Patent Publication Number: US-9889867-B2

Title: Railroad switch machine

Description:
FIELD OF THE INVENTION 
     The present invention concerns a railroad switch machine. 
     BACKGROUND 
     Railroad switch machines are key elements of a railroad installation. A railroad switch machine has to operate over extreme temperatures and is subject to intense shock and vibration. Maintenance of a switch machine may take place in difficult and potentially dangerous conditions, and is a significant expense for a railroad operator because of the large number of switch machines used. Therefore it is important to minimize the required maintenance for the switch machines, while optimizing their reliability and maintaining their performance over time. 
     Switch machines typically utilize a DC permanent magnet motor to drive a gearbox, whose output is coupled to a mechanical apparatus for moving switch blades of the track. Such switch machines are typically powered by a DC voltage source from a railroad wayside equipment station, and comprise high current contactors powering and driving the DC permanent magnet motor. 
     The switch machines presented above include, in the gear box, a mechanical torque-limiting clutch. The torque-limiting clutch is adjusted to slip when motor torque exceeds a certain predetermined limit, in order to maintain a good and secured operating of the motor and more generally of the switch machine. One of the main reasons a mechanical clutch is required in switch machines is that track obstacles such as rocks or ice could block normal rail switch motion, in which case the motor could stall. Since the motors in the switch machines presented above are driven by a fixed voltage, the lack of counter electromotive force in the motor windings during a motor stall condition will cause motor current and corresponding motor torque to increase to a very high level. This could damage the motor and/or the switch machine mechanism. 
     However the use of a torque-limiting clutch has drawbacks. Indeed, the torque-limiting clutch does not provide a precise motor torque control and is also relatively large and expensive. The performance of the mechanical clutch will change with environmental conditions (e.g. temperature and humidity), and will also change over time as the mechanical parts wear and corrosion occurs. 
     Moreover, periodic maintenance is required for the high current contactors and for the torque-limiting clutch to ensure proper operation of the switch machine. 
     Besides, it is known from the U.S. Pat. No. 6,366,041 B1 a switch machine comprising switch blades, a motor for moving the switch blades, and a regulation unit to control the electric power supplied to the motor so as to command the movement of the switch blades from an initial position to a final position. In such switch machine, the regulation unit includes high current contactors and a control device of the contactors. 
     However, in such a switch machine, the regulation unit does not provide a precise control of the motor and a periodic maintenance of the contactors is required. Indeed the contactors are subject to significant mechanical wear because of arcing. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention provide a response to the drawbacks mentioned above. 
     In one embodiment, a railroad switch machine comprises:
         a pair of switch blades,   an electric motor, the electric motor being a reversible motor having two phases, the electric motor being mechanically coupled to the pair of switch blades to move the switch blades relative to a pair of stationary rails,   a pair of input terminals, connected to a DC power supply for receiving a power signal,   a regulation unit to adapt the power signal between the input terminals into a motor current applied to the electric motor, said regulation unit comprising several switch elements connected in an H bridge configuration between the pair of input terminals and the two phases of the electric motor,   wherein the switch elements are transistors and the regulation unit comprises a control device adapted to drive each transistor with a respective command signal to adjust a value and a direction of an intensity of the motor current.       

