Abstract:
A motor vehicle electrical power system includes a light source powered from an electrical power source. At key on the light source is tested to determine operational readiness and the type of the light source. At key on the switch is cycled to apply a pulse width modulated energization to the light source. A reference copy of the pulse width modulated signal is available. A comparator having first and second inputs provides a comparison of the pulse width modulated signal applied to the light source and reference. Variation in the rate of change of voltage across the light source may be compared with the reference to characterize the light source as a light emitting diode or another type of source, usually an incandescent bulb.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The technical field relates generally to impedance measurement and more specifically to application of impedance measurement to identify which from a known set of possible loads is connected to a power supply circuit. 
         [0003]    2. Description of the Problem 
         [0004]    Motor vehicle lighting systems may employ different types of light sources, including incandescent bulbs, arc lamps and light emitting diodes, among other devices. Low voltage light sources such as some types of incandescent bulbs and light emitting diodes can be directly energized from a body controller, allowing easy implementation of electronic switching and bulb monitoring. However, doing so introduces the possibility that the character of the load supported by the body controller may change over the life of a given vehicle or from vehicle to vehicle equipped with similarly programmed body controllers. 
         [0005]    Incandescent lamps are energized by connecting the bulbs to a voltage source. Their service life may be adversely affected by application of an over voltage. Light emission in terms of lumens radiated may be adjusted by connecting additional radiators to the circuit. Light emission from light emitting diodes may be adjusted by changing the current sourced to the device. Incandescent lamps have been viewed as resistive loads and their operational status has been readily confirmed by detection of current flow through their circuit. If just operational availability is at issue light emitting diodes may be checked the same way. Lighting circuit integrity has been verified on vehicles by application of an electrical voltage pulse to each lighting circuit at least at key-on of a vehicle. To date this is believed to have been limited to simply verifying current flow commensurate with operational availability of known device. 
         [0006]    However, incandescent lamps and light emitting diodes are to some extent complex devices with an imaginary axis component in their response to application of a voltage. An incandescent bulb is a hot coil and a light emitting diode is a cold PN junction. Thus an incandescent lamp exhibits some inductance. Application of a voltage across a light emitting diode results in generation of a static electric field and thus the device should exhibit some characteristics of a capacitor. The complex load vectors for the respective devices have distinctive, detectable components. 
         [0007]    U.S. Pat. No. 7,030,627 to Ashley teaches that complex impedances are commonly measured with electronic test equipment. The complex impedance at any specific frequency consists of a real resistive component and a reactive portion. 
       SUMMARY 
       [0008]    A motor vehicle electrical power system includes a light source powered from an electrical power source. A control switch provides for connection of the light source to the electrical power source. Control over the connection may be implemented in a way to deliver power as a pulse width modulated signal in order to control the total current delivered and thus the illumination intensity. At key-on of the vehicle ignition or some other defined start point a vehicle&#39;s light sources are tested to determine operational readiness and the types of the light sources. At key-on the light sources are cycled by application of pulse width modulated energization signal. Reference copies of the pulse width modulated signals are available. A comparator having first and second inputs provides a comparison of the pulse width modulated signal applied to a given light source and its reference signal. Variation in the rate of change of voltage across the light source may be compared with the reference to characterize the light source as a light emitting diode or another type of source, usually an incandescent bulb. Light source operation may be automatically adjusted to allow for changes in the type of light source installed on the vehicle, including provision of pulse width modulated energization to provide illumination level control. The system is implemented in digital format and resolution control for analog to digital conversion is affected by selection of the duration of the pulse width modulated signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a high level schematic of a vehicle electrical power generation, storage and distribution system. 
           [0010]      FIG. 2  is detailed schematic of a light emitting diode switching circuit. 
           [0011]      FIG. 3  is a detailed schematic of an incandescent bulb switching circuit. 
           [0012]      FIG. 4  is a high level flow chart illustrating operation of an embodiment of the system. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Referring to  FIG. 1 , a high level schematic of elements of a vehicle electrical control system  10  related to control over a plurality of lamps  12  is illustrated. External lamps are more typically monitored for operational integrity than internal lamps, however the principals of the system disclosed here are applicable to diverse systems as long as the qualitative operating characteristics of possible loads are known. The elements of the vehicle electrical control system  10  shown include a body controller  30 , an engine controller  40  and a serial data link  60  over which the body controller  30  and the engine controller  40 , among other controllers, communicate data using controller area network (CAN) interfaces  44  and  43 , respectively. The body controller  30  and the engine controller  40  each include a programmable microcontroller. For the engine controller  40  this is microcontroller  41 . For the body computer  30  it is microcontroller  31 . 
         [0014]    Body controller  30  is a high level controller which, among other functions, provides for switching control over the vehicle lamps  12  including, by group: the low beam headlight filaments  61 ; the high beam headlight filaments  48 ; the parking marker lights  18 ; identification (ID) lights  38 ; the left front turn signal lamps; the right front turn signal lamps; the right rear turn signal lamps; and the left rear turn signal lamps; etc. 
