2. Related Art Motor vehicle headlight systems generally allow a set of headlamps to be illuminated in a low beam mode or a high beam mode. Usually, each headlamp includes a low beam filament and a high beam filament, wherein the low beam filaments are energized in the low beam mode and the high beam filaments are energized in the high beam mode. Such a system usually includes two relays and four fuses. One relay is used to energize both low beam filaments at full intensity and the other relay is used to energize both high beam filaments at full intensity. Each fuse is associated with, and provides overload protection, for a particularfilament.

The system described above has a number of åisadvantages. First, such a system does not ai ! ow pufsed power to he applied across the headlamp filaments because pulsed voltage cannot be continuously applied across a relay.

Instead, the filaments are continuously energized at a particular voltage level; usL'atty battery voltage minus small voltage drops from the system. Continuously energizing the filaments at high intensity causes high power consumption and shortens the life of the filaments. Also, fuses are mechanical devices that must be manually replace in the event of a short to ground. When a fuse blows, the vehicle is usually taken to a dealer to replace the fuse for a cost or under warranty. As r,, achanical devices, fuses can be less reliable than semiconductor devices. Additionally, fuses do not allow the system to sense whether there is an open or short circuit condition in the headlight system and to automatically switch power to an operable filament when a fault condition is present with a particular filament.

Therefore, it is desirable to provide-i veh, cle headlight system that allows pulsed power to be applied across selected headlight filaments. It is also desirable to provide a vehicle headlight system which includes solid state switching devices to connect the filaments to the power source, and a control element to detect the presence of a fault condition and automatically adjust the headlight system to overcome detected fault conditions.

SUMMARY OF THE INVENTION The present invention is a headlight system that provides pulsed power to the headlamp filaments through a set of solid state switching devices controlled by a control element. The present headligi-,. system is capable of supplying pulsed power to maintain a constant RMS value across the selected headlamp filaments.

Using so ! id state switching devices reduces the need to have a fuse for each filament and thus the requirement of fuse serviceability. The present headlight system also recognizes when a fault condition exists for a particular filament and automatically switches to an operable filament. The present vehicle headlight system is also configured to adjust a characteristic of the pulsed power, for example, by adjusting the duty cycle of a PWM drive signal, in order to adjust the brightness of the filaments in either headlamp. For example, it may be desirable to compensate for the loss of low beam filament in one headlamp by energizing the high beam filament and adjusting the pulsed power across the high beam filament so that the brightness of that high beam filament approximates the brightness of the low beam filaments of the unaffected headlamp.

In one embodiment, the present vehicle headlight system comprises: a first lamp comprising first and second filaments; a second lamp comprising third and fourth filaments; a power supply; first and second switches, each of the switches having an input coupled to the power supply and an output coupled to a respective one of the first and second lamps, each switch output providing one of a constant output and a pulsed output in response to a control signal; first and second sensors respectively operatively coupled to one of the first and second switches, the first and second sensors providing a fault indication when a respective one of the first, second, third and fourth filaments are inoperable; a control unit coupled to the first and second switches and ddapted to provide the control signal, the control signal automatically causing one of the first and second switches to provide a first pulsed output so that the brightness of the second and fourth filaments is approximately equai to the brightness of the first and third filaments when the fault indication is present.

In another embodiment of the present invention, each of the headlamps include one filament and a control unit that controls a characteristic of the pulsed power across a particular filament to provide both low and high beam brightness levels from the filament. Here, only one filament is required to provide both leveis of brightness. For examp'_, the headlight system may be configured to provide PWM signal to the headlight filaments and the control unit may adjust the brightness of the filaments by adjusting the duty cycle of the PWM signal. In another embodiment, a range of duty cycles may be provided in order to provide a range of headlamp brightness levels rather than only low beam and high beam levels.

Further details and advantages of the invention will be apparent from the accompanying drawing and following description.

BRIEF DESCRIPTION OF THE DRAWING Fig. 1 shows a circuit diagram of an exemplary embodiment of the present invention.

DETAILED DESCRIPTION Circuit 10 is a portion of the headlight system in accordance with the present invention. The upper and lower portions of circuit 10 are mirror images of each other illustrating the portions associated with the left and right portions of the headlight system. Each portion inc's a headlamp having a high beam filament and a low beam filament contained therein, and switch elements and relay elements for energizing the filaments. The upper portion of circuit 10, which relates to the left headlamp, is now described. This description is equally applicable to the lower portion of circuit 10.

Micro controller 13 is connected to input connection 12 and controls the generation of a pulse width modulated (PWM) drive signal. Micro controller 13 may be programmed to provide various control functions as is known in the art.

