阅读说明：本技术 一种基于双电机驱动的高精度快速伺服控制方法 (High-precision rapid servo control method based on dual-motor drive ) 是由 陈龙淼 孙乐 邹权 高鹏 于 2021-07-12 设计创作，主要内容包括：本发明公开了一种基于模型预测控制的双电机高精度快速伺服控制方法。以模型预测控制为基础,在有齿轮侧隙的双电机伺服全闭环控制系统中,采用分段控制方法,对高速运行和精确到位两段控制分别采用不同的协调控制方法。通引入协调电流i～(*)-(co)：一方面,在大误差带高速运行中动态调节双电机转矩分配,使双电机运行转速保持同步,避免高速运行抖动；另一方面,在伺服接近到位的过程中,采用恒定协调电流提高系统刚度,避免侧隙造成的震荡。本发明实现了双电机伺服系统的快速、准确、稳定控制。(The invention discloses a double-motor high-precision rapid servo control method based on model predictive control. Based on model prediction control, a sectional control method is adopted in a dual-motor servo full-closed loop control system with gear backlash, and different coordination control methods are respectively adopted for high-speed operation and accurate-in-place two-stage control. Introduction of a coordinated current i * co : on the one hand, the torque distribution of the double motors is dynamically adjusted in the high-speed operation of the large-error belt, so thatThe running rotating speeds of the double motors are kept synchronous, so that high-speed running jitter is avoided; on the other hand, in the process that the servo is close to the in-place position, the constant coordination current is adopted to improve the system rigidity, and the oscillation caused by the backlash is avoided. The invention realizes the rapid, accurate and stable control of the dual-motor servo system.)

1. A high-precision rapid servo control method based on dual-motor drive is characterized by comprising the following specific steps:

step 1, calculating a displacement error through a displacement instruction and load displacement feedback obtained by measurement of an encoder, and generating a motor speed ring instruction rotating speed through a proportion link;

step 2, judging whether the motor and the load enter a small error band, wherein the judging method comprises the following steps: if the displacement error is larger than the error threshold, the motor and the load do not enter a small error band; otherwise, entering a small error band;

step 3, the speed loop model prediction controller plans a reference rotating speed track of N sampling periods of the future motor according to speed loop speed feedback and instruction rotating speed calculation, predicts a rotating speed prediction value of K sampling periods of the future motor through a model prediction algorithm, establishes a loss function based on a difference value of the reference rotating speed and the predicted rotating speed, and calculates a current reference value through searching a loss function extreme value;

step 4, taking the reference current and the rotating speed of the double motors as the input of an extended observer, estimating a damping coefficient through the extended observer, and outputting the reference current;

step 5, determining a coordination current according to the reference current, and controlling the double motors to operate by controlling the coordination current through the current distributor;

step 6, the permanent magnet synchronous motor current controller respectively controls the currents of the permanent magnet motor 1 and the permanent magnet motor 2 according to the reference current, and respectively performs the step i_dControl, i ═ 0_dUnder a dq axis coordinate system, d-axis current of current is fed back to a speed loop model prediction controller through actually measured q-axis current.

2. The high-precision fast servo control method based on the dual-motor drive as claimed in claim 1, wherein the calculation process of the current reference value is as follows:

the method comprises the following steps of constructing a motion equation of a speed loop model predictive controller control object, wherein the speed loop model predictive controller control object consists of two motors and a load, and the motion equation specifically comprises the following steps:

in the formula, C_m、C_LRepresents the coefficient of friction, T_e1、T_e2Which represents the motoring torque generated by the motor,andrespectively representing the rotational speeds of the two motors and the load,andrepresenting the acceleration of the two motors and the acceleration of the load, J_mRepresenting the moment of inertia of the machine, J_LRepresenting the moment of inertia of the load, T_LRepresenting the load torque, T_s1、T_s2Representing shaft torque, ω₁And omega₂At a dual motor speed, ω_LFor speed loop speed feedback, N_sIs the transmission ratio at the motor end.

