The present invention relates to an electronic system for remotely controlling a locomotive. The system is particularly suitable for use in switching yard assignments.

Economic constraints have led railway companies to develop portable units allowing a ground based operator to remotely control a locomotive in a switching yard. The unit is essentially a transmitter communicating with a slave controller on the locomotive by way of a radio link. Typically, the operator carries this unit and can perform duties such as coupling and uncoupling cars while remaining in control of the locomotive movement at all times. This allows for placing the point of control at the point of movement thereby potentially enhancing safety, accuracy and efficiency.

Remote locomotive controllers currently used in the industry are relatively simple devices that enable the operator to manually regulate the throttle and brake in order to accelerate, decelerate and/or maintain a desired speed. The operator is required to judge the speed of the locomotive and modulate the throttle and/or brake levers to control the movement of the locomotive. Therefore, the operator must posses a good understanding of the track dynamics, the braking characteristics of the train, etc. in order to remotely operate the locomotive in a safe manner.

An object of the invention is to provide a remote control system allowing the operator to command a desired speed and responding by appropriately controlling the throttle or brake to achieve and maintain that speed.

Another object of the invention is to provide a remote locomotive control system allowing for control of the locomotive from one of two different transmitters.

Yet another object of the invention is to provide a remote locomotive control system having the ability to perform a number of safety verifications in order to automatically default the locomotive to a safe state should a malfunction be detected.

As embodied and broadly described herein the invention provides a locomotive remote control system. The system has

• • a transmitter capable of generating a binary coded radio frequency signal representing commands to be executed by the locomotive and
  • a slave controller for mounting on-board the locomotive.

The slave controller has

• • a) a receiver for sensing the radio frequency signal;
  • b) a processor for receiving the radio frequency signal; and
  • c) a velocity sensor for generating data representing velocity of the locomotive. The processor responds to the velocity sensor and to the RF signal to actuate either one of a brake of a locomotive or a tractive power of the locomotive in order to attempt maintaining a requested speed.

As embodied and broadly described herein the invention also provides a locomotive control system which has

• • a) a transmitter for generating a binary coded RF signal; and
  • b) a slave controller mounted on-board the locomotive for receiving that signal, the slave controller selectively accepting commands from a first transmitter or from a second transmitter.

As embodied and broadly described herein the invention further provides a remote control system for a locomotive which has

• • a) a transmitter for generating an RF binary coded signal; and
  • b) a slave controller mounted on-board the locomotive.

The slave controller includes

• • a first sensor responsive to pressure of compressed air in a main tank of the locomotive; and
  • a second sensor responsive to flow of compressed air in a pneumatic brake line. The slave controller responds to output of the sensors to enable application of tractive power to the locomotive only when a pressure in the main tank is above a predetermined level and a flow of air in the brake line is below a predetermined level.

With reference to the annexed drawings, the locomotive control system in accordance with the invention includes a portable transmitter 10 which generates a digitally encoded radio frequency (RF) signal to convey commands to a slave controller mounted on-board the locomotive. The slave controller decodes the transmission and operates various actuators on the locomotive to carry into effect the commands remotely issued by the operator.

FIGS. 1 to 4 illustrate the physical layout of the portable transmitter 10. The unit comprises a housing 12 enclosing the electronic circuitry and a battery supplying electric power to operate the system. A plurality of manually operable levers and switches projecting outside the housing 12 are provided to dial-in locomotive speed, brake and horn settings, among others. The various controls on the portable transmitter are defined in the following table:

A detailed description of the various functions summarized in the above table is provided later in this specification.

On the top surface of the housing 12 is provided a display panel 34 that visually echoes the control settings of the portable transmitter 10. The display panel 34 includes an array of individual light sources 36, such as light emitting diodes (LED), corresponding to the various operative conditions of the locomotive that can be selected by the operator. Hence, a simple visual observation of the active LED's 36 allows the operator to determine the current position of the controls.

FIG. 5 provides a functional diagram of the portable transmitter 10. The various manually operable switches and levers briefly described above are constituted by electric contacts whose state of conduction is altered when the an opened condition. The multi-position levers 14 and 16, and the multi-position switches 20 and 26, have a set of electric contact pairs, only a single pair being closed at each position of the lever or switch. By reading the conduction state of the individual electric contact pairs, the commands issued by the operator can be determined.

