A control method for adjusting the mechanical characteristics of a three-phase permanent magnet brushless DC motor system

Abstract

The present invention is a control method for regulating the mechanical characteristics of a three-phase bridge permanent magnet brushless DC motor system by means of current negative feedback. In this control method, a negative feedback current I proportional to the electromagnetic torque is selected, and the acquisition path for measuring and calculating the current I is given; a negative feedback cycle control period synchronized with the pulsation of the current I is set, and a timing method synchronized with the pulsation of the current I is given; according to the requirements for regulating the mechanical characteristics of the system, the system current negative feedback voltage U f = f(I) is set, and two preferred design methods for the function relationship of f(I) and its parameters are provided; the control flow of the digital microcontroller in the system for implementing the current negative feedback control cycle is given. The invention can enable the system to obtain the desired mechanical characteristics, improve the operation experience of users, and limit the input electric power of the system.

Description

Technical Field

[0001] The present invention belongs to the technical field of brushless DC motor control, and particularly relates to a control method for adjusting the mechanical characteristics of a three-phase permanent magnet brushless DC motor system. Background Art

[0002] Since the series-wound motor has soft mechanical characteristics, it is widely used in driving power tools. However, for battery-powered portable power tools, the series-wound motor is too heavy, too large, and has too low efficiency, wasting battery capacity. Moreover, since the series-wound motor is a brushed motor, its service life and reliability are significantly insufficient, and it will also generate radio interference.

[0003] With the progress of technology, the application scope of permanent magnet brushless DC motors and their electronic drive systems, which are small in size, high in efficiency, large in power, and convenient for control and adjustment, has been continuously expanding. Replacing the series-wound motor with it to drive power tools has become a typical new technology application case with a bright future.

[0004] The mechanical characteristic of the system is the curve of its rotational speed varying with the electromagnetic torque. Currently, the permanent magnet brushless DC motor system for driving battery-powered portable power tools adopts open-loop control. The mechanical characteristic of the system is hard. The system rotational speed does not decrease significantly as the load torque increases, weakening the operator's experience of the change in the processing load; the hard characteristic causes the input electric power of the system to increase significantly as the load torque increases, accelerating the battery discharge speed and significantly shortening the usage time of the battery for one charge.

[0005] It is necessary to introduce a control method for adjusting the mechanical characteristics of the permanent magnet brushless DC motor system for driving portable power tools. By adjusting the output mechanical characteristics of the system, the operator's operation experience can be improved, and the input electric power of the system can be restricted. Summary of the Invention

[0006] The present invention aims to solve the above existing technical problems and provides a control method for adjusting the mechanical characteristics of a three-phase permanent magnet brushless DC motor system. The digital microcontroller is used to control the three-phase bridge to cyclically conduct and drive the three-phase permanent magnet brushless DC motor to operate, and the current negative feedback control is used to adjust the mechanical characteristics of the system. The steps are as follows:

[0007] S1 Determine the control period of the current negative feedback as T f : To make the control period match the pulsation of the motor current, the current negative feedback control period T of the digital microcontroller in the system f is an integer multiple of 1 to 3 of the cycle of the switching tubes in the three-phase bridge cyclically conducting;

[0008] S2 Obtain the negative feedback current I within each control period T f The system negative feedback current I is the series-conduction current of the two-phase windings of the motor within a control period T fThe average value within, which is measured from the input side of the DC power supply of the three-phase bridge and then referred to the motor side;

[0009] S3 sets the negative feedback voltage U f The functional relationship with the negative feedback current I: Set the functional relationship f(I) according to the requirements of the target mechanical characteristic of the system. The negative feedback voltage U of the current negative feedback control implemented by the digital microcontroller in the system f = f(I);

[0010] S4 The control period of the system is T f Current negative feedback control: The digital microcontroller calculates the negative feedback voltage U of this control period with the current I obtained by measurement and calculation in the previous control period f = f(I), implements the current negative feedback control of this control period, and measures and calculates to obtain the current I of this control period.