When the motor does not enter the small error band, C_m、C_LTo 0, the equation of motion reduces to:

and constructing a time dispersion equation by using the simplified equation, and predicting the motion state by using the time dispersion equation:

in the formula, theta₁(k)、θ₂(k)、θ_L(k) Respectively representing the position of the two motors and the load at time k, theta₁(k+1)、θ₂(k+1)、θ_L(k +1) represents the positions of the two motors and the load, respectively, at the time k + 1. Omega₁(k)、ω₂(k)、ω_L(k) Representing the rotational speeds, omega, of the two motors and of the load, respectively, at the moment k₁(k+1)、ω₂(k+1)、ω_L(k +1) respectively represents the rotating speeds of the two motors and the load at the moment of k + 1; the load being the sum of shaft torques T_s＝T_s1+T_s2；T_e1(k)、T_e2(k) Representing the electric torques, T, of the two motors at time k_s1(θ₁(k))、T_s2(θ₂(k) Represents the shaft torque of both motors at time k; m_Lcosθ_L(k) Represents the load at time k;

when the motor enters a small error band, a damping coefficient C is introduced into a motion equation_coReconstructing the motion equation as follows:

the discrete time equation for the load is as follows:

ω_L(k) representing the speed of rotation, ω, of the load at time k_L(k +1) represents the rotational speed of the load at the time k + 1; t is_s1(θ₁(k))、T_s2(θ₂(k) Represents the shaft torque of both motors at time k; m_Lcosθ_L(k) Represents the load at time k;

the motion equation of the load is reconstructed asΔC_coRepresents C_coThe uncertainty of (a) is determined,

calculating reference rotating speed track omega of future N sampling periods of motor_r(1)...ω_rThe (N) is specifically as follows:

wherein the content of the first and second substances,T_sis the rotational speed loop sampling time, T_rIs the speed loop dynamic response time, ω (k) is the current actual speed feedback, ω^*(k) Indicating a speed command, ω_r(k + i) represents the reference speed at k + i, and P represents the predicted number of steps;

the loss function based on the difference between the reference and predicted speeds is:

the error in the predicted rotational speed is defined as,is the current reference value, ω, at time k_r(k + i) is the reference speed trajectory at time k + i, ω_m(k + i) is a predicted value of the rotation speed at the time k + i, and e (k + i) is ω_r(k+i)-ω_m(k+i)；

Predicting a current reference value i based on the loss function model_q ^*Comprises the following steps:

i.e. in the loss function J_P(i_q ^*) The motor reference value when the minimum value is obtained is i_q ^*。

3. The high-precision fast servo control method based on the dual motor drive as claimed in claim 2, wherein the shaft torque is defined as follows:

in the formula, theta_d1、θ_d2Is the difference angle between the angle of the two motors and the load angle, b₁、b₂Is the backlash of the motor, D_sIndicating damping.

4. The high-precision fast servo control method based on dual-motor drive as claimed in claim 1, wherein the extended observer structure is:

wherein x₁Is the rotation speed omega of the double motors₁And omega₂Average value of (1), x₂Is composed ofω_LIs omega₁And omega₂Average value of (1), J_LU is the moment of inertia of the load, and T_s1+T_s2-M_Lcosθ_L，Respectively represent a pair x₁，x₂And (6) carrying out derivation.

5. The high-precision rapid servo control method based on dual-motor driving as claimed in claim 1, wherein the specific method for controlling the dual-motor operation by the current distributor through controlling the coordinated current is as follows:

in the formula i_co ^*To coordinate the currents, i^* _q1And i^* _q2For dual-motor reference currents, i_q ^*Is a motor reference value.

6. High-precision fast servo control method based on dual motor drive as claimed in claim 1, characterized in that the current i is coordinated before the load enters the small error band_co ^*And calculating by a PI link to obtain:

in the formula, ω₁、ω₂Respectively the rotational speeds of two motors, K_pco、K_icoThe proportional coefficient and the integral coefficient of the PI link are respectively.

Technical Field

The invention belongs to a servo control technology, and particularly relates to a high-precision rapid servo control method based on dual-motor driving.

Background

An electric servo system composed of a permanent magnet motor, a speed reducer and other transmission mechanisms is widely applied to industrial servo and military products. Because the speed reducer and other mechanisms have transmission gaps, dead zones exist in the position feedback of the load end and the position of the motor shaft, and the torque of the motor cannot be effectively transmitted to the load side in the dead zone range, the transmission efficiency can be reduced, more importantly, the error of the servo control in place can be increased, and even after the servo is in place, low-quality performance such as jitter, shaking and the like is generated, so that the high-quality control has important significance.

