Operating method for a BLDC motor of an electrical machine tool

Abstract

The invention relates to a method for operating a drive unit of a machine tool, wherein the drive unit is supplied by a mains voltage and the drive unit has a brushless DC motor and an inverter upstream of the DC motor, and wherein during the method a back-emf voltage, which is induced in a respective stator winding of the brushless DC motor, is determined, wherein the inverter is operated with field-oriented control and the stator current setpoint of the control is amplitude-modulated.

Description

[0001] Operating procedure for a BLDC motor of an electric machine tool

[0002] The present invention relates to a method for operating a drive unit of a machine tool, wherein the drive unit is supplied by a mains voltage and comprises a brushless DC motor (BLDC motor) and an inverter connected upstream of the DC motor. Within the scope of the method, a back EMF voltage induced in a respective stator winding of the brushless DC motor is determined.

[0003] Such methods are basically known from the state of the art.

[0004] It is an object of the present invention to provide a method for operating a drive unit of a machine tool which creates the basis for reduced switching losses and for improved EMC behavior.

[0005] The task is solved by operating the inverter with field-oriented control, whose stator current setpoint is amplitude-modulated. This can be generated depending on the line voltage curve, the back EMF voltage, and the manipulated variable of a speed controller.

[0006] It has proven advantageous if the brushless DC motor is controlled in a field-oriented manner within the scope of the method. In a particularly preferred embodiment, the drive unit has a slim DC link connected upstream of the DC motor. A slim DC link is understood, in particular, to be an DC link with a comparatively low storage capacity. This can be achieved using compact film capacitors and / or small electrolytic capacitors with low capacitance. In addition, the motor current amplitude can be modeled to result in an approximately sinusoidal mains current waveform with a high power factor.

[0007] The invention incorporates the finding that operating a BLDC motor with block commutation—as is the case in the prior art—typically generates current harmonics in the inverter input current at six times the electrical stator frequency and integer multiples thereof. These inverter harmonics interact with the grid impedance and excite the RLC resonant circuit, consisting of the grid internal resistance, grid inductance, and intermediate circuit capacitance of a lean DC electronics (lean intermediate circuit). If six times the electrical motor rotation frequency coincides with the resonant frequency of the RLC resonant circuit, the harmonic amplitudes of the inverter input current are greatly amplified and become visible in the grid input current of the lean DC electronics. These oscillations cause the power factor to become very poor, and can drop to a level of approximately A=0.7.

[0008] The method according to the invention creates the basis for reduced switching losses and control with low current harmonics for significantly higher power factors. This is especially true when the brushless DC motor is controlled in a field-oriented manner.

[0009] In a particularly preferred embodiment, the inverter is controlled in control mode based on a setpoint value that takes into account both the mains voltage and the back EMF voltage. It has proven advantageous if, as part of the setpoint determination, the back EMF voltage is subtracted from an amount of the mains voltage, thus obtaining a first differential voltage. In a particularly preferred embodiment, the first differential voltage is limited to zero for its negative voltage values.

[0010] In a further preferred embodiment, it is provided that, as part of the setpoint determination, the back EMF voltage is subtracted from a mains peak voltage of the mains voltage, thus obtaining a second differential voltage. It has proven advantageous if the first differential voltage is divided by the second differential voltage, and the quotient thus obtained is preferably normalized to an amplitude value of one. In a further preferred embodiment, the quotient, in particular the normalized quotient, is multiplied by a speed control variable originating from a speed controller of the drive unit, thus generating a stator current setpoint.

[0011] In other words, the invention and its embodiments are based on the concept of operating the BLDC motor using field-oriented control in order to significantly reduce fluctuations in the instantaneous power of the motor and, consequently, fluctuations or current harmonics of the inverter input current. Furthermore, the setpoint of the motor current control can be generated such that the motor current is actively driven to zero when the time interval at the mains zero crossing is reached. The width of the time interval at the mains zero crossing, at which no motor current flows (switch-off mode), is advantageously defined by the induced voltage of the brushless motor and the intersection point of the instantaneous value of the mains voltage. The advantage of the setpoint generation described above is that even distorted, non-sinusoidal input voltages are represented in the motor current curve or, as a result, in the input current curve.This allows a good power factor to be achieved even with distorted, non-sinusoidal input voltages. In a particularly preferred embodiment, the method is free of block commutation of the BLDC motor.