An encoder 38 scans at short intervals the state of conduction of each pair of contacts. The scan results allow the encoder to assemble a binary locomotive status word that represent the requested operative state of the locomotive being controlled. The following table provides the number of bits in the locomotive status word required for each function:

The locomotive status word also contains an identifier segment that uniquely represents the transmitter designated to control the locomotive. The purpose of this feature is to ensure that the locomotive will only accept the commands issued by the transmitter generating the proper identifier.

Most preferably, the encoder 38 includes a microprocessor programmed to intelligently assemble the locomotive status word. The microprocessor continuously scans the electric contacts of the transmitter controls and records their state of conduction. On the basis of the identity of the closed contacts, the program will produce the function component of the locomotive status word which is the string of bits that uniquely represents the functions to be performed by the locomotive. The program then appends to the function component the locomotive identifier component and preferably a data security code enabling the receiver on-board the locomotive to check for transmission errors.

In a different form of construction, the encoder may be constituted by an array of hardwired logic gates that generate the locomotive status word upon actuation of the controls.

A transmitter 40 receives the locomotive status word and generates an RF signal for transmission of the coded sequence by frequency shift keying. In essence, the frequency of a carrier is shifted to a first value to signal a logical 1 and to a second value to signal a logical 0. The transmission protocol is best shown in FIG. 6. Each transmission begins with a burst of the carrier frequency 42 for a duration of eight (8) bits (the actual time frame is established on the basis of the transmission baud rate allowed by the equipment). Each bit of the data stream is then sent by shifting the frequency to the first or the second value depending on the value of the bit, during a predetermined time slot 44.

The transmitter 40 sends out the locomotive status word in repetition at a fixed rate selected in the range from two (2) to five (5) times per second. By providing the transmitter with a unique repetition rate, the likelihood of transmission errors is reduced when several portable transmitters in close proximity broadcast control signals to individual locomotives, as described below.

FIG. 7 provides a diagrammatic representation of the slave controller mounted on board the locomotive. The slave controller identified comprehensively by the reference numeral 46 has three main components, namely a receiver unit 48, a processing unit 50 and a driver unit 52. More particularly, the receiver unit 48 senses the locomotive status word sent out from the portable transmitter 10, decodes the transmission and supplies the resulting binary sequence to the processing unit 50. To achieve a reliable communication link, the receiver 48 is synchronized with the transmitter 40 at three different levels. First, the receiver circuitry defines a signal acceptance window that opens itself at the rate at which the locomotive status word is sent out by the respective controlling transmitter 40. Second, the receiver 48 will observe the frequency value of the transmission in order to decode the binary sequence at intervals precisely corresponding to the time slots 44. Third, the acceptance window opens in phase with the signal transmission.

The first two levels of synchronization are established through hardware design, by setting the transmitter 40 and the receiver 48 to the same period of transmission/reception. On the other hand, the phasing of the receiver to the incoming locomotive status word transmission is effected through observation of the burst of carrier frequency 42 that begins each transmission cycle. The diagram in FIG. 8 graphically illustrates the relationship between the signal transmission and the signal reception. The time line 54 shows the successive transmission of the locomotive status word as a series of blocks 56. The activity of the receiver 48 is shown on the time line 58. The hatched areas correspond to the time intervals during which the receiver is not listening. At time t=0 the first locomotive status word is sent out by the transmitter 40. The burst of the carrier frequency 42 is sensed by the receiver 48 which then activates the sequence of opening and closing of the signal acceptance window which is fully synchronized (in period and phase) with the signal transmission.

This characteristic is particularly advantageous when several transmitters broadcast simultaneously control signals to different locomotives in close proximity to one another. By setting each transmitter (and the companion receiver) at a unique transmission/reception period, secure communication links can be maintained even when all the transmitters use the same carrier frequency. FIG. 9 illustrates this feature. Time line 60 shows the transmission pattern of a first portable transmitter. The time line 62 depicts the window of acceptance of the companion receiver. The numeral 64 identifies the transmission pattern of a second portable transmitter. Assuming that both portable transmitters are actuated exactly at t=0, the signal received during the first opening of the window of acceptance will be corrupted since two locomotive status word transmissions are concurrent in time. However, the third and the seventh locomotive status word transmissions from the first portable transmitter will be clearly received since there is no overlap with the locomotive status words sent out by the second portable transmitter. Hence the purpose of providing each transmitter with a unique signal repetition rate reduces the likelihood of transmission conflicts.

