Abstract:
A two-wire active sensor interface circuit includes a constant current circuit adapted to be coupled to a two-wire active sensor for receipt of a sensor current signal indicating one of two sensor states. The constant current circuit provides a preselected constant current amount positioned between the two sensor states that vary the sensor current signal thereby generating a current level indicator signal. Additionally, the two-wire active sensor interface circuit includes a digital interface circuit operably coupled to the constant current circuit for receipt of the current level indicator signal and produces an interface output indicating which of the two sensor states is present.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a two-wire active sensor providing an output in a form of an electrical current. More specially, it relates to a two-wire active sensor interface circuit receiving and transforming a current signal sent by the two-wire active sensor into a voltage signal output for an information gathering system. 
     BACKGROUND OF THE INVENTION 
     One conventional system used to communicate information generated by a two-wire active sensor to an information gathering system utilizes the two-wire active sensor to send a sensor current signal to a current limiting circuit that protects remaining portions of the two-wire active sensor interface circuit against faults and/or high voltage conditions, such as a short to battery. For example, if the short to battery occurs, the current limiting circuit interrupts the current flow to a current-to-voltage converter circuit by drastically increasing the input impedance. Otherwise, the current limiting circuit sends the sensor current signal to the current-to-voltage converter circuit. The current-to-voltage converter circuit transforms the sensor current signal into a voltage signal. A comparator circuit compares the voltage signal against a voltage threshold to determine a state represented by the sensor current signal output of the two-wire active sensor. The comparator circuit sends an output signal representing the state to a digital interface circuit. The digital interface circuit converts the output signal into a signal recognized by the information gathering system. 
     In order to reduce the number of components in a two-wire active sensor interface circuit and minimize costs, it may be desirable to provide an alternative two-wire active sensor interface circuit to transform the current signal produced by a two-wire active sensor into an output recognized by an information gathering system. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a two-wire active sensor interface circuit is provided. One aspect of the present invention includes a constant current circuit adapted to be coupled to a two-wire active sensor for receipt of a sensor current signal indicating one of two sensor states. The constant current circuit provides a preselected constant current positioned between the two sensor state current levels that varies the sensor current signal thereby generating a current level indicator signal. Additionally, the two-wire active sensor interface circuit comprises a digital interface circuit operably coupled to the constant current circuit for receipt of the current level indicator signal and operative to produce an interface output indicating which of the two sensor states is present. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a block diagram of a prior art two-wire active sensor interface circuit in a vehicle; 
         FIG. 2  is a detailed schematic of a two-wire active sensor interface circuit arranged in accordance with the principles of the invention; and, 
         FIG. 3  is a detailed schematic of an alternative embodiment of a two-wire active sensor interface circuit arranged in accordance with the principles of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  sets forth a block diagram of a prior art two-wire active sensor interface circuit  10  and a two-wire active sensor  12  for converting a sensor current signal into a voltage output for an information gathering system (not shown). The two-wire active sensor interface circuit  10  includes a current limiting circuit  14 , a current-to-voltage converter circuit  16 , a comparator circuit  18 , and a digital interface circuit  20 . 
     An input  12   a  of a two-wire active sensor  12  is coupled to a dynamic source (not shown). An output  12   b  of a two-wire active sensor  12  is coupled to the two-wire active sensor interface circuit  10  via the current limiting circuit  14 . The current limiting circuit  14  is, in turn, coupled to the current-to-voltage converter circuit  16 . The current-to-voltage converter circuit  16  is, in turn, coupled to the comparator circuit  18 . In addition, the comparator circuit  18  is coupled to the digital interface circuit  20 . 
     The two-wire active sensor  12  includes any conventional two-wire active sensor that is operational to sense a change in value or in a physical quality (e.g. temperature, pressure, flow rate, vehicle speed, wheel speed, illumination levels etc.) through the input  12   a  produced by the dynamic source. For exemplary purposes only, the two-wire active sensor  12  may comprise an active wheel speed sensor manufactured by Robert Bosch Corp., Farmington Hills, Mich. Additionally, the two-wire active sensor  12  converts that change into a current signal produced via the output  12   b  for use by an information gathering system. The current signal comprises a current pulse train. The two-wire active sensor  12  modulates the current pulse train signal between a high current level and a low current level to indicate the change in value or in the physical quality. 
     The dynamic source may include any source which changes in value or physical quality that may be detected and/or measured by the two-wire active sensor  12 . 
