POWER TOOL, METHOD AND COMPUTER PROGRAM PRODUCT

Description

[0001] The invention relates to a power tool, in particular a handheld power tool, for example a polisher, a grinder and / or a saw, comprising a tool, an electric motor for driving the tool and a control unit for controlling the electric motor. The power tool is designed for connection to a mains voltage and includes a rectifier arrangement with an intermediate circuit for providing an intermediate circuit voltage based on the mains voltage.

[0002] The DC link is, for example, a capacitor that serves primarily to smooth the rectified mains voltage. There are various approaches to dimensioning the DC link. One approach is to dimension the DC link large enough so that the DC link voltage is nearly constant. This allows, in particular, nearly constant phase currents for powering the electric motor and a resulting nearly constant torque curve of the electric motor. However, a large DC link leads to a poor power factor and high harmonic content in the input current. A second approach is to dimension the DC link small so that the DC link voltage follows the mains voltage, especially the rectified mains voltage.However, with such a small DC link, the ohmic power loss in the motor windings is generally higher, necessitating a larger electric motor. Furthermore, such a small DC link can lead to situations where, with conventional motor control, the induced back EMF of the motor becomes greater than or equal to the DC link voltage, preventing current from flowing into the motor. This behavior can result in torque ripple at twice the mains frequency.

[0003] EP 3 068 032 A1 relates to a power tool with a smoothing capacitor configured to generate a pulsating voltage from a rectified voltage.

[0004] EP 2 949 036 A1 concerns an electric motor control system.

[0005] US 2014 / 028237 A1 concerns an inverter control unit.

[0006] Saren H et al: "DTC Driven Single Phase Fed Voltage Source Inverter with Small DC-Link Capacitor", Industrial Electronics, 2005. ISIE 2005. Proceedings of the IEEE Inter National Symposium on Dubrovnik, Croatia June 20-23, 2005, Piscataway, NJ, USA, IEEE, Vol. 2, June 20, 2005, pages 411-416, concerns the use of a relatively small film capacitor.

[0007] EP 3 386 096 A1 concerns waveform shaping in a power tool.

[0008] EP 2 765 703 A2 concerns an engine control unit.

[0009] One object of the invention is to provide an improved power tool, in particular a power tool that achieves a good power factor with low ohmic power loss.

[0010] The problem is solved by an electric tool according to claim 1.

[0011] In the power tool, the DC link voltage consists of multiple successive voltage half-waves. Specifically, the DC link, for example a capacitor, is dimensioned so small that the DC link voltage follows the rectified mains voltage, thereby generating the voltage half-waves. The rectified mains voltage comprises, in particular, a sequence of positive sine wave half-waves. For example, the DC link voltage follows the rectified mains voltage over at least 50% of the amplitude of the rectified mains voltage. For instance, the DC link voltage follows the rectified mains voltage in a range of 140 V to 320 V. The small DC link advantageously achieves a good power factor, for example, a power factor of at least 0.70 or at least 0.75.

[0012] Furthermore, the control unit of the power tool is designed to provide, and in particular calculate, a corresponding torque half-wave for each voltage half-wave for controlling the electric motor. Each voltage half-wave is expediently assigned a corresponding torque half-wave, which in particular has the same period and / or the same phase angle as the voltage half-wave. The waveform of the voltage half-wave is flattened compared to the waveform of the (each assigned) voltage half-wave. This means that the torque half-wave is expediently more uniform than the voltage half-wave. For example, the ratio of the maximum to the average of the torque half-wave is lower than the ratio of the maximum to the average of the voltage half-wave. Preferably, the standard deviation of the respective torque half-wave is lower than the standard deviation of the respective assigned voltage half-wave.

[0013] A flattened torque half-wave curve improves the ratio of torque to ohmic power loss. The current supplied to the electric motor is proportional to the specified torque – i.e., to the torque half-wave. The ohmic power loss is a function of the square of the current. Consequently, higher currents lead to a disproportionate increase in ohmic power loss (relative to the achieved torque). A flattened torque half-wave prevents this disproportionate increase in ohmic power loss. In particular, the flattened curve results in lower ohmic power loss when the electric motor is energized.The flattened curve shape results in lower ohmic power loss when energizing the electric motor, especially at the same average torque (as with a non-flattened curve shape, such as a sine wave). "Average torque" refers to the average torque.

