown, an adaptive hole-making control system based on the equivalent voltage of an ultrasonic vibration device includes: an ultrasonic vibration assisted drilling device, an ultrasonic generator, a signal acquisition circuit, a data acquisition card, and a host computer control system. Among them, the ultrasonic vibration assisted drilling device includes a piezoelectric transducer, a horn, etc., and the structure of the signal acquisition circuit is as Figure 2 shown; First, the signal acquisition circuit extracts the original electrical signals inside the device during the ultrasonic vibration assisted drilling process, and the data acquisition card performs A / D conversion. After being transmitted to the host computer control system, through median filtering and fast Fourier transform, the current I and voltage V are obtained; Secondly, according to the current and voltage signals, the equivalent voltage U eq is calculated; The equivalent voltage U eqInput it into the model reference adaptive controller, compare it with the reference equivalent voltage output by the reference model to obtain the real-time error e0. Subsequently, based on the error e0 of the equivalent voltage, calculate the excitation signal value u(k) at this moment according to the model reference adaptive control rate. Finally, the ultrasonic generator amplifies the excitation signal u(k) given by the host computer control system and applies it to the ultrasonic device, so that the amplitude makes an adaptive adjustment.

[0043] As Figure 1 shown, an adaptive hole-making method based on the equivalent voltage of an ultrasonic vibration device includes the following

[0044] steps:

[0045] (1) The signal acquisition circuit extracts the original electrical signal inside the device during the ultrasonic vibration-assisted drilling process. After the data acquisition card performs A / D conversion and transmits it to the host computer control system, through median filtering and fast Fourier transform, the amplitudes and phases of the electrical signal parameters V1 and V2 are obtained. As Figure 2 shown, connect the piezoelectric transducer inside the ultrasonic vibration device to the acquisition circuit, and collect the voltage and current of the piezoelectric transducer through the resistors Rm1 and Rm3 respectively. The calculation formulas are as follows:

[0046]

[0047] V t = V - V2 (13)

[0048] (2) Calculate the equivalent voltage U eq , based on the electromechanical conversion characteristics of the piezoelectric transducer, the induced voltage of the transducer can reflect the trend of force change during the entire processing process. However, its dual functions as driving and sensing make the voltage's characterization of force affected by the coupling effect of the piezoelectric effect. On the one hand, according to the above analysis, the external force change will cause the voltage to change; on the other hand, only by changing the amplitude of the excitation voltage, within a certain range, the excitation voltage will also proportionally affect the induced voltage of the transducer. And the excitation voltage needs to be adjusted in real time according to the control strategy as a control quantity in the control system. In this case, simply from the transducer voltage, the change of the external force cannot be characterized, and it needs to be converted into an equivalent voltage for decoupling, so that the amplitude of the equivalent voltage is not affected by the excitation voltage and only characterizes the change of the external force:

[0049]

[0050] At the same time, in order to facilitate the construction of the subsequent control system, construct an equivalent voltage form that is positively correlated with the external force:

[0051] U eq = U′0 - U′ (15)

[0052] (3) Input the equivalent voltage U eq into the model reference adaptive controller, compare it with the reference equivalent voltage output by the reference model, and obtain the real-time error e0. The calculation method is as follows:

[0053] e0(t) = y p (t) - y M (t) (16)

[0054]

[0055] where y p is the equivalent voltage U calculated from the measured electrical signal during the machining process eq , y M is the reference equivalent voltage calculated by the reference model according to the reference input, r is the reference input, and A p , b p , h T are all model parameters identified in the reference model.

[0056] The overall process includes that the piezoelectric transducer generates vibration displacement under the excitation voltage, which is amplified by the horn of the device and acts on the tool, affecting the axial force during the machining process and being feedback as an equivalent voltage signal. The identified model structure is as Figure 3 shown.

[0057] (4) Based on the error e0 of the equivalent voltage, calculate the excitation signal value u(k) at this moment according to the model reference adaptive control law. The ultrasonic generator amplifies the excitation signal u(k) given by the host computer control system and applies it to the ultrasonic device, so that the amplitude makes an adaptive adjustment.

[0058] The calculation method of u(k) is as follows:

[0059]

[0060]

[0061] where, ω(k) are respectively the adaptive rate vector and the system input-output vector inside the control system at time k, and Γ, k1 are controller parameters, and their values are determined according to numerical simulation.

[0062] An adaptive hole-making control method based on the equivalent voltage of an ultrasonic vibration device realizes the adaptive change of the amplitude according to the trend of the target axial force during the ultrasonic vibration-assisted drilling process, thus compensating for the axial force fluctuations caused by processing external load fluctuation factors such as tool wear, machine tool vibration, workpiece deformation, and reduction of workpiece stiffness during the processing. By establishing an equivalent voltage model representing the drilling axial force, the electrical signal is collected in real time during the processing to calculate the equivalent voltage. The model reference adaptive controller updates the excitation signal at this time according to the difference between the actual equivalent voltage and the ideal model equivalent voltage, so that the ultrasonic amplitude changes adaptively. This method enhances the stability of the ultrasonic vibration-assisted drilling process, thereby improving the hole-making quality. At the same time, both the actuator and the sensor of the control system are ultrasonic devices, which simplifies the control system, effectively reduces the cost, and improves the feasibility.

