Windscreen wiper comprising a protection device for the electric motor thereof

A windscreen wiper, including a protection device for an electric motor for drive thereof, and corresponding method. The windscreen wiper includes a DC motor with first, second, and third carbon contacts rubbing on a rotor thereof. The first carbon contact is ground, the second is selectively connected to a DC current supply, and the third is connected to neither the ground nor the DC current supply. A sub-assembly for detection of sticking of the electric motor provides information on the sticking of the motor. A voltage measuring device between the first and the third carbon contacts permits taking a first measured voltage value that is compared to a first threshold value to generate further second information on the sticking of the electric motor if the voltage is lower than the first threshold value.

TECHNICAL FIELD

The invention relates to a windshield wiper, particularly for a motor vehicle, this windshield wiper comprising a direct current electric drive motor for driving a windshield wiper mechanism. The motor comprises a rotor, a first, a second and a third carbon brush brushing on its rotor, the first carbon brush being grounded, the second carbon brush being connected selectively to a direct current source by means of a contactor, the third carbon brush being neither grounded nor connected to the direct current source, a subassembly for detecting the sticking of the electric motor in order to deliver an item of information on the sticking of the motor and means for delivering a command signal for the contactor which interrupts the connection with the direct current source.

It also relates to a device for protecting a direct current motor, the motor comprising a first, a second and a third carbon brush brushing on its rotor, the first carbon brush being grounded, the second being connected selectively to a direct current source by means of a contactor for the direct current electric supply, the third carbon brush being neither grounded nor connected to the direct current source.

Finally, the invention relates to a method for protecting a direct current motor of the type described above against destruction by overheating if the motor for driving the windshield wiper mechanism sticks.

A motor vehicle windshield wiper comprises an electric motor for driving a mechanism of the connecting rod/crank type that makes it possible to transform a rotary movement into an alternating movement so as to produce alternating back-and-forth movements of the windshield wiper blades on the windshield of the vehicle in the event of rain. These mechanisms can usually operate in a low speed mode and in a high speed mode in order to adapt to the rain conditions, the difference between the two modes being at least 15 back-and-forth movements per minute. The starting of the windshield wiper may be commanded manually by a contactor, for example situated on the steering wheel, actuated by the driver of the vehicle. The windshield wiper may also be switched on automatically by a rain sensor that commands the operation of the windshield wiper in low speed or high speed mode. When a wiping request is sent, it is transmitted to a logic computing unit that commands power relays in order to cause the motor to operate in low speed mode or in high speed mode, depending on the case.

However, it may happen that the electric motor sticks. In this case, the intensity of the current in the windings of the rotor increases, which leads to a heating that may result in the destruction of the motor. That is why, in order to protect the electric motor if the windshield wiper blades stick, it is normal to provide a protective device.

U.S. Pat. No. 5,630,009 describes a method and a device for commanding a direct current electric motor of a windshield wiper that comprises a heat protection device comprising a bimetallic strip that disconnects the supply of the motor in the case of overheating.

Also known are devices for protecting the electric motor comprising a cam mounted on the motor output shaft. This cam controls the opening and closure of an electric contact so that a rectangular electric signal is generated whose frequency is the same as that of the motor. If the motor sticks, the signal no longer varies. The onboard computer (the arithmetic and logic unit) reacts and disconnects the motor supply. A program managed by the arithmetic and logic unit (on/off test loop) protects the motor from overheating when it sticks and allows it to return to normal operation when the sticking has disappeared.

However, if the detection of the rectangular signal fails, for example if the wire that connects the cam to the arithmetic and logic unit is disconnected, or else if the cam is defective, the arithmetic and logic unit will react in the same manner as if the motor were stuck whereas in reality the latter is rotating normally. As is understood, the result of this is a serious risk if the vehicle is traveling, particularly at high speed, in rain, because the arithmetic unit will interrupt the supply of the electric motor. With the windshield not being wiped, the driver will no longer have visibility, which may result in an accident.

