Method for determining the rotor position of an electric motor of a power tool and power tool

Abstract

The present invention relates to a method for determining the rotor position of an electric motor (2) of a power tool (10), in particular an electric screwdriving tool, comprising the steps of: checking (S4) whether a first electric motor current (I1) is below a threshold (SW), and, in response to the first electric motor current (I1) obtained from the checking being below the threshold (SW), determining the rotor position of the electric motor (2) based on the first electric motor current (I1) and / or a second electric motor current.

Description

Field of the Invention

[0001] The present invention relates to a method for determining the rotor position of an electric motor, in particular of an electric screwdriving tool, of a power tool. Background Art

[0002] EP0579694B1, EP1051801B1, EP1746718B1 and DE102016222754A1 each describe a method for detecting the rotor position based on the electric motor current. This method is based on the correlation between the winding inductance of the electric motor and the rotor position. A change in the rotor position causes a change in the winding inductance, which in turn causes a change in the electric motor current.

[0003] The detection of the rotor position based on the electric motor current should also be referred to as a sensorless detection of the rotor position. Summary of the Invention

[0004] The object of the present invention is to provide a method by means of which a reliable determination of the rotor position can be achieved in an effective manner.

[0005] This object is solved by the method according to claim 1. The method comprises the steps of checking whether a first electric motor current is below a threshold value and, in response to the first electric motor current being below the threshold value as a result of said check, determining the rotor position of the electric motor based on the first electric motor current and / or a second electric motor current.

[0006] By detecting the rotor position based on the electric motor current (i.e., in particular sensorlessly), a corresponding sensing mechanism for detecting the rotor position, such as a signal emitter, can be dispensed with. Advantageously, the power tool does not include a sensor for detecting, in particular directly detecting, the rotor position. In particular, within the scope of this method, a sensor for detecting, in particular directly detecting, the rotor position is not used.

[0007] Furthermore, by checking whether the electric motor current is below the threshold value, it can be ensured that the determination of the rotor position is reliable. The electric motor current is advantageously used as an indicator for the magnetic saturation of the electric motor. In the case of magnetic saturation of the electric motor, the correlation between the winding inductance and the rotor position is usually only very small or no longer given. Therefore, in the case of magnetic saturation of the electric motor, the rotor position can usually no longer be reliably deduced from the electric motor current (which depends on the winding inductance).

[0008] To avoid this problem, it is common practice to dimension the electric motor so large that magnetic saturation does not occur during normal operation, thus avoiding the influence of magnetic saturation on the detection of the rotor position based on the electric motor current.

[0009] In contrast, in the present method, magnetic saturation is suitably allowed during a defined operation. The electric motor of the power tool does not have to be dimensioned accordingly so large that magnetic saturation does not occur during the defined operation.

[0010] Nevertheless, a reliable detection of the rotor position can be carried out with the power tool. By checking whether the electric motor current is higher than a threshold value before detecting the rotor position and detecting the rotor position in response to the electric motor current being lower than the threshold value, it can be suitably ensured that the detection of the rotor position takes place in a state in which there is no magnetic saturation of the electric motor and the detection of the rotor position is suitably not affected. Therefore, a reliable detection of the rotor position can be achieved in an effective manner.

[0011] Suitably, the electric motor is operated in non-overlapping operating phases that are successive in time. The operating phases particularly include a position determination phase in which the rotor position is measured and in which a first electric motor current is less than the threshold value. Suitably, the operating phases furthermore include a torque phase in which the rotor position is not measured and in which the first electric motor current is greater than the threshold value. Suitably, the power tool uses the rotor position measured in the position determination phase to control the electric motor during the torque phase.

[0012] Advantageous refinements are the subject matter of the dependent claims.

[0013] The invention furthermore relates to a power tool, in particular an electric screwdriving tool, having an electric motor. The power tool is configured to determine the rotor position of the electric motor based on the electric motor current. The power tool is configured to check whether a first electric motor current is lower than a threshold value and, in response to the first electric motor current being lower than the threshold value determined by the check, to determine the rotor position of the electric motor based on the first electric motor current and / or a second electric motor current.

