Digital electrochromic mirror system

Abstract: A vehicular electrochromic rearview mirror system (10) includes an interior electrochromic rearview mirror assembly (11) and at least one exterior electrochromic rearview mirror assembly. Each mirror assembly includes an electrochromic reflective element (18) that assumes a partial reflectance level in response to a signal applied thereto. A drive circuit applies a drive signal to each of the electrochromic reflective elements. The drive circuit (13) may include a digital controller, a master drive circuit responsive to the digital controller to apply a first drive signal to one of the electrochromic reflective elements (18) and a slave drive circuit responsive to the digital controller to apply a second drive signal to a second of the electrochromic reflective elements (18). The second drive signal may be derived from the first drive signal. The drive circuit (13) may include a digital controller and a switching power supply controlled by the digital controller to produce a drive signal.

Description: BACKGROUND OF THE INVENTION This invention relates generally to vehicle rearview mirror systems and, more particularly, to electrochromic mirror systems, including an interior electrochromic rearview mirror assembly and at least one exterior electrochromic rearview mirror assembly. An electrochromic rearview mirror assembly includes an electrochromic reflective element that responds to a signal applied across a pair of terminals by varying the light reflected by the reflective element. In this manner, the reflectance level of the reflective element can be varied by varying the signal applied to the electrochromic cell. The electrochromic reflective element has characteristics which makes control of its reflectance level difficult. The electrochromic reflective element operates at a relatively low voltage, which typically may not exceed approximately 2 volts DC, more typically not more than about 1.5 volts DC, for more than a brief period of time or else useful life of the reflective element is compromised. Furthermore, the amount of voltage necessary to establish a particular value of the light transmissivity of the cell varies with the temperature of the cell, the amount of change in light transmittance undertaken, and the surface area of the cell. Therefore, optimum control of the electrochromic reflective element may require more than merely applying a DC voltage corresponding to the desired reflectance level. Electrochromic mirror systems have incorporated therein many additional electronic functions in addition to mirror drive. These include compass functions; namely, determination and display of vehicle heading, display of the interior and/or exterior temperature as well as other functions as disclosed in United States Patent No. 5,798,575. By combining multiple functions in a single system, it is possible to utilize the computational power of a microcomputer in order to perform a wide range of functions. Although a microcomputer has extensive computational capability, its ability to supply a drive signal to external components, such as an electrochromic cell, is limited. Therefore, it has been necessary to provide an interface circuit between the microprocessor and the electrochromic cell in order to supply the electrochromic cell with a desired reflectance level established by the microcomputer. United States Patent No. 5,451,822 integrates a Pulse-Width Modulated (PWM) output of a microcomputer and amplifies the integrated PWM signal utilizing a conventional analog DC amplifier. The use of analog components to amplify the output of the microcomputer not only increases the cost of the system, but, additionally, increases its power consumption. The increase in power consumption requires larger components, which increases the physical size of the assembly and also increases the necessity for enhanced heat dissipation capabilities. In published International Publication No. WO 99/14619, a plurality of electrochromic mirror elements used in an automobile are controlled by individual drive circuits for each of the electrochromic elements in accordance with the pre-characterising features of claim 1. The individual drive circuits are supplied with PWM signals generated by a microcomputer from photosensors positioned inside the automobile. Additionally, separate glare signals are supplied to each of the electrochromic elements such that the reflected light at a predetermined reference point is relatively constant. The system described in Publication No. WO 99/14619 multiplies the number of analog components and the increase in energy consumption resulting therefrom. Thereby, not only is the advantage derived from the generation of digital signals by the microcomputer not realized, the problems associated with analog circuitry are multiplied. In United States Patent 5,675,438 complementary push-pull transistors, connected between positive and negative voltages of