[0014] Each torque half-wave has torque minima that together extend over at least 10% of the period of the torque half-wave.

[0015] Advantageous further training is the subject of the sub-claims.

[0016] The invention further relates to a method for operating a power tool, in particular a handheld power tool, for example a polisher, grinder and / or saw, comprising a tool, an electric motor for driving the tool and a control unit for controlling the electric motor, comprising the steps of: providing, by means of a rectifier arrangement with an intermediate circuit, an intermediate circuit voltage based on a mains voltage to which the power tool is connected, wherein the intermediate circuit voltage has a plurality of successive voltage half-waves, and, for each voltage half-wave, providing a corresponding torque half-wave for controlling the electric motor, the waveform of which is flattened compared to the waveform of the voltage half-wave. Each torque half-wave has torque minima which together extend over at least 10% of the period of the torque half-wave.

[0017] Preferably, an arrangement is provided comprising an electric motor for driving a tool and a control unit for controlling the electric motor. The control unit is configured to provide a corresponding torque half-wave for controlling the electric motor for each voltage half-wave of an intermediate circuit voltage, the waveform of which is flattened compared to the waveform of the voltage half-wave.

[0018] The invention further relates to a computer program product comprising commands that cause the power tool to perform the aforementioned process steps.

[0019] Preferably, a computer-readable medium is provided on which the computer program is stored.

[0020] Preferably, a procedure is carried out comprising the step: installing the computer program product onto a power tool.

[0021] Further exemplary details and embodiments are explained below with reference to the figures. Figure 1 is a schematic representation of a power tool designed as a polishing device, Figure 2 is a schematic representation of a power tool designed as a sawing device, Figure 3 is a schematic representation of the rectifier arrangement, the control unit and the electric motor of the power tool, Figure 4 is a time course of an intermediate circuit voltage, Figure 5 is a time course of a torque and Figure 6 is a block diagram of a signal processing system.

[0022] The Figures 1 and 2 Figure 1 shows exemplary designs of a power tool 1. The power tool 1 is shown as a handheld power tool. The power tool 1 can be gripped, carried, and / or guided by a user. The power tool 1 can be used in particular as a polishing device 1A (see Figure 1). Figure 1) or as a sawing device 1B (see Figure 2 ) be implemented. The polishing device 1A, for example, is a polishing machine, in particular a rotary polisher. The sawing device 1B, for example, is a plunge saw. Alternatively, the power tool 1 can also be implemented as another power tool, for example as a grinding device, in particular as a renovation sander.

[0023] The power tool 1 comprises a tool 2. The tool 2 is exemplified as a polishing pad 2A or a saw blade 2B. Alternatively, the tool 2 can also be a different tool, for example, a grinding pad. The tool 2 serves in particular to machine a workpiece, especially in a state in which the tool 2 is set into a working motion, in particular a rotary motion, by an electric motor 3.

[0024] The power tool 1 comprises the electric motor 3 for driving the tool 2. The electric motor 3 is specifically designed to provide the drive rotary motion on the basis of which the tool 2 is driven. The electric motor 3 is, for example, designed as an EC motor – that is, as an electronically commutated motor. The electric motor 3 is, in particular, a brushless motor, preferably a brushless DC motor (BLDC motor).

[0025] The power tool 1 further comprises a control unit 4 for controlling the electric motor 3. The control unit 4 includes, for example, a microcontroller and / or power electronics. The control unit 4 is specifically designed to supply the electric motor 3 with several motor currents I1, I2, I3, which are preferably phase-shifted relative to each other, in order to expediently cause the electric motor 3 to perform the drive rotational movement. The motor currents I1, I2, I3 can also be referred to as phase currents. The control unit 4 expediently has an inverter for providing the motor currents I1, I2, I3.

[0026] By way of example, the power tool 1 further comprises a handle 5 with which a user can manually grip, carry and / or guide the power tool 1. The power tool 1 further comprises an operating device 6, for example a button and / or a switch, by means of which the user can control the drive of the tool 2, in particular start and / or stop it. By way of example, the operating device 6 is arranged on the handle 5.