     According to further aspects of the invention, which are advantageous but not compulsory, such a railroad switch machine may incorporate one or several of the following features:
         each respective command signal is a pulse width modulation signal;   the regulation unit comprises a motor current sensor configured to measure the intensity of the motor current applied to the electric motor and the control device drives each transistor in function of the intensity of the motor current, to regulate the intensity of the motor current supplied to the electric motor;   the control device is adapted to drive each transistor also in function of a predetermined motor current limit setpoint, to limit the intensity of the motor current below the predetermined motor current limit setpoint, providing a feedback control of the intensity of the motor current;   the electric motor is intended to move the switch blades relative to stationary rails from an initial position to a final position, the regulation unit comprising identifying means to identify an instantaneous state of the railroad switch machine among:
           a starting state, during which the motor starts to spin and the switch blades have traveled relative to the stationary rails on a first distance comprised in a first interval,   a normal operating state, during which the switch blades have traveled relative to the stationary rails on a second distance comprised in a second interval following the first interval, and   an ending state, during which the switch blades have traveled relative to the stationary rails on a third distance comprised in a third interval following the second interval,   and the control device is configured to adapt the value of the predetermined motor current limit setpoint, depending on the instantaneous state of the electric motor;   
           during the normal operating state the control device drives each transistor to connect the electric motor to the input terminals continuously, in order to apply a voltage of maximum absolute value to the electric motor;   during the starting and ending operating states the control device drives each transistor to limit motor current to the predetermined motor current limit setpoint;   the regulation unit comprises a supply current sensor configured to measure an intensity of the power signal received on the input terminals, the control device drives each transistor also in function of the intensity of the power signal, the regulation unit comprising a first feedback loop on the intensity of the motor current and a second feedback loop on the intensity of the power signal, the regulation unit thus forming a cascade controller;   the railroad switch machine includes a capacitor connected in parallel with the input terminals;   the regulation unit, the motor and the capacitor forms a buck DC-DC converter devoid of independent inductor and an internal inductance of the electric motor is used as an inductor for the buck DC-DC converter; and   the railroad switch machine is devoid of a mechanical torque-limiting clutch.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  represents schematically a railroad installation comprising a railroad track and a railroad switch machine according to a first embodiment of the invention; 
         FIG. 2  represents a motor assembly of the railroad switch machine of  FIG. 1  comprising an electric motor and a regulation unit for the electric motor; 
         FIG. 3  is a block diagram representing the regulation of the intensity of a motor current going through the electric motor of  FIG. 2 ; 
         FIG. 4  is a flowchart of an example of method for controlling the motor of  FIG. 2 ; 
         FIG. 5  shows four graphs representing the number of rotation per minute of the electric motor of  FIG. 2 , the motor current across the electric motor, the supply current supplied to the motor assembly of  FIG. 2  and a pulse width modulation duty cycle of a transistor driving the electric motor, both in function of time, while switch blades of the railroad switch machine are moved from a deviated position towards a direct position, 
         FIG. 6  is similar to  FIG. 1  and represents a railroad installation comprising a railroad track and a railroad switch machine according to a second embodiment of the invention; 
         FIG. 7  is similar to  FIG. 2  and represents a motor assembly of the railroad switch machine of  FIG. 6 ; and 
         FIG. 8  is a block diagram representing the regulation of the intensity of a motor current going through an electric motor of the motor assembly of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a railroad switch installation  10  which comprises a railroad track  12  and a railroad switch machine  14  installed on the railroad track. 
     The railroad track  12  comprises a stationary left rail  16  and a stationary right rail  18 . 
     The stationary left rail  16  defines a deviated way for a vehicle traveling on the railroad track  12  in a predetermined flow direction F and the stationary right rail  18  defines a direct way for a vehicle traveling on the railroad track  12  in the predetermined flow direction F. 
     The switch machine  14  includes a left switch blade  20  and a right switch blade  22  which are linked by one tie rod  24 . 
     The switch machine  14  comprises also a motor assembly  26  and a displacement element  28  linking the motor assembly  26  to the tie rod  24 . The motor assembly  26  and the displacement element  28  are configured to move the tie rod  24  and therefore the left  16  and right  18  switch blades to guide a non-represented vehicle, such as a train, crossing the switch blades in the predetermined flow direction F, either in the direct way or in the deviated way. 
     More especially, the motor assembly  26  and the displacement element  28  are intended to move the left  20  and right  22  switch blades relative to the left  16  and right  18  stationary rails between a direct position, wherein a vehicle crossing the switch blades  20 ,  22  is guided to the direct way and a deviated position, wherein a vehicle crossing the switch blades  20 ,  22  is guided to the deviated way. 
     In the direct position, illustrated on  FIG. 1 , the left switch blade  20  is positioned against the stationary left rail  16 , and the right switch blade  22 , is away from the stationary right rail  18 , i.e. separated from the stationary right rail  18  by a free space of, for example, 10 cm. 
     In the direct position, the vehicle is directed to the direct way, via the right stationary rail  18  and the left switch blade  20 , which is extended by a straight left rail  30  extending along the direct way sensibly parallel to the right stationary rail  18 . 
     In the deviated position, not shown, the left switch blade  20  and the right switch blade  22  are moved to the right with respect to  FIG. 1 , the left switch blade  20  moving away from the stationary left rail  16  and the right switch blade  22  moving to a position against the stationary right rail  18 . 
     In the deviated position, the vehicle is directed to the deviated way, via the left stationary rail  16  and the right switch blade  22 , which is extended by a deviated right rail  32  extending along the deviated way, sensibly parallel to the left stationary rail  16 . 
     As represented on  FIGS. 1 and 2 , the motor assembly  26  comprises a reversible electric motor  34 , whose output is connected to the displacement element  28  through a non-represented gear box, two input terminals  36 A,  36 B connected to a direct current (DC) power supply  41 , a regulation unit  38  configured to drive the motor  34  and a capacitor  39  connected in parallel with the input terminals  36 A,  36 B. 