[0011] Use current negative feedback control to adjust the mechanical characteristic of the system. Set the period T of digital control when the digital microcontroller performs current negative feedback control f , so that the control period is in sync with the pulsation of the system current. Provide the control period T that is in sync with the current pulsation f Timing method.

[0012] The current I as the feedback variable should not only be proportional to the electromagnetic torque of the motor, but also be convenient for measurement and calculation processing. Select the current I and provide the way to measure and calculate to obtain it.

[0013] On the basis of clarifying the target mechanical characteristic required by the system, determine the negative feedback voltage U of the current negative feedback control implemented by the digital microcontroller f = f(I), where f(I) is the functional relationship set according to the target mechanical characteristic required by the system.

[0014] According to the target mechanical characteristic required by the system, provide two preferred functional relationships: U f = f(I) = K0I + K1(I - I1)ε(I - I1) + K2(I - I2)ε(I - I2) + K m (I - I m )ε(I - I m ) and U f = f(I) = K0I + K1I 2 + K m (I - I m )ε(I - I m ), where ε(I - I1), ε(I - I2) and ε(I - I m ) are unit step functions that generate steps at I = I1, I = I2 and I = I m respectively, and I1, I2 and I mThe characteristic current value for the target mechanical characteristics. Provide the parameters K0, K1, K2, and K in the functional relationship m Design method

[0015] The digital microcontroller uses the numerical control cycle T f Implement a current negative feedback control loop for the system, and give the control flow of the current negative feedback control loop. The control flow is as follows:

[0016] The digital microcontroller obtains the set input voltage U of the system, obtains the DC power supply voltage U of the previous control cycle d and the negative feedback current I, and calculates the current negative feedback voltage U of this control cycle f = f(I), calculates the terminal voltage U of the two series-connected windings of the motor a = U - U f , calculates the duty cycle ρ = U for implementing PWM control on the upper transistors of the bridge arms of the three-phase bridge a / U d , outputs a PWM control pulse with a duty cycle of ρ through the isolation drive circuit, implements PWM control on the upper transistors of the bridge arms, and makes the terminal voltage U of the two series-connected windings of the motor a = ρU d , collects and calculates the DC power supply voltage U d and the negative feedback current I. At the end of this cycle, the next control cycle starts

[0017] The beneficial effects of the present invention are:

[0018] Connect the feedback voltage U f = f(I) current negative feedback to the three-phase bridge permanent magnet brushless DC motor system. By reasonably setting the functional relationship of f(I) and the parameters therein, the desired system mechanical characteristics can be obtained, the operation experience of the user can be improved, and the input electric power of the system can be limited Description of the Drawings

[0019] Figure 1 Is the structural diagram of the digital control system of the three-phase bridge permanent magnet brushless DC motor

[0020] Figure 2 Is the control block diagram of the three-phase bridge permanent magnet brushless DC motor system

[0021] Figure 3 Is the mechanical characteristic curve of the present invention

[0022] Figure 4 Is the control flow chart of the present invention

[0023] Markings in the figure: 1. Three-phase bridge; 2. Motor; 3. Circuit measurement circuit; 4. Voltage measurement circuit; 5. Digital microcontroller; 6. Isolation drive circuit Detailed Implementation Manner

[0024] The present invention will be described below in conjunction with the embodiments shown in the accompanying drawings:

[0025] Figure 1 As shown, it is a digital control system for a three-phase bridge permanent magnet brushless DC motor, including a three-phase bridge 1, a motor 2, a circuit measurement circuit 3, a voltage measurement circuit 4, a digital microcontroller 5, and an isolation drive circuit 6. Its working principle is as follows: According to the measured rotor magnetic pole position of the motor 2, the digital microcontroller 5 and the isolation drive circuit 6 drive and control the upper tube of a certain bridge arm and the lower tube of another bridge arm of the three-phase bridge 1 to conduct, and a certain two phases in the three-phase windings of the motor 2 are connected in series to the power supply voltage U d , so that the energized winding interacts with the rotor magnetic pole to generate the maximum electromagnetic torque and drive the motor 2 to rotate. Then, according to the rotating and changing rotor magnetic pole position, the conducting tubes are changed to control the cyclic change of the two series-connected energized windings, such as AB → AC → BC → BA → CA → CB → AB, to make the motor 2 continue to operate. In the figure, U is the system-set input voltage not greater than U d , and i is the current on the DC power input side of the three-phase bridge 1. The current and voltage measured by the current measurement circuit 3 and the voltage measurement circuit 4 in the figure are used for system control.

