METHOD FOR CONTROLLING A HEAD-UP DISPLAY AND HEAD-UP DISPLAY

Description

TECHNICAL FIELD TO WHICH THE INVENTION RELATES

[0001] The present invention relates generally to the field of head-up displays.

[0002] It applies in particular to a head-up display for motor vehicles, comprising: a housing, an image generation unit disposed in the housing, which includes a light source and is adapted to emit a light beam carrying images, a projection system adapted to project said images into the field of vision of the driver of the motor vehicle, a light beam intensity attenuation unit, and a control unit for the attenuation unit and the image generation unit.

[0003] It relates more specifically to a method for controlling such a head-up display, which includes: a step of developing a control instruction for the light source, a step of developing a control instruction for the attenuation unit, and a step of controlling said light source and said attenuation unit according to the two control instructions. TECHNOLOGICAL BACKGROUND

[0004] To make driving a motor vehicle easier and safer, we want to avoid the driver being forced to take their eyes off the road they are driving on.

[0005] For this purpose, it is known to use a head-up display adapted to display information (vehicle speed, direction to follow, presence of obstacle, ...) at the driver's eye level.

[0006] Such a head-up display typically includes an image generation unit that produces a light beam carrying images, and a combiner that projects these images into the driver's field of vision.

[0007] The head-up display must be usable day and night. However, the image brightness specifications are not the same for daytime driving as for nighttime driving.

[0008] Indeed, for the displayed information to be visible during daytime driving, the image brightness must be high, typically exceeding 15,000 cd / m². Conversely, during nighttime driving, the image brightness must be very low to avoid dazzling the driver.

[0009] To reduce this light intensity, it is known, for example, to decrease the supply current to the light source of the image generation unit.

[0010] This method has two major drawbacks.

[0011] The first drawback is that it does not allow the light intensity to be reduced as much as desired for night driving.

[0012] The second drawback is that when LEDs are used as light sources in the image generation unit, the emitted beam color changes when the supply current is low. This is because the colorimetry of LEDs depends on the intensity of their supply current.

[0013] To address these problems, document FR3015704 describes a system comprising a laser image generation unit, an active polarizer designed to vary the polarization orientation of the light beam, and a passive polarizer. The two polarizers together reduce the intensity of the light beam passing through them.

[0014] The disadvantage of this system is that it generates significant heat. SUBJECT OF THE INVENTION

[0015] In order to remedy the aforementioned drawback of the prior art, the present invention proposes a control method as defined in claim 1.

[0016] By attenuating the intensity of the light beam emitted by the light source using an attenuation unit, this light intensity is controlled independently of the current supply to the light source, thus providing greater possibilities for reducing light intensity. Consequently, it is possible to have both high daytime brightness and low nighttime brightness.

[0017] By decreasing the electrical power supply to the light source and also reducing the light intensity emitted by the light source, we can best reduce the light intensity of the displayed images without loss of color, and by best reducing heating problems.

[0018] Other advantageous and non-limiting features of the piloting method according to the invention are as follows: when said parameter is equal to said minimum threshold value, said attenuation rate has a second value which varies according to the external brightness; the control setpoint for the light source is designed so that said light beam is emitted by the image generation unit with a luminous intensity which is constant when said parameter is equal to said minimum threshold value, and which varies when said parameter is greater than said minimum threshold value; the control setpoint for the attenuation unit allows control of a variation in the polarization orientation of the light beam within the attenuation unit; said parameter is relative to or equal to the intensity of the electrical current supplying the light source;the control instruction for the light source includes a duty cycle relative to a pulse width of an electrical current supplying the light source, and the minimum threshold value is a duty cycle value; and the duty cycle being controlled by a current regulation means,;

[0019] the electrical current at the input of said regulating means is less than a maximum value recommended for the current supply of the light source.

[0020] The invention also proposes a head-up display as defined in the introduction, whose control unit is adapted to implement a process as defined above. DETAILED DESCRIPTION OF A PROJECT EXAMPLE

[0021] The description that follows, with regard to the attached drawings, given by way of non-limiting examples, will make it clear what the invention consists of and how it can be carried out.

