The FLYWAY Propulsion Cars

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Everything should be as simple as possible, but not simpler than that.
(Albert Einstein, 1879-1955)

FlyWay is SwedeTrack System´s own solution to the urban public transportation problem

Anfang his page describes the FLYWAY® propulsion car. Let us start by defining our terminology. The propulsion car is the controlling and driving unit for every vehicle. It is the "invisible" part that is inside the hollow beam.

The carriage is the part that hangs underneath the beam and is transported along by the propulsion carriage.
This carriage could be a passenger cabin if it is meant to take people. Or it could be a flatcar for carrying goods, or some other suitable carrying equipment. Together, the propulsion car and the carriage constitute the beam vehicle or beamcar.

On this page we will look at:

  1. General
  2. What the FLYWAY propulsion car has to do
  3. Connecting the propulsion cars
  4. The Traction motor
  5. Methodes of propulsion
  6. The Swiveling function
  7. Regulating the speed
  8. Braking the car
  9. Design considerations

1. General

Anfang he FLYWAY® propulsion car (figures 1 and 2) can have the following attributes and equipment:

  • A small cross-sectional area relative to the beam, to minimize air-resistance while travelling.

  • Propulsion by means of an electrical motor, which feeds the energy back into the power conduits whenever it is slowing down on its own volition.

  • A mechanical emergency break, which clamps around the bottom flange of the beam.
    It would be controlled automatically by the beamcar´s computer, from the Network Control Center and by the passengers by means of an on-board emergency brake.
  • An elevator motor.

  • A hydraulic or spring-loaded device for pressing the elevator assembly's upper girder attachment against the botttom of the beam.

  • Control mechanism for the twisting arms for those cars that can swivel the carriage horisontally 90 degrees in either direction.

  • A motor that raises and lowers the wheels of the propulsion carriage at the shunting points.

  • A transceiver antenna for communication with the next upcoming node and with the central computer by way of waveguides or (more likely) Bluetooth-technology inside the beam.
Figure 1:1: Cross-sectional view of beam and propulsion car

  • 2 transponders to communicate with sensors in the beams.
  • An addressable computer which controls all the actions of the vehicle, including the automativ doors.

Figure 1:2

The FLYWAY propulsion car is longer than is strictly necessary, and has a slimmed waist (see figure 1:2 above) which is also flexibly jointed. The purpose is of course to:

  • better negotiate the twist and turns of the beam
  • better distribute the weight of the carriage.

The FLYWAY® propulsion cars rely on radar signals and on detectors in the beam to alert them about obstacles. The FLYWAY cabins for PRT-operation are not planned to be longer than about 6 meters. Cabins of 10 meters or more in length (for GRT-operation) might need 2 beam attachments (and thus 2 propulsion cars) in order to keep the cabin properly balanced, as shown to the left in figure 3:1 below. This arrangement makes it trickier to keep the cabin level when the beam slopes.
Figure 1:3

2. What the FLYWAY propulsion car has to do

This is a (incomplete) list of controls and measures that has to be programmed into the propulsion car.
  1. When starting; check that:
    • the lift is retracted
    • the carriage is swivelled forward
    • it´s cleared by the node computer for take-off
    Thereafter, release the parking brake
  2. When accelerating: respecting speed limits that apply on each beam segment
  3. When passing a booking point (in the point-synchronous system); report to the node and abide by its directives
  4. When arriving at its destination:
    • ensure that it is positioned right (with the aid of the sensor in the beam)
    • apply the parking brake
    • lower the lift, and
    • if so directed; swivel the carriage
    • when ground contact is established, release the door locks
  5. If ground is detected at the wrong height or no ground is detected; alert the node; expect human intervention
  6. When at a stop; wait for "clear" signal from the passengers before closing doors
  7. If no "clear" signal at a stop-site comes within 2 minutes, close the doors, up with the carriage and proceed to a human-manned check site nearby (During this uplifting, it should be possible for a human in a cabin to countermand this procedure, whereupon the car will settle at the berth again for another 2 minutes).
  8. If the lift goes down too fast for some reason, this will be detected, and an emergency lift wire will take on the load and brake the lift to "normal" speed.
  1. If the carriage is found to be too heavy when the uplift starts, the lift motor will be cut, and the node will be alerted. The car will wait at the berth for human intervention.
  2. When emergency braking, a procedure that consists of 3 (or 4) simultaneous steps is followed:
    • power to the propulsion engine is cut; the engine starts performing as a generator with a strong braking force
    • if required: the upper girder attachment is clamped onto the underside of the beam
    • if required: the wheels of the car are mechanically braked
    • the passenger cabin might be tipped forward if there are passengers in it.