Claims

1. An adaptive hole-making method based on the equivalent voltage of an ultrasonic vibration device, characterized in that It includes the following processes: The hole-making system includes an ultrasonic vibration-assisted drilling device, an ultrasonic generator, a signal acquisition circuit, a data acquisition card, a host computer, and a numerical control machine tool. Among them, the ultrasonic vibration-assisted drilling device includes a piezoelectric transducer and a horn, which are installed on the electric spindle of the numerical control machine tool to achieve basic rotation and feeding functions. By applying an alternating voltage to the piezoelectric transducer, ultrasonic vibration is applied on the basis of ordinary drilling to achieve ultrasonic vibration-assisted drilling. Within a certain range of excitation voltage amplitudes, the excitation amplitude is positively correlated with the voltage. The ultrasonic generator performs D / A conversion on the received digital excitation signal and amplifies the signal amplitude through a voltage amplifier, thereby exciting the ultrasonic device to generate vibration. During the signal acquisition process, the voltage and current signals of the ultrasonic device are real-time acquired through the signal acquisition circuit during processing, and are converted by the data acquisition card through A / D conversion, and the converted digital signal is transmitted to the host computer. In the host computer, the digital signal first removes the process noise through median filtering, and then extracts the amplitude, frequency, and phase frequency domain characteristics through Fourier transform, and calculates the equivalent voltage. The reference input is formulated by the pre-determined optimal drilling strategy, and its value reflects the expected change trend of the axial force during the three drilling stages of drilling-in - drilling-middle - drilling-out, and serves as the control target of the control system. The adaptive control rate is designed based on the transfer function of the ultrasonic vibration-assisted drilling process obtained by off-line identification. Based on this, the real-time equivalent voltage signal is input, and the updated excitation signal is calculated to achieve feedback control. The specific steps are as follows: Step 1: The signal acquisition circuit extracts the additional voltage and current generated inside the device due to external load changes during the ultrasonic vibration-assisted drilling process. After A / D conversion by the data acquisition card and transmission of the converted digital signal to the host computer, the current I and voltage V are obtained through median filtering and fast Fourier transform. The signal acquisition circuit consists of three resistors R m1 , R m2 , R m3 . Among them, R m3 is connected in series with the piezoelectric transducer inside the ultrasonic vibration device. R m1 and R m2 are connected in series to form another branch, and are connected in parallel with the branch formed by connecting R m3 in series with the piezoelectric transducer. The voltage and current signals at both ends of the piezoelectric transducer usually exceed the range of the data acquisition card. In order to achieve acquisition, since the current signal flowing through the piezoelectric transducer is equal to the current signal of R m3 , the current is calculated by collecting the voltage at both ends of R m3 . The resistance ratio of R m1 to R m2 is K. By dividing the voltage of R m2 , the voltage signal at both ends of R m1 is measured. Combining Kirchhoff's voltage law, the voltage signal at both ends of the piezoelectric transducer is calculated according to the resistance ratio, so as to achieve the acquisition of current and voltage signals; Step 2: Calculate the equivalent voltage U based on the current and voltage signals eq , and the model expression is: Among them, R m1 , R m2 are the resistance values of the signal acquisition circuit resistors. are respectively the voltage signals at both ends of resistors R m1 , R m3 . is the voltage across the piezoelectric transducer. is the overall voltage of the signal acquisition circuit branch. are both in the form of phasors. U′ is the voltage of the transducer after excitation decoupling, and U′0 is the static transducer voltage value. The equivalent voltage obtained after calculations (1)-(4) is used to characterize the changing trend of the axial force value during the drilling process. Step 3: Input the equivalent voltage U eq into the model reference adaptive controller, compare it with the reference equivalent voltage output by the reference model to obtain the real-time error e0, and the calculation method is as follows: e0(t) = y p (t) - y M (t) (5) y M (s) = G M (s)r p (s) (6) where y p (t) is the equivalent voltage U calculated from the measured electrical signal at time t during the processing eq , y M (t) is the reference equivalent voltage calculated by the reference model according to the reference input at time t, r p (s) is the reference input, G M (s) is the designed transfer function of the reference model; Step 4: Based on the error e0 of the equivalent voltage, a model reference adaptive control rate is designed. Since the signal type is a digital signal, a discrete form needs to be adopted when designing the control rate. For the excitation signal value u(k) of the k-th sequence, the calculation method is: Where Γ is the adaptive rate adjustment parameter, and k1 is the controller adjustment parameter. The values of both are determined according to the results of numerical simulation; ω(k) are the adaptive rate and the system input-output vector inside the control system under the k sequence respectively, and ω(k) is constituted as shown in the following formula: ω T (k) = [r(k), v1 T (k), y p (k), v2 T (k)] (9) where r(k), y p (k) are the reference input and the measured equivalent voltage U eq under the k sequence respectively, and v1(k), v2(k) are two state variables in the model reference adaptive controller under the k sequence, and their calculation formulas are as follows: v1(k) = Λv1(k - 1) + bu(k - 1) (10) v2(k) = Λv2(k - 1) + by p (k - 1) (11) where Λ and b are the model parameters obtained by system identification. Step 5: The ultrasonic generator amplifies the excitation signal u(k) given by the host computer control system and applies it to the ultrasonic device, so that the amplitude makes an adaptive adjustment.

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