Circuit arrangement for operating an electric motor

The invention relates to a circuit arrangement for operating an electric motor, in particular an electric motor for a windshield wiper. The electric motor has a first voltage connection, a ground connection, and a return connection. The ground connection is connected to a battery ground connection of a battery. Furthermore, a switch is provided, which is designed to open or disconnect a conductive connection between the ground connection and the return connection depending on an angle position of a shaft driven by the electric motor. Additionally, a filter member is arranged between the return connection and the ground connection.

BACKGROUND OF THE INVENTION

The invention relates to a circuit arrangement for operating an electric motor.

Electric motors are known in a multiplicity of designs and are used in various technical fields. In motor vehicles, electric motors are used for driving windshield wipers, for example. It is known that electric motors emit electromagnetic interference on account of the electrical and magnetic fields which arise inside them and which vary over time. It is likewise known practice to reject such interference in the useful ranges of the electromagnetic spectrum by using filter elements. The increasingly continuous use of the electromagnetic spectrum by new applications, such as digital radio broadcasting, means an increase in the demands on the electromagnetic compatibility (EMC) of technical appliances. In this case, the focus is on rarely but regularly recurring interference pulses in relatively high frequency ranges. It has been found that such interference pulses are not adequately rejected by the measures known hitherto in the case of wiper motors.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an improved circuit arrangement for operating an electric motor which meets increased EMC demands.

In a circuit arrangement according to the invention for operating an electric motor, particularly an electric motor for a windshield wiper, the electric motor has a first voltage connection, a ground connection and a return line connection. In this case, the ground connection is connected to a battery ground connection on a battery. In addition, a switch is provided which is designed to open or break a conductive connection between the ground connection and the return line connection on the basis of an angular position of a shaft driven by the electric motor. Furthermore, the return line connection and the ground connection have a filter element arranged between them. Advantageously, this filter element eliminates or reduces high-frequency interference pulses from the switch. As a result, the circuit arrangement also meets intensified EMC demands.

Preferably, the filter element is in the form of a capacitive electrical component. Theoretical considerations and experimental trials have shown that such a filter element brings about advantageous rejection of electromagnetic interference.

With particular preference, the filter element is a capacitor having a capacitance of between 1 nF and 100 nF. Advantageously, this capacitance range has been found to be particularly effective.

Expediently, the electric motor is in the form of a rotary motor. Such electric motors are highly suited to driving windshield wipers.

According to one embodiment of the circuit arrangement, the electric motor has a limit stop connection which is connected to a battery voltage contact, wherein the internal switch is designed to connect the return line connection either to the ground connection or to the limit stop connection on the basis of the angular position of the shaft. Advantageously, the filter element then rejects electromagnetic interference pulses caused by the periodic toggling.

DETAILED DESCRIPTION

FIG. 1shows a schematic illustration of a circuit arrangement100based on a first embodiment. The circuit arrangement100is used for operating an electric motor110, which, by way of example, may be provided for the purpose of driving a windshield wiper on a motor vehicle. The motor110has a first voltage connection53and a second voltage connection53b. Furthermore, the motor110has a ground connection31, which may be connected to a housing of the motor110or to a housing of the circuit arrangement100. The circuit arrangement100additionally has a return line connection31band a limit stop connection53a.

The circuit arrangement100is connected to a battery300which has a battery voltage contact153and a battery ground connection131. The ground connection31is connected to the battery ground connection131. The limit stop connection53ais connected to the battery voltage contact153.

In addition, an internal switch200is provided. The internal switch200comprises a disk-shaped parking position disk240which is rigidly connected to a shaft130driven by the motor110. The shaft130may be a shaft of the motor110or a shaft of a transmission driven by the motor110. Rotation of the shaft130brings about rotation of the parking position disk240about an axis perpendicular to the parking position disk240. The parking position disk240has an essentially disk-shaped contact region270arranged on it centrally about the axis of rotation of the parking position disk240. The contact region270is electrically conductive and may be in the form of a metal coating on the parking position disk240, for example. In a narrow angular range of the contact region270, the contact region270has a contact interruption250in a radially inner section of the contact region270and has a contact extension260in a radially outer section of the contact region270.