     The motor assembly  26 , and more generally the railroad switch machine  14 , is devoid of a mechanical torque-limiting clutch adapted to slip when a motor torque produced by the electric motor  34  exceed a predetermined value. 
     The motor assembly  26 , i.e. the regulation unit  38 , the motor  34  and the capacitor  39 , forms a buck DC-DC converter devoid of independent inductor. More especially, an internal inductance of the electric motor is used as an inductor for the buck DC-DC converter. 
     The electric motor  34  moves the displacement element  28  while the electric motor  34  is spinning. 
     The electric motor  34  is a two-phase motor and is, for example, a DC brush motor. 
     The electric motor  34  comprises a first phase  40 A and a second phase  40 B. 
     Alternatively, the electric motor  34  is a brushless DC servomotors or a field-wound motor. 
     As presented above, the electric motor  34  is mechanically coupled to the switch blades  20 ,  22 . 
     The input terminals  36 A,  36 B are connected to the DC power supply  41  via two supplying electric cables  42 A,  42 B, corresponding, for example, to a phase conductor  42 A and a neutral conductor  42 B, and to receive a power signal from the DC power supply  41 . The power supply is, for example, located several hundred meters distant from the motor assembly  26 . The power supply comprises, for example, a bank of battery and delivers a supply current IS 1  to the motor assembly  26 . 
     The regulation unit  38  generates, from the power signal on the input terminals  36 A,  36 B, a motor current applied to the electric motor  34 . 
     The regulation unit  38  is configured to adapt the power signal between the input terminals  36 A,  36 B into the motor current applied to the electric motor  34 . 
     As shown in  FIG. 2 , the regulation unit  38  comprises four transistors  44 A,  44 B,  44 C,  44 D connected in an H bridge configuration between the input terminals  36 A,  36 B and the phases  40 A,  40 B of the electric motor  34 . 
     The regulation unit  38  comprises a motor current sensor  48 , located on a central branch of the H bridge in series with the first phase  40 A of the electric motor  34 , to measure the intensity IM 1  of a motor current supplied to the electric motor  34 . 
     The regulation unit  38  comprises also identifying means  50 , such as an identification sensor, configured to identify an instantaneous state of the railroad switch machine during the movement of the switch blades  20 ,  22  from an initial position, corresponding to the direct position or the deviated position, to a final position, corresponding to the deviated position or the direct position. 
     The regulation unit  38  comprises a control device  52  driving each transistor  44 A,  44 B,  44 C,  44 D with a respective command signal SA, SB, SC, SD. 
     The respective command signals SA, SB, SC, SD are advantageously pulse width modulation signals. 
     The transistors  44 A,  44 B,  44 C,  44 D connect the motor  34  to the input terminals  36 A,  36 B based on the respective command signal SA, SB, DC, SD. 
     The transistors  44 A,  44 B,  44 C,  44 D are driven between a closed state in which they are equivalent to a close switch and an open state in which they are equivalent to an open switch. 
     The transistors  44 A,  44 B,  44 C,  44 D are, for example, metal-oxide-semiconductor field-effect transistors (MOSFET). 
     According to their close or open state the transistors  44 A,  44 B,  44 C,  44 D command the passage of the current through the motor  34 , said motor current being characterized by an intensity IM 1  defined by an absolute value and a direction through the motor through the motor  34 . 
     More especially, when the transistors  44 A,  44 C are in the closed state and the transistors  44 B,  44 D are in the open state, the intensity of the motor current IM 1  flows in a first direction and command the rotation of the electric motor  34  in a first way. Thus, the switch blades  20 ,  22  move, for example, towards the direct position. 
     When the transistors  44 B,  44 D are in the closed state and the transistors  44 A,  44 C are in the open state, the intensity of the motor current IM 1  flows in a second direction or reverse direction, opposite to the first direction, and command the rotation of the electric motor in an other way. Thus, the switch blades  20 ,  22  move, for example, towards the deviated position. 
     The alternate command of the pairs of switches  44 A,  44 C and  44 B,  44 D according to different values of a duty cycle will allow the regulation of the value of the intensity of the motor current. 
     When the transistors  44 A,  44 B,  44 C,  44 D are all in the open state, the intensity of the motor current IM 1  is null and the motor is motionless. 