Methods for improving the transmission stiffness researched by current researchers can be divided into two categories: 1. the single motor-based drive control method compensates in-place errors and disturbances caused by air gaps by combining control methods such as internal model control and sliding mode control with a model observer. 2. Based on the drive control method of the double motors, the transmission clearance is actively eliminated by manufacturing unbalanced torque in the driving process of the two motors. The single-motor control method needs a large amount of parameter support including setting and measuring of controller parameters, model parameters and the like, is very inconvenient in actual use, and has poor anti-interference capability in variable parameters and variable environment states. The double-motor method can realize active clearance elimination, however, unbalanced clearance elimination can cause the system, on one hand, deformation torque generated by clearance elimination can cause extra load to influence the motion speed of the load, and on the other hand, the transmission system can accelerate fatigue and aging when being in a high deformation state for a long time, thereby influencing the service life.

Disclosure of Invention

The invention aims to provide a high-precision quick servo control method based on dual-motor driving so as to inhibit jitter and in-place oscillation caused by a transmission gap.

The technical scheme for realizing the aim of the invention is a high-precision quick servo control method based on double-motor drive, which comprises the following specific steps:

step 1, calculating a displacement error through a displacement instruction and load displacement feedback obtained by measurement of an encoder, and generating a motor speed ring instruction rotating speed through a proportion link;

step 2, judging whether the motor and the load enter a small error band, wherein the judging method comprises the following steps: if the displacement error is larger than the error threshold, the motor and the load do not enter a small error band; otherwise, entering a small error band;

step 3, the speed loop model prediction controller plans a reference rotating speed track of N sampling periods of the future motor according to speed loop speed feedback and instruction rotating speed calculation, predicts a rotating speed prediction value of K sampling periods of the future motor through a model prediction algorithm, establishes a loss function based on a difference value of the reference rotating speed and the predicted rotating speed, and calculates a current reference value through searching a loss function extreme value;

step 4, taking the reference current and the rotating speed of the double motors as the input of an extended observer, estimating a damping coefficient through the extended observer, and outputting the reference current;

step 5, determining a coordination current according to the reference current, and controlling the double motors to operate by controlling the coordination current through the current distributor;

step 6, the permanent magnet synchronous motor current controller respectively controls the currents of the permanent magnet motor 1 and the permanent magnet motor 2 according to the reference current, and the currents are respectively made as i_dControl, i ═ 0_dUnder a dq axis coordinate system, d-axis current of current is fed back to a speed loop model prediction controller through actually measured q-axis current.

Preferably, the calculation process of the current reference value is as follows:

the method comprises the following steps of constructing a motion equation of a speed loop model predictive controller control object, wherein the speed loop model predictive controller control object consists of two motors and a load, and the motion equation specifically comprises the following steps:

in the formula, C_m、C_LRepresents the coefficient of friction, T_e1、T_e2Which represents the motoring torque generated by the motor,andrespectively representing the rotational speeds of the two motors and the load,andrepresenting the acceleration of the two motors and the acceleration of the load, J_mRepresenting the moment of inertia of the machine, J_LRepresenting the moment of inertia of the load, T_LRepresenting the load torque, T_s1、T_s2Representing shaft torque, ω₁And omega₂At a dual motor speed, ω_LFor speed loop speed feedback, N_sIs the transmission ratio at the motor end.

When the motor does not enter the small error band, C_m、C_LTo 0, the equation of motion reduces to:

and constructing a time dispersion equation by using the simplified equation, and predicting the motion state by using the time dispersion equation:

in the formula, theta₁(k)、θ₂(k)、θ_L(k) Respectively representing the position of the two motors and the load at time k, theta₁(k+1)、θ₂(k+1)、θ_L(k +1) represents the positions of the two motors and the load, respectively, at the time k + 1. Omega₁(k)、ω₂(k)、ω_L(k) Representing the rotational speeds, omega, of the two motors and of the load, respectively, at the moment k₁(k+1)、ω₂(k+1)、ω_L(k +1) respectively represents the rotating speeds of the two motors and the load at the moment of k + 1; the load being the sum of shaft torques T_s＝T_s1+T_s2；T_e1(k)、T_e2(k) Representing the electric torques, T, of the two motors at time k_s1(θ₁(k))、T_s2(θ₂(k) Represents the shaft torque of both motors at time k; m_Lcosθ_L(k) Represents the load at time k;

when the motor enters a small error band, a damping coefficient C is introduced into a motion equation_coReconstructing the motion equation as follows:

the discrete time equation for the load is as follows:

