Method and device for the indirect field-oriented control of a brushless DC motor

Abstract

The invention relates to a method and an electronic device for the indirect field-oriented control (FOC) of a brushless DC motor (1) using digital Hall sensors (3a-3c) for detecting the instantaneous rotor position of the DC motor (1) for the purpose of torque control, wherein an electrical revolution of the rotor (2) by a plurality (n) of digital Hall sensors (3a-3c) arranged equidistantly to one another in relation to the rotor circumference is divided into twice (2n) as many sectors, wherein each sector corresponds to a defined Hall pattern and a change in the Hall signal is interpreted as an exceeding of the limit of a sector caused by the rotation of the rotor (2) in order to apply an incrementally constant rotor angle (α) for determining the instantaneous rotor position for the field-oriented control.

Description

[0001] Method and device for indirect field-oriented control of a brushless DC motor

[0002] The present invention relates to a method and an electronic device for field-oriented control of a brushless DC motor using digital HALL sensors for detecting the current rotor position of the DC motor for the purpose of torque control.

[0003] The field of application of the invention extends in particular to power tools that are subject to a braking torque during use, such as an electric screwdriver when loosening a screw or a hammer drill when loosening a jammed drill bit. All of these power tools are subject to highly fluctuating loads during operation, unlike, for example, an electric hammer, which operates at a relatively constant speed and drive torque.

[0004] State of the art

[0005] The field-oriented control (FOC) of interest here is one of the most commonly used control strategies for driving brushless direct current (BLDC) motors, which is also used in the field of power tools. The well-known FOC improves the dynamic behavior of the drive and delivers optimal torque, especially for a uniform drive. The basic concept of FOC is the consideration of instantaneous values ​​over time. Thus, the alternating electrical quantities of a three-phase motor are not treated as spatially fixed, but rotate with the rotor. This means that the three-phase currents measured at the stator are converted into rotor coordinates. Since the reference system is static, the controller can operate with direct current quantities instead of alternating quantities.Unlike an alternative block commutation control, in which the commutation states are switched every 60 electrical degrees, the FOC continuously monitors the rotor position and constantly switches the excitation of the three stator windings so that an optimal rotor torque is generated.

[0006] There are basically two general methods of FOC: direct FOC and indirect FOC. Both methods differ in the way the rotor angle / position detection is determined. With direct FOC, the rotor angle is calculated using the terminal voltage and currents, while with indirect FOC, which is of interest in the context of the present invention, the rotor position is measured. Thus, additional position detection is required, which is the subject of the present invention.

[0007] A well-known method for measuring rotor position is based on a position sensor, such as an anisotropic magnetoresistive (AMR) sensor, resolver, or encoder. Despite the high measurement accuracy of an AMR sensor, it is quite complex and requires a relatively large amount of additional space within a BLDC motor.

[0008] For power tools of the type of interest here, where the compactness of the tool housing plays an important role, the use of an AMR sensor is therefore unfavorable.

[0009] Alternatively, the digital HALL sensors used in the present invention can also be used to discretely detect the rotor position. Three digital HALL sensors typically measure the rotor position with an accuracy of 60 electrical degrees.

[0010] Attempts have already been made to use digital Hall sensors to calculate the motor speed in the electronic control unit based on the frequency of the Hall interrupts and to estimate the continuous rotor position, which is then fed into the FOC algorithm. A phase-locked loop (PLL) technique can be used to linearize the angle of the signal from the Hall sensors, which appears as a stepped waveform. The PLL technique is usually implemented in the electronic control unit's software to obtain an estimated continuous rotor angle as a function of the pulse-wide modulation (PWM) frequency. The PLL technique receives digital Hall interrupts as input and estimates the quasi-continuous rotor angles, which are used as the FOC input.The rotor angle estimated in this way is still quantized, but the significantly higher resolution compared to the otherwise step-like waveform results in a quasi-continuous rotor angle.