     The identifying means  50  are configured to identify the instantaneous state of the switch machine among at least three different states of the railroad switch machine  14  that successively occurred during the movement of the switch blades  20 ,  22  from the initial position to the final position. The path followed by the switch blades  20 ,  22  relative to the corresponding stationary blades  16 ,  18  is subdivided in three successive intervals, i.e. a first, a second and a third intervals, between the initial and the final position. Then the three possible states are:
         A starting state, during which the motor  34  starts to spin and the switch blades  20 ,  22 , have traveled relative to the stationary rails  16 ,  18 , on a first distance comprised in the first interval. In the starting state a voltage, is applied to the motor  34 , i.e. to terminals of the motor  34 , and as the motor begins turning there is very little counter electromotive force generated by the motor to oppose the voltage at the terminals of motor  34 .   A normal operating state, during which the switch blades  20 ,  22  have traveled relative to the stationary rails  16 ,  18  on a second distance comprised in the second interval. In the normal operating state, the motor  34  is spinning rapidly, at a nominal angular velocity. The counter electromotive force generated by the spinning motor  34  will limit the voltage applied across the motor  34 , i.e. across winding of the motor  34 , and therefore limit motor current and motor torque.   An ending state, during which the switch blades  20 ,  22  have traveled relative to the stationary rails  16 ,  18  on a third distance comprised in the third interval. In the ending state, the switch blades are close to the final position, the motor load increases compared to the normal operating state, and the motor  34  slows down to stop when the switch blades  20 ,  22  are in the final position. In the ending state, as the motor  34  slows down, there is little counter electromotive force to oppose the voltage of the motor current.       

     The identification sensor  50  can be, for example, a position sensor configured to measure the position of the switch blades  20 ,  22  relative to the stationary rails  16 ,  18  and to transmit position information to the control device  52 . The position of the switch blades  20 ,  22  is typically determined by limit switches which detect the position of displacement element  28 . 
     The control device  52  comprises a reception organ  55  and a calculating unit  56 . 
     The reception organ  55  receives command instructions of moving the switch blades  20 ,  22  from a remote central controller (not shown on the figure). 
     Calculating unit  56  is implemented in hardware and comprises programmable logic components. 
     Alternatively calculating unit  56  comprises dedicated integrated circuits. 
     Alternatively calculating unit  56  is implemented by means of a programmable computer comprising a processor and an information medium storing programming code instructions. In this alternative, the processor executes the programming code instructions saved by the information medium and the programming code instructions form a computer program configured to be executed by the processor. 
     The calculation unit  56  is configured to limit the motor current below a predetermined motor current limit setpoint CLSM 1  of the railroad switch machine  14 . 
     The predetermined motor current limit setpoint CLSM 1  corresponds to a predetermined desired maximal absolute value of the intensity of the motor current IM, which directly corresponds to a maximal motor torque output by the DC electrical motor  34 . In the proposed invention, motor torque is limited to a predetermined maximal value thanks to the calculation unit  56 . 
     The value of the predetermined motor current limit setpoint CLSM 1  is, for example, equal to 20 Amperes. 
     Calculating unit  56  is configured to drive each transistor  44 A,  44 B,  44 C,  44 D, i.e. to generate each respective command signal SA, SB, SC, SD, in function of the intensity measured by the motor current sensor  48  and, advantageously, also in function of the predetermined motor current limit setpoint CLSM 1 , to regulate the value of the intensity of the motor current IM 1  delivered to the electric motor  34  to be below the current limit setpoint CLSM 1 . 
     More especially, calculating unit  56  is configured to identify the instantaneous state of the railroad switch machine  14  in function of the positions of the switch blades  20 ,  22  detected by the identifying means  50  and to drive each transistor  44 A,  44 B,  44 C,  44 D in order to limit the value of the motor current through motor  34  below the value of the predetermined motor current limit setpoint CLSM 1 . 
     More precisely, as shown on  FIG. 3 , the calculating unit  56  calculates and generates the respective command signals SA, SB, SC, SD of the transistors  44 A,  44 B,  44 C,  44 D in function of a difference between the intensity of the motor current IM 1  measured by the motor current sensor  48  and the value of the predetermined motor current limit setpoint CLSM 1 . The respective command signals SA, SB, SC, SD are calculated using, for example, a proportional-integral-derivative controller  62  (PID controller) and are applied to the transistors  44 A,  44 B,  44 C,  44 D. 
     Calculating unit  56  is also configured to stop the motor  34  if, while the switch blades  20 ,  22  are commanded to move towards the direct position, the position detector detects that the left switch blade  20  is already in the direct position. Inversely calculating unit  56  is configured to stop the motor  34  if, while the switch blades  20 ,  22  are commanded to move towards the deviated position, the position detector detects that the right switch blade  22  is already in the deviated position. 