[0027] The power tool 1 expediently comprises a shaft 7 via which the tool 2 is coupled to the electric motor 3, so that the tool 2 can be driven by the electric motor 3. Optionally, the power tool 1 can comprise a gearbox, in particular a bevel gearbox 8, via which the tool 2 is coupled to the electric motor 3. In the Figure 1The power tool 1 includes, for example, a coupling shaft 9 via which the electric motor 3 is coupled to the bevel gear 8. The bevel gear 8 is, for example, coupled to the tool 2 via the shaft 7. Alternatively, the electric motor 3 can be coupled directly to the tool 2, for example via the shaft 7.

[0028] The power tool 1 comprises, by way of example, an outer housing 10 in which the electric motor 3, the control unit 4 and / or a rectifier assembly 12 are expediently arranged. For example, the handle 5 is arranged on the outer housing 10. Alternatively, the handle 5 can be part of the outer housing 10.

[0029] The power tool 1 is designed for connection to a mains voltage V1 (see Figure 2The mains voltage V1 is an alternating voltage. The mains voltage V1 is, in particular, sinusoidal and, for example, has an RMS value of 230 V and / or a mains frequency of 50 Hz. Furthermore, the mains voltage V1 can have an RMS value of 120 V and / or a mains frequency of 60 Hz. The power tool 1 includes a connection 11, for example, a mains plug and / or a mains plug connector, via which the power tool 1 can be connected to the mains voltage, for example, to a wall socket.

[0030] The power tool 1 has the rectifier assembly 12, which is shown by way of example in the outer housing 10. The rectifier assembly 12 is, for example, in the Figure 3The rectifier arrangement 12 is designed to provide an intermediate circuit voltage V2 based on the mains voltage V1. The rectifier arrangement 12 comprises a rectifier 14, which is implemented as a bridge rectifier. The rectifier 14 expediently includes four diodes, which are connected as a bridge. The mains voltage V1 is supplied to the rectifier 14, which provides a rectified mains voltage based on the mains voltage V1. The rectifier arrangement 12 further comprises an intermediate circuit 15, which is implemented as a capacitor. The intermediate circuit 15 is connected to the output of the rectifier 14. The intermediate circuit 15 serves to smooth the rectified mains voltage. The smoothed rectified mains voltage is also referred to as the intermediate circuit voltage V2. The intermediate circuit voltage V2 drops across the intermediate circuit 15, in particular across the capacitor.

[0031] The intermediate circuit 15, in particular the capacitor, is, for example, smaller than 100 µ F less than 50 µ F less than 30 µ F less than 20 µ F or less than 10 µ F.

[0032] An exemplary time course of the intermediate circuit voltage V2 is shown in the Figure 4The DC link voltage V2 comprises a plurality of successive voltage half-waves 16. By way of example, each voltage half-wave 16 comprises a sinusoidal section 17, which has the waveform of a partial segment of a sinusoidal half-wave. The sinusoidal section 17, in particular the partial segment, expediently comprises the maximum 20 of the sinusoidal half-wave. By way of example, the DC link voltage V2 follows the rectified mains voltage in the sinusoidal section 17. The voltage half-waves 16 also each comprise, by way of example, two transition sections 18, which are arranged before and after the sinusoidal section 17. The transition sections 18 comprise the minima 19 of the DC link voltage V2. In the transition sections 18, the voltage half-waves 16 do not, by way of example, have the form of a sinusoidal half-wave. In particular, the intermediate circuit voltage does not drop to zero in the transition sections 18.For example, the intermediate circuit voltage V2 does not follow the rectified mains voltage in the transition sections 18.

[0033] The minima 19 of each voltage half-wave 16 expediently amount to at least 20%, preferably at least 30%, of the maximum 20. For example, the minima 19 of each voltage half-wave 16 amount to at least 90 V or at least 100 V. Furthermore, the minima 19 of each voltage half-wave 16 expediently amount to at most 70%, in particular at most 50%, preferably at most 40% or at most 35%, of the maximum 20. For example, the minima 19 amount to at most 120 V or 110 V.

