Abstract:
A circuit  17  for interfacing with a sensor  18  having a sensor input current and a modulated sensor current signal corresponding to a sensed condition. A control module  20  is coupled to the sensor  18  and receives the sensor current signal. The control module  20  converts the sensor current signal to a modulated signal having a pulse width with a duration corresponding to the sensed condition. The control module  20  counts a time corresponding to the pulse width. The time corresponds to the sensed condition.

Description:
TECHNICAL FIELD 
     The present invention relates generally to sensors particularly suited for automotive vehicles, and more particularly, to a circuit for interfacing with a sensor. 
     BACKGROUND OF THE INVENTION 
     Automotive vehicles typically provide a number of sensors that are used to sense various operating conditions of the vehicle. Systems that are sensor intensive include vehicle handling systems such as anti-lock brakes and traction control, and safety systems such as airbag systems. 
     Sensor based systems typically use a microcontroller to read multiple asynchronous remote sensor signals with serial state machines. Serial state machines such as a universal asynchronous receive transmitter (UART) are typically employed as an interface device. Typically, two UARTs are provided per sensor; one in the controller as well as one UART at each remote sensor. However, many systems have multiple sensors and therefore require multiple UARTs. 
     Previous systems use a digital word to transmit data between the sensor and central controller. The digital word corresponds to the sensed condition at the sensor. The digital word operates only when the sensor is to send a signal. Previous systems often generate noise emissions due to the sharp on and off transitions of the digital communication signal. 
     It would therefore be desirable to provide an interface for receiving signals from a remote sensor that, when implemented, uses a reduced number of components from presently known systems synchronizes remote sensor data acquisition using readily available hardware. 
     SUMMARY OF THE INVENTION 
     In one aspect of the invention, a circuit has a sensor having a sensor and a modulated sensor current signal corresponding to a sensed condition. A control module is coupled to the sensor and receives the sensor current signal. The control module converts the sensor current signal to a pulse width with a duration corresponding to the sensed condition. The control module measures a time corresponding to the pulse width. The time corresponds to the sensed condition. 
     In a further aspect of the invention, a method for communicating a sensed condition of a sensor comprises the steps of: 
     modulating a sensor current signal corresponding to a sensed condition; 
     generating a pulse width corresponding to the sensor current signal; 
     monitoring a time corresponding to said pulse width; and 
     converting the time into a digital value, wherein the time corresponds to a sensed condition. 
     One advantage of the invention is that a current modulated signal from the sensor circuit to the central controller has reduced electromagnetic interference than previously known sensing circuits due to the ability of use of a substantially triangular signal with rounded transitions rather than sharp transitions. Another advantage of the invention is that drift in the remote sensor&#39;s quiescent current due to age, temperature and tolerances are tracked by the voltage comparator which uses the average current for comparison. 
     Other objects and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an automotive vehicle having a sensor interface circuit according to the present invention. 
     FIG. 2 is a block diagram of a sensor interface circuit according to the present invention. 
     FIG. 3 is a block diagram of the sensor interface circuit of FIG.  2 . 
     FIG. 4A is a plot of sensor current versus time of the present invention. 
     FIG. 4B is an enlarged plot of a portion of the output sensor signal of FIG.  4 A. 
     FIG. 5 is a block diagram of a interface circuit for a controller according to the present invention. 
     FIG. 6 is a plot of a sensor output and SYNC signal according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the following figures the same reference numerals are used to identify identical components in the various figures. Although the present invention is described with respect to a sensor system for airbag deployment, the present invention may be applied to various other automotive applications such as anti-lock brakes and to non-automotive sensor applications. 
     Referring to FIG. 1, an automotive vehicle  10  is shown having a control module  12  coupled to a sensor  14 . Control module  12  may be used to deploy an airbag  16  based on a sensed condition at sensor  14 . Sensor  14  may, for example, be an accelerometer. 
     Referring now to FIG.  2 . the present invention is particularly suited for use in a circuit  17  employing multiple sensors in a plurality of sensor circuits  18 . Sensor circuit  18  is coupled to control module  20 . Control module  20  has a current-to-voltage converter  22  coupled to each sensor circuit  15 . Each current-to-voltage converter  22  is coupled to a divide-by-n counter  24 . Each divide-by-n counter  24  is coupled to a microcontroller  26 . More specifically, microcontroller  26 , is coupled to divide-by-n counter  24  through a timer input pin  28 . One timer input pin  28  is provided for each divide-by-n counter  24 . Timer input pins  28  are commonly found on microprocessors. Microcontroller  26  has a SYNC output  30  that is coupled to a CLR input  32  on each divide-by-n counter  24 . 
     In the preferred implementation current-to-voltage converter  22  and divide-by-n counter  24  may be implemented in an application specific integrated circuit (ASIC). 
     Each sensor circuit  18  may be located in various positions in automotive vehicle or around any other product to which circuit  17  is applied. 