There are two points that might need elaboration here:

  • When the car is waiting for boarding passengers, it will detect the smart cards that these passengers carry, and it will announce its destination on an internal screen. When a passenger presses the OK-button, the doors will close and the car will take off. If nobody presses this button within 2 minutes, it will close the doors and take off anyhow, towards the announced destination. If the car, before taking off, should be asked manually (by somebody pressing a button) to go down again, it will do so. But it will report this event to the node. Should this be repeated a second (or third) time, the car will take off anyhow, but going to a check point for manual control of the car. It could be children playing with the car, or something similar.

  • If the carriage is found to be too heavy when the uplift starts, the internal screen will announce the situation, and ask excess passengers to get off. Every time somebody prsses the "OK"-button, the lift will make a new attempt to lift the cabin. Each failed attempt will be reported to the node.

3. Connecting the propulsion cars

Two propulsion cars driving the same vehicle should be connected inside the beam
Figure 3:1

Anfang here could be 2 situations when the propulsion cars need to be coupled together (and please note that we are talking about the propulsion cars here, not the beam vehicles):
  • When a cabin or flatcar has 2 suspension points (A in the figure above).

  • When two or more cars are joined together, forming a train (B in the figure).

Long cars might, for stability reasons, need two suspension points, each controlled by a propulsion vehicle. The reasons why those propulsion cars should be joined together inside the beam are mainly two:

  • If the two propulsion cars, for some reason, do not pull together (or breake equally hard), the resulting mechanical strain should be taken up by those propulsion vehicles, rather than being transferred to the cabin (or flatcar) below. This makes for better design, and also simplifies the interface between the propulsion car and the cabin (or flatcar) below.

  • In sloping beams the traction against the beam might be uneven between the two cars. Here, again, that strain should not be transferred to the cabin (or flatcar) below.

Regarding the train situation, the same two reasons would apply. In addition, one could conceive of a situation where one car pulls some other cars that are not equipped with proper propulsion engines. The reason for that might be economics; such cars would be cheaper to manufacture.

While the connection point at A in the figure above is permanent, the connection at B should be automatically controlled by the cars' computers. There would have to be strain indicators mounted at those connection points, signalling whether a proper connection has been attained or the connection has been disengaged, as the case may be. When passageways for passengers are used between the cabins, as in the illustration, the same kind of safety measures would have to apply there. As a rule, however, when there are no passageways between cars, the cabins should not come within touching distance of one another.

Another safety aspect comes into play if these cars are equipped with elevators to lower them to the ground. Then, obviously, all elevators have to operate in concert with one another. This would have to be closely monitored, with safety mechanisms to stop the operation of the elevators if they should get out of synchronization.

4. The Traction Motor

The demands on this motor are:
  • It should be electrical
  • It should be at least 85 % efficient at all times
  • It should be able to accelerate a beamcar at least 4 meters/second2 with maximum load
  • It should be silent
  • It should feed at least 80 % of the braking energy back into the powerlines at normal braking
  • It should be dimensioned to handle the emergency braking required.
(You can read more about propulsion motors on this page)

The beamcar engines are required to vary their speeds all the time during travel. To use gears between the motor and the wheels degrade efficiency, but might have to be used in FLYWAY anyway. What is really needed is a supply voltage that varies it frequency in tune with the motor´s rotational speed, so much so that the motor can cover the whole range of speeds for the vehicle without having to use gears.

