Abstract:
A sensor for monitoring the current flowing in a wire of a circuit is provided. The sensor includes a variable inductance contactless current detector to sense the current flowing in the wire, serially connected to an internal power source and a resistor to form a voltage divider circuit. The sensor also includes a voltage detector to monitor the voltage level across the resistor and generate an output signal. A system is also provided.

Description:
BACKGROUND 
       [0001]    The present invention concerns a safety device for the railroad industry. 
         [0002]    With the signing of the Rail Safety Improvement Act, in October 2008, the railroad industry will need to be “Positive Train Control” compliant throughout the United States by 2015. 
         [0003]    The term “Positive Train Control” (PTC) means that a system must be designed to prevent: train to train collisions; over speed derailments; incursions into established work zone limits; and movement of a train through a switch left in the improper position. 
         [0004]    To satisfy these requirements, a number of new types of devices are needed to provide complete PTC systems. 
         [0005]    In particular, there is a need for a device to capable report to an on-board locomotive subsystem the status of a wayside signal supplied to a wayside signal lamp. The wayside signal allows the determination whether the locomotive movement is in agreement with the condition of the railroad. The report of the status of this wayside signal is necessary to satisfy the fundamental requirements of any PTC solution. 
       SUMMARY OF THE INVENTION 
       [0006]    An object of the present invention is to provide a response to this need. 
         [0007]    The present invention provides a current sensor for monitoring the current flowing in a wire of a circuit, the current sensor includes a variable inductance contactless current detector to sense the current flowing in said wire, serially connected to an internal power source and a resistor to form a voltage divider circuit and a voltage detector to monitor the voltage level across the resistor, and generate an output signal. 
         [0008]    The variable inductance contactless current detector according to the invention, called VCS for “Vital Current Sensor” throughout this document, is a stand-alone device used to monitor, in an “overlay” configuration, the status of the wayside signal of an associated wayside signal lamp. 
         [0009]    This may be accomplished by measuring the current drawn by the wayside signal lamp, which corresponds to the wayside signal. 
         [0010]    The output signal of the VCS represents the status of the wayside signal. 
         [0011]    The output signal of the VCS is intended to drive an input of a wayside interface unit (WIU), which converts the analog output signal of the VCS into a communication message, which is eventually delivered to an onboard locomotive subsystem. The details of the operation of the WIU are outside the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    A preferred embodiment of the present invention will be elucidated with reference to the drawings, in which: 
           [0013]      FIG. 1  represents a Vital Current Sensor according to a preferred embodiment of the invention; 
           [0014]      FIG. 2  shows graphs illustrating the various types of current waveforms the VCS will respond to (both AC and DC, steady state and modulated); 
           [0015]      FIG. 3  is a circuit illustrating the major components of the output circuit responsible for generating the DC output voltage; 
           [0016]      FIG. 4  is a graph representing the output signal of the VCS of  FIG. 1  relative to the current flowing in the monitored wayside lamp; 
           [0017]      FIG. 5  is a graph illustrating the general relationship of DC current on the control winding of the sensing inductor to its inductance value; 
           [0018]      FIG. 6  is a graph illustrating the Current Sensor&#39;s transfer function of input sensed current to output voltage; and 
           [0019]      FIG. 7  is a graph illustrating the general relationship of AC current on the control winding of the sensing inductor to its inductance value. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    As shown in  FIG. 1 , the VCS  1  comprises a housing  3 , for example made of plastic, which is provided with: 
         [0021]    power input connectors  5  and  7 , to be connected to an external power source  8 , for supplying a nominal 12V DC to the VCS ; signal output connectors  9  and  11 , to be connected with corresponding input connectors of a WIU  12 , for the exchange of an output signal S generated by VCS  1 ; and, a wire passage  15 . 
         [0022]    The passage  15  extents between two through holes  17  and  19 , provided on two opposite walls of the housing  3 . The passage  15  is realized by a tubular sheath  21 , made for example of Garolite (a paper-based material that is lighter than metal but denser and stronger), whose ends are maintained in said through holes  17  and  19  and which connects one external face  23  of the housing  3  to the opposite external face  25 . 
         [0023]    Inside the housing, the VCS  1  comprises: 
         [0024]    a voltage source  31 ; 
         [0025]    a magnetic core  33 ; 
         [0026]    a fixed resistor  35 , whose resistance is R1; and, 
         [0027]    an output circuit  37 . 
         [0028]    The voltage source  31  is a quadrupole, whose first and second input terminals,  45  and  47 , are respectively connected to the power input connectors  5  and  7 . The voltage source  31  has first and second onput terminals  46  and  48 . 
         [0029]    The magnetic core  33  surrounds the sheath  21  of the passage  15 , so that the passage goes through the magnetic core center. 
