Abstract:
Electrical power distribution circuits for motor vehicles incorporate a switching element for controlling the energization of the circuits. Current metering elements associated with each switching element indicate the current drawn by the respective electrical circuits. A microcontroller is provided which provides an activation signal for the switching elements, often in accord with a pulse width modulated duty cycle. The microcontroller implements a circuit protective algorithm which takes as inputs the indication of current drawn by a particular electrical circuit and the duty cycle. An equivalent D.C. current is estimated for determining a heat index for a hypothetical fuse suitable for protecting the circuit. When the heat index exceeds the rating for the fuse the fuse melts.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to motor vehicle electrical systems and more particularly to a programmable system protecting such electrical systems from overcurrent conditions. 
     2. Description of the Problem 
     The number of electrical circuits in automotive vehicles has increased over the years. In today&#39;s motor vehicles there are numerous electrical devices which are used for various purposes such as illumination, control, power, and instrumentation. While the advent of electronics has given rise to major changes in automotive electrical systems, conventional circuit protection devices, e.g. fuses and circuit breakers, continue to be used, and in increasing numbers as the number of circuits in the electrical systems increases. The common technique for providing protection against shorts, overloads, and other types of electrical problems or conditions is to include a circuit breaker or fuse connected in series, with the wiring circuit to be protected. 
     With increasing numbers of circuits, and the correlative need for an increased number of protective devices, today&#39;s typical automotive vehicle or truck requires a panel devoted essentially exclusively to the mounting of most of these protective devices in a single location. The panel, or fuse block as it is sometimes called, comprises multiple compartments for the individual protective devices. Associated with these compartments are receptacles to provide for the replaceable mounting of the protective devices in the associated circuits. Accordingly, the panel comprises a large number of individual parts in assembled relationship, and it occupies a certain amount of space in an area of the vehicle where space is typically at a premium. A large number of wires attach to the panel to carry current to and from the various protective devices, and in order to serve the grouping of the protective devices in the panel, complexities are introduced into the associated wiring harnesses and cost is added to the vehicle. In addition, the variability of commercial vehicles may result in different valued fuses or circuit breakers being installed at the same physical location on different vehicles of the same model truck, resulting in assembly errors. 
     There are several ways to protect an electrical device without a circuit breaker or fuse, but most of the ways add several parts to the circuit and typically degrade the performance of the electrical circuit, such as by added voltage drop, higher power dissipation, etc. These protection methods are not known to enjoy any significant commercial use because of disadvantages such as those just mentioned. Providing a substitute device for a fuse can pose other complications. Devices for interrupting a circuit based on detection of a simple overcurrent condition do not mimic fuse behavior, which is characterized by opening after passage of an overcurrent of a sufficient time duration to cause the fusible element to melt. Fuses thus tolerate transient, non fault related, overcurrent conditions, sometimes greatly exceeding the rated tolerance of the fuse, such as occur when a lamp is turned on. Fuses also tolerate other types of brief overcurrent excursions such as peaks occurring in alternating current circuits, where the root mean square value for the current remains below the direct current rating for the fuse. It is often desirable to use fuses in circuits for just this feature. 
     U.S. Pat. No. 4,799,126 to Kruse, et al., which is assigned to the assignee of the present invention, recognized that the fuse and circuit breaker panel concept of protection could be eliminated, thereby reducing the large number of individual circuit devices (i.e., fuses and circuit breakers) required to provide the protective function, and at the same time, freeing space because there is no longer a need for a separate panel. The circuit breaker function is provided by using a particular type of power MOSFETs, which also serve for circuit switching. MOSFETs comprise an internal, controlled conduction path the conductivity of which is controlled by an external control input. The type of MOSFET used by Kruse comes with built in protection, contained in another internal portion which monitors current flow through the main controlled conduction path and serves to internally interrupt the flow through the path in response to incipiency of current or temperature exceeding the rating of the main controlled conduction path. When one of these MOSFETs is incorporated into a circuit, it is selected on the basis of a close match in the amount of current to be allowed to be drawn by a circuit and the tolerances of the MOSFET. This final aspect of Kruse&#39;s teaching necessitated manufacturing vehicles using MOSFETs of a number of different capacities. 