It should be noted that the receiver 48 can, and probably will, correctly receive from time to time a locomotive status word from an unrelated transmitter. This status word will be rejected, however, because the transmitter identifier will not match the value stored in the memory of the slave controller.

The transmitter/receiver gear of the remote locomotive control system has been described above in terms of function of the principal parts of the system and their interaction. The components and interconnections of the electric network necessary to carry into effect the desired functions are not being specified because such details are well within the reach of a man skilled in the art.

FIG. 10 provides a functional diagram of the processing unit 50. A central processing unit (CPU) 66 communicates with a memory through a bus 70. A reserved portion memory 68 contains the programm that directs the CPU 66 to control the locomotive depending on the several inputs that will be discussed later. The memory also contains a section allowing temporary storage of data used by the CPU when handling hardware events.

The current locomotive status and the commands issued from the remote transmitter are directed to the CPU through an interface 72 communicating with the bus 70. The interface 72 receives input signals from the following sources:

• • a) A speed direction sensor 74 providing locomotive velocity and direction of movement data;
  • b) A speed sensor 76 providing solely locomotive velocity data. The speed sensor 76 provides the CPU 66 with redundant velocity data allowing the CPU 66 to detect a possible failure of the main speed sensor 74.
  • c) A pressure sensor 78 observing the air pressure in the locomotive brake system;
  • d) A pressure sensor 79 observing the air pressure in the main reservoir;
  • e) A pressure sensor 80 observing the air pressure in the train brake system;
  • f) A sensor 82 observing the flow rate of air in the brake system of the train; and
  • g) The decoded locomotive status word generated by the receiver 48.

The structure of the speed/direction sensor 74 is illustrated in FIGS. 11 and 12. The sensor includes a disk 84 mounted to an axle 86 of the locomotive. When the locomotive is moving the disk 84 turns at the same angular speed as the axle 86. The disk 84 is provided with a layer of reflective coating 85 deposited to form on the periphery of the disk equidistant and alternating reflective zones 87 and substantially non-reflective zones 89. A pair of opto-electric sensors 92 and 94 are mounted in a spaced apart relationship adjacent the periphery of the disk 84. The sensor 92 comprises an emitter 92a generating a light beam perpendicular to the plane of the disk 84, and a receiver 92b producing an electrical signal when sensing the reflection of the light beam on the reflective zones 87. However, when a substantially non-reflective surface 89 registers with the sensor 92, the output of the receiver is null or very low. The structure and operation of the opto-electric sensor 94 is identical to the sensor 92. Thus, the sensor 94 comprises an emitter 94a and a receiver 94b.

The spacing between the opto-electric sensors 92 and 94 is such that they generate output pulses due to the periodic change in reflectivity of the disk surface, occurring at different instants in time. As best shown in FIG. 10, and assuming that the disk 84 rotates in the counter clockwise direction, when the sensor 92 switches on as a result of a reflective zone 87 registering with the emitter 92a and the receiver 92b, the sensor 94 is still in a stable on condition and can be caused to switch off only by further rotating the disk 84.

Preferably, the disk 84 and the sensors 92 and 94 are mounted in a hermetically sealed housing to protect the assembly against contamination by water or dirt.

FIG. 13 illustrates the signal waveforms produced by the opto-electric sensors 92 and 94. Both outputs are pulse trains having the same frequency but out of phase by an angle α which depends upon the spacing of the sensors 92 and 94. When the locomotive moves forward the disk 84 rotates in a given direction, say clockwise. In this case, the pulse train from sensor 94 leads the pulse train from sensor 92 by angle α. When the locomotive is in reverse, then the output of sensor 92 leads the output of sensor 94 by angle α (this possibility is not shown in FIG. 13). The processing unit 50 observes the occurrence of the leading pulse edges from the sensors 92 and 94 with relation to time to determine the identity of the leading signal, which allows derivation of the direction of movement of the locomotive.