     The functioning of the control device  52  will be explained in more details below using  FIGS. 3 to 5 . 
     As shown on  FIG. 4 , the method for controlling the motor comprises first an initial step  100 , then further measuring  102 , starting  104 , operating  106 , ending  108  and final  110  steps. 
       FIG. 5  shows four curves  112 ,  114 ,  116  and  118  corresponding respectively to the number of rotation per minute (RPM) of the motor  34 , the value of the motor current IM 1 , the value of the supply current IS 1  and the duty cycle DCC of the command signal SC in function of time, during steps  100  to  108 . On  FIG. 5 , initial step  100 , then further starting  104 , operating  106 , ending  108  and final  110  steps are indicated on the time line, i.e. on the horizontal axis of the graphs. 
     During the initial step  100 , the control device  52  receives, through the reception organ  55  and from the central controller, a command instruction of moving the switch blades  20 ,  22  towards the deviated or the direct position. In our example, the method for controlling the motor will be presented in the case where the control device  52  receives a command instruction of moving the switch blades  20 ,  22  into the direct position, whereas the switch blades  20 ,  22  are initially in the deviated position. 
     In the further measuring step  102 , the motor current sensor  48  measures the intensity of the motor current IM 1  and the identifying means  50  measures the position of the switch blades  20 ,  22 . This step is, for example, performed periodically with a period, for example, equal to 0.5 second. 
     Then, during the starting step  104 , calculating unit  56  identifies that the switch machine  14  is in the starting state and calculates the respective command signals SA, SB, SC, SD, in function of the difference between the measured intensity of the motor current IM 1  and the value of the predetermined motor current limit setpoint CLSM 1 . During the starting step, the motor begins turning slowly and then accelerates as shown on curve  112 . As the motor begins to turn, there is minimum counter electromotive force, and if motor  34  was connected to input terminals  36 A,  36 B continuously, motor current and torque would quickly increase to a large value that could damage the motor or the switch machine mechanism. In this mode of operation the control device  52  commands each transistor  44 A,  44 B,  44 C,  44 D, utilizing pulse width modulation to limit the intensity of the motor current IM 1  to the value of the predetermined motor current limit setpoint CLSM 1 , as shown on curves  114  and  116 . There are several different well-known timing schemes for pulse width modulation of H bridge switches  44 A, 44 B, 44 C, 44 D for bidirectional motor drivers, including two-quadrant chopping, four-quadrant chopping, and enable chopping. Below is a description of operation using two-quadrant chopping, although other timing schemes could be utilized. 
     More especially, during the starting step  104 , the command signal SC of transistor  44 C is a pulse width modulation signal, having a duty cycle less than 100%, and the command signal SA is set to keep transistor  44 A closed. The command signals SB and SD are set to keep transistors  44 B and  44 D open. 
     As shown on  FIG. 5 , on curve  118 , during the starting step  104 , the value of duty cycle of the command signal SC starts from value 0 and increases to reach the value 100%, where the switch machine is in the normal operating state. 
     After the motor has accelerated enough so that counter electromotive force limits the current and torque delivered by the motor, operating step  106  begins. More especially, the calculating unit  56  identifies that the switch machine  14  is in the normal operating state and calculates the respective command signals SA, SB, SC, SD. In this mode the motor current is below the motor current limit setpoint CLSM 1  and is, for example, equal to 3 A as shown on curve  114 . In this case the control device  52  drives each transistor to connect the electric motor  34  to the input terminals  36 A,  36 B continuously, in order to apply a voltage of maximum absolute value to the electric motor  34 . In other words, during the operating step  106 , if the motor  34  is commanded to move the switch blades towards the direct position, the calculating unit  56  maintains the transistors  44 A and  44 C in the closed state and transistors  44 B and  44 D in the open state, i.e. the duty cycle of the command signals SA and SC is equal to 100%; and if the motor  34  is commanded to move the switch blades towards the deviated position, the calculating unit  56  is configured to maintain the transistors  44 B and  44 D in the closed state and the transistors  44 A and  44 C in the open state. 
