VI
THE UNIT CARRIER
Having
established that we are firmly in the camp of those who favor pallets (that is
the last time we will use this term), it is time to examine this creature.
A.
General
Characteristics
The
unit carrier, or simply carrier, is the heart of the system. As we indicated in Section II, each carrier
will:
a.
Have the means to propel itself along the guideway and know where it is
within the system. It will know how to
get from any entrance station to any exit station on the system, including
alternate routes; and have the means to effect transfer and exit switching.
b.
Posses a means to communicate with nearby vehicles, as well as the
transported vehicle. The vehicle will
also be able to communicate directly with each of the several sector monitors
tracking all activity along the system.
c. Have appropriate sensors to know
its position relative to others, including relative velocities. The carrier will also possess a longer-range
sensor to sweep ahead and insure that no obstacles or obstructions are present
on the guideway.
Clearly these are not separate,
individual activities - they must be effectively coordinated and
controlled. Thus an onboard computer is
required for this task. We anticipate
the central processor of almost any present-day PC would be more than
adequate. It is not our intention to
detail the inner workings and relationship between the computer and the
activities it controls; but rather to list a general description and qualifying
factors of these activities. We begin
with a description of the mechanical properties including the traction
motors. We then consider guidance and
switching, including how and when the carrier effects it. And finally we consider the communications
and sensors that are required for the intelligent
control of the carrier.
All carriers will have identical
equipment and capabilities, even though
some may be employed only when the carrier is acting as a packet leader,
i.e., the lead carrier in the packet.
B. Mechanical
As
for basic mechanical construction, there would seem to be no need for any
special consideration beyond present medium-duty truck or light-rail
practice. The planform of the carrier
is only slightly larger than the size of a full size sedan, or van, and need
only be rugged enough to carry that type of load. It is not anticipated that anything special in the way of a
shock or suspension system is required, as this would seem merely to be
duplicating that of the transported vehicle.
It is expected that the lifetime of the basic carrier would be 30 years
or more, with electronics and traction motors being repaired or replaced
several times in that interval.
a. Traction Unlike present light-rail practice, the traction, or
support, wheels will no doubt be
separate from the guide wheels or other means of guidance. Accordingly, to
facilitated switching, both on the main line and stations, the traction wheels
would not be equipped with standard flanges.
The
power for propulsion would be electric, with distribution either direct
current, or standard 60 Hz alternating current; depending on the type of
traction motor chosen. In either
instance, we anticipate a type of third rail distribution system. To minimize contact with high voltage power,
the power for operation within stations is supplied from an onboard battery.
The
choice of a traction motor is not critical to system operation. It can be a standard d-c traction motor. Or
alternatively, it can be a synchronous a-c motor which would require an on
board variable frequency inverter.
In
these, tractive forces are transmitted through the support wheels. The normal limitation to a 3% grade found in
heavy railroad applications is not operative here; all the wheels are traction
wheels. In that sense, this is a light
rail operation. Moreover, the contact
surface of neither the support wheels, nor the guideway is limited to
steel. We see no critical need for
pneumatic tires, however.
Alternatively,
some favor the use of a linear synchronous motor (LSM) for this task. This has the attractive capability of
providing the tractive force directly by the electro-magnetic field between the
two halves of the motor; one in the carrier, the other as part of the
guideway. Unfortunately, this would
most likely be prohibitively expensive.
Nevertheless, developments in this area should e monitored
carefully.
b. Brakes In normal operation, the requirements for these are no more demanding than of other transit vehicles. The motor can provide some dynamic breaking capability, whether rotary of linear.
In emergency application, the friction forces can be augmented by means of the guidance apparatus. When pulled into contact with the guidance rail, considerable additional braking forces can be provided.
c.
Coupler and Impact Absorbers
These provide the mechanism for coupling carriers together in packet
operation. It is essential that these provide
only a longitudinal force. This, so
when exiting, In the unlikely occasion that an exit distance has not been
established, these devices must nevertheless accommodate a departure of the
carrier from the main line without the imposition of any appreciable transverse
force.