[0014] The invention furthermore relates to a control mechanism for a power tool having an electric motor. The control mechanism is configured to check whether a first electric motor current is lower than a threshold value and, in response to the first electric motor current being lower than the threshold value determined by the check, to determine the rotor position of the electric motor based on the first electric motor current and / or a second electric motor current. Description of the Drawings

[0015] Further exemplary details and exemplary embodiments are subsequently explained with reference to the figures. Here,

[0016] Figure 1 a schematic illustration of the power tool is shown,

[0017] Figure 2 a sectional view of the electric motor is shown,

[0018] Figure 3 A circuit diagram showing the control circuit,

[0019] Figure 4 A flow chart showing the control procedure,

[0020] Figure 5 The temporal course of the electric motor current is shown. DETAILED DESCRIPTION

[0021] Figure 1 A power tool 10 according to an exemplary embodiment is shown. The power tool 10 is exemplarily embodied here as an electric screwdriver, in particular as a battery-powered screwdriver. Alternatively, the power tool 10 can be embodied as another power tool, in particular as a power tool with a rotating tool. The power tool can be embodied, for example, as a saw, grinder, drill, and / or milling cutter. The power tool 10 is in particular a manually guided power tool. Alternatively, the power tool 10 can be a stationary or semi-stationary power tool.

[0022] The power tool 10 comprises an electric motor 2. The power tool 10 is designed to check whether a first electric motor current I1 is below a threshold value SW. An exemplary temporal profile of the electric motor current I1 together with the threshold value SW is shown in FIG. Figure 5 The electric tool 10 is configured to determine the rotor position of the electric motor 2 based on the first electric motor current I1 and / or the second electric motor current in response to the first electric motor current I1 obtained from the check being lower than the threshold value SW. The second electric motor current includes, for example, Figure 3 One or more of the branch currents IZ1, IZ2, IZ3 shown in FIG and / or Figure 3 The total current IS is shown in FIG.

[0023] The power tool 10 is particularly configured to perform a determination of the rotor position based on the first and / or second electric motor current in response to the electric motor current I1 determined by the check being below a threshold value SW. Preferably, the power tool 10 is configured not to perform a determination of the rotor position based on the first and / or second electric motor current in response to the electric motor current I1 determined by the check being above the threshold value SW. The power tool 10 is particularly configured to perform a determination of the rotor position based on the first and / or second electric motor current only if the check indicates that the electric motor current I1 is below the threshold value SW.

[0024] As mentioned at the outset, the first motor current I1 serves as an indicator for the magnetic saturation of the electric motor 2. By determining the rotor position when the motor current I1 is below the threshold value SW (i.e., in particular when it can be assumed that no (significant) magnetic saturation is present), it can be ensured that the determination of the rotor position is not influenced by the magnetic saturation of the electric motor 2.

[0025] Subsequently, further exemplary details are explained.

[0026] First, about power tools:

[0027] The power tool 10 includes an electric drive 1. The electric drive 1 includes an electric motor 2. Optionally, the electric drive 1 also includes a transmission mechanism (not shown in the figure). The electric motor 2 is preferably configured as a brushless DC motor. In particular, the electric motor 2 is a three-phase synchronous motor excited by permanent magnets. Preferably, the electric motor 2 is an EC motor (electronically commutated motor).

[0028] The power tool 10 further comprises a tool 3, for example a screwdriver, which can be driven by the electric motor 2. The tool 3 can in particular be set into rotation by the electric motor 2. The tool 3 is coupled to the electric motor 2, for example, via a shaft 15 (and / or an optionally present transmission).

[0029] Furthermore, the power tool 10 expediently comprises a control device 4 and an operating device 5. Optionally, the power tool 10 also comprises an energy storage device 6, for example a battery.

