A compound control method suitable for a low-speed running permanent magnet direct drive servo motor

Abstract

The application discloses a compound control method suitable for a low-speed operation of a permanent magnet direct drive servo motor, and is applied to the field of permanent magnet direct drive servo motors.The control method mainly consists of rotor position tracking control and deadbeat current prediction control.The rotor position tracking control converts a speed instruction in a speed outer loop into a real-time rotor position track, effectively reduces the error of speed detection, improves the anti-interference ability of the system, and avoids the complicated parameter setting of a traditional controller.The deadbeat current prediction control calculates the control voltage vector to be applied at the next moment through a prediction model and current voltage sampling values in a current inner loop, eliminates the control delay, improves the response speed of the current loop, and guarantees the dynamic performance of the system.

Description

TECHNICAL FIELD

[0001] The application relates to the field of permanent magnet synchronous motors, in particular to a compound control method suitable for a permanent magnet direct drive servo motor running at low speed. BACKGROUND

[0002] A servo system is an important system in many applications such as industry and aerospace. Among them, a permanent magnet synchronous motor is favored and becomes a mainstream of high-performance servo motors due to a series of advantages such as high power density, fast response and high efficiency. Compared with a traditional servo system structure, a permanent magnet direct drive servo system cancels a transmission mechanism, and a servo motor is directly connected with a load. This structure enables the direct drive servo system to achieve higher efficiency and control accuracy, and reduces maintenance cost of the system. However, after the direct drive structure is adopted, the motor can only maintain a low speed to drive a low-speed load. Under the low-speed working condition, on the one hand, disturbances such as a non-ideal factor inherent in the motor, for example, a tooth slot torque and a friction torque, will present a lower frequency, and since the motor has a low-pass filter property, the above factors have a greater impact on the motor compared with high speed. On the other hand, for a control system under the low-speed working condition, detection error in a speed detection process and current loop delay caused by a digital controller will cause greater torque and speed fluctuation through closed-loop control, and seriously affect stability of speed control. Therefore, a low-speed running control method of the permanent magnet direct drive servo motor becomes a difficult problem to be solved urgently.

[0003] Many scholars at home and abroad have studied the control method of permanent magnet direct drive servo motor at low speed, which is mainly divided into two categories, namely current control and speed control. Current control mainly includes PI control, finite set model predictive control and deadbeat current predictive control. The current loop control of the traditional vector control method generally uses PI control, which has the advantages of easy implementation and easy adjustment, but its current response speed is slow and the control performance needs to be improved, which is not suitable for low speed control. The finite set model predictive control method has strong adaptability and good control performance when there are constraints, but it only acts on one voltage vector in one switching period, which will cause large current harmonics. The deadbeat current predictive control method has small calculation amount, fixed switching frequency, fast response ability and easy digital implementation, and has been applied in many occasions. Speed control mainly includes PI control, active disturbance rejection control and rotor position tracking control method. PI control has limited bandwidth and needs accurate speed feedback information, which cannot meet the speed tracking ability and anti-interference ability at the same time. Active disturbance rejection control can effectively suppress nonlinear disturbances of the system, but the structure of nonlinear active disturbance rejection control is often complex, which is difficult to use in practical engineering. The rotor position tracking control converts the speed control into the rotor position control, which does not need accurate speed feedback and can effectively suppress periodic and non-periodic torque disturbances, so that the PMSM has good speed performance under low speed working condition, but the method does not consider the influence of current control at low speed. In summary, the existing control methods of permanent magnet direct drive servo motor at low speed have their own advantages and disadvantages, and have certain limitations, such as large detection error, poor anti-interference performance and large control delay, which will further affect the control performance of the motor. SUMMARY

[0004] Based on the above problems, the present application provides a compound control method of permanent magnet direct drive servo motor suitable for low speed operation, which comprises the following steps:

[0005] S1, in the speed outer loop, the rotor position tracking control method is adopted to convert the speed given instruction into real-time rotor position trajectory, so that the rotor position can be smoothly tracked. In addition, the robust stability and anti-interference ability are important indicators for the stable operation of the motor, and the parameters of the controller are designed according to the indicators;

[0006] S2, the deadbeat current predictive control method is adopted in the current inner loop, the mathematical model of permanent magnet synchronous motor is constructed, and the given voltage vector at the next moment is predicted by using the mathematical model combined with the current voltage sampling value of the motor, so as to compensate the control delay of the system;

[0007] S3, the rotor position tracking control and the deadbeat current predictive control are combined, the system transfer function is derived, and the compound control of permanent magnet direct drive servo motor under low speed working condition is realized.