     Kato et al., U.S. Pat. No. 5,856,711 provides a circuit interrupt device capable of being set for different current-time characteristics without physical modification of the device itself. In addition, Kato appears to provide a device which mimics the time delay in breaking inherent to fuses operating under overcurrent conditions. Kato teaches switches (relays) having a control input; current detection functionality; and a load drive line for connection to loads to which electric power is supplied from a battery. A device controller includes data processing capacity and memory, on which is stored the desired current/breaking time characteristics data. The device controller opens a switch by supplying a control input signal to the switch when the breaking time in the memory has elapsed. This is effected by starting a timer immediately after detection of an overcurrent condition and running it against a time out threshold stored in memory for the value of the current. Memory is programmable for the desired time-current values. The &#39;711 patent does not appear to vary the breaking time period for changes in current once a timer has been started. Thus, so long as an overcurrent condition continues to exist, the timer continues to run against the initial time out period matched with the initially detected overcurrent condition. The timer stops only if current falls below a minimum threshold level. This aspect of Kato&#39;s control algorithm presents difficulties in applying the system to circuits other than those designed for use with clean direct current loads. Kato et al. do not address these problems. 
     Power MOSFETs are popular switching devices in contemporary vehicle electronics. Among other applications, power MOSFETS can be used to implement pulse width modulation (PWM) switching, which allows precise control over vehicle features such as varying the illumination level of running lights and changing the operating speed of electric motors to change the sweep speed of windshield wipers. PWM is, in effect, an alternating current signal with a direct current offset, or unipolarity A.C. In PWM switching systems, peak values in current drawn may vary, for example changing with the changing load associated with windshield motor operation under conditions windshield icing. Circuit protection devices used with such systems, in order to be effective, must operate accurately in such a quasi or unipolarity alternating current (A.C.) environment. Peak pulse current values may safely exceed the current rating for the circuit without being symptomatic of a dangerous condition or indicative of a short, so long as the root mean square (RMS) value of the current remains below the maximum current rating. Conversely, current drawn may be excessive, but a system such as proposed by Kato et al. would miss detection of it because the duty cycle is to short for the timer to expire. 
     The Kruse et al. and Kato et al. patents do not address environments where the circuit current has A.C. components, but instead appear limited to D.C. applications. Kato et al. apply data processing capacity to the determination of when to trip a relay in response to excessive current being drawn by a circuit. Though the algorithm employed by Kato et al. appears tolerant of transient overcurrent situations, it does explicitly deal with quasi A.C. conditions. 
     In addition, MOSFET devices require protection. During high levels of overload, any field effect transistor (FET) will be rapidly heated and cooled as temperature protection mechanisms of the FET limit the power dissipated in the FET. Over time, such heating and cooling of the FET reduce the useful life of the device, an effect known as the Coffin-Manson acceleration. 
     It would be advantageous in vehicle manufacture to dispense with fuses for circuit protection and implement circuit overcurrent protection directly in the switches used to control the circuits. It would be still more advantageous if the switches were standardized and if implementation of their response characteristics could be introduced to the vehicle by programming. Such a feature would simplify manufacture and repair. It would be still more advantageous if the devices could be programmed to handle a wide variety of different operating conditions, including unipolarity A.C. operation. 
     SUMMARY OF THE INVENTION 
     According to the invention there is provided a motor vehicle comprising electrical power distribution circuits. Switching elements are incorporated in the electrical circuits for controlling the energization thereof. Current metering elements associated with each switching element indicate the current drawn by the respective electrical circuits. A microcontroller is provided which provides an activation signal for the switching elements, often in accord with a pulse width modulated duty cycle. The microcontroller implements a circuit protective algorithm which takes as inputs the indication of current drawn by a particular electrical circuit and the duty cycle. An equivalent D.C. current is developed for determining a heat index for a hypothetical fuse suitable for protecting the circuit. When the accumulated heat index exceeds the heat index rating for the hypothetical fuse the circuit is opened. 
     Additional effects, features and advantages will be apparent in the written description that follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a perspective view of a truck including features energized by electrical power such as lamps and horns; 
     FIG. 2 is a high level block diagram illustrating operation of the invention; 
     FIG. 3 is a graph illustrating typical tolerances for an automotive fuse in terms of current versus time; 
     FIG. 4 is a block diagram of a vehicle control and power system environment wherein the invention is applied; 
     FIG. 5 is a detailed schematic of a lighting circuit illustrating a preferred embodiment of the present circuit protection system; and 
     FIG. 6 is a flow chart of an algorithm used to implement the invention on a microprocessor. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a perspective view of a truck tractor  10 . Tractor  10  includes the conventional major systems of a vehicle, including an engine, a starter system for the engine, brakes, and a transmission. Tractor  10  also includes a number of electrical systems including interior and exterior lamps by which the vehicle provides light for its driver to see by and means to be seen, particularly at night, by others. On the front of tractor  10  are headlights  11 , front corner turn signal lamps  13 , and fog lamps  12 . Identification or running lights  14  are installed on the roof of tractor  10 . A lamp box  15  installed on the rear end of tractor  10  carries additional turn signal lights, reverse lights and brake lights. As is common, the forward and tail end turn signal lights have a hazard function and can be cycled on and off together (generally the forward pair together and then the tail end pair together) to provide warning to passing motorists. A pair of electrically activated horns  16  are installed on the roof of tractor  10 . The protection of circuits used to energize the lamps and horns, among other devices, is achieved as described below. 
