Method for controlling an electric motor with a mechanical commutator

Abstract

A method for controlling an electric motor (10) having a mechanical commutator (12), wherein the moment of commutation is determined by means of a sensor (36) or without a sensor. Furthermore, the electric motor (10) is controlled by means of a supply voltage signal (16) having a pulse sequence. Finally, the supply voltage signal (16) is modulated by means of a modulation signal to reduce its magnitude at the commutation moment (24).

Description

Technical Field

[0001] The present invention relates to a method for controlling an electric motor having a mechanical commutator. Background Technology

[0002] Electric motors (used for direct current and alternating current) have commutators, which are either implemented mechanically and therefore have brushes, or implemented electronically, thus making the motor brushless. With mechanical commutators that have brushes, sparks form on the brushes, causing them to wear down over time due to corrosion. Furthermore, this so-called "brushing" generates noise and unwanted electromagnetic radiation.

[0003] DE-A-10 2010 017 835 discloses a method for processing motor variables of a DC motor in a motor vehicle servo drive, and a servo device for this purpose. In this known method and in this known device, it should be ensured that supply voltage fluctuations are compensated.

[0004] DE-A-10 2008 018 818 describes a method for controlling an electric motor with load torque adaptation, while DE-A-10 2016 101 905 discloses a method for controlling an electric motor for controlling an adjustment element of a motor vehicle. Summary of the Invention

[0005] The objective of this invention is to provide a method for operating an electric motor with a mechanical commutator, wherein the brushes of the mechanical commutator do not wear out significantly.

[0006] To address this task, this invention proposes a method for controlling an electric motor with a mechanical commutator, wherein in this method...

[0007] - Determine the timing of commutation using sensors or without sensors.

[0008] - The motor is controlled by means of a supply voltage signal with a pulse sequence, and

[0009] - The supply voltage signal is modulated by a modulation signal to reduce its magnitude at the commutation moment.

[0010] The electric motor with a mechanical commutator according to the invention is controlled by means of a supply voltage signal having a pulse sequence. This signal is typically a pulse width modulated (PWM) or pulse density modulated (PDM) signal. Now, according to the invention, this supply voltage signal is additionally modulated by means of a modulation signal, such that the effective magnitude of the supply voltage at the commutation moment is reduced, and substantially reduced to zero volts (see below for the meaning of "substantially zero volts"). This reduces spark formation, noise generation, and electromagnetic radiation at the commutation moment. In particular, it reduces the risk of brush wear due to spark corrosion.

[0011] In addition to the two modulation methods mentioned above, pulse amplitude modulation (PMA), pulse frequency modulation (PFM), pulse width modulation (PWM), pulse pause modulation (PPM), pulse phase modulation (PPM), and pulse position modulation (PPM) such as in Manchester code are also suitable.

[0012] The method according to the invention requires knowledge of the commutation time. This can be done by means of a sensor or without a sensor, methods known in principle in the prior art. Hall sensors are advantageously suited as sensors. Examples of sensorless determination of the commutation time based on the current ripple in the probed armature current signal are described in DE 42 17 265C2 and DE 195 11 307 C1.

[0013] As described above, the supply voltage of the motor is provided by means of a supply voltage signal, for example, through pulse width modulation or pulse density modulation. Now, in order for this pulsed supply voltage signal to result in a reduced supply voltage at the commutation moment, the pulse width or pulse density is modulated by means of a modulation signal, so that ideally a supply voltage of essentially zero volts is generated at the commutation moment. "Effectively zero volts" here means, for example, that the magnitude of the supply voltage is less than 20%, 15%, 10%, or 5% of the nominal supply voltage. Although the magnitude of the supply voltage at the commutation moment is reduced, the average magnitude of the supply voltage should continue to remain at the level required for the current operation of the motor. In the case of a DC motor, this means that the DC voltage component of the supply voltage signal remains unchanged by the additional modulation of the modulation signal and the resulting pulsed supply voltage signal. This is advantageously achieved in this case by the modulation signal being an alternating signal, for example, a sinusoidal signal. The phase angle of the modulation signal is selected such that the modulation signal takes a minimum value at the commutation moment.

[0014] Advantageous in the method according to the invention is that minimal ripple appears in the armature current signal during sensorless commutation moment detection. However, this is not necessarily permanent. That is, the ripple may also disappear completely, at least temporarily, because regulation is only designed to be permanently "idealized" in very rare cases—which results in no more ripple in the armature current signal, and thus ripple will always reappear from time to time. However, if commutation moment detection is performed without sensors, this in itself is advantageous for the method according to the invention, because regulation can then be "activated" repeatedly.

[0015] The situation is different when a corresponding sensor system is used to detect the commutation moment.

[0016] Other advantageous designs of the invention are the subject of the dependent claims.

[0017] The present invention allows for the upgrading of existing electric motors with mechanical commutators, particularly DC motors, through the control method according to the invention. The control method according to the invention also enables the use of cost-effective electric motors with mechanical commutators in areas where brushless motors have historically been used due to brush wear, interfering noise generation, and EMV interference. Attached Figure Description

[0018] The invention will now be explained in more detail with reference to embodiments and the accompanying drawings. Specifically, here:

[0019] Figure 1 A block diagram schematically illustrates the main components for controlling a DC motor according to the present invention.

