Abstract:
A two-wire adapter circuit comprises a current shunt monitor and a circuit for sensing an abrupt change in the output of the current shunt monitor, a power supply, and a two-wire electrical conduit connecting the power supply to a load via the current shunt monitor, the circuit for sensing the abrupt change in the output of the current shunt monitor configured to output a defined digital signal on a third wire upon sensing the abrupt change.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an electrical circuit for generating a defined digital signal in response to a change in the load in a remote sensing circuit and has application, for example, in mass transit vehicles that have passenger presence sensing circuits associated with vehicle doors. The electrical circuit generates a defined digital input to the vehicle control computer. 
     2. Description of Related Art 
     This application relates to sensing of the presence of passengers at the doors of mass transit vehicles. The prior art is described in our co-pending application Ser. No. 11/084,789 entitled “Capacitance Activated Switch Device” incorporated herein by reference. As explained therein, it is common for vehicle operators to be able to place certain doors in a condition enabling passengers to open such doors by contacting a switch mounted on the side of the door, such as a mechanically-operated touch bar. In our co-pending application, we describe a capacitance-activated switch device attached to the door that senses the presence of a passenger entering an alternating electric field. Electrical conduits supply power to the switch. The electrical conduits are used to signal the presence of a passenger at the door. The conduits are fed through a limited space along the hinged edge of the door. Space is also limited for circuitry attached to the door itself. It is an advantage, according to this invention, to address these space limitations by using the two-wire power leads to the door mounted or remote sensor circuit to signal the passenger presence at the door. 
     SUMMARY OF THE INVENTION 
     Briefly, according to one aspect of this invention, there is provided a two-wire adapter. A two-wire electrical conduit connects a power supply to a load via a current shunt monitor. A circuit is provided for sensing an abrupt change in the output of the current shunt monitor due to a change in the current drawn by the load. The circuit for sensing an abrupt change is configured to output a defined digital signal on a third wire upon sensing an abrupt change. 
     Briefly, according to another aspect of this invention, there is provided an annunciator circuit having a remote circuit including a voltage regulator and sensing circuit and a two-wire adapter circuit comprising a current shunt monitor and a circuit for sensing an abrupt change in the output of the current shunt monitor. A two-wire electrical conduit connects a power supply to the voltage regulator of the remote circuit via a resistor in parallel with the current shunt monitor. The sensing circuit in the remote circuit connects the output of the voltage regulator to a shunt resistor to place an additional drain on the voltage regulator upon detection of a sensed condition. The circuit for sensing an abrupt change in the output of the current shunt monitor due to the additional drain on the voltage regulator is configured to output a defined digital signal on sensing an abrupt change. Preferably, the annunciator circuit further comprises high-side and low-side drives connected to receive the defined digital output signal. 
     According to yet another aspect of this invention, an annunciator circuit comprises an electric field-sensing circuit comprising a remote circuit including a voltage regulator and circuit for sensing the change in capacitance in an ac electric field. A two-wire adapter circuit comprises a current shunt monitor and a circuit for sensing an abrupt change in the output of the current shunt monitor. A two-wire electrical conduit connects a power supply to the voltage regulator of the remote circuit via a resistor in parallel with the current shunt monitor. The sensing circuit in the remote circuit connects the output of the voltage regulator to a shunt resistor to place an additional drain on the voltage regulator upon detection of a sensed condition. The circuit for sensing an abrupt change in the output of the current shunt monitor due to the additional drain on the voltage regulator is configured to output a defined digital signal on a third wire upon sensing an abrupt change. 
