AUTOSHUTTLE
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RENDEZVOUS-MANOEUVRE  WITH  SYNCHRONOUS  LONG-STATOR-MOTOR

Dr. Andreas Steingröver  /  Dr. Rasmus Krevet

1 INTRODUCTION,    2 ARCHITECTURE,    3 OPERATION,    4 CONCLUSION,    AUTHORS,    REFERENCES

The rendezvous-manoeuvre is a prerequisite for effective operation of a track guided system involving transportation of road vehicles in individual cabins. Feasibility is demonstrated on the basis of a long-stator-synchronous-linear-drive. Vehicle position monitoring is achieved with wayside magnetic field sensors communicating data to a vital motor control unit MCU via a bus. A MCU controls the respective section of the linear drive. The vital control system is entirely wayside based and achieves high levels of performance, safety and reliability. A simulation model including the dynamics of the motor current control is presented. Copyright 2000 IFAC

Keywords: Traffic control, Train control, Vehicle dynamics, Automated guided vehicles, Traction control, Synchronous motors system, Linear motors, Magnetic Suspension, Safety analysis.

1 INTRODUCTION

In track guided transport research the rendezvous-manoeuvre, i.e. controlled approach and coupling of individually travelling vehicles at speed has been proposed as a means of enhancing operations for a long time. So far, this has proved to be commercially and technically infeasible. A newly proposed application of the rendezvous-manoeuvre provides transportation of road vehicles in individual track guided cabins which form convoys at speed in order to reduce air resistance as shown in Fig. 1 (Krevet, et al., 1999). The financial and technical feasibility is now highly promising based on the principles outlined below.

Fig. 1. Illustration of Autoshuttle – the application of the rendezvous-manoeuvre

The evolution in railway safety and control systems is directed towards a further reduction of the distance between trains. The rendezvous-manoeuvre is the last step in this development in that at the end of the manoeuvre this distance is null. Approval of such a system is feasible based on the CENELEC standards EN 50126, EN 50128 and EN 50129 with a quantified analysis of the risks and hazard probabilities.

The main safety requirement of the safety system controlling the rendezvous-manoeuvre is that in case of an unexpected delay of the vehicle participating in the rendezvous and running in front of another vehicle, this delay must be transferred within a specified time to the back vehicle if the front vehicle cannot regain its scheduled position and speed. In the case of absence of position information from the front vehicle for longer than a specified tolerance interval, the back vehicle must be decelerated.

2 ARCHITECTURE

The proposed architecture provides long-stator-synchronous-linear-drive which could conveniently be combined with a magnetic levitation system for suspension and guidance (Weh, et al., 1993). The long-stator is divided into individually controlled sections (Dreimann, 1989), where the length of the motor sections varies as necessary for the rendezvous-manoeuvre. The length of the motor sections at the location of the rendezvous is less than the length of a vehicle. In the last phase before closing up, just a fraction of the vehicle’s length covers energised motor sections. In non-rendezvous areas the sections might be much longer, limited only by the headway and the train or convoy length.

The principle is schematically demonstrated in Fig. 2. In a rendezvous-manoeuvre a vehicle 1 runs behind another vehicle 2 on a track 3 equipped with long-stator motor segments. The segments are shown as rectangular boxes. At speed, both vehicles 1 and 2 will approach each other and combine to form a unified train or convoy.
 

Fig. 2. Phases of the rendezvous-manoeuvre


 The rendezvous-manoeuvre can be divided in phases a) to d) as depicted in Fig. 2. Phase a) is the start of the rendezvous, when both vehicles have reached their initial positions. In phase b) an approach of the vehicles is made with full coverage of the vehicle’s reactive motor parts by active motor sections. However, in phase c) only fractions of the vehicle are covered ba active motor sections until the rendezvous takes place in phase d).

Due to the advantageous wayside control of vehicle speed in a long-stator configuration the safety system can be installed entirely wayside as well (Cießow, et al., 1989). Magnetic field sensors 4 are mounted in regular intervals along the motor sections including one sensor at the beginning and the end of the motor section respectively. The maximum separation between these sensors is the minimum length of a vehicle. The sensors 4 transmit the received signal to a motor control unit MCU 5 via direct cables. All MCUs are interconnected by a bus 6.