[0020] Figures 2a to 2e Different signal variation curves (Verlauf) are shown to illustrate the modulation control of a DC motor for reducing brush wear according to the present invention. Sine curves and modulation via pulse width modulation (i.e., by means of pulse width modulation) are used exemplarily here. Figure 2d The implementation of ) and

[0021] Figure 3a and Figure 3b The curves showing the variation of the alternating modulated signals are illustrated. Detailed Implementation

[0022] Figure 1A block diagram illustrating the operation of a DC motor 10 is shown. The motor 10 is equipped with a mechanical commutator 12, which typically has two brushes 14 to which a supply voltage U is applied. The supply voltage U is provided by means of a pulsed supply voltage signal 16, which in this embodiment is generated by a PWM generator 18. The PWM generator 18 generates a pulse-width modulated pulse sequence that results in a DC voltage component used to control the motor 10 based on current load requirements.

[0023] The pulse sequence of the power supply voltage signal 16 in Figure 2a This is illustrated exemplarily. Without the measures according to the invention, if it is assumed that the load requirements do not change, the pulse width of each pulse 20 is substantially static. Figure 2b The exemplary curve shown in the figure illustrates the armature current variation curve 22 encountered, when according to Figure 2a This curve appears when the motor 10 is controlled by the supply voltage signal 16. At the commutation moment 24, a current ripple 26 is formed in the armature current signal 22, which causes wear and interference, which should be avoided in this invention.

[0024] Therefore, according to the present invention, the supply voltage signal 16 having a pulse sequence is further modulated, and by means of the modulation signal 28 generated by the modulator 30. Figure 2c In this embodiment, the modulation signal 28 is a sinusoidal signal, and its amplitude 32 is substantially the same as the magnitude of the DC voltage component 34 of the supply voltage U. The position and frequency of the modulation signal 28 are now selected such that the minimum value of the sinusoidal signal (modulation signal 28) is in phase with the commutation time 24. This modulation of the supply voltage signal 16 does not change the DC voltage component 34 of the supply voltage signal 16, but the magnitude of the supply voltage U changes at the commutation time 24, and here it is exemplarily changed to zero volts.

[0025] In this embodiment, the determination of the commutation time 24 is based on Figure 1 The circuit block diagram is constructed using sensor 36, whose output signal 38 can also be fed to control unit 40, such as a microcontroller, which in turn outputs output signal 42 to modulator 30. Alternatively, the output signal 38 of sensor 36 can also be fed directly to modulator 30; the sensor is typically a Hall sensor. If the commutation time 24 is determined without sensors, it may be necessary to evaluate a computing unit, such as a microcontroller. Corresponding techniques and methods for determining the commutation time 24 of the motor without sensors are known in principle and will not be described further here.

[0026] The modulation result of the supply voltage signal is in Figure 2dThis is illustrated exemplarily. The time-varying curve of the supply voltage U thus switches between essentially zero volts at commutation moment 24 and twice the DC voltage component 34, which is located in the time interval between two consecutive commutation moments 24, as shown in the example. Figure 2e As shown, thus based on Figure 2a Compared to the described situation, the DC voltage component 34 remains unchanged on average.

[0027] exist Figure 3a and Figure 3b Two other examples are illustrated, showing the variation curves of alternating modulated signals 28' and 28".

[0028] List of reference numerals

[0029] 10 Electric motors

[0030] 12. Commutator

[0031] 14 Brushes

[0032] 16. Power supply voltage signal

[0033] 18 PWM generator

[0034] 20. Pulse of the power supply voltage signal

[0035] 22 Armature current signal

[0036] 24. Reversal Moments

[0037] 26. Current ripple in armature current signal

[0038] 28 Modulation signal

[0039] 28' Modulation signal

[0040] 28" Modulation signal

[0041] 30 Modulator

[0042] 32. Amplitude of the modulated signal

[0043] 34 DC voltage components

[0044] 36 sensors

[0045] 38. Sensor output signal

[0046] 40 Control Unit

[0047] 42 Output signal of the control unit

[0048] U power supply voltage

Claims

1. A method for controlling an electric motor (10) having a mechanical commutator (12), wherein in the method -The timing of commutation is determined by means of a sensor (36) or without a sensor. - The motor (10) is controlled by means of a supply voltage signal (16) with a pulse sequence, and - The supply voltage signal (16) is modulated by means of a modulation signal (28) to reduce its magnitude at the commutation time (24), wherein the modulation signal (28) is in phase with the commutation time (24) such that the minimum value of the modulation signal (28) coincides with the commutation time (24).

2. The method according to claim 1, characterized in that, The commutation time (24) is determined by means of a sensor (36), and the commutation time is determined by analyzing the armature current signal (22) when the commutation time (24) is determined without a sensor.

3. The method according to claim 1 or 2, characterized in that, The supply voltage signal (16) is subjected to pulse width modulation or pulse density modulation, and the pulse width or pulse density is modulated by means of the modulation signal (28) to reduce the magnitude of the supply voltage signal (16) at the commutation time (24).

4. The method according to claim 1 or 2, characterized in that, The magnitude of the supply voltage signal (16) at the commutation moment (24) is reduced to essentially zero volts by means of the modulation signal (28).

5. The method according to claim 1 or 2, characterized in that, The motor (10) is a DC motor, the pulse width of the supply voltage signal (16) having the pulse sequence has a constant component (34) and the modulation signal (28) of the pulse width is a sine signal, the amplitude (32) of the sine signal is substantially equal to the constant component (34) of the pulse width of the supply voltage signal (16) and takes a minimum value at the commutation time (24).

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