     Then, during the ending step  108 , following the normal operating step  106  and identified by the calculating unit  56 , displacement element  28  has moved the switch blades close to the ending point of travel. In this interval the torque load to the motor increases as the displacement element  28  compresses the switch blades against the stationary rails to reach the final position, the motor slows down as shown on curve  112 , and the motor current IM 1  must be limited to avoid damage to the motor  34  or switch machine  14 . During the ending step  108 , the control device commands each transistor  44 A,  44 B,  44 C,  44 D to adjust the intensity of the motor current IM 1  to the value of the predetermined motor current limit setpoint CLSM 1 , as shown on curve  114 . In this case the H bridge switching control is the same as in step  104 . More especially, the command signals SC of transistor  44 C is a pulse width modulation signal having, for example, a duty cycle shown on curve  118  starting from value 100% and decreases to reach the value 0, where the final step  110  is reached and the switch blades are in the direct position. The command signal SA is set to keep transistor  44 A in the closed state and the command signals SB and SD are set to keep transistors  44 B and  44 D in the open state. 
     Alternatively, if the control device  52  receives a command instruction of moving the switch blades  20 ,  22  into the deviated position, whereas the switch blades  20 ,  22  are initially in the direct position, the transistors  44 A,  44 B,  44 C,  44 D are driven to reverse the direction of current flow through the motor. In this case the command signal SB of transistor  44 B is a pulse width modulation signal and the command signal SD is set to keep transistor  44 D closed. In this case command signals SA and SC are set to leave transistors  44 A and  44 C open. 
     As shown on  FIG. 3 , during the starting  104 , normal operating  106  and ending  108  steps, the calculating unit  56  could implement a control feedback loop: the PID controller  62  generates the respective command signals SA, SB, SC, SD in function of the measured intensity of the motor current IM 1  and the value of the predetermined motor current limit setpoint CLSM 1 . In other words, the control device  52  comprises a first feedback loop on the intensity of the motor current. It has to be noted that calculating unit  56  is configured so that if the measured motor current IM 1  is less than the motor current limit setpoint CLSM 1 , calculating unit  56  will drive each transistor to connect the electric motor to the input terminals  36 A,  36 B continuously. 
     During the starting step  104  and the ending  108  steps, the control of the transistors  44 A,  44 B,  44 C,  44 D in function of the value of the predetermined motor current limit setpoint CLSM 1  allows to limit the torque produced by the motor  34 , to avoid developing excessive motor torque which could damage the gear box and/or the displacement element  28  and/or the tie rod  24 . Indeed the control of the transistors  44 A,  44 B,  44 C,  44 D allows controlling the motor current and it is known that the motor torque of an electric motor  34  is proportional to the value of the intensity of the motor current IM 1  going through the motor  34 . Such a control is essential because, when the switch machine  14  is in the starting state  104  the motor torque is limited only by the resistance of the motor  34  and can increase in a dangerous manner for the switch machine  14 . In the same manner, when the switch machine is in the ending state  108 , the motor load typically increases significantly as the switch blades  20 ,  22  are driven to their final position and the motor torque could become excessive and damage the motor  34 . 
     During the operating step  106 , the control of the transistors  44 A,  44 B,  44 C,  44 D is not directly used to limit the motor torque and the transistors  44 A,  44 C are maintained in the closed state, because the motor is spinning rapidly and the counter electromotive force generated by the spinning motor will limit the intensity of the motor current IM 1  and consequently the motor torque. 
     Regulation unit  38 , the motor  34 , and the capacitor  39  form a buck DC-DC converter. The benefits of the buck converter are significant. As shown on curves  114  and  116  of  FIG. 5 , in the proposed invention, the intensity of the supply current IS 1  drawn is less than 6 A and this allows delivering a motor current IM 1  with an intensity of 20 A. A buck converter produces the same power at the output as is consumed at the input, neglecting relatively small losses in efficiency due to non-ideal components. In the example, the power drawn from the power supply when the motor reaches end of travel is equal to 6 A×24V=144 W. Neglecting buck converter losses, the motor power is thus 144 W. In this case, since the motor current is 20 A, the voltage across the motor terminals is 144 W/20 A=7.2V. In other words, when motor  34  slows down due to increased torque load, e.g. at end of travel, the counter electromotive force developed by the motor is relatively small, the voltage output from the buck converter is reduced, and the current drawn from the power supply is reduced. 
     As previously explained, the supply current is typically supplied by batteries, and the buck DC-DC converter allows reducing the economic and environmental cost of buying and maintaining the power supply. Indeed the buck DC-DC converter allows to reduce the value of the supply current required to operate the switch machine, and therefore reducing the amount of batteries required for supplying the supply current. 
     In addition, supplying electric cables capable of supplying  20 A which must be relatively thick, e.g. AWG6, and are expensive are not necessary. Indeed the use of the buck DC-DC converter allows to use supplying electric cables  42 A,  42 B configured to supply only 6 A and not 20 A, i.e. less thick and less expensive. 
