CN117767838A - Motor rotating speed acquisition method - Google Patents

Abstract

The invention provides a motor rotating speed acquisition method, which comprises the following steps: a. providing an encoder whose output is a first motor angle θ _i The method comprises the steps of carrying out a first treatment on the surface of the b. Providing a digital phase-locked loop, and setting the first motor angle theta _i Transmitting to the digital phase-locked loop; c. determining the first motor angle θ _i A difference delta theta from the output of the digital phase locked loop; d. calculating the motor rotation speed n according to the difference delta theta; e. and calculating a second motor angle theta according to the motor rotating speed n, wherein the second motor angle theta is the output of the digital phase-locked loop.

Description

Motor rotating speed acquisition method

Technical Field

The invention relates to a motor rotating speed acquisition method, in particular to a method for calculating the motor rotating speed by using a digital phase-locked loop method and utilizing an angle fed back by an encoder.

Background

The permanent magnet synchronous motor has the characteristics of small volume, light weight, high efficiency, quick response and the like. As an energy conversion device and a power device, the device is widely applied to the production and living fields of household appliances, industrial control, robots and the like. In order to obtain a higher performance control effect, an encoder is required to obtain rotor position and rotational speed information of the motor.

Encoders for detecting the rotational speed and position of a permanent magnet synchronous motor are widely used and are mainly classified into incremental encoders and absolute encoders.

For incremental encoders, incremental photoelectric encoders are currently used in many cases. This type of encoder converts rotor angular displacement into a pulse signal output. The larger the angular displacement, the more pulses are output, and the amount of change in the angular displacement of the rotor, that is, the increment, can be calculated from the number of pulses, so that the incremental encoder is called. The incremental encoder can obtain the speed by M method, T method, M/T method, etc. The method is to acquire the pulse variation M in unit time T to calculate the rotating speed n of the motor:

wherein Z is the number of pulses output by the motor in one rotation. n is the rotation speed of the motor, and the unit is r/min

These methods are susceptible to encoder quantization errors and measurement noise, and the differential form of calculation also plays a role in amplifying errors, resulting in inaccurate speed estimation.

For absolute encoders, more magnetic encoders, rotary transformers, and absolute photoelectric encoders are currently used. Each position of the motor rotor rotation corresponds to a digital quantity to represent that position. The different numbers of positions are different and are therefore called absolute encoders. The way in which the rotational speed is obtained is also the same as in the incremental encoder.

The speed is calculated by passing the change amount delta theta of the angle in the unit time T.

Wherein n is the motor rotation speed, in r/min.

The digital signal output by the absolute encoder also contains noise and higher harmonics. In fact, in any encoder, the rotation speed is obtained by performing differential operation on angle information fed back by the encoder. The angle obtained has high-frequency noise due to the quantization error of the encoder and the measurement noise, and meanwhile, the differential calculation mode plays a role of amplifying the error, so that the calculated speed is inaccurate. Although a part of noise can be filtered by low-pass filtering, phase lag is introduced at the same time, and dynamic response of the motor is weakened.

Therefore, there is a need for a motor rotation speed acquisition method with small calculation errors and high accuracy.

Disclosure of Invention

The invention provides a motor rotating speed acquisition method for reducing the calculating error of the motor rotating speed and improving the calculating accuracy of the motor rotating speed.

The motor rotation speed acquisition method comprises the following steps:

a. providing an encoder whose output is a first motor angle θ _i ；

b. Providing a digital phase-locked loop, and setting the first motor angle theta _i Transmitting to the digital phase-locked loop;

c. determining the first motor angle θ _i A difference delta theta from the output of the digital phase locked loop;

d. calculating the motor rotation speed n according to the difference delta theta;

e. and calculating a second motor angle theta according to the motor rotating speed n, wherein the second motor angle theta is the output of the digital phase-locked loop.

In one embodiment, the first motor angle θ _i Including the true value theta _iN And measuring error theta _iC 。

In one embodiment, step d calculates the motor speed n according to the following formula:

wherein Kp is a proportional coefficient, ki is an integral coefficient, and T is a time interval during which the digital phase-locked loop performs computation.