         [0015]    The lamps  12  are usually light emitting diodes (LEDs) or incandescent bulbs. Here, by way of example only, the park marker lights  18  and ID lights  38  are LEDs and the dual filament headlamp bulbs  48  and  61  are incandescent bulbs. The headlamps or course are not limited to being incandescent bulbs. Light radiators  32 ,  33 ,  34  and  35 , either LED or incandescent in character, may be used to provide the turn signal lamps or other exterior lights. The park marker lights  18 , ID lights  38 , low beam filaments  61 , high beam filaments  48  and the light radiators  32 - 35  are turned on and off by switching of the conductive state of a plurality of switches/switch circuits incorporated into the body controller  30 . The plurality of switches may be implemented in field effect transistor (FET) switch circuits  52 ,  53 ,  54 ,  55 ,  56 ,  57  and  58  under the control of the microcontroller  31 . 
         [0016]    Electrical power may be supplied to lamps  12  from an electrical power system including a battery  14  and an engine driven alternator  20 . Voltage levels on the battery and power output from the alternator  20  may be monitored by the engine controller. 
         [0017]    Body controller  30  may receive a signal from an ignition switch  22  directly over the controller area network serial data link  60  from a gauge controller (not shown) or over the serial data link  60  from engine controller  40 . The body computer includes a microcontroller  31  which may be programmed to test exterior lamps connected to body controller  30  following a state change of the ignition from off to on. Microcontroller  31  may be configured to apply an electrical pulse to each external lamp upon occurrence of ignition on and to check for current flow in response thereto (See  FIGS. 2 and 3 ). 
         [0018]      FIGS. 2 and 3  provide increased detail of the FET switch circuits  52 - 58 . Using FET switch circuits  56 ,  57  as representative examples the connections between FET switching circuits  52 - 58  to microcontroller  31  and to a light radiator  33 ,  34  are illustrated. FET switch circuits  52 - 58  provide power to lamps  12  and can be cycled to provide a pulse width modulated (PWM) signal which produces characteristic responses from a load depending upon whether the load is an incandescent bulb, a diode, a ballast for a florescent device, some other type of load or whether the load has failed operationally. The response can be compared to a reference signal and used to generate signals for return to the microcontroller  31  indicative of the character and operational readiness of each light radiator connected to an FET switch circuits. 
         [0019]    Referring particularly to FET switch circuit  56  (FET switch circuit  57  is similar except for connection to a incandescent light radiator  34 ), an LED based light radiator  33  is connected to the source of MOSFET  82  and receives energization through the MOSFET  82  from a direct current power source. A second MOSFET  84  is connected by its drain to the same power source and is connected by its source to the reference input of a comparator  78 . The source of MOSFET  82  is connected to the second input of comparator  78  and by way of a resistor. 
         [0020]    FET switch circuit  56  receives input signals from microcontroller  31  over a control input line  24  and a clock input line  26 . FET switch circuit  56  includes a logic circuit  76  which receives power from a battery input and which is connected to receive the control input and the clock signal input from microcontroller  31 . Logic circuit  76  operates on the signals to provide the gate signal which in turn controls the conductive state of two power switching MOSFETs  82 ,  84  and to provide a clock signal for comparator  78  over clock line  88 . The source of MOSFET  84  is relatively isolated from the source of MOSFET  82  for short duration PWM signals and the signal level on the source of MOSFET  82  will reflect the response of the load, be it LED light radiator  33  or incandescent light radiator  34 . The output of MOSFET  84  becomes a reference signal against which the response of the light source load to the cyclic signal is compared. 
         [0021]    The output of comparator  78  is connected to the gate of a field effect transistor (FET)  86 . The drain of FET  86  is connected to the source of MOSFET  84  and thus the signal level on the drain of FET  86  tracks the signal level on the reference input of comparator  78 . The drain of FET  86  is connected a feedback line  64 , which includes a resistor, to microcontroller  31 . Collectively comparator  78  and FET  86  form an analog to digital (A/D) converter  70  which provides in a serial output a digitized representation of the response of light radiator  33  to energization. 
         [0022]    Connected between feedback line  64  and ground is an A/D gain control circuit  62  which comprises a pair of resistors  72 ,  74 , connected in parallel between the feedback line  64  and a gain control MOSFET  68  connected in series with resistor  74  which controls conduction through resistor  74 . In other words, there are two gain values for A/D converter  70 , and the gain is changed by reducing the resistance between the feedback line  64  and ground by placing gain control MOSFET  68  into conduction to allow current flow through resistor  74  which reduces the resistance of the A/D gain control circuit  62 . Microcontroller  31  selects the gain for gain control circuit  62  by setting the value on gain control line  66 . 