     More generally, the value of the intensity of the motor current is globally equal to the inverse of the duty cycle of the command signal SC when the switch blades  20 ,  22  are moved in the direct position and to the inverse of duty cycle of the command signal SB when the switch blades  20 ,  22  are moved in the deviated position. 
     Therefore the control device  52  allows a precise control of the intensity of the motor current IM 1  and therefore of the motor torque to avoid any damage of the switch machine  14 . 
     Alternatively, the identifying means  50  comprises, for example, a speed sensor configured to measure the electric motor  34  angular velocity and the control device  52  is configured to identify the instantaneous state of the railroad switch machine  14  in function of the measured motor angular velocity. Indeed the angular velocity of the motor is different according to the state of the railroad switch machine  14 , because the load presented to the motor  34  varies according to the position of the switch blades, i.e. according to the state of the railroad switch  14 . 
     Alternatively the identifying means  50  are counter electromotive force sensors. 
     In field operation, obstructions on the railroad track, such as rocks or ice, could cause the switch blades  20 ,  22  to jam, which will cause the motor to stall. In this stall condition, the control device  52  is configured to drive the transistors  44 A,  44 B,  44 C,  44 D in function of the value of the predetermined motor current limit setpoint CLSM 1  to limit the value of the intensity of the motor current IM 1  and prevent damage of the switch machine  14 . 
     In the following, a second embodiment of the invention, as presented on  FIGS. 6 to 8  will be described. 
       FIG. 6  represents a railroad installation  200  comprising a railroad track  12  and a railroad switch machine  202  according to the second embodiment of the invention. 
     The railroad installation  200  represented in  FIG. 6  is globally similar to the one represented on  FIG. 1  and the similar elements have the same references. 
     The railroad switch machine  202  includes a left switch blade  20  and a right switch blade  22  which are linked by one tie rod  24 , a motor assembly  210  and a displacement element  28  linking the motor assembly  210  to the tie rod  24   
     The railroad switch machine  202  is globally similar to the railroad switch machine  14 . Such a railroad switch machine  202  differs from the railroad switch machine  14  according to the first embodiment only by its motor assembly  210 . 
     The motor assembly  210 , represented on  FIG. 7 , is globally similar to the motor assembly  26  and the similar elements between the two motor assemblies  210 ,  26  will have the same reference numbers. 
     In the following, only the difference between the first embodiment and the second embodiment will be presented. 
     The motor assembly  210  comprises an electric motor  34 , input terminals  36 A,  36 B, a capacitor  39  and a regulation unit  212 . 
     The regulation unit  212  comprises transistors  44 A,  44 B,  44 C,  44 D, a motor current sensor  48  located on a central branch of the H bridge in series with the first phase  40 A of the electric motor to measure the intensity of the motor current IM 1 , identifying means  50 , a control device  213  driving each transistor  44 A,  44 B,  44 C,  44 D with a respective command signal SA, SB, DC, SD and a supply current sensor  214  located on a feed branch of the H bridge, just behind the input terminal  36 A to measure the intensity of a supply current IS 1  received on the input terminals. 
     The control device  213  comprises a reception organ  55  and a calculating unit  57  which is different than in the first embodiment. As with the first embodiment, calculating unit  57  may be implemented in hardware, or by means of a programmable computer. 
     The supply current sensor  214  is configured to measure the power signal received on the input terminals and notably the intensity of power signal, i.e. the intensity of the supply current IS 1 . In the following, the intensity of the power signal and the intensity of the supply current IS 1  corresponds to the same thing. 
     Calculating unit  57  is similar to calculating unit  56  and differs only in that it further takes into account the measured intensity of the supply current IS 1 , to provide optimized control of the transistors  44 A,  44 B,  44 C,  44 D. 
     Calculating unit  57  is configured to drive each transistor  44 A,  44 B,  44 C,  44 D, i.e. to generate each respective command signal SA, SB, SC, SD, in function of the intensity measured by the motor current sensor  48  and of the intensity measured by the supply current sensor  214  and, advantageously, also in function of the predetermined motor current limit setpoint CLSM 1 , to regulate the value of the intensity of the motor current IM 1  supplied to the electric motor  34 . 
     More especially, as shown on  FIG. 7 , calculating unit  57  is configured to drive each transistor  44 A,  44 B,  44 C,  44 D in function of the intensity of the motor current IM 1  and of the intensity of the supply current IS 1 , providing a feedback control of the motor through a first feedback loop based on the intensity of the motor current IM 1  and a second feedback loop based on the intensity of the supply current IS 1 . The control device  213  is thus a cascade controller. 
