Abstract:
A signal integration hub for automotive sensors measures battery voltage and provides an indication of that voltage to a processing microcontroller. A program executed on the microcontroller distinguishes between convenience and required function sensors and input circuits. When battery voltage is low, the microprocessor substitutes only for the convenience sensors their last debounced value during a time when battery voltage was not low. Inputs from required function sensors are not altered.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     BACKGROUND OF THE INVENTION 
     The present invention relates to electronic circuits for use in automobiles and in particular, to an apparatus and method for processing automotive electronic signals during periods of low system voltage. 
     In recent years, the number of electronic signals in an automobile has increased dramatically. This large number of signals is managed through the use of signal integration hubs having connectors for receiving conductors from a wide variety of sensors and actuators, and a microprocessor for processing such signals. The automobile instrument cluster is one site used as a signal integration hub because a large number of actuators, such as electronic gauges, lamps and chimes that are found there and because it is in a protected location in the instrument panel near the engine compartment. Other locations for signal integration hub locations are also possible. 
     While starting the automobile engine or at other times of high current demand, the standard system voltage of approximately twelve volts may drop. A low system voltage may affect digital input circuits and the output of automotive sensors that rely on this voltage as an implicit reference. In these input circuits and sensors as the system voltage drops, the state of the digital input or the output from the sensor may change. System voltage induced changes may be interpreted by a microcontroller at the signal integration hub as a change of state in the sensed quantity and may result in the erroneous control of actuators. 
     The electronic signals in an automobile may be divided loosely into “convenience signals” intended to enhance or simplify the operation of the automobile or make its use more enjoyable, and “required function signals” associated with safety or legally mandated equipment. Whereas the convenience signals may tolerate momentary interruption during low system voltage conditions, the required function signals are less accommodating of interruption and are designed to operate even at low system voltages. 
     Normally, the cost of providing error-free low-voltage operation is not justified for convenience signals. Nevertheless, because both convenience and required function signals are typically mixed at a single integration hub, manufacturers face the prospect of either providing low voltage compensation circuitry for all types of signals, or accepting occasional erroneous signals from unprotected convenience signals during periods of low system voltage. These spurious signals may produce harmless but false activations of chimes, bulbs, gauges and the like. 
     BRIEF SUMMARY OF THE INVENTION 
     The present inventors have recognized that the microcontroller at the integration hub may be enlisted to distinguish between convenience and required function signals and to lock the former against changes during periods of low system voltage without the need for additional circuitry. The microprocessor may monitor the system voltage to detect low system voltage. By “locking” signals from convenience sensors or their input circuits during periods of low system voltage, the last legitimate value of those signals is preserved. 
     Specifically, the present invention provides an integration hub having connections receiving sensor signals and electrical power from an automotive battery/charger system. A voltage sensor communicates with the connection to the electrical power to provide a measure of the voltage of the electrical power to the sensors. Generally that measure may be a digitized analog value or a single digital bit indicating the relative magnitudes of the voltage and a threshold. A microcontroller evaluates the measure of a voltage to determine whether the voltage is above a predetermined threshold and when it is, employs the sensor signals for control functions and stores the values of the sensor signals. When the measure of the voltage is not above the predetermined threshold, the microcontroller employs the stored values of the previous sensor signals for the control functions. 
     Thus it is one object of the invention to reduce the effect of erroneous signals from sensors or their input circuits caused by low voltage conditions. Implicit in the present invention is the recognition that the true value of the sensor is likely to be its last value prior to the low voltage condition. 
     It is another object of the invention to provide a method of reducing the effect of an erroneous signal that may be implemented in a microcontroller without the cost of additional circuitry and yet which may be applied selectively to convenience signals and not to required function signals. 
     The microcontroller may “debounce” the sensor signals prior to using them for control functions or storing their values. Bounce refers to a condition of a fluctuating state occurring immediately after a change of state of a sensor, typically but not always, associated with mechanical vibration in a set of making and breaking contacts. The debouncing may obtain periodic samples of the sensor signals and test for a predetermined number of successor samples of a consistent value. 
     Thus it is another object of the invention to provide a system that may be readily integrated with conventional debounce techniques and algorithms. 
     The microcontroller may initialize the stored values of the sensor signals to predetermined values. The predetermined values may be ones which minimize false actuations under most circumstances. 
     Thus it is another object of the invention to provide effectively two estimates of the state of signals from convenience sensors that may be used during low system voltage conditions, one estimate based on the last measure of the signal and one default estimate when no measure has yet been determined based on a default signal. 
     The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessary represent the full scope of the invention, however, and reference must be made to the claims herein for interpreting the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is an exploded perspective view of an instrument cluster and its associated printed circuit board used as a signal integration hub; 
     FIG. 2 is a schematic diagram of the circuitry of the integration hub of FIG. 1 showing a microcontroller with input circuits receiving signals from various convenience and required function sensors to provide control signals to various actuators; and 
     FIG. 3 is a flowchart of a program executed by the microcontroller of FIGS. 1 and 2 according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, a signal integration hub  10  may be provided on a printed circuit board  12  such as forms a part of the vehicle instrument cluster  14 . The printed circuit board  12  may be supported behind an instrument cluster cowling  28  having apertures  30  for receiving and displaying gauges  22  and lamps  26 . 