[0030] The control device 4 advantageously includes a computing unit, such as a microcontroller. The control device 4 is used, in particular, to perform the aforementioned check of whether the first electric motor current I1 is below a threshold value SW and / or to determine the rotor position based on the first and / or second electric motor current. The control device 4 advantageously includes a control circuit 8 for providing a control voltage for the electric motor 2. An exemplary embodiment of the control circuit 8 is shown in FIG. Figure 3 Shown in.

[0031] The operating mechanism 5 exemplarily includes a button, in particular a trigger button. Via the operating mechanism 5 , a user can expediently control the drive of the tool 3 via the electric motor 2 , in particular, the speed and / or torque at which the tool 3 is driven. The control mechanism 4 is expediently configured to detect an actuation of the operating mechanism 5 and to control the electric motor 2 based on the actuation.

[0032] The energy for operating the electric tool 10 , in particular the control device 4 and / or the electric motor 2 , is expediently provided by the energy storage device 6 .

[0033] The power tool 10 comprises, by way of example, a handle 7 , by means of which the power tool 10 can be grasped and carried by a user, in particular with one hand.

[0034] By way of example, the power tool 10 comprises a housing in which the electric drive 1, the control mechanism 4 and / or the energy storage device 6 are arranged. Preferably, the operating mechanism 5 is arranged externally on the housing. Expediently, the handle 7 forms part of the housing.

[0035] Then, you should refer to Figure 2 Let's discuss an exemplary design of the electric motor 2:

[0036] The electric motor 2 includes a stator 9 and a rotor 11. The stator 9 includes a plurality of windings 12 distributed around the rotation axis of the rotor 11. The rotor 11 includes a plurality of permanent magnets 14 distributed around the rotation axis of the rotor 11.

[0037] The rotor 11 is preferably designed to be non-radially symmetrical, particularly with respect to its magnetic properties. The rotor's influence on the inductance of the winding 12 changes depending on the rotor position of the rotor 11. The inductance of the winding 12 is expediently dependent on the rotor position of the rotor 11. The term "rotor position" refers in particular to the rotational position of the rotor 11 about its axis of rotation. According to an alternative embodiment, the rotor 11 is designed to be radially symmetrical.

[0038] Expediently, winding 12 is energized by control circuit 7 in such a way that a rotating field is provided which rotates about the rotation axis of rotor 11 and which, through interaction with permanent magnets 14 , drives rotor 11 in a rotational movement about its rotation axis.

[0039] The stator 9 and / or the rotor 11 advantageously comprise a magnetizable, in particular ferromagnetic, material, such as iron. Above a certain current intensity of the current flowing through the winding 12, the magnetizable material enters magnetic saturation. In the event of magnetic saturation, the dependence of the inductance of the winding 12 (and thus the current flowing through the winding 12) on the rotor position is reduced and / or eliminated.

[0040] The magnetic saturation of the aforementioned magnetizable material of the electric motor 2, in particular the stator 9 and / or the rotor 11, is advantageously indicated by a threshold value SW. Advantageously, the threshold value SW corresponds to the current intensity of the first electric motor current I1 at which the magnetic saturation of the aforementioned magnetizable material of the electric motor 2, in particular the stator 9 and / or the rotor 11, is achieved. The threshold value SW is advantageously determined in advance and stored in the electric tool 10, in particular in the control device 4.

[0041] The first electric motor current I1 suitably comprises one, more or all of the currents flowing through the winding 12. For example, the first electric motor current I1 is the sum of the currents flowing through the winding 12. In particular, the first electric motor current I1 is the sum of the currents flowing into or out of the winding 12.

[0042] Subsequently, reference should be made to Figure 3 a more detailed discussion of the control of the electric motor 2:

[0043] Figure 3 An exemplary design of the control circuit 8 is shown. The control circuit 8 is suitably configured as an inverter. The control circuit 8 is particularly used to provide a plurality of control signals for controlling the electric motor 2, in particular the winding 12. Exemplarily, the control circuit 8 provides three control signals. The control circuit 8 exemplary has a first output U for providing a first control signal, a second output V for providing a second control signal, and a third output W for providing a third control signal.