[0022] Regarding the attached drawings: there figure 1 is a schematic view of a head-up display according to the invention, the figures 2A et 2B These are schematic views of an image generation unit and a head-up display attenuation unit. figure 1 represented in two distinct states.

[0023] On the figure 1 We have schematically represented a head-up display 10 which is intended to equip a vehicle V, for example a motor vehicle, to display images in the field of vision of the driver of this vehicle V.

[0024] As a preliminary point, it should be noted that the terms "light beam" and "image" will be used interchangeably. Indeed, the light beam carries the images and has a luminous intensity that defines the luminous intensity of the image.

[0025] The terms "upstream" and "downstream" will also be used taking into account the direction of propagation of the light beam, from the area in which it is generated to the outside of the head-up display 10.

[0026] The head-up display 10 includes a housing 34 which houses an image generation unit 12, an attenuation unit 14, an optical projection assembly 16 and a computer 17 adapted to drive the image generation unit 12 and the attenuation unit 14.

[0027] The image generation unit 12 includes a light source 12A which emits a light beam 18 (shown on the figures 2A et 2B Here, the light source includes at least one light-emitting diode (LED). In practice, it comprises several light-emitting diodes distributed in the same plane, across several rows and several columns.

[0028] The image generation unit 12 further includes an image formation unit.

[0029] The image formation unit includes a screen. The screen here is a liquid crystal display (or LCD for " Liquid Crystal Display " à thin-film transistors (or TFTs for " Thin-Film Transistor " . This screen, hereinafter referred to as the TFT 20 screen, is placed parallel to the plane in which the light-emitting diodes are distributed.

[0030] The 20-inch TFT screen allows the light beam 18 to be modified so that it "carries" images.

[0031] In a classic way, as shown by figures 2A et 2B The TFT screen 20 polarizes the light along a polarization axis A TFT. In this example, the orientation of the polarization axis A TFT of the TFT screen 20 is vertical, but alternatively, it could of course be horizontal, or have an intermediate orientation. The light beam 18 exiting the image generation unit 12 therefore has vertical polarization.

[0032] The attenuation unit 14 is placed downstream of the image generation unit 12.

[0033] It is designed to be able, if necessary, to reduce the light intensity of the light beam 18 when it passes through it.

[0034] In the example shown in the figures, the attenuation unit 14 includes an active polarizer 24 and a single passive polarizer 26 placed here downstream of the active polarizer 24.

[0035] The active polarizer 24 is capable of modifying the polarization orientation of the light beam 18 within the attenuation unit 14, under the control of an electrical signal. The active polarizer 24 here comprises a liquid crystal cell controlled by the computer 17.

[0036] The passive polarizer 26 is configured to transmit only one component of the polarization of the light beam 18. The passive polarizer 26 here has a polarization axis AP parallel to that of the TFT screen 20, i.e., vertical. The passive polarizer 26 includes, for example, an inert linear polarizing film.

[0037] As shown by figure 2A In the absence of any change in the initial polarization orientation 22 of the light beam by the active polarizer 24, the attenuation unit 14 allows for a transmission close to 100% of the luminous intensity of the light beam 18. The luminous intensity of the light beam 18 is therefore not attenuated. The attenuation rate is thus considered to be zero.

[0038] As shown by figure 2B , when the initial polarization orientation 22 of the light beam 18 is modified by the active polarizer 24, the attenuation unit 14 makes it possible to reduce the luminous intensity of the light beam 18. The rate of attenuation of the luminous intensity is then greater the more the initial polarization orientation 22 of the light beam 18 is modified.

[0039] It should be noted here that the passive polarizer 26 helps to protect the TFT screen 20 from the sun by partially filtering the sunlight entering the housing 34.

[0040] The optical projection assembly 16 is here placed downstream of the attenuation unit 14.

[0041] Each image Img generated by the image generation unit 12 then propagates to the optical projection assembly 16 so as to be projected into the driver's field of vision when the driver's gaze is turned towards the road.