[0012] According to a further advantageous embodiment, it may be possible for the inverter to be controlled in a switch-off mode during a time interval in which the mains voltage is less than or equal to the back EMF voltage, and otherwise in a control mode. A switch-off mode is understood in particular to be a mode in which the inverter is switched off (i.e. in particular floating / high-impedance). This preferably applies to all commutation blocks of the inverter. It has proven advantageous if the inverter control signals are set to zero in the switch-off mode. A control mode is understood in particular to be a mode in which the inverter is controlled by a current controller, in particular in a field-oriented manner.

[0013] The problem is also solved by a drive unit for a machine tool, wherein the drive unit is or can be supplied by a mains voltage and the drive unit has a brushless DC motor and an inverter connected upstream of the DC motor, and wherein the drive unit is designed to determine a back EMF voltage induced in a respective stator winding of the brushless DC motor. The drive unit is designed to control the DC motor in braking mode based on a motor current amplitude and a reference variable that is dependent on an intermediate circuit voltage of the intermediate circuit.

[0014] The drive unit is preferably part of an electric power tool, in particular a handheld power tool. The drive unit and the power tool can be further developed in a corresponding manner using the features described with reference to the method.

[0015] Further advantages will become apparent from the following description of the figures. Particularly preferred embodiments of the present invention are illustrated in the figures. The figures, the description, and the claims contain numerous features in combination. Those skilled in the art will expediently consider the features individually and combine them into useful further combinations.

[0016] In the figures, identical and similar components are numbered with the same reference numerals.

[0017] It shows:

[0018] Fig. 1 is a block diagram for field-oriented control of a BLDC motor;

[0019] Fig. 2 shows a block diagram for generating the stator current setpoint; Fig. 3 shows exemplary curves of various intermediate signals during setpoint generation according to the block diagram of Fig. 2; and

[0020] Fig. 4 Example curves of mains voltage, motor currents and mains current during field-oriented control of the BLDC motor.

[0021] Examples of implementation:

[0022] A preferred embodiment of the method is described below using a field-oriented motor control system with a rotor-fixed d / q coordinate system (Park transformation). Figure 1 shows a block diagram 100 with which a drive unit (not shown in detail here) can be controlled, which comprises a brushless direct current motor (BLDC motor), an inverter, and an intermediate circuit connected upstream of the inverter.

[0023] The block diagram 100 has a switching block 10, whose six output signals are fed to an inverter (not shown here). The switching block 10 can be operated in a switch-off mode ASM ("upper" path) and in a control mode ATM ("lower path"). In the switch-off mode ASM, all inverter control signals are set to zero. The switch-off mode ASM is active during the time interval in which the mains voltage u Netz less than or equal to the back EMF voltage u EMK is.

[0024] Otherwise, ie the mains voltage u Netz is greater than the back EMF voltage u EMK, the ATM control mode is in effect, whereby the inverter (not shown here) is controlled in a field-oriented manner by the current controllers 21, 23. For this purpose, the block diagram 100 further includes a PWM block 20 connected upstream of the switching block 10 for pulse-width modulated control of the BLDC motor. In the ATM control mode ("lower path"), the signals from the PWM block 20 are passed to the inverter via the switching block 10.

[0025] The input variables for PWM block 20 are provided by the aforementioned d-current controller 21 and the q-current controller 23. Connected upstream of the d-current controller 21 and the q-current controller 23 is the MTPA (Maximum Torque per Ampere) block 30, which outputs a d-current setpoint and a q-current setpoint. The MTPA block 30, in turn, receives its setpoint current from a calculation block 40. The input variable for the calculation block 40 is a speed controller 50.

[0026] A block diagram 200 for generating the stator current setpoint i S rcis shown in Fig. 2. The block diagram 200 represents, by way of example, the calculation block 40 of Fig. 1. The task of the calculation block 40 is to modulate the motor current in such a way that the inverter input current approximately follows the course of the mains voltage in order to obtain a good power factor, on the other hand, the motor current should also be guided to the value zero when the mains voltage u Netz the back EMF voltage u EMK falls below.

[0027] First, the mains voltage u Netz through a magnitude block 201 into a rectified mains voltage |ujv etz | From this rectified mains voltage |ujv etz | the back EMF voltage u is calculated by means of a first subtraction block 203 EMK and their negative values ​​are limited to zero by a limiter block 205. From a network peak voltage ü detected by a peak detection block 207 Netz the EMF voltage uEMK subtracted. This is done by a second subtraction block 209. The subsequent division by a division block 211 generates the waveform of a current reference signal, which is normalized to an amplitude of one. The stator current setpoint i S re / , which is also the output of the calculation block 40 shown in Fig. 1, is finally obtained by multiplying the output of the division block 211 by the desired amplitude i s This is achieved by means of the multiplication block 213. The desired amplitude l s , which is the manipulated variable of the speed controller 50 shown in Fig. 1, changes only slowly with respect to the mains voltage u Netz , so that the line voltage curve of i S rc / is not distorted.