[0044] Suitably, the control circuit 8 provides control signals based on the supply voltage Ud present between two connection points AP1, AP2. The supply voltage Ud is suitably a DC voltage.

[0045] The three control signals can also be referred to as phases. The control signals preferably relate to voltage signals.

[0046] The control circuit 8 exemplary includes corresponding circuit branches SZ1, SZ2, SZ3 for each of the outputs U, V, W. The circuit branches SZ1, SZ2, SZ3 are correspondingly connected between the two connection points AP1, AP2. The circuit branches SZ1, SZ2, SZ3 are exemplary implemented as half-bridges respectively. Each circuit branch SZ1, SZ2, SZ3 suitably includes two switches, namely a first switch S11, S21, S31 and a second switch S12, S22, S32. The respective outputs U, V, W can be connected to the first connection point AP1 via the respective first switches S11, S21, S31 and to the second connection point AP2 via the respective second switches S12, S22, S33.

[0047] By controlling the first and second switches S11, S21, S31, S12, S22, S32, it is suitably possible to provide alternating signals, in particular alternating signals phase-shifted relative to each other, as control signals in order to drive the rotor 11.

[0048] The power tool 10 is configured to detect a first electric motor current I1 and / or a second electric motor current. The first electric motor current I1 and / or the second electric motor current is the current flowing through one, more or all of the windings 12 of the electric motor 2. The first electric motor current I1 and the second electric motor current can be the same current or can be different currents.

[0049] Exemplarily, the first electric motor current I1 is the total current flowing through the winding 12. The current is exemplarily detected as the total current IS flowing into the second connection point AP2. Alternatively or additionally thereto, the total current can also be detected as the total current IS flowing into or out of the first connection point AP1.

[0050] The power tool 10, in particular the control circuit 8, exemplarily includes a total current measuring unit SM for detecting the total current IS. The total current measuring unit SM is exemplarily connected between the second connection point AP2 and the second switches S12, S22, S32.

[0051] The first electric motor current I1 can furthermore include one, more or all of the branch currents IZ1, IZ2, IZ3 flowing in the circuit branches SZ1, SZ2, SZ3. Suitably, the power tool 10 is configured to compare one, more or all of the branch currents IZ1, IZ2, IZ3 with a threshold value SW respectively and to perform the determination of the rotor position in response to each of the compared branch currents IZ1, IZ2, IZ3 being less than the threshold value SW.

[0052] The power tool 10, in particular the control circuit 8, exemplarily includes a plurality of branch current measuring units ZM1, ZM2, ZM3 for detecting the respective branch currents IZ1, IZ2, IZ3. The branch current measuring units ZM1, ZM2, ZM3 are respectively connected in the respective circuit branches SZ1, SZ2, SZ3.

[0053] As an alternative to the shown design (wherein there is a respective branch current measuring unit ZM1, ZM2, ZM3 for each circuit branch SZ1, SZ2, SZ3), there can also be fewer branch current measuring units than circuit branches. For example, there can be no branch current measuring unit for one circuit branch. Then, the branch current of the circuit branch is suitably calculated by the power tool 10, in particular based on the other branch currents and the total current.

[0054] In the following, the determination of the rotor position based on the first and / or second electric motor current should be discussed.

[0055] Suitably, the power tool 10 is configured to load the electric motor 2 with a test signal and determine the rotor position based on the response of the first and / or second electric motor current to the test signal. Suitably, the rotor 11 is not driven by loading it with the test signal. The test signal suitably includes the sequence of connection states and / or the voltage values for the outputs U, V, W, which are obtained by actuating the first and second switches S11, S21, S31, S12, S22, S32.

[0056] The specific test signal for actuating the electric motor 2 to determine the rotor position is described in the prior art mentioned at the beginning. The test signal is especially known from the so-called "notification process". Suitably, the power tool 10 is configured to actuate the electric motor 2 with a test signal according to the notification process.