positive 1.6 volts DC and negative 1.6 volts DC, respectively, are operated by an oscillation circuit to supply drive pulses to an electrochromic mirror. The duty factor of the pulses are adjusted by the oscillation circuit in response to light levels sensed by forward and rearward facing light sensors. Power supplies convert positive 12 volt vehicle battery voltage to the positive 1.6 volt and negative 1.6 volt levels needed to operate the push-pull transistors. The power supplies add components to the circuit and increases the power consumption thereof. SUMMARY OF THE INVENTION The present invention provides a vehicular electrochromic rearview mirror system characterised in that the drive circuitry comprises a digital controller, a first drive circuit is a master drive circuit responsive to the digital controller to apply a first drive signal to a first electrochromic reflective element, a second drive circuit is a slave drive circuit responsive to the digital controller to apply a second drive signal to a second electrochromic reflective element, the second drive signal being derived from the first drive signal. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a side elevation of a vehicle incorporating an electrochromic mirror system according to the invention; Fig. 2 is a side elevation of a portion of an electrochromic mirror system according to the invention; Fig. 3 is a block diagram of a master/slave drive circuit according to the invention; Fig. 4 is a generalized circuit diagram of a slave drive circuit according to the invention; and Fig. 5 is a detailed schematic diagram of the master/slave drive circuit in Fig. 3. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now specifically to the drawings, and the illustrative embodiments depicted therein, a vehicle 9 includes an electrochromic mirror system 10 having an electrochromic interior rearview mirror assembly 11 and one or more exterior rearview mirror assemblies 19 including a driver side rearview mirror assembly 19 on the driver side of the vehicle and/or a passenger side rearview mirror system on an opposite side of vehicle 9 (Fig. 1). Electrochromic mirror system 10 includes an electronic control 12 which, preferably, is positioned within a housing 14 of interior rearview mirror assembly 11 (Fig. 2). Electronic control 12 includes a drive circuit 13 which receives a first input from a rearward-facing light sensor 20, which faces rearwardly in the direction of travel of the vehicle, and a second input from forward-facing light sensor 22, which faces forward with respect to the direction of travel of vehicle 9. Drive circuit 13 supplies a drive signal to electrochromic reflective element 18 included with interior rearview mirror assembly 11. It may also supply a drive signal to other electrochromic reflective elements, such as electrochromic reflective element 24 associated with exterior rearview mirror assembly 19. In the illustrative embodiment, electronic control 12 may include a display that may be positioned behind reflective element 18 and viewed by the driver through the electrochromic reflective element as disclosed in United States Patent No. 5,285,060. Alternatively, the display may be positioned on a lip portion of housing 14 below reflective element 18 or on any other portion of the housing visible to the driver as illustrated in United States Patent No. 5,786,772. Alternatively, the display could be in the form of a heads-up display projected from housing 14 on the vehicle windshield. Electronic control 12 may additionally include a heading sensor, or compass, which produces outputs indicative of the heading of the vehicle. Such heading sensor may be of the magneto-resistive type, such as disclosed in United States Patent No. 5,255,442, or may be of the magneto-inductive type, such as disclosed in United States Patent No. 5,924,212 , or may be of the flux-gate type, or may be of the magneto-capacitive type. Electrochromic reflective elements 18 and 24 may be of any type such as disclosed in United States Patent No. 4,902,108 issued to Byker; United States Patent No. 5,140,455; United States Patent No. 5,910,854; and United States patent No. 5,724,187. Electrochromic reflective elements 18 and 24 include electrochromic units, or cells, that decrease in transmissivity in response to a drive signal applied thereto. The decrease in transmissivity of the electrochromic unit, or cell, results in a decrease in reflectance level of the associated reflective element. Light sensors 20 and 22 are conventional and may be applied as described in United States Patent 4,917,477 issued to Bechtel et al.; United States Patent No. 3,601,614 issued to Platzer, Jr.; and United States Patent Nos. 4,793,690 issued to Graham et al. and 5,715,093 issued to Schierbeek et al.. Outputs of the light sensors are supplied as inputs to a microcomputer controller 26. For solution-phase, single-compartment, self-erasing electrochromic mirror elements commonly used