[0008] The speed outer loop in step S1 adopts a rotor position tracking control method, and the transfer function of the rotor position tracking controller is:

[0009]

[0010] where J is the rotational inertia of the motor, B a is the active damping coefficient of the motor, P n is the number of pole pairs, ψ f is the flux, r is the filter order, and T f is the filter time constant:

[0011] Therefore, by selecting appropriate values of T f and r in G(s), stable operation at low speed can be achieved.

[0012] First, the value of r is discussed. According to the analysis of the closed-loop transfer function of the system, the filter is selected as a 2nd-order filter, i.e., r = 2. Further analysis is performed on the value of T f .

[0013] In order to analyze the following performance of the rotor position tracking control method at low speed, the closed-loop transfer function of the system is written as shown in equation 2.2:

[0014]

[0015] Under the same given conditions, gradually reduce the value of T f , the tracking ability of the system first increases and then decreases, so the value of T f cannot be too small.

[0016] Further analysis of the anti-interference performance of the rotor position tracking control method at low speed, the transfer function between the disturbance output by the control object and the actual speed is written as shown in equation 2.3:

[0017]

[0018] By analyzing the Bode diagram of T(s), it is found that as T f gradually decreases, the amplitude of T(s) also gradually decreases, indicating that the influence of the motor speed on the disturbance gradually decreases, and the anti-interference performance of the system gradually increases.

[0019] From the above analysis, it can be concluded that after the value of T f is selected, the rotor position tracking control can simultaneously satisfy strong anti-interference ability and speed tracking ability. It can not only suppress the inherent torque disturbance such as cogging torque when the permanent magnet direct drive motor runs at low speed, but also avoid the great influence brought by speed detection delay or error, thereby improving the robustness of the permanent magnet direct drive motor when running at low speed.

[0020] The current inner loop in step S2 adopts a deadbeat current predictive control method:

[0021] The deadbeat current predictive control equation is as follows:

[0022] u * (k) = B -1 (i * (k) - Ai(k) - C(k))(2.4)

[0023] Wherein

[0024]

[0025] u * (k) is the given d, q-axis voltage, L s is the stator inductance, R s is the stator resistance, ω e is the motor electrical angular velocity, T s is the sampling period, only the given current output by the speed loop and the feedback current obtained by sampling are combined in each switching period, and the given voltage value of the motor can be calculated to realize the deadbeat tracking of the current, in the case of ensuring the accuracy of the motor parameters, combined with the mathematical model of the motor.

[0026] However, in the actual application process, a certain time is consumed for A / D conversion and control algorithm execution, and the actual voltage vector can act on the motor only at the next moment, that is, there is a one-beat delay in the control system. Therefore, the control delay needs to be compensated, that is, the voltage vector u(k+1) of the next period is calculated in advance at k moment, and acts at k+1 moment. The specific calculation method is as follows:

[0027] In order to further obtain the predicted voltage u(k+1), it is necessary to predict one more period, and the two-step prediction of current is adopted in this paper

[0028] i(k+2) = Ai(k+1) + Bu(k+1) + C(k+1)(2.5)

[0029] According to the control timing of DSP, when the voltage value at k+1 moment is calculated at k moment, and acts on the motor at k+1 moment, A / D sampling will be performed at k+2 moment to obtain the measured feedback current value. Therefore, the given current and the feedback current are different by two periods. Therefore, it can be obtained that:

[0030] i(k+2) = i * (k)(2.6)

[0031] Then the expression of the reference voltage required in the next period can be obtained

[0032] u * (k+1) = B -1 (i* (k)-A(Ai(k)+Bu(k)+C(k))-C(k+1))(2.7)

[0033] Wherein, u * (k+1) is the given d, q axis voltage at k+1 time. The control delay of permanent magnet direct drive motor at low speed is effectively solved, and the response ability of current loop is improved.

[0034] The rotor position tracking control is combined with the dead-beat current predictive control in step S3:

[0035] From the derivation principle of the dead-beat current predictive control, the actual current follows the given current after two periods, so that the closed-loop transfer function of the current loop is obtained:

[0036]

[0037] The system transfer function of the method combining the rotor position tracking control and the dead-beat current predictive control is written as:

[0038]

[0039] The compound control method for realizing the low-speed operation of the permanent magnet direct drive servo motor is realized.

[0040] Compared with the prior art, the beneficial effects of the present application are:

[0041] (1) The present application first improves the speed outer ring, introduces the rotor position control method, converts the speed control into the rotor position control, reduces the speed detection error, and improves the anti-interference ability of the system.

[0042] (2) In order to eliminate the control delay of the system, the present application introduces the dead-beat current predictive control, predicts the voltage vector of the next period through the two-step prediction method of the current, and improves the response speed of the system.