This is why this method of protecting the electric motor against overheating if it sticks is activated only in the following two conditions:vehicle traveling at less than 7 km/h;the manual control is switched off before the vehicle reaches 7 km/h.

Except for these two cases, the protective device is not activated. If there is really a sticking, and if this sticking persists during travel, the motor burns out. That is the reason why an additional heat protection is frequently installed, for example a protection by bimetallic strip as described in American U.S. Pat. No. 5,630,009. However, this solution requires the use of an additional component in the electric circuit, hence an increase in cost. Furthermore, it has the effect of increasing the on-line resistance, and hence the energy losses on line.

The subject of the present invention is a windshield wiper and a device for protecting an electric motor that remedy these disadvantages.

With respect to the windshield wiper, these aims are achieved by the fact that it comprises means for measuring the voltage between the first and the third carbon brushes in order to obtain a first measured voltage value; a comparator for comparing the first measured voltage with a first threshold value in order to generate a second item of information on the sticking of the electric motor if the comparison has determined that the first measured voltage is less than the first threshold value.

The second item of information on the sticking of the electric motor forms an item of redundant information as a control of the information supplied by the subassembly for detecting the sticking of the electric motor.

Advantageously, the windshield wiper or the device for protecting the motor may also comprise means for measuring the voltage between the first and the second carbon brushes in order to obtain a second measured value supplied to the comparator and in order to compare this second measured voltage value with a second threshold value.

This second measured value may be another item of redundant information making it possible to confirm that the motor is sticking.

According to the method, in a first step, with the electric motor of the windshield wiper operating normally, on the one hand, the voltage between the first and the second carbon brush is measured in order to obtain a first threshold value and, on the other hand, the voltage between the first and the third carbon brushes is measured in order to obtain a second threshold value.

These values having been measured and recorded in a memory register of the arithmetic and logic unit of the vehicle, during the normal operation of the windshield wiper, on the one hand, the voltage between the first carbon brush and the second carbon brush is measured and the measured value is compared with the first threshold value. On the other hand, and optionally, the voltage between the first carbon brush and the third carbon brush is measured and the value thus measured is compared with the second threshold value. The electric supply of the motor is interrupted if the comparison has determined that the first measured value is less than the first threshold value and/or that the second measured value is less than the second threshold value.

The windshield wiper assembly represented inFIG. 1comprises an electric motor2, a first arithmetic and logic unit4, called the passenger compartment control unit (UCH) and a second arithmetic and logic unit6, called the motor slave unit (USM).

The motor2is a direct current motor. In a conventional manner it comprises a fixed portion, the stator, and a rotating portion, the rotor. The stator consists of permanent magnets that generate a magnetic field Φ. The rotor is an elongated circular metal part. It comprises a number of notches in which electric conductor wires forming turns are wound. The electric motor2is a motor with two rotation speeds. It may operate at a first rotation speed, called “low speed” and at a second rotation speed, higher than the first, called “high speed”. Only the rotor is electrically supplied. Accordingly, a first carbon brush or brush10, constituting a brushing contact, is connected by electric connections12to a terminal of the electric current source, the negative terminal in the present case forming the ground. A second carbon brush14is connected selectively to the other terminal16of the electric current source17, for example the accumulator battery of the vehicle. A low speed relay18and a high speed relay20are interposed between the positive terminal16and the second carbon brush14. An armature22of the low speed relay18is in contact either with a terminal24that is grounded, or with a terminal26that is connected by means of an electric conductor28to the terminal16. A protective fuse30is optionally placed in series on the conductor28. An armature32of the high speed relay20is in contact either with a terminal34connected via a conductor35to the second carbon brush14, or with a terminal36connected via a conductor37to a third carbon brush40. In this example, the low speed relay18and the high speed relay20are placed in series. When the armature22of the low speed relay is connected to ground, the relay is in the “off” position. On the other hand, when the armature22is in contact with the terminal26, the relay18is in the “on” position. In the same manner when the armature32of the high speed relay20is in contact with the terminal34, the relay is in the “off” position and when the armature32is in contact with the terminal36, the relay20is in the “on” position. In order for the direct current electric motor2to be supplied at low speed, it is necessary for the low speed relay18to be in the “on” position and for the high speed relay20to be in the “off” position. For the motor2to be supplied at high speed, it is necessary for the relays18and20both to be in the “on” position.