[0011] This method works particularly well at high speeds, but is less effective at low speeds, for example when the DC motor starts up under heavy load, as occurs with an electric screwdriver loosening a screw or a hammer drill loosening jammed drill bits, etc. This is because during motor start-up, the information available for a reliable estimation of the rotor angle using the PLL method is very limited, as the number of incoming HALL interrupts is very small. Estimating the rotor position requires a precise calculation of the motor speed, and the incoming HALL interrupts are not enough to achieve sufficient accuracy. The inaccurate speed estimation ultimately leads to an unreliable estimation of the rotor position. When the motor starts up under load, it is a very dynamic process, and the motor speed can fluctuate greatly.This also leads to unreliable estimates of the rotor position.

[0012] If an inaccurate rotor angle / position is input into an FOC algorithm, the magnetic field generated by the coil's excitation cannot efficiently build motor torque. If the DC motor fails to build sufficient torque within the first few revolutions, it will not start.

[0013] It is therefore the object of the present invention to enable the implementation of an indirect FOC in a brushless DC motor using digital HALL sensors to detect the current rotor position in order to create an optimal drive for electrical machines with high starting torque and dynamic speed behavior. Disclosure of the Invention

[0014] The problem is solved in terms of process technology by claim 1. The subordinate claim 10 describes a corresponding electronic device for carrying out the method. Claim 11 is directed to an electric hand tool with such an electronic device.

[0015] The invention includes the process engineering teaching that an electrical revolution of the rotor of a BLDC motor is divided into twice as many sectors by several digital Hall sensors arranged equidistant from one another with respect to the rotor circumference, wherein each sector corresponds to a defined Hall pattern and a change in the Hall signal is interpreted as an exceeding of the boundary of a sector caused by the rotation of the rotor in order to apply a stepwise constant rotor angle a to determine the current rotor position for the field-oriented control.

[0016] In other words, the inventive solution estimates the rotor position, particularly during the first few rotor revolutions from standstill, when the motor speed cannot be accurately estimated due to a lack of information and the dynamic load torque. By feeding the FOC algorithm with the stepwise constant rotor angle signal obtained according to the invention as an input value, the FOC becomes more reliable than with a conventionally inaccurate estimate of the motor speed and thus a correspondingly inaccurate estimate of the rotor position.

[0017] Electric hand tools in which the solution according to the invention is implemented thereby achieve a starting torque that is at least as high as that of a conventional square-wave control and, at high speeds, exhibit a comparably high efficiency as conventional field-oriented controlled DC motors in which the rotor position is measured with a high-resolution encoder. According to a preferred embodiment, the selection of the stepwise constant rotor angle can be optimized such that the averaged motor torque of a sector is highest and the torque ripple is lowest when switching from one sector to another.

[0018] According to a preferred embodiment of the invention, one revolution of the rotor is divided into six sectors of 60° each using three digital HALL sensors, each sector corresponding to a defined HALL pattern, preferably with the following binary values: 101, 001, 011, 010, 110, 100. The change in the HALL signal indicates that the rotor exceeds the boundary of a sector.

[0019] Preferably, the current rotor position determined in the manner according to the invention is used in the control of the brushless DC motor in a low speed range between 0 to 15% of an idle speed in order to achieve a functional field-oriented control during the start-up phase of, for example, an electric screwdriver.

[0020] Preferably, the brushless DC motor is operated with a FOC across its entire speed range, with control in the low speed range based on the gradually constant rotor angle and in the subsequent higher speed range based on a conventionally estimated continuous rotor angle. This allows the DC motor to achieve the same or even higher starting torque from standstill than with a conventional control system. Furthermore, the high efficiency at high speeds is utilized when the FOC receives the precise rotor position and performs optimal control.