A first sliding contact210, a second sliding contact220and a third sliding contact230are in contact with the parking position disk240. The first sliding contact210is electrically conductively connected to the ground connection31. The second sliding contact220is electrically conductively connected to the return line connection31b. The third sliding contact220is electrically conductively connected to the limit stop connection53a. The sliding contacts210,220,230are arranged on the parking position disk240such that in almost all angular positions of the parking position disk240the second sliding contact220and the third sliding contact230are electrically conductively connected to one another via the contact region270, and the first sliding contact210is electrically insulated from the second sliding contact220and from the third sliding contact230. Only in a narrow angular position range of the parking position disk240is the first sliding contact210arranged above the contact extension260and the third sliding contact230arranged above the contact interruption250. In this angular position of the parking position disk240, the first sliding contact210is conductively connected to the second sliding contact220, while the third sliding contact230is electrically insulated from the first sliding contact210and from the second sliding contact220.

In one alternative embodiment, the internal switch200may also be in a different form. It is merely critical that the internal switch200connects the return line connection31beither to the ground connection31or to the limit stop connection53aon the basis of the angular position of the shaft130.

In addition, the circuit arrangement100has a steering column switch120which is provided for the purpose of switching on, switching off and toggling the windshield wiper. By way of example, the steering column switch120may be arranged in a passenger compartment of the motor vehicle. The steering column switch120can adopt three different positions. In a first position I, the steering column switch120connects the first voltage connection53to the battery voltage contact153. In a second position II, the steering column switch120connects the battery voltage contact153to the second voltage connection53b. In a third position O, the steering column switch120connects the first voltage connection53to the return line connection31b. This position of the steering column switch120is shown inFIG. 1.

When the steering column switch is in position I, the first voltage connection53of the motor110is connected to the battery voltage contact153. The ground connection31of the motor110is connected to the battery ground connection131. As a result, the motor110runs at a first speed and drives the windshield wiper connected to it at a first wiping speed. When the steering column switch120is in position II, the second voltage connection53bof the motor110is connected to the battery voltage contact153, and the ground connection31of the motor110is connected to the battery ground connection131. In this position of the steering column switch120, the motor110runs at a second speed and drives the windshield wiper connected to the motor110at a second wiping speed. The second speed may be higher than the first speed, for example. In certain embodiments of the circuit arrangement100, the second position II of the steering column switch120and the second voltage connection53bof the motor110may also be dispensed with. In this case, the motor110can be operated only at one speed. In further embodiments, the motor110may also have further voltage connections in order to operate the motor110at further speeds. In this embodiment, the steering column switch120accordingly has further switching positions.

When the steering column switch120is moved from position I or from position II to position O, the parking position disk240of the internal switch200is generally in an angular position in which the second sliding contact220is conductively connected to the third sliding contact230, and the first sliding contact210is insulated from the second sliding contact220and the third sliding contact230. As a result, the first voltage connection53of the motor110is connected to the return line connection31bby means of the steering column switch120, the return line connection31bis connected to the limit stop connection53aby means of the second sliding contact220and the third sliding contact230of the internal switch200, and the limit stop connection53ais connected to the battery voltage contact153. In addition, the ground connection31of the motor110is also connected to the battery ground connection131. Subsequently, the motor110continues to run at the first speed until the parking position disk240of the internal switch200has reached that angular position in which the first sliding contact210is electrically conductively connected to the second sliding contact220, and the third sliding contact is electrically insulated from the first sliding contact210and the second sliding contact220. In this angular position of the parking position disk240, the first voltage connection53of the motor110is connected to the return line connection31bby means of the steering column switch120, and the return line connection31bis connected to the ground connection31by means of the first sliding contact210and the second sliding contact220of the internal switch200. In addition, the ground connection31of the motor110is connected to the battery ground connection131. In this angular position of the parking position disk240of the internal switch200, the first voltage connection53of the motor110is therefore pulled to ground, which stops the motor110. The associated angular position of the parking position disk240has been chosen such that it is obtained when the windshield wiper driven by the motor110is in a parking position. When the steering column switch120is moved from position I or from position II to position O, the motor110and the windshield wiper driven by it therefore initially continue to run until the windshield wiper reaches a parking position. In the parking position, the motor110is switched off.