The bus 6 extends over the track sections in which a rendezvous-manoeuvre will take place. The actual extent of this controlled area must cover at least the worst case stopping distance backwards from the front vehicle 2 in order to be able to resume a safe state in all considered operational conditions, i.e. stop the rear vehicle 1 in time in the case for a standing front vehicle 2. Typical forms of the controlled area are linear shapes or Y-shapes in the case for a station with a passing track. The vehicles 1 and 2 come from different origins and then enter a merging track.

3 OPERATION

In actual operation the sensors 4 detect the magnetic field of the reactive motor part at the front ends of  the vehicles 1 and 2 travelling over the sensors 4. Detection of a vehicle is reported to the MCU 5, which controls the motor current according to the movement of the vehicle. The MCU 5 transmits the vehicles 1 and 2 position, speed and acceleration information to the adjacent MCUs. The adjacent MCU 5 in the direction travel calculates what the required initial speed and acceleration must be.

The motor is deactivated as long as neither vehicle 1 nor 2 passes over it. On detection of a vehicle at the edge positioned sensors 4 the long-stator is activated. On detection of a second vehicle group entering a motor section at different speed from the first vehicle group the motor section is deactivated.

The rendezvous takes place according to the following control schemes:

Parameter limitation:

Standard operation: Actions in case of deviations:


3.1 Simulation of the vehicle position and speed
 

Fig. 3. Simplified simulation model of a vehicle, the motor and the control loops


 Fig. 3 shows the simplified simulation model of a vehicle control in form of a control block diagram. The corresponding differential equations with addional control algorithms were used to investigate the rendezvous control of the two vehicles drive by a long stator motor in regular, irregular and failure scenarios.

The vehicle dynamic including the aerodynamic drag is considered by the integrators, where the friction force Ffr which includes the aerodynamic effects is subtracted from the motor thrust Fm.

The detailed motor structure including the motor current control of the different sections is represented by the non-linear block in the centre. However, its dynamic characteristics including position sensors, current control and power inverter is much faster than the vehicle and the speed and position controllers. It has therefore an insignificant influence on the rendezvous-manoeuvre.

The cascaded con-trollers for speed and position are fed by a reference position generator for sref which is triggered by the position of the other vehicle respectively

Fig. 4. Position time diagram s(t) of a regular rendezvous manoeuvre

Fig. 4 depicts the standard operation of the rendezvous-manoeuvre in a position-time diagram s(t) of the vehicles 1 and 2 as a result of a simulation. At the beginning of the manoeuvre the back of the front vehicle is positioned at the reference position 1. On detection of the passage of the front of the rear vehicle at reference position 2 the front vehicle starts with a predetermined acceleration value. The rendezvous takes place at reference position 3.

The horizontal lines mark the motor sections which are individually controlled. The length of the motor section become shorter in the area of approach and have standard length behind the reference position 3.

Shaded areas symbolise the time interval when the respective motor sections are activated. The number of motor sections is shown for a minimum convoy length of 200 m for clarity purposes. Shorter minimum convoy lengths require more and shorter motor sections.

Activation of a motor section is performed just by sensing the number of vehicles running on the respective motor section and is independent from the control system which sets the acceleration values of the vehicles. On sensing exactly one vehicle group running on the motor section the respective motor section is activated. From this control scheme evolves the non shaded areas between two activated motor sections. These motor sections sense two vehicle groups travelling at different speeds.

Fig. 5. Position time diagram of a rendezvous manoeuvre with failed motor section

Fig. 5 depicts the results of a simulation of an operation with a failed motor section in a position-time diagram s(t). The corresponding speed-time diagram v(t) is printed in Fig. 6  The movement of the front vehicle 1 shows a non-accelerated phase. For clarity purposes it is shown as an immediate interruption of propulsion.

The MCU at this location reports a delay of the front vehicle 1 to the MCU controlling the motor section of the rear vehicle 2. After a reaction time of the safety system the rear vehicle starts to decelerate until it also reaches the defective motor section. A small deceleration in the area of the failed motor section occurs due to the aerodynamic drag.