     In other words, the control device  213  comprises a first feedback loop on the intensity of the motor current and a second feedback loop on the intensity of the supply current so as to form a cascade controller. 
     Calculating unit  57  is configured to calculate a supply current setpoint CS 1  in function of the measured intensity of the motor current IM 1  and of the predetermined motor current setpoint CM 1 , and to command each transistor  44 A,  44 B,  44 C,  44 D in function of the measured intensity of the supply current IS 1  and the supply current setpoint CS 1 , in order to maintain the value of the intensity of the supply current IS 1  equal to the value of the supply current setpoint CS 1  and to limit the value of the intensity of the motor current IM 1  to the value of the predetermined motor current setpoint limit CLSM 1 . 
     The supply current setpoint CS 1  and the command signals SA, SB, SC, SD are calculated using, for example, two proportional-integral-derivative controllers  218 ,  219  (PID controller) shown on  FIG. 8 . 
     More precisely, the proportional-integral-derivative controller  218  calculates the value of the supply current setpoint CS 1  as a function of the measured value of the intensity of the motor current IM 1  and the value of the predetermined motor current limit setpoint CLSM 1 . 
     The proportional-integral-derivative controller  219  calculates the command signals SA, SB, SC, SD as function of the measured value of the intensity of the supply current IS 1  and of the value of the supply current setpoint CS 1 . 
     In both embodiments of the invention, the control of transistors  44 A,  44 B,  44 C,  44 D to control the motor current allows limiting the motor torque applied to the displacement element  28  to avoid any damage of the switch machine  14 . 
     Besides, the fact to control the intensity of the motor current IM 1  with transistors  44 A,  44 B,  44 C,  44 D which are switching provides a fast and reliable control of the motor current IM 1 . 
     Moreover, the use of the internal inductance of the electric motor  34  as an inductor for the buck DC-DC converter, allows to avoid the use of a supplementary independent inductor in the motor assembly  26 , which for the current required, e.g. 20 A, is generally a large, heavy, expensive and relatively fragile inductor. 
     Furthermore the use of the regulation unit  38  allows an optimal functioning of the switch machine  14  with a limited maintenance. More especially it allows a switch machine  14  to be built without a mechanical torque-limiting clutch adapted to slip when a torque produced by the electric motor  34  exceed a predetermined value. Such a clutch is expensive, subject to wear, and requires regular maintenance. 
     Finally, the use of a pulse width modulation command signal having a duty cycle different than 100% in the ending state allows increasing the current going through the motor  34  while limiting the intensity of the supply current IS 1  supplied to the motor assembly  26 . In the example of  FIGS. 2 and 7 , when the duty cycle of the command signal SA is equal to 0.3, the intensity of the supply current IS 1  is, for example, equal to 6 A whereas the intensity of the motor current IM 1  is equal to 20 A. Therefore the current going from the DC power supply  41  to the motor assembly  26  is limited which allows to optimize the operating life and to minimize the required maintenance of the DC power supply  41 . 
     In the second embodiment, the use of two feedback loops allows to smoothly control the supply current IS 1  as well as motor current IM 1 . The two feedback loops are, for example, proportional-integral-derivative loops. The two feedback loops are a supply current control loop also called the inner loop and a motor current control loop also called the outer loop. 
     Indeed, it is important to smoothly control the supply current IS 1 , because the supplying cables  42 A,  42 B are generally long cables, up to hundreds of meters in length, which implies that the installation could present a relatively large inductance between the power supply  41  and the input terminals  36 A,  36 B. In this case, rapid fluctuations in supply current applied across the large cable inductance would produce large voltage fluctuations at the input terminals  36 A,  36 B, which could cause instability in the control loop or damage the controller. As shown on  FIG. 8 , the supply current control loop regulates the current to supply current setpoint CS 1 , which limits the fluctuations of supply current IS 1 . 
     The second embodiment of the invention uses a cascade control loop to perform the two important functions required of the controller, which are limiting the intensity of the motor current to a predetermined setpoint CLSM 1 , and regulating the intensity of the supply current to a relatively slowly varying setpoint CS 1 . The invention described uses a simple cascade control loop to achieve these functions. 
     Other well-known classical and modern control techniques could also be utilized to implement these functions. For example a state-space controller could be used in place of the cascade controller. 
     The embodiments and variants discussed above are suitable for being combined with one another wholly or partially to give rise to other embodiments of the invention.