Velocity data is derived by measuring the rate of fluctuation of the signal from any one of sensors 92 and 94. It has been found practical to determine the velocity at low locomotive speeds by measuring the period of the signal. However, at higher speeds the frequency of the signal is being measured since the period shortens which may introduce non-negligible measurement errors.

The speed sensor 76 is similar to sensor 74 described above with two exceptions. First, a single opto-electric sensor may be used since all that is required is velocity data. Second, the speed sensor 76 is mounted to a different axle of the locomotive.

The pressure sensors 78 and 79 are switches mounted to the main reservoir and to the pneumatic line that supplies working fluid to the locomotive independent braking mechanism, and produce an electric signal in response to pressure. These sensors merely indicate the presence of pressure, not its magnitude. In essence, each sensor produces an output when the air pressure exceeds a preset level, indicating whether the reserve of compressed air is sufficient for reliable braking. Unlike the sensors 78 and 79, the pressure sensor 80 is a transducer that generates a signal indicative of presence and magnitude of pressure in the train brake air line.

The airflow sensor 82 observes the volume of air circulating in the pneumatic lines of the train brake system. The results of this measurement along with the output of pressure sensor 78 provide an indication of the state of charge of the pneumatic network. It is considered normal for a long pneumatic path to experience some air leaks due primarily to imperfect unions in pipe couplings between cars of the train. However, when a considerable volume of air leaks, the airflow sensor 82 enables the processing unit to sense such condition and to implement corrective measures, as will be discussed later.

The interface 72 receives the signals produced by the sensors 74, 76, 78, 79, 80, and 82 and digitizes them where required so they can be directly processed by the CPU 66. The locomotive status word issued by the receiver 48 requires no conversion since it is already in the proper binary format.

The binary signals generated by the CPU 66 that control the various functions of the locomotive are supplied through the bus 70 and the interface 72. The following control signals are being issued:

• • a) A signal 98 to set the lights of the locomotive to off/low intensity/high intensity. The signal is constituted by one (1) bit, each operative condition of the locomotive lights being represented by a different bit state;
  • b) A two (2) bit signal 100 to operate the bell or the horn of the locomotive;
  • c) A five (5) bit signal 102 for traction control. Four bits are used to communicate the throttle settings (only eight (8) settings are possible) and one bit for the power contacts of the electric traction motors;
  • d) An eight (8) bit signal 104 for train brake control. The number of bits used allows 256 possible brake settings; and
  • e) A seven (7) bit signal 106 for independent brake control. The number of bits used allows 128 possible brake settings.

The interface 72 will convert at least some of the signals 98, 100, 102, 104, and 106 from the binary form to a different form that the devices at which the signals are directed can handle. This is described in more detail below.

The actuators for the lights and bell/horn are merely switches such as relays or solid state devices that energize or de-energize the desired circuit. The interface 72, in response to the CPU 66 instruction to set the lights/bell/horn in the desired operative position, will generate an electric signal that is amplified by the driver unit 52 and then directed to the respective relay or solid state switch.

With regard to the traction control it should be noted that most locomotive manufacturers will install on the diesel/electric engine as original equipment a series of actuators that control the fuel injection, power contracts and brakes among others, hence the tractive power that the locomotive develops. This feature permits coupling several locomotives under control of one driver. By electrically and pneumatically interconnecting the actuators of all the locomotives, the throttle commands the driver issues in the cab of the mother engine are duplicated in all the slave locomotives. The locomotive remote control system in accordance with the invention makes use of the existing throttle/brake actuators in order to control power. The interface 72 converts the binary throttle settings issued by the CPU 66 to the standard signal protocol established by the industry for controlling throttle/brake actuators. This feature is particularly advantageous because the locomotive remote control system does not require the installation of any throttle/brake actuators. As in the case of the lights and bell/horn signals 98 and 100, respectively, the traction control signal 102 incoming from the interface 72 is amplified in the driver unit 52 before being directed to the throttle/brake actuators.