[0026] Figure 2 In the control block diagram of the three-phase bridge permanent magnet brushless DC motor system shown, when U f = f(I) = 0, the three-phase bridge permanent magnet brushless DC motor digital control system is in open-loop control. The voltage measurement circuit 4 measures the DC power supply voltage U d , and the digital microcontroller 5 calculates the duty cycle ρ = U / U d of the PWM control implemented on the three-phase bridge 1 based on the system-set input voltage U and the DC power supply voltage U d obtained by it, and outputs a PWM control pulse with a duty cycle of ρ through the isolation drive circuit 6 to implement PWM control on the originally conducting upper tube of the bridge arm, so that the terminal voltage U a of the two series-connected energized windings of the motor 2 = U = ρU d .

[0027] When the system operates stably in open loop, the armature voltage equation of the motor 2 is U = 2RI + E, where R is the resistance of one-phase winding, I is the average value of the current during the conduction of the two-phase windings in series, and E is the average value of the sum of the back electromotive forces of the two series-connected windings during this period. E = C e n, where n is the motor speed and C e is the electromotive force constant. The electromagnetic torque T = C t I, C t is the torque constant. When operating stably, T = T L , and T L is the load torque when the motor 2 operates stably. The mechanical characteristic of the system in open loop is n = (U / Ce )-(2R / C e )I, where the slope of the rotational speed n decreasing as the current I increases is 2R / C e . Due to the small winding resistance R, this slope is small, and the rotational speed decreases slowly as the current increases. The system characteristic in the open-loop state is a hard characteristic, as shown in Figure 3 Characteristic curve 1. In the figure, I m is the threshold current for motor overload protection.

[0028] Aiming at the hard characteristic defect of the existing open-loop controlled permanent magnet brushless DC motor system, the present invention proposes a control method for adjusting the mechanical characteristics of a three-phase permanent magnet brushless DC motor system. The specific technical measures include:

[0029] Using a current negative feedback control to adjust the mechanical characteristics of the system. The control block diagram of the three-phase bridge permanent magnet brushless DC motor system is as shown in Figure 2 shown.

[0030] To make the control period match the pulsation of the current, the period T of digital control by the digital microcontroller 5 during feedback control f is an integer multiple of 1 to 3 of the cycle of the switch tubes in the three-phase bridge 1 conducting cyclically. In one conduction cycle of the three-phase bridge 1, the lower tubes of the three bridge arms receive a total of 3 trigger pulse rising edges evenly. Using the count value k of the trigger pulse rising edge of the lower tube of the bridge arm, the negative feedback control period T f is timed. When the count value k is equal to the set value k1, the period T f times out. The count set value k1 should be 3 or 6 or 9.

[0031] The feedback variable I of the system current negative feedback control is the average value of the series conduction current of the two-phase windings of the motor 2 within one T f period. The method for obtaining the current I is: the current measuring circuit 3 measures the current i on the DC power input side of the three-phase bridge 1, filters out the high-order harmonics, and the digital microcontroller 5 collects and calculates its average value within one T f period, and then divides it by the duty cycle ρ of the PWM control of the upper tube of the bridge arm of the three-phase bridge 1 within this T f period.

[0032] The voltage measuring circuit 4 measures the DC power supply voltage U d , and the digital microcontroller 5 collects and calculates its average value within one T f period.

