Method for operating brushless dc motor for electric machining tool

Abstract

A method for operating a drive unit of a machining tool, in which the drive unit has a brushless DC motor and an intermediate circuit connected upstream of the DC motor, in which the DC motor is controlled by a speed controller and operates at a nominal speed in an operating mode, and in which the intermediate circuit is connected upstream of the DC motor. The method has an operating mode of braking the DC motor to a braking speed below the nominal speed, wherein the DC motor is controlled in the braking mode with a reference variable based on an intermediate circuit voltage of an intermediate circuit.

Description

Technical Field

[0001] The invention relates to a method for operating a drive unit of a machining tool, wherein the drive unit has a brushless DC motor (BLDC motor) and an intermediate circuit connected upstream of the BLDC motor. The BLDC motor is controlled by a speed controller in an operating mode and operates at a nominal speed. The method further comprises a braking mode in which the BLDC motor is braked to a braking speed below the nominal speed. Background Art

[0002] Methods of this type are known in principle from the prior art. In order to brake a BLDC motor quickly to a standstill, the rotational energy stored in the BLDC motor and in the driven mechanical overall system must be extracted or converted. It is usually technically impossible to feed the rotational energy back into the voltage source, or the instantaneous power and duration of the fed-back energy are limited, so that the typically required rapid braking times cannot be achieved.

[0003] In so-called short-circuit braking, the BLDC motor is short-circuited at its terminals. This can be achieved by switching on all low-side power semiconductors of the motor inverter simultaneously. An initially higher initial short-circuit current is formed, which decays into a continuous short-circuit current. The short-circuit current generates a braking torque in the motor. Another braking mode of the prior art can be provided using a brake chopper with a brake resistor. Here, the BLDC motor is braked in a controlled manner, thereby becoming a generator. The energy output is fed back into the intermediate circuit, thereby increasing the intermediate circuit voltage. The brake chopper with a brake resistor periodically switches the resistor to the intermediate circuit via a power semiconductor switch in order to convert the energy in the brake resistor into heat. Summary of the invention

[0004] The object of the present invention is to provide a method for operating a drive unit of a machining tool which provides the basis for a simple and fast braking mode.

[0005] This object is achieved by controlling the BLDC motor in braking mode with a reference variable based on an intermediate circuit voltage of the intermediate circuit. It has been found to be advantageous to control the BLDC motor in braking mode with the reference variable in such a way that the generator power fed back into the intermediate circuit during braking of the BLDC motor is equal to the resistive power losses converted in the BLDC motor.

[0006] The invention includes the recognition that in the case of short-circuit braking mentioned at the outset, it is typically not possible to influence the braking torque or the braking time. In particular, the level of the short-circuit current is determined solely by the motor parameters. The electronics must also be able to carry the initially higher initial short-circuit current. Solutions based on brake choppers with brake resistors known from the prior art do have more degrees of freedom in achieving the desired braking torque or braking time. However, additional hardware components are required here (MOSFET / IGBT, gate drivers, discrete brake resistors, connecting wires), which require more installation space and incur additional costs. Braking resistors must be specified in particular for high pulse powers and contribute significantly to installation space and costs.

[0007] In contrast thereto, 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 main advantage of the invention is that a fast braking process is facilitated by the best possible utilization of the power electronics / inverter without additional components. The desired braking behavior can be precisely defined in the described braking method by means of parameters. The minimum possible braking time depends on the current carrying capacity of the inverter of the power electronics, but faster braking processes are possible simply by corresponding scaling of the inverter for high currents.

[0009] In a particularly preferred embodiment, a controller is used in the braking mode, the control input variable of which is the 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 means of vector control and the reference variable represents the current space vector angle. In a further particularly preferred embodiment, the brushless DC motor is controlled by means of block commutation and the reference variable represents the commutation angle.

[0011] It has been found to be advantageous that the motor current amplitude is constant in braking mode up to a lower braking speed threshold greater than zero. For example, the lower braking speed threshold may be 100 rpm. Advantageously, the motor current amplitude in braking mode may correspond to the current carrying capacity of an inverter associated with the DC motor.

[0012] In a further particularly preferred embodiment, the DC motor is controlled in braking mode with a reference variable in such a way that the generator power fed back into the intermediate circuit during braking of the DC motor is equal to the resistive power losses converted in the DC motor.