In one embodiment, the first term kp (θ in equation (4) _iN -θ)+ki*T*∑(θ _iN - θ) is the true value of the motor speed, the second term kpθ _ic +ki*T*∑θ _ic For the electricityAnd a rotational speed error of the engine rotational speed.

In one embodiment, the method further comprises cyclically performing steps c-e until the second motor angle θ is equal to the first motor angle θ _i Equal.

In one embodiment, the motor speed n is an intermediate product that is necessarily generated during the output of the second motor angle θ by the digital phase-locked loop.

In one embodiment, the encoder is an incremental encoder or an absolute encoder.

In one embodiment, the second motor angle θ is calculated according to the following equation:

θ＝T*∑n

wherein T is a time interval during which the digital phase locked loop performs the computation.

In one embodiment, step c is performed by a phase discrimination module of the digital phase locked loop.

In one embodiment, step d is performed by a proportional-integral controller of the digital phase-locked loop.

In one embodiment, step e is performed by an integral controller of the digital phase locked loop.

The motor rotating speed obtaining method adopts a digital phase-locked loop method to construct an angle theta between the motor rotating speed obtaining method and an encoder _i The same quantity θ, which has an intermediate variable n during construction, is used as the motor speed. The motor rotation speed is obtained through a PI controller of the digital phase-locked loop, and the PI controller has a certain filtering effect, so that high-frequency noise can be filtered, and the speed stability is improved. Meanwhile, the whole digital phase-locked loop is an I-type system. The system can realize error-free tracking on step signals and linear signals, and the rotating speed and the angular phase delay of a motor obtained by a digital phase-locked loop are greatly reduced. Compared with the prior art, the method and the device can acquire more stable, effective and accurate speed information under the condition of not improving the precision of the encoder, realize the efficient operation of the motor and have certain economic effects.

Drawings

The foregoing summary of the invention, as well as the following detailed description of the invention, will be better understood when read in conjunction with the accompanying drawings. It is to be noted that the drawings are merely examples of the claimed invention. In the drawings, like reference numbers indicate identical or similar elements.

Fig. 1 shows a block diagram of a digital phase-locked loop for a motor rotation speed acquisition method according to an embodiment of the present invention;

fig. 2 shows a block diagram of a digital phase-locked loop for a motor rotation speed acquisition method according to still another embodiment of the present invention;

fig. 3 shows a flowchart of a motor rotation speed acquisition method according to an embodiment of the present invention.

Detailed Description

The detailed features and advantages of the present invention will be readily apparent to those skilled in the art from the following detailed description, claims, and drawings that follow. While the description of the invention will be presented in connection with a preferred embodiment, it is not intended to limit the inventive features to that embodiment. Rather, the purpose of the invention described in connection with the embodiments is to cover other alternatives or modifications, which may be extended by the claims based on the invention. The following description contains many specific details for the purpose of providing a thorough understanding of the present invention. The invention may be practiced without these specific details. Furthermore, some specific details are omitted from the description in order to avoid obscuring the invention.

In the description of the present invention, it should be noted that, unless explicitly specified and limited otherwise, the terms "connected," "connected," and "connected" are to be construed broadly, and may be either fixedly connected, detachably connected, or integrally connected, for example; can be mechanically or electrically connected; can be directly connected or indirectly connected through an intermediate medium, and can be communication between two elements. The specific meaning of the above terms in the present invention will be understood in specific cases by those of ordinary skill in the art.

It will be understood that, although the terms "first," "second," "third," etc. may be used herein to describe various elements, channels, components, regions, layers and/or sections, these elements, channels, components, regions, layers and/or sections should not be limited by these terms, and these terms are used solely to distinguish between different elements, channels, components, regions, layers and/or sections. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance.

As used in this application and in the claims, the terms "a," "an," "the," and/or "the" are not specific to the singular, but may include the plural, unless the context clearly dictates otherwise. In general, the terms "comprises" and "comprising" merely indicate that the steps and elements are explicitly identified, and they do not constitute an exclusive list, as other steps or elements may be included in a method or apparatus.

