Operating method for a brushless direct current motor of an electric power tool

Abstract

The invention relates to a method for operating a drive unit of a power tool, the drive unit comprising a brushless direct current motor and a DC link connected upstream of the direct current motor, wherein: in a working mode, the direct current motor is controlled by a speed controller and is operated with a nominal speed; the method has a braking mode, in which the direct current motor is braked to a braking speed lower than the nominal speed; in the braking mode, the direct current motor is controlled with a reference variable which is based on a DC link voltage of the DC link.

Description

[0001] Operating method for a brushless direct current 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 comprises a brushless direct current (BLDC) motor and an intermediate circuit connected upstream of the BLDC motor. In a working mode, the BLDC motor is controlled by a speed controller and operated at a nominal speed. The method further comprises a braking mode in which the BLDC motor is braked to a braking speed lower than the nominal speed.

[0003] Such methods are generally known from the state of the art. To quickly decelerate a BLDC motor to a standstill, the rotational energy stored in the BLDC motor and the overall driven mechanical system must be extracted or converted. Feeding the rotational energy back into the electrical power source is often technically impossible, or the regenerative energy is limited in instantaneous power and duration, making it impossible to achieve the typically required rapid deceleration time.

[0004] In what is known as short-circuit braking, the BLDC motor is short-circuited at its terminals. This can be achieved by switching on all of the motor inverter's low-side power semiconductors simultaneously. This creates an initially high short-circuit current, which then decays to a continuous short-circuit current. The short-circuit current generates a braking torque in the motor. Another state-of-the-art braking mode can be achieved using a brake chopper with a braking resistor. In this case, the BLDC motor is braked in a controlled manner and thus becomes a generator. The energy released is fed back into the DC link, causing the DC link voltage to rise. The brake chopper with a braking resistor periodically switches a resistor to the DC link via a power semiconductor switch, thus converting the energy in the braking resistor into heat.

[0005] The object of the present invention is to provide a method for operating a drive unit of a machine tool that creates the basis for a simple and fast braking mode. This object is achieved by controlling the BLDC motor in braking mode with a reference variable that is based on an intermediate circuit voltage of the intermediate circuit. It has proven advantageous if the BLDC motor is controlled in braking mode with the reference variable in such a way that the generator power fed back into the intermediate circuit when the BLDC motor is braking is equal to the ohmic power loss converted in the BLDC motor.

[0006] The invention incorporates the insight that, with the short-circuit braking mentioned above, typically no influence can be exerted on the braking torque or braking time. The magnitude of the short-circuit current is determined, in particular, solely by motor parameters. Furthermore, the electronics must be able to handle the initially higher short-circuit current. The prior art solution based on a brake chopper with a braking resistor does offer more degrees of freedom with regard to achieving a desired braking torque or braking time. However, additional hardware components (MOSFET / IGBT, gate driver, a discrete braking resistor, connecting leads) are required, which require more installation space and incur additional costs. The braking resistor, in particular, must be specified for high pulse power and contributes significantly to installation space and costs.

[0007] In contrast to this, the method according to the invention is based on the idea of ​​converting the braking energy into ohmic losses in the BLDC motor itself in braking mode.

[0008] The key advantage of the invention is that it promotes rapid braking with optimal utilization of the power electronics / inverter without the need for additional components. The desired braking behavior can be precisely defined using parameters in the described braking process. The minimum possible braking time depends on the current-carrying capacity of the power electronics inverter; however, faster braking is easily achieved by scaling the inverter to higher currents.

[0009] In a particularly preferred embodiment, a controller is used in the braking mode, the control input variable of which is a control deviation of the intermediate circuit voltage and / or the control output variable of which is the reference variable.

[0010] In a further particularly preferred embodiment, the brushless DC motor is controlled by vector control, and the reference variable represents a current space vector angle. In another particularly preferred embodiment, the brushless DC motor is controlled by block commutation, and the reference variable represents a commutation angle. It has proven advantageous if the motor current amplitude in braking mode is constant up to a lower braking speed threshold that is greater than zero. The lower braking speed threshold can, for example, be 100 rpm. The motor current amplitude in braking mode can advantageously correspond to a current-carrying capacity of an inverter assigned to the DC motor.

[0011] In a further particularly preferred embodiment, the control of the DC motor in braking mode is carried out with the reference variable in such a way that a generator power fed back into the intermediate circuit when braking the DC motor is equal to an ohmic power loss converted in the DC motor.