[0013] It has been found to be advantageous if, within the scope of the method, a switching block is used for switching between the operating mode and the braking mode. In a further particularly preferred embodiment, the intermediate circuit voltage is determined based on the voltage induced in the stator winding of the brushless DC motor due to the back electromotive force. The intermediate circuit voltage can also be measured directly.

[0014] It has been found to be advantageous if the brushless DC motor is fully braked in braking mode, ie brought down to speed 0.

[0015] The object is also achieved by a drive unit for a machining 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 can 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 below the nominal speed. According to the invention, the drive unit is designed to control the DC motor in the braking mode based on the motor current amplitude and a reference variable that depends on the intermediate circuit voltage of the intermediate circuit. It has proven to be advantageous to use a controller in the braking mode, the control input variable of which is the control deviation of the intermediate circuit voltage and / or the control output variable of which is the reference variable.

[0016] The drive unit is preferably part of a working tool, in particular a handheld working tool.

[0017] Further advantages will become apparent from the following description of the drawings. In the drawings, particularly preferred exemplary embodiments of the present invention are shown. The drawings, the description and the claims contain many combined features. It will also be convenient for a person skilled in the art to consider these features individually and to combine these features to form reasonable further combinations. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In the figures, identical and functionally identical parts are denoted by the same reference numerals, in which:

[0019] Figure 1 shows a block diagram of field oriented control for a BLDC motor;

[0020] Figure 2 shows the motor characteristics of the BLDC motor in the d / q coordinate system with the rotor fixed;

[0021] Figure 3 Shown in accordance with Figure 1 The current space vector angle over time in the method of ; and

[0022] Figure 4 A block diagram of block commutation control for a BLDC motor is shown. DETAILED DESCRIPTION

[0023] In the following, a preferred embodiment of the method is described based on field-oriented motor control using a rotor-fixed d / q coordinate system (Park transformation). For this purpose, Figure 1 A block diagram 100 is shown, with the use of which a drive unit (not shown here) can be controlled, which has a brushless DC motor (BLDC motor) and an intermediate circuit connected upstream of the BLDC motor.

[0024] The block diagram 100 has a PWM block 10 for pulse width modulation control of a BLDC motor. The input variables of the PWM block 10 are provided by a d current controller 11 and a q current controller 13. Upstream of the d current controller 11 and the q current controller 13 is a common switching block 20 for switching between the operating mode and the braking mode of the method.

[0025] The MTPA (maximum torque / ampere) block 30 is connected upstream of the switching block 20, in the "upper" path, and outputs a d setpoint current and a q setpoint current. The MTPA block 30 in turn receives its setpoint current from the speed controller 40. This path represents an operating mode in which the BLDC motor is controlled by the speed controller 40 and operates at a nominal speed. The nominal speed may be specified, for example, by an operating knob of a machining tool.

[0026] The block diagram 100 further has a "lower" path, which represents a preferred change in braking mode. In braking mode, the BLDC motor is braked to a braking speed less than the nominal speed. In this path, the calculation block 40 is connected upstream of the switching block 20, and the calculation block outputs the d set point current and the q set point current to the switching block 20.

[0027] The calculation block 50 has as input variable the motor current magnitude I S (See Figure 2 The magnitude of the stator current space vector SRZ) and the reference variable β 制动 , which indicates the displacement of the stator current space vector relative to the d-axis (see Figure 2 ). Reference variable β 制动 is the control output variable of the current space vector angle controller 60. Reference variable β 制动 The limiter of the current space vector angle controller 60 can be limited downward and / or upward. The control input variable of the current space vector angle controller 60, which is designed as a PI controller, for example, is a specified setpoint intermediate circuit voltage U of the intermediate circuit connected upstream of the BLDC motor (not shown here). DC，设定点 The actual intermediate circuit voltage U DC，实际 Therefore, the reference variable β 制动The motor current amplitude I as input variable of the calculation block 50 is dependent on the intermediate circuit voltage of the intermediate circuit in a controlled manner. S In the present exemplary embodiment, it is constant in braking mode and corresponds, for example, to the current-carrying capacity of the inverter assigned to the BLDC motor.