[0057] The first and / or second electric motor current can be changed in response to the test signal. The power tool 10 is configured to detect the first and / or second electric motor current and determine the rotor position based on this. Suitably, the power tool 10 detects the branch currents IZ1, IZ2, IZ3 as the second electric motor current and determines the rotor position based on the branch currents IZ1, IZ2, IZ3, especially based on the temporal course and / or certain signal characteristics of the branch currents IZ1, IZ2, IZ3.

[0058] Alternatively or additionally, the power tool 10 detects two of the branch currents IZ1, IZ2, IZ3 as the second electric motor current and determines the rotor position based on the two detected branch currents, especially based on the temporal course and / or certain signal characteristics of the two detected branch currents.

[0059] Alternatively or additionally, the power tool 10 detects the sum current IS as the second electric motor current and determines the rotor position based on the sum current IS, especially based on the temporal course and / or certain signal characteristics of the sum current IS.

[0060] The specific mathematical method for determining the rotor position based on the branch currents is described in the prior art mentioned at the beginning. The mathematical method is especially known from the notification process. Suitably, the power tool 10 is configured to determine the rotor position by a mathematical method according to the notification process.

[0061] The power tool 10 is in particular configured to perform the control, in particular commutation, of the electric motor 2 based on the determined rotor position. The power tool 10 is in particular configured to perform sensorless commutation of the electric motor 2 in the case of applying the determined rotor position. The power tool 10 is in particular configured to provide control signals provided at the outputs U, V, W based on the determined rotor position. Suitably, the power tool 10 provides the control signals provided at the outputs U, V, W based on the determined rotor position and based on the user input, such as the required rotational speed and / or the required torque, input via the operating mechanism 5.

[0062] The power tool 10 is suitably configured to continue the control, in particular commutation, of the electric motor 2 based on the finally determined rotor position for as long as there is a newly determined rotor position. When, for example, the first electric motor current I1 is higher than the threshold SW and the power tool 10 does not perform the determination of the rotor position for this reason at this time, the power tool 10 will thereby additionally apply the finally determined rotor position to the control of the electric motor 2 at this time. Suitably, the power tool 10 electrically performs a 90-degree rotation of the rotor 11 by means of the control of the electric motor 2 based on the determined first rotor position and continues the control based on the first rotor position as long as a new rotor position of the rotor 11 cannot be determined. This can cause the rotor 11 to remain stationary after the 90-degree electrical rotation until the power tool 10 has determined a new rotor position and performs the control based on the new rotor position. The 90-degree electrical rotation corresponds, for example, to a 45-degree mechanical rotation of the rotor in the case of two pole pairs and a 30-degree mechanical rotation in the case of three pole pairs.

[0063] Subsequently, reference should be made to Figure 4 and 5 the exemplary control program AP for controlling the electric motor 2 will be discussed in more detail. The control program is an embodiment of a method for determining the rotor position of the electric motor 2 of the power tool 10.

[0064] Suitably, the control program AP is implemented by the power tool 10, in particular by the control mechanism 4.

[0065] The control program AP starts with an optional step S1, in which the rotor position is determined based on the first and / or second electric motor current, suitably in a state in which the rotor 11 has not yet been driven and suitably has no movement. After determining the rotor position, an optional step S2 is performed, in which the electric motor 2 is started; that is, in particular, the first electric motor current I1 is increased such that the rotor 11 is driven and suitably set in motion.

[0066] If the control program AP is started in the following state, in which the electric motor 2 is already running, steps S1 and S2 are not necessary and are suitably absent.

[0067] The control program AP continues with step S3, in which the electric motor 2 is controlled, in particular the winding 12 is energized, in order to generate a torque with which the tool 3 is loaded. In step S3, the winding 12 can be energized so strongly that the first electric motor current I1 rises above the threshold SW or the first electric motor current I1 remains below the threshold SW.