commercially today, the maximum voltage tolerable for a sustained period is in the 1.0 volt to 2.0 volt range, typically less than 1.5 volts, and most typically about 1.4 volts. For solid-film electrochromic devices that utilize a layer, such as a tungsten oxide thin-filmed layer, the maximum voltage tolerable for a sustained period is in the 1.0 volt to the 3.0 volt range, typically in the 1.3 volt to 1.5 volt range. Usually, application of a voltage much in excess of such maximum tolerable voltage to the electrochromic cell for a sustained period, typically at least several seconds, may cause change to the electrochromic medium in the electrochromic cell. Electrochromic reflective element 24 can be modeled as having a series resistance shown as R1 and a shunt resistance shown as R2 (Fig. 5). The series resistance R1 in the model arises from resistance of the electrodes and is on the order of 1 to 2 ohms. Shunt resistance R2 in the model arises from internal losses of the electrochromic cell. When the cell is fully colored, the internal shunt resistance is approximately 10 ohms. The electrochromic element 24 typically develops a voltage, which is the potential stored on the cell. Upon application of external voltage thereto, the electrochromic element temporarily retains said potential even when the external voltage potential is removed and the cell is open-circuited. This property of electrochromic element 24 can be represented in the model by a capacitance C cell which is on the order of a half of farad for an interior mirror electrochromic reflective element. The capacitance will vary as a function of the surface area of the electrochromic reflective element. In an electrochromic mirror system for a vehicle, an electronic control has a drive circuit 13' including a master drive circuit 40 and a slave drive circuit 42 (Figs. 3, 4 and 5). Master drive circuit 40, which is illustrated in Fig. 4 as a voltage source, produces a drive signal at its output 32 which may be supplied to an electrochromic reflective element 24. Slave drive circuit 42 derives from output 32 a second signal which is supplied to another rearview mirror electrochromicreflective element 18. Although described in more detail later, it will be seen that other commonly available drive circuits, such as the type disclosed in Bechtel et al. United States Patent 5,451,822, could be utilized as a master drive circuit. In the preferred embodiment, slave drive circuit 42 includes a first electronic switch Q3 in electrical series connection between master drive circuit 40 and electrochromic reflective element 18 and a second electronic switch Q4 which shunts electrochromic reflective element 18 when closed. Electronic switch Q4 is for the purpose of rapidly discharging accumulated charge on electrochromic reflective element 18 when it is desired to rapidly increase the reflectivity of the associated reflective element such as when the vehicle is replaced in reverse gear. Electronic switch Q3 is opened and closed in a PWM fashion in order to supply a pulsed DC voltage to slave driven electrochromic reflective element 18 that is derived from the drive signal produced by the master drive circuit 32. The drive signal supplied to slave electrochromic reflective element 18 is at a level that is less than or equal to the master drive signal at 32. If transistor Q3 is closed 100 percent of the time, then the slave drive signal supplied to slave electrochromic element 18 will be equal to the master drive signal supplied to master cell 24. As the duty cycle of electronic switch Q3 being in a saturated state decreases below 100 percent, the slave voltage supplied to electrochromic element 18 will decrease as a proportion of master drive signal 44 supplied to electrochromic element 24. In the illustrated embodiment, switch Q3 is operated at a duty cycle that is greater than or equal to 65 percent and preferably in a range of between 75 percent and 100 percent. Advantageously, transistor Q3 can be operated at a relatively slow repetition rate without producing undesirable ripple in the voltage supplied to slave electrochromic element 18. This is accomplished by utilizing the significant capacitance CCELL of electrochromic element 18. Preferably, electronic switch Q3 is operated at a pulse repetition rate that is at the low end of the audio perception of humans, but above a rate that may produce noticeable flicker; namely, below 200 hertz and preferably less than or equal to 100 hertz. Most preferably, the pulse repetition rate of electronic switch Q3 is approximately 50 hertz. In addition to allowing the repetition rate of electronic switch Q3 to be relatively low, the large capacitance of electrochromic cell 15 allows the change in reflectance level thereof to be changed in relatively coarse steps. For example, the change in duty cycle of electronic switch Q3 could be changed in 5 percent steps, 10 percent steps, or even 15 percent steps. This results in a