[0034] The control unit 4 and / or the electric motor 3 are preferably supplied from the intermediate circuit 15. The control unit 4 is configured to generate motor currents I1, I2, I3 based on the electrical energy provided by the intermediate circuit 15, in particular based on the intermediate circuit voltage V2, and to supply these currents to the electric motor 3. For example, the control unit 4 provides three motor currents I1, I2, I3. The motor currents I1, I2, I3 flow from the intermediate circuit 15.

[0035] The electric motor 3 comprises, by way of example, a stator 43 and a rotor 21. The rotor 21 is coupled to the tool 2. By supplying the electric motor 3 with the motor currents I1, I2, I3, the rotor 21 is set into rotational motion relative to the stator 43.

[0036] Optionally, the electric motor 3 has a position sensor device 22, which serves to detect the position and / or movement, in particular the current angle, of the rotor 21. The position sensor device 22 comprises, for example, a magnetic sensor, in particular a Hall sensor. Advantageously, the control unit 4 is configured to detect the current angle of the rotor 21 using the position sensor device 22. The control unit 4 can further be configured to detect the current rotational speed of the electric motor 3 using the position sensor device 22.

[0037] Alternatively or additionally, the control unit 4 can be configured to detect the current angle of the rotor 21 and / or the current speed of the electric motor 3 without sensors. In this case, a position sensor device 22 is advantageously omitted. The control unit 4 is specifically configured to determine a back EMF induced in the electric motor and to calculate the current angle of the rotor 21 and / or the current speed of the electric motor 3 based on this back EMF. The control unit 4 is specifically configured to calculate the current angle of the rotor 21 and / or the current speed of the electric motor 3 using the back EMF principle, where "EMF" stands for "Electromotive Force".

[0038] The Figure 6 shows an exemplary block diagram of the signal processing carried out by the control unit 4 for controlling the electric motor 3.

[0039] The control unit 4 is advantageously configured to control the speed of the electric motor 3. In particular, the control unit 4 is configured to provide the motor currents I1, I2, I3 based on a target speed 23 and an actual speed 24. The target speed 23 is set by the control unit 4, for example, according to a user input made via the operating device 6. Alternatively, the target speed 23 can be pre-stored in the control unit 4 and / or calculated by the control unit 4. The actual speed 24 is the current speed of the electric motor 3 and is advantageously obtained as described above – in particular by means of the position sensor device 22 and / or on the basis of a sensorless principle, especially based on the induced back EMF. The actual speed 24 describes how fast the rotor 21 rotates relative to the stator 43.

[0040] The control unit 4 includes a speed controller 25, which receives the target speed 23 and the actual speed 24. Based on the target speed 23 and the actual speed 24, and in particular on a comparison between the target speed 23 and the actual speed 24, the speed controller 25 calculates a torque setpoint 26. The torque setpoint 26 specifies the torque with which the rotor 21 is to be acted upon by energizing the electric motor 3 in order to achieve a change in the actual speed value towards the speed setpoint.

[0041] The control unit 4 further comprises a torque curve calculation unit 27, which is configured to calculate a torque curve 28 over time based on the torque setpoint 26. The torque curve 28 can also be referred to as a torque signal. An exemplary torque curve 28 is shown in the Figure 5 shown. As an example, the torque curve 28 exhibits a trapezoidal shape.

[0042] The control unit 4 further comprises a motor current supply unit 31, which is configured to calculate the motor currents I1, I2, I3 based on the torque curve 28. By way of example, the motor current supply unit 31 includes a target current setpoint unit 32, which is configured to calculate a q-current and a d-current, in particular based on the torque curve 28. The d-current and the q-current are currents in a rotor-related d / q system that rotates according to the rotation of the rotor 21. The d-current represents the flux-generating component, and the q-current represents the torque-generating component. The d-current can be referred to as the d-component or the field-weakening current, and the q-current can be referred to as the q-component or the torque-generating current. The target current setpoint unit 32 calculates the q-current and the d-current in such a way as to achieve the torque curve 28 when the electric motor 3 is energized according to the q-current and the d-current.For example, the time course of the calculated q-current corresponds to the torque curve 28. In particular, the q-current has the same curve shape as the torque curve 28.