[0012] It has proven advantageous to use a switching block for switching between working mode and braking mode within the method. In another particularly preferred embodiment, the intermediate circuit voltage is determined based on the voltages induced in the stator windings of the brushless DC motor, which are caused by the counter-electromotive force. The intermediate circuit voltage can also be measured directly.

[0013] It has been found to be advantageous if the brushless DC motor is braked completely in braking mode, i.e. to 0 speed.

[0014] The problem is also solved by a drive unit for a machine tool, wherein the drive unit has a brushless DC motor and an intermediate circuit connected upstream of the DC motor, wherein the drive unit is designed to control the DC motor in a working mode by means of a speed controller and to be able to operate it at a nominal speed, and the drive unit is designed to be able to operate the DC motor in a braking mode in which the DC motor is braked to a braking speed that is lower than the nominal speed. According to the invention, the drive unit is designed to control the DC motor in the braking mode based on a motor current amplitude and a reference variable that is dependent on an intermediate circuit voltage of the intermediate circuit.It has proven advantageous to use a controller in braking mode whose control input variable is a control deviation of the intermediate circuit voltage and / or whose control output variable is the reference variable.

[0015] The drive unit is preferably part of a machine tool, in particular a handheld power tool. 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 Motor characteristics of a BLDC motor in the rotor-fixed d / q coordinate system; Fig. 3 a temporal course of the current space vector angle in the method according to Fig. 1; and

[0020] Fig. 4 shows a block diagram for block-commutated control of a BLDC motor.

[0021] Example:

[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). Fig. 1 shows a block diagram 100 with which a drive unit (not shown here) can be controlled, which has a brushless direct current motor (BLDC motor) and an intermediate circuit connected upstream of the BLDC motor.

[0023] Block diagram 100 includes a PWM block 10 for pulse-width modulated control of the BLDC motor. Input variables for PWM block 10 are provided by a d-current controller 11 and a q-current controller 13. A common switching block 20 for switching between a working mode and a braking mode of the process is connected upstream of the d-current controller 11 and the q-current controller 13.

[0024] An MTPA (Maximum Torque per Ampere) block 30 is connected upstream of the switching block 20 in an "upper" path, which outputs a d-set current and a q-set current. The MTPA block 30, in turn, receives its set current from a speed controller 40. This path represents the operating mode in which the BLDC motor is controlled by the speed controller 40 and operated at a nominal speed. The nominal speed can be specified, for example, by a control knob on a machine tool.

[0025] Block diagram 100 further includes a "lower" path, which represents a preferred variant of the braking mode. In braking mode, the BLDC motor is braked to a braking speed lower than the rated speed. In this path, a calculation block 40 is connected upstream of the switching block 20, which outputs a d-set current and a q-set current to the switching block 20.

[0026] The calculation block 50 has as input variables a motor current amplitude Is (cf. Fig. 2 magnitude of the stator current space vector SRZ) and a reference variable ßbrems, which specifies the displacement of the stator current space vector relative to the d-axis (cf. Fig. 2). The reference variable ßbrems is the control output variable of a current space vector angle controller 60. The reference variable ßbrems can be limited downwards and / or upwards in a limiter of the current space vector angle controller 60. The control input variable of the current space vector angle controller 60, which is designed as a PI controller, for example, is a control deviation between the specified target intermediate circuit voltage UDC,SOII and an actual intermediate circuit voltage UDO,ist of the intermediate circuit connected upstream of the BLDC motor (not shown here). The reference variable ßbrems is thus dependent on an intermediate circuit voltage of the intermediate circuit in a controlled manner.The motor current amplitude Is as input variable for the calculation block 50 is constant in the braking mode in the embodiment shown here and corresponds, for example, to a current carrying capacity of an inverter assigned to the BLDC motor.

[0027] The aforementioned switching block 20 thus determines whether the d-current controller 11 and the q-current controller 13 receive their setpoints from the speed controller 40 via MTPA block 30 or whether the setpoints are provided by the braking process represented in the "lower" path. A switching request signal for the switching block 20 can be triggered, for example, by "releasing" a control button on a machine tool.