In some embodiments, numbers describing the components, number of attributes are used, it being understood that such numbers being used in the description of embodiments are modified in some examples by the modifier "about," approximately, "or" substantially. Unless otherwise indicated, "about," "approximately," or "substantially" indicate that the number allows for a 20% variation. Accordingly, in some embodiments, numerical parameters set forth in the specification and claims are approximations that may vary depending upon the desired properties sought to be obtained by the individual embodiments. In some embodiments, the numerical parameters should take into account the specified significant digits and employ a method for preserving the general number of digits. Although the numerical ranges and parameters set forth herein are approximations that may be employed in some embodiments to confirm the breadth of the range, in particular embodiments, the setting of such numerical values is as precise as possible.

Meanwhile, the present application uses specific words to describe embodiments of the present application. Reference to "one embodiment," "an embodiment," and/or "some embodiments" means that a particular feature, structure, or characteristic is associated with at least one embodiment of the present application. Thus, it should be emphasized and should be appreciated that two or more references to "an embodiment" or "one embodiment" or "an alternative embodiment" in various positions in this specification are not necessarily referring to the same embodiment. Furthermore, certain features, structures, or characteristics of one or more embodiments of the present application may be combined as suitable.

The invention uses a digital phase-locked loop method to calculate the motor rotation speed by utilizing the feedback angle of the encoder. The method is characterized in that the angle obtained by the encoder is quickly tracked through the angle output by the digital phase-locked loop, and the intermediate product of the digital phase-locked loop (namely the output of the PI controller) is the motor rotating speed.

Fig. 1 shows a block diagram of a digital phase-locked loop for a motor rotation speed acquisition method according to an embodiment of the present invention.

The digital phase-locked loop of the invention is connected with the output of the encoder, namely, the first motor angle theta obtained by the encoder _i Input to a digital phase locked loop.

The digital phase locked loop includes three parts: phase discrimination module 101, speed output module 102 and angle output module 103.

The phase detection module 101 is used for calculating a first motor angle θ obtained by the encoder _i And the difference delta theta of the second motor angle theta calculated by the digital phase-locked loop.

The speed output module 102 is configured to calculate a motor rotational speed n (i.e., an angular speed of the motor) based on the angular difference Δθ output. The input quantity is the angle difference delta theta and the output quantity is n.

The speed output module 102 calculates the motor speed n according to the following equation (1):

n＝Kp*Δθ+Ki*T*∑Δθ (1)

where Kp is a proportional coefficient and Ki is an integral coefficient. T is the time interval for performing the digital phase locked loop computation.

In one embodiment, the speed output module 102 is a proportional-integral controller PI.

The angle output module 103 is configured to calculate a second motor angle θ according to the motor rotation speed n. The input quantity is n, and the output is an angle theta.

The angle output module 103 calculates the second motor angle θ according to the following well known (2):

θ＝T*∑n (2)

the method is a pure integration link, and the integration coefficient is 1.

In one embodiment, the angle output module 103 is an integral controller I.

The digital phase-locked loop has the function of enabling the output signal of the phase-locked loop to be consistent with the input signal through internal calculation and adjustment, namely, the error of the output signal and the input signal is 0.

For the present invention, the input signal to the phase locked loop is the encoder-derived angle θ _i (first motor angle θ) _i ) The internal calculation and adjustment are realized through a proportional integral controller of the speed output module and an integral controller of the angle output module. The digital phase-locked loop of the invention can enable the output signal (the second motor angle theta) of the digital phase-locked loop to quickly follow the input signal (the first motor angle thetai obtained by the encoder).

When θi is greater than θ, the phase detection module 101 outputs Δθ as a positive value, the proportional-integral controller output n of the speed output module 102 increases (the effect of integration is equal to accumulation, the positive number is accumulated, the result is gradually increased), and the digital phase-locked loop output θ also increases by the pure integration of the angle output module 103, so that θ _i The difference delta theta between theta and theta gradually decreases.

When theta is as _i When the value is smaller than theta, the phase discrimination module 101 outputs delta theta as a negative value, the output n of the proportional-integral controller of the speed output module is reduced, and then the digital phase-locked loop output theta is reduced under the pure integral action of the angle output module 103, so that theta _i The difference delta theta from theta will also decrease gradually. By this feedback action, θ and θ are reduced _i The difference between them is until the two are completely consistent.