lessening of the software requirement for controlling the slave drive. Alternatively, switches Q1 and D1 of master drive 40 could operate at a repetition rate below human audible reception and switches Q4 of slave drive 42 operate at a rate above human audible perception. In the illustrated embodiment, slave drive circuit 42 supplies a drive signal to electrochromic element 18 that is part of interior electrochromic mirror assembly 11 and master drive circuit 40 supplies a drive signal to electrochromic element 24 that is a part of exterior electrochromic rearview mirror assembly 19 of which two are preferred on a vehicle. This is based upon a preference that the reflectance level of the interior rearview mirror should be no less than the reflectance level of the exterior rearview mirrors. This arrangement is particularly advantageous in providing a relationship between the reflectivity of the interior rearview mirror assembly and exterior rearview mirror assemblies that takes into account the configuration of the vehicle as disclosed in UK Patent Application No. GB2,342,467A . However, it may be desirable for particular applications to drive the interior electrochromic reflective element with the master drive circuit and one or both exterior electrochromic reflective elements with the slave drive circuit. Although a master/slave drive circuit according to the invention has more design constraints than other known electrochromic mirror systems, which provide independent drive signals for each mirror controlled, it has been discovered that any differences are not perceived by the driver. A detailed embodiment of drive circuit 13' is illustrated in Fig. 9 in which an inductor L4 and capacitor C4 filter noise for a voltage source 33 to the emitter of transistor Q1. A diode D2 is connected from output 34 of microcomputer 26 to a voltage source, such as 5 volts. The purpose of diode D2 is to clip any voltage spikes that may be fed to line 34 such as from inductor L6. Otherwise, the master drive of drive circuit 13' is identical with switching power supply 28 in Fig. 6. Microcomputer 26 has an output 46 which supplies a PWM drive signal to an NPN transistor Q3. The emitter of transistor Q3 is supplied at 48 to a terminal of slave electrochromic element 18. Transistor Q3 selectively interconnects line 48, which supplies a drive signal to slave electrochromic element 18, with line 32 which supplies a drive signal to master electrochromic cell 24. Transistor Q3 is selectively switched between an open state and a saturated state in response to an output 46 from microcomputer 26. By varying the duty cycle of the PWM signal at output 46, microcontroller 26 selectively adjusts the relative proportion between the drive signal supplied to slave electrochromic element 18 and the drive signal supplied to master electrochromic element 24. Microcomputer 26 additionally has an output 50 which is supplied to a base of a transistor Q4. Transistor Q4 is connected between line 48 and ground. When microcomputer 26 switches transistor Q4 to a saturated state, electrical charge on slave electrochromic reflective element 18 is rapidly dissipated. Transistors Q3 and Q4 are placed in saturation at different times. Microcomputer 26 additionally receives an input at 52 which is connected through a resistor R29 to line 48. Input 52 supplies to microcomputer 26 a signal representative of the voltage across slave electrochromic element 18. In operation, microcomputer 26 determines a desired reflectance level for interior electrochromic rearview mirror assembly 11 and electrochromic exterior mirror assembly 19. The values of desired reflectance level may be established, by way of example, from the respective light levels sensed by light sensors 20, 22 as well as the configuration of the vehicle as disclosed in United Kingdom Patent Application No. GB2,342,467A . The microcomputer controller then selects a duty cycle for the PWM signal supplied at output 34 to transistor Q1. The selected duty cycle may take into account, by way of example, the voltage across master electrochromic cell 24 as monitored by input 38 from line 44 and the degree of change in reflectance level desired. The PWM signal supplied at output 34 produces a voltage level at line 32 which is monitored by the microcomputer at input 38. If master electrochromic element 24 is being switched to a substantially lower transmissivity, it may be desirable to momentarily increase the voltage supplied to the master electrochromic cell in order to rapidly modify the transmissivity of the cell. During such transitions, microcomputer 38 may more frequently monitor the voltage at line 44 in order to ensure that any temporary over voltage condition of master electrochromic element 24 is kept within a prescribed maximum voltage level. Values of maximum voltage level, as well as PWM duty cycles, may be selected according to the principles disclosed in United States Patent No. US6,089,721. If microcomputer