[0028] To further explain the process, Fig. 2 shows motor characteristics of an exemplary BLDC motor in a rotor-fixed d / q coordinate system with four quadrants I to IV. (The rotor angle required for the transformation can be determined, for example, using Hall sensors or by evaluating the EMF forces on the stator windings.) In the first quadrant I, the d-current and the q-current are positive. In the third quadrant III, the d-current and the q-current are negative. In the second quadrant II, the d-current is negative and the q-current is positive. In the fourth quadrant IV, the d-current is positive and the q-current is negative.

[0029] The stator current space vector SRZ has the motor current amplitude Is as its magnitude. The current space vector angle ßbrems indicates the displacement of the stator current space vector relative to the d-axis. The current space vector angle ßbrems corresponds to the reference variable ßbrems in Fig. 1. The current space vector angle ß bremsis calculated by a controller, for example a PI controller, whose control input variable is the intermediate circuit voltage u DC is as shown in Fig. 1.

[0030] As already mentioned, the invention is based on the concept of converting the braking energy into ohmic losses in the motor itself. For this purpose, the ohmic losses in the BLDC motor (P v = R • i 2 ) advantageously in balance with the power fed back by the BLDC motor during braking (P M = M brems • co mech ) to prevent an uncontrolled increase in the DC link voltage. The braking torque of the motor M bremscan be adjusted by changing the current space vector angle ßbrems while maintaining a constant motor current amplitude Is. The motor current amplitude Is is selected in braking mode so that the current measuring ranges and the current carrying capacity of the inverter are not exceeded. The inverter's current limit SLI is shown as a circle in Fig. 2. The motor current amplitude Is is preferably kept constant to maximize the inverter's performance and achieve the shortest braking time.

[0031] An angle ß brems = 0 corresponds to a stator current space vector lying on the positive d-axis. With increasing current space vector angle ß brems This results in a negative q-current component and a simultaneously decreasing positive d-current. It should be noted that the current space vector angle ß bremsshould not fall below a minimum value, as this would cause the generator braking mode to transition to motor mode. This limit depends on the BLDC motor used. The control variable of the current controller must therefore be limited to a minimum permissible value ß bre ms,min, which is approximately 47° for example (see Fig.3). At ßbrems = 90° the quadrant is finally changed until the MTPA (Maximum Torque per Ampere) operating point is reached with negative d- and q-current.

[0032] In Fig. 2, the time at which the switch from working mode to braking mode occurs is denoted by ti. The current space vector angle ß brems is approximately 57° (see Fig.3). During the braking process, the current space vector angle ß moves brems in its operating range towards larger angles up to a time t2 with low braking speed. The current space vector angle ß bremsis now approximately 117°. At such a low braking speed, the ohmic losses can exceed the generator-generated power, so that the difference would have to be covered by the voltage source. If no power is to be drawn from the voltage source, the motor current amplitude Is of the stator current space vector SRZ can optionally be shortened along the MTPA characteristic curve. At time ts, the BLDC motor is immediately before its standstill. A typical braking process can, for example, have the temporal progression of the current space vector angle ß shown in Fig. 3. brems During the braking process, the angular frequency of the BLDC motor is reduced, and accordingly the angular frequency of the rotor-fixed d / q coordinate system, in which the BLDC motor is controlled by means of the stator current space vector SRZ.

[0033] Fig. 4 shows another embodiment of the method using block commutation. Fig. 4 shows a block diagram 200 with which a drive unit (not shown here) can be controlled, which has a brushless direct current motor (BLDC motor) and an intermediate circuit connected upstream of the BLDC motor.

[0034] Block diagram 200 includes a PWM block 10 for pulse-width modulated control of the BLDC motor. Input variables for PWM block 10 are provided by a current controller 15 and an angle setting block 17. A common switching block 20 for switching between a working mode and a braking mode of the process is connected upstream of the current controller 15 and the angle setting block 17.

[0035] A speed controller 40 and a constant angle block 70 for outputting a constant commutation angle are connected upstream of the switching block 20 in an "upper" path. This path represents the operating mode in which the BLDC motor is controlled by the speed controller 40 and operated at a nominal speed. The nominal speed can be specified, for example, by a control knob on a machine tool.