[0043] The motor current supply unit 31 expediently comprises a current controller 33 configured to supply the motor currents I1, I2, I3. In particular, the current controller 33 is configured to regulate the motor currents I1, I2, I3. The motor currents I1, I2, and I3 are preferably pulse-width modulated. The current controller 33 supplies the motor currents I1, I2, I3 based on the q-current and the d-current, in particular by performing a transformation from a two-axis coordinate system to a three-axis coordinate system. The current controller 33 supplies the motor currents I1, I2, I3 such that the calculated q-current and the calculated d-current are achieved. The electric motor 3, in particular the motor windings of the stator 43, are energized by the motor currents I1, I2, I3.The rotor 21 has an exemplary permanent magnet and is set into drive rotation by an interaction of the permanent magnet with the magnetic field generated by the current flowing to the motor windings.

[0044] The following section will discuss the torque curve 28 in more detail. Figure 5 Figure 28 shows an exemplary torque curve (as a solid line). The torque curve 28 exhibits a plurality of successive torque half-waves 36. For example, the torque half-waves 36 each have a trapezoidal curve shape. Alternatively, the torque half-waves can have a different curve shape, for example, a rectangular curve shape.

[0045] The control unit 4 is configured to provide, and in particular calculate, a corresponding torque half-wave 36 for each voltage half-wave 16 for controlling the electric motor 3. The waveform of the respective torque half-wave is flattened compared to the waveform of the voltage half-wave 16. This flattened waveform results in a lower ohmic power loss when energizing the electric motor 3, advantageously at the same average torque.

[0046] The control unit 4 supplies current to the electric motor 3 expediently in proportion to the torque half-wave. When the torque half-wave 36 assumes larger values, the control unit 4 increases the current supplied to the electric motor 3. The ohmic power loss is a function of the square of the current supplied to the electric motor 3, so that larger currents lead to a disproportionately higher ohmic power loss. The flattened curve of the torque half-waves 36 reduces the current maxima, which are particularly significant for the ohmic power loss (due to the quadratic relationship), thus reducing the ohmic power loss, especially when there is no or only a slight reduction in torque. The flattened curve can also be described as a smoothed curve.In particular, each torque half-wave 36 has a more uniform curve shape than the respective voltage half-wave 16 and / or a respective (imaginary) sine half-wave 37 of the same period and / or the same area.

[0047] Preferably, the standard deviation of the respective torque half-wave 36 is less than the standard deviation of the respective voltage half-wave 16 and / or a respective (imaginary) sine half-wave 37 of the same period and / or the same area.

[0048] In particular, the flattened curve shape is a curve shape that, for the same area under the curve shape, has a smaller maximum, especially relative to an (imaginary) sine half-wave 37. The Figure 5Figure 1 shows a reference curve 38 (as a dashed line) which has the shape of a rectified sine wave and comprises successive sine half-waves 37. The sine half-waves 37 each have the same period as the torque half-waves 36. The area under each sine half-wave 37 – i.e., the integral over the period of the sine half-wave 37 – is conveniently equal to the area under a respective torque half-wave 36 – i.e., equal to the integral over the period of a torque half-wave 36. The maximum of each torque half-wave 36 is conveniently smaller, in particular at least 10% smaller or at least 20% smaller, than the maximum of the respective sine half-wave 37. The sine half-wave 37 serves here for the mathematical definition of the flattened shape of the torque half-wave and does not need to be provided by the power tool 1.The flattened curve shape of each torque half-wave 36 is preferably mathematically defined such that, with the same period and area as a (particularly imaginary) sine half-wave 37, it has a smaller maximum than the sine half-wave 37.

[0049] For example, in the case of a flattened curve shape, the ratio between the maximum and average values ​​of the curve shape is reduced. Advantageously, the ratio between the maximum and average values ​​of the torque half-wave 36 is smaller than the ratio between the maximum and average values ​​of the voltage half-wave 16.

[0050] Preferably, the control unit 4 is designed to provide the torque half-shaft 36 with a trapezoidal curve shape or a rectangular curve shape.