[0068] Preferably, in step S3, the electric motor 2 is controlled based on the rotor position determined in the previous steps (for example S2, S5 or S8).

[0069] In step S4, it is checked whether the first electric motor current I1 is less than the threshold SW. As long as it is determined during the check that the first electric motor current I1 is less than the threshold SW, the control program continues with step S5, in which the rotor position is determined based on the first and / or second electric motor current. Suitably, in step S5, the electric motor 2, in particular the winding 12, is controlled with a test signal. Furthermore, in step S5, the branch currents IZ1, IZ2, IZ3 resulting from the test signal are suitably detected and the rotor position is determined based on the branch currents IZ1, IZ2, IZ3. The control program AP then suitably returns to step S3.

[0070] As long as it is determined during the check that the first electric motor current I1 is greater than the threshold SW, the control program continues with steps S6 and S7.

[0071] Suitably, the power tool 10 is configured not to perform the determination of the rotor position based on the first and / or second electric motor current in response to the electric motor current I1 being higher than the threshold SW determined by the check. The power tool 10 is in particular configured not to perform the control of the electric motor 2 with a test signal and / or not to perform the detection of the branch currents IZ1, IZ2, IZ3 resulting from the test signal and / or not to perform the determination of the rotor position based on the branch currents IZ1, IZ2, IZ3 when the check determines that the electric motor current I1 is higher than the threshold SW. Accordingly, in steps S6 and S7, the determination of the rotor position based on the first and / or second electric motor current is not performed.

[0072] Suitably, the power tool 10 is configured to reduce the electric motor current I1 until the electric motor current I1 is below the threshold value SW in response to the electric motor current I1 resulting from the check being above the threshold value SW. This is done in the control program AP, exemplarily, by stopping the control of the electric motor 2 in step S6. For example, the motor phase of the electric motor 2, in particular the control signal, is disconnected. Exemplarily, the control circuit 8 is connected in such a way that the current flowing in the winding 12 is reduced. For example, all first switches S11, S21, S31 are opened (that is, set to "non-conductive") and all second switches S21, S22, S32 are closed (that is, set to "conductive"). Alternatively, all first switches S11, S21, S31 are closed and all second switches S21, S22, S32 are opened.

[0073] In step S7 , for example, while the activation of electric motor 2 continues to be stopped, a wait is carried out until the first electric motor current I1 falls below a threshold value SW.

[0074] The power tool 10 is expediently designed to determine the rotor position based on the first and / or second electric motor current when the first electric motor current I1 falls below a threshold value SW. The rotor position is determined, for example, in step S8.

[0075] The control program then continues with step S3 . The electric tool is expediently designed to increase the first electric motor current I1 above the threshold value SW after the rotor position has been determined, for example within the scope of step S3 .

[0076] You should then refer to Figure 5 The different phases which are passed through when actuating the electric motor, in particular when executing the actuation program AP, are explained.

[0077] Figure 5 The temporal profile of the first electric motor current I1 is shown together with the successively executed operating phases of the electric tool 10. The operating phases comprise, by way of example, position determination phases PP1, PP2, PP3, torque phases DP1, DP2 and reduction phases RP1, RP2.

[0078] Exemplarily, the operating phases are sequentially executed in the following order: position determination phase, torque phase, lowering phase. The operating phases are expediently repeated multiple times, in particular continuously, in the aforementioned order. In the example shown, the operating phases are sequentially executed in the following order: first position determination phase PP1, first torque phase DP1, first lowering phase RP1, second position determination phase PP2, second torque phase DP2, second lowering phase RP2, and third position determination phase PP3.

[0079] Suitably, the power tool 10 is configured to start with a first position determination phase PP1. In the first position determination phase PP1, the first electric motor current I1 is lower than a threshold SW. The power tool 10 is configured to determine the rotor position based on the first and / or second electric motor current during the first position determination phase PP1.