     FIG. 2 a functional block diagram overview of the invention implemented on a microcontroller  48 . Those skilled in the art will recognize that the functions grouped within microcontroller  48 , such as data processing, data storage and gate control signal generation, may be associated with one another in some other fashion, for example, by controllers interacting over a network. A switching output device  43 , preferably comprising an FET having known over temperature shut down characteristics and which produces an analog representation of a load current passing through the device provides direct control over a load  44 . While output device  43  is preferably a power MOSFET, it can also be a bi-polar transistor, a relay, or some other equivalent device, with some loss of functionality, as such other switching elements may not inherently provide a secondary output proportional to the load current drawn through the device. Micro-controller  48  provides an ON/OFF binary output for controlling the conduction of output device  43  and an analog input  45  for taking as an input the secondary output reflecting load current through the FET. Output device  43  is connected by its drain to a source of power VB and at its source to a load device  44 . Microcontroller  48  is conventional and includes a central processing unit and memory for storing permanent and temporary data, including programs. Here the microcontroller  48  is illustrated as relating its functional blocks to one another as they interact to provide control of the output device  43 . Load device  44  has a function which is normally turned on and off in accordance with instructions issued by a function block  41 . The function may include cycling output device  43  on and off in accordance with PWM control of the load device  44 . Absent overcurrent or overheating conditions, the control signal from function ON/OFF block  41  is passed by current limiter  42  to output device  43  as a gate signal. Load device  44  can be any one of a number of vehicle systems. One system where the invention is advantageously employed is with system lights the illumination levels of which are controlled by varying the duration of pulses in a PWM control scheme. 
     Output device  43 , when implemented in a MOSFET can incorporate, as an inherent function, means to produce an output current sense signal which is proportional to the load current drawn by load device  44  and the energization circuit in which load device  44  is connected. An output current block  45  takes this output current sense signal, I S , and converts it a digital signal to provide as an input to a root mean square calculation function  46 . RMS calculator  46  also takes as an input the basic ON/OFF signal from the function block, or, equivalently, the duty cycle of the output, and uses the two inputs to determine the RMS value of the output. In a typical PWM application this involves sampling I S  when the output device  43  is conducting (in effect sampling the peak value each cycle), averaging the samples and multiplying the result by the fraction of the time that the output device  43  is conducting to develop an estimate of the RMS equivalent current. It is not strictly necessary to determine the exact RMS equivalent as long as the algorithm used produces a result falls within published tolerances for appropriate fuses for the application. 
     RMS calculator  46  provides an RMS estimate for load current to a heat estimator block  47 , which in turn determines if a hypothetical fuse would be progressing toward melting. If the RMS current is to high, than points are added to a running total which, if it exceeds a threshold, indicates an appropriate time for a fuse opening event. If the RMS current is below a selected minimum threshold (typically 110% of the rating for a fuse for the circuit) the running total is reduced. Once an accumulated count exceeds a desired level heat estimator overrides function block  41  and causes current limiter to apply a gate signal to the output device  43  opening the device. 
     To reduce the effects of Coffin-Manson acceleration, simple counting methods limit the number of thermal cycles impressed on the power MOSFETs. An ignition cycles block  49  is tied to current limiter  42  and allows non-safety devices only one fuse event per ignition cycle. Safety related systems can be allowed multiple fusing events per ignition cycle. This arrangement can be varied on a vehicle by vehicle basis by programming. Other schemes can be used if the criticality of a particular system changes. Appropriate flags may be set upon starting a vehicle to implement this feature. 
     The time to opening characteristics programmed for MOSFET switches are set to fall within normal tolerances of standard fuses. The Society of Automotive Engineers has published test limits on the current/time functionality of fuses for vehicular use. The present invention implements an algorithm designed to produce results falling within the tolerances for particular fuses, as illustrated in FIG.  3 . 
     FIG. 3 is a graph illustrating tolerances for times to melt for a fuse for a given application as a function of D.C. equivalent current. The Society of Automotive Engineers publishes recommended fuse operating characteristics in terms of minimum and maximum recommended times to melting at various percentages of rated current. A fuse carrying the equivalent of 100% of its rated current should never melt. A fuse should tolerate up to at least 110% of its rated value. At most, such a fuse should tolerate 200% of its rated value. Actual fuse times versus current fits between the recommended minimum and maximum melting time curves. In the present invention the operation of the switching element is to be similar to that of an automatic reset circuit breaker. The switch can be returned to operation by using an algorithm that allows the conductors sufficient time to cool during the non-conductive periods. The cooling time required depends upon the starting temperature of the switch, and the device programmed on the basis of empirical testing on representative circuits. 