The front vehicle starts to accelerate again but is no longer able to reach the cruising speed at reference position 3. The rear vehicle then remains for a while at the lower speed until it has to accelerate to the new rendezvous speed.

Fig. 6. Speed time diagram of a rendezvous manoeuvre with failed motor section

The acceleration time is calculated in a way that the new rendezvous speed of the front vehicle 1 at reference position 3 is met. After rendezvous the whole convoy continues to accelerate to the cruising speed.

Principally, a malfunction of a motor section is not harmful in the area where the rendezvous takes place because of the low lengths of the motor sections. The decentralized structure privides a high level of fault tolerance and therefore enables reliable operation.

3.2 Simulation of the motor control

The motor thrust is proportional to the applied effective motor current, if a vector control for opti-mum pole angle is assumed. The speed controller commands current amplification on increasing travel resistance until the allowable maximum current is met.

The current capacity of the motor has to be dimen-sioned so as to guarantee the nominal acceleration value even with the maximum travelling resistance, i.e. maximum vehicle mass and maximum front wind. This control scheme allows maintaining the precalculated regular vehicle motion in virtually all circumstances.

Also, applied thrust is proportional to the vehicle’s length covering activated motor sections. This relation becomes evident during the last phases of the rendezvous when the distance between the two vehicles is less than the length of the motor sections in this area.

Fig. 7. Motor currents of long stator sections in the area of rendezvous

Fig. 7 schematically shows the effective motor currents of the different sections against time during the last movement phases before the rendezvous takes place. The motor sections in this area are then activated and deactivated according to the principles explained above.

The necessary thrust is only applied to a fraction of the vehicle’s length and the motor current has to be increased accordingly. The required factor is commanded from the MCU which detects two vehicles on its section to the neighbouring MCUs. These increase the motor currents accordingly so that a constant thrust for the vehicles is a achieved.

Inversely, the decrease of the vehicle’s acceleration value in this phase diminishes the motor current demand. Consequently the amplitude of the applied motor current is controlled by the MCUs dependent on the vehicle length coverage ratio, the actual speed and the required acceleration value. So the rear vehicle 2 requires only a little motor current to maintain cruising speed, whereas the front vehicle 1 operates at maximum motor current for full acceleration.

4 CONCLUSION

The presented architecture of the long-stator-synchronous-linear-drive based safety and control system provides safe and reliable control of the rendezvous principle. No communication with the vehicles is necessary. The control schemes allow the rendezvous to take place at a fixed track section so that the area with the short motor sections can be restricted. An economically and ecologically promising application thus can be based on the principles for a rendezvous-manoeuvre based on a synchronous long-stator motor as outlined above.

AUTHORS

Dr. Ing. Andreas Steingröver:
Born in 1962. Studies and PhD in electrical engineering in Braunschweig, Germany at the Institute for Electric Machines, Drives and Track Guided Systems. Realisation of a magnetically levitated experimental vehicle with transversal flux levitation and guidance system. Since 1996 working in railway signalling.

Dr. rer.nat. Rasmus Krevet:
Born in 1963. Studies and PhD in Physics in Mayence and Braunschweig, Germany. Studies and publications on innovative track guided systems since 1994. Since 1997 working in railway signalling.

REFERENCES

Cießow G., R. Friederich, H. Hochbruck and G. Holzinger. Der Linearmotor und seine Energieversorgung. In: Magnetbahn Transrapid, 50-59. Hestra-Verlag, Darmstadt

Dreimann K. M-Bahn Maglev Transit System Experience, Status, Application. Proceedings of 11th International Conference on MAGLEV-Yokohama, MAGLEV 1989

Krevet R., A. Steingröver. Magnetbahn mit starrer Weiche. Offenlegungsschrift DE19923161A1, Deutsches Patent- und Markenamt 1999

Weh H., A. Steingröver and H. Hupe. Maglev Transportation with Controlled Permanent Magnets and Linear Synchronous Motors. Proceedings of 13th International Conference on MAGLEV-Argonne, MAGLEV 1993.