[0036] Block diagram 200 further includes a "lower" path, which represents a preferred variant of the braking mode. In braking mode, the BLDC motor is braked to a braking speed that is lower than the nominal speed. In this path, a commutation angle controller 80, which is embodied, for example, as a PI controller, is connected upstream of the switching block 20. The commutation angle controller 80 outputs a reference variable ßKommbrem Sto the switching block 20, wherein a control input variable of the commutation angle controller 80 (as in the embodiment of Fig. 1) is a control deviation between the specified setpoint intermediate circuit voltage UDC.SOII and an actual intermediate circuit voltage UDO, of the intermediate circuit connected upstream of the BLDC motor (not shown here). The reference variable ßKommbrems is thus dependent on a controlled intermediate circuit voltage of the intermediate circuit. The motor current amplitude Is as the input variable for the switching block 20 is constant in the braking mode in the embodiment shown here and corresponds, for example, to a current carrying capacity of an inverter assigned to the BLDC motor. The switching block 20 determines here whether a current controller amplitude with constant commutation angles is provided by the speed controller (working mode) or a constant current controller setpoint (motor current amplitude) with variable commutation angles (braking mode) is used.

[0037] List of reference symbols

[0038] 10 PWM block

[0039] 11 d-current regulator

[0040] 13 q-current controller

[0041] 15 current controller (block commutation)

[0042] 17 Angle setting block (block commutation)

[0043] 20 switching block

[0044] 30 MTPA block

[0045] 40 speed controllers

[0046] 50 Calculation block

[0047] 60 current space vector angle controllers

[0048] 70 Constant angle block

[0049] 80 commutation angle controllers

[0050] 100 Block diagram for field-oriented motor control

[0051] 200 Block diagram for block-commutated motor control ßbrake command variable (vector control) ßcommbrake command variable (block commutation)

[0052] ID, lq Motor currents in the d / q coordinate system

[0053] Is motor current amplitude

[0054] UDO, is actual intermediate circuit voltage

[0055] UDO, target intermediate circuit voltage

[0056] SLI inverter current limit

[0057] SRZ stator current space vector

[0058] I..IV quadrants

Claims

Patent claims 1. A method for operating a drive unit of a machine tool, wherein the drive unit has a brushless DC motor and an intermediate circuit connected upstream of the DC motor, wherein the DC motor is controlled in a working mode by a speed controller and operated at a nominal speed, and wherein the method has a braking mode in which the DC motor is braked to a braking speed which is lower than the nominal speed, characterized in that the DC motor is controlled in the braking mode on the basis of a motor current amplitude and a command variable, wherein the command variable is dependent on an intermediate circuit voltage of the intermediate circuit.

2. Method according to claim 1, characterized in that in the braking mode a controller is used whose control input variable is an intermediate circuit voltage or a control deviation of the intermediate circuit voltage and / or whose control output variable is the reference variable for controlling the DC motor.

3. Method according to claim 1 or 2, characterized in that the motor current amplitude in braking mode is constant up to a lower braking speed threshold which is greater than zero.

4. Method according to claim 3, characterized in that the motor current amplitude in braking mode corresponds to the current carrying capacity of an inverter assigned to the DC motor.

5. Method according to one of the preceding claims, characterized in that the control of the DC motor in braking mode with the reference variable is carried out in such a way that a generator power fed back into the intermediate circuit when braking the DC motor is equal to an ohmic power loss converted in the DC motor. Method according to one of the preceding claims, characterized in that, within the scope of the method, a switching block is used to switch between working mode and braking mode. Method according to one of the preceding claims, characterized in that the intermediate circuit voltage is determined on the basis of the voltages induced in the stator windings of the brushless DC motor, which are caused by the counter-electromotive force. Method according to one of the preceding claims, characterized in that the brushless DC motor is completely braked in braking mode. Method according to one of the preceding claims, characterized in that the brushless DC motor is controlled by means of vector control and the reference variable represents a current space vector angle.Method according to one of claims 1 to 8, characterized in that the brushless DC motor is controlled by means of block commutation and the reference variable represents a commutation angle.Drive unit for a machine tool, wherein the drive unit has a brushless DC motor and an intermediate circuit connected upstream of the DC motor, wherein the drive unit is designed to control the DC motor in a working mode by means of a speed controller and to be able to operate it at a nominal speed, and the drive unit is designed to be able to operate the DC motor in a braking mode in which the DC motor is braked to a braking speed which is lower than the nominal speed, characterized in that the drive unit is designed to control the DC motor in the braking mode on the basis of a motor current amplitude and a reference variable which is dependent on an intermediate circuit voltage of the intermediate circuit.

12. Machine tool, in particular hand-held machine tool with a drive unit according to claim 11.

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