controller 26 determines that the reflectance level of master electrochromic element 24 should be substantially increased, then the PWM signal would be disconnected from output 34 and output 36 energized in order to place transistor Q2 in a saturated condition. This will rapidly dissipate charge on master electrochromic element 24 to increase the transmissivity thereof. Microcomputer controller 26 additionally selects a duty cycle for a PWM signal supplied at output 46. The PWM signal will be selected at least as a function of the desired reflectance level for slave electrockomic element 18. The selected duty cycle may take into account the current reflectance level of master electrochromic element 24 represented at least in part by the voltage across slave electrochromic element 18 on line 48, as monitored by input 52 to the microcomputer. Once a duty cycle is selected microcomputer 26 produces a PWM signal at output 46. Microcomputer 26 may choose to dwell on a particular duty cycle at output 46 unless the calculated duty cycle change is more than a given percent. In the illustrated embodiment, the duty cycle is not changed by microcomputer 26 unless the calculated duty cycle change is at least 5 percent, more preferably at least 10 percent and most preferably at least 15 percent. If the calculated duty cycle of slave electrochromic reflective element 18 results in a substantial increase in the transmissivity of reflective element 18, then microcomputer 26 removes the PWM signal from output 46 and supplies an output at 50 to cause transistor Q 4 to enter a saturated mode which quickly dissipates any charge from line 48 thereby rapidly increasing the transmissivity of slave electrochromic element 18. Drive signal 48 applied to interior electrochromic reflective element 18 is compensated for the thermal environment of integer electrochromic mirror assembly 11 as well as the surface area, present reflectance level, and the like, of interior electrochromic reflective element 18. This is accomplished by drive circuit 13' monitoring voltage across electrochromic reflective element 18 as supplied to input 52 of microcomputer 26. Drive signal 32 applied to exterior electrochromic reflective element 24 is compensated for the thermal environment of exterior electrochromic mirror assembly 19, as well as the surface area, present reflectance level, and the like, of exterior electrochromic reflective element 24. This is accomplished by drive circuit 13' monitoring voltage across electrochromic reflective element 24 as supplied to input 38 of microcomputer 26. In this manner, drive circuit 13 can compensate for different thermal environments of interior and exterior electrochromic mirror assemblies 11, 19. Additionally, microcomputer 26 may receive temperature sensor inputs for any of the interior and/or electrochromic exterior rearview mirror assemblies to further compensate for different thermal environments. This additional input or inputs may assist microcomputer 26 in reducing effects created by non-linearity in the relationship between voltage across the electrochromic reflective element and its reflectance level for different temperatures. Although transistor Q 3 is illustrated as operated in a saturated mode at a particular PWM, the slave drive could alternatively be a transistor drive in a non-saturated state either continuously or at a PWM. The present invention, although illustrated with one master electrochromic reflective element and one slave electrochromic reflective element, could be utilized with two master drive circuits to drive two electrochromic reflective elements, such as for two exterior rearview mirror assemblies and one slave electrochromic element which is slave to one of the master drive circuits. Alternatively, an electrochromic mirror system may utilize one master electrochromic element to drive one exterior electrochromic rearview mirror and two slave electrochromic elements, each driven to an independently or tandemly controlled partial reflectance level that is higher than the partial reflectance level of the master rearview assembly. Other variations may suggest themselves to the skilled artisan; for example, the two electrochromic reflective elements driven by the master and slave drive circuits may both be interior or exterior mirror systems. The functions performed by microcomputer 26 could be implemented by any circuit capable of generating a digital signal including a programmable computer, a dedicated digital logic circuit, or the like. Preferably, the components making up electronic control 13, 13' are commonly mounted in housing 14 of interior electrochromic mirror assembly I 1 as disclosed in United States Patent No. 5,424,898. However, it should be understood that the components could alternatively be located in exterior electrochromic mirror assembly 19 or in a location separate from the mirror assemblies as disclosed in commonly assigned United States Patent No. 5,659,423.