[0080] In the first position determination phase PP1, step S2 is particularly executed and optionally step S1 is executed before it.

[0081] Suitably, the power tool 10 is configured to execute a first torque phase DP1 after the position determination phase and to increase the first electric motor current I1 above the threshold SW during the first torque phase DP1. Suitably, the power tool 10 is configured not to perform the determination of the rotor position based on the first and / or second electric motor current during the first torque phase DP1.

[0082] In the first torque phase DP1, steps S3 and S4 are particularly executed.

[0083] Suitably, the power tool 10 applies the rotor position determined in the first position determination phase PP1 to control the electric motor 2 in the first torque phase DP1.

[0084] Suitably, the power tool 10 is configured to execute a first reduction phase RP1 after the first torque phase DP1 and to reduce the first electric motor current I1 below the threshold SW during the first reduction phase RP1.

[0085] In the first reduction phase, steps S6 and S7 are particularly executed.

[0086] Suitably, then, particularly when the power tool 10 determines that the first electric motor current I1 is less than the threshold SW within the inspected range, the power tool 10 continues with a second position determination phase PP2. The power tool 10 executes the second position determination phase PP2 as in the first position determination phase PP1 and then continues with the subsequent operating phase in the manner explained above.

Claims

1. A method for determining the rotor position of an electric motor (2) of a power tool (10), comprising the following steps: Check (S4) whether a first electric motor current (I1) of the electric motor (2) is below a threshold value (SW), and determine (S5) a rotor position of a rotor (11) of the electric motor (2) based on the first electric motor current (I1) and / or a second electric motor current of the electric motor (2) in response to the first electric motor current (I1) obtained from the check being below the threshold value (SW), wherein, Loading the electric motor (2) with a test signal and determining the rotor position based on the response of the first electric motor current (I1) and / or the second electric motor current (I2) to the test signal, wherein the rotor (11) is not driven by the loading of the test signal; Furthermore, it includes: in response to the first electric motor current (I1) obtained from the check being higher than the threshold (SW), reducing (S6, S7) the first electric motor current (I1) until the electric motor current (I1) is lower than the threshold (SW); Wherein, the first electric motor current (I1) is used as an indicator of the magnetic saturation of the electric motor (2), and when the first electric motor current (I1) is lower than the threshold (SW), magnetic saturation of the electric motor (2) is not reached; furthermore, it includes: after reducing the first electric motor current (I1) to be lower than the threshold (SW), determining (S8) the rotor position based on the first and / or second electric motor current.

2. The method according to claim 1, wherein, Magnetic saturation of the electric motor (2) indicates that the first electric motor current (I1) exceeds the threshold (SW).

3. The method according to claim 1 or 2, further comprising: Checking (S4) whether the first electric motor current (I1) is lower than the threshold (SW), and not determining the rotor position based on the first electric motor current (I1) and / or the second electric motor current in response to the first electric motor current (I1) obtained from the check being higher than the threshold (SW).

4. The method according to claim 1 or 2, further comprising: After determining the rotor position, raising (S3) the first electric motor current (I1) to be higher than the threshold (SW).

5. The method according to claim 1 or 2, wherein When controlling the electric motor (10), a position determination phase (PP1, PP2, PP3), a torque phase (DP1, DP2), and a reduction phase (RP1, RP2) are sequentially executed.

6. The method according to claim 5, wherein, Executing the position determination phase (PP1, PP2, PP3) in response to the first electric motor current (I1) being lower than the threshold (SW), and determining the rotor position based on the first and / or second electric motor current within the position determination phase (PP1, PP2, PP3).

7. The method according to claim 5, wherein, Executing the torque phase (DP1, DP2) after the position determination phase (PP1, PP2, PP3) and raising the first electric motor current (I1) to be higher than the threshold (SW) within the torque phase (DP1, DP2), and not performing the determination of the rotor position based on the first and / or second electric motor current within the position determination phase (PP1, PP2, PP3).