Inverted rectifiers for speed regulation have been around for some time. But the technology for integrating this frequency regulation with the motor in a small and handy format has been available only since 1997. VFD (Variable-Frequency Drives) units are relatively expensive, but unit size reduction and mass production are gradually lowering costs.

Modern VFDs produce the variable frequency output by a process called Pulse Width Modulation (PWM). It converts intermediate DC voltage to the synthesized AC voltage that drives the motor. Unfortunately, it is tricky to digitally produce perfect sinus waves. The PWM wave does not come anywhere near resembling a sinus wave, as can be seen in figure 4:1. The Amplitude (A) is adapted to the motor´s requirements, and the pulse width (W) is varied according to the speed requirements of the motor, and that´s it! Feeding the motor this kind of current causes great strain to the motor, and has other drawbacks as well.

Digital components can of course produce sinus waves, but it is difficult to make them perfect. They generate radio frequency electrical energy because of their "jagged" appearance, as can be seen in A in figure 4:2. Radio Frequency Interference (RFI) along the cable from the VFD to the motor may exceed FCC requirements for digital equipment, especially if the installation is not carefully balanced and grounded. Imperfect sinus waves also cause strain on electrical components and wear down the insulation of motor wirings.
Figure 4:1
Figure 4:2
The FLYWAY® system uses asynchronous motors controlled by a speed regulator which uses VFD and generates superior-quality sinus waves, as shown in B in figure 4:2. It is a Swedish patented invention from NFO Drives that successfully deals with the problems mentioned above. Thus, it:
  • Does not create electro-magnetical interference
  • Is more efficient than other VFD-systems
  • Is actually cheaper than other VFD-systems
  • Provides more durable service than other VFD-systems

5. Methodes of Propulsion

T he FLYWAY beamcars can be propelled by 3 alternative means:
  1. Asynchronous electrical motors with variable-frequency control driving on 4 rubber-tire wheels, as outlined in the foregoing chapter
  2. LIM, using magnets to propel the car. The propulsion car has 4 rubber-tire wheels, but they are not used for traction
  3. LIM, in combination with Magnetic Levitation, where the propulsion car has no wheels for running. The shunting is in this case performed in a different manner than that outlined on this page.
FLYWAY´s propulsion cars use rubber wheels, as they provide for better traction and less noise than steel wheels. They have to negotiate sloping beams, and the steepness allowed for those beams is determined by motor strength and possibly traction.		 simple illustration of the LIM propulsion principle Traction, then, is only a limiting factor in the first case, i.e. when asynchronous motors for propelling the wheels are used. In this case, the slope of the beams are limited to 5o. If linear motors (LIMs) are used, traction is not a limiting factor, and the slope could be increased to 10o, possibly more.

Magnetic Levitation is a technology which probably has a great future. It is as yet rather expensive. The FLYWAY® system will include MagLev if customers so desire. In this case, there are a couple of US patents that are quite promising, as regards performance and affordability.

6. The Swiveling Function

Anfang t is often advantageous to be able to swivel the carriage sideways 90 degrees or maybe even more. Take, for instance, the loading of motorcars at a terminus or along highways (figure 6:1), to allow motorists to hike a few miles towards their destination, while saving gas. It would take a long time of vigorous construction before the beam network reaches everywhere in a metropolitan area. So, motorists will need to take their cars along to carry them the extra miles back and forth, along the roads which the beam network does not cover.
Figure 6:1
 Figure 6:1 shows 4 beamcars (depicted in red) lowering, swivelling, and taking on motorcars (blue) in parallel, thus speeding up the loading/unloading process. You can read about its technical functionality on this page.

The FLYWAY® system will use a patented technology to swivel the cabins and other types of loads 180 degrees around. Thus, passenger could always travel facing forwards (or backwards) regardless in which direction the propulsion car is travelling.

7. Regulating the Speed

The following factors influence the speed of the beamcar:
  1. The maximum allowable speed on the current beam section (info from local node).

  2. Any restrictions that apply for this car during this journey (info from central computer and/or locally stored information in the car´s computer).

  3. Whether it has to negotiate a shunt, with consequential changing of direction (info from beam sensor and from stored information regarding this trip).