[0051] The description of a torque half-wave 36 preferably applies to each torque half-wave 36. The torque half-wave 36 has a plateau section 39 that comprises or represents the maximum of the torque half-wave 36. The plateau section 39 preferably has a slope of 0 and extends, in particular, over at least 40%, at least 50%, at least 60%, at least 70%, or at least 80% of the period of the torque half-wave 36. By way of example, the maximum 42 of the sine half-wave 37 is higher than the plateau section 39. The torque half-wave 36 further has two flank sections 40 that surround the plateau section 39. The flank sections 40 have a higher slope (in magnitude) than the plateau section 39, in particular a higher slope than the sine half-wave 37 and / or the voltage half-wave 16.According to one possible embodiment, the flank sections 40 can be vertical – that is, they can have a slope of (in absolute value) infinity. The torque half-wave 37 also expediently has two torque minima 41, which represent the beginning and the end of the torque half-wave 36. For example, the torque is zero at the torque minima 41. A torque half-wave 36 thus comprises the following sections, which follow each other in the aforementioned order, in particular directly one after the other: a first torque minimum 41 (preferably zero), a first flank section 40 (preferably with a positive slope), the plateau section 39 (preferably with a slope of zero), a second flank section 40 (preferably with a negative slope), and a second torque minimum (preferably zero).The torque minima 41 together extend over a period of time of at least 10%, at least 20% or at least 30% of the period of the torque half-wave 36.

[0052] The control unit 4 is advantageously configured to provide each torque half-wave 36 with a period equal to the period of a respective voltage half-wave 16. Preferably, the frequency of the torque curve 28 is twice as high as the frequency of the mains voltage V1. In particular, the frequency of the torque curve 28 is 100 Hz or 120 Hz.

[0053] For example, the control unit 4 detects the period of the voltage half-waves 16 and / or the time interval of adjacent zero crossings of the mains voltage V1 and uses the detected period and / or the time interval as the period for the torque half-waves 36. Advantageously, the control unit 4 synchronizes the torque half-waves 36 with the mains voltage V1 and / or the voltage half-waves 16. For example, the control unit 4 synchronizes the minima of the torque curve 28 with the zero crossings of the mains voltage V1, so that the minima of the torque curve 28 occur simultaneously with the zero crossings of the mains voltage V2. Furthermore, the control unit 4 can synchronize the minima of the torque curve 28 with the minima 19 of the DC link voltage V2, so that the minima of the torque curve 28 occur simultaneously with the minima of the DC link voltage V2.For example, the control unit 4 is configured to determine a phase angle 30 of the mains voltage V1 and / or the DC link voltage V2 and to provide the torque half-wave 36 according to the phase angle 30, in particular such that the torque half-wave 36 is synchronous with the voltage half-wave 16 and / or with the mains voltage V1. The torque curve 28 expediently has the same phase angle as the DC link voltage V2.

[0054] The control unit 4, in particular the torque curve calculation unit 27, is preferably configured to calculate the torque curve 28 based on a curve profile 29. The curve profile is advantageously predefined in the control unit 4 and / or provided by the control unit 4. The curve profile 29 defines the flattened curve shape of the torque half-wave 36. For example, the curve profile 29 defines a trapezoidal or rectangular curve shape.

[0055] The control unit 4 is configured to calculate the torque curve 28 taking into account the phase angle 30. In particular, the control unit 4 is configured to calculate the period for a respective torque half-wave 36 based on the phase angle 30 and to stretch or compress the curve profile 29 according to the calculated period, so that the period of the curve profile 29 is equal to the calculated period.

[0056] Advantageously, the control unit 4 is further configured to calculate the respective torque half-wave 36 based on the torque setpoint 26. In particular, the control unit 4 is configured to calculate the respective torque half-wave 36 such that the average value of the torque half-wave 36 is equal to the torque setpoint 26. In particular, the control unit 4 is configured to calculate the respective torque half-wave 36 by scaling the curve profile 29 according to the torque setpoint 26. By way of example, the control unit 4 is configured to scale the curve profile 29 with the torque setpoint 26 or with a scaling factor that depends on the torque setpoint 26.

[0057] As mentioned above, the control unit 4 has a speed controller 25 for providing the torque setpoint 26, on the basis of which the control unit 4 provides the respective torque half-wave 36. The control unit 4 is specifically designed to energize the electric motor 3 according to the torque half-wave.

[0058] The provision of the torque half-wave 36 can also be referred to as torque shaping. Through torque shaping and the associated current shaping, particularly in the form of a trapezoid or a rectangle, the ohmic power loss – i.e., the copper losses – can be reduced, so that the electric motor 3 can be made smaller.