The use and programming of a control unit such as micro controller 13 to control the various elements described below, as well as suitable types of micro controllers, is well known in the art and will not be discussed in detail here. A suitable micro controller includes, but is not limited to, Motorola MC688CO8.

Manual control of the present headlight system by operation of a headlamp switch (not shown) is provided for via input connection 14.

The control signal from input connection 12 is coupled to the base of Darlington transistor Q5 resistor R1 and capacitor C1. Resistor R1 and capacitor C1 determine the frequency and duty cycle of the PWM signal input to transistor Q5. The combination of resistor R1 and capacitor C1 also provide isolation of micro controller 13 from the downstream elements of circuit 10 in the event micro controller 13 faits. Resistor R1 and Capacitor C1 AC couple the signal from input connection 12 to Darlington transistor Q5. This AC coupling allows low beam operation of the headlight using the manual control signal from input connection 14 in the event of micro controller 13 failure. Diode CR1 is an isolation diode that provides isolation for elements coupled to input connection 14. Diode CR2 provides negative transient spike protection for transistor Q5 maintaining the base emitter voltage C1 clamped at about at least-0.7 volts. Diode CR2 ensures proper discharge of capacitor C1 to generate the PWM signal.

The base of transistor Q5 is coupled to pull down resistor R3 which ensures that transistor Q5 remains OFF when the filaments in headlamp 30 are de-energized. The collector of transistor Q5 is connected to the gate input of PROFET Q1 via current limiting resistor R4. The battery, or another suitable power source, is connected to the gate input of PROFET Q1 via resistor R5 to set the quiescent point of PROFET Q1. The drain of PROFET Q1 is coupled to the battery and the source of PROFET Q1 is coupled to steering relay 28. It can be seen that the application of q PWN signal to the gate input of PROFET Q1 controls PROFET Q1 to provide puised power t. selected filaments 31 and 32 via PROFET Q1 and steering relay 23.

PROFET Q1 further includes current sensing line 21 that provides an output that is proportional to the current provided to steering relay 28. The output of current sense line 21 is a signal which is proportional to the output of PROFET Q1, but reduced by a factor of about 12 to 15 thousand. Since the output from current sensing line 21 is proportional to the current applied to the filaments, zero current from current sense line 21 indicates an open circuit condition and a relatively high current from current sense line 21 indicates a short circuit condition. The output of current sense line 21 is provided to a voltage detect circuit (not shown) via output connection 20 through current limiting resistor R6.

The output of the voltage detect circuit may be provided to micro controller 13 in a manner known in the art. Alternatively, the voltage detect circuit may be incorporated into micro controller 13.

Steenng relay 28 is a dual relay package comprising relay 37 associated with left side headlamp 30 and relay 38. >sãociated with, ynt side headlamp 35.

Steering relay 28 includes switches 40 and 41 associated with respective relays 37 and 38. For example, when relay 37 is de-energized, switch 40 connects the output of PROFET Q1 to low beam filament 32, and when relay 37 is energized, switch 40 connects the output of PROFET Q 1 to high beam filament 31. Relay 37 is energized and de-energized by a high beam control circuit (not shown) connected to input connection 24. The high beam control circuit may also be incorporated into micro controller 13, wherein micro controller is programmed to turn ON Darlington transistor Q3 when high beam filament 31 must be energized.

Relay 37 is energized and de-energized by Darlington transistor Q3 that is connected to input connection 24 via current limiting resistor R15. Darlington Transistor Q3 is also coupled to Zener c: ode CR5 which protects transistor Q3 against transient conditions and to pull down resistor R16 which ensures that transistor Q3 remains in the OFF state when headlamp 30 highbeam is de- energized.

The operation of the circuit 10 is now described. During normal operation, steering relay 28 connects the outputs of PROFETs Q1 and Q2 to one of the desired filament pairs 32 and 34 or 31 and 33. PROFETs Q1 and Q2 are driven by respective transistors Q5 and Q6 based on control signals from input connections 12,14,16 and 18. Based on the control signals, PROFETs Q 1 and Q2 may provide either constant DC or pulsed power to the selected filament pair.

When a fault condition exists, the present headlight system automatically detects the presence of the fault and switches filaments in order to de-energize the faulty filament and to energize the unaffected filament. The present headlight system also adjusts the brightness of the unaffected filament by adjusting the ! VYIVI signal applied thereto in order to approximate the brightness of the filament in the unaffected headlamp. The presence of a faulty condition is detected by monitoring the output of current sensing lines 21 and 23. Zero current from current sensing lines 21 and 23 indicate the presence of an open circuit condition and high current from current sensing lines 2 i and 23 indicated the presence of a short circuit condition.