  4. Whether it goes through a sharp turn (info from beam sensor).

  5. Whether it´s about to stop (info from beam sensor and from stored information regarding this trip).

  6. If it receives an alert from local node, central computer or other vehicle that immediately influenses its speed (complementary stored information regarding this trip).

  7. If the radar on the carriage notes an unexpected obstacle in its path.

  8. If running in a timeslot; directives from the local node computer.

  9. If the radar on the propulsion car detects a vehicle ahead (illustration at right).
Propulsion cars with obstacle detectors
Figure 7:1

The internal radar inside the beam functions as an electronic bumper. It also has a doppler function, enabling the car to calculate the speed of an encountered car by measuring their relative speeds and knowing its own speed. The beamcar then behaves as a human driver; it regulates its own speed so that it keeps a safety distance commensurate with the current speed, i.e. the higher the speed, the longer the required distance to the car up ahead. The doppler function will in all likelihood be implemented by comparing successive measurements.

For trunkbeams, we calculate with speeds upwards of 140 km/hour (corresponding to 90 miles/hour). Depending on how the obstacle detection is implemented, it has been calculated that the safety distance for that speed should be at least 120 meters. Generally, this internal radar should be able to see far enough. Long, straight beams should not present any impediment to these 120 meters.

Regulating the speed when the beam bends
Figure 7:2
 Anfang urving beams usually means that the speed will have to be reduced, thus lowering the requirements as to how far the radar has to see. Obstacle detection for FLYWAY is examined more in detail on another page.

It could be that, as soon as the beam bends, the allowable maximum speed would be reduced to that commensurate with visibility inside the beam. The beamcar could be told by sensors whenever this allowed speed is altered, or the inside of the outer wall of the bending beam segment could have reflectors, telling the car´s radar that the beam is bending, and how far away this is. This speed reduction is a policy matter, however, since there is also an obstacle detction system on the carriages themselves.

8. Braking the Car

Anfang he beamcar must (of course!) be able to brake, both in order to regulate speed and to stop at stations, and in emergency situations. There are situations such as:
  1. When passing a booking point and being ordered by the node to regulate speed
  2. Approaching a shunt
  3. Going through a curve
  4. Passing a downward-sloping beam
  5. Stopping at a station
  6. Handle an emergency.
At such times, the motor power is reduced or, in the case of an VFD-controlled Synchronous AC-motor, the frequency of the supplied power is reduced.		 The ensuing mechanical torque is fed back to the power rail. The propulsion motor should be designed to generate enough braking torque to suffice in most cases. This not only conserves energy (by feeding it back into the power rails) but also saves on the wear of the mechanical brakes.

Under certain circumstances, however, the beamcar might be required to brake at maximum force to prevent a possible accident. This "maximum force" has to take due consideration to possible passengers; an empty car could brake even harder. It is stated above about the traction motor that "It should be dimensioned to handle the emergency braking required". This means that if the electrical power is cut altogether, the beamcar will breake at a rate of approximately 2g (= 20 meters/second2). To complement this, there is also a mechanical brake, functioning in principle like the illustration at right, and applied on all four wheels of the propulsion car.

Illustration of mechanical wheel brake
Figure 8:1: Mechanical brake assembly
When the beamcar has to emergency-brake, the slow-down speed is monitored. If it isn´t sufficiently quick, the mechanical brakes will be applied. The braking force would be applied at F in the above figure by means of, for instance, in electrical relay.

9. Design Considerations

Anfang he propulsion car cannot be designed as a rectangular wagon, with a wheel in each corner, as a regular railway bogey car.
Because the beam bends and slopes here and there,
  • the front and back axis have to be mobile relative to each other
  • the "waist" has to be a bit slimmer than a straight beam would allow
Were it not for the lift, the propulsion car could probably be short enough to be constructed with one pivot point. But with a lift machinery to carry, more space is needed. With two pivot points the car can be longer and thus carry a bigger load, while still allowing sharp turns in the beam, in all directions; vertically and horizontally.

One could make 3 general designs for the propulsion car.