Claims: A vehicular electrochromic rearview mirror system, comprising: a plurality of electrochromic rearview mirror assemblies (11, 19) and a plurality of electrochromic reflective elements (18, 24), each associated with one of said electrochromic rearview mirror assemblies, wherein each of said electrochromic reflective elements assumes a partial reflectance level in response to a drive signal applied thereto; and drive circuit means (13') applying a drive signal to each of said electrochromic reflective elements, said drive circuit means comprises a first drive circuit (40') to apply a first drive signal (32) to a first of said electrochromic reflective elements (24) and a second drive circuit (42) to apply a second drive signal (48) to a second of said electrochromic reflective elements (18), characterised in that : said drive circuit means comprises a digital controller (26), said first drive circuit (40) is a master drive circuit responsive to said digital controller (26) to apply said first drive signal (32) to said first electrochromic reflective element (24), said second drive circuit (42) is a slave drive circuit responsive to said digital controller (26) to apply said second drive signal (48) to said second electrochromic reflective element (18), said second drive signal being derived from said first drive signal. The vehicular electrochromic rearview mirror system claimed in claim 1 wherein said master drive circuit comprises a switching power supply (28). The vehicular electrochromic rearview mirror system claimed in claim 1 wherein said slave drive circuit comprises a proportional controller. The vehicular electrochromic rearview mirror system claimed in claim 1 wherein said master drive circuit comprises a first electronic switch and said slave drive circuit comprises a second electronic switch. The vehicular electrochromic rearview mirror system claimed in claim 4 wherein said controller operates said first electronic switch at a first repetition rate and said second electronic switch at a second repetition rate, one said repetition rate being at a high end of human hearing frequencies and the other said repetition rate being at a low end of human hearing frequencies. The vehicular electrochromic rearview mirror system claimed in claim 4 wherein said controller operates said first electronic switch during steady-state reflectance levels at a duty cycle of less than or equal to 65%. The vehicular electrochromic rearview mirror system claimed in claim 6 wherein said controller operates said second electronic switch during steady-state reflectance levels at a duty cycle greater than or equal to 65%. The vehicular electrochromic rearview mirror system claimed in claim 2 wherein said switching power supply comprises a buck converter. The vehicular electrochromic rearview mirror system claimed in claim 8 wherein said switching power supply comprises an impedance device and an electronic switch in series electrical connection between said electrochromic reflective element and a voltage potential. The vehicular electrochromic rearview mirror system in claim 9 wherein said impedance device has energy storing capability. The vehicular electrochromic rearview mirror system in claim 9 wherein said impedance device comprises an inductor. The vehicular electrochromic rearview mirror system claimed in claim 9 wherein said voltage potential is in the range from 5 volts DC to 18 volts DC. The vehicular electrochromic rearview mirror system claimed in claim 4 wherein said first and second electronic switches comprise transistors. The vehicular electrochromic rearview mirror system in claim 13 wherein said first and second electronic switches comprise one of a bipolar transistor and a field-effect transistor. The vehicular electrochromic rearview mirror system in claim 4 wherein said first and second electronic switches are operated in saturated modes. The vehicular electrochromic rearview mirror system claimed in claim 4 wherein said drive circuit monitors a voltage across at least one of said reflective elements and controls at least one of said first and second switches at least in part as a function of monitored voltage. The vehicular electrochromic rearview mirror system claimed in any preceding claim, wherein said one of said electrochromic reflective elements is associated with an exterior electrochromic rearview mirror assembly and said second of said electrochromic reflective elements is associated with an interior electrochromic rearview mirror assembly.

Inventors: HOEKSTRA ERIC J, SCHIERBEEK KENNETH L, HOEKSTRA, ERIC J., SCHIERBEEK, KENNETH L.
Applicants: DONNELLY CORP, DONNELLY CORPORATION
Application year: 2000
Main classification: B60R1
IPC codes: B60R1