The brightness of the energized filament is controlled by adjusting the duty cycle of the PWM drive signal applied to the filaments. For example, duty cycle of the PWM signal via input connections 12 and 16. For example, in the event that low beam filaments 32 and 34 are energized and a short circuit condition appears for low beam filament 32, micro controller 13 uetects the short circuit concition by detecting the high current output form current sensing line 21.

In response, relay 37 is energized so that switch 40 connects the output of PROFET Q1 to high beam filament 31 and the duty cycle of PWM signal form PROFET Q1 is reduced to reduce the brightness of filament 31 to thereby approximate the brightness of low beam filament 34. In this manner, the present headlight system automatically detects a fault condition and compensates for the loss of a low beam filament by energizing the high beam filament at a lower brightness level. It can be seen that the present invention provides a driver with two low-beam headlights even if a low beam filament is unavailable from one headlamp. Previously, the loss of a low-beam filament would force the driver to use a single headlamp or the high beam.

In an alternative embodiment, although not shown, it can be appreciated that the present invention may be modified so that a fault condition in a high beam filament may be compensated for by energizinq the low beam filament and adjusting the brightness of the low beam figent to approximate the high beam filament in the other headlamp. It can also be appreciated that a fault indication may be provided to the driver, for example by a flashing light on the dashboard, if the outputs from current sensing lines 21 and 23 irdicate a fault condition. The detecíion and indication of a fault condition by the present headlight system allows the driver to immediately recognize that a fault condition exists where otherwise the drive may not become aware of the fault condition until much later.

Further, the present invention may use the automatic turn OFF feature of PROFETs Q1 and Q2 to determine whether a short or an open circuit condition resulteca in the shutoff of a particular PROFET. As is known in the art, a PROFET device automatically turn OFF in the event of high output current through it. As such, when a PROFET fails to provide any output current, it may be difficult to initially determine whether the PROFET turn OFF was due to a short circuit or an open circuit condition. Therefore, micro controller 13 may be programmed to periodically turn ON the effected PROFET and check the output of the associated current sense line. If no output is present at the current sense line, it can be assumed that an open condition exists whereas if a rapid increase in the output of the current sense line followed by zero current through the PROFET is detected immediately after the PROFET is turned ON. In either event, micro controller 13 may be programmed to automatically provide an indication to the driver and switch the power to an operable filament, that is, a filament not associated with the fault condition, via steering relay 28.

One of ordinary skill can further appreciate that the circuitry of the present invention can be readily adapted to provide daytime running (GTR) lamps. DTR lamps involve the headlights being i !) umhated even when the user has not actuated the manual headlamp switch. During day times, the DTR lamps are believed to improve the visibility ot trie vehicle to other drivers. In a similar fashion, the invention is also readily suitable for automatic headlamps, in which a light sensor is used to determine when it is sufficiently dark to warrant the illnination of the headlamps. In both these situations, one of ordinary skill can appreciate that the present invention provides the flexibility of illuminating any of the filaments to a variety of illumination levels through the use of PWM.

In a similar vein, another alternative embodiment of the present invention includes a flash-to-pass feature, wherein the operator can signal to the vehicle ahead of him/ner using headlamps 30 and 35. In some flash-to-pass features, a flash-to-pass input to a headtighl system may cause all four filaments in both headlamps to be energized momentarily. The present headlight system implements a flash-to-pass feature by momentarily causing the low and high beam filaments to be alternately energized. Such a feature may be implemented by programming micro controller 13 as reqjireu to control relays 37 and 38 of steering relay 28 thereby causing the headlamps to flash. This flash-to-pass strategy is effective regardless of whether the headlamps are energized in the low beam or high beam mode. These filaments may further be illuminated at any of the various illumination intensities described earlier.

In another alternative embodiment, the present invention may be modified by providing a PROFET for each filament. Such a configuration would obviate the need for steering relay 28 because each filament is driven by a respective PROFET, which is controlled by micro controller 13. In such a case, any combination of the filaments may be energized, wherein each filament is energized by a respective DC or PWM signal. The presence of a fault condition in a particular filament is detected as described above by monitoring the output of the current sensing lines. Further, micro controller 13 may be programmed to adjust the combination of filaments and their respective drive signals to provide an approximately balanced output from the headlamps as described above.

In another alternative embodiment, the present invention may be modified by providing a single filament in each headlamp. The brightness level of each filament is controlled by micro controlier 13 by adjusting the duty cycle of the PWM drive signal as described above.

One of ordinary skill also appreciates that, while it may be desirable for performance reasons to include a driver circuit for each low-high beam filament combination, it may be desirable for cost reasons to have a single driver. In such situations, a detected low beam filament failure would result in both high beam filaments being operated in a reduced intensity mode through PWM.

Although exemplary embodiments of the present invention have been shown and described herein, it will be understood that various changes, alterations and modifications may be made to the system without departing from the spirit and scope of the appended claims.