     The current limiting circuit  14  is operative to receive the current signal produced by the two-wire active sensor  12  and protect a remaining portion of the two-wire active sensor interface circuit from faults including high voltage conditions, such as a short to battery. For example, if the short to battery occurs, the current signal will exceed a maximum current threshold defined by the current limiting circuit  14 , the current limiting circuit  14  interrupts the current flow to the current-to-voltage converter circuit  16  from the two-wire active sensor  12 . 
     During normal operation, the current-to-voltage converter circuit  16  is operative to receive the current signal from the current limiting circuit  14  and convert the current signal into the voltage signal. The voltage signal is then received by the comparator circuit  18 . 
     The comparator circuit  18  receives the voltage signal from the current-to-voltage converter circuit  16  to determine the sensor state indicated by the sensor current signal. The comparator circuit  18  compares the voltage signal to a voltage threshold. Based on the comparison, the comparator circuit  18  produces an output signal for the digital interface circuit  20 . The digital interface circuit  20  is operative to receive the output signal and produce a voltage output recognized by the information gathering system. More specifically, the digital interface circuit  20  includes a switching device that is operable to switch from ON to OFF, or vice versa, based on the voltage output. For example, if the comparator circuit  18  determines that the voltage signal is below the voltage threshold, the digital interface circuit  20  produces a high voltage signal, for example 5V, as an input for the information gathering system. On the other hand, if the comparator circuit  18  determines that the voltage signal is above the voltage threshold, the digital interface circuit  20  outputs a low voltage signal, for example 0V, to the information gathering system. 
       FIG. 2  shows a two-wire active sensor interface circuit  100  for converting the current signal from the two-wire active sensor  12  into the output signal for the information gathering system in accordance with the present invention. Elements in common with  FIG. 1  will be identified with like reference numerals increased by  100  and the discussion of the two-wire active sensor interface circuit  100  will focus on the differences between the two-wire active sensor interface circuit  100  and the two-wire active sensor interface circuit  10 . 
     Referring to  FIG. 2 , the two-wire active sensor interface circuit  100  includes a constant current circuit  122  and a digital interface circuit  120 . While the functionality of the digital interface circuit  20  of  FIG. 1  and the digital interface circuit  120  are the same, the digital interface circuit  120  may include additional elements in order to perform and operate with the constant current circuit  122 . Additionally, the two-wire active sensor interface circuit  100  includes the constant current circuit  122  in lieu of the current limiting circuit  14 , the current-to-voltage converter circuit  16 , and the comparator circuit  18  of  FIG. 1 . 
     The two-wire active sensor  12  is coupled in parallel to the constant current circuit  122  and the digital interface circuit  120 . 
     The constant current circuit  122 , such as a current sink circuit, is operative to receive the current signal from an output  12   b  of the two-wire active sensor  12 . The constant current circuit  122  is configured to drain a predetermined constant current amount. The constant current amount is indicative of a current signal positioned between the low current level and the high current level. More specifically, the constant current amount is positioned approximately halfway between the low current level and the high current level. Additionally, the constant current circuit drains an amount of current equal to the constant current amount independent of the sensor current signal as the sensor current signal between its high and low current level states. 
     For example, if the sensor current signal is equal to or less than the constant current amount, the constant current circuit  122  sinks the sensor current signal to ground. Additionally, if the sensor current signal is less than the constant current amount, the constant current circuit pulls additional current from the digital interface circuit  120 , until the constant current circuit drains a total amount of current equal to the constant current amount. On the other hand, if the sensor current signal is greater than the constant current amount, the constant current circuit  122  drains a portion of the sensor current signal equal to the constant current amount to ground. 
     The constant current circuit  122  is further operative to protect any circuit coupled to the two-wire active sensor  12  from faults or high voltage conditions, such as the short to battery. During the high voltage conditions, the constant current circuit  122  limits power dissipation to the remaining portions of the two-wire active sensor interface circuit. In other words, by draining the sensor current signal, the constant current circuit  122  protects the digital interface circuit  120  from high voltage conditions by reducing the amount of current received by the digital interface circuit  120 . The remaining current is indicative of the sensor current signal sent by the two-wire active sensor  12  minus the amount of current drained by the constant current circuit  122 . 
     As shown in  FIG. 2 , the constant current circuit  122  includes an NPN transistor Q 1 . A collector of transistor Q 1  is coupled to the output  12   b  of the two-wire active sensor  12 . An emitter of transistor Q 1  is coupled to one side of a resistor R 1 . The other side of resistor R 1  is, in turn, coupled to ground. A base of transistor Q 1  is coupled to power supply V CC1 . 