         [0023]    MOSFET  82  is used to connect LED light radiator  33  through the source of the MOSFET to a direct current power, typically the vehicle battery or alternator, to generate light or to disconnect the light radiator from the direct current power to cease the generation of light. In addition, MOSFET  82  connects to one input of a comparator  78  and signals received on this input reflect the response of light radiator  33  as a load. MOSFET  84  is connected by its source to a reference input of comparator  78  and by its drain to the source of direct current power. In the conductive state of MOSFET  84  the direct current power supply is connected to the reference input of comparator  78  to the source of direct current power. If light radiator  33  is non-conductive, potentially due to its failure, than the signals appearing at the sources of power by MOSFETs  82 ,  84  (and to the inputs of compartor  78 ) when the power MOSFETs are in conduction should be synchronous and identical. The conduction of power MOSFETs  82 ,  84  may be driven by use of pulse width modulated signals over a gate output line  94  from control logic  76  for the purpose of determining if the load represented by the light radiator has an inductive component or a capacitive component. Where there is an inductive component associated with an incandescent load initial current flow should be low and voltage high when compared with initial current flow through a light emitting diode, which has a capacitive component. Serialization of the response of the light radiator to a gating pulse applied to power MOSFET  82  is compared with a reference value passed by power MOSFET  84  with a sufficient degree of resolution to allow incandescent loads to be distinguished from diode loads without determining quantitative values for inductive and capacitive loads. Variation of the duration of the pulse (in other words PWM) may be used for isolating particular types of loads, particularly depending upon the output characteristics for light radiators used for particular applications. 
         [0024]    A zener diode  42  connected between the drains of power MOSFETs  82 ,  84  to a protective circuit  98  protects the MOSFETs and control logic circuit  76  from overvoltage conditions. The drain of power MOSFET  82  is connected by a resistor  96  to the protective circuit  98 . The feedback line  64  includes a resistor and capacitor for pulse shaping network  67 . A capacitor  36  supports voltage levels from the direct current power source when the power MOSFETs  82 ,  84  switch on. 
         [0025]      FIG. 4  is a high level flow chart which illustrates execution of a routine after a change in keyswitch/IGN position to ON. For purposes of illustration it is assumed that each illumination position to be checked is illustrated as referenced by an index number and that at least two different types of light radiator can be installed at some position. The types of light radiators exhibit differing complex impedances. Upon initialization of the routine the index value is set equal to 1 (step  100 ) and the process begins. At step  102  a pulse width value appropriate for testing the possible complex loads at a given location (referenced by the index value). The selection step  102  also comprehends selection of a gain value for the A/D converter  62 . This may involve a simple state value for control of gain control MOSFET  68 . More than pulse width value (and gain value) may be used if the test for a given location is run more than once to generate additional data for evaluating the load. Next, at step  104 , the test signal(s) (and gains) are applied to the FET switch circuit  56  and the A/D gain adjustment circuit  62 . At step  106  the result(s) (the output of the A/D converter) are collected. 
         [0026]    Step  108  provides for failure detection. Typically a failure will be indicated by a string of all “1&#39;s” or all “0&#39;s” from the A/D converter  70 . If this occurs a failure is indicated (step  110 ). Thereupon the index is incremented and the routine returned to step  102 . Another result is likely indication of data which can be used to determine what kind of light radiator is installed at the index location. 
         [0027]    At step  112  the data string(s) are analyzed to determine what kind of light radiator is attached. Here two possibilities are given, incandescent and light emitting diode. Steps  114  and  116  provide for storing the type of light radiator to storage. Power MOSFET  82  may be operated differently depending upon the type of radiator attached to it. Generally an incandescent source is energized using a voltage source, and during operation power MOSFET  82  is simply held on. However, an LED device is characterized by a constant voltage drop and is energized by a current source. In order to allow control over current sourced to the LEDs the power MOSFET  82  may be cycled on and off, hence the character of the device is stored to inform later control over power MOSFET  82 . After storing the results the index is incremented (step  118 ) and it is determined if the operation is completed (step  120 ). If not the process returns to step  102 . 
         [0028]    Complex impedances associated with particular types of loads can be qualitatively detected using numerous techniques. While a particular method, amenable to a digital control environment, has been discussed, other techniques may be employed and under circumstances where the qualitative character of the load is known may be used for quantitative evaluation. 
         [0029]    A complex impedance expected expected to exhibit capacitive characteristics, such as exhibited by a light emitting diode, may be detected using a multiplexer and series resistor to make a Thevenin sine wave source. The phase relationship of current to voltage across a capacitor is well known and may be detected across the series resistor. 
         [0030]    Analog quantitative analysis of detection of a filament (incandescent) lamp may be implemented by including a resistor of known resistance in the controller module in series with the lamp and varying the frequency to find the circuit&#39;s series RL resonant frequency. Finding the resonant frequency will allow determination of the maximum through current and will indicate the lamp inductance. Similarly a known inductance may be place in series with a LED and the resonant frequency for the series RLC circuit found to provide the current maximum and thereby indicate the capacitance of the LED. 
         [0031]    Parallel or series resonant frequency measurements may allow switch in of a step up transformer to supply internal 110 or 220 volts AC where the appropriate load is detected. The body controller may be used to provide an AC inverter operating at 60 or 50 Hz. 
         [0032]    Tunable capacitors and inductors may be used to implement the resonant frequency detection described above. A tunable inductance like characteristic may be implemented using a gyrator (voltage adjustable complex impedance). 
         [0033]    Electrically erasable programmable memory (EEPROM) or other long term storage systems may be used to record measurements for future reference. Life testing may be implemented by comparing present values for components over properties exhibited upon installation.