         [0030]    A primary winding  53  of the magnetic core  33  has a first terminal  56  connected to the second output terminal  48  of the voltage source  31  and a second terminal  58  connected to a first terminal  66  of the fixed resistor  35 . 
         [0031]    The second terminal  68  of the fixed resistor  35  is connected to the first output terminal  46  of the voltage source  31 . 
         [0032]    The output circuit  37  is a quadrupole. Its first and second input terminals,  76  and  78 , are connected respectively to the first and second terminals  66  and  68  of the fixed resistor  35 . Its first and second output terminals,  77  and  79 , are connected respectively to the output connectors  9  and  11 . 
         [0033]    In a typical application, the VCS  1  is used in combination with a WIU  12 . Consequently, the output connectors  9  an  11  provided on the housing  3  are connected to input connectors provided on the WIU  12 . The output signal S generated by the VCS  1  is thus transmitted to the WIU  12 . 
         [0034]    The VCS  1  is able to sense the current I flowing through a wire  80  of a wayside circuit  82  connecting a lamp driving unit  84  to a wayside signal lamp  85 . In the typical setup the wayside signal lamp  85  is an 18W or a 25W lamp. 
         [0035]    The lamp driving unit  84  may drive the wayside signal lamp with either a Direct Current or an Alternating Current. Both currents can be controlled either to be ON steady, or modulated ON and OFF to produce a flashing indication, typically at a 1 Hz rate. 
         [0036]      FIG. 2  depicts the various types of current the VCS  1  can detect. 
         [0037]    The first graph G 1  of  FIG. 2  depicts a DC current transitioning from the OFF state to the ON state. 
         [0038]    The second graph G 2  of  FIG. 2  depicts an AC current transitioning from the OFF state to the ON state. 
         [0039]    The third graph G 3  of  FIG. 2  depicts a modulated DC current cycling between the OFF state and the ON state. 
         [0040]    The fourth graph G 4  of  FIG. 2  depicts a modulated AC current cycling between the OFF state and the ON state. 
         [0041]    The wire  80  is threaded through the VCS  1 , in the passage  15 . 
         [0042]    For the installation, the wire  80  is disconnected from at least one of the connection points in the wayside circuit  82 , inserted through the passage  15  of the VCS  1 , and then reconnected to an original connection point. 
         [0043]    The core  33  is used as a variable impedance component, whose impedance L 1  is controlled by a secondary “winding”. This secondary winding is realized by the wire  80  being passed through the magnetic core center. 
         [0044]    Thus inductor L 1  and fixed resistor R1, connected in series, compose a voltage divider circuit supplied by voltage source  31 . 
         [0045]    The output circuit  37  monitors the voltage V developed across the fixed resistor  35 , and generates an output signal S when the monitored voltage V exceeds a preset threshold V 0 , corresponding to a preset threshold I 0  for the current I in wire  80 . This threshold V 0  is defined by the passive components selected to make the output circuit  37 . 
         [0046]    An illustrative example of a preferred embodiment of output circuit  37  is shown in  FIG. 3 . Output circuit  37  consists of a driver stage  130 , configured as a bridge driver, a series resonant L-C tuned circuit, made of a capacitor  140  and a transformer  150 , and an output block made of a rectified DC output  160 , sufficient to energize an input circuit of the WIU. 
         [0047]    The AC voltage V developed across the fixed resistor  35  is applied to the input  78  of the driver stage  130 , resulting in both sides of the transformer  150  primary and series capacitor  140  being driven between +V_DRIVE and COMMON. The voltage produced across the transformer  150  secondary is the product of twice the input voltage and the amplification factor of the series resonant L-C tuned circuit at resonance divided by the turns ratio of transformer  150 . If the input frequency departs from the resonant frequency of the series resonant L-C tuned circuit, the amplification factor rapidly decreases and the output voltage reduces accordingly, de-energizing the WIU input circuit. 
         [0048]    In the preferred embodiment, the output signal S generated by the output circuit  37  is a DC output voltage: when the current I is above the threshold I 0 , the output of circuit  37  is driven to an ON (permissive) state. In this state, the output circuit  37  provides a nominal output signal S of 12V DC; on the contrary, when the current I falls below the threshold I 0 , the output is driven to an OFF (non-permissive) state. In this state, the output circuit  37  provides a nominal output signal S of 0V DC. 
         [0049]    During operation, when the current I flowing in the wire  80  is null (i.e. the lamp  85  is de-energized), the impedance L 1  of the magnetic core  33  is relatively high with respect to the fixed resistance R 1 . The majority of the signal amplitude from the voltage source  31  is divided primarily across L 1  (i.e. the core). The output circuit  37  monitors the voltage across the fixed resistor R 1 , and since this voltage is below the voltage threshold V 0  there is no output from the VCS  1 . 