[0033] On the basis of clarifying the target mechanical characteristics required by the system, the current negative feedback voltage U of the current negative feedback control implemented by the digital microcontroller 5 f = f(I), where f(I) is the functional relationship set according to the target mechanical characteristics required by the system.

[0034] After connecting the current negative feedback, the digital microcontroller 5 obtains the DC power supply voltage U of the previous control cycle d and the feedback current I, and calculates the current negative feedback voltage U of this control cycle f = f(I), calculates the terminal voltage U of the two-phase windings of the motor 2 connected in series and energized a = U - U f , calculates the duty cycle ρ = U for implementing PWM control on the upper transistor of the bridge arm of the three-phase bridge 1 a / U d , outputs a PWM control pulse with a duty cycle of ρ through the isolation drive circuit 6 to implement PWM control on the upper transistor of the bridge arm, so that the terminal voltage U of the two-phase windings of the motor 2 connected in series and energized a = ρU d , collects the current i measured by the current measurement circuit 3 and calculates the average value i av , collects the voltage measured by the voltage measurement circuit 4 and calculates the average value U d , until the end of this control cycle and the start of the next control cycle

[0035] The current negative feedback voltage U f = f(I), the mechanical characteristic of the closed-loop system is n = (U / C e ) - (2R / C e )I - f(I) / C e , when f(I) = 0, it is the mechanical characteristic of the open-loop system. The rate of change of the rotational speed of the closed-loop system with respect to the current is -[2R + f / (I)] / C e , where f / (I) is the derivative of f(I). If the condition f / (I) > 0 is satisfied, then after the system connects the current negative feedback of U f = f(I), the speed at which the system speed decreases with the increase of current is faster than that in the open-loop state, that is, the mechanical characteristic is softer than that in the open-loop state, and the system stability can be ensured

[0036] Example 1

[0037] In this embodiment, it is required that after connecting the current negative feedback, the mechanical characteristic of the system is as Figure 3 shown by characteristic line 2: there are currents I1 and I2, and 0 < I1 < I2 < I m , the section corresponding to the current 0 to I1 is the characteristic of the light load section, and the slope of its rotational speed n decreasing with the increase of the current I is 5 to 10 times the slope 2R / C e in the open-loop state; the section corresponding to the current I1 to I2 is the characteristic of the medium load section, and the slope of its rotational speed n decreasing with the increase of the current I is 1 to 2 times the slope 2R / C e in the open-loop state; the section corresponding to the current I2 to I mThe section is the overload section characteristic, and the slope of the speed n decreasing with the increase of the current I is 5 to 10 times the slope 2R / C in the open loop; when I = I e , the speed n = (0.1 - 0.3)(U / C m ), where (U / C e ) is the ideal no-load speed of the motor 2; when I > I e , the speed n drops sharply with the increase of the current I, making overload protection, and the current I m at stall = (1.05 - 1.15)I st . m .

[0038] According to the requirements of the system target mechanical characteristic, set U f = f(I) = K0I + K1(I - I1)ε(I - I1) + K2(I - I2)ε(I - I2) + K m (I - I m )ε(I - I m ), where ε(I - I1), ε(I - I2) and ε(I - I m ) are unit step functions that generate steps at I = I1, I = I2 and I = I m respectively. When 0 ≤ I < I1, U f = K0I; when I1 ≤ I < I2, U f = K0I + K1(I - I1); when I2 ≤ I < I m , U f = K0I + K1(I - I1) + K2(I - I2); when I ≥ I m , U f = K0I + K1(I - I1) + K2(I - I2) + K m (I - I m ).

[0039] When 0 ≤ I < I1, the mechanical characteristic of the closed-loop system is n = (U / C e ) - [(2R + K0) / C e I, and the rate of change of speed with current is -(2R + K0) / C e ; when I1 ≤ I < I2, the system mechanical characteristic is n = (U / C e ) - [(2R + K0) / C e I - (K1 / C e )(I - I1), and the rate of change of speed with current is -(2R + K0 + K1) / C e ; when I2 ≤ I < I m , the system mechanical characteristic is n = (U / C e ) - [(2R + K0) / C e I - (K1 / Ce )(I - I1) - (K2 / C e )(I - I2), and the rate of change of speed with current is -(2R + K0 + K1 + K2) / C e .