ω_L(k) representing the speed of rotation, ω, of the load at time k_L(k +1) represents the rotational speed of the load at the time k + 1; t is_s1(θ₁(k))、T_s2(θ₂(k) Represents the shaft torque of both motors at time k; m_L cosθ_L(k) Represents the load at time k;

the motion equation of the load is reconstructed asΔC_coRepresents C_coThe uncertainty of (a) is determined,

calculating reference rotating speed track omega of future N sampling periods of motor_r(1)...ω_rThe (N) is specifically as follows:

ω_r(k+i)＝ω^*(k)-α_r ⁱ[ω^*(k)-ω(k)]p, wherein,T_sis the rotational speed loop sampling time, T_rIs the speed loop dynamic response time, ω (k) is the current actual speed feedback, ω^*(k) Indicating a speed command, ω_r(k + i) represents the reference speed at k + i, and P represents the predicted number of steps;

the loss function based on the difference between the reference and predicted speeds is:

the error in the predicted rotational speed is defined as,is the current reference value, ω, at time k_r(k + i) is the reference speed trajectory at time k + i, ω_m(k + i) is a predicted value of the rotation speed at the time k + i, and e (k + i) is ω_r(k+i)-ω_m(k+i)；

Predicting a current reference value i based on the loss function model_q ^*Comprises the following steps:

i.e. in the loss function J_P(i_q ^*) The motor reference value when the minimum value is obtained is i_q ^*。

Preferably, the shaft torque is defined as follows:

θ_d1、θ_d2is the difference angle between the angle of the two motors and the load angle, b₁、b₂Is the backlash of the motor, D_sIndicating damping.

Preferably, the extended observer structure is:

wherein x₁Is the rotation speed omega of the double motors₁And omega₂Average value of (1), x₂Is composed ofω_LIs omega₁And omega₂Average value of (1), J_LU is the moment of inertia of the load, and T_s1+T_s2-M_L cosθ_L，Respectively represent a pair x₁，x₂And (6) carrying out derivation.

Preferably, the specific method for controlling the operation of the double motors by the current distributor through controlling the coordinated current is as follows:

in the formula i_co ^*To coordinate the currents, i^* _q1And i^* _q2For dual-motor reference currents, i_q ^*Is a motor reference value.

Preferably, the current i is coordinated before the load enters a small error band_co ^*And calculating by a PI link to obtain:

in the formula, ω₁、ω₂Respectively the rotational speeds of two motors, K_pco、K_icoThe proportional coefficient and the integral coefficient of the PI link are respectively.

The invention has the following beneficial effects:

1) when the double-motor servo is in a large error band, the double motors realize high-speed synchronous operation, and system jitter and noise are obviously inhibited;

2) in the accurate in-place process, the double motors are interlocked, the aim of improving the system rigidity in a short time is fulfilled, and in-place vibration caused by transmission gaps is inhibited.

Drawings

Fig. 1 is a basic principle diagram of the present invention.

Fig. 2 is a transmission dead zone characteristic caused by transmission backlash.

Fig. 3 compares the load position curve with the current curve of each of the two motors using conventional control and the proposed solution of the present invention.

Fig. 4 is a graph comparing the dynamic transmission clearance curve and the dual motor respective speed curves using conventional control and the proposed solution of the present invention.

Detailed Description

The present invention will be further described with reference to the accompanying drawings.

As shown in fig. 1, a high-precision fast servo control method based on dual-motor drive includes the following specific steps:

step 1, calculating reference rotating speed: by displacement command theta^* _refFeedback of load displacement theta measured by encoder_LCalculating the displacement error e_θThe displacement error is subjected to a proportional link to generate a motor speed ring instruction rotating speed omega^*。

Step 2, judging whether the motor and the load enter a small error band, wherein the judging method comprises the following steps: if the displacement error e_θ>Epsilon, motor and load do not enter the small error band; otherwise, entering a small error band;

step 3, a speed loop model prediction controller: according to the speed feedback and instruction speed omega of the speed loop^*Calculating and planning reference rotating speed track of N sampling periods of future motor, and predicting K sampling periods of future motor through model prediction algorithmPredicting the rotation speed, establishing a loss function based on the difference value between the reference rotation speed and the predicted rotation speed, and calculating a current reference value i by searching an extreme value of the loss function^* _q. Specifically, the speed feedback of the speed loop adopts a double-motor rotating speed omega₁And omega₂Average value of the rotational speed of (1).