     Referring to FIG. 2, the printed circuit board  12  may include a microcontroller  32  communicating via circuit traces  20 ′ with various actuators  44  including vehicle gauges  22 , electronic chimes  24 , and lamps  26  which may be mounted on and connected directly to the printed circuit board  12 . The microcontroller  32  may also communicate with convenience sensors  34  and required function  36  located off of the printed circuit board  12 . These sensors  34  and  36  are attached to the microcontroller  32  via conductors  20  attached to connectors  18  received by corresponding connectors  16  on the printed circuit board  12 . The signals from convenience sensors  34  and required function sensors  36  may pass through input circuits  40 , prior to being received by the microcontroller  32 , providing battery voltage reference, basic filtering or voltage clamping such as is well known in the art. 
     The microcontroller  32  generally includes a microprocessor  31  and associated memory  29  and may include various other components such as oscillators, timers, multiplexers input circuits and A to D converters and D to A converters such as allow it to receive and process various electrical input signals from the convenience sensors  34  and required function sensors  36  and to generate output signals to the actuators  44 . Microcontrollers suitable for this purpose are commercially available from a number of suppliers. 
     Once received by the microcontroller, the inputs from the convenience sensors  34  and required function sensors  36  are processed according to a control program  47  contained in memory  29 . The control program  47  may respond to the inputs from convenience sensors  32  and required function sensors  34  and its own control logic to actuate actuators  44 . Additional output processing circuitry, for example buffer amplifiers  46 , may be interposed along conductors  20  and  20 ′ between the microcontroller  32  and the actuators  44 . 
     The present invention also provides a voltage measure  48  input to the microcontroller  32  measuring a system voltage of the battery  38  as provided to the printed circuit board  12  by a conductor  20 . This voltage measure  48  may be a multi-bit value provided by a dedicated analog to digital converter  50  (as shown) external or internal to the microcontroller  32  or may be a single bit indicating the result of a comparison of a voltage reference to the battery voltage. The battery voltage is also provided to the microcontroller  32  for powering the same but only after regulation, filtering and clamping provided by circuitry  51  well understood in the art. The voltage ultimately received by the microcontroller  32  for power is isolated and thus often unrelated to the system voltage. 
     The convenience sensors  34  and required function sensors  36  may receive the system voltage as measured by the voltage measure  48  and use it as a reference for their outputs as communicated to the microcontroller  34  or may use the system voltage as a reference for their input circuitry  40 . A lowering of the system voltage either through lack of charging of battery  38  or high current demand from other devices such as the vehicle starter can therefore cause erroneously low output signals to the microcontroller  34  from convenience input circuits  40  or sensors  34 . Generally, the required function sensors  36  are much more indifferent to the system voltage either by design of the sensor or by special preprocessing circuitry  41  not used with the convenience sensors  34 . 
     The present invention addresses the problem of erroneous states from input circuit  40  or erroneous output signals from the convenience sensors  34  by a modification of the control program  47  contained in memory  29 . Referring now to FIG. 3, at an initial step of control program  47  indicated by process block  49 , default values are placed in variables in memory  29  corresponding to each of the convenience sensors  34 . These default values are selected generally so as to provide no alarm condition for gauges  22 , chimes  24  or lamps  26 , but may be any default value considered desirable under the anticipated circumstances in which a low system voltage will occur. 
     At a first decision block  51  of the control program  47 , the voltage measure  48  is interrogated and the microcontroller  32  determines whether the system voltage to the various sensors  34  and  36  is above or below a predetermined threshold, typically nine volts. If the system voltage is below the threshold, then a low voltage flag is set indicated by process block  52 . The low voltage flag consists of a bit in memory  29 . Alternatively if the voltage is above the threshold, the low voltage flag is cleared at process block  54 . 
     In either case, the control program  47  then proceeds to process block  56  where the inputs on conductors  20  from the convenience sensors  34  and required function sensors  36  are sampled. At succeeding decision block  58 , the low voltage flag is interrogated and if it is not set, indicating that adequate voltage is being provided to the convenience sensors  34 , the control program  47  proceeds to process block  60  and the input signals are debounced and stored. Debouncing involves obtaining successive samples of the inputs until a predetermined number of successive samples shows a consistent value and adopting that consistent value as the debounced input. 
     On the other hand if at decision block  58  the low voltage flag is set, indicating that the outputs from the convenience sensors  34  may be erroneous, then at succeeding process block  62 , the last stored values for each of the convenience sensors  34  is read. Ideally, the last stored values are those values saved in process block  60  in a previous cycle of the control program  47 . However, they may also be the default values established in process block  49  if no previous cycle of the program has occurred. 
     At succeeding process block  64 , the inputs for the convenience sensors  34  are replaced with their last stored debounced values. As indicated by next process block  66  for the required function sensors  36 , current debounced values of their signals are used under the assumption that the low system voltage does not affect these inputs for reasons described above. 
     At process block  70 , the normal processing of the inputs from the convenience sensors  34  and the required function sensors  36 , as modified above, are processed according to the normal operation of the control program. During conditions of normal system voltage, all inputs are debounced and used directly while in conditions of low system voltage, the last debounced inputs during a state of normal system voltage are used to replace the raw input values for the convenience sensors  34  only. The inputs from the required function sensors  36  which are used directly after debouncing. 
     The above description has been that of a preferred embodiment of the present invention. It will occur to those that practice the art that many modifications may be made without departing from the spirit and scope of the invention. In order to apprise the public of the various embodiments that may fall within the scope of the invention, the following claims are made.