     The digital interface circuit  120  includes a resistor R 2  coupled at one side to the constant current circuit  122  and the two-wire active sensor  12  at node I. The other side of resistor R 2  is coupled at an anode of a diode D 1  and one side of resistor R 3  at node J. Additionally, coupled to node J is a base electrode of a switching transistor Q 2 , which opens and closes to produce a voltage output based on the amount of current received from the two-wire active sensor  12 . A cathode of diode D 1 , the other side of resistor R 3  and an emitter of transistor Q 2  are coupled at node H to power supply V CC2 . A collector of transistor Q 2  is coupled to one side of resistor R 4 . The other side of resistor R 4  is coupled to ground. Output  120   a  is coupled to the collector of transistor Q 2  and provides an output signal to an information gathering system coupled thereto. 
     In operation, the power source V CC1  supplies base drive for transistor Q 1 . Transistor Q 1  is biased in forward-active mode. This means that the base-emitter junction of transistor Q 1  is forward-biased and the base-collector is reversed-biased. Since the collector current of a linear transistor, in the active mode, is independent of the base-collector voltage (as long as the base-junction is reversed biased), the collector of transistor Q 1  behaves as an ideal current sink. More specifically, the predetermined voltage amount of power supply V CC1  and the value of resistor R 1  are used to set an emitter current of transistor Q 1 . Additionally, a collector current is defined to be approximately equal to the emitter current of transistor Q 1  in a forward-active mode. This means that regardless of any voltage at the collector of transistor Q 1 , the collector current of transistor Q 1  remains constant. 
     For example, if two-wire active sensor  12  sends the sensor current signal equal to 7 mA and transistor Q 1  is set to draw 10 mA (the constant current amount), transistor Q 1  will draw substantially all of the sensor current signal or 7 mA sent from the two-wire active sensor  12 . Additionally, transistor Q 1  will drain the remaining current or 3 mA from power supply V CC2  through a base-emitter of transistor Q 2  and resistor R 2 . This causes transistor Q 2  to turn ON and produce the high voltage output at the collector of transistor Q 2  that is equal to an emitter voltage of transistor Q 2 . The high voltage is received by the information gathering system for an appropriate processing. 
     If the sensor current signal generated by the two-wire active sensor  12  is greater than the constant current amount, transistor Q 1  will draw an amount of current equal to the constant current amount. Additionally, if only a portion of the constant current amount is drained, any remaining flows to the digital interface circuit  120 . For example, if the two-wire active sensor  12  outputs a sensor current signal equal to 14 mA, and transistor Q 1  is set to draw 10 mA (the constant current amount), transistor Q 1  will not take any more current than the constant current amount. The remaining current equal to or about 4 mA will travel through the resistor R 2  and diode D 1 . A voltage across diode D 1  (approximately 0.7 V) is added to power supply V CC2  voltage causing transistor Q 2  to operate in the cutoff region and switch from ON to OFF. When transistor Q 2  turns OFF, the low voltage output is transmitted to the information gathering system. 
     The resultant pulse train having pulses transitioning between high and low voltage outputs of digital interface circuit  20  is coupled to an information gathering system for determination of the physical quality being monitored by sensor  12 . For example, when sensor  12  is used as a vehicle wheel speed sensor, the frequency of the pulse train output by interface circuit  20  is indicative of the angular speed of a vehicle wheel being monitored. 
     In a moderate ambient temperature environment, the performance of the constant current circuit  122  is consistent and accurate. In hot and cold ambient temperatures, however, the constant current circuit  122  may include temperature sensitive parameters that may affect the performance of the constant current circuit  122 . For example, when using transistor Q 1 , the base-emitter voltage varies as ambient temperatures enter into hot and cold temperatures. Referring to  FIG. 3 , an alternative embodiment of the two-wire active sensor interface module  100  is provided. The two-wire active sensor interface module  100  may further comprise a temperature compensator  140  used to stabilize the performance of the constant current circuit  122  during hot and cold ambient temperatures. The temperature compensator  140  is operative to stabilize the amount of current pulled to ground by the constant current circuit  122  as temperature fluctuates to hot and cold ambient temperatures. 
     The temperature compensator  140  includes an NPN transistor. The NPN transistor is an identical transistor in features and function to transistor Q 1 . Additionally, the temperature compensator  140  converts the constant current circuit into a current mirror circuit. Power supply V CC1  is removed from the base of transistor Q 1  of the constant current circuit  122 , such that the base of transistor Q 1  is coupled to a base of transistor Q 3  of the temperature compensator  140  at point L. A collector of transistor Q 3  is coupled to its base and is coupled to one side of a resistor R 5 . The other side of resistor R 5  is coupled to a power supply V CC3 . An emitter of transistor Q 3  is coupled to a power supply V CC4 . 
     In operation, the emitter current of transistor Q 1  is set by the voltage value of resistor R 1 , V BEQ1  of transistor Q 1 , V BEQ3  of transistor Q 3 , and an amount of voltage produced by power supply V CC4 . Using Kirchhoff&#39;s voltage law equation around the B-E loop of transistors Q 1  and Q 3 :
 