The method comprises the following steps of constructing a motion equation of a speed loop model predictive controller control object, wherein the speed loop model predictive controller control object consists of two motors and a load, and the motion equation specifically comprises the following steps:

C_m、C_Lrepresents the coefficient of friction, T_e1、T_e2Which represents the motoring torque generated by the motor,andrespectively representing the rotational speeds of the two motors and the load,andrepresenting the acceleration of the two motors and the acceleration of the load, J_mRepresenting the moment of inertia of the machine, J_LRepresenting the moment of inertia of the load, T_LRepresenting the load torque, T_s1、T_s2Representing shaft torque, ω₁And omega₂At a dual motor speed, ω_LFor speed loop speed feedback, N_sIs the transmission ratio at the motor end; load torque T_LIn relation to the inclination of the load, T_L＝M_Lcosθ_L，M_LIs the weight of the load.

T_s1、T_s2Representing shaft torque, which is defined as follows:

θ_d1、θ_d2is the difference angle between the angle of the two motors and the load angle, b₁、b₂Is the backlash of the motor, and when the differential angle is smaller than the backlash, the shaft torque is 0.

When the motor does not enter the small error band, C_m、C_LTo 0, the equation of motion reduces to:

and constructing a time dispersion equation by using the simplified equation, and predicting the motion state by using the time dispersion equation:

θ₁(k)、θ₂(k)、θ_L(k) respectively representing the position of the two motors and the load at time k, theta₁(k+1)、θ₂(k+1)、θ_L(k +1) represents the positions of the two motors and the load, respectively, at the time k + 1. Omega₁(k)、ω₂(k)、ω_L(k) Representing the rotational speeds, omega, of the two motors and of the load, respectively, at the moment k₁(k+1)、ω₂(k+1)、ω_LAnd (k +1) respectively represents the rotating speeds of the two motors and the load at the moment of k + 1. Load by sum of shaft torques T_s＝T_s1+T_s2Instead. T is_e1(k)、T_e2(k) Representing the electric torques, T, of the two motors at time k_s1(θ₁(k))、T_s2(θ₂(k) Represents the shaft torque of both motors at time k. M_L cosθ_L(k) Representing the load at time k.

After the motor enters a small error band (e)_θ<Epsilon), in order to improve the system rigidity, a larger constant coordination current is adopted, and a larger damping coefficient C is introduced into the motion equation_coReconstructing the motion equation as follows:

the discrete time equation for the load is as follows:

ω_L(k) representing the speed of rotation, ω, of the load at time k_L(k +1) represents the rotational speed of the load at the time k + 1. T is_s1(θ₁(k))、T_s2(θ₂(k) Represents the shaft torque of both motors at time k. M_L cosθ_L(k) Representing the load at time k. Taking into account the damping coefficient C_coThe equation of motion of the load can be reconstructed asΔC_coRepresents C_coThe uncertainty of (a) is determined,

reference rotating speed track omega for future N sampling periods of motor_r(1)...ω_rThe specific calculation method of (N) is as follows:

ω_r(k+i)＝ω^*(k)-α_r ⁱ[ω^*(k)-ω(k)]p, wherein,T_sis the rotational speedLoop sample time, T_rIs the speed loop dynamic response time, and ω (k) is the current actual speed feedback, i.e., ω₁And omega₂Average value of (a), ω^*(k) Indicating a speed command, ω_r(k + i) denotes the reference speed at k + i, and P denotes the predicted number of steps.

The loss function based on the difference between the reference and predicted speeds is:

the error in the predicted rotational speed is defined as,is the current reference value, ω, at time k_r(k + i) is the reference value of the speed of rotation at the moment k + i, ω_m(k + i) is a predicted value of the rotation speed at the time k + i, and e (k + i) is ω_r(k+i)-ω_m(k+i)。

Model prediction current reference value i based on the loss function_q ^*The calculation method comprises the following steps:

i.e. in the loss function J_P(i_q ^*) The motor reference value when the minimum value is obtained is i_q ^*。

And 4, expanding the anti-interference control of the observer: will refer to the current i_refAnd the rotation speed omega of the double motors₁And omega₂As input, the damping coefficient C is estimated by an extended observer_coAnd outputs a reference current i_ref。

The extended observer structure is:

wherein x₁Is omega₁And omega₂Average value of (1), x₂Is composed ofω_LIs omega₁And omega₂Average value of (1), J_LU is the moment of inertia of the load, and T_s1+T_s2-M_L cosθ_L，Respectively represent a pair x₁，x₂And (6) carrying out derivation.