     A preferred embodiment of the invention will now be described in connection with FIGS. 4-6. FIG. 4 illustrates schematically electronic control and electrical power distribution system for vehicle  11 . Electronic System Controller (ESC)  24  is a body controller computer which communicates with several autonomous controllers over a SAE J1939 data link  18 , including a gauge cluster  94 , a transmission controller  96 , an antilock brake system controller  22  and an engine controller  20 . One or more other controllers  37  may be attached to the bus  18 . Each controller includes data processing capability allowing programming and functional control to be distributed across the network. Each of these local autonomous controllers may in turn receive data directly from switches and sensors, as ESC  24  does from a switch bank  48  and discrete input section  50 . Discrete inputs may include ignition key switch position and start button position. Each local controller may provide control or informational signals to local discretely controllable components, as ESC  24  does with discrete output section  52 . Vehicle power system  30  includes batteries and the alternator system, and distributes unipolarity power over a power cable  31  to each of the major vehicle systems. Line  32  is chassis ground. 
     FIG. 5 is a circuit schematic of a engine controller EGC  20  which receives operator requests for illumination of headlamps, ESC  24 , and a plurality of lamps energized under the control of the ESC. Headlamp requests are routed through the engine controller  20  because it is required that the ignition be on before use of the headlamps is permitted. ESC  24  is a programmable body systems computer used to control many vehicle electrical system functions, and the functions it performs relating to lamp control could readily be distributed to lower functionality microcontrollers. Typically, however, the operation of lamps is handled by ESC  24 . ESC  24  is based on a microprocessor  61  which executes programs and which controls switching of a plurality of power FETs used to actuate vehicle exterior lights and the horn. EGC  20  communicates with ESC  24  over an SAE J1939 data link (bus  18 ) and CAN controllers  63  and  64 . EGC  20  is based on a microprocessor  65  which includes only limited and typically fixed programming. EGC  20  handles lamp microswitch  66  inputs providing manual control over headlights and enablement of the headlamps. Another source of switch inputs may by provided by a switch pack  68  which is connected to microprocessor over an SAE J1708 bus and controller  69  or through switches associated with brake pedals, turn signal levers and other similar systems. Illumination levels of the lamps  67  may be selected using switch pack  68 . 
     Microprocessor  61  can apply activation signals to a Power FET  70  for the control of lamps  67 . Microprocessor  61  is connected to provide an activation signal to the gate of a power FET  70  which in turn energizes the lamps  67 . The gate signal may be pulse width modulated to control illumination intensity. Microprocessor is further connected to power FET  70  to receive a output current sense signal I S , which is applied across a resistor  72  to produce a voltage signal which is applied to an A/D converter port  71  on microprocessor  61 . 
     FIG. 6 is a flow chart of the control routine executed by microprocessor  61  of ESC  24 . A proxy for I S , the digitally converted signal developed from the voltage level generated for I S , is sampled at times indicated by the duty cycle when a pulse will have closed power FET  70 . A set of consecutive samples will than be averaged and multiplied by the percentage of time that the duty cycle indicates that the FET  70  is conducting by an RMS estimate routine  82 . The RMS estimate generated is passed to a direct current equivalent compare step  83 , which compares this result with 110% of the desired rated value for a fuse for the lamp illumination circuit of FIG.  5 . If the RMS current estimate exceeds the threshold the value is used as an input to a heat index function  85 . Heat index function  85  may be an empirically developed look up table indexed by RMS current estimates or it may be an equation using the RMS estimates as an input variable. A result is obtained which is passed to a summer  86  which accumulates the results from prior executions of the step. When the threshold compare step indicates that the RMS estimate is less than the minimum threshold the heat index function  85  generates a number to be subtracted from the accumulated heat index result. With each cycle of sampling, the output of summer  86  is subjected to a compare operation at compare step  87  to a fuse temperature threshold  88 . When the accumulated result exceeds this second threshold a gate cutoff signal is generated. The routine is repeated for successive collections of samples with the accumulated heat index result carried over from sample set to sample set. 
     The present invention allows vehicle manufactures to dispense with fuses for many circuits and to substantially reduce the size of the fuse block. A single type of power MOSFET may be used for to implement switching and protective functions for various circuits, simplifying assembly. The use of programmable components allows easy reconfiguration of vehicles. The invention is also usable with signals having substantial A.C. components. 
     While the invention is shown in only one of its forms, it is not thus limited but is susceptible to various changes and modifications without departing from the spirit and scope of the invention.