[0040] Design the parameters K0, K1, and K2 according to the relationship that the slope of the speed n decreasing with the increase of current I in the target mechanical characteristic is the slope of the characteristic in the open-loop case: Since 2R + K0 = (5 - 10)2R, then K0 = (4 - 9)2R; since 2R + K0 + K1 = (1 - 2)2R, then K1 = (0 - 1)2R - K0; since 2R + K0 + K1 + K2 = (5 - 10)2R, then K2 = (4 - 9)2R - K0 - K1. After designing and calculating the parameters, check that when I = I m , the speed n = (0.1 - 0.3)(U / C e ), that is, it is required that (2R + K0)I m + K1(I m - I1) + K2(I m - I2) = (0.9 - 0.7)U, otherwise, adjust the values of K0, K1, and K2 within the value range.

[0041] Design the parameter K m according to the requirements of the target mechanical characteristic: When I > I m , the mechanical characteristic of the closed-loop system is n = (U / C e ) - [(2R + K0) / C e I - (K1 / C e )(I - I1) - (K2 / C e )(I - I2) - (K m / C e )(I - I m ). Since when n = 0, I = I st = (1.05 - 1.15)I m , then K m = [U - 2RI st - K0I st - K1(I st - I1) - K2(I st - I2)] / (I st - I m ).

[0042] When the system is blocked, the voltage at both ends of the motor 2 is the minimum value U amin , and its calculation formula is U amin = U - K0I st - K1(I st - I1) - K2(I st - I2) - K m (I st-I m )。The digital microcontroller 5 performs negative feedback control with a numerically controlled period T f , that is, numerical control hysteresis will occur, and the calculated U a may be less than U amin . At this time, a correction needs to be made, and let U a =U amin .

[0043] Due to the decrease in the battery voltage U d that powers the system, the duty cycle ρ = U a / U d calculated by the digital microcontroller (5) may be greater than 1. At this time, a correction needs to be made, and let ρ = 1.

[0044] Figure 4 The following shows the process of the digital microcontroller 5 of the system in Embodiment 1 performing current negative feedback control. The control process is as follows:

[0045] The system is powered on, and the digital microcontroller 5 is initialized. U = set value, k1 = set value, I1 = set value, I2 = set value, I m = set value, I = I m , k = -1, U d = 1.1U, K0 = design value, K1 = design value, K2 = design value, K m = design value, U amin = design value; enter the current negative feedback control loop with a period of T f . For each rising edge of the trigger pulse received by the lower transistor of the bridge arm of the three-phase bridge 1, the rising edge count value k = k + 1. When k = k1, obtain the average value i f of the current i in this T av period, obtain the average value U f of the power supply voltage in this T d period, calculate the current I = i av / ρ, let k = 0, this control period ends, and the next period starts.

[0046] The digital microcontroller 5 enters the current negative feedback control loop with a period of T f . When the count value k of the rising edge of the trigger pulse received by the lower transistor of the bridge arm is k = k1, obtain the average value i f of the current i in this T av period, obtain the average value U f of the power supply voltage in this T d period, calculate the current I = i av / ρ, let k = 0, this T f control period ends, and the next period starts; calculate the feedback voltage U f , when I < I1, U f= K0I, when I1 ≤ I < I2, U f = K0I + K1(I - I1), when I2 ≤ I < I m when U f = K0I + K1(I - I1) + K2(I - I2), when I ≥ I m when U f = K0I + K1(I - I1) + K2(I - I2) + K m (I - I m ); Calculate the motor terminal voltage U a = U - U f , if U a < U amin then set U a = U amin ; Calculate the duty cycle ρ = U a / U d , if ρ > 1 then set ρ = 1; Implement PWM control on the upper transistor of the first leg of the three-phase bridge to make U a = ρU d ; Collect the current i measured by the current measurement circuit 3 and calculate the average value i av , collect the voltage measured by the voltage measurement circuit 4 and calculate the average value U d ; For each rising edge of the trigger pulse received by the lower transistor of the leg, the rising edge count value k = k + 1. When k = k1, obtain the average values i av and U d , calculate the current I = i av / ρ, set k = 0, the current cycle ends, and the next T f control cycle starts.