[0043] (3) The present application further combines the rotor position control with the dead-beat current predictive control, derives the transfer function of the system, and realizes the compound control method for realizing the low-speed operation of the permanent magnet direct drive servo motor. BRIEF DESCRIPTION OF DRAWINGS

[0044] Figure 1 is the system control block diagram;

[0045] Figure 2 is the step response diagram of Φ θ

[0046] Figure 3 is the logarithmic amplitude-frequency characteristic curve of T(s);

[0047] Figure 4 is the timing diagram of the DSP controller;​

[0048] Figure 5 This is a system structure diagram. Detailed implementation method:

[0049] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings: This application discloses a composite control method for a permanent magnet direct-drive servo motor suitable for low-speed operation, and its system control structure block diagram is as follows. Figure 1 As shown, the method includes the following steps:

[0050] In step S1, the outer speed loop employs a rotor position tracking control method. The transfer function of the rotor position tracking controller is as follows:

[0051]

[0052] Where J is the moment of inertia of the motor, B a P is the active damping coefficient of the motor. n Let ψ be the extreme logarithm. f Let T be the flux linkage, r be the filter order, and T be the filter order. f It is the filter time constant:

[0053] Therefore, by selecting an appropriate T in G(s) f The r value enables stable operation at low speeds.

[0054] First, we discuss the value of r. Based on the analysis of the system's closed-loop transfer function, a second-order filter is chosen, i.e., r = 2. Further, we consider the value of T... f We will analyze the possible values ​​of .

[0055] To analyze the tracking performance of the rotor position tracking control method at low speeds, the closed-loop transfer function of the system is given in Equation 3.2:

[0056]

[0057] Under the same given conditions, gradually decrease T f The system's tracking capability first increases and then decreases with the value of , such as Figure 2 As shown, T c Because it is the system control cycle, therefore T f The value cannot be too small.

[0058] Further analysis of the anti-interference performance of the rotor position tracking control method at low speed is conducted, and the transfer function between the disturbance output by the controlled object and the actual speed is written, as shown in Equation 3.3:

[0059]

[0060] Analysis of the Bode plot of T(s) reveals that as T...f The amplitude of T(s) also gradually decreases, which indicates that the influence of the disturbance on the motor speed gradually decreases, and the anti-disturbance performance of the system gradually increases, as shown in Figure 3

[0061] From the above analysis, it can be concluded that T f After the compromise selection of the value, the rotor position tracking control can simultaneously satisfy strong anti-interference ability and speed tracking ability. It can not only suppress the inherent torque disturbance such as slotting torque of the permanent magnet direct drive motor at low speed, but also avoid the great influence brought by the speed detection delay or error, and improve the robustness of the permanent magnet direct drive motor at low speed.

[0062] In step S2, the deadbeat current prediction control method is used in the current inner loop:

[0063] The deadbeat current prediction control equation is as follows:

[0064] u * (k)=B -1 (i * (k)-Ai(k)-C(k))(3.4)

[0065] Wherein

[0066]

[0067] u * (k) is the given d, q-axis voltage, L s is the stator inductance, R s is the stator resistance, ω e is the motor electrical angular velocity, T s is the sampling period, T s is the sampling period. As long as the given current output by the speed loop and the feedback current obtained by sampling are combined in the case of ensuring the accuracy of motor parameters, the given voltage value of the motor can be calculated according to the mathematical model of the motor, and the current can be tracked without error.

[0068] However, in the actual application process, a certain time is consumed for A / D conversion and control algorithm execution, and the actual voltage vector can act on the motor at the next moment, that is, there is a one-beat delay in the control system. Therefore, the control delay needs to be compensated, that is, the voltage vector u(k+1) of the next period is calculated in advance at k moment, and acts at k+1 moment. The specific calculation method is as follows:

[0069] In order to further obtain the predicted voltage u(k+1), it is necessary to predict one more period, and this paper adopts two-step current prediction

[0070] ​i(k+2) = Ai(k+1) + Bu(k+1) + C(k+1) (3.5)

[0071] From the control timing of DSP, as shown in Fig. 3, when the voltage value of k+1 time is calculated at k time, and after acting on the motor at k+1 time, the measured feedback current value will be obtained by A / D sampling at k+2 time. Therefore, the given current and the feedback current are different by two periods. Therefore, we can get: Figure 4

[0072] i(k+2) = i * (k)(3.6)

[0073] Then the expression of the reference voltage required in the next period can be obtained

[0074] u * (k+1) = B -1 (i * (k) - A(Ai(k) + Bu(k) + C(k)) - C(k+1)) (3.7)

[0075] Where, u * (k+1) is the given d, q-axis voltage at k+1 time. It effectively solves the control delay of permanent magnet direct drive motor at low speed and improves the response ability of current loop.

