Abstract:
A multiplexing system is provided for controlling and monitoring all the electrical functions of a vehicle. The system includes a switch control unit which receives inputs from various sources and converts the inputs into digital signals which are decoded in a microprocessor. The decoded signals are then sent to a power distribution unit, which sends out, shuts off, increases or decreases the power to different parts of the vehicle as needed.

Description:
BACKGROUND OF THE INVENTION 
     Over the years, fire trucks have evolved from simple vehicles for transporting hoses, pumps, and hooks and ladders to sophisticated command centers for controlling all aspects of the firefighting operation, from communications to nozzle pressure regulation. The increased capability of these vehicles is to a large part due to advances in the electronics industry, which have enabled complex and cumbersome mechanical controls to be replaced with more streamlined electronic devices. The conversion from mechanical to electronic controls has been rather haphazard, however, with each electronic device developed independently of the others, each having its own separate set of wires. Because no attempt has been made to integrate the various electronic devices into a single unit, the wiring within a typical fire truck has come to resemble a maze consisting of hundreds of wires. The huge number of wires becomes a problem when an electrical malfunction occurs, forcing a technician to test each of the lines individually in order to diagnose, monitor, and repair the problem. 
     Accordingly, the object of this invention is to provide a multiplexing system for integrating the electronic controls of a fire truck or similar vehicle in order to reduce sensor duplication, increase flexibility, simplify the wiring and facilitate diagnosis of electronic problems. 
     SUMMARY OF THE INVENTION 
     The aforementioned and other objects of the invention are accomplished by an apparatus multiplexing system which controls and monitors all the electrical functions of a vehicle through a common two-wire data link. The system comprises a switch control unit which receives inputs from a number of sources such as switches and gauges on the dash panel of the vehicle, and converts these inputs into digital signals which are sent through two pairs of redundant wires to a microprocessor which decodes the signals and sends them to a power distribution unit. The power distribution unit then responds to the signals by sending out, shutting off, increasing or decreasing the power to different parts of the vehicle as needed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and further and more specific objects and advantages of the instant invention will become readily apparent to those skilled in the art from the following detailed description of preferred embodiments thereof taken in conjunction with the drawings in which: 
     FIG. 1 is a schematic diagram of the multiplexing system according to the present invention; and 
     FIG. 2 is a circuit diagram showing the switch control unit. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning now to the drawings in which like reference numerals indicate corresponding parts throughout the various views, attention is first directed to FIG. 1, which shows the multiplexing system according to the present invention indicated in its entirety by the numeral  10 . The multiplexing system comprises a switch control unit  12  receiving inputs from a number of sources such as switches, gauges, etc. on the dash panel of a fire truck. The switch control unit  12  is coupled by two pairs of redundant wires  13 ,  14  to a microprocessor  15  which in turn is coupled by another pair of wires  16  to a power distribution unit  17 , which distributes power to the relay boards and various other loads  18  such as a pressure governor, lights, and instrumentation. The network consists of the parallel connection of a supply voltage or ground  19  and a number of loads L 1 , L 2  . . . L x , which may include brake, compartment, warning and turning lights, pumps, instrumentation, air conditioning, fans, transmissions and the like. Each load L 1 , L 2  . . . L x  is connected via a relay or solid state switch S 1 , S 2  . . . S x  in series with a parallel connection of a resistor R 1 , R 2  . . . R x  and a capacitor C 1 , C 2  . . . C x . Each RC circuit is connected to a multiplexer  20  which in turn feeds the output of an amplifier  21  to the microprocessor  15 . 
     The system  10  monitors vehicle functions in the following fashion: 
     When the system  10  is first installed on a vehicle and power is applied, a distinct signal is sent to the microprocessor  15 . The signal instructs the microprocessor  15  to measure and “learn” or record in its memory the transient and steady state currents for each load as that load is applied. Until the readings are taken, the excess current limit for each load is set at maximum. After the readings are taken, the learned levels remain in memory and are used as the “normal” limits for the loads. Lower and upper limits, e.g. normal ±10%, normal ±20%, normal ±50%, etc. can then be set according to the application. The limits can also be preset in memory by instructing the microprocessor via data bus. In addition, the system can be reprogrammed at any time by reinitializing the learn cycle. 
     Once the current limits have been established, the microprocessor  15  multiplexes the voltage through each circuit in turn, measures its current, and determines whether the current is within the limits, taking into account any momentary allowable excesses due to transient conditions. If the current in any circuit is too high, the microprocessor  15  sends a signal to the power distribution unit  17  to open that circuit and another signal to the system user via a visual or audible alarm. The microprocessor  15  will also notify the user if the current is too low. All circuits, including inactive ones, can be measured in this way to test for possible failed relays or solid state switches. 
     The microprocessor  15  cycles through the entire system in a few milliseconds and repeats the procedure as many times as determined necessary for a given application. The cycle rate, duration of measurement and sequence are also functions of the application. 
     The measurement cycle consists of two phases for each load—a transient phase and a steady state phase. When a circuit is being closed, the microprocessor  15  can be devoted to that circuit until the transient phase has ended to ensure the transient current does not exceed the learned or preset levels. If a load, such as a heavily loaded motor, has a long start-up cycle, the transient phase of the cycle can be interrupted provided the transient is within learned limits, while the steady state phase can continue for the remaining circuits. 
     In addition to monitoring the output currents associated with each individual load, the microprocessor  15  also sums up the various output currents to determine if the total current is excessive. If so, a signal is sent to the power distribution unit  17  to disengage the loads which have been determined and preset by the user to be less critical. Similarly, if the input voltage is determined to be too low, loads will be shed in a predetermined sequence depending on priorities set by the user. Simultaneously, a signal will be issued to increase the supply voltage by increasing engine speed or similar measures. When the supply voltage increases, the disengaged loads can be reapplied. These loads will be applied sequentially to prevent drastic changes in loading. To prevent unwanted system oscillations, time delays can be incorporated in the disengagement and reconnection of the loads. 
     Another function of the microprocessor  15  is to acquire and monitor data associated with various devices on the vehicle, such as the brake and turning lights, pumps, instrumentation, air conditioning and the like. A real time clock can be included to allow the data to be recorded in a memory internal to the unit. Alternatively, the microprocessor  15  can be connected to an external computer or modem and the data broadcast either as events occur or when queried through a diagnostic terminal. 
     Various other modifications and variations to the embodiments herein chosen for purposes of illustration will readily occur to those skilled in the art. For instance, the invention could be very easily adapted for use in non-automotive applications, including but not limited to, emergency systems where the system status must be monitored and checked at all times. To the extent that such variations and modifications do not depart from the spirit of the invention, they are intended to be included within the scope thereof which is assessed only by a fair interpretation of the following claims.