8. The method according to claim 5, wherein, After the torque phase (DP1, DP2), executing the reduction phase (RP1, RP2) and reducing the first electric motor current (I1) to be lower than the threshold (SW) within the reduction phase (RP1, RP2).

9. The method according to claim 5, wherein The position determination phases (PP1, PP2, PP3), the torque phases (DP1, DP2), and the reduction phases (RP1, RP2) are repeatedly carried out in the mentioned sequence a plurality of times.

10. The method according to claim 1 or 2, wherein, The electric motor (2) is controlled based on the determined rotor position.

11. The method according to claim 1 or 2, wherein, The electric motor (2) is a brushless DC motor.

12. The method according to claim 1 or 2, further comprising: The electric motor (2) is loaded with a test signal and the rotor position is determined based on the response of the first and / or second electric motor current to the test signal.

13. The method according to claim 1, wherein, The power tool is an electric screwdriving tool.

14. The method according to claim 10, wherein, Rectification of the electric motor (2) is carried out based on the determined rotor position.

15. The method according to claim 2, wherein, The threshold value (SW) corresponds to the current intensity of the first electric motor current (I1) at which magnetic saturation of the electric motor (2) occurs.

16. Power tool (10), having an electric motor (2), wherein, The power tool (10) is configured to check whether the first electric motor current (I1) of the electric motor (2) is below a threshold value (SW), and, in response to the first electric motor current (I1) being below the threshold value (SW) resulting from the check, to determine the rotor position of the rotor (11) of the electric motor (2) based on the first electric motor current (I1) and / or the second electric motor current of the electric motor (2), wherein the power tool (10) is configured to load the electric motor (2) with a test signal and to determine the rotor position based on the response of the first electric motor current (I1) and / or the second electric motor current (I2) to the test signal, wherein the rotor (11) is not driven by the loading with the test signal; wherein the power tool (10) is furthermore configured to reduce (S6, S7) the first electric motor current (I1) until the electric motor current (I1) is below the threshold value (SW) in response to the first electric motor current (I1) resulting from the check being above the threshold value (SW); wherein the first electric motor current (I1) serves as an indicator for magnetic saturation of the electric motor (2), wherein magnetic saturation of the electric motor (2) does not occur when the first electric motor current (I1) is below the threshold value (SW); wherein the power tool (10) is furthermore configured to determine (S8) the rotor position based on the first and / or second electric motor current after reducing the first electric motor current (I1) to below the threshold value (SW).

17. The power tool (10) according to claim 16, wherein, The power tool (10) is configured to carry out the method according to any one of claims 1 to 15.

18. The power tool (10) according to claim 16, wherein, The power tool is an electric screwdriving tool.

19. A control mechanism (4) for a power tool (10) having an electric motor (2), wherein, The control mechanism (4) is configured to check whether a first electric motor current (I1) of the electric motor (2) is below a threshold value (SW), and to determine a rotor position of a rotor (11) of the electric motor (2) based on the first electric motor current (I1) and / or a second electric motor current of the electric motor (2) in response to the first electric motor current (I1) being below the threshold value (SW) as a result of the check, wherein the control mechanism (4) is arranged to load the electric motor (2) with a test signal and to determine the rotor position based on a response of the first electric motor current (I1) and / or the second electric motor current (I2) to the test signal, wherein the rotor (11) is not driven by the loading with the test signal; wherein the control mechanism (4) is furthermore configured to reduce (S6, S7) the first electric motor current (I1) until the electric motor current (I1) is below the threshold value (SW) in response to the first electric motor current (I1) being above the threshold value (SW) as a result of the check; wherein the first electric motor current (I1) serves as an indicator for magnetic saturation of the electric motor (2), wherein magnetic saturation of the electric motor (2) is not present when the first electric motor current (I1) is below the threshold value (SW); wherein the control mechanism (4) is furthermore configured to determine (S8) the rotor position based on the first and / or second electric motor current after reducing the first electric motor current (I1) to below the threshold value (SW).

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