In the invention, theta is an output signal of the digital phase-locked loop, and an input signal of the digital phase-locked loop is a motor rotating shaft angle signal theta sent by an encoder _i . By regulating actions theta and theta of digital phase-locked loop _i And the two are completely consistent, so theta is the motor rotating shaft angle signal sent by the encoder. In a digital phase-locked loopIn the structure, it can be seen that θ is n obtained by integration over time. θ is the angle of the motor, and in a physical sense, only the angular velocity can be integrated over time to obtain the angle. So n is the angular speed of the motor, i.e. the motor speed.

As a similar example, on a straight line motion, the velocity times the time equals the distance. If there is a quantity n, it is multiplied by time to obtain this distance. Then this n is numerically equal to the speed. It can replace the speed.

A similar approach is used here as well. n can be seen as an intermediate variable of the digital phase locked loop. n is integrated to obtain the output theta of the digital phase-locked loop. Through the adjusting function of the phase-locked loop, θ is the angle of the motor, and n is the rotation speed of the motor naturally.

The advantages of the present invention over the prior art are set forth below.

An encoder is a physical device. It is able to characterize an angular position of the motor rotor by means of output pulses or binary data. It is not possible to obtain an accurate true value for any measurement of the physical quantity, and the true value plus a measurement error is obtained (e.g., measuring the length of a book with a ruler, obtaining 10cm. This 10cm is just a measurement value that is equal to the true value plus the measurement error. Measurement errors include errors in the ruler itself, reading errors when a person reads a value, etc.). The angle information output by the encoder is no exception, and the angle information contains both a true angle and a measurement error (the errors are mainly due to the accuracy errors of the encoder, errors of data or pulses in the transmission process, calculation errors when an upper computer processes the signals, and the like). In the prior art, when the motor rotation speed is calculated by utilizing the angle output by the encoder, the absolute encoder or the incremental encoder is usually obtained in a differential mode. For example, assume θ ₁ Is the angle of the current encoder output and contains the angle true value θ _1N And measuring error theta _1C . Suppose θ ₂ Is the angle output by the encoder after the T time, and comprises the angle true value theta _2N And measuring error theta _2C . The motor rotational speed obtained by the differential method is as follows:

the first term in the above equation is a true value of the rotational speed. The latter term is rotational speed error. Typically, the time interval T is 10 ^-3 Is a small number for dimension. The second term in the above equation corresponds to the amplification of the error. The rotational speed obtained in the differential system contains a large amount of error components.

The method of the invention adopts a digital phase-locked loop method to construct a phase-locked loop with the encoder angle theta _i The same quantity θ, which is the same as the motor speed, is used as the motor speed during construction as there is an intermediate variable n. The calculation of n is obtained from equation (1). Let θ _i Is the angle of the current encoder output and contains the angle true value θ _iN And measuring error theta _iC . θ is the angle of the phase locked loop output. The rotational speed n obtainable according to formula (1) is as follows:

n＝kp(θ _i -θ)+ki*T*∑(θ _i -θ)

＝[kp(θ _iN -θ)+ki*T*∑(θ _iN -θ)]+[kpθ _ic +ki*T*∑θ _ic ] (4)

the first term kp (θ) in formula (4) _iN -θ)+ki*T*∑(θ _iN - θ) is the true value of the rotational speed, the second term kpθ _ic +ki*T*∑θ _ic Is the rotational speed error. Due to measurement error theta _iC The method has positive and negative randomness, so that the error can be effectively reduced by accumulating the errors, T is a small number, and the effect of reducing the errors is achieved by multiplying the accumulated result with T. The motor rotation speed obtained by the method is superior to the existing mode because the differential mode is not used, and the measurement error is not amplified.

Fig. 2 shows a block diagram of a digital phase-locked loop for a motor rotation speed acquisition method according to still another embodiment of the present invention. The digital phase locked loop structure of fig. 2 is similar to that of fig. 1. The digital phase locked loop includes three parts: phase discrimination module 201, speed output module 202 and angle output module 203.

Unlike fig. 1, the phase discrimination module 201 uses an angle heterodyne method to calculate the angle difference.