Flexible, compliant
impact absorbers, or bumpers, are integral to this assembly. These should provide sufficient protection
such that a low-speed impact should cause no appreciable damage to the
transported vehicle or its occupants.
d.
Packet operation As detailed in
Section IV packet operation is central to the efficient utilization of the
carrier. While operating as a member of
a packet, both the traction motor and the brake system are under the control of
the packet leader. When appending to
the packet, control is retained by the carrier; it surrenders control only
after the coupling process is completed.
Either the forward or following carrier can initiate a disconnect,
whereupon the carrier automatically regains control. This is essential for departure from the packet.
e.
Vehicle Restraint And finally,
some sort of automatic wheel chocks or other restraining device would be required to secure the transported
vehicle.
C. Guidance and Switching
As
we have indicated previously, unlike conventional rail practice, control of the
carrier along the guideway is a separate function from support of the
carrier. This is necessitated by the
zero-headway contemplated within the packet.
Accordingly, the main line guideway is entirely static; it would
physically impossible to move the track from one destination to another between
successive carriers.
As
a carrier approaches a switching point, an auxiliary guidance rail is provided
by the system. It is incumbent upon the
carrier to know where it is and whether or not it wishes to utilize it. This property of the carrier to know when
and how to effect a transfer is critical.
This
separate guidance rail is provided to guide the carrier to alternate destinations.
As shown schematically in Fig 1 a guidance arms extend downward from the
carrier and have the means to “grasp” the rail. We use the term grasp in quotation marks because, in normal
operation, there is no physical contact with the rail.
In practice, the lower part
of the arm is maintained in proximity to the guidance rail with its position
relative to the rail sensed electronically.
The position of the guidance arm does not directly control steering. The arm is allowed to follow the normal
variations in track position, while the high frequency movement is damped out
by the steering system, providing a smooth ride to the passengers.
Fig 1 Plan view of the relationship between the main
and transfer guidance rails
This
“grasping” action is backed up by rollers that under normal conditions do not
come into contact with the rail. Should
the electronic sensors fail, these limit the displacement relative to the
rail. In addition to insuring that the
arm remains in proximity to the guidance rail, this provides two important
safety-related functions. As we
indicated, if for whatever reason, additional braking is required, applying an
upward force through the apparatus brings the brake pads in contact with the
rail and thus provides the additional friction, both directly and through the
normal brake system. Moreover, in the
unlikely event of an accident, this will work to retain the carrier on the
guideway.
a.
Switching Sequence As shown, initially
the auxiliary rail runs parallel to the main rail, and thus, in principal,
grasping either would result in maintaining the carrier on the guideway. However, as the auxiliary rail will
eventually diverge and guide the carrier to an alternate destination, It is
essential that either one or the other be in control; never both. Each of the two halves of the grasper can be
operated independently. As described in
the following, this proves useful in the switching operation. The sequence of release and grasp occurs in
the following order. Please refer to
Fig. 2. Please also note that Fig. 2 is
intended to indicate the sequence of switching, not the details of construction.
Fig 2 a Stage 0 Carrier under control of main arm
Stage 0: Initially,
both the left and right halves of the main arm grasp the rail, and the carrier
is under control of the main rail
.
Fig 2 b Stage 1 Initial contact with transfer
arm
Stage
1: The left half of the auxiliary arm
engages the auxiliary rail. If for some
reason the sequence stopped, when the auxiliary departs, this will not over
define control.
Fig 2 c Stage 2 Control of carrier shared with main and transfer arms
Stage
2: With the left half of the auxiliary arm
in place, arm the left half of the main arm will release. During the parallel phase of the operation,
this condition will maintain carrier direction. In addition, should the subsequent action not occur, a dummy
transfer rail is provided beyond the transfer point to maintain directional
control till an emergency stop is effected.
.