[0028] Thus, the above-mentioned switching block 20 determines whether the d current controller 11 and the q current controller 13 receive their set point values ​​from the speed controller 40 using the MTPA block 30, or whether these set point values ​​are provided by the braking method represented in the "lower" path. The switching request signal for the switching block 20 can be triggered, for example, by the "release" of the operating button of the processing tool.

[0029] To further explain the method, Figure 2 The motor characteristics of an exemplary BLDC motor in a fixed rotor d / q coordinate system with four quadrants I to IV are shown. (The rotor angle required for the transformation can be determined, for example, by Hall sensors or by evaluating the EMF forces at 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.

[0030] The stator current space vector SRZ has the motor current amplitude I S As its magnitude. The current space vector angle β 制动 Indicates the displacement of the stator current space vector relative to the d-axis. Current space vector angle β 制动 Corresponds to Figure 1 The reference variable β in 制动 . Current space vector angle β 制动 is calculated by a controller (e.g. a PI controller) whose control input variable is Figure 1 The intermediate circuit voltage u DC .

[0031] As mentioned above, the present invention is based on the concept of converting the braking energy into ohmic losses in the motor itself. For this purpose, the ohmic losses (P V =R·I 2 ) is advantageously combined with the power (P M =M 制动 ·ω 机械 ) to avoid an uncontrolled increase in the intermediate circuit voltage. This can be achieved by changing the current space vector angle β 制动 To adjust the motor's braking torque M 制动 , while maintaining the motor current amplitude IS Constant. Motor current amplitude I S In braking mode, the current limit SLI of the inverter is selected so that the current measuring range and the current carrying capacity of the inverter are not exceeded. Figure 2 Preferably, the motor current amplitude I S Keep it constant in order to make the best possible use of the inverter and achieve the shortest possible braking time.

[0032] Angle β 制动 = 0 corresponds to the stator current space vector being on the positive d-axis. As the current space vector angle β 制动 The increase of the current space vector β results in a negative q current component and a decrease in the positive d current. 制动 should not fall below a minimum value, since generator braking operation will then change to motor operation. This limit depends on the BLDC motor used. Therefore, the manipulated variable of the current controller is limited to the minimum permissible value β 制动，最小 , the minimum permissible value is, for example, about 47° (see Figure 3 ). 制动 =90°, eventually changing quadrant until the MTPA (maximum torque / ampere) operating point is reached with negative d and q currents.

[0033] exist Figure 2 In the figure, the time of transition from working mode to braking mode is represented by t1. Current space vector angle β 制动 For example, about 57° (see Figure 3 ). During the braking process, the current space vector angle β 制动 It moves in the direction of a larger angle within its working range until time t2 with a low braking speed. As an example, the current space vector angle β 制动 It is now about 117°. At such low braking speeds, the ohmic losses can exceed the power produced by the generator, so that the difference must be covered by the voltage source. If no power is drawn from the voltage source, the motor current amplitude I of the stator current space vector SRZ can optionally be shortened along the MTPA characteristic S At time t3, the BLDC motor is just before it comes to rest. A typical braking process may have, for example, Figure 3 The current space vector angle β is shown 制动 During braking, the angular frequency of the BLDC motor decreases, and accordingly, the angular frequency of the d / q coordinate system to which the rotor is fixed decreases, wherein the BLDC motor is controlled by means of the stator current space vector SRZ.

[0034] Figure 4 Another exemplary embodiment of a method using block commutation is shown. For this purpose, Figure 4A block diagram 200 is shown, with the use of which a drive unit (not shown here) can be controlled, which has a brushless DC motor (BLDC motor) and an intermediate circuit connected upstream of the BLDC motor.

[0035] The block diagram 200 has a PWM block 10 for pulse width modulation control of a BLDC motor. The input variables of the PWM block 10 are provided by a current controller 15 and an angle preset block 17. A common switching block 20 for switching between the working mode and the braking mode of the method is connected upstream of the current controller 15 and the angle preset block 17.

[0036] The speed controller 40 and the constant angle block 70 for outputting a constant commutation angle are connected upstream of the switching block 20, in the "upper" path. This path represents an operating mode in which the BLDC motor is controlled by the speed controller 40 and operates at a nominal speed. The nominal speed may be specified, for example, by an operating knob of a machining tool.