In positions I and II of the steering column switch120, the parking position disk240is rotated by the shaft130. As a result, during normal operation of the motor110, the internal switch200connects the return line connection31balternately to the ground connection31and to the limit stop connection53aconnected to the battery voltage contact153. Such toggling takes place periodically upon every revolution of the parking position disk240. Subsequently, the return line connection31band lines connected to the return line connection31bexperience periodic short voltage pulses.

In accordance with the invention, it has been recognized that these voltage pulses produce wideband noise in a frequency range up to several hundred MHz. Therefore, a filter element400is arranged between the return line connection31band the ground connection31. Preferably, the filter element400is a capacitive filter element, for example a capacitor. The capacitance of the capacitor should preferably be in a range between 1 nF and 100 nF. However, the capacitance of the filter element400can also be chosen to be lower or higher. The optimum capacitance value should be determined experimentally. Preferably, the filter element400should be arranged as close as possible to the sliding contacts210,220,230of the internal switch200. If the internal switch200is of a different design than that described, the filter element400should nevertheless be provided as close as possible to the internal switch200. It has been found that the filter element400improves the emission spectrum of the circuit arrangement100and of the motor110in a very advantageous manner.

FIG. 2shows a further view of the circuit arrangement100. Components which correspond to those inFIG. 1are denoted via the same reference symbols.FIG. 2shows that the circuit arrangement100may have further interference suppression elements. Thus, the second voltage connection53band the ground connection31have a first capacitor500arranged between them. The motor110and the second voltage connection53bhave a first inductor505arranged between them. The first voltage connection and the ground connection31have a second capacitor510arranged between them. The motor110and the first voltage connection53have a second inductor515arranged between them. In addition, the first voltage connection53and the second voltage connection53bhave a third capacitor520arranged between them. The motor110and the ground connection31have a third inductor535between them. The provision of the capacitors500,510,520and the inductors505,515,535is already known from the prior art.

FIG. 3shows a circuit arrangement1100based on a second embodiment. The circuit arrangement1100is used for operating an electric motor1110, which may likewise be a motor for driving a windshield wiper on a motor vehicle, for example. The motor1110again has a first voltage connection53, a second voltage connection53band a ground connection31. In addition, a return line connection31bis again provided.

The motor1110has an internal switch1200, which again comprises a parking position disk1240which is rotated by a shaft130. A first sliding contact1210and a second sliding contact1220are in contact with the parking position disk1240. The parking position disk1240has a contact area1250arranged on it which electrically conductively connects the first sliding contact1210to the second sliding contact1220in a particular angular position of the parking position disk1240. In all other angular positions of the parking position disk1240, the first sliding contact1210and the second sliding contact1220are electrically insulated from one another. The first sliding contact1210is electrically conductively connected to the ground connection31. The second sliding contact1220is electrically conductively connected to the return line connection31b. The internal switch1200may also be of a different design, but should make or break a conductive connection between the ground connection31and the return line connection31bon the basis of the angular position of the shaft130. In particular, the internal switch1200may also be designed such that the first sliding contact1210is insulated from the second sliding contact1220only in one particular angular position of the parking position disk1240and is electrically conductively connected to the second sliding contact1220in all other angular positions of the parking position disk1240.

Again, a battery300is provided which has a battery voltage contact153and a battery ground connection131. The battery ground connection131is connected to the ground connection31.

The circuit arrangement1100also comprises a controller1120. By way of example, the controller1120may be in the form of a microcontroller or microcomputer. The controller1120may also be integrated in another controller in a motor vehicle. The controller1120is connected to the battery voltage contact153of the battery300. In addition, the controller1120is connected to the first voltage connection53, to the second voltage connection53band to the return line connection31b. The controller1120can adopt different switching positions. By way of example, the switching positions can be prescribed for the controller1120by a steering column switch which is connected to the controller1120. In a first position I, the controller1120connects the battery voltage contact153to the first voltage connection53of the motor1110. As a result, the motor1110runs at a first speed and drives the windshield wiper connected to the motor1110at a first wiping speed. In a second switching position II of the controller1120, the controller1120connects the battery voltage contact153to the second voltage connection53bof the motor1110. In the second switching position II, the motor1110therefore runs at a second speed, which may be higher than the first speed. Further switching states for further speeds may also be provided.

