A method for adjusting current loop PI parameters using bandwidth and damping ratio

Abstract

A method for adjusting the current loop PI parameters using bandwidth and damping ratio belongs to the field of motor control technology, and includes the following steps: S1 calculates the current loop PI parameters based on a theoretical model to obtain specific PI parameters; S2 uses a constant stator current method to obtain the current loop step response waveform under specific PI parameters; S3 fits the current waveform based on the mathematical model of the standard second-order system unit step response to obtain performance parameters; S4 determines whether the current PI parameters are appropriate based on the bandwidth and damping ratio, and uses them if they are appropriate; adjusts them if they are not appropriate, and then returns to step S2. The present invention obtains the natural frequency of the current loop closed-loop transfer function by fitting the current loop step response waveform, thereby obtaining parameters such as its bandwidth and damping ratio that directly characterize the rapidity and stability of the current loop. The method provided by the present invention can realize the visualization of the control effect of any PI parameter, indicate the direction of subsequent adjustment of the PI parameter, and save adjustment time.

Description

Technical Field

[0001] The present invention belongs to the technical field of motor control, and relates to a method for adjusting motor current loop PI parameters, and specifically to a method for adjusting current loop PI parameters by using bandwidth and damping ratio. Background Art

[0002] In the field of industrial motor control, motor control generally includes three control loops, which are current loop, speed loop and position loop from the inside to the outside. Existing PI parameter adjustment patents, such as CN102426417B, CN104993764B, CN108631674B, CN109802609A, CN113517836B and CN113824375B, mainly involve the setting of one or two PI parameters of speed loop and position loop. Current loop control is a crucial link. Current loop control aims to ensure that the current output by the motor runs stably near the set value, so as to achieve the reliability and stability of the motor system under various load conditions. The load is large and the load changes frequently, represented by AGV (Automated Guided Vehicle), which has high requirements for torque stability and fast response. These two are mutually exclusive to a certain extent, and higher requirements are put forward for current loop control. In practical applications, a PI (proportional-integral) controller is usually used to implement current loop control because the PI controller has a simple structure, good stability and is easy to implement.

[0003] Although PI controllers are widely used in industrial motor control systems, there are still some challenges in parameter adjustment. In current industrial practice, the traditional blind adjustment strategy is often used for PI parameter adjustment, but this method has some obvious problems. One of the main disadvantages of the blind adjustment strategy is that it is impossible to quantitatively evaluate the control effect. The debuggers often adjust the parameters based on their understanding and recognition of the control effect of parameters such as P (proportional) and I (integral). The debuggers often select PI parameters through parameters such as rise time and overshoot, but the stability of the current loop cannot be directly quantified. This adjustment method is easily affected by factors such as the debugger's personal experience, subjective judgment, and working environment, resulting in uncertainty and reliability issues in the adjustment results.

[0004] Compared with the blind adjustment strategy, the parameter adjustment method based on the theoretical model has solved the problems caused by blind adjustment to a certain extent, but there are still some challenges. The motor system is often affected by nonlinearity and time-varying properties. The theoretical model often cannot accurately describe the system behavior, resulting in insufficient accuracy of parameter adjustment. In addition, the adjustment method of the theoretical model often ignores many links and interference factors, such as friction and inertia, which may cause a large difference between the adjustment result and the actual situation. The parameters calculated theoretically often need to be re-debugged in actual application.

[0005] The above two methods of debugging the PI parameters of the current loop in motor control are ultimately based on actual conditions and blindly adjusted to ensure the reliable operation of the motor. The above two methods of debugging PI parameters are not only inefficient, but also unable to quantify the rapidity and stability of the current loop at the same time. Summary of the invention

[0006] In order to solve the problems existing in the prior art: the PI parameters based on theoretical calculations cannot be used directly, and still need to be re-debugged in combination with actual working conditions; the PI parameters are blindly adjusted, and the adjustment effect and adjustment time mostly depend on the experience of the debugger, and the control effect of the PI parameters cannot be quantified. The purpose of the present invention is to provide a method for adjusting the PI parameters of the current loop using bandwidth and damping ratio, firstly obtaining the performance parameters of the closed-loop transfer function of the current loop, and using the obtained performance parameters to characterize the rapidity and stability of the current loop under the corresponding PI parameters, thereby adjusting and optimizing the PI parameters. The purpose of the present invention is achieved through the following specific technical solutions.