  1. one-pivot with a central platform (figure 9:1)
  2. one-pivot with two platforms (figure 9:2)
  3. two-pivot with one platform in between (figure 9:3)
The one-pivot design in figure 9:1 is the cheapest and simplest, and it regulates itself so that the platform with the lift machinery (shown in orange) always stays in the middle of the beam (i.e. it is always positioned over the slit). This platform cannot be bigger than shown, since the beam bends both ways (i.e. both left and right). It takes some calculations to find the optimum size. The length of the shafts A relative the width of the beam Wb has an optimum where the width of the platform Wp is as it widest. Providing the platform with rounded corners would make it larger, but it still is not deemed possible to make it big enough.

The one-pivot design with 2 platforms, as shown in figure 9:2 is better than the foregoing, since it provides more platform space. The lift machinery would then have to be placed on either one of these platforms.

The two-pivot design, as shown in figure 9:3, is an even better solution. The optimum length L of this platform would be the length that provides the largest area, and this length is limited by:

  • its width relative to the width of the beam
  • its width relative to the shortest radius of curvature of the beam.
The only problem with this design, compared to the one-pivot design, is that the center of the platform x does not stay over the beam´s slit in curves.

We want the Area A = L * Wp to be as large as possible. Let´s apply some trigonometric thinking and see how big we can make A as a function of bending radius Rb and beamwidth Wb. The beam´s bending radius is properly calculated from the center of the beam, where the slit is.

From the triangle in figure 9:3 we can see that:

(Lp)2/4 + (2Rb - Wb + Wp)2/4 = (2Rb)2/4

which comes to:

(Lp)2= 4RbWb - 4RbWp - Wb2 - Wp2 + 2WbWp

Assuming a smallest bending radius of 6 meters and one of the smallest FLYWAY beamwidths of 0.80 meters, we get:

(Lp)2= 19.2 - 24*Wp - 0.64 - Wp2 + 1.6*Wp

=> (Lp)2= 18.56 - 22.4*Wp - Wp2

deriving dA/dWp = 0, one gets Wp = 0.54 meters as an optimal platform width.

From the equations above we get the otimum dimensions as:

Lp = 2.48 m, Wp = 0.54 m and A = 2.48 * 0.54 = 1.34 m2
The propulsion car in a bending beam; One pivot point, one platform
Figure 9:1: One pivot point, one platform
The propulsion car in a bending beam; One pivot point, two platforms
Figure 9:2: One pivot point, two platforms
The propulsion car in a bending beam; Two pivot points, one platform
Figure 9:3: Two pivot points, one platform
The propulsion car in a bending beam; trigonometrical calculations
Figure 9:4: Trigonometry
Now, Wp has to take account for the fact that the beamsdes has some thickness which we disregarded here. But 0.54 m is sufficiently small to a handle that. Another matter is that L is larger than Lp, since the pivot points are overlapped by the platform. How much that overlap is depends in part on whether the platform has rounded corners (which seems like a good idea), but if we add 0.26 m at each end, we would get L = 2*0.26 + 2.48 = 3 meters, and the Area comes to about 1.5 m2.

Slit in the Platform
Anfang he platform center in figure 9:2 does not stay centered over the beam slit when going through curves. If only one vertical suspender is used for carrying the carriage below, this has to be compensated for by a transverse slit in the platform, to allow the elevator cables to hold on to the cabin or carriage without undue strain, as shown in figure 9:4. If two vertcal suspenders are used (each positioned near the wheel axis) then this slit will of course not be required.

To top of Page If the free length of the elevator cables inside the beam is sufficient, they can move of their own volition, back and forth, and need only be protected from abrasive forces from the slit linings by a round holder underneath the propulsion car. If the free cable length is not sufficient for this, the round holder has to move the cables be following the slit as it moves back and forth relative the car. This can be achieved mechanically by letting the holder travel on its own wheels underneath the car (but still inside the beam.

 The propulsion car in a bending beam; accomodating a platform slit
Figure 9:5: The slit in the Platform

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Last Updated: 2007-01-17
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