Since the working principle of the digital phase locked loop of fig. 2 is similar to that of fig. 1, the description thereof will not be repeated. The person skilled in the art will be able to derive the working mode of fig. 2 on the basis of the foregoing description.

Fig. 3 shows a flowchart of a motor rotation speed acquisition method according to an embodiment of the present invention. The method includes, but is not limited to, the steps of:

step 301: providing an encoder whose output is a first motor angle θ _i ；

Step 302: providing a digital phase-locked loop, and setting the first motor angle theta _i Transmitting to the digital phase-locked loop;

step 303: determining the first motor angle θ _i A difference delta theta from the output of the digital phase locked loop;

step 304: calculating the motor rotation speed n according to the difference delta theta;

step 305: and calculating a second motor angle theta according to the motor rotating speed n, wherein the second motor angle theta is the output of the digital phase-locked loop.

In one embodiment, the first motor angle θ _i Including the true value theta _iN And measuring error theta _iC 。

In one embodiment, step 304 calculates the motor speed n according to the following formula:

n＝kp(θ _i -θ)+ki*T*∑(θ _i -θ)

＝[kp(θ _iN -θ)+ki*T*∑(θ _iN -θ)]+[kpθ _ic +ki*T*∑θ _ic ] (4)

wherein Kp is a proportional coefficient, ki is an integral coefficient, and T is a time interval during which the digital phase-locked loop performs computation.

In one embodiment, the first term kp (θ in equation (4) _iN -θ)+ki*T*∑(θ _iN - θ) is the true value of the motor speed, the second term kpθ _ic +ki*T*∑θ _ic A rotation speed error for the rotation speed of the motorAnd (3) difference.

In one embodiment, the motor rotation speed obtaining method further includes performing steps 303-305 in a loop until the second motor angle θ is equal to the first motor angle θ _i Equal.

In one embodiment, the motor speed n is an intermediate product that is necessarily generated during the output of the second motor angle θ by the digital phase-locked loop.

In one embodiment, the encoder is an incremental encoder or an absolute encoder.

In one embodiment, the second motor angle θ is calculated according to the following equation:

θ＝T*∑n

wherein T is a time interval during which the digital phase locked loop performs the computation.

In one embodiment, step 303 is performed by a phase discrimination module of the digital phase locked loop.

In one embodiment, step 304 is performed by a proportional-integral controller of the digital phase-locked loop.

In one embodiment, step 305 is performed by an integral controller of the digital phase locked loop.

In summary, the motor rotation speed acquisition method of the invention quickly tracks the angle obtained by the encoder through the angle output by the digital phase-locked loop, and the output of the speed part obtained in the process is the motor rotation speed. The motor rotation speed is obtained through a PI controller of the digital phase-locked loop, and the PI controller has a certain filtering effect, so that high-frequency noise can be filtered, and the speed stability is improved. Meanwhile, the whole digital phase-locked loop is an I-type system. The system can realize error-free tracking on step signals and linear signals, and the rotating speed and the angular phase delay of a motor obtained by a digital phase-locked loop are greatly reduced. Compared with the prior art, the method and the device can acquire more stable, effective and accurate speed information under the condition of not improving the precision of the encoder, realize the efficient operation of the motor and have certain economic effects.

Those of skill in the art will appreciate that the various illustrative components, modules, blocks, units, circuits, systems, and steps described in connection with the embodiments disclosed herein may be implemented as hardware, software (including firmware, resident software, micro-code, etc.), or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, modules, blocks, units, circuits, systems, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present invention.

Flowcharts are used in this application to describe the operations or steps performed by systems according to embodiments of the present application. It should be understood that the preceding or following operations or steps are not necessarily performed in order precisely. Rather, the various operations or steps may be processed in reverse order or simultaneously. At the same time, other operations or steps are added to or removed from these processes.

The order in which elements and sequences are processed, the use of numerical letters, or other designations in the various elements of the application are disclosed is not intended to limit the order in which the processes and methods of the application are performed unless explicitly recited in the claims.

Furthermore, aspects of the present application may take the form of a computer product, comprising computer-readable program code, embodied in one or more computer-readable media.