[0007] A method for adjusting current loop PI parameters using bandwidth and damping ratio comprises the following steps:

[0008] S1 calculates the current loop PI parameters based on the theoretical model to obtain specific PI parameters;

[0009] S2 uses a constant stator current method to obtain the current loop step response waveform under specific PI parameters;

[0010] S3 fits the current waveform to obtain performance parameters based on the mathematical model of the standard second-order system unit step response;

[0011] S4 determines whether the current PI parameters are appropriate based on the bandwidth and the damping ratio. If appropriate, the PI parameters are used; if not appropriate, the PI parameters are adjusted, and then the process returns to step S2.

[0012] The main steps are described in detail below.

[0013] (1) Obtaining specific PI parameters

[0014] Combined with engineering design methods, the open-loop transfer function of the current loop is adjusted to a typical Type I system. The theoretical PI parameters of the current loop can be determined only by the motor's own parameters and the switching cycle time. The theoretical values ​​are calculated as the initial given values ​​of the current loop PI parameters.

[0015] (2) Obtaining the current loop step response waveform

[0016] When the motor is at zero position, among the three-phase currents, if the current of phase A is of amplitude I, then the currents of phases B and C are half of the amplitude. At this time, the direction of the synthetic current will be the same as that of phase A, that is, the direction of the stator magnetomotive force is the same as that of phase A. If the stator current vector remains constant, under the action of the stator magnetic field, the direction of the rotor magnetic field will coincide with phase A. Based on Clark transformation and Park transformation, the current in the dq coordinate system can be obtained:

[0017]

[0018] Therefore, when the d-axis current of the current loop is constant and the q-axis current is 0, the A-phase current can be directly observed. At this time, the current waveform of phase A is the step response under the current loop PI parameters.

[0019] (3) Waveform fitting to obtain performance parameters

[0020] The control object of the current loop can be considered as a double inertia link. Generally, the PI regulator and the large time constant of the control object are used for zero-pole cancellation. The open-loop transfer function of the current loop is adjusted to a typical type I system, and the closed-loop transfer function can be equivalent to a standard second-order system. The standard form of the closed-loop transfer function of the second-order system is:

[0021]

[0022] Where, ζ is the damping ratio, unitless; ω n is the natural frequency, measured in Hz, and cannot be measured experimentally.

[0023] When the input signal is 1 / s, the output of the system can be expressed as:

[0024]

[0025] in, is the damped oscillation frequency in Hz.

[0026] By taking the inverse Laplace transform, the unit step response can be obtained as:

[0027]

[0028] in, No units.

[0029] The amplitude-frequency characteristics of the second-order system can be expressed as:

[0030]

[0031] According to the definition of bandwidth and amplitude-frequency characteristics, the bandwidth frequency can be expressed as:

[0032]

[0033] Based on formula (4) and formula (6), the step response waveform of the current loop can be fitted and the corresponding key performance parameters such as natural frequency, bandwidth and damping ratio can be obtained.

[0034] (4) PI parameter selection and adjustment

[0035] From formula (6), we can see that the bandwidth frequency ω BW Proportional to the natural frequency ω n , which is inversely proportional to the damping ratio ζ. In engineering, there is a relatively reasonable range for the selection of damping ratio, which is 0.4 to 0.8. The bandwidth frequency and damping ratio can characterize the rapidity and stability of the current loop. The appropriate PI parameters can be selected based on the requirements for the rapidity and stability of the current loop in actual use of the motor. If the damping ratio is in the range of 0.4 to 0.8, and the bandwidth frequency and damping ratio meet the requirements for the rapidity and stability of the current loop in the use of the motor, then this PI parameter is used; otherwise, according to the principle that the natural frequency and bandwidth are proportional to P and inversely proportional to I, and the damping ratio is inversely proportional to the bandwidth, the PI parameters are adjusted and refitted.

[0036] The current loop PI parameter setting method provided by the present invention can obtain the natural frequency of the current loop closed-loop transfer function by fitting the current loop step response waveform, thereby obtaining its bandwidth and damping ratio and other parameters that can directly characterize the rapidity and stability of the current loop. Compared with the traditional blind adjustment or the method of adjusting PI parameters based on theoretical calculation, the method provided by the present invention can realize the visualization of the control effect of any PI parameter, indicate the direction of subsequent adjustment of the PI parameter, and save adjustment time. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is a flow chart for adjusting PI parameters based on waveform fitting.

[0038] Figure 2 It is the corresponding diagram of the original waveform of the step response and the fitted waveform. DETAILED DESCRIPTION

[0039] The technical solution of the present invention is described clearly and completely below in conjunction with the accompanying drawings of the specification. Obviously, the described implementation is only a part of the implementation of the present invention, not all of the implementations. Based on the implementation of the present invention, all other implementations obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0040] The current loop PI parameter setting and optimization method proposed in the present invention is as follows: Figure 1 shown.