The passenger compartment control unit4is connected to a control switch44, placed for example beneath the steering wheel. The switch44has four positions: an “off” position, a low speed “on” position, a high speed “on” position and an automatic position. When the switch is in the automatic position, the switching on and the switching off of the windshield wiper blades are commanded by a rain sensor46connected to the passenger compartment control unit4. The unit4is connected via electric conductors to the coils of the low speed relay18and high speed relay20in order to control the armatures of each of these relays and, consequently, the switching on and off of the windshield wiper blades. The motor slave unit6is also capable, as will be explained later, of commanding the relays18and20.

It may happen that the electric motor sticks, for example, because the windshield wiper blades encounter an obstacle on the windshield. In this case, the intensity of the current traveling through the coils of the winding channels of the rotor increases, which leads to its overheating and possibly its destruction.

In order to prevent this, the windshield wiper assembly ofFIG. 1is fitted with a subassembly for detecting the sticking of the motor. In a known manner, this subassembly, represented in detail inFIG. 4, comprises a cam50that is mounted on the shaft52of the rotor of the electric motor. The cam50commands the opening and closure of an electric contact54connected to one of its terminals, to ground12and to its other terminal at the positive pole of the accumulator battery17via a set of electronic components56allowing the filtering and regulation at the terminals of the switch54that is also connected to the motor slave unit6. In this manner, this subassembly generates a rectangular-shaped voltage having the same frequency as the rotation of the motor. This voltage is represented inFIG. 5. When the contact54is closed, the voltage is equal to 0. When the contact is open, the voltage U perceived by the unit6is substantially equal to the voltage of the accumulator battery17.

If the motor sticks, the voltage U no longer varies.

It is permanently equal to 0 or to the slave voltage (the voltage of the accumulator battery17), depending on the position in which the motor is stuck. When the unit6detects this situation, it commands the low speed relay18in order to make it switch to its “off” position that interrupts the supply of the electric motor. A program managed by the motor slave unit6(“on/off” test loop) protects the motor from overheating during the sticking and allows it to operate normally when the sticking disappears.

However, it may happen that the subassembly for detecting the sticking of the motor supplies an incorrect indication because the wire that connects it to the arithmetic and logic unit6is disconnected or else because the cam is defective (worn). In this case, the unit6will interpret the absence of variation of the signal as a sticking of the electric motor2and will interrupt its supply. If the vehicle is traveling at a high speed in heavy rain, the result of this will be a sudden loss of visibility and consequently a serious risk of an accident. This is why, to ensure the safety of operation of the windshield wiper, the device for detecting sticking is activated only in the following conditions:the vehicle is traveling at a speed of less than 7 km/h;the manual control switch44of the windshield wiper has been placed by the driver of the vehicle in the “off” position before a speed greater than 7 km/h.

Except for these cases, the device for detecting the sticking of the motor will not disconnect the supply of the latter. Consequently, if the detection made corresponds to an actual sticking, the motor will burn out.

It is known practice to remedy this disadvantage by installing an additional heat protection, for example a circuit-breaker comprising a bimetallic element, that will interrupt the supply of electric current to the motor if its temperature rises. However, the use of this heat protection has a cost. Also, it increases the on-line electric losses.