[0021] The transition between control based on a stepwise constant rotor angle and an estimated continuous rotor angle is preferably carried out taking into account a hysteresis to avoid unwanted switching back and forth in the transition range. Detailed description based on drawing

[0022] Further measures improving the invention are described in more detail below, together with the description of a preferred embodiment of the invention, with reference to the figures. It shows:

[0023] Fig. 1 is a schematic representation of components of a brushless DC motor with several Hall sensors,

[0024] Fig. 2 a graphical representation of sectors of one revolution of the rotor with associated binary values ​​of a defined Hall pattern with three Hall sensors,

[0025] Fig. 3 is a graphical representation of the course of a stepwise constant rotor angle a specified as a result of the method according to the invention in connection with the associated phase currents and the motor torque of a brushless DC motor controlled by this,

[0026] Fig. 4 is a graphical representation of a transition between a discrete rotor angle and a continuous rotor angle as hysteresis-affected FOC input values,

[0027] Fig. 5 is a graphical representation for determining a switching point from an initial stepwise constant rotor angle to a subsequent estimated continuous rotor angle, and

[0028] Fig. 6 is a block diagram illustrating the operation of the electronic device for implementing the method according to the invention. According to Fig. 1, the brushless DC motor 1, shown here in highly schematic form, comprises three digital HALL sensors 3a to 3c arranged equidistant from one another with respect to the circumference of a rotor 2. The three digital HALL sensors 3a to 3c are known to divide one electrical revolution of the rotor 2 into twice as many, i.e., six sectors of 60° each. The rotor 2 comprises a permanent magnet with a south pole S and a north pole N. Electrical coil windings A, B, and C are also arranged on the stator side, circumferentially equidistant from one another.

[0029] According to Fig. 2, the configuration described above results in the illustrated HALL pattern over a 360° rotation. The binary number 1 represents the north pole, whereas the binary number 2 represents the south pole of the permanent magnet. The signals from the three digital HALL sensors 3a to 3c in each of the six 60° sectors result in the illustrated HALL pattern 101, 001, 011, 010, 110, 100. This HALL pattern is transmitted as a high or low voltage level to the electronic device for indirect field-oriented control. The HALL pattern defined in this way consists of (angular) sectors with an opening angle of 60°, in which the rotor of the brushless DC motor is currently located, allowing the electronic device to energize the corresponding motor coils to build up torque.

[0030] Due to the presence of only three HALL sensors, this HALL system only resolves the angle in 60° increments. Each of the six sectors thus corresponds to a defined HALL pattern, and according to the invention, a change in the HALL signal is interpreted as a sector boundary being exceeded due to the rotation of the rotor, in order to apply a gradually constant rotor angle to determine the current rotor position for field-oriented control (FOC).

[0031] According to Fig. 3, the said stepwise constant rotor angle a is shown in the upper area over a starting time considered from the start, in connection with the phase currents IA, IB and lc shown in the middle area of ​​the figure as well as the curve of the motor torque MM shown in the lower area. From this it can be seen that even the curve of the rotor angle quantized to 60° steps and currents calculated according to the rules of the FOC generate a sufficiently large torque.

[0032] According to Fig. 4, the transition between an initial control with a stepwise constant rotor angle a and an estimated continuous rotor angle a', which is already linearized from a PLL algorithm, which also works correctly due to the higher speed, is carried out taking into account a shown hysteresis with regard to the motor speed n in order to avoid unwanted switching back and forth in this transition area of ​​different control strategies.

[0033] With reference to Fig. 5, the rotor angle a, a' is smoothed for several revolutions from 0° to 360° from the start according to different criteria. According to the invention, the brushless DC motor is operated across its entire speed range with field-oriented control, whereby in the low speed range - here purely schematically the first 1.5 revolutions - control is carried out according to the stepwise constant rotor angle a, and in the higher speed range according to a conventionally estimated continuous rotor angle a'. The estimated continuous rotor angle a' is determined by applying a PLL method for linearizing a stepped measured value curve of the current motor position detected by the digital HALL sensors.At a switching point S, the system switches from an FOC with a stepwise constant rotor angle a to an FOC with an estimated continuous rotor angle a' in order to change the control strategies after the rotor has started up.