     According to still another aspect of this invention, an annunciator circuit for generating an input to a control computer on a mass transit vehicle comprises an electric field-sensing circuit for sensing the presence of a passenger at a mass transit vehicle door, including a circuit having a voltage regulator and circuit for sensing the change in capacitance in an ac electric field associated with the vehicle door. A two-wire adapter circuit is positioned above the vehicle door comprising a current shunt monitor and a circuit for sensing an abrupt change in the output of the current shunt monitor. A two-wire electrical conduit is connectable to a power supply and extends from above the door and to the voltage regulator of the sensing circuit via a resistor in parallel with the current shunt monitor. The sensing circuit connects the output of the voltage regulator to a shunt resistor to place an additional drain on the voltage regulator upon detection of a passenger at the vehicle door. The circuit for sensing an abrupt change in the output of the current shunt monitor due to the additional drain on the voltage regulator is configured to output a defined digital signal on a third wire upon sensing an abrupt change. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and other objects and advantages will become apparent from the following detailed description made with reference to the drawings in which: 
         FIG. 1  is a circuit diagram for a two-wire adapter circuit according to one embodiment of this invention; 
         FIG. 2  is a circuit diagram for a remote capacitance-sensing circuit according to one embodiment of this invention; and 
         FIG. 3  is a simplified flow diagram of the program stored in the microcontroller U 13  of  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 2 , there is illustrated a remote capacitance touch sensor circuit based upon an electric field imaging device that senses objects entering a low level electric field and measures the capacitive loading of the field caused by objects moving into the field. A two-wire input at P 1  and P 2  supplies power to the remote circuit via voltage controller U 21 . Associated with integrated circuit U 21  are capacitors C 21 , C 24  and C 25  and resistors R 25  and R 26 . An integrated circuit U 22  (MC33794 sold by Freescale) provides most of the function of the touch sensor circuit. A low frequency sine wave is generated by the integrated circuit U 22  which is applied to a volume by electrodes E 1  and E 2 . Associated with integrated circuit U 22  are capacitors C 211 , C 212 , C 213 , C 214  and C 23  and resistor R 21 . The digitized capacitance detected by U 22  is processed by microcontroller U 23  to detect an abrupt change in capacitance. Associated with integrated circuit U 23  are capacitors C 26  and C 29  and inductor L 21 . Up to this point, the circuit of  FIG. 2  is basically as described in our co-pending application identified above. When the presence of an object is detected, the output of the voltage regulator U 21  is shunted through resistor R 23 . The resistance of the resistor R 23  is chosen to provide a small but significant increased load on the voltage regulator. The presence of resistor R 23  is a feature new to this application. In the specific embodiment described with reference to  FIG. 2 , the resistance of resistor R 23  is set at 475 ohms so that the increased load will be about 20 mA. 
     Referring now to  FIG. 1 , there is shown an embodiment of a two-wire adapter circuit according to this invention. A power supply is indicated at Vbat. The power supply feeds a voltage regulator U 11  that powers microcontroller U 13  and shunt monitor IC U 12 . Associated with voltage regulator U 11  are capacitors C 11  and C 13 . Associated with microcontroller U 13  are capacitors C 15  and C 16  and inductor L 11 . Associated with shunt monitor U 12  are capacitors C 17  and C 18 . A current shunt monitor U 12  is connected to each side of a small resistor R 13 . The power supply is connected to terminals TB 1 + and TB 2 +, for example, which are then connected through a two-wire conduit (TWC) to respective voltage regulators of the sensing circuits shown in  FIG. 2 . The terminals TB 1 + and TB 1 − correspond to a sensing circuit for one side of a double swinging door of a transit vehicle, for example. The terminals TB 2 + and TB 2 − correspond to the other door. 
     In the preferred embodiment, U 16  is placed between R 13  and TB 1 + and TB 2 +. If the current draw through R 13  is excessive due to an accidental short at TB 1 + or TB 2 +, the microcontroller U 13  can disconnect R 13  by pulling up enable pin  2  EN or U 16  preventing R 13  from being destroyed. 
     As indicated in  FIG. 1 , the output voltage of the current shunt monitor in this particular embodiment is twenty times the voltage drop sensed across the resistor R 13 . For a 20 mA change in current through the resistor R 13 , the output voltage would change 400 mV. 
     As shown in  FIG. 3 , the analog output of the shunt monitor is repeatedly digitized by microcontroller U 13  and stored in a rolling memory so that the trend over a short period can be observed and abrupt changes can be recognized. Gradual changes in the current through resistor R 13  due to various factors, such as drift in the battery voltage, the temperature of the resistor R 13 , or the effects of temperature on the circuit shown in  FIG. 2  will not result in an output from the microcontroller. When the microcontroller does produce an output, it activates both the high-side and low-side drivers U 14  and U 15  having outputs OH and OL respectively. Typically, only one of those will be connected to the vehicle computer (depending on the vehicle computer&#39;s logical input requirements). 
     Referring to  FIG. 3 , Steps S 1  to S 6  comprise a setup loop in which the controller internals are setup at S 1  and initial samples and averages are detected and computed at S 2 . Thereafter, a timer loop comprising steps S 3  to S 6  at spaced time intervals calls the detect function at S 5  transferring control to Step S 7  to get the current values at the analog to digital converter. A test for a massive change in current is made at S 8 . If such change has taken place, averaging is stopped at S 9  and the output drivers are activated at S 10 . If, however, no massive change in current is detected at S 8 , a test is made at S 16  to determine if there is a rolling average for determining differential change in current. If so, the change is checked multiple times to verify the change and if verified the output drivers are activated at S 17 . If, however, there is no average for determining differential change in current at S 12 , a test is made at S 13  to determine if there has been a decrease in current. If so, the drivers are deactivated at S 15 . If not, output timers are reset after time out at S 14  and the averages function is performed at S 11 . Finally, if verification cannot be made at S 16 , the averages function is performed at S 11 . 
     We have described the two-wire adapter circuit in the setting of a capacitance-activated switch used with doors on a transit vehicle. However, it should be clear that any device (proximity switch, Hall switch, etc.) may use the two-wire adapter circuit. 
     Having thus defined our invention in the detail and particularity required by the Patent Laws, what is desired protected by Letters Patent is set forth in the following claims.