The train brake control signal 104 issued by the interface 72 is an eight (8) bit binary sequence applied to a valve mounted in the train brake circuit to modulate the air pressure in the train line that controls the braking mechanism. The working fluid is supplied from a main reservoir whose integrity is monitored by the pressure sensor 79 described above. The independent locomotive brake is controlled in the same fashion with binary signal 106.

The operation of the locomotive control system will now be described with more detail.

SPEED CONTROL TASK

The flowchart of the speed control logic is shown in FIGS. 14a to 14d. The program execution begins by reading the velocity data generated from sensors 74 and 76 that are mounted at different axles of the locomotive. The data gathered from each sensor is stored in the memory 68 and then compared at step 124. If both sensors are functioning properly they should generate identical or nearly identical velocity values. In the event a significant difference is noted the CPU 66 concludes that a malfunction exists and issues a command (step 126) to fully apply the independent brake in order to bring the locomotive to a complete stop.

Assuming that no mismatch between the readings of sensors 74 and 76 is detected, the CPU 66 will compare the observed locomotive speed with the speed requested by the operator. The later variable is represented by a string of three (3) bits in the locomotive status word (the flowchart of FIGS. 14a to 14d assumes that the locomotive status word has been correctly received, has the proper identifier and has been stored in the memory 68). The operator can select on the portable transmitter 10 eight possible speed settings, each setting being represented by a different binary sequence. The speed settings are as follows:

• • 1) STOP
  • 2) COAST WITH BRAKE
  • 3) COAST
  • 4) COUPLE (1 MILE PER HOUR (MPH))
  • 5) 4 MPH
  • 6) 7 MPH
  • 7) 10 MPH
  • 8) 15 MPH

If any one of settings 4 to 8 have been selected, which require the locomotive to positively maintain a certain speed, the CPU 66 will effect a certain number of comparisons at steps 128 and 130 to determine if there is a variation between the actual speed and the selected speed along with the sign of the variation, i.e. whether the locomotive is overspeeding or moving too slowly. More particularly, if at step 128 the CPU 66 determines that the observed speed is in line with the desired speed no corrective measure is taken and the program execution initiates a new cycle. On the other hand, if the actual speed differs from the setting, the conditional test 130 is applied to determine the sign of the difference. Under a negative sign, i.e. the locomotive is moving too slowly, the program execution branches to processing thread A (shown in FIG. 14b). In this program segment the CPU 66 will determine at step 132 the velocity error by subtracting the actual velocity from the set point contained in the locomotive status word. A proportional plus derivative plus integral algorithm is then applied for calculating throttle setting intended for reducing the velocity error to zero. Essentially the CPU 66 will calculate the sum of the integral of the velocity error signal (calculated in step 145), of the derivative of the velocity error signal (calculated in step 147), and of a proportional factor (calculated in step 143). The latter is the velocity error signal multiplied by a predetermined constant. The result of this calculation provides a control signal that is used for modulating the throttle actuator of the locomotive through output signal 102 of the interface 72.

FIG. 15a is a diagram illustrating the variation of the current velocity signal, the set point, the velocity error, the velocity error integral, the velocity error derivative and velocity error proportional with respect to time.

With reference to FIG. 14d, when the new throttle setting has been implemented the program execution continues to steps 134 and 136 when the current direction of movement and speed of the locomotive are determined from the reading of sensor 74. In the event the CPU 66 observes a zero speed value for a time period of more than 20 seconds in spite of the fact that a tractive effort is being applied (step 138), it declares a malfunction and fully applies the independent locomotive brake. Normally, when a tractive effort is applied it causes the locomotive to accelerate. The movement, however, may occur after a certain delay following the application of the tractive effort especially if the locomotive is pulling a heavy consist. Still, if after a certain time period no movement is observed, some sort of malfunction is probably present. One possibility is that both sensors 74 and 76 have failed and register zero speed even when the locomotive is rolling. This is highly unlikely but not impossible. When such condition is encountered the CPU 66 immobilizes the locomotive immediately upon determination that a fault is present.