[0076] In step S3, the rotor position tracking control is combined with the dead-beat current predictive control:

[0077] From the derivation principle of dead-beat current predictive control, the actual current follows the given current after two periods, so the closed-loop transfer function of the current loop is obtained:

[0078]

[0079] As shown in Fig. 3, the open-loop transfer function of the method of combining rotor position tracking control with dead-beat current predictive control is written: Figure 5

[0080]

[0081] The composite control method for permanent magnet direct drive servo motor at low speed is realized.​​

Claims

1. A compound control method suitable for a low-speed operation of a permanent magnet direct drive servo motor, characterized in that, The traditional PI control method is improved to a combination method of rotor position tracking control and deadbeat current prediction control, including the following three steps: S1, in the speed outer loop, the rotor position tracking control method is adopted to convert the speed given instruction into real-time rotor position trajectory, which can smoothly track the rotor position. In addition, robust stability and anti-interference ability are important indicators for the stable operation of the motor, and the parameters of the controller are designed according to the indicators; S2, in the current inner loop, the deadbeat current prediction control method is adopted, the mathematical model of the permanent magnet synchronous motor is constructed, and the given voltage vector at the next moment is predicted by using the mathematical model combined with the current voltage sampling value of the motor, to compensate the control delay of the system; S3, the rotor position tracking control and the deadbeat current prediction control are combined, the system transfer function is derived, and the compound control of the permanent magnet direct drive servo motor under low speed working condition is realized; The transfer function of the rotor position tracking controller in step S1 is: (1.1) where J is the moment of inertia of the motor, B a is the active damping coefficient of the motor, P n is the number of pole pairs, ψ f is the flux linkage, r is the filter order, T f is the filter time constant: Thus, by choosing appropriate T f and r values in G(s), stable operation at low speeds can be achieved; Firstly, the value of r is discussed. According to the analysis of the closed-loop transfer function of the system, the filter is selected as a second-order filter, i.e. r = 2; further, the value of T f is analyzed; In order to analyze the following performance of the rotor position tracking control method under low speed, the closed-loop transfer function of the system is written as shown in equation 1.2: (1.2) Under the same given conditions, gradually reducing the value of T f , the tracking ability of the system first increases and then decreases, so the value of T f cannot be too small; Further analysis of the anti-interference performance of the rotor position tracking control method under low speed, the transfer function between the disturbance output by the control object and the actual speed is written as shown in equation 1.3: (1.3) By analyzing the Bode diagram of T(s), as T f gradually decreases, the amplitude of T(s) also gradually decreases, which indicates that the influence of disturbance on motor speed gradually decreases, and the anti-disturbance performance of the system gradually increases. From the above analysis, it can be concluded that T f After the compromise selection of the value, the rotor position tracking control can simultaneously satisfy the strong anti-interference ability and speed tracking ability, can suppress the inherent cogging torque disturbance of the permanent magnet direct drive motor at low speed, and can avoid the great influence brought by the speed detection delay or error, and improves the robustness of the permanent magnet direct drive motor at low speed.

2. The compound control method for a permanent magnet direct drive servo motor suitable for low speed operation according to claim 1, characterized in that, The deadbeat current prediction control equation in step S2 is as follows: (1.4) Wherein ， ， ， ， ， u * (k) is given d, q-axis voltage, L s is the stator inductance, R s is the stator resistance, ω e is the motor electrical angular velocity, T s is the sampling period, only through the given current output by the speed loop and the feedback current obtained by sampling, in the case of ensuring the accuracy of motor parameters, combined with the mathematical model of the motor, the given voltage value of the motor can be calculated, realizing the current tracking without error; But in the actual application process, due to the A / D conversion and the time required for the execution of the control algorithm, the actual voltage vector can only act on the motor at the next moment, that is, there is one beat delay in the control system; Therefore, the control delay needs to be compensated, that is, the voltage vector u(k+1) of the next period is calculated in advance at k moment, and acts at k+1 moment; The specific calculation method is as follows: in order to further obtain the predicted voltage u(k+1), it is necessary to predict one more period, and two-step prediction of current is adopted; (1.5) According to the control timing of DSP, when the voltage value at k+1 moment is calculated at k moment, and acts on the motor at k+1 moment, A / D sampling will be performed at k+2 moment to obtain the measured feedback current value; Therefore, the given current and the feedback current are different by two periods; Therefore, the expression of the reference voltage required for the next period can be obtained (1.6) The rotor position tracking control and the deadbeat current prediction control are combined in step S3: (1.7) wherein u * (k+1) is the given d, q-axis voltage at k+1 time; effectively solves the control delay of permanent magnet direct drive motor at low speed, and improves the response ability of current loop.

3. The compound control method for a permanent magnet direct drive servo motor suitable for low speed operation according to claim 2, characterized in that, According to the derivation principle of the deadbeat current prediction control, the actual current follows the given current after two periods, so the closed-loop transfer function of the current loop is obtained: The system transfer function of the combination method of rotor position tracking control and deadbeat current prediction control is written as: (1.8) The compound control method of permanent magnet direct drive servo motor under low speed is realized. (1.9) ​

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