And the speed feedback adopted by the extended observer in the step 3 is the average value of the rotating speeds of the two motors.

Step 5, the current distributor: before the loaded position enters a small error band (e)_θ>Epsilon) introducing a coordinating current i_co ^*Inhibiting the difference of the rotating speeds, enabling the two motors to be in a synchronous driving mode, adopting synchronous operation control, and if the two motors refer to a current i^* _q1And i^* _q2Are respectively a reference current i_refHalf of the system, the rotation speed of the double motors is asynchronous due to the influence of the backlash; entering a small error band (e) in the loaded position_θ<E) by applying a constant, large amplitude coordination current i_co ^*The rigidity of the system is improved, and in-place vibration is restrained.

Current distributor capable of coordinating current i_co ^*The method comprises the following steps of controlling the double motors to operate, and achieving different control effects in different modes:

before the load enters a small error band (e)_θ>ε) by controlling i_co ^*To equalize the rotational speeds of the two motors i_co ^*And calculating by a PI link to obtain:

in the formula, ω₁、ω₂Respectively the rotational speeds of two motors, K_pco、K_icoThe PI is a proportional coefficient and an integral coefficient of the PI, respectively, and s denotes a complex frequency.

After the load enters the error band (e)_θ<Epsilon) to provide a constant and large value to increase system stiffness to avoid in-place oscillations.

Step 6, the current controller: the current controllers of the permanent magnet synchronous motors are respectively according to the reference current i^* _q1And i^* _q2Controlling the current of the permanent magnet motor 1 and the permanent magnet motor 2 to respectively do i_dControl, i ═ 0_dIs d-axis current of current under dq-axis coordinate system, and measured q-axis current i_q1And i_q2And feeding back to the model predictive controller.

The invention is different from the existing double-motor gap eliminating technology, only applies constant large current coordination control when the load is deformed in place, does not influence the system load in the motion process, and dynamically adjusts the coordination current in high-speed motion to ensure the synchronization of the rotating speeds of the double motors.

The invention is suitable for the motion control occasions with high requirement on servo precision and high servo in-place speed, so as to inhibit the out-of-synchronization and in-place oscillation of double motors of the electric servo control system caused by transmission gaps and improve the servo in-place precision.

In the embodiment of the invention, two permanent magnet synchronous motors with rated torque of 3Nm are driven, and one permanent magnet synchronous motor with inertia of 0.1 kg.m is dragged through two speed reducers with transmission ratio of 20²Within 0.5s, from 0 degrees to 90 degrees, and the in-place error is within 1 degree. The sum of the clearances without torsional deformation of the two sets of speed reducers is 6 degrees.

Fig. 3 compares the load position curve with the current curve of each of the two motors using conventional control and the proposed solution of the present invention. It can be seen from the figure that, by adopting the scheme provided by the invention, the load position is converged quickly after reaching 90 degrees, and if a conventional control algorithm is adopted, the load is vibrated continuously after being in place. The current waveforms of the two motors can also be seen, the driving currents of the two motors adopting the scheme are not completely the same, which is caused by the existence of the coordination current; the coordinated current is shifted to a constant value of larger amplitude, especially as the load approaches the target position.

Fig. 4 is a graph comparing the dynamic transmission clearance curve and the dual motor respective speed curves using conventional control and the proposed solution of the present invention. It can be seen from the figure that by adopting the method provided by the invention, the fluctuation of the sum of the gaps of the two motors is always small, the sum is close to 0 in a large error band, and is close to 6 degrees in a small error band, and in the whole process, the rotating speeds of the two motors are basically the same, but have a difference value of small amplitude fluctuation after being in place. By adopting the conventional method, the sum of the gaps of the two motors continuously fluctuates greatly, the difference of the rotating speeds of the two motors is also large, and the two motors are in an obviously asynchronous state.

In the description herein, references to the description of the term "one embodiment," "some embodiments," "an illustrative embodiment," "an example," "a specific example," or "some examples" or the like mean that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the invention. In this specification, the schematic representations of the terms used above do not necessarily refer to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

While embodiments of the invention have been shown and described, it will be understood by those of ordinary skill in the art that: various changes, modifications, substitutions and alterations can be made to the embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the claims and their equivalents.