The 20 seconds waiting period before application of the independent brake is implemented by verifying the velocity data from sensor 74 during a certain number of program execution cycles. For instance, the current velocity value is compared to the velocity value observed during the previous execution cycle that has been stored in the memory 68. If a change is noted, i.e. the locomotive moves, then the step 138 is considered to have been successively passed. If, however, after 200 execution cycles that require about 20 seconds to be completed, no change with the previously observed velocity value is noted, the independent brake is fully applied.

Assuming that motion of the locomotive is detected at step 138, the program proceeds to step 140 where the direction of movement of the locomotive read from the output of sensor 74 is compared to the direction of movement specified by the operator. This value is represented by a four (4) bit string in the locomotive status word. If the locomotive is moving rearwardly while the operator has specified a forward movement, the CPU 66 detects a condition known as “rollback”. Such condition may occur when the locomotive is starting to move upwardly on a grade while pulling a heavy consist. Under the effect of gravity the train may move backward for a certain distance until the traction system of the locomotive has been able to build-up the pulling force necessary to reverse the movement. During a rollback condition the electric current in the traction motors of the locomotive increase beyond safe levels. Hence it is desirable to limit the rollback in order to avoid damaging the hardware. The program is designed to tolerate a rollback condition for no longer than 20 seconds. If the condition persists beyond this time period the independent brake is fully applied. The 20 seconds delay is implemented by comparing the evolution of the results of the comparison step 140 with the results obtained during the previous execution cycle; if the results do not change for 200 program execution cycles that require about 20 seconds of running time on the CPU 66, a fault is declared and the brake applied.

In the case where both tests 136 and 140 are successively passed, i.e. the locomotive is moving in the selected direction, the program execution returns to the beginning of the cycle as shown in FIG. 14a.

Referring back to step 130, if the conditional branch points toward processing thread B (see FIGS. 14a and 14c), which means that the locomotive is overspeeding, then the CPU 66 will calculate at step 142 the difference between the selected speed and the observed speed. The resulting error signal is then processed by using the proportional plus derivative plus integral algorithm described above to derive a new throttle setting. If by controlling the throttle (reducing the tractive effort developed by the engine) speed correction cannot be achieved, the brake is applied. The brake is modulated by using a proportional plus derivative plus integral algorithm, FIG. 15b illustrates the brake response, along with the actual brake, error, proportional, derivative, and integral signals with relation to time. The calculated brake setting is issued as binary signal 106 (see FIG. 10) that is directed to the braking mechanism on the locomotive.

The STOP, COAST WITH BRAKE and COAST settings will now be briefly described. The STOP setting, as the name implies, intends to bring and maintain the locomotive stationary. When the CPU 66 receives a locomotive status word containing a speed setting corresponding to STOP it immediately terminates the tractive effort and applies the independent locomotive brake at a controlled rate.

The program logic to implement the COAST and COAST WITH BRAKE services is illustrated as flowcharts in FIGS. 16a and 16b, respectively. When the multi-position lever 14 is set to the COAST setting the program reads the velocity data from sensor 74 at step 144 and then compares it at step 146 to the velocity value recorded during the previous program execution cycle. If the consist accelerates under the effect of gravity down a grade (no tractive effort is applied by the system in the COAST and COAST WITH BRAKE settings) the observed velocity will show an increase. The CPU 66 will then apply the independent locomotive brake to slow the consist at step 148. The brake is modulated by using a proportional plus integral plus derivative (PID) algorithm. In the event that no velocity increase is observed the CPU 66 may set (depending upon the control signal resulting from the PID calculation) the independent brake to the release position at step 150 or keep the brake at the current setting.

The next step in the program execution is a test 152 which determines if the speed of the consist is below 0.5 MPH. In the affirmative the movement is stopped by full application of the independent brake at step 154. If the speed of the consist exceeds or is equal to 0.5 MPH then the program returns to step 144.

The COAST WITH BRAKE function, depicted in FIG. 16b is very similar to the COAST service described above. The only difference is that a minimum independent brake pressure of 15 pounds per square inch (psi) is always maintained. At step 156 the acceleration of the consist is determined by comparison of the current velocity with a previous velocity value. If a positive acceleration is observed, such as when the consist moves down a grade, the brake pressure is increased at step 158 (the control is made by a PID algorithm). During the next program execution cycle the acceleration is determined again. If no positive acceleration is sensed the brake pressure is returned to 15 psi at step 160. At step 162 the velocity of the consist is tested against the 0.5 MPH value. If the current speed is less than this limit a full independent brake application is effected in order to stop the consist, otherwise the program execution initiates a new cycle.