         [0050]    As the current level increases in the wire  80 , the magnetic core  33  saturates and its impedance L 1  decreases. This in turn increases the voltage level across the fixed resistor  35 . Once this voltage V is of a sufficient level, the output circuit  37  activates and generates an output signal S. 
         [0051]      FIG. 4  shows schematically the operational structure of the magnetic core  33 , with the field lamp wire  80  represented as a control winding on the left, and a inductance winding  82  on the right. 
         [0052]    As DC current I in the control winding increases, the inductance L 1  of the inductance winding remains relatively stable until the magnetic core enters into saturation. Once in saturation, the inductance L 1  of the inductance winding, and hence its corresponding impedance, drops dramatically as illustrated in  FIG. 5 . 
         [0053]    When applied in the VCS, the signal lamp current I is used as the control winding current. As the lamp current I increase, the inductance L 1  decrease once the core goes into saturation and the VCS output is enabled as described in the preceding paragraphs above. 
         [0054]    In order to optimize the effect, the magnetic core  33  is designed to switch from a non-saturate state to a saturate state when the monitored current I moves above the predefined threshold I 0 . 
         [0055]      FIG. 6  graphically shows the relationship between the current I in the wayside signal lamp  85  and the voltage of the output signal S generated by the VCS  1 . 
         [0056]    During operation, when the current I is in the range from 0 to 0.5A, the output signal S voltage must not exceed 3.4V (i.e. the OFF-state) (zone  100  in  FIG. 2 ). 
         [0057]    When the lamp current I exceeds 1.3A, the output signal S voltage can be any value between 9V DC and 16.5V DC (i.e. the ON-state) (zone  110  in  FIG. 2 ). 
         [0058]    In the range of the current I between 500 mA and 1.3A, the output signal S is indeterminate and can be anywhere between 0V and 16.5V DC (zone  120  in  FIG. 2 ). 
         [0059]    With this behavior, the VCS  1  complies with the safety requirements for a device intended to be integrated in a PTC system, and, as such is considered as a “fail-safe” device. Indeed, under no circumstances the output signal S exceeds 3.4V DC when the current being monitored is below 0.5A DC or 0.5Arms; under no circumstances the output signals “flashes” (i.e. oscillates between the ON state and the OFF state) when the current being monitored is either constantly below or constantly above the detection threshold (this requirement originating from the fact that, in North American signal applications, a flashing signal aspect is considered to be more permissive than a steady, i.e. non-flashing, signal aspect); a failure of the VCS  1  which generates an output signal when the monitored current is above the preset threshold is considered to be an acceptable failure (i.e. safe side). 
         [0060]    Any current I that causes the saturable inductor made of the magnetic core to change its impedance will cause the VCS output circuit to energize. 
         [0061]    When the VCS senses an AC current I in the wire  80 , the AC current waveform travels from 0 current, to the positive peak current, back to zero current, then to peak negative current, finally returning to 0 current. This sequence is repeated for as long as the AC current is present. At both the positive and negative peaks, the saturable inductor is in the saturation state. During the transition time, the saturable inductor is in various states of intermediary saturation, including not saturated at all. At this time, the VCS output circuit turns off. 
         [0062]    The relationship between the inductance L 1  and AC current I is shown in  FIG. 7 . The rate at which the core goes in and out of saturation is directly proportional to the frequency of the AC current I. 
         [0063]    However, this is sufficient filtering in order the output block of the output circuit to maintain a value of the output voltage during this time. The end result is that the final output voltage from the VCS appears to be on steady when detecting AC current. 
         [0064]    Unlike the condition when the lamp is driven with an AC current, when the lamp current is DC, the sense core is driven into a continuous state of saturation. 
         [0065]    This allows the VCS to be used to detect the state of any signal lamp, to be either steady ON, FLASHING, or OFF. In the case where a railroad system uses flashing aspects, signals will typically flash at a rate of 1 Hz with a nominal duty cycle of 50. 
         [0066]    In combination with a WIU, a single VCS is used for each wayside signal lamp to be monitored. 
         [0067]    The VCS is a unique device suitable for use in “fail-safe” railways applications. In addition, any failure of the VCS will have no impact on the operation or performance of the wayside signal lamp being monitored. The isolation between the monitored system and the VCS is extremely high. 
         [0068]    The VCS is a contactless monitoring component, able to detect current on a wire without the need of a physical connection. The installation of the VCS does not require any electrical connection to the circuit to be monitored. So, it is very easy to put in place. 
         [0069]    Compared to the prior art, the design of the present sensor is simpler and only uses analog components. There is no dedicated active means, such as a processor, for the checking of the threshold.