FIG. 6shows a windshield wiper assembly according to the present invention that remedies the disadvantages listed above. It is generally similar to the windshield wiper assembly that has been described with reference toFIGS. 1 to 4. In the same manner, it comprises a direct current electric motor2, a passenger compartment control unit4, a motor slave unit6, a direct current source17, a low speed relay18and a high speed relay20. Three brushes or carbon brushes10,14,40are in brushing contact with the electric conductors wound on the rotor of the motor2. The operation of the device is identical. Consequently, its description will not be repeated.

According to the invention, the windshield wiper comprises means for measuring the voltage between the first carbon brush10and the third carbon brush40in order to obtain a first measured voltage value. In the example shown, these means consist in a recorder60capable of measuring and recording the voltage between the carbon brushes10and40. The recorder60supplies the measured voltage to the motor slave unit6. In this unit, the first measured voltage value is compared with a first threshold value in order to generate an item of information on the sticking of the electric motor2, if the comparison has determined that the first measured voltage is less than the first threshold value. As explained above, the item of information on the sticking of the motor2acts so as to interrupt the electric current supply of the motor2.

This system can operate in isolation. Or else, preferably, it can operate in parallel with a conventional device for detecting the sticking of the motor, such as the system with a cam and switch described with reference toFIGS. 4 and 5. In this case, the item of information on the sticking of the electric motor2that results from comparing the first measured voltage value with a threshold value constitutes a second item of information on the sticking of the electric motor that confirms or invalidates the first item of information on the sticking of the electric motor delivered by the system with a cam and switch. In this way there is an item of redundant information that increases the reliability of detection. If the motor sticks, the sticking information supplied by the cam is confirmed by the measured voltage level. In degraded mode, that is to say in the event of a false detection by the cam of sticking of the motor due, as explained above, to a disconnection of the wire connecting to the motor slave unit6or due to wearing of the cam, the level of the first measured voltage makes it possible to detect the false indication and consequently that there is no sticking of the motor. Consequently, in such a situation, the supply of the motor is not interrupted. In this mode of operation, the sticking detection system using the measurement of the voltage takes priority over the item of information supplied by the cam.

This system has the advantage of dispensing with a heat protection component, for example a circuit-breaker, in the electric circuit supplying the motor2. The on-line resistance is reduced. Also, safety of operation is increased thanks to the redundancy of information.

InFIG. 6, the windshield wiper assembly has been shown in the low speed operating mode. In this mode, the relay18is in the “on” position whereas the relay20is in the “off” position. The carbon brush14is thus connected to the direct current source17while the third carbon brush40is not supplied.

The device for detecting the sticking of the motor that has just been described also applies when the windshield wiper operates in high speed mode. In this case, each of the two relays18and20is in the “on” position so that the third carbon brush40is connected to the direct current source17and the second carbon brush14is not supplied. In this case, the roles of the carbon brushes14and40are inverted. Instead of measuring the voltage between the carbon brush10and the carbon brush40, the voltage between the carbon brush10and the carbon brush14is measured. This voltage is compared with a second threshold value, different from the first, in order to generate an item of information on sticking of the electric motor, in the same manner as above.

In low speed mode as represented inFIG. 2, it is the carbon brush14, diametrically opposed to the first carbon brush10, that is connected to the electric current source. For a direct current motor with constant excitation or flux such as the electric motor2, the basic formulas are:

In these formulas, U is the supply voltage of the rotor, R is the resistance of the rotor windings, n is the speed of rotation of the rotor in radians per second, Φ is the average flux under a pole, p is the number of pairs of poles, a is the number of pairs of winding channels and N is the number of active conductors per winding channel. The active conductors form the portion of the winding that is in contact with the carbon brushes10and14. The values a and p being constant (K=p/a), the speed of rotation of the motor is expressed by the following formula:

n=U-R×Ik×N×Φ
When the position of a supply carbon brush is moved, as shown inFIG. 3(carbon brush40), the resistance R reduces because the winding of the rotor is no longer supplied symmetrically. The current increases proportionally because the product R×I is constant. The active conductors moved relative to the main flux are subjected to a lesser flux. The product N×Φ diminishes so that the speed of rotation of the motor increases. Consequently, the speed of rotation of the motor n1(FIG. 2) is less than the speed of rotation n2(FIG. 3).