Fig 2 d Stage 3 Carrier under control of transfer
arm
Stage
3: With the completion of stage 2, the
right half of the auxiliary arm will engage the auxiliary rail. With this action, the auxiliary rail is in
control. No conflict will occur, even
if the next does not take place.
Fig 2 e Stage 4 Main arm completely withdrawn
Stage 4 With the auxiliary arm in control, the right half of the main arm will release, and the carrier is under control of the transfer rail.
The
carrier is now completely under control of the auxiliary rail and is guided to
an alternate guideway. It should be
noted that this entire action will take place in considerable less time that it
takes to describe it,
After
the carrier has completely left the main line, the sequence is reversed and the
main rail of the new guideway reclaims control. The carrier is now ready for any subsequent transfers. However, for a guideway that serves merely
as a bridge between to main guideways, it may be simpler to effect the
transition to the new guideway by maintaining control under the auxiliary rail.
This
same sequence could also effect a left exit with an auxiliary rail to the
left. The carrier is equipped with an
equivalent set of graspers on the left as well.
b.
Location Knowledge For all this
to be effective, the carrier must know at all times where it is. We suggest a system to augment the Global
Positioning System (GPS). Each carrier
would have a GPS receiver. The
positional accuracy (although recently improved for civilian users) may not be
sufficient. Thus we suggest of system
of markers, either passive or active, to supplement this system. In this, it would be essential that these
markers be sufficiently far apart that the identity, and thus the exact
location, of each would be unambiguous.
By this means the carrier can update his exact position to within a few
millimeters. Moreover, as the distance
between successive markers would be known, it would then be possible to
continuously update the calibration of an onboard odometer. In this way, the carrier could remain
certain of its location within centimeters.
D. Carrier Communications
a.
Intra-packet Communication
Reliable intra-packet communication is
essential to system operation.
As noted above, in packet operation, the packet leader controls both
motor and brakes of each member of the packet.
Moreover, when leaving, individual carriers must notify the packet to
provide an exit distance. When
appending to the packet, individual carriers must have a means to notify the
packet of its intention. All emergency
instructions initiated by the packet leader are also communicated to the packet
by this system..
It
is essential that any communication within a packet is not intercepted and
acted on by another packet. Thus,
intra-packet communications should probably be by means of a full-duplex
digital bucket brigade. If the packet
leader needs to signal the packet, it initiates the signal and passes it to the
next carrier behind it. The second carrier
will pass to the carrier behind it and so forth. The last carrier will acknowledge the signal and pass the
original signal forward, with each carrier in turn passing forward. The lead carrier then compares the signals
and (assuming they are identical) knows that all have received and acknowledged. The same means of communication will be used
in an exiting situation except that the exiting carrier will originate the
signal.
The
specific means for transmission between carriers can be any of several,
possibly infrared or high frequency microwave.
The primary requirement is the sending and receiving antennae (these
will undoubtedly be physically the same, but will alternate functions) be in
close proximity for contiguous carriers, providing negligible leakage
radiation. In any event transmission
delay should be of the order of milliseconds, at most. Thus signals can be exchanged and acknowledged
in times considerably shorter than that required for any mechanical
action.
b.
Interaction with the Transported Vehicle There are some special considerations required for interacting
with the transported vehicle - the vehicle must indicate where it wants to
go. This can be accomplished either
directly or by indirect means such as low power radio, or other means. Communicating between the carrier and
vehicle, while on board, pose no particular difficulty or restriction. However, if we also wish to use the same
port for local communication between the vehicle and the station before loading
onto the carrier, this suggests an emphasis on low power radio, as in cordless
telephones.
However,
there is a need for direct contact. As
previously indicated, we propose an on board charging unit for electric
vehicles. Moreover, during weather
extremes, we anticipate a need to supply energy for heating and/or air conditioning. If we require the transported vehicle to
supply its own, then we partially defeat our environmental objectives, at least
for internal combustion powered vehicles.