When the controller1120is toggled to a third switching state O, in order to disconnect the motor1110and the windshield wiper driven by the motor, the parking position disk1240is generally in an angular position in which the first sliding contact1210and the second sliding contact1220are electrically insulated from one another. In this angular position of the parking position disk1240, the return line connection31bis thus not connected to the ground connection31. This can be determined by the controller1120. So long as the return line connection31bis not connected to the ground connection31, the controller1120maintains an electrical connection between the battery voltage contact153and the first voltage connection53of the motor1110. This has the result that the motor1110continues to run and continues to drive the shaft130, so that the parking position disk1240continues to rotate until the first sliding contact1210and the second sliding contact1220of the internal switch1200come into contact with the contact area1250and are thereby shorted. In this angular position of the parking position disk1240, the return line connection31bis concomitantly pulled to the potential of the ground connection31. This is determined by the controller1120, which interrupts the connection between the battery voltage contact153and the first voltage connection53. As a result, the motor1110and the windshield wiper driven by the motor stop. The contact area1250is arranged on the parking position disk1240such that the angular position of the parking position disk1240in which the sliding contacts1210,1220are shorted arises precisely when the windshield wiper driven by the motor1110is in a parking position. When the windshield wiper is disconnected, it is therefore first of all put into the parking position before it is actually disconnected. If the internal switch1200is designed such that the sliding contacts1210,1220are shorted together in all positions of the parking position disk1240apart from the parking position, the controller1120is accordingly designed to recognize when the short circuit is broken.

During operation of the motor1110in switching position I or II of the controller1120, the parking position disk1240results in an electrical connection being periodically made and broken between the return line connection31band the ground connection31, as a result of which the return line connection31band lines connected to the return line connection experience periodic voltage pulses. Such a voltage pulse arises once for every revolution of the parking position disk1240.

In accordance with the invention, it has been recognized that these periodic voltage pulses on the return line connection31bresult in emission of electromagnetic interference in a frequency range up to a few hundred MHz. Therefore, the return line connection31band the ground connection31again have a filter element400provided between them which is preferably in the form of a capacitor having a capacitance of a few nF. Experiments have shown that the filter element400can effectively suppress the interference emissions described. Again, it is advantageous if the filter element400is arranged as close as possible to the internal switch200, and as close as possible to the sliding contacts1210,1220when the internal switch1200is designed with a parking position disk1240and sliding contacts1210,1220.

FIG. 4shows a further view of the circuit arrangement1100in the second embodiment.FIG. 4shows that the circuit arrangement100may have further interference suppression elements. Thus, a first capacitor500is again provided between the second voltage connection53band the ground connection33. The motor1110and the second voltage connection53have a first inductor505arranged between them. The first voltage connection and the ground connection33have a second capacitor510provided between them. A second inductor515is between the motor1110and the first voltage connection53. A third capacitor520connects the first voltage connection53and the second voltage connection53b. A third inductor535is provided between the motor1110and the ground connection31. The provision of the capacitors500,510,520and the inductors505,515,535is already known from the prior art.

FIG. 5shows a schematic illustration of a noise spectrum for the circuit arrangement100in the first embodiment without the filter element400. Frequencies in the range between 30 MHz and 200 MHz are plotted on the right. A noise amplitude in arbitrary units is shown upwards. By way of example, the graph inFIG. 5can be recorded using a spectrum analyzer or an RF test receiver.FIG. 5shows that the circuit arrangement100without the filter element400causes severe noise in the range between 50 MHz and upwards of 200 MHz. This becomes clear from a few spikes in the graph shown. A more precise analysis of the noise spectrum also shows that the interference is distributed continuously over the entire frequency range and is not of a discrete nature.

FIG. 6shows a schematic illustration of a noise spectrum100for the first embodiment with the filter element400. Again, frequencies in the range between 30 MHz and 200 MHz are plotted on the right. The measured noise amplitude in arbitrary units, but units which are comparable with those inFIG. 5, is shown upwards.FIG. 6clearly shows that the noise pulses in the range between 50 MHz and 200 MHz are greatly reduced by the use of the filter element400.