     Referring now to FIG. 3, sensor circuit  18  includes sensor  14 . Sensor circuit  18  is coupled between a voltage input  40  and voltage return  42 . A sensor transmitter circuit  44  is coupled to sensor  14 , voltage input  40  and voltage return  42 . Sensor transmitter circuit  44  may include a voltage regulator  46  that is used to control the voltage to sensor  14  within predetermined limits. Commonly, sensor  14  operates at 5 volts DC. 
     Sensor transmitter circuit  44  includes a voltage controlled oscillator  48  and a communications output stage  50 . Communication output stage  50  is coupled between voltage input  40  and voltage return  42 . As will be further discussed below, voltage controlled oscillator  48  controls communication output stage  50  to modulate the transient sensor current I Tx  with a period proportional to the output voltage of sensor  14 . The input current to the sensor circuit  18  is I Q . One skilled in the art would recognize frequency modulation could also be employed. 
     A diagnostic state machine  52  is coupled to sensor  14  and voltage controlled oscillator  48 . Diagnostic state machine  52  may be used to verify proper connections of the sensor circuitry. Diagnostic state machine  52  may also be used to sense faults with the sensor circuitry. Diagnostic state machines  52  may be implemented in numerous ways as would be evident to those skilled in the art. 
     Referring now to FIG. 4A, the current output signal  54  of communications output stage  50  of FIG. 3 is illustrated. The current output signal sinks current which is added to the quiescent current draw I Q  of the sensor circuit  18 . Current output signal  54  is continuous and has an average current I avg  and peaks  56  and valleys  57 . The upper limit of signal  54  is thus I Q +I Tx . The lower limit of signal  54  is I Q  The change in time between peaks (ΔT) corresponds to the output of voltage controlled oscillator  48 . 
     Referring now to FIG. 4B, an enlarged portion of a peak  56  of current output signal  54  is illustrated. Peak  56  has a rounded portion  58  to reduce the amount of electromagnetic interference generated from the current output signal  54 . Valleys  57  (of FIG. 4A) are also preferably rounded in a similar manner. 
     Referring now to FIG. 5, a more detailed schematic of control module  20  is illustrated. Generally, current-to-voltage converter  22  is coupled to a comparator circuit  60 . Comparator circuit  60  is coupled to divide-by-n counter  24 . Divide-by-n counter  24  has a clear CLR input  32 . Divide-by-n counter  24  is coupled to input pin  28  of microcontroller shown above in FIG.  2 . The microcontroller also has a system clock  62  and a counter  63 . The output from microcontroller is coupled to a microcontroller register  64 . Microcontroller register  64  stores a value that corresponds to the sense condition at the sensor. The value stored in register  64  may be used by the system to deploy an airbag if the sensor is an accelerometer for an airbag circuit or change other vehicle parameters. The value may, for example, be a count from counter  63  of the number of clock cycles within a pulse width. 
     Current-to-voltage converter  22  has a sensor current input  66  that is coupled to the output of sensor transmitter circuit  44  shown above in FIG.  3 . Sensor current input  66  receives a signal such as that shown in FIG.  4 A. Current-to-voltage converter may include an operational amplifier  70 . A feedback component such as a resistor  68  is coupled to sensor current input  66  and output  70 C to convert the current signal into a voltage signal. 
     Comparator circuit  60  includes a comparator  72  that is coupled to output  70 C of operational amplifier  70  and to the average current I avg  of the signal of FIG.  4 A. The I avg  signal may be obtained by feeding the signal of FIG. 4A through a low pass filter as would be evident to those skilled in the art. The quiescent current of a sensor has a tendency to change with age, temperature and tolerances. By using the I avg  current, the voltage differences over time are thereby tracked by comparator circuit  60 . Comparator circuit  72  may also include circuit components  74  and  76  to obtain the desired output signal from comparator  72 . 
     The output of comparator circuit  72  is coupled to divide-by-n counter  24 . Divide-by-n counter  24  is used to synchronize the sampling of data with the microcontroller system clock  62 . 
     Referring now to FIG. 6, signal  80  is the output of divide-by-n counter  24 . Signal  80  has a pulse  82  having a width  84  that corresponds to the sensed condition at the sensor. Signal  80  is coupled to the input pin  28  of the microcontroller. SYNC signal  86  allows the microcontroller to synchronize the sampling of data to its software execution timing. The number of system clock pulses within pulse width  84  is counted by a counter  63  within the microcontroller. The number of clock pulses present within the pulse width  84  of pulse  82  corresponds to the sensed condition at sensor  14 . The count is stored within register  64 . The system into which this circuit is employed may then monitor register  64  and adjust operation accordingly. 
     Advantageously, because many standard microcontrollers contain several input timer pins, no UARTs are required by the microcontroller. This reduces the overall system cost. Also, one SYNC signal may be used to synchronize data from several sensors. This reduces the number of asynchronous events that the software of the microcontroller must handle. This increases the software throughput for analysis of the remote sensor signals. 
     While particular embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Accordingly, it is intended that the invention be limited only in terms of the appended claims.