The computer program code necessary for operation of portions of the present application may be written in any one or more programming languages, including an object oriented programming language such as Java, scala, smalltalk, eiffel, JADE, emerald, C ++, c#, vb net, python, etc., a conventional programming language such as C language, visual Basic, fortran 2003, perl, COBOL 2002, PHP, ABAP, dynamic programming languages such as Python, ruby and Groovy, or other programming languages, etc. The program code may execute entirely on the user's computer or as a stand-alone software package, partly on the user's computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user's computer through any form of network, such as a Local Area Network (LAN) or a Wide Area Network (WAN), or the connection may be made to an external computer (for example, through the Internet), or the use of services such as software as a service (SaaS) in a cloud computing environment.

The steps of a method or algorithm described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. An exemplary storage medium is coupled to the processor such the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. In the alternative, the processor and the storage medium may reside as discrete components in a user terminal.

The terms and expressions which have been employed above are used as terms of description and not of limitation. The use of these terms and expressions is not meant to exclude any equivalents of the features shown and described (or portions thereof), and it is recognized that various modifications are possible and are intended to be included within the scope of the claims. Other modifications, variations, and alternatives are also possible. Accordingly, the claims should be looked to in order to cover all such equivalents.

Likewise, it should be noted that in order to simplify the presentation disclosed herein and thereby aid in understanding one or more inventive embodiments, various features are sometimes grouped together in a single embodiment, figure, or description thereof. This method of disclosure, however, is not intended to imply that more features than are presented in the claims are required for the subject application.

Also, it should be noted that while the present invention has been described with reference to the particular embodiments presently, it will be appreciated by those skilled in the art that the above embodiments are provided for illustration only and that various equivalent changes or substitutions may be made without departing from the spirit of the invention, and therefore, the changes and modifications to the above embodiments shall fall within the scope of the claims of the present application as long as they are within the true spirit of the invention.

Claims (10)

1. A motor speed acquisition method, the method comprising:

a. providing an encoder whose output is a first motor angle θ _i ；

b. Providing a digital phase-locked loop, and setting the first motor angle theta _i Transmitting to the digital phase-locked loop;

c. determining the first motor angle θ _i A difference delta theta from the output of the digital phase locked loop;

d. calculating the motor rotation speed n according to the difference delta theta;

e. and calculating a second motor angle theta according to the motor rotating speed n, wherein the second motor angle theta is the output of the digital phase-locked loop.

2. The motor rotation speed obtaining method according to claim 1, wherein the first motor angle θ _i Including the true value theta _iN And measuring error theta _iC 。

3. The motor rotation speed acquisition method according to claim 2, wherein step d calculates the motor rotation speed n according to the following formula:

n＝kp(θ _i -θ)+ki*T*∑(θ _i -θ)

＝[kp(θ _iN -θ)+ki*T*∑(θi _N -θ)]+[kpθ _ic +ki*T*∑θ _ic ] (4)

wherein Kp is a proportional coefficient, ki is an integral coefficient, and T is a time interval during which the digital phase-locked loop performs computation.

4. A motor rotation speed obtaining method according to claim 3, wherein the first term kp (θ _iN -θ)+ki*T*∑(θ _iN - θ) is the true value of the motor speed, the second term kpθ _ic +ki*T*∑θ _ic Is the rotational speed error of the rotational speed of the motor.

5. The motor rotation speed acquisition method according to claim 1, characterized by further comprising:

c-e, circularly executing the steps until the second motor angle theta and the first motor angle theta _i Equal.

6. The motor rotation speed obtaining method according to claim 1, wherein the motor rotation speed n is an intermediate product necessarily produced in the process of outputting the second motor angle θ by the digital phase-locked loop.

7. The motor rotation speed acquisition method according to claim 1, wherein the second motor angle θ is calculated according to the following formula:

θ＝T*∑n

wherein T is a time interval during which the digital phase locked loop performs the computation.

8. The motor speed acquisition method according to claim 1, wherein step c is performed by a phase discrimination module of the digital phase locked loop.

9. The motor speed acquisition method according to claim 1, wherein step d is performed by a proportional-integral controller of the digital phase-locked loop.

10. The motor speed acquisition method according to claim 1, wherein step e is performed by an integral controller of the digital phase-locked loop.