EXCHANGE OF COMMAND AUTHORITY BETWEEN REMOTE TRANSMITTERS

In some instances a single operator may effectively and safely control a consist that includes a limited number of cars remaining at all times well within the visual range of the operator. However, when the consist is long two operators may be required, each person being physically close to and monitoring one end of the train. The present invention provides a locomotive control system capable of receiving inputs from the selected one of two or more remote transmitters. In a two-operator arrangement, each person is provided with a portable transmitter 10 able to generate the complete range of locomotive control commands. In order to avoid confusion, however, the slave controller on-board the locomotive will accept at any point in time commands from a single designated transmitter. The only exception is a limited set of emergency and signalling commands that are available to both operators. The control function can be transferred from one transmitter to the other by following the logic depicted in the flowchart of FIGS. 17a and 17b.

Upon reception of a locomotive status word, the CPU will compare the identifier in the word to a list of two or more possible identifiers stored in the memory 68. The list of acceptable identifiers contains the identifiers of all the remote transmitters permitted to assume control of the locomotive. If the identifier in the locomotive status word does not correspond to any one of the identifiers in the list, then the system rejects the word and takes no action. Otherwise, the system will determine what are the requested functions that the locomotive should perform. If the locomotive status word requests application of the emergency brake or sounding the bell or horn, then the system complies with the request. Otherwise (step 179), if a new speed setting is requested for example, the system will comply only if the identifier in the locomotive status word matches a specific identifier in the list that designates the remote transmitter currently holding the command authority. If this step is verified, then the locomotive executes the command unless the command is a request to transfer command authority to another remote controller. The CPU 66 recognizes this request by checking the state of the bit reserved for this function in the locomotive status word. If the state of the bit is 1 (command transfer requested) the program execution continues at step 180 where the CPU 66 will perform a certain number of safety checks to determine if the command transfer can be made in a safe manner. More particularly, the CPU will determine if the locomotive is stopped and if the brake safety checks (to be described later) are verified. If the locomotive is moving or the brake safety checks fail, then no action is taken and the command remains with the portable transmitter currently in control. If this test is passed, then the CPU will monitor the reset bit of all the locomotive status words received that carry an identifier in the list stored in the memory 68 (the reset bit issued by the transmitter currently holding the controls is not considered). If within 10 seconds of the reception of the request to transfer control from the current transmitter the CPU observes a reset bit in the high position, which means that the operator of a remote transmitter in the pool of candidates able to acquire control has depressed the reset button, then the CPU 66 shifts in memory the identifier associated with the reset bit at high to the position of the current control holder. From now on the CPU 66 will accept commands (except the safety related functions of emergency brake and sounding the bell/horn) only from the new authority. The procedure of checking the reset bit is used for safety purposes in order to transfer the control of the locomotive only when the target remote controller has effectively acknowledged acceptance of the control.

If within the 10 seconds no reset bit is set to the high position, the CPU 66 will abort the transfer function and resume normal execution of the program.

BRAKE SAFETY CHECKS

FIG. 18 is a flow chart of a program segment used to identify the state of readiness of the braking system before authorizing movement of the locomotive. When a command is received to move the locomotive forward, the CPU 66 will check the pressure in the main tank that supplies compressed air to both the independent locomotive and to the train brake. If the pressure is below a preset level, the command to move the locomotive forward is aborted and no action is taken. A second verification step is required to allow movement of a locomotive which is a measurement of the flow rate of compressed air in the train brake line. The traction control signal 102 is issued only when the compressed air flow rate is below a predetermined level. As briefly discussed earlier, it is normal for a train brake line to exhibit a certain leakage due to imperfect couplings in unions between cars. However, when this leakage exceeds a predetermined level, either there is a major leak or the system is discharged and it is currently being pumped with air. In both cases the train should not be operated for obvious safety reasons.

The scope of the present invention is not limited by the description, examples and suggestive uses herein as modifications and refinements can be made without departing from the spirit of the invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.