[0037] The block diagram 200 further has a "lower" path, which represents a preferred change in braking mode. In braking mode, the BLDC motor is braked to a braking speed that is less than the nominal speed. In this path, the commutation angle controller 80 (illustrated as a PI controller) is connected upstream of the switching block 20. The commutation angle controller 80 sets the reference variable β 换向制动 Output to the switching block 20, where the control input variable of the commutation angle controller 80 (such as Figure 1 In the exemplary embodiment of ) is a specified setpoint intermediate circuit voltage U of an intermediate circuit connected upstream of a BLDC motor (not shown here). DC，设定点 The actual intermediate circuit voltage U DC，实际 Therefore, the reference variable β 换向制动 The motor current amplitude I as an input variable of the switching block 20 is dependent on the intermediate circuit voltage of the intermediate circuit in a controlled manner. S In the present exemplary embodiment, it is constant in braking mode and corresponds, for example, to the current carrying capacity of the inverter assigned to the BLDC motor. The switching block 20 here determines whether a current controller amplitude with a constant commutation angle is provided by the speed controller (operating mode) or whether a constant current controller setpoint (motor current amplitude) with a variable commutation angle is used (braking mode).

[0038] Reference numerals list

[0039] 10 PWM Block

[0040] 11d Current controller

[0041] 13 q Current Controller

[0042] 15 Current controller (block commutation)

[0043] 17 Angle preset block (block commutation)

[0044] 20 Switch Block

[0045] 30 MTPA Block

[0046] 40 Speed ​​Controller

[0047] 50 calculation blocks

[0048] 60 Current Space Vector Angle Controller

[0049] 70 Constant Angle Block

[0050] 80 Commutation angle controller

[0051] 100 Block diagram for field-oriented motor control

[0052] 200 Block diagram for block commutation motor control

[0053] β 制动 Reference variable (vector control)

[0054] β 换向制动 Reference variable (block commutation)

[0055] I D ,I q Motor current in d / q coordinate system

[0056] I S Motor current amplitude

[0057] U DC，实际 Actual intermediate circuit voltage

[0058] U DC，设定点 Setpoint intermediate circuit voltage

[0059] SLI inverter current limit

[0060] SRZ stator current space vector

[0061] Quadrants I to IV

Claims

1. A method for operating a drive unit of a machining 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 by a speed controller in an operating mode and operates at a nominal speed, and wherein the method has an operating mode in which the DC motor is braked to a braking speed below the nominal speed, It is characterized in that the DC motor is controlled in the braking mode on the basis of the motor current amplitude and a reference variable, wherein the reference variable depends on the intermediate circuit voltage of the intermediate circuit.

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

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

4. The method according to claim 3, It is characterized in that The motor current amplitude corresponds to the current carrying capacity of the inverter associated with the DC motor in the braking mode.

5. The method according to any one of the preceding claims, It is characterized in that The DC motor is controlled in braking mode with the reference variable in such a way that the generator power fed back into the intermediate circuit during braking of the DC motor is equal to the resistive power losses converted in the DC motor.

6. The method according to any one of the preceding claims, It is characterized in that Within the scope of the method, a switching block is used to switch between an operating mode and a braking mode.

7. The method according to any one of the preceding claims, It is characterized in that The intermediate circuit voltage is determined based on the voltage induced in the stator winding of the brushless DC motor due to back electromotive force.

8. The method according to any one of the preceding claims, It is characterized in that The brushless DC motor is fully braked in brake mode.

9. The method according to any one of the preceding claims, It is characterized in that The brushless DC motor is controlled by means of vector control, and the reference variable represents the current space vector angle.

10. The method according to any one of claims 1 to 8, It is characterized in that The brushless DC motor is controlled by means of block commutation, and the reference variable represents the commutation angle.

11. A drive unit for a processing 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 an operating mode by means of a speed controller and can 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 below the nominal speed, It is characterized in that the drive unit is designed to control the DC motor in the braking mode on the basis of the motor current amplitude and a reference variable which is dependent on the intermediate circuit voltage of the intermediate circuit. 12 . A processing tool, in particular a hand-held processing tool, comprising a drive unit as claimed in claim 11 .

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