[0042] The optical projection assembly 16 is more specifically designed to project a virtual image Img' into the driver's field of vision, at a distance from the driver that is greater than that separating the driver from the windshield 1 (so that the driver's eyes do not have to perform accommodation work to perceive the projected information).

[0043] The optical projection assembly 16 includes for this purpose an optical referral system 30 and a combiner 32 placed in the field of vision of the vehicle driver.

[0044] The optical deflection system 30 here comprises only a folding mirror, housed in the casing 34. The folding mirror 30 can be flat or can have a complex aspheric type surface.

[0045] The folding mirror 30 allows the image Img generated by the image generation unit 12 to be sent back to the combiner 32.

[0046] The housing 34 has for this purpose a window 36 configured to allow the image Img to propagate to the combiner 32.

[0047] To protect the TFT screen 20, for example from dust, the window 36 can be covered with a glass cover (commonly called a "cover window"). Alternatively, this glass cover can be located between the TFT screen 20 and the optical transfer system 30.

[0048] However, in the example shown in the figures, the attenuation unit 14 replaces the glass cover. It is positioned to completely close the window 36. Reducing the number of optical elements in the head-up display helps to limit transmission losses of the light beam 18.

[0049] The combiner 32 allows the image Img to be reflected in such a way that it appears to the driver as if it were displayed at a great distance. In the example illustrated on the figure 1 The combiner 32 has a partially reflective plate.

[0050] Here, this combiner 32 is preferably located in the passenger compartment of the motor vehicle, between the windshield 1 of the vehicle V and the driver's eyes.

[0051] Alternatively, the windscreen 1 itself could act as a combiner.

[0052] The computer 17 of the head-up display 10 forms a control unit for the attenuation unit 14 and the image generation unit 12.

[0053] This control unit 17 is capable of generating and issuing control instructions to vary the light intensity of the light beam 18 seen by the driver. To achieve this, the control unit 17 is configured to modify, firstly, the supply current of the light source 12A, and secondly, the control current of the active polarizer 24.

[0054] This calculator includes for this purpose a processor, memory and various input and output interfaces.

[0055] Thanks to its input interfaces, the computer 17 is adapted to receive input signals. Typically, it can receive a signal relating to external brightness (i.e., relating to the intensity of light coming from outside the vehicle, in particular sunlight).

[0056] Thanks to its output interfaces, the calculator 17 is suitable for controlling the light source 12A and the active polarizer 24.

[0057] Thanks to its memory, the computer can store a computer application, consisting of computer programs including instructions whose execution by the processor allows the computer to implement the process that will be described below.

[0058] When the head-up display 10 is started, the light beam 18 is emitted by the light source 12A, it passes through the TFT screen 20 so as to carry images Img, it passes through the attenuation unit 14 so that its light intensity may be reduced, and then it is projected into the driver's field of vision.

[0059] The calculator 17 is configured to vary the light intensity of the light beam 18 seen by the driver.

[0060] The goal is for this light intensity to be lower in night driving conditions than in daytime driving conditions.

[0061] We could plan to play for this solely on the supply intensity of the 12A light source, which, as a reminder, is composed of light-emitting diodes.

[0062] However, a light-emitting diode (LED) has usage recommendations corresponding to its nominal operation. These recommendations include, for example, a recommended supply current range outside of which the nominal operation of the LED is not guaranteed.

[0063] The range is defined between a maximum value (above which the LED risks damage) and a minimum value (below which the LED's performance is degraded). For example, the minimum supply current is 250mA.

[0064] Within the framework of the invention, the nominal colorimetry of the LEDs is the nominal operating characteristic of greatest interest. However, since the emission wavelength of the light-emitting diode varies below the recommended supply current range, the nominal colorimetry is no longer maintained below this range.

[0065] It is therefore understood that it is not possible to guarantee proper operation of the head-up display 10 with a supply current intensity of less than 250mA.

[0066] However, with this electrical intensity, the light beam 18 exhibits excessive brightness under nighttime driving conditions. Therefore, an additional attenuation unit 14 is provided to further reduce the brightness of the images Img seen by the driver of vehicle V.