[0047] Embodiment 2

[0048] In this embodiment, after connecting the current negative feedback, the mechanical characteristics of the system are required to be as Figure 3 shown by characteristic line 3: When the light load I → 0, the maximum slope of the rotational speed n decreasing with the increase of the current I is 6 - 12 times the slope 2R / C e in the open-loop case; When the heavy load I → I m , the minimum slope of the rotational speed n decreasing with the increase of the current I is 2 - 3 times the slope 2R / C e in the open-loop case; When I = I m , the rotational speed n = (0.1 - 0.3)(U / C e ); When I > I m , the rotational speed n decreases rapidly with the increase of the current I, and an overload protection is made. The current I st at stall = (1.05 - 1.15)I m .

[0049] According to the requirements of the system target mechanical characteristics, set Uf = f(I) = K0I + K1I 2 + K m (I − I m )ε(I − I m )。When 0 ≤ I < I m , U f = K0I + K1I 2 ; When I ≥ I m , U f = K0I + K1I 2 + K m (I − I m ).

[0050] When 0 ≤ I < I m , the mechanical characteristic of the closed-loop system is n = (U / C e ) − [(2R + K0) / C e I − (K1 / C e )I 2 , and the rate of change of the rotational speed with respect to the current is −(2R + K0 + 2K1I) / C e ; When I ≥ I m , the mechanical characteristic of the closed-loop system is n = (U / C e ) − [(2R + K0) / C e I − (K1 / C e )I 2 − (K m / C e )(I − I m ).

[0051] According to the relationship that the slope of the rotational speed n of the target mechanical characteristic decreases with the increase of the current I, which is the slope of the open-loop characteristic, design the parameters K0, K1 and K m : When the light load I → 0, since 2R + K0 = (6 - 12)2R, so K0 = (5 - 11)2R; When the heavy load I → I m , since 2R + K0 + 2K1I m = (2 - 3)2R, so K1 = [(1 - 2)2R - K0] / (2I m ). After calculating K0 and K1 by design, check the rotational speed n = (0.1 - 0.3)(U / C m ) when I = I e , that is, it is required that (2R + K0)I m + K1I m 2 = (0.9 - 0.7)U, otherwise adjust the values of K0 and K1. Since when n = 0, I = I st = (1.05 - 1.15)I m , so the parameter K m = (U - 2RIst -K0I st -K1I st 2 ) / (I st -I m )。

[0052] When the system is blocked, the voltage at both ends of the motor 2 is the minimum value U amin , and its calculation formula is U amin =U - K0I st -K1I st 2 -K m (I st -I m ). The digital microcontroller 5 performs negative feedback control with a numerical control period T f . The calculated U a may be less than U amin . At this time, a correction needs to be made, and let U a =U amin .

[0053] Due to the decrease in the battery voltage supplying power to the system, the duty cycle ρ = U a / U d calculated by the digital microcontroller 5 is greater than 1. At this time, let ρ = 1.

[0054] The process of the digital microcontroller 5 of the system in Embodiment 2 implementing current negative feedback control is as follows:

[0055] The system is powered on, and the digital microcontroller 5 is initialized. U = set value, k1 = set value, I m = set value, I = I m , k = -1, U d = 1.1U, K0 = design value, K1 = design value, K m = design value, U amin = design value; enter the current negative feedback control loop with a period of T f . For each rising edge of the trigger pulse received by the lower tube of the bridge arm of the three-phase bridge 1, the rising edge count value k = k + 1. When k = k1, obtain the average value i f of the current i in this T av period, obtain the average value U f of the power supply voltage in this T d period, calculate the current I = i av / ρ, let k = 0, and this control period ends and the next period begins.