[0034] According to Fig. 6, an electronic device designed according to the invention for carrying out the method described above uses a control unit 10 with various control strategies, between which it is possible to switch using a switch S depending on the speed and load of the brushless DC motor. These include the field-oriented control according to the invention in accordance with a stepwise constant rotor angle I for the lower speed range, a field-oriented control (FOC) in accordance with a conventionally estimated continuous rotor angle according to a second control strategy II, and a control strategy III which linearizes the angle using a PLL algorithm. In the smallest speed range (approximately 0 to 5% of the idle speed), strategy I is used; in range II (5 to 15% of the idle speed), the Hall signals are linearly extrapolated and in the third range they are linearized via a PLL.

Claims

Patent claims 1. Method for indirect field-oriented control (FOC) of a brushless DC motor (1) using digital Hall sensors (3a - 3c) to detect the current rotor position of the DC motor (1) for the purpose of torque control, wherein one electrical revolution of the rotor (2) is divided into twice (2n) as many sectors by a plurality (n) of digital Hall sensors (3a - 3c) arranged equidistant from one another with respect to the rotor circumference, characterized in that each sector corresponds to a defined Hall pattern and a change in the Hall signal is interpreted as an exceeding of the boundary of a sector caused by the rotation of the rotor (2) in order to apply a stepwise constant rotor angle (a) to determine the current rotor position for the field-oriented control.

2. Method according to claim 1, characterized in that the determination of the stepwise constant rotor angle (a) for a sector takes place at a maximum of the averaged motor torque of a sector in conjunction with a minimum of the torque ripple when switching from one sector to the other sector.

3. Method according to claim 1, characterized in that exactly three digital Hall sensors (3a - 3c) are used to determine the stepwise constant rotor angle (a) for subdivision into six sectors.

4. Method according to claim 3, characterized in that the six sectors correspond to a defined Hall pattern with the following binary values: 101, 001, 011, 010, 110, 100.

5. Method according to claim 1, characterized in that the current rotor position determined therefrom is used in the control of the brushless DC motor (1) in a low speed range between 0 to 15% of an idle speed.

6. Method according to claim 5, characterized in that the brushless DC motor (1) is operated over its entire speed range with a field-oriented control (FOC), wherein in the low speed range control is carried out in accordance with the stepwise constant rotor angle (a) and in the higher speed range control is carried out in accordance with a conventionally estimated continuous rotor angle (a').

7. Method according to claim 6, characterized in that the transition between a control according to a stepwise constant rotor angle (a) and an estimated continuous rotor angle (a') is carried out taking into account a hysteresis in order to avoid unwanted switching back and forth in the transition range.

8. Method according to claim 6, characterized in that the estimated continuous rotor angle (a') is determined by applying a PLL method for linearizing a graduated measured value curve of the current rotor position detected by the digital Hall sensors (3a - 3c).

9. Method according to claim 6, characterized in that a switching point (S) from an FOC according to the stepwise constant rotor angle (a) to an FOC according to the estimated continuous rotor angle (a') is defined.

10. Electronic device for carrying out the method according to one of the preceding claims, comprising digital Hall sensors (3a - 3c) for detecting the current rotor position of the DC motor (1), wherein the digital Hall sensors (3a - 3c) are arranged such that one electrical revolution of the rotor (2) is divided into twice (2n) as many sectors by a plurality (n) of digital Hall sensors () arranged equidistant from one another with respect to the rotor circumference, characterized in that an electronic control unit (10) is provided which is designed to assign a defined Hall pattern to each sector and to interpret a change in the Hall signal as an exceeding of the boundary of a sector caused by the rotation of the rotor (2) in order to use a stepwise constant rotor angle (α) for a sector as the current rotor position for the field-oriented control.

11. Electric hand tool with a drive unit designed as a brushless DC motor, comprising an electronic device according to claim 10.

12. Electric hand tool according to claim 11, wherein it is designed as an electric screwdriver.

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