[0059] As explained above, a small intermediate circuit 15 is advantageously used so that the intermediate circuit voltage V2 corresponds mainly to the (rectified) mains voltage. A small intermediate circuit advantageously saves space and costs. To reduce copper losses – i.e., the ohmic power loss – in the electric motor 3, the torque and thus the current flowing through the electric motor 3, in particular the q-current, is advantageously regulated to a trapezoidal or rectangular waveform, which preferably has twice the mains frequency.

[0060] Since the copper losses are calculated using I² < *R, the plateau section 39 of the trapezoidal torque half-wave 36 should have the smallest possible torque value. This is preferably achieved by maximizing the time per half-wave during which current can flow into the electric motor 3, i.e., by choosing a large temporal extent of the plateau section 39. The electric motor 3 is advantageously designed for a lower voltage than the mains voltage V1. Furthermore, the induced back EMF of the electric motor 3 is advantageously varied by varying the field-weakening current depending on the DC link voltage V2.

[0061] Preferably, the control unit 4 is configured to determine a back EMF 34 induced in the electric motor 3 and, in response to the fact that the back EMF 34 is greater than the DC link voltage V2, to reduce a current torque value of the torque half-wave 36, in particular to set it to zero, exemplified by the torque curve calculation unit 27. Exemplarily, the control unit 4 is configured to determine the induced back EMF 34 based on the rotational speed of the rotor 21. In particular, the control unit 4 is configured, in response to the fact that the back EMF 34 is greater than the DC link voltage V2, to reduce the q-current and preferably the d-current, in particular to set it to zero, preferably to regulate it to zero. In this way, undesirable braking torques, which can occur when the induced back EMF 34 is greater than the DC link voltage V2, can be avoided or reduced.

[0062] The control unit 4 is specifically designed to regulate the torque-generating current to zero when the induced back EMF 34 of the electric motor 3 is greater than or equal to the DC link voltage V2. The regulation of the torque-generating current remains active and is not switched off when the induced back EMF 34 of the electric motor 3 is greater than or equal to the DC link voltage V2.

[0063] Preferably, the control unit 4, in particular the setpoint current control unit 32, is configured to calculate the d-current taking into account the DC link voltage V2. In particular, the control unit 4 is configured to adjust the d-current Id according to the DC link voltage V2. For example, the control unit 4 is configured to reduce the magnitude of the d-current Id at a higher DC link voltage V2 and to increase it at a lower DC link voltage V2. In particular, the control unit 4 is configured to adjust the d-current Id based on the DC link voltage V2 such that the induced back EMF is reduced, especially below the DC link voltage V2. The adjustment of the d-current Id is, for example, inversely proportional to the DC link voltage V2.With an increasing DC link voltage V2, the d-current Id is reduced in magnitude, and with a decreasing DC link voltage V2, the d-current Id is increased in magnitude.

[0064] In particular, the control unit 4 is designed to always supply current to the electric motor, according to the d-current and / or the q-current, so that braking torques are prevented and in particular so that the power factor is dynamically adapted to the load.

[0065] In particular, the control unit 4 is designed to perform a variation of d-current and q-current depending on the intermediate circuit voltage V2 and / or mains voltage V1 in order to always control the motor optimally in order to require as little current as possible.

[0066] The power tool 1 can be operated in particular according to the method described below. The method comprises a first step in which the DC link voltage V2 is provided by means of the rectifier arrangement 12 with the DC link 15, based on the mains voltage V1 to which the power tool 1 is connected. The DC link voltage V2 has a majority of successive voltage half-waves 16. The method comprises a second step in which a corresponding torque half-wave is provided for each voltage half-wave 16 for controlling the electric motor 3. The waveform of the respective torque half-wave is flattened compared to the waveform of the voltage half-wave.

[0067] Advantageously, the method includes a further step in which the electric motor 3 is energized according to the torque half-waves, so that the rotor 21 is set into the drive rotary motion and thereby the tool 2 is set into the working motion.

[0068] Advantageously, the method includes a further step in which a workpiece is machined, in particular polished, ground or sawn, using tool 2.