[0056] The digital microcontroller 5 enters the current negative feedback control loop with a period of T f : When the count value k of the rising edge of the trigger pulse received by the lower tube of the bridge arm is k1, obtain the current i in this Tf Average value i of the period av , obtain the power supply voltage during this T f Average value U of the period d , calculate the current I = i av / ρ, let k = 0, this T f The control period ends and the next period begins; calculate the feedback voltage U f , when I < I m then U f = K0I + K1I 2 , when I ≥ I m then U f = K0I + K1I 2 + K m (I - I m ); calculate the motor terminal voltage U a = U - U f , if U a < U amin then let U a = U amin ; calculate the duty cycle ρ = U a / U d , if ρ > 1 then let ρ = 1; implement PWM control on the upper transistor of the 1st arm of the three-phase bridge to make U a = ρU d ; collect the current i measured by the current measurement circuit 3 and calculate the average value i av , collect the voltage measured by the voltage measurement circuit 4 and calculate the average value U d ; for each rising edge of the trigger pulse received by the lower transistor of the arm, the rising edge count value k = k + 1, when k = k1, obtain the average value i av and U d of this period, calculate the current I = i av / ρ, let k = 0, this period ends, and the next T f control period begins.

[0057] The specific embodiments described in the text are only illustrative of the spirit of the present invention. Those skilled in the technical field to which the present invention pertains can make various modifications or supplements to the described specific embodiments or use similar methods for substitution, but will not deviate from the spirit of the present invention or exceed the scope defined by the appended claims.

Claims

1. A control method for adjusting the mechanical characteristics of a three-phase permanent magnet brushless DC motor system, characterized in that, A digital microcontroller is used to control the operation of a three-phase permanent magnet brushless DC motor by driving a three-phase bridge to conduct cyclically, and the mechanical characteristics of the system are adjusted by current negative feedback control. The steps are as follows: S1 determines that the control period of the current negative feedback is T f : To make the control period in sync with the pulsation of the motor current, the control period of the current negative feedback of the digital microcontroller (5) in the system T f is an integer multiple of 1 to 3 of the cycle of the switch tubes in the three-phase bridge (1) conducting cyclically; S2 Obtain each control cycle T f the negative feedback current within I : the system negative feedback current I is the average value of the series conduction current of the two-phase windings of the motor (2) within one control cycle T f and this current is measured from the DC power input side of the three-phase bridge (1) and then referred to the motor side; S3 Set the negative feedback voltage U f and the negative feedback current I Functional relationship: Set the functional relationship according to the requirements of the target mechanical characteristics of the system f ( I ) The negative feedback voltage of the current negative feedback control implemented by the digital microcontroller (5) in the system U f = f ( I ) The implementation control cycle of the S4 system is T f Current negative feedback control: The digital microcontroller (5) uses the current obtained by measurement and calculation in the previous control cycle I to calculate the negative feedback voltage of this control cycle U f = f ( I ) to implement the current negative feedback control of this control cycle, measure and calculate the current obtained in this control cycle I ; The control period T f is timed by the count value of the rising edge of the trigger pulse of the lower transistor of the three-phase bridge (1) arm k , and when the timing is full, the count value k is equal to the set value k 1, and the set value k 1 is equal to 3 or 6 or 9 ； The negative feedback current I is obtained by measuring the current on the DC power input side of the three-phase bridge (1) with a current measurement circuit (3) i , filtering out high-order harmonics, and collecting and calculating the average value of the current within one T f cycle by a digital microcontroller (5), and then dividing it by the T f duty cycle of the PWM control of the upper transistor of the bridge arm of the three-phase bridge (1) within the cycle ρ; The set negative feedback voltage U f and the negative feedback current I have the following functional relationship: The open-loop linear mechanical characteristic of the system where the rotational speed decreases slowly as the current increases is adjusted to a broken-line mechanical characteristic with different slopes. The system has a feedback current I 1, I 2, and I m where 0 < I 1 < I 2 < I m It is required that the slope of the rotational speed decrease as the current increases in the light-load section of the system characteristic when the current is from 0 to I 1 is 5 to 10 times the slope in the open-loop case. When the current is I 1 to I 2, the slope of the rotational speed decrease in the medium-load section of the characteristic is 1 to 2 times the slope in the open-loop case. When the current is I 2 to I m , the slope of the rotational speed decrease in the heavy-load section of the characteristic is 5 to 10 times the slope in the open-loop case. When the current is I > I m , the rotational speed in the overload protection section of the characteristic decreases rapidly as the current increases.