FIG. 7shows the principle of the measurement of the first voltage. In this figure, the motor2is supplied at low speed. The carbon brush14is connected to the direct current source17. The supply voltage U causes the motor to rotate at the speed n1. The formulas explained previously with reference toFIG. 3apply. On the other hand, the circuit that is not supplied behaves like a generator. Between the carbon brush10and the carbon brush40, there appears a voltage U2=E′2−(R2×I2). Given that this circuit is open, I2=0. Consequently, U2=E′2=K×N2×Φ2×n1.

InFIG. 8, the electric motor2is supplied in high speed mode. The third carbon brush40is connected to the direct current source17at a voltage U which causes the motor to rotate at the speed n2. The circuit that is not supplied, namely in this case the low speed circuit, behaves like a generator. On the carbon brush14there appears a voltage U1=E′1=k×N1×Φ1×n2.

FIG. 9represents the voltage readout in the high speed circuit, respectively in high speed mode (left portion ofFIG. 9) and in low speed mode (central portion ofFIG. 9).

For a voltage of the direct current source17of 13.85 volts, the average voltage in the high speed circuit in high speed mode is substantially 12.62 volts. This voltage is variable between +9 volts and +16 volts approximately.

On the other hand, the voltage in the low speed circuit is on average 8.33 volts. This voltage is a voltage induced by the rotation of the electric motor, the low speed circuit behaving like a generator. This voltage is the first measured value corresponding to the case ofFIG. 7when the motor rotates normally. Consequently, the first threshold value is therefore 8.33 volts for a supply voltage of 13.85 volts. Naturally, if the supply voltage varies, the threshold value will also change.

Finally, the right portion ofFIG. 9(zone B) shows the voltage in the high speed circuit in the event of sticking of the motor. As can be seen, this voltage is stable, unlike the two previous cases. Its value is approximately 6 volts. It can be seen consequently that it is less than the first threshold value of 8.33 volts, which characterizes a situation of sticking of the electric motor.

FIG. 10represents the readout of the voltage in the low speed circuit respectively in high speed mode (left portion ofFIG. 10) and in low speed mode (central portion ofFIG. 10).

For a supply voltage of 13.85 volts of the current source17, the average voltage in high speed mode in the low speed circuit is 17.22 volts. This voltage is an induced voltage, as explained with reference toFIG. 8described above. It is found that this voltage is extremely variable since it varies substantially between +10 volts and +35 volts. In addition, as can be seen, its level is higher than the voltage induced in the high speed circuit in the low speed operating mode. This is explained by the fact that the rotation speed of the motor is higher. Consequently, the electromotive force generated by the generator is greater. It even exceeds the supply voltage of 13.85 volts. The average voltage of 17.22 volts is consequently a first threshold value in high speed operating mode.

The central portion ofFIG. 10represents the voltage in the low speed circuit for the low speed mode. This voltage is on average 12.96 volts, still for a supply voltage of 13.85 volts. It is variable, but its variations are not very great (+10 volts to +15 volts). Finally, the right portion ofFIG. 10(zone B) shows the readout of the voltage in the low speed circuit in the case of sticking of the motor. This voltage is approximately 12 volts. It can be seen that it is less than the threshold value defined for the high speed operating mode, namely 17.22 volts as explained above. The detection of this voltage by the motor slave unit6will therefore trigger the interruption of the supply of the electric motor.

Additionally, it is possible to compare, according to the invention, not only the value of the voltage but also its variability since it is noted that the induced voltages, whether in the low speed circuit or in the high speed circuit, vary greatly whereas, on the contrary, in the case of sticking of the motor, the voltage readout in these same circuits is strictly constant.