[0067] The control method for the head-up display 10 is then implemented in three steps, including a first step of developing a control instruction for the light source 12A, a second step of developing a control instruction for the attenuation unit 14, and a third step of controlling the light source 12A and the attenuation unit 14 according to these two control instructions.

[0068] According to the invention, it is planned to use the attenuation unit 14 only when the light source is no longer able to reduce the light intensity emitted by the light-emitting diodes (except when outside the aforementioned supply current range).

[0069] For this purpose, according to a particularly advantageous feature of the invention, it is planned to acquire a parameter which is related to the luminous intensity of the light beam 18 (for example at its output of the light source 12A), and then to develop the control setpoint of the attenuation unit 14 differently depending on whether this parameter is greater or less than a minimum threshold value.

[0070] More specifically (considering that the aforementioned parameter increases as light intensity increases): when the parameter is greater than a minimum threshold value, it is planned at least to minimize, or even cancel, the effect of the attenuation unit 14 on the light beam 18, and it is planned otherwise to drive the attenuation unit 14 so that it attenuates the light intensity of the light beam 18.

[0071] We can now describe more precisely how this process is implemented.

[0072] During the first stage of developing the control instruction for the light source 12A of the image generation unit 12, the instruction developed by the computer 17 includes an intensity instruction for the supply current of the light source 12A, so that the light-emitting diodes emit a more or less intense light.

[0073] It could be expected that the 12A light source would be controlled via linear regulation of the supply current intensity.

[0074] However, here we will consider that the 12A light source is controlled via current intensity regulation by pulse width modulation.

[0075] In this case, the control signal for the light intensity of the light beam 18 emitted by the LEDs is determined by a duty cycle corresponding to the electrical pulse width over one period. By varying this duty cycle, the average current supplying the LEDs is varied. It is therefore possible to vary the light intensity of the light beam 18 emitted by the LEDs.

[0076] The duty cycle is controlled by a current regulator. To ensure that the electrical current supplying the light source (12A) is less than the maximum value within the recommended range (beyond which the light source risks being damaged), the electrical current at the input of this regulator is maintained at a constant value below this maximum. Thus, for a 100% duty cycle (corresponding to direct current), the electrical current supplying the light source is equal to the electrical current at the input of the regulator, and is therefore less than the maximum value.

[0077] It should be noted that, generally speaking, light-emitting diodes (LEDs) have a minimum pulse duration (and therefore a minimum duty cycle) below which their nominal operation is not guaranteed. This minimum duty cycle limits the reduction in light intensity that can be achieved solely through the LED's control signal.

[0078] The aforementioned "parameter" will then be formed by this duty cycle. The minimum threshold is then chosen taking into account the minimum duty cycle. It can thus be chosen to be equal to the latter.

[0079] Thus, according to the invention, as long as the duty cycle is above this minimum threshold, the computer 17 will only adjust the control setpoint of the light source 12A to vary the luminous intensity of the light beam 18. The attenuation unit 14 will be controlled in such a way that the active polarizer 24 does not change the polarization orientation of the light beam (case of the figure 2A ).

[0080] To achieve this, the voltage across the active polarizer 24 will be maintained at zero. Thus, the active polarizer 24 will not alter the initial polarization orientation 22 of the light beam.

[0081] The minimum intensity obtained in this way is therefore limited, on the one hand, by the minimum value of the supply current intensity, and, on the other hand, by the minimum duty cycle.

[0082] In fact, adjusting only this control setting allows us to achieve a light intensity of, for example, 30 cd / m². To further reduce this light intensity, the attenuation unit 14 is intended to be used.

[0083] More specifically, when the duty cycle reaches the minimum threshold, the controller only adjusts the control setpoint of the attenuation unit 14 to vary the luminous intensity of the light beam 18. The light source 12A, on the other hand, is controlled so that the duty cycle remains equal to the minimum threshold (case of the figure 2B ).

[0084] More specifically, the control command of the attenuation unit 14 allows a variation in the orientation of the polarization of the light beam within the attenuation unit 14.

[0085] To do this, the control command of the attenuation unit 14 allows the voltage across the terminals of the liquid crystal cell 24 to be varied.