(We could just be mean, and let them suffer; but in the interest of
gaining public acceptance, providing this additional service is probably a
useful concept. Particularly, since we
can effect the means to charge for it.)
Accordingly,
for regular commuters, it seems reasonable that minor modifications could be
made to the private vehicle to provide both a communication port and a standard
power connector. The heating/air
conditioning units might be either separate or a modification to existing ones. A standard would be required only for the connector;
leaving for the commuter, or any cottage industry that develops, the specifics
of the modification.
For
the occasional user, perhaps a separate auxiliary apparatus which connects to
the carrier and attaches to the vehicle much like the speaker in a drive-in
movie theater.
c. Communication
with the Station and the Larger System
We also see a need for the carrier to be able to communicate with the
local station. A station-to-carrier net
provides communication both directly to the station, while the carrier is
actually in the station; and to the appropriate sector monitor when it is
not. This would also be used to
communicate with the launching, separation, and transfer functions.
Because
of this somewhat multi-faceted role, and in order to minimize spectrum, we
would anticipate that this station-to-carrier net would operate very much like
a digital cellular telephone system.
The traffic should be relatively light; using only cryptic commands that
are expanded and acted upon by the receiver.
The system would not directly involve the driver, and involves no voice
communication in and of itself. However
it might direct the communication port with the transported vehicle to inform
the driver of any changes or other pertinent information. In the event of any unusual circumstances or
emergency, information or instructions could be sent to either to the entire
system, segments of it, or to a specific carrier.
E. Position Sensors and Collision Avoidance
There are
two, more or less, separate requirements for position sensors. These are those required for packet
operations, and those required to insure that the guideway ahead in unimpeded.
a.
Packet Sensors These are
essential for safe and effective packet operation. Each carrier is provided with the means to know the relative
position and velocity of adjacent carriers.
When forming or re-forming the packet, the carrier, as it approaches a
forward carrier to effect a coupling, must have accurate position data accurate
to within millimeters. Thus, sensor
capability that extends out a few tens of meters may have to be augmented with
a close-in capability to be used when actually making the coupling.
As the carrier is capable of operating in either direction, these
are required both forward and backward.
This backward sensing also is used as a redundant check on appending
carriers; sounding an alarm if the approach velocity is too rapid.
A likely candidate for this
application would be millimeter microwaves.
b.
A Longer Range Capability and Collision Avoidance It is imperative that the carrier, more
properly the packet leader in most instances, be provided with the means to
insure that there are no obstacles or other impediments to safe passage. To some degree this involves proving a
negative. Thus it would be insufficient for merely the absence of a
return signal indicating danger. There
must be a positive signal indicating this absence.
Accordingly, this suggests some sort of
artificial intelligence, or more accurately, an expert system may be
required. In this we can ease the
burden somewhat by painting either the guideway itself, or adjacent markers,
with a highly reflective surface. We
can also consider the possibility of infrared but this would involve providing a temperature
differential, and possibly a large consumption of power.
At any rate, a camera of some sort
that provides an uninterrupted view of the guideway out to something of the
order of 100 meters, coupled with some intelligent signal processing might be
sufficient. Of course, the camera would
need to be sufficiently elevated such that looking out ahead to the guideway,
elevated obstructions (i.e., above the track) would also be detectable.
In addition, the relative velocity of
any thing that is detected must also
be determined.[1] In particular, it should always be possible
to detect the preceding packet (if one exists). If an appropriate headway exists, and the forward packet is
proceeding at system speed, no action is required. In other words, the packet leader must be provided with
sufficient information to make the intelligent decision, rather than a drastic
one.
It is
also a requirement that this capability can be applied to an adjacent guideway.
That is, as a carrier is about to merge
into the main (or other) line, it must be possible to determine that there is
adequate room.
Providing
an adequate collision avoidance system is not a trivial task. A number of academic and commercial
laboratories are working on this problem, and we are confident that a solution
will be forthcoming. However, we are
not sufficiently confident, at this time, to identify a particular
approach. It is clearly an area that
deserves early investigation.