[0041] Combined with the engineering design method, the current loop open-loop transfer function is adjusted to a typical type I system. The theoretical PI parameters of the current loop can be determined only by the motor's own parameters and the switching cycle time. The theoretical value is calculated as the initial setting of the current loop PI parameters. Based on the PI parameters, the motor is controlled only by the current loop, the q-axis current is kept at 0, and the d-axis current is given a constant value. The waveform of the A-phase current at this time is recorded using an oscilloscope.

[0042] Based on the unit step response formula of the second-order system, use Python (or other tools) to fit the collected current waveform, obtain key performance parameters such as natural frequency, damping ratio and bandwidth, and record them. Combined with the actual working conditions for the current loop, determine whether the PI parameters meet the needs of the current loop's rapidity and stability from the bandwidth and damping ratio. If the PI parameters do not meet the requirements, it is necessary to adjust the PI parameters and refit according to the influence (natural frequency and bandwidth are proportional to P and inversely proportional to I; damping ratio is inversely proportional to bandwidth). If satisfied, use this PI parameter.

[0043] The given PI parameters of a motor and its corresponding performance parameters are shown in the following table.

[0044]

[0045] K cp is the current loop proportional parameter (P), K ci is the current loop integral parameter (I).

[0046] By changing K cp and K ci , measure the current waveform, and then fit to get the columns of parameters on the right side of the table.

[0047] To ensure the stability of the current loop and the fastest response, the damping ratio should be close to 0.4 and the bandwidth should be as large as possible. Based on this principle, P is 0.15 and I is 0.008 as the best parameters. The step response waveform and the fitting waveform are as follows: Figure 2 As shown. At this time, the damping ratio is 0.43, the motor switching frequency is 16KHz, and the bandwidth is about 6.8KHz. While the current loop is stable, the rise time takes less than 3 cycles. The above proves that the bandwidth and damping ratio parameters of the current loop can be obtained by fitting, and based on this parameter, the rapidity and stability of the current loop can be characterized, and the visualization of the PI parameter control effect can be realized.

[0048] Although the embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and cannot be understood as limiting the present invention. A person skilled in the art may change, modify, replace and modify the above embodiments within the scope of the present invention without departing from the principles and purpose of the present invention. The protection scope of the present invention is defined by the claims and their equivalent technical solutions.

Claims

1. A method for adjusting current loop PI parameters using bandwidth and damping ratio, characterized in that: The following steps are involved: S1 calculates the current loop PI parameters based on the theoretical model to obtain specific PI parameters; S2 uses a constant stator current method to obtain the current loop step response waveform under specific PI parameters; S3 Based on the mathematical model of the standard second-order system unit step response, the current loop step response waveform is fitted to obtain performance parameters; the specific method is: The standard form of the closed-loop transfer function of a second-order system is: in, ζ is the damping ratio, unitless; ω n is the natural frequency, in Hz; When the input signal is 1 / S When , the output of the system is expressed as: in, , is the damped oscillation frequency, in Hz; Obtain the inverse Laplace transform and obtain the unit step response formula: ， in, , no unit; The amplitude-frequency characteristic of the second-order system is expressed as: According to the definition of bandwidth and amplitude-frequency characteristics, the bandwidth-frequency formula is expressed as: Based on the above unit step response formula and bandwidth frequency formula, the step response waveform of the current loop is fitted to obtain the corresponding key performance parameters: natural frequency, bandwidth and damping ratio; S4: judging whether the current PI parameters are appropriate based on the bandwidth and the damping ratio, and using the PI parameters if appropriate; adjusting the PI parameters if inappropriate, and then returning to step S2.

2. The method according to claim 1, characterized in that The specific method of step S1 is: the open-loop transfer function of the current loop is adjusted to a typical type I system, the theoretical PI parameters of the current loop are only determined by the motor's own parameters and the switching cycle time, and the theoretical values ​​are calculated as the initial given values ​​of the current loop PI parameters.

3. The method according to claim 1, characterized in that The specific method of step S2 is: the motor is controlled by the current loop only, the q-axis current is kept at 0, the d-axis current is given a constant value, and the waveform of the A-phase current at this time is recorded by using a recorder.

4. The method according to claim 1, characterized in that The specific method of step S4 is: if the damping ratio is in the range of 0.4 to 0.8, and the bandwidth frequency and the damping ratio meet the requirements of the rapidity and stability of the current loop during the use of the motor, then this PI parameter is used; otherwise, the PI parameter is adjusted according to the principle that the natural frequency and bandwidth are proportional to P and inversely proportional to I, and the damping ratio is inversely proportional to the bandwidth, and then return to step S2.

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