 V   CC4   +V   BEQ3   −V   BEQ1   −V   EQ1 =0
 
where V CC4  is a voltage produced by power supply V CC4 , V BEQ3  is a voltage of the base-emitter junction regarding transistor Q 3 , V BEQ1  a voltage of base-emitter junction regarding transistor Q 1 , and V BEQ1  is a voltage at the emitter of transistor Q 1 .
 
     As shown above, if the V BEQ3  of transistor Q 3  and the V BEQ1  of transistor Q 1  change with temperature in the same way, the net effect on the emitter current of transistor Q 1  is nominal. This causes the constant current to remains unchanged as hot and cold ambient temperatures occur. 
     Typical values and identifications of the described elements are listed as follows, these being typical only: 
     
       
         
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Device 
                 Value or Part number 
               
               
                   
               
             
             
               
                 Wheel Speed Two-Wire Active Sensor 
                 P/N: 0265007601 
               
               
                   
                 Low current signal = 7 mA 
               
               
                   
                 High current signal = 14 mA 
               
               
                 Constant Current Circuit 
                 Current Threshold = 10 mA 
               
               
                 Q1 
                 2N4401 
               
               
                 Q2 
                 2N4403 
               
               
                 Q3 
                 2N4401 
               
               
                 R1 
                  150 Ω 
               
               
                 R2 
                   10 kΩ 
               
               
                 R3 
                   10 kΩ 
               
               
                 R4 
                  330 Ω 
               
               
                 Power Supply V cc1   
                   5 V 
               
               
                 Power Supply V cc2   
                  1.5 V 
               
               
                 Power Supply V cc3   
                  5.0 V 
               
               
                 Power Supply V cc4   
                  1.5 V 
               
               
                   
               
             
          
         
       
     
     As shown above, the two-wire active sensor interface circuit  100  of  FIG. 2  or  3  is advantageous over the prior art two-wire active sensor interface circuit  10  of  FIG. 1  in reducing the number of components and minimizing cost to produce a more robust and cost effective two-wire active sensor interface circuit. 
     The description of the present invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. For example, the constant current module may include a current source circuit having a PNP transistor. Therefore, such variations are not to be regarded as a departure from the spirit and scope of the invention.