[0086] This variation in voltage allows the initial polarization orientation 22 of the light beam passing through the active polarizer 24 to be gradually rotated.

[0087] This orientation can be predetermined. Alternatively, it can be predicted to vary depending on the external light conditions (sunny weather, cloudy weather, foggy weather, night) or depending on the time of day.

[0088] The new orientation obtained 42 at the output of the liquid crystal cell 24 is inclined with respect to the polarization axis AP of the polarizer by an angle between 0 and 90 degrees, excluding terminals.

[0089] This method of controlling the head-up display 10 makes it possible to achieve a very low light intensity, for example 1cd / m2, while maintaining the nominal colorimetry of the light source and minimizing heat generation.

Claims

1. Method for controlling a head-up display (10) for a motor vehicle (V) comprising a housing (34) which includes an image generation unit (12), comprising a light source (12A) which is at least one light-emitting diode and an image formation unit which is a thin-film transistor liquid crystal display (20), adapted to emit a light beam (18) carrying images (Img), and an attenuation unit (14) for attenuating the light intensity of said light beam (18), said driving method comprising: - a step of developing a control setpoint for a light source (12A) of the image generation unit (12), - a step of developing a control setpoint for the attenuation unit (14), - a step of controlling said light source (12A) and said attenuation unit (14) according to the two control setpoints, when a parameter relating to the light intensity of said light beam (18) is greater than a minimum threshold value, the control setpoint for the attenuation unit (14) is developed so that the attenuation rate of the light intensity of the light beam (18) by the attenuation unit (14) has a first minimum value, when said parameter is equal to said minimum threshold value, the control setpoint for the attenuation unit (14) is calculated so that said attenuation rate has a second value strictly greater than the first value, characterised in that the attenuation unit (14) is placed downstream of the image generation unit (12), and said attenuation unit (14) completely closes a window (36) provided in the housing (34).

2. Control method according to the previous claim, in which, when said parameter is equal to said minimum threshold value, said attenuation rate has a second value which varies according to the external brightness.

3. A control method according to one of the preceding claims, wherein the control setpoint for the light source (12A) is calculated such that said light beam (18) is emitted by the image generation unit (12) with a light intensity that is constant when said parameter is equal to said minimum threshold value, and that varies when said parameter is greater than said minimum threshold value.

4. A control method according to one of the preceding claims, wherein the control command for the attenuation unit (14) controls a variation in the polarisation orientation of the light beam (18) within the attenuation unit (14).

5. Control method according to one of the preceding claims, wherein said parameter is related to or is equal to the intensity of the electric current supplying the light source (12A).

6. Control method according to the preceding claim, wherein the control setpoint for the light source (12A) comprises a duty cycle relative to a pulse width of an electric current supplying the light source (12A), and wherein the minimum threshold value is a duty cycle value.

7. Control method according to the previous claim, wherein the duty cycle is controlled by a current regulation means, the electrical current input to said regulation means is less than a maximum value recommended for the power supply to the light source (12A).

8. Head-up display (10) for a motor vehicle (V), comprising: - a housing (34), - an image generation unit (12) disposed in the housing (34), which comprises a light source (12A), which is at least one light-emitting diode, and which is adapted to emit a light beam (18) carrying images (Img), and an image formation unit which is a thin-film transistor liquid crystal display (20), - a projection system (16) adapted to project said images (Img) into the field of vision of the driver of the motor vehicle (V), - a unit (14) for attenuating the light intensity of the light beam (18), and - a control unit (17) for the attenuation unit (14) and the image generation unit (12), characterised in that the attenuation unit (14) is located downstream of the image generation unit (12) and in that said control unit (17) is adapted to implement a method in accordance with one of the preceding claims, and said attenuation unit (14) completely closes a window (36) formed in the housing (34).

9. Head-up display (10) according to one of the two preceding claims, wherein said attenuation unit (14) comprises a single passive polariser (26) and an active polariser (24) which is capable of modifying the orientation of the polarisation of the light beam (18) within the attenuation unit (14).