Claims

1. Power tool (1), in particular a hand-held power tool, for example a polishing device (1A), grinding device and / or sawing device (1B), with a tool (2), an electric motor (3) for driving the tool (2) and a control unit (4) for controlling the electric motor (3), wherein the power tool (1) is designed for connection to a mains voltage (V1) and comprises a rectifier arrangement (12) with an intermediate circuit (15) for providing an intermediate circuit voltage (V2) based on the mains voltage (V1), the intermediate circuit voltage (V2) having a plurality of successive voltage half-waves (16), characterized in that the control unit (4) is configured to provide, for each voltage half-wave (16), a respective torque half-wave (36) for driving the electric motor (3), the waveform of which torque half-wave (36) is flattened with respect to the waveform of the voltage half-wave (16), wherein each torque half-wave (36) has torque minima (41), which together extend temporally over at least 10% of the period of the torque half-wave (36).

2. Power tool (1) according to claim 1, wherein the control unit (4) is configured to provide the torque half-wave (36) with a waveform that has, for the same area as a sine half-wave (37), a lower maximum than the sine half-wave (37).

3. Power tool (1) according to a preceding claim, wherein the flattened waveform results, for the same average torque, in a lower ohmic power loss when the electric motor (3) is energized.

4. Power tool (1) according to a preceding claim, wherein the control unit (4) is configured to provide the torque half-wave (36) with a trapezoidal waveform or a rectangular waveform.

5. Power tool (1) according to a preceding claim, wherein the control unit (4) is adapted to calculate the torque half-wave (36) using a waveform profile (29).

6. Power tool (1) according to a preceding claim, wherein the control unit (4) is configured to calculate the torque half-wave (36) on the basis of a torque setpoint (26).

7. Power tool (1) according to a preceding claim, wherein the control unit (4) is configured to calculate the torque half-wave (36) by scaling a / the waveform profile (29) according to a / the torque setpoint (26).

8. Power tool (1) according to a preceding claim, wherein the control unit (4) is configured to determine a phase angle (30) of the mains voltage (V1) and / or the intermediate circuit voltage (V2) and to provide the torque half-wave according to the phase angle (30).

9. Power tool (1) according to a preceding claim, wherein the control unit (4) has a closed-loop rotational speed controller (25) for providing a / the torque setpoint (26) on the basis of which the control unit (4) provides the respective torque half-wave (36).

10. Power tool (1) according to a preceding claim, wherein the control unit (4) is configured to energize the electric motor (3) in accordance with the torque half-wave (36).

11. Power tool (1) according to a preceding claim, wherein the control unit (4) is configured to calculate a d current (Id) and a q current (Iq) for controlling the electric motor (3) and to set the d current (Id) in accordance with the intermediate circuit voltage (V2), wherein preferably the control unit (4) is configured to reduce the d current (Id) in absolute value when the intermediate circuit voltage (V2) is higher and to increase the d current (Id) in absolute value when the intermediate circuit voltage (V2) is lower.

12. Power tool (1) according to one of the preceding claims, wherein the control unit (4) is configured to determine a countervoltage (34) induced in the electric motor (3) and, in response to the fact that the countervoltage (34) is greater than the intermediate circuit voltage (V2), to reduce a current torque value of the torque half-wave (36), in particular to set it to zero.

13. Method for operating a power tool (1), in particular a hand-held power tool, for example a polishing device (1A), grinding device and / or sawing device (1B), with a tool (2), an electric motor (3) for driving the tool (2) and a control unit (4) for controlling the electric motor (3), comprising the steps: - providing, by means of a rectifier arrangement (12) comprising an intermediate circuit (15), an intermediate circuit voltage (V2) based on a mains voltage (V1) to which the power tool (1) is connected, the intermediate circuit voltage (V2) comprising a plurality of successive voltage half-waves (16), and - for each voltage half-wave (16), providing a respective torque half-wave (36) for driving the electric motor (3), the waveform of which is flattened relative to the waveform of the voltage half-wave (16), wherein each torque half-wave (36) has torque minima (41), which together extend temporally over at least 10% of the period of the torque half-wave (36).

14. Method according to claim 13, wherein the power tool (1) is adapted in accordance with one of claims 1 to 12.

15. Computer program product comprising instructions which cause the power tool (1) according to claim 1 to carry out the method steps in accordance with one of claims 13 to 14.