[0028] With the block diagram shown in Fig. 2, the setpoint of the motor current control can advantageously be generated in such a way that the motor current is actively driven to zero when the time interval at the mains zero crossing is reached.

[0029] Exemplary time courses of various intermediate signals during setpoint generation are shown in Fig. 3. The numbers shown to the left of the diagrams correspond to the measuring points 1 to 6 shown in Fig. 2 (each circled).

[0030] Diagram 1 shows the time course of the rectified mains voltage |uj etz |. Diagram 2 shows the back EMF voltage u EMK . Diagram 3 shows the difference between mains voltage luivetzl un d back EMF voltage u EMK, whereby this difference excludes negative values ​​(see Fig. 2 limiter block 205). Diagram 4 shows the quotient of the first differential voltage and the second differential voltage normalized to the value one. Diagram 5 shows the manipulated variable l s , which comes from the speed controller 50 shown in Fig. 1. Diagram 6 finally shows the time course of the stator current setpoint which is the output variable of the calculation block 40 shown in Fig. 1.

[0031] Fig. 4 shows typical line voltage curves (diagram A), motor phase currents (diagram B), and line current curves (diagram c) resulting from the exemplary embodiment of the invention. In the example shown in Fig. 5, a power factor of A=0.97 is advantageously achieved. 10 Switching block

[0032] 20 PWM block

[0033] 21 d-current regulator

[0034] 23 q-current controller

[0035] 30 MTPA block

[0036] 40 Calculation block

[0037] 50 speed controllers

[0038] 100 Block diagram for field-oriented motor control

[0039] 200 Block diagram for block-commutated motor control

[0040] 201 Amount block

[0041] 203 first subtraction block

[0042] 205 limiter block

[0043] 207 Vertex detection block

[0044] 209 second subtraction block

[0045] 211 Division Block

[0046] 213 Multiplication block i s Control variable from the speed controller i s> ref Stator current setpoint u EMK Back EMF voltage u Netz Mains voltage fi Netz Grid peak voltage

Claims

Patent claims Method for operating a drive unit of a machine tool, wherein the drive unit is supplied by a mains voltage and the drive unit has a brushless DC motor and an inverter connected upstream of the DC motor, and wherein, within the scope of the method, a back EMF voltage induced in a respective stator winding of the brushless DC motor is determined, characterized in that the inverter is operated with field-oriented control and its stator current setpoint is amplitude-modulated. Method according to claim 1, characterized in that the inverter is controlled in control mode on the basis of a setpoint that takes both the mains voltage and the back EMF voltage into account. Method according to claim 2, characterized in that, within the scope of the setpoint determination, the back EMF voltage is subtracted from an amount of the mains voltage and a first differential voltage is thus obtained.Method according to claim 3, characterized in that the first differential voltage is limited to zero for its negative voltage values. Method according to claim 3 or 4, characterized in that, within the scope of the setpoint determination, the back EMF voltage is subtracted from a network peak voltage of the network voltage, thus obtaining a second differential voltage. Method according to claim 5, characterized in that, within the scope of the setpoint determination, the first differential voltage is divided by the second differential voltage, and the quotient thus obtained is preferably normalized to one. Method according to claim 6, characterized in that, within the scope of the setpoint determination, the quotient, in particular the standardized quotient, is multiplied by a speed control variable originating from a speed controller of the drive unit, thus generating a stator current setpoint. Method according to claim 7, characterized in that, within the scope of the method, the brushless DC motor is controlled in a field-oriented manner based on the stator current setpoint. Method according to claim 8, characterized in that, within the scope of the method, the stator current setpoint is passed through an MTPA block. Method according to at least one of the preceding claims, characterized in that the drive unit has a slim intermediate circuit connected upstream of the DC motor.Method according to at least one of the preceding claims, characterized in that the inverter is controlled in a switch-off mode during a time interval in which the mains voltage is less than or equal to the back EMF voltage, and otherwise in a control mode. Drive unit for a machine tool, wherein the drive unit is or can be supplied by a mains voltage and the drive unit has a brushless DC motor and an inverter connected upstream of the DC motor, and wherein the drive unit is designed to determine a back EMF voltage that is induced in a respective stator winding of the brushless DC motor, characterized in that the drive unit is designed to control the DC motor in braking mode based on a motor current amplitude and a reference variable that is dependent on an intermediate circuit voltage of the intermediate circuit.

13. Machine tool, in particular hand-held machine tool with a drive unit according to claim 12.

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