2. A control method for adjusting the mechanical characteristics of a three-phase permanent magnet brushless DC motor system according to claim 1, characterized in that, The set negative feedback voltage U f and the negative feedback current I The functional relationship: It is required that I = I m When the rotational speed n = (0.1~0.3)( U / C e ), where ( U / C e ) is the ideal no-load rotational speed of the motor (2). It is required that the current I st = (1.05~1.15) I m , U is The system-set input voltage, C e is the electromotive force constant.

3. A control method for adjusting the mechanical characteristics of a three-phase permanent magnet brushless DC motor system according to claim 1, characterized in that, The set negative feedback voltage U f and the negative feedback current I have the following functional relationship: Set U f = f ( I ) = K 0 I + K 1( I − I 1) ε ( I − I 1) + K 2( I − I 2) ε ( I − I 2) + K m ( I − I m ) ε ( I − I m ), where ε ( I − I 1), ε ( I − I 2) and ε ( I − I m ) are unit step functions that generate steps at I = I 1, I = I 2 and I = I m respectively. According to the relationship between the slope of the target mechanical characteristic speed decreasing with the increase of current and the slope of the open-loop characteristic, the parameters K 0 = (4~9)2 R , K 1 = (0~1)2 R − K 0, K 2 = (4~9)2 R − K 0 − K 1. In the parameter calculation formula, R is the resistance of one-phase winding of the motor (2).

4. A control method for adjusting the mechanical characteristics of a three-phase permanent magnet brushless DC motor system according to claim 3, characterized in that The set negative feedback voltage U f and the negative feedback current I The functional relationship: Because I = I m When the rotational speed n = (0.1~0.3)( U / C e ), it is required that (2 R + K 0) I m + K 1( I m - I 1)+ K 2( I m - I 2) = (0.9~0.7) U , otherwise, the values of K 0, K 1 and K 2 should be adjusted within the value range. Because when n = 0, I = I st = (1.05~1.15) I m , so the parameter K m = U -2R I st - K 0 I st - K 1( I st - I 1)- K 2( I st - I 2)] / ( I st - I m ).

5. A control method for adjusting the mechanical characteristics of a three-phase permanent magnet brushless DC motor system according to claim 1, characterized in that The control period implemented by the system is T f The process of current negative feedback control: The digital microcontroller (5) obtains the set input voltage of the system U , obtains the DC power supply voltage of the previous control period U d and the negative feedback current I , let k = 0, the current control period starts, and calculates the current negative feedback voltage of the current control period U f = f ( I ), calculates the terminal voltage of the two series-connected energized windings of the motor (2) U a = U - U f , calculates the duty cycle for implementing PWM control on the upper transistors of the three-phase bridge (1) ρ = U a / U d , outputs a PWM control pulse with a duty cycle of ρ through the isolation drive circuit (6) to implement PWM control on the upper transistors of the bridge arm, so that the terminal voltage of the two series-connected energized windings of the motor (2) U a = ρU d , collects the current measured by the current measurement circuit (3) i and calculates the average value i av , collects the voltage measured by the voltage measurement circuit (4) and calculates the average value U d , for each rising edge of the trigger pulse received by the lower transistor of the bridge arm, the rising edge count value k = k + 1, when k = k 1, obtains the average values i av and U d , calculates the